Experimental and numerical investigation on two-phase flow instabilities
Energy Technology Data Exchange (ETDEWEB)
Ruspini, Leonardo Carlos
2013-03-01
Two-phase flow instabilities are experimentally and numerically studied within this thesis. In particular, the phenomena called Ledinegg instability, density wave oscillations and pressure drop oscillations are investigated. The most important investigations regarding the occurrence of two-phase flow instabilities are reviewed. An extensive description of the main contributions in the experimental and analytical research is presented. In addition, a critical discussion and recommendations for future investigations are presented. A numerical framework using a hp-adaptive method is developed in order to solve the conservation equations modelling general thermo-hydraulic systems. A natural convection problem is analysed numerically in order to test the numerical solver. Moreover, the description of an adaptive strategy to solve thermo-hydraulic problems is presented. In the second part of this dissertation, a homogeneous model is used to study Ledinegg, density wave and pressure drop oscillations phenomena numerically. The dynamic characteristics of the Ledinegg (flow excursion) phenomenon are analysed through the simulation of several transient examples. In addition, density wave instabilities in boiling and condensing systems are investigated. The effects of several parameters, such as the fluid inertia and compressibility volumes, on the stability limits of Ledinegg and density wave instabilities are studied, showing a strong influence of these parameters. Moreover, the phenomenon called pressure drop oscillations is numerically investigated. A discussion of the physical representation of several models is presented with reference to the obtained numerical results. Finally, the influence of different parameters on these phenomena is analysed. In the last part, an experimental investigation of these phenomena is presented. The designing methodology used for the construction of the experimental facility is described. Several simulations and a non
Experimental studies of magnetorotational instability in differentially rotating cylindrical flows
Brawn, Barbara; Lathrop, Daniel
2006-11-01
Given the ubiquity of rotating disks in the observable universe (e.g., galaxies, planetary rings, protoplanetary disks and accretion disks around compact objects), understanding differentially rotating, electrically conducting flows is of considerable astrophysical interest. Theoretical and numerical studies indicate that infall and accretion of orbiting material can result from a so-called magnetorotational instability (MRI) arising in such flows. Recent experimental work suggests that MRI is observable in a laboratory setting; inspired by these observations, we are building a sodium Taylor-Couette experiment, comprised of a stationary 30 cm diameter outer cylinder and a rotating 15 cm diameter inner cylinder, with liquid sodium filling the gap between the cylinders. Numerical studies indicate that MRI arises in this geometry in the presence of an external magnetic field; we will impose on the sodium flow a uniform axial magnetic field produced by Helmholtz coils at either end of the experiment. We will use ultrasound Doppler velocimetry to examine the turbulent sodium flow, and a Hall probe array to examine the induced magnetic field of the system, and will relate our observations to theoretical and numerical expectations.
Experimental research on flow instability in vertical narrow annuli
Institute of Scientific and Technical Information of China (English)
WU Geping; QIU Suizheng; SU Guanghui; JIA Dounan
2007-01-01
A narrow annular test section of 1.5mm gap and 1800mm length was designed and manufactured, with good tightness and insulation. Experiments were carried out to investigate characteristics of flow instability of forced-convection in vertical narrow annuli. Using distilled water as work fluid, the experiments were conducted at pressures of 1.0～3.0 MPa, mass flow rates of 3.0～25 kg/h, heating power of 3.0～ 6.5kW and inlet fluid temperature of 20 ℃, 40 ℃ or 60℃. It was found that flow instability occured with fixed inlet condition and heating power when mass flow rate was below a special value. Effects of inlet subcooling, system pressure and mass flow rate on the system behavior were studied and the instability region was given.
Experimental study of natural circulation flow instability in rectangular channels
Energy Technology Data Exchange (ETDEWEB)
Zhou, Tao; Qi, Shi; Song, Mingqiang [North China Electric Power Univ., Beijing (China). School of Nuclear Science and Engineering; Passive Nuclear Safety Technology, Beijing (China). Beijing Key Lab.; Xiao, Zejun [Nuclear, Reactor Thermal Hydraulics Technology, Chengdu (China). CNNC Key Lab.
2017-05-15
Experiments of natural circulation flow instability were conducted in rectangular channels with 5 mm and 10 mm wide gaps. Results for different heating powers were obtained. The results showed that the flow will tend to be instable with the growing of heating power. The oscillation period of pressure D-value and volume flow are the same, but their phase positions are opposite. They both can be described by trigonometric functions. The existence of edge position and secondary flow will strengthen the disturbance of fluid flow in rectangle channels, which contributes to heat transfer. The disturbance of bubble and fluid will be strengthened, especially in the saturated boiling section, which make it possible for the mixing flow. The results also showed that the resistance in 5 mm channel is bigger than that in 10 mm channel, it is less likely to form stable natural circulation in the subcooled region.
Experimental evidence of a helical, supercritical instability in pipe flow of shear thinning fluids
Picaut, L.; Ronsin, O.; Caroli, C.; Baumberger, T.
2017-08-01
We study experimentally the flow stability of entangled polymer solutions extruded through glass capillaries. We show that the pipe flow becomes linearly unstable beyond a critical value (Wic≃5 ) of the Weissenberg number, via a supercritical bifurcation which results in a helical distortion of the extrudate. We find that the amplitude of the undulation vanishes as the aspect ratio L /R of the capillary tends to zero, and saturates for large L /R , indicating that the instability affects the whole pipe flow, rather than the contraction or exit regions. These results, when compared to previous theoretical and experimental works, lead us to argue that the nature of the instability is controlled by the level of shear thinning of the fluids. In addition, we provide strong hints that the nonlinear development of the instabiilty is mitigated, in our system, by the gradual emergence of gross wall slip.
A Experimental and Computational Study of Flow Instability in a Helical Coil.
Webster, Donald Robert
This study concerns the transitional regime between laminar and turbulent flow states in a helically coiled pipe of circular cross-section. The study consists of complementary experimental measurements and numerical calculations in a coil with a radius of curvature to pipe radius ratio (R_{rm c}/a) equal to 18.2. A new test section and flow apparatus were constructed. The streamwise pressure drop measurements agreed very well with those of previous investigations. Laser-Doppler velocimetry (LDV) measurements of two instantaneous velocity components were obtained along the midplane of the pipe cross-section at a nominally fully developed location in the coil. Thirteen Reynolds numbers were examined in the range 3800 laminar, transitional, and turbulent flow regimes. In the range 5060 flow oscillations (at St = 0.25 and 0.5) in the inner half of the pipe cross-section due to a traveling wave instability. Similar low frequency unsteadiness was not observed near the outer wall. A significant local maximum in the rms velocity was observed near the pipe center for Re > 5060 and is attributed to the velocity perturbation of the traveling wave instability. Still photographs and video images of the flow visualization by means of a dye streak in the range 5060 flow is turbulent beyond this range. The CUTEFLOWS algorithm was modified to solve numerically the finite difference approximation of the Navier-Stokes equations formulated for the toroidal coordinate system. The unsteady three-dimensional calculations were performed for Re = 5480 (De = 1280). The mean characteristics of the flow predicted by the calculations agree very well with the experimental data. The flow perturbation due to the traveling wave agrees qualitatively for all three grid refinements and with the experimental data. The calculated results indicate that energy is transferred to the traveling wave from the mean flow through a complex interaction between the centripetal acceleration in the inner half of
Institute of Scientific and Technical Information of China (English)
无
1996-01-01
An experimental investigation is described for the characteristics of convective boiling flow instabilities in horizontally helically coiled tubes using a steam-water two-phase closed circulation test loop at pressure from 0.5 MPa to 3.5MPa.Three kinds of oscillation are reported.density waves;pressure drop excorsions;thermal fluctuations.We describe their dependence on main system parameters such as system pressure,mass flowrate,inlet subcooling,compressible volume and heat flux.Utilising the experimental data together with conservation constraints,a dimensionless correlation is proposed for the occurrence of density waves.
Directory of Open Access Journals (Sweden)
Hiong Yap Gan
2012-12-01
Full Text Available Viscoelastically induced flow instabilities, via a simple planar microchannel, were previously used to produce rapid mixing of two dissimilar polymeric liquids (i.e. at least a hundredfold different in shear viscosity even at a small Reynolds number. The unique advantage of this mixing technology is that viscoelastic liquids are readily found in chemical and biological samples like organic and polymeric liquids, blood and crowded proteins samples; their viscoelastic properties could be exploited. As such, an understanding of the underlying interactions will be important especially in rapid microfluidic mixing involving multiple-stream flow of complex (viscoelastic fluids in biological assays. Here, we use the same planar device to experimentally show that the elasticity ratio (i.e. the ratio of stored elastic energy to be relaxed between two liquids indeed plays a crucial role in the entire flow kinematics and the enhanced mixing. We demonstrate here that the polymer stretching dynamics generated in the upstream converging flow and the polymer relaxation events occurring in the downstream channel are not exclusively responsible for the transverse flow mixing, but the elasticity ratio is also equally important. The role of elasticity ratio for transverse flow instability and the associated enhanced mixing were illustrated based on experimental observations. A new parameter Deratio = Deside / Demain (i.e. the ratio of the Deborah number (De of the sidestream to the mainstream liquids is introduced to correlate the magnitude of energy discontinuity between the two liquids. A new Deratio-Demain operating space diagram was constructed to present the observation of the effects of both elasticity and energy discontinuity in a compact manner, and for a general classification of the states of flow development.
Experimental investigation of three-dimensional flow instabilities in a rotating lid-driven cavity
DEFF Research Database (Denmark)
Sørensen, Jens Nørkær; Naumov, I.; Mikkelsen, Robert Flemming
2006-01-01
The flow between a rotating lid and a stationary cylinder is studied experimentally. The flow is governed by two parameters: The ratio of container height to disk radius, h, and the Reynolds number, Re, based on the disk angular velocity, cylinder radius and kinematic viscosity of the working...
Experimental investigation of three-dimensional flow instabilities in a rotating lid-driven cavity
DEFF Research Database (Denmark)
Sørensen, Jens Nørkær; Naumov, I.; Mikkelsen, Robert Flemming
2006-01-01
The flow between a rotating lid and a stationary cylinder is studied experimentally. The flow is governed by two parameters: The ratio of container height to disk radius, h, and the Reynolds number, Re, based on the disk angular velocity, cylinder radius and kinematic viscosity of the working...... liquid. For the first time the onset of three-dimensionality and transition are analysed by combining the high spatial resolution of Particle Image Velocimetry (PIV) and the temporal accuracy of Laser Doppler Anemometry (LDA). A detailed mapping of the transition from steady and axisymmetric flow...
DEFF Research Database (Denmark)
Sørensen, Jens Nørkær; Gelfgat, A. Yu; Naumov, I. V.;
2009-01-01
The three-dimensional axisymmetry-breaking instability of axisymmetric flow between a rotating lid and a stationary cylinder is analyzed both numerically and experimentally for the case of tall cylinders with the height/radius aspect ratio between 3.3 and 5.5. A complete stability diagram...... for each mode. The onset of three-dimensional flow behavior is measured by combining the high spatial resolution of particle image velocimetry and the temporal accuracy of laser Doppler anemometry. The results are compared to the numerical stability analysis. The measured onset of three dimensionality...... is in a good agreement with the numerical results. Disagreements observed in supercritical regimes can be explained by secondary bifurcations that are not accounted for by linear stability analysis of the primary base flow. ©2009 American Institute of Physics...
Experimental investigation of three-dimensional flow instabilities in a rotating lid-driven cavity
Energy Technology Data Exchange (ETDEWEB)
Soerensen, Jens Noerkaer; Mikkelsen, Robert [Technical University of Denmark, Department of Mechanical Engineering, Lyngby (Denmark); Naumov, Igor [Technical University of Denmark, Department of Mechanical Engineering, Lyngby (Denmark); SB RAS, Institute of Thermophysics, Novosibirsk (Russian Federation)
2006-09-15
The swirling flow between a rotating lid and a stationary cylinder is studied experimentally. The flow is governed by two parameters: the ratio of container height to disk radius, h, and the Reynolds number, Re, based on the disk angular velocity, cylinder radius and kinematic viscosity of the working liquid. For the first time, the onset of three-dimensional flow behavior is measured by combining the high spatial resolution of particle image velocimetry and the temporal accuracy of laser Doppler anemometry. A detailed mapping of the transition scenario from steady and axisymmetric flow to unsteady and three-dimensional flow is investigated for 1 {>=}h{>=} 3.5. The flow is characterized by the development of azimuthal modes of different wave numbers. A range of different modes is detected and critical Reynolds numbers and associated frequencies are identified. The results are compared to the numerical stability analysis of Gelfgat et al. (J Fluid Mech 438:363-377, 2001). In most cases, the measured onset of three-dimensionality is in good agreement with the numerical results and disagreements can be explained by bifurcations not accounted for by the numerical stability analysis. (orig.)
Omeroglu, Gokhan; Comakli, Omer; Karagoz, Sendogan; Sahin, Bayram
2013-01-01
The aim of this study is to experimentally investigate the effect of the coiled wire insertions on dynamic instabilities and to compare the results with the smooth tube for forced convection boiling. The experiments were conducted in a circular tube, and water was used as the working fluid. Two different pitch ratios (H/D = 2.77 and 5.55) of coiled wire with circular cross-sections were utilised. The constant heat flux boundary condition was applied to the outer side of the test tube, and the constant exit restriction was used at the tube outlet. The mass flow rate changed from 110 to 20 g/s in order to obtain a detailed idea about the density wave and pressure drop oscillations, and the range of the inlet temperature was 15-35°C. The changes in pressure drop, inlet temperature, amplitude, and the period with mass flow rate are presented. For each configuration, it is seen that density wave and pressure drop oscillations occur at all inlet temperatures. Analyses show that the decrease in the mass flow rate and inlet temperature causes the amplitude and the period of the density wave and the pressure drop oscillations to decrease separately.
Directory of Open Access Journals (Sweden)
Gokhan Omeroglu
2013-01-01
Full Text Available The aim of this study is to experimentally investigate the effect of the coiled wire insertions on dynamic instabilities and to compare the results with the smooth tube for forced convection boiling. The experiments were conducted in a circular tube, and water was used as the working fluid. Two different pitch ratios (H/D=2.77 and 5.55 of coiled wire with circular cross-sections were utilised. The constant heat flux boundary condition was applied to the outer side of the test tube, and the constant exit restriction was used at the tube outlet. The mass flow rate changed from 110 to 20 g/s in order to obtain a detailed idea about the density wave and pressure drop oscillations, and the range of the inlet temperature was 15–35°C. The changes in pressure drop, inlet temperature, amplitude, and the period with mass flow rate are presented. For each configuration, it is seen that density wave and pressure drop oscillations occur at all inlet temperatures. Analyses show that the decrease in the mass flow rate and inlet temperature causes the amplitude and the period of the density wave and the pressure drop oscillations to decrease separately.
Experimental observation of capillary instabilities of two phase flow in a microfluidic T-junction
CSIR Research Space (South Africa)
Mbanjwa, MB
2010-01-01
Full Text Available . Table 1 lists the volume and surface force ratios represented in terms of the important dimensionless numbers, for a characteristic two-phase flow in a microchannel with a hydraulic diameter Dh and average velocity V, where ? and ? are fluid density... of transparent PDMS elastomer using soft lithography techniques. The fluids were fed into the microchannel using syringe pumps, which were independently controlled. 10?l/ml (1% v/v) of sorbitan monolaurate (Span 20) surfactant was dissolved in the oil phase...
Elastic instability in stratified core annular flow.
Bonhomme, Oriane; Morozov, Alexander; Leng, Jacques; Colin, Annie
2011-06-01
We study experimentally the interfacial instability between a layer of dilute polymer solution and water flowing in a thin capillary. The use of microfluidic devices allows us to observe and quantify in great detail the features of the flow. At low velocities, the flow takes the form of a straight jet, while at high velocities, steady or advected wavy jets are produced. We demonstrate that the transition between these flow regimes is purely elastic--it is caused by the viscoelasticity of the polymer solution only. The linear stability analysis of the flow in the short-wave approximation supplemented with a kinematic criterion captures quantitatively the flow diagram. Surprisingly, unstable flows are observed for strong velocities, whereas convected flows are observed for low velocities. We demonstrate that this instability can be used to measure the rheological properties of dilute polymer solutions that are difficult to assess otherwise.
Elastic instability in stratified core annular flow
Bonhomme, Oriane; Leng, Jacques; Colin, Annie
2010-01-01
We study experimentally the interfacial instability between a layer of dilute polymer solution and water flowing in a thin capillary. The use of microfluidic devices allows us to observe and quantify in great detail the features of the flow. At low velocities, the flow takes the form of a straight jet, while at high velocities, steady or advected wavy jets are produced. We demonstrate that the transition between these flow regimes is purely elastic -- it is caused by viscoelasticity of the polymer solution only. The linear stability analysis of the flow in the short-wave approximation captures quantitatively the flow diagram. Surprisingly, unstable flows are observed for strong velocities, whereas convected flows are observed for low velocities. We demonstrate that this instability can be used to measure rheological properties of dilute polymer solutions that are difficult to assess otherwise.
Patterns and instability of grannular flow
Energy Technology Data Exchange (ETDEWEB)
Ecke, Robert E [Los Alamos National Laboratory; Borzsonyi, Tamas [NON LANL; Mcelwaine, Jim N [U. CAMBRIDGE
2009-01-01
Dense granular flows are often observed to become unstable and form inhomogeneous structures in nature or industry. Although recently significant advances have been made in understanding simple flows, instabilities are often not understood in detail. We present experimental and numerical results that show the formation of longitudinal stripes. These arise from instability of the uniform flowing state of granular media on a rough inclined plane. The form of the stripes depends critically on the mean density of the flow with a robust form of stripes at high density that consists of fast sliding plug-like regions (stripes) on top of highly agitated boiling material -- a configuration reminiscent of the Leidenfrost effect when a droplet of liquid lifted by its vapor is hovering above a hot surface.
Two-phase flow instabilities in a vertical annular channel
Energy Technology Data Exchange (ETDEWEB)
Babelli, I.; Nair, S.; Ishii, M. [Purdue Univ., West Lafayette, IN (United States)
1995-09-01
An experimental test facility was built to study two-phase flow instabilities in vertical annular channel with emphasis on downward flow under low pressure and low flow conditions. The specific geometry of the test section is similar to the fuel-target sub-channel of the Savannah River Site (SRS) Mark 22 fuel assembly. Critical Heat Flux (CHF) was observed following flow excursion and flow reversal in the test section. Density wave instability was not recorded in this series of experimental runs. The results of this experimental study show that flow excursion is the dominant instability mode under low flow, low pressure, and down flow conditions. The onset of instability data are plotted on the subcooling-Zuber (phase change) numbers stability plane.
Secondary instability of wall-bounded shear flows
Orszag, S. A.; Patera, A. T.
1983-01-01
The present analysis of a secondary instability in a wide class of wall-bounded parallel shear flows indicates that two-dimensional, finite amplitude waves are exponentially unstable to infinitessimal three-dimensional disturbances. The instability appears to be the prototype of transitional instability in such flows as Poiseuille flow, Couette flow, and flat plate boundary layers, in that it has the convective time scales observed in the typical transitions. The energetics and vorticity dynamics of the instability are discussed, and it is shown that the two-dimensional perturbation without directly providing energy to the disturbance. The three-dimensional instability requires that a threshold two-dimensional amplitude be achieved. It is found possible to identify experimental features of transitional spot structure with aspects of the nonlinear two-dimensional/linear three-dimensional instability.
Taylor instability in rhyolite lava flows
Baum, B. A.; Krantz, W. B.; Fink, J. H.; Dickinson, R. E.
1989-01-01
A refined Taylor instability model is developed to describe the surface morphology of rhyolite lava flows. The effect of the downslope flow of the lava on the structures resulting from the Taylor instability mechanism is considered. Squire's (1933) transformation is developed for this flow in order to extend the results to three-dimensional modes. This permits assessing why ridges thought to arise from the Taylor instability mechanism are preferentially oriented transverse to the direction of lava flow. Measured diapir and ridge spacings for the Little and Big Glass Mountain rhyolite flows in northern California are used in conjunction with the model in order to explore the implications of the Taylor instability for flow emplacement. The model suggests additional lava flow features that can be measured in order to test whether the Taylor instability mechanism has influenced the flows surface morphology.
Instabilities of flows and transition to turbulence
Sengupta, Tapan K
2012-01-01
Introduction to Instability and TransitionIntroductionWhat Is Instability?Temporal and Spatial InstabilitySome Instability MechanismsComputing Transitional and Turbulent FlowsFluid Dynamical EquationsSome Equilibrium Solutions of the Basic EquationBoundary Layer TheoryControl Volume Analysis of Boundary LayersNumerical Solution of the Thin Shear Layer (TSL) EquationLaminar Mixing LayerPlane Laminar JetIssues of Computing Space-Time Dependent FlowsWave Interaction: Group Velocity and Energy FluxIssues of Space-Time Scale Resolution of FlowsTemporal Scales in Turbulent FlowsComputing Time-Averag
Instability in electromagnetically driven flows Part II
Imazio, Paola Rodriguez
2016-01-01
In a previous paper, we have reported numerical simulations of the MHD flow driven by a travelling magnetic field (TMF) in an annular channel, at low Reynolds number. It was shown that the stalling of such induction pump is strongly related to magnetic flux expulsion. In the present article, we show that for larger hydrodynamic Reynolds number, and with more realistic boundary conditions, this instability takes the form of a large axisymmetric vortex flow in the (r,z)-plane, in which the fluid is locally pumped in the direction opposite to the one of the magnetic field. Close to the marginal stability of this vortex flow, a low-frequency pulsation is generated. Finally, these results are compared to theoretical predictions and are discussed within the framework of experimental annular linear induction electromagnetic pumps.
Instability in electromagnetically driven flows. II
Rodriguez Imazio, Paola; Gissinger, Christophe
2016-03-01
In a previous paper, we have reported numerical simulations of the magnetohydrodynamic flow driven by a travelling magnetic field in an annular channel, at low Reynolds number. It was shown that the stalling of such induction pump is strongly related to magnetic flux expulsion. In the present article, we show that for larger hydrodynamic Reynolds number, and with more realistic boundary conditions, this instability takes the form of a large axisymmetric vortex flow in the (r, z)-plane, in which the fluid is locally pumped in the direction opposite to the one of the magnetic field. Close to the marginal stability of this vortex flow, a low-frequency pulsation is generated. Finally, these results are compared to theoretical predictions and are discussed within the framework of experimental annular linear induction electromagnetic pumps.
