Shear-free flows of a perfect fluid
International Nuclear Information System (INIS)
Barnes, A.
1984-01-01
Previous results on shear-free fluid flows are reviewed. Attention is then restricted to flows which satisfy the additional condition that the Weyl tensor is purely electric: Hsub(ac)=*Csub(abcd)usup(b)usup(d)=O where usup(a) is the velocity vector of the fluid. The analysis is also applied to Einstein spaces admitting a shear-free timelike vector field usup(a). If the Weyl tensor is purely electric or purely magnetic, it is shown that usup(a) is necessarily irrotational unless the spacetime has constant curvature. (author)
Homogeneous purely buoyancy driven turbulent flow
Arakeri, Jaywant; Cholemari, Murali; Pawar, Shashikant
2010-11-01
An unstable density difference across a long vertical tube open at both ends leads to convection that is axially homogeneous with a linear density gradient. We report results from such tube convection experiments, with driving density caused by salt concentration difference or temperature difference. At high enough Rayleigh numbers (Ra) the convection is turbulent with zero mean flow and zero mean Reynolds shear stresses; thus turbulent production is purely by buoyancy. We observe different regimes of turbulent convection. At very high Ra the Nusselt number scales as the square root of the Rayleigh number, giving the so-called "ultimate regime" of convection predicted for Rayleigh-Benard convection in limit of infinite Ra. Turbulent convection at intermediate Ra, the Nusselt number scales as Ra^0.3. In both regimes, the flux and the Taylor scale Reynolds number are more than order of magnitude larger than those obtained in Rayleigh-Benard convection. Absence of a mean flow makes this an ideal flow to study shear free turbulence near a wall.
Agius, Matthew R.; Lebedev, Sergei
2017-09-01
Of the two debated, end-member models for the late-Cenozoic thickening of Tibetan crust, one invokes 'channel flow' (rapid viscous flow of the mid-lower crust, driven by topography-induced pressure gradients and transporting crustal rocks eastward) and the other 'pure shear' (faulting and folding in the upper crust, with viscous shortening in the mid-lower crust). Deep-crustal deformation implied by each model is different and would produce different anisotropic rock fabric. Observations of seismic anisotropy can thus offer a discriminant. We use broad-band phase-velocity curves-each a robust average of tens to hundreds of measurements-to determine azimuthal anisotropy in the entire lithosphere-asthenosphere depth range and constrain its amplitude. Inversions of the differential dispersion from path pairs, region-average inversions and phase-velocity tomography yield mutually consistent results, defining two highly anisotropic layers with different fast-propagation directions within each: the middle crust and the asthenosphere. In the asthenosphere beneath central and eastern Tibet, anisotropy is 2-4 per cent and has an NNE-SSW fast-propagation azimuth, indicating flow probably driven by the NNE-ward, shallow-angle subduction of India. The distribution and complexity of published shear wave splitting measurements can be accounted for by the different anisotropy in the mid-lower crust and asthenosphere. The estimated splitting times that would be accumulated in the crust alone are 0.25-0.8 s; in the upper mantle-0.5-1.2 s, depending on location. In the middle crust (20-45 km depth) beneath southern and central Tibet, azimuthal anisotropy is 3-5 and 4-6 per cent, respectively, and its E-W fast-propagation directions are parallel to the current extension at the surface. The rate of the extension is relatively low, however, whereas the large radial anisotropy observed in the middle crust requires strong alignment of mica crystals, implying large finite strain and
Bhateja, Ashish; Khakhar, Devang V.
2018-06-01
We consider the rheology of steady two-dimensional granular flows, in different geometries, using discrete element method-based simulations of soft spheres. The flow classification parameter (ψ ), which defines the local flow type (ranging from pure rotation to simple shear to pure extension), varies spatially, to a significant extent, in the flows. We find that the material behaves as a generalized Newtonian fluid. The μ -I scaling proposed by Jop et al. [Nature (London) 441, 727 (2006), 10.1038/nature04801] is found to be valid in both two-dimensional and unidirectional flows, as observed in previous studies; however, the data for each flow geometry fall on a different curve. The results for the two-dimensional silo flow indicate that the viscosity does not depend directly on the flow type parameter, ψ . We find that the scaling based on "granular fluidity" [Zhang and Kamrin, Phys. Rev. Lett. 118, 058001 (2017), 10.1103/PhysRevLett.118.058001] gives good collapse of the data to a single curve for all the geometries. The data for the variation of the solid faction with inertial number show a reasonable collapse for the different geometries.
International Nuclear Information System (INIS)
Ware, A.S.; Diamond, P.H.
1993-01-01
The effects of a poloidally asymmetric ionization source on both dissipative toroidal drift wave stability and the generation of mean sheared parallel flow are examined. The first part of this work extends the development of a local model of ionization-driven drift wave turbulence [Phys. Fluids B 4, 877 (1992)] to include the effects of magnetic shear and poloidal source asymmetry, as well as poloidal mode coupling due to both magnetic drifts and the source asymmetry. Numerical and analytic investigation confirm that ionization effects can destabilize collisional toroidal drift waves. However, the mode structure is determined primarily by the magnetic drifts, and is not overly effected by the poloidal source asymmetry. The ionization source drives a purely inward particle flux, which can explain the anomalously rapid uptake of particles which occurs in response to gas puffing. In the second part of this work, the role poloidal asymmetries in both the source and turbulent particle diffusion play in the generation of sheared mean parallel flow is examined. Analysis indicates that predictions of sonic parallel shear flow [v parallel (r)∼c s ] are an unphysical result of the assumption of purely parallel flow (i.e., v perpendicular =0) and the neglect of turbulent parallel momentum transport. Results indicate that the flow produced is subcritical to the parallel shear flow instability when diamagnetic effects are properly considered
Degradation of homogeneous polymer solutions in high shear turbulent pipe flow
Elbing, B. R.; Winkel, E. S.; Solomon, M. J.; Ceccio, S. L.
2009-12-01
This study quantifies degradation of polyethylene oxide (PEO) and polyacrylamide (PAM) polymer solutions in large diameter (2.72 cm) turbulent pipe flow at Reynolds numbers to 3 × 105 and shear rates greater than 105 1/s. The present results support a universal scaling law for polymer chain scission reported by Vanapalli et al. (2006) that predicts the maximum chain drag force to be proportional to Re 3/2, validating this scaling law at higher Reynolds numbers than prior studies. Use of this scaling gives estimated backbone bond strengths from PEO and PAM of 3.2 and 3.8 nN, respectively. Additionally, with the use of synthetic seawater as a solvent the onset of drag reduction occurred at higher shear rates relative to the pure water solvent solutions, but had little influence on the extent of degradation at higher shear rates. These results are significant for large diameter pipe flow applications that use polymers to reduce drag.
Steady flow on to a conveyor belt - Causal viscosity and shear shocks
Syer, D.; Narayan, Ramesh
1993-01-01
Some hydrodynamical consequences of the adoption of a causal theory of viscosity are explored. Causality is introduced into the theory by letting the coefficient of viscosity go to zero as the flow velocity approaches a designated propagation speed for viscous signals. Consideration is given to a model of viscosity which has a finite propagation speed of shear information, and it is shown that it produces two kinds of shear shock. A 'pure shear shock' corresponds to a transition from a superviscous to a subviscous state with no discontinuity in the velocity. A 'mixed shear shock' has a shear transition occurring at the same location as a normal adiabatic or radiative shock. A generalized version of the Rankine-Hugoniot conditions for mixed shear shocks is derived, and self-consistent numerical solutions to a model 2D problem in which an axisymmetric radially infalling stream encounters a spinning star are presented.
Multifractal spectra in shear flows
Keefe, L. R.; Deane, Anil E.
1989-01-01
Numerical simulations of three-dimensional homogeneous shear flow and fully developed channel flow, are used to calculate the associated multifractal spectra of the energy dissipation field. Only weak parameterization of the results with the nondimensional shear is found, and this only if the flow has reached its asymptotic development state. Multifractal spectra of these flows coincide with those from experiments only at the range alpha less than 1.
Vesicle dynamics in shear and capillary flows
International Nuclear Information System (INIS)
Noguchi, Hiroshi; Gompper, Gerhard
2005-01-01
The deformation of vesicles in flow is studied by a mesoscopic simulation technique, which combines multi-particle collision dynamics for the solvent with a dynamically triangulated surface model for the membrane. Shape transitions are investigated both in simple shear flows and in cylindrical capillary flows. We focus on reduced volumes, where the discocyte shape of fluid vesicles is stable, and the prolate shape is metastable. In simple shear flow at low membrane viscosity, the shear induces a transformation from discocyte to prolate with increasing shear rate, while at high membrane viscosity, the shear induces a transformation from prolate to discocyte, or tumbling motion accompanied by oscillations between these two morphologies. In capillary flow, at small flow velocities the symmetry axis of the discocyte is found not to be oriented perpendicular to the cylinder axis. With increasing flow velocity, a transition to a prolate shape occurs for fluid vesicles, while vesicles with shear-elastic membranes (like red blood cells) transform into a coaxial parachute-like shape
Non-homogeneous flow profiles in sheared bacterial suspensions
Samanta, Devranjan; Cheng, Xiang
Bacterial suspensions under shear exhibit interesting rheological behaviors including the remarkable ``superfluidic'' state with vanishing viscosity at low shear rates. Theoretical studies have shown that such ``superfluidic'' state is linked with non-homogeneous shear flows, which are induced by coupling between nematic order of active fluids and hydrodynamics of shear flows. However, although bulk rheology of bacterial suspensions has been experimentally studied, shear profiles within bacterial suspensions have not been explored so far. Here, we experimentally investigate the flow behaviors of E. coli suspensions under planar oscillatory shear. Using confocal microscopy and PIV, we measure velocity profiles across gap between two shear plates. We find that with increasing shear rates, high-concentration bacterial suspensions exhibit an array of non-homogeneous flow behaviors like yield-stress flows and shear banding. We show that these non-homogeneous flows are due to collective motion of bacterial suspensions. The phase diagram of sheared bacterial suspensions is systematically mapped as functions of shear rates an bacterial concentrations. Our experiments provide new insights into rheology of bacterial suspensions and shed light on shear induced dynamics of active fluids. Chemical Engineering and Material Science department.
Acoustic waves in unbounded shear flows
International Nuclear Information System (INIS)
Chagelishvili, G.D.; Khujadze, G.R.; Lominadze, J.G.; Rogava, A.D.
1996-05-01
The linear evolution of acoustic waves in fluid flow with constant density and uniform shear of velocity is investigated. The process of the mean flow energy extraction by the three-dimensional acoustic waves which is due to the non-normality of linear dynamics in shear flows is analyzed. The thorough examination of the dynamics of different physical quantities, specifying the wave evolution, is outlined. The revealing of the behaviour becomes possible owing to the nonmodal approach that has been extensively used in the study of the perturbations evolution in shear flows since the beginning of the nineties. In addition, a detailed analyses of the physics of shear energy gain by vortex and acoustic perturbations is presented. (author). 28 refs, 7 figs
Edge Sheared Flows and Blob Dynamics
Energy Technology Data Exchange (ETDEWEB)
Myra, J.; D' Ippolito, D.; Russell, D., E-mail: jrmyra@lodestar.com [Lodestar Research Corporation, Boulder (United States); Davis, W. M.; Zweben, S. [Princeton Plasma Physics Laboratory, Princeton (United States); Terry, J.; LaBombard, B. [Massachusetts Institute of Technology, Cambridge (United States)
2012-09-15
Full text: A study of sheared flows in the edge and scrape-off layer (SOL) and their interaction with blob-filaments is presented. Edge sheared flows are believed to be important for the L-H, and H-L transitions. Blob generation and dynamics impacts both the (near-separatrix) scrape-off-layer (SOL) width critical for power handling in the divertor, and the interaction of plasma in the far SOL with plasma-facing components. These topics are critical for ITER and future devices. A fluid-based 2D curvature-interchange model embedded in the SOLT code is employed to study these issues. Sheared binormal flows both regulate the power flux crossing the separatrix and control the character of emitted turbulence structures such as blob-filaments. At a critical power level (depending on parameters) the laminar flows containing intermittent, but bound, structures give way to full-blown blob emissions signifying a transition from quasi-diffusive to convective transport. In order to diagnose sheared flows in experiments and assess their interaction with blobs, a blob-tracking algorithm has been developed and applied to both NSTX and Alcator C-Mod data. Blob motion and ellipticity can be affected by sheared flows, and are diagnosed and compared with seeded blob simulations. A picture of the interaction of blobs and sheared flows is emerging from advances in the theory and simulation of edge turbulence, combined with ever-improving capabilities for edge diagnostics and their analysis. (author)
Exponential Shear Flow of Linear, Entangled Polymeric Liquids
DEFF Research Database (Denmark)
Neergaard, Jesper; Park, Kyungho; Venerus, David C.
2000-01-01
A previously proposed reptation model is used to interpret exponential shear flow data taken on an entangled polystyrenesolution. Both shear and normal stress measurements are made during exponential shear using mechanical means. The model iscapable of explaining all trends seen in the data......, and suggests a novel analysis of the data. This analysis demonstrates thatexponential shearing flow is no more capable of stretching polymer chains than is inception of steady shear at comparableinstantaneous shear rates. In fact, all exponential shear flow stresses measured are bounded quantitatively...
How shear increments affect the flow production branching ratio in CSDX
Li, J. C.; Diamond, P. H.
2018-06-01
The coupling of turbulence-driven azimuthal and axial flows in a linear device absent magnetic shear (Controlled Shear Decorrelation Experiment) is investigated. In particular, we examine the apportionment of Reynolds power between azimuthal and axial flows, and how the azimuthal flow shear affects axial flow generation and saturation by drift wave turbulence. We study the response of the energy branching ratio, i.e., ratio of axial and azimuthal Reynolds powers, PzR/PyR , to incremental changes of azimuthal and axial flow shears. We show that increasing azimuthal flow shear decreases the energy branching ratio. When axial flow shear increases, this ratio first increases but then decreases to zero. The axial flow shear saturates below the threshold for parallel shear flow instability. The effects of azimuthal flow shear on the generation and saturation of intrinsic axial flows are analyzed. Azimuthal flow shear slows down the modulational growth of the seed axial flow shear, and thus reduces intrinsic axial flow production. Azimuthal flow shear reduces both the residual Reynolds stress (of axial flow, i.e., ΠxzR e s ) and turbulent viscosity ( χzDW ) by the same factor |⟨vy⟩'|-2Δx-2Ln-2ρs2cs2 , where Δx is the distance relative to the reference point where ⟨vy⟩=0 in the plasma frame. Therefore, the stationary state axial flow shear is not affected by azimuthal flow shear to leading order since ⟨vz⟩'˜ΠxzR e s/χzDW .
SHEAR ACCELERATION IN EXPANDING FLOWS
Energy Technology Data Exchange (ETDEWEB)
Rieger, F. M. [ZAH, Institut für Theoretische Astrophysik, Universität Heidelberg, Philosophenweg 12, D-69120 Heidelberg (Germany); Duffy, P., E-mail: frank.rieger@mpi-hd.mpg.de, E-mail: peter.duffy@ucd.ie [University College Dublin, Belfield, Dublin 4 (Ireland)
2016-12-10
Shear flows are naturally expected to occur in astrophysical environments and potential sites of continuous non-thermal Fermi-type particle acceleration. Here we investigate the efficiency of expanding relativistic outflows to facilitate the acceleration of energetic charged particles to higher energies. To this end, the gradual shear acceleration coefficient is derived based on an analytical treatment. The results are applied to the context of the relativistic jets from active galactic nuclei. The inferred acceleration timescale is investigated for a variety of conical flow profiles (i.e., power law, Gaussian, Fermi–Dirac) and compared to the relevant radiative and non-radiative loss timescales. The results exemplify that relativistic shear flows are capable of boosting cosmic-rays to extreme energies. Efficient electron acceleration, on the other hand, requires weak magnetic fields and may thus be accompanied by a delayed onset of particle energization and affect the overall jet appearance (e.g., core, ridge line, and limb-brightening).
On the self-organizing process of large scale shear flows
Energy Technology Data Exchange (ETDEWEB)
Newton, Andrew P. L. [Department of Applied Maths, University of Sheffield, Sheffield, Yorkshire S3 7RH (United Kingdom); Kim, Eun-jin [School of Mathematics and Statistics, University of Sheffield, Sheffield, Yorkshire S3 7RH (United Kingdom); Liu, Han-Li [High Altitude Observatory, National Centre for Atmospheric Research, P. O. BOX 3000, Boulder, Colorado 80303-3000 (United States)
2013-09-15
Self organization is invoked as a paradigm to explore the processes governing the evolution of shear flows. By examining the probability density function (PDF) of the local flow gradient (shear), we show that shear flows reach a quasi-equilibrium state as its growth of shear is balanced by shear relaxation. Specifically, the PDFs of the local shear are calculated numerically and analytically in reduced 1D and 0D models, where the PDFs are shown to converge to a bimodal distribution in the case of finite correlated temporal forcing. This bimodal PDF is then shown to be reproduced in nonlinear simulation of 2D hydrodynamic turbulence. Furthermore, the bimodal PDF is demonstrated to result from a self-organizing shear flow with linear profile. Similar bimodal structure and linear profile of the shear flow are observed in gulf stream, suggesting self-organization.
Transport reduction via shear flow modification of the cross phase
International Nuclear Information System (INIS)
Ware, A.S.; Terry, P.W.; Diamond, P.H.; Carreras, B.A.
1996-01-01
As a model example of the effect of E x B shear flow on the cross phase between electrostatic potential and pressure fluctuations, a nonlinear theory of resistive pressure gradient driven turbulence (RPGDT) in a shear flow is presented. This work builds on numerical studies of RPGDT, which have shown that both flow shear and curvature can affect the cross phase as well as the fluctuation levels. In this work, we show that the effect of shear flow on transport can be expressed through the temporal response of pressure to potential. It is shown heuristically that even in the case where the fluctuation levels are not modified, the flow shear still acts to reduce the phase angle between potential and pressure fluctuations, thereby suppressing transport. The scaling of the cross phase with flow shear and flow curvature is presented. (author)
Microalga propels along vorticity direction in a shear flow
Chengala, Anwar; Hondzo, Miki; Sheng, Jian
2013-05-01
Using high-speed digital holographic microscopy and microfluidics, we discover that, when encountering fluid flow shear above a threshold, unicellular green alga Dunaliella primolecta migrates unambiguously in the cross-stream direction that is normal to the plane of shear and coincides with the local fluid flow vorticity. The flow shear drives motile microalgae to collectively migrate in a thin two-dimensional horizontal plane and consequently alters the spatial distribution of microalgal cells within a given suspension. This shear-induced algal migration differs substantially from periodic rotational motion of passive ellipsoids, known as Jeffery orbits, as well as gyrotaxis by bottom-heavy swimming microalgae in a shear flow due to the subtle interplay between torques generated by gravity and viscous shear. Our findings could facilitate mechanistic solutions for modeling planktonic thin layers and sustainable cultivation of microalgae for human nutrition and bioenergy feedstock.
Modeling and flow analysis of pure nylon polymer for injection molding process
International Nuclear Information System (INIS)
Nuruzzaman, D M; Kusaseh, N; Basri, S; Hamedon, Z; Oumer, A N
2016-01-01
In the production of complex plastic parts, injection molding is one of the most popular industrial processes. This paper addresses the modeling and analysis of the flow process of the nylon (polyamide) polymer for injection molding process. To determine the best molding conditions, a series of simulations are carried out using Autodesk Moldflow Insight software and the processing parameters are adjusted. This mold filling commercial software simulates the cavity filling pattern along with temperature and pressure distributions in the mold cavity. In the modeling, during the plastics flow inside the mold cavity, different flow parameters such as fill time, pressure, temperature, shear rate and warp at different locations in the cavity are analyzed. Overall, this Moldflow is able to perform a relatively sophisticated analysis of the flow process of pure nylon. Thus the prediction of the filling of a mold cavity is very important and it becomes useful before a nylon plastic part to be manufactured. (paper)
Modeling and flow analysis of pure nylon polymer for injection molding process
Nuruzzaman, D. M.; Kusaseh, N.; Basri, S.; Oumer, A. N.; Hamedon, Z.
2016-02-01
In the production of complex plastic parts, injection molding is one of the most popular industrial processes. This paper addresses the modeling and analysis of the flow process of the nylon (polyamide) polymer for injection molding process. To determine the best molding conditions, a series of simulations are carried out using Autodesk Moldflow Insight software and the processing parameters are adjusted. This mold filling commercial software simulates the cavity filling pattern along with temperature and pressure distributions in the mold cavity. In the modeling, during the plastics flow inside the mold cavity, different flow parameters such as fill time, pressure, temperature, shear rate and warp at different locations in the cavity are analyzed. Overall, this Moldflow is able to perform a relatively sophisticated analysis of the flow process of pure nylon. Thus the prediction of the filling of a mold cavity is very important and it becomes useful before a nylon plastic part to be manufactured.
Chirality-specific lift forces of helix under shear flows: Helix perpendicular to shear plane.
Zhang, Qi-Yi
2017-02-01
Chiral objects in shear flow experience a chirality-specific lift force. Shear flows past helices in a low Reynolds number regime were studied using slender-body theory. The chirality-specific lift forces in the vorticity direction experienced by helices are dominated by a set of helix geometry parameters: helix radius, pitch length, number of turns, and helix phase angle. Its analytical formula is given. The chirality-specific forces are the physical reasons for the chiral separation of helices in shear flow. Our results are well supported by the latest experimental observations. © 2016 Wiley Periodicals, Inc.
Energy Technology Data Exchange (ETDEWEB)
Bagherpour, E., E-mail: e.bagherpour@semnan.ac.ir [Faculty of Metallurgical and Materials Engineering, Semnan University, Semnan (Iran, Islamic Republic of); Department of Mechanical Engineering, Doshisha University, Kyotanabe, Kyoto 610–0394 (Japan); Qods, F., E-mail: qods@semnan.ac.ir [Faculty of Metallurgical and Materials Engineering, Semnan University, Semnan (Iran, Islamic Republic of); Ebrahimi, R., E-mail: ebrahimy@shirazu.ac.ir [Department of Materials Science and Engineering, School of Engineering, Shiraz University, Shiraz (Iran, Islamic Republic of); Miyamoto, H., E-mail: hmiyamot@mail.doshisha.ac.jp [Department of Mechanical Engineering, Doshisha University, Kyotanabe, Kyoto 610–0394 (Japan)
2016-06-01
In the present paper the role of shear reversal on microstructure, texture and mechanical properties of pure copper during a single pass of the simple shear extrusion (SSE) process was investigated. For SSE processing an appropriate die with a linear die profile was designed and constructed, which imposes forward shear in the first half and reverse shear in the second half channels. Electron back-scattering diffraction (EBSD), transmission electron microscopy (TEM) and scanning transmission electron microscopy (STEM) were used to evaluate the microstructure of the deformed samples. The geometrical nature of this process imposes a distribution of strain results in the inhomogeneous microstructure and the hardness throughout the plane perpendicular to the extrusion direction. Strain reversal during the process results in a slight reduction in dislocation density, the hardness and mean disorientation angle of the samples, and an increase in the grain size. After a complete pass of SSE, dislocation density decreased by ~14% if compared to the middle of the process. This suggests that the dislocation annihilation occurred by the reversal of the shear strain. The simple shear textures were formed gradually and the strongest simple shear textures were observed on the middle of the SSE channel. The degree of the simple shear textures decreases with the distance from the middle plane where the shear is reversed, but the simple shear textures are still the major components after exit of the channel. Hardness variation was modeled by contributions from dislocation strengthening and grain boundary strengthening, where dislocation density is approximated by the misorientation angle of LAGBs which are regarded as dislocation cell boundaries. As a result, the hardness can be predicted successfully by the microstructural features, i.e. the low-angle boundaries, the mean misorientation angle and the fraction of high-angle grain boundaries.
Hydrodynamical fluctuations in smooth shear flows
International Nuclear Information System (INIS)
Chagelishvili, G.D.; Khujadze, G.R.; Lominadze, J.G.
1999-11-01
Background of hydrodynamical fluctuations in a intrinsically/stochastically forced, laminar, uniform shear flow is studied. The employment of so-called nonmodal mathematical analysis makes it possible to represent the background of fluctuations in a new light and to get more insight into the physics of its formation. The basic physical processes responsible for the formation of vortex and acoustic wave fluctuation backgrounds are analyzed. Interplay of the processes at low and moderate shear rates is described. Three-dimensional vortex fluctuations around a given macroscopic state are numerically calculated. The correlation functions of the fluctuations of physical quantities are analyzed. It is shown that there exists subspace D k in the wave-number space (k-space) that is limited externally by spherical surface with radius k ν ≡ A/ν (where A is the velocity shear parameter, ν - the kinematic viscosity) in the nonequilibrium open system under study. The spatial Fourier harmonics of vortex as well as acoustic wave fluctuations are strongly subjected by flow shear (by the open character of the system) at wave-numbers satisfying the condition k ν . Specifically it is shown that in D k : The fluctuations are non-Markovian; the spatial spectral density of energy of the vortex fluctuations by far exceeds the white-noise; the term of a new type associated to the hydrodynamical fluctuation of velocity appears in the correlation function of pressure; the fluctuation background of the acoustic waves is completely different at low and moderate shear rates (at low shear rates it is reduced in D k in comparison to the uniform (non-shear) flow; at moderate shear rates it it comparable to the background of the vortex fluctuations). The fluctuation background of both the vortex and the acoustic wave modes is anisotropic. The possible significance of the fluctuation background of vortices for the subcritical transition to turbulence and Brownian motion of small macroscopic
Local particle flux reversal under strongly sheared flow
International Nuclear Information System (INIS)
Terry, P.W.; Newman, D.E.; Ware, A.S.
2003-01-01
The advection of electron density by turbulent ExB flow with linearly varying mean yields a particle flux that can reverse sign at certain locations along the direction of magnetic shear. The effect, calculated for strong flow shear, resides in the density-potential cross phase. It is produced by the interplay between the inhomogeneities of magnetic shear and flow shear, but subject to a variety of conditions and constraints. The regions of reversed flux tend to wash out if the turbulence consists of closely spaced modes of different helicities, but survive if modes of a single helicity are relatively isolated. The reversed flux becomes negligible if the electron density response is governed by electron scales while the eigenmode is governed by ion scales. The relationship of these results to experimentally observe flux reversals is discussed
Precessing rotating flows with additional shear: stability analysis.
Salhi, A; Cambon, C
2009-03-01
We consider unbounded precessing rotating flows in which vertical or horizontal shear is induced by the interaction between the solid-body rotation (with angular velocity Omega(0)) and the additional "precessing" Coriolis force (with angular velocity -epsilonOmega(0)), normal to it. A "weak" shear flow, with rate 2epsilon of the same order of the Poincaré "small" ratio epsilon , is needed for balancing the gyroscopic torque, so that the whole flow satisfies Euler's equations in the precessing frame (the so-called admissibility conditions). The base flow case with vertical shear (its cross-gradient direction is aligned with the main angular velocity) corresponds to Mahalov's [Phys. Fluids A 5, 891 (1993)] precessing infinite cylinder base flow (ignoring boundary conditions), while the base flow case with horizontal shear (its cross-gradient direction is normal to both main and precessing angular velocities) corresponds to the unbounded precessing rotating shear flow considered by Kerswell [Geophys. Astrophys. Fluid Dyn. 72, 107 (1993)]. We show that both these base flows satisfy the admissibility conditions and can support disturbances in terms of advected Fourier modes. Because the admissibility conditions cannot select one case with respect to the other, a more physical derivation is sought: Both flows are deduced from Poincaré's [Bull. Astron. 27, 321 (1910)] basic state of a precessing spheroidal container, in the limit of small epsilon . A Rapid distortion theory (RDT) type of stability analysis is then performed for the previously mentioned disturbances, for both base flows. The stability analysis of the Kerswell base flow, using Floquet's theory, is recovered, and its counterpart for the Mahalov base flow is presented. Typical growth rates are found to be the same for both flows at very small epsilon , but significant differences are obtained regarding growth rates and widths of instability bands, if larger epsilon values, up to 0.2, are considered. Finally
Flow shear stabilization of hybrid electron-ion drift mode in tokamaks
International Nuclear Information System (INIS)
Bai, L.
1999-01-01
In this paper, a model of sheared flow stabilization on hybrid electron-ion drift mode is proposed. At first, in the presence of dissipative trapped electrons, there exists an intrinsic oscillation mode in tokamak plasmas, namely hybrid dissipative trapped electron-ion temperature gradient mode (hereafter, called as hybrid electron-ion drift mode). This conclusion is in agreement with the observations in the simulated tokamak experiment on the CLM. Then, it is found that the coupling between the sheared flows and dissipative trapped electrons is proposed as the stabilization mechanism of both toroidal sheared flow and poloidal sheared flow on the hybrid electron-ion drift mode, that is, similar to the stabilizing effect of poloidal sheared flow on edge plasmas in tokamaks, in the presence of both dissipative trapped electrons and toroidal sheared flow, large toroidal sheared flow is always a strong stabilizing effect on the hybrid electron-ion drift mode in internal transport barrier location, too. This result is consistent with the experimental observations in JT-60U. (author)
Flow shear stabilization of hybrid electron-ion drift mode in tokamaks
International Nuclear Information System (INIS)
Bai, L.
2001-01-01
In this paper, a model of sheared flow stabilization on hybrid electron-ion drift mode is proposed. At first, in the presence of dissipative trapped electrons, there exists an intrinsic oscillation mode in tokamak plasmas, namely hybrid dissipative trapped electron-ion temperature gradient mode (hereafter, called as hybrid electron-ion drift mode). This conclusion is in agreement with the observations in the simulated tokamak experiment on the CLM. Then, it is found that the coupling between the sheared flows and dissipative trapped electrons is proposed as the stabilization mechanism of both toroidal sheared flow and poloidal sheared flow on the hybrid electron-ion drift mode, that is, similar to the stabilizing effect of poloidal sheared flow on edge plasmas in tokamaks, in the presence of both dissipative trapped electrons and toroidal sheared flow, large toroidal sheared flow is always a strong stabilizing effect on the hybrid electron-ion drift mode in internal transport barrier location, too. This result is consistent with the experimental observations in JT-60U. (author)
Shear flow stabilization of the hydromagnetic Rayleigh-Taylor instability
International Nuclear Information System (INIS)
Roderick, N.F.; Shumlak, U.; Douglas, M.; Peterkin, R.E. Jr.; Ruden, E.
1997-01-01
Numerical simulations have indicated that shear flow may help stabilize the hydromagnetic Rayleigh-Taylor instability in imploding plasma z-pinches. A simple extension to a model presented in Chandrasekhar has been developed to study the linear stability of incompressible plasma subjected to both a shear flow and acceleration. The model has been used to investigate the stability plasma implosion schemes using externally imposed velocity shear which develops from the plasma flow itself. Specific parameters were chosen to represent plasma implosions driven by the Saturn and PBFA-Z, pulsed power generators at Sandia National Laboratories. Results indicate a high shear is necessary to stabilize the z-pinch implosions studied
Boiko, Andrey V; Grek, Genrih R; Kozlov, Victor V
2012-01-01
Starting from fundamentals of classical stability theory, an overview is given of the transition phenomena in subsonic, wall-bounded shear flows. At first, the consideration focuses on elementary small-amplitude velocity perturbations of laminar shear layers, i.e. instability waves, in the simplest canonical configurations of a plane channel flow and a flat-plate boundary layer. Then the linear stability problem is expanded to include the effects of pressure gradients, flow curvature, boundary-layer separation, wall compliance, etc. related to applications. Beyond the amplification of instability waves is the non-modal growth of local stationary and non-stationary shear flow perturbations which are discussed as well. The volume continues with the key aspect of the transition process, that is, receptivity of convectively unstable shear layers to external perturbations, summarizing main paths of the excitation of laminar flow disturbances. The remainder of the book addresses the instability phenomena found at l...
Influence of Equilibrium Perpendicular Shear Flow on Peeling-ballooning Instabilities
Xi, P. W.; Xu, X. Q.
2011-10-01
The influence of perpendicular ExB shear flow on peeling-ballooning instabilities is investigated with BOUT++ code. In our simulation, a set of reduced MHD equations are solved for a very unstable equilibrium and a marginal unstable equilibrium in shifted-circular tokamak geometry. For ideal MHD cases without diamagnetic terms and resistivity, we find that flow shear shows dramatic stabilizing effect on peeling-ballooning modes and the stabilizing degree increases with mode number. When the flow shear is large enough, we find the curvature of growth rate verse mode number has the same shape like that for the case with only diamagnetic term, and this implies that diamagnetic term and the shear flow have the same mechanism acting on peeling-ballooning instabilities. The role of Kelvin-Helmholtz term is also investigated and we find it is destabilizing and the effect depends on both flow shear and mode number. For cases with both diamagnetic term and the applied shear flow, modes with intermediate mode number are strongest stabilized while high n and low n mode keep unstable. Based on these results, an ELM trigger sketch is proposed. Performed for USDoE by LLNL Contract DE-AC52-07NA27344.
Interaction of equal-size bubbles in shear flow.
Prakash, Jai; Lavrenteva, Olga M; Byk, Leonid; Nir, Avinoam
2013-04-01
The inertia-induced forces on two identical spherical bubbles in a simple shear flow at small but finite Reynolds number, for the case when the bubbles are within each other's inner viscous region, are calculated making use of the reciprocal theorem. This interaction force is further employed to model the dynamics of air bubbles injected to a viscous fluid sheared in a Couette device at the first shear flow instability where the bubbles are trapped inside the stable Taylor vortex. It was shown that, during a long time scale, the inertial interaction between the bubbles in the primary shear flow drives them away from each other and, as a result, equal-size bubbles eventually assume an ordered string with equal separation distances between all neighbors. We report on experiments showing the dynamic evolution of various numbers of bubbles. The results of the theory are in good agreement with the experimental observations.
Effects of shear flow on phase nucleation and crystallization.
Mura, Federica; Zaccone, Alessio
2016-04-01
Classical nucleation theory offers a good framework for understanding the common features of new phase formation processes in metastable homogeneous media at rest. However, nucleation processes in liquids are ubiquitously affected by hydrodynamic flow, and there is no satisfactory understanding of whether shear promotes or slows down the nucleation process. We developed a classical nucleation theory for sheared systems starting from the molecular level of the Becker-Doering master kinetic equation and we analytically derived a closed-form expression for the nucleation rate. The theory accounts for the effect of flow-mediated transport of molecules to the nucleus of the new phase, as well as for the mechanical deformation imparted to the nucleus by the flow field. The competition between flow-induced molecular transport, which accelerates nucleation, and flow-induced nucleus straining, which lowers the nucleation rate by increasing the nucleation energy barrier, gives rise to a marked nonmonotonic dependence of the nucleation rate on the shear rate. The theory predicts an optimal shear rate at which the nucleation rate is one order of magnitude larger than in the absence of flow.
Directory of Open Access Journals (Sweden)
Mahesh Varpe
2013-01-01
Full Text Available This paper explores the effect of inlet shear flow on the tip leakage flow in an axial flow compressor cascade. A flow with a high shear rate is generated in the test section of an open circuit cascade wind tunnel by using a combination of screens with a prescribed solidity. It is observed that a stable shear flow of shear rate 1.33 is possible and has a gradual decay rate until 15 times the height of the shear flow generator downstream. The computational results obtained agree well with the available experimental data on the baseline configuration. The detailed numerical analysis shows that the tip clearance improves the blade loading near the tip through the promotion of favorable incidence by the tip leakage flow. The tip clearance shifts the centre of pressure on the blade surface towards the tip. It, however, has no effect on the distribution of end wall loss and deviation angle along the span up to 60% from the hub. In the presence of a shear inflow, the end wall effects are considerable. On the other hand, with a shear inflow, the effects of tip leakage flow are observed to be partly suppressed. The shear flow reduces the tip leakage losses substantially in terms of kinetic energy associated with it.
Starch-zein beldns formed by shear flow
Habeych Narvaez, E.A.; Dekkers, B.; Goot, van der A.J.; Boom, R.M.
2008-01-01
A newly in-house developed shearing device was used to explore the formation of new types of microstructures in concentrated starch¿zein blends. The device allowed processing of the biopolymer blends under homogeneous, simple shear flow conditions. Water and glycerol were added as plasticizers.
Effect of sheared flows on neoclassical tearing modes
Energy Technology Data Exchange (ETDEWEB)
Sen, A [Institute for Plasma Research, Bhat, Gandhinagar (India); Chandra, D; Kaw, P [Institute for Plasma Research, Bhat, Gandhinagar (India); Bora, M P [Physics Dept., Gauhati University, Guwahati (India); Kruger, S [Tech-X, Boulder, CO (United States); Ramos, J [Plasma Science and Fusion Center, MIT, Cambridge, MA (United States)
2005-01-01
The influence of toroidal sheared equilibrium flows on the nonlinear evolution of classical and neoclassical tearing modes (NTMs) is studied through numerical solutions of a set of reduced generalized MHD equations that include viscous force effects based on neoclassical closures. In general, differential flow is found to have a strong stabilizing influence leading to lower saturated island widths for the classical (m/n = 2/1) mode and reduced growth rates for the (m/n = 3/1) neoclassical mode. Velocity shear on the other hand is seen to make a destabilizing contribution. An analytic model calculation, consisting of a generalized Rutherford island evolution equation that includes shear flow effects is also presented and the numerical results are discussed in the context of this model. (author)
Torsional shear flow of granular materials: shear localization and minimum energy principle
Artoni, Riccardo; Richard, Patrick
2018-01-01
The rheological properties of granular matter submitted to torsional shear are investigated numerically by means of discrete element method. The shear cell is made of a cylinder filled by grains which are sheared by a bumpy bottom and submitted to a vertical pressure which is applied at the top. Regimes differing by their strain localization features are observed. They originate from the competition between dissipation at the sidewalls and dissipation in the bulk of the system. The effects of the (i) the applied pressure, (ii) sidewall friction, and (iii) angular velocity are investigated. A model, based on the purely local μ (I)-rheology and a minimum energy principle is able to capture the effect of the two former quantities but unable to account the effect of the latter. Although, an ad hoc modification of the model allows to reproduce all the numerical results, our results point out the need for an alternative rheology.
Generation of rotation and shear flow in an imploding liner
Energy Technology Data Exchange (ETDEWEB)
Hammer, J H; Ryutov, D D [Lawrence Livermore National Lab., Livermore, CA (United States)
1997-12-31
There exist several techniques that can set the liner into rotation and/or excite an embedded shear flow at any desired depth of the liner material. A common element of all these techniques is the use of properly used left-right asymmetric structures, situated either on the liner surface or embedded in the shell. Both rotation and shear flow get enhanced in the course of the liner implosion because of the angular momentum conservation. While fast enough rotation should stabilize the Rayleigh-Taylor instability near the turn-around point, the shear flow can also have a stabilizing effect on the interface. The specific model presented in the paper shows that a strong enough shear causes stabilization of a broad class of Rayleigh-Taylor perturbations. Thus, the use of left-right asymmetric structure for generation of rotation and shear flow is an interesting new option for improvement of the quality of the liner implosions. (J.U.). 4 figs., 12 refs.
Ballooning instabilities in tokamaks with sheared toroidal flows
International Nuclear Information System (INIS)
Waelbroeck, F.L.; Chen, L.
1990-11-01
The stability of ballooning modes in the presence of sheared toroidal flows is investigated. The eigenmodes are shown to be related by a Fourier transformation to the non-exponentially growing Floquet solutions found by Cooper. It is further shown that the problem cannot be reduced further than to a two dimensional partial differential equation. Next, the generalized ballooning equation is solved analytically for a circular tokamak equilibrium with sonic flows, but with a small rotation shear compared to the sound speed. With this ordering, the centrifugal forces are comparable to the pressure gradient forces driving the instability, but coupling of the mode with the sound wave is avoided. A new stability criterion is derived which explicitly demonstrates that flow shear is stabilizing at constant centrifugal force gradient. 34 refs
Shear flow generation by Reynolds stress and suppression of resistive g modes
International Nuclear Information System (INIS)
Sugama, H.; Horton, W.
1993-01-01
The authors have investigated suppression of the resistive g mode turbulence by background shear flow produced by the external source and by the fluctuation-induced Reynolds stress. For that purpose, the authors used the model consisting of the equations describing the electrostatic potential φ≡(φ 0 +φ) and the pressure fluctuation p of the resistive g mode, and the equation for the background poloidal flow. They have done the single-helicity nonlinear simulations using the model equations in the sheared slab configuration. They find that, in the nonlinear turbulent regime, significant suppression of the turbulent transport is realized only when the shear flow v' E exceeds that which makes the fastest-growing linear modes marginally stable. With the shear flow which decreases the fastest linear growth rates by about a half, the turbulent transport in the saturated state is about the same as in the case of no shear flow. As seen from the equation for the background flow v E , the relative efficiency of the external flow and the Reynolds stress for producing shear flow depends on the parameter ν. For large ν, the external flow is a dominant contribution to the total background poloidal shear flow although its strength predicted by the neoclassical theory is not enough to suppress the turbulence significantly. On the other hand, for small ν, they observe that, as the fluctuations grow, the Reynolds stress becomes large and suddenly at some critical point in time shear flow much larger than the external one is generated and leads to the significant reduction of the turbulent transport just like that of the L-H transition in tokamak experiments. It is remarkable that the Reynolds stress due to the resistive g mode fluctuations works not as a conventional viscosity term weakening the shear flow but as a negative viscosity term enhancing it
Generation of sheared poloidal flows via Reynolds stress and transport barrier physics
International Nuclear Information System (INIS)
Hidalgo, C.; Pedrosa, M.A.; Sanchez, E.; Balbin, R.; Lopez-Fraguas, A.; Milligen, B. van; Silva, C.; Fernandes, H.; Varandas, C.A.F.; Riccardi, C.; Carrozza, R.; Fontanesi, M.; Carreras, B.A.; Garcia, L.
2000-01-01
A view of the latest experimental results and progress in the understanding of the role of poloidal flows driven by fluctuations via Reynolds stress is given. Reynolds stress shows a radial gradient close to the velocity shear layer location in tokamaks and stellarators, indicating that this mechanism may drive significant poloidal flows in the plasma boundary. Observation of the generation of ExB sheared flows via Reynolds stress at the ion Bernstein resonance layer has been noticed in toroidal magnetized plasmas. The experimental evidence of sheared ExB flows linked to the location of rational surfaces in stellarator plasmas might be interpreted in terms of Reynolds stress sheared driven flows. These results show that ExB sheared flows driven by fluctuations can play an important role in the generation of transport barriers. (author)
Shear flow generation by Reynolds stress and suppression of resistive g-modes
International Nuclear Information System (INIS)
Sugama, H.; Horton, W.
1993-08-01
Suppression of resistive g-mode turbulence by background shear flow generated from a small external flow source and amplified by the fluctuation-induced Reynolds stress is demonstrated and analyzed. The model leads to a paradigm for the low-to-high (L-H) confinement mode transition. To demonstrate the L-H transition model, single-helicity nonlinear fluid simulations using the vorticity equation for the electrostatic potential, the pressure fluctuation equation and the background poloidal flow equation are used in the sheared slab configuration. The relative efficiency of the external flow and the Reynolds stress for producing shear flow depends on the poloidal flow damping parameter ν which is given by neoclassical theory. For large ν, the external flow is a dominant contribution to the total background poloidal shear flow and its strength predicted by the neoclassical theory is not enough to suppress the turbulence significantly. In contrast, for small ν, we show that the fluctuations drive a Reynolds stress that becomes large and suddenly, at some critical point in time, shear flow much larger than the external flow is generated and leads to an abrupt, order unity reduction of the turbulent transport just like that of the L-H transition in tokamak experiments. It is also found that, even in the case of no external flow, the shear flow generation due to the Reynolds stress occurs through the nonlinear interaction of the resistive g-modes and reduces the transport. To supplement the numerical solutions we derive the Landau equation for the mode amplitude of the resistive g-mode taking into account the fluctuation-induced shear flow and analyze the opposite action of the Reynolds stress in the resistive g turbulence compared with the classical shear flow Kelvin-Helmholtz (K-H) driven turbulence
Sphere interaction in bounded shear flow of Oldroyd-B fluids
Chiu, Shang-Huan; Pan, Tsorng-Whay; Glowinski, Roland
2017-11-01
It is well-known that, up to the initial sphere displacement, binary encounters of spheres in bounded shear flow of a Newtonian fluid can have either swapping or non-swapping trajectories under creeping flow conditions. The motion of dilute sphere suspensions in bounded shear flow of Oldroyd-B fluids at zero Reynolds number has been studied. The pass and return trajectories of the two ball mass centers in a two wall driven shear flow are similar to those in a Newtonian fluid; but they lose the symmetry due to the effect of elastic force arising from viscoelastic fluids. A tumbling chain of two balls (a dipole) may occur, depending on the value of the Weissenberg number and the initial vertical displacement of the ball mass center to the middle plane between two walls. The two ball tumbling motion has also been compared with that of an ellipsoid in bounded shear flow Oldroyd-B fluids. This work was supported by NSF (Grant DMS-1418308).
Hydrodynamic of a deformed bubble in linear shear flow
International Nuclear Information System (INIS)
Adoua, S.R.
2007-07-01
This work is devoted to the study of an oblate spheroidal bubble of prescribed shape set fixed in a linear shear flow using direct numerical simulation. The three dimensional Navier-Stokes equations are solved in orthogonal curvilinear coordinates using a finite volume method. The bubble response is studied over a wide range of the aspect ratio (1-2.7), the bubble Reynolds number (50-2000) and the non-dimensional shear rate (0.-1.2). The numerical simulations shows that the shear flow imposes a plane symmetry of the wake whatever the parameters of the flow. The trailing vorticity is organized into two anti-symmetrical counter rotating tubes with a sign imposed by the competition of two mechanisms (the Lighthill mechanism and the instability of the wake). Whatever the Reynolds number, the lift coefficient reaches the analytical value obtained in an inviscid, weakly sheared flow corresponding to a lift force oriented in the same direction as that of a spherical bubble. For moderate Reynolds numbers, the direction of the lift force reverses when the bubble aspect ratio is large enough as observed in experiments. This reversal occurs for aspect ratios larger than 2.225 and is found to be directly linked to the sign of the trailing vorticity which is concentrated within two counter-rotating threads which propel the bubble in a direction depending of their sign of rotation. The behavior of the drag does not revel any significant effect induced by the wake structure and follows a quadratic increase with the shear rate. Finally, the torque experienced by the bubble also reverses for the same conditions inducing the reversal of the lift force. By varying the orientation of the bubble in the shear flow, a stable equilibrium position is found corresponding to a weak angle between the small axis of the bubble and the flow direction. (author)
Sheared flow layer formation in tokamak plasmas with reversed magnetic shear
International Nuclear Information System (INIS)
Dong, J.Q.; Long, Y.X.; Mou, Z.Z.; Zhang, J.H.; Li, J.Q.
2005-01-01
Sheared flow layer (SFL) formation due to magnetic energy release through tearing-reconnections in tokamak plasmas is investigated. The characteristics of the SFLs created in the development of double tearing mode, mediated by electron viscosity in configurations with non-monotonic safety factor q profiles and, therefore, two rational flux surfaces of same q value, are analyzed in detail as an example. Quasi-linear simulations demonstrate that the sheared flows induced by the mode have desirable characteristics (lying at the boundaries of the magnetic islands), and sufficient levels required for internal transport barrier (ITB) formation. A possible correlation of the SFLs with experimental observations, that double transport barrier structures are preferentially formed in proximity of the two rational surfaces, is also proffered. (author)
Cross flow response of a cylindrical structure under local shear flow
Directory of Open Access Journals (Sweden)
Yoo-Chul Kim
2009-12-01
Full Text Available The VIV (Vortex-Induced Vibration analysis of a flexible cylindrical structure under locally strong shear flow is presented. The model is made of Teflon and has 9.5m length, 0.0127m diameter, and 0.001m wall thickness. 11 2-dimensional accelerometers are installed along the model. The experiment has been conducted at the ocean engineering basin in the University of Tokyo in which uniform current can be generated. The model is installed at about 30 degree of slope and submerged by almost overall length. Local shear flow is made by superposing uniform current and accelerated flow generated by an impeller. The results of frequency and modal analysis are presented.
Shear flow effects on ion thermal transport in tokamaks
International Nuclear Information System (INIS)
Tajima, T.; Horton, W.; Dong, J.Q.; Kishimoto, Y.
1995-03-01
From various laboratory and numerical experiments, there is clear evidence that under certain conditions the presence of sheared flows in a tokamak plasma can significantly reduce the ion thermal transport. In the presence of plasma fluctuations driven by the ion temperature gradient, the flows of energy and momentum parallel and perpendicular to the magnetic field are coupled with each other. This coupling manifests itself as significant off-diagonal coupling coefficients that give rise to new terms for anomalous transport. The authors derive from the gyrokinetic equation a set of velocity moment equations that describe the interaction among plasma turbulent fluctuations, the temperature gradient, the toroidal velocity shear, and the poloidal flow in a tokamak plasma. Four coupled equations for the amplitudes of the state variables radially extended over the transport region by toroidicity induced coupling are derived. The equations show bifurcations from the low confinement mode without sheared flows to high confinement mode with substantially reduced transport due to strong shear flows. Also discussed is the reduced version with three state variables. In the presence of sheared flows, the radially extended coupled toroidal modes driven by the ion temperature gradient disintegrate into smaller, less elongated vortices. Such a transition to smaller spatial correlation lengths changes the transport from Bohm-like to gyrobohm-like. The properties of these equations are analyzed. The conditions for the improved confined regime are obtained as a function of the momentum-energy deposition rates and profiles. The appearance of a transport barrier is a consequence of the present theory
Flow rate dependency of critical wall shear stress in a radial-flow cell
DEFF Research Database (Denmark)
Detry, J.G.; Jensen, Bo Boye Busk; Sindic, M.
2009-01-01
In the present work, a radial-flow cell was used to study the removal of starch particle aggregates from several solid substrates (glass, stainless steel, polystyrene and PTFE) in order to determine the critical wall shear stress value for each case. The particle aggregates were formed by aspersion...... of a water or ethanol suspension of starch granules on the surfaces. Depending on the substrate and on the suspending liquid, the aggregates differed in size and shape. Aggregate removal was studied at two flow rates. At the lower flow rate (Re-inlet = 955), the values of critical wall shear stress...... for the different surfaces suggested that capillary forces were, for all of them, playing an important role in aggregate adhesion since aqueous based aggregates were always more difficult to remove. At the higher flow rate (Re-inlet = 2016) the critical wall shear stress increased as a result of the change...
Coexistence and transition between shear zones in slow granular flows.
Moosavi, Robabeh; Shaebani, M Reza; Maleki, Maniya; Török, János; Wolf, Dietrich E; Losert, Wolfgang
2013-10-04
We report experiments on slow granular flows in a split-bottom Couette cell that show novel strain localization features. Nontrivial flow profiles have been observed which are shown to be the consequence of simultaneous formation of shear zones in the bulk and at the boundaries. The fluctuating band model based on a minimization principle can be fitted to the experiments over a large variation of morphology and filling height with one single fit parameter, the relative friction coefficient μ(rel) between wall and bulk. The possibility of multiple shear zone formation is controlled by μ(rel). Moreover, we observe that the symmetry of an initial state, with coexisting shear zones at both side walls, breaks spontaneously below a threshold value of the shear velocity. A dynamical transition between two asymmetric flow states happens over a characteristic time scale which depends on the shear strength.
Self-diffusion in dense granular shear flows.
Utter, Brian; Behringer, R P
2004-03-01
Diffusivity is a key quantity in describing velocity fluctuations in granular materials. These fluctuations are the basis of many thermodynamic and hydrodynamic models which aim to provide a statistical description of granular systems. We present experimental results on diffusivity in dense, granular shear flows in a two-dimensional Couette geometry. We find that self-diffusivities D are proportional to the local shear rate gamma; with diffusivities along the direction of the mean flow approximately twice as large as those in the perpendicular direction. The magnitude of the diffusivity is D approximately gamma;a(2), where a is the particle radius. However, the gradient in shear rate, coupling to the mean flow, and strong drag at the moving boundary lead to particle displacements that can appear subdiffusive or superdiffusive. In particular, diffusion appears to be superdiffusive along the mean flow direction due to Taylor dispersion effects and subdiffusive along the perpendicular direction due to the gradient in shear rate. The anisotropic force network leads to an additional anisotropy in the diffusivity that is a property of dense systems and has no obvious analog in rapid flows. Specifically, the diffusivity is suppressed along the direction of the strong force network. A simple random walk simulation reproduces the key features of the data, such as the apparent superdiffusive and subdiffusive behavior arising from the mean velocity field, confirming the underlying diffusive motion. The additional anisotropy is not observed in the simulation since the strong force network is not included. Examples of correlated motion, such as transient vortices, and Lévy flights are also observed. Although correlated motion creates velocity fields which are qualitatively different from collisional Brownian motion and can introduce nondiffusive effects, on average the system appears simply diffusive.
Shear flows induced by nonlinear evolution of double tearing modes
International Nuclear Information System (INIS)
Wang Zhengxiong; Kishimoto, Y.; Li, J. Q.; Wang Xiaogang; Dong, J. Q.
2008-01-01
Shear flows induced by nonlinear evolution of double tearing modes are investigated in a resistive magnetohydrodynamic model with slab geometry. It is found that intensive and thin poloidal shear flow layers are generated in the magnetic island region driven by coupled reconnection process at both rational surfaces. The structure of the flow layers keeps evolving after the merging of magnetic separatrices and forms a few narrow vortices along the open field lines in the final stage of magnetic reconnection. The effects of the distance between both rational surfaces and the initial magnetic shear on the nonlinear evolution of the plasma flows are also taken into consideration and the relevant mechanism is discussed
Wang, Junxia; Cao, Changlin; Yu, Dingshan; Chen, Xudong
2018-02-01
In this paper, the effect of varying extensional-shear couple loading on deformation and stress response of Carbon Nanotubes/ ultra-high molecular weight polyethylene (CNTs/UHMWPE) composites was investigated using finite element numerical simulation, with expect to improve the manufacturing process of UHMWPE-based composites with reduced stress and lower distortion. When applying pure extensional loading and pure X-Y shear loading, it was found that the risk of a structural breakage greatly rises. For identifying the coupling between extensional and shear loading, distinct generations of force loading were defined by adjusting the magnitude of extensional loading and X-Y shear loading. It was shown that with the decrement of X-Y shear loading the deformation decreases obviously where the maximal Mises stress in Z-direction at 0.45 m distance is in the range from 24 to 10 MPa and the maximal shear stress at 0.61 m distance is within the range from 0.9 to 0.3 MPa. In addition, all the stresses determined were clearly below the yield strength of CNTs/UHMWPE composites under extensional-shear couple loading.
Instabilities and vortex dynamics in shear flow of magnetized plasmas
International Nuclear Information System (INIS)
Tajima, T.; Horton, W.; Morrison, P.J.; Schutkeker, J.; Kamimura, T.; Mima, K.; Abe, Y.
1990-03-01
Gradient-driven instabilities and the subsequent nonlinear evolution of generated vortices in sheared E x B flows are investigated for magnetized plasmas with and without gravity (magnetic curvature) and magnetic shear by using theory and implicit particle simulations. In the linear eigenmode analysis, the instabilities considered are the Kelvin-Helmholtz (K-H) instability and the resistive interchange instability. The presence of the shear flow can stabilize these instabilities. The dynamics of the K-H instability and the vortex dynamics can be uniformly described by the initial flow pattern with a vorticity localization parameter ε. The observed growth of the K-H modes is exponential in time for linearly unstable modes, secular for marginal mode, and absent until driven nonlinearly for linearly stable modes. The distance between two vortex centers experiences rapid merging while the angle θ between the axis of vortices and the external shear flow increases. These vortices proceed toward their overall coalescence, while shedding small-scale vortices and waves. The main features of vortex dynamics of the nonlinear coalescence and the tilt or the rotational instabilities of vortices are shown to be given by using a low dimension Hamiltonian representation for interacting vortex cores in the shear flow. 24 refs., 19 figs., 1 tab
Sensor for Boundary Shear Stress in Fluid Flow
Bao, Xiaoqi; Badescu, Mircea; Sherrit, Stewart; Bar-Cohen, Yoseph; Lih, Shyh-Shiuh; Chang, Zensheu; Trease, Brian P.; Kerenyi, Kornel; Widholm, Scott E.; Ostlund, Patrick N.
2012-01-01
The formation of scour patterns at bridge piers is driven by the forces at the boundary of the water flow. In most experimental scour studies, indirect processes have been applied to estimate the shear stress using measured velocity profiles. The estimations are based on theoretical models and associated assumptions. However, the turbulence flow fields and boundary layer in the pier-scour region are very complex and lead to low-fidelity results. In addition, available turbulence models cannot account accurately for the bed roughness effect. Direct measurement of the boundary shear stress, normal stress, and their fluctuations are attractive alternatives. However, most direct-measurement shear sensors are bulky in size or not compatible to fluid flow. A sensor has been developed that consists of a floating plate with folded beam support and an optical grid on the back, combined with a high-resolution optical position probe. The folded beam support makes the floating plate more flexible in the sensing direction within a small footprint, while maintaining high stiffness in the other directions. The floating plate converts the shear force to displacement, and the optical probe detects the plate s position with nanometer resolution by sensing the pattern of the diffraction field of the grid through a glass window. This configuration makes the sensor compatible with liquid flow applications.
Energy Technology Data Exchange (ETDEWEB)
Choi, Se Bin; Lee, Joon Sang [Dept. of Mechanical Engineering, Yonsei Unversity, Seoul (Korea, Republic of)
2015-08-15
We simulate an emulsion system under simple shear rates to analyze its rheological characteristics using the lattice Boltzmann method (LBM). We calculate the relative viscosity of an emulsion under a simple shear flow along with changes in temperature, shear rate, and surfactant concentration. The relative viscosity of emulsions decreased with an increase in temperature. We observed the shear-thinning phenomena, which is responsible for the inverse proportion between the shear rate and viscosity. An increase in the interfacial tension caused a decrease in the relative viscosity of the decane-in-water emulsion because the increased deformation caused by the decreased interfacial tension significantly influenced the wall shear stress.
Thermal convection of viscoelastic shear-thinning fluids
International Nuclear Information System (INIS)
Albaalbaki, Bashar; Khayat, Roger E; Ahmed, Zahir U
2016-01-01
The Rayleigh–Bénard convection for non-Newtonian fluids possessing both viscoelastic and shear-thinning behaviours is examined. The Phan-Thien–Tanner (PTT) constitutive equation is implemented to model the non-Newtonian character of the fluid. It is found that while the shear-thinning and viscoelastic effects could annihilate one another for the steady roll flow, presence of both behaviours restricts the roll stability limit significantly compared to the cases when the fluid is either inelastic shear-thinning or purely viscoelastic with constant viscosity. (paper)
Nonlinear drift waves in a dusty plasma with sheared flows
Energy Technology Data Exchange (ETDEWEB)
Vranjes, J. [K.U. Leuven (Belgium). Center for Plasma Astrophysics; Shukla, R.K. [Ruhr-Univ. Bochum (Germany). Inst. fuer Theoretische Physik IV
2002-01-01
Nonlinear properties of dust-modified drift waves and dust-drift waves in a dusty magnetoplasma with equilibrium sheared flows are examined. For this purpose, the relevant nonlinear equations for drift waves are analyzed for various profiles of the perpendicular and parallel plasma flows, and a variety of nonlinear solutions (viz. single and double vortex chains accompanied with zonal flows, tripolar and global vortices), which are driven by nommiform shear flows and nommiform dust density, is presented.
Nonlinear drift waves in a dusty plasma with sheared flows
International Nuclear Information System (INIS)
Vranjes, J.; Shukla, R.K.
2002-01-01
Nonlinear properties of dust-modified drift waves and dust-drift waves in a dusty magnetoplasma with equilibrium sheared flows are examined. For this purpose, the relevant nonlinear equations for drift waves are analyzed for various profiles of the perpendicular and parallel plasma flows, and a variety of nonlinear solutions (viz. single and double vortex chains accompanied with zonal flows, tripolar and global vortices), which are driven by nommiform shear flows and nommiform dust density, is presented
Shear flow in smectic A liquid crystals
International Nuclear Information System (INIS)
Stewart, I W; Stewart, F
2009-01-01
This paper considers the onset of a shear-induced instability in a sample of smectic A liquid crystal. Unlike many previous models, the usual director n need not necessarily coincide with the local smectic layer normal a; the traditional Oseen constraint (∇xa=0) is not imposed when flow is present. A recent dynamic theory for smectic A (Stewart 2007 Contin. Mech. Thermodyn. 18 343-60) will be used to examine a stationary instability in a simple model when the director reorientation and smectic layer distortions are, firstly, assumed not to be coupled to the velocity and, secondly, are supposed coupled to the velocity. A critical shear rate at which the onset of the instability occurs will be identified, together with an accompanying critical director tilt angle and critical wavenumber for the associated smectic layer undulations. Despite some critical phenomena being largely unaffected by any coupling to the flow, it will be shown that the influence of some material parameters, especially the smectic layer compression constant B 0 and the coupling constant B 1 , upon the critical shear rate and critical tilt angle can be greatly affected by flow.
Ultrafine-Grained Pure Ti Processed by New SPD Scheme Combining Drawing with Shear
Raab, A. G.; Bobruk, E. V.; Raab, G. I.
2018-05-01
The paper displays the results of the studies and analysis of a promising severe plastic deformation scheme that implements the conditions of a non-monotonous impact during shear drawing of long-length bulk metal materials. The paper describes the efficiency of the proposed severe plastic deformation technique to form a gradient ultrafine-grained state in rod-shaped billets on the example of commercially pure Ti and its further development for future industrial applications.
International Nuclear Information System (INIS)
Min Kim, Jung; Kate Gurnon, A.; Wagner, Norman J.; Eberle, Aaron P. R.; Porcar, Lionel
2014-01-01
The microstructure-rheology relationship for a model, thermoreversible nanoparticle gel is investigated using a new technique of time-resolved neutron scattering under steady and time-resolved large amplitude oscillatory shear (LAOS) flows. A 21 vol. % gel is tested with varying strength of interparticle attraction. Shear-induced structural anisotropy is observed as butterfly scattering patterns and quantified through an alignment factor. Measurements in the plane of flow show significant, local anisotropy develops with alignment along the compressional axis of flow, providing new insights into how gels flow. The microstructure-rheology relationship is analyzed through a new type of structure-Lissajous plot that shows how the anisotropic microstructure is responsible for the observed LAOS response, which is beyond a response expected for a purely viscous gel with constant structure. The LAOS shear viscosities are observed to follow the “Delaware-Rutgers” rule. Rheological and microstructural data are successfully compared across a broad range of conditions by scaling the shear rate by the strength of attraction, providing a method to compare behavior between steady shear and LAOS experiments. However, important differences remain between the microstructures measured at comparatively high frequency in LAOS experiments and comparable steady shear experiments that illustrate the importance of measuring the microstructure to properly interpret the nonlinear, dynamic rheological response
Energy Technology Data Exchange (ETDEWEB)
Min Kim, Jung; Kate Gurnon, A.; Wagner, Norman J., E-mail: wagnernj@udel.edu [Department of Chemical and Biomolecular Engineering and Center for Neutron Science, University of Delaware, Newark, Delaware 19716 (United States); Eberle, Aaron P. R. [NIST Center for Neutron Research, National Institute of Standards and Technology, Gaithersburg, Maryland 20899 (United States); Porcar, Lionel [NIST Center for Neutron Research, National Institute of Standards and Technology, Gaithersburg, Maryland 20899 and Institut Laue-Langevin, BP 156, F-38042 Grenoble Cedex 9 (France)
2014-09-01
The microstructure-rheology relationship for a model, thermoreversible nanoparticle gel is investigated using a new technique of time-resolved neutron scattering under steady and time-resolved large amplitude oscillatory shear (LAOS) flows. A 21 vol. % gel is tested with varying strength of interparticle attraction. Shear-induced structural anisotropy is observed as butterfly scattering patterns and quantified through an alignment factor. Measurements in the plane of flow show significant, local anisotropy develops with alignment along the compressional axis of flow, providing new insights into how gels flow. The microstructure-rheology relationship is analyzed through a new type of structure-Lissajous plot that shows how the anisotropic microstructure is responsible for the observed LAOS response, which is beyond a response expected for a purely viscous gel with constant structure. The LAOS shear viscosities are observed to follow the “Delaware-Rutgers” rule. Rheological and microstructural data are successfully compared across a broad range of conditions by scaling the shear rate by the strength of attraction, providing a method to compare behavior between steady shear and LAOS experiments. However, important differences remain between the microstructures measured at comparatively high frequency in LAOS experiments and comparable steady shear experiments that illustrate the importance of measuring the microstructure to properly interpret the nonlinear, dynamic rheological response.
Shear layer flame stabilization sensitivities in a swirling flow
Directory of Open Access Journals (Sweden)
Christopher Foley
2017-03-01
Full Text Available A variety of different flame configurations and heat release distributions exist in high swirl, annular flows, due to the existence of inner and outer shear layers as well a vortex breakdown bubble. Each of these different configurations, in turn, has different thermoacoustic sensitivities and influences on combustor emissions, nozzle durability, and liner heating. This paper presents findings on the sensitivities of the outer shear layer- stabilized flames to a range of parameters, including equivalence ratio, bulkhead temperature, flow velocity, and preheat temperature. There is significant hysteresis for flame attachment/detachment from the outer shear layer and this hysteresis is also described. Results are also correlated with extinction stretch rate calculations based on detailed kinetic simulations. In addition, we show that the bulkhead temperature near the flame attachment point has significant impact on outer shear layer detachment. This indicates that understanding the heat transfer between the edge flame stabilized in the shear layer and the nozzle hardware is needed in order to predict shear layer flame stabilization limits. Moreover, it shows that simulations cannot simply assume adiabatic boundary conditions if they are to capture these transitions. We also show that the reference temperature for correlating these transitions is quite different for attachment and local blow off. Finally, these results highlight the deficiencies in current understanding of the influence of fluid mechanic parameters (e.g. velocity, swirl number on shear layer flame attachment. For example, they show that the seemingly simple matter of scaling flame transition points with changes in flow velocities is not understood.