Asymmetric Vesicle Instability in Extensional Flow
Spann, Andrew; Zhao, Hong; Shaqfeh, Eric
2012-11-01
Previous researchers have chronicled the breakup of drops in an extensional flow as they stretch into a dumbbell shape with a long thin neck. Motivated by recent experimental observations, we study an apparently similar problem with vesicles, which are deformable but incompressible membranes that conserve area and volume. First, we simulate vesicles in an unbounded uniaxial extensional flow which are given general radial perturbations from an initially stable symmetric equilibrium state. For sufficiently low reduced volume (outer viscosity) there exists a capillary number at which an asymmetric perturbation mode will grow, resulting in the formation of an asymmetric dumbbell shape with a thin connecting cylindrical bridge analogous to the shapes associated with drop breakup. Our simulations help elucidate a mechanism for this instability based on a competition between internal pressure differentials in the vesicle resulting from the membrane bending force and ambient flow. We compare and contrast this transition to the ``standard'' drop breakup transition. Funded by NSF GRFP and Stanford Graduate Fellowship.
Hydrodynamic instability of nanofluids in a channel flow
Energy Technology Data Exchange (ETDEWEB)
Lin, Jianzhong; Xia, Yi [Department of Mechanics, Zhejiang University, Hangzhou 310027, People’s Republic of China (China); Bao, Fubing, E-mail: mecjzlin@zju.edu.cn [Institution of Fluid Mechanics, China Jiliang University, Hangzhou 310018, People’s Republic of China (China)
2014-10-01
A linear hydrodynamic instability of nanofluids in a channel flow is investigated. The instability equations of nanofluids are derived and solved numerically. The validity of the numerical formulations and schemes is tested by comparing the present results with the available experimental data and theoretical results. The results show that the existence of particles suppresses the flow instability, but cannot completely eliminate it. As particle mass loading is increased, the region of unstable wavenumbers is reduced from that of the pure Newtonian flow and the largest growth rate that governs the flow instability is reduced. The Stokes number has an effect on the instability behavior of the nanofluids. As the Stokes number and Knudsen number decrease, the critical Reynolds numbers increase and the unstable regions of small perturbations decrease, along with a decrease in the largest growth rates that govern the flow instability, therefore reinforcing the flow stability. Larger particles reduce the peak value of the velocity disturbance and hence attenuate the flow instability. (paper)
Secondary instability in boundary-layer flows
Nayfeh, A. H.; Bozatli, A. N.
1979-01-01
The stability of a secondary Tollmien-Schlichting wave, whose wavenumber and frequency are nearly one half those of a fundamental Tollmien-Schlichting instability wave is analyzed using the method of multiple scales. Under these conditions, the fundamental wave acts as a parametric exciter for the secondary wave. The results show that the amplitude of the fundamental wave must exceed a critical value to trigger this parametric instability. This value is proportional to a detuning parameter which is the real part of k - 2K, where k and K are the wavenumbers of the fundamental and its subharmonic, respectively. For Blasius flow, the critical amplitude is approximately 29% of the mean flow, and hence many other secondary instabilities take place before this parametric instability becomes significant. For other flows where the detuning parameter is small, such as free-shear layer flows, the critical amplitude can be small, thus the parametric instability might play a greater role.
International Conference on Instability and Control of Massively Separated Flows
Soria, Julio
2015-01-01
This book contains the outcome of the international meeting on instability, control and noise generated by massive flow separation that was organized at the Monash Center, in Prato, Italy, September 4-6, 2013. The meeting served as the final review of the EU-FP7 Instability and Control of Massively Separated Flows Marie Curie travel grant and was supported by the European Office of Aerospace Research and Development. Fifty leading specialists from twelve countries reviewed the progress made since the 50s of the last century and discussed modern analysis techniques, advanced experimental flow diagnostics, and recent developments in active flow control techniques from the incompressible to the hypersonic regime. Applications involving massive flow separation and associated instability and noise generation mechanisms of interest to the aeronautical, naval and automotive industries have been addressed from a theoretical, numerical or experimental point of view, making this book a unique source containing the stat...
Nuclear-Coupled Flow Instabilities and Their Effects on Dryout
Energy Technology Data Exchange (ETDEWEB)
M. Ishii; X. Sunn; S. Kuran
2004-09-27
Nuclear-coupled flow/power oscillations in boiling water reactors (BWRs) are investigated experimentally and analytically. A detailed literature survey is performed to identify and classify instabilities in two-phase flow systems. The classification and the identification of the leading physical mechanisms of the two-phase flow instabilities are important to propose appropriate analytical models and scaling criteria for simulation. For the purpose of scaling and the analysis of the nonlinear aspects of the coupled flow/power oscillations, an extensive analytical modeling strategy is developed and used to derive both frequency and time domain analysis tools.
Two-phase flow instability in a parallel multichannel system
Institute of Scientific and Technical Information of China (English)
HOU Suxia
2009-01-01
The two-phase flow instabilities observed in through parallel multichannel can be classified into three types, of which only one is intrinsic to parallel multichannel systems. The intrinsic instabilities observed in parallel multichannel system have been studied experimentally. The stable boundary of the flow in such a parallel-channel system are sought, and the nature of inlet flow oscillation in the unstable region has been examined experimentally under various conditions of inlet velocity, heat flux, liquid temperature, cross section of channel and entrance throttling. The results show that parallel multichannel system possess a characteristic oscillation that is quite independent of the magnitude and duration of the initial disturbance, and the stable boundary is influenced by the characteristic frequency of the system as well as by the exit quality when this is low, and upon raising the exit quality and reducing the characteristic frequency, the system increases its instability, and entrance throttling effectively contributes to stabilization of the system.
Stochastically driven instability in rotating shear flows
Mukhopadhyay, Banibrata
2012-01-01
Origin of hydrodynamic turbulence in rotating shear flows is investigated. The particular emphasis is the flows whose angular velocity decreases but specific angular momentum increases with increasing radial coordinate. Such flows are Rayleigh stable, but must be turbulent in order to explain observed data. Such a mismatch between the linear theory and observations/experiments is more severe when any hydromagnetic/magnetohydrodynamic instability and then the corresponding turbulence therein is ruled out. The present work explores the effect of stochastic noise on such hydrodynamic flows. We essentially concentrate on a small section of such a flow which is nothing but a plane shear flow supplemented by the Coriolis effect. This also mimics a small section of an astrophysical accretion disk. It is found that such stochastically driven flows exhibit large temporal and spatial correlations of perturbation velocities, and hence large energy dissipations of perturbation, which presumably generate instability. A ra...
Experimental Replication of an Aeroengine Combustion Instability
Cohen, J. M.; Hibshman, J. R.; Proscia, W.; Rosfjord, T. J.; Wake, B. E.; McVey, J. B.; Lovett, J.; Ondas, M.; DeLaat, J.; Breisacher, K.
2000-01-01
Combustion instabilities in gas turbine engines are most frequently encountered during the late phases of engine development, at which point they are difficult and expensive to fix. The ability to replicate an engine-traceable combustion instability in a laboratory-scale experiment offers the opportunity to economically diagnose the problem (to determine the root cause), and to investigate solutions to the problem, such as active control. The development and validation of active combustion instability control requires that the causal dynamic processes be reproduced in experimental test facilities which can be used as a test bed for control system evaluation. This paper discusses the process through which a laboratory-scale experiment was designed to replicate an instability observed in a developmental engine. The scaling process used physically-based analyses to preserve the relevant geometric, acoustic and thermo-fluid features. The process increases the probability that results achieved in the single-nozzle experiment will be scalable to the engine.
Instability in flow boiling in microchannels
Saha, Sujoy Kumar
2016-01-01
This Brief addresses the phenomena of instability in flow boiling in microchannels occurring in high heat flux electronic cooling. A companion edition in the SpringerBrief Subseries on Thermal Engineering and Applied Science to “Critical Heat Flux in Flow Boiling in Microchannels,” and "Heat Transfer and Pressure Drop in Flow Boiling in Microchannels,"by the same author team, this volume is idea for professionals, researchers, and graduate students concerned with electronic cooling.
Rotating polygon instability of a swirling free surface flow
DEFF Research Database (Denmark)
Tophøj, Laust Emil Hjerrild; Bohr, Tomas; Mougel, J.;
2013-01-01
an analytically soluble model, which, together with estimates of the circulation based on angular momentum balance, reproduces the main features of the experimental phase diagram. The generality of our arguments implies that the instability should not be limited to flows with a rotating bottom (implying singular...
Instability in electromagnetically driven flows Part I
Gissinger, Christophe; Fauve, Stephan
2016-01-01
The MHD flow driven by a travelling magnetic field (TMF) in an annular channel is investigated numerically. For sufficiently large magnetic Reynolds number Rm, or if a large enough pressure gradient is externally applied, the system undergoes an instability in which the flow rate in the channel dramatically drops from synchronism with the wave to much smaller velocities. This transition takes the form of a saddle-node bifurcation for the time-averaged quantities. In this first paper, we characterize the bifurcation, and study the stability of the flow as a function of several parameters. We show that the bifurcation of the flow involves a bistability between Poiseuille-like and Hartman-like regimes, and relies on magnetic flux expulsion. Based on this observation, new predictions are made for the occurrence of this stalling instability.
Mutabazi, Innocent; Bai, Yang; Crumeyrolle, Olivier
2015-11-01
The analogy between viscoelastic instability in the Taylor-Couette flow and the magnetorotational instability (MRI) has been found by Ogilvie & Potter. It relies on the similarity between the governing equations of viscoelastic flows of constant viscosity (Oldroyd-B model equations)and those of Magnetohydrodynamics (MHD). We have performed linear stability analysis of the Taylor-Couette flow with a polymer solution obeying the Oldroyd-B model. A diagram of critical states shows the existence of stationary and helicoidal modes depending on the elasticity of the polymer solution. A generalized Rayleigh criterion determines the potentially unstable zone to pure elasticity-driven perturbations. Experimental results yield four type of modes : one pure elasticity mode and three elastorotational modes that are the MRI-analog modes. Anti-Keplerian case has also been investigated. There is a good agreement between experimental and theoretical results. Work supported by the CPER and ANR-LABEX EMC3.
Numerical simulation of the hydrodynamic instability experiments and flow mixing
Institute of Scientific and Technical Information of China (English)
BAI JingSong; WANG Tao; LI Ping; ZOU LiYong; LIU CangLi
2009-01-01
Based on the numerical methods of volume of fluid (VOF) and piecewise parabolic method (PPM) and parallel circumstance of Message Passing Interface (MPI), a parallel multi-viscosity-fluid hydrodynamic code MVPPM (Multi-Viscosity-Fluid Piecewise Parabolic Method) is developed and performed to study the hydrodynamic instability and flow mixing. Firstly, the MVPPM code is verified and validated by simulating three instability cases: The first one is a Riemann problem of viscous flow on the shock tube;the second one is the hydrodynamic instability and mixing of gaseous flows under re-shocks; the third one is a half height experiment of interfacial instability, which is conducted on the AWE's shock tube. By comparing the numerical results with experimental data, good agreement is achieved. Then the MVPPM code is applied to simulate the two cases of the interfacial instabilities of jelly models accelerated by explosion products of a gaseous explosive mixture (GEM), which are adopted in our experiments. The first is implosive dynamic interfacial instability of cylindrical symmetry and mixing. The evolving process of inner and outer interfaces, and the late distribution of mixing mass caused by Rayleigh-Taylor (RT) instability in the center of different radius are given. The second is jelly layer experiment which is initialized with one periodic perturbation with different amplitude and wave length. It reveals the complex processes of evolution of interface, and presents the displacement of front face of jelly layer, bubble head and top of spike relative to initial equilibrium position vs. time. The numerical results are in excellent agreement with that experimental images, and show that the amplitude of initial perturbations affects the evolvement of fluid mixing zone (FMZ) growth rate extremely, especially at late times.
Numerical simulation of the hydrodynamic instability experiments and flow mixing
Institute of Scientific and Technical Information of China (English)
无
2009-01-01
Based on the numerical methods of volume of fluid (VOF) and piecewise parabolic method (PPM) and parallel circumstance of Message Passing Interface (MPI),a parallel multi-viscosity-fluid hydrodynamic code MVPPM (Multi-Viscosity-Fluid Piecewise Parabolic Method) is developed and performed to study the hydrodynamic instability and flow mixing. Firstly,the MVPPM code is verified and validated by simulating three instability cases:The first one is a Riemann problem of viscous flow on the shock tube; the second one is the hydrodynamic instability and mixing of gaseous flows under re-shocks; the third one is a half height experiment of interfacial instability,which is conducted on the AWE’s shock tube. By comparing the numerical results with experimental data,good agreement is achieved. Then the MVPPM code is applied to simulate the two cases of the interfacial instabilities of jelly models acceler-ated by explosion products of a gaseous explosive mixture (GEM),which are adopted in our experi-ments. The first is implosive dynamic interfacial instability of cylindrical symmetry and mixing. The evolving process of inner and outer interfaces,and the late distribution of mixing mass caused by Rayleigh-Taylor (RT) instability in the center of different radius are given. The second is jelly layer ex-periment which is initialized with one periodic perturbation with different amplitude and wave length. It reveals the complex processes of evolution of interface,and presents the displacement of front face of jelly layer,bubble head and top of spike relative to initial equilibrium position vs. time. The numerical results are in excellent agreement with that experimental images,and show that the amplitude of initial perturbations affects the evolvement of fluid mixing zone (FMZ) growth rate extremely,especially at late times.
Experimental investigation of unstrained diffusion flames and their instabilities
Robert, Etienne
2009-01-01
In this thesis, thermal-diffusive instabilities are studied experimentally in diffusion flames. The novel species injector of a recently developed research burner, consisting of an array of hypodermic needles, which allows to produce quasi one-dimensional unstrained diffusion flames has been improved. It is used in a new symmetric design with fuel and oxidizer injected through needle arrays which allows to independently choose both the magnitude and direction of the bulk flow through the flam...
RELAP5 investigation on subchannel flow instability
Energy Technology Data Exchange (ETDEWEB)
Wang, S.; Yang, B.W.; Liu, A.; Liu, X. [Xi' an Jiaotong Univ., Shaanxi (China). Science and Technology Center for Advanced Nuclear Fuel Research
2016-07-15
Two-phase flow instability is a vitally important area of study for a large number of industrial systems. Density Wave Oscillation (DWO) is the most common type of flow instability caused by the change in flow rate or power in boiling systems. The code RELAP5 is used to simulate single channel, 2 x 2 subchannels, and 3 x 3 subchannels with typical BWR subchannel geometry. The onset of flow instability determinating criterion and the results of simulations are utilized to create a stable boundary. The stable boundary of a single channel is compared with those from results of other researchers. Some conclusions are made as follows. 3 x 3 subchannels are more stable than single channel and 2 x 2 subchannels. Open subchannels possess a larger stable region than close channels. The heating model is analyzed determining that asymmetrical heating has negative effect on stability as compared to symmetric heating. With the analysis of transit time, period and subcooling number, there is a positive linear relationship between the subcooling number and oscillation period.
Instability in electromagnetically driven flows Part II
Imazio, Paola Rodriguez; Gissinger, Christophe
2016-01-01
In a previous paper, we have reported numerical simulations of the MHD flow driven by a travelling magnetic field (TMF) in an annular channel, at low Reynolds number. It was shown that the stalling of such induction pump is strongly related to magnetic flux expulsion. In the present article, we show that for larger hydrodynamic Reynolds number, and with more realistic boundary conditions, this instability takes the form of a large axisymmetric vortex flow in the (r,z)-plane, in which the flui...
Oscillatory and electrohydrodynamic instabilities in flow over a viscoelastic gel
Indian Academy of Sciences (India)
R M Thaokar
2015-05-01
The stability of oscillatory flows over compliant surfaces is studied analytically and numerically. The type of compliant surfaces studied is the incompressible viscoelastic gel model. The stability is determined using the Floquet analysis, where amplitude of perturbations at time intervals separated by one time period is examined to determine whether perturbations grow or decay. Oscillatory flows pas viscoelastic gels exhibit an instability in the limit of zero Reynolds number, and the transition amplitude of the oscillatory velocity increases with the frequency of oscillations. The transition amplitude has a minimum at a finite wavenumber for the viscoelastic gel model. The instability is found to depend strongly on the gel viscosity $\\eta_{g}$, and the effect of oscillations on the continuation of viscous modes at intermediate Reynolds number shows a complicated dependence on the oscillation frequency. Experimental studies are carried out on the stability of an oscillatory flow past a viscoelastic gel at zero Reynolds number, and these confirm the theoretical predictions.
Topographic instability of flow in a rotating fluid
Directory of Open Access Journals (Sweden)
K. I. Patarashvili
2006-01-01
Full Text Available Here are presented the results of experimental and theoretical studies on a stability of zonal geostrophic flows in the rotating layer of the shallow water. In the experiments, a special apparatus by Abastumani Astrophysical Observatory Georgian Academy of Science was used. This apparatus represents a paraboloid of rotation, which can be set in a regulable rotation around the vertical axis. Maximal diameter of the paraboloid is 1.2 m, radius of curvature in the pole is 0.698 m. In the paraboloid, water spreads on walls as a layer uniform on height under the period of rotation 1.677 s. Against a background of the rotating fluid, the zonal flows are formed by the source-sink system. It consists of two concentric circular perforations on the paraboloid bottom (width is 0.3 cm, radiuses are 8.4 and 57.3 cm, respectively; water can be pumped through them with various velocities and in all directions. It has been established that under constant vertical depth of the rotating fluid the zonal flows are stable. There are given the measurements of the radial profiles for the water level and velocity in the stationary regime. It has been found that zonal flows may lose stability under the presence of the radial gradient of full depth formed by a change of angular velocity of paraboloid rotation. An instability origin results in the loss of flow axial symmetry and in the appearance of self-excited oscillations in the zonal flow. At the given angular velocity of rotation, instability is observed only in the definite range of intensities of the source-sink system. The theoretical estimations are performed in the framework of the equations of the shallow water theory, including the terms describing the bottom friction. It has been shown that the instability of zonal flows found experimentally has a topographical nature and is related with non-monotone dependence of the potential vorticity on radius.
Patterns, Instabilities, Colors, and Flows in Vertical Foam Films
Yilixiati, Subinuer; Wojcik, Ewelina; Zhang, Yiran; Pearsall, Collin; Sharma, Vivek
2015-03-01
Foams find use in many applications in daily life, industry and biology. Examples include beverages, firefighting foam, cosmetics, foams for oil recovery and foams formed by pollutants. Foams are collection of bubbles separated by thin liquid films that are stabilized against drainage by the presence of surfactant molecules. Drainage kinetics and stability of the foam are strongly influenced by surfactant type, addition of particles, proteins and polymers. In this study, we utilize the thin film interference colors as markers for identifying patterns, instabilities and flows within vertical foam films. We experimentally study the emergence of thickness fluctuations near the borders and within thinning films, and study how buoyancy, capillarity and gravity driven instabilities and flows, are affected by variation in bulk and interfacial physicochemical properties dependent on the choice of constituents.
THE LINEAR HOMOGENEOUS FLOW MODEL FOR TWO-PHASE FLOW INSTABILITY IN BOILING CHANNELS
Institute of Scientific and Technical Information of China (English)
无
2002-01-01
This paper presents liner homogeneous model describing two-phase flow instability. Dimensionless parameter η was derived by using the linear homogeneous model. Using parameter η the stability of a system could be easily judged. The calculated results agree with the experimental data well.
Instabilities developed in stratified flows over pronounced obstacles
Varela, J.; Araújo, M.; Bove, I.; Cabeza, C.; Usera, G.; Martí, Arturo C.; Montagne, R.; Sarasúa, L. G.
2007-12-01
In the present work we study numerical and experimentally the flow of a two-layer stratified fluid over a topographic obstacle. The problem reflects a wide number of oceanographic and meteorological situations, where the stratification plays an important role. We identify the different instabilities developed by studying the pycnocline deformation due to a pronounced obstacle. The numerical simulations were made using the model caffa3D.MB which works with a numerical model of Navier-Stokes equations with finite volume elements in curvilinear meshes. The experimental results are contrasted with numerical simulations. Linear stability analysis predictions are checked with particle image velocimetry (PIV) measurements.
Experimental model of bladder instability in rabbits
Directory of Open Access Journals (Sweden)
Balasteghin K.T.
2003-01-01
Full Text Available OBJECTIVE: Propose a new experimental model of bladder instability in rabbits after partial bladder obstruction. MATERIALS AND METHODS: Thirty North Folk male rabbits, weighting 1,700 to 2,820 g (mean: 2,162 g were studied. The animals were distributed in 2 experimental groups, formed by 15 rabbits each: Group 1 - clinical control. In this group there was no surgical intervention; Group 2 - bladder outlet obstruction. In this group, after anesthetizing the animal, urethral cannulation with Foley catheter 10F was performed and then an adjustable plastic bracelet was passed around the bladder neck. It was then adjusted in order to not constrict the urethra. The following parameters were studied in M1 - pre-operative period; M2 - 4 weeks post-operatively moments: 1- urine culture; 2- cystometric study; 3- serum creatinine and BUN. RESULTS: Bladder weight was 2.5 times larger in the group with obstruction than in the control group. Cystometric evaluation showed a significant increase in maximal vesical volume in the final moment at Group G2. However, there was no statistically significant difference among the groups studied. There was no statistically significant difference between maximal detrusor pressure and vesical compliance in the different moments or in the studied groups. There was an absence of uninhibited detrusor contractions in all the animals in group 1, and involuntary contractions were detected in 93% of group 2 animals. There was no significant variation in BUN and serum creatinine either among the groups or in the same group. CONCLUSIONS: We observed in the group with obstruction a bladder weight 2.5 higher than normal bladders. We detected involuntary contractions in 93% of the animals in group 2, establishing this experimental model as appropriate to secondary bladder instability and partial bladder outlet obstruction.
Large-scale instabilities of helical flows
Cameron, Alexandre; Brachet, Marc-Étienne
2016-01-01
Large-scale hydrodynamic instabilities of periodic helical flows are investigated using $3$D Floquet numerical computations. A minimal three-modes analytical model that reproduce and explains some of the full Floquet results is derived. The growth-rate $\\sigma$ of the most unstable modes (at small scale, low Reynolds number $Re$ and small wavenumber $q$) is found to scale differently in the presence or absence of anisotropic kinetic alpha (\\AKA{}) effect. When an $AKA$ effect is present the scaling $\\sigma \\propto q\\; Re\\,$ predicted by the $AKA$ effect theory [U. Frisch, Z. S. She, and P. L. Sulem, Physica D: Nonlinear Phenomena 28, 382 (1987)] is recovered for $Re\\ll 1$ as expected (with most of the energy of the unstable mode concentrated in the large scales). However, as $Re$ increases, the growth-rate is found to saturate and most of the energy is found at small scales. In the absence of \\AKA{} effect, it is found that flows can still have large-scale instabilities, but with a negative eddy-viscosity sca...
Energy Technology Data Exchange (ETDEWEB)
Gerschuetz, W.
2006-07-01
Experimental investigations at two different three-stage low-pressure (LP) model turbines operated at a very low or even zero flow rate will be presented. Experience shows that in this operating range, also called windage, the running blades of the last stage undergo the highest alternating stresses. The high stress caused by centrifugal and gas forces is superposed with these extreme dynamic stresses and thus form the critical load mechanism of the long and slender blades. Experimental investigations on the first of the turbines considered here showed high forced vibrations at resonant frequency in the 2{sup nd} mode. This excitation only occurs in a narrow band of the operating range at low volumetric flow numbers. To comprehend the aerodynamic stimulus the unsteady part of the flow-field in the last stage was determined with high-response pitot tubes. The analysis of the experimental data shows distinct disturbances in the flow field at a critical volumetric flow number, especially in the axial spacing between the guide vanes and the rotor at the outer casing. Frequency spectra gained in the axial gap especially in the area of blade tips of the rotor show a significant broadband level rise between 500 and 2000 Hz with several discrete, evenly spaced peaks superimposed. Detailed analysis shows some disturbances with particular high amplitudes occur in the flow field at distinct frequencies. These pressure fluctuations rotate in circumferential direction at a fraction of the rotor speed. In the second turbine with more recent blading, no vibrations at resonant frequency could be detected in the foremost four modes in this operating range. However, rotating flow instabilities comparable with the phenomenon discovered in the first turbine can also be found in the axial gap of the last stage. Comparing the pressure signals measured at two circumferential locations a so called 'rotating instability' can be observed. This unsteady phenomenon has previously
Linear instability of plane Couette and Poiseuille flows
Chefranov, S. G.; Chefranov, A. G.
2016-05-01
It is shown that linear instability of plane Couette flow can take place even at finite Reynolds numbers Re > Reth ≈ 139, which agrees with the experimental value of Reth ≈ 150 ± 5 [16, 17]. This new result of the linear theory of hydrodynamic stability is obtained by abandoning traditional assumption of the longitudinal periodicity of disturbances in the flow direction. It is established that previous notions about linear stability of this flow at arbitrarily large Reynolds numbers relied directly upon the assumed separation of spatial variables of the field of disturbances and their longitudinal periodicity in the linear theory. By also abandoning these assumptions for plane Poiseuille flow, a new threshold Reynolds number Reth ≈ 1035 is obtained, which agrees to within 4% with experiment—in contrast to 500% discrepancy for the previous estimate of Reth ≈ 5772 obtained in the framework of the linear theory under assumption of the "normal" shape of disturbances [2].
Magnetorotational Instability of Dissipative MHD Flows
Energy Technology Data Exchange (ETDEWEB)
HERRON, ISOM H
2010-07-10
Executive summary Two important general problems of interest in plasma physics that may be addressed successfully by Magnetohydrodynamics (MHD) are: (1) Find magnetic field configurations capable of confining a plasma in equilibrium. (2) Study the stability properties of each such an equilibrium. It is often found that the length scale of many instabilities and waves that are able to grow or propagate in a system, are comparable with plasma size, such as in magnetically confined thermonuclear plasmas or in astrophysical accretion disks. Thus MHD is able to provide a good description of such large-scale disturbances. The Magnetorotational instability (MRI) is one particular instance of a potential instability. The project involved theoretical work on fundamental aspects of plasma physics. Researchers at the Princeton Plasma Physics Laboratory (PPPL) began to perform a series of liquid metal Couette flow experiments between rotating cylinders. Their purpose was to produce MRI, which they had predicted theoretically 2002, but was only observed in the laboratory since this project began. The personnel on the project consisted of three persons: (1) The PI, who was partially supported on the budget during each of four summers 2005-2008. (2) Two graduate research assistants, who worked consecutively on the project throughout the years 2005-2009. As a result, the first student, Fritzner Soliman, obtained an M.S. degree in 2006; the second student, Pablo Suarez obtained the Ph.D. degree in 2009. The work was in collaboration with scientists in Princeton, periodic trips were made by the PI as part of the project. There were 4 peer-reviewed publications and one book produced.
Studies of fluid instabilities in flows of lava and debris
Fink, Jonathan H.
1987-01-01
At least two instabilities have been identified and utilized in lava flow studies: surface folding and gravity instability. Both lead to the development of regularly spaced structures on the surfaces of lava flows. The geometry of surface folds have been used to estimate the rheology of lava flows on other planets. One investigation's analysis assumed that lava flows have a temperature-dependent Newtonian rheology, and that the lava's viscosity decreased exponentially inward from the upper surface. The author reviews studies by other investigators on the analysis of surface folding, the analysis of Taylor instability in lava flows, and the effect of surface folding on debris flows.
Kelvin-Helmholtz versus Hall magnetoshear instability in astrophysical flows.
Gómez, Daniel O; Bejarano, Cecilia; Mininni, Pablo D
2014-05-01
We study the stability of shear flows in a fully ionized plasma. Kelvin-Helmholtz is a well-known macroscopic and ideal shear-driven instability. In sufficiently low-density plasmas, also the microscopic Hall magnetoshear instability can take place. We performed three-dimensional simulations of the Hall-magnetohydrodynamic equations where these two instabilities are present, and carried out a comparative study. We find that when the shear flow is so intense that its vorticity surpasses the ion-cyclotron frequency of the plasma, the Hall magnetoshear instability is not only non-negligible, but it actually displays growth rates larger than those of the Kelvin-Helmholtz instability.
Magnetorotational Instability in Liquid Metal Couette Flow
Noguchi, K; Colgate, S A; Nordhaus, J; Beckley, H F
2002-01-01
Despite the importance of the magnetorotational instability (MRI) as a fundamental mechanism for angular momentum transport in magnetized accretion disks, it has yet to be demonstrated in the laboratory. A liquid sodium alpha-omega dynamo experiment at the New Mexico Institute of Mining and Technology provides an ideal environment to study the MRI in a rotating metal annulus (Couette flow). A local stability analysis is performed as a function of shear, magnetic field strength, magnetic Reynolds number, and turbulent Prandtl number. The later takes into account the minimum turbulence induced by the formation of an Ekman layer against the rigidly rotating end walls of a cylindrical vessel. Stability conditions are presented and unstable conditions for the sodium experiment are compared with another proposed MRI experiment with liquid gallium. Due to the relatively large magnetic Reynolds number achievable in the sodium experiment, it should be possible to observe the excitation of the MRI for a wide range of w...
Effect of Running Parameters on Flow Boiling Instabilities in Microchannels.
Zong, Lu-Xiang; Xu, Jin-Liang; Liu, Guo-Hua
2015-04-01
Flow boiling instability (FBI) in microchannels is undesirable because they can induce the mechanical vibrations and disturb the heat transfer characteristics. In this study, the synchronous optical visualization experimental system was set up. The pure acetone liquid was used as the working fluid, and the parallel triangle silicon microchannel heat sink was designed as the experimental section. With the heat flux ranging from 0-450 kW/m2 the microchannel demand average pressure drop-heater length (Δp(ave)L) curve for constant low mass flux, and the demand pressure drop-mass flux (Δp(ave)G) curve for constant length on main heater surface were obtained and studied. The effect of heat flux (q = 188.28, 256.00, and 299.87 kW/m2), length of main heater surface (L = 4.5, 6.25, and 8.00 mm), and mass flux (G = 188.97, 283.45, and 377.94 kg/m2s) on pressure drops (Ap) and temperatures at the central point of the main heater surface (Twc) were experimentally studied. The results showed that, heat flux, length of the main heater surface, and mass flux were identified as the important parameters to the boiling instability process. The boiling incipience (TBI) and critical heat flux (CHF) were early induced for the lower mass flux or the main heater surface with longer length. With heat flux increasing, the pressure drops were linearly and slightly decreased in the single liquid region but increased sharply in the two phase flow region, in which the flow boiling instabilities with apparent amplitude and long period were more easily triggered at high heat flux. Moreover, the system pressure was increased with the increase of the heat flux.
Instability of Taylor-Couette Flow of Electrorheological Fluid
Institute of Scientific and Technical Information of China (English)
PENG Jie; ZHU Ke-Qin
2004-01-01
A linearized instability analysis of Taylor-Couette flow between two rotating concentric cylinders of an electrorheological (ER) fluid is carried out. The ER fluid exhibits a yield stress in addition to the plastic viscosity when an extra electric-field is applied. It can be found that the yield stress plays a dual role in the flow instability.The possibility of the yield surface falling between the cylinders is analysed. Although small waves appeared on the yielded surface is considered, the yielded surface, which has been treated as a free surface, has little effect on the flow instability. The effects of axisymmetric perturbation on the flow instability are presented due to the axisymmetric of the basic flow. The parameterβ in the yield stress formula of the ER fluid is shown to have distinct effects on the flow instability characteristics.
Investigation on two-phase flow instability in steam generator of integrated nuclear reactor
Institute of Scientific and Technical Information of China (English)
无
1996-01-01
In the pressure range of 3-18MPa,high pressure steam-water two-phase flow density wave instability in vertical upward parallel pipes with inner diameter of 12mm is studied experimentally.The oscillation curves of two-phase flow instability and the effects of several parameters on the oscillation threshold of the system are obtained.Based on the small pertubation linearization method and the stability principles of automatic control system,a mathematical model is developed to predict the characteristics of density wave instability threshold.The predictions of the model are in good agreement with the experimental results.
DVI Film Flow Instability Based on the Normal Mode Analysis
Energy Technology Data Exchange (ETDEWEB)
Lee, Jae Young; Euh, Dong Jin [Handong Global Univ., Pohang (Korea, Republic of)
2013-10-15
In the present paper, as the first step of nonlinear studies, the appearance of the third order spatial differentiation of the film thickness in the wave propagation equation is to be derived. The two-fluid model in the adiabatic condition is employed and normal mode analysis. Interfacial pressure forces between steam and water need to be modeled for this purpose. n the present study, we developed a theoretical basis to study nonlinear wave phenomena on the DVI film flow which highly affect the DVI penetration and liquid droplets entrainment out. We set the hyperbolicity breaking condition by providing the interfacial pressure difference considering the curvature of the reactor vessel. The interfacial pressure difference could generate nonlinear wave such as the horseshoe wave which has been believed as a main source of film break up to produce huge amount of droplets to be entrained out. As a safety injection method, the direct vessel injection has been intensively developed in Korea and employed in the APR1400. The developing efforts were made from the determination of the number, location and size of DVI nozzles to the experimental demonstrations. Experimental facilities with various scales have shown its complicated phenomena due to highly nonlinear interaction between the steam and injected water flow. The injected DVI water forms a film type flow but very unstable due to the unsteady energetic steam flow which find the exits around the shell of the downcomer of the reactor vessel. This steam and liquid film interaction leads to the instability on the surface of the film flow and the waves are highly nonlinear to form undercutting, roll over, and finally droplet releasing. The entrained droplets causes a difficulty in the analysis to estimate the water penetrated into the reactor core to cool the nuclear fuels heated up. Unfortunately, these instabilities on the interface of the DVI liquid film have not been studied appropriately and the conservative
Static flow instability in subcooled flow boiling in parallel channels
Energy Technology Data Exchange (ETDEWEB)
Siman-Tov, M.; Felde, D.K.; McDuffee, J.L.; Yoder, G.L. Jr.
1995-04-01
A series of tests for static flow instability or flow excursion (FE) at conditions applicable to the proposed Advanced Neutron Source reactor was completed in parallel rectangular channels configuration with light water flowing vertically upward at very high velocities. True critical heat flux experiments under similar conditions were also conducted. The FE data reported in this study considerably extend the velocity range of data presently available worldwide. Out of the three correlations compared, the Saha and Zuber correlation had the best fit with the data. However, a modification was necessary to take into account the demonstrated dependence of the Stanton (St) and Nusselt (Nu) numbers on subcooling levels, especially in the low subcooling regime.
Experimental study of absolute instability over a rotating disk
Othman Bekhit, Hesham Abdel Ghafar
2005-07-01
A series of experiments were performed to study the absolute instability of Type I traveling cross-flow modes in the boundary layer on a smooth disk rotating at constant speed. The basic flow agreed with analytic theory, and the growth of natural disturbances matched linear theory predictions. Controlled temporal disturbances were introduced by a short-duration air pulse from a hypodermic tube located above the disk and outside the boundary layer. The air pulse was positioned just outboard of the critical radius for Type I cross-flow modes. A hot-wire sensor primarily sensitive to the azirnuthal velocity component, was positioned at different spatial locations on the disk to document the growth of disturbances produced by the air pulses. Ensemble averages conditioned on the air pulses revealed wave packets that evolved in time and space. Two amplitudes of air pulses were used. The lower amplitude produced wave packets with linear amplitude characteristics that agreed with linear-theory wall-normal eigenfunction distributions and spatial growth rates. The higher amplitude pulse produced wave packets that had nonlinear amplitude characteristics. The space-time evolution of the leading and trailing edges of the wave packets were followed well past the critical radius for the absolute instability based on Lingwood (1995). With the linear amplitudes, the absolute instability was dominated by the convective modes, agreeing with the linear DNS simulations of Davies and Carpenter (2003). With the nonlinear amplitudes, larger temporal growth of the wave packets existed which supports the finite amplitude analysis of Pier (2003), and more closely resembles the wave packet evolution in the experimental study of Lingwood (1996). This suggests that the disturbance levels in the experiment that was intended to demonstrate the linear analysis, were likely fuite.
Numerical investigation of the mechanism of two-phase flow instability in parallel narrow channels
Energy Technology Data Exchange (ETDEWEB)
Hu, Lian [Key Laboratory of Low-grade Energy Utilization Technologies and Systems, Chongqing University (China); Chen, Deqi, E-mail: chendeqi@cqu.edu.cn [Key Laboratory of Low-grade Energy Utilization Technologies and Systems, Chongqing University (China); CNNC Key Laboratory on Nuclear Reactor Thermal Hydraulics Technology, Nuclear Power Institute of China, Chengdu 610041 (China); Huang, Yanping, E-mail: hyanping007@163.com [CNNC Key Laboratory on Nuclear Reactor Thermal Hydraulics Technology, Nuclear Power Institute of China, Chengdu 610041 (China); Yuan, Dewen; Wang, Yanling [CNNC Key Laboratory on Nuclear Reactor Thermal Hydraulics Technology, Nuclear Power Institute of China, Chengdu 610041 (China); Pan, Liangming [Key Laboratory of Low-grade Energy Utilization Technologies and Systems, Chongqing University (China)
2015-06-15
Highlights: • A mathematical model is proposed to predict the two-phase flow instability. • The mathematical model predicted result agrees well with the experimental result. • Oscillation characteristics of the two-phase flow instability is discussed in detail. - Abstract: In this paper, the mechanism of two-phase flow instability in parallel narrow channels is studied theoretically, and the characteristic of the flow instability is discussed in detail. Due to the significant confining effect of the narrow channel on the vapor–liquid interface, the two-phase flow resistance in the narrow channel is probably different from that in conventional channel. Therefore, the vapor confined number (N{sub conf}), defined by the size of narrow channel and bubble detachment diameter, is considered in the “Chisholm B model” to investigate the two-phase flow pressure drop. The flow instability boundaries are plotted in parameter plane with phase-change-number (N{sub pch}) and subcooling-number (N{sub sub}) under different working conditions. It is found that the predicted result agrees well with the experimental result. According to the predicted result, the oscillation behaviors near the flow instability boundary indicate that the Supercritical Hopf bifurcation appears in high sub-cooled region and the Subcritical Hopf bifurcation appears in low sub-cooled region. Also, a detailed analysis about the effects of key parameters on the characteristic of two-phase flow instability and the flow instability boundary is proposed, including the effects of inlet subcooling, heating power, void distribution parameter and drift velocity.
Instability criteria for steady flows of a perfect fluid.
Friedlander, Susan; Vishik, Misha M.
1992-07-01
An instability criterion based on the positivity of a Lyapunov-type exponent is used to study the stability of the Euler equations governing the motion of an inviscid incompressible fluid. It is proved that any flow with exponential stretching of the fluid particles is unstable. In the case of an arbitrary axisymmetric steady integrable flow, a sufficient condition for instability is exhibited in terms of the curvature and the geodesic torsion of a stream line and the helicity of the flow.
Taylor-Couette flow instabilities in neutrally-buoyant suspensions
Majji, Madhu; Banerjee, Sanjoy; Morris, Jeffrey F.
2016-11-01
Experimentally-determined instabilities and flow states of a neutrally-buoyant suspension are described. The flow is studied in a concentric-cylinder device with inner-to-outer cylinder ratio of 0.877 with inner cylinder rotating and outer stationary. The cylinder length to annular gap ratio is 20, while the gap to particle size ratio is approximately 30, for spherical particles of 250 μm diameter. Using a slowly increasing or decreasing Re ramp, the flow agrees with all expectations for the pure fluid, while a slowly decreasing (quasi-static) ramp is used for the suspension flow, which is found to be unstable at lower Reynolds number Re (based on the effective viscosity) than pure fluid, and exhibits spiraling and ribbon states not found for a pure fluid with only inner cylinder rotating. Strikingly, the suspension at solid fraction ϕ >= 0 . 05 goes unstable first to a nonaxisymetric state rather than axisymmetric Taylor vortices. At 0 . 1 states during quais-static ramping of Re , while for ϕ = 0 . 3 , the base state Couette flow gives way to wavy spirals (WS) at Re 80 and exhibits only the WS state up to Re = 150 . Transient behavior on sudden change of Re and particle tracking will also be presented.
Non-axisymmetric instabilities in discs with imposed zonal flows
Vanon, R.; Ogilvie, G. I.
2016-12-01
We conduct a linear stability calculation of an ideal Keplerian flow on which a sinusoidal zonal flow is imposed. The analysis uses the shearing sheet model and is carried out both in isothermal and adiabatic conditions, with and without self-gravity (SG). In the non-SG regime, a structure in the potential vorticity (PV) leads to a non-axisymmetric Kelvin-Helmholtz (KH) instability; in the short-wavelength limit its growth rate agrees with the incompressible calculation by Lithwick, which only considers perturbations elongated in the streamwise direction. The instability's strength is analysed as a function of the structure's properties, and zonal flows are found to be stable if their wavelength is ≳8 H, where H is the disc's scaleheight, regardless of the value of the adiabatic index γ. The non-axisymmetric KH instability can operate in Rayleigh-stable conditions, and it therefore represents the limiting factor to the structure's properties. Introducing SG triggers a second non-axisymmetric instability, which is found to be located around a PV maximum, while the KH instability is linked to a PV minimum, as expected. In the adiabatic regime, the same gravitational instability is detected even when the structure is present only in the entropy (not in the PV) and the instability spreads to weaker SG conditions as the entropy structure's amplitude is increased. This eventually yields a non-axisymmetric instability in the non-SG regime, albeit of weak strength, localized around an entropy maximum.
Nonaxisymmetric linear instability of cylindrical magnetohydrodynamic Taylor-Couette flow
Child, Adam; Hollerbach, Rainer
2015-01-01
We consider the nonaxisymmetric modes of instability present in Taylor-Couette flow under the application of helical magnetic fields, mainly for magnetic Prandtl numbers close to the inductionless limit, and conduct a full examination of marginal stability in the resulting parameter space. We allow for the azimuthal magnetic field to be generated by a combination of currents in the inner cylinder and fluid itself, and introduce a parameter governing the relation between the strength of these currents. A set of governing eigenvalue equations for the nonaxisymmetric modes of instability are derived and solved by spectral collocation with Chebyshev polynomials over the relevant parameter space, with the resulting instabilities examined in detail. We find that by altering the azimuthal magnetic field profiles the azimuthal magnetorotational instability, nonaxisymmetric helical magnetorotational instability, and Tayler instability yield interesting dynamics, such as different preferred mode types, and modes with a...
Rolie-Poly fluid flowing through constrictions: Two distinct instabilities
Reis, T.
2013-05-01
Elastic instabilities of entangled polymer melts are common in industrial processes but the physics responsible is not well understood. We present a numerical linear stability study of a molecular based constitutive model which grants us physical insight into the underlying mechanics involved. Two constriction flows are considered - one shear dominated, the other extension dominated - and two distinct instabilities are found. The influence of the molecular structure and the behaviour of the polymer dynamics are investigated and in both cases chain relaxation and orientation play a crucial role. This suggests a molecular-based physical interpretation of the underlying mechanisms responsible for flow instabilities. © 2013 Elsevier B.V.
Energy Technology Data Exchange (ETDEWEB)
Sung, Chang Kyung [Korea Electric Power Research Institute, Taejon (Korea, Republic of)
1997-12-31
This paper presents a theoretical approach of the instability criterion from stratified to nonstratified flow in horizontal pipe at cocurrent flow conditions. The new theoretical instability criterion for the stratified and nonstratified flow transition in horizontal pipe has been developed by hyperbolic equations in two-phase flow. Critical flow condition criterion and onset of slugging at cocurrent flow condition correspond to zero and imaginary characteristics which occur when the hyperbolicity of a stratified two-phase flow is broken, respectively. Through comparison between results predicted by the present flow is broken, respectively. Through comparison between results predicted by the present theory and the Kukita et al. [1] experimental data of pipes, it is shown that they are in good agreement with data. 4 refs., 2 figs. (Author)
The effect of spacer ribs on Ledinegg type flow instabilities
Energy Technology Data Exchange (ETDEWEB)
Coutts, D.A.
1993-09-09
An experimental program has been completed which evaluated the effect of a flow obstruction in a heated channel on the onset of flow instability (OBI). The test channel was rectangular (80 {times} 3 mm), heated on one surface, and equipped with view ports. Tests were conducted in a flow controlled mode at heat fluxes of 370 kW/M{sup 2}, and 610 kW/m{sup 2}. Direct comparisons were made between the demand curve minimum for the unobstructed channel and a channel equipped with a 2.07 mm wide rib that was parallel to the flow and in contact with the heated surface. Data at OFI is presented in the nondimensional terms Of Q{sub ratio} (ratio of heat flux applied to heat flux required to achieve saturated liquid conditions at the exit), and the local Stanton number at the channel exit for each channel arrangement. The Q{sub ratio} and Stanton number values for the unobstructed channel and the rib equipped channel are then compared to produce an estimate of the rib effect.
Analysis of flow instabilities in forced-convection steam generator
Institute of Scientific and Technical Information of China (English)
无
2006-01-01
Because of the practical importance of two-phase instabilities, substantial efforts have been made to date to understand the physical phenomena governing such instabilities and to develop computational tools to model the dynamics. The purpose of this study is to present a numerical model for the analysis of flow-induced instabilities in forced-convection steam generator. The model is based on the assumption of homogeneous two-phase flow and thermodynamic equilibrium of the phases. The thermal capacity of the heater wall has been included in the analysis. The model is used to analyze the flow instabilities in the steam generator and to study the effects of system pressure, mass flux, inlet temperature and inlet/outlet restriction, gap size, the ratio of do /di, and the ratio of qi/qo on the system behavior.
Magnetorotational Instability in a Couette Flow of Plasma
Noguchi, K; Noguchi, Koichi; Pariev, Vladimir I.