Influence of equilibrium shear flow on peeling-ballooning instability and edge localized mode crash
International Nuclear Information System (INIS)
Xi, P. W.; Xu, X. Q.; Wang, X. G.; Xia, T. Y.
2012-01-01
The E × B shear flow plays a dual role on peeling-ballooning modes and their subsequently triggered edge localized mode (ELM) crashes. On one hand, the flow shear can stabilize high-n modes and twist the mode in the poloidal direction, constraining the mode's radial extent and reducing the size of the corresponding ELM. On the other hand, the shear flow also introduces the Kelvin-Helmholtz drive, which can destabilize peeling-ballooning modes. The overall effect of equilibrium shear flow on peeling-ballooning modes and ELM crashes depends on the competition between these two effects. When the flow shear is either small or very large, it can reduce ELM size. However, for moderate values of flow shear, the destabilizing effect from the Kelvin-Helmholtz term is dominant and leads to larger ELM crashes.
Kraus, Emma; Kraus, Kristina; Obser, Tobias; Oyen, Florian; Klemm, Ulrike; Schneppenheim, Reinhard; Brehm, Maria A
2014-12-01
The multimeric form of von Willebrand factor (VWF), is the largest soluble protein in mammals and exhibits a multidomain structure resulting in multiple functions. Upon agonist stimulation endothelial cells secrete VWF multimers from Weibel-Palade bodies into the blood stream where VWF plays an essential role in platelet-dependent primary hemostasis. Elongation of VWF strings on the cells' surface leads to accessibility of VWF binding sites for proteins, such as platelet membrane glycoprotein Ib. The prothrombotic strings are size-regulated by the metalloprotease ADAMTS13 by shear force-activated proteolytic cleavage. VWF string formation was induced by histamine stimulation of HUVEC cells under unidirectional shear flow and VWF strings were detected employing the VWF binding peptide of platelet glycoprotein Ib coupled to latex beads. VWF strings were then used as substrate for kinetic studies of recombinant and plasma ADAMTS13. To investigate specific aspects of the shear-dependent functions of VWF and ADAMTS13, we developed a shear flow assay that allows observation of VWF string formation and their degradation by ADAMTS13 without the need for isolated platelets. Our assay specifically detects VWF strings, can be coupled with fluorescent applications and allows semi-automated, quantitative assessment of recombinant and plasma ADAMTS13 activity. Our assay may serve as a valuable research tool to investigate the biochemical characteristics of VWF and ADAMTS13 under shear flow and could complement diagnostics of von Willebrand Disease and Thrombotic Thrombocytopenic Purpura as it allows detection of shear flow-dependent dysfunction of VWD-associated VWF mutants as well as TTP-associated ADAMTS13 mutants. Copyright © 2014 Elsevier Ltd. All rights reserved.
Pulsatile blood flow, shear force, energy dissipation and Murray's Law
Directory of Open Access Journals (Sweden)
Bengtsson Hans-Uno
2006-08-01
Full Text Available Abstract Background Murray's Law states that, when a parent blood vessel branches into daughter vessels, the cube of the radius of the parent vessel is equal to the sum of the cubes of the radii of daughter blood vessels. Murray derived this law by defining a cost function that is the sum of the energy cost of the blood in a vessel and the energy cost of pumping blood through the vessel. The cost is minimized when vessel radii are consistent with Murray's Law. This law has also been derived from the hypothesis that the shear force of moving blood on the inner walls of vessels is constant throughout the vascular system. However, this derivation, like Murray's earlier derivation, is based on the assumption of constant blood flow. Methods To determine the implications of the constant shear force hypothesis and to extend Murray's energy cost minimization to the pulsatile arterial system, a model of pulsatile flow in an elastic tube is analyzed. A new and exact solution for flow velocity, blood flow rate and shear force is derived. Results For medium and small arteries with pulsatile flow, Murray's energy minimization leads to Murray's Law. Furthermore, the hypothesis that the maximum shear force during the cycle of pulsatile flow is constant throughout the arterial system implies that Murray's Law is approximately true. The approximation is good for all but the largest vessels (aorta and its major branches of the arterial system. Conclusion A cellular mechanism that senses shear force at the inner wall of a blood vessel and triggers remodeling that increases the circumference of the wall when a shear force threshold is exceeded would result in the observed scaling of vessel radii described by Murray's Law.
Fast calculation of microphone array steering vectors with shear flow
Sijtsma, P.
2018-01-01
This paper proposes a fast method for calculating the acoustic time delay between an observer and a receiver in a shear flow. This method is applied to an outdoor microphone array measurement on a large-scale wind turbine. In such a set-up, a shear flow represents the actual wind field better than a
Assembly of vorticity-aligned hard-sphere colloidal strings in a simple shear flow
Cheng, X.; Xu, X.; Rice, S. A.; Dinner, A. R.; Cohen, I.
2011-01-01
under shear, there are conflicting predictions about whether particles link up into string-like structures along the shear flow direction. Here, using confocal microscopy, we measure the shear-induced suspension structure. Surprisingly, rather than flow
Radially sheared azimuthal flows and turbulent transport in a cylindrical helicon plasma device
International Nuclear Information System (INIS)
Tynan, G R; Burin, M J; Holland, C; Antar, G; Diamond, P H
2004-01-01
A radially sheared azimuthal flow is observed in a cylindrical helicon plasma device. The shear flow is roughly azimuthally symmetric and contains both time-stationary and slowly varying components. The turbulent radial particle flux is found to peak near the density gradient maximum and vanishes at the shear layer location. The shape of the radial plasma potential profile associated with the azimuthal E x B flow is predicted accurately by theory. The existence of the mean shear flow in a plasma with finite flow damping from ion-neutral collisions and no external momentum input implies the existence of radial angular momentum transport from the turbulent Reynolds-stress
Multifractal spectra in homogeneous shear flow
Deane, A. E.; Keefe, L. R.
1988-01-01
Employing numerical simulations of 3-D homogeneous shear flow, the associated multifractal spectra of the energy dissipation, scalar dissipation and vorticity fields were calculated. The results for (128) cubed simulations of this flow, and those obtained in recent experiments that analyzed 1- and 2-D intersections of atmospheric and laboratory flows, are in some agreement. A two-scale Cantor set model of the energy cascade process which describes the experimental results from 1-D intersections quite well, describes the 3-D results only marginally.
Propagation of waves in shear flows
Fabrikant, A L
1998-01-01
The state of the art in a theory of oscillatory and wave phenomena in hydrodynamical flows is presented in this book. A unified approach is used for waves of different physical origins. A characteristic feature of this approach is that hydrodynamical phenomena are considered in terms of physics; that is, the complement of the conventionally employed formal mathematical approach. Some physical concepts such as wave energy and momentum in a moving fluid are analysed, taking into account induced mean flow. The physical mechanisms responsible for hydrodynamic instability of shear flows are conside
Upward swimming of a sperm cell in shear flow.
Omori, Toshihiro; Ishikawa, Takuji
2016-03-01
Mammalian sperm cells are required to swim over long distances, typically around 1000-fold their own length. They must orient themselves and maintain a swimming motion to reach the ovum, or egg cell. Although the mechanism of long-distance navigation is still unclear, one possible mechanism, rheotaxis, was reported recently. This work investigates the mechanism of the rheotaxis in detail by simulating the motions of a sperm cell in shear flow adjacent to a flat surface. A phase diagram was developed to show the sperm's swimming motion under different shear rates, and for varying flagellum waveform conditions. The results showed that, under shear flow, the sperm is able to hydrodynamically change its swimming direction, allowing it to swim upwards against the flow, which suggests that the upward swimming of sperm cells can be explained using fluid mechanics, and this can then be used to further understand physiology of sperm cell navigation.
Liangjie, Mao; Qingyou, Liu; Shouwei, Zhou
2014-01-01
A considerable number of studies for VIV under the uniform flow have been performed. However, research on VIV under shear flow is scarce. An experiment for VIV under the shear flow with the same shear parameter at the two different Reynolds numbers was conducted in a deep-water offshore basin. Various measurements were obtained by the fiber bragg grating strain sensors. Experimental data were analyzed by modal analysis method. Results show several valuable features. First, the corresponding maximum order mode of the natural frequency for shedding frequency is the maximum dominant vibration mode and multi-modal phenomenon is appeared in VIV under the shear flow, and multi-modal phenomenon is more apparent at the same shear parameter with an increasing Reynolds number under the shear flow effect. Secondly, the riser vibrates at the natural frequency and the dominant vibration frequency increases for the effect of the real-time tension amplitude under the shear flow and the IL vibration frequency is the similar with the CF vibration frequency at the Reynolds number of 1105 in our experimental condition and the IL dominant frequency is twice the CF dominant frequency with an increasing Reynolds number. In addition, the displacement trajectories at the different locations of the riser appear the same shape and the shape is changed at the same shear parameter with an increasing Reynolds number under the shear flow. The diagonal displacement trajectories are observed at the low Reynolds number and the crescent-shaped displacement trajectories appear with an increasing Reynolds number under shear flow in the experiment.
Directory of Open Access Journals (Sweden)
Mao Liangjie
Full Text Available A considerable number of studies for VIV under the uniform flow have been performed. However, research on VIV under shear flow is scarce. An experiment for VIV under the shear flow with the same shear parameter at the two different Reynolds numbers was conducted in a deep-water offshore basin. Various measurements were obtained by the fiber bragg grating strain sensors. Experimental data were analyzed by modal analysis method. Results show several valuable features. First, the corresponding maximum order mode of the natural frequency for shedding frequency is the maximum dominant vibration mode and multi-modal phenomenon is appeared in VIV under the shear flow, and multi-modal phenomenon is more apparent at the same shear parameter with an increasing Reynolds number under the shear flow effect. Secondly, the riser vibrates at the natural frequency and the dominant vibration frequency increases for the effect of the real-time tension amplitude under the shear flow and the IL vibration frequency is the similar with the CF vibration frequency at the Reynolds number of 1105 in our experimental condition and the IL dominant frequency is twice the CF dominant frequency with an increasing Reynolds number. In addition, the displacement trajectories at the different locations of the riser appear the same shape and the shape is changed at the same shear parameter with an increasing Reynolds number under the shear flow. The diagonal displacement trajectories are observed at the low Reynolds number and the crescent-shaped displacement trajectories appear with an increasing Reynolds number under shear flow in the experiment.
Nonlinear vortex structures and Rayleigh instability condition in shear flow plasmas
International Nuclear Information System (INIS)
Haque, Q.; Saleem, H.; Mirza, A.M.
2009-01-01
Full text: It is shown that the shear flow produced by externally applied electric field can unstable the drift waves. Due to shear flow, the Rayleigh instability condition is modified, which is obtained for both electron-ion and electron-positron-ion plasmas. These shear flow driven drift waves can be responsible for large amplitude electrostatic fluctuations in tokamak edges. In the nonlinear regime, the stationary structures may appear in electron-positron-ion plasmas similar to electron-ion plasmas. The nonlinear vortex structures like counter rotating dipole vortices and vortex chains can be formed with the aid of special type of shear flows. The positrons can be used as a probe in laboratory plasmas, which make it a multi-component plasma. The presence of positrons in electron-ion plasma system can affect the speed and amplitude of the nonlinear vortex structures. This investigation can have application in both laboratory and astrophysical plasmas. (author)
Modeling of the blood rheology in steady-state shear flows
International Nuclear Information System (INIS)
Apostolidis, Alex J.; Beris, Antony N.
2014-01-01
We undertake here a systematic study of the rheology of blood in steady-state shear flows. As blood is a complex fluid, the first question that we try to answer is whether, even in steady-state shear flows, we can model it as a rheologically simple fluid, i.e., we can describe its behavior through a constitutive model that involves only local kinematic quantities. Having answered that question positively, we then probe as to which non-Newtonian model best fits available shear stress vs shear-rate literature data. We show that under physiological conditions blood is typically viscoplastic, i.e., it exhibits a yield stress that acts as a minimum threshold for flow. We further show that the Casson model emerges naturally as the best approximation, at least for low and moderate shear-rates. We then develop systematically a parametric dependence of the rheological parameters entering the Casson model on key physiological quantities, such as the red blood cell volume fraction (hematocrit). For the yield stress, we base our description on its critical, percolation-originated nature. Thus, we first determine onset conditions, i.e., the critical threshold value that the hematocrit has to have in order for yield stress to appear. It is shown that this is a function of the concentration of a key red blood cell binding protein, fibrinogen. Then, we establish a parametric dependence as a function of the fibrinogen and the square of the difference of the hematocrit from its critical onset value. Similarly, we provide an expression for the Casson viscosity, in terms of the hematocrit and the temperature. A successful validation of the proposed formula is performed against additional experimental literature data. The proposed expression is anticipated to be useful not only for steady-state blood flow modeling but also as providing the starting point for transient shear, or more general flow modeling
Formation of structural steady states in lamellar/sponge phase-separating fluids under shear flow
Panizza, P.; Courbin, L.; Cristobal, G.; Rouch, J.; Narayanan, T.
2003-05-01
We investigate the effect of shear flow on a lamellar-sponge phase-separating fluid when subjected to shear flow. We show the existence of two different steady states (droplets and ribbons structures) whose nature does not depend on the way to reach the two-phase unstable region of the phase diagram (temperature quench or stirring). The transition between ribbons and droplets is shear thickening and its nature strongly depends on what dynamical variable is imposed. If the stress is fixed, flow visualization shows the existence of shear bands at the transition, characteristic of coexistence in the cell between ribbons and droplets. In this shear-banding region, the viscosity oscillates. When the shear rate is fixed, no shear bands are observed. Instead, the transition exhibits a hysteretic behavior leading to a structural bi-stability of the phase-separating fluid under flow.
Khabaz, Fardin; Cloitre, Michel; Bonnecaze, Roger T.
2018-03-01
In a recent study [Khabaz et al., Phys. Rev. Fluids 2, 093301 (2017), 10.1103/PhysRevFluids.2.093301], we showed that jammed soft particle glasses (SPGs) crystallize and order in steady shear flow. Here we investigate the rheology and microstructures of these suspensions in oscillatory shear flow using particle-dynamics simulations. The microstructures in both types of flows are similar, but their evolutions are very different. In both cases the monodisperse and polydisperse suspensions form crystalline and layered structures, respectively, at high shear rates. The crystals obtained in the oscillatory shear flow show fewer defects compared to those in the steady shear. SPGs remain glassy for maximum oscillatory strains less than about the yield strain of the material. For maximum strains greater than the yield strain, microstructural and rheological transitions occur for SPGs. Polydisperse SPGs rearrange into a layered structure parallel to the flow-vorticity plane for sufficiently high maximum shear rates and maximum strains about 10 times greater than the yield strain. Monodisperse suspensions form a face-centered cubic (FCC) structure when the maximum shear rate is low and hexagonal close-packed (HCP) structure when the maximum shear rate is high. In steady shear, the transition from a glassy state to a layered one for polydisperse suspensions included a significant induction strain before the transformation. In oscillatory shear, the transformation begins to occur immediately and with different microstructural changes. A state diagram for suspensions in large amplitude oscillatory shear flow is found to be in close but not exact agreement with the state diagram for steady shear flow. For more modest amplitudes of around one to five times the yield strain, there is a transition from a glassy structure to FCC and HCP crystals, at low and high frequencies, respectively, for monodisperse suspensions. At moderate frequencies, the transition is from glassy to HCP via
Stimulated bioluminescence by fluid shear stress associated with pipe flow
Energy Technology Data Exchange (ETDEWEB)
Cao Jing; Wang Jiangan; Wu Ronghua, E-mail: caojing981@126.com [Col. of Electronic Eng., Naval University of Engineering, Wuhan 430033 (China)
2011-01-01
Dinoflagellate can be stimulated bioluminescence by hydrodynamic agitation. Two typical dinoflagellate (Lingulodinium polyedrum and Pyrocystis noctiluca) was choosed to research stimulated bioluminescence. The bioluminescence intensity and shear stress intensity were measured using fully developed pipe flow. There is shear stress threshold to agitate organism bioluminescence. From these experiment, the response thresholds of the stimulated bioluminscence always occurred in laminar flows at a shear stress level of 0.6-3 dyn/cm{sup 2}. At the same time, the spectral characteristc of dinoflagellate was recorded, the wavelength of them is about 470nm, and the full width at half maximum is approximate 30nm.
Rheometry-PIV of shear-thickening wormlike micelles.
Marín-Santibañez, Benjamín M; Pérez-Gonzalez, José; de Vargas, Lourdes; Rodríguez-Gonzalez, Francisco; Huelsz, Guadalupe
2006-04-25
The shear-thickening behavior of an equimolar semidilute aqueous solution of 40 mM/L cetylpyridinium chloride and sodium salicylate was studied in this work by using a combined method of rheometry and particle image velocimetry (PIV). Experiments were conducted at 27.5 degrees C with Couette, vane-bob, and capillary rheometers in order to explore a wide shear stress range as well as the effect of boundary conditions and time of flow on the creation and destruction of shear-induced structures (SIS). The use of the combined method of capillary rheometry with PIV allowed the detection of fast spatial and temporal variations in the flow kinematics, which are related to the shear-thickening behavior and the dynamics of the SIS but are not distinguished by pure rheometrical measurements. A rich-in-details flow curve was found for this solution, which includes five different regimes. Namely, at very low shear rates a Newtonian behavior was found, followed by a shear thinning one in the second regime. In the third, shear banding was observed, which served as a precursor of the SIS and shear-thickening. The fourth and fifth regimes in the flow curve were separated by a spurtlike behavior, and they clearly evidenced the existence of shear-thickening accompanied by stick-slip oscillations at the wall of the rheometer, which subsequently produced variations in the shear rate under shear stress controlled flow. Such a stick-slip phenomenon prevailed up to the highest shear stresses used in this work and was reflected in asymmetric velocity profiles with spatial and temporal variations linked to the dynamics of creation and breakage of the SIS. The presence of apparent slip at the wall of the rheometer provides an energy release mechanism which leads to breakage of the SIS, followed by their further reformation during the stick part of the cycles. In addition, PIV measurements allowed the detection of apparent slip at the wall, as well as mechanical failures in the bulk of the
Transport barriers with and without shear flows in a magnetized plasma
International Nuclear Information System (INIS)
Martinell, Julio J.
2014-01-01
Different ways of producing a transport barrier in a toroidal magnetized plasma are discussed and the properties of the barriers are analyzed. The first mechanism is associated with the presence of a sheared plasma flow that is present in a limited region of the plasma, which creates a zonal flow. In contrast to the usual paradigm stating that the sheared flow reduces the turbulence correlation length and leads to suppression of the fluctuation driven transport in the region of highest shear, it is shown that from the perspective of chaotic transport of plasma particles in the fluctuation fields, the transport barrier is formed in the region of zero shear and it can be destroyed when the fluctuation level is high enough. It is also shown that finite gyroradius effects modify the dynamics and introduces new conditions for barrier formation. The second mechanism considers a method in which radio-frequency waves injected into the plasma can stabilize the drift waves and therefore the anomalous transport is reduced, creating a barrier. This process does not involve the presence of sheared flows and depends only on the effect of the RF wave field on the drift waves. The stabilizing effect in this case is due to the nonlinear ponderomotive force which acts in a way that offsets the pressure gradient destabilization. Finally, a mechanism based on the ponderomotive force of RF waves is described which produces poloidal plasma rotation around the resonant surface due to the asymmetry of induced transport; it creates a transport barrier by shear flow stabilization of turbulence
EFFECT OF ION ∇ B DRIFT DIRECTION ON TURBULENCE FLOW AND FLOW SHEAR
International Nuclear Information System (INIS)
FENZI, C; McKEE, G.R; BURRELL, K.H; CARLSTROM, T.N; FONCK, R.J; GROEBNER, R.J
2003-01-01
The divertor magnetic geometry has a significant effect on the poloidal flow and resulting flow shear of turbulence in the outer region of L-mode tokamak plasmas, as determined via two-dimensional measurements of density fluctuations with Beam Emission Spectroscopy on DIII-D. Plasmas with similar parameters, except that in one case the ion (del)B drift points towards the divertor X-point (lower single-null, LSN), and in the other case, the ion (del)B drift points away from the divertor X-point (upper single-null, USN), are compared. Inside of r/a=0.9, the turbulence characteristics (amplitude, flow direction, correlation lengths) are similar in both cases, while near r/a=0.92, a dramatic reversal of the poloidal flow of turbulence relative to the core flow direction is observed in plasmas with the ion (del)B drift pointing towards the divertor X-point. No such flow reversal is observed in plasmas with the ion (del)B drift pointing away from the divertor X-point. This poloidal flow reversal results in a significantly larger local shear in the poloidal turbulence flow velocity in plasmas with the ion (del)B drift pointing towards the divertor X-point. Additionally, these plasmas locally exhibit significant dispersion, with two distinct and counter-propagating turbulence modes. Likewise, the radial correlation length of the turbulence is reduced in these plasmas, consistent with biorthogonal decomposition measurements of dominant turbulence structures. The naturally occurring turbulence flow shear in these LSN plasmas may facilitate the LH transition that occurs at an input power of roughly one-half to one-third that of corresponding plasmas with the ion (del)B drift pointing away from the X-point
Deformation of a Capsule in a Power-Law Shear Flow
Directory of Open Access Journals (Sweden)
Fang-Bao Tian
2016-01-01
Full Text Available An immersed boundary-lattice Boltzmann method is developed for fluid-structure interactions involving non-Newtonian fluids (e.g., power-law fluid. In this method, the flexible structure (e.g., capsule dynamics and the fluid dynamics are coupled by using the immersed boundary method. The incompressible viscous power-law fluid motion is obtained by solving the lattice Boltzmann equation. The non-Newtonian rheology is achieved by using a shear rate-dependant relaxation time in the lattice Boltzmann method. The non-Newtonian flow solver is then validated by considering a power-law flow in a straight channel which is one of the benchmark problems to validate an in-house solver. The numerical results present a good agreement with the analytical solutions for various values of power-law index. Finally, we apply this method to study the deformation of a capsule in a power-law shear flow by varying the Reynolds number from 0.025 to 0.1, dimensionless shear rate from 0.004 to 0.1, and power-law index from 0.2 to 1.8. It is found that the deformation of the capsule increases with the power-law index for different Reynolds numbers and nondimensional shear rates. In addition, the Reynolds number does not have significant effect on the capsule deformation in the flow regime considered. Moreover, the power-law index effect is stronger for larger dimensionless shear rate compared to smaller values.
An experimental investigation of pure-substance, adiabatic two-phase flow in a vertical pipe
International Nuclear Information System (INIS)
Nikitopoulos, D.E.; Maeder, P.F.
1994-01-01
Two-phase flows of pure substances are of particular importance for a wide range of applications in the thermo-hydraulic components of nuclear power generations systems, heat exchangers, geothermal wells, refrigeration systems, etc. Measurements of pressure drop, temperature, and average void fraction are presented for adiabatic, vertical-upwards, two-phase flow of Refrigerant 114 in a pipe. An experimental method has been developed according to which the evolution of flow states occurring in long pipes can be realized in a test section of limited length. The experiments cover the range of the flow from flashing to near choking. The measurements indicate existence of macroscopic thermodynamic equilibrium, except in the immediate neighborhood of flashing. Compressibility due to phase change is shown to play a very important role in the development of the flow. Three regions are recognized based on the measured energetics of the flow. Each region is dominated by potential energy changes, dissipation, and kinetic energy changes, respectively. The evolution of the flow is governed by hydrostatic effects in the initial region after flashing and by high, phase-change-induced kinetic energy increases far downstream as the flow approaches choking. In the intermediate region, viscous, inertial and gravitational effects play a role of comparable importance. The interfacial and wall shear forces have also been calculated from the measurements. The former dominate the initial regions of the flow, while the latter are strongest at high vapor contents
Laboratory observation of magnetic field growth driven by shear flow
Energy Technology Data Exchange (ETDEWEB)
Intrator, T. P., E-mail: intrator@lanl.gov; Feng, Y.; Sears, J.; Weber, T. [Los Alamos National Laboratory, M.S. E526, Los Alamos, New Mexico 87545 (United States); Dorf, L. [Applied Materials, Inc., Santa Clara, CA 95054 (United States); Sun, X. [University of Science and Technology, Hefei (China)
2014-04-15
Two magnetic flux ropes that collide and bounce have been characterized in the laboratory. We find screw pinch profiles that include ion flow v{sub i}, magnetic field B, current density J, and plasma pressure. The electron flow v{sub e} can be inferred, allowing the evaluation of the Hall J×B term in a two fluid magnetohydrodynamic Ohm's Law. Flux ropes that are initially cylindrical are mutually attracted and compress each other, which distorts the cylindrical symmetry. Magnetic field is created via the ∇×v{sub e}×B induction term in Ohm's Law where in-plane (perpendicular) shear of parallel flow (along the flux rope) is the dominant feature, along with some dissipation and magnetic reconnection. We predict and measure the growth of a quadrupole out-of-plane magnetic field δB{sub z}. This is a simple and coherent example of a shear flow driven dynamo. There is some similarity with two dimensional reconnection scenarios, which induce a current sheet and thus out-of-plane flow in the third dimension, despite the customary picture that considers flows only in the reconnection plane. These data illustrate a general and deterministic mechanism for large scale sheared flows to acquire smaller scale magnetic features, disordered structure, and possibly turbulence.
Interfacial shear stress in stratified flow in a horizontal rectangular duct
International Nuclear Information System (INIS)
Lorencez, C.; Kawaji, M.; Murao, Y.
1995-01-01
Interfacial shear stress has been experimentally examined for both cocurrent and countercurrent stratified wavy flows in a horizontal interfacial shear stress from the measurements were examined and the results have been compared with existing correlations. Some differences were found in the estimated interfacial shear stress from the measurements were examined and the results have been compared with existing correlations. Some differences were found in the estimated interfacial shear stress values at high gas flow rates which could be attributed to the assumptions and procedures involved in each method. The interfacial waves and secondary motions were also found to have significant effects on the accuracy of Reynolds stress and turbulence kinetic energy extrapolation methods
Interfacial shear stress in stratified flow in a horizontal rectangular duct
Energy Technology Data Exchange (ETDEWEB)
Lorencez, C.; Kawaji, M. [Univ. of Toronto (Canada); Murao, Y. [Tokushima Univ. (Japan)] [and others
1995-09-01
Interfacial shear stress has been experimentally examined for both cocurrent and countercurrent stratified wavy flows in a horizontal interfacial shear stress from the measurements were examined and the results have been compared with existing correlations. Some differences were found in the estimated interfacial shear stress from the measurements were examined and the results have been compared with existing correlations. Some differences were found in the estimated interfacial shear stress values at high gas flow rates which could be attributed to the assumptions and procedures involved in each method. The interfacial waves and secondary motions were also found to have significant effects on the accuracy of Reynolds stress and turbulence kinetic energy extrapolation methods.
Another look at zonal flows: Resonance, shearing, and frictionless saturation
Li, J. C.; Diamond, P. H.
2018-04-01
We show that shear is not the exclusive parameter that represents all aspects of flow structure effects on turbulence. Rather, wave-flow resonance enters turbulence regulation, both linearly and nonlinearly. Resonance suppresses the linear instability by wave absorption. Flow shear can weaken the resonance, and thus destabilize drift waves, in contrast to the near-universal conventional shear suppression paradigm. Furthermore, consideration of wave-flow resonance resolves the long-standing problem of how zonal flows (ZFs) saturate in the limit of weak or zero frictional drag, and also determines the ZF scale. We show that resonant vorticity mixing, which conserves potential enstrophy, enables ZF saturation in the absence of drag, and so is effective at regulating the Dimits up-shift regime. Vorticity mixing is incorporated as a nonlinear, self-regulation effect in an extended 0D predator-prey model of drift-ZF turbulence. This analysis determines the saturated ZF shear and shows that the mesoscopic ZF width scales as LZ F˜f3 /16(1-f ) 1 /8ρs5/8l03 /8 in the (relevant) adiabatic limit (i.e., τckk‖2D‖≫1 ). f is the fraction of turbulence energy coupled to ZF and l0 is the base state mixing length, absent ZF shears. We calculate and compare the stationary flow and turbulence level in frictionless, weakly frictional, and strongly frictional regimes. In the frictionless limit, the results differ significantly from conventionally quoted scalings derived for frictional regimes. To leading order, the flow is independent of turbulence intensity. The turbulence level scales as E ˜(γL/εc) 2 , which indicates the extent of the "near-marginal" regime to be γLcase of avalanche-induced profile variability. Here, εc is the rate of dissipation of potential enstrophy and γL is the characteristic linear growth rate of fluctuations. The implications for dynamics near marginality of the strong scaling of saturated E with γL are discussed.
Exact coherent structures in an asymptotically reduced description of parallel shear flows
Beaume, Cédric; Knobloch, Edgar; Chini, Gregory P.; Julien, Keith
2015-02-01
A reduced description of shear flows motivated by the Reynolds number scaling of lower-branch exact coherent states in plane Couette flow (Wang J, Gibson J and Waleffe F 2007 Phys. Rev. Lett. 98 204501) is constructed. Exact time-independent nonlinear solutions of the reduced equations corresponding to both lower and upper branch states are found for a sinusoidal, body-forced shear flow. The lower branch solution is characterized by fluctuations that vary slowly along the critical layer while the upper branch solutions display a bimodal structure and are more strongly focused on the critical layer. The reduced equations provide a rational framework for investigations of subcritical spatiotemporal patterns in parallel shear flows.
Exact coherent structures in an asymptotically reduced description of parallel shear flows
International Nuclear Information System (INIS)
Beaume, Cédric; Knobloch, Edgar; Chini, Gregory P; Julien, Keith
2015-01-01
A reduced description of shear flows motivated by the Reynolds number scaling of lower-branch exact coherent states in plane Couette flow (Wang J, Gibson J and Waleffe F 2007 Phys. Rev. Lett. 98 204501) is constructed. Exact time-independent nonlinear solutions of the reduced equations corresponding to both lower and upper branch states are found for a sinusoidal, body-forced shear flow. The lower branch solution is characterized by fluctuations that vary slowly along the critical layer while the upper branch solutions display a bimodal structure and are more strongly focused on the critical layer. The reduced equations provide a rational framework for investigations of subcritical spatiotemporal patterns in parallel shear flows. (paper)
Shape oscillations of elastic particles in shear flow.
Subramaniam, Dhananjay Radhakrishnan; Gee, David J
2016-09-01
Particle suspensions are common to biological fluid flows; for example, flow of red- and white-blood cells, and platelets. In medical technology, current and proposed methods for drug delivery use membrane-bounded liquid capsules for transport via the microcirculation. In this paper, we consider a 3D linear elastic particle inserted into a Newtonian fluid and investigate the time-dependent deformation using a numerical simulation. Specifically, a boundary element technique is used to investigate the motion and deformation of initially spherical or spheroidal particles in bounded linear shear flow. The resulting deformed shapes reveal a steady-state profile that exhibits a 'tank-treading' motion for initially spherical particles. Wall effects on particle trajectory are seen to include a modified Jeffrey׳s orbit for spheroidal inclusions with a period that varies inversely with the strength of the shear flow. Alternately, spheroidal inclusions may exhibit either a 'tumbling' or 'trembling' motion depending on the initial particle aspect ratio and the capillary number (i.e., ratio of fluid shear to elastic restoring force). We find for a capillary number of 0.1, a tumbling mode transitions to a trembling mode at an aspect ratio of 0.87 (approx.), while for a capillary number of 0.2, this transition takes place at a lower aspect ratio. These oscillatory modes are consistent with experimental observations involving similarly shaped vesicles and thus serves to validate the use of a simple elastic constitutive model to perform relevant physiological flow calculations. Copyright © 2016 Elsevier Ltd. All rights reserved.
Estimation of shear stress in counter-current gas-liquid annular two-phase flow
International Nuclear Information System (INIS)
Abe, Yutaka; Akimoto, Hajime; Murao, Yoshio
1991-01-01
The accuracy of the correlations of the friction factor is important for the counter-current flow (CCF) analysis with two-fluid model. However, existing two fluid model codes use the correlations of friction factors for co-current flow or correlation developed based on the assumption of no wall shear stress. The assessment calculation for two fluid model code with those existing correlations of friction factors shows the falling water flow rate is overestimated. Analytical model is developed to calculate the shear stress distribution in water film at CCF in order to get the information on the shear stress at the interface and the wall. The analytical results with the analysis model and Bharathan's CCF data shows that the wall shear stress acting on the falling water film is almost same order as the interfacial shear stress and the correlations for co-current flow cannot be applied to the counter-current flow. Tentative correlations of the interfacial and the wall friction factors are developed based on the results of the present study. (author)
International Nuclear Information System (INIS)
Derks, Didi; Wisman, Hans; Blaaderen, Alfons van; Imhof, Arnout
2004-01-01
We report on novel possibilities for studying colloidal suspensions in a steady shear field in real space. Fluorescence confocal microscopy is combined with the use of a counter-rotating cone-plate shear cell. This allows imaging of individual particles in the bulk of a sheared suspension in a stationary plane. Moreover, this plane of zero velocity can be moved in the velocity gradient direction while keeping the shear rate constant. The colloidal system under study consists of rhodamine labelled PMMA spheres in a nearly density and refractive index matched mixture of cyclohexylbromide and cis-decalin. We show measured flow profiles in both the fluid and the crystalline phase and find indications for shear banding in the case of a sheared crystal. Furthermore, we show that, thanks to the counter-rotating principle of the cone-plate shear cell, a layer of particles in the bulk of a sheared crystalline suspension can be imaged for a prolonged time, with the result that their positions can be tracked
Computational analysis of integrated biosensing and shear flow in a microfluidic vascular model
Wong, Jeremy F.; Young, Edmond W. K.; Simmons, Craig A.
2017-11-01
Fluid flow and flow-induced shear stress are critical components of the vascular microenvironment commonly studied using microfluidic cell culture models. Microfluidic vascular models mimicking the physiological microenvironment also offer great potential for incorporating on-chip biomolecular detection. In spite of this potential, however, there are few examples of such functionality. Detection of biomolecules released by cells under flow-induced shear stress is a significant challenge due to severe sample dilution caused by the fluid flow used to generate the shear stress, frequently to the extent where the analyte is no longer detectable. In this work, we developed a computational model of a vascular microfluidic cell culture model that integrates physiological shear flow and on-chip monitoring of cell-secreted factors. Applicable to multilayer device configurations, the computational model was applied to a bilayer configuration, which has been used in numerous cell culture applications including vascular models. Guidelines were established that allow cells to be subjected to a wide range of physiological shear stress while ensuring optimal rapid transport of analyte to the biosensor surface and minimized biosensor response times. These guidelines therefore enable the development of microfluidic vascular models that integrate cell-secreted factor detection while addressing flow constraints imposed by physiological shear stress. Ultimately, this work will result in the addition of valuable functionality to microfluidic cell culture models that further fulfill their potential as labs-on-chips.
Gyrokinetic analysis of ion temperature gradient modes in the presence of sheared flows
International Nuclear Information System (INIS)
Artun, M.; Tang, W.M.
1992-01-01
The linearized gyrokinetic equation governing electrostatic microinstabilities in the presence of sheared equilibrium flow in both the z and y directions has been systematically derived for a sheared slab geometry, where in the large aspect ratio limit z and y directions correspond to the toroidal and poloidal directions respectively. In the familiar long perpendicular wavelength regime (κ perpendicular ρi > 1), the analysis leads to a comprehensive kinetic differential eigenmode equation which is solved numerically. The numerical results have been successfully cross-checked against analytic estimates in the fluid limit. For typical conditions, the Ion Temperature Gradient (ηi) modes are found to be stabilized for y-direction flows with a velocity shear scale comparable to that of the ion temperature gradient and velocities of a few percent of the sound speed. Sheared flows in the z-direction taken along are usually destabilizing, with the effect being independent of the sign of the flow. However, when both types are simultaneously considered, it is found that in the presence of shared z-direction flow, sheared y-direction flow can be either stabilizing or destabilizing depending on the relative sign of these flows. However, for sufficiently large values of υ' y the mode is completely stabilized regardless of the sign of υ' z υ' y . The importance of a proper kinetic treatment of this problem is supported by comparisons with fluid estimates. In particular, when such effects are favorable, significantly smaller values of sheared y-direction flow are required for stability than fluid estimates would indicate
Eigenmode characteristics of the double tearing mode in the presence of shear flows
International Nuclear Information System (INIS)
Mao Aohua; Li Jiquan; Kishimoto, Y.; Liu Jinyuan
2013-01-01
The double tearing mode (DTM) is characterized by two eigen states with antisymmetric or symmetric magnetic island structure, referred to as the even or odd DTM. In this work, we systematically revisit the DTM instabilities in the presence of an antisymmetric shear flow with a focus on eigenmode characteristics as well as the stabilization or destabilization mechanism in a wide parameter region. Both initial value simulation and eigenvalue analysis are performed based on reduced resistive MHD model in slab geometry. A degenerated eigen state is found at a critical flow amplitude v c . The even (or odd) DTM is stabilized (or destabilized) by weak shear flow below v c through the distortion of magnetic islands mainly due to the global effect of shear flow rather than the local flow shear. The distortion can be quantified by the phase angles of the perturbed flux, showing a perfect correspondence to the growth rates. As the shear flow increases above v c , the degenerated eigen state bifurcates into two eigen modes with the same growth rate but opposite propagating direction, resulting in an oscillatory growth of fluctuation energy. It is identified that two eigen modes show the single tearing mode structure due to the Alfvén resonance (AR) occurring on one current sheet. Most importantly, the AR can destabilize the DTMs through enhancing the plasma flow exerting on the remaining island. Meanwhile, the local flow shear plays a remarkable stabilizing role in this region. In addition, the eigenmode characteristic of the electromagnetic Kelvin-Helmholtz instability is also discussed.
An Experimental Investigation of an Airfoil Traversing Across a Shear Flow
Hamedani, Borhan A.; Naguib, Ahmed; Koochesfahani, Manoochehr
2017-11-01
While the aerodynamics of an airfoil in a uniform approach flow is well understood, less attention has been paid to airfoils in non-uniform flows. An aircraft encounters such flow, for example, during landing through the air wake of an aircraft carrier. The present work is focused on investigating the fundamental aerodynamics of airfoils in such an environment using canonical flow experiments. To generate a shear approach flow, a shaped honeycomb block is employed in a wind tunnel setup. Direct force measurements are performed on a NACA 0012 airfoil, with an aspect ratio of 1.8, as the airfoil traverses steadily across the shear region. Measurements are conducted at a chord Reynolds number Rec 75k, based on the mean approach stream velocity at the center of the shear zone, for a range of airfoil traverse velocities and angles of attack (0 - 12 degree). The results are compared to those obtained for the same airfoil when placed statically at different points along the traverse path inside the shear zone. The comparison enables examination of the applicability of quasi-steady analysis in computing the forces on the moving airfoil. This work is supported by ONR Grant Number N00014-16-1-2760.
Influence of equilibrium shear flow in the parallel magnetic direction on edge localized mode crash
Energy Technology Data Exchange (ETDEWEB)
Luo, Y.; Xiong, Y. Y. [College of Physical Science and Technology, Sichuan University, 610064 Chengdu (China); Chen, S. Y., E-mail: sychen531@163.com [College of Physical Science and Technology, Sichuan University, 610064 Chengdu (China); Key Laboratory of High Energy Density Physics and Technology of Ministry of Education, Sichuan University, Chengdu 610064 (China); Southwestern Institute of Physics, Chengdu 610041 (China); Huang, J.; Tang, C. J. [College of Physical Science and Technology, Sichuan University, 610064 Chengdu (China); Key Laboratory of High Energy Density Physics and Technology of Ministry of Education, Sichuan University, Chengdu 610064 (China)
2016-04-15
The influence of the parallel shear flow on the evolution of peeling-ballooning (P-B) modes is studied with the BOUT++ four-field code in this paper. The parallel shear flow has different effects in linear simulation and nonlinear simulation. In the linear simulations, the growth rate of edge localized mode (ELM) can be increased by Kelvin-Helmholtz term, which can be caused by the parallel shear flow. In the nonlinear simulations, the results accord with the linear simulations in the linear phase. However, the ELM size is reduced by the parallel shear flow in the beginning of the turbulence phase, which is recognized as the P-B filaments' structure. Then during the turbulence phase, the ELM size is decreased by the shear flow.
Nonlinear dynamics and anisotropic structure of rotating sheared turbulence.
Salhi, A; Jacobitz, F G; Schneider, K; Cambon, C
2014-01-01
Homogeneous turbulence in rotating shear flows is studied by means of pseudospectral direct numerical simulation and analytical spectral linear theory (SLT). The ratio of the Coriolis parameter to shear rate is varied over a wide range by changing the rotation strength, while a constant moderate shear rate is used to enable significant contributions to the nonlinear interscale energy transfer and to the nonlinear intercomponental redistribution terms. In the destabilized and neutral cases, in the sense of kinetic energy evolution, nonlinearity cannot saturate the growth of the largest scales. It permits the smallest scale to stabilize by a scale-by-scale quasibalance between the nonlinear energy transfer and the dissipation spectrum. In the stabilized cases, the role of rotation is mainly nonlinear, and interacting inertial waves can affect almost all scales as in purely rotating flows. In order to isolate the nonlinear effect of rotation, the two-dimensional manifold with vanishing spanwise wave number is revisited and both two-component spectra and single-point two-dimensional energy components exhibit an important effect of rotation, whereas the SLT as well as the purely two-dimensional nonlinear analysis are unaffected by rotation as stated by the Proudman theorem. The other two-dimensional manifold with vanishing streamwise wave number is analyzed with similar tools because it is essential for any shear flow. Finally, the spectral approach is used to disentangle, in an analytical way, the linear and nonlinear terms in the dynamical equations.
International Nuclear Information System (INIS)
Wan Liang; Wang Shaoqing
2010-01-01
The deformation behavior of Cu bicrystals with the symmetric tilt grain boundary (STGB) under pure shear has been studied by atomistic simulation method with the embedded atom method (EAM) interatomic potentials. By using an energy minimization method, it shows that there are two optimized structures of this grain boundary (GB) which correspond to two local energy minima on the potential energy surface of the GB. The structure with lower energy is the stable one while the other is a metastable structure. The pure shear process of the bicrystals at ambient temperature has been studied by molecular dynamics (MD) simulation method. The simulated results indicate that there are three structure transformation modes of this GB depending on the shear direction: (1) pure GB sliding; (2) GB atomic shuffling accompanied by dislocation emission from GB; (3) GB migration coupled GB sliding, namely, GB coupling motion. In addition, an analysis of the structure evolution of the GB shows that, there are two mechanisms for GB coupling motion depending on the shear direction. One is the collective motion of GB atoms and the other is structure transformation realized by uncorrelated atomic shuffling processes. The former mechanism can induce structure transition of GB between the stable one and the metastable one, while the latter introduces faceting of the GB. (authors)
Ideal flow theory for the double - shearing model as a basis for metal forming design
Alexandrov, S.; Trung, N. T.
2018-02-01
In the case of Tresca’ solids (i.e. solids obeying the Tresca yield criterion and its associated flow rule) ideal flows have been defined elsewhere as solenoidal smooth deformations in which an eigenvector field associated everywhere with the greatest principal stress (and strain rate) is fixed in the material. Under such conditions all material elements undergo paths of minimum plastic work, a condition which is often advantageous for metal forming processes. Therefore, the ideal flow theory is used as the basis of a procedure for the preliminary design of such processes. The present paper extends the theory of stationary planar ideal flow to pressure dependent materials obeying the double shearing model and the double slip and rotation model. It is shown that the original problem of plasticity reduces to a purely geometric problem. The corresponding system of equations is hyperbolic. The characteristic relations are integrated in elementary functions. In regions where one family of characteristics is straight, mapping between the principal lines and Cartesian coordinates is determined by linear ordinary differential equations. An illustrative example is provided.
Observation of Droplet Size Oscillations in a Two-Phase Fluid under Shear Flow
Courbin, Laurent; Panizza, Pascal; Salmon, Jean-Baptiste
2004-01-01
Experimental observations of droplet size sustained oscillations are reported in a two-phase flow between a lamellar and a sponge phase. Under shear flow, this system presents two different steady states made of monodisperse multilamellar droplets, separated by a shear-thinning transition. At low and high shear rates, the droplet size results from a balance between surface tension and viscous stress, whereas for intermediate shear rates it becomes a periodic function of time. A possible mechanism for such kinds of oscillations is discussed.
Coherent structures in compressible free-shear-layer flows
Energy Technology Data Exchange (ETDEWEB)
Aeschliman, D.P.; Baty, R.S. [Sandia National Labs., Albuquerque, NM (United States). Engineering Sciences Center; Kennedy, C.A.; Chen, J.H. [Sandia National Labs., Livermore, CA (United States). Combustion and Physical Sciences Center
1997-08-01
Large scale coherent structures are intrinsic fluid mechanical characteristics of all free-shear flows, from incompressible to compressible, and laminar to fully turbulent. These quasi-periodic fluid structures, eddies of size comparable to the thickness of the shear layer, dominate the mixing process at the free-shear interface. As a result, large scale coherent structures greatly influence the operation and efficiency of many important commercial and defense technologies. Large scale coherent structures have been studied here in a research program that combines a synergistic blend of experiment, direct numerical simulation, and analysis. This report summarizes the work completed for this Sandia Laboratory-Directed Research and Development (LDRD) project.
International Nuclear Information System (INIS)
Pedrosa, M.A.; Hidalgo, C.; Alonso, A.; Calderon, E.; Orozco, O.; Pablos, J.L. de
2005-01-01
Experimental results have shown that the generation of spontaneous perpendicular sheared flow (i.e. the naturally occurring shear layer) requires a minimum plasma density or gradient in the TJ-II stellarator. This finding has been observed by means of multiple plasma diagnostics, including probes, fast cameras, reflectometry and HIBP. The obtained shearing rate of the naturally occurring shear layer results in general comparable to the one observed during biasing-improved confinement regimes. It has been found that there is a coupling between the onset of sheared flow development and an increase in the level of plasma edge fluctuations pointing to turbulence as the main ingredient of the radial electric field drive; once the shear flow develops the level of turbulence tends to decrease. The link between the development of sheared flows and plasma density in TJ-II has been observed in different magnetic configurations and plasma regimes. Preliminary results show that the threshold density value depends on the iota value and on the magnetic ripple (plasma volume). Recent experiments carried out in the LHD stellarator have shown that edge sheared flows are also affected by the magnitude of edge magnetic ripple: the threshold density to trigger edge sheared flows increases with magnetic ripple . Those results have been interpreted as an evidence of the importance of neoclassical effect in the physics of ExB sheared flows. For some TJ-II magnetic configurations with higher edge iota (ι/2π≥ 1.8) there is a sharp increase in the edge density gradient simultaneous to a strong reduction of fluctuations and transport and a slight increase of the shearing rate and perpendicular rotation (≥2 km/s) as density increases above the threshold. The role of the edge ripple, the presence of edge rational surfaces and properties of turbulent transport are considered as possible ingredients to explain the spontaneous development of edge sheared flows in TJ-II. (author)
Nonlinear coupling of the resistive tearing modes under the unperturbed shear flow
International Nuclear Information System (INIS)
Urata, Kazuhiro
1990-01-01
The influence of the unperturbed shear flow on the nonlinear evolution of the tearing mode is studied. In the case of single helicity, the shear flow activates the unstable mode which finally saturates to a rigid rotor state. In the case of multiple helicity, a variety of flow patterns is created depending on parameters, and always forms the current bubble soon after the collapse of the 3/2 magnetic island. (author)
International Nuclear Information System (INIS)
Guzdar, P.N.; Drake, J.F.
1993-01-01
The generation of shear flow by resistive ballooning modes and resistive interchange modes is compared and contrasted using a 3-D fluid code. The resistive ballooning modes give rise to poloidally asymmetric transport and hence drive poloidal rotation due to the Reynold's Stress as well as the anomalous Stringer/Winsor mechanism. On the other hand the resistive interchange mode can drive shear flow only through the Reynold's Stress. The studies show that if the self-consistent sheared flow is suppressed, the resistive ballooning modes give rise to a larger anomalous transport than produced by the resistive interchange modes. Furthermore the shear flow generated by the resistive ballooning modes is larger than that driven by the resistive interchange modes due to the combined effect of the dual mechanisms stated earlier. As a consequence strong suppression of the fluctuations as well as reduction of the transport occurs for resistive ballooning modes. On the other hand, for the resistive interchange modes the level of fluctuation as well as the anomalous transport is not reduced by the self consistent shear flow generated by the Reynold's Stress. This latter result is in agreement with some earlier 3-D simulation of resistive interchange modes
Jia, Yali; Bagnaninchi, Pierre O.; Yang, Ying; Haj, Alicia El; Hinds, Monica T.; Kirkpatrick, Sean J.; Wang, Ruikang K.
2009-05-01
Establishing a relationship between perfusion rate and fluid shear stress in a 3D cell culture environment is an ongoing and challenging task faced by tissue engineers. We explore Doppler optical coherence tomography (DOCT) as a potential imaging tool for in situ monitoring of local fluid flow profiles inside porous chitosan scaffolds. From the measured fluid flow profiles, the fluid shear stresses are evaluated. We examine the localized fluid flow and shear stress within low- and high-porosity chitosan scaffolds, which are subjected to a constant input flow rate of 0.5 ml.min-1. The DOCT results show that the behavior of the fluid flow and shear stress in micropores is strongly dependent on the micropore interconnectivity, porosity, and size of pores within the scaffold. For low-porosity and high-porosity chitosan scaffolds examined, the measured local fluid flow and shear stress varied from micropore to micropore, with a mean shear stress of 0.49+/-0.3 dyn.cm-2 and 0.38+/-0.2 dyn.cm-2, respectively. In addition, we show that the scaffold's porosity and interconnectivity can be quantified by combining analyses of the 3D structural and flow images obtained from DOCT.
Direct test of a nonlinear constitutive equation for simple turbulent shear flows using DNS data
Schmitt, François G.
2007-10-01
Several nonlinear constitutive equations have been proposed to overcome the limitations of the linear eddy-viscosity models to describe complex turbulent flows. These nonlinear equations have often been compared to experimental data through the outputs of numerical models. Here we perform a priori analysis of nonlinear eddy-viscosity models using direct numerical simulation (DNS) of simple shear flows. In this paper, the constitutive equation is directly checked using a tensor projection which involves several invariants of the flow. This provides a 3 terms development which is exact for 2D flows, and a best approximation for 3D flows. We provide the quadratic nonlinear constitutive equation for the near-wall region of simple shear flows using DNS data, and estimate their coefficients. We show that these coefficients have several common properties for the different simple shear flow databases considered. We also show that in the central region of pipe flows, where the shear rate is very small, the coefficients of the constitutive equation diverge, indicating the failure of this representation for vanishing shears.
Cosmic Shear With ACS Pure Parallels
Rhodes, Jason
2002-07-01
Small distortions in the shapes of background galaxies by foreground mass provide a powerful method of directly measuring the amount and distribution of dark matter. Several groups have recently detected this weak lensing by large-scale structure, also called cosmic shear. The high resolution and sensitivity of HST/ACS provide a unique opportunity to measure cosmic shear accurately on small scales. Using 260 parallel orbits in Sloan textiti {F775W} we will measure for the first time: beginlistosetlength sep0cm setlengthemsep0cm setlengthopsep0cm em the cosmic shear variance on scales Omega_m^0.5, with signal-to-noise {s/n} 20, and the mass density Omega_m with s/n=4. They will be done at small angular scales where non-linear effects dominate the power spectrum, providing a test of the gravitational instability paradigm for structure formation. Measurements on these scales are not possible from the ground, because of the systematic effects induced by PSF smearing from seeing. Having many independent lines of sight reduces the uncertainty due to cosmic variance, making parallel observations ideal.
A film-based wall shear stress sensor for wall-bounded turbulent flows
Amili, Omid; Soria, Julio
2011-07-01
In wall-bounded turbulent flows, determination of wall shear stress is an important task. The main objective of the present work is to develop a sensor which is capable of measuring surface shear stress over an extended region applicable to wall-bounded turbulent flows. This sensor, as a direct method for measuring wall shear stress, consists of mounting a thin flexible film on the solid surface. The sensor is made of a homogeneous, isotropic, and incompressible material. The geometry and mechanical properties of the film are measured, and particles with the nominal size of 11 μm in diameter are embedded on the film's surface to act as markers. An optical technique is used to measure the film deformation caused by the flow. The film has typically deflection of less than 2% of the material thickness under maximum loading. The sensor sensitivity can be adjusted by changing the thickness of the layer or the shear modulus of the film's material. The paper reports the sensor fabrication, static and dynamic calibration procedure, and its application to a fully developed turbulent channel flow at Reynolds numbers in the range of 90,000-130,000 based on the bulk velocity and channel full height. The results are compared to alternative wall shear stress measurement methods.
Measuring Advection and Diffusion of Colloids in Shear Flow
Duits, Michael H.G.; Ghosh, Somnath; Mugele, Friedrich Gunther
2015-01-01
An analysis of the dynamics of colloids in shear flow can be challenging because of the superposition of diffusion and advection. We present a method that separates the two motions, starting from the time-dependent particle coordinates. The restriction of the tracking to flow lanes and the
Tripolar vortices of dust-drift waves in dusty plasma with shear flow
International Nuclear Information System (INIS)
Chen Yinhua; Wang Ge
2002-01-01
Nonlinear equations governing dust-drift waves in magnetized dusty plasma with transverse shear flow are derived. For the specific profiles of flow and the plasma equilibrium density, a new type of solution in the form of tripolar vortices is found. The results show that the peak magnitude of tripolar vortices increases with increasing shear intensity and dust content
Energy Technology Data Exchange (ETDEWEB)
Pedrosa, M. A.; Hidalgo, C.; Alonso, A.; Calderon, E.; Orozco, R. O.; Pablos, J. L. de
2005-07-01
It is well known the importance of the shear as a stabilizing mechanism to control plasma fluctuations in magnetically confined plasmas [1]. It has been clearly established that Ex B shear stabilization mechanisms are an important piece for the improvement of confinement on fusion devices. In particular both edge and core transport barriers are related to a large increase in the Ex B sheared flow. As a consequence clarifying the driving mechanisms of sheared flow in fusion plasmas is a main issue. The existence of parallel and perpendicular sheared flows at the plasma edge, and the interplay between them in different plasma conditions has been studied in the TJ-II [2]. Recent experiments carried out by means of different approaches in the TJ-II stellarator have shown that the generation of spontaneous edge perpendicular sheared flow can be externally controlled by means of plasma density with good reproducibility and reliability [3, 4]. Although experimentally the plasma density has been used as an external control knob, it would be more appropriate to characterize experimental results in terms of edge plasma gradient (e.g. ion saturation current gradient) [3]. It has also been found that there exists a coupling between the onset of sheared flow development and an increase in the level of plasma edge turbulence; once sheared flow is fully developed the level of fluctuations and turbulent transport slightly decreases whereas edge gradients and plasma density increase. It has been experimentally established that the minimum plasma density (or/and minimum level of plasma turbulence) essential for the development of the shear layer depends on the plasma magnetic configuration [5, 6]. For some plasma magnetic configurations with high iota value a sheared flow-induced regime with characteristics resembling those of an improved confinement one has been found. The similarity in the structure of the velocity shear layer and in the turbulence characteristics [7] in different
The non-monotonic shear-thinning flow of two strongly cohesive concentrated suspensions
Buscall, Richard; Kusuma, Tiara E.; Stickland, Anthony D.; Rubasingha, Sayuri; Scales, Peter J.; Teo, Hui-En; Worrall, Graham L.
2014-01-01
The behaviour in simple shear of two concentrated and strongly cohesive mineral suspensions showing highly non-monotonic flow curves is described. Two rheometric test modes were employed, controlled stress and controlled shear-rate. In controlled stress mode the materials showed runaway flow above a yield stress, which, for one of the suspensions, varied substantially in value and seemingly at random from one run to the next, such that the up flow-curve appeared to be quite irreproducible. Th...
Experimental investigation of single small bubble motion in linear shear flow in water
International Nuclear Information System (INIS)
Li, Zhongchun; Zhao, Yang; Song, Xiaoming; Yu, Hongxing; Jiang, Shengyao; Ishii, Mamoru
2016-01-01
Highlights: • The bubble motion in simple linear shear flow was experimentally investigated. • The bubble trajectories, bubble velocity and drag and lift force were obtained using image process routine. • The bubble trajectory was coupled with a zigzag motion and incline path. • The lift force was kept negative and it decreased when bubble diameter and shear flow magnitude increased. - Abstract: The motion of small bubble in a simple shear flow in water was experimental studied. Stable shear flow with low turbulence level was achieved with curved screen and measured using LDV. The bubbles were captured by high speed camera and the captured images were processed with digital image routine. The bubble was released from a capillary tube. The instantaneous bubble position, bubble velocity and forces were obtained based on the captured parameters. The quasi-steady lift coefficient was determined by the linear fitting of the bubble trajectory of several cycles. The results indicated that the lateral migration was coupled with the zigzag motion of bubble in the present experiment. The bubble migrated to the left side and its quasi-steady lift coefficient was negative. Good repeatable results were observed by measurements of 18 bubbles. The bubble motion in shear flow in water was first experimental studied and negative lift force was observed in the present study condition. The lift coefficient decreased when shear stress magnitude or bubble diameter increased in the present experiment condition.
Assembly of vorticity-aligned hard-sphere colloidal strings in a simple shear flow
Cheng, X.
2011-12-23
Colloidal suspensions self-assemble into equilibrium structures ranging from face- and body-centered cubic crystals to binary ionic crystals, and even kagome lattices. When driven out of equilibrium by hydrodynamic interactions, even more diverse structures can be accessed. However, mechanisms underlying out-of-equilibrium assembly are much less understood, though such processes are clearly relevant in many natural and industrial systems. Even in the simple case of hard-sphere colloidal particles under shear, there are conflicting predictions about whether particles link up into string-like structures along the shear flow direction. Here, using confocal microscopy, we measure the shear-induced suspension structure. Surprisingly, rather than flow-aligned strings, we observe log-rolling strings of particles normal to the plane of shear. By employing Stokesian dynamics simulations, we address the mechanism leading to this out-of-equilibrium structure and show that it emerges from a delicate balance between hydrodynamic and interparticle interactions. These results demonstrate a method for assembling large-scale particle structures using shear flows.
Constitutive Curve and Velocity Profile in Entangled Polymers during Start-Up of Steady Shear Flow
Hayes, Keesha A.
2010-05-11
Time-dependent shear stress versus shear rate, constitutive curve, and velocity profile measurements are reported in entangled polymer solutions during start-up of steady shear flow. By combining confocal microscopy and particle image velocimetry (PIV), we determine the time-dependent velocity profile in polybutadiene and polystyrene solutions seeded with fluorescent 150 nm silica and 7.5 μm melamine particles. By comparing these profiles with time-dependent constitutive curves obtained from experiment and theory, we explore the connection between transient nonmonotonic regions in the constitutive curve for an entangled polymer and its susceptibility to unstable flow by shear banding [Adams et al. Phys. Rev. Lett. 2009, 102, 067801-4]. Surprisingly, we find that even polymer systems which exhibit transient, nonmonotonic shear stress-shear rate relationships in bulk rheology experiments manifest time-dependent velocity profiles that are decidedly linear and show no evidence of unstable flow. We also report that interfacial slip plays an important role in the steady shear flow behavior of entangled polymers at shear rates above the reciprocal terminal relaxation time but has little, if any, effect on the shape of the velocity profile. © 2010 American Chemical Society.
Winkel, Leah C; Hoogendoorn, Ayla; Xing, Ruoyu; Wentzel, Jolanda J; Van der Heiden, Kim
2015-07-01
Atherosclerosis is a chronic inflammatory disease of the arterial tree that develops at predisposed sites, coinciding with locations that are exposed to low or oscillating shear stress. Manipulating flow velocity, and concomitantly shear stress, has proven adequate to promote endothelial activation and subsequent plaque formation in animals. In this article, we will give an overview of the animal models that have been designed to study the causal relationship between shear stress and atherosclerosis by surgically manipulating blood flow velocity profiles. These surgically manipulated models include arteriovenous fistulas, vascular grafts, arterial ligation, and perivascular devices. We review these models of manipulated blood flow velocity from an engineering and biological perspective, focusing on the shear stress profiles they induce and the vascular pathology that is observed. Copyright © 2015 Elsevier Ireland Ltd. All rights reserved.
Rational surfaces, ExB sheared flows and transport interplay in fusion plasmas
International Nuclear Information System (INIS)
Hidalgo, C.; Pedrosa, M.A.; Erents, K.
2002-01-01
Experimental evidence of a strong interplay between magnetic topology (rational surfaces) and the generation of ExB sheared flows has been observed in the plasma edge region of stellarator (TJ-II) and tokamak (JET) devices. Both constant and varying in time ExB sheared flows are close to the critical value to trigger the transition to improved confinement regimes, but below the power threshold to trigger the formation of transport barriers. Flows driven by fluctuations are candidates to explain these experimental results. (author)
Sheared-flow induced confinement transition in a linear magnetized plasma
Zhou, S.; Heidbrink, W. W.; Boehmer, H.; McWilliams, R.; Carter, T. A.; Vincena, S.; Friedman, B.; Schaffner, D.
2012-01-01
A magnetized plasma cylinder (12 cm in diameter) is induced by an annular shape obstacle at the Large Plasma Device [W. Gekelman, H. Pfister, Z. Lucky, J. Bamber, D. Leneman, and J. Maggs, Rev. Sci. Instrum. 62, 2875 (1991)]. Sheared azimuthal flow is driven at the edge of the plasma cylinder through edge biasing. Strong fluctuations of density and potential (δn /n~eδφ/kTe~0.5) are observed at the plasma edge, accompanied by a large density gradient (Ln=|∇lnn |-1~2cm) and shearing rate (γ ~300kHz). Edge turbulence and cross-field transport are modified by changing the bias voltage (Vbias) on the obstacle and the axial magnetic field (Bz) strength. In cases with low Vbias and large Bz, improved plasma confinement is observed, along with steeper edge density gradients. The radially sheared flow induced by E ×B drift dramatically changes the cross-phase between density and potential fluctuations, which causes the wave-induced particle flux to reverse its direction across the shear layer. In cases with higher bias voltage or smaller Bz, large radial transport and rapid depletion of the central plasma density are observed. Two-dimensional cross-correlation measurement shows that a mode with azimuthal mode number m =1 and large radial correlation length dominates the outward transport in these cases. Linear analysis based on a two-fluid Braginskii model suggests that the fluctuations are driven by both density gradient (drift wave like) and flow shear (Kelvin-Helmholtz like) at the plasma edge.
Sheared-flow induced confinement transition in a linear magnetized plasma
International Nuclear Information System (INIS)
Zhou, S.; Heidbrink, W. W.; Boehmer, H.; McWilliams, R.; Carter, T. A.; Vincena, S.; Friedman, B.; Schaffner, D.
2012-01-01
A magnetized plasma cylinder (12 cm in diameter) is induced by an annular shape obstacle at the Large Plasma Device [W. Gekelman, H. Pfister, Z. Lucky, J. Bamber, D. Leneman, and J. Maggs, Rev. Sci. Instrum. 62, 2875 (1991)]. Sheared azimuthal flow is driven at the edge of the plasma cylinder through edge biasing. Strong fluctuations of density and potential (δn/n∼eδφ/kT e ∼0.5) are observed at the plasma edge, accompanied by a large density gradient (L n =∇lnn -1 ∼2cm) and shearing rate (γ∼300kHz). Edge turbulence and cross-field transport are modified by changing the bias voltage (V bias ) on the obstacle and the axial magnetic field (B z ) strength. In cases with low V bias and large B z , improved plasma confinement is observed, along with steeper edge density gradients. The radially sheared flow induced by ExB drift dramatically changes the cross-phase between density and potential fluctuations, which causes the wave-induced particle flux to reverse its direction across the shear layer. In cases with higher bias voltage or smaller B z , large radial transport and rapid depletion of the central plasma density are observed. Two-dimensional cross-correlation measurement shows that a mode with azimuthal mode number m=1 and large radial correlation length dominates the outward transport in these cases. Linear analysis based on a two-fluid Braginskii model suggests that the fluctuations are driven by both density gradient (drift wave like) and flow shear (Kelvin-Helmholtz like) at the plasma edge.
Directory of Open Access Journals (Sweden)
Yu Du
2015-11-01
Full Text Available Blood cell aggregation and adhesion to endothelial cells under shear flow are crucial to many biological processes such as thrombi formation, inflammatory cascade, and tumor metastasis, in which these cellular interactions are mainly mediated by the underlying receptor–ligand bindings. While theoretical modeling of aggregation dynamics and adhesion kinetics of interacting cells have been well studied separately, how to couple these two processes remains unclear. Here we develop a combined model that couples cellular aggregation dynamics and adhesion kinetics under shear flow. The impacts of shear rate (or shear stress and molecular binding affinity were elucidated. This study provides a unified model where the action of a fluid flow drives cell aggregation and adhesion under the modulations of the mechanical shear flow and receptor–ligand interaction kinetics. It offers an insight into understanding the relevant biological processes and functions.
Theory of the mechanical response of focal adhesions to shear flow
International Nuclear Information System (INIS)
Biton, Y Y; Safran, S A
2010-01-01
The response of cells to shear flow is primarily determined by the asymmetry of the external forces and moments that are sensed by each member of a focal adhesion pair connected by a contractile stress fiber. In the theory presented here, we suggest a physical model in which each member of such a pair of focal adhesions is treated as an elastic body subject to both a myosin-activated contractile force and the shear stress induced by the external flow. The elastic response of a focal adhesion complex is much faster than the active cellular processes that determine the size of the associated focal adhesions and the direction of the complex relative to the imposed flow. Therefore, the complex attains its mechanical equilibrium configuration which may change because of the cellular activity. Our theory is based on the experimental observation that focal adhesions modulate their cross-sectional area in order to attain an optimal shear. Using this assumption, our elastic model shows that such a complex can passively change its orientation to align parallel to the direction of the flow.