2003-01-01
All experiments, which have been proposed so far to model the magnetorotational instability (MRI) in the laboratory, involve a Couette flow of liquid metals in a rotating annulus. All liquid metals have small magnetic Prandtl numbers, Pm, of about 10^{-6} (the ratio of kinematic viscosity to magnetic diffusivity). With plasmas both large and small Pm are achievable by varying the temperature and the density of plasma. Compressibility and fast rotation of the plasma result in radial stratification of the equilibrium plasma density. Evolution of perturbations in radially stratified viscous and resistive plasma permeated by an axial uniform magnetic field is considered. The differential rotation of the plasma is induced by the ExB drift in applied radial electric field. Global unstable eigenmodes are calculated by our newly developed matrix code. The plasma is shown to be MRI unstable for parameters easily achievable in experimental setup.
Instability of Taylor-Couette Flow between Concentric Rotating Cylinders
Dou, H S; Phan-Thien, N; Yeo, K S; Dou, Hua-Shu; Khoo, Boo Cheong; Phan-Thien, Nhan; Yeo, Koon Seng
2005-01-01
The energy gradient theory is used to study the instability of Taylor-Couette flow between concentric rotating cylinders. In our previous studies, the energy gradient theory was demonstrated to be applicable for wall bounded parallel flows. It was found that the critical value of the energy gradient parameter K at subcritical transition is about 370-389 for wall bounded parallel flows (which include plane Poiseuille flow, pipe Poiseuille flow and plane Couette flow) below which no turbulence occurs. In this paper, the detailed derivation for the calculation of the energy gradient parameter in the flow between concentric rotating cylinders is provided. The theoretical results for the critical condition of primary instability obtained are in very good agreement with the experiments found in literature. The mechanism of spiral vortices generation for counter-rotating of two cylinders is also explained using the energy gradient theory. The energy gradient theory can also serve to relate the condition of flow tran...
Mechanism of Secondary Instability of Flow around a Circular Cylinder
Dou, Hua-Shu; Ben, An-Qing; Fluid Mechanics Research Team Team
2016-11-01
Flow around a circular cylinder in infinite domain is simulated with large eddy simulation at Re =200, and the mechanism of the origin of secondary vortex street is analyzed. The simulation results show that the vortex street generated in the cylinder near wake disappears as the flow moving downstream. Secondary instability occurs in far wake of the cylinder after the primary vortex street dying away. The processes of first instability and secondary instability in the cylinder wake are recorded in the simulation. The instability of the entire flow field is studied with the energy gradient theory. It is found that it is the high value of the energy gradient function generated by the zero velocity gradients that leads to the instability. As the vortex developing at rear of the cylinder, the value of the energy gradient function becomes low downstream, which leads to the vortex dying away. At further downstream, the value of the energy gradient function is enlarged again because of the role of perturbation, which leads to the secondary instability. It can be concluded that the interaction of the variation of the value of the energy gradient function and the perturbation leads to the occurrence of secondary instability.
Energy Technology Data Exchange (ETDEWEB)
Lee, Juhyung; Chang, Soon Heung [Korea Advanced Institute of Science and Technology, Daejeon (Korea, Republic of); Jo, Daeseong [Korea Atomic Energy Research Institute, Daejeon (Korea, Republic of)
2013-10-15
Subcooled flow boiling in a vertical rectangular channel was experimentally investigated to enhance the understanding of the CHF and the effect of the two-phase flow instability on it under low pressure conditions, especially for downward flow which was adopted for Jordan Research and Training Reactor (JRTR) and Kijang research reactor (KJRR) to achieve easier fuel and irradiation rig loading. In addition, visual observations of subcoold flow boiling was conducted by using high-speed video (HSV) for a clear understanding of both phenomena. We concluded that flow excursion (which is static instability) could be induced due to the OPDF (which is dynamic instability) when a system has no resistibility to the pressure drop perturbation, which is caused by the coalescence of facing bubbles on opposing heated surfaces. In more stable system with throttling applied, flow rate could be maintained and stable CHF could be reached. The static flow instability (FI) and critical heat flux (CHF) for subcooled flow boiling in a vertical narrow channels under low pressure condition are fairly crucial phenomena relative to thermal-hydraulic design and safety analysis for pool-type research reactors. It has been recommended that RRs and MTRs be designed to have sufficient margins for CHF and the onset of FI as well, since unstable flow could leads to premature CHF under very low wall heat flux in comparison to stable CHF. Even the fact and previous studies, however, the understanding of relationship among FI, premature CHF and stable CHF is not sufficient to date.
Solutal Marangoni instability in layered two-phase flows
Picardo, Jason R; Pushpavanam, S
2015-01-01
In this paper, the instability of layered two-phase flows caused by the presence of a soluble surfactant (or a surface active solute) is studied. The fluids have different viscosities, but are density matched to focus on Marangoni effects. The fluids flow between two flat plates, which are maintained at different solute concentrations. This establishes a constant flux of solute from one fluid to the other in the base state. A linear stability analysis is performed, using a combination of asymptotic and numerical methods. In the creeping flow regime, Marangoni stresses destabilize the flow, provided a concentration gradient is maintained across the fluids. One long wave and two short wave Marangoni instability modes arise, in different regions of parameter space. A well-defined condition for the long wave instability is determined in terms of the viscosity and thickness ratios of the fluids, and the direction of mass transfer. Energy budget calculations show that the Marangoni stresses that drive long and shor...
How Polarization and Political Instability affect Learning through Experimentation
I.V. Ossokina (Ioulia); O.H. Swank (Otto)
2001-01-01
textabstractIn a multiperiod setting, decision-makers can learn about the consequences of their decisions through experimentation. In this paper we examine how in a two-party system polarization and political instability affect learning through experimentation. We distinguish two cases: the decisi
Flow Instabilities in Cavitating and Non-Cavitating Pumps
2006-11-01
stall and surge may occur in non-cavitating turbomachines at flow rates smaller than design. Rotating stall is a local instability at the...turbomachinery which is basically not dependent on the hydraulic system in which the turbomachine is installed. The stalled region rotates faster than impeller...rate and without significant head decrease. So, it is required to confirm the absence of cavitation instabilities, whenever the turbomachine is
Instabilities of uniform filtration flows with phase transition
Il'Ichev, A. T.; Tsypkin, G. G.
2008-10-01
New mechanisms of instability are described for vertical flows with phase transition through horizontally extended two-dimensional regions of a porous medium. A plane surface of phase transition becomes unstable at an infinitely large wavenumber and at zero wavenumber. In the latter case, the unstable flow undergoes reversible subcritical bifurcations leading to the development of secondary flows (which may not be horizontally uniform). The evolution of subcritical modes near the instability threshold is governed by the Kolmogorov-Petrovskii-Piskunov equation. Two examples of flow through a porous medium are considered. One is the unstable flow across a water-bearing layer above a layer that carries a vapor-air mixture under isothermal conditions in the presence of capillary forces at the phase transition interface. The other is the vertical flow with phase transition in a high-temperature geothermal reservoir consisting of two high-permeability regions separated by a low-permeability stratum.
Instability of diverging and converging flows in an annulus
Ilin, Konstantin
2012-01-01
The stability of two-dimensional diverging and converging flows in an annulus between two permeable cylinders is examined. The basic flow is irrotational and has both the radial and azimuthal components. It is shown that for a wide range of the parameters of the problem, the basic flow is unstable to small two-dimensional perturbations. The instability is inviscid and oscillatory and persists if the viscosity of the fluid is taken into consideration.
Modeling of fluidelastic instability in tube bundle subjected to two-phase cross-flow
Energy Technology Data Exchange (ETDEWEB)
Sawadogo, T.P.; Mureithi, N.W.; Azizian, R.; Pettigrew, M.J. [Ecole Polytechnique, Dept. of Mechanical Engineering, BWC/AECL/NSERC Chair of Fluid-Structure Interaction, Montreal, Quebec (Canada)
2009-07-01
Tube arrays in steam generators and heat exchangers operating in two-phase cross-flow are subjected sometimes to strong vibration due mainly to turbulence buffeting and fluidelastic forces. This can lead to tube damage by fatigue or fretting wear. A computer implementation of a fluidelastic instability model is proposed to determine with improved accuracy the fluidelastic forces and hence the critical instability flow velocity. Usually the fluidelastic instability is 'predicted', using the Connors relation with K=3. While the value of K can be determined experimentally to get an accurate prediction of the instability, the Connors relation does not allow good estimation of the fluid forces. Consequently the RMS value of the magnitude of vibration of the tube bundle, necessary to evaluate the work rate and the tube wear is only poorly estimated. The fluidelastic instability analysis presented here is based on the quasi-steady model, originally developed for single phase flow. The fluid forces are expressed in terms of the quasi-static drag and lift force coefficients and their derivatives which are determined experimentally. The forces also depend on the tube displacement and velocity. In the computer code ABAQUS, the fluid forces are provided in the user subroutines VDLOAD or VUEL. A typical simulation of the vibration of a single flexible tube within an array in two phase cross-flow is done in ABAQUS and the results are compared with the experimental measurements for a tube with similar physical properties. For a cantilever tube, in two phase cross-flow of void fraction 60%, the numerical critical flow velocity was 2.0 m/s compared to 1.8 m/s obtained experimentally. The relative error was 5% compared to 26.6% for the Connors relation with K=3. The simulation of the vibration of a typical tube in a steam generator is also presented. The numerical results show good agreement with experimental measurements. (author)
Flow Instability of Molten GaAs in the Czochralski Configuration
Institute of Scientific and Technical Information of China (English)
Shuxian CHEN; Mingwei LI
2007-01-01
The flow and heat transfer of molten GaAs under the interaction of buoyancy, Marangoni and crystal rotation in the Czochralski configuration are numerically studied by using a time-dependent and three-dimensional turbulent flow model for the first time. The transition from axisymmetric flow to non-axisymmetric flow and then returning to axisymmetric flow again with increasing centrifugal and coriolis forces by increasing the crystal rotation rate was numerically observed. The origin of the transition to non-axisymmetric flow has been proved to be baroclinic instability. Several important characteristics of baroclinic instability in the CZ GaAs melt have been predicted. These characteristics are found to be in agreement with experimental observations.
Global Instability and Control of Low-Pressure Turbine Flows
2006-03-31
Methodos Numericos in Ingenieria 2005, Granada Y 3 rd Symposium on Global Flow Instability and Control September 2005 Crete, Greece. * AIAA San...valid OMB control number. PLEASE DO NOT RETURN YOUR FORM TO THE ABOVE ADDRESS. 1. REPORT DATE (DD-MM-YYYY) 2. REPORT TYPE 3. DATES COVERED (From - To...31-03-2006 Final 01-02-2003 To 31-12-2005 4. TITLE AND SUBTITLE 5a. CONTRACT NUMBER Global Instability and Control of Low Pressure Turbine Flows 5b
Shear Flow instability in a strongly coupled dusty plasma
Banerjee, D; Chakrabarti, N
2013-01-01
Linear stability analysis of strongly coupled incompressible dusty plasma in presence of shear flow has been carried out using Generalized Hydrodynamical(GH) model. With the proper Galilean invariant GH model, a nonlocal eigenvalue analysis has been done using different velocity profiles. It is shown that the effect of elasticity enhances the growth rate of shear flow driven Kelvin- Helmholtz (KH) instability. The interplay between viscosity and elasticity not only enhances the growth rate but the spatial domain of the instability is also widened. The growth rate in various parameter space and the corresponding eigen functions are presented.
Numerical Simulation of Flow Instabilities in High Speed Multistage Compressors
Institute of Scientific and Technical Information of China (English)
JunHu; ThomasPeters; 等
1999-01-01
In the present paper,a nonlinear multi“actuator disk” model is proposed to analyze the dynamic behavior of flow instabilities,including rotating stall and surge,in high speed multistage axial compressors.The model describes the duct flow fields using two dimensional,compressible and unsteady Euler equations,and accounts for the influences of downstream plenum and throttle in the system as well.It replaces each blade row of multistage compressore with a disk.For numerical calculations,the time marching procedure,using MacCormack two steps scheme,is used.The main pupose of this paper is to predict the mechanism of two dimensional short wavelength rotating stall inception and the interation between blade rows in high speed multistage compressors.It has been demonstrated that the model has the ability to predict those phenomena,and the results show that some system parameters have a strong effect on the stall features as well.Results for a five stage high speed compressor are analyzed in detail,and comparison with the experimental data demonstrates that the model and calculating results are reliable.
Transverse electron-scale instability in relativistic shear flows
Alves, E P; Fonseca, R A; Silva, L O
2015-01-01
Electron-scale surface waves are shown to be unstable in the transverse plane of a shear flow in an initially unmagnetized plasma, unlike in the (magneto)hydrodynamics case. It is found that these unstable modes have a higher growth rate than the closely related electron-scale Kelvin-Helmholtz instability in relativistic shears. Multidimensional particle-in-cell simulations verify the analytic results and further reveal the emergence of mushroom-like electron density structures in the nonlinear phase of the instability, similar to those observed in the Rayleigh Taylor instability despite the great disparity in scales and different underlying physics. Macroscopic ($\\gg c/\\omega_{pe}$) fields are shown to be generated by these microscopic shear instabilities, which are relevant for particle acceleration, radiation emission and to seed MHD processes at long time-scales.
Flow instability in laminar jet flames driven by alternating current electric fields
Kim, Gyeong Taek
2016-10-13
The effect of electric fields on the instability of laminar nonpremixed jet flames was investigated experimentally by applying the alternating current (AC) to a jet nozzle. We aimed to elucidate the origin of the occurrence of twin-lifted jet flames in laminar jet flow configurations, which occurred when AC electric fields were applied. The results indicated that a twin-lifted jet flame originated from cold jet instability, caused by interactions between negative ions in the jet flow via electron attachment as O +e→O when AC electric fields were applied. This was confirmed by conducting systematic, parametric experiment, which included changing gaseous component in jets and applying different polarity of direct current (DC) to the nozzle. Using two deflection plates installed in parallel with the jet stream, we found that only negative DC on the nozzle could charge oxygen molecules negatively. Meanwhile, the cold jet instability occurred only for oxygen-containing jets. A shedding frequency of jet stream due to AC driven instability showed a good correlation with applied AC frequency exhibiting a frequency doubling. However, for the applied AC frequencies over 80Hz, the jet did not respond to the AC, indicating an existence of a minimum flow induction time in a dynamic response of negative ions to external AC fields. Detailed regime of the instability in terms of jet velocity, AC voltage and frequency was presented and discussed. Hypothesized mechanism to explain the instability was also proposed.
Studies on flow instability of helical tube steam generator with Nyquist criterion
Energy Technology Data Exchange (ETDEWEB)
Niu, Fenglei, E-mail: niufenglei@ncepu.edu.cn [State Key Laboratory of Alternate Electrical Power System with Renewable Energy Sources, North China Electric Power University, Beijing 102206 (China); Tian, Li; Yu, Yu [State Key Laboratory of Alternate Electrical Power System with Renewable Energy Sources, North China Electric Power University, Beijing 102206 (China); Li, Rizhu [Institute of Nuclear and New Energy Technology, Tsinghua University, Beijing 100084 (China); Norman, Timothy L. [Westinghouse Electric Company, Madison, PA 15663 (United States)
2014-01-15
Highlights: • Density-wave oscillation in helical-tube steam generators was studied. • The multi-variable frequency domain method was used for the modeling. • The flow stability was evaluated by the Nyquist stability criterion. • The calculated results are consistent with the experimental results. -- Abstract: The steam generator of the 10 MW High Temperature Gas-Cooled Reactor (HTR-10) in China consists of a series of helical tubes where water/steam flows inside and helium flows outside. It operates under middle pressure, which tends to cause the flow instability. Density-wave oscillation is the most common type of two-phase flow instability in the steam generators. This paper presents the research on flow instability for the HTR-10 steam generator. The drift flux model was used for two-phase flow analysis. The transfer matrix was obtained by using linearized perturbation and Laplace transformation on the conservation equations. The flow stability was evaluated by the Nyquist stability criterion. The results obtained from frequency domain method were compared and discussed with the results from the time domain method and the experimental results.
Experimental Investigation of Aerodynamic Instability of Iced Bridge Cable Sections
DEFF Research Database (Denmark)
Koss, Holger; Lund, Mia Schou Møller
2013-01-01
of bridge cables under wind action. This paper describes the experimental simulation of ice accretion on a real bridge cable sheet HDPE tube segment (diameter 160mm) and its effect on the aerodynamic load. Furthermore, aerodynamic instability will be estimated with quasi-steady theory using the determined...
The Inveterate Tinkerer: 2. Instability of Kolmogorov Flow
Indian Academy of Sciences (India)
Aditi Kambli; Chirag Kalelkar
2017-04-01
In this series of articles, the authors discuss various phenomenain fluid dynamics, which may be investigated via tabletopexperiments using low-cost or home-made instruments.The second article in this series is about a simple set-up fordemonstrating the instability of Kolmogorov Flow.
Experimental demonstration of bow-shock instability and its numerical analysis
Kikuchi, Y.; Ohnishi, N.; Ohtani, K.
2016-07-01
An experimental demonstration was carried out in a ballistic range at high Mach numbers with the low specific heat ratio gas hydrofluorocarbon HFC-134a to observe the unstable bow-shock wave generated in front of supersonic blunt objects. The shadowgraph images obtained from the experiments showed instability characteristics, in which the disturbances grow and flow downstream and the wake flow appears wavy because of the shock oscillation. Moreover, the influence of the body shape and specific heat ratio on the instability was investigated for various experimental conditions. Furthermore, the observed features, such as wave structure and disturbance amplitude, were captured by numerical simulations, and it was demonstrated that computational fluid dynamics could effectively simulate the physical instability. In addition, it was deduced that the shock instability is induced by sound emissions from the edge of the object. This inference supports the dependence of the instability on the specific heat ratio and Mach number because the shock stand-off distance is affected by these parameters and limits the sound wave propagation.
Nonaxisymmetric linear instability of cylindrical magnetohydrodynamic Taylor-Couette flow.
Child, Adam; Kersalé, Evy; Hollerbach, Rainer
2015-09-01
We consider the nonaxisymmetric modes of instability present in Taylor-Couette flow under the application of helical magnetic fields, mainly for magnetic Prandtl numbers close to the inductionless limit, and conduct a full examination of marginal stability in the resulting parameter space. We allow for the azimuthal magnetic field to be generated by a combination of currents in the inner cylinder and fluid itself and introduce a parameter governing the relation between the strength of these currents. A set of governing eigenvalue equations for the nonaxisymmetric modes of instability are derived and solved by spectral collocation with Chebyshev polynomials over the relevant parameter space, with the resulting instabilities examined in detail. We find that by altering the azimuthal magnetic field profiles the azimuthal magnetorotational instability, nonaxisymmetric helical magnetorotational instability, and Tayler instability yield interesting dynamics, such as different preferred mode types and modes with azimuthal wave number m>1. Finally, a comparison is given to the recent WKB analysis performed by Kirillov et al. [Kirillov, Stefani, and Fukumoto, J. Fluid Mech. 760, 591 (2014)JFLSA70022-112010.1017/jfm.2014.614] and its validity in the linear regime.
Experimental study of parametric subharmonic instability for internal waves
Bourget, Baptiste; Joubaud, Sylvain; Odier, Philippe
2013-01-01
Internal waves are believed to be of primary importance as they affect ocean mixing and energy transport. Several processes can lead to the breaking of internal waves and they usually involve non linear interactions between waves. In this work, we study experimentally the parametric subharmonic instability (PSI), which provides an efficient mechanism to transfer energy from large to smaller scales. It corresponds to the destabilization of a primary plane wave and the spontaneous emission of two secondary waves, of lower frequencies and different wave vectors. Using a time-frequency analysis, we observe the time evolution of the secondary waves, thus measuring the growth rate of the instability. In addition, a Hilbert transform method allows the measurement of the different wave vectors. We compare these measurements with theoretical predictions, and study the dependence of the instability with primary wave frequency and amplitude, revealing a possible effect of the confinement due to the finite size of the be...
Goertler instability. [for boundary layer flow over curved walls
Ragab, S. A.; Nayfeh, A. H.
1981-01-01
Goertler instability for boundary-layer flows over generally curved walls is considered. The full-linearized disturbance equations are obtained in an orthogonal curvilinear coordinate system. A perturbation procedure to account for second-order effects is used to determine the effects of the displacement thickness and the variation of the streamline curvature on the neutral stability of the Blasius flow. The streamwise pressure gradient in the mean flow is accounted for by solving the nonsimilar boundary-layer equations. Growth rates are obtained for the actual mean flow and compared with those for the Blasius flow and the Falkner-Skan flows. The results demonstrate the strong influence of the streamwise pressure gradient and the nonsimilarity of the basic flow on the stability characteristics.
Observation of dual-mode, Kelvin-Helmholtz instability vortex merger in a compressible flow
Wan, W. C.; Malamud, G.; Shimony, A.; Di Stefano, C. A.; Trantham, M. R.; Klein, S. R.; Shvarts, D.; Drake, R. P.; Kuranz, C. C.
2017-05-01
We report the first observations of Kelvin-Helmholtz vortices evolving from well-characterized, dual-mode initial conditions in a steady, supersonic flow. The results provide the first measurements of the instability's vortex merger rate and supplement data on the inhibition of the instability's growth rate in a compressible flow. These experimental data were obtained by sustaining a shockwave over a foam-plastic interface with a precision-machined seed perturbation. This technique produced a strong shear layer between two plasmas at high-energy-density conditions. The system was diagnosed using x-ray radiography and was well-reproduced using hydrodynamic simulations. Experimental measurements imply that we observed the anticipated vortex merger rate and growth inhibition for supersonic shear flow.
THREE-DIMENSIONAL INSTABILITY OF AN OSCILLATING VISCOUS FLOW PAST A CIRCULAR CYLINDER
Institute of Scientific and Technical Information of China (English)
陆夕云; 凌国灿
2003-01-01
A systematically numerical study of the sinusoidally oscillating viscous flowaround a circular cylinder was performed to investigate vortical instability by solving thethree-dimensional incompressible Navier-Stokes equations. The transition from two- to three-dimensional flow structures along the axial direction due to the vortical instability appears,and the three-dimensional structures lie alternatively on the two sides of the cylinder.Numerical study has been taken for the Keulegan-Carpenter(KC) numbers from 1 to 3.2and frequency parameters from 1O0 to 600. The force behaviors are also studied by solvingthe Morison equation. Calculated results agree well with experimental data and theoreticalprediction.
Experimental investigation of cavity flows
Energy Technology Data Exchange (ETDEWEB)
Loeland, Tore
1998-12-31
This thesis uses LDV (Laser Doppler Velocimetry), PIV (Particle Image Velocimetry) and Laser Sheet flow Visualisation to study flow inside three different cavity configurations. For sloping cavities, the vortex structure inside the cavities is found to depend upon the flow direction past the cavity. The shape of the downstream corner is a key factor in destroying the boundary layer flow entering the cavity. The experimental results agree well with numerical simulations of the same geometrical configurations. The results of the investigations are used to find the influence of the cavity flow on the accuracy of the ultrasonic flowmeter. A method to compensate for the cavity velocities is suggested. It is found that the relative deviation caused by the cavity velocities depend linearly on the pipe flow. It appears that the flow inside the cavities should not be neglected as done in the draft for the ISO technical report on ultrasonic flowmeters. 58 refs., 147 figs., 2 tabs.
The Physics of Flow Instability and Turbulent Transition in Shear Flows
Dou, H S
2006-01-01
In this paper, the physics of flow instability and turbulent transition in shear flows is studied by analyzing the energy variation of fluid particles under the interaction of base flow with a disturbance. It is shown that it is the transverse energy gradient that leads to the disturbance amplification while the disturbance is damped by the energy loss due to viscosity along the streamline. For the first time, a theory derived strictly from physics, is used to show that the flow instability under finite amplitude disturbance leads to turbulent transition. It is also shown that flow instability in shear flows is a nonlinear phenomenon and it has a threshold related to the disturbance amplitude. The mechanism for velocity inflection and hairpin vortex formation are explained with reference to analytical results. The inverse Reynolds number dependence of the disturbance threshold, observed in recent experiments, is well explained. Following from this analysis, it can be demonstrated that the critical value of th...
Interfacial instabilities in a stratified flow of two superposed fluids
Schaflinger, Uwe
1994-06-01
Here we shall present a linear stability analysis of a laminar, stratified flow of two superposed fluids which are a clear liquid and a suspension of solid particles. The investigation is based upon the assumption that the concentration remains constant within the suspension layer. Even for moderate flow-rates the base-state results for a shear induced resuspension flow justify the latter assumption. The numerical solutions display the existence of two different branches that contribute to convective instability: long and short waves which coexist in a certain range of parameters. Also, a range exists where the flow is absolutely unstable. That means a convectively unstable resuspension flow can be only observed for Reynolds numbers larger than a lower, critical Reynolds number but still smaller than a second critical Reynolds number. For flow rates which give rise to a Reynolds number larger than the second critical Reynolds number, the flow is absolutely unstable. In some cases, however, there exists a third bound beyond that the flow is convectively unstable again. Experiments show the same phenomena: for small flow-rates short waves were usually observed but occasionally also the coexistence of short and long waves. These findings are qualitatively in good agreement with the linear stability analysis. Larger flow-rates in the range of the second critical Reynolds number yield strong interfacial waves with wave breaking and detached particles. In this range, the measured flow-parameters, like the resuspension height and the pressure drop are far beyond the theoretical results. Evidently, a further increase of the Reynolds number indicates the transition to a less wavy interface. Finally, the linear stability analysis also predicts interfacial waves in the case of relatively small suspension heights. These results are in accordance with measurements for ripple-type instabilities as they occur under laminar and viscous conditions for a mono-layer of particles.
Experimental, Numerical and Analytical Studies of the MHD-driven plasma jet, instabilities and waves
Zhai, Xiang
This thesis describes a series of experimental, numerical, and analytical studies involving the Caltech magnetohydrodynamically (MHD)-driven plasma jet experiment. The plasma jet is created via a capacitor discharge that powers a magnetized coaxial planar electrodes system. The jet is collimated and accelerated by the MHD forces. We present three-dimensional ideal MHD finite-volume simulations of the plasma jet experiment using an astrophysical magnetic tower as the baseline model. A compact magnetic energy/helicity injection is exploited in the simulation analogous to both the experiment and to astrophysical situations. Detailed analysis provides a comprehensive description of the interplay of magnetic force, pressure, and flow effects. We delineate both the jet structure and the transition process that converts the injected magnetic energy to other forms. When the experimental jet is sufficiently long, it undergoes a global kink instability and then a secondary local Rayleigh-Taylor instability caused by lateral acceleration of the kink instability. We present an MHD theory of the Rayleigh-Taylor instability on the cylindrical surface of a plasma flux rope in the presence of a lateral external gravity. The Rayleigh-Taylor instability is found to couple to the classic current-driven instability, resulting in a new type of hybrid instability. The coupled instability, produced by combination of helical magnetic field, curvature of the cylindrical geometry, and lateral gravity, is fundamentally different from the classic magnetic Rayleigh-Taylor instability occurring at a two-dimensional planar interface. In the experiment, this instability cascade from macro-scale to micro-scale eventually leads to the failure of MHD. When the Rayleigh-Taylor instability becomes nonlinear, it compresses and pinches the plasma jet to a scale smaller than the ion skin depth and triggers a fast magnetic reconnection. We built a specially designed high-speed 3D magnetic probe and
Energy Technology Data Exchange (ETDEWEB)
Andronov, V.A.; Zhidov, I.G.; Meskov, E.E.; Nevmerzhitskii, N.V.; Nikiforov, V.V.; Razin, A.N.; Rogatchev, V.G.; Tolshmyakov, A.I.; Yanilkin, Yu.V. [Russian Federal Nuclear Center (Russian Federation)
1995-02-01
This report describes an extensive program of investigations conducted at Arzamas-16 in Russia over the past several decades. The focus of the work is on material interface instability and the mixing of two materials. Part 1 of the report discusses analytical and computational studies of hydrodynamic instabilities and turbulent mixing. The EGAK codes are described and results are illustrated for several types of unstable flow. Semiempirical turbulence transport equations are derived for the mixing of two materials, and their capabilities are illustrated for several examples. Part 2 discusses the experimental studies that have been performed to investigate instabilities and turbulent mixing. Shock-tube and jelly techniques are described in considerable detail. Results are presented for many circumstances and configurations.
On the instability of hypersonic flow past a flat plate
Blackaby, Nicholas; Cowley, Stephen; Hall, Philip
1990-01-01
The instability of hypersonic boundary-layer flows over flat plates is considered. The viscosity of the fluid is taken to be governed by Sutherland's law, which gives a much more accurate representation of the temperature dependence of fluid viscosity at hypersonic speeds than Chapman's approximate linear law; although at lower speeds the temperature variation of the mean state is less pronounced so that the Chapman law can be used with some confidence. Attention is focussed on the so-called (vorticity) mode of instability of the viscous hypersonic boundary layer. This is thought to be the fastest growing inviscid disturbance at hypersonic speeds; it is also believed to have an asymptotically larger growth rate than any viscous or centrifugal instability. As a starting point the instability of the hypersonic boundary layer which exists far downstream from the leading edge of the plate is investigated. In this regime the shock that is attached to the leading edge of the plate plays no role, so that the basic boundary layer is non-interactive. It is shown that the vorticity mode of instability of this flow operates on a significantly different lengthscale than that obtained if a Chapman viscosity law is assumed. In particular, it is found that the growth rate predicted by a linear viscosity law overestimates the size of the growth rate by O(M(exp 2). Next, the development of the vorticity mode as the wavenumber decreases is described, and it is shown that acoustic modes emerge when the wavenumber has decreased from it's O(1) initial value to O(M (exp -3/2). Finally, the inviscid instability of the boundary layer near the leading edge in the interaction zone is discussed and particular attention is focussed on the strong interaction region which occurs sufficiently close to the leading edge. It is found that the vorticity mode in this regime is again unstable, and that it is concentrated in the transition layer at the edge of the boundary layer where the temperature
Rossby Wave Instability in the Accretion Flows around Black Holes
Gholipour, Mahmoud
2017-01-01
The roles of the Rossby wave instability (RWI) have been significantly developed in some important processes, such as planet formation and angular momentum transport through thin accretion disks. However, their development on accretion flows with advection is insignificant. In this paper, we investigate the effect of advection in the occurrence of RWI through accretion flows around black holes (BHs). In the absence of advection, the occurrence of RWI is extremely low because of high viscosity in the accretion flows around BHs. The results of this paper show that there is a significant chance for the occurrence of RWI in some wavelengths if we consider advection even in low amounts. Therefore, the RWI can be a suitable candidate for angular momentum transport in the accretion flows around BHs. Also, the results show that the advection parameter and the ratio of heat capacity, which are special characters of advection flows, play important roles in the occurrence of RWI.
Study of flow instability in off design operation of a multistage centrifugal pump
Energy Technology Data Exchange (ETDEWEB)
Shibata, Akiha; Maeda, Manabu; Kamei, Shun; Hazama, Ryota; Sano, Takeshi; Iino, Masamichi [Mitsubishi Heavy Industries, Ltd., Tokyo (Japan); Hiramatsu, Hideto; Komaki, Shutaro; Miyagawa, Kazuyoshi [Waseda University, Okubo (Japan)
2016-02-15
In recent years, attempts have been made to make multistage centrifugal pumps smaller in size and more efficient. However, such designs are known to cause positive-slope phenomena in the Q-H curve, especially under low-flow conditions. These phenomena, which have thus far been studied experimentally and numerically, stem from flow instability in the pump. However, their mechanisms have not yet been clarified because it depends on various parameters. In this study, we focused on diffuser rotating stall, observed in positive Q-H characteristics. This study elucidates the mechanism of positive-slope generation through experimental results and two-dimensional numerical analysis.
Matsuzaka, R.; Nakashima, T.; Miyagawa, K.
2016-11-01
A swirling flow in a diffuser such as a draft tube of a hydro turbine may induce the flow instabilities accompanied by pressure fluctuations known as vortex rope behaviour and cavitation surge. Cavitation surge is the self-excited oscillation, which induces the large flow rate fluctuation that results from the change of the cavity volume. In this research, the investigation of the effect of the pipe length and the swirl intensity on the flow instabilities in a diffuser was performed by experiments and numerical analyses using the draft tube component experimental facility. The length of the pipe was modified by up to about 25 times as long as the diameter of the throat in order to validate the one-dimensional analyses. In addition, the swirl intensity was changed by replacing another swirl generator. The frequency of cavitation surge was changed with regard to the swirl intensity as the one-dimensional analyses in the previous study has predicted it. Unsteady numerical simulations of the swirling flow with cavitation in the diffuser was performed. The results of experiments and numerical analyses correspond qualitatively with the result of the one-dimensional analyses, which suggested that the coupling with the experiments, CFD analyses and the one-dimensional analyses is the more effective way in order to predict the flow instabilities in the diffuser.
Observation of Single-Mode, Kelvin-Helmholtz Instability in a Supersonic Flow.
Wan, W C; Malamud, G; Shimony, A; Di Stefano, C A; Trantham, M R; Klein, S R; Shvarts, D; Kuranz, C C; Drake, R P
2015-10-02
We report the first observation, in a supersonic flow, of the evolution of the Kelvin-Helmholtz instability from a single-mode initial condition. To obtain these data, we used a novel experimental system to produce a steady shock wave of unprecedented duration in a laser-driven experiment. The shocked, flowing material creates a shear layer between two plasmas at high energy density. We measured the resulting interface structure using radiography. Hydrodynamic simulations reproduce the large-scale structures very well and the medium-scale structures fairly well, and imply that we observed the expected reduction in growth rate for supersonic shear flow.
Non-axisymmetric instabilities in discs with imposed zonal flows
Vanon, R
2016-01-01
We conduct a linear stability calculation of an ideal Keplerian flow on which a sinusoidal zonal flow is imposed. The analysis uses the shearing sheet model and is carried out both in isothermal and adiabatic conditions, with and without self-gravity (SG). In the non-SG regime a structure in the potential vorticity (PV) leads to a non-axisymmetric Kelvin-Helmholtz (KH) instability; in the short-wavelength limit its growth rate agrees with the incompressible calculation by Lithwick (2007), which only considers perturbations elongated in the streamwise direction. The instability's strength is analysed as a function of the structure's properties, and zonal flows are found to be stable if their wavelength is $\\gtrsim 8H$, where $H$ is the disc's scale height, regardless of the value of the adiabatic index $\\gamma$. The non-axisymmetric KH instability can operate in Rayleigh-stable conditions, and it therefore represents the limiting factor to the structure's properties. Introducing SG triggers a second non-axisym...
Elliptical flow instability in a conducting fluid triggered by an external magnetic field.
Bajer, Konrad; Mizerski, Krzysztof
2013-03-08
We demonstrate that arbitrarily weak magnetic field may cause violent instability of an anticyclonic, recirculating flow with uniform mean angular velocity. This magnetohydrodynamic instability would trigger turbulence in the cores of vortices where neither centrifugal, exchange instability, nor magnetorotational instability is effective. In the accretion disk vortices this can be an important mechanism of enhanced outward transport of angular momentum.
Institute of Scientific and Technical Information of China (English)
Xu Wan-hai; Yu Jian-xing; Du Jie; CHENG An-kang; KANG Hao
2012-01-01
The streamwise flow-induced vibration of a circular cylinder with symmetric vortex shedding in the first instability range is investigated,and a wake oscillator model for the dynamic response prediction is proposed.An approach is applied to calibrate the empirical parameters in the present model; the numerical and experimental results are compared to validate the proposed model.It can be found that the present prediction model is accurate and sufficiently simple to be easily applied in practice.
Linear Instability of the Plane Couette and Plane Poiseuille Flows
Chefranov, Sergey G
2015-01-01
We show possibility of the Plane Couette (PC) flow instability for Reynolds number Re>Reth=140. This new result of the linear hydrodynamic stability theory is obtained on the base of refusal from the traditionally used assumption on longitudinal periodicity of the disturbances along the direction of the fluid flow. We found that earlier existing understanding on the linear stability of this flow for any arbitrary large Reynolds number is directly related with an assumption on the separation of the variables of the spatial variability for the disturbance field and their periodicity in linear theory of stability. By the refusal from the pointed assumptions also for the Plane Poiseuille (PP) flow, we get a new threshold Reynolds value Reth=1040 that with 4% accuracy agrees with the experiment contrary to more than 500% discrepancy for the earlier known estimate Reth=5772 obtained in the frame of the linear theory but when using the "normal" disturbance form (S. A. Orszag, 1971).
Flow Instability and Wall Shear Stress Ocillation in Intracranial Aneurysms
Baek, Hyoungsu; Jayamaran, Mahesh; Richardson, Peter; Karniadakis, George
2009-11-01
We investigate the flow dynamics and oscillatory behavior of wall shear stress (WSS) vectors in intracranial aneurysms using high-order spectral/hp simulations. We analyze four patient- specific internal carotid arteries laden with aneurysms of different characteristics : a wide-necked saccular aneurysm, a hemisphere-shaped aneurysm, a narrower-necked saccular aneurysm, and a case with two adjacent saccular aneurysms. Simulations show that the pulsatile flow in aneurysms may be subject to a hydrodynamic instability during the decelerating systolic phase resulting in a high-frequency oscillation in the range of 30-50 Hz. When the aneurysmal flow becomes unstable, both the magnitude and the directions of WSS vectors fluctuate. In particular, the WSS vectors around the flow impingement region exhibit significant spatial and temporal changes in direction as well as in magnitude.
Comprehensive experimental and numerical analysis of instability phenomena in pump turbines
Gentner, Ch; Sallaberger, M.; Widmer, Ch; Bobach, B.-J.; Jaberg, H.; Schiffer, J.; Senn, F.; Guggenberger, M.
2014-03-01
The changes in the electricity market have led to changed requirements for the operation of pump turbines. Utilities need to change fast and frequently between pumping and generating modes and increasingly want to operate at off-design conditions for extended periods. Operation of the units in instable areas of the machine characteristic is not acceptable and may lead to self-excited vibration of the hydraulic system. In turbine operation of pump turbines unstable behaviour can occur at low load off-design operation close to runaway conditions (S-shape of the turbine characteristic). This type of instability may impede the synchronization of the machine in turbine mode and thus increase start-up and switch over times. A pronounced S-shaped instability can also lead to significant drop of discharge in the event of load rejection. Low pressure on the suction side and in the tail-race tunnel could cause dangerous separation of the water column. Understanding the flow features that lead to the instable behaviour of pump turbines is a prerequisite to the design of machines that can fulfil the growing requirements relating to operational flexibility. Flow simulation in these instability zones is demanding due to the complex and highly unsteady flow patterns. Only unsteady simulation methods are able to reproduce the governing physical effects in these operating regions. ANDRITZ HYDRO has been investigating the stability behaviour of pump turbines in turbine operation in cooperation with several universities using simulation and measurements. In order to validate the results of flow simulation of unstable operating points, the Graz University of Technology (Austria) performed detailed experimental investigations. Within the scope of a long term research project, the operating characteristics of several pump turbine runners have been measured and flow patterns in the pump turbine at speed no load and runaway have been examined by 2D Laser particle image velocimetry (PIV
Experimental studies on the flow through soft tubes and channels
Indian Academy of Sciences (India)
V Kumaran
2015-05-01
Experiments conducted in channels/tubes with height/diameter less than 1 mm with soft walls made of polymer gels show that the transition Reynolds number could be significantly lower than the corresponding value of 1200 for a rigid channel or 2100 for a rigid tube. Experiments conducted with very viscous fluids show that there could be an instability even at zero Reynolds number provided the surface is sufficiently soft. Linear stability studies show that the transition Reynolds number is linearly proportional to the wall shear modulus in the low Reynolds number limit, and it increases as the 1/2 and 3/4 power of the shear modulus for the ‘inviscid’ and ‘wall mode’ instabilities at high Reynolds number. While the inviscid instability is similar to that in the flow in a rigid channel, the mechanisms of the viscous and wall mode instabilities are qualitatively different. These involve the transfer of energy from the mean flow to the fluctuations due to the shear work done at the interface. The experimental results for the viscous instability mechanism are in quantitative agreement with theoretical predictions. At high Reynolds number, the instability mechanism has characteristics similar to the wall mode instability. The experimental transition Reynolds number is smaller, by a factor of about 10, than the theoretical prediction for the parabolic flow through rigid tubes and channels. However, if the modification in the tube shape due to the pressure gradient, and the consequent modification in the velocity profile and pressure gradient, are incorporated, there is quantitative agreement between theoretical predictions and experimental results. The transition has important practical consequences, since there is a significant enhancement of mixing after transition.
The limiting form of symmetric instability in geophysical flows
Griffiths, Stephen
2017-04-01
The stability of parallel flow with vertical shear, density stratification and background rotation is of fundamental importance in geophysical fluid dynamics. For a flow with vertical shear Uz and buoyancy frequency N, the dominant instability is typically a symmetric instability (sometimes known as slantwise convection) when 1/4 linear stability problem has been well studied for the case of constant Uz and N, and has some interesting mathematical properties (e.g., non-separable governing PDE, an absence of normal mode solutions in rectangular domains). Here, for the first time, a general theory of symmetric instability is given when Ri varies smoothly with height, thinking of the more realistic case where an unstable layer with Ri 1. The mathematical theory is developed for horizontally periodic disturbances to a basic state with arbitrary smooth N(z), but constant Uz. An asymptotic analysis is used to derive expressions for the most unstable mode, which occurs in the limit of large cross-isentropic wavenumber and takes the form of solutions trapped within the unstable layer; the same result is derived using an interesting generalised parcel dynamics argument, which explicitly shows how the trapping is linked to vertical variations of the potential vorticity. A separate asymptotic analysis is given for the small wavenumber limit, where only one such trapped mode may exist, as expected from the spectral theory of the Schrödinger equation. These two limiting results are shown to be consistent with an exact solution of the linear stability problem that can be obtained for a special choice of N(z). The asymptotic analysis can be extended to allow for weak diffusion at arbitrary Prandtl number, yielding an explicit diffusive scale selection at large wavenumber. Numerical simulations show that these weakly diffusive modes dominate the early stages of the nonlinear evolution of the symmetric instability.
Chefranov, Sergey; Chefranov, Alexander
2016-04-01
Linear hydrodynamic stability theory for the Hagen-Poiseuille (HP) flow yields a conclusion of infinitely large threshold Reynolds number, Re, value. This contradiction to the observation data is bypassed using assumption of the HP flow instability having hard type and possible for sufficiently high-amplitude disturbances. HP flow disturbance evolution is considered by nonlinear hydrodynamic stability theory. Similar is the case of the plane Couette (PC) flow. For the plane Poiseuille (PP) flow, linear theory just quantitatively does not agree with experimental data defining the threshold Reynolds number Re= 5772 ( S. A. Orszag, 1971), more than five-fold exceeding however the value observed, Re=1080 (S. J. Davies, C. M. White, 1928). In the present work, we show that the linear stability theory conclusions for the HP and PC on stability for any Reynolds number and evidently too high threshold Reynolds number estimate for the PP flow are related with the traditional use of the disturbance representation assuming the possibility of separation of the longitudinal (along the flow direction) variable from the other spatial variables. We show that if to refuse from this traditional form, conclusions on the linear instability for the HP and PC flows may be obtained for finite Reynolds numbers (for the HP flow, for Re>704, and for the PC flow, for Re>139). Also, we fit the linear stability theory conclusion on the PP flow to the experimental data by getting an estimate of the minimal threshold Reynolds number as Re=1040. We also get agreement of the minimal threshold Reynolds number estimate for PC with the experimental data of S. Bottin, et.al., 1997, where the laminar PC flow stability threshold is Re = 150. Rogue waves excitation mechanism in oppositely directed currents due to the PC flow linear instability is discussed. Results of the new linear hydrodynamic stability theory for the HP, PP, and PC flows are published in the following papers: 1. S.G. Chefranov, A
The Instability in Accretion Flows: GvMRI
Yardimci, Melis; Ebru Devlen, Doç.
2016-07-01
In this study, we discuss the physical instability defining the expected turbulence in Radiatively Inefficient Accretion Flows (RIAFs) around the supermassive black holes (e.g., Sagittarius A* in the center of our Galaxy). These flows, with a high probability, include weakly collisional hot, optically thin and dilute plasmas. Within these flows, gravitational potential energy brought about by turbulent stresses is trapped as heat energy. Thus, in order accretion to be realized, outward transport of heat as well as angular momentum is required. This outward heat transport may reduce the mass inflow rate on black hole. We solve MHD equations including variation of viscosity coefficients with pressure in the momentum conservation equation. We plot the wave number-frequency diagrams for the wave modes. We show that one of the most probable candidates for definition of mass accretion and the source of excess heat energy in RIAFs is the gyroviscous modified magnetorotational instabilitiy (GvMRI).
Turbulent patterns in wall-bounded flows: a Turing instability?
Manneville, Paul
2012-01-01
In their way to/from turbulence, plane wall-bounded flows display an interesting transitional regime where laminar and turbulent oblique bands alternate, the origin of which is still mysterious. In line with Barkley's recent work about the pipe flow transition involving reaction-diffusion concepts, we consider plane Couette flow in the same perspective and transform Waleffe's classical four-variable model of self-sustaining process into a reaction-diffusion model. We show that, upon fulfillment of a condition on the relative diffusivities of its variables, the featureless turbulent regime becomes unstable against patterning as the result of a Turing instability. A reduced two-variable model helps us to delineate the appropriate region of parameter space. An {\\it intrinsic} status is therefore given to the pattern's wavelength for the first time. Virtues and limitations of the model are discussed, calling for a microscopic support of the phenomenological approach.