The interaction of two spheres in a simple-shear flow of complex fluids
Firouznia, Mohammadhossein; Metzger, Bloen; Ovarlez, Guillaume; Hormozi, Sarah
2017-11-01
We study the interaction of two small freely-moving spheres in a linear flow field of Newtonian, shear thinning and yield stress fluids. We perform a series of experiments over a range of shear rates as well as different shear histories using an original apparatus and with the aid of conventional rheometry, Particle Image Velocimetry and Particle Tracking Velocimetry. Showing that the non-Newtonian nature of the suspending fluid strongly affects the shape of particle trajectories and the irreversibility. An important point is that non-Newtonian effects can be varied and unusual. Depending on the shear rate, nonideal shear thinning and yield stress suspending fluids might show elasticity that needs to be taken into account. The flow field around one particle is studied in different fluids when subjected to shear. Then using these results to explain the two particle interactions in a simple-shear flow we show how particle-particle contact and non-Newtonian behaviors result in relative trajectories with fore-aft asymmetry. Well-resolved velocity and stress fields around the particles are presented here. Finally, we discuss how the relative particle trajectories may affect the microstructure of complex suspensions and consequently the bulk rheology. NSF (Grant No. CBET-1554044-CAREER).
Ribbon phase in a phase-separated lyotropic lamellar-sponge mixture under shear flow
Cristobal, G.; Rouch, J.; Panizza, P.; Narayanan, T.
2001-07-01
We report the effect of shear flow on a phase-separated system composed of lyotropic lamellar (Lα) and sponge (L3) phases in a mixture of brine, surfactant, and cosurfactant. Optical microscopy, small-angle light, and x-ray scattering measurements are consistent with the existence of a steady state made of multilamellar ribbonlike structures aligned in the flow direction. At high shear rates, these ribbonlike structures become unstable and break up into monodisperse droplets resulting in a shear-thickening transition.
Vortex-induced vibrations of a square cylinder under linear shear flow
Energy Technology Data Exchange (ETDEWEB)
Sun, Wenjuan; Zhou, Dai; Han, Zhaolong [School of Naval Architecture, Ocean and Civil Engineering, Shanghai Jiao Tong University, Shanghai 200240 (China); Tu, Jiahuang, E-mail: tujiahuang1982@163.com, E-mail: han.arkey@gmail.com [College of Civil Engineering and Mechanics, Xiangtan University, Xiangtan, Hunan 411105 (China)
2017-04-15
This paper investigates the numerical vortex-induced vibration (VIV) of a square cylinder which is connected to a 2-DOF mass-spring system and is immersed in the planar shear flow by employing a characteristic-based split (CBS) finite element method (FEM). The reduced mass of the square cylinder is M {sub r} = 2, while the reduced velocity, U {sub r}, is changed from 3 to 12 with an increment of Δ U {sub r} = 1. The effects of some key parameters on the cylinder dynamic responses, vibrating frequencies, the flow patterns as well as the energy transferred between the fluid and cylinder are revealed. In this study, the key parameters are selected as follows: shear ratio ( k = 0, 0.05 and 0.1) and Reynolds numbers ( Re = 80 and 160). Numerical results demonstrate that the X – Y trajectories of the cylinder mainly appear as a symmetrical figure ‘8’ in uniform flow ( k = 0) and an unsymmetrical figure ‘8’ and ‘O’ in shear flows ( k = 0.05 and 0.1). The maximum oscillation amplitudes of the square cylinder in both the inline and transverse directions have distinct characteristics compared to that of a circular cylinder. Two kinds of flow patterns, ‘2S’ and ‘P + S’, are mainly observed under the shear flow. Also, the mean values of the energy of the cylinder system increase with the reduced velocity, while the root mean square (rms) of the energy reaches its peak value at reduced velocity U {sub r} = 5. (paper)
Oligomerized backbone pilin helps piliated Lactococcus lactis to withstand shear flow.
Castelain, Mickaël; Duviau, Marie-Pierre; Oxaran, Virginie; Schmitz, Philippe; Cocaign-Bousquet, Muriel; Loubière, Pascal; Piard, Jean-Christophe; Mercier-Bonin, Muriel
2016-09-01
The present work focuses on the role of pili present at the cell surface of Lactococcus lactis in bacterial adhesion to abiotic (hydrophobic polystyrene) and biotic (mucin-coated polystyrene) surfaces. Native pili-displaying strains and isogenic derivatives in which pilins or sortase C structural genes had been modified were used. Surface physico-chemistry, morphology and shear-flow-induced detachment of lactococcal cells were evaluated. The involvement of pili in L. lactis adhesion was clearly demonstrated, irrespective of the surface characteristics (hydrophobic/hydrophilic, presence or not of specific binding sites). The accessory pilin, PilC, and the backbone pilin, PilB, were revealed to play a major role in adhesion, provided that the PilB was present in its polymerized form. Within the population fraction that remained attached to the surface under increasing shear flow, different association behaviors were observed, showing that pili could serve as anchoring sites thus hampering the effect of shear flow on cell orientation and detachment.
Nature of turbulent transport across sheared zonal flows: insights from gyrokinetic simulations
International Nuclear Information System (INIS)
Sanchez, R; Newman, D E; Leboeuf, J-N; Decyk, V K
2011-01-01
The traditional view regarding the reduction of turbulence-induced transport across a stable sheared flow invokes a reduction of the characteristic length scale in the direction perpendicular to the flow as a result of the shearing and stretching of eddies caused by the differential pull exerted in the direction of the flow. A reduced effective transport coefficient then suffices to capture the reduction, that can then be readily incorporated into a transport model. However, recent evidence from gyrokinetic simulations of the toroidal ion-temperature-gradient mode suggests that the dynamics of turbulent transport across sheared flows changes in a more fundamental manner, and that the use of reduced effective transport coefficients fails to capture the full dynamics that may exhibit both subdiffusion and non-Gaussian statistics. In this contribution, after briefly reviewing these results, we propose some candidates for the physical mechanisms responsible for endowing transport with such non-diffusive characteristics, backing these proposals with new numerical gyrokinetic data.
Rational surfaces, ExB sheared flows and transport interplay in fusion plasmas
International Nuclear Information System (INIS)
Hidalgo, Carlos; Pedrosa, Maria A.; Erents, Kevin
2001-01-01
Experimental evidence of a strong interplay between magnetic topology (rational surfaces) and the generation of ExB sheared flows has been observed in the plasma edge region of stellarator (TJ-II) and tokamak (JET) devices. Constant and varying in time ExB sheared flows are close to the critical value to trigger the transition to improved confinement regimes. The plasma conditions where this has been observed are clearly below the power threshold to trigger the formation of transport barriers. Flows driven by fluctuations are candidates to explain these experimental results. (author)
Optimal energy growth in a stably stratified shear flow
Jose, Sharath; Roy, Anubhab; Bale, Rahul; Iyer, Krithika; Govindarajan, Rama
2018-02-01
Transient growth of perturbations by a linear non-modal evolution is studied here in a stably stratified bounded Couette flow. The density stratification is linear. Classical inviscid stability theory states that a parallel shear flow is stable to exponentially growing disturbances if the Richardson number (Ri) is greater than 1/4 everywhere in the flow. Experiments and numerical simulations at higher Ri show however that algebraically growing disturbances can lead to transient amplification. The complexity of a stably stratified shear flow stems from its ability to combine this transient amplification with propagating internal gravity waves (IGWs). The optimal perturbations associated with maximum energy amplification are numerically obtained at intermediate Reynolds numbers. It is shown that in this wall-bounded flow, the three-dimensional optimal perturbations are oblique, unlike in unstratified flow. A partitioning of energy into kinetic and potential helps in understanding the exchange of energies and how it modifies the transient growth. We show that the apportionment between potential and kinetic energy depends, in an interesting manner, on the Richardson number, and on time, as the transient growth proceeds from an optimal perturbation. The oft-quoted stabilizing role of stratification is also probed in the non-diffusive limit in the context of disturbance energy amplification.
Shear-flow coupling in non-planar rock joints
International Nuclear Information System (INIS)
Makurat, A.; Barton, N.
1985-01-01
Crystalline rock masses are regarded as a possible host rock for permanent nuclear waste disposal. During the excavation of the required shafts and tunnels, the initial state of stress will be changed and cause a deformation of the rock mass and discontinuities. During the lifetime of the nuclear repository joint apertures may change due to thermally induced stress variations during the heating and cooling phase. As the conductivity of a joint is very sensitive to its aperture, fluid flow from and towards a repository, as well as the potential transport times of radionuclides are highly dependent on the deformability of the joints. Theoretical calculations of coupled flow in rock joints (Barton et al. 1984) predict an increase of conductivity of several orders of magnitude for the first few millimeters for shear displacement. The shear-dilation-conductivity coupling for two block sizes at two effective stress levels is shown
The role of flow shear in the ballooning stability of tokamak transport barriers
International Nuclear Information System (INIS)
Webster, A.J.; Wilson, H.R.; Scaife, A.M.M.
2004-01-01
A tokamak's economic performance is strongly affected by the plasma pressure that it may sustain, which in turn is limited by the maximum pressure gradients that may be supported. Ballooning modes are typically driven unstable by increasing the pressure gradient, and because they can radially extend across many rational surfaces, they can seriously reduce a plasma's energy confinement. Here an eigenmode formulation is used to study the stability of ballooning modes in internal transport barriers ('ITBs'), in which very strong pressure gradients and flow shears may be found. This extends previous studies that used an 'eikonal' formulation, as it enables the study of: ballooning modes with a finite toroidal mode-number n (finite wavelength perpendicular to the magnetic field), to find new solution branches, to obtain the eigenmode structures, and to investigate the effects of a radially varying equilibrium. The structure of a finite n ballooning mode in flow shear is found to be significantly affected by a radially varying equilibrium, and at low flow shears the growth rates are increased above those of modes studied in the limit of n→∞. The different solution branches can couple as the flow shear is increased, leading to a pair of asymmetric mode structures with complex conjugate growth rates. These effects are shown to be a consequence of the mode trying to localize at the most unstable radial location, and its desire to rotate with the flow. In addition, closer to marginal stability a sufficiently strong flow-shear can (at least for some cases), destabilize a previously stable mode
Effects of flow shear and Alfven waves on two-dimensional magnetohydrodynamic turbulence
International Nuclear Information System (INIS)
Douglas, Jamie; Kim, Eun-jin; Thyagaraja, A.
2008-01-01
The suppression of turbulent transport by large scale mean shear flows and uniform magnetic fields is investigated in two-dimensional magnetohydrodynamic turbulence driven by a small-scale forcing with finite correlation time. By numerical integration the turbulent magnetic diffusivity D T is shown to be significantly quenched, with a scaling D T ∝B -2 Ω 0 -5/4 , which is much more severe than in the case of a short or delta correlated forcing typified by white noise, studied in E. Kim and B. Dubrulle [Phys. Plasmas 8, 813 (2001)]. Here B and Ω 0 are magnetic field strength and flow shear rate, respectively. The forcing with finite correlation time also leads to much stronger suppression of momentum transport through the cancellation of the Reynolds stress by the Maxwell stress with a positive small value of turbulent viscosity, ν T >0. While fluctuating kinetic and magnetic energies are unaffected by the magnetic field just as in the case of a delta correlated forcing, they are much more severely quenched by flow shear than in that of a delta correlated forcing. Underlying physical mechanisms for the reduction of turbulent transport and turbulence level by flow shear and magnetic field are discussed
Reynolds stress and shear flow generation
DEFF Research Database (Denmark)
Korsholm, Søren Bang; Michelsen, Poul; Naulin, V.
2001-01-01
The so-called Reynolds stress may give a measure of the self-consistent flow generation in turbulent fluids and plasmas by the small-scale turbulent fluctuations. A measurement of the Reynolds stress can thus help to predict flows, e.g. shear flows in plasmas. This may assist the understanding...... of improved confinement scenarios such as H-mode confinement regimes. However, the determination of the Reynolds stress requires measurements of the plasma potential, a task that is difficult in general and nearly impossible in hot plasmas in large devices. In this work we investigate an alternative method......, based on density measurements, to estimate the Reynolds stress, and demonstrate the validity range of this quantity, which we term the pseudo-Reynolds stress. The advantage of such a quantity is that accurate measurements of density fluctuations are much easier to obtain experimentally. Prior...
Momentum-energy transport from turbulence driven by parallel flow shear
International Nuclear Information System (INIS)
Dong, J.Q.; Horton, W.; Bengtson, R.D.; Li, G.X.
1994-04-01
The low frequency E x B turbulence driven by the shear in the mass flow velocity parallel to the magnetic field is studied using the fluid theory in a slab configuration with magnetic shear. Ion temperature gradient effects are taken into account. The eigenfunctions of the linear instability are asymmetric about the mode rational surfaces. Quasilinear Reynolds stress induced by such asymmetric fluctuations produces momentum and energy transport across the magnetic field. Analytic formulas for the parallel and perpendicular Reynolds stress, viscosity and energy transport coefficients are given. Experimental observations of the parallel and poloidal plasma flows on TEXT-U are presented and compared with the theoretical models
Turbulent flows over superhydrophobic surfaces with shear-dependent slip length
Khosh Aghdam, Sohrab; Seddighi, Mehdi; Ricco, Pierre
2015-11-01
Motivated by recent experimental evidence, shear-dependent slip length superhydrophobic surfaces are studied. Lyapunov stability analysis is applied in a 3D turbulent channel flow and extended to the shear-dependent slip-length case. The feedback law extracted is recognized for the first time to coincide with the constant-slip-length model widely used in simulations of hydrophobic surfaces. The condition for the slip parameters is found to be consistent with the experimental data and with values from DNS. The theoretical approach by Fukagata (PoF 18.5: 051703) is employed to model the drag-reduction effect engendered by the shear-dependent slip-length surfaces. The estimated drag-reduction values are in very good agreement with our DNS data. For slip parameters and flow conditions which are potentially realizable in the lab, the maximum computed drag reduction reaches 50%. The power spent by the turbulent flow on the walls is computed, thereby recognizing the hydrophobic surfaces as a passive-absorbing drag-reduction method, as opposed to geometrically-modifying techniques that do not consume energy, e.g. riblets, hence named passive-neutral. The flow is investigated by visualizations, statistical analysis of vorticity and strain rates, and quadrants of the Reynolds stresses. Part of this work was funded by Airbus Group. Simulations were performed on the ARCHER Supercomputer (UKTC Grant).
Laboratory investigation of nonlinear flow characteristics in rough fractures during shear process
Rong, Guan; Yang, Jie; Cheng, Long; Zhou, Chuangbing
2016-10-01
To understand the influence of shear behavior on the transporting properties of fluid through a single fracture, splitting fractures were made in the laboratory and shear flow tests were carried out under constant normal load conditions. The applied normal stress is in the range of 0.5-3.0 MPa. Before the physical test, the fracture's morphology is measured for identification of the roughness. At each shear step, we performed 5-8 high precise hydraulic tests with different hydraulic gradient. The relationship between pressure gradient and volume flow rate demonstrates to be nonlinear and fits very well with Forchheimer's and Izbash's laws. The linear and nonlinear coefficients in Forchheimer's law are quite sensitive to shear deformation (closure or dilation), experienced 1-2 and 1-3 orders of magnitude reduction during shear, respectively. An empirical equation is proposed to quantify the relationship between linear coefficient and nonlinear coefficient based on the experimental observations. The two coefficients in Izbash's law are quantified. The m value is in the range between 1.06 and 1.41 and the λ value experiences a reduction of 1-2 orders of magnitude during shear. In addition, the studied critical Reynolds number exhibits a decreasing and increasing variation corresponding to shear contraction and shear dilation of rock fracture. For all the cases in this study, the critical Reynolds number ranges between 1.5 and 13.0.
The effect of shear flow on the rotational diffusivity of a single axisymmetric particle
Leahy, Brian; Koch, Donald; Cohen, Itai
2014-11-01
Colloidal suspensions of nonspherical particles abound in the world around us, from red blood cells in arteries to kaolinite discs in clay. Understanding the orientation dynamics of these particles is important for suspension rheology and particle self-assembly. However, even for the simplest case of dilute suspensions in simple shear flow, the orientation dynamics of Brownian nonspherical particles are poorly understood at large shear rates. Here, we analytically calculate the time-dependent orientation distributions of particles confined to the flow-gradient plane when the rotary diffusion is small but nonzero. For both startup and oscillatory shear flows, we find a coordinate change that maps the convection-diffusion equation to a simple diffusion equation with an enhanced diffusion constant, simplifying the orientation dynamics. For oscillatory shear, this enhanced diffusion drastically alters the quasi-steady orientation distributions. Our theory of the unsteady orientation dynamics provides an understanding of a nonspherical particle suspension's rheology for a large class of unsteady flows. For particles with aspect ratio 10 under oscillatory shear, the rotary diffusion and intrinsic viscosity vary with amplitude by a factor of ~ 40 and ~ 2 , respectively.
Numerical simulation of systems of shear bands in ductile metal with inclusions
Plohr, Jeeyeon
2017-06-01
We develop a method for numerical simulations of high strain-rate loading of mesoscale samples of ductile metal with inclusions. Because of its small-scale inhomogeneity, the composite material is prone to localized shear deformation. This method employs the Generalized Method of Cells to ensure that the micro mechanical behavior of the metal and inclusions is reflected properly in the behavior of the composite at the mesoscale. To find the effective plastic strain rate when shear bands are present, we extend and apply the analytic and numerical analysis of shear bands of Glimm, Plohr, and Sharp. Our tests of the method focus on the stress/strain response in uniaxial-strain flow, both compressive and tensile, of depleted uranium metal containing silicon carbide inclusions. In results, we verify the elevated temperature and thermal softening at shear bands in our simulations of pure DU and DU/SiC composites. We also note that in composites, due the asymmetry caused by the inclusions, shear band form at different times in different subcells. In particular, in the subcells near inclusions, shear band form much earlier than they do in pure DU.
Turbulent characteristics of shear-thinning fluids in recirculating flows
Energy Technology Data Exchange (ETDEWEB)
Pereira, A.S. [Inst. Superior de Engenharia do Porto (Portugal). Dept. de Engenharia Quimica; Pinho, F.T. [Centro de Estudos de Fenomenos de Transporte, Departamento de Engenharia Mecanica e Gestao Industrial, Faculdade de Engenharia da Universidade do Porto, Rua dos Bragas, 4050-123 Porto (Portugal)
2000-03-01
A miniaturised fibre optic laser-Doppler anemometer was used to carry out a detailed hydrodynamic investigation of the flow downstream of a sudden expansion with 0.1-0.2% by weight shear-thinning aqueous solutions of xanthan gum. Upstream of the sudden expansion the pipe flow was fully-developed and the xanthan gum solutions exhibited drag reduction with corresponding lower radial and tangential normal Reynolds stresses, but higher axial Reynolds stress near the wall and a flatter axial mean velocity profile in comparison with Newtonian flow. The recirculation bubble length was reduced by more than 20% relative to the high Reynolds number Newtonian flow, and this was attributed to the occurrence further upstream of high turbulence for the non-Newtonian solutions, because of advection of turbulence and earlier high turbulence production in the shear layer. Comparisons with the measurements of Escudier and Smith (1999) with similar fluids emphasized the dominating role of inlet turbulence. The present was less anisotropic, and had lower maximum axial Reynolds stresses (by 16%) but higher radial turbulence (20%) than theirs. They reported considerably longer recirculating bubble lengths than we do for similar non-Newtonian fluids and Reynolds numbers. (orig.)
Hemolysis in a laminar flow-through Couette shearing device: an experimental study.
Boehning, Fiete; Mejia, Tzahiry; Schmitz-Rode, Thomas; Steinseifer, Ulrich
2014-09-01
Reducing hemolysis has been one of the major goals of rotary blood pump development and in the investigational phase, the capability of hemolysis estimation for areas of elevated shear stresses is valuable. The degree of hemolysis is determined by the amplitude of shear stress and the exposure time, but to date, the exact hemolytic behavior at elevated shear stresses and potential thresholds for subcritical shear exposure remain vague. This study provides experimental hemolysis data for a set of shear stresses and exposure times to allow better estimations of hemolysis for blood exposed to elevated shearing. Heparinized porcine blood with a hematocrit of 40% was mechanically damaged in a flow-through laminar Couette shear flow at a temperature of 23°C. Four levels of shear stress, 24, 592, 702, and 842 Pa, were replicated at two exposure times, 54 and 873 ms. For the calculation of the shear stresses, an apparent viscosity of 5 mPas was used, which was verified in an additional measurement of the blood viscosity. The hemolysis measurements were repeated four times, whereby all conditions were measured once within the same day and with blood from the same source. Samples were taken at the inlet and outlet of the shear region and an increase in plasma-free hemoglobin was measured. An index of hemolysis (IH) was thereby calculated giving the ratio of free to total hemoglobin. The results are compared with data from previously published studies using a similar shearing device. Hemolysis was found to increase exponentially with shear stress, but high standard deviations existed at measurements with elevated IH. At short exposure times, the IH remained low at under 0.5% for all shear stress levels. For high exposure times, the IH increased from 0.84% at 592 Pa up to 3.57% at the highest shear stress level. Hemolysis was significant for shear stresses above ∼600 Pa at the high exposure time of 873 ms. Copyright © 2014 International Center for Artificial
Regional cerebral blood flow in pure dysarthria. A 3D-SSP study
International Nuclear Information System (INIS)
Okamoto, Kensho; Kamogawa, Kenji; Okuda, Bungo; Kawabata, Keita; Tachibana, Hisao
2007-01-01
Pure dysarthria from brain infarction is a rare condition, and its pathophysiology remains unclear. To clarify the underlying mechanism of pure dysarthria, we investigated the lesion sites and regional cerebral blood flow in patients with pure dysarthria. We examined 18 consecutive patients with pure dysarthria (9 men and 9 women; mean age, 71 years) who underwent MRI and cerebral blood flow studies. To visualize the regional cerebral blood flow, we generated Z score images using the three-dimensional stereotactic surface projection (3D-SSP) method with single-photon emission computed tomography (SPECT) and N-isopropyl-p [ 123 I]iodoamphetamine. Data on the brain surface perfusion extracted by the 3D-SSP analysis were compared between the pure dysarthria (PD) patients and 9 control subjects. MRI revealed multiple lacunar infarctions involving the internal capsule and/or corona radiata in 11 patients, left internal capsule-corona radiata infarction in 4 patients, and pontine infarction in 3 patients. SPECT with 3D-SSP demonstrated bilateral frontal cortical hypoperfusion in all patients, particularly in the anterior opercular region. Based on intergroup comparisons, the PD group exhibited pronounced cortical hypoperfusion in the opercular and medial frontal regions, left more than right. In conclusion, pure dysarthria is considered to originate from frontal cortical hypoperfusion, mainly in the anterior opercular and medial frontal regions, which is probably due to interruption of the corticosubcortical neural networks relevant to speech expression and articulation. In addition, it is suggested that left hemispheric lesions may make a greater contribution to the development of pure dysarthria than do right ones. (author)
Shear flow simulations of biaxial nematic liquid crystals
Sarman, Sten
1997-08-01
We have calculated the viscosities of a biaxial nematic liquid crystal phase of a variant of the Gay-Berne fluid [J. G. Gay and B. J. Berne, J. Chem. Phys. 74, 3316 (1981)] by performing molecular dynamics simulations. The equations of motion have been augmented by a director constraint torque that fixes the orientation of the directors. This makes it possible to fix them at different angles relative to the stream lines in shear flow simulations. In equilibrium simulations the constraints generate a new ensemble. One finds that the Green-Kubo relations for the viscosities become linear combinations of time correlation function integrals in this ensemble whereas they are complicated rational functions in the conventional canonical ensemble. We have evaluated these Green-Kubo relations for all the shear viscosities and all the twist viscosities. We have also calculated the alignment angles, which are functions of the viscosity coefficients. We find that there are three real alignment angles but a linear stability analysis shows that only one of them corresponds to a stable director orientation. The Green-Kubo results have been cross checked by nonequilibrium shear flow simulations. The results from the different methods agree very well. Finally, we have evaluated the Miesowicz viscosities [D. Baalss, Z. Naturforsch. Teil A 45, 7 (1990)]. They vary by more than 2 orders of magnitude. The viscosity is consequently highly orientation dependent.
Shear-thickening behavior of Fe-ZSM5 zeolite slurry and its removal with alumina/boehmites
Liu, Xiao-guang; Li, Yan; Xue, Wen-dong; Sun, Jia-lin; Tang, Qian
2018-06-01
A cryogenic scanning electron microscopy (cryo-SEM) technique was used to explore the shear-thickening behavior of Fe-ZSM5 zeolite pastes and to discover its underlying mechanism. Bare Fe-ZSM5 zeolite samples were found to contain agglomerations, which may break the flow of the pastes and cause shear-thickening behaviors. However, the shear-thickening behaviors can be eliminated by the addition of halloysite and various boehmites because of improved particle packing. Furthermore, compared with pure Fe-ZSM5 zeolite samples and its composite samples with halloysite, the samples with boehmite (Pural SB or Disperal) additions exhibited network structures in their cryo-SEM images; these structures could facilitate the storage and release of flow water, smooth paste flow, and avoid shear-thickening. By contrast, another boehmite (Versal 250) formed agglomerations rather than network structures after being added to the Fe-ZSM5 zeolite paste and resulted in shear-thickening behavior. Consequently, the results suggest that these network structures play key roles in eliminating the shear-thickening behavior.
Microturbulence and Flow Shear in High-performance JET ITB Plasma; TOPICAL
International Nuclear Information System (INIS)
R.V. Budny; A. Andre; A. Bicoulet; C. Challis; G.D. Conway; W. Dorland; D.R. Ernst; T.S. Hahm; T.C. Hender; D. McCune; G. Rewoldt; S.E. Sharapov
2001-01-01
The transport, flow shear, and linear growth rates of microturbulence are studied for a Joint European Torus (JET) plasma with high central q in which an internal transport barrier (ITB) forms and grows to a large radius. The linear microturbulence growth rates of the fastest growing (most unstable) toroidal modes with high toroidal mode number are calculated using the GS2 and FULL gyrokinetic codes. These linear growth rates, gamma (subscript lin) are large, but the flow-shearing rates, gamma (subscript ExB) (dominated by the toroidal rotation contribution) are also comparably large when and where the ITB exists
International Nuclear Information System (INIS)
Sotnikov, V.I.; Paraschiv, I.; Makhin, V.; Bauer, B.S.; Leboeuf, J.N.; Dawson, J.M.
2002-01-01
A systematic study of the linear stage of sheared flow stabilization of Z-pinch plasmas based on the Hall fluid model with equilibrium that contains sheared flow and an axial magnetic field is presented. In the study we begin with the derivation of a general set of equations that permits the evaluation of the combined effect of sheared flow and axial magnetic field on the development of the azimuthal mode number m=0 sausage and m=1 kink magnetohydrodynamic (MHD) instabilities, with the Hall term included in the model. The incorporation of sheared flow, axial magnetic field, and the Hall term allows the Z-pinch system to be taken away from the region in parameter space where ideal MHD is applicable to a regime where nonideal effects tend to govern stability. The problem is then treated numerically by following the linear development in time of an initial perturbation. The numerical results for linear growth rates as a function of axial sheared flow, an axial magnetic field, and the Hall term are reported
Influence of steady shear flow on dynamic viscoelastic properties of ...
Indian Academy of Sciences (India)
Unknown
temporary network formed by the fibres, their entangle- ment etc. The structural density is also a function of vol- ume fraction of reinforcing fibres (Amari et al 1992). The complex flow pattern encountered during moulding/ stamping are generally far from simple steady or oscilla- tory shear flow. Therefore, it is important to ...
Effect of stable-density stratification on counter gradient flux of a homogeneous shear flow
Energy Technology Data Exchange (ETDEWEB)
Lida, Oaki; Nagano, Yasutaka [Nagoya Institute of Technology, Gokiso-cho, Showa-ku, Nagoya (Japan). Department of Mechanical Engineering
2007-01-15
We performed direct numerical simulations of homogeneous shear flow under stable-density stratification to study the buoyancy effects on the heat and momentum transfer. These numerical data were compared with those of a turbulent channel flow to investigate the similarity between the near-wall turbulence and the homogeneous shear flow. We also investigated the generation mechanism of the persistent CGFs (counter gradient fluxes) appearing at the higher wavenumbers of the cospectrum, and lasting over a long time without oscillation. Spatially, the persistent CGFs are associated with the longitudinal vortical structure, which is elongated in the streamwise direction and typically observed in both homogeneous shear flow and near-wall turbulence. The CGFs appear at both the top and bottom of this longitudinal vortical structure, and expand horizontally with an increase in the Richardson number. It was found that the production and turbulent-diffusion terms are responsible for the distribution of the Reynolds shear stress including the persistent CGFs. The buoyancy term, combined with the swirling motion of the vortex, contributes to expand the persistent CGF regions and decrease the down gradient fluxes. (author)
High strength semi-active energy absorbers using shear- and mixedmode operation at high shear rates
Becnel, Andrew C.
This body of research expands the design space of semi-active energy absorbers for shock isolation and crash safety by investigating and characterizing magnetorheological fluids (MRFs) at high shear rates ( > 25,000 1/s) under shear and mixed-mode operation. Magnetorheological energy absorbers (MREAs) work well as adaptive isolators due to their ability to quickly and controllably adjust to changes in system mass or impact speed while providing fail-safe operation. However, typical linear stroking MREAs using pressure-driven flows have been shown to exhibit reduced controllability as impact speed (shear rate) increases. The objective of this work is to develop MREAs that improve controllability at high shear rates by using pure shear and mixed shear-squeeze modes of operation, and to present the fundamental theory and models of MR fluids under these conditions. A proof of concept instrument verified that the MR effect persists in shear mode devices at shear rates corresponding to low speed impacts. This instrument, a concentric cylinder Searle cell magnetorheometer, was then used to characterize three commercially available MRFs across a wide range of shear rates, applied magnetic fields, and temperatures. Characterization results are presented both as flow curves according to established practice, and as an alternate nondimensionalized analysis based on Mason number. The Mason number plots show that, with appropriate correction coefficients for operating temperature, the varied flow curve data can be collapsed to a single master curve. This work represents the first shear mode characterization of MRFs at shear rates over 10 times greater than available with commercial rheometers, as well as the first validation of Mason number analysis to high shear rate flows in MRFs. Using the results from the magnetorheometer, a full scale rotary vane MREA was developed as part of the Lightweight Magnetorheological Energy Absorber System (LMEAS) for an SH-60 Seahawk helicopter
Estimation of gas wall shear stress in horizontal stratified gas-liquid pipe flow
International Nuclear Information System (INIS)
Newton, C.H.; Behnia, M.
1996-01-01
Two-phase pipe flows occur in many industrial applications, such as condensers and evaporators, chemical processing equipment, nuclear reactors, and oil pipelines. A variety of basic mechanistic flow models for predicting the pressure gradient and liquid loading characteristics of these types of flows to assist in design calculations has emerged over the past two decades, especially for the stratified and slug flow regimes. These models generally rely on a number of basic assumptions and empirical closure equations. Possibly the most notable of these relates to the evaluation of interfacial shear stresses. However, one of the most important yet least discussed assumptions used in most of these models is that the phase wall shear stresses can be accurately estimated from correlations developed for single-phase pipe flows. The object of this article is to present measurements of gas wall shear up to locations in close proximity to the gas-liquid interface for a variety of interface conditions in developed flow, and to determine the effects of the interface on average gas wall friction factors. In this context the interface may be smooth, rippled or wavy
Huang, Bin; Chen, Chang-Ting; Chen, Chi-Shia; Wang, Yun-Ming; Hsieh, Hsyue-Jen; Wang, Danny Ling
2015-09-04
Laminar shear flow triggers a signaling cascade that maintains the integrity of endothelial cells (ECs). Hydrogen sulfide (H2S), a new gasotransmitter is regarded as an upstream regulator of nitric oxide (NO). Whether the H2S-generating enzymes are correlated to the enzymes involved in NO production under shear flow conditions remains unclear as yet. In the present study, the cultured ECs were subjected to a constant shear flow (12 dyn/cm(2)) in a parallel flow chamber system. We investigated the expression of three key enzymes for H2S biosynthesis, cystathionine-γ-lyase (CSE), cystathionine-β-synthase (CBS), and 3-mercapto-sulfurtransferase (3-MST). Shear flow markedly increased the level of 3-MST. Shear flow enhanced the production of H2S was determined by NBD-SCN reagent that can bind to cysteine/homocystein. Exogenous treatment of NaHS that can release gaseous H2S, ECs showed an increase of phosphorylation in Akt(S473), ERK(T202/Y204) and eNOS(S1177). This indicated that H2S can trigger the NO-production signaling cascade. Silencing of CSE, CBS and 3-MST genes by siRNA separately attenuated the phosphorylation levels of Akt(S473) and eNOS(S1177) under shear flow conditions. The particular mode of shear flow increased H2S production. The interplay between H2S and NO-generating enzymes were discussed in the present study. Copyright © 2015 Elsevier Inc. All rights reserved.
Molecular characteristics of stress overshoot for polymer melts under start-up shear flow.
Jeong, Sohdam; Kim, Jun Mo; Baig, Chunggi
2017-12-21
Stress overshoot is one of the most important nonlinear rheological phenomena exhibited by polymeric liquids undergoing start-up shear at sufficient flow strengths. Despite considerable previous research, the fundamental molecular characteristics underlying stress overshoot remain unknown. Here, we analyze the intrinsic molecular mechanisms behind the overshoot phenomenon using atomistic nonequilibrium molecular dynamics simulations of entangled linear polyethylene melts under shear flow. Through a detailed analysis of the transient rotational chain dynamics, we identify an intermolecular collision angular regime in the vicinity of the chain orientation angle θ ≈ 20° with respect to the flow direction. The shear stress overshoot occurs via strong intermolecular collisions between chains in the collision regime at θ = 15°-25°, corresponding to a peak strain of 2-4, which is an experimentally well-known value. The normal stress overshoot appears at approximately θ = 10°, at a corresponding peak strain roughly equivalent to twice that for the shear stress. We provide plausible answers to several basic questions regarding the stress overshoot, which may further help understand other nonlinear phenomena of polymeric systems.
Flow shear suppression of turbulence using externally driven ion Bernstein and Alfven waves
International Nuclear Information System (INIS)
Biglari, H.; Ono, M.
1992-01-01
The utilization of externally-launched radio-frequency waves as a means of active confinement control through the generation of sheared poloidal flows is explored. For low-frequency waves, kinetic Alfven waves are proposed, and are shown to drive sheared E x B flows as a result of the radial variation in the electromagnetic Reynolds stress. In the high frequency regime, ion Bernstein waves are considered, and shown to generate sheared poloidal rotation through the pondermotive force. In either case, it is shown that modest amounts of absorbed power (∼ few 100 kW) are required to suppress turbulence in a region of several cm radial width
The Effect of a Shear Flow on the Uptake of LDL and Ac-LDL by Cultured Vascular Endothelial Cells
Niwa, Koichi; Karino, Takeshi
The effects of a shear flow on the uptake of fluorescence-labeled low-density lipoprotein (DiI-LDL), acetylated LDL (DiI-Ac-LDL), and lucifer yellow (LY; a tracer of fluid-phase endocytosis) by cultured bovine aortic ECs were studied using a rotating-disk shearing apparatus. It was found that 2hours’ exposure of ECs to a laminar shear flow that imposed ECs an area-mean shear stress of 10dynes/cm2 caused an increase in the uptake of DiI-LDL and LY. By contrast, the uptake of DiI-Ac-LDL was decreased by exposure of the ECs to a shear flow. Addition of dextran sulfate (DS), a competitive inhibitor of scavenger receptors, reversed the effect of a shear flow on the uptake of DiI-Ac-LDL, resulting in an increase by the imposition of a shear flow, while the uptake of DiI-LDL and LY remained unaffected. It was concluded that a shear flow promotes the endocytosis of DiI-LDL and LY by ECs, but suppresses the uptake of DiI-Ac-LDL by ECs by inhibiting scavenger receptor-mediated endocytosis.
Sheared flow amplification by vacuum magnetic islands in stellarator plasmas
International Nuclear Information System (INIS)
Garcia, L.; Carreras, B. A.; Lynch, V. E.; Pedrosa, M. A.; Hidalgo, C.
2001-01-01
There is some experimental evidence that the E x B flows have radial structure that may be linked to rational surfaces. This flow structure may result from a self-organization process involving nonlinear flow amplification through Reynolds stress and fluctuation reduction by sheared flows. In stellarators, a large contribution to the Reynolds stress comes from the coupling of the magnetic field component of a vacuum field island with a plasma instability. In this process, the self-organization principle seems to be marginal stability for the fluctuations driving the flow
ExB flow shear and enhanced confinement in the Madison Symmetric Torus reversed-field pinch
International Nuclear Information System (INIS)
Chapman, B.E.; Almagri, A.F.; Anderson, J.K.; Chiang, C.; Craig, D.; Fiksel, G.; Lanier, N.E.; Prager, S.C.; Sarff, J.S.; Stoneking, M.R.; Terry, P.W.
1998-01-01
Strong ExB flow shear occurs in the edge of three types of enhanced confinement discharge in the Madison Symmetric Torus [Dexter et al., Fusion Technol. 19, 131 (1991)] reversed-field pinch. Measurements in standard (low confinement) discharges indicate that global magnetic fluctuations drive particle and energy transport in the plasma core, while electrostatic fluctuations drive particle transport in the plasma edge. This paper explores possible contributions of ExB flow shear to the reduction of both the magnetic and electrostatic fluctuations and, thus, the improved confinement. In one case, shear in the ExB flow occurs when the edge plasma is biased. Biased discharges exhibit changes in the edge electrostatic fluctuations and improved particle confinement. In two other cases, the flow shear emerges (1) when auxiliary current is driven in the edge and (2) spontaneously, following sawtooth crashes. Both edge electrostatic and global magnetic fluctuations are reduced in these discharges, and both particle and energy confinement improve. copyright 1998 American Institute of Physics
Steady shear flow properties of Cordia myxa leaf gum as a function of concentration and temperature.
Chaharlang, Mahmood; Samavati, Vahid
2015-08-01
The steady shear flow properties of dispersions of Cordia myxa leaf gum (CMLG) were determined as a function of concentration (0.5-2.5%, w/w), and temperature (10-50 °C). The CMLG dispersions exhibited strong shear-thinning behavior at all concentrations and temperatures. The Power-law (Ostwald-Waele's) and Herschel-Bulkley models were employed to characterize flow behavior of CMLG solutions at 0.1-100 s(-1) shear rate. Non-Newtonian shear-thinning behavior was observed at all temperatures and concentrations. While increase in temperature decreased the viscosity and increased the flow behavior indices, adverse effect was obtained by increasing the concentration. The Power-law model was found the best model to describe steady shear flow behavior of CMLG. The pseudoplasticity of CMLG increased markedly with concentration. An Arrhenius-type model was also used to describe the effect of temperature. The activation energy (Ea) appeared in the range of 5.972-18.104 kJ/mol, as concentration increased from 0.5% to 2.5%, at a shear rate of 10 s(-1). Copyright © 2015 Elsevier B.V. All rights reserved.
Bounded energy states in homogeneous turbulent shear flow: An alternative view
Bernard, Peter S.; Speziale, Charles G.
1990-01-01
The equilibrium structure of homogeneous turbulent shear flow is investigated from a theoretical standpoint. Existing turbulence models, in apparent agreement with physical and numerical experiments, predict an unbounded exponential time growth of the turbulent kinetic energy and dissipation rate; only the anisotropy tensor and turbulent time scale reach a structural equilibrium. It is shown that if vortex stretching is accounted for in the dissipation rate transport equation, then there can exist equilibrium solutions, with bounded energy states, where the turbulence production is balanced by its dissipation. Illustrative calculations are present for a k-epsilon model modified to account for vortex stretching. The calculations indicate an initial exponential time growth of the turbulent kinetic energy and dissipation rate for elapsed times that are as large as those considered in any of the previously conducted physical or numerical experiments on homogeneous shear flow. However, vortex stretching eventually takes over and forces a production-equals-dissipation equilibrium with bounded energy states. The validity of this result is further supported by an independent theoretical argument. It is concluded that the generally accepted structural equilibrium for homogeneous shear flow with unbounded component energies is in need of re-examination.
Lattice Boltzmann Study of Bubbles on a Patterned Superhydrophobic Surface under Shear Flow
Chen, Wei; Wang, Kai; Hou, Guoxiang; Leng, Wenjun
2018-01-01
This paper studies shear flow over a 2D patterned superhydrophobic surface using lattice Boltzmann method (LBM). Single component Shan-Chen multiphase model and Carnahan-Starling EOS are adopted to handle the liquid-gas flow on superhydrophobic surface with entrapped micro-bubbles. The shape of bubble interface and its influence on slip length under different shear rates are investigated. With increasing shear rate, the bubble interface deforms. Then the contact lines are depinned from the slot edges and move downstream. When the shear rate is high enough, a continuous gas layer forms. If the protrusion angle is small, the gas layer forms and collapse periodically, and accordingly the slip length changes periodically. While if the protrusion angle is large, the gas layer is steady and separates the solid wall from liquid, resulting in a very large slip length.
Zeegers, J.C.H.; Zeegers, Jos; van den Ende, Henricus T.M.; Blom, C.; Altena, E.G.; Beukema, Gerrit J.; Beukema, G.J.; Mellema, J.
1995-01-01
A new instrument to carry out complex viscosity measurements in equilibrium and in a steady shear flow has been developed. A small amplitude harmonic excitation is superimposed orthogonally to the steady shear rate component. It is realized by a thin-walled cylinder, which oscillates in the axial
Electromotive force and large-scale magnetic dynamo in a turbulent flow with a mean shear.
Rogachevskii, Igor; Kleeorin, Nathan
2003-09-01
An effect of sheared large-scale motions on a mean electromotive force in a nonrotating turbulent flow of a conducting fluid is studied. It is demonstrated that in a homogeneous divergence-free turbulent flow the alpha effect does not exist, however a mean magnetic field can be generated even in a nonrotating turbulence with an imposed mean velocity shear due to a "shear-current" effect. A mean velocity shear results in an anisotropy of turbulent magnetic diffusion. A contribution to the electromotive force related to the symmetric parts of the gradient tensor of the mean magnetic field (the kappa effect) is found in nonrotating turbulent flows with a mean shear. The kappa effect and turbulent magnetic diffusion reduce the growth rate of the mean magnetic field. It is shown that a mean magnetic field can be generated when the exponent of the energy spectrum of the background turbulence (without the mean velocity shear) is less than 2. The shear-current effect was studied using two different methods: the tau approximation (the Orszag third-order closure procedure) and the stochastic calculus (the path integral representation of the solution of the induction equation, Feynman-Kac formula, and Cameron-Martin-Girsanov theorem). Astrophysical applications of the obtained results are discussed.
On changing the size of the atmosphere of a vortex pair embedded in a periodic external shear flow
International Nuclear Information System (INIS)
Ryzhov, E.A.
2011-01-01
The dynamics of fluid particles in the vicinity of a self-propagating vortex pair, embedded in a nonstationary shear flow, is studied. When the shear flow is steady, the vicinity of the pair, which is called as a vortex atmosphere, consists of closed stream-lines, which coincide with fluid particles' trajectories. When the shear flow is nonstationary, the trajectories' behaviour changes drastically, then chaotic advection occurs. It is shown in the Letter that the vortex pair propagation velocity varies with the parameters (amplitude, and frequency) of the nonstationary shear flow. It is demonstrated, that changing of the mean velocity leads to changing of the size of the atmosphere. -- Highlights: → A three-layered model of an inviscid incompressible geophysical flow is formulated. → A vortex pair is studied in the middle layer when a periodic shear flow is superimposed. → Dynamics of fluid particles inside the vortex atmosphere of the pair on it is studied. → When the external flow is nonstationary, then chaotic advection of fluid particles emerges. → Vortex pair's mean velocity of self-propelling changes depending on amplitude and frequency.
Cox, Christopher; Plesniak, Michael W.
2017-11-01
One of the most physiologically relevant factors within the cardiovascular system is the wall shear stress. The wall shear stress affects endothelial cells via mechanotransduction and atherosclerotic regions are strongly correlated with curvature and branching in the human vasculature, where the shear stress is both oscillatory and multidirectional. Also, the combined effect of curvature and pulsatility in cardiovascular flows produces unsteady vortices. In this work, our goal is to assess the correlation between multiple vortex pairs and wall shear stress. To accomplish this, we use an in-house high-order flux reconstruction Navier-Stokes solver to simulate pulsatile flow of a Newtonian blood-analog fluid through a rigid 180° curved artery model. We use a physiologically relevant flow rate and generate results using both fully developed and uniform entrance conditions, the latter motivated by the fact that flow upstream to a curved artery may not be fully developed. Under these two inflow conditions, we characterize the evolution of various vortex pairs and their subsequent effect on several wall shear stress metrics. Supported by GW Center for Biomimetics and Bioinspired Engineering.
Sensor for direct measurement of the boundary shear stress in fluid flow
Bao, Xiaoqi; Badescu, Mircea; Bar-Cohen, Yoseph; Lih, Shyh-Shiuh; Sherrit, Stewart; Chang, Zensheu; Chen, Beck; Widholm, Scott; Ostlund, Patrick
2011-04-01
The formation of scour patterns at bridge piers is driven by the forces at the boundary of the water flow. In most experimental scour studies, indirect processes have been applied to estimate the shear and normal stress using measured velocity profiles. The estimations are based on theoretical models and associated assumptions. However, the turbulence flow fields and boundary layer in the pier-scour region are very complex. In addition, available turbulence models cannot account accurately for the bed roughness effect. Direct measurement of the boundary shear and normal stress and their fluctuations are attractive alternatives. However, this approach is a challenging one especially for high spatial resolution and high fidelity measurements. The authors designed and fabricated a prototype miniature shear stress sensor including an EDM machined floating plate and a high-resolution optical encoder. Tests were performed both in air as well as operation in water with controlled flow. The sensor sensitivity, stability and signal-to-noise level were measured and evaluated. The detailed test results and a discussion of future work will be presented in this paper.
The mitigation effect of sheared axial flow on the rayleigh-taylor instability in Z-pinch plasma
International Nuclear Information System (INIS)
Zhang Yang
2005-01-01
A magnetohydrodynamic formulation is derived to investigate the mitigation effects of the sheared axial flow on the Rayleigh-Taylor (RT) instability in Z-pinch plasma. The dispersion relation of the compressible model is given. The mitigation effects of sheared axial flow on the Rayleigh-Taylor instability of Z-pinch plasma in the compressible and incompressible models are compared respectively, and the effect of compressible on the instability of system with sheared axial flow is discussed. It is found that, compressibility effects can stabilize the Rayleigh-Taylor/Kelvin-Helmholtz (RT/KH) instability, and this allows the sheared axial flow mitigate the RT instability far more effectively. The authors also find that, at the early stage of the implosion, if the temperature of the plasma is not very high, the compressible model is much more suitable to describing the state of system than the incompressible one. (author)
Hard wall - soft wall - vorticity scattering in shear flow
Rienstra, S.W.; Singh, D.K.
2014-01-01
An analytically exact solution, for the problem of lowMach number incident vorticity scattering at a hard-soft wall transition, is obtained in the form of Fourier integrals by using theWiener-Hopf method. Harmonic vortical perturbations of inviscid linear shear flow are scattered at the wall
Hard wall - soft wall - vorticity scattering in shear flow
Rienstra, S.W.; Singh, D.K.
2014-01-01
An analytically exact solution, for the problem of low Mach number incident vorticity scattering at a hard-soft wall transition, is obtained in the form of Fourier integrals by using the Wiener-Hopf method. Harmonic vortical perturbations of inviscid linear shear flow are scattered at the wall
Rheological State Diagrams for Rough Colloids in Shear Flow.
Hsiao, Lilian C; Jamali, Safa; Glynos, Emmanouil; Green, Peter F; Larson, Ronald G; Solomon, Michael J
2017-10-13
To assess the role of particle roughness in the rheological phenomena of concentrated colloidal suspensions, we develop model colloids with varying surface roughness length scales up to 10% of the particle radius. Increasing surface roughness shifts the onset of both shear thickening and dilatancy towards lower volume fractions and critical stresses. Experimental data are supported by computer simulations of spherical colloids with adjustable friction coefficients, demonstrating that a reduction in the onset stress of thickening and a sign change in the first normal stresses occur when friction competes with lubrication. In the quasi-Newtonian flow regime, roughness increases the effective packing fraction of colloids. As the shear stress increases and suspensions of rough colloids approach jamming, the first normal stresses switch signs and the critical force required to generate contacts is drastically reduced. This is likely a signature of the lubrication films giving way to roughness-induced tangential interactions that bring about load-bearing contacts in the compression axis of flow.
Rheological State Diagrams for Rough Colloids in Shear Flow
Hsiao, Lilian C.; Jamali, Safa; Glynos, Emmanouil; Green, Peter F.; Larson, Ronald G.; Solomon, Michael J.
2017-10-01
To assess the role of particle roughness in the rheological phenomena of concentrated colloidal suspensions, we develop model colloids with varying surface roughness length scales up to 10% of the particle radius. Increasing surface roughness shifts the onset of both shear thickening and dilatancy towards lower volume fractions and critical stresses. Experimental data are supported by computer simulations of spherical colloids with adjustable friction coefficients, demonstrating that a reduction in the onset stress of thickening and a sign change in the first normal stresses occur when friction competes with lubrication. In the quasi-Newtonian flow regime, roughness increases the effective packing fraction of colloids. As the shear stress increases and suspensions of rough colloids approach jamming, the first normal stresses switch signs and the critical force required to generate contacts is drastically reduced. This is likely a signature of the lubrication films giving way to roughness-induced tangential interactions that bring about load-bearing contacts in the compression axis of flow.
Shear-induced structural transitions in Newtonian non-Newtonian two-phase flow
Cristobal, G.; Rouch, J.; Colin, A.; Panizza, P.
2000-09-01
We show the existence under shear flow of steady states in a two-phase region of a brine-surfactant system in which lyotropic dilute lamellar (non-Newtonian) and sponge (Newtonian) phases are coexisting. At high shear rates and low sponge phase-volume fractions, we report on the existence of a dynamic transition corresponding to the formation of a colloidal crystal of multilamellar vesicles (or ``onions'') immersed in the sponge matrix. As the sponge phase-volume fraction increases, this transition exhibits a hysteresis loop leading to a structural bistability of the two-phase flow. Contrary to single phase lamellar systems where it is always 100%, the onion volume fraction can be monitored continuously from 0 to 100 %.
Numerical and Theoretical Studies of Turbulence and Transport with E x B Shear Flows
International Nuclear Information System (INIS)
Krommes, J.A.; Chance, M.S.; Hahm, T.S.; Lin, Z.
1999-01-01
This paper reports: (1) substantial transport reduction by turbulence-driven E x B flows observed in 3D nonlinear gyrokinetic simulations of microturbulence in magnetically-confined toroidal plasmas; (2) analytical derivation of the effective shearing rate for the time-dependent E x B flow; (3) interpretation of experimental data using linear gyrokinetic microinstability rotation models of E x B shear; and (4) other developments in gyrokinetic theory and simulation
Hybrid simulations of current-carrying instabilities in Z-pinch plasmas with sheared axial flow
International Nuclear Information System (INIS)
Sotnikov, Vladimir I.; Makhin, Volodymyr; Bauer, Bruno S.; Hellinger, Petr; Travnicek, Pavel; Fiala, Vladimir; Leboeuf, Jean-Noel
2002-01-01
The development of instabilities in z-pinch plasmas has been studied with three-dimensional (3D) hybrid simulations. Plasma equilibria without and with sheared axial flow have been considered. Results from the linear phase of the hybrid simulations compare well with linear Hall magnetohydrodynamics (MHD) calculations for sausage modes. The hybrid simulations show that sheared axial flow has a stabilizing effect on the development of both sausage and kink modes
Statistical properties of wall shear stress fluctuations in turbulent channel flows
International Nuclear Information System (INIS)
Keirsbulck, L.; Labraga, L.; Gad-el-Hak, M.
2012-01-01
Highlights: ► Accurate measurements of instantaneous wall shear stress are conducted. ► LDA is used to measure near-wall streamwise velocity. ► Electrochemical probe is used to measure wall shear stress. ► Frequency response and non-uniform correction methods were used to provide an accurate, well-resolved wall-statistics database. ► Reynolds number dependency of the statistical wall quantities is investigated. - Abstract: Instantaneous velocity and wall shear stress measurements are conducted in a turbulent channel flow in the Kármán number range of Re τ = 74–400. A one-dimensional LDA system is used to measure the streamwise velocity fluctuations, and an electrochemical technique is utilized to measure the instantaneous wall shear stress. For the latter, frequency response and nonuniform correction methods are used to provide an accurate, well-resolved wall statistics database. The Reynolds number dependency of the statistical wall quantities is carefully investigated. The corrected relative wall shear stress fluctuations fit well with the best DNS data available and meet the need for clarification of the small discrepancy observed in the literature between the experimental and numerical results of such quantities. Higher-order statistics of the wall shear stress, spectra, and the turbulence kinetic energy budget at the wall are also investigated. The present paper shows that the electrochemical technique is a powerful experimental method for hydrodynamic studies involving highly unsteady flows. The study brings with it important consequences, especially in the context of the current debate regarding the appropriate scaling as well as the validation of new predictive models of near-wall turbulence.
Skin-friction drag reduction in turbulent channel flow based on streamwise shear control
International Nuclear Information System (INIS)
Kim, Jung Hoon; Lee, Jae Hwa
2017-01-01
Highlights: • We perform DNSs of fully developed turbulent channel flows to explore an active flow control concept using streamwise velocity shear control at the wall. • The structural spacing and wall amplitude parameters are systematically changed to achieve a high-efficient drag reduction rate for longitudinal control surface. • Significant drag reduction is observed with an increase in the two parameters with an accompanying reduction of the Reynolds stresses and vorticity fluctuations. • The generation and evolution of the turbulent vortices in the absence of velocity shear and how they contribute to DR have been examined. - Abstract: It is known that stretching and intensification of a hairpin vortex by mean shear play an important role to create a hairpin vortex packet, which generates the large Reynolds shear stress associated with skin-friction drag in wall-bounded turbulent flows. In order to suppress the mean shear at the wall for high efficient drag reduction (DR), in the present study, we explore an active flow control concept using streamwise shear control (SSC) at the wall. The longitudinal control surface is periodically spanwise-arranged with no-control surface while varying the structural spacing, and an amplitude parameter for imposing the strength of the actuating streamwise velocity at the wall is introduced to further enhance the skin-friction DR. Significant DR is observed with an increase in the two parameters with an accompanying reduction of the Reynolds stresses and vorticity fluctuations, although a further increase in the parameters amplifies the turbulence activity in the near-wall region. In order to study the direct relationship between turbulent vortical structures and DR under the SSC, temporal evolution with initial eddies extracted by conditional averages for Reynolds-stress-maximizing Q2 events are examined. It is shown that the generation of new vortices is dramatically inhibited with an increase in the parameters
Sarkarinejad, Khalil; Sarshar, Maryam Asadi; Adineh, Sadegh
2018-02-01
One of the main characteristic of the Zagros foreland fold-and-thrust belt and the Zagros foreland folded belt are wide distributions of surface extrusion from the Hormuz salt diapirs. This study examines the structure and kinematic of channel flow in the Karmostaj salt diapir in the southwestern part of the Zagros foreland folded belt. This diapir has reached the surface as a result of the channel flow mechanism and has extruded in the southern limb of the Kuh-Gach anticline which is an asymmetric décollement fold with convergence to the south. Structural and microstructural studies and quantitative finite strain (Rs) and kinematic vorticity number (Wk) analyses were carried out within this salt diapir and its namakier. This was in order to investigate the structural evolution in the salt diapiric system, the characteristics and mechanism of the salt flow and the distribution of flow regimes within the salt diapir and interaction of regional tectonics and salt diaprism. The extruded salt has developed a flow foliation sub-parallel to the remnant bedding recorded by different colors, a variety of internal folds including symmetrical and asymmetrical folds and interference fold patterns, shear zones, and boudins. These structures were used to analyze mechanisms and history of diapiric flow and extrusion. The microstructures, reveal various deformation mechanisms in various parts of salt diapir. The measurements of finite strain show that Rs values in the margin of salt diapir are higher than within its namakier which is consistent with the results of structural studies. Mean kinematic vorticity number (Wm) measured in steady state deformation of diapir and namakier is Wm = 0.45-0.48 ± 0.13. The estimated mean finite deformation (Wm) values indicate that 67.8% pure shear and 32.2% simple shear deformation were involved; the implications of which are discussed. The vorticity of flow indicates that in the early stage of growth, Poiseuille flow was the dominate
Yielding and flow of sheared colloidal glasses
International Nuclear Information System (INIS)
Petekidis, G; Vlassopoulos, D; Pusey, P N
2004-01-01
We have studied some of the rheological properties of suspensions of hard-sphere colloids with particular reference to behaviour near the concentration of the glass transition. First we monitored the strain on the samples during and after a transient step stress. We find that, at all values of applied step stress, colloidal glasses show a rapid, apparently elastic, recovery of strain after the stress is removed. This recovery is found even in samples which have flowed significantly during stressing. We attribute this behaviour to 'cage elasticity', the recovery of the stress-induced distorted environment of any particle to a more isotropic state when the stress is removed. Second, we monitored the stress as the strain rate dot γ of flowing samples was slowly decreased. Suspensions which are glassy at rest show a stress which becomes independent of dot γ as dot γ →0. This limiting stress can be interpreted as the yield stress of the glass and agrees well both with the yield stress deduced from the step stress and recovery measurements and that predicted by a recent mode coupling theory of sheared suspensions. Thus, the behaviours under steady shearing and transient step stress both support the idea that colloidal glasses have a finite yield stress. We note however that the samples do exhibit a slow accumulation of strain due to creep at stresses below the yield stress
International Nuclear Information System (INIS)
Bagherpour, E.; Qods, F.; Ebrahimi, R.; Miyamoto, H.
2016-01-01
In the present paper commercially pure copper was processed by simple shear extrusion (SSE) technique up to 12 passes using the so-called route C. For SSE processing an appropriate die with a linear die profile was designed and constructed. Effect of SSE passes on isotropy and uniformity of microstructures are focused. Electron back-scattering diffraction (EBSD) was used to evaluate the microstructure of the deformed samples in three orthogonal planes. To investigate the microstructural uniformity EBSD maps were taken from center to periphery of the extrusion direction plane (ED-plane) samples. Significant evolution in grain refinement was achieved down to sub-micron grain size in all planes. Hardness measurements show a considerable increase in hardness of the material after the processing, which confirms the microstructural evolutions. EBSD scans revealed a homogeneous ultrafine grained microstructure after 12 passes. Micro-shear bands were found as potential sites for accelerating the formation of new grains by fragmentation of the initial grains. The total frequency of coincidence site lattice (CSL) boundaries including Σ3 boundaries increased by the increasing of SSE passes. The higher fraction of low to high angle grain boundaries of SSE compared to equal channel angular pressing is an evidence for the cyclic behavior of SSE technique.
Energy Technology Data Exchange (ETDEWEB)
Bagherpour, E., E-mail: e.bagherpour@semnan.ac.ir [Faculty of Metallurgical and Materials Engineering, Semnan University, Semnan (Iran, Islamic Republic of); Department of Mechanical Engineering, Doshisha University, Kyotanabe, Kyoto 610-0394 (Japan); Qods, F., E-mail: qods@semnan.ac.ir [Faculty of Metallurgical and Materials Engineering, Semnan University, Semnan (Iran, Islamic Republic of); Ebrahimi, R., E-mail: ebrahimy@shirazu.ac.ir [Department of Materials Science and Engineering, School of Engineering, Shiraz University, Shiraz (Iran, Islamic Republic of); Miyamoto, H., E-mail: hmiyamot@mail.doshisha.ac.jp [Department of Mechanical Engineering, Doshisha University, Kyotanabe, Kyoto 610-0394 (Japan)
2016-09-30
In the present paper commercially pure copper was processed by simple shear extrusion (SSE) technique up to 12 passes using the so-called route C. For SSE processing an appropriate die with a linear die profile was designed and constructed. Effect of SSE passes on isotropy and uniformity of microstructures are focused. Electron back-scattering diffraction (EBSD) was used to evaluate the microstructure of the deformed samples in three orthogonal planes. To investigate the microstructural uniformity EBSD maps were taken from center to periphery of the extrusion direction plane (ED-plane) samples. Significant evolution in grain refinement was achieved down to sub-micron grain size in all planes. Hardness measurements show a considerable increase in hardness of the material after the processing, which confirms the microstructural evolutions. EBSD scans revealed a homogeneous ultrafine grained microstructure after 12 passes. Micro-shear bands were found as potential sites for accelerating the formation of new grains by fragmentation of the initial grains. The total frequency of coincidence site lattice (CSL) boundaries including Σ3 boundaries increased by the increasing of SSE passes. The higher fraction of low to high angle grain boundaries of SSE compared to equal channel angular pressing is an evidence for the cyclic behavior of SSE technique.
Directory of Open Access Journals (Sweden)
Frank C G van Bussel
Full Text Available Flow-mediated dilation is aimed at normalization of local wall shear stress under varying blood flow conditions. Blood flow velocity and vessel diameter are continuous and opposing influences that modulate wall shear stress. We derived an index FMDv to quantify wall shear stress normalization performance by flow-mediated dilation in the brachial artery. In 22 fasting presumed healthy men, we first assessed intra- and inter-session reproducibilities of two indices pFMDv and mFMDv, which consider the relative peak and relative mean hyperemic change in flow velocity, respectively. Second, utilizing oral glucose loading, we evaluated the tracking performance of both FMDv indices, in comparison with existing indices [i.e., the relative peak diameter increase (%FMD, the peak to baseline diameter ratio (Dpeak/Dbase, and the relative peak diameter increase normalized to the full area under the curve of blood flow velocity with hyperemia (FMD/shearAUC or with area integrated to peak hyperemia (FMD/shearAUC_peak]. Inter-session and intra-session reproducibilities for pFMDv, mFMDv and %FMD were comparable (intra-class correlation coefficients within 0.521-0.677 range. Both pFMDv and mFMDv showed more clearly a reduction after glucose loading (reduction of ~45%, p≤0.001 than the other indices (% given are relative reductions: %FMD (~11%, p≥0.074; Dpeak/Dbase (~11%, p≥0.074; FMD/shearAUC_peak (~20%, p≥0.016 and FMD/shearAUC (~38%, p≤0.038. Further analysis indicated that wall shear stress normalization under normal (fasting conditions is already far from ideal (FMDv << 1, which (therefore does not materially change with glucose loading. Our approach might be useful in intervention studies to detect intrinsic changes in shear stress normalization performance in conduit arteries.
Analysis of Zero Reynolds Shear Stress Appearing in Dilute Surfactant Drag-Reducing Flow
Directory of Open Access Journals (Sweden)
Weiguo Gu
2011-01-01
Full Text Available Dilute surfactant solution of 25 ppm in the two-dimensional channel is investigated experimentally compared with water flow. Particle image velocimetry (PIV system is used to take 2D velocity frames in the streamwise and wall-normal plane. Based on the frames of instantaneous vectors and statistical results, the phenomenon of zero Reynolds shear stress appearing in the drag-reducing flow is discussed. It is found that 25 ppm CTAC solution exhibits the highest drag reduction at Re = 25000 and loses drag reduction completely at Re = 40000. When drag reduction lies in the highest, Reynolds shear stress disappears and reaches zero although the RMS of the velocity fluctuations is not zero. By the categorization in four quadrants, the fluctuations of 25 ppm CTAC solution are distributed in all four quadrants equally at Re = 25000, which indicates that turnaround transportation happens in drag-reducing flow besides Reynolds shear stress transportation. Moreover, the contour distribution of streamwise velocity and the fluctuations suggests that turbulence transportation is depressed in drag-reducing flow. The viscoelasticity is possible to decrease the turbulence transportation and cause the turnaround transportation.
Shear flow driven counter rotating vortices in an inhomogeneous dusty magnetoplasma
Masood, W.; Mirza, Arshad M.; Ijaz, Aisha; Haque, Q.
2014-02-01
The coupling of Shukla-Varma (SV) and convective cell modes is discussed in the presence of non-Boltzmannian electron response and parallel equilibrium shear flow. In the linear case, a new dispersion relation is derived and analyzed. It is found that the coupled SV and convective cell modes destabilize in the presence of electron shear flow. On the other hand, in the nonlinear regime, it is shown that Shukla-Varma mode driven counter rotating vortices can be formed for the system under consideration. It is found that these vortices move slowly by comparison with the ion acoustic or electron drift-wave driven counter rotating vortices. The relevance of the present investigation with regard to space plasmas is also pointed out.
Dagan, Yuval; Ghoniem, Ahmed
2017-11-01
Recent experimental observations show that the dynamic response of a reactive flow is strongly impacted by the fuel chemistry. In order to gain insight into some of the underlying mechanisms we formulate a new linear stability model that incorporates the impact of finite rate chemistry on the hydrodynamic stability of shear flows. Contrary to previous studies which typically assume that the velocity field is independent of the kinetic rates, the velocity field in our study is coupled with the temperature field. Using this formulation, we reproduce previous results, e.g., most unstable global modes, obtained for non-reacting shear flow. Moreover, we show that these modes are significantly altered in frequency and gain by the presence of a reaction region within the shear layer. This qualitatively agrees with results of our recent experimental and numerical studies, which show that the flame surface location relative to the shear layer influences the stability characteristics in combustion tunnels. This study suggests a physical explanation for the observed impact of finite rate chemistry on shear flow stability.
Discrete element simulation studies of angles of repose and shear flow of wet, flexible fibers.
Guo, Y; Wassgren, C; Ketterhagen, W; Hancock, B; Curtis, J
2018-04-18
A discrete element method (DEM) model is developed to simulate the dynamics of wet, flexible fibers. The angles of repose of dry and wet fibers are simulated, and the simulation results are in good agreement with experimental results, validating the wet, flexible fiber model. To study wet fiber flow behavior, the model is used to simulate shear flows of wet fibers in a periodic domain under Lees-Edwards boundary conditions. Significant agglomeration is observed in dilute shear flows of wet fibers. The size of the largest agglomerate in the flow is found to depend on a Bond number, which is proportional to liquid surface tension and inversely proportional to the square of the shear strain rate. This Bond number reflects the relative importance of the liquid-bridge force to the particle's inertial force, with a larger Bond number leading to a larger agglomerate. As the fiber aspect ratio (AR) increases, the size of the largest agglomerate increases, while the coordination number in the largest agglomerate initially decreases and then increases when the AR is greater than four. A larger agglomerate with a larger coordination number is more likely to form for more flexible fibers with a smaller bond elastic modulus due to better connectivity between the more flexible fibers. Liquid viscous force resists pulling of liquid bridges and separation of contacting fibers, and therefore it facilitates larger agglomerate formation. The effect of liquid viscous force is more significant at larger shear strain rates. The solid-phase shear stress is increased due to the presence of liquid bridges in moderately dense flows. As the solid volume fraction increases, the effect of fiber-fiber friction coefficient increases sharply. When the solid volume fraction approaches the maximum packing density, the fiber-fiber friction coefficient can be a more dominant factor than the liquid bridge force in determining the solid-phase shear stress.
Cosmic Shear With ACS Pure Parallels. Targeted Portion.
Rhodes, Jason
2002-07-01
Small distortions in the shapes of background galaxies by foreground mass provide a powerful method of directly measuring the amount and distribution of dark matter. Several groups have recently detected this weak lensing by large-scale structure, also called cosmic shear. The high resolution and sensitivity of HST/ACS provide a unique opportunity to measure cosmic shear accurately on small scales. Using 260 parallel orbits in Sloan i {F775W} we will measure for the first time: the cosmic shear variance on scales Omega_m^0.5, with signal-to-noise {s/n} 20, and the mass density Omega_m with s/n=4. They will be done at small angular scales where non-linear effects dominate the power spectrum, providing a test of the gravitational instability paradigm for structure formation. Measurements on these scales are not possible from the ground, because of the systematic effects induced by PSF smearing from seeing. Having many independent lines of sight reduces the uncertainty due to cosmic variance, making parallel observations ideal.
Shear induced hexagonal ordering observed in an ionic viscoelastic fluid in flow past a surface
International Nuclear Information System (INIS)
Hamilton, W.A.; Butler, P.D.; Baker, S.M.; Smith, G.S.; Hayter, J.B.; Magid, L.J.; Pynn, R.
1994-01-01
We present the first clear evidence of a shear induced hexagonal phase in a polyionic fluid in flow past a plane quartz surface. The dilute surfactant solution studied is viscoelastic due to the formation and entanglement of highly extended charged threadlike micelles many thousands of A long, which are known to align along the flow direction under shear. Small-angle neutron diffraction data show that in the high shear region within a few tens of microns of the surface these micelles not only align, but form a remarkably well ordered hexagonal array separated by 370 A, 8 times their 46 A diameter
Bartel, Esther Maria; Neubauer, Franz; Heberer, Bianca; Genser, Johann
2014-12-01
Based on structural and fabric analyses at variable scales we investigate the evaporitic gypsum-dominated Comeglians-Paularo shear zone in the Southern Alps (Friuli). It represents the lateral western termination of the brittle Fella-Sava Fault. Missing dehydration products of gypsum and the lack of annealing indicate temperatures below 100 °C during development of the shear zone. Despite of such low temperatures the shear zone clearly exhibits mylonitic flow, thus evidencing laterally coeval activity of brittle and viscous deformation. The dominant structures within the gypsum rocks of the Lower Bellerophon Formation are a steeply to gently S-dipping foliation, a subhorizontal stretching lineation and pure shear-dominated porphyroclast systems. A subordinate simple shear component with dextral displacement is indicated by scattered σ-clasts. Both meso- and microscale structures are characteristic of a subsimple shear type of deformation with components of both coaxial and non-coaxial strain. Shortening in a transpressive regime was accommodated by right-lateral displacement and internal pure shear deformation within the Comeglians-Paularo shear zone. The shear zone shows evidence for a combination of two stretching faults, where stretching occurred in the rheologically weaker gypsum member and brittle behavior in enveloping lithologies.
The Effect of Shear Flow on the Isotropic-Nematic Transition in Liquid Crystals.
Olmsted, Peter David
1991-08-01
In this thesis I will discuss the effects of shear flow on the Isotropic-Nematic phase transition in liquid crystals. Shear flow has dramatic orienting effects on the rod-like constituents of nematic liquid crystals, with the general effects of (1) inducing order in the high-temperature isotropic phase, and (2) dictating a direction of alignment for the low-temperature nematic phase. Shear flow also imposes a biaxial symmetry on both the high and low temperature phases, thereby changing the nature of the symmetry-breaking at the transition. We develop coupled deterministic dynamical equations for the 5-component nematic order parameter and the fluid velocity, which may be considered generalizations of the Leslie-Ericksen and Navier-Stokes equations, respectively. We examine the stable stationary solutions to these equations to determine the nature of the non-equilibrium phases, and discuss the analogies and differences between this system and equilibrium systems. From homogeneous solutions we obtain a state diagram analogous to that of a Van der Waals fluid, including a two-state region and a discontinuous transition which terminates at a critical point. To resolve the question of the analog of the Maxwell construction to distinguish locally stable states, we construct stable inhomogeneous interfacial states. From an analysis of these states we determine a coexistence line and find exponents characterizing the shape of the coexistence curve and the interface thickness as the critical point is approached. We find mean-field critical behavior, and comment on the possibility of the analogs of spinodal decomposition and nucleation. Finally, we develop a formalism for describing light scattering from biaxial steady state, and investigate the Gaussian level fluctuations about these states. In the vicinity of the critical point we find singular behavior analogous to critical opalescence of a simple fluid at its critical point. We also find anisotropic correlations at the
International Nuclear Information System (INIS)
Choi, Young Joon; Djilali, Ned
2016-01-01
Colloidal agglomeration of nanoparticles in shear flow is investigated by solving the fluid-particle and particle-particle interactions in a 2D system. We use an extended finite element method in which the dynamics of the particles is solved in a fully coupled manner with the flow, allowing an accurate description of the fluid-particle interfaces without the need of boundary-fitted meshes or of empirical correlations to account for the hydrodynamic interactions between the particles. Adaptive local mesh refinement using a grid deformation method is incorporated with the fluid-structure interaction algorithm, and the particle-particle interaction at the microscopic level is modeled using the Lennard-Jones potential. Motivated by the process used in fabricating fuel cell catalysts from a colloidal ink, the model is applied to investigate agglomeration of colloidal particles under external shear flow in a sliding bi-periodic Lees-Edwards frame with varying shear rates and particle fraction ratios. Both external shear and particle fraction are found to have a crucial impact on the structure formation of colloidal particles in a suspension. Segregation intensity and graph theory are used to analyze the underlying agglomeration patterns and structures, and three agglomeration regimes are identified
Numerical simulation of systems of shear bands in ductile metal with inclusions
Energy Technology Data Exchange (ETDEWEB)
Plohr, JeeYeon N., E-mail: jplohr@lanl.gov; Plohr, Bradley J. [Los Alamos National Laboratory, Theoretical Division, Los Alamos, NM 87545 (United States)
2016-02-15
We develop a method for numerical simulations of high strain-rate loading of mesoscale samples of ductile metal with inclusions. Because of its small-scale inhomogeneity, the composite material is prone to localized shear deformation (adiabatic shear bands). This method employs the Generalized Method of Cells of Paley and Aboudi [Mech. Materials, vol. 14, pp. 127–139, 1992] to ensure that the micro mechanical behavior of the metal and inclusions is reflected properly in the behavior of the composite at the mesoscale. To find the effective plastic strain rate when shear bands are present, we extend and apply the analytic and numerical analysis of shear bands of Glimm, Plohr, and Sharp [Mech. Materials, vol. 24, pp. 31–41, 1996]. Our tests of the method focus on the stress/strain response in uniaxial-strain flow, both compressive and tensile, of depleted uranium metal containing silicon carbide inclusions. We use the Preston-Tonks-Wallace viscoplasticity model [J. Appl. Phys., vol. 93, pp. 211–220, 2003], which applies to the high strain-rate regime of an isotropic viscoplastic solid. In results, we verify the elevated temperature and thermal softening at shear bands in our simulations of pure DU and DU/SiC composites. We also note that in composites, due the asymmetry caused by the inclusions, shear band form at different times in different subcells. In particular, in the subcells near inclusions, shear band form much earlier than they do in pure DU.
Numerical simulation of systems of shear bands in ductile metal with inclusions
Directory of Open Access Journals (Sweden)
JeeYeon N. Plohr
2016-02-01
Full Text Available We develop a method for numerical simulations of high strain-rate loading of mesoscale samples of ductile metal with inclusions. Because of its small-scale inhomogeneity, the composite material is prone to localized shear deformation (adiabatic shear bands. This method employs the Generalized Method of Cells of Paley and Aboudi [Mech. Materials, vol. 14, pp. 127–139, 1992] to ensure that the micro mechanical behavior of the metal and inclusions is reflected properly in the behavior of the composite at the mesoscale. To find the effective plastic strain rate when shear bands are present, we extend and apply the analytic and numerical analysis of shear bands of Glimm, Plohr, and Sharp [Mech. Materials, vol. 24, pp. 31–41, 1996]. Our tests of the method focus on the stress/strain response in uniaxial-strain flow, both compressive and tensile, of depleted uranium metal containing silicon carbide inclusions. We use the Preston-Tonks-Wallace viscoplasticity model [J. Appl. Phys., vol. 93, pp. 211–220, 2003], which applies to the high strain-rate regime of an isotropic viscoplastic solid. In results, we verify the elevated temperature and thermal softening at shear bands in our simulations of pure DU and DU/SiC composites. We also note that in composites, due the asymmetry caused by the inclusions, shear band form at different times in different subcells. In particular, in the subcells near inclusions, shear band form much earlier than they do in pure DU.
International Nuclear Information System (INIS)
Wang, A.K.; Dong, J.Q.; Qu, W.X.; Qiu, X.M.
2002-01-01
A new suppression mechanism of turbulent transport, characteristic of the synergism between safety factor and shear flows, is proposed to explain the internal transport barriers (ITBs) observed in neutral-beam-heated tokamak discharges with reversed magnetic shear. It is shown that the evolution of turbulent transport with the strength of the suppression mechanism reproduces the basic features of the formation and development of ITBs observed in experiments. In addition, the present analyses predict the possibility of global ion and electron heat transport barriers
Metamorphosis of plasma turbulence-shear-flow dynamics through a transcritical bifurcation
International Nuclear Information System (INIS)
Ball, R.; Dewar, R.L.; Sugama, H.
2002-01-01
The structural properties of an economical model for a confined plasma turbulence governor are investigated through bifurcation and stability analyses. A close relationship is demonstrated between the underlying bifurcation framework of the model and typical behavior associated with low- to high-confinement transitions such as shear-flow stabilization of turbulence and oscillatory collective action. In particular, the analysis evinces two types of discontinuous transition that are qualitatively distinct. One involves classical hysteresis, governed by viscous dissipation. The other is intrinsically oscillatory and nonhysteretic, and thus provides a model for the so-called dithering transitions that are frequently observed. This metamorphosis, or transformation, of the system dynamics is an important late side-effect of symmetry breaking, which manifests as an unusual nonsymmetric transcritical bifurcation induced by a significant shear-flow drive
Kuiroukidis, Ap.; Throumoulopoulos, G. N.
2015-08-01
We construct nonlinear toroidal equilibria of fixed diverted boundary shaping with reversed magnetic shear and flows parallel to the magnetic field. The equilibria have hole-like current density and the reversed magnetic shear increases as the equilibrium nonlinearity becomes stronger. Also, application of a sufficient condition for linear stability implies that the stability is improved as the equilibrium nonlinearity correlated to the reversed magnetic shear gets stronger with a weaker stabilizing contribution from the flow. These results indicate synergetic stabilizing effects of reversed magnetic shear, equilibrium nonlinearity and flow in the establishment of Internal Transport Barriers (ITBs).
Possible ionospheric preconditioning by shear flow leading to equatorial spread F
Directory of Open Access Journals (Sweden)
D. L. Hysell
2005-10-01
Full Text Available Vertical shear in the zonal plasma drift speed is apparent in incoherent and coherent scatter radar observations of the bottomside F region ionosphere made at Jicamarca from about 1600–2200 LT. The relative importance of the factors controlling the shear, which include competition between the E and F region dynamos as well as vertical currents driven in the E and F regions at the dip equator, is presently unknown. Bottom-type scattering layers arise in strata where the neutral and plasma drifts differ widely, and periodic structuring of irregularities within the layers is telltale of intermediate-scale waves in the bottomside. These precursor waves appear to be able to seed ionospheric interchange instabilities and initiate full-blown equatorial spread F. The seed or precursor waves may be generated by a collisional shear instability. However, assessing the viability of shear instability requires measurements of the same parameters needed to understand shear flow quantitatively - thermospheric neutral wind and off-equatorial conductivity profiles. Keywords. Ionosphere (Equatorial ionosphere; ionospheric irregularities – Space plasma physics (Waves and instabilities
Numerical investigations of two-degree-of-freedom vortex-induced vibration in shear flow
Energy Technology Data Exchange (ETDEWEB)
Zhang, Hui; Liu, Mengke; Han, Yang; Li, Jian; Gui, Mingyue; Chen, Zhihua, E-mail: zhanghui1902@hotmail.com [Science and Technology on Transient Physics Laboratory, Nanjing University of Science and Technology, Nanjing 210094 (China)
2017-06-15
Exponential-polar coordinates attached to a moving cylinder are used to deduce the stream function-vorticity equations for two-degree-of-freedom vortex-induced vibration, the initial and boundary conditions, and the distribution of the hydrodynamic force, which consists of the vortex-induced force, inertial force, and viscous damping force. The fluid-structure interactions occurring from the motionless cylinder to the steady vibration are investigated numerically, and the variations of the flow field, pressure, lift/drag, and cylinder displacement are discussed. Both the dominant vortex and the cylinder shift, whose effects are opposite, affect the shear layer along the transverse direction and the secondary vortex along the streamwise direction. However, the effect of the cylinder shift is larger than that of the dominant vortices. Therefore, the former dominates the total effects of the flow field. Moreover, the symmetry of the flow field is broken with the increasing shear rate. With the effect of the background vortex, the upper vortices are strengthened, and the lower vortices are weakened; thus, the shear layer and the secondary vortices induced by the upper shedding vortices are strengthened, while the shear layer and the secondary vortices induced by the lower shedding vortices are weakened. Therefore, the amplitudes of the displacement and drag/lift dominated by the upper vortex are larger than those of the displacement and drag/lift dominated by the lower vortex. (paper)
Shear flow instability in a partially-ionized plasma sheath around a fast-moving vehicle
International Nuclear Information System (INIS)
Sotnikov, V. I.; Mudaliar, S.; Genoni, T. C.; Rose, D. V.; Oliver, B. V.; Mehlhorn, T. A.
2011-01-01
The stability of ion acoustic waves in a sheared-flow, partially-ionized compressible plasma sheath around a fast-moving vehicle in the upper atmosphere, is described and evaluated for different flow profiles. In a compressible plasma with shear flow, instability occurs for any velocity profile, not just for profiles with an inflection point. A second-order differential equation for the electrostatic potential of excited ion acoustic waves in the presence of electron and ion collisions with neutrals is derived and solved numerically using a shooting method with boundary conditions appropriate for a finite thickness sheath in contact with the vehicle. We consider three different velocity flow profiles and find that in all cases that neutral collisions can completely suppress the instability.
Vescovi, Dalila; Berzi, Diego; Richard, Patrick; Brodu, Nicolas
2014-01-01
International audience; We use existing 3D Discrete Element simulations of simple shear flows of spheres to evaluate the radial distribution function at contact that enables kinetic theory to correctly predict the pressure and the shear stress, for different values of the collisional coefficient of restitution. Then, we perform 3D Discrete Element simulations of plane flows of frictionless, inelastic spheres, sheared between walls made bumpy by gluing particles in a regular array, at fixed av...
DEFF Research Database (Denmark)
Søgaard, Søren Vinter; Pedersen, Troels; Allesø, Morten
2014-01-01
Powder flow in small-scale equipment is challenging to predict. To meet this need, the impact of consolidation during powder flow characterization, the level of consolidation existing during discharge of powders from a tablet press hopper and the uncertainty of shear and wall friction measurements...... normal stress were approximately 200Pa and 114Pa, respectively, in the critical transition from the converging to the lower vertical section of the hopper. The lower limit of consolidation for the shear and wall friction test was approximately 500Pa and 200Pa, respectively. At this consolidation level......, the wall and shear stress resolution influences the precision of the measured powder flow properties. This study highlights the need for an improved experimental setup which would be capable of measuring the flow properties of powders under very small consolidation stresses with a high shear stress...
Constitutive Curve and Velocity Profile in Entangled Polymers during Start-Up of Steady Shear Flow
Hayes, Keesha A.; Buckley, Mark R.; Qi, Haibo; Cohen, Itai; Archer, Lynden A.
2010-01-01
-4]. Surprisingly, we find that even polymer systems which exhibit transient, nonmonotonic shear stress-shear rate relationships in bulk rheology experiments manifest time-dependent velocity profiles that are decidedly linear and show no evidence of unstable flow
Evolution of symmetric reconnection layer in the presence of parallel shear flow
Energy Technology Data Exchange (ETDEWEB)
Lu Haoyu [Space Science Institute, School of Astronautics, Beihang University, Beijing 100191 (China); Sate Key Laboratory of Space Weather, Chinese Academy of Sciences, Beijing 100190 (China); Cao Jinbin [Space Science Institute, School of Astronautics, Beihang University, Beijing 100191 (China)
2011-07-15
The development of the structure of symmetric reconnection layer in the presence of a shear flow parallel to the antiparallel magnetic field component is studied by using a set of one-dimensional (1D) magnetohydrodynamic (MHD) equations. The Riemann problem is simulated through a second-order conservative TVD (total variation diminishing) scheme, in conjunction with Roe's averages for the Riemann problem. The simulation results indicate that besides the MHD shocks and expansion waves, there exist some new small-scale structures in the reconnection layer. For the case of zero initial guide magnetic field (i.e., B{sub y0} = 0), a pair of intermediate shock and slow shock (SS) is formed in the presence of the parallel shear flow. The critical velocity of initial shear flow V{sub zc} is just the Alfven velocity in the inflow region. As V{sub z{infinity}} increases to the value larger than V{sub zc}, a new slow expansion wave appears in the position of SS in the case V{sub z{infinity}} < V{sub zc}, and one of the current densities drops to zero. As plasma {beta} increases, the out-flow region is widened. For B{sub y0} {ne} 0, a pair of SSs and an additional pair of time-dependent intermediate shocks (TDISs) are found to be present. Similar to the case of B{sub y0} = 0, there exists a critical velocity of initial shear flow V{sub zc}. The value of V{sub zc} is, however, smaller than the Alfven velocity of the inflow region. As plasma {beta} increases, the velocities of SS and TDIS increase, and the out-flow region is widened. However, the velocity of downstream SS increases even faster, making the distance between SS and TDIS smaller. Consequently, the interaction between SS and TDIS in the case of high plasma {beta} influences the property of direction rotation of magnetic field across TDIS. Thereby, a wedge in the hodogram of tangential magnetic field comes into being. When {beta}{yields}{infinity}, TDISs disappear and the guide magnetic field becomes constant.
Analytical modeling for heat transfer in sheared flows of nanofluids
Ferrari, C.; Kaoui, B.; L'vov, V.S.; Procaccia, I.; Rudenko, O.; Thije Boonkkamp, ten J.H.M.; Toschi, F.
2012-01-01
We developed a model for the enhancement of the heat flux by spherical and elongated nanoparticles in sheared laminar flows of nanofluids. Besides the heat flux carried by the nanoparticles, the model accounts for the contribution of their rotation to the heat flux inside and outside the particles.
Yu, Guihua; Kushwaha, Amit; Lee, Jungkyu K; Shaqfeh, Eric S G; Bao, Zhenan
2011-01-25
DNA has been recently explored as a powerful tool for developing molecular scaffolds for making reproducible and reliable metal contacts to single organic semiconducting molecules. A critical step in the process of exploiting DNA-organic molecule-DNA (DOD) array structures is the controlled tethering and stretching of DNA molecules. Here we report the development of reproducible surface chemistry for tethering DNA molecules at tunable density and demonstrate shear flow processing as a rationally controlled approach for stretching/aligning DNA molecules of various lengths. Through enzymatic cleavage of λ-phage DNA to yield a series of DNA chains of various lengths from 17.3 μm down to 4.2 μm, we have investigated the flow/extension behavior of these tethered DNA molecules under different flow strengths in the flow-gradient plane. We compared Brownian dynamic simulations for the flow dynamics of tethered λ-DNA in shear, and found our flow-gradient plane experimental results matched well with our bead-spring simulations. The shear flow processing demonstrated in our studies represents a controllable approach for tethering and stretching DNA molecules of various lengths. Together with further metallization of DNA chains within DOD structures, this bottom-up approach can potentially enable efficient and reliable fabrication of large-scale nanoelectronic devices based on single organic molecules, therefore opening opportunities in both fundamental understanding of charge transport at the single molecular level and many exciting applications for ever-shrinking molecular circuits.
International Nuclear Information System (INIS)
Di Stefano, C. A.; Kuranz, C. C.; Klein, S. R.; Drake, R. P.; Malamud, G.; Henry de Frahan, M. T.; Johnsen, E.; Shimony, A.; Shvarts, D.; Smalyuk, V. A.; Martinez, D.
2014-01-01
In this work, we examine the hydrodynamics of high-energy-density (HED) shear flows. Experiments, consisting of two materials of differing density, use the OMEGA-60 laser to drive a blast wave at a pressure of ∼50 Mbar into one of the media, creating a shear flow in the resulting shocked system. The interface between the two materials is Kelvin-Helmholtz unstable, and a mixing layer of growing width develops due to the shear. To theoretically analyze the instability's behavior, we rely on two sources of information. First, the interface spectrum is well-characterized, which allows us to identify how the shock front and the subsequent shear in the post-shock flow interact with the interface. These observations provide direct evidence that vortex merger dominates the evolution of the interface structure. Second, simulations calibrated to the experiment allow us to estimate the time-dependent evolution of the deposition of vorticity at the interface. The overall result is that we are able to choose a hydrodynamic model for the system, and consequently examine how well the flow in this HED system corresponds to a classical hydrodynamic description
Investigation of particle-laden turbulent flow in free shear turbulent combustion
International Nuclear Information System (INIS)
Buckingham, A.C.; Siekhaus, W.J.; Ellzey, J.; Daily, J.W.
1983-01-01
Explicit numerical mixed phase simulations are described which couple random gasdynamic motions to inertiallly interactive gas borne particles. Theses simulations are numerical experiments intended to provide data for investigating the interaction between a developing turbulent free shear layer and gas borne solid particles it entrains. The simulations predict most probable distributions of dispersed phase trajectories, standard deviations, and gas phase mixing dynamics which include the concomitant back-influences of the particle phase on the carrier gas flow. Data for refinement of the computational scheme and physical verification are provided by experiment. The experimental evidence is developed in a splitter plate divided, two-channel free shear mixing combustion tube. A variety of particle concentrations and particle size distributions are admitted into non-combusting or combusting flows with selected heat release levels. The computations, in turn, provide guidance on design and selection of new experiments
Review Article: Advances in modeling of bed particle entrainment sheared by turbulent flow
Dey, Subhasish; Ali, Sk Zeeshan
2018-06-01
Bed particle entrainment by turbulent wall-shear flow is a key topic of interest in hydrodynamics because it plays a major role to govern the planetary morphodynamics. In this paper, the state-of-the-art review of the essential mechanisms governing the bed particle entrainment by turbulent wall-shear flow and their mathematical modeling is presented. The paper starts with the appraisal of the earlier multifaceted ideas in modeling the particle entrainment highlighting the rolling, sliding, and lifting modes of entrainment. Then, various modeling approaches of bed particle entrainment, such as deterministic, stochastic, and spatiotemporal approaches, are critically analyzed. The modeling criteria of particle entrainment are distinguished for hydraulically smooth, transitional, and rough flow regimes. In this context, the responses of particle size, particle exposure, and packing condition to the near-bed turbulent flow that shears the particles to entrain are discussed. From the modern experimental outcomes, the conceptual mechanism of particle entrainment from the viewpoint of near-bed turbulent coherent structures is delineated. As the latest advancement of the subject, the paper sheds light on the origin of the primitive empirical formulations of bed particle entrainment deriving the scaling laws of threshold flow velocity of bed particle motion from the perspective of the phenomenological theory of turbulence. Besides, a model framework that provides a new look on the bed particle entrainment phenomenon stemming from the stochastic-cum-spatiotemporal approach is introduced. Finally, the future scope of research is articulated with open questions.
On the link between ExB sheared flows and rational surfaces in fusion plasmas
International Nuclear Information System (INIS)
Hidalgo, C.; Erents, K.; Matthews, G.
2000-11-01
Experimental evidence of flattening in plasma profiles has been observed in the edge region of the JET tokamak. This observation has been interpreted in terms of the influence of rational surfaces on plasma profiles. In the framework of this interpretation, significant ExB sheared flows linked to rational surfaces have been identified. These ExB sheared flows are close to the critical value to trigger the transition to improved confinement regimes. These results can explain the link between the magnetic topology and the generation of transport barriers reported in fusion devices. (author)
International Nuclear Information System (INIS)
Kim, Eun-jin; Diamond, P.H.
2003-01-01
The dynamics of and an interplay among structures (mean shear flows, zonal flows, and generalized Kelvin-Helmholtz modes) are studied in drift wave turbulence. Mean shear flows are found to inhibit the nonlinear generation of zonal flows by weakening the coherent modulation response of the drift wave spectrum. Based on this result, a minimal model for the L→H (low- to high-confinement) transition is proposed, which involves the amplitude of drift waves, zonal flows, and the density gradient. A transition to quiescent H-mode sets in as the profile becomes sufficiently steep to completely damp out drift waves, following an oscillatory transition phase where zonal flows regulate drift wave turbulence. The different roles of mean flows and zonal flows are elucidated. Finally, the effect of poloidally nonaxisymmetric structures (generalized Kelvin-Helmholtz mode) on anomalous transport is investigated, especially in reference to damping of collisionless zonal flows. Results indicate that nonlinear excitation of this structure can be potentially important in enhancing anomalous transport as well as in damping zonal flows
On Shear Stress Distributions for Flow in Smooth or Partially Rough Annuli
Energy Technology Data Exchange (ETDEWEB)
Kjellstroem, B; Hedberg, S
1966-08-15
It is commonly assumed that for turbulent flow in annuli the radii of zero shear and maximum velocity are coincident. By inspection of the differential equations for such flow and by an integral analysis it is shown that this is not necessarily true. To check whether important differences could occur, experiments were made in which velocity and shear stress distributions were measured in one smooth and two partially rough annuli. The results show no difference in the radii for the smooth annulus, but for the partially rough annuli there was a small but significant difference. This difference explains the breakdown of Hall's transformation theory reported by other investigators. The error introduced by use of Hall's theory is however small, of the order of 10 % or less.
On Shear Stress Distributions for Flow in Smooth or Partially Rough Annuli
International Nuclear Information System (INIS)
Kjellstroem, B.; Hedberg, S.
1966-08-01
It is commonly assumed that for turbulent flow in annuli the radii of zero shear and maximum velocity are coincident. By inspection of the differential equations for such flow and by an integral analysis it is shown that this is not necessarily true. To check whether important differences could occur, experiments were made in which velocity and shear stress distributions were measured in one smooth and two partially rough annuli. The results show no difference in the radii for the smooth annulus, but for the partially rough annuli there was a small but significant difference. This difference explains the breakdown of Hall's transformation theory reported by other investigators. The error introduced by use of Hall's theory is however small, of the order of 10 % or less
International Nuclear Information System (INIS)
Olsen, Michael G
2009-01-01
An analytical model for the microscopic particle image velocimetry (microPIV) correlation signal peak in a purely shearing flow was derived for the case of in-plane shearing (out-of-plane shearing was not considered). This model was then used to derive equations for the measured velocity weighting functions for the two velocity components, and the weighting functions were in turn used to define the depths of correlation associated with the two measured velocity components. The depth of correlation for the velocity component perpendicular to the shear was found to be unaffected by the shear rate. However, the depth of correlation for the velocity component in the direction of the shear was found to be highly dependent on the shear rate, with the depth of correlation increasing as the shear rate increased. Thus, in a flow with shear, there is not a single value for the depth of correlation within an interrogation region. Instead, the depth of correlation exhibits directional dependence, with a different depth of correlation for each of the two measured velocity components. The increase in the depth of correlation due to the shear rate is greater for large numerical aperture objectives than for small numerical aperture objectives. This increase in the depth of correlation in a shearing flow can be quite large, with increases in the depth of correlation exceeding 100% being very possible for high numerical aperture objectives. The effects of out-of-plane shear are beyond the capabilities of this analysis, although the possible consequences of out-of-plane shear are discussed
Effects of flow unsteadiness on the wall shear stress
International Nuclear Information System (INIS)
Amiri, K; Cervantes, M J; Raisee, M
2012-01-01
Measurements were performed on pulsating fully turbulent flows in a pipe test rig with a diameter of 100 mm. Sinusoidal oscillatory flow at different frequencies was superimposed on a mean flow of averaged Reynolds number Re=20000 based on the pipe diameter. The measurements have been performed at different forcing frequencies (0.001 + < 0.08) covering all the oscillatory regimes; quasi-steady, relaxation, quasi laminar and high frequency. The amplitude of the flow oscillation was small enough to allow a linear response in the measurements, i.e., all flow parameters showed an oscillatory behavior at the frequency of the flow. The amplitude of the oscillatory flow was about 10% of the mean velocity in all cases. The results include mean and phase averaged values of different parameters. The centerline velocity was measured by a 2D LDA system. Hot film and constant temperature anemometry system was used to determine the wall shear stress. Bulk velocity and pressure gradient along the pipe were also acquired. The results showed a good agreement with the previous analytical, experimental and numerical results available in the literature.
Shear thinning and shear thickening of a confined suspension of vesicles
Nait Ouhra, A.; Farutin, A.; Aouane, O.; Ez-Zahraouy, H.; Benyoussef, A.; Misbah, C.
2018-01-01
Widely regarded as an interesting model system for studying flow properties of blood, vesicles are closed membranes of phospholipids that mimic the cytoplasmic membranes of red blood cells. In this study we analyze the rheology of a suspension of vesicles in a confined geometry: the suspension, bound by two planar rigid walls on each side, is subject to a shear flow. Flow properties are then analyzed as a function of shear rate γ ˙, the concentration of the suspension ϕ , and the viscosity contrast λ =ηin/ηout , where ηin and ηout are the fluid viscosities of the inner and outer fluids, respectively. We find that the apparent (or effective viscosity) of the suspension exhibits both shear thinning (decreasing viscosity with shear rate) or shear thickening (increasing viscosity with shear rate) in the same concentration range. The shear thinning or thickening behaviors appear as subtle phenomena, dependant on viscosity contrast λ . We provide physical arguments on the origins of these behaviors.
Directory of Open Access Journals (Sweden)
K. Kondoh
2009-06-01
Full Text Available Three-dimensional configuration of earthward fast flow in the near-Earth plasma sheet is studied using three-dimensional magnetohydrodynamics (MHD simulations on the basis of the spontaneous fast reconnection model. In this study, the sheared magnetic field in the plasma sheet is newly considered in order to investigate the effects of it to the earthward fast flow, and the results are discussed in comparison with no-shear simulations. The virtual probes located at different positions in our simulation domain in shear/no-shear cases could explain different behavior of fast flows in the real observations.
Engel, Frank; Rhoads, Bruce L.
2016-01-01
Compound meander bends with multiple lobes of maximum curvature are common in actively evolving lowland rivers. Interaction among spatial patterns of mean flow, turbulence, bed morphology, bank failures and channel migration in compound bends is poorly understood. In this paper, acoustic Doppler current profiler (ADCP) measurements of the three-dimensional (3D) flow velocities in a compound bend are examined to evaluate the influence of channel curvature and hydrologic variability on the structure of flow within the bend. Flow structure at various flow stages is related to changes in bed morphology over the study timeframe. Increases in local curvature within the upstream lobe of the bend reduce outer bank velocities at morphologically significant flows, creating a region that protects the bank from high momentum flow and high bed shear stresses. The dimensionless radius of curvature in the upstream lobe is one-third less than that of the downstream lobe, with average bank erosion rates less than half of the erosion rates for the downstream lobe. Higher bank erosion rates within the downstream lobe correspond to the shift in a core of high velocity and bed shear stresses toward the outer bank as flow moves through the two lobes. These erosion patterns provide a mechanism for continued migration of the downstream lobe in the near future. Bed material size distributions within the bend correspond to spatial patterns of bed shear stress magnitudes, indicating that bed material sorting within the bend is governed by bed shear stress. Results suggest that patterns of flow, sediment entrainment, and planform evolution in compound meander bends are more complex than in simple meander bends. Moreover, interactions among local influences on the flow, such as woody debris, local topographic steering, and locally high curvature, tend to cause compound bends to evolve toward increasing planform complexity over time rather than stable configurations.
Producing High-Performance, Stable, Sheared-Flow Z-Pinches in the FuZE project
Golingo, R. P.; Shumlak, U.,; Nelson, B. A.; Claveau, E. L.; Forbes, E. G.; Stepanov, A. D.; Weber, T. R.; Zhang, Y.; McLean, H. S.; Tummel, K. K.; Higginson, D. P.; Schmidt, A. E.; University of Washington (UW) Collaboration; Lawrence Livermore National Laboratory (LLNL) Collaboration
2017-10-01
The Fusion Z-Pinch Experiment (FuZE) has made significant strides towards generating high-performance, stable Z-pinch plasmas with goals of ne = 1018 cm-3 and T =1 keV. The Z-pinch plasmas are stabilized with a sheared axial flow that is driven by a coaxial accelerator. The new FuZE device has been constructed and reproduces the major scientific achievements the ZaP project at the University of Washington; ne = 1016 cm-3,T = 100 eV, r20 μs. These parameters are measured with an array of magnetic field probes, spectroscopy, and fast framing cameras. The plasma parameters are achieved using a small fraction of the maximum energy storage and gas injection capability of the FuZE device. Higher density, ne = 5×1017 cm-3, and temperature, T = 500 eV, Z-pinch plasmas are formed by increasing the pinch current. At the higher voltages and currents, the ionization rates in the accelerator increase. By modifying the neutral gas profile in the accelerator, the plasma flow from the accelerator is maintained, driving the flow shear. Formation and sustainment of the sheared-flow Z-pinch plasma will be discussed. Experimental data demonstrating high performance plasmas in a stable Z-pinches will be shown. This work is supported by an award from US ARPA-E.
Structural predictor for nonlinear sheared dynamics in simple glass-forming liquids.
Ingebrigtsen, Trond S; Tanaka, Hajime
2018-01-02
Glass-forming liquids subjected to sufficiently strong shear universally exhibit striking nonlinear behavior; for example, a power-law decrease of the viscosity with increasing shear rate. This phenomenon has attracted considerable attention over the years from both fundamental and applicational viewpoints. However, the out-of-equilibrium and nonlinear nature of sheared fluids have made theoretical understanding of this phenomenon very challenging and thus slower to progress. We find here that the structural relaxation time as a function of the two-body excess entropy, calculated for the extensional axis of the shear flow, collapses onto the corresponding equilibrium curve for a wide range of pair potentials ranging from harsh repulsive to soft and finite. This two-body excess entropy collapse provides a powerful approach to predicting the dynamics of nonequilibrium liquids from their equilibrium counterparts. Furthermore, the two-body excess entropy scaling suggests that sheared dynamics is controlled purely by the liquid structure captured in the form of the two-body excess entropy along the extensional direction, shedding light on the perplexing mechanism behind shear thinning.
Structural predictor for nonlinear sheared dynamics in simple glass-forming liquids
Ingebrigtsen, Trond S.; Tanaka, Hajime
2018-01-01
Glass-forming liquids subjected to sufficiently strong shear universally exhibit striking nonlinear behavior; for example, a power-law decrease of the viscosity with increasing shear rate. This phenomenon has attracted considerable attention over the years from both fundamental and applicational viewpoints. However, the out-of-equilibrium and nonlinear nature of sheared fluids have made theoretical understanding of this phenomenon very challenging and thus slower to progress. We find here that the structural relaxation time as a function of the two-body excess entropy, calculated for the extensional axis of the shear flow, collapses onto the corresponding equilibrium curve for a wide range of pair potentials ranging from harsh repulsive to soft and finite. This two-body excess entropy collapse provides a powerful approach to predicting the dynamics of nonequilibrium liquids from their equilibrium counterparts. Furthermore, the two-body excess entropy scaling suggests that sheared dynamics is controlled purely by the liquid structure captured in the form of the two-body excess entropy along the extensional direction, shedding light on the perplexing mechanism behind shear thinning.
Flow modelling of a newtonian fluid by two regions- the region of pure fluid and porous region
International Nuclear Information System (INIS)
Sampaio, R.; Gama, R.M.S. da
1983-01-01
A model of flow with two regions is presented using mixture theory. One region contains only pure fluid and the other a mixture of fluid and porous rigid solid. Compatibility conditons on the pure fluid-mixture interface are carefully discussed. The theory is used to solve a problem of a flow induced by pressure gradient and helicoidal motion of an impermeable cylinder on two rings one of pure fluid and another of mixture. (Author) [pt
MHD shear flows with non-constant transverse magnetic field
International Nuclear Information System (INIS)
Núñez, Manuel
2012-01-01
Viscous conducting flows parallel to a fixed plate are studied. In contrast with the Hartmann setting, the problem is not linearized near a fixed transverse magnetic field, although the field tends to be transversal far from the wall. While general solutions may be formally obtained for all cases, their behavior is far more clear when the magnetic Prandtl number equals one. We consider two different instances: a fixed magnetic field at the wall, or an insulating sheet. The evolution of the flow and the magnetic field both near the plate and far from it are detailed, analyzing the possibility of reverse flow and instability of the solutions. -- Highlights: ► A conducting shear flow does not leave a transverse magnetic field invariant. ► Solutions are found for all cases, but these are more useful when kinetic and magnetic diffusivities coincide. ► Dirichlet and Neumann conditions on the magnetic field are studied. ► Reverse flow, and eventual instability, are possible.
Boersen, Johannes T; Groot Jebbink, Erik; Versluis, Michel; Slump, Cornelis H; Ku, David N; de Vries, Jean-Paul P M; Reijnen, Michel M P J
2017-12-01
Endovascular aneurysm repair (EVAR) with a modular endograft has become the preferred treatment for abdominal aortic aneurysms. A novel concept is endovascular aneurysm sealing (EVAS), consisting of dual endoframes surrounded by polymer-filled endobags. This dual-lumen configuration is different from a bifurcation with a tapered trajectory of the flow lumen into the two limbs and may induce unfavorable flow conditions. These include low and oscillatory wall shear stress (WSS), linked to atherosclerosis, and high shear rates that may result in thrombosis. An in vitro study was performed to assess the impact of EVAR and EVAS on flow patterns and WSS. Four abdominal aortic aneurysm phantoms were constructed, including three stented models, to study the influence of the flow divider on flow (Endurant [Medtronic, Minneapolis, Minn], AFX [Endologix, Irvine, Calif], and Nellix [Endologix]). Experimental models were tested under physiologic resting conditions, and flow was visualized with laser particle imaging velocimetry, quantified by shear rate, WSS, and oscillatory shear index (OSI) in the suprarenal aorta, renal artery (RA), and common iliac artery. WSS and OSI were comparable for all models in the suprarenal aorta. The RA flow profile in the EVAR models was comparable to the control, but a region of lower WSS was observed on the caudal wall compared with the control. The EVAS model showed a stronger jet flow with a higher shear rate in some regions compared with the other models. Small regions of low WSS and high OSI were found near the distal end of all stents in the common iliac artery compared with the control. Maximum shear rates in each region of interest were well below the pathologic threshold for acute thrombosis. The different stent designs do not influence suprarenal flow. Lower WSS is observed in the caudal wall of the RA after EVAR and a higher shear rate after EVAS. All stented models have a small region of low WSS and high OSI near the distal outflow
Cross-flow shearing effects on the trajectory of highly buoyant bent-over plumes
Tohidi, Ali; Kaye, Nigel Berkeley; Gollner, Michael J.
2017-11-01
The dynamics of highly buoyant plumes in cross-flow is ubiquitous throughout both industrial and environmental phenomena. The rise of smoke from a chimney, wastewater discharge into river currents, and dispersion of wildfire plumes are only a few instances. There have been many previous studies investigating the behavior of jets and highly buoyant plumes in cross-flow. So far, however, very little attention has been paid to the role of shearing effects in the boundary layer on the plume trajectory, particularly on the rise height. Numerical simulations and dimensional analysis are conducted to characterize the near- and far-field behavior of a highly buoyant plume in a boundary layer cross-flow. The results show that shear in the cross-flow leads to large differences in the rise height of the plume in relation to a uniform cross-flow, especially at far-field. This material is based upon work supported by the National Science Foundation under Grant No.1200560. Any opinions, findings, and conclusions or recommendations expressed in the material are of the authors and do not necessarily reflect the views of NSF.
Probabilistic approach to diffusion in shear flows of generalized viscoelastic second-grade fluids
International Nuclear Information System (INIS)
Wafo Soh, C
2010-01-01
We study diffusion in point-source-driven shear flows of generalized second-grade fluids. We start by obtaining exact solutions of shear flows triggered by point sources under various boundary conditions. For unrestricted flows, we demonstrate that the velocity distribution is the probability density function of a coupled or uncoupled continuous-time random walk. In the first instance, the motion is described by a compound Poisson process with an explicit probability density function corresponding to the velocity distribution. The average waiting time in this situation is finite and is identified with the structural relaxation time. In the second case, we obtain an explicit formula for the probability density function in terms of special functions. In both cases, the probability density functions of the associated stochastic processes are leptokurtic at all finite times with variances linear in time. By using the method of images, we infer velocity fields for restricted flows from those of unrestricted flows. Equipped with some exact expressions of the velocity field, we analyze advection–diffusion via the Feynman–Kac formula, which lends itself naturally to Monte Carlo simulation
Effects of magnetic field, sheared flow and ablative velocity on the Rayleigh-Taylor instability
International Nuclear Information System (INIS)
Li, D.; Zhang, W.L.; Wu, Z.W.
2005-01-01
It is found that magnetic field has a stabilization effect whereas the sheared flow has a destabilization effect on the RT instability in the presence of sharp interface. RT instability only occurs in the long wave region and can be completely suppressed if the stabilizing effect of magnetic field dominates. The RT instability increases with wave number and flow shear, and acts much like a Kelvin-Helmholtz instability when destabilizing effect of sheared flow dominates. It is shown that both of ablation velocity and magnetic filed have stabilization effect on RT instability in the presence of continued interface. The stabilization effect of magnetic field takes place for whole waveband and becomes more significant for the short wavelength. The RT instability can be completely suppressed by the cooperated effect of magnetic field and ablation velocity so that the ICF target shell may be unnecessary to be accelerated to very high speed. The growth rate decreases as the density scale length increases. The stabilization effect of magnetic field is more significant for the short density scale length. (author)
Fibrillization kinetics of insulin solution in an interfacial shearing flow
Balaraj, Vignesh; McBride, Samantha; Hirsa, Amir; Lopez, Juan
2015-11-01
Although the association of fibril plaques with neurodegenerative diseases like Alzheimer's and Parkinson's is well established, in-depth understanding of the roles played by various physical factors in seeding and growth of fibrils is far from well known. Of the numerous factors affecting this complex phenomenon, the effect of fluid flow and shear at interfaces is paramount as it is ubiquitous and the most varying factor in vivo. Many amyloidogenic proteins have been found to denature upon contact at hydrophobic interfaces due to the self-assembling nature of protein in its monomeric state. Here, fibrillization kinetics of insulin solution is studied in an interfacial shearing flow. The transient surface rheological response of the insulin solution to the flow and its effect on the bulk fibrillization process has been quantified. Minute differences in hydrophobic characteristics between two variants of insulin- Human recombinant and Bovine insulin are found to result in very different responses. Results presented will be in the form of fibrillization assays, images of fibril plaques formed, and changes in surface rheological properties of the insulin solution. The interfacial velocity field, measured from images (via Brewster Angle Microscopy), is compared with computations. Supported by NNX13AQ22G, National Aeronautics and Space Administration.
Experimental observation of shear thickening oscillation
DEFF Research Database (Denmark)
Nagahiro, Shin-ichiro; Nakanishi, Hiizu; Mitarai, Namiko
2013-01-01
We report experimental observations of the shear thickening oscillation, i.e. the spontaneous macroscopic oscillation in the shear flow of severe shear thickening fluid. Using a density-matched starch-water mixture, in the cylindrical shear flow of a few centimeters flow width, we observed...
DEFF Research Database (Denmark)
Rasmussen, Henrik K.; Bejenariu, Anca Gabriela; Hassager, Ole
2010-01-01
to the flow in the non-linear flow regime. This has allowed highly elastic measurements within the limit of pure orientational stress, as the time of the flow was considerably smaller than the Rouse time. A Doi-Edwards [J. Chem. Soc., Faraday Trans. 2 74, 1818-1832 (1978)] type of constitutive model...... with the assumption of pure configurational stress was accurately able to predict the startup as well as the reversed flow behavior. This confirms that this commonly used theoretical picture for the flow of polymeric liquids is a correct physical principle to apply. c 2010 The Society of Rheology. [DOI: 10.1122/1.3496378]...
Modeling of the reactant conversion rate in a turbulent shear flow
Frankel, S. H.; Madnia, C. K.; Givi, P.
1992-01-01
Results are presented of direct numerical simulations (DNS) of spatially developing shear flows under the influence of infinitely fast chemical reactions of the type A + B yields Products. The simulation results are used to construct the compositional structure of the scalar field in a statistical manner. The results of this statistical analysis indicate that the use of a Beta density for the probability density function (PDF) of an appropriate Shvab-Zeldovich mixture fraction provides a very good estimate of the limiting bounds of the reactant conversion rate within the shear layer. This provides a strong justification for the implementation of this density in practical modeling of non-homogeneous turbulent reacting flows. However, the validity of the model cannot be generalized for predictions of higher order statistical quantities. A closed form analytical expression is presented for predicting the maximum rate of reactant conversion in non-homogeneous reacting turbulence.
Stationary shear flows in CGL anisotropic toroidal plasmas
International Nuclear Information System (INIS)
Pastukhov, V.P.; Ilgisonis, V.I.
1996-01-01
Recently a general structure of stationary shear flows in toroidal plasmas was obtained in the frame of ideal isotropic-pressure MHD model. The structure of the stationary plasma flows was shown to be determined by a hidden symmetry of MHD equations inherent in the toroidal systems with nested magnetic surfaces. However, the characteristic frequencies of the stationary plasma motion can considerably exceed the collisional frequencies in real plasma experiments. In this case the CGL collisionless MHD model seems to be more adequate than the simplified isotropic-pressure MHD model to describe the stationary plasma flows. In this paper we have generalized our approach to analyze the stationary plasma flows in the frame of the collisionless CGL model. We have found again that the hidden symmetry inherent in the toroidal topology results in two integral invariants which depend on two independent surface functions. The structure of stationary flows for CGL model is still the same as for isotropic MHD, however, the pressure tensor components satisfy a appreciably modifies the steady state force-balance equation. These results are applied to analyze the generalized equilibrium in axisymmetric (tokamak-like) magnetic confinement systems
Vescovi, D.; Berzi, D.; Richard, P.; Brodu, N.
2014-05-01
We use existing 3D Discrete Element simulations of simple shear flows of spheres to evaluate the radial distribution function at contact that enables kinetic theory to correctly predict the pressure and the shear stress, for different values of the collisional coefficient of restitution. Then, we perform 3D Discrete Element simulations of plane flows of frictionless, inelastic spheres, sheared between walls made bumpy by gluing particles in a regular array, at fixed average volume fraction and distance between the walls. The results of the numerical simulations are used to derive boundary conditions appropriated in the cases of large and small bumpiness. Those boundary conditions are, then, employed to numerically integrate the differential equations of Extended Kinetic Theory, where the breaking of the molecular chaos assumption at volume fraction larger than 0.49 is taken into account in the expression of the dissipation rate. We show that the Extended Kinetic Theory is in very good agreement with the numerical simulations, even for coefficients of restitution as low as 0.50. When the bumpiness is increased, we observe that some of the flowing particles are stuck in the gaps between the wall spheres. As a consequence, the walls are more dissipative than expected, and the flows resemble simple shear flows, i.e., flows of rather constant volume fraction and granular temperature.
International Nuclear Information System (INIS)
Vescovi, D.; Berzi, D.; Richard, P.; Brodu, N.
2014-01-01
We use existing 3D Discrete Element simulations of simple shear flows of spheres to evaluate the radial distribution function at contact that enables kinetic theory to correctly predict the pressure and the shear stress, for different values of the collisional coefficient of restitution. Then, we perform 3D Discrete Element simulations of plane flows of frictionless, inelastic spheres, sheared between walls made bumpy by gluing particles in a regular array, at fixed average volume fraction and distance between the walls. The results of the numerical simulations are used to derive boundary conditions appropriated in the cases of large and small bumpiness. Those boundary conditions are, then, employed to numerically integrate the differential equations of Extended Kinetic Theory, where the breaking of the molecular chaos assumption at volume fraction larger than 0.49 is taken into account in the expression of the dissipation rate. We show that the Extended Kinetic Theory is in very good agreement with the numerical simulations, even for coefficients of restitution as low as 0.50. When the bumpiness is increased, we observe that some of the flowing particles are stuck in the gaps between the wall spheres. As a consequence, the walls are more dissipative than expected, and the flows resemble simple shear flows, i.e., flows of rather constant volume fraction and granular temperature
Electromagnetic transport components and sheared flows in drift-Alfven turbulence
DEFF Research Database (Denmark)
Naulin, V.
2003-01-01
Results from three-dimensional numerical simulations of drift-Alfven turbulence in a toroidal geometry with sheared magnetic field are presented. The simulations show a relation between self-generated poloidal shear flows and magnetic field perturbations. For large values of the plasma beta we ob...... with a folded Gaussian, while the PDFs of the spatially averaged transport are in good agreement with an extreme value distribution. (C) 2003 American Institute of Physics....... related to magnetic flutter is proposed. The characteristics of the ExB flux are investigated using probability density distribution functions (PDFs). Although they are not Gaussian, no signs of algebraic tails in the PDFs are observed. The PDFs of the pointwise transport are found to agree well...
Shear flow over a plane wall with an axisymmetric cavity or a circular orifice of finite thickness
International Nuclear Information System (INIS)
Pozrikidis, C.
1994-01-01
Shear flow over a plane wall that contains an axisymmetric depression or pore is studied using a new boundary integral method which is suitable for computing three-dimensional Stokes flow within axisymmetric domains. Numerical results are presented for cavities in the shape of a section of a sphere or a circular cylinder of finite length, and for a family of pores or orifices with finite thickness. The results illustrate the distribution of shear stresses over the plane wall and inside the cavities or pores. It is found that in most cases, the distribution of shear stresses over the plane wall, around the depressions, is well approximated with that for flow over an orifice of infinitesimal thickness for which an exact solution is available. The kinematic structure of the flow is discussed with reference to eddy formation and three-dimensional flow reversal. It is shown that the thickness of a circular orifice or depth of a pore play an important role in determining the kinematical structure of the flow underneath the orifice in the lower half-space
Magnetic field correlations in random flow with strong steady shear
International Nuclear Information System (INIS)
Kolokolov, I. V.; Lebedev, V. V.; Sizov, G. A.
2011-01-01
We analyze the magnetic kinematic dynamo in a conducting fluid where a stationary shear flow is accompanied by relatively weak random velocity fluctuations. The diffusionless and diffusion regimes are described. The growth rates of the magnetic field moments are related to the statistical characteristics of the flow describing divergence of the Lagrangian trajectories. The magnetic field correlation functions are examined, and their growth rates and scaling behavior are established. General assertions are illustrated by the explicit solution of a model where the velocity field is short-correlated in time.
Wang, Yan-Xia; Xiang, Cheng; Liu, Bo; Zhu, Yong; Luan, Yong; Liu, Shu-Tian; Qin, Kai-Rong
2016-12-28
In vivo studies have demonstrated that reasonable exercise training can improve endothelial function. To confirm the key role of wall shear stress induced by exercise on endothelial cells, and to understand how wall shear stress affects the structure and the function of endothelial cells, it is crucial to design and fabricate an in vitro multi-component parallel-plate flow chamber system which can closely replicate exercise-induced wall shear stress waveforms in artery. The in vivo wall shear stress waveforms from the common carotid artery of a healthy volunteer in resting and immediately after 30 min acute aerobic cycling exercise were first calculated by measuring the inner diameter and the center-line blood flow velocity with a color Doppler ultrasound. According to the above in vivo wall shear stress waveforms, we designed and fabricated a parallel-plate flow chamber system with appropriate components based on a lumped parameter hemodynamics model. To validate the feasibility of this system, human umbilical vein endothelial cells (HUVECs) line were cultured within the parallel-plate flow chamber under abovementioned two types of wall shear stress waveforms and the intracellular actin microfilaments and nitric oxide (NO) production level were evaluated using fluorescence microscope. Our results show that the trends of resting and exercise-induced wall shear stress waveforms, especially the maximal, minimal and mean wall shear stress as well as oscillatory shear index, generated by the parallel-plate flow chamber system are similar to those acquired from the common carotid artery. In addition, the cellular experiments demonstrate that the actin microfilaments and the production of NO within cells exposed to the two different wall shear stress waveforms exhibit different dynamic behaviors; there are larger numbers of actin microfilaments and higher level NO in cells exposed in exercise-induced wall shear stress condition than resting wall shear stress condition
International Nuclear Information System (INIS)
Anon.
1989-01-01
Papers on turbulent shear flows are presented, covering topics such as the structure of pressure fluctuations, fossil two-dimensional turbulence in the ocean, turbulence production and eddy structure in wall turbulence, bypass transition in a heated boundary layer, a turbulent spot in plane Poiseuille flow, the evolution of an axisymmetric jet, plane mixing layer development, vortex models of a pseudoturbulent shear flow, numerical techniques for turbulence studies, Reynolds stress in the wall region of turbulent pipe flow, the turbulent structure of a momentumless wake, the near field of the transverse jet. Additional topics include a turbulent boundary layer disturbed by a cylinder, evolving mixing layers, flow analysis in a vortex flowmeter, ejections and bursts in pulsatile turbulent wall flow measurements, a flat plate oscillating in pitch, turbulent buoyant flows, isothermal lobed mixer flows, flow distortion on a turbulent scalar field, two phase flows. In addition, papers on the applications of turbulent shear flow studies are given, including air pollutant deposition, closures, oceanography, instrumentation, heat transfer, rotating flows, combustion, coherent structures, turbulence control, and scalar transport modeling
Liu, Richeng; Li, Bo; Jiang, Yujing; Yu, Liyuan
2018-01-01
Hydro-mechanical properties of rock fractures are core issues for many geoscience and geo-engineering practices. Previous experimental and numerical studies have revealed that shear processes could greatly enhance the permeability of single rock fractures, yet the shear effects on hydraulic properties of fractured rock masses have received little attention. In most previous fracture network models, single fractures are typically presumed to be formed by parallel plates and flow is presumed to obey the cubic law. However, related studies have suggested that the parallel plate model cannot realistically represent the surface characters of natural rock fractures, and the relationship between flow rate and pressure drop will no longer be linear at sufficiently large Reynolds numbers. In the present study, a numerical approach was established to assess the effects of shear on the hydraulic properties of 2-D discrete fracture networks (DFNs) in both linear and nonlinear regimes. DFNs considering fracture surface roughness and variation of aperture in space were generated using an originally developed code DFNGEN. Numerical simulations by solving Navier-Stokes equations were performed to simulate the fluid flow through these DFNs. A fracture that cuts through each model was sheared and by varying the shear and normal displacements, effects of shear on equivalent permeability and nonlinear flow characteristics of DFNs were estimated. The results show that the critical condition of quantifying the transition from a linear flow regime to a nonlinear flow regime is: 10-4 〈 J hydraulic gradient. When the fluid flow is in a linear regime (i.e., J reduce the equivalent permeability significantly in the orientation perpendicular to the sheared fracture as much as 53.86% when J = 1, shear displacement Ds = 7 mm, and normal displacement Dn = 1 mm. By fitting the calculated results, the mathematical expression for δ2 is established to help choose proper governing equations when
Sustained turbulence and magnetic energy in non-rotating shear flows
DEFF Research Database (Denmark)
Nauman, Farrukh; Blackman, Eric G.
2017-01-01
From numerical simulations, we show that non-rotating magnetohydrodynamic shear flows are unstable to finite amplitude velocity perturbations and become turbulent, leading to the growth and sustenance of magnetic energy, including large scale fields. This supports the concept that sustained...... magnetic energy from turbulence is independent of the driving mechanism for large enough magnetic Reynolds numbers....
Wall shear stress characterization of a 3D bluff-body separated flow
Fourrié, Grégoire; Keirsbulck, Laurent; Labraga, Larbi
2013-10-01
Efficient flow control strategies aimed at reducing the aerodynamic drag of road vehicles require a detailed knowledge of the reference flow. In this work, the flow around the rear slanted window of a generic car model was experimentally studied through wall shear stress measurements using an electrochemical method. The mean and fluctuating wall shear stress within the wall impact regions of the recirculation bubble and the main longitudinal vortex structures which develop above the rear window are presented. Correlations allow a more detailed characterization of the recirculation phenomenon within the separation bubble. In the model symmetry plane the recirculation structure compares well with simpler 2D configurations; specific lengths, flapping motion and shedding of large-scale vortices are observed, these similarities diminish when leaving the middle plane due to the strong three-dimensionality of the flow. A specific attention is paid to the convection processes occurring within the recirculation: a downstream convection velocity is observed, in accordance with 2D recirculations from the literature, and an upstream convection is highlighted along the entire bubble length which has not been underlined in some previous canonical configurations.
Prediction of stably stratified homogeneous shear flows with second-order turbulence models
International Nuclear Information System (INIS)
Pereira, J C F; Rocha, J M P
2010-01-01
The present study investigated the role of pressure-correlation second-order turbulence modelling schemes on the predicted behaviour of stably stratified homogeneous vertical-sheared turbulence. The pressure-correlation terms were modelled with a nonlinear formulation (Craft 1991), which was compared with a linear pressure-strain model and the 'isotropization of production' model for the pressure-scalar correlation. Two additional modelling issues were investigated: the influence of the buoyancy term in the kinetic energy dissipation rate equation and the time scale in the thermal production term in the scalar variance dissipation equation. The predicted effects of increasing the Richardson number on turbulence characteristics were compared against a comprehensive set of direct numerical simulation databases. The linear models provide a broadly satisfactory description of the major effects of the Richardson number on stratified shear flow. The buoyancy term in the dissipation equation of the turbulent kinetic energy generates excessively low levels of dissipation. For moderate and large Richardson numbers, the term yields unrealistic linear oscillations in the shear and buoyancy production terms, and therefore should be dropped in this flow (or at least their coefficient c ε3 should be substantially reduced from its standard value). The mechanical dissipation time scale provides marginal improvements in comparison to the scalar time scale in the production. The observed inaccuracy of the linear model in predicting the magnitude of the effects on the velocity anisotropy was demonstrated to be attributed mainly to the defective behaviour of the pressure-correlation model, especially for stronger stratification. The turbulence closure embodying a nonlinear formulation for the pressure-correlations and specific versions of the dissipation equations failed to predict the tendency of the flow to anisotropy with increasing stratification. By isolating the effects of the
Snijkers, F.; Kirkwood, K. M.; Vlassopoulos, D.; Leal, L. G.; Nikopoulou, A.; Hadjichristidis, Nikolaos; Coppola, S.
2016-01-01
We report upon the characterization of the steady-state shear stresses and first normal stress differences as a function of shear rate using mechanical rheometry (both with a standard cone and plate and with a cone partitioned plate) and optical rheometry (with a flow-birefringence setup) of an entangled solution of asymmetric exact combs. The combs are polybutadienes (1,4-addition) consisting of an H-skeleton with an additional off-center branch on the backbone. We chose to investigate a solution in order to obtain reliable nonlinear shear data in overlapping dynamic regions with the two different techniques. The transient measurements obtained by cone partitioned plate indicated the appearance of overshoots in both the shear stress and the first normal stress difference during start-up shear flow. Interestingly, the overshoots in the start-up normal stress difference started to occur only at rates above the inverse stretch time of the backbone, when the stretch time of the backbone was estimated in analogy with linear chains including the effects of dynamic dilution of the branches but neglecting the effects of branch point friction, in excellent agreement with the situation for linear polymers. Flow-birefringence measurements were performed in a Couette geometry, and the extracted steady-state shear and first normal stress differences were found to agree well with the mechanical data, but were limited to relatively low rates below the inverse stretch time of the backbone. Finally, the steady-state properties were found to be in good agreement with model predictions based on a nonlinear multimode tube model developed for linear polymers when the branches are treated as solvent.
Snijkers, F.
2016-03-31
We report upon the characterization of the steady-state shear stresses and first normal stress differences as a function of shear rate using mechanical rheometry (both with a standard cone and plate and with a cone partitioned plate) and optical rheometry (with a flow-birefringence setup) of an entangled solution of asymmetric exact combs. The combs are polybutadienes (1,4-addition) consisting of an H-skeleton with an additional off-center branch on the backbone. We chose to investigate a solution in order to obtain reliable nonlinear shear data in overlapping dynamic regions with the two different techniques. The transient measurements obtained by cone partitioned plate indicated the appearance of overshoots in both the shear stress and the first normal stress difference during start-up shear flow. Interestingly, the overshoots in the start-up normal stress difference started to occur only at rates above the inverse stretch time of the backbone, when the stretch time of the backbone was estimated in analogy with linear chains including the effects of dynamic dilution of the branches but neglecting the effects of branch point friction, in excellent agreement with the situation for linear polymers. Flow-birefringence measurements were performed in a Couette geometry, and the extracted steady-state shear and first normal stress differences were found to agree well with the mechanical data, but were limited to relatively low rates below the inverse stretch time of the backbone. Finally, the steady-state properties were found to be in good agreement with model predictions based on a nonlinear multimode tube model developed for linear polymers when the branches are treated as solvent.
Flow of a non-Newtonian fluid through channels with permeable wall
Energy Technology Data Exchange (ETDEWEB)
Martins-Costa, Maria Laura [Universidade Federal Fluminense, Niteroi, RJ (Brazil). Dept. de Engenharia Mecanica. Lab. de Matematica Teorica e Aplicada]. E-mail: laura@mec.uff.br; Gama, Rogerio M. Saldanha da [Laboratorio Nacional de Computacao Cientifica (LNCC), Petropolis, RJ (Brazil)]. E-mail: rsgama@domain.com.br; Frey, Sergio [Rio Grande do Sul Univ., Porto Alegre, RS (Brazil). Dept. de Engenharia Mecanica. Grupo de Estudos Termicos e Energeticos
2000-07-01
In the present work the momentum transport in two adjacent flow regions is described by means of a continuum theory of mixtures, specially developed to model multiphase phenomena. A generalized Newtonian fluid flows through the permeable wall channel, originating a pure fluid region and a mixture region - where the fluid saturates the porous matrix. The fluid and the porous matrix are treated as continuous constituents of a binary mixture coexisting superposed, each of them occupying simultaneously the whole volume of the mixture. An Ostwald-de Waele behavior is assumed for both the fluid constituent (in the mixture region) and the fluid (in the so-called pure fluid region), while the porous matrix, represented by the solid constituent, is assumed rigid, homogeneous, isotropic and at rest. Compatibility conditions at the interface (pure fluid-mixture) for momentum transfer are proposed and discussed. Assuming no flow across the interface, the velocity should be zero on the solid parts of the boundary and should match the fluid diffusing velocity on the fluid parts of the boundary. Also the shear stress at the pure fluid region is to be balanced by a multiple of the partial shear stress at the mixture region. A minimum principle for the above-described problem, assuming fully developed flow in both regions, is presented, providing an easy and reliable way for carrying out numerical simulations. (author)
Nonlinear inertial Alfven waves in plasmas with sheared magnetic field and flow
International Nuclear Information System (INIS)
Chen Yinhua; Wang Ge; Tan Liwei
2004-01-01
Nonlinear equations describing inertial Alfven waves in plasmas with sheared magnetic field and flow are derived. For some specific parameters chosen, authors have found a new type of electromagnetic coherent structures in the tripolar vortex-like form
Spinning and tumbling of micron-sized triangles in a micro-channel shear flow
Fries, J.; Kumar, M. Vijay; Mihiretie, B. Mekonnen; Hanstorp, D.; Mehlig, B.
2018-03-01
We report on measurements of the angular dynamics of micron-sized equilaterally triangular platelets suspended in a micro-channel shear flow. Our measurements confirm that such particles spin and tumble like a spheroid in a simple shear. Since the triangle has corners, we can observe the spinning directly. In general, the spinning frequency is different from the tumbling frequency and the spinning is affected by tumbling. This gives rise to doubly periodic angular dynamics.
Zalm, van der E.E.J.; Goot, van der A.J.; Boom, R.M.