Experimental investigation on flow modes of electrospinning
Institute of Scientific and Technical Information of China (English)
Ting Si; Guang-Bin Li; Xing-Xing Chen; Rui-Jun Tian; Xie-Zhen Yin
2012-01-01
Electrospinning experiments are performed by using a set of experimental apparatus,a stroboscopic system is adopted for capturing instantaneous images of the conejet configuration.The cone and the jet of aqueous solutions of polyethylene oxide (PEO) are formed from an orifice of a capillary tube under the electric field.The viscoelastic constitutive relationship of the PEO solution is measured and discussed.The phenomena owing to the jet instability are described,five flow modes and corresponding structures are obtained with variations of the fluid flow rate Q,the electric potential U and the distance h from the orifice of the capillary tube to the collector.The flow modes of the cone-jet configuration involves the steady bending mode,the rotating bending mode,the swinging rotating mode,the blurring bending mode and the branching mode.Regimes in the Q-U plane of the flow modes are also obtained.These results may provide the fundamentals to predict the operating conditions expected in practical applications.
Flow Instability and Its Control in Compression Systems
Institute of Scientific and Technical Information of China (English)
Jingyi Chen
2003-01-01
This paper reviews the development in the research of flow instability and its control over the recent ten or more years. This development was largely stimulated by the novel idea of active control of the aerodynamic instability in compressors. Three topics are covered in the paper, which appeared as the major themes towards the goal of stability enhancement. The first topic is the pre-stall behavior of rotating stall, which plays a vital role in designing the control scheme and discovering the convenient route to find the causal factors of flow disturbances potentially leading to stall. The second topic is the mechanism of blade passage flow during stall and its inception, which is the basic knowledge needed to manipulate the blade design for the stability improvement and eventually to predict the unsteady performance of the compressor system. The third topic is the recent trend of the control strategy based on the learning of active vs. passive methods. To introduce to the discussion of these topics, a brief description of the history of the recent development is given at the beginning of the paper. In discussing each topic, future works are also highlighted to enhance the further development of this long-standing problem in turbomachinery research and application.
The Effect of Fin Pitch on Fluid Elastic Instability of Tube Arrays Subjected to Cross Flow of Water
Desai, Sandeep Rangrao; Pavitran, Sampat
2016-07-01
Failure of tubes in shell and tube exchangers is attributed to flow induced vibrations of such tubes. There are different excitations mechanisms due to which flow induced vibration occurs and among such mechanisms, fluid elastic instability is the most prominent one as it causes the most violent vibrations and may lead to rapid tube failures within short time. Fluid elastic instability is the fluid-structure interaction phenomenon which occurs when energy input by the fluid force exceeds energy expended in damping. This point is referred as instability threshold and corresponding velocity is referred as critical velocity. Once flow velocity exceeds critical flow velocity, the vibration amplitude increases very rapidly with flow velocity. An experimental program is carried out to determine the critical velocity at instability for plain and finned tube arrays subjected to cross flow of water. The tube array geometry is parallel triangular with cantilever end condition and pitch ratios considered are 2.6 and 2.1. The objective of research is to determine the effect of increase in pitch ratio on instability threshold for plain tube arrays and to assess the effect of addition of fins as well as increase in fin density on instability threshold for finned tube arrays. Plain tube array with two different pitch ratios; 2.1 and 2.6 and finned tube arrays with same pitch ratio; 2.6 but with two different fin pitches; such as fine (10 fpi) and coarse (4 fpi) are considered for the experimentation. Connors' equation that relates critical velocity at instability to different parameters, on which instability depends, has been used as the basis for analysis and the concept of effective diameter is used for the present investigation. The modal parameters are first suitably modified using natural frequency reduction setup that is already designed and developed to reduce natural frequency and hence to achieve experimental simulation of fluid elastic instability within the limited
Effect of flux flow on self-field instability
Energy Technology Data Exchange (ETDEWEB)
Dresner, L.
1977-08-01
Flux flow causes type II superconductors to develop resistance continuously rather than suddenly as transport current increases. This means that the distribution of current among the filaments in a composite conductor is determined not only by their inductive coupling but also by the longitudinal resistance they develop as they begin to carry current. The current distribution is calculated in two cases, taking flux flow into account: a composite clamped suddenly across a constant-current source and a composite charged with current at a uniform rate. The results of the latter problem are used to show that slowly charged conductors will be much more stable against self-field instability than is indicated by purely inductive calculations.
Standing Shock Instability in Advection-Dominated Accretion Flows
Le, Truong; Wolff, Michael T; Becker, Peter A; Putney, Joy
2015-01-01
Depending on the values of the energy and angular momentum per unit mass in the gas supplied at large radii, inviscid advection-dominated accretion flows can display velocity profiles with either pre-shock deceleration or pre-shock acceleration. Nakayama has shown that these two types of flow configurations are expected to have different stability properties. By employing the Chevalier & Imamura linearization method and the Nakayama instability boundary conditions, we discover that there are regions of parameters space where disk/shocks with outflows can be stable or unstable. In region of instability, we find that pre-shock deceleration is always unstable to the zeroth mode with zero frequency of oscillation, but is always stable to the fundamental and overtones. Furthermore, we also find that pre-shock acceleration is always unstable to the zeroth mode, and that the fundamental and overtones become increasingly less stable as the shock location moves away from the horizon when the disk half-height expan...
Energy Technology Data Exchange (ETDEWEB)
Mercier, J.F.
1997-10-22
Fluid flows in which liquid layers are submitted to temperature gradient applied horizontally are studied. The thermo capillary and buoyancy effects are thus described. In the experiments, the form of the cell which contains the fluid is different and the fluid heating position too. Each of these experiments reveals a different aspect of the mechanism which is responsible of the flow instability. In the first experiment, the fluid is contained in a rectangular cell longer than larger and whose two longer vertical walls are differentially heated. The waves lengths, frequencies, propagation directions and instability thresholds of stationary, unsteady or oscillating modes obtained theoretically are compared to the structures observed experimentally. The instability mechanisms are essentially bound to the temperature vertical profile in the fluid and the heat exchanges between the fluid and the ambient air are particularly described. In the second experiment, the fluid is contained in an annular cell whose vertical cylindrical walls are differentially heated. The results obtained in the rectangular cell can be transposed to the annular cell substituting the constant thermal gradient of the rectangular geometry by those of the annular geometry, inversely proportional to the radial distance. The introduction of local parameters allows to show that the instability is developed at first near the inside cylinder. In the last experiment, the fluid layer is heated by an electric wire immersed in a parallel direction to the free surface. The development of an ascending vertical flow above the wire induces a deformation of the free surface which can be added to the instability mechanisms of the previous cells. (O.M.) 58 refs.
Experimental investigation of weir instability in main vessel cooling system of 1/4 FBR model
Energy Technology Data Exchange (ETDEWEB)
Thirumalai, M., E-mail: mtl@igcar.gov.i [Fast Reactor Technology Group, Department of Atomic Energy, Indira Gandhi Centre for Atomic Research, Kalpakkam, Tamil Nadu (India); Anandaraj, M.; Kumar, P. Anup [Fast Reactor Technology Group, Department of Atomic Energy, Indira Gandhi Centre for Atomic Research, Kalpakkam, Tamil Nadu (India); Prakash, V., E-mail: prakash@igcar.gov.i [Fast Reactor Technology Group, Department of Atomic Energy, Indira Gandhi Centre for Atomic Research, Kalpakkam, Tamil Nadu (India); Anandbabu, C.; Kalyanasundaram, P.; Vaidyanathan, G. [Fast Reactor Technology Group, Department of Atomic Energy, Indira Gandhi Centre for Atomic Research, Kalpakkam, Tamil Nadu (India)
2010-01-15
The 500 MWe Prototype Fast Breeder Reactor (PFBR) is under construction at Kalpakkam, India. The main vessel of this pool type reactor acts as the primary containment in the reactor assembly. In order to keep the main vessel temperature below creep range and to reduce high temperature embrittlement and also to ensure its healthiness for 40 years of reactor life, a small fraction of core flow (0.5 m{sup 3}/s) is sent through an annular space formed between the main vessel and a cylindrical baffle (primary thermal baffle) to cool the vessel. The sodium after cooling the main vessel overflows the primary baffle (weir shell) and falls into another concentric pool of sodium separated from the cold pool by the secondary thermal baffle and then returned to cold pool. The weir shell, where the overflow of liquid sodium takes place, is a thin shell prone to flow induced vibrations due to instability caused by sloshing and fluid-structure interaction. A similar vibration phenomenon was first observed during the commissioning of Super-Phenix reactor. In order to understand the phenomenon and provide necessary experimental back up to validate the analytical models, weir instability experiments were conducted in a 1:4 scale stainless steel (SS) model installed in a water loop. The experiments were conducted with flow rate and fall height as the varying parameters. The experimental results showed that the instability of the weir shell was caused due to fluid structure interaction. This paper discusses the details of the model, the modeling laws, similitude criteria adopted, analytical prediction, the experimental results and conclusion.
Observation of magnetic field generation via the Weibel instability in interpenetrating plasma flows
Energy Technology Data Exchange (ETDEWEB)
Huntington, C. M. [Lawrence Livermore National Lab. (LLNL), Livermore, CA (United States); Fiuza, F. [Lawrence Livermore National Lab. (LLNL), Livermore, CA (United States); Ross, J. S. [Lawrence Livermore National Lab. (LLNL), Livermore, CA (United States); Zylstra, A. B. [MIT (Massachusetts Inst. of Technology), Cambridge, MA (United States); Drake, R. P. [Univ. of Michigan, Ann Arbor, MI (United States). Dept. of Atmospheric, Oceanic, and Space Sciences; Froula, D. H. [Univ. of Rochester, NY (United States). Physics Dept. and Lab. for Laser Energetics; Gregori, G. [Univ. of Oxford (United Kingdom). Dept. of Physics; Kugland, N. L. [Lam Research Corp., Fremont, CA (United States); Kuranz, C. C. [Univ. of Michigan, Ann Arbor, MI (United States). Dept. of Atmospheric, Oceanic, and Space Sciences; Levy, M. C. [Lawrence Livermore National Lab. (LLNL), Livermore, CA (United States); Li, C. K. [MIT (Massachusetts Inst. of Technology), Cambridge, MA (United States); Meinecke, J. [Univ. of Oxford (United Kingdom). Dept. of Physics; Morita, T. [Osaka Univ. (Japan). Inst. of Laser Engineering; Petrasso, R. [MIT (Massachusetts Inst. of Technology), Cambridge, MA (United States); Plechaty, C. [Lawrence Livermore National Lab. (LLNL), Livermore, CA (United States); Remington, B. A. [Lawrence Livermore National Lab. (LLNL), Livermore, CA (United States); Ryutov, D. D. [Lawrence Livermore National Lab. (LLNL), Livermore, CA (United States); Sakawa, Y. [Osaka Univ. (Japan). Inst. of Laser Engineering; Spitkovsky, A. [Princeton Univ., NJ (United States). Dept. of Astrophysical Sciences; Takabe, H. [Osaka Univ. (Japan). Inst. of Laser Engineering; Park, H.-S. [Lawrence Livermore National Lab. (LLNL), Livermore, CA (United States)
2015-01-19
Collisionless shocks can be produced as a result of strong magnetic fields in a plasma flow, and therefore are common in many astrophysical systems. The Weibel instability is one candidate mechanism for the generation of su fficiently strong fields to create a collisionless shock. Despite their crucial role in astrophysical systems, observation of the magnetic fields produced by Weibel instabilities in experiments has been challenging. Using a proton probe to directly image electromagnetic fields, we present evidence of Weibel-generated magnetic fields that grow in opposing, initially unmagnetized plasma flows from laser-driven laboratory experiments. Three-dimensional particle-in-cell simulations reveal that the instability effi ciently extracts energy from the plasma flows, and that the self-generated magnetic energy reaches a few percent of the total energy in the system. Furthermore, this result demonstrates an experimental platform suitable for the investigation of a wide range of astrophysical phenomena, including collisionless shock formation in supernova remnants, large-scale magnetic field amplification, and the radiation signature from gamma-ray bursts.
Mind the gap: a flow instability controlled by particle-surface distance
Driscoll, Michelle; Delmotte, Blaise; Youssef, Mena; Sacanna, Stefano; Donev, Aleksandar; Chaikin, Paul
2016-11-01
Does a rotating particle always spin in place? Not if that particle is near a surface: rolling leads to translational motion, as well as very strong flows around the particle, even quite far away. These large advective flows strongly couple the motion of neighboring particles, giving rise to strong collective effects in groups of rolling particles. Using a model experimental system, weakly magnetic colloids driven by a rotating magnetic field, we observe that driving a compact group of microrollers leads to a new kind of flow instability. First, an initially uniformly-distributed strip of particles evolves into a shock structure, and then it becomes unstable, emitting fingers with a well-defined wavelength. Using 3D large-scale simulations in tandem with our experiments, we find that the instability wavelength is controlled not by the driving torque or the fluid viscosity, but a geometric parameter: the microroller's distance above the container floor. Furthermore, we find that the instability dynamics can be reproduced using only one ingredient: hydrodynamic interactions near a no-slip boundary.
Xin, Bo; Sun, Dakun; Jing, Xiaodong; Sun, Xiaofeng
2016-07-01
Lined ducts are extensively applied to suppress noise emission from aero-engines and other turbomachines. The complex noise/flow interaction in a lined duct possibly leads to acoustic instability in certain conditions. To investigate the instability, the full linearized Navier-Stokes equations with eddy viscosity considered are solved in frequency domain using a Galerkin finite element method to compute the sound transmission in shear flow in the lined duct as well as the flow perturbation over the impedance wall. A good agreement between the numerical predictions and the published experimental results is obtained for the sound transmission, showing that a transmission peak occurs around the resonant frequency of the acoustic liner in the presence of shear flow. The eddy viscosity is an important influential factor that plays the roles of both providing destabilizing and making coupling between the acoustic and flow motions over the acoustic liner. Moreover, it is shown from the numerical investigation that the occurrence of the sound amplification and the magnitude of transmission coefficient are closely related to the realistic velocity profile, and we find it essential that the actual variation of the velocity profile in the axial direction over the liner surface be included in the computation. The simulation results of the periodic flow patterns possess the proper features of the convective instability over the liner, as observed in Marx et al.'s experiment. A quantitative comparison between numerical and experimental results of amplitude and phase of the instability is performed. The corresponding eigenvalues achieve great agreement.
Instability analysis on drainage flow over a complex terrain
Yi, C.
2007-12-01
The turbulent transport processes that occur within canopies are extremely complex and have not been accurately represented in past models, especially for ecosystems with hilly or mountainous terrain. The stability analysis on the terrain-induced canopy flows is the key to understanding the introduction of pollutants into the atmosphere and the transfer of water from soil and vegetation to the atmosphere. We applied the Computational Fluid Dynamics (CFD) approach to forest environments to simulate airflows within and above canopy. The results of the CFD experiments show three different dynamic regimes of topographic drainage flow that were simulated under different thermal-dynamic conditions: (1) Cold inflow induces drainage flow in the lower part of canopy and strong stratification of airflows within entire canopy; additionally, the model predicts that there is a super stable layer around the maximum LAD level, which is consistent with our canopy flow theory. This super stable layer minimizes vertical land-atmosphere exchange around the middle level of canopy. (2) Warm inflow causes the rapid flushing of land-atmosphere exchange at the location where two opposite air motions meet, this is called the 'chimney phenomenon'. (3) The oscillation of canopy flow occurs as the inflow temperature is close to the environmental temperature. These CFD simulations are based on fully derived thermal and fluid dynamic equations. In order to clearly understand the physical mechanisms for the transfer between the different dynamic regimes, I utilized the nonlinear dynamics approach to derive the analytical instability conditions of terrain-induced flows from the simplified thermal-hydro-mechanical equations. The analytical derivations are tested against the CFD simulations. These analytical conditions provide a better understanding of transport problem in ecosystem- atmosphere exchanges of water, carbon dioxide, and energy over complex terrain.
Buoyancy Effects on Flow Structure and Instability of Low-Density Gas Jets
Pasumarthi, Kasyap Sriramachandra
2004-01-01
A low-density gas jet injected into a high-density ambient gas is known to exhibit self-excited global oscillations accompanied by large vortical structures interacting with the flow field. The primary objective of the proposed research is to study buoyancy effects on the origin and nature of the flow instability and structure in the near-field of low-density gas jets. Quantitative rainbow schlieren deflectometry, Computational fluid dynamics (CFD) and Linear stability analysis were the techniques employed to scale the buoyancy effects. The formation and evolution of vortices and scalar structure of the flow field are investigated in buoyant helium jets discharged from a vertical tube into quiescent air. Oscillations at identical frequency were observed throughout the flow field. The evolving flow structure is described by helium mole percentage contours during an oscillation cycle. Instantaneous, mean, and RMS concentration profiles are presented to describe interactions of the vortex with the jet flow. Oscillations in a narrow wake region near the jet exit are shown to spread through the jet core near the downstream location of the vortex formation. The effects of jet Richardson number on characteristics of vortex and flow field are investigated and discussed. The laminar, axisymmetric, unsteady jet flow of helium injected into air was simulated using CFD. Global oscillations were observed in the flow field. The computed oscillation frequency agreed qualitatively with the experimentally measured frequency. Contours of helium concentration, vorticity and velocity provided information about the evolution and propagation of vortices in the oscillating flow field. Buoyancy effects on the instability mode were evaluated by rainbow schlieren flow visualization and concentration measurements in the near-field of self-excited helium jets undergoing gravitational change in the microgravity environment of 2.2s drop tower at NASA John H. Glenn Research Center. The jet
STANDING SHOCK INSTABILITY IN ADVECTION-DOMINATED ACCRETION FLOWS
Energy Technology Data Exchange (ETDEWEB)
Le, Truong [Department of Physics, Astronomy and Geology, Berry College, Mount Berry, GA 30149 (United States); Wood, Kent S.; Wolff, Michael T. [High Energy Space Environment Branch, Space Science Division, Naval Research Laboratory, Washington, DC 20375 (United States); Becker, Peter A. [Department of Physics and Astronomy, George Mason University, Fairfax, VA 22030 (United States); Putney, Joy, E-mail: tle@berry.edu [Department of Physics and Engineering, Washington and Lee University, Lexington, VA 24450 (United States)
2016-03-10
Depending on the values of the energy and angular momentum per unit mass in the gas supplied at large radii, inviscid advection-dominated accretion flows can display velocity profiles with either preshock deceleration or preshock acceleration. Nakayama has shown that these two types of flow configurations are expected to have different stability properties. By employing the Chevalier and Imamura linearization method and the Nakayama instability boundary conditions, we discover that there are regions of parameter space where disks/shocks with outflows can be stable or unstable. In regions of instability, we find that preshock deceleration is always unstable to the zeroth mode with zero frequency of oscillation, but is always stable to the fundamental mode and overtones. Furthermore, we also find that preshock acceleration is always unstable to the zeroth mode and that the fundamental mode and overtones become increasingly less stable as the shock location moves away from the horizon when the disk half-height expands above ∼12 gravitational radii at the shock radius. In regions of stability, we demonstrate the zeroth mode to be stable for the velocity profiles that exhibit preshock acceleration and deceleration. Moreover, for models that are linearly unstable, our model suggests the possible existence of quasi-periodic oscillations (QPOs) with ratios 2:3 and 3:5. These ratios are believed to occur in stellar and supermassive black hole candidates, for example, in GRS 1915+105 and Sgr A*, respectively. We expect that similar QPO ratios also exist in regions of stable shocks.
Goldstein, M. E.
1984-01-01
Attention is given to the sound produced by artificially excited, spatially growing instability waves on subsonic shear layers. Real flows that always diverge in the downstream direction allow sound to be produced by the interaction of the instability waves with the resulting streamwise variations of the flow. The upstream influence, or feedback, can interact with the splitter plate lip to produce a downstream-propagating instability wave that may under certain conditions be the same instability wave that originally generated the upstream influence. The present treatment is restricted to very low Mach number flows, so that compressibility effects can only become important over large distances.
Pinning effects on flux flow instability in epitaxial Nb thin films
Dobrovolskiy, Oleksandr V.; Shklovskij, Valerij A.; Hanefeld, Marc; Zörb, Markus; Köhs, Lukas; Huth, Michael
2017-08-01
The flux flow properties of epitaxial niobium films with different pinning strengths are investigated by dc electrical resistance measurements and mapped to results derived within the framework of a theoretical model. The cases of weak random pinning in as-grown films, strong random pinning in Ga ion-irradiated films, and strong periodic pinning induced by a nanogroove array milled by a focused ion beam are investigated. The generic feature of the current-voltage curves of the films consists of instability jumps to the normal state at some instability current density j * as the vortex lattice reaches its critical velocity v *. While {v}* (B) monotonically decreases for as-grown films, the irradiated films exhibit a non-monotonic dependence {v}* (B) attaining a maximum in the low-field range. In the case of nanopatterned films, this broad maximum is accompanied by a much sharper maximum in both {v}* (B) and {j}* (B), which we attribute to the commensurability effect when the spacing between the vortex rows coincides with the location of the grooves. We argue that the observed behavior of {v}* (B) can be explained by the pinning effect on the vortex flow instability and support our claims by fitting the experimental data to theoretical expressions derived within a model accounting for the field dependence of the depinning current density.
Experimental study on modulational instability and evolution of crescent waves
Directory of Open Access Journals (Sweden)
Ya-long ZHOU
2012-12-01
Full Text Available A series of experiments on the instability of steep water wave trains in water with finite water depths and infinite water depths in a wide wave basin were performed. It was found that under the coupled development of modulational instability and class-II instability, the initial two-dimensional steep wave trains evolved into three-dimensional crescent waves, followed by the occurrence of disordered water surfaces, and that the wave energy transferred to sidebands in the amplitude spectrum of the water surface elevation. The results also show that water depth has a significant effect on the growth of modulational instability and the evolution of crescent waves. The larger the water depth, the more quickly the modulational instability suppresses class-II instability.
Experimental investigation of transitional flow in a toroidal pipe
Kühnen, J; Hof, B; Kuhlmann, H
2015-01-01
The flow instability and further transition to turbulence in a toroidal pipe (torus) with curvature (tube-to-coiling diameter) 0.049 is investigated experimentally. The flow inside the toroidal pipe is driven by a steel sphere fitted to the inner pipe diameter. The sphere is moved with constant azimuthal velocity from outside the torus by a moving magnet. The experiment is designed to investigate curved pipe flow by optical measurement techniques. Using stereoscopic particle image velocimetry, laser Doppler velocimetry and pressure drop measurements, the flow is measured for Reynolds numbers ranging from 1000 to 15000. Time- and space-resolved velocity fields are obtained and analysed. The steady axisymmetric basic flow is strongly influenced by centrifugal effects. On an increase of the Reynolds number we find a sequence of bifurcations. For Re=4075 a supercritical bifurcation to an oscillatory flow is found in which waves travel in the streamwise direction with a phase velocity slightly faster than the mean...