2010-01-01
Wheat dough can be separated into a starch-rich and a gluten-rich fraction by subjecting the dough to curvilinear shear flow. This paper presents the effect of salt (NaCl) addition on the shear-induced separation process. The separation (defined as the changes in protein concentration in the various
Energy Technology Data Exchange (ETDEWEB)
Sharma, S.L., E-mail: sharma55@purdue.edu [School of Nuclear Engineering, Purdue University, West Lafayette, IN (United States); Hibiki, T.; Ishii, M. [School of Nuclear Engineering, Purdue University, West Lafayette, IN (United States); Schlegel, J.P. [Department of Mining and Nuclear Engineering, Missouri University of Science and Technology, Rolla, MO (United States); Buchanan, J.R.; Hogan, K.J. [Bettis Laboratory, Naval Nuclear Laboratory, West Mifflin, PA (United States); Guilbert, P.W. [ANSYS UK Ltd, Oxfordshire (United Kingdom)
2017-02-15
Highlights: • Closure form of the interfacial shear term in three-dimensional form is investigated. • Assessment against adiabatic upward bubbly air–water flow data using CFD. • Effect of addition of the interfacial shear term on the phase distribution. - Abstract: In commercially available Computational Fluid Dynamics (CFD) codes such as ANSYS CFX and Fluent, the interfacial shear term is missing in the field momentum equations. The derivation of the two-fluid model (Ishii and Hibiki, 2011) indicates the presence of this term as a momentum source in the right hand side of the field momentum equation. The inclusion of this term is considered important for proper modeling of the interfacial momentum coupling between phases. For separated flows, such as annular flow, the importance of the shear term is understood in the one-dimensional (1-D) form as the major mechanism by which the wall shear is transferred to the gas phase (Ishii and Mishima, 1984). For gas dispersed two-phase flow CFD simulations, it is important to assess the significance of this term in the prediction of phase distributions. In the first part of this work, the closure of this term in three-dimensional (3-D) form in a CFD code is investigated. For dispersed gas–liquid flow, such as bubbly or churn-turbulent flow, bubbles are dispersed in the shear layer of the continuous phase. The continuous phase shear stress is mainly due to the presence of the wall and the modeling of turbulence through the Boussinesq hypothesis. In a 3-D simulation, the continuous phase shear stress can be calculated from the continuous fluid velocity gradient, so that the interfacial shear term can be closed using the local values of the volume fraction and the total stress of liquid phase. This form also assures that the term acts as an action-reaction force for multiple phases. In the second part of this work, the effect of this term on the volume fraction distribution is investigated. For testing the model two
Effect of shear-thinning behaviour on liquid-liquid plug flow in microchannels
Roumpea, Evangelia; Chinaud, Maxime; Weheliye, Weheliye Hashi; Angeli, Panagiota; Kahouadji, Lyes; Matar, Omar K.
2016-11-01
The present work investigates the dynamics of plug formation of shear-thinning solutions in a 200 μm microchannel using a two-colour micro-PIV system. Measurements, including phase-averaged velocity fields, have been conducted both at the T-junction inlet and the main channel to enhance understanding of non-Newtonian liquid-liquid flows. Two aqueous glycerol solutions containing xanthan gum are used as the non-Newtonian fluids while 5 cSt silicone oil is the Newtonian phase. The current experimental results revealed a pronounced impact of the xanthan gum (shear-thinning behaviour) on the flow pattern transition boundaries, and enhance the fluid flowrates where plug flow occurred. The addition of polymer resulted also in different hydrodynamic characteristics such as a bullet-shaped plug and an increased film thickness between the plug and the wall. In the present work, the technique allows to capture the velocity field of both phases simultaneously. Experimental results are compared with the numerical simulations provided by the code BLUE. Project funded under the UK Engineering and Physical Sciences Research Council (EPSRC) Programme Grant MEMPHIS.
Energy Technology Data Exchange (ETDEWEB)
Im, Sang Hyuk; Lee, Su Jin [Kyung Hee University, Yongin (Korea, Republic of); Suh, Duck Jong; Park, O Ok [Korea Advanced Institute of Science and Technology (KAIST), Daejeon (Korea, Republic of); Kwon, Moo Hyun [Woosuk University, Wanju (Korea, Republic of)
2015-07-15
We investigated whether a graded-index profile, specified by the polymer compositional gradient, could be formed using shear-induced polymer migration phenomenon in a polymer solution. For the presented model system, we generated a shear flow by rotating a glass rod at the center of a polystyrene/methylmethacrylate (PS/MMA) solution and measured the degree of polymer migration by the shear flow field by examining the concentration of polymer solution along the radial direction from the rotating axis to the periphery. Through model experiments, we formed a compositional gradient and controlled its profile in the solution by varying the concentration of polymer solution, molecular weight of polymer, and shear rate. Finally, we solidified the gradient profiles by the polymerization of the PS/MMA solution and confirmed that the gradient profiles were maintained with a compositional gradient twice larger than the mother PS/MMA solution.
International Nuclear Information System (INIS)
Kwon, H.; Park, G. C.
2000-01-01
The object of experiment is improved model of evaporative heat transfer coefficient using interfacial friction factor on evaporation. Experiments have been conducted with near-vertical(87 .deg.) flat plate on evaporation for air-water countercurrent stratified flow. Experiment facility is consisted of 1.7m length and 0.2 X 0.005m cross section, the one side direct heating system which have 10kw power capacity. The interfacial shear stress, pressure drop and temperatures in test section were measured. These parameters were measured by DP-103 pressure transducer, K-type thermocouple, RTD and Hot Wire Anemometer(HWA). Experimental results were inclination as increased interfacial shear stress with increased the evaporation rate. Interfacial shear stress was increased as increased water flow rate and air flow rate too. For the evaluation of the measured evaporative heat transfer coefficients and physical understanding of the evaporation phenomena, the evaporative heat transfer coefficients were obtained through the simple calculation process by the use of mass transfer coefficient correlation and the experimental data of wavy film surface effect on shear and on evaporation
Steady state drift vortices in plasmas with shear flow in equilibrium
DEFF Research Database (Denmark)
Chakrabarti, N.
1999-01-01
The Hasegawa-Mima equation in the presence of sheared poloidal flow is solved for two-dimensional steady state vortex. It is shown that when the phase velocity of the vortex is the same as the diamagnetic drift velocity, an exact solution in the form of counter-rotating vortices may appear...
Spectral and evolutionary analysis of advection-diffusion equations and the shear flow paradigm
International Nuclear Information System (INIS)
Thyagaraja, A.; Loureiro, N.; Knight, P.J.
2002-01-01
Advection-diffusion equations occur in a wide variety of fields in many contexts of active and passive transport in fluids and plasmas. The effects of sheared advective flows in the presence of irreversible processes such as diffusion and viscosity are of considerable current interest in tokamak and astrophysical contexts, where they are thought to play a key role in both transport and the dynamical structures characteristic of electromagnetic plasma turbulence. In this paper we investigate the spectral and evolutionary properties of relatively simple, linear, advection-diffusion equations. We apply analytical approaches based on standard Green's function methods to obtain insight into the nature of the spectra when the advective and diffusive effects occur separately and in combination. In particular, the physically interesting limit of small (but finite) diffusion is studied in detail. The analytical work is extended and supplemented by numerical techniques involving a direct solution of the eigenvalue problem as well as evolutionary studies of the initial value problem using a parallel code, CADENCE. The three approaches are complementary and entirely consistent with each other when appropriate comparison is made. They reveal different aspects of the properties of the advection-diffusion equation, such as the ability of sheared flows to generate a direct cascade to high wave numbers transverse to the advection and the consequent enhancement of even small amounts of diffusivity. The invariance properties of the spectra in the low diffusivity limit and the ability of highly sheared, jet-like flows to 'confine' transport to low shear regions are demonstrated. The implications of these properties in a wider context are discussed and set in perspective. (author)
Directory of Open Access Journals (Sweden)
Kyung Min Kim
Full Text Available Shear stress activates cellular signaling involved in cellular proliferation, differentiation, and migration. However, the mechanisms of mesenchymal stem cell (MSC differentiation under interstitial flow are not fully understood. Here, we show the increased osteogenic differentiation of MSCs under exposure to constant, extremely low shear stress created by osmotic pressure-induced flow in a microfluidic chip. The interstitial level of shear stress in the proposed microfluidic system stimulated nuclear localization of TAZ (transcriptional coactivator with PDZ-binding motif, a transcriptional modulator of MSCs, activated TAZ target genes such as CTGF and Cyr61, and induced osteogenic differentiation. TAZ-depleted cells showed defects in shear stress-induced osteogenic differentiation. In shear stress induced cellular signaling, Rho signaling pathway was important forthe nuclear localization of TAZ. Taken together, these results suggest that TAZ is an important mediator of interstitial flow-driven shear stress signaling in osteoblast differentiation of MSCs.
Effect of pulse pressure on borehole stability during shear swirling flow vibration cementing.
Directory of Open Access Journals (Sweden)
Zhihua Cui
Full Text Available The shear swirling flow vibration cementing (SSFVC technique rotates the downhole eccentric cascade by circulating cementing fluid. It makes the casing eccentrically revolve at high speed around the borehole axis. It produces strong agitation action to the annulus fluid, makes it in the state of shear turbulent flow, and results in the formation of pulse pressure which affects the surrounding rock stress. This study was focused on 1 the calculation of the pulse pressure in an annular turbulent flow field based on the finite volume method, and 2 the analysis of the effect of pulse pressure on borehole stability. On the upside, the pulse pressure is conducive to enhancing the liquidity of the annulus fluid, reducing the fluid gel strength, and preventing the formation of fluid from channeling. But greater pulse pressure may cause lost circulation and even formation fracturing. Therefore, in order to ensure smooth cementing during SSFVC, the effect of pulse pressure should be considered when cementing design.
Energy Technology Data Exchange (ETDEWEB)
Liang, Edison; Fu, Wen [Rice University, Houston, TX 77005 (United States); Böttcher, Markus [North-West University, Potchefstroom, 2520 (South Africa)
2017-10-01
We present particle-in-cell simulation results of relativistic shear boundary layers between electron–ion and electron–positron plasmas and discuss their potential applications to astrophysics. Specifically, we find that in the case of a fast electron–positron spine surrounded by a slow-moving or stationary electron–ion sheath, lepton acceleration proceeds in a highly anisotropic manner due to electromagnetic fields created at the shear interface. While the highest-energy leptons still produce a beaming pattern (as seen in the quasi-stationary frame of the sheath) of order 1/Γ, where Γ is the bulk Lorentz factor of the spine, for lower-energy particles, the beaming is much less pronounced. This is in stark contrast to the case of pure electron–ion shear layers, in which anisotropic particle acceleration leads to significantly narrower beaming patterns than 1/Γ for the highest-energy particles. In either case, shear-layer acceleration is expected to produce strongly angle-dependent lepton (hence, emanating radiation) spectra, with a significantly harder spectrum in the forward direction than viewed from larger off-axis angles, much beyond the regular Doppler boosting effect from a co-moving isotropic lepton distribution. This may solve the problem of the need for high (and apparently arbitrarily chosen) minimum Lorentz factors of radiating electrons, often plaguing current blazar and GRB jet modeling efforts.
International Nuclear Information System (INIS)
Hau, Jan-Niklas; Oberlack, Martin; Chagelishvili, George; Khujadze, George; Tevzadze, Alexander
2015-01-01
Aerodynamic sound generation in shear flows is investigated in the light of the breakthrough in hydrodynamics stability theory in the 1990s, where generic phenomena of non-normal shear flow systems were understood. By applying the thereby emerged short-time/non-modal approach, the sole linear mechanism of wave generation by vortices in shear flows was captured [G. D. Chagelishvili, A. Tevzadze, G. Bodo, and S. S. Moiseev, “Linear mechanism of wave emergence from vortices in smooth shear flows,” Phys. Rev. Lett. 79, 3178-3181 (1997); B. F. Farrell and P. J. Ioannou, “Transient and asymptotic growth of two-dimensional perturbations in viscous compressible shear flow,” Phys. Fluids 12, 3021-3028 (2000); N. A. Bakas, “Mechanism underlying transient growth of planar perturbations in unbounded compressible shear flow,” J. Fluid Mech. 639, 479-507 (2009); and G. Favraud and V. Pagneux, “Superadiabatic evolution of acoustic and vorticity perturbations in Couette flow,” Phys. Rev. E 89, 033012 (2014)]. Its source is the non-normality induced linear mode-coupling, which becomes efficient at moderate Mach numbers that is defined for each perturbation harmonic as the ratio of the shear rate to its characteristic frequency. Based on the results by the non-modal approach, we investigate a two-dimensional homentropic constant shear flow and focus on the dynamical characteristics in the wavenumber plane. This allows to separate from each other the participants of the dynamical processes — vortex and wave modes — and to estimate the efficacy of the process of linear wave-generation. This process is analyzed and visualized on the example of a packet of vortex modes, localized in both, spectral and physical, planes. Further, by employing direct numerical simulations, the wave generation by chaotically distributed vortex modes is analyzed and the involved linear and nonlinear processes are identified. The generated acoustic field is anisotropic in the wavenumber
Cavanagh, J. P.; Lampkin, D. J.; Moon, T.
2017-12-01
The impact of meltwater injection into the shear margins of Jakobshavn Isbræ via drainage from water-filled crevasses on ice flow is examined. We use Landsat-8 Operational Land Imager panchromatic, high-resolution imagery to monitor the spatiotemporal variability of seven water-filled crevasse ponds during the summers of 2013 to 2015. The timing of drainage from water-filled crevasses coincides with an increase of 2 to 20% in measured ice velocity beyond Jakobshavn Isbræ shear margins, which we define as extramarginal ice velocity. Some water-filled crevasse groups demonstrate multiple drainage events within a single melt season. Numerical simulations show that hydrologic shear weakening due to water-filled crevasse drainage can accelerate extramarginal flow by as much as 35% within 10 km of the margins and enhance mass flux through the shear margins by 12%. This work demonstrates a novel mechanism through which surface melt can influence regional ice flow.
Reverse flow and vortex breakdown in a shear-thinning fluid
International Nuclear Information System (INIS)
Cabeza, C; Sarasua, G; Barrere, N; Marti, A C
2011-01-01
The effect of polymer concentration on the development of reverse secondary flow and vortex breakdown was studied using a viscoelastic solution of polyacrlylamide in water. The fluid was contained in cylindrical containers of two different radii, the top end wall of which rotated at a varying speed, thus, imparting a circulating motion to the fluid. Whereas using a newtonian fluid, streamlines will occupy the entire container, the flow of a shear-thinning fluid may divide into two cells of opposite circulating motion. The curve of critical Reynolds and elasticity numbers (Re, E) values corresponding to the development of reverse flow was obtained over a wide range of Re values. Vortex breakdown was found to occur at extremely low Re values.
Closed compact Taylor's droplets in a phase-separated lamellar-sponge mixture under shear flow
Courbin, L.; Cristobal, G.; Rouch, J.; Panizza, P.
2001-09-01
We have studied by optical microscopy, small-angle light scattering, and rheology, the behavior under shear flow of a phase-separated lamellar-sponge (Lα - L3) ternary mixture. We observe in the Lα-rich region (ΦLα > 80%) the existence of a Newtonian assembly made of closed compact monodisperse lamellar droplets immersed in the sponge phase. Contrary to the classical onion glassy texture obtained upon shearing Lα phases, the droplet size scales herein as dot gamma-1, the inverse of the shear rate. This result is in good agreement with Taylor's picture. Above a critical shear rate, dot gammac, the droplets organize to form a single colloidal crystal whose lattice size varies as dot gamma-1/3. To the memory of Tess Melissa P.
Laser reflection method for determination of shear stress in low density transitional flows
Sathian, Sarith P.; Kurian, Job
2006-03-01
The details of laser reflection method (LRM) for the determination of shear stress in low density transitional flows are presented. The method is employed to determine the shear stress due to impingement of a low density supersonic free jet issuing out from a convergent divergent nozzle on a flat plate. The plate is smeared with a thin oil film and kept parallel to the nozzle axis. For a thin oil film moving under the action of aerodynamic boundary layer, the shear stress at the air-oil interface is equal to the shear stress between the surface and air. A direct and dynamic measurement of the oil film slope generated by the shear force is done using a position sensing detector (PSD). The thinning rate of the oil film is directly measured which is the major advantage of the LRM. From the oil film slope history, calculation of the shear stress is done using a three-point formula. The range of Knudsen numbers investigated is from 0.028 to 0.516. Pressure ratio across the nozzle varied from 3,500 to 8,500 giving highly under expanded free jets. The measured values of shear, in the overlapping region of experimental parameters, show fair agreement with those obtained by force balance method and laser interferometric method.
Jia, Yali; Bagnaninchi, Pierre O.; Wang, Ruikang K.
2008-02-01
Mechanical stimuli can be introduced to three dimensional (3D) cell cultures by use of perfusion bioreactor. Especially in musculoskeletal tissues, shear stress caused by fluid flow generally increase extra-cellular matrix (ECM) production and cell proliferation. The relationship between the shear stress and the tissue development in situ is complicated because of the non-uniform pore distribution within the cell-seeded scaffold. In this study, we firstly demonstrated that Doppler optical coherence tomography (DOCT) is capable of monitoring localized fluid flow and shear stress in the complex porous scaffold by examining their variation trends at perfusion rate of 5, 8, 10 and 12 ml/hr. Then, we developed the 3D porous cellular constructs, cell-seeded chitosan scaffolds monitored during several days by DOCT. The fiber based fourier domain DOCT employed a 1300 nm superluminescent diode with a bandwidth of 52 nm and a xyz resolution of 20×20×15 μm in free space. This setup allowed us not only to assess the cell growth and ECM deposition by observing their different scattering behaviors but also to further investigate how the cell attachment and ECM production has the effect on the flow shear stress and the relationship between flow rate and shear stress in the developing tissue construct. The possibility to monitor continuously the constructs under perfusion will easily indicate the effect of flow rate or shear stress on the cell viability and cell proliferation, and then discriminate the perfusion parameters affecting the pre-tissue formation rate growth.
Observation of Droplet Size Oscillations in a Two Phase Fluid under Shear Flow
Courbin, Laurent; Panizza, Pascal
2004-11-01
It is well known that complex fluids exhibit strong couplings between their microstructure and the flow field. Such couplings may lead to unusual non linear rheological behavior. Because energy is constantly brought to the system, richer dynamic behavior such as non linear oscillatory or chaotic response is expected. We report on the observation of droplet size oscillations at fixed shear rate. At low shear rates, we observe two steady states for which the droplet size results from a balance between capillary and viscous stress. For intermediate shear rates, the droplet size becomes a periodic function of time. We propose a phenomenological model to account for the observed phenomenon and compare numerical results to experimental data.
Nonequilibrium statistical mechanics of shear flow: invariant quantities and current relations
International Nuclear Information System (INIS)
Baule, A; Evans, R M L
2010-01-01
In modeling nonequilibrium systems one usually starts with a definition of the microscopic dynamics, e.g., in terms of transition rates, and then derives the resulting macroscopic behavior. We address the inverse question for a class of steady state systems, namely complex fluids under continuous shear flow: how does an externally imposed shear current affect the microscopic dynamics of the fluid? The answer can be formulated in the form of invariant quantities, exact relations for the transition rates in the nonequilibrium steady state, as discussed in a recent letter (Baule and Evans, 2008 Phys. Rev. Lett. 101 240601). Here, we present a more pedagogical account of the invariant quantities and the theory underlying them, known as the nonequilibrium counterpart to detailed balance (NCDB). Furthermore, we investigate the relationship between the transition rates and the shear current in the steady state. We show that a fluctuation relation of the Gallavotti–Cohen type holds for systems satisfying NCDB
International Nuclear Information System (INIS)
Fedorczak, N; Manz, P; Chakraborty Thakur, S; Xu, M; Tynan, G R
2013-01-01
The consequences of vorticity conservation on the spatio-temporal interaction of a E × B zonal shear with a generic pattern of plasma potential modes are investigated in a magnetized plasma environment. Eddies organized on a chain along the zonal direction are locally depleted, resulting in what appears to be a radial decorrelation by the shear flow in the absence of dissipation. The eddy depletion occurs due to a transfer of enstrophy from the chain to the shear flow during the progressive growth in the chain anisotropy. The rate of zonal shear acceleration is derived analytically and its expression is validated by numerical simulations. The rate is proportional to the chain amplitude in the weak shear regime and to the shearing rate in the strong shear regime. Basic properties of the model are validated with fast visible imaging data collected on a magnetized plasma column experiment. A characteristic vorticity flux across the edge shear layer of tokamak plasmas is associated with the model predictions. The dependence of the interaction rate with turbulence amplitude and shearing rate could be an important ingredient of the low to high confinement mode transition. (paper)
Yan, Z; McKee, G R; Fonck, R; Gohil, P; Groebner, R J; Osborne, T H
2014-03-28
Comprehensive 2D turbulence and eddy flow velocity measurements on DIII-D demonstrate a rapidly increasing turbulence-driven shear flow that develops ∼100 μs prior to the low-confinement (L mode) to high-confinement (H mode) transition and appears to trigger it. These changes are localized to a narrow layer 1-2 cm inside the magnetic boundary. Increasing heating power increases the Reynolds stress, the energy transfer from turbulence to the poloidal flow, and the edge flow shearing rate that then exceeds the decorrelation rate, suppressing turbulence and triggering the transition.
Energy Technology Data Exchange (ETDEWEB)
Adoua, S.R
2007-07-15
This work is devoted to the study of an oblate spheroidal bubble of prescribed shape set fixed in a linear shear flow using direct numerical simulation. The three dimensional Navier-Stokes equations are solved in orthogonal curvilinear coordinates using a finite volume method. The bubble response is studied over a wide range of the aspect ratio (1-2.7), the bubble Reynolds number (50-2000) and the non-dimensional shear rate (0.-1.2). The numerical simulations shows that the shear flow imposes a plane symmetry of the wake whatever the parameters of the flow. The trailing vorticity is organized into two anti-symmetrical counter rotating tubes with a sign imposed by the competition of two mechanisms (the Lighthill mechanism and the instability of the wake). Whatever the Reynolds number, the lift coefficient reaches the analytical value obtained in an inviscid, weakly sheared flow corresponding to a lift force oriented in the same direction as that of a spherical bubble. For moderate Reynolds numbers, the direction of the lift force reverses when the bubble aspect ratio is large enough as observed in experiments. This reversal occurs for aspect ratios larger than 2.225 and is found to be directly linked to the sign of the trailing vorticity which is concentrated within two counter-rotating threads which propel the bubble in a direction depending of their sign of rotation. The behavior of the drag does not revel any significant effect induced by the wake structure and follows a quadratic increase with the shear rate. Finally, the torque experienced by the bubble also reverses for the same conditions inducing the reversal of the lift force. By varying the orientation of the bubble in the shear flow, a stable equilibrium position is found corresponding to a weak angle between the small axis of the bubble and the flow direction. (author)
López-Barrón, Carlos R; Gurnon, A Kate; Eberle, Aaron P R; Porcar, Lionel; Wagner, Norman J
2014-04-01
We present direct measurements of the evolution of the segmental-level microstructure of a stable shear-banding polymerlike micelle solution during flow startup and cessation in the plane of flow. These measurements provide a definitive, quantitative microstructural understanding of the stages observed during flow startup: an initial elastic response with limited alignment that yields with a large stress overshoot to a homogeneous flow with associated micellar alignment that persists for approximately three relaxation times. This transient is followed by a shear (kink) band formation with a flow-aligned low-viscosity band that exhibits shear-induced concentration fluctuations and coexists with a nearly isotropic band of homogenous, highly viscoelastic micellar solution. Stable, steady banding flow is achieved only after approximately two reptation times. Flow cessation from this shear-banded state is also found to be nontrivial, exhibiting an initial fast relaxation with only minor structural relaxation, followed by a slower relaxation of the aligned micellar fluid with the equilibrium fluid's characteristic relaxation time. These measurements resolve a controversy in the literature surrounding the mechanism of shear banding in entangled wormlike micelles and, by means of comparison to existing literature, provide further insights into the mechanisms driving shear-banding instabilities in related systems. The methods and instrumentation described should find broad use in exploring complex fluid rheology and testing microstructure-based constitutive equations.
Velocity and shear stress distribution downstream of mechanical heart valves in pulsatile flow.
Giersiepen, M; Krause, U; Knott, E; Reul, H; Rau, G
1989-04-01
Ten mechanical valves (TAD 27 mm): Starr-Edwards Silastic Ball, Björk-Shiley Standard, Björk-Shiley Concave-Convex, Björk-Shiley Monostrut, Hall-Kaster (Medtronic-Hall), OmniCarbon, Bicer Val, Sorin, Saint-Jude Medical and Hemex (Duromedics) are investigated in a comparative in vitro study. The velocity and turbulent shear stress profiles of the valves were determined by Laser Doppler anemometry in two different downstream axes within a model aortic root. Depending on the individual valve design, velocity peaks up to 1.5 m/s and turbulent shear stress peaks up to 150 N/m2 were measured during the systolic phase. These shear stress peaks mainly occurred in areas of flow separation and intense momentum exchange. Directly downstream of the valves (measuring axis 0.55.dAorta) turbulent shear stress peaks occurred at peak systole and during the deceleration phase, while in the second measuring axis (1.5.dAorta) turbulence levels were lower. Shear stress levels were high at the borders of the fluid jets. The results are discussed from a fluid-dynamic point of view.
Wei, Guoguang; Mangal, Sharad; Denman, John; Gengenbach, Thomas; Lee Bonar, Kevin; Khan, Rubayat I; Qu, Li; Li, Tonglei; Zhou, Qi Tony
2017-10-01
This study has investigated the surface coating efficiency and powder flow improvement of a model cohesive acetaminophen powder by high-shear processing with pharmaceutical lubricants through 2 common equipment, conical comil and high-shear mixer. Effects of coating materials and processing parameters on powder flow and surface coating coverage were evaluated. Both Carr's index and shear cell data indicated that processing with the lubricants using comil or high-shear mixer substantially improved the flow of the cohesive acetaminophen powder. Flow improvement was most pronounced for those processed with 1% wt/wt magnesium stearate, from "cohesive" for the V-blended sample to "easy flowing" for the optimally coated sample. Qualitative and quantitative characterizations demonstrated a greater degree of surface coverage for high-shear mixing compared with comilling; nevertheless, flow properties of the samples at the corresponding optimized conditions were comparable between 2 techniques. Scanning electron microscopy images demonstrated different coating mechanisms with magnesium stearate or l-leucine (magnesium stearate forms a coating layer and leucine coating increases surface roughness). Furthermore, surface coating with hydrophobic magnesium stearate did not retard the dissolution kinetics of acetaminophen. Future studies are warranted to evaluate tableting behavior of such dry-coated pharmaceutical powders. Copyright © 2017 American Pharmacists Association®. Published by Elsevier Inc. All rights reserved.
Droplet breakup driven by shear thinning solutions in a microfluidic T-junction
Chiarello, Enrico; Gupta, Anupam; Mistura, Giampaolo; Sbragaglia, Mauro; Pierno, Matteo
2017-12-01
Droplet-based microfluidics turned out to be an efficient and adjustable platform for digital analysis, encapsulation of cells, drug formulation, and polymerase chain reaction. Typically, for most biomedical applications, the handling of complex, non-Newtonian fluids is involved, e.g., synovial and salivary fluids, collagen, and gel scaffolds. In this study, we investigate the problem of droplet formation occurring in a microfluidic T-shaped junction, when the continuous phase is made of shear thinning liquids. At first, we review in detail the breakup process, providing extensive, side-by-side comparisons between Newtonian and non-Newtonian liquids over unexplored ranges of flow conditions and viscous responses. The non-Newtonian liquid carrying the droplets is made of Xanthan solutions, a stiff, rodlike polysaccharide displaying a marked shear thinning rheology. By defining an effective Capillary number, a simple yet effective methodology is used to account for the shear-dependent viscous response occurring at the breakup. The droplet size can be predicted over a wide range of flow conditions simply by knowing the rheology of the bulk continuous phase. Experimental results are complemented with numerical simulations of purely shear thinning fluids using lattice Boltzmann models. The good agreement between the experimental and numerical data confirm the validity of the proposed rescaling with the effective Capillary number.
Inertial shear flow of assemblies of frictionless polygons: Rheology and microstructure.
Azéma, Émilien; Radjaï, Farhang; Roux, Jean-Noël
2018-01-05
Motivated by the understanding of shape effects in granular materials, we numerically investigate the macroscopic and microstructural properties of anisotropic dense assemblies of frictionless polydisperse rigid pentagons in shear flow, and compare them with similar systems of disks. Once subjected to large cumulative shear strains their rheology and microstructure are investigated in uniform steady states, depending on inertial number I, which ranges from the quasistatic limit ([Formula: see text]) to 0.2. In the quasistatic limit both systems are devoid of Reynolds dilatancy, i.e., flow at their random close packing density. Both macroscopic friction angle [Formula: see text], an increasing function of I , and solid fraction [Formula: see text], a decreasing function of I, are larger with pentagons than with disks at small I, but the differences decline for larger I and, remarkably, nearly vanish for [Formula: see text]. Under growing I , the depletion of contact networks is considerably slower with pentagons, in which increasingly anisotropic, but still well-connected force-transmitting structures are maintained throughout the studied range. Whereas contact anisotropy and force anisotropy contribute nearly equally to the shear strength in disk assemblies, the latter effect dominates with pentagons at small I, while the former takes over for I of the order of 10 -2 . The size of clusters of grains in side-to-side contact, typically comprising more than 10 pentagons in the quasistatic limit, very gradually decreases for growing I.
Domino structures evolution in strike-slip shear zones; the importance of the cataclastic flow
Moreira, N.; Dias, R.
2018-05-01
The Porto-Tomar-Ferreira do Alentejo dextral Shear Zone is one of the most important structures of the Iberian Variscides. In its vicinity, close to Abrantes (Central Portugal), a localized heterogeneous strain pattern developed in a decimetric metamorphic siliceous multilayer. This complex pattern was induced by the D2 dextral shearing of the early S0//S1 foliation in brittle-ductile conditions, giving rise to three main shear zone families. One of these families, with antithetic kinematics, delimits blocks with rigid clockwise rotation surrounded by coeval cataclasites, generating a local domino structure. The proposed geometrical and kinematic analysis, coupled with statistical studies, highlights the relation between subsidiary shear zones and the main shear zone. Despite the heterogeneous strain pattern, a quantitative approach of finite strain was applied based on the restoration of the initial fracture pattern. This approach shows the importance of the cataclastic flow coupled with the translational displacement of the domino domain in solving space problems related to the rigid block rotation. Such processes are key in allowing the rigid block rotation inside shear zones whenever the simple shear component is a fundamental mechanism.
Buoyant miscible displacement flow of shear-thinning fluids: Experiments and Simulations
Ale Etrati Khosroshahi, Seyed Ali; Frigaard, Ian
2017-11-01
We study displacement flow of two miscible fluids with density and viscosity contrast in an inclined pipe. Our focus is mainly on displacements where transverse mixing is not significant and thus a two-layer, stratified flow develops. Our experiments are carried out in a long pipe, covering a wide range of flow-rates, inclination angles and viscosity ratios. Density and viscosity contrasts are achieved by adding Glycerol and Xanthan gum to water, respectively. At each angle, flow rate and viscosity ratio are varied and density contrast is fixed. We identify and map different flow regimes, instabilities and front dynamics based on Fr , Re / Frcosβ and viscosity ratio m. The problem is also studied numerically to get a better insight into the flow structure and shear-thinning effects. Numerical simulations are completed using OpenFOAM in both pipe and channel geometries and are compared against the experiments. Schlumberger, NSERC.
Energy Technology Data Exchange (ETDEWEB)
Vechiato-Filho, Aljomar José, E-mail: aljomarvechiatoflo@gmail.com [Department of Dental Materials and Prosthodontics, Aracatuba Dental School, Univ. Estadual Paulista — UNESP, Aracatuba, Sao Paulo (Brazil); Silva Vieira Marques, Isabella da [Department of Prosthodontics and Periodontology, Piracicaba Dental School, University of Campinas (UNICAMP), Piracicaba, Sao Paulo (Brazil); Santos, Daniela Micheline dos [Department of Dental Materials and Prosthodontics, Aracatuba Dental School, Univ. Estadual Paulista — UNESP, Aracatuba, Sao Paulo (Brazil); Oliveira Matos, Adaias [Department of Prosthodontics and Periodontology, Piracicaba Dental School, University of Campinas (UNICAMP), Piracicaba, Sao Paulo (Brazil); Rangel, Elidiane Cipriano; Cruz, Nilson Cristino da [Laboratory of Technological Plasmas (LaPTec), Engineering College, Univ. Estadual Paulista — UNESP, Sorocaba, Sao Paulo (Brazil); Barão, Valentim Adelino Ricardo [Department of Prosthodontics and Periodontology, Piracicaba Dental School, University of Campinas (UNICAMP), Piracicaba, Sao Paulo (Brazil)
2016-03-01
The effect of nonthermal plasma on the surface characteristics of commercially pure titanium (cp-Ti), and on the shear bond strength between an autopolymerizing acrylic resin and cp-Ti was investigated. A total of 96 discs of cp-Ti were distributed into four groups (n = 24): Po (no surface treatment), SB (sandblasting), Po + NTP and SB + NTP (methane plasma). Surface characterization was performed through surface energy, surface roughness, scanning microscopy, energy dispersive spectroscopy, and X-ray diffraction tests. Shear bond strength test was conducted immediately and after thermocycling. Surface treatment affected the surface energy and roughness of cp-Ti discs (P < .001). SEM–EDS showed the presence of the carbide thin film. XRD spectra revealed no crystalline phase changes. The SB + NTP group showed the highest bond strength values (6.76 ± 0.70 MPa). Thermocycling reduced the bond strength of the acrylic resin/cp-Ti interface (P < .05), except for Po group. NTP is an effective treatment option for improving the shear bond strength between both materials. - Highlights: • We tested the bond strength between two widely used materials in dentistry (acrylic and titanium). • We performed an innovative surface treatment with nonthermal plasma. • Increasing adhesion will avoid complications of full-arch implant-retained prostheses.
Exploiting similarity in turbulent shear flows for turbulence modeling
Robinson, David F.; Harris, Julius E.; Hassan, H. A.
1992-12-01
It is well known that current k-epsilon models cannot predict the flow over a flat plate and its wake. In an effort to address this issue and other issues associated with turbulence closure, a new approach for turbulence modeling is proposed which exploits similarities in the flow field. Thus, if we consider the flow over a flat plate and its wake, then in addition to taking advantage of the log-law region, we can exploit the fact that the flow becomes self-similar in the far wake. This latter behavior makes it possible to cast the governing equations as a set of total differential equations. Solutions of this set and comparison with measured shear stress and velocity profiles yields the desired set of model constants. Such a set is, in general, different from other sets of model constants. The rational for such an approach is that if we can correctly model the flow over a flat plate and its far wake, then we can have a better chance of predicting the behavior in between. It is to be noted that the approach does not appeal, in any way, to the decay of homogeneous turbulence. This is because the asymptotic behavior of the flow under consideration is not representative of the decay of homogeneous turbulence.
Exploiting similarity in turbulent shear flows for turbulence modeling
Robinson, David F.; Harris, Julius E.; Hassan, H. A.
1992-01-01
It is well known that current k-epsilon models cannot predict the flow over a flat plate and its wake. In an effort to address this issue and other issues associated with turbulence closure, a new approach for turbulence modeling is proposed which exploits similarities in the flow field. Thus, if we consider the flow over a flat plate and its wake, then in addition to taking advantage of the log-law region, we can exploit the fact that the flow becomes self-similar in the far wake. This latter behavior makes it possible to cast the governing equations as a set of total differential equations. Solutions of this set and comparison with measured shear stress and velocity profiles yields the desired set of model constants. Such a set is, in general, different from other sets of model constants. The rational for such an approach is that if we can correctly model the flow over a flat plate and its far wake, then we can have a better chance of predicting the behavior in between. It is to be noted that the approach does not appeal, in any way, to the decay of homogeneous turbulence. This is because the asymptotic behavior of the flow under consideration is not representative of the decay of homogeneous turbulence.
Balaguru, Uma Maheswari; Sundaresan, Lakshmikirupa; Manivannan, Jeganathan; Majunathan, Reji; Mani, Krishnapriya; Swaminathan, Akila; Venkatesan, Saravanakumar; Kasiviswanathan, Dharanibalan; Chatterjee, Suvro
2016-06-01
Disturbed fluid flow or modulated shear stress is associated with vascular conditions such as atherosclerosis, thrombosis, and aneurysm. In vitro simulation of the fluid flow around the plaque micro-environment remains a challenging approach. Currently available models have limitations such as complications in protocols, high cost, incompetence of co-culture and not being suitable for massive expression studies. Hence, the present study aimed to develop a simple, versatile model based on Computational Fluid Dynamics (CFD) simulation. Current observations of CFD have shown the regions of modulated shear stress by the disturbed fluid flow. To execute and validate the model in real sense, cell morphology, cytoskeletal arrangement, cell death, reactive oxygen species (ROS) profile, nitric oxide production and disturbed flow markers under the above condition were assessed. Endothelium at disturbed flow region which had been exposed to low shear stress and swirling flow pattern showed morphological and expression similarities with the pathological disturbed flow environment reported previously. Altogether, the proposed model can serve as a platform to simulate the real time micro-environment of disturbed flow associated with eccentric plaque shapes and the possibilities of studying its downstream events.
Sheared Electroconvective Instability
Kwak, Rhokyun; Pham, Van Sang; Lim, Kiang Meng; Han, Jongyoon
2012-11-01
Recently, ion concentration polarization (ICP) and related phenomena draw attention from physicists, due to its importance in understanding electrochemical systems. Researchers have been actively studying, but the complexity of this multiscale, multiphysics phenomenon has been limitation for gaining a detailed picture. Here, we consider electroconvective(EC) instability initiated by ICP under pressure-driven flow, a scenario often found in electrochemical desalinations. Combining scaling analysis, experiment, and numerical modeling, we reveal unique behaviors of sheared EC: unidirectional vortex structures, its size selection and vortex propagation. Selected by balancing the external pressure gradient and the electric body force, which generates Hagen-Poiseuille(HP) flow and vortical EC, the dimensionless EC thickness scales as (φ2 /UHP)1/3. The pressure-driven flow(or shear) suppresses unfavorably-directed vortices, and simultaneously pushes favorably-directed vortices with constant speed, which is linearly proportional to the total shear of HP flow. This is the first systematic characterization of sheared EC, which has significant implications on the optimization of electrodialysis and other electrochemical systems.
Experiments in a flighted conveyor comparing shear rates in compressed versus free surface flows
Pohlman, Nicholas; Higgins, Hannah; Krupiarz, Kamila; O'Connor, Ryan
2017-11-01
Uniformity of granular flow rate is critical in industry. Experiments in a flighted conveyor system aim to fill a gap in knowledge of achieving steady mass flow rate by correlating velocity profile data with mass flow rate measurements. High speed images were collected for uniformly-shaped particles in a bottom-driven flow conveyor belt system from which the velocity profiles can be generated. The correlation of mass flow rates from the velocity profiles to the time-dependent mass measurements will determine energy dissipation rates as a function of operating conditions. The velocity profiles as a function of the size of the particles, speed of the belt, and outlet size, will be compared to shear rate relationships found in past experiments that focused on gravity-driven systems. The dimension of the linear shear and type of decaying transition to the stationary bed may appear different due to the compression versus dilation space in open flows. The application of this research can serve to validate simulations in discrete element modeling and physically demonstrate a process that can be further developed and customized for industry applications, such as feeding a biomass conversion reactor. Sponsored by NIU's Office of Student Engagement and Experiential Learning.
International Nuclear Information System (INIS)
Irace, C.; Tamburini, S.; Bertucci, B.; Franceschi, M.S. de; Gnasso, A.
2006-01-01
The aim of our study was to evaluate the effect of the intravenous contrast media iomeprol on wall shear stress, blood flow and vascular parameters in the common carotid and brachial artery. Thirty outpatients undergoing thoracic or abdominal spiral CT scans were studied. The internal diameter and flow velocity of the common carotid and brachial artery were evaluated by ultrasound, and blood viscosity was measured before and after low osmolality iomeprol (Iomeron 350) injection. The wall shear stress, blood flow and pulsatility index were calculated. To test the differences between groups, the Wilcoxon rank test and Mann Whitney U test were applied. Blood viscosity decreased slightly, but significantly after contrast media (4.6±0.7 vs. 4.5±0.7 mPa.s, P=0.02). Contrarily, blood flow and wall shear stress did not change in the common carotid artery, but significantly decreased in the brachial artery (0.9±0.4 vs. 0.6±0.3 ml/s, P<0.0001, and 41.5±13.9 vs. 35.3±11.0 dynes/cm2, P<0.002, respectively), whereas the pulsatility index significantly increased in the brachial artery (5.0±3.3 vs. 7.5±5.3, P<0.001). Iomeprol injection causes blood flow and wall shear stress reduction of the brachial artery; the rise in the pulsatility index suggests an increase in peripheral vascular resistance. Further investigation is needed to evaluate whether these modifications can be clinically relevant. (orig.)
Shear localization and effective wall friction in a wall bounded granular flow
Artoni, Riccardo; Richard, Patrick
2017-06-01
In this work, granular flow rheology is investigated by means of discrete numerical simulations of a torsional, cylindrical shear cell. Firstly, we focus on azimuthal velocity profiles and study the effect of (i) the confining pressure, (ii) the particle-wall friction coefficient, (iii) the rotating velocity of the bottom wall and (iv) the cell diameter. For small cell diameters, azimuthal velocity profiles are nearly auto-similar, i.e. they are almost linear with the radial coordinate. Different strain localization regimes are observed : shear can be localized at the bottom, at the top of the shear cell, or it can be even quite distributed. This behavior originates from the competition between dissipation at the sidewalls and dissipation in the bulk of the system. Then we study the effective friction at the cylindrical wall, and point out the strong link between wall friction, slip and fluctuations of forces and velocities. Even if the system is globally below the sliding threshold, force fluctuations trigger slip events, leading to a nonzero wall slip velocity and an effective wall friction coefficient different from the particle-wall one. A scaling law was found linking slip velocity, granular temperature in the main flow direction and effective friction. Our results suggest that fluctuations are an important ingredient for theories aiming to capture the interface rheology of granular materials.
International Nuclear Information System (INIS)
Mirza, Arshad M.; Masood, W.
2011-01-01
Nonlinear equations governing the dynamics of finite amplitude drift-ion acoustic-waves are derived by taking into account sheared ion flows parallel and perpendicular to the ambient magnetic field in a quantum magnetoplasma comprised of electrons and ions. It is shown that stationary solution of the nonlinear equations can be represented in the form of a tripolar vortex for specific profiles of the equilibrium sheared flows. The tripolar vortices are, however, observed to form on very short scales in dense quantum plasmas. The relevance of the present investigation with regard to dense astrophysical environments is also pointed out.
Mirza, Arshad M.; Masood, W.
2011-12-01
Nonlinear equations governing the dynamics of finite amplitude drift-ion acoustic-waves are derived by taking into account sheared ion flows parallel and perpendicular to the ambient magnetic field in a quantum magnetoplasma comprised of electrons and ions. It is shown that stationary solution of the nonlinear equations can be represented in the form of a tripolar vortex for specific profiles of the equilibrium sheared flows. The tripolar vortices are, however, observed to form on very short scales in dense quantum plasmas. The relevance of the present investigation with regard to dense astrophysical environments is also pointed out.
Energy Technology Data Exchange (ETDEWEB)
Mirza, Arshad M. [Theoretical Plasma Physics Group, Department of Physics, Quaid-i-Azam University, Islamabad 45320 (Pakistan); Masood, W. [TPPD, PINSTECH, P.O. Nilore, Islamabad (Pakistan) and National Centre for Physics (NCP), Shahdara Valley Road, 44000 Islamabad (Pakistan)
2011-12-15
Nonlinear equations governing the dynamics of finite amplitude drift-ion acoustic-waves are derived by taking into account sheared ion flows parallel and perpendicular to the ambient magnetic field in a quantum magnetoplasma comprised of electrons and ions. It is shown that stationary solution of the nonlinear equations can be represented in the form of a tripolar vortex for specific profiles of the equilibrium sheared flows. The tripolar vortices are, however, observed to form on very short scales in dense quantum plasmas. The relevance of the present investigation with regard to dense astrophysical environments is also pointed out.
Large-scale dynamo action due to α fluctuations in a linear shear flow
Sridhar, S.; Singh, Nishant K.
2014-12-01
We present a model of large-scale dynamo action in a shear flow that has stochastic, zero-mean fluctuations of the α parameter. This is based on a minimal extension of the Kraichnan-Moffatt model, to include a background linear shear and Galilean-invariant α-statistics. Using the first-order smoothing approximation we derive a linear integro-differential equation for the large-scale magnetic field, which is non-perturbative in the shearing rate S , and the α-correlation time τα . The white-noise case, τα = 0 , is solved exactly, and it is concluded that the necessary condition for dynamo action is identical to the Kraichnan-Moffatt model without shear; this is because white-noise does not allow for memory effects, whereas shear needs time to act. To explore memory effects we reduce the integro-differential equation to a partial differential equation, valid for slowly varying fields when τα is small but non-zero. Seeking exponential modal solutions, we solve the modal dispersion relation and obtain an explicit expression for the growth rate as a function of the six independent parameters of the problem. A non-zero τα gives rise to new physical scales, and dynamo action is completely different from the white-noise case; e.g. even weak α fluctuations can give rise to a dynamo. We argue that, at any wavenumber, both Moffatt drift and Shear always contribute to increasing the growth rate. Two examples are presented: (a) a Moffatt drift dynamo in the absence of shear and (b) a Shear dynamo in the absence of Moffatt drift.
Interfacial shear modeling in two-phase annular flow
International Nuclear Information System (INIS)
Kumar, R.; Edwards, D.P.
1996-11-01
A new interfacial shear stress model called the law of the interface model, based on the law of the wall approach in turbulent flows, has been developed and locally applied in a fully developed, adiabatic, two-phase annular flow in a duct. Numerical results have been obtained using this model in conjunction with other models available in the literature that are required for the closure of the continuity and momentum equations. These results have been compared with droplet velocity data (using laser Doppler velocimetry and hot film anemometry), void fraction data (using gamma densitometry) and pressure drop data obtained in a R-134A refrigerant test facility. Droplet velocity results match the experimental data well, however, the prediction of the void fraction is less accurate. The poor prediction of void fraction, especially for the low void fraction cases, appears to be due to the lack of a good mechanistic model for entrainment
Interfacial shear modeling in two-phase annular flow
International Nuclear Information System (INIS)
Kumar, R.; Edwards, D.P.
1996-07-01
A new interfacial shear stress model called the law of the interface model, based on the law of the wall approach in turbulent flows, has been developed and locally applied in a fully developed, adiabatic, two-phase annular flow in a duct. Numerical results have been obtained using this model in conjunction with other models available in the literature that are required for the closure of the continuity and momentum equations. These results have been compared with droplet velocity data (using laser Doppler velocimetry and hot film anemometry), void fraction data (using gamma densitometry) and pressure drop data obtained in a R-134A refrigerant test facility. Droplet velocity results match the experimental data well, however, the prediction of the void fraction is less accurate. The poor prediction of void fraction, especially for the low void fraction cases, appears to be due to the lack of a good mechanistic model for entrainment
A 3D spectral anelastic hydrodynamic code for shearing, stratified flows
Barranco, Joseph A.; Marcus, Philip S.
2006-11-01
We have developed a three-dimensional (3D) spectral hydrodynamic code to study vortex dynamics in rotating, shearing, stratified systems (e.g., the atmosphere of gas giant planets, protoplanetary disks around newly forming protostars). The time-independent background state is stably stratified in the vertical direction and has a unidirectional linear shear flow aligned with one horizontal axis. Superposed on this background state is an unsteady, subsonic flow that is evolved with the Euler equations subject to the anelastic approximation to filter acoustic phenomena. A Fourier Fourier basis in a set of quasi-Lagrangian coordinates that advect with the background shear is used for spectral expansions in the two horizontal directions. For the vertical direction, two different sets of basis functions have been implemented: (1) Chebyshev polynomials on a truncated, finite domain, and (2) rational Chebyshev functions on an infinite domain. Use of this latter set is equivalent to transforming the infinite domain to a finite one with a cotangent mapping, and using cosine and sine expansions in the mapped coordinate. The nonlinear advection terms are time-integrated explicitly, the pressure/enthalpy terms are integrated semi-implicitly, and the Coriolis force and buoyancy terms are treated semi-analytically. We show that internal gravity waves can be damped by adding new terms to the Euler equations. The code exhibits excellent parallel performance with the message passing interface (MPI). As a demonstration of the code, we simulate the merger of two 3D vortices in the midplane of a protoplanetary disk.
Mulchrone, Kieran F.; Meere, Patrick A.
2015-09-01
Shape fabrics of elliptical objects in rocks are usually assumed to develop by passive behavior of inclusions with respect to the surrounding material leading to shape-based strain analysis methods belonging to the Rf/ϕ family. A probability density function is derived for the orientational characteristics of populations of rigid ellipses deforming in a pure shear 2D deformation with both no-slip and slip boundary conditions. Using maximum likelihood a numerical method is developed for estimating finite strain in natural populations deforming for both mechanisms. Application to a natural example indicates the importance of the slip mechanism in explaining clast shape fabrics in deformed sediments.
Flow through internal elastic lamina affects shear stress on smooth muscle cells (3D simulations).
Tada, Shigeru; Tarbell, John M
2002-02-01
We describe a three-dimensional numerical simulation of interstitial flow through the medial layer of an artery accounting for the complex entrance condition associated with fenestral pores in the internal elastic lamina (IEL) to investigate the fluid mechanical environment around the smooth muscle cells (SMCs) right beneath the IEL. The IEL was modeled as an impermeable barrier to water flow except for the fenestral pores, which were assumed to be uniformly distributed over the IEL. The medial layer was modeled as a heterogeneous medium composed of a periodic array of cylindrical SMCs embedded in a continuous porous medium representing the interstitial proteoglycan and collagen matrix. Depending on the distance between the IEL bottom surface and the upstream end of the proximal layer of SMCs, the local shear stress on SMCs right beneath the fenestral pore could be more than 10 times higher than that on the cells far removed from the IEL under the conditions that the fenestral pore diameter and area fraction of pores were kept constant at 1.4 microm and 0.05, respectively. Thus these proximal SMCs may experience shear stress levels that are even higher than endothelial cells exposed to normal blood flow (order of 10 dyn/cm(2)). Furthermore, entrance flow through fenestral pores alters considerably the interstitial flow field in the medial layer over a spatial length scale of the order of the fenestral pore diameter. Thus the spatial gradient of shear stress on the most superficial SMC is noticeably higher than computed for endothelial cell surfaces.
Energy Technology Data Exchange (ETDEWEB)
Worth Longest, P. [Virginia Commonwealth University, Richmond, VA (United States). Dept. of Mechanical Engineering; Kleinstreuer, C. [North Carolina State University, Raleigh, NC (United States). Dept. of Mechanical and Aerospace Engineering
2004-10-01
Individual and interacting effects of uniform flow, plane shear, and near-wall proximity on spherical droplet heat and mass transfer have been assessed for low Reynolds number conditions beyond the creeping flow regime. Validated resolved volume simulations were used to compute heat and mass transfer surface gradients of two-dimensional axisymmetric droplets and three-dimensional spherical droplets near planar wall boundaries for conditions consistent with inhalable aerosols (5 {<=} d {<=} 300 {mu}m) in the upper respiratory tract. Results indicate that planar shear significantly impacts droplet heat and mass transfer for shear-based Reynolds numbers greater than 1, which occur for near-wall respiratory aerosols with diameters in excess of 50 {mu}m. Wall proximity is shown to significantly enhance heat and mass transfer due to conduction and diffusion at separation distances less than five particle diameters and for small Reynolds numbers. For the Reynolds number conditions of interest, significant non-linear effects arise due to the concurrent interaction of uniform flow and shear such that linear superposition of Sherwood or Nusselt number terms is not allowable. Based on the validated numeric simulations, multivariable Sherwood and Nusselt number correlations are provided to account for individual flow characteristics and concurrent non-linear interactions of uniform flow, planar shear, and near-wall proximity. These heat and mass transfer correlations can be applied to effectively compute condensation and evaporation rates of potentially toxic or therapeutic aerosols in the upper respiratory tract, where non-uniform flow and wall proximity are expected to significantly affect droplet transport, deposition, and vapor formation. (author)
Masood, W.; Mirza, Arshad M.
2014-04-01
A set of nonlinear equations governing the dynamics of finite amplitude drift-ion acoustic-waves is derived for sheared ion flows parallel and perpendicular to the ambient magnetic field in the presence of Cairns and Kappa distributed electrons. It is shown that stationary solution of the nonlinear equations can be represented in the form of a tripolar vortex for specific profiles of the equilibrium sheared flows. The tripolar vortices are, however, observed to form on a scale of the order of ion Larmor radius ρ i which is calculated to be around a Kilometer for the plasma parameters found in the Saturn's E-ring. The relevance of the present investigation in planetary environments is also pointed out.
Shear Flow Instabilities and Droplet Size Effects on Aerosol Jet Printing Resolution
Chen, Guang; Gu, Yuan; Hines, Daniel; Das, Siddhartha; LaboratoryPhysical Science Collaboration; Soft Matter, Interfaces, Energy Laboratory Collaboration
2017-11-01
Aerosol Jet printing (AJP) is an additive technology utilizing aerodynamic focusing to produce fine feature down to 10 micrometers that can be used in the manufacture of wearable electronics and biosensors. The main concern of the current technology is related to unstable printing resolution, which is usually assessed by effective line width, edge smoothness, overspray and connectivity. In this work, we perform a 3D CFD model to study the aerodynamic instabilities induced by the annular shear flow (sheath gas flow or ShGF) trapped with the aerosol jet (carried gas flow or CGF) with ink droplets. Extensive experiments on line morphology have shown that by increasing ShGF, one can first obtain thinner line width, and then massive overspray is witnessed at very large ShGF/ CGF ratio. Besides the fact that shear-layer instabilities usually trigger eddy currents at comparatively low Reynolds number 600, the tolerance of deposition components assembling will also propagate large offsets of the deposited feather. We also carried out detailed analysis on droplet size and deposition range on the printing resolution. This study is intended to come up with a solution on controlling the operating parameters for finer printed features, and offer an improvement strategy on next generation.
Numerical simulation of stratified shear flow using a higher order Taylor series expansion method
Energy Technology Data Exchange (ETDEWEB)
Iwashige, Kengo; Ikeda, Takashi [Hitachi, Ltd. (Japan)
1995-09-01
A higher order Taylor series expansion method is applied to two-dimensional numerical simulation of stratified shear flow. In the present study, central difference scheme-like method is adopted for an even expansion order, and upwind difference scheme-like method is adopted for an odd order, and the expansion order is variable. To evaluate the effects of expansion order upon the numerical results, a stratified shear flow test in a rectangular channel (Reynolds number = 1.7x10{sup 4}) is carried out, and the numerical velocity and temperature fields are compared with experimental results measured by laser Doppler velocimetry thermocouples. The results confirm that the higher and odd order methods can simulate mean velocity distributions, root-mean-square velocity fluctuations, Reynolds stress, temperature distributions, and root-mean-square temperature fluctuations.
Directory of Open Access Journals (Sweden)
Nemati Hasan
2011-01-01
Full Text Available A numerical investigation of the two-dimensional laminar flow and heat transfer a rotating circular cylinder with uniform planar shear, where the free-stream velocity varies linearly across the cylinder using Multi-Relaxation-Time Lattice Boltzmann method is conducted. The effects of variation of Reynolds number, rotational speed ratio at shear rate 0.1, blockage ratio 0.1 and Prandtl number 0.71 are studied. The Reynolds number changing from 50 to 160 for three rotational speed ratios of 0, 0.5, 1 is investigated. Results show that flow and heat transfer depends significantly on the rotational speed ratio as well as the Reynolds number. The effect of Reynolds number on the vortex-shedding frequency and period-surface Nusselt numbers is overall very strong compared with rotational speed ratio. Flow and heat conditions characteristics such as lift and drag coefficients, Strouhal number and Nusselt numbers are studied.
Hoda, Nazish; Kumar, Satish
2007-12-01
The adsorption of single polyelectrolyte molecules in shear flow is studied using Brownian dynamics simulations with hydrodynamic interaction (HI). Simulations are performed with bead-rod and bead-spring chains, and electrostatic interactions are incorporated through a screened Coulombic potential with excluded volume accounted for by the repulsive part of a Lennard-Jones potential. A correction to the Rotne-Prager-Yamakawa tensor is derived that accounts for the presence of a planar wall. The simulations show that migration away from an uncharged wall, which is due to bead-wall HI, is enhanced by increases in the strength of flow and intrachain electrostatic repulsion, consistent with kinetic theory predictions. When the wall and polyelectrolyte are oppositely charged, chain behavior depends on the strength of electrostatic screening. For strong screening, chains get depleted from a region close to the wall and the thickness of this depletion layer scales as N1/3Wi2/3 at high Wi, where N is the chain length and Wi is the Weissenberg number. At intermediate screening, bead-wall electrostatic attraction competes with bead-wall HI, and it is found that there is a critical Weissenberg number for desorption which scales as N-1/2κ-3(lB∣σq∣)3/2, where κ is the inverse screening length, lB is the Bjerrum length, σ is the surface charge density, and q is the bead charge. When the screening is weak, adsorbed chains are observed to align in the vorticity direction at low shear rates due to the effects of repulsive intramolecular interactions. At higher shear rates, the chains align in the flow direction. The simulation method and results of this work are expected to be useful for a number of applications in biophysics and materials science in which polyelectrolyte adsorption plays a key role.
A new energy transfer model for turbulent free shear flow
Liou, William W.-W.
1992-01-01
A new model for the energy transfer mechanism in the large-scale turbulent kinetic energy equation is proposed. An estimate of the characteristic length scale of the energy containing large structures is obtained from the wavelength associated with the structures predicted by a weakly nonlinear analysis for turbulent free shear flows. With the inclusion of the proposed energy transfer model, the weakly nonlinear wave models for the turbulent large-scale structures are self-contained and are likely to be independent flow geometries. The model is tested against a plane mixing layer. Reasonably good agreement is achieved. Finally, it is shown by using the Liapunov function method, the balance between the production and the drainage of the kinetic energy of the turbulent large-scale structures is asymptotically stable as their amplitude saturates. The saturation of the wave amplitude provides an alternative indicator for flow self-similarity.
Biomechanics of P-selectin PSGL-1 bonds: Shear threshold and integrin-independent cell adhesion
Energy Technology Data Exchange (ETDEWEB)
Xiao, Zhihua; Goldsmith, Harry L.; MacIntosh, Fiona A.; Shankaran, Harish; Neelamegham, Sriram
2006-03-01
Platelet-leukocyte adhesion may contribute to thrombosis and inflammation. We examined the heterotypic interaction between unactivated neutrophils and either thrombin receptor activating peptide (TRAP) stimulated platelets or P-selectin bearing beads (Ps-beads) in suspension. Cone-plate viscometers were used to apply controlled shear rates from 14-3000/s. Platelet-neutrophil and bead-neutrophil adhesion analysis was performed using both flow cytometry and high-speed videomicroscopy. We observed that while blocking antibodies against either P-selectin or P-selectin glycoprotein ligand-1 (PSGL-1) alone inhibited platelet-neutrophil adhesion by ~60% at 140/s, these reagents completely blocked adhesion at 3000/s. Anti-Mac-1 alone did not alter platelet-neutrophil adhesion rates at any shear rate, though in synergy with selectin antagonists it abrogated cell binding. Unstimulated neutrophils avidly bound Ps-beads and activated platelets in an integrin-independent manner, suggesting that purely selectin-dependent cell adhesion is possible. In support of this, antagonists against P-selectin or PSGL-1 dissociated previously formed platelet-neutrophil and Ps-bead neutrophil aggregates under shear in a variety of experimental systems, including in assays performed with whole blood. In studies where medium viscosity and shear rate were varied, a subtle shear threshold for P-selectin PSGL-1 binding was also noted at shear rates<100/s and at force loading rates of ~300pN/sec. Results are discussed in light of biophysical computations that characterize the collision between unequal size particles in linear shear flow. Overall, our studies reveal an integrin-independent regime for cell adhesion that may be physiologically relevant.
Nishida, Masahiro; Yamane, Takashi; Tsukamoto, Yuki; Ito, Kazuyuki; Konishi, Yoshiaki; Masuzawa, Toru; Tsukiya, Tomonori; Endo, Seiko; Taenaka, Yoshiyuki
To clarify the correlation between high-shear flow and hemolysis in blood pumps, detail shear velocity distribution was quantified by an experimental method with a model centrifugal blood pump that has a series data of hemolysis tests and computational fluid dynamic analyses. Particular attention was paid to the shear velocity near the casing surface in the volute where the high shear causes in circumferentially wide region that is considerable to cause high hemolysis. Three pump models were compared concern with the radial gap width between the impeller and casing (the radial volute width) also with the outlet position whereas the impeller geometry was identical. These casing geometries were as follows: model 1-the gap width is standard 3mm and the outlet locates to make a smooth geometrical connection with the volute, model 2-the gap width is small 0.5mm and the outlet locates to make the smooth geometrical connection with the volute, and model 3-the gap width is small 0.5mm and the outlet locates to hardly make the smooth geometrical connection with the volute but be similar radial position with that of model 1. Velocity was quantified with a particle tracking velocimetry that is one of the quantitative flow visualization techniques, and the shear velocity was calculated. Results showed that all large shear velocity existed within the layers of about 0.1mm from the casing surface and that those layers were hardly affected by a vane passage even if the gap width is 0.5mm. They also showed that the maximum shear velocity appeared on the casing surface, and the shear velocities of models 2 and 3 were almost twice as large as that of model 1. This finding is in full corresponding with the results of hemolysis tests which showed that the hemolysis levels of both models 2 and 3 were 1.5 times higher than that of model 1. These results suggest that detailed high-shear evaluation near the casing surface in the volute is one of the most important keys in estimating the
Localized modelling and feedback control of linear instabilities in 2-D wall bounded shear flows
Tol, Henry; Kotsonis, Marios; de Visser, Coen
2016-11-01
A new approach is presented for control of instabilities in 2-D wall bounded shear flows described by the linearized Navier-Stokes equations (LNSE). The control design accounts both for spatially localized actuators/sensors and the dominant perturbation dynamics in an optimal control framework. An inflow disturbance model is proposed for streamwise instabilities that drive laminar-turbulent transition. The perturbation modes that contribute to the transition process can be selected and are included in the control design. A reduced order model is derived from the LNSE that captures the input-output behavior and the dominant perturbation dynamics. This model is used to design an optimal controller for suppressing the instability growth. A 2-D channel flow and a 2-D boundary layer flow over a flat plate are considered as application cases. Disturbances are generated upstream of the control domain and the resulting flow perturbations are estimated/controlled using wall shear measurements and localized unsteady blowing and suction at the wall. It will be shown that the controller is able to cancel the perturbations and is robust to unmodelled disturbances.
Kefayati, Sarah; Poepping, Tamie L
2010-01-01
The carotid artery bifurcation is a common site of atherosclerosis which is a major leading cause of ischemic stroke. The impact of stenosis in the atherosclerotic carotid artery is to disturb the flow pattern and produce regions with high shear rate, turbulence, and recirculation, which are key hemodynamic factors associated with plaque rupture, clot formation, and embolism. In order to characterize the disturbed flow in the stenosed carotid artery, stereoscopic PIV measurements were performed in a transparent model with 50% stenosis under pulsatile flow conditions. Simulated ECG gating of the flowrate waveform provides external triggering required for volumetric reconstruction of the complex flow patterns. Based on the three-component velocity data in the lumen region, volumetric shear-stress patterns were derived.
Non-Newtonian stress tensor and thermal conductivity tensor in granular plane shear flow
Alam, Meheboob; Saha, Saikat
2014-11-01
The non-Newtonian stress tensor and the heat flux in the plane shear flow of smooth inelastic disks are analysed from the Grad-level moment equations using the anisotropic Gaussian as a reference. Closed-form expressions for shear viscosity, pressure, first normal stress difference (N1) and the dissipation rate are given as functions of (i) the density or the area fraction (ν), (ii) the restitution coefficient (e), (iii) the dimensionless shear rate (R), (iv) the temperature anisotropy [ η, the difference between the principal eigenvalues of the second moment tensor] and (v) the angle (ϕ) between the principal directions of the shear tensor and the second moment tensor. Particle simulation data for a sheared hard-disk system is compared with theoretical results, with good agreement for p, μ and N1 over a large range of density. In contrast, the predictions from a Navier-Stokes order constitutive model are found to deviate significantly from both the simulation and the moment theory even at moderate values of e. We show that the gradient of the deviatoric part of the kinetic stress drives a heat current and the thermal conductivity is characterized by an anisotropic 2nd rank tensor for which explicit expressions are derived.
Adiabatic shear behaviors in rolled and annealed pure titanium subjected to dynamic impact loading
Energy Technology Data Exchange (ETDEWEB)
Kuang, Lianjun; Chen, Zhiyong, E-mail: czysh@netease.com; Jiang, Yanghui; Wang, Zhiming; Wang, Renke; Liu, Chuming
2017-02-08
The hat-shaped samples cut from rolled and annealed titanium plates were prepared to explore the adiabatic shear behaviors subjected to high-strain-rate deformation operated via Split Hopkinson Pressure Bar. The dynamic shear response calculation reveals that dynamic deformation processes of both state samples can be divided in similar three stages but rolled sample shows a higher susceptibility of adiabatic shear localization compared with the annealed one. Optical microscopy and electronic backscatter diffraction technique (EBSD) were used to systematically analyze the microstructure and texture characteristics. The results show that adiabatic shear bands form in both state samples and rotational dynamic recrystallization (RDRX) occurs within shear area and results in the formation of ultrafine equiaxed grains. Furthermore, ultrafine equiaxed grains within adiabatic shear bands have the same texture feature that <11–20> direction and {10-10} plane parallel to macro local shear direction and shear plane respectively. In the deformation region around the shear band, {10–12} <–1011> tensile and {11–22} <11-2-3> compressive two types twins are observed in both state samples and {10–12} <–1011> tensile twins are more frequently observed in rolled sample. In the rolled sample, {10–12} <–1011> tensile twins are more likely to happen in the hat-brim side than the hat-body side due to the difference of stress state in two sides.
Laminar flow of a shear-thickening fluid in a 90∘ pipe bend
Marn, Jure; Ternik, Primož
2006-05-01
The non-Newtonian fluid flow in a sharp 90∘ curved pipe is studied numerically to obtain the pressure loss coefficient prompted by disagreement between the existing empirical correlations and results obtained by computer codes. This disagreement results from presumption of fully developed flow throughout the curvature (correlations) while the actual flow is partially developed for the Newtonian and sharp 90∘ curved bend non-Newtonian flows, and fully developed for slightly bent 90∘ curvature non-Newtonian flow. The Quadratic model is employed to accommodate the shear-thickening behavior of an electrostatic ash and water mixture. Numerical results are obtained for different values of Reynolds number. Finally, results for local pressure loss coefficient are compared with values obtained for the Power law rheological model.
Granular shear flows of flexible rod-like particles
Guo, Y.; Curtis, J.; Wassgren, C.; Ketterhagen, W.; Hancock, B.
2013-06-01
Flexible particles are widely encountered in nature, e.g., stalks of plants, fiberglass particles, and ceramic nanofibers. Early studies indicated that the deformability of particles has a significant impact on the properties of granular materials and fiber suspensions. In this study, shear flows of flexible particles are simulated using the Discrete Element Method (DEM) to explore the effect of particle flexibility on the flow behavior and constitutive laws. A flexible particle is formed by connecting a number of constituent spheres in a straight line using elastic bonds. The forces/moments due to the normal, tangential, bending, and torsional deformation of a bond resist the relative movement between two bonded constituent spheres. The bond stiffness determines how difficult it is to make a particle deform, and the bond damping accounts for the energy dissipation in the particle vibration process. The simulation results show that elastically bonded particles have smaller coefficients of restitution compared to rigidly connected particles, due to the fact that kinetic energy is partially converted to potential energy in a contact between flexible particles. The coefficient of restitution decreases as the bond stiffness decreases and the bond damping coefficient increases. As a result, smaller stresses are obtained for granular flows of the flexible particles with smaller bond stiffness and larger bond damping coefficient.
Stability of unstably stratified shear flow between parallel plates
Energy Technology Data Exchange (ETDEWEB)
Fujimura, Kaoru; Kelly, R E
1987-09-01
The linear stability of unstably stratified shear flows between two horizontal parallel plates was investigated. Eigenvalue problems were solved numerically by making use of the expansion method in Chebyshev polynomials, and the critical Rayleigh numbers were obtained accurately in the Reynolds number range of (0.01, 100). It was found that the critical Rayleigh number increases with an increase of the Reynolds number. The result strongly supports previous stability analyses except for the analysis by Makino and Ishikawa (J. Jpn. Soc. Fluid Mech. 4 (1985) 148 - 158) in which a decrease of the critical Rayleigh number was obtained.
Stability of unstably stratified shear flow between parallel plates
International Nuclear Information System (INIS)
Fujimura, Kaoru; Kelly, R.E.
1987-01-01
The linear stability of unstably stratified shear flows between two horizontal parallel plates was investigated. Eigenvalue problems were solved numerically by making use of the expansion method in Chebyshev polynomials, and the critical Rayleigh numbers were obtained accurately in the Reynolds number range of [0.01, 100]. It was found that the critical Rayleigh number increases with an increase of the Reynolds number. The result strongly supports previous stability analyses except for the analysis by Makino and Ishikawa [J. Jpn. Soc. Fluid Mech. 4 (1985) 148 - 158] in which a decrease of the critical Rayleigh number was obtained. (author)
A Piezoelectric Shear Stress Sensor
Kim, Taeyang; Saini, Aditya; Kim, Jinwook; Gopalarathnam, Ashok; Zhu, Yong; Palmieri, Frank L.; Wohl, Christopher J.; Jiang, Xiaoning
2016-01-01
In this paper, a piezoelectric sensor with a floating element was developed for shear stress measurement. The piezoelectric sensor was designed to detect the pure shear stress suppressing effects of normal stress generated from the vortex lift-up by applying opposite poling vectors to the: piezoelectric elements. The sensor was first calibrated in the lab by applying shear forces and it showed high sensitivity to shear stress (=91.3 +/- 2.1 pC/Pa) due to the high piezoelectric coefficients of PMN-33%PT (d31=-1330 pC/N). The sensor also showed almost no sensitivity to normal stress (less than 1.2 pC/Pa) because of the electromechanical symmetry of the device. The usable frequency range of the sensor is 0-800 Hz. Keywords: Piezoelectric sensor, shear stress, floating element, electromechanical symmetry
Direct numerical simulation of solidification microstructures affected by fluid flow
International Nuclear Information System (INIS)
Juric, D.
1997-12-01
The effects of fluid flow on the solidification morphology of pure materials and solute microsegregation patterns of binary alloys are studied using a computational methodology based on a front tracking/finite difference method. A general single field formulation is presented for the full coupling of phase change, fluid flow, heat and solute transport. This formulation accounts for interfacial rejection/absorption of latent heat and solute, interfacial anisotropies, discontinuities in material properties between the liquid and solid phases, shrinkage/expansion upon solidification and motion and deformation of the solid. Numerical results are presented for the two dimensional dendritic solidification of pure succinonitrile and the solidification of globulitic grains of a plutonium-gallium alloy. For both problems, comparisons are made between solidification without fluid flow and solidification within a shear flow
Padé approximant for normal stress differences in large-amplitude oscillatory shear flow
Poungthong, P.; Saengow, C.; Giacomin, A. J.; Kolitawong, C.; Merger, D.; Wilhelm, M.
2018-04-01
Analytical solutions for the normal stress differences in large-amplitude oscillatory shear flow (LAOS), for continuum or molecular models, normally take the inexact form of the first few terms of a series expansion in the shear rate amplitude. Here, we improve the accuracy of these truncated expansions by replacing them with rational functions called Padé approximants. The recent advent of exact solutions in LAOS presents an opportunity to identify accurate and useful Padé approximants. For this identification, we replace the truncated expansion for the corotational Jeffreys fluid with its Padé approximants for the normal stress differences. We uncover the most accurate and useful approximant, the [3,4] approximant, and then test its accuracy against the exact solution [C. Saengow and A. J. Giacomin, "Normal stress differences from Oldroyd 8-constant framework: Exact analytical solution for large-amplitude oscillatory shear flow," Phys. Fluids 29, 121601 (2017)]. We use Ewoldt grids to show the stunning accuracy of our [3,4] approximant in LAOS. We quantify this accuracy with an objective function and then map it onto the Pipkin space. Our two applications illustrate how to use our new approximant reliably. For this, we use the Spriggs relations to generalize our best approximant to multimode, and then, we compare with measurements on molten high-density polyethylene and on dissolved polyisobutylene in isobutylene oligomer.
Comparison of erythrocyte dynamics in shear flow under different stress-free configurations
Cordasco, Daniel; Yazdani, Alireza; Bagchi, Prosenjit
2014-04-01
An open question that has persisted for decades is whether the cytoskeleton of a red blood cell is stress-free or under a stress. This question is important in the context of theoretical modeling of cellular motion under a flowing condition where it is necessary to make an assumption about the stress-free state. Here, we present a 3D numerical study to compare the cell dynamics in a simple shear flow under two different stress-free states, a biconcave discocyte representing the resting shape of the cell, and a nearly spherical oblate shape. We find that whether the stress-free states make a significant difference or not depends on the viscosity of the suspending medium. If the viscosity is close to that of blood plasma, the two stress-free states do not show any significant difference in cell dynamics. However, when the suspending medium viscosity is well above that of the physiological range, as in many in vitro studies, the shear rate separating the tank-treading and tumbling dynamics is observed to be higher for the biconcave stress-free state than the spheroidal state. The former shows a strong shape oscillation with repeated departures from the biconcave shape, while the latter shows a nearly stable biconcave shape. It is found that the cell membrane in the biconcave stress-free state is under a compressive stress and a weaker bending force density, leading to a periodic compression of the cell. The shape oscillation then leads to a higher energy barrier against membrane tank-tread leading to an early transition to tumbling. However, if the cells are released with a large off-shear plane angle, the oscillations can be suppressed due to an azimuthal motion of the membrane along the vorticity direction leading to a redistribution of the membrane points and lowering of the energy barrier, which again results in a nearly similar behavior of the cells under the two different stress-free states. A variety of off-shear plane dynamics is observed, namely, rolling
Dilute suspensions in annular shear flow under gravity: simulation and experiment
Directory of Open Access Journals (Sweden)
Schröer Kevin
2017-01-01
Full Text Available A dilute suspension in annular shear flow under gravity was simulated using multi-particle collision dynamics (MPC and compared to experimental data. The focus of the analysis is the local particle velocity and density distribution under the influence of the rotational and gravitational forces. The results are further supported by a deterministic approximation of a single-particle trajectory and OpenFOAM CFD estimations of the overcritical frequency range. Good qualitative agreement is observed for single-particle trajectories between the statistical mean of MPC simulations and the deterministic approximation. Wall contact and detachment however occur earlier in the MPC simulation, which can be explained by the inherent thermal noise of the method. The multi-particle system is investigated at the point of highest particle accumulation that is found at 2/3 of the particle revolution, starting from the top of the annular gap. The combination of shear flow and a slowly rotating volumetric force leads to strong local accumulation in this section that increases the particle volume fraction from overall 0.7% to 4.7% at the outer boundary. MPC simulations and experimental observations agree well in terms of particle distribution and a close to linear velocity profile in radial direction.
Onset of shear thinning in glassy liquids: Shear-induced small reduction of effective density.
Furukawa, Akira
2017-01-01
We propose a simple mechanism for describing the onset of shear thinning in a high-density glassy liquid. In a shear flow, along the compression axis, the overlap between neighboring particles is more enhanced than that at equilibrium, meaning that the "effective" size is reduced along this axis. On the other hand, along the extension axis perpendicular to the compression axis, the average structural configurations are stretched, but it does not indicate the expansion of the "effective" size itself. This asymmetric shear flow effect for particles results in a small reduction of the "effective" density. Because, in glass-forming liquids, the structural relaxation time τ_{α} strongly depends on the density ρ, even a very small reduction of the effective density should lead to a significant decrease of the relaxation time under shear flow. We predict that the crossover shear rate from Newtonian to non-Newtonian flow behaviors is given by γ[over ̇]_{c}=[ρ(∂τ_{α}/∂ρ)]^{-1}, which can be much smaller than 1/τ_{α} near the glass transition point. It is shown that this prediction is consistent with the results of molecular dynamics simulations.
Directory of Open Access Journals (Sweden)
Zheng-Shou Chen
2012-03-01
Full Text Available This article presents a numerical investigation concerning the effect of two kinds of axially progressing internal flows (namely, upward and downward on fluid–structure interaction (FSI dynamics about a marine riser model which is subject to external shear current. The CAE technology behind the current research is a proposed FSI solution, which combines structural analysis software with CFD technology together. Efficiency validation for the CFD software was carried out first. It has been proved that the result from numerical simulations agrees well with the observation from relating model test cases in which the fluidity of internal flow is ignorable. After verifying the numerical code accuracy, simulations are conducted to study the vibration response that attributes to the internal progressive flow. It is found that the existence of internal flow does play an important role in determining the vibration mode (/dominant frequency and the magnitude of instantaneous vibration amplitude. Since asymmetric curvature along the riser span emerges in the case of external shear current, the centrifugal and Coriolis accelerations owing to up- and downward internal progressive flows play different roles in determining the fluid–structure interaction response. The discrepancy between them becomes distinct, when the velocity ratio of internal flow against external shear current is relatively high.
Directory of Open Access Journals (Sweden)
Tianfeng Shen
2016-12-01
Full Text Available The N1,N1ʹ-(ethane-1,2-diylbis(N2-phenyloxalamide (OXA is a soluble-type nucleator with a dissolving temperature of 230 °C in poly(l-lactic acid (PLLA matrix. The effect of thermal history and shear flow on the crystallization behavior of the PLLA/OXA samples was investigated by rheometry, polarized optical microscopy (POM, differential scanning calorimetry (DSC, wide angle X-ray diffraction (WAXD, and scanning electron microscopy (SEM. The crystallization process of the PLLA/OXA-240 sample (i.e., pre-melted at 240 °C was significantly promoted by applying a shear flow, e.g., the onset crystallization time (tonset of the PLLA at 155 °C was reduced from 1600 to 200 s after shearing at 0.4 rad/s for even as short as 1.0 s, while the crystallinity (Xc was increased to 40%. Moreover, the tonset of the PLLA/OXA-240 sample is 60%–80% lower than that of the PLLA/OXA-200 sample (i.e., pre-melted at 200 °C with a total shear angle of 2 rad, indicating a much higher crystallization rate of the PLLA/OXA-240 sample. A better organization and uniformity of OXA fibrils can be obtained due to a complete pre-dissolution in the PLLA matrix followed by shear and oscillation treatments. The well dispersed OXA fibrils and flow-induced chain orientation are mainly responsible for the fast crystallization of the PLLA/OXA-240 samples. In addition, the shear flow created some disordered α′-form crystals in the PLLA/OXA samples regardless of the thermal history (200 or 240 °C.
Hurychová, Hana; Lebedová, Václava; Šklubalová, Zdenka; Dzámová, Pavlína; Svěrák, Tomáš; Stoniš, Jan
Flowability of powder excipients is directly influenced by their size and shape although the granulometric influence of the flow and shear behaviour of particulate matter is not studied frequently. In this work, the influence of particle size on the mass flow rate through the orifice of a conical hopper, and the cohesion and flow function was studied for four free-flowable size fractions of sorbitol for direct compression in the range of 0.080-0.400 mm. The particles were granulometricaly characterized using an optical microscopy; a boundary fractal dimension of 1.066 was estimated for regular sorbitol particles. In the particle size range studied, a non-linear relationship between the mean particle size and the mass flow rate Q10 (g/s) was detected having amaximum at the 0.245mm fraction. The best flow properties of this fraction were verified with aJenike shear tester due to the highest value of flow function and the lowest value of the cohesion. The results of this work show the importance of the right choice of the excipient particle size to achieve the best flow behaviour of particulate material.Key words: flowability size fraction sorbitol for direct compaction Jenike shear tester fractal dimension.
Hu, Wei; Xu, Qiang; Wang, Gonghui; Scaringi, Gianvito; Mcsaveney, Mauri; Hicher, Pierre-Yves
2017-11-01
We present results of ring shear frictional resistance for mudstone granules of different size obtained from a landslide shear zone. Little rate dependency of shear resistance was observed in sand-sized granules in any wet or dry test, while saturated gravel-sized granules exhibited significant and abrupt reversible rate-weakening (from μ = 0.6 to 0.05) at about 2 mm/s. Repeating resistance variations occurred also under constant shear displacement rate. Mudstone granules generate mud as they are crushed and softened. Shear-thinning and thixotropic behavior of the mud can explain the observed behavior: with the viscosity decreasing, the mud can flow through the coarser soil pores and migrate out from the shear zone. This brings new granules into contact which produces new mud. Thus, the process can start over. Similarities between experimental shear zones and those of some landslides in mudstone suggest that the observed behavior may play a role in some landslide kinematics.
Cardinaels, Ruth; Verhulst, Kristof; Renardy, Yuriko; Moldenaers, Paula
2008-07-01
The transient droplet deformation and droplet orientation after inception of shear, the shape relaxation after cessation of shear and droplet breakup during shear, are microscopically studied, both under bulk and confined conditions. The studied blends contain one viscoelastic Boger fluid phase. A counter rotating setup, based on a Paar Physica MCR300, is used for the droplet visualisation. For bulk shear flow, it is shown that the droplet deformation during startup of shear flow and the shape relaxation after cessation of shear flow are hardly influenced by droplet viscoelasticity, even at moderate to high capillary and Deborah numbers. The effects of droplet viscoelasticity only become visible close to the critical conditions and a novel break-up mechanism is observed. Matrix viscoelasticity has a more pronounced effect, causing overshoots in the deformation and significantly inhibiting relaxation. However, different applied capillary numbers prior to cessation of shear flow, with the Deborah number fixed, still result in a single master curve for shape retraction, as in fully Newtonian systems. The long tail in the droplet relaxation can be qualitatively described with a phenomenological model for droplet deformation, when using a 5-mode Giesekus model for the fluid rheology. It is found that the shear flow history significantly affects the droplet shape evolution and the breakup process in blends with one viscoelastic component. Confining a droplet between two plates accelerates the droplet deformation kinetics, similar to fully Newtonian systems. However, the increased droplet deformation, due to wall effects, causes the steady state to be reached at a later instant in time. Droplet relaxation is less sensitive to confinement, leading to slower relaxation kinetics only for highly confined droplets. For the blend with a viscoelastic droplet, a non-monotonous trend is found for the critical capillary number as a function of the confinement ratio. Finally
International Nuclear Information System (INIS)
Cardinaels, Ruth; Verhulst, Kristof; Moldenaers, Paula; Renardy, Yuriko
2008-01-01
The transient droplet deformation and droplet orientation after inception of shear, the shape relaxation after cessation of shear and droplet breakup during shear, are microscopically studied, both under bulk and confined conditions. The studied blends contain one viscoelastic Boger fluid phase. A counter rotating setup, based on a Paar Physica MCR300, is used for the droplet visualisation. For bulk shear flow, it is shown that the droplet deformation during startup of shear flow and the shape relaxation after cessation of shear flow are hardly influenced by droplet viscoelasticity, even at moderate to high capillary and Deborah numbers. The effects of droplet viscoelasticity only become visible close to the critical conditions and a novel break-up mechanism is observed. Matrix viscoelasticity has a more pronounced effect, causing overshoots in the deformation and significantly inhibiting relaxation. However, different applied capillary numbers prior to cessation of shear flow, with the Deborah number fixed, still result in a single master curve for shape retraction, as in fully Newtonian systems. The long tail in the droplet relaxation can be qualitatively described with a phenomenological model for droplet deformation, when using a 5-mode Giesekus model for the fluid rheology. It is found that the shear flow history significantly affects the droplet shape evolution and the breakup process in blends with one viscoelastic component. Confining a droplet between two plates accelerates the droplet deformation kinetics, similar to fully Newtonian systems. However, the increased droplet deformation, due to wall effects, causes the steady state to be reached at a later instant in time. Droplet relaxation is less sensitive to confinement, leading to slower relaxation kinetics only for highly confined droplets. For the blend with a viscoelastic droplet, a non-monotonous trend is found for the critical capillary number as a function of the confinement ratio. Finally
Wall shear stress fixed points in blood flow
Arzani, Amirhossein; Shadden, Shawn
2017-11-01
Patient-specific computational fluid dynamics produces large datasets, and wall shear stress (WSS) is one of the most important parameters due to its close connection with the biological processes at the wall. While some studies have investigated WSS vectorial features, the WSS fixed points have not received much attention. In this talk, we will discuss the importance of WSS fixed points from three viewpoints. First, we will review how WSS fixed points relate to the flow physics away from the wall. Second, we will discuss how certain types of WSS fixed points lead to high biochemical surface concentration in cardiovascular mass transport problems. Finally, we will introduce a new measure to track the exposure of endothelial cells to WSS fixed points.
International Nuclear Information System (INIS)
Paraschiv, I.; Bauer, B. S.; Lindemuth, I. R.; Makhin, V.
2010-01-01
The effect of sheared axial flow on the Z-pinch sausage instability has been examined with two-dimensional magnetohydrodynamic simulations. Diffuse Bennett equilibria in the presence of axial flows with parabolic and linear radial profiles have been considered, and a detailed study of the linear and nonlinear development of small perturbations from these equilibria has been performed. The consequences of both single-wavelength and random-seed perturbations were calculated. It was found that sheared flows changed the internal m=0 mode development by reducing the linear growth rates, decreasing the saturation amplitude, and modifying the instability spectrum. High spatial frequency modes were stabilized to small amplitudes and only long wavelengths continued to grow. Full stability was obtained for supersonic plasma flows.
Viallat, Annie; Abkarian, Manouk; Dupire, Jules
2015-11-01
The analytical model presented by Keller and Skalak on the dynamics of red blood cells in shear flow described the cell as a fluid ellipsoid of fixed shape. It was extended to introduce shear elasticity of the cell membrane. We further extend the model when the cell discoid physiological shape is not a stress-free shape. We show that spheroid stress-free shapes enables fitting experimental data with values of shear elasticity typical to that found with micropipettes and optical tweezers. For moderate shear rates (when RBCs keep their discoid shape) this model enables to quantitatively determine an effective cell viscosity, that combines membrane and hemoglobin viscosities and an effective shear modulus of the membrane that combines shear modulus and stress-free shape. This model allows determining RBC mechanical parameters both in the tanktreading regime for cells suspended in a high viscosity medium, and in the tumbling regime for cells suspended in a low viscosity medium. In this regime,a transition is predicted between a rigid-like tumbling motion and a fluid-like tumbling motion above a critical shear rate, which is directly related to the mechanical parameters of the cell. A*MIDEX (n ANR-11-IDEX-0001-02) funded by the ''Investissements d'Avenir'', Region Languedoc-Roussillon, Labex NUMEV (ANR-10-LABX-20), BPI France project DataDiag.
Scaling of turbulence spectra measured in strong shear flow near the Earth’s surface
DEFF Research Database (Denmark)
Mikkelsen, Torben Krogh; Larsen, Søren Ejling; Ejsing Jørgensen, Hans
2017-01-01
Within the lowest kilometer of the Earth's atmosphere, in the so-called atmospheric boundary layer, winds are often gusty and turbulent. Nearest to the ground, the turbulence is predominately generated by mechanical wall-bounded wind shear, whereas at higher altitudes turbulent mixing of heat...... subrange with a distinct inverse-linear power law for turbulence in a strongly sheared high-Reynolds number wall-bounded flow, as is encountered in the lowest sheared part of the atmospheric boundary layer, also known as the eddy surface layer. This paper presents observations of spectra measured...... and moisture also play a role. The variance (square of the standard deviation) of the fluctuation around the mean wind speed is a measure of the kinetic energy content of the turbulence. This kinetic energy can be resolved into the spectral distributions, or spectra, as functions of eddy size, wavenumber...
Turbulence suppression by E x B shear in JET optimized shear pulses
International Nuclear Information System (INIS)
Beer, M.A.; Budny, R.V.; Challis, C.D.; Conway, G.
2000-01-01
The authors calculate microinstability growth rates in JET optimized shear plasmas with a comprehensive gyrofluid model, including sheared E x B flows, trapped electrons, and all dominant ion species in realistic magnetic geometry. They find good correlation between E x B shear suppression of microinstabilities and both the formation and collapse of the internal transport barrier
Nested separatrices in simple shear flows: the effect of localized disturbances on stagnation lines
Wilson, M.C.T.; Gaskell, P.H.; Savage, M.D.
2005-01-01
The effects of localized two-dimensional disturbances on the structure of shear flows featuring a stagnation line are investigated. A simple superposition of a planar Couette flow and Moffatt's [J. Fluid Mech. 18, 1--18 (1964)] streamfunction for the decay of a disturbance between infinite stationary parallel plates shows that in general the stagnation line is replaced by a chain of alternating elliptic and hyperbolic stagnation points with a separation equal to 2.78 times the half-gap betwee...
Direct measurement of wall shear stress in a reattaching flow with a photonic sensor
International Nuclear Information System (INIS)
Ayaz, U K; Ioppolo, T; Ötügen, M V
2013-01-01
Wall shear stress measurements are carried out in a planar backward-facing step flow using a micro-optical sensor. The sensor is essentially a floating element system and measures the shear stress directly. The transduction method to measure the floating element deflection is based on the whispering gallery optical mode (WGM) shifts of a dielectric microsphere. This method is capable of measuring floating element displacements of the order of a nanometer. The floating element surface is circular with a diameter of ∼960 µm, which is part of a beam that is in contact with the dielectric microsphere. The sensor is calibrated for shear stress as well as pressure sensitivity yielding 7.3 pm Pa −1 and 0.0236 pm Pa −1 for shear stress and pressure sensitivity, respectively. Hence, the contribution by the wall pressure is less than two orders of magnitude smaller than that of shear stress. Measurements are made for a Reynolds number range of 2000–5000 extending to 18 step heights from the step face. The results are in good agreement with those of earlier reports. An analysis is also carried out to evaluate the performance of the WGM sensor including measurement sensitivity and bandwidth. (paper)
Molar exergy and flow exergy of pure chemical fuels
International Nuclear Information System (INIS)
Zanchini, Enzo; Terlizzese, Tiziano
2009-01-01
Expressions of the molar exergy and of the molar flow exergy of a pure chemical fuel are deduced rigorously from the basic principles of thermodynamics. It is shown that molar exergy and molar flow exergy coincide when the temperature T and the pressure p of the fuel are equal to the temperature T B and the pressure p B of the environment; a general relation between exergy and flow exergy is proved as a consequence. The deduction of the expression of the molar exergy of a chemical fuel for non-standard values of T B and p B is clarified. For hydrogen, carbon dioxide and several hydrocarbons, tables are reported to allow a simple calculation of the molar exergy of the fuel for any value of the temperature T B and the relative humidity φ B of the environment, in the range 268.15 K ≤ T B ≤ 313.15 K and 0.1 ≤ φ B ≤ 1, with reference to the standard atmospheric pressure. Additional tables are provided to evaluate the difference between the exergy or the flow exergy of the fuel in its given initial state and the exergy at T = T B and p = p B . In these tables, it is assumed that fuel and environment have the same temperature and that the fuel pressure varies in the range 1.01325 bar ≤ p ≤ 200 bar; the fuel may be gas or liquid.
Shear flow generation and transport barrier formation on rational surface current sheets in tokamaks
International Nuclear Information System (INIS)
Wang Xiaogang; Xiao Chijie; Wang Jiaqi
2009-01-01
Full text: A thin current sheet with a magnetic field component in the same direction can form the electrical field perpendicularly pointing to the sheet, therefore an ExB flow with a strong shear across the current sheet. An electrical potential well is also found on the rational surface of RFP as well as the neutral sheet of the magnetotail with the E-field pointing to the rational (neutral) surface. Theoretically, a current singularity is found to be formed on the rational surface in ideal MHD. It is then very likely that the sheet current on the rational surfaces will generate the electrical potential well in its vicinity so the electrical field pointing to the sheet. It results in an ExB flow with a strong shear in the immediate neighborhood of the rational surface. It may be the cause of the transport barrier often seen near the low (m, n) rational surfaces with MHD signals. (author)
A topological method for vortex identification in turbulent flows
Energy Technology Data Exchange (ETDEWEB)
Zhong, Qiang; Chen, Huai; Li, Danxun [State Key Laboratory of Hydroscience and Engineering, Tsinghua University, Beijing 100084 (China); Chen, Qigang, E-mail: lidx@mail.tsinghua.edu.cn [School of Civil Engineering, Beijing Jiaotong University, Beijing 100044 (China)
2017-02-15
We present a novel vortex identification method based on structured vorticity ( ω {sub s}) of the direction field of flow (velocity vectors set to unit magnitude). As a direct measure of streamline curvature is insensitive to vortex strength, ω {sub s} is effective in detecting vortices of various strengths. The effectiveness has been tested against both analytical flows (pure shear flow, Oseen vortex flow, strong outward spiraling motion, straining flow, Taylor–Green flow) and experimental flows (closed cavity flow, closed and open channel flow). Comparison of the new method with the swirling-strength method indicates that the new method shows promise as being a simple and effective criterion for vortex identification. (paper)
1991-09-01
opposite Fance mode at streaks in the spanwise direction and in the direction per- time when ri is maximum- V, Fv; 9 , F.; and a , F, pendicular to the...space, Fig, lbIII, flow, see Hassa et al. (1988). resp., requires that the filter input sig- By the use of a monodisperse di plet gene - nals have to be...of turbulent shear flows". V K I Lecture .me Series 1989-3 on "Tuibuleti Shear Flows", Rhode-Saint- SWIM Genese (Belgique), February 6-10, 1989 to" 13
Elliptic Flow at Finite Shear Viscosity in a Kinetic Approach at RHIC
International Nuclear Information System (INIS)
Greco, V.; Colonna, M.; Di Toro, M.; Ferini, G.
2010-01-01
Within a covariant parton cascade, we discuss the impact of both finite shear viscosity η and freeze-out dynamics on the elliptic flow generated at RHIC. We find that the enhancement of η/s in the cross-over region of the QGP phase transition cannot be neglected in order to extract the information from the QGP phase. We also point out that the elliptic flow v 2 (p T ) for a fluid at η/s∼0.1-0.2 is consistent with the one needed by quark number scaling drawing a nice consistency between the nearly perfect fluid property of QGP and the coalescence process.
Comparison of turbulent particle dispersion models in turbulent shear flows
Directory of Open Access Journals (Sweden)
S. Laín
2007-09-01
Full Text Available This work compares the performance of two Lagrangian turbulent particle dispersion models: the standard model (e.g., that presented in Sommerfeld et al. (1993, in which the fluctuating fluid velocity experienced by the particle is composed of two components, one correlated with the previous time step and a second one randomly sampled from a Wiener process, and the model proposed by Minier and Peirano (2001, which is based on the PDF approach and performs closure at the level of acceleration of the fluid experienced by the particle. Formulation of a Langevin equation model for the increments of fluid velocity seen by the particle allows capturing some underlying physics of particle dispersion in general turbulent flows while keeping the mathematical manipulation of the stochastic model simple, thereby avoiding some pitfalls and simplifying the derivation of macroscopic relations. The performance of both dispersion models is tested in the configurations of grid-generated turbulence (Wells and Stock (1983 experiments, simple shear flow (Hyland et al., 1999 and confined axisymmetric jet flow laden with solids (Hishida and Maeda (1987 experiments.
Schwarz, Janina C. V.; Duivenvoorden, Raphaël; Nederveen, Aart J.; Stroes, Erik S. G.; VanBavel, Ed
2015-01-01
Endothelial shear stress (ESS) dynamics are a major determinant of atherosclerosis development. The frequently used Poiseuille method to estimate ESS dynamics has important limitations. Therefore, we investigated whether Womersley flow may provide a better alternative for estimation of ESS while
International Nuclear Information System (INIS)
1998-10-01
The topics discussed comprise the onset of instability in heated free jets and jets with density gradients, flow past heated/cooled boundaries, atmospheric shear flow, and mathematical modeling of laminar-turbulent transition phenomena. Three contributions have been input to INIS. (P.A.)
Continuous media theory for MR fluids in non-shearing flows
International Nuclear Information System (INIS)
Ruiz-López, J A; Hidalgo-Alvarez, R; Vicente, J de
2013-01-01
The enhanced mechanical response of magnetorheological fluids under slow compression has been investigated by means of experiments, theory and particle-level simulations. A wide range of magnetic field strengths (0–354 kA/m), dispersing medium viscosities (20–500 mPa·s) and particle concentrations (5–30 vol%) were investigated. Plastic media theory in compressive flow was in good agreement with experimental data. Slight deviations from the theory were associated to the so-called strengthening effect as the yield shear stress could increase during compression. Particle-level simulations were in good agreement with both experiments and simulations.
Nonlinear interaction of Rayleigh--Taylor and shear instabilities
International Nuclear Information System (INIS)
Finn, J.M.
1993-01-01
Results on the nonlinear behavior of the Rayleigh--Taylor instability and consequent development of shear flow by the shear instability [Phys. Fluids B 4, 488 (1992)] are presented. It is found that the shear flow is generated at sufficient amplitude to reduce greatly the convective transport. For high viscosity, the time-asymptotic state consists of an equilibrium with shear flow and vortex flow (with islands, or ''cat's eyes''), or a relaxation oscillation involving an interplay between the shear instability and the Rayleigh--Taylor instability in the presence of shear. For low viscosity, the dominant feature is a high-frequency nonlinear standing wave consisting of convective vortices localized near the top and bottom boundaries. The localization of these vortices is due to the smaller shear near the boundary regions. The convective transport is largest around these convective vortices near the boundary and there is a region of good confinement near the center. The possible relevance of this behavior to the H mode and edge-localized modes (ELM's) in the tokamak edge region is discussed
Directory of Open Access Journals (Sweden)
G. G. Didebulidze
2008-06-01
Full Text Available The formation of the mid-latitude sporadic E layers (E_{s} layers by an atmospheric vortical perturbation excited in a horizontal shear flow (horizontal wind with a horizontal linear shear is investigated. A three-dimensional atmospheric vortical perturbation (atmospheric shear waves, whose velocity vector is in the horizontal plane and has a vertical wavenumber k_{z}≠0, can provide a vertical shear of the horizontal wind. The shear waves influence the vertical transport of heavy metallic ions and their convergence into thin and dense horizontal layers. The proposed mechanism takes into account the dynamical influence of the shear wave velocity in the horizontal wind on the vertical drift velocity of the ions. It also can explain the multi-layer structure of E_{s} layers. The pattern of the multi-layer structure depends on the value of the shear-wave vertical wavelength, the ion-neutral collision frequency and the direction of the background horizontal wind. The modelling of formation of sporadic E layers with a single and a double peak is presented. Also, the importance of shear wave coupling with short-period atmospheric gravity waves (AGWs on the variations of sporadic E layer ion density is examined and discussed.
Shear flow effect on ion temperature gradient vortices in plasmas with sheared magnetic field
DEFF Research Database (Denmark)
Chakrabarti, N.; Juul Rasmussen, J.
1999-01-01
The effect of velocity shear on ion temperature gradient (ITG) driven vortices in a nonuniform plasma in a curved, sheared magnetic field is investigated. In absence of parallel ion dynamics, vortex solutions for the ITG mode are studied analytically. It is shown that under certain conditions...... and ultimately lead to a dominating monopolar form. The effects of magnetic shear indicate it may destroy these structures. (C) 1999 American Institute of Physics....
Ghardashi Afousi, Alireza; Izadi, Mohammad Reza; Rakhshan, Kamran; Mafi, Farnoosh; Biglari, Soheil; Gandomkar Bagheri, Habibalah
2018-06-22
What is the central question of this study? Endothelial function is impaired because of increased oscillatory and retrograde shear in patients with type 2 diabetes. It is unclear whether low-volume high-intensity interval training and continuous moderate intensity exercise can modulate oscillatory and retrograde shear, blood flow and flow-mediated arterial dilation in these patients. What is the main finding and its importance? We found that low-volume high-intensity interval training, by increasing anterograde shear and decreasing retrograde shear and oscillatory index, can increase nitric oxide production and consequently result in increased flow-mediated dilation and outward arterial remodelling in patients with type 2 diabetes. Atherosclerosis in patients with type 2 diabetes is characterized by endothelial dysfunction associated with impaired flow-mediated dilation (FMD) and increases retrograde and oscillatory shear. The present study investigated endothelium-dependent vasodilation and shear rate in patients with type 2 diabetes at baseline and follow-up after 12 weeks of low-volume high-intensity interval training (LV-HIIT) or continuous moderate intensity training (CMIT). Seventy five sedentary patients with type 2 diabetes and untreated pre- or stage I hypertension were randomly divided into LV-HIIT, CMIT and control groups. The LV-HIIT group intervention was 12 intervals of 1.5 min at 85%-90% HR max and 2 min at 55%-60% HR max . The CMIT group intervention was 42 min of exercise at 70% HR max for 3 sessions per week during 12 weeks. High-resolution Doppler ultrasound was used to measure FMD, arterial diameter, anterograde and retrograde blood flow and shear rate patterns. Brachial artery FMD increased significantly in the LV-HIIT group (3.83 ± 1.13 baseline, 7.39 ± 3.6% follow-up), whereas there were no significant increase in the CMIT group (3.45 ± 0.97 baseline, 4.81 ± 2.36% follow-up) compared to the control group (3.16 ± 0
Analytical modeling for heat transfer in sheared flows of nanofluids.
Ferrari, Claudio; Kaoui, Badr; L'vov, Victor S; Procaccia, Itamar; Rudenko, Oleksii; ten Thije Boonkkamp, J H M; Toschi, Federico
2012-07-01
We developed a model for the enhancement of the heat flux by spherical and elongated nanoparticles in sheared laminar flows of nanofluids. Besides the heat flux carried by the nanoparticles, the model accounts for the contribution of their rotation to the heat flux inside and outside the particles. The rotation of the nanoparticles has a twofold effect: it induces a fluid advection around the particle and it strongly influences the statistical distribution of particle orientations. These dynamical effects, which were not included in existing thermal models, are responsible for changing the thermal properties of flowing fluids as compared to quiescent fluids. The proposed model is strongly supported by extensive numerical simulations, demonstrating a potential increase of the heat flux far beyond the Maxwell-Garnett limit for the spherical nanoparticles. The road ahead, which should lead toward robust predictive models of heat flux enhancement, is discussed.
Shear stress from hot-film sensors in unsteady gas flow
International Nuclear Information System (INIS)
Cole, K.D.
1991-01-01
In this paper a data analysis procedure is proposed for obtaining unsteady wall shear stress from flush-mounted hot-film anemometer measurements. The method is based on a two-dimensional heat transfer model of the unsteady heat transfer in both the hot-film sensor and in the gas flow. The sensor thermal properties are found from preliminary calibration experiments at zero flow. Numerical experiments are used to demonstrate the data analysis method using simulated sensor signals that are corrupted with noise. The numerical experiments show that noise in the data propagates into the results so that data smoothing may be important in analyzing experimental data. Because the data analysis procedure is linear, a linear digital filter is constructed that could be used for processing large amounts of experimental data. However, further refinements will be needed before the method can be applied to experimental data
A purely nonlinear route to transition approaching the edge of chaos in a boundary layer
International Nuclear Information System (INIS)
Cherubini, S; De Palma, P; Robinet, J-Ch; Bottaro, A
2012-01-01
The understanding of transition in shear flows has recently progressed along new paradigms based on the central role of coherent flow structures and their nonlinear interactions. We follow such paradigms to identify, by means of a nonlinear optimization of the energy growth at short time, the initial perturbation which most easily induces transition in a boundary layer. Moreover, a bisection procedure has been used to identify localized flow structures living on the edge of chaos, found to be populated by hairpin vortices and streaks. Such an edge structure appears to act as a relative attractor for the trajectory of the laminar base state perturbed by the initial finite-amplitude disturbances, mediating the route to turbulence of the flow, via the triggering of a regeneration cycle of Λ and hairpin structures at different space and time scales. These findings introduce a new, purely nonlinear scenario of transition in a boundary-layer flow. (paper)
A stationary bulk planar ideal flow solution for the double shearing model
Lyamina, E. A.; Kalenova, N. V.; Date, P. P.
2018-04-01
This paper provides a general ideal flow solution for the double shearing model of pressure-dependent plasticity. This new solution is restricted to a special class of stationary planar flows. A distinguished feature of this class of solutions is that one family of characteristic lines is straight. The solution is analytic. The mapping between Cartesian and principal lines based coordinate systems is given in parametric form with characteristic coordinates being the parameters. A simple relation that connects the scale factor for one family of coordinate curves of the principal lines based coordinate system and the magnitude of velocity is derived. The original ideal flow theory is widely used as the basis for inverse methods for the preliminary design of metal forming processes driven by minimum plastic work. The new theory extends this area of application to granular materials.
van der Beek, M.H.E.; Peters, G.W.M.; Meijer, H.E.H.
2006-01-01
The influence of shear flow on the temperature evolution of the specific volume and the crystalline morphology of two iPP's, differing in weight-averaged molar mass w, was investigated at nonisothermal conditions and elevated pressures, using a custom-designed dilatometer. These conditions are
Wall shear stress from a rotating cylinder in cross flow using the electrochemical technique
International Nuclear Information System (INIS)
Labraga, L.; Bourabaa, N.; Berkah, T.
2002-01-01
The wall shear rate from a rotating cylinder in a uniform flow was measured with flush-mounted electrochemical mass transfer probes. The experiments were performed using two rectangular electrodes in a sandwich arrangement. Initially, the frequency response of that probe was numerically studied using an inverse mass transfer method in order to restore the whole wall shear stress in the time domain starting from the measured transfer coefficients given by the split probe. The experiments were performed in the range of velocity ratios 0 4, points of zero shear stress on the rotating cylinder vanish, which is in fact consistent with the previous arguments that the cylinder is surrounded by a set of closed streamlines. This experimental study shows that, when their dynamic behaviour is known, the electrochemical probes are able to sense complex fine structures not observed up to now by previous analytical, numerical or experimental methods, even when non-linear effects are not negligible. (orig.)
Shear flows at the tokamak edge and their interaction with edge-localized modes
International Nuclear Information System (INIS)
Aydemir, A. Y.
2007-01-01
Shear flows in the scrape-off layer (SOL) and the edge pedestal region of tokamaks are shown to arise naturally out of transport processes in a magnetohydrodynamic model. In quasi-steady-state conditions, collisional resistivity coupled with a simple bootstrap current model necessarily leads to poloidal and toroidal flows, mainly localized to the edge and SOL. The role of these flows in the grad-B drift direction dependence of the power threshold for the L (low) to H (high) transition, and their effect on core rotation, are discussed. Theoretical predictions based on symmetries of the underlying equations, coupled with computational results, are found to be in agreement with observations in Alcator C-Mod [Phys. Plasmas 12, 056111 (2005)]. The effects of these self-consistent flows on linear peeling/ballooning modes and their nonlinear consequences are also examined
Dynamics of a confined dusty fluid in a sheared ion flow
Energy Technology Data Exchange (ETDEWEB)
Laishram, Modhuchandra; Sharma, Devendra; Kaw, Predhiman K. [Institute for Plasma Research, Bhat, Gandhinagar 382428 (India)
2014-07-15
Dynamics of an isothermally driven dust fluid is analyzed which is confined in an azimuthally symmetric cylindrical setup by an effective potential and is in equilibrium with an unconfined sheared flow of a streaming plasma. Cases are analyzed where the confining potential constitutes a barrier for the driven fluid, limiting its spatial extension and boundary velocity. The boundary effects entering the formulation are characterized by applying the appropriate boundary conditions and a range of solutions exhibiting single and multiple vortex are obtained. The equilibrium solutions considered in the cylindrical setup feature a transition from single to multiple vortex state of the driven flow. Effects of (i) the variation in dust viscosity, (ii) coupling between the driving and the driven fluid, and (iii) a friction determining the equilibrium dynamics of the driven system are characterized.
The effect of viscosity on the resistive tearing mode with the presence of shear flow
International Nuclear Information System (INIS)
Chen, X.L.; Morrison, P.J.
1990-01-01
The effect of small isotropic viscosity on the ''constant ψ'' tearing mode in the presence of shear flow, is analyzed by the boundary layer approach. It is found that the influence of viscosity depends upon the parameter (G'(0)/F'(0)), where G'(0) and F'(0) denote that shear and magnetic field shear at the magnetic null plane, respectively. When |(G'(0)/F'(0))| much-lt 1, the tearing mode growth rate is suppressed by the viscosity, but not completely stabilized. When |(G'(0)/F'(0))| ∼ in the order of (1) and the viscosity is comparable with the resistivity, the growth rate vanishes as ((1 - G'(0) 2 /F'(0) 2 ) 1/3 ), when G'(0) 2 → F'(0) 2 from below. In the case where (1 - G'(0) 2 /F'(0) 2 ) < 0 matching cannot be achieved. 8 refs
Chowdhury, Sugata; Simpson, Jeffrey; Einstein, T. L.; Walker, Angela R. Hight
2D-materials with controllable optical, electronic and magnetic properties are desirable for novel nanodevices. Here we studied these properties for both pristine and hydrogenated TaSe2 (TaSe2-H) monolayer (ML) in the framework of DFT using the PAW method. We considered uniaxial and biaxial tensile strain, as well as shear strain along the basal planes in the range between 1% and 16%. Previous theoretical works (e.g.) considered only symmetrical biaxial tensile. Pristine ML is ferromagnetic for uniaxial tensile strain along ◯ or ŷ. For tensile strain in ŷ, the calculated magnetic moments of the Ta atoms are twice those for the same strain in ◯. Under pure shear strain (expansion along ŷ and compression along ◯), a pristine ML is ferromagnetic, but becomes non-magnetic when the strain directions are interchanged. Due to carrier-mediated double-exchange, the pristine ML is ferromagnetic when the Se-Ta-Se bond angle is < 82° and the ML thickness is < 3.25Å. We find that all Raman-active phonon modes show obvious red-shifting due to bond elongation and the E2 modes degeneracy is lifted as strain increases. For a TaSe2-H ML, the same trends were observed. Results show the ability to tune the properties of 2D-materials.
Directory of Open Access Journals (Sweden)
Noirez Laurence
2017-03-01
Full Text Available This work points out the importance of the substrate boundary conditions to lower the dissipation in the dynamic measurement and access the closest dynamic characteristics of liquids, in particular to access the low frequency shear elasticity. The liquid/surface interface is a source of dissipation that enters and impacts the measurement. Examples of steady-state shear flows or flow birefringence are presented to highlight the non-universality of the behavior with respect to the nature of the substrate or the sheared thickness. Additionally the present development completes and extends the identification of low frequency shear elasticity made at sub-millimeter gaps in various one-component liquids to salt-free aqueous solutions (CTAB-water (Hexadecyl-TrimethylAmmonium Bromide.
Flow and bed shear stresses in scour protections around a pile in a current
DEFF Research Database (Denmark)
Nielsen, Anders Wedel; Liu, Xiaofeng; Sumer, B. Mutlu
2013-01-01
on it in an unfavourable manner. Using physical models and 3D computational fluid dynamic (CFD) numerical simulations, the velocity and bed shear stresses are investigated in complex scour protections around mono piles in steady current. In the physical model the scour protections consisted of an upper cover layer...... simulations are capable of calculating the flow velocities when the scour protection is represented by regular arranged spheres, while the turbulence in general is underestimated. The velocity can also be calculated using porous media flow approach, but the accuracy is not as good as for spheres...
Shafer, M. W.; McKee, G. R.; Schlossberg, D. J.; Austin, M. E.; Burrell, K. H.
2008-11-01
Long-wavelength turbulence (kρiITBs) may form. Application of off-axis ECH slows the q-profile evolution and increases ρqmin, both of which enhance turbulence measurements using a new high-sensitivity large-area (8x,8) 2D BES array. The measured transient turbulence suppression is localized to the low-order rational surface (qmin= 2, 5/2, 3, etc.). Measured poloidal flow shear transiently exceeds the turbulence decorrelation rate, which is consistent with shear suppression. The localized suppression zone propagates radially outward, nearly coincident with the low-order surface.
International Nuclear Information System (INIS)
Fakori-Monazah, M.R.; Todreas, N.E.
1977-08-01
A simulated model of triangular array rods with pitch to diameter ratio of 1.10 (as a test section) and air as the fluid flow was used to study the LMFBR hydraulic parameters. The wall shear stress distribution around the rod periphery, friction factors, static pressure distributions and turbulence intensity corresponding to various Reynolds numbers ranging from 4140 to 36170 in the central subchannel were measured. Various approaches for measurement of wall shear stress were compared. The measurement was performed using the Preston tube technique with the probe outside diameter equal to 0.014 in
Duddu, Ravindra; Chopp, David L.; Moran, Brian
2009-01-01
of the biofilm. The model considers fluid flow around the biofilm surface, the advection-diffusion and reaction of substrate, variable biomass volume fraction and erosion due to the interfacial shear stress at the biofilm-fluid interface. The key assumptions
Flow accelerated organic coating degradation
Zhou, Qixin
Applying organic coatings is a common and the most cost effective way to protect metallic objects and structures from corrosion. Water entry into coating-metal interface is usually the main cause for the deterioration of organic coatings, which leads to coating delamination and underfilm corrosion. Recently, flowing fluids over sample surface have received attention due to their capability to accelerate material degradation. A plethora of works has focused on the flow induced metal corrosion, while few studies have investigated the flow accelerated organic coating degradation. Flowing fluids above coating surface affect corrosion by enhancing the water transport and abrading the surface due to fluid shear. Hence, it is of great importance to understand the influence of flowing fluids on the degradation of corrosion protective organic coatings. In this study, a pigmented marine coating and several clear coatings were exposed to the laminar flow and stationary immersion. The laminar flow was pressure driven and confined in a flow channel. A 3.5 wt% sodium chloride solution and pure water was employed as the working fluid with a variety of flow rates. The corrosion protective properties of organic coatings were monitored inline by Electrochemical Impedance Spectroscopy (EIS) measurement. Equivalent circuit models were employed to interpret the EIS spectra. The time evolution of coating resistance and capacitance obtained from the model was studied to demonstrate the coating degradation. Thickness, gloss, and other topography characterizations were conducted to facilitate the assessment of the corrosion. The working fluids were characterized by Fourier Transform Infrared Spectrometer (FTIR) and conductivity measurement. The influence of flow rate, fluid shear, fluid composition, and other effects in the coating degradation were investigated. We conclude that flowing fluid on the coating surface accelerates the transport of water, oxygen, and ions into the coating, as
Interfacial shear stress and hold-up in an air-water annular two-phase flow
International Nuclear Information System (INIS)
Fukano, T.; Ousaka, A.; Kawakami, Y.; Tominaga, A.
1991-01-01
This paper reports on an experimental investigation that was made into hold-up, frictional pressure drop and interfacial shear stress of an air-water two-phase annular flow in horizontal and vertical up- and downward flows to make clear the effects of tube diameter and flow direction on them. The tube diameters examined are 10mm, 16mm and 26mm. Both the hold-up and the pressure drop considerably changed with time. Especially, the amplitude of the variation of the hold-up was quite larger in comparison with its averaged value in the cause of disturbance wave flow. for the time averaged hold-up and interfacial friction factor, we got new correlations, by which we can estimate them within an accuracy of ±20% and ±30%, respectively, independent of the flow direction and the tube diameter
Velocity-pressure correlation measurements in complex free shear flows
International Nuclear Information System (INIS)
Naka, Yoshitsugu; Obi, Shinnosuke
2009-01-01
Simultaneous measurements of fluctuating velocity and pressure were performed in various turbulent free shear flows including a turbulent mixing layer and the wing-tip vortex trailing from a NACA0012 half-wing. Two different methods for fluctuating static pressure measurement were considered: a direct method using a miniature Pitot tube and an indirect method where static pressure was calculated from total pressure. The pressure obtained by either of these methods was correlated with the velocity measured by an X-type hot-wire probe. The results from these two techniques agreed with each other in the turbulent mixing layer. In the wing-tip vortex case, however, some discrepancies were found, although overall characteristics of the pressure-related statistics were adequately captured by both methods.
Chun, Byoungjin; Kwon, Ilyoung; Jung, Hyun Wook; Hyun, Jae Chun
2017-12-01
The shear-induced migration of concentrated non-Brownian monodisperse suspensions in combined plane Couette-Poiseuille (C-P) flows is studied using a lattice Boltzmann simulation. The simulations are mainly performed for a particle volume fraction of ϕbulk = 0.4 and H/a = 44.3, 23.3, where H and a denote the channel height and radius of suspended particles, respectively. The simulation method is validated in two simple flows, plane Poiseuille and plane Couette flows. In the Poiseuille flow, particles migrate to the mid-plane of the channel where the local concentration is close to the limit of random-close-packing, and a random structure is also observed at the plane. In the Couette flow, the particle distribution remains in the initial uniform distribution. In the combined C-P flows, the behaviors of migration are categorized into three groups, namely, Poiseuille-dominant, Couette-dominant, and intermediate regimes, based on the value of a characteristic force, G, where G denotes the relative magnitude of the body force (P) against the wall-driving force (C). With respect to the Poiseuille-dominant regime, the location of the maximum concentration is shifted from the mid-plane to the lower wall moving in the same direction as the external body force, when G decreases. With respect to the Couette-dominant regime, the behavior is similar to that of a simple shear flow with the exception that a slightly higher concentration of particles is observed near the lower wall. However, with respect to the intermediate value of G, several layers of highly ordered particles are unexpectedly observed near the lower wall where the plane of maximum concentration is located. The locally ordered structure is mainly due to the lateral migration of particles and wall confinement. The suspended particles migrate toward a vanishingly small shear rate at the wall, and they are consequently layered into highly ordered two-dimensional structures at the high local volume fraction.
International Nuclear Information System (INIS)
Sauga, Ako; Laas, Katrin; Mankin, Romi
2015-01-01
Highlights: • Cross-correlation (CC) of coordinates of particles in viscoelastic shear flows is discussed. • Expressions for CC functions subjected to both internal and external noises are presented. • Impact of internal and external noises on CC functions are compared. • Memory-induced reentrant sign reversals of the spatial cross-moment are established. - Abstract: The behavior of shear-induced cross-correlation functions between particle fluctuations along orthogonal directions in the shear plane for harmonically trapped Brownian particles in a viscoelastic shear flow is studied. A generalized Langevin equation with a power-law-type memory kernel is used to model the complex structure of the viscoelastic media. Interaction with fluctuations of environmental parameters is modeled by a multiplicative white Gaussian noise, by an internal fractional Gaussian noise, and by an additive external white noise. It is shown that the presence of a memory has a profound effect on the behavior of the cross-correlation functions. Particularly, memory-induced reentrant sign reversals of the spatial cross-moment between orthogonal random displacements of a particle are established, i.e., an increase of the memory exponent can cause the sign reversal from positive to negative, but by a further increase of the memory exponent a reentrant transition from negative to positive values appears. Similarities and differences between the behavior of the models with additive internal and external noises are considered. It is shown that additive external and internal noises cause qualitatively different dependencies of the cross-correlation functions on the time lag. The occurrence of energetic instability due to the influence of multiplicative noise is also discussed.
Gandía-Barberá, Sergio; Hoyas, Sergio; Oberlack, Martin; Kraheberger, Stefanie
2018-04-01
The length and width of the long and wide structures appearing in turbulent Couette flows are studied by means of a new dataset of direct numerical simulation covering a stepped transition from pure Couette flow to pure Poiseuille one, at Reτ ≈ 130, based on the stationary wall. The existence of these structures is linked to the averaged Reynolds stress, u v ¯ : as soon as in any part of the channel u v ¯ changes its sign, the structures disappear. The length and width of the rolls are found to be, approximately, 50h and 2.5h, respectively. For this Reynolds number, simulations with a domain shorter than 100h cannot properly describe the behaviour of the longest structures of the flow.
International Nuclear Information System (INIS)
Zhongchun, Li; Xiaoming, Song; Shengyao, Jiang; Jiyang, Yu
2014-01-01
Highlights: • A VOF simulation of bubble in low viscosity fluid was conducted. • Lift force in different viscosity fluid had different lateral migration characteristics. • Bubble with different size migrated to different direction. • Shear stress triggered the bubble deformation process and the bubble deformation came along with the oscillation behaviors. - Abstract: Two phase flow systems have been widely used in industrial engineering. Phase distribution characteristics are vital to the safety operation and optimization design of two phase flow systems. Lift force has been known as perpendicular to the bubbles’ moving direction, which is one of the mechanisms of interfacial momentum transfer. While most widely used lift force correlations, such as the correlation of Tomiyama et al. (2002), were obtained by experimentally tracking single bubble trajectories in high viscosity glycerol–water mixture, the applicability of these models into low viscosity fluid, such as water in nuclear engineering system, needs to be further evaluated. In the present paper, bubble in low viscosity fluid in shear flow was investigated in a full 3D numerical simulation and the volume of fluid (VOF) method was applied to capture the interface. The fluid parameter: fluid viscosity, bubble parameter: diameter and external flow parameters: shear stress magnitude and liquid velocity were examined. Comparing with bubble in high viscosity shear flow and bubble in low viscosity still flow, relative large bubble in low viscosity shear flow keep an oscillation way towards the moving wall and experienced a shape deformation process. The oscillation amplitude increased as the viscosity of fluid decreased. Small bubble migrated to the static wall in a line with larger migration velocity than that in high viscosity fluid and no deformation occurred. The shear stress triggered the oscillation behaviors while it had no direct influence with the behavior. The liquid velocity had no effect on
Effect of short-chain branching on interfacial polymer structure and dynamics under shear flow.
Jeong, Sohdam; Kim, Jun Mo; Cho, Soowon; Baig, Chunggi
2017-11-22
We present a detailed analysis on the effect of short-chain branches on the structure and dynamics of interfacial chains using atomistic nonequilibrium molecular dynamics simulations of confined polyethylene melts in a wide range of shear rates. The intrinsically fast random motions of the short branches constantly disturb the overall chain conformation, leading to a more compact and less deformed chain structure of the short-chain branched (SCB) polymer against the imposed flow field in comparison with the corresponding linear polymer. Moreover, such highly mobile short branches along the backbone of the SCB polymer lead to relatively weaker out-of-plane wagging dynamics of interfacial chains, with highly curvy backbone structures in the intermediate flow regime. In conjunction with the contribution of short branches (as opposed to that of the backbone) to the total interfacial friction between the chains and the wall, the SCB polymer shows a nearly constant behavior in the degree of slip (d s ) with respect to shear rate in the weak-to-intermediate flow regimes. On the contrary, in the strong flow regime where irregular chain rotation and tumbling dynamics occur via intensive dynamical collisions between interfacial chains and the wall, an enhancement effect on the chain detachment from the wall, caused by short branches, leads to a steeper increase in d s for the SCB polymer than for the linear polymer. Remarkably, the SCB chains at the interface exhibit two distinct types of rolling mechanisms along the backbone, with a half-dumbbell mesoscopic structure at strong flow fields, in addition to the typical hairpin-like tumbling behavior displayed by the linear chains.
Influence of Base Oil Polarity on the Transient Shear Flow of Biodegradable Lubricating Greases
Directory of Open Access Journals (Sweden)
Martin Fiedler
2015-09-01
Full Text Available The scope of this study is to elucidate the physical mechanisms influencing the transient flow behavior of lubricating greases based on biogenic oleochemicals from a polarity point of view. This includes the mutually interacting influence of base oil polarity and thickening agents on the rheologically-measured mechanical structural degradation in transient shear flow. Due to the high temperature dependence of Keesom forces in the background of polar-active bond mechanisms, the analysis of the transient flow response as a function of temperature allows to attribute the observed influences to differences in base oil polarity. In general, clay-thickened greases show a greater tendency to be rheologically influenced by base oil polarities than soap-thickened lubricating greases.
Rivulet flow round a horizontal cylinder subject to a uniform surface shear stress
Paterson, C.
2014-09-14
© 2014 © The Author, 2014. Published by Oxford University Press; all rights reserved. For Permissions, please email: journals.permissions@oup.com. The steady flow of a slowly varying rivulet with prescribed flux in the azimuthal direction round a large stationary horizontal cylinder subject to a prescribed uniform azimuthal surface shear stress is investigated. In particular, we focus on the case where the volume flux is downwards but the shear stress is upwards, for which there is always a solution corresponding to a rivulet flowing down at least part of one side of the cylinder. We consider both a rivulet with constant non-zero contact angle but slowly varying width (that is, de-pinned contact lines) and a rivulet with constant width but slowly varying contact angle (that is, pinned contact lines), and show that they have qualitatively different behaviour. When shear is present, a rivulet with constant non-zero contact angle can never run all the way from the top to the bottom of the cylinder, and so we consider the scenario in which an infinitely wide two-dimensional film of uniform thickness covers part of the upper half of the cylinder and \\'breaks\\' into a single rivulet with constant non-zero contact angle. In contrast, a sufficiently narrow rivulet with constant width can run all the way from the top to the bottom of the cylinder, whereas a wide rivulet can do so only if its contact lines de-pin, and so we consider the scenario in which the contact lines of a wide rivulet de-pin on the lower half of the cylinder.
Shear of ordinary and elongated granular mixtures
Hensley, Alexander; Kern, Matthew; Marschall, Theodore; Teitel, Stephen; Franklin, Scott
2015-03-01
We present an experimental and computational study of a mixture of discs and moderate aspect-ratio ellipses under two-dimensional annular planar Couette shear. Experimental particles are cut from acrylic sheet, are essentially incompressible, and constrained in the thin gap between two concentric cylinders. The annular radius of curvature is much larger than the particles, and so the experiment is quasi-2d and allows for arbitrarily large pure-shear strains. Synchronized video cameras and software identify all particles and track them as they move from the field of view of one camera to another. We are particularly interested in the global and local properties as the mixture ratio of discs to ellipses varies. Global quantities include average shear rate and distribution of particle species as functions of height, while locally we investigate the orientation of the ellipses and non-affine events that can be characterized as shear transformational zones or possess a quadrupole signature observed previously in systems of purely circular particles. Discrete Element Method simulations on mixtures of circles and spherocylinders extend the study to the dynamics of the force network and energy dissipated as the system evolves. Supported by NSF CBET #1243571 and PRF #51438-UR10.
Moore, Johnathan; Crandall, Dustin; Gill, Magdalena; Brown, Sarah; Tennant, Bryan
2018-04-01
Fluid flow in the subsurface is not well understood in the context of "impermeable" geologic media. This is especially true of formations that have undergone significant stress fluctuations due to injection or withdrawal of fluids that alters the localized pressure regime. When the pressure regime is altered, these formations, which are often already fractured, move via shear to reduce the imbalance in the stress state. While this process is known to happen, the evolution of these fractures and their effects on fluid transport are still relatively unknown. Numerous simulation and several experimental studies have been performed that characterize the relationship between shearing and permeability in fractures; while many of these studies utilize measurements of fluid flow or the starting and ending geometries of the fracture to characterize shear, they do not characterize the intermediate stages during shear. We present an experimental apparatus based on slight modifications to a commonly available Hassler core holder that allows for shearing of rocks, while measuring the hydraulic and mechanical changes to geomaterials during intermediate steps. The core holder modification employs the use of semi-circular end caps and structural supports for the confining membrane that allow for free movement of the sheared material while preventing membrane collapse. By integrating this modified core holder with a computed tomography scanner, we show a new methodology for understanding the interdependent behavior between fracture structure and flow properties during intermediate steps in shearing. We include a case study of this device function which is shown here through shearing of a fractured shale core and simultaneous observation of the mechanical changes and evolution of the hydraulic properties during shearing.
Energy Technology Data Exchange (ETDEWEB)
Lee, Inchul; Kim, Dohun; Koo, Jaye [Korea Aerospace Univ., Goyang (Korea, Republic of)
2013-07-15
To verify the effect of inner- and outer-stage gas jets, a shear coaxial injector was designed to analyze the axial velocity profile and breakup phenomenon with an increase in the measurement distance. When the measurement position was increased to Z/d=100, the axial flow showed a fully developed shape due to the momentum transfer, aerodynamic drag effect, and viscous mixing. An inner gas injection, which induces a higher momentum flux ratio near the nozzle, produces the greater shear force on atomization than an outer gas injection. Inner- and Outer-stage gas injection do not affect the mixing between the inner and outer gas flow below Z/d=5. The experiment results showed that the main effect of liquid jet breakup was governed by the gas jet of an inner stage. As the nozzle exit of the outer-stage was located far from the liquid column, shear force and turbulence breaking up of the liquid jets do not fully affect the liquid column. In the case of an inner-stage gas injection momentum flux ratio within 0.84, with the increase in the outer gas momentum flux ratio, the Smd decreases. However, at an inner-stage gas jet momentum flux ratio over 1.38, the Smd shows the similar distribution.
Shear Elasticity and Shear Viscosity Imaging in Soft Tissue
Yang, Yiqun
In this thesis, a new approach is introduced that provides estimates of shear elasticity and shear viscosity using time-domain measurements of shear waves in viscoelastic media. Simulations of shear wave particle displacements induced by an acoustic radiation force are accelerated significantly by a GPU. The acoustic radiation force is first calculated using the fast near field method (FNM) and the angular spectrum approach (ASA). The shear waves induced by the acoustic radiation force are then simulated in elastic and viscoelastic media using Green's functions. A parallel algorithm is developed to perform these calculations on a GPU, where the shear wave particle displacements at different observation points are calculated in parallel. The resulting speed increase enables rapid evaluation of shear waves at discrete points, in 2D planes, and for push beams with different spatial samplings and for different values of the f-number (f/#). The results of these simulations show that push beams with smaller f/# require a higher spatial sampling rate. The significant amount of acceleration achieved by this approach suggests that shear wave simulations with the Green's function approach are ideally suited for high-performance GPUs. Shear wave elasticity imaging determines the mechanical parameters of soft tissue by analyzing measured shear waves induced by an acoustic radiation force. To estimate the shear elasticity value, the widely used time-of-flight method calculates the correlation between shear wave particle velocities at adjacent lateral observation points. Although this method provides accurate estimates of the shear elasticity in purely elastic media, our experience suggests that the time-of-flight (TOF) method consistently overestimates the shear elasticity values in viscoelastic media because the combined effects of diffraction, attenuation, and dispersion are not considered. To address this problem, we have developed an approach that directly accounts for all
Orientational dynamics of a triaxial ellipsoid in simple shear flow: Influence of inertia.
Rosén, Tomas; Kotsubo, Yusuke; Aidun, Cyrus K; Do-Quang, Minh; Lundell, Fredrik
2017-07-01
The motion of a single ellipsoidal particle in simple shear flow can provide valuable insights toward understanding suspension flows with nonspherical particles. Previously, extensive studies have been performed on the ellipsoidal particle with rotational symmetry, a so-called spheroid. The nearly prolate ellipsoid (one major and two minor axes of almost equal size) is known to perform quasiperiodic or even chaotic orbits in the absence of inertia. With small particle inertia, the particle is also known to drift toward this irregular motion. However, it is not previously understood what effects from fluid inertia could be, which is of highest importance for particles close to neutral buoyancy. Here, we find that fluid inertia is acting strongly to suppress the chaotic motion and only very weak fluid inertia is sufficient to stabilize a rotation around the middle axis. The mechanism responsible for this transition is believed to be centrifugal forces acting on fluid, which is dragged along with the rotational motion of the particle. With moderate fluid inertia, it is found that nearly prolate triaxial particles behave similarly to the perfectly spheroidal particles. Finally, we also are able to provide predictions about the stable rotational states for the general triaxial ellipsoid in simple shear with weak inertia.
Augmentative effect of pulsatility on the wall shear stress in tube flow.