Nonmodal Growth Of Kelvin-Helmholtz Instability In Compressible Flows
Karimi, Mona; Girimaji, Sharath
2016-11-01
Kelvin-helmholtz instability (khi) is central to the vertical mixing in shear flows and is known to be suppressed in compressible flows. To understand the inhibition of mixing under the influence of compressibility, we analyze the linear growth of khi in the short-time limit using initial value analysis. The evolution of perturbations is studied from a nonmodal standpoint. As the underlying suppression mechanism can be understood by considering primarily linear physics, the effect of compressibility on khi is scrutinized by linear analysis. Then its inferences are verified against direct numerical simulations. It has been demonstrated that compressibility forces the dominance of dilatational, rather than shear, dynamics at the interface of two fluids of different velocities. Within the dilatiatonal interface layer, pressure waves cause the velocity perturbation to become oscillatory [karimi and girimaji, 2016]. Thereupon, the focus is to examine the effect of the initial perturbation wavenumber on the formation of this layer and eventually the degree of khi suppression in compressible flows. We demonstrate that the degree of suppression decreases with the increase the wavenumbers of the initial perturbation of dilatational, rather than shear, dynamics at the interface of two fluids of different velocities. Within the dilatiatonal interface layer, pressure waves cause the velocity perturbation to become oscillatory [karimi and girimaji, 2016]. Thereupon, the focus is to examine the effect of the initial perturbation wavenumber on the formation of this layer and eventually the degree of khi suppression in compressible flows. We demonstrate that the degree of suppression decreases with the increase the wavenumbers of the initial perturbation.
Magnetic flux concentration and zonal flows in magnetorotational instability turbulence
Energy Technology Data Exchange (ETDEWEB)
Bai, Xue-Ning [Institute for Theory and Computation, Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, MS-51, Cambridge, MA 02138 (United States); Stone, James M., E-mail: xbai@cfa.harvard.edu [Department of Astrophysical Sciences, Peyton Hall, Princeton University, Princeton, NJ 08544 (United States)
2014-11-20
Accretion disks are likely threaded by external vertical magnetic flux, which enhances the level of turbulence via the magnetorotational instability (MRI). Using shearing-box simulations, we find that such external magnetic flux also strongly enhances the amplitude of banded radial density variations known as zonal flows. Moreover, we report that vertical magnetic flux is strongly concentrated toward low-density regions of the zonal flow. Mean vertical magnetic field can be more than doubled in low-density regions, and reduced to nearly zero in high-density regions in some cases. In ideal MHD, the scale on which magnetic flux concentrates can reach a few disk scale heights. In the non-ideal MHD regime with strong ambipolar diffusion, magnetic flux is concentrated into thin axisymmetric shells at some enhanced level, whose size is typically less than half a scale height. We show that magnetic flux concentration is closely related to the fact that the turbulent diffusivity of the MRI turbulence is anisotropic. In addition to a conventional Ohmic-like turbulent resistivity, we find that there is a correlation between the vertical velocity and horizontal magnetic field fluctuations that produces a mean electric field that acts to anti-diffuse the vertical magnetic flux. The anisotropic turbulent diffusivity has analogies to the Hall effect, and may have important implications for magnetic flux transport in accretion disks. The physical origin of magnetic flux concentration may be related to the development of channel flows followed by magnetic reconnection, which acts to decrease the mass-to-flux ratio in localized regions. The association of enhanced zonal flows with magnetic flux concentration may lead to global pressure bumps in protoplanetary disks that helps trap dust particles and facilitates planet formation.
The instability of counter-propagating kernel gravity waves in a constant shear flow
Umurhan, O M; Harnik, N; Lott, F
2007-01-01
The mechanism describing the recently developed notion of kernel gravity waves (KGWs) is reviewed and such structures are employed to interpret the unstable dynamics of an example stratified plane parallel shear flow. This flow has constant vertical shear, is infinite in the vertical extent, and characterized by two density jumps of equal magnitude each decreasing successively with height, in which the jumps are located symmetrically away from the midplane of the system. We find that for a suitably defined bulk-Richardson number there exists a band of horizontal wavenumbers which exhibits normal-mode instability. The instability mechanism closely parallels the mechanism responsible for the instability seen in the problem of counter-propagating Rossby waves. In this problem the instability arises out of the interaction of counter-propagating gravity waves. We argue that the instability meets the Hayashi-Young criterion for wave instability. We also argue that the instability is the simplest one that can arise ...
Strong electron-scale instability in relativistic shear flows
Alves, Eduardo Paulo; Grismayer, Thomas; Fonseca, Ricardo; Silva, Luis
2013-10-01
Collisionless shear-driven plasma instabilities have recently been shown to be capable of generating strong and large-scale magnetic fields and may therefore play an important role in relativistic astrophysical outflows. We present a new collisionless shear-driven plasma instability, which operates in the plane transverse to the Kelvin Helmholtz instability (KHI). We develop the linear stability analysis of electromagnetic modes in the transverse plane and find that the growth rate of this instability is greater than the competing KHI in relativistic shears. The analytical results are confirmed with 2D particle-in-cell (PIC) simulations. Simulations also reveal the nonlinear evolution of the instability which leads to the development of mushroom-like electron-density structures, similar to the Rayleigh Taylor instability. Finally, the interplay between the competing instabilities is investigated in 3D PIC simulations.
Ferguson, Kevin; Sewell, Everest; Krivets, Vitaliy; Greenough, Jeffrey; Jacobs, Jeffrey
2016-11-01
Initial conditions for the Richtmyer-Meshkov instability (RMI) are measured in three dimensions in the University of Arizona Vertical Shock Tube using a moving magnet galvanometer system. The resulting volumetric data is used as initial conditions for the simulation of the RMI using ARES at Lawrence-Livermore National Laboratory (LLNL). The heavy gas is sulfur hexafluoride (SF6), and the light gas is air. The perturbations are generated by harmonically oscillating the gasses vertically using two loudspeakers mounted to the shock tube which cause Faraday resonance, producing a random short wavelength perturbation on the interface. Planar Mie scattering is used to illuminate the flow field through the addition of propylene glycol particles seeded in the heavy gas. An M=1.2 shock impulsively accelerates the interface, initiating instability growth. Images of the initial condition and instability growth are captured at a rate of 6 kHz using high speed cameras. Comparisons between experimental and simulation results, mixing diagnostics, and mixing zone growth are presented.
Experimental investigation of the Richtmyer-Meshkov instability.
Energy Technology Data Exchange (ETDEWEB)
Weber, Christopher R. (University of Wisconsin-Madison, Madison, WI)
2011-09-01
The Richtmyer-Meshkov instability (RMI) is experimentally investigated using several different initial conditions and with a range of diagnostics. First, a broadband initial condition is created using a shear layer between helium+acetone and argon. The post-shocked turbulent mixing is investigated using planar laser induced fluorescence (PLIF). The signature of turbulent mixing is present in the appearance of an inertial range in the mole fraction energy spectrum and the isotropy of the late-time dissipation structures. The distribution of the mole fraction values does not appear to transition to a homogeneous mixture, and it is possible that this effect may be slow to develop for the RMI. Second, the influence of the RMI on the kinetic energy spectrum is investigated using particle image velocimetry (PIV). The influence of the perturbation is visible relatively far from the interface when compared to the energy spectrum of an initially flat interface. Closer to the perturbation, an increase in the energy spectrum with time is observed and is possibly due to a cascade of energy from the large length scales of the perturbation. Finally, the single mode perturbation growth rate is measured after reshock using a new high speed imaging technique. This technique produced highly time-resolved interface position measurements. Simultaneous measurements at the spike and bubble location are used to compute a perturbation growth rate history. The growth rates from several experiments are compared to a new reshock growth rate model.
Huntington, Channing; Fiuza, Frederico; Ross, James Steven; Zylstra, Alex; Pollock, Brad; Drake, R. Paul; Froula, Dustin; Gregori, Gianluca; Kugland, Nathan; Kuranz, Carolyn; Levy, Matthew; Li, Chikang; Meinecke, Jena; Petrasso, Richard; Remington, Bruce; Ryutov, Dmitri; Sakawa, Youichi; Spitkovsky, Anatoly; Takabe, Hideke; Turnbull, David; Park, Hye-Sook
2015-08-01
Astrophysical collisionless shocks are often associated with the presence of strong magnetic fields in a plasma flow. The magnetic fields required for shock formation may either be initially present, for example in supernova remnants or young galaxies, or they may be self-generated in systems such as gamma-ray bursts (GRBs). In the case of GRB outflows, the intense magnetic fields are greater than those seeded by the GRB progenitor or produced by misaligned density and temperature gradients in the plasma flow (the Biermann-battery effect). The Weibel instability is one candidate mechanism for the generation of sufficiently strong fields to create a collisionless shock. Despite their crucial role in astrophysical systems, observation of the magnetic fields produced by Weibel instabilities in experiments has been challenging. Using a proton probe to directly image electromagnetic fields, we present evidence of Weibel-generated magnetic fields that grow in opposing, initially unmagnetized plasma flows from laser-driven laboratory experiments. Three-dimensional particle-in-cell simulations reveal that the instability efficiently extracts energy from the plasma flows, and that the self-generated magnetic energy reaches a few percent of the total energy in the system. This result demonstrates an experimental platform suitable for the investigation of a wide range of astrophysical phenomena, including collisionless shock formation in supernova remnants, large-scale magnetic field amplification, and the radiation signature from gamma-ray bursts.This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344.
Tam, C. K. W.; Burton, D. E.
1984-01-01
An investigation is conducted of the phenomenon of sound generation by spatially growing instability waves in high-speed flows. It is pointed out that this process of noise generation is most effective when the flow is supersonic relative to the ambient speed of sound. The inner and outer asymptotic expansions corresponding to an excited instability wave in a two-dimensional mixing layer and its associated acoustic fields are constructed in terms of the inner and outer spatial variables. In matching the solutions, the intermediate matching principle of Van Dyke and Cole is followed. The validity of the theory is tested by applying it to an axisymmetric supersonic jet and comparing the calculated results with experimental measurements. Very favorable agreements are found both in the calculated instability-wave amplitude distribution (the inner solution) and the near pressure field level contours (the outer solution) in each case.
Olivine Instability: An Experimental View of Mechanism of Deep Earthquakes
Long, H.; Weidner, D.; Li, L.; Chen, J.; Wang, L.
2007-12-01
Olivine (¦Á-(Mg,Fe)2SiO4) is the major constituent of the upper mantle and the ocean lithosphere. In subduction zone, where the earthquakes happen, the rheology of slab is mainly controlled by that of olivine. Several different mechanisms for deep focus earthquakes have been suggested, which include olivine instability (Bridgman, 1936; Orowan, 1960; Post, 1977; Ogawa, 1987; Hobbs and Ord, 1988; Kao and Chen, 1995), shear-induced melting (Griggs, 1954, 1972; Griggs and Handin, 1960; Griggs and Baker, 1969), phase transformation (Bridgman, 1945; Benioff, 1963; Meade and Jeanloz, 1989), dehydration of hydrous specimens (Meade and Jeanloz, 1991), and olivine metastability-induced anticrack (Green and Houston, 1995). Since the low temperature of the ¡°cold¡± slab, which can be as low as 600¡ãC in transition zone, olivine may still exist there and thus its shear instability may still be the possible mechanism for the deep-focus earthquakes. In our experimental study on deformation of San Carlos olivine at subduction zone conditions carried out on a D-DIA apparatus, Sam85 at X17B2, NSLS, we observed that the transitional temperature between regimes of insensitive to temperature and sensitive to temperature can be as high as 900¡ãC or even higher for the annealed polycrystal olivine sample, while that for unannealed sample can be as low as 450¡ãC. Our results for the unannealed sample are consistent to the result of Raterron et al (2004), which is concluded from the relaxation experiments. The annealed and unannealed olivine can be present the natural olivine in non-fault systems and that in pre-existing fault systems in subduction zone, respectively. We therefore propose a new olivine instability model with a ¡°sandwich¡± formation for the deep focus earthquakes: In this model the pre-existing fault system is surrounded by the no-fault systems. When the slab dives down, the olivine in both systems undergoes a stress- build-up process and can hold very high
Coronary Artery Stenosis Flow: Experimental and Computational Investigation
Egelhoff, Carla; Budwig, Ralph; Hansen, Byron; Foster, Jonathan
2000-11-01
The effects of symmetry, flowrate, wall roughness and size are investigated using realistic pulsatile waveforms for flow viz and LDV experimental models as well as CFD models using original code. Distal to the stenosis flow is characterized by a high speed jet which is central for symmetric models and attached to the wall for eccentric models. The jet is accompanied by a low speed recirculation zone which persists while lengthening and shortening during most of the cardiac cycle. Of particular note is the downstream onset of flow instability and turbulence for high flow rate conditions in symmetric and eccentric severely occluded stenoses. The location and extent of the unstable flow region continually changes throughout the cycle, which may be a factor contributing to the thrombogenesis which coronary arteries experience.
Institute of Scientific and Technical Information of China (English)
HUANG Lin; JIAN Guang-de; QIU Xiao-ming
2007-01-01
The synergistic stabilizing effect of gyroviscosity and sheared axial flow on the Rayleigh-Taylor instability in Z-pinch implosions is studied by means of the incompressible viscid magneto-hydrodynamic equations. The gyroviscosity (or finite Larmor radius) effects are introduced in the momentum equation through an anisotropic ion stress tensor. Dispersion relation with the effect of a density discontinuity is derived. The results indicate that the short-wavelength modes of the Rayleigh-Taylor instability are easily stabilized by the gyroviscosity effects. The long wavelength modes are stabilized by the sufficient sheared axial flow. However, the synergistic effects of the finite Larmor radius and sheared axial flow can heavily mitigate the Rayleigh-Taylor instability. This synergistic effect can compress the Rayleigh-Taylor instability to a narrow wave number region. Even with a sufficient gyroviscosity and large enough flow velocity, the synergistic effect can completely suppressed the Rayleigh-Taylor instability in whole wave number region.
Instabilities of Shercliff and Stewartson layers in spherical Couette flow
Wei, Xing
2010-01-01
We explore numerically the flow induced in a spherical shell by differentially rotating the inner and outer spheres. The fluid is also taken to be electrically conducting (in the low magnetic Reynolds number limit), and a magnetic field is imposed parallel to the axis of rotation. If the outer sphere is stationary, the magnetic field induces a Shercliffe layer on the tangent cylinder, the cylinder just touching the inner sphere and parallel to the field. If the magnetic field is absent, but a strong overall rotation is present, Coriolis effects induce a Stewartson layer on the tangent cylinder. The non-axisymmetric instabilities of both types of layer separately have been studied before; here we consider the two cases side by side, as well as the mixed case, and investigate how magnetic and rotational effects interact. We find that if the differential rotation and the overall rotation are in the same direction, the overall rotation may have a destabilizing influence, whereas if the differential rotation and t...
Experimental studies on the photoelectron instability in the Beijing Electron Positron Collider
Directory of Open Access Journals (Sweden)
Z. Y. Guo
2002-12-01
Full Text Available A vertical coupled-bunch instability was observed for a positron beam at the Beijing Electron Positron Collider (BEPC. The experimental results show that the instability has similar characteristics as that observed in the Photon Factory of KEK several years ago. The instability at BEPC can be explained by the effect of an electron cloud which is produced in the beam chamber by synchrotron light hitting the wall.
Electrostatic plasma instabilities driven by neutral gas flows in the solar chromosphere
Gogoberidze, G; Poedts, S; De Keyser, J
2013-01-01
We investigate electrostatic plasma instabilities of Farley-Buneman (FB) type driven by quasi-stationary neutral gas flows in the solar chromosphere. The role of these instabilities in the chromosphere is clarified. We find that the destabilizing ion thermal effect is highly reduced by the Coulomb collisions and can be ignored for the chromospheric FB-type instabilities. On the contrary, the destabilizing electron thermal effect is important and causes a significant reduction of the neutral drag velocity triggering the instability. The resulting threshold velocity is found as function of chromospheric height. Our results indicate that the FB type instabilities are still less efficient in the global chromospheric heating than the Joule dissipation of the currents driving these instabilities. This conclusion does not exclude the possibility that the FB type instabilities develop in the places where the cross-field currents overcome the threshold value and contribute to the heating locally. Typical length-scales...
Hydrodynamic instabilities in the developing region of an axially rotating pipe flow
Energy Technology Data Exchange (ETDEWEB)
Miranda-Barea, A; Fabrellas-García, C; Parras, L; Pino, C del, E-mail: cpino@uma.es [Universidad de Málaga, Escuela Técnica Superior de Ingeniería Industrial, Ampliación Campus de Teatinos, 29071, Málaga, España (Spain)
2015-06-15
We conduct experiments in a rotating Hagen–Poiseuille flow (RHPF) through flow visualizations when the flow becomes convectively and absolutely unstable at low-to-moderate Reynolds numbers, Re. We characterize periodic patterns at a very high swirl parameter, L, when the flow overcomes the absolutely unstable region. These non-steady helical filaments wrapped around the axis appear in the developing region of the pipe. Experimentally, we compute the onset of these oscillations in the (L, Re)-plane finding that the rotation rate decreases as the Reynolds number increases in the process of achieving the time-dependent state. Additionally, we report information regarding frequencies and wavelengths that appear downstream of the rotating pipe for convectively and absolutely unstable flows, even for very high swirl parameters at which the flow becomes time-dependent in the developing region. We do not observe variations in the trends of these parameters, so these hydrodynamic instabilities in the developing region do not affect the unstable travelling waves downstream of the pipe. (paper)
Thermal-hydraulic instabilities in natural circulation flow loops under supercritical conditions
Jain, Rachna
In recent years, a growing interest has been generated in investigating the thermal hydraulics and flow stability phenomenon in supercritical natural circulation loops. These flow conditions are relevant to some of the innovative passive safety designs proposed for the Gen-IV Supercritical Water Reactor (SCWR) concepts. A computational model has been developed at UW Madison which provides a good basic simulation tool for the steady state and transient analysis of one dimensional natural circulation flow, and can be applied to conduct stability analysis. Several modifications and improvements were incorporated in an earlier numerical scheme before applying it to investigate the transient behavior of two experimental loops, namely, the supercritical water loop at UW-Madison and the supercritical carbon-dioxide (SCCO2) loop at Argonne National Laboratories. Although the model predicted development of instabilities for both SCW and SCCO2 loop which agrees with some previous work, the experiments conducted at SCCO2 loop exhibited stable behavior under similar conditions. To distinguish between numerical effects and physical processes, a linear stability approach has also been developed to investigate the stability characteristics associated with the natural circulation loop systems for various inlet conditions, input powers and geometries. The linear stability results for the SCW and SCCO2 loops exhibited differences with the corresponding transient simulations. This linear model also predicted the presence of instability in the SCCO 2 loop for certain high input powers contradictory to the experimental findings. Dimensionless parameters were proposed which would generalize the stability characteristics of the natural circulation flow loops under supercritical conditions.
Effects of two-phase pressure drop on the self-sustained oscillatory instability in condensing flows
Energy Technology Data Exchange (ETDEWEB)
Bhatt, B.L.; Wedekind, G.L.; Jung, K. (Software Support Corp., Birmingham, MI (United States))
1989-05-01
This paper presents the results of an extension of an experimental and theoretical investigation of an unstable flow phenomenon that leads to self-sustained limit-cycle-type oscillations of large amplitude, and which, under certain conditions, can involve flow reversals. The influence of two-phase pressure drop is examined and shown to have a stabilizing effect on the instability. Inclusion of the two-phase pressure drop as part of the downstream throttling allows the utilization of a previously developed linearized analysis, based on the system mean void fraction model, to predict successfully the experimentally observed stability boundary.
Drift-Alfven instabilities of a finite beta plasma shear flow along a magnetic field
Mikhailenko, V. V.; Mikhailenko, V. S.; Lee, Hae June
2016-02-01
It was derived that the drift-Alfven instabilities with the shear flow parallel to the magnetic field have significant difference from the drift-Alfven instabilities of a shearless plasma when the ion temperature is comparable with electron temperature for a finite plasma beta. The velocity shear not only modifies the frequency and the growth rate of the known drift-Alfven instability, which develops due to the inverse electron Landau damping, but also triggers a combined effect of the velocity shear and the inverse ion Landau damping, which manifests the development of the ion kinetic shear-flow-driven drift-Alfven instability. The excited unstable waves have the phase velocities along the magnetic field comparable with the ion thermal velocity, and the growth rate is comparable with the frequency. The development of this instability may be the efficient mechanism of the ion energization in shear flows.
Short-wave vortex instability in stratified flow
Bovard, Luke
2014-01-01
In this paper we investigate a new instability of the Lamb-Chaplygin dipole in a stratified fluid. Through numerical linear stability analysis, a secondary peak in the growth rate emerges at vertical scales about an order of magnitude smaller than the buoyancy scale $L_{b}=U/N$ where $U$ is the characteristic velocity and $N$ is the Brunt-V\\"{a}is\\"{a}l\\"{a} frequency. This new instability exhibits a growth rate that is similar to, and even exceeds, that of the zigzag instability, which has the characteristic length of the buoyancy scale. This instability is investigated for a wide range of Reynolds $Re=2000-20000$ and horizontal Froude numbers $F_{h}=0.05-0.2$, where $F_{h}=U/NR$, $Re=UR/\
Experimental investigation of drift instabilities in E×B discharges
Gascon, Nicolas; Young, Chris V.; Lucca Fabris, Andrea; Ito, Tsuyohito; Cappelli, Mark A.
2014-10-01
Drift plasma instabilities are characterized in three E×B discharges operating on noble gases: two Hall-type plasma thrusters with insulating channel walls (70 mm outer diameter, 20 mm long, and 90 mm outer diameter, 80 mm long), and a small magnetron discharge (5 mm diameter). Plasma instabilities in the E×B discharges are investigated using arrays of electrostatic probes. The signals from the probes arrays are processed with wavelet filtering, and frequency-wavelength dispersion analysis tools. Results are compared to hybrid PIC-fluid axial azimuthal simulations and analyzed in light of recent theories of gradient-driven drift instabilities, in an effort to better understand the relation between drift instabilities and anomalous electron transport in these discharges. This work is sponsored by the U.S. Air Force Office of Scientific Research with Dr. Mitat Birkan as program manager. CVY acknowledges support from the DOE NNSA Stewardship Science Graduate Fellowship under Contract DE-FC52-08NA28752.