Nakata, M; Tatsumi, E; Tsukiya, T; Taenaka, Y; Nishimura, T; Nishinaka, T; Takano, H; Masuzawa, T; Ohba, K
1999-08-01
Wall shear stress (WSS) has been considered to play an important role in the physiological and metabolic functions of the vascular endothelial cells. We investigated the effects of the pulse rate and the maximum flow rate on the WSS to clarify the influence of pulsatility. Water was perfused in a 1/2 inch transparent straight cylinder with a nonpulsatile centrifugal pump and a pulsatile pneumatic ventricular assist device (VAD). In nonpulsatile flow (NF), the flow rate was changed 1 to 6 L/min by 1 L/min increments to obtain standard values of WSS at each flow rate. In pulsatile flow (PF), the pulse rate was controlled at 40, 60, and 80 bpm, and the maximum flow rate was varied from 3.3 to 12.0 L/min while the mean flow rate was kept at 3 L/min. The WSS was estimated from the velocity profile at measuring points using the laser illuminated fluorescence method. In NF, the WSS was 12.0 dyne/cm2 at 3 L/min and 33.0 dyne/cm2 at 6 L/min. In PF, the pulse rate change with the same mean, and the maximum flow rate did not affect WSS. On the other hand, the increase in the maximum flow rate at the constant mean flow rate of 3 L/min augmented the mean WSS from 13.1 to 32.9 dyne/cm2. We concluded that the maximum flow rate exerted a substantial augmentative effect on WSS, and the maximum flow rate was a dominant factor of pulsatility in this effect.
Approximate Dispersion Relations for Waves on Arbitrary Shear Flows
Ellingsen, S. À.; Li, Y.
2017-12-01
An approximate dispersion relation is derived and presented for linear surface waves atop a shear current whose magnitude and direction can vary arbitrarily with depth. The approximation, derived to first order of deviation from potential flow, is shown to produce good approximations at all wavelengths for a wide range of naturally occuring shear flows as well as widely used model flows. The relation reduces in many cases to a 3-D generalization of the much used approximation by Skop (1987), developed further by Kirby and Chen (1989), but is shown to be more robust, succeeding in situations where the Kirby and Chen model fails. The two approximations incur the same numerical cost and difficulty. While the Kirby and Chen approximation is excellent for a wide range of currents, the exact criteria for its applicability have not been known. We explain the apparently serendipitous success of the latter and derive proper conditions of applicability for both approximate dispersion relations. Our new model has a greater range of applicability. A second order approximation is also derived. It greatly improves accuracy, which is shown to be important in difficult cases. It has an advantage over the corresponding second-order expression proposed by Kirby and Chen that its criterion of accuracy is explicitly known, which is not currently the case for the latter to our knowledge. Our second-order term is also arguably significantly simpler to implement, and more physically transparent, than its sibling due to Kirby and Chen.Plain Language SummaryIn order to answer key questions such as how the ocean surface affects the climate, erodes the coastline and transports nutrients, we must understand how waves move. This is not so easy when depth varying currents are present, as they often are in coastal waters. We have developed a modeling tool for accurately predicting wave properties in such situations, ready for use, for example, in the complex oceanographic computer models. Our
Hooyer, T.S.; Iverson, N.R.; Lagroix, F.; Thomason, J.F.
2008-01-01
Wet-based portions of ice sheets may move primarily by shearing their till beds, resting in high sediment fluxes and the development of subglacial landforms. This model of glacier movement, which requires high bed shear strains, can be tested using till microstructural characteristics that evolve during till deformation. Here we examine the development of magnetic fabric using a ring shear device to defom two Wisconsin-age basal tills to shear strains as high as 70. Hysteresis experiments and the dependence of magnetic susceptibility of these tills on temperature demonstrate that anisotropy of magnetic susceptibility (AMS) develops during shear due to the rotation of primarily magnetite particles that are silt sized or smaller. At moderate shear strains (???6-25), principal axes of maximum magnetic susceptibility develop a strong fabric (S1 eignevalues of 0.83-0.96), without further strengthening at higher strains, During deformation, directions of maximum susceptibility cluster strongly in the direction of shear and plunge 'up-glacier,' consistent with the behavior of pebbles and sand particles studied in earlier experiments. In contrast, the magnitude of AMS does not vary systematically with strain and is small relative to its variability among samples; this is because most magnetite grains are contained as inclusions in larger particles and hence do not align during shear. Although processes other than pervasive bed deformation may result in strong flow parallel fabrics, AMS fabrics provide a rapid and objective means of identifying basal tills that have not been sheared sufficiently to be compatible with the bed deformation model. Copyright 2008 by the American Geophysical Union.
Freezing of a colloidal liquid subject to shear flow
International Nuclear Information System (INIS)
Bagchi, B.; Thirumalai, D.
1988-01-01
A nonequilibrium generalization of the density-functional theory of freezing is proposed to investigate the shear-induced first-order phase transition in colloidal suspensions. It is assumed that the main effect of a steady shear is to break the symmetry of the structure factor of the liquid and that for small shear rate, the phenomenon of a shear-induced order-disorder transition may be viewed as an equilibrium phase transition. The theory predicts that the effective density at which freezing takes place increases with shear rate. The solid (which is assumed to be a bcc lattice) formed upon freezing is distorted and specifically there is less order in one plane compared with the order in the other two perpendicular planes. It is shown that there exists a critical shear rate above which the colloidal liquid does not undergo a transition to an ordered (or partially ordered) state no matter how large the density is. Conversely, above the critical shear rate an initially formed bcc solid always melts into an amorphous or liquidlike state. Several of these predictions are in qualitative agreement with the light-scattering experiments of Ackerson and Clark. The limitations as well as possible extensions of the theory are also discussed
Wang, C. R.; Hingst, W. R.; Porro, A. R.
1991-01-01
The properties of 2-D shock wave/turbulent boundary layer interaction flows were calculated by using a compressible turbulent Navier-Stokes numerical computational code. Interaction flows caused by oblique shock wave impingement on the turbulent boundary layer flow were considered. The oblique shock waves were induced with shock generators at angles of attack less than 10 degs in supersonic flows. The surface temperatures were kept at near-adiabatic (ratio of wall static temperature to free stream total temperature) and cold wall (ratio of wall static temperature to free stream total temperature) conditions. The computational results were studied for the surface heat transfer, velocity temperature correlation, and turbulent shear stress in the interaction flow fields. Comparisons of the computational results with existing measurements indicated that (1) the surface heat transfer rates and surface pressures could be correlated with Holden's relationship, (2) the mean flow streamwise velocity components and static temperatures could be correlated with Crocco's relationship if flow separation did not occur, and (3) the Baldwin-Lomax turbulence model should be modified for turbulent shear stress computations in the interaction flows.
Mean wall-shear stress measurements using the micro-pillar shear-stress sensor MPS3
International Nuclear Information System (INIS)
Große, S; Schröder, W
2008-01-01
A new sensor to measure the mean turbulent wall-shear stress in turbulent flows is described. The wall-shear stress sensor MPS 3 has been tested in a well-defined fully developed turbulent pipe flow at Reynolds numbers Re b based on the bulk velocity U b and the pipe diameter D in the range of Re b = 10 000–20 000. The results demonstrate a convincing agreement of the mean wall-shear stress obtained with the new sensor technique with analytical and experimental results from the literature. The sensor device consists of a flexible micro-pillar that extends from the wall into the viscous sublayer. Bending due to the exerting fluid forces, the pillar-tip deflection serves as a measure for the local wall-shear stress. The sensor concept, calibration techniques, the achievable accuracy and error estimates, the fields of application and the sensor limits will be discussed. Furthermore, a first estimate of the pillar dynamic response will be derived showing the potential of the sensor to also measure the turbulent fluctuating wall-shear stress
Adhesion behavior of endothelial progenitor cells to endothelial cells in simple shear flow
Gong, Xiao-Bo; Li, Yu-Qing; Gao, Quan-Chao; Cheng, Bin-Bin; Shen, Bao-Rong; Yan, Zhi-Qiang; Jiang, Zong-Lai
2011-12-01
The adhesion of endothelial progenitor cells (EPCs) on endothelial cells (ECs) is one of the critical physiological processes for the regenesis of vascular vessels and the prevention of serious cardiovascular diseases. Here, the rolling and adhesion behavior of EPCs on ECs was studied numerically. A two-dimensional numerical model was developed based on the immersed boundary method for simulating the rolling and adhesion of cells in a channel flow. The binding force arising from the catch bond of a receptor and ligand pair was modeled with stochastic Monte Carlo method and Hookean spring model. The effect of tumor necrosis factor alpha (TNF- α) on the expression of the number of adhesion molecules in ECs was analyzed experimentally. A flow chamber system with CCD camera was set up to observe the top view of the rolling of EPCs on the substrate cultivated with ECs. Numerical results prove that the adhesion of EPC on ECs is closely related to membrane stiffness of the cell and shear rate of the flow. It also suggests that the adhesion force between EPC and EC by P-selectin glycoprotein ligand-1 only is not strong enough to bond the cell onto vessel walls unless contributions of other catch bond are considered. Experimental results demonstrate that TNF- α enhanced the expressions of VCAM, ICAM, P-selectin and E-selectin in ECs, which supports the numerical results that the rolling velocity of EPC on TNF- α treated EC substrate decreases obviously compared with its velocity on the untreated one. It is found that because the adhesion is affected by both the rolling velocity and the deformability of the cell, an optimal stiffness of EPC may exist at a given shear rate of flow for achieving maximum adhesion rates.
The rheological properties of shear thickening fluid reinforced with SiC nanowires
Directory of Open Access Journals (Sweden)
Jianhao Ge
Full Text Available The rheological properties of shear thickening fluid (STF reinforced with SiC nanowires were investigated in this paper. Pure STF consists of 56 vol% silica nano-particles and polyethylene glycol 400 (PEG 400 solvent was fabricated; and a specific amount of SiC nanowires were dispersed into this pure STF, and then the volume fraction of PEG400 was adjusted to maintain the volume fraction of solid phase in the STF at a constant of 56%. The results showed there was almost 30% increase in the initial and shear thickening viscosity of the STF reinforced with SiC nanowires compared to the pure STF. Combining with the hydrodynamic cluster theory, the effect of the mechanism of SiC nanowire on the viscosity of STF was discussed, and based on the experimental results, an analytical model of viscosity was used to describe the rheological properties of STF, which agreed with the experimental results. Keywords: Shear thickening fluid (STF, Nanowire, Rheology, Viscosity, Analytical model
Ren, Zhiqiang; Harshe, Yogesh M; Lattuada, Marco
2015-06-02
In this work we build on our previous paper (Harshe, Y. M.; Lattuada, M. Langmuir 2012, 28, 283-292) and compute the breakage rate of colloidal aggregates under the effect of shear forces by means of Stokesian dynamics simulations. A library of clusters made of identical spherical particles covering a broad range of masses and fractal dimension values (from 1.8 to 3.0) was generated by means of a combination of several Monte Carlo methods. DLVO theory has been used to describe the interparticle interactions, and contact forces have been introduced by means of the discrete element method. The aggregate breakage process was investigated by exposing them to well-defined shear forces, generated under both simple shear and uniaxial extensional flow conditions, and by recording the time required to reach the first breakage event. It has been found that the breakage rate of clusters was controlled by the potential well between particles as described by DLVO theory. A semiempirical Arrhenius-type exponential equation that relates the potential well to the breakage rate has been used to fit the simulation results. The dependence of the breakage process on the radius of gyration, on the external shear strength, and on the fractal dimension has been obtained, providing a very general relationship for the breakage rate of clusters. It was also found that the fragment mass distribution is insensitive to the presence of electrostatic repulsive interactions. We also clarify the physical reason for the large difference in the breakage rate of clusters between simple shear and the uniaxial extensional flow using a criterion based on the energy dissipation rate. Finally, in order to answer the question of the minimum cluster size that can break under simple shear conditions, a critical rotation number has been introduced, expressing the maximum number of rotations that a cluster exposed to simple shear could sustain before breakage.
Nonlinear modeling and testing of magneto-rheological fluids in low shear rate squeezing flows
International Nuclear Information System (INIS)
Farjoud, Alireza; Ahmadian, Mehdi; Craft, Michael; Mahmoodi, Nima; Zhang, Xinjie
2011-01-01
A novel analytical investigation of magneto-rheological (MR) fluids in squeezing flows is performed and the results are validated with experimental test data. The squeeze flow of MR fluids has recently been of great interest to researchers. This is due to the large force capacity of MR fluids in squeeze mode compared to other modes (valve and shear modes), which makes the squeeze mode appropriate for a wide variety of applications such as impact dampers and engine mounts. Tested MR fluids were capable of providing a large range of controllable force along a short stroke in squeeze mode. A mathematical model was developed using perturbation techniques to predict closed-form solutions for velocity field, shear rate distribution, pressure distribution and squeeze force. Therefore, the obtained solutions greatly help with the design process of intelligent devices that use MR fluids in squeeze mode. The mathematical model also reduces the need for complicated and computationally expensive numerical simulations. The analytical results are validated by performing experimental tests on a novel MR device called an 'MR pouch' in an MR squeeze mode rheometer, both designed and built at CVeSS
Motion of cells sedimenting on a solid surface in a laminar shear flow.
Tissot, O; Pierres, A; Foa, C; Delaage, M; Bongrand, P
1992-01-01
Cell adhesion often occurs under dynamic conditions, as in flowing blood. A quantitative understanding of this process requires accurate knowledge of the topographical relationships between the cell membrane and potentially adhesive surfaces. This report describes an experimental study made on both the translational and rotational velocities of leukocytes sedimenting of a flat surface under laminar shear flow. The main conclusions are as follows: (a) Cells move close to the wall with constant velocity for several tens of seconds. (b) The numerical values of translational and rotational velocities are inconsistent with Goldman's model of a neutrally buoyant sphere in a laminar shear flow, unless a drag force corresponding to contact friction between cells and the chamber floor is added. The phenomenological friction coefficient was 7.4 millinewton.s/m. (c) Using a modified Goldman's theory, the width of the gap separating cells (6 microns radius) from the chamber floor was estimated at 1.4 micron. (d) It is shown that a high value of the cell-to-substrate gap may be accounted for by the presence of cell surface protrusions of a few micrometer length, in accordance with electron microscope observations performed on the same cell population. (e) In association with previously reported data (Tissot, O., C. Foa, C. Capo, H. Brailly, M. Delaage, and P. Bongrand. 1991. Biocolloids and Biosurfaces. In press), these results are consistent with the possibility that cell-substrate attachment be initiated by the formation of a single molecular bond, which might be considered as the rate limiting step.
Collisional Rayleigh-Taylor instability and shear-flow in equatorial Spread-F plasma
Directory of Open Access Journals (Sweden)
N. Chakrabarti
2003-05-01
Full Text Available Collisional Rayleigh-Taylor (RT instability is considered in the bottom side of the equatorial F-region. By a novel nonmodal calculation it is shown that for an applied shear flow in equilibrium, the growth of the instability is considerably reduced. Finite but small amounts of diffusion enhances the stabilization process. The results may be relevant to the observations of long-lived irregularities at the bottom-side of the F-layer.Key words. Ionosphere (ionospheric irregularities, equatorial ionosphere, plasma waves and instabilities
Nordgaard, Håvard; Swillens, Abigail; Nordhaug, Dag; Kirkeby-Garstad, Idar; Van Loo, Denis; Vitale, Nicola; Segers, Patrick; Haaverstad, Rune; Lovstakken, Lasse
2010-12-01
Competitive flow from native coronary vessels is considered a major factor in the failure of coronary bypass grafts. However, the pathophysiological effects are not fully understood. Low and oscillatory wall shear stress (WSS) is known to induce endothelial dysfunction and vascular disease, like atherosclerosis and intimal hyperplasia. The aim was to investigate the impact of competitive flow on WSS in mammary artery bypass grafts. Using computational fluid dynamics, WSS was calculated in a left internal mammary artery (LIMA) graft to the left anterior descending artery in a three-dimensional in vivo porcine coronary artery bypass graft model. The following conditions were investigated: high competitive flow (non-significant coronary lesion), partial competitive flow (significant coronary lesion), and no competitive flow (totally occluded coronary vessel). Time-averaged WSS of LIMA at high, partial, and no competitive flow were 0.3-0.6, 0.6-3.0, and 0.9-3.0 Pa, respectively. Further, oscillatory WSS quantified as the oscillatory shear index (OSI) ranged from (maximum OSI = 0.5 equals zero net WSS) 0.15 to 0.35, OSI similar to the no competitive flow condition. Graft flow is highly dependent on the degree of competitive flow. High competitive flow was found to produce unfavourable WSS consistent with endothelial dysfunction and subsequent graft narrowing and failure. Partial competitive flow, however, may be better tolerated as it was found to be similar to the ideal condition of no competitive flow.
International Nuclear Information System (INIS)
Xu, Y.; Shesterikov, I.; Berte, M.; Dumortier, P.; Van Schoor, M.; Vergote, M.; Hidalgo, C.; Krämer-Flecken, A.; Koslowski, R.
2013-01-01
Direct measurements of residual stress (force) have been executed at the edge of the TEXTOR tokamak using multitip Langmuir and Mach probes, together with counter-current NBI torque to balance the existing toroidal rotation. Substantial residual stress and force have been observed at the plasma boundary, confirming the existence of a finite residual stress as possible mechanisms to drive the intrinsic toroidal rotation. In low-density discharges, the residual stress displays a quasi-linear dependence on the local pressure gradient, consistent with theoretical predictions. At high-density shots the residual stress and torque are strongly suppressed. The results show close correlation between the residual stress and the E r × B flow shear rate, suggesting a minimum threshold of the E × B flow shear required for the k ∥ symmetry breaking. These findings provide the first experimental evidence of the role of E r × B sheared flows in the development of residual stresses and intrinsic rotation. (letter)
Determination of wall shear stress from mean velocity and Reynolds shear stress profiles
Volino, Ralph J.; Schultz, Michael P.
2018-03-01
An analytical method is presented for determining the Reynolds shear stress profile in steady, two-dimensional wall-bounded flows using the mean streamwise velocity. The method is then utilized with experimental data to determine the local wall shear stress. The procedure is applicable to flows on smooth and rough surfaces with arbitrary pressure gradients. It is based on the streamwise component of the boundary layer momentum equation, which is transformed into inner coordinates. The method requires velocity profiles from at least two streamwise locations, but the formulation of the momentum equation reduces the dependence on streamwise gradients. The method is verified through application to laminar flow solutions and turbulent DNS results from both zero and nonzero pressure gradient boundary layers. With strong favorable pressure gradients, the method is shown to be accurate for finding the wall shear stress in cases where the Clauser fit technique loses accuracy. The method is then applied to experimental data from the literature from zero pressure gradient studies on smooth and rough walls, and favorable and adverse pressure gradient cases on smooth walls. Data from very near the wall are not required for determination of the wall shear stress. Wall friction velocities obtained using the present method agree with those determined in the original studies, typically to within 2%.
Roosjen, A; Boks, NP; van der Mei, HC; Busscher, HJ; Norde, W
2005-01-01
Microbial adhesion to surfaces often occurs despite high wall shear rates acting on the adhering microorganisms. In this paper, we compare the wall shear rates needed to prevent microbial adhesion to bare glass and poly(ethylene oxide) (PEO)-brush coated glass in a parallel plate flow chamber.
Barranco, Joseph
2006-03-01
We have developed a three-dimensional (3D) spectral hydrodynamic code to study vortex dynamics in rotating, shearing, stratified systems (eg, the atmosphere of gas giant planets, protoplanetary disks around newly forming protostars). The time-independent background state is stably stratified in the vertical direction and has a unidirectional linear shear flow aligned with one horizontal axis. Superposed on this background state is an unsteady, subsonic flow that is evolved with the Euler equations subject to the anelastic approximation to filter acoustic phenomena. A Fourier-Fourier basis in a set of quasi-Lagrangian coordinates that advect with the background shear is used for spectral expansions in the two horizontal directions. For the vertical direction, two different sets of basis functions have been implemented: (1) Chebyshev polynomials on a truncated, finite domain, and (2) rational Chebyshev functions on an infinite domain. Use of this latter set is equivalent to transforming the infinite domain to a finite one with a cotangent mapping, and using cosine and sine expansions in the mapped coordinate. The nonlinear advection terms are time integrated explicitly, whereas the Coriolis force, buoyancy terms, and pressure/enthalpy gradient are integrated semi- implicitly. We show that internal gravity waves can be damped by adding new terms to the Euler equations. The code exhibits excellent parallel performance with the Message Passing Interface (MPI). As a demonstration of the code, we simulate vortex dynamics in protoplanetary disks and the Kelvin-Helmholtz instability in the dusty midplanes of protoplanetary disks.
A feedback control model for network flow with multiple pure time delays
Press, J.
1972-01-01
A control model describing a network flow hindered by multiple pure time (or transport) delays is formulated. Feedbacks connect each desired output with a single control sector situated at the origin. The dynamic formulation invokes the use of differential difference equations. This causes the characteristic equation of the model to consist of transcendental functions instead of a common algebraic polynomial. A general graphical criterion is developed to evaluate the stability of such a problem. A digital computer simulation confirms the validity of such criterion. An optimal decision making process with multiple delays is presented.
Thin liquid films with time-dependent chemical reactions sheared by an ambient gas flow
Bender, Achim; Stephan, Peter; Gambaryan-Roisman, Tatiana
2017-08-01
Chemical reactions in thin liquid films are found in many industrial applications, e.g., in combustion chambers of internal combustion engines where a fuel film can develop on pistons or cylinder walls. The reactions within the film and the turbulent outer gas flow influence film stability and lead to film breakup, which in turn can lead to deposit formation. In this work we examine the evolution and stability of a thin liquid film in the presence of a first-order chemical reaction and under the influence of a turbulent gas flow. Long-wave theory with a double perturbation analysis is used to reduce the complexity of the problem and obtain an evolution equation for the film thickness. The chemical reaction is assumed to be slow compared to film evolution and the amount of reactant in the film is limited, which means that the reaction rate decreases with time as the reactant is consumed. A linear stability analysis is performed to identify the influence of reaction parameters, material properties, and environmental conditions on the film stability limits. Results indicate that exothermic reactions have a stabilizing effect whereas endothermic reactions destabilize the film and can lead to rupture. It is shown that an initially unstable film can become stable with time as the reaction rate decreases. The shearing of the film by the external gas flow leads to the appearance of traveling waves. The shear stress magnitude has a nonmonotonic influence on film stability.
Prominence Bubble Shear Flows and the Coupled Kelvin-Helmholtz — Rayleigh-Taylor Instability
Berger, Thomas; Hillier, Andrew
2017-08-01
Prominence bubbles are large arched structures that rise from below into quiescent prominences, often growing to heights on the order of 10 Mm before going unstable and generating plume upflows. While there is general agreement that emerging flux below pre-existing prominences causes the structures, there is lack of agreement on the nature of the bubbles and the cause of the instability flows. One hypothesis is that the bubbles contain coronal temperature plasma and rise into the prominence above due to both magnetic and thermal buoyancy, eventually breaking down via a magnetic Rayleigh-Taylor (RT) instability to release hot plasma and magnetic flux and helicity into the overlying coronal flux rope. Another posits that the bubbles are actually just “arcades” in the prominence indicating a magnetic separator line between the bipole and the prominence fields with the observed upflows and downflows caused by reconnection along the separator. We analyze Hinode/SOT, SDO/AIA, and IRIS observations of prominence bubbles, focusing on characteristics of the bubble boundary layers that may discriminate between the two hypotheses. We find speeds on the order of 10 km/s in prominence plasma downflows and lateral shear flows along the bubble boundary. Inflows to the boundary gradually increase the thickness and brightness of the layer until plasma drains from there, apparently around the dome-like bubble domain. In one case, shear flow across the bubble boundary develops Kelvin-Helmholtz (KH) vortices that we use to infer flow speeds in the low-density bubble on the order of 100 km/sec. IRIS spectra indicate that plasma flows on the bubble boundary at transition region temperatures achieve Doppler speeds on the order of 50 km/s, consistent with this inference. Combined magnetic KH-RT instability analysis leads to flux density estimates of 10 G with a field angle of 30° to the prominence, consistent with vector magnetic field measurements. In contrast, we find no evidence
A mathematical model for fluid shear-sensitive 3D tissue construct development.
Liu, Dan; Chua, Chee-Kai; Leong, Kah-Fai
2013-01-01
This research studies dynamic culture for 3D tissue construct development with computational fluid dynamics. It proposes a mathematical model to evaluate the impact of flow rates and flow shear stress on cell growth in 3D constructs under perfusion. The modeling results show that dynamic flow, even at flow rate as low as 0.002 cm/s, can support much better mass exchange, higher cell number, and more even cell and nutrient distribution compared to static culture. Higher flow rate can further improve nutrient supply and mass exchange in the construct, promoting better nutritious environment and cell proliferation compared to lower flow rate. In addition, consideration of flow shear stress predicts much higher cell number in the construct compared to that without shear consideration. While the nutrient can dominate shear stress in influencing cell proliferation, the shear effect increases with flow rate. The proposed model helps tissue engineers better understand the cell-flow relationship at the molecular level during dynamic culture.
Edge-Induced Shear Banding in Entangled Polymeric Fluids.
Hemingway, Ewan J; Fielding, Suzanne M
2018-03-30
Despite decades of research, the question of whether solutions and melts of highly entangled polymers exhibit shear banding as their steady state response to a steadily imposed shear flow remains controversial. From a theoretical viewpoint, an important unanswered question is whether the underlying constitutive curve of shear stress σ as a function of shear rate γ[over ˙] (for states of homogeneous shear) is monotonic, or has a region of negative slope, dσ/dγ[over ˙]<0, which would trigger banding. Attempts to settle the question experimentally via velocimetry of the flow field inside the fluid are often confounded by an instability of the free surface where the sample meets the outside air, known as "edge fracture." Here we show by numerical simulation that in fact even only very modest edge disturbances-which are the precursor of full edge fracture but might well, in themselves, go unnoticed experimentally-can cause strong secondary flows in the form of shear bands that invade deep into the fluid bulk. Crucially, this is true even when the underlying constitutive curve is monotonically increasing, precluding true bulk shear banding in the absence of edge effects.
Havens, Vance; Ragaller, Dana
1988-01-01
Management of two-phase fluid and control of the heat transfer process in microgravity is a technical challenge that must be addressed for an orbital Organic Rankine Cycle (ORC) application. A test program was performed in 1-g that satisfactorily demonstrated the two-phase management capability of the rotating fluid management device (RFMD) and shear-flow condenser. Operational tests of the RFMD and shear flow condenser in adverse gravity orientations, confirmed that the centrifugal forces in the RFMD and the shear forces in the condenser were capable of overcoming gravity forces. In a microgravity environment, these same forces would not have to compete against gravity and would therefore be dominant. The specific test program covered the required operating range of the Space Station Solar Dynamic Rankine Cycle power system. Review of the test data verified that: fluid was pumped from the RFMD in all attitudes; subcooled states in the condenser were achieved; condensate was pushed uphill against gravity; and noncondensible gases were swept through the condenser.
International Nuclear Information System (INIS)
Marín-Santibáñez, Benjamín M.; Pérez-González, José; Rodríguez-González, Francisco
2014-01-01
The origin of shear thickening in an equimolar semidilute wormlike micellar solution of cetylpyridinium chloride and sodium salicylate was investigated in this work by using Couette rheometry, flow visualization, and capillary Rheo-particle image velocimetry. The use of the combined methods allowed the discovery of gradient shear banding flow occurring from a critical shear stress and consisting of two main bands, one isotropic (transparent) of high viscosity and one structured (turbid) of low viscosity. Mechanical rheometry indicated macroscopic shear thinning behavior in the shear banding regime. However, local velocimetry showed that the turbid band increased its viscosity along with the shear stress, even though barely reached the value of the viscosity of the isotropic phase. This shear band is the precursor of shear induced structures that subsequently give rise to the average increase in viscosity or apparent shear thickening of the solution. Further increase in the shear stress promoted the growing of the turbid band across the flow region and led to destabilization of the shear banding flow independently of the type of rheometer used, as well as to vorticity banding in Couette flow. At last, vorticity banding disappeared and the flow developed elastic turbulence with chaotic dynamics
Ultra-Pure Water and Extremophilic Bacteria interactions with Germanium Surfaces
Sah, Vasu R.
Supported by a consortium of semiconductor industry sponsors, an international "TIE" project among 5 National Science Foundation (NSF) Industry/university Cooperative Research Centers discovered that a particular extremophilic microbe, Pseudomonas syzygii, persists in the UltraPure Water (UPW) supplies of chip fabrication facilities (FABs) and can bio-corrode germanium wafers to produce microbe-encased optically transparent crystals. Considered as potentially functional "biochips", this investigation explored mechanisms for the efficient and deliberate production of such microbe-germania adducts as a step toward later testing of their properties as sensors or switches in bioelectronic or biophotonic circuits. Recirculating UPW (Ultra-Pure Water) and other purified water, laminar-flow loops were developed across 50X20x1mm germanium (Ge) prisms, followed by subsequent examination of the prism surfaces using Multiple Attenuated Internal Reflection InfraRed (MAIR-IR) spectroscopy, Contact Potential measurements, Differential Interference Contrast Light Microscopy (DICLM), Scanning Electron Microscopy (SEM), Energy Dispersive X-Ray Analysis (EDS), and Electron Spectroscopy for Chemical Analysis (ESCA; XPS). P. syzygii cultures originally obtained from a working FAB at University of Arizona were successfully grown on R2A minimal nutrient media. They were found to be identical to the microbes in stored UPW from the same facility, such microbes routinely capable of nucleation and entrapment within GeO2 crystals on the Ge flow surfaces. Optimum flow rates and exposure times were 1 ml/minute (3.2 s-1 shear rate) for 4 days at room temperature, producing densest crystal arrays at the prism central zones 2-3 cm from the flow inlets. Other flow rates and exposure times have higher shear rate which induces a different nucleation mechanism and saturation of crystal formation. Nucleation events began with square and circular oxide deposits surrounding active attached bacteria
Air gun near the sea floor as shear-wave source?
Drijkoningen, G.G.; Dieulangard, D.; Holicki, M.E.
2015-01-01
The feasibility of using an air gun near the sea floor as shear-wave source has been investigated. With an air gun near the sea floor, an evanescent P-wave in the water becomes a propagating S-wave in the sea floor, such that it seems that a pure shear-wave source has been used at the sea floor.
Zhang, Yuzhou; Xu, Junbo; He, Xianfeng
2018-07-01
The behaviour of a single droplet in shear flow is a fundamental problem in immiscible liquid-liquid multiphase fluid systems. In this article, the deformation and inclination angle of single droplet covered with surfactants in shear flow at moderate Reynolds number, when both the inertial effects and interfacial tension are the key governing factors, were simulated by dissipative particle dynamics (DPD). Weber number We was adopted to indicate the force state of the droplet and a linear relationship between the deformation parameter D and We was found when Reynolds number Re is about 1-10, which is similar to the relation of D and Capillary number Ca when Re ≪ 1. When the surfactant concentration is lower than the critical micelle concentration (CMC), the distribution of surfactants, the droplet inclination angle θ and the droplet deformation parameter D were investigated at different surfactant density at interface ds and shear rate ?. When the droplet size is close to the characteristic size of surfactant molecules, phase interfaces of water in oil (W/O) and oil in water (O/W) systems have different microstructures, which result in differences in the surfactant distribution, the droplet inclination angle and deformation of the two systems.
Ballooning mode stabilization by moderate sheared rotation
International Nuclear Information System (INIS)
Hameiri, E.
1996-01-01
Sheared toroidal plasma rotation has been known for some time to have a stabilizing effect on the ballooning modes. A recent calculation showed that a large flow shear, with dΩ/dq of the order of the Alfven toroidal frequency, can stabilize the ballooning modes. This latest result is, in fact, not so optimistic. For observed flows with Mach number of order unity one gets dΩ/dq smaller by a factor O(√β) from the required level (if the flow shear length is of the same order as the magnetic shear length). Moreover, the calculation does not take into account a possibly large transient growth of the mode amplitude due to its Floquet structures We show here that, in fact, there is a general tendency of the ballooning mode to stabilize as soon as the flow shear dΩ/dq exceeds the (O√β smaller) open-quotes slowclose quotes magnetosonic wave frequency. Our analysis is perturbative, where the small parameter is related to the small coupling between the slow and Alfven waves-as is the case in a high aspect-ratio tokamak. (In the perturbation it is important to take the Hamiltonian nature of the governing equations into account.) Moreover, our results apply to the relevant transient growth of the mode amplitude
Atomic simulation of bcc niobium Σ5〈001〉{310} grain boundary under shear deformation
International Nuclear Information System (INIS)
Huang, Bo-Wen; Shang, Jia-Xiang; Liu, Zeng-Hui; Chen, Yue
2014-01-01
The shear behaviors of grain boundaries are investigated using molecular dynamics simulations. The Σ5〈001〉{310} symmetric tilt grain boundary (GB) of body-centered cubic (bcc) Nb is investigated and the simulations are conducted under a series of shear directions at a wide range of temperatures. The results show that the GB shearing along [13 ¯ 0], which is perpendicular to the tilt axis, has a coupled motion behavior. The coupling factor is predicted using Cahn’s model. The critical stress of the coupling motion is found to decrease exponentially with increasing temperature. The GB under shear deformation along the [001 ¯ ] direction, which is parallel to the tilt axis, has a pure sliding behavior at most of the temperatures investigated. The critical stress of sliding is found to be much larger than that of the coupled motion at the same temperature. At very low temperatures, pure sliding is not observed, and dislocation nucleating and extending is found on GBs. We observed mixed behaviors when the shear direction is between [13 ¯ 0] and [001 ¯ ]. The transition region between GB coupled motion and pure sliding is determined. If the shear angles between the shear direction and the tilt axis are larger than a certain value, the GB has a coupled motion behavior similar to the [13 ¯ 0] direction. A GB with a shear angle smaller than the critical angle exhibits mixed mechanisms at low temperatures, such as dislocation, atomic shuffle and GB distortion, whereas for the [001 ¯ ]-like GB pure sliding is the dominating mechanism at high temperatures. The stresses to activate the coupling and gliding motions are analyzed for shear deformations along different directions at various temperatures
Bounded energy states in homogeneous turbulent shear flow - An alternative view
Bernard, P. S.; Speziale, C. G.
1992-01-01
The equilibrium structure of homogeneous turbulent shear flow is investigated from a theoretical standpoint. Existing turbulence models, in apparent agreement with physical and numerical experiments, predict an unbounded exponential time growth of the turbulent kinetic energy and dissipation rate; only the anisotropy tensor and turbulent time scale reach a structural equilibrium. It is shown that if a residual vortex stretching term is maintained in the dissipation rate transport equation, then there can exist equilibrium solutions, with bounded energy states, where the turbulence production is balanced by its dissipation. Illustrative calculations are presented for a k-epsilon model modified to account for net vortex stretching.
Turbulent shear layers in confining channels
Benham, Graham P.; Castrejon-Pita, Alfonso A.; Hewitt, Ian J.; Please, Colin P.; Style, Rob W.; Bird, Paul A. D.
2018-06-01
We present a simple model for the development of shear layers between parallel flows in confining channels. Such flows are important across a wide range of topics from diffusers, nozzles and ducts to urban air flow and geophysical fluid dynamics. The model approximates the flow in the shear layer as a linear profile separating uniform-velocity streams. Both the channel geometry and wall drag affect the development of the flow. The model shows good agreement with both particle image velocimetry experiments and computational turbulence modelling. The simplicity and low computational cost of the model allows it to be used for benchmark predictions and design purposes, which we demonstrate by investigating optimal pressure recovery in diffusers with non-uniform inflow.
Grigioni, Mauro; Daniele, Carla; D'Avenio, Giuseppe; Barbaro, Vincenzo
2002-05-01
Turbulent flow generated by prosthetic devices at the bloodstream level may cause mechanical stress on blood particles. Measurement of the Reynolds stress tensor and/or some of its components is a mandatory step to evaluate the mechanical load on blood components exerted by fluid stresses, as well as possible consequent blood damage (hemolysis or platelet activation). Because of the three-dimensional nature of turbulence, in general, a three-component anemometer should be used to measure all components of the Reynolds stress tensor, but this is difficult, especially in vivo. The present study aimed to derive the maximum Reynolds shear stress (RSS) in three commercially available prosthetic heart valves (PHVs) of wide diffusion, starting with monodimensional data provided in vivo by echo Doppler. Accurate measurement of PHV flow field was made using laser Doppler anemometry; this provided the principal turbulence quantities (mean velocity, root-mean-square value of velocity fluctuations, average value of cross-product of velocity fluctuations in orthogonal directions) needed to quantify the maximum turbulence-related shear stress. The recorded data enabled determination of the relationship, the Reynolds stresses ratio (RSR) between maximum RSS and Reynolds normal stress in the main flow direction. The RSR was found to be dependent upon the local structure of the flow field. The reported RSR profiles, which permit a simple calculation of maximum RSS, may prove valuable during the post-implantation phase, when an assessment of valve function is made echocardiographically. Hence, the risk of damage to blood constituents associated with bileaflet valve implantation may be accurately quantified in vivo.
Orr, Matthew W.
This investigation concerns the design, analysis, and initial testing of a new, two-component wall shear gage for 3D, high-temperature flows. This gage is a direct-measuring, non-nulling design with a round head surrounded by a small gap. Two flexure wheels are used to allow small motions of the floating head. Fiber-optic displacement sensors measure how far the polished faces of counterweights on the wheels move in relation to a fixed housing as the primary measurement system. No viscous damping was required. The gage has both fiber-optic instrumentation and strain gages mounted on the flexures for validation of the newer fiber optics. The sensor is constructed of Haynes RTM 230RTM, a high-temperature nickel alloy. The gage housing is made of 316 stainless steel. All components of the gage in pure fiber-optic form can survive to a temperature of 1073 K. The bonding methods of the backup strain gages limit their maximum temperature to 473 K. The dynamic range of the gage is from 0--500 Pa (0--10g) and higher shears can be measured by changing the floating head size. Extensive use of finite element modeling was critical to the design and analysis of the gage. Static structural, modal, and thermal analyses were performed on the flexures using the ANSYS finite element package. Static finite element analysis predicted the response of the flexures to a given load, and static calibrations using a direct force method confirmed these results. Finite element modal analysis results were within 16.4% for the first mode and within 30% for the second mode when compared with the experimentally determined modes. Vibration characteristics of the gage were determined from experimental free vibration data after the gage was subjected to an impulse. Uncertainties in the finished geometry make this level of error acceptable. A transient thermal analysis examined the effects of a very high heat flux on the exposed head of the gage. The 100,000 W/m2 heat flux used in this analysis is
Sinha, Kumari Priti; Thaokar, Rochish M.
2018-03-01
Vesicles or biological cells under simultaneous shear and electric field can be encountered in dielectrophoretic devices or designs used for continuous flow electrofusion or electroporation. In this work, the dynamics of a vesicle subjected to simultaneous shear and uniform alternating current (ac) electric field is investigated in the small deformation limit. The coupled equations for vesicle orientation and shape evolution are derived theoretically, and the resulting nonlinear equations are handled numerically to generate relevant phase diagrams that demonstrate the effect of electrical parameters on the different dynamical regimes such as tank treading (TT), vacillating breathing (VB) [called trembling (TR) in this work], and tumbling (TU). It is found that while the electric Mason number (Mn), which represents the relative strength of the electrical forces to the shear forces, promotes the TT regime, the response itself is found to be sensitive to the applied frequency as well as the conductivity ratio. While higher outer conductivity promotes orientation along the flow axis, orientation along the electric field is favored when the inner conductivity is higher. Similarly a switch of orientation from the direction of the electric field to the direction of flow is possible by a mere change of frequency when the outer conductivity is higher. Interestingly, in some cases, a coupling between electric field-induced deformation and shear can result in the system admitting an intermediate TU regime while attaining the TT regime at high Mn. The results could enable designing better dielectrophoretic devices wherein the residence time as well as the dynamical states of the vesicular suspension can be controlled as per the application.
Courbin, L.; Benayad, A.; Panizza, P.
2006-01-01
By means of several rheophysics techniques, we report on an extensive study of the couplings between flow and microstructures in a two-phase fluid made of lamellar (Lα) and sponge (L3) phases. Depending on the nature of the imposed dynamical parameter (stress or shear rate) and on the experimental conditions (brine salinity or temperature), we observe several different structural steady states consisting of either multilamellar droplets (with or without a long range order) or elongated (L3) phase domains. Two different astonishing phenomena, shear-induced phase inversion and relaxation oscillations, are observed. We show that (i) phase inversion is related to a shear-induced topological change between monodisperse multilamellar droplets and elongated structures and (ii) droplet size relaxation oscillations result from a shear-induced change of the surface tension between both coexisting (Lα) and (L3) phases. To explain these relaxation oscillations, we present a phenomenological model and compare its numerical predictions to our experimental results.
International Nuclear Information System (INIS)
Anon.
1987-01-01
This symposium includes topics on wall flows, unsteady flows, scalar and buoyant transport, instrumentation and techniques, combustion, aerodynamic flows, free flows, geophysical flows, complex flows, separated flows, coherent structures, closures, numerical simulation, and two-phase flows. Papers are presented on the effect of favorable pressure gradients on turbulent boundary layers, the models of hydrodynamic resonances in separated shear flows, the transport of passive scalars in a turbulent channel flow, a pulsed hot-wire probe for near-wall measurements, and vortex dynamics in diffusion flames. Consideration is also given to time-dependent structure in wing-body junction flows, bifurcating air jets at high subsonic speeds, the wake of an axisymmetric body with or without tail separation, coherent structures in quasi-geostrophic jets, and separated flow predictions using a new turbulence model. Additional papers are on stochastic estimation of organized structures in turbulent channel flow, a comparative study of eleven models of turbulence, and a numerical study of a stably stratified mixing layer
Approximation of wave action flux velocity in strongly sheared mean flows
Banihashemi, Saeideh; Kirby, James T.; Dong, Zhifei
2017-08-01
Spectral wave models based on the wave action equation typically use a theoretical framework based on depth uniform current to account for current effects on waves. In the real world, however, currents often have variations over depth. Several recent studies have made use of a depth-weighted current U˜ due to [Skop, R. A., 1987. Approximate dispersion relation for wave-current interactions. J. Waterway, Port, Coastal, and Ocean Eng. 113, 187-195.] or [Kirby, J. T., Chen, T., 1989. Surface waves on vertically sheared flows: approximate dispersion relations. J. Geophys. Res. 94, 1013-1027.] in order to account for the effect of vertical current shear. Use of the depth-weighted velocity, which is a function of wavenumber (or frequency and direction) has been further simplified in recent applications by only utilizing a weighted current based on the spectral peak wavenumber. These applications do not typically take into account the dependence of U˜ on wave number k, as well as erroneously identifying U˜ as the proper choice for current velocity in the wave action equation. Here, we derive a corrected expression for the current component of the group velocity. We demonstrate its consistency using analytic results for a current with constant vorticity, and numerical results for a measured, strongly-sheared current profile obtained in the Columbia River. The effect of choosing a single value for current velocity based on the peak wave frequency is examined, and we suggest an alternate strategy, involving a Taylor series expansion about the peak frequency, which should significantly extend the range of accuracy of current estimates available to the wave model with minimal additional programming and data transfer.
The effect of sheared axial flow on nonlinear Z-pinch dynamics
International Nuclear Information System (INIS)
Kassapakis, N.
2000-01-01
A two dimensional Eulerian fluid code has been used to study three problems related to Z-pinch and laser produced plasmas. a) The nonlinear evolution of a localised m=0 MHD mode neck is studied in order to extract some scaling laws for the size and form of the artificial neck. We examine whether the ubiquitous m=0 instability could be beneficially used to assist in the formation of a transient localised dense plasma. The results obtained were in satisfactory agreement with experiments and other theoretical work where available. b) The development of the m=0 instability on a Z-pinch although beneficial in the previous case, is detrimental from a stability point of view and thus to the utilisation of the device as a fusion reactor by itself. This is because the timescales of the instability development are faster than the confinement time needed for fusion to occur. Sheared axial flow is a proposed mechanism for the non-linear saturation of this particular instability. Indeed the linear growth rate also can be substantially reduced. It is hoped that it can inhibit the growth of the instabilities or at least delay their development sufficiently for fusion to take place. The numerical study of the effect of sheared axial flow on the nonlinear dynamics of the Z-pinch carried out, demonstrates that sheared flow with velocity u z z >4 Alfven speed other modes, of the Kelvin-Helmholtz type, are excited which take over from the fastest growing mode in the static case. c) The expansion of the ablated plasma in laser-solid interactions is an important phenomenon for a plethora of reasons one of which is ICF. The simulations were in direct agreement with previous experimental work regarding the bulk properties of the ablation surface. They also provided justification for some assumptions made during the analysis of the observations and helped to confirm the calibration of the diagnostics timewise. The most striking feature of the experiments, namely the density dip on the
The impact of shearing flows on electroactive biofilm formation, structure, and current generation
Jones, A.-Andrew; Buie, Cullen
2016-11-01
A special class of bacteria exist that directly produce electricity. First explored in 1911, these electroactive bacteria catalyze hydrocarbons and transport electrons directly to a metallic electron acceptor forming thicker biofilms than other species. Electroactive bacteria biofilms are thicker because they are not limited by transport of oxygen or other terminal electron acceptors. Electroactive bacteria can produce power in fuel cells. Power production is limited in fuel cells by the bacteria's inability to eliminate protons near the insoluble electron acceptor not utilized in the wild. To date, they have not been successfully evolved or engineered to overcome this limit. This limitation may be overcome by enhancing convective mass transport while maintaining substantial biomass within the biofilm. Increasing convective mass transport increases shear stress. A biofilm may respond to increased shear by changing biomass, matrix, or current production. In this study, a rotating disk electrode is used to separate nutrient from physical stress. This phenomenon is investigated using the model electroactive bacterium Geobacter sulfurreducens at nutrient loads comparable to flow-through microbial fuel cells. We determine biofilm structure experimentally by measuring the porosity and calculating the tortuosity from confocal microscope images. Biofilm adaptation for electron transport is quantified using electrical impedance spectroscopy. Our ultimate objective is a framework relating biofilm thickness, porosity, shear stress and current generation for the optimization of bioelectrochemical systems The Alfred P Sloan Foundation MPHD Program.
Larsen, Laurel G.; Harvey, Judson; Crimaldi, John P.
2009-01-01
Entrainment of sediment by flowing water affects topography, habitat suitability, and nutrient cycling in vegetated floodplains and wetlands, impacting ecosystem evolution and the success of restoration projects. Nonetheless, restoration managers lack simple decision-support tools for predicting shear stresses and sediment redistribution potential in different vegetation communities. Using a field-validated numerical model, we developed state-space diagrams that provide these predictions over a range of water-surface slopes, depths, and associated velocities in Everglades ridge and slough vegetation communities. Diminished bed shear stresses and a consequent decrease in bed sediment redistribution are hypothesized causes of a recent reduction in the topographic and vegetation heterogeneity of this ecosystem. Results confirmed the inability of present-day flows to entrain bed sediment. Further, our diagrams showed bed shear stresses to be highly sensitive to emergent vegetation density and water-surface slope but less sensitive to water depth and periphyton or floating vegetation abundance. These findings suggested that instituting a pulsing flow regime could be the most effective means to restore sediment redistribution to the Everglades. However, pulsing flows will not be sufficient to erode sediment from sloughs with abundant spikerush, unless spikerush density first decreases by natural or managed processes. Our methods provide a novel tool for identifying restoration parameters and performance measures in many types of vegetated aquatic environments where sediment erosion and deposition are involved.
PIV Measurement of Wall Shear Stress and Flow Structures within an Intracranial Aneurysm Model
Chow, Ricky; Sparrow, Eph; Campbell, Gary; Divani, Afshin; Sheng, Jian
2012-11-01
The formation and rupture of an intracranial aneurysm (IA) is a debilitating and often lethal event. Geometric features of the aneurysm bulb and upstream artery, such as bulb size, bulb shape, and curvature of the artery, are two groups of factors that define the flow and stresses within an IA. Abnormal flow stresses are related to rupture. This presentation discusses the development of a quasi-3D PIV technique and its application in various glass models at Re = 275 and 550 to experimentally assess at a preliminary level the impact of geometry and flow rate. Some conclusions are to be drawn linking geometry of the flow domain to rupture risk. The extracted results also serve as the baseline case and as a precursor to a companion presentation by the authors discussing the impact of flow diverters, a new class of medical devices. The PIV experiments were performed in a fully index-matched flow facility, allowing for unobstructed observations over complex geometry. A reconstruction and analysis method was devised to obtain 3D mean wall stress distributions and flow fields. The quasi 3D measurements were reconstructed from orthogonal planes encompassing the entire glass model, spaced 0.4mm apart. Wall shear stresses were evaluated from the near-wall flow viscous stresses.
Bruyn, De P.; Chen, Dongju; Moldenaers, P.; Cardinaels, R.M.
2014-01-01
The effects of geometrical confinement and viscosity ratio on droplet coalescence in shear flow are experimentally investigated by means of a counter rotating parallel plate device, equipped with a microscope. The ratio of droplet diameter to gap spacing is varied between 0.03 and 0.33 to study both
International Nuclear Information System (INIS)
Khabbouchi, Imed; Guellouz, Mohamed Sadok; Tavoularis, Stavros
2009-01-01
Synchronised hot-film and hot-wire measurements were made in the narrower region of a rectangular channel containing a cylindrical rod. The hot-film probe was mounted flush with the channel bottom wall to measure the wall shear stress, while the hot-wire probe was placed at a fixed position, selected in order to easily detect the passage of coherent structures. Mean and rms profiles of the wall shear stress show the influence of the gap to diameter ratio on their respective distributions. The latter presented peculiarities that could only be explained by the presence of coherent structures in the flow between the rod and the wall. Evidence of this presence is seen in the velocity power spectra. The strong influence of the coherent structures on the wall shear stress spatial and temporal distributions is established through velocity-wall shear stress cross-correlations functions and through conditionally sampled measurements
Shear-induced anisotropic plastic flow from body-centred-cubic tantalum before melting
Wu, Christine J.; Söderlind, Per; Glosli, James N.; Klepeis, John E.
2009-03-01
There are many structural and optical similarities between a liquid and a plastic flow. Thus, it is non-trivial to distinguish between them at high pressures and temperatures, and a detailed description of the transformation between these phenomena is crucial to our understanding of the melting of metals at high pressures. Here we report a shear-induced, partially disordered viscous plastic flow from body-centred-cubic tantalum under heating before it melts into a liquid. This thermally activated structural transformation produces a unique, one-dimensional structure analogous to a liquid crystal with the rheological characteristics of Bingham plastics. This mechanism is not specific to Ta and is expected to hold more generally for other metals. Remarkably, this transition is fully consistent with the previously reported anomalously low-temperature melting curve and thus offers a plausible resolution to a long-standing controversy about melting of metals under high pressures.
Self-oscillations of a two-dimensional shear flow with forcing and dissipation
López Zazueta, A.; Zavala Sansón, L.
2018-04-01
Two-dimensional shear flows continuously forced in the presence of dissipative effects are studied by means of numerical simulations. In contrast with most previous studies, the forcing is confined in a finite region, so the behavior of the system is characterized by the long-term evolution of the global kinetic energy. We consider regimes with 1 limited to develop only one vortical instability by choosing an appropriate width of the forcing band. The most relevant regime is found for Reλ > 36, in which the energy maintains a regular oscillation around a reference value. The flow configuration is an elliptical vortex tilted with respect to the forcing axis, which oscillates steadily also. Second, the flow is allowed to develop two Kelvin-Helmholtz billows and eventually more complicated structures. The regimes of the one-vortex case are observed again, except for Reλ > 135. At these values, the energy oscillates chaotically as the two vortices merge, form dipolar structures, and split again, with irregular periodicity. The self-oscillations are explained as a result of the alternate competition between forcing and dissipation, which is verified by calculating the budget terms in the energy equation. The relevance of the forcing-vs.-dissipation competition is discussed for more general flow systems.
Velocity shear generated Alfven waves in electron-positron plasmas
International Nuclear Information System (INIS)
Rogava, A.D.; Berezhiani, V.I.; Mahajan, S.M.
1996-01-01
Linear MHD modes in cold, nonrelativistic electron-positron plasma shear flow are considered. The general set of differential equations, describing the evolution of perturbations in the framework of the nonmodal approach is derived. It is found, that under certain circumstances, the compressional and shear Alfven perturbations may exhibit large transient growth fuelled by the mean kinetic energy of the shear flow. The velocity shear also induces mode coupling allowing the exchange of energy as well as the possibility of a strong mutual transformation of these modes into each other. The compressional Alfven mode may extract the energy of the mean flow and transfer it to the shear Alfven mode via this coupling. The relevance of these new physical effects to provide a better understanding of the laboratory e + e - plasma is emphasized. It is speculated that the shear-induced effects in the electron-positron plasmas could also help solve some astrophysical puzzles (e.g., the generation of pulsar radio emission). Since most astrophysical plasma are relativistic, it is shown that the major results of the study remain valid for weakly sheared relativistic plasmas. (author). 21 refs, 4 figs
International Nuclear Information System (INIS)
Krishnamurthy, V.V.; Bhandar, A.S.; Piao, M.; Zoto, I.; Lane, A.M.; Nikles, D.E.; Wiest, J.M.; Mankey, G.J.; Porcar, L.; Glinka, C.J.
2003-01-01
Small-angle neutron scattering experiments have been performed to investigate orientational ordering of a dispersion of rod-shaped ferromagnetic nanoparticles under the influence of shear flow and static magnetic field. In this experiment, the flow and flow gradient directions are perpendicular to the direction of the applied magnetic field. The scattering intensity is isotropic in zero-shear-rate or zero-applied-field conditions, indicating that the particles are randomly oriented. Anisotropic scattering is observed both in a shear flow and in a static magnetic field, showing that both flow and field induce orientational order in the dispersion. The anisotropy increases with the increase of field and with the increase of shear rate. Three states of order have been observed with the application of both shear flow and magnetic field. At low shear rates, the particles are aligned in the field direction. When increasing shear rate is applied, the particles revert to random orientations at a characteristic shear rate that depends on the strength of the applied magnetic field. Above the characteristic shear rate, the particles align along the flow direction. The experimental results agree qualitatively with the predictions of a mean field model
Shear and shearless Lagrangian structures in compound channels
Enrile, F.; Besio, G.; Stocchino, A.
2018-03-01
Transport processes in a physical model of a natural stream with a composite cross-section (compound channel) are investigated by means of a Lagrangian analysis based on nonlinear dynamical system theory. Two-dimensional free surface Eulerian experimental velocity fields of a uniform flow in a compound channel form the basis for the identification of the so-called Lagrangian Coherent Structures. Lagrangian structures are recognized as the key features that govern particle trajectories. We seek for two particular class of Lagrangian structures: Shear and shearless structures. The former are generated whenever the shear dominates the flow whereas the latter behave as jet-cores. These two type of structures are detected as ridges and trenches of the Finite-Time Lyapunov Exponents fields, respectively. Besides, shearlines computed applying the geodesic theory of transport barriers mark Shear Lagrangian Coherent Structures. So far, the detection of these structures in real experimental flows has not been deeply investigated. Indeed, the present results obtained in a wide range of the controlling parameters clearly show a different behaviour depending on the shallowness of the flow. Shear and Shearless Lagrangian Structures detected from laboratory experiments clearly appear as the flow develops in shallow conditions. The presence of these Lagrangian Structures tends to fade in deep flow conditions.
Pore Fluid Effects on Shear Modulus for Sandstones with Soft Anisotropy
International Nuclear Information System (INIS)
Berryman, J G
2004-01-01
A general analysis of poroelasticity for vertical transverse isotropy (VTI) shows that four eigenvectors are pure shear modes with no coupling to the pore-fluidmechanics. The remaining two eigenvectors are linear combinations of pure compression and uniaxial shear, both of which are coupled to the fluid mechanics. After reducing the problem to a 2x2 system, the analysis shows in a relatively elementary fashion how a poroelastic system with isotropic solid elastic frame, but with anisotropy introduced through the poroelastic coefficients, interacts with the mechanics of the pore fluid and produces shear dependence on fluid properties in the overall mechanical system. The analysis shows, for example, that this effect is always present (though sometimes small in magnitude) in the systems studied, and can be quite large (up to a definite maximum increase of 20 per cent) in some rocks--including Spirit River sandstone and Schuler-Cotton Valley sandstone
Experimental study of shear rate dependence in perpetually sheared granular matter
Liu, Sophie Yang; Guillard, François; Marks, Benjy; Rognon, Pierre; Einav, Itai
2017-06-01
We study the shear behaviour of various granular materials by conducting novel perpetual simple shear experiments over four orders of magnitude of relatively low shear rates. The newly developed experimental apparatus employed is called "3D Stadium Shear Device" which is an extended version of the 2D Stadium Shear Device [1]. This device is able to provide a non-radial dependent perpetual shear flow and a nearly linear velocity profile between two oppositely moving shear walls. Using this device, we are able to test a large variety of granular materials. Here, we demonstrate the applicability of the device on glass beads (diameter 1 mm, 3 mm, and 14 mm) and rice. We particularly focus on studying these materials at very low inertial number I ranging from 10-6 to 10-2. We find that, within this range of I, the friction coefficient μ of glass beads has no shear rate dependence. A particularly appealing observation comes from testing rice, where the attainment of critical state develops under much longer duration than in other materials. Initially during shear we find a value of μ similar to that found for glass beads, but with time this value decreases gradually towards the asymptotic critical state value. The reason, we believe, lies in the fact that rice grains are strongly elongated; hence the time to achieve the stable μ is primarily controlled by the time for particles to align themselves with respect to the shear walls. Furthermore, the initial packing conditions of samples also plays a role in the evolution of μ when the shear strain is small, but that impact will eventually be erased after sufficient shear strain.
Ring-Sheared Drop (RSD): Microgravity Module for Containerless Flow Studies
Gulati, Shreyash; Raghunandan, Aditya; Rasheed, Fayaz; McBride, Samantha A.; Hirsa, Amir H.
2017-02-01
Microgravity is potentially a powerful tool for investigating processes that are sensitive to the presence of solid walls, since fluid containment can be achieved by surface tension. One such process is the transformation of protein in solution into amyloid fibrils; these are protein aggregates associated with neurodegenerative diseases such as Alzheimer's and Parkinson's. In addition to solid walls, experiments with gravity are also subject to influences from sedimentation of aggregates and buoyancy-driven convection. The ring-sheared drop (RSD) module is a flow apparatus currently under development to study formation of amyloid fibrils aboard the International Space Station (ISS). A 25 mm diameter drop of protein solution will be contained by surface tension and constrained by a pair of sharp-edged tubes, forming two contact rings. Shear can be imparted by rotating one ring with the other ring kept stationary. Here we report on parabolic flights conducted to test the growth and pinning of 10 mm diameter drops of water in under 10 s of microgravity. Finite element method (FEM) based fluid dynamics computations using a commercial package (COMSOL) assisted in the design of the parabolic flight experiments. Prior to the parabolic flights, the code was validated against experiments in the lab (1 g), on the growth of sessile and pendant droplets. The simulations show good agreement with the experiments. This modeling capability will enable the development of the RSD at the 25 mm scale for the ISS.
Groebner Basis Methods for Stationary Solutions of a Low-Dimensional Model for a Shear Flow
Pausch, Marina; Grossmann, Florian; Eckhardt, Bruno; Romanovski, Valery G.
2014-10-01
We use Groebner basis methods to extract all stationary solutions for the nine-mode shear flow model described in Moehlis et al. (New J Phys 6:56, 2004). Using rational approximations to irrational wave numbers and algebraic manipulation techniques we reduce the problem of determining all stationary states to finding roots of a polynomial of order 30. The coefficients differ by 30 powers of 10, so that algorithms for extended precision are needed to extract the roots reliably. We find that there are eight stationary solutions consisting of two distinct states, each of which appears in four symmetry-related phases. We discuss extensions of these results for other flows.
Energy Technology Data Exchange (ETDEWEB)
Galizia, Mauricio S.; Barker, Alex; Collins, Jeremy; Carr, James [Northwestern University, Department of Radiology, Feinberg School of Medicine, Chicago, IL (United States); Liao, Yihua [Northwestern University' s Feinberg School of Medicine, Department of Preventive Medicine, Chicago, IL (United States); McDermott, Mary M. [Northwestern University' s Feinberg School of Medicine, Department of Preventive Medicine, Chicago, IL (United States); Northwestern University' s Feinberg School of Medicine, Department of Medicine, Chicago, IL (United States); Markl, Michael [Northwestern University, Department of Radiology, Feinberg School of Medicine, Chicago, IL (United States); Northwestern University, Department Biomedical Engineering, McCormick School of Engineering, Chicago, IL (United States)
2014-04-15
To investigate the influence of atherosclerotic plaques on femoral haemodynamics assessed by two-dimensional (2D) phase-contrast (PC) magnetic resonance imaging (MRI) with three-directional velocity encoding. During 1 year, patients with peripheral artery disease and an ankle brachial index <1.00 were enrolled. After institutional review board approval and written informed consent, 44 patients (age, 70 ± 12 years) underwent common femoral artery MRI. Patients with contra-indications for MRI were excluded. Sequences included 2D time-of-flight, proton-density, T1-weighted and T2-weighted MRI. Electrocardiogram (ECG)-gated 2D PC-MRI with 3D velocity encoding was acquired. A radiologist classified images in five categories. Blood flow, velocity and wall shear stress (WSS) along the vessel circumference were quantified from the PC-MRI data. The acquired images were of good quality for interpretation. There were no image quality problems related to poor ECG-gating or slice positioning. Velocities, oscillatory shear stress and total flow were similar between patients with normal arteries and wall thickening/plaque. Patients with plaques demonstrated regionally increased peak systolic WSS and enhanced WSS eccentricity. Combined multi-contrast morphological imaging of the peripheral arterial wall with PC-MRI with three-directional velocity encoding is a feasible technique. Further study is needed to determine whether flow is an appropriate marker for altered endothelial cell function, vascular remodelling and plaque progression. (orig.)
Klein, D. Harley; Leal, L. Gary; García-Cervera, Carlos J.; Ceniceros, Hector D.
2007-02-01
We consider the behavior of the Doi-Marrucci-Greco (DMG) model for nematic liquid crystalline polymers in planar shear flow. We found the DMG model to exhibit dynamics in both qualitative and quantitative agreement with experimental observations reported by Larson and Mead [Liq. Cryst. 15, 151 (1993)] for the Ericksen number and Deborah number cascades. For increasing shear rates within the Ericksen number cascade, the DMG model displays three distinct regimes: stable simple shear, stable roll cells, and irregular structure accompanied by disclination formation. In accordance with experimental observations, the model predicts both ±1 and ±1/2 disclinations. Although ±1 defects form via the ridge-splitting mechanism first identified by Feng, Tao, and Leal [J. Fluid Mech. 449, 179 (2001)], a new mechanism is identified for the formation of ±1/2 defects. Within the Deborah number cascade, with increasing Deborah number, the DMG model exhibits a streamwise banded texture, in the absence of disclinations and roll cells, followed by a monodomain wherein the mean orientation lies within the shear plane throughout the domain.
Dimitrakopoulos, P; Kuriakose, S
2015-04-14
Determination of the elastic properties of the membrane of artificial capsules is essential for the better design of the various devices that are utilized in their engineering and biomedical applications. However this task is complicated owing to the combined effects of the shear and area-dilatation moduli on the capsule deformation. Based on computational investigation, we propose a new methodology to determine a membrane's shear modulus, independent of its area-dilatation modulus, by flowing strain-hardening capsules in a converging micro-capillary of comparable size under Stokes flow conditions, and comparing the experimental measurements of the capsule elongation overshooting with computational data. The capsule prestress, if any, can also be determined with the same methodology. The elongation overshooting is practically independent of the viscosity ratio for low and moderate viscosity ratios, and thus a wide range of capsule fluids can be employed. Our proposed experimental device can be readily produced via glass fabrication while owing to the continuous flow in the micro-capillary, the characterization of a large number of artificial capsules is possible.
Flow behavior at different shear rates for dry powders
Singh, A.; Singh, A.; Luding, Stefan; Nürnberg Messe GmbH,
2010-01-01
Using Discrete Element Simulations (DEM), an effort is made to study the so called “Split bottom ring shear cell” where a slow, quasi-static deformation leads to wide shear bands. Density, velocity and deformation gradients as well as structure and stress tensors, can be computed by a single
Shear-induced partial translational ordering of a colloidal solid
Ackerson, B. J.; Clark, N. A.
1984-08-01
Highly charged submicrometer plastic spheres suspended in water at low ionic strength will order spontaneously into bcc crystals or polycrystals. A simple linear shear orients and disorders these crystals by forcing (110) planes to stack normal to the shear gradient and to slide relative to each other with a direction parallel to the solvent flow. In this paper we analyze in detail the disordering and flow processes occurring beyond the intrinsic elastic limit of the bcc crystal. We are led to a model in which the flow of a colloidal crystal is interpreted as a fundamentally different process from that found in atomic crystals. In the colloidal crystal the coupling of particle motion to the background fluid forces a homogeneous flow, where every layer is in motion relative to its neighboring layers. In contrast, the plastic flow in an atomic solid is defect mediated flow. At the lowest applied stress, the local bcc order in the colloidal crystal exhibits shear strains both parallel and perpendicular to the direction of the applied stress. The magnitude of these deformations is estimated using the configurational energy for bcc and distorted bcc crystals, assuming a screened Coulomb pair interaction between colloidal particles. As the applied stress is increased, the intrinsic elastic limit of the crystal is exceeded and the crystal begins to flow with adjacent layers executing an oscillatory path governed by the balance of viscous and screened Coulomb forces. The path takes the structure from the bcc1 and bcc2 twins observed at zero shear to a distorted two-dimensional hcp structure at moderate shear rates, with a loss of interlayer registration as the shear is increased. This theoretical model is consistent with other experimental observations, as well.
Importance of Tensile Strength on the Shear Behavior of Discontinuities
Ghazvinian, A. H.; Azinfar, M. J.; Geranmayeh Vaneghi, R.