Suppression of Instability in Strongly Coupled Dusty Plasmas with Ion Flow
Institute of Scientific and Technical Information of China (English)
贺凯芬; 谢柏松; 刘克富
2001-01-01
The instability of low-frequency longitudinal modes in strongly coupled dusty plasmas with an ion flow is investigated. The dust charging relaxation is taken into account. It is found that when the ion flow is strong enough,the suppression, even disappearance. of instability can occur. Similar to that of the real frequency of waves, the imaginary part of waves also exhibits a transition, which arises from the sensitive dependences on the system parameters and their competition.
Dynamic instability of shallow shells in three-dimensional incompressible inviscid potential flow
Avramov, K. V.; Papazov, S. V.; Breslavsky, I. D.
2017-04-01
The system of the hypersingular integral equations with respect to the aerodynamic derivatives of the shell pressure drop is obtained to analyze the interaction of the shallow shell with three-dimensional incompressible potential air flow. This system of the integral equations is very applicable to analyze aeroelastic vibrations of thin-walled structures. The numerical approach based on the discrete vortices method is suggested to solve the system of the hypersingular integral equations. Using the assumed-mode method, the finite degrees of freedom dynamical system is derived to analyze the shallow shell dynamic instability. The dynamic instability of the shallow shell equilibrium in the subsonic air flow is analyzed numerically. This type of instability results in flutter. The influence of the structure parameters on the dynamic instability is analyzed. The parameters of the dynamic instability are compared with the data, which are calculated by the software ANSYS.
Experimental investigation of stator flow in diagonal flow fan
Wang, Jie; Kinoue, Yoichi; Shiomi, Norimasa; Setoguchi, Toshiaki; Kaneko, Kenji; Jin, Yingzi
2008-12-01
Experimental investigations were conducted for the internal flow of the stator of the diagonal flow fan. Corner separation near the hub surface and the suction surface of the stator blade are focused on. At the design flow rate, the values of the axial velocity and the total pressure at stator outlet decrease near the suction surface at around the hub surface by the influence of the corner wall. At low flow rate of 80-90 % of the design flow rate, the corner separation between the suction surface and the hub surface can be found, which become widely spread at 80 % of the design flow rate.
Moortgat, J.; Amooie, M. A.; Soltanian, M. R.
2016-12-01
Problems in hydrogeology and hydrocarbon reservoirs generally involve the transport of solutes in a single solvent phase (e.g., contaminants or dissolved injection gas), or the flow of multiple phases that may or may not exchange mass (e.g., brine, NAPL, oil, gas). Often, flow is viscously and gravitationally unstable due to mobility and density contrasts within a phase or between phases. Such instabilities have been studied in detail for single-phase incompressible fluids and for two-phase immiscible flow, but to a lesser extent for multiphase multicomponent compressible flow. The latter is the subject of this presentation. Robust phase stability analyses and phase split calculations, based on equations of state, determine the mass exchange between phases and the resulting phase behavior, i.e., phase densities, viscosities, and volumes. Higher-order finite element methods and fine grids are used to capture the small-scale onset of flow instabilities. A full matrix of composition dependent coefficients is considered for each Fickian diffusive phase flux. Formation heterogeneity can have a profound impact and is represented by realistic geostatistical models. Qualitatively, fingering in multiphase compositional flow is different from single-phase problems because 1) phase mobilities depend on rock wettability through relative permeabilities, and 2) the initial density and viscosity ratios between phases may change due to species transfer. To quantify mixing rates in different flow regimes and for varying degrees of miscibility and medium heterogeneities, we define the spatial variance, scalar dissipation rate, dilution index, skewness, and kurtosis of the molar density of introduced species. Molar densities, unlike compositions, include compressibility effects. The temporal evolution of these measures shows that, while transport at the small-scale (cm) is described by the classical advection-diffusion-dispersion relations, scaling at the macro-scale (> 10 m) shows
Energy Technology Data Exchange (ETDEWEB)
Richard W. Johnson; Hugh M. McIlroy
2010-08-01
The U. S. Department of Energy (DOE) is supporting the development of a next generation nuclear plant (NGNP), which will be based on a very high temperature reactor (VHTR) design. The VHTR is a single-phase helium-cooled reactor wherein the helium will be heated initially to 750 °C and later to temperatures approaching 1000 °C. The high temperatures are desired to increase reactor efficiency and to provide a heat source for the manufacture of hydrogen and other applications. While computational fluid dynamics (CFD) has not been used in the past to design or license nuclear reactors in the U. S., it is expected that CFD will be used in the design and safety analysis of forthcoming designs. This is partly because of the maturity of CFD and partly because detailed information is desired of the flow and heat transfer inside the reactor to avoid hot spots and other conditions that might compromise reactor safety. Numerical computations of turbulent flow should be validated against experimental data for flow conditions that contain some or all of the physics expected in the thermal fluid machinery of interest. To this end, a scaled model of a narrow slice of the lower plenum of the prismatic VHTR was constructed and installed in the Idaho National Laboratory’s (INL) matched index of refraction (MIR) test facility and data were taken. The data were then studied and compared to CFD calculations to help determine their suitability for validation data. One of the main findings was that the inlet data, which were measured and controlled by calibrated mass flow rotameters and were also measured using detailed stereo particle image velocimetry (PIV) showed considerable discrepancies in mass flow rate between the two methods. The other finding was that a randomly unstable recirculation zone occurs in the flow. This instability has a very significant effect on the flow field in the vicinity of the inlet jets. Because its time scale is long and because it is apparently a
Experimental Observations on a Low Strain Counter-Flow Diffusion Flame: Flow and Bouyancy Effects
Sutula, J. A.; Torero, J. L.; Ezekoye, O. A.
1999-01-01
Diffusion flames are of great interest in fire safety and many industrial processes. The counter-flow configuration provides a constant strain flow, and therefore is ideal to study the structure of diffusion flames. Most studies have concentrated on the high velocity, high strain limit, since buoyantly induced instabilities will disintegrate the planar flame as the velocity decreases. Only recently, experimental studies in microgravity conditions have begun to explore the low strain regimes. Numerical work has shown the coupling between gas phase reaction rates, soot reaction rates, and radiation. For these programs, size, geometry and experimental conditions have been chosen to keep the flame unaffected by the physical boundaries. When the physical boundaries can not be considered infinitely far from the reaction zone discrepancies arise. A computational study that includes boundary effects and accounts for the deviations occurring when the major potential flow assumptions are relaxed was presented by Borlik et al. This development properly incorporates all heat loss terms and shows the possibility of extinction in the low strain regime. A major constraint of studying the low strain regime is buoyancy. Buoyant instabilities have been shown to have a significant effect on the nature of reactants and heat transport, and can introduce instabilities on the flow that result in phenomena such as flickering or fingering. The counter-flow configuration has been shown to provide a flame with no symmetry disrupting instabilities for inlet velocities greater than 50 mm/s. As the velocity approaches this limit, the characteristic length of the experiment has to be reduced to a few millimetres so as to keep the Rayleigh number (Ra(sub L) = (Beta)(g(sub 0))(L(exp 3) del T)/(alpha(v))) below 2000. In this work, a rectangular counter-flow burner was used to study a two-dimensional counter-flow diffusion flame. Flow visualisation and Particle Image Velocimetry served to describe
Flow instability of buoyant-Marangoni convection in the LEC GaAs melt
Institute of Scientific and Technical Information of China (English)
2008-01-01
Flow transitions and instabilities have significant effects on the quality of the crystals. The flow and heat transfer in the LEC GaAs melt are numerically studied by a time-dependent and three-dimensional turbulent flow model. The effects of the change of the buoyancy and Marangoni force on the flow state are analyzed by changing the temperature difference between the crystal and the crucible walls. The results show that the flow will transform from axisymmetric steady flow to non-axisymmetric oscillatory flow when the temperature difference exceeds the critical value, and that the mechanism of the transition is attributed to the Marangoni instability. The critical temperature differences for the flow transitions corresponding to different melt depth H are numerically predicted. Several important characteristics of the non-axisymmetric buoyant-Marangoni convection are numerically observed and compared with that of the non-axisymmetric mixed convection coupled with crystal rotation.
Theoretical and Experimental Studies of Magneto-Rayleigh-Taylor Instabilities
Energy Technology Data Exchange (ETDEWEB)
Lau, Yue Ying [University of Michigan, Ann Arbor, MI (United States); Gilgenbach, Ronald [University of Michigan, Ann Arbor, MI (United States)
2013-07-07
Magneto-Rayleigh-Taylor instability (MRT) is important to magnetized target fusion, wire-array z-pinches, and equation-of-state studies using flyer plates or isentropic compression. It is also important to the study of the crab nebula. The investigators performed MRT experiments on thin foils, driven by the mega-ampere linear transformer driver (LTD) facility completed in their laboratory. This is the first 1-MA LTD in the USA. Initial experiments on the seeding of MRT were performed. Also completed was an analytic study of MRT for a finite plasma slab with arbitrary magnetic fields tangential to the interfaces. The effects of magnetic shear and feedthrough were analyzed.
A Unified View of Global Instability of Compressible Flow over Open Cavities
2006-03-28
predict instability in Hagen- Poiseuille (pipe) flow , aggravated by the erroneous predictions of BiGlobal theory of stability of pressure-gradient driven... flow in a square duct (Tatsumi and Yoshimura 1990) and wall-bounded Couette flow . It thus becomes of interest, before investing efforts in the... Couette flow at φ = π / 2 which is (erroneously predicted by BiGlobal analysis to be) stable. The introduction of a third velocity component by the lid
Experimental Study of Rayleigh-Taylor Instability Using Paramagnetic Fluids
Tsiklashvili, Vladimer; Likhachev, Oleg; Jacobs, Jeffry
2009-11-01
Experiments that take advantage of the properties of paramagnetic liquids are used to study Rayleigh-Taylor instability. A gravitationally unstable combination of a paramagnetic salt solution and a nonmagnetic solution is initially stabilized by a magnetic field gradient that is produced by the contoured pole-caps of a large electromagnet. Rayleigh-Taylor instability originates with the rapid removal of current from the electromagnet, which results in the heavy liquid falling into the light liquid due to gravity and, thus, mixing with it. The mixing zone is visualized by back-lit photography and is recorded with a digital video camera. For visualization purposes, a blue-green dye is added to the magnetic fluid. The mixing rate of the two liquids is determined from an averaged dye concentration across the mixing layer by means of the Beer-Lambert law. After removal of the suspending magnetic field, the initially flat interface between the two liquids develops a random surface pattern with the dominant length scale well approximated by the fastest growing wavelength in accordance with the viscous linear stability theory. Several combinations of paramagnetic and nonmagnetic solutions have been considered during the course of the research. A functional dependence of the mixing layer growth constant, α, on the properties of the liquids is a primary subject of the present study.
Noise-sustained convective instability in a magnetized Taylor-Couette flow
Energy Technology Data Exchange (ETDEWEB)
Liu, Wei [Los Alamos National Laboratory
2008-01-01
The helical magnetorotational instability of the magnetized Taylor-Couette flow is studied numerically in a finite cylinder. A distant upstream insulating boundary is shown to stabilize the convective instability entirely while reducing the growth rate of the absolute instability. The reduction is less severe with larger height. After modeling the boundary conditions properly, the wave patterns observed in the experiment turn out to be a noise-sustained convective instability. After the source of the noise resulted from unstable Ekman and Stewartson layers is switched off, a slowly-decaying inertial oscillation is observed in the simulation. We reach the conclusion that the experiments completed to date have not yet reached the regime of absolute instability.
Noise-Sustained Convective Instability in a Magnetized Taylor-Couette Flow
Liu, Wei
2008-01-01
The helical magnetorotational instability of the magnetized Taylor-Couette flow is studied numerically in a finite cylinder. A distant upstream insulating boundary is shown to stabilize the convective instability entirely while reducing the growth rate of the absolute instability. The reduction is less severe with larger height. After modeling the boundary conditions properly, the wave patterns observed in the experiment turn out to be a noise-sustained convective instability. After the source of the noise resulted from unstable Ekman and Stewartson layers is switched off, a slowly-decaying inertial oscillation is observed in the simulation. We reach the conclusion that the experiments completed to date have not yet reached the regime of absolute instability.
Noise-Sustained Convective Instability in a Magnetized Taylor-Couette Flow
Energy Technology Data Exchange (ETDEWEB)
W. Liu
2009-02-20
The helical magnetorotational instability of the magnetized Taylor-Couette flow is studied numerically in a finite cylinder. A distant upstream insulating boundary is shown to stabilize the convective instability entirely while reducing the growth rate of the absolute instability. The reduction is less severe with larger height. After modeling the boundary conditions properly, the wave patterns observed in the experiment turn out to be a noise-sustained convective instability. After the source of the noise resulted from unstable Ekman and Stewartson layers is switched off, a slowly-decaying inertial oscillation is observed in the simulation. We reach the conclusion that the experiments completed to date have not yet reached the regime of absolute instability.
Role of viscoelasticity in instability in plane shear flow over a deformable solid
Indian Academy of Sciences (India)
Paresh Chokshi
2015-05-01
The stability of the flow of a viscoelastic fluid over a deformable elastic solid medium is reviewed focusing on the role played by the fluid elasticity on the earlier known instability modes for the Newtonian fluids. In particular, two classes of modes are emphasized: the viscous mode for the creeping flow, and the wall mode for high Reynolds number flow. The flow geometry is restricted to plane Couette flow of fluid supported on elastic substrate of finite thickness. The viscoelastic fluid is described using the Oldroyd-B model and the dynamics of the deformable solid continuum is described by either Hookean or neo-Hookean elastic model. In the limit of $Re \\to 0$, the introduction of fluid elasticity delays the onset of instability and for sufficiently viscoelastic fluid with dilute polymer concentration, the instability is suppressed rendering the flow stable. For concentrated solution and polymer melt, the instability persists, but with higher value of critical shear rate than for the Newtonian fluid, indicating stabilizing role of fluid elasticity in creeping flow regime. However, for high Reynolds number flow of dilute polymer solution, the polymer addition plays a destabilizing role for wall modes, indicated by reduction in critical Reynolds number by an order of magnitude.
Energy Technology Data Exchange (ETDEWEB)
Fujita, K.; Morikazu, H.; Shintani, A. [Osaka Prefectural Univ., Mechanical Systems Engineering, Graduate School of Engineering (Japan)
2004-07-01
The dynamic behavior of an axisymmetric elastic beam subjected to axial leakage flow is investigated numerically and experimentally. The coupled equations of motion for a fluid and a beam structure are derived using the Navier-Stokes equation for an axial leakage flow-path and the Euler-Bernoulli beam theory. The variation in the dynamic behavior during pre- and post-instability is investigated with respect to increasing axial leakage flow velocity. The experiment was performed to determine the critical velocity of the unstable dynamic behavior of an axisymmetric elastic beam confined in a concentric cylinder subjected to axial leakage flow through a small annulus, and to measure the variation of the dynamic behavior during pre- and post-instability when the unstable phenomenon with the lower predominant frequency is shifted to the higher. The relationships between the axial flow velocities and the unstable phenomena are clarified for the transition from the lower mode to the higher mode by comparing the numerical simulation results with experimental observations. (authors)
Absolute versus convective helical magnetorotational instability in a Taylor-Couette flow
Priede, JÄnis
2008-01-01
We analyze numerically the magnetorotational instability of a Taylor-Couette flow in a helical magnetic field (HMRI) using the inductionless approximation defined by a zero magnetic Prandtl number (Pm=0). The Chebyshev collocation method is used to calculate the eigenvalue spectrum for small amplitude perturbations. First, we carry out a detailed conventional linear stability analysis with respect to perturbations in the form of Fourier modes that corresponds to the convective instability which is not in general self-sustained. The helical magnetic field is found to extend the instability to a relatively narrow range beyond its purely hydrodynamic limit defined by the Rayleigh line. There is not only a lower critical threshold at which HMRI appears but also an upper one at which it disappears again. The latter distinguishes the HMRI from a magnetically-modified Taylor vortex flow. Second, we find an absolute instability threshold as well. In the hydrodynamically unstable regime before the Rayleigh line, the t...
Overview and Issues of Experimental Observation of Microbunching Instabilities
Lumpkin, Alex H
2014-01-01
The generation of the ultra-bright beams required by modern accelerators and drivers of free-electron lasers (FELs) has generally relied on chicane-based bunch compressions that often result in the microbunching instability. Following compression, spectral enhancements extend even into the visible wavelengths through the longitudinal space charge (LSC) impedances. Optical transition radiation (OTR) screens have been extensively used for transverse electron beam size measurements for the bright beams, but the presence of such longitudinal microstructures (microbunching) in the electron beam or the leading edge spikes can result in strong, localized coherent enhancements (COTR) that mask the actual beam profile. Generally, we have observed effects in rf photocathode (PC) injected linacs with chicane compressions since an R56 term is needed. In the past COTR had been only reported in S-band and L-band photoinjected based linacs with single or double bunch compression. Drive laser modulations and charge shot nois...
Large-Scale Flow and Spiral Core Instability in Rayleigh-Benard Convection
Aranson, I S; Steinberg, V; Tsimring, L S; Aranson, Igor; Assenheimer, Michel; Steinberg, Victor; Tsimring, Lev S.
1996-01-01
The spiral core instability, observed in large aspect ratio Rayleigh-Benard convection, is studied numerically in the framework of the Swift-Hohenberg equation coupled to a large-scale flow. It is shown that the instability leads to non-trivial core dynamics and is driven by the self-generated vorticity. Moreover, the recently reported transition from spirals to hexagons near the core is shown to occur only in the presence of a non-variational nonlinearity, and is triggered by the spiral core instability. Qualitative agreement between the simulations and the experiments is demonstrated.
Global instabilities and transient growth in Blasius boundary-layer flow over a compliant panel
Indian Academy of Sciences (India)
K Tsigklifis; A D Lucey
2015-05-01
We develop a hybrid of computational and theoretical approaches suited to study the fluid–structure interaction (FSI) of a compliant panel, flush between rigid upstream and downstream wall sections, with a Blasius boundary-layer flow. The ensuing linear-stability analysis is focused upon global instability and transient growth of disturbances. The flow solution is developed using a combination of vortex and source boundary-element sheets on a computational grid while the dynamics of a plate-spring compliant wall are couched in finite-difference form. The fully coupled FSI system is then written as an eigenvalue problem and the eigenvalues of the various flow- and wall-based instabilities are analysed. It is shown that coalescence or resonance of a structural eigenmode with either a flow-based Tollmien–Schlichting Wave (TSW) or wall-based travelling-wave flutter (TWF) modes can occur. This can render the nature of these well-known convective instabilities to become global for a finite compliant wall giving temporal growth of system disturbances. Finally, a non-modal analysis based on the linear superposition of the extracted temporal modes is presented. This reveals a high level of transient growth when the flow interacts with a compliant panel that has structural properties which render the FSI system prone to global instability. Thus, to design stable finite compliant panels for applications such as boundary-layer transition postponement, both global instabilities and transient growth must be taken into account.
Observation of magnetic field generation via the Weibel instability in interpenetrating plasma flows
Huntington, C M; Ross, J S; Zylstra, A B; Drake, R P; Froula, D H; Gregori, G; Kugland, N L; Kuranz, C C; Levy, M C; Li, C K; Meinecke, J; Morita, T; Petrasso, R; Plechaty, C; Remington, B A; Ryutov, D D; Sakawa, Y; Spitkovsky, A; Takabe, H; Park, H -S
2013-01-01
As the ejecta from supernovae or other energetic astrophysical events stream through the interstellar media, this plasma is shaped by instabilities that generate electric and magnetic fields. Among these instabilities, the Weibel filamentation instability plays a particularly important role, as it can generate significant magnetic fields in an initially un-magnetized medium. It is theorized that these Weibel fields are responsible for the observed gamma-ray burst light curve, particle acceleration in shock waves, and for providing seed fields for larger-scale cosmological magnetic structures. While the presence of these instability-generated fields has been inferred from astrophysical observation and predicted in simulation, observation in experiments is challenging. Here we report direct observation of well-organized, large-amplitude, filamentary magnetic fields associated with the Weibel instability in a scaled laboratory experiment. The experimental images, captured with proton radiography, are shown to be...
An experimental platform for generating Richtmyer-Meshkov instabilities on Z.
Energy Technology Data Exchange (ETDEWEB)
Harding, Eric; Martin, Matthew
2013-04-01
The Richtmyer-Meshkov (RM) instability results when a shock wave crosses a rippled interface between two different materials. The shock deposited vorticity causes the ripples to grow into long spikes. Ultimately this process encourages mixing in many warm dense matter and plasma flows of interest. However, generating pure RM instabilities from initially solid targets is difficult because longlived, steady shocks are required. As a result only a few relevant experiments exist, and current theoretical understanding is limited. Here we propose using a flyer-plate driven target to generate RM instabilities with the Z machine. The target consists of a Be impact layer with sinusoidal perturbations and is followed by a low-density carbon foam. Simulation results show that the RM instability grows for 60 ns before release waves reach the perturbation. This long drive time makes Z uniquely suited for generating the high-quality data that is needed by the community.
Stromberger, Jorg Hermann
Numerous experimental and theoretical investigations on two-phase flow instability and burnout in heated microchannels have been reported in the literature. However none of these investigations deals with the possible effects of wall vibrations on such flow boiling processes within microchannels. Fluid-structure interaction in ultra high power density systems cooled by high velocity single phase forced convection in microchannels may result in vibration amplitudes that are a significant fraction of the diameter of the channel. Such vibrations may significantly impact vapor bubble dynamics at the wall and, hence, the limiting heat fluxes corresponding to the onset of flow instability and/or burnout. The primary purpose of this research was to experimentally quantify the effect of forced wall vibration on the onset of flow instability (OFI) and the critical heat flux (CHF) in uniformly-heated annular microchannels. The secondary interest of this investigation was to compare the experimental data collected in the single-phase regime to commonly used single-phase forced convection correlations. Experimental data acquired in the flow boiling regime were to be utilized to confirm the validity of common flow boiling correlations for microchannel flow. The influence of forced wall vibration on subcooled single-phase forced convection and flow boiling was examined. The Georgia Tech Microchannel Test Facility (GTMTF) was modified to allow such experiments to be conducted at controlled values of transverse wall vibration amplitudes and accelerations for a range of frequencies. The channel demand curves were obtained for various inner and outer surface heat fluxes. Experiments were conducted for broad ranges of transverse wall vibration amplitudes over a range of frequencies. The experiments conducted in this investigation provide designers of high power density systems cooled by forced convection in microchannels with the appropriate data and correlations to confidently
Meandering instability of air flow in a granular bed: self-similarity and fluid-solid duality
Yoshimura, Yuki; Okumura, Ko
2016-01-01
Meandering instability is familiar to everyone through river meandering or small rivulets of rain flowing down a windshield. However, its physical understanding is still premature, although it could inspire researchers in various fields, such as nonlinear science, fluid mechanics and geophysics, to resolve their long-standing problems. Here, we perform a small-scale experiment in which air flow is created in a thin granular bed to successfully find a meandering regime, together with other remarkable fluidized regimes, such as a turbulent regime. We discover that phase diagrams of the flow regimes