2012-05-01
In this study, the shear behavior of discontinuities possessing two different rock wall types with distinct separate compressive strengths was investigated. The designed profiles consisted of regular artificial joints molded by five types of plaster mortars, each representing a distinct uniaxial compressive strength. The compressive strengths of plaster specimens ranged from 5.9 to 19.5 MPa. These specimens were molded considering a regular triangular asperity profile and were designed so as to achieve joint walls with different strength material combinations. The results showed that the shear behavior of discontinuities possessing different joint wall compressive strengths (DDJCS) tested under constant normal load (CNL) conditions is the same as those possessing identical joint wall strengths, but the shear strength of DDJCS is governed by minor joint wall compressive strength. In addition, it was measured that the predicted values obtained by Barton's empirical criterion are greater than the experimental results. The finding indicates that there is a correlation between the joint roughness coefficient (JRC), normal stress, and mechanical strength. It was observed that the mode of failure of asperities is either pure tensile, pure shear, or a combination of both. Therefore, Barton's strength criterion, which considers the compressive strength of joint walls, was modified by substituting the compressive strength with the tensile strength. The validity of the modified criterion was examined by the comparison of the predicted shear values with the laboratory shear test results reported by Grasselli (Ph.D. thesis n.2404, Civil Engineering Department, EPFL, Lausanne, Switzerland, 2001). These comparisons infer that the modified criterion can predict the shear strength of joints more precisely.
Experimental study of shear rate dependence in perpetually sheared granular matter
Directory of Open Access Journals (Sweden)
Liu Sophie Yang
2017-01-01
Full Text Available We study the shear behaviour of various granular materials by conducting novel perpetual simple shear experiments over four orders of magnitude of relatively low shear rates. The newly developed experimental apparatus employed is called “3D Stadium Shear Device” which is an extended version of the 2D Stadium Shear Device [1]. This device is able to provide a non-radial dependent perpetual shear flow and a nearly linear velocity profile between two oppositely moving shear walls. Using this device, we are able to test a large variety of granular materials. Here, we demonstrate the applicability of the device on glass beads (diameter 1 mm, 3 mm, and 14 mm and rice. We particularly focus on studying these materials at very low inertial number I ranging from 10−6 to 10−2. We find that, within this range of I, the friction coefficient μ of glass beads has no shear rate dependence. A particularly appealing observation comes from testing rice, where the attainment of critical state develops under much longer duration than in other materials. Initially during shear we find a value of μ similar to that found for glass beads, but with time this value decreases gradually towards the asymptotic critical state value. The reason, we believe, lies in the fact that rice grains are strongly elongated; hence the time to achieve the stable μ is primarily controlled by the time for particles to align themselves with respect to the shear walls. Furthermore, the initial packing conditions of samples also plays a role in the evolution of μ when the shear strain is small, but that impact will eventually be erased after sufficient shear strain.
Dynamics and rheology under continuous shear flow studied by x-ray photon correlation spectroscopy
Energy Technology Data Exchange (ETDEWEB)
Fluerasu, Andrei [Brookhaven National Laboratory, NSLS-II, Upton, NY 11973 (United States); Kwasniewski, Pawel; Caronna, Chiara; Madsen, Anders [European Synchrotron Radiation Facility, ID10 (Troika), Grenoble 38043 (France); Destremaut, Fanny; Salmon, Jean-Baptiste [LOF, UMR 5258 CNRS-Rhodia Bordeaux 1, 33608 Pessac (France)], E-mail: fluerasu@bnl.gov
2010-03-15
X-ray photon correlation spectroscopy (XPCS) has emerged as a unique technique allowing the measurement of dynamics of materials on mesoscopic lengthscales. One of the most common problems associated with the use of bright x-ray beams is beam-induced radiation damage, and this is likely to become an even more limiting factor at future synchrotron and free-electron laser sources. Flowing the sample during data acquisition is one of the simplest methods allowing the radiation damage to be limited. In addition to distributing the dose over many different scatterers, the method also enables new functionalities such as time-resolved studies. Here, we further develop a recently proposed experimental technique that combines XPCS and continuously flowing samples. More specifically, we use a model colloidal suspension to show how the macroscopic advective response to flow and the microscopic dissipative dynamics (diffusion) can be quantified from the x-ray data. Our results show very good quantitative agreement with a Poisseuille-flow hydrodynamical model combined with Brownian mechanics. The method has many potential applications, e.g. in the study of dynamics of glasses and gels under continuous shear/flow, protein aggregation processes and the interplay between dynamics and rheology in complex fluids.
Electro—magnetic control of shear flow over a cylinder for drag reduction and lift enhancement
International Nuclear Information System (INIS)
Zhang Hui; Fan Bao-Chun; Chen Zhi-Hua; Chen Shuai; Li Hong-Zhi
2013-01-01
In this paper, the electro—magnetic control of a cylinder wake in shear flow is investigated numerically. The effects of the shear rate and Lorentz force on the cylinder wake, the distribution of hydrodynamic force, and the drag/lift phase diagram are discussed in detail. It is revealed that Lorentz force can be classified into the field Lorentz force and the wall Lorentz force and they affect the drag and lift forces independently. The drag/lift phase diagram with a shape of ''8'' consists of two closed curves, which correspond to the halves of the shedding cycle dominated by the upper and lower vortices respectively. The free stream shear (K > 0) induces the diagram to move downward and leftward, so that the average lift force directs toward the downside. With the upper Lorentz force, the diagram moves downwards and to the right by the field Lorentz force, thus resulting in the drag increase and the lift reduction, whereas it moves upward and to the left by the wall Lorentz force, leading to the drag reduction and the lift increase. Finally the diagram is dominated by the wall Lorentz force, thus moving upward and leftward. Therefore the upper Lorentz force, which enhances the lift force, can be used to overcome the lift loss due to the free stream shear, which is also obtained in the experiment. (electromagnetism, optics, acoustics, heat transfer, classical mechanics, and fluid dynamics)
Critical wall shear stress for the EHEDG test method
DEFF Research Database (Denmark)
Jensen, Bo Boye Busk; Friis, Alan
2004-01-01
In order to simulate the results of practical cleaning tests on closed processing equipment, based on wall shear stress predicted by computational fluid dynamics, a critical wall shear stress is required for that particular cleaning method. This work presents investigations that provide a critical...... wall shear stress of 3 Pa for the standardised EHEDG cleaning test method. The cleaning tests were performed on a test disc placed in a radial flowcell assay. Turbulent flow conditions were generated and the corresponding wall shear stresses were predicted from CFD simulations. Combining wall shear...... stress predictions from a simulation using the low Re k-epsilon and one using the two-layer model of Norris and Reynolds were found to produce reliable predictions compared to empirical solutions for the ideal flow case. The comparison of wall shear stress curves predicted for the real RFC...
Padilla, Jaume; Johnson, Blair D; Newcomer, Sean C; Wilhite, Daniel P; Mickleborough, Timothy D; Fly, Alyce D; Mather, Kieren J; Wallace, Janet P
2008-09-04
Normalization of brachial artery flow-mediated dilation (FMD) to individual shear stress area under the curve (peak FMD:SSAUC ratio) has recently been proposed as an approach to control for the large inter-subject variability in reactive hyperemia-induced shear stress; however, the adoption of this approach among researchers has been slow. The present study was designed to further examine the efficacy of FMD normalization to shear stress in reducing measurement variability. Five different magnitudes of reactive hyperemia-induced shear stress were applied to 20 healthy, physically active young adults (25.3 +/- 0. 6 yrs; 10 men, 10 women) by manipulating forearm cuff occlusion duration: 1, 2, 3, 4, and 5 min, in a randomized order. A venous blood draw was performed for determination of baseline whole blood viscosity and hematocrit. The magnitude of occlusion-induced forearm ischemia was quantified by dual-wavelength near-infrared spectrometry (NIRS). Brachial artery diameters and velocities were obtained via high-resolution ultrasound. The SSAUC was individually calculated for the duration of time-to-peak dilation. One-way repeated measures ANOVA demonstrated distinct magnitudes of occlusion-induced ischemia (volume and peak), hyperemic shear stress, and peak FMD responses (all p index of endothelial function.
Duddu, Ravindra
2009-05-01
We present a two-dimensional biofilm growth model in a continuum framework using an Eulerian description. A computational technique based on the eXtended Finite Element Method (XFEM) and the level set method is used to simulate the growth of the biofilm. The model considers fluid flow around the biofilm surface, the advection-diffusion and reaction of substrate, variable biomass volume fraction and erosion due to the interfacial shear stress at the biofilm-fluid interface. The key assumptions of the model and the governing equations of transport, biofilm kinetics and biofilm mechanics are presented. Our 2D biofilm growth results are in good agreement with those obtained by Picioreanu et al. (Biotechnol Bioeng 69(5):504-515, 2000). Detachment due to erosion is modeled using two continuous speed functions based on: (a) interfacial shear stress and (b) biofilm height. A relation between the two detachment models in the case of a 1D biofilm is established and simulated biofilm results with detachment in 2D are presented. The stress in the biofilm due to fluid flow is evaluated and higher stresses are observed close to the substratum where the biofilm is attached. © 2008 Wiley Periodicals, Inc.
Flexible Micropost Arrays for Shear Stress Measurement
Wohl, Christopher J.; Palmieri, Frank L.; Hopkins, John W.; Jackson, Allen M.; Connell, John W.; Lin, Yi; Cisotto, Alexxandra A.
2015-01-01
Increased fuel costs, heightened environmental protection requirements, and noise abatement continue to place drag reduction at the forefront of aerospace research priorities. Unfortunately, shortfalls still exist in the fundamental understanding of boundary-layer airflow over aerodynamic surfaces, especially regarding drag arising from skin friction. For example, there is insufficient availability of instrumentation to adequately characterize complex flows with strong pressure gradients, heat transfer, wall mass flux, three-dimensionality, separation, shock waves, and transient phenomena. One example is the acoustic liner efficacy on aircraft engine nacelle walls. Active measurement of shear stress in boundary layer airflow would enable a better understanding of how aircraft structure and flight dynamics affect skin friction. Current shear stress measurement techniques suffer from reliability, complexity, and airflow disruption, thereby compromising resultant shear stress data. The state-of-the-art for shear stress sensing uses indirect or direct measurement techniques. Indirect measurements (e.g., hot-wire, heat flux gages, oil interferometry, laser Doppler anemometry, small scale pressure drag surfaces, i.e., fences) require intricate knowledge of the studied flow, restrictive instrument arrangements, large surface areas, flow disruption, or seeding material; with smaller, higher bandwidth probes under development. Direct measurements involve strain displacement of a sensor element and require no prior knowledge of the flow. Unfortunately, conventional "floating" recessed components for direct measurements are mm to cm in size. Whispering gallery mode devices and Fiber Bragg Gratings are examples of recent additions to this type of sensor with much smaller (?m) sensor components. Direct detection techniques are often single point measurements and difficult to calibrate and implement in wind tunnel experiments. In addition, the wiring, packaging, and installation
Theory of ion Bernstein wave induced shear suppression of turbulence
Craddock, G. G.; Diamond, P. H.; Ono, M.; Biglari, H.
1994-06-01
The theory of radio frequency induced ion Bernstein wave- (IBW) driven shear flow in the edge is examined, with the goal of application of shear suppression of fluctuations. This work is motivated by the observed confinement improvement on IBW heated tokamaks [Phys. Fluids B 5, 241 (1993)], and by previous low-frequency work on RF-driven shear flows [Phys. Rev. Lett. 67, 1535 (1991)]. It is found that the poloidal shear flow is driven electrostatically by both Reynolds stress and a direct ion momentum source, analogous to the concepts of helicity injection and electron momentum input in current drive, respectively. Flow drive by the former does not necessarily require momentum input to the plasma to induce a shear flow. For IBW, the direct ion momentum can be represented by direct electron momentum input, and a charge separation induced stress that imparts little momentum to the plasma. The derived Er profile due to IBW predominantly points inward, with little possibility of direction change, unlike low-frequency Alfvénic RF drive. The profile scale is set by the edge density gradient and electron dissipation. Due to the electrostatic nature of ion Bernstein waves, the poloidal flow contribution dominates in Er. Finally, the necessary edge power absorbed for shear suppression on Princeton Beta Experiment-Modified (PBX-M) [9th Topical Conference on Radio Frequency Power in Plasmas, Charleston, SC, 1991 (American Institute of Physics, New York, 1991), p. 129] is estimated to be 100 kW distributed over 5 cm.
High Resolution Shear Profile Measurements in Entangled Polymers
Hayes, Keesha A.
2008-11-17
We use confocal microscopy and particle image velocimetry to visualize motion of 250-300 nm. fluorescent tracer particles in entangled polymers subject to a rectilinear shear flow. Our results show linear velocity profiles in polymer solutions spanning a wide range of molecular weights and number of entanglements (8≤Z≤56), but reveal large differences between the imposed and measured shear rates. These findings disagree with recent reports that shear banding is a characteristic flow response of entangled polymers, and instead point to interfacial slip as an important source of strain loss. © 2008 The American Physical Society.
Shear thinning behaviors in magmas
Vetere, F. P.; Cassetta, M.; Perugini, D.
2017-12-01
Studies on magma rheology are of fundamental importance to understanding magmatic processes from depth to surface. Since viscosity is one of the most important parameter controlling eruption mechanisms, as well as lava flow emplacement, a comprehensive knowledge on the evolution of magma viscosities during crystallization is required. We present new viscosity data on partly crystalized basalt, andesite and analogue lavas comparable to those erupted on Mercury's northern volcanic plains. High-temperature viscosity measurements were performed using a rotational Anton Paar RheolabQC viscometer head at the PVRG labs, in Perugia (Italy) (http://pvrg.unipg.it). The relative proportion of phases in each experimental run were determined by image analysis on BS-SEM images at different magnifications; phases are glasses, clinopyroxene, spinel, plagioclase for the basalt, plagioclase and spinel for the andesite and pure enstatite and clinopyroxenes, for the analogue Mercury's composition. Glass and crystalline fractions determined by image analysis well correlate with compositions of residual melts. In order to constrain the viscosity (η) variations as a function of crystallinity, shear rate (γ) was varied from 0.1 to 5 s-1. Viscosity vs. time at constant temperature shows a typical S-shape curve. In particular, for basaltic composition η vary from 3.1-3.8 Pa s [log η] at 1493 K and crystallinity of 19 area % as γ vary from 1.0 to 0.1 s-1; the andesite viscosity evolution is 3.2 and 3.7 Pa s [log η] as γ varies from 1 to 0.1 at 1493 K and crystal content of 17 area %; finally, Mercury's analogue composition was investigated at different temperature ranging from 1533 to 1502 K (Vetere et al., 2017). Results, for γ = 0.1, 1.0 and 5.0 s-1, show viscosity variation between 2.7-4.0, 2.5-3.4 and 2.0-3.0 [log η inPa s] respectively while crystallinity vary from 9 to 27 (area %). As viscosity decreases as shear rate increases, these data points to a shear thinning behaviour
Axisymmetric wave propagation in gas shear flow confined by a rigid-walled pipeline
International Nuclear Information System (INIS)
Chen Yong; Huang Yi-Yong; Chen Xiao-Qian; Bai Yu-Zhu; Tan Xiao-Dong
2015-01-01
The axisymmetric acoustic wave propagating in a perfect gas with a shear pipeline flow confined by a circular rigid wall is investigated. The governing equations of non-isentropic and isentropic acoustic assumptions are mathematically deduced while the constraint of Zwikker and Kosten is relaxed. An iterative method based on the Fourier–Bessel theory is proposed to semi-analytically solve the proposed models. A comparison of numerical results with literature contributions validates the present contribution. Meanwhile, the features of some high-order transverse modes, which cannot be analyzed based on the Zwikker and Kosten theory, are analyzed (paper)
Analysis of zonal flow bifurcations in 3D drift wave turbulence simulations
International Nuclear Information System (INIS)
Kammel, Andreas
2012-01-01
The main issue of experimental magnetic fusion devices lies with their inherently high turbulent transport, preventing long-term plasma confinement. A deeper understanding of the underlying transport processes is therefore desirable, especially in the high-gradient tokamak edge which marks the location of the drift wave regime as well as the outer boundary of the still badly understood high confinement mode. One of the most promising plasma features possibly connected to a complete bifurcation theory for the transition to this H-mode is found in large-scale phenomena capable of regulating radial transport through vortex shearing - i.e. zonal flows, linearly stable large-scale poloidal vector E x vector B-modes based on radial flux surface averages of the potential gradient generated through turbulent self-organization. Despite their relevance, few detailed turbulence studies of drift wave-based zonal flows have been undertaken, and none of them have explicitly targeted bifurcations - or, within a resistive sheared-slab environment, observed zonal flows at all. In this work, both analytical means and the two-fluid code NLET are used to analyze a reduced set of Hasegawa-Wakatani equations, describing a sheared collisional drift wave system without curvature. The characteristics of the drift waves themselves, as well as those of the drift wave-based zonal flows and their retroaction on the drift wave turbulence are examined. The single dimensionless parameter ρ s proposed in previous analytical models is examined numerically and shown to divide the drift wave scale into two transport regimes, the behavioral characteristics of which agree perfectly with theoretical expectations. This transport transition correlates with a transition from pure drift wave turbulence at low ρ s into the high-ρ s zonal flow regime. The associated threshold has been more clearly identified by tracing it back to a tipping of the ratio between a newly proposed frequency gradient length at
International Nuclear Information System (INIS)
Jiquan Li; Kishimoto, Y.; Tuda, T.
2000-01-01
The separate structure of two branches of the sheared slab η i mode near the minimum-q magnetic surface is analysed and the effects of plasma rotation shears are considered in the weak magnetic shear region. Results show that the separation condition depends on the non-monotonous q profile and the deviation of rational surface from the minimum-q surface. Furthermore, it is found that the diamagnetic rotation shear may suppress the perturbation of the sheared slab η i mode at one side of the minimum-q surface, the poloidal rotation shear from the sheared E-vector x B-vector flow has a similar role to the slab mode structure when it possesses a direction same as the diamagnetic shear. A plausible interrelation between the separate structures of the two branches of the sheared slab mode and the discontinuity or gap of the radially global structure of the drift wave near the minimum-q surface observed in the toroidal particle simulation (Kishimoto Y et al 1998 Plasma Phys. Control. Fusion 40 A663) is discussed. It seems to support such a viewpoint: the double or/and global branches of the sheared slab η i mode near the minimum-q surface may become a bridge to connect the radially global structures of the drift wave at two sides of the minimum-q surface and the discontinuity may originate from the separate structures of these slab modes for a flatter q profile. (author)
Evidence for modified transport due to sheared E x B flows in high-temperature plasmas
International Nuclear Information System (INIS)
Groebner, R.J.; Burrell, K.H.; Austin, M.E.
1994-11-01
Sheared mass flows are generated in many fluids and are often important for the dynamics of instabilities in these fluids. Similarly, large values of the E x B velocity have been observed in magnetic confinement machines and there is theoretical and experimental evidence that sufficiently large shear in this velocity may stabilize important instabilities. Two examples of this phenomenon have been observed in the DIII-D tokamak. In the first example, sufficient heating power can lead to the L-H transition, a rapid improvement in confinement in the boundary layer of the plasma. For discharges with heating power close to the threshold required to get the transition, changes in the edge radial electric field are observed to occur prior to the transition itself. In the second example, certain classes of discharges with toroidal momentum input from neutral beam injection exhibit a further improvement of confinement in the plasma core leading to a regime called the VH-mode. In both examples, the region of improved confinement is characterized by an increase of shear in the radial electric field E r , reduced levels of turbulence and increases in gradients of temperatures and densities. These observations are consistent with the hypothesis that the improved confinement is caused by an increase in shear of the E x B velocity which leads to a reduction of turbulence. For the VH-mode, the dominant term controlling E r is the toroidal rotation v φ , indicating that the E r profile is controlled by the source and transport of toroidal momentum
Yan, Zheng; McKee, George; Gohil, Punit; Schmitz, Lothar; Eldon, David; Grierson, Brian; Kriete, Matt; Rhodes, Terry; Petty, Craig
2017-10-01
Measurements of long wavelength density fluctuation characteristics have been obtained in the edge of Deuterium (D) plasmas across the L-H transition on DIII-D during density and q95 scans. The relative density fluctuation amplitude measured by Beam Emission Spectroscopy (BES) increases with higher q95. The power threshold is found to increase with plasma current (i.e., lower q95) but with complex density dependence: the largest increase of PLH is seen at ne 3.2e19 m-3. Interestingly, a dual counter-propagating mode is observed for cases when PLH is low. The existence of the dual mode is correlated with increasing flow shear. Estimation of the turbulence kinetic energy transfer from turbulence to the flow increases prior to the transition. The complex behaviors of the turbulence characteristics and dual frequency modes interactions impact the flow shear generation, the transition process and the power threshold scaling. Work supported by the US Department of Energy under DE-FG02-08ER54999, DE-AC02-09CH11466, DE-FC02-04ER54698, and DE-AC52-07NA27344.
Seismic cycle feedbacks in a mid-crustal shear zone
Melosh, Benjamin L.; Rowe, Christie D.; Gerbi, Christopher; Smit, Louis; Macey, Paul
2018-07-01
Mid-crustal fault rheology is controlled by alternating brittle and plastic deformation mechanisms, which cause feedback cycles that influence earthquake behavior. Detailed mapping and microstructural observations in the Pofadder Shear Zone (Namibia and South Africa) reveal a lithologically heterogeneous shear zone core with quartz-rich mylonites and ultramylonites, plastically overprinted pseudotachylyte and active shear folds. We present evidence for a positive feedback cycle in which coseismic grain size reduction facilitates active shear folding by enhancing competency contrasts and promoting crystal plastic flow. Shear folding strengthens a portion of a shear zone by limb rotation, focusing deformation and promoting plastic flow or brittle slip in resulting areas of localized high stress. Using quartz paleopiezometry, we estimate strain and slip rates consistent with other studies of exhumed shear zones and modern plate boundary faults, helping establish the Pofadder Shear Zone as an ancient analogue to modern, continental-scale, strike-slip faults. This feedback cycle influences seismicity patterns at the scale of study (10s of meters) and possibly larger scales as well, and contributes to bulk strengthening of the brittle-plastic transition on modern plate boundary faults.
Lundahl, P. Johan; Kitts, Catherine C.; Nordé n, Bengt
2011-01-01
This article presents a new design of flow-orientation device for the study of bio-macromolecules, including DNA and protein complexes, as well as aggregates such as amyloid fibrils and liposome membranes, using Linear Dichroism (LD) spectroscopy. The design provides a number of technical advantages that should make the device inexpensive to manufacture, easier to use and more reliable than existing techniques. The degree of orientation achieved is of the same order of magnitude as that of the commonly used concentric cylinders Couette flow cell, however, since the device exploits a set of flat strain-free quartz plates, a number of problems associated with refraction and birefringence of light are eliminated, increasing the sensitivity and accuracy of measurement. The device provides similar shear rates to those of the Couette cell but is superior in that the shear rate is constant across the gap. Other major advantages of the design is the possibility to change parts and vary sample volume and path length easily and at a low cost. © 2011 The Royal Society of Chemistry.
Energy Technology Data Exchange (ETDEWEB)
Nofar, M. [Center for High Performance Polymer and Composite Systems (CREPEC), Chemical Engineering Department, Polytechnique Montreal, Montreal, Quebec H3T 1J4, Canada and CREPEC, Department of Chemical Engineering, McGill University, Montreal, Quebec H3A 2B2 (Canada); Heuzey, M. C.; Carreau, P. J., E-mail: pierre.carreau@polymtl.ca [Center for High Performance Polymer and Composite Systems (CREPEC), Chemical Engineering Department, Polytechnique Montreal, Montreal, Quebec H3T 1J4 (Canada); Kamal, M. R. [CREPEC, Department of Chemical Engineering, McGill University, Montreal, Quebec H3A 2B2 (Canada); Randall, J. [NatureWorks LLC, 15305 Minnetonka Boulevard, Minnetonka, Minnesota 55345 (United States)
2016-07-15
Blends containing 75 wt. % of an amorphous polylactide (PLA) with two different molecular weights and 25 wt. % of a poly[(butylene adipate)-co-terephthalate] (PBAT) were prepared using either a Brabender batch mixer or a twin-screw extruder. These compounds were selected because blending PLA with PBAT can overcome various drawbacks of PLA such as its brittleness and processability limitations. In this study, we investigated the effects of varying the molecular weight of the PLA matrix and of two different mixing processes on the blend morphology and, further, on droplet coalescence during shearing. The rheological properties of these blends were investigated and the interfacial properties were analyzed using the Palierne emulsion model. Droplet coalescence was investigated by applying shear flows of 0.05 and 0.20 s{sup −1} at a fixed strain of 60. Subsequently, small amplitude oscillatory shear tests were conducted to investigate changes in the viscoelastic properties. The morphology of the blends was also examined using scanning electron microscope (SEM) micrographs. It was observed that the PBAT droplets were much smaller when twin-screw extrusion was used for the blend preparation. Shearing at 0.05 s{sup −1} induced significant droplet coalescence in all blends, but coalescence and changes in the viscoelastic properties were much more pronounced for the PLA-PBAT blend based on a lower molecular weight PLA. The viscoelastic responses were also somehow affected by the thermal degradation of the PLA matrix during the experiments.
International Nuclear Information System (INIS)
Nofar, M.; Heuzey, M. C.; Carreau, P. J.; Kamal, M. R.; Randall, J.
2016-01-01
Blends containing 75 wt. % of an amorphous polylactide (PLA) with two different molecular weights and 25 wt. % of a poly[(butylene adipate)-co-terephthalate] (PBAT) were prepared using either a Brabender batch mixer or a twin-screw extruder. These compounds were selected because blending PLA with PBAT can overcome various drawbacks of PLA such as its brittleness and processability limitations. In this study, we investigated the effects of varying the molecular weight of the PLA matrix and of two different mixing processes on the blend morphology and, further, on droplet coalescence during shearing. The rheological properties of these blends were investigated and the interfacial properties were analyzed using the Palierne emulsion model. Droplet coalescence was investigated by applying shear flows of 0.05 and 0.20 s"−"1 at a fixed strain of 60. Subsequently, small amplitude oscillatory shear tests were conducted to investigate changes in the viscoelastic properties. The morphology of the blends was also examined using scanning electron microscope (SEM) micrographs. It was observed that the PBAT droplets were much smaller when twin-screw extrusion was used for the blend preparation. Shearing at 0.05 s"−"1 induced significant droplet coalescence in all blends, but coalescence and changes in the viscoelastic properties were much more pronounced for the PLA-PBAT blend based on a lower molecular weight PLA. The viscoelastic responses were also somehow affected by the thermal degradation of the PLA matrix during the experiments.
Calanoid Copepod Behavior in Thin Layer Shear Flows: Freshwater Versus Marine
Skipper, A. N.; Webster, D. R.; Yen, J.
2015-11-01
Marine copepods have been shown to behaviorally respond to vertical gradients of horizontal velocity and aggregate around thin layers. The current study addresses whether a freshwater copepod from an alpine lake demonstrates similar behavior response. Hesperodiaptomus shoshone is often the greatest biomass in alpine lakes and is the dominant zooplankton predator within its environment. The hypothesis is that H. shoshone responds to vertical gradients of horizontal velocity, which are associated with river outflows from alpine lakes, with fine-scale changes in swimming kinematics. The two calanoid copepods studied here, H. shoshone (freshwater) and Calanus finmarchicus(marine), are of similar size (2 - 4 mm), have similar morphologies, and utilize cruising as their primary swimming mode. The two animals differ not only in environment, but also in diet; H. shoshone is a carnivore, whereas C. finmarchicusis an herbivore. A laminar, planar jet (Bickley) was used in the laboratory to simulate a free shear flow. Particle image velocimetry (PIV) quantified the flow field. The marine species changed its swimming behavior significantly (increased swimming speed and turning frequency) and spent more time in the layer (40% vs. 70%) from control to treatment. In contrast, the freshwater species exhibited very few changes in either swimming behavior or residence time. Swimming kinematics and residence time results were also similar between males and females. Unlike the marine copepod, the results suggest the environmental flow structure is unimportant to the freshwater species.
Directory of Open Access Journals (Sweden)
Mickleborough Timothy D
2008-09-01
Full Text Available Abstract Background Normalization of brachial artery flow-mediated dilation (FMD to individual shear stress area under the curve (peak FMD:SSAUC ratio has recently been proposed as an approach to control for the large inter-subject variability in reactive hyperemia-induced shear stress; however, the adoption of this approach among researchers has been slow. The present study was designed to further examine the efficacy of FMD normalization to shear stress in reducing measurement variability. Methods Five different magnitudes of reactive hyperemia-induced shear stress were applied to 20 healthy, physically active young adults (25.3 ± 0. 6 yrs; 10 men, 10 women by manipulating forearm cuff occlusion duration: 1, 2, 3, 4, and 5 min, in a randomized order. A venous blood draw was performed for determination of baseline whole blood viscosity and hematocrit. The magnitude of occlusion-induced forearm ischemia was quantified by dual-wavelength near-infrared spectrometry (NIRS. Brachial artery diameters and velocities were obtained via high-resolution ultrasound. The SSAUC was individually calculated for the duration of time-to-peak dilation. Results One-way repeated measures ANOVA demonstrated distinct magnitudes of occlusion-induced ischemia (volume and peak, hyperemic shear stress, and peak FMD responses (all p AUC (p = 0.785. Conclusion Our data confirm that normalization of FMD to SSAUC eliminates the influences of variable shear stress and solidifies the utility of FMD:SSAUC ratio as an index of endothelial function.
Metal release behavior of surface oxidized stainless steels into flowing high temperature pure water
International Nuclear Information System (INIS)
Fujiwara, Kazuo; Tomari, Haruo; Nakayama, Takenori; Shimogori, Kazutoshi; Ishigure, Kenkichi; Matsuura, Chihiro; Fujita, Norihiko; Ono, Shoichi.
1987-01-01
In order to clarify the effect of oxidation treatment of Type 304 SS on the inhibition of metal release into high temperature pure water, metal release rate of individual alloying element into flowing deionized water containing 50 ppb dissolved oxygen was measured as the function of exposure time on representative specimens oxidized in air and steam. The behavior of metal release was also discussed in relation to the structure of surface films. Among the alloying elements the amount of Fe ion, Cr ion and Fe crud in high temperature pure water tended to saturate with the exposure time and that of Ni ion and Co ion tended to increase monotonously with the exposure time for all specimens tested. And the treatment of steam-oxidation was the most effective to decrease the metal release of alloying elements and the treatment by air-oxidation also decreased the metal release. These tendencies were confirmed to correlate well with the structure of the surface films as it was in the results in the static autoclave test. (author)
Helzel, Christiane; Tzavaras, Athanasios
2016-01-01
We consider a kinetic model, which describes the sedimentation of rod-like particles in dilute suspensions under the influence of gravity, presented in Helzel and Tzavaras (submitted for publication). Here we restrict our considerations to shear flow and consider a simplified situation, where the particle orientation is restricted to the plane spanned by the direction of shear and the direction of gravity. For this simplified kinetic model we carry out a linear stability analysis and we derive two different nonlinear macroscopic models which describe the formation of clusters of higher particle density. One of these macroscopic models is based on a diffusive scaling, the other one is based on a so-called quasi-dynamic approximation. Numerical computations, which compare the predictions of the macroscopic models with the kinetic model, complete our presentation.
Helzel, Christiane
2016-07-22
We consider a kinetic model, which describes the sedimentation of rod-like particles in dilute suspensions under the influence of gravity, presented in Helzel and Tzavaras (submitted for publication). Here we restrict our considerations to shear flow and consider a simplified situation, where the particle orientation is restricted to the plane spanned by the direction of shear and the direction of gravity. For this simplified kinetic model we carry out a linear stability analysis and we derive two different nonlinear macroscopic models which describe the formation of clusters of higher particle density. One of these macroscopic models is based on a diffusive scaling, the other one is based on a so-called quasi-dynamic approximation. Numerical computations, which compare the predictions of the macroscopic models with the kinetic model, complete our presentation.
Local parametric instability near elliptic points in vortex flows under shear deformation
Energy Technology Data Exchange (ETDEWEB)
Koshel, Konstantin V., E-mail: kvkoshel@poi.dvo.ru [Pacific Oceanological Institute, FEB RAS, 43, Baltiyskaya Street, Vladivostok 690041 (Russian Federation); Institute of Applied Mathematics, FEB RAS, 7, Radio Street, Vladivostok 690022 (Russian Federation); Far Eastern Federal University, 8, Sukhanova Street, Vladivostok 690950 (Russian Federation); Ryzhov, Eugene A., E-mail: ryzhovea@gmail.com [Pacific Oceanological Institute, FEB RAS, 43, Baltiyskaya Street, Vladivostok 690041 (Russian Federation)
2016-08-15
The dynamics of two point vortices embedded in an oscillatory external flow consisted of shear and rotational components is addressed. The region associated with steady-state elliptic points of the vortex motion is established to experience local parametric instability. The instability forces the point vortices with initial positions corresponding to the steady-state elliptic points to move in spiral-like divergent trajectories. This divergent motion continues until the nonlinear effects suppress their motion near the region associated with the steady-state separatrices. The local parametric instability is then demonstrated not to contribute considerably to enhancing the size of the chaotic motion regions. Instead, the size of the chaotic motion region mostly depends on overlaps of the nonlinear resonances emerging in the perturbed system.
High Resolution Shear Profile Measurements in Entangled Polymers
Hayes, Keesha A.; Buckley, Mark R.; Cohen, Itai; Archer, Lynden A.
2008-01-01
spanning a wide range of molecular weights and number of entanglements (8≤Z≤56), but reveal large differences between the imposed and measured shear rates. These findings disagree with recent reports that shear banding is a characteristic flow response
On the behaviour of gelled fibre suspensions in steady shear
Energy Technology Data Exchange (ETDEWEB)
Wolf, Bettina [Unilever Corporate Research, Bedford (United Kingdom); University of Nottingham, Division of Food Sciences, Loughborough (United Kingdom); White, Duncan; Melrose, John R.; Frith, William J. [Unilever Corporate Research, Bedford (United Kingdom)
2007-03-15
The shear rheological properties of suspensions of gelled agar fibres in a low viscosity Newtonian matrix fluid were investigated. Two classes of fibres, low aspect ratio fibres and high aspect ratio fibres with an aspect ratio of the order of 10 and 100 respectively were included in the investigations. For all fibre phase volumes investigated, from as low as 0.01 upwards, the flow curves are characterised by an apparent yield stress followed by shear-thinning which was independent of the fibre aspect ratio. Based on our analysis of the flow curves, we conclude that the high aspect ratio fibres behave like flexible threads in contrast to the low aspect ratio fibres whose high shear relative viscosity is successfully described by a relation for long rigid rods. These findings are supported by flow visualisation using an optical shearing stage coupled to a light microscope. (orig.)
International Nuclear Information System (INIS)
Colgate, Stirling A.; Beckley, Howard; Si, Jiahe; Martinic, Joe; Westpfahl, David; Slutz, James; Westrom, Cebastian; Klein, Brianna; Schendel, Paul; Scharle, Cletus; McKinney, Travis; Ginanni, Rocky; Bentley, Ian; Mickey, Timothy; Ferrel, Regnar; Li, Hui; Pariev, Vladimir; Finn, John
2011-01-01
The Ω phase of the liquid sodium α-Ω dynamo experiment at New Mexico Institute of Mining and Technology in cooperation with Los Alamos National Laboratory has demonstrated a high toroidal field B φ that is ≅8xB r , where B r is the radial component of an applied poloidal magnetic field. This enhanced toroidal field is produced by the rotational shear in stable Couette flow within liquid sodium at a magnetic Reynolds number Rm≅120. Small turbulence in stable Taylor-Couette flow is caused by Ekman flow at the end walls, which causes an estimated turbulence energy fraction of (δv/v) 2 ∼10 -3 .
Effect of shear stress on the migration of hepatic stellate cells.
Sera, Toshihiro; Sumii, Tateki; Fujita, Ryosuke; Kudo, Susumu
2018-01-01
When the liver is damaged, hepatic stellate cells (HSCs) can change into an activated, highly migratory state. The migration of HSCs may be affected by shear stress due not only to sinusoidal flow but also by the flow in the space of Disse because this space is filled with blood plasma. In this study, we evaluated the effects of shear stress on HSC migration in a scratch-wound assay with a parallel flow chamber. At regions upstream of the wound area, the migration was inhibited by 0.6 Pa and promoted by 2.0 Pa shear stress, compared to the static condition. The platelet-derived growth factor (PDGF)-BB receptor, PDGFR-β, was expressed in all conditions and the differences were not significant. PDGF increased HSC migration, except at 0.6 Pa shear stress, which was still inhibited. These results indicate that another molecular factor, such as PDGFR-α, may act to inhibit the migration under low shear stress. At regions downstream of the wound area, the migration was smaller under shear stress than under the static condition, although the expression of PDGFR-β was significantly higher. In particular, the migration direction was opposite to the wound area under high shear stress; therefore, migration might be influenced by the intercellular environment. Our results indicate that HSC migration was influenced by shear stress intensity and the intercellular environment.
Microstructures and mechanical properties of pure Mg processed by rotary swaging
International Nuclear Information System (INIS)
Gan, W.M.; Huang, Y.D.; Wang, R.; Wang, G.F.; Srinivasan, A.; Brokmeier, H.-G.; Schell, N.; Kainer, K.U.; Hort, N.
2014-01-01
Highlights: • Grain size of pure Mg can be effectively reduced by rotary swaging processing. • The dominated texture of the swaged pure Mg was a basal fibre. • Twinning and non-basal plane sliding accommodated the swaging process. • Gradient texture distribution was observed through the rod diameter. • There existed a slight shear deformation on the surface of the swaged rod. - Abstract: Microstructures and tensile properties of commercial pure magnesium processed by rotary swaging (RS) technique were investigated. Bulk and gradient textures in the RS processed Mg were characterised by neutron and synchrotron diffractions, respectively. Grains of the pure Mg were gradually refined with increase in the RS passes, which largely contributed to an increase in the tensile yield strength. A dominated basal fibre texture was observed in the RS processed pure Mg. Accommodated twinning deformation was also observed. Both the optical observations and texture analyses through the diameter of the swaged rod showed a gradient evolution in microstructure
DEFF Research Database (Denmark)
Sedaghatizadeh, N.; Atefi, G.; Fardad, A. A.
2011-01-01
In this investigation, semiempirical and numerical studies of blood flow in a viscoelastic artery were performed using the Cosserat continuum model. The large-amplitude oscillatory shear deformation model was used to quantify the nonlinear viscoelastic response of blood flow. The finite differenc...... method was used to solve the governing equations, and the particle swarm optimization algorithm was utilized to identify the non-Newtonian coefficients (kυ and γυ). The numerical results agreed well with previous experimental results....
CSIR Research Space (South Africa)
Yong Song, H
2017-11-01
Full Text Available -1 Korea-Australia Rheology Journal A comparative study of the effects of cone-plate and parallel- plate geometries on rheological properties under oscillatory shear flow Hyeong Yong Song1, Reza Salehiyan2, Xiaolei Li1, Seung Hak Lee1 and Kyu Hyun1...
Growth and detachment of single hydrogen bubbles in a magnetohydrodynamic shear flow
Baczyzmalski, Dominik; Karnbach, Franziska; Mutschke, Gerd; Yang, Xuegeng; Eckert, Kerstin; Uhlemann, Margitta; Cierpka, Christian
2017-09-01
This study investigates the effect of a magnetohydrodynamic (MHD) shear flow on the growth and detachment of single sub-millimeter-sized hydrogen gas bubbles. These bubbles were electrolytically generated at a horizontal Pt microelectrode (100 μ m in diameter) in an acidic environment (1 M H2SO4 ). The inherent electric field was superimposed by a homogeneous electrode-parallel magnetic field of up to 700 mT to generate Lorentz forces in the electrolyte, which drive the MHD flow. The growth and motion of the hydrogen bubble was analyzed by microscopic high-speed imaging and measurements of the electric current, while particle tracking velocimetry (μ PTV ) and particle image velocimetry (μ PIV ) were applied to measure the surrounding electrolyte flow. In addition, numerical flow simulations were performed based on the experimental conditions. The results show a significant reduction of the bubble growth time and detachment diameter with increasing magnetic induction, which is known to improve the efficiency of water electrolysis. In order to gain further insight into the bubble detachment mechanism, an analysis of the forces acting on the bubble was performed. The strong MHD-induced drag force causes the bubble to slowly slide away from the center of the microelectrode before its detachment. This motion increases the active electrode area and enhances the bubble growth rate. The results further indicate that at large current densities the coalescence of tiny bubbles formed at the foot of the main bubble might play an important role for the bubble detachment. Moreover, the occurrence of Marangoni stresses at the gas-liquid interface is discussed.
International Nuclear Information System (INIS)
Choedkiatsakul, I.; Ngaosuwan, K.; Assabumrungrat, S.; Tabasso, S.; Cravotto, G.
2015-01-01
A new simple flow system which is made up of a multi-rotor high-shear mixer connected to a multimode microwave reactor has been assembled. This simple loop reactor has been successfully used in the NaOH-catalyzed transesterification of refined palm oil in methanol. Thanks to optimal mass/heat transfer, full conversion was achieved within 5 min (biodiesel yield of 99.80%). High-quality biodiesel was obtained that is in accordance with international specifications and analytical ASTM standards. The procedure's high efficiency and low energy consumption should pave the way for process scale up. - Highlights: • The combination of HSM-MW flow system for biodiesel production has been proposed. • Highly efficient mass and heat transfer in transesterification reaction. • The hybrid reactor enables a complete conversion in 5 min reaction time. • The new system halved the energy consumption of conventional processes
Energy transport in a shear flow of particles in a two-dimensional dusty plasma.
Feng, Yan; Goree, J; Liu, Bin
2012-11-01
A shear flow of particles in a laser-driven two-dimensional (2D) dusty plasma is observed in a study of viscous heating and thermal conduction. Video imaging and particle tracking yields particle velocity data, which we convert into continuum data, presented as three spatial profiles: mean particle velocity (i.e., flow velocity), mean-square particle velocity, and mean-square fluctuations of particle velocity. These profiles and their derivatives allow a spatially resolved determination of each term in the energy and momentum continuity equations, which we use for two purposes. First, by balancing these terms so that their sum (i.e., residual) is minimized while varying viscosity η and thermal conductivity κ as free parameters, we simultaneously obtain values for η and κ in the same experiment. Second, by comparing the viscous heating and thermal conduction terms, we obtain a spatially resolved characterization of the viscous heating.
Shear-Induced Amyloid Formation in the Brain: I. Potential Vascular and Parenchymal Processes.
Trumbore, Conrad N
2016-09-06
Shear distortion of amyloid-beta (Aβ) solutions accelerates amyloid cascade reactions that may yield different toxic oligomers than those formed in quiescent solutions. Recent experiments indicate that cerebrospinal fluid (CSF) and interstitial fluid (ISF) containing Aβ flow through narrow brain perivascular pathways and brain parenchyma. This paper suggests that such flow causes shear distortion of Aβ molecules involving conformation changes that may be one of the initiating events in the etiology of Alzheimer's disease. Aβ shearing can occur in or around brain arteries and arterioles and is suggested as the origin of cerebral amyloid angiopathy deposits in cerebrovascular walls. Comparatively low flow rates of ISF within the narrow extracellular spaces (ECS) of the brain parenchyma are suggested as a possible initiating factor in both the formation of neurotoxic Aβ42 oligomers and amyloid fibrils. Aβ42 in slow-flowing ISF can gain significant shear energy at or near the walls of tortuous brain ECS flow paths, promoting the formation of a shear-distorted, excited state hydrophobic Aβ42* conformation. This Aβ42* molecule could possibly be involved in one of two paths, one involving rapid adsorption to a brain membrane surface, ultimately forming neurotoxic oligomers on membranes, and the other ultimately forming plaque within the ECS flow pathways. Rising Aβ concentrations combined with shear at or near critical brain membranes are proposed as contributing factors to Alzheimer's disease neurotoxicity. These hypotheses may be applicable in other neurodegenerative diseases, including tauopathies and alpha-synucleinopathies, in which shear-distorted proteins also may form in the brain ECS.
Development of shear bands in amorphous-crystalline metallic alloys
International Nuclear Information System (INIS)
Pozdnyakov, V.A.
2004-01-01
A theoretical study is made into conditions of shear band evolution in amorphous-crystalline alloys with various morphological types of structural constituents. The condition of shear band evolution in thin amorphous alloys in the interior of the crystalline matrix is obtained. It is shown that a scale effect exists which manifests itself in suppression of the process of localized plastic flow with amorphous alloy thickness decreasing down to the limit. The analysis of the condition for shear band evolution in an amorphous alloy with nanocrystalline inclusions is accomplished. The relationship of a critical stress of shear band evolution to a volume fraction of disperse crystal inclusions is obtained. A consideration is also given to the evolution of shear bands in the material containing amorphous and crystalline areas of micro meter size. For the alloy with the structure of this type conditions for propagation of localized flows by a relay race type mechanism are determined [ru
Shear-coupled grain-boundary migration dependence on normal strain/stress
Combe, N.; Mompiou, F.; Legros, M.
2017-08-01
In specific conditions, grain-boundary (GB) migration occurs in polycrystalline materials as an alternative vector of plasticity compared to the usual dislocation activity. The shear-coupled GB migration, the expected most efficient GB based mechanism, couples the GB motion to an applied shear stress. Stresses on GB in polycrystalline materials seldom have, however, a unique pure shear component. This work investigates the influence of a normal strain on the shear coupled migration of a Σ 13 (320 )[001 ] GB in a copper bicrystal using atomistic simulations. We show that the yield shear stress inducing the GB migration strongly depends on the applied normal stress. Beyond, the application of a normal stress on this GB qualitatively modifies the GB migration: while the Σ 13 (320 )[001 ] GB shear couples following the 〈110 〉 migration mode without normal stress, we report the observation of the 〈010 〉 mode under a sufficiently high tensile normal stress. Using the nudge elastic band method, we uncover the atomistic mechanism of this 〈010 〉 migration mode and energetically characterize it.
Thermomechanical characterization of pure polycrystalline tantalum
International Nuclear Information System (INIS)
Rittel, D.; Bhattacharyya, A.; Poon, B.; Zhao, J.; Ravichandran, G.
2007-01-01
The thermomechanical behavior of pure polycrystalline tantalum has been characterized over a wide range of strain rates, using the recently developed shear compression specimen [D. Rittel, S. Lee, G. Ravichandran, Experimental Mechanics 42 (2002) 58-64]. Dynamic experiments were carried out using a split Hopkinson pressure bar, and the specimen's temperature was monitored throughout the tests using an infrared radiometer. The results of the mechanical tests confirm previous results on pure Ta. Specifically, in addition to its significant strain rate sensitivity, it was observed that pure Ta exhibits very little strain hardening at high strain rates. The measured temperature rise in the specimen's gauge was compared to theoretical predictions which assume a total conversion of the mechanical energy into heat (β = 1) [G.I. Taylor, H. Quinney, Proceedings of the Royal Society of London, vol. A, 1934, pp. 307-326], and an excellent agreement was obtained. This result confirms the previous result of Kapoor and Nemat-Nasser [R. Kapoor, S. Nemat-Nasser, Mech. Mater. 27 (1998) 1-12], while a different experimental approach was adopted here. The assumption that β = 1 is found to be justified in this specific case by the lack of dynamic strain hardening of pure Ta. However, this assumption should be limited to non-hardening materials, to reflect the fact that strain hardening implies that part of the mechanical energy is stored into the material's microstructure
Directory of Open Access Journals (Sweden)
Zhang De-Sheng
2015-01-01
Full Text Available The prediction accuracies of partially-averaged Navier-Stokes model and improved shear stress transport k-ω turbulence model for simulating the unsteady cavitating flow around the hydrofoil were discussed in this paper. Numerical results show that the two turbulence models can effectively reproduce the cavitation evolution process. The numerical prediction for the cycle time of cavitation inception, development, detachment, and collapse agrees well with the experimental data. It is found that the vortex pair induced by the interaction between the re-entrant jet and mainstream is responsible for the instability of the cavitation shedding flow.
Scaling of turbulence spectra measured in strong shear flow near the Earth’s surface
Mikkelsen, T.; Larsen, S. E.; Jørgensen, H. E.; Astrup, P.; Larsén, X. G.
2017-12-01
Within the lowest kilometer of the Earth’s atmosphere, in the so-called atmospheric boundary layer, winds are often gusty and turbulent. Nearest to the ground, the turbulence is predominately generated by mechanical wall-bounded wind shear, whereas at higher altitudes turbulent mixing of heat and moisture also play a role. The variance (square of the standard deviation) of the fluctuation around the mean wind speed is a measure of the kinetic energy content of the turbulence. This kinetic energy can be resolved into the spectral distributions, or spectra, as functions of eddy size, wavenumber, or frequency. Spectra are derived from Fourier transforms of wind records as functions of space or time corresponding to wavenumber and frequency spectra, respectively. Atmospheric spectra often exhibit different subranges that can be distinguished and scaled by the physical parameters responsible for: (1) their generation; (2) the cascade of energy across the spectrum from large- to small-scale; and (3) the eventual decay of turbulence into heat owing to viscosity effects on the Kolmogorov microscale, in which the eddy size is only a fraction of a millimeter. This paper addresses atmospheric turbulence spectra in the lowest part of the atmospheric boundary layer—the so-called surface layer—where the wind shear is strong owing to the nonslip condition at the ground. Theoretical results dating back to Tchen’s early work in 1953 ‘on the spectrum of energy in turbulent shear flow’ led Tchen to predict a shear production subrange with a distinct inverse-linear power law for turbulence in a strongly sheared high-Reynolds number wall-bounded flow, as is encountered in the lowest sheared part of the atmospheric boundary layer, also known as the eddy surface layer. This paper presents observations of spectra measured in a meteorological mast at Høvsøre, Denmark, that support Tchen’s prediction of a shear production subrange following a distinct power law of degree -1
Non-Newtonian behavior and molecular structure of Cooee bitumen under shear flow
DEFF Research Database (Denmark)
Lemarchand, Claire; Bailey, Nicholas; Daivis, Peter
2015-01-01
The rheology and molecular structure of a model bitumen (Cooee bitumen) under shear are investigated in the non-Newtonian regime using non-equilibrium molecular dynamics simulations. The shear viscosity, normal stress differences, and pressure of the bitumen mixture are computed at different shear...... rates and different temperatures. The model bitumen is shown to be a shear-thinning fluid at all temperatures. In addition, the Cooee model is able to reproduce experimental results showing the formation of nanoaggregates composed of stacks of flat aromatic molecules in bitumen. These nanoaggregates...
State diagram for adhesion dynamics of deformable capsules under shear flow.
Luo, Zheng Yuan; Bai, Bo Feng
2016-08-17
Due to the significance of understanding the underlying mechanisms of cell adhesion in biological processes and cell capture in biomedical applications, we numerically investigate the adhesion dynamics of deformable capsules under shear flow by using a three-dimensional computational fluid dynamic model. This model is based on the coupling of the front tracking-finite element method for elastic mechanics of the capsule membrane and the adhesion kinetics simulation for adhesive interactions between capsules and functionalized surfaces. Using this model, three distinct adhesion dynamic states are predicted, such as detachment, rolling and firm-adhesion. Specifically, the effects of capsule deformability quantified by the capillary number on the transitions of these three dynamic states are investigated by developing an adhesion dynamic state diagram for the first time. At low capillary numbers (e.g. Ca state no longer appears, since capsules exhibit large deviation from the spherical shape.
The response of dense dry granular material to the shear reversal
Zhang, Jie; Ren, Jie; Farhadi, Somayeh; Behringer, Robert
2008-11-01
We have performed two dimensional granular experiments under pure shear using bidisperse photo-elastic disks. Starting from a stress free state, a square box filled with granular particles is subject to shear. The forward shears involved various number of steps, leading to maximum strains between 0.1 and 0.3. The area is kept constant during the shear. The network of force chains gradually built up as the strain increased, leading to increased pressure and shear stress. Reverse shear was then applied to the system. Depending on the initial packing fraction and the strain at which the shear is reversed, the force chain network built prior to the shear reversal may be destroyed completely or partially destroyed. Following the force chain weakening, when the reserve shear is continuously applied to the system, there is a force chain strengthening. Following each change of the system, contact forces of individual disks were measured by applying an inverse algorithm. We also kept track of the displacement and angle of rotation of every particle from frame to frame. We present the results for the structure failure and reconstruction during shear reversals. We also present data for stresses, contact force distributions and other statistical measures.
Verhulst, Kristof; Cardinaels, Ruth; Renardy, Yuriko; Moldenaers, Paula
2008-07-01
The steady deformation and orientation of droplets in shear flow, both under bulk and confined conditions, is microscopically studied for blends with one viscoelastic phase and a viscosity ratio of 1.5. The experiments are performed with a Linkam shearing cell and a counter rotating setup, based on a Paar Physica MCR300. For bulk shear flow, it is shown that matrix viscoelasticity suppresses droplet deformation and promotes droplet orientation towards the flow direction. Interestingly, these effects saturate at Deborah numbers above 2. For ellipsoidal droplets, viscoelasticity of the droplet fluid hardly affects the droplet deformation and droplet orientation, even up to Deborah numbers as high as 16. When the droplet is confined between two plates, the droplet deformation and the orientation towards the flow direction increase with confinement ratio, as in fully Newtonian systems. At a Deborah number of 1, the effect of component viscoelasticity under confined conditions remains qualitatively the same as under bulk conditions, at least up to a confinement ratio 2R/H of 0.6. The experiments under bulk conditions are compared with the predictions of phenomenological models, such as the Maffettone-Minale model, for droplet deformation. The Shapira-Haber model, which analytically describes the effects of the walls on the droplet deformation for fully Newtonian systems, is used to describe the experimental results under confinement. Here, this model is combined with the bulk phenomenological models to include bulk viscoelasticity effects. Under the present conditions, the adapted Shapira-Haber model describes the steady droplet deformation under confinement rather well. Finally, the experimentally obtained droplet shapes are compared with the results of 3D simulations, performed with a volume-of-fluid algorithm.
Structure-rheology relationship in a sheared lamellar fluid.
Jaju, S J; Kumaran, V
2016-03-01
The structure-rheology relationship in the shear alignment of a lamellar fluid is studied using a mesoscale model which provides access to the lamellar configurations and the rheology. Based on the equations and free energy functional, the complete set of dimensionless groups that characterize the system are the Reynolds number (ργL(2)/μ), the Schmidt number (μ/ρD), the Ericksen number (μγ/B), the interface sharpness parameter r, the ratio of the viscosities of the hydrophilic and hydrophobic parts μ(r), and the ratio of the system size and layer spacing (L/λ). Here, ρ and μ are the fluid density and average viscosity, γ is the applied strain rate, D is the coefficient of diffusion, B is the compression modulus, μ(r) is the maximum difference in the viscosity of the hydrophilic and hydrophobic parts divided by the average viscosity, and L is the system size in the cross-stream direction. The lattice Boltzmann method is used to solve the concentration and momentum equations for a two dimensional system of moderate size (L/λ=32) and for a low Reynolds number, and the other parameters are systematically varied to examine the qualitative features of the structure and viscosity evolution in different regimes. At low Schmidt numbers where mass diffusion is faster than momentum diffusion, there is fast local formation of randomly aligned domains with "grain boundaries," which are rotated by the shear flow to align along the extensional axis as time increases. This configuration offers a high resistance to flow, and the layers do not align in the flow direction even after 1000 strain units, resulting in a viscosity higher than that for an aligned lamellar phase. At high Schmidt numbers where momentum diffusion is fast, the shear flow disrupts layers before they are fully formed by diffusion, and alignment takes place by the breakage and reformation of layers by shear, resulting in defects (edge dislocations) embedded in a background of nearly aligned layers
Shear modulus and damping ratio of natural rubber containing carbon nanotubes
Ismail, R.; Ibrahim, A.; Rusop, M.; Adnan, A.
2018-05-01
This paper presents the results of an investigation into the potential application of Natural rubber (NR) containing Carbon Nanotubes (CNTs) by measuring its shear modulus and damping ratio. Four different types of rubber specimens which fabricated with different MWCNT loadings: 0 wt% (pure natural rubber), 1 wt%, 3 wt%, and 5 wt%. It is observed that the shear modulus and damping ratio of CNTs filled rubber composites are remarkably higher than that of raw rubber indicating the inherent reinforcing potential of CNTs.
Circulatory shear flow alters the viability and proliferation of circulating colon cancer cells
Fan, Rong; Emery, Travis; Zhang, Yongguo; Xia, Yuxuan; Sun, Jun; Wan, Jiandi
2016-06-01
During cancer metastasis, circulating tumor cells constantly experience hemodynamic shear stress in the circulation. Cellular responses to shear stress including cell viability and proliferation thus play critical roles in cancer metastasis. Here, we developed a microfluidic approach to establish a circulatory microenvironment and studied circulating human colon cancer HCT116 cells in response to a variety of magnitude of shear stress and circulating time. Our results showed that cell viability decreased with the increase of circulating time, but increased with the magnitude of wall shear stress. Proliferation of cells survived from circulation could be maintained when physiologically relevant wall shear stresses were applied. High wall shear stress (60.5 dyne/cm2), however, led to decreased cell proliferation at long circulating time (1 h). We further showed that the expression levels of β-catenin and c-myc, proliferation regulators, were significantly enhanced by increasing wall shear stress. The presented study provides a new insight to the roles of circulatory shear stress in cellular responses of circulating tumor cells in a physiologically relevant model, and thus will be of interest for the study of cancer cell mechanosensing and cancer metastasis.
Dilatancy induced ductile-brittle transition of shear band in metallic glasses
Zeng, F.; Jiang, M. Q.; Dai, L. H.
2018-04-01
Dilatancy-generated structural disordering, an inherent feature of metallic glasses (MGs), has been widely accepted as the physical mechanism for the primary origin and structural evolution of shear banding, as well as the resultant shear failure. However, it remains a great challenge to determine, to what degree of dilatation, a shear banding will evolve into a runaway shear failure. In this work, using in situ acoustic emission monitoring, we probe the dilatancy evolution at the different stages of individual shear band in MGs that underwent severely plastic deformation by the controlled cutting technology. A scaling law is revealed that the dilatancy in a shear band is linearly related to its evolution degree. A transition from ductile-to-brittle shear bands is observed, where the formers dominate stable serrated flow, and the latter lead to a runaway instability (catastrophe failure) of serrated flow. To uncover the underlying mechanics, we develop a theoretical model of shear-band evolution dynamics taking into account an atomic-scale deformation process. Our theoretical results agree with the experimental observations, and demonstrate that the atomic-scale volume expansion arises from an intrinsic shear-band evolution dynamics. Importantly, the onset of the ductile-brittle transition of shear banding is controlled by a critical dilatation.
Ordering fluctuations in a shear-banding wormlike micellar system
DEFF Research Database (Denmark)
Angelico, R.; Rossi, C. Oliviero; Ambrosone, L.
2010-01-01
We present a first investigation about the non-linear flow properties and transient orientational-order fluctuations observed in the shear-thinning lecithin–water–cyclohexane wormlike micellar system at a concentration near to the zero-shear isotropic–nematic phase transition. From rheological...
Liao, G. K.; Long, Z. L.; Zhao, M. S. Z.; Peng, L.; Chai, W.; Ping, Z. H.
2018-04-01
This paper presents the research on the evolution of shear transformation zone (STZ) in a Pd-based bulk metallic glass (BMG) during serrated flow under nanoindentation. A novel method of estimating the STZ volume through statistical analysis of the serrated flow behavior was proposed for the first time. Based on the proposed method, the STZ volume of the studied BMG at various peak loads have been systematically investigated. The results indicate that the measured STZ volumes are in good agreement with that documented in literature, and the STZ size exhibits an increasing trend during indentation. Moreover, the correlation between the serrated flow dynamics and the STZ activation has also been evaluated. It is found that the STZ activation can promote the formation of self-organized critical (SOC) state during serrated flow.
International Nuclear Information System (INIS)
Satoh, Akira; Hayasaka, Ryo; Majima, Tamotsu
2008-01-01
We have treated a dilute dispersion composed of ferromagnetic rodlike particles with a magnetic moment normal to the particle axis, such as hematites, to investigate the influences of the magnetic field strength, shear rate, and random forces on the orientational distribution of rodlike particles and also on transport coefficients, such as viscosity and diffusion coefficient. In the present analysis, these rodlike particles are assumed to conduct the rotational Brownian motion in a simple shear flow as well as an external magnetic field. The results obtained here are summarized as follows. In the case of a strong magnetic field and a smaller shear rate, the rodlike particle can freely rotate in the xy-plane with the magnetic moment continuing to point the magnetic field direction. On the other hand, for a strong shear flow, the particle has a tendency to incline in the flow direction with the magnetic moment pointing to the magnetic field direction. In the case of the magnetic field applied normal to the direction of the sedimentation, the diffusion coefficient gives rise to smaller values than expected, since the rodlike particle sediments with the particle axis inclining toward directions normal to the movement direction and, of course, toward the direction along that direction
In vivo wall shear measurements within the developing zebrafish heart.
Directory of Open Access Journals (Sweden)
R Aidan Jamison
Full Text Available Physical forces can influence the embryonic development of many tissues. Within the cardiovascular system shear forces resulting from blood flow are known to be one of the regulatory signals that shape the developing heart. A key challenge in investigating the role of shear forces in cardiac development is the ability to obtain shear force measurements in vivo. Utilising the zebrafish model system we have developed a methodology that allows the shear force within the developing embryonic heart to be determined. Accurate wall shear measurement requires two essential pieces of information; high-resolution velocity measurements near the heart wall and the location and orientation of the heart wall itself. We have applied high-speed brightfield imaging to capture time-lapse series of blood flow within the beating heart between 3 and 6 days post-fertilization. Cardiac-phase filtering is applied to these time-lapse images to remove the heart wall and other slow moving structures leaving only the red blood cell movement. Using particle image velocimetry to calculate the velocity of red blood cells in different regions within the heart, and using the signal-to-noise ratio of the cardiac-phase filtered images to determine the boundary of blood flow, and therefore the position of the heart wall, we have been able to generate the necessary information to measure wall shear in vivo. We describe the methodology required to measure shear in vivo and the application of this technique to the developing zebrafish heart. We identify a reduction in shear at the ventricular-bulbar valve between 3 and 6 days post-fertilization and demonstrate that the shear environment of the ventricle during systole is constantly developing towards a more uniform level.
In vivo wall shear measurements within the developing zebrafish heart.
Jamison, R Aidan; Samarage, Chaminda R; Bryson-Richardson, Robert J; Fouras, Andreas
2013-01-01
Physical forces can influence the embryonic development of many tissues. Within the cardiovascular system shear forces resulting from blood flow are known to be one of the regulatory signals that shape the developing heart. A key challenge in investigating the role of shear forces in cardiac development is the ability to obtain shear force measurements in vivo. Utilising the zebrafish model system we have developed a methodology that allows the shear force within the developing embryonic heart to be determined. Accurate wall shear measurement requires two essential pieces of information; high-resolution velocity measurements near the heart wall and the location and orientation of the heart wall itself. We have applied high-speed brightfield imaging to capture time-lapse series of blood flow within the beating heart between 3 and 6 days post-fertilization. Cardiac-phase filtering is applied to these time-lapse images to remove the heart wall and other slow moving structures leaving only the red blood cell movement. Using particle image velocimetry to calculate the velocity of red blood cells in different regions within the heart, and using the signal-to-noise ratio of the cardiac-phase filtered images to determine the boundary of blood flow, and therefore the position of the heart wall, we have been able to generate the necessary information to measure wall shear in vivo. We describe the methodology required to measure shear in vivo and the application of this technique to the developing zebrafish heart. We identify a reduction in shear at the ventricular-bulbar valve between 3 and 6 days post-fertilization and demonstrate that the shear environment of the ventricle during systole is constantly developing towards a more uniform level.
Nelson, Arif Z.; Ewoldt, Randy H.
2017-11-01
Design in fluid mechanics often focuses on optimizing geometry (airfoils, surface textures, microfluid channels), but here we focus on designing fluids themselves. The dramatically shear-thinning ``yield-stress fluid'' is currently the most utilized non-Newtonian fluid phenomenon. These rheologically complex materials, which undergo a reversible transition from solid-like to liquid-like fluid flow, are utilized in pedestrian products such as paint and toothpaste, but also in emerging applications like direct-write 3D printing. We present a paradigm for yield-stress fluid design that considers constitutive model representation, material property databases, available predictive scaling laws, and the many ways to achieve a yield stress fluid, flipping the typical structure-to-rheology analysis to become the inverse: rheology-to-structure with multiple possible materials as solutions. We describe case studies of 3D printing inks and other flow scenarios where designed shear-thinning enables performance remarkably beyond that of Newtonian fluids. This work was supported by Wm. Wrigley Jr. Company and the National Science Foundation under Grant No. CMMI-1463203.
Development of tearing instability in a current sheet forming by sheared incompressible flow
Tolman, Elizabeth A.; Loureiro, Nuno F.; Uzdensky, Dmitri A.
2018-02-01
Sweet-Parker current sheets in high Lundquist number plasmas are unstable to tearing, suggesting they will not form in physical systems. Understanding magnetic reconnection thus requires study of the stability of a current sheet as it forms. Formation can occur due to sheared, sub-Alfvénic incompressible flows which narrow the sheet. Standard tearing theory (Furth et al. Phys. Fluids, vol. 6 (4), 1963, pp. 459-484, Rutherford, Phys. Fluids, vol. 16 (11), 1973, pp. 1903-1908, Coppi et al. Fizika Plazmy, vol. 2, 1976, pp. 961-966) is not immediately applicable to such forming sheets for two reasons: first, because the flow introduces terms not present in the standard calculation; second, because the changing equilibrium introduces time dependence to terms which are constant in the standard calculation, complicating the formulation of an eigenvalue problem. This paper adapts standard tearing mode analysis to confront these challenges. In an initial phase when any perturbations are primarily governed by ideal magnetohydrodynamics, a coordinate transformation reveals that the flow compresses and stretches perturbations. A multiple scale formulation describes how linear tearing mode theory (Furth et al. Phys. Fluids, vol. 6 (4), 1963, pp. 459-484, Coppi et al. Fizika Plazmy, vol. 2, 1976, pp. 961-966) can be applied to an equilibrium changing under flow, showing that the flow affects the separable exponential growth only implicitly, by making the standard scalings time dependent. In the nonlinear Rutherford stage, the coordinate transformation shows that standard theory can be adapted by adding to the stationary rates time dependence and an additional term due to the strengthening equilibrium magnetic field. Overall, this understanding supports the use of flow-free scalings with slight modifications to study tearing in a forming sheet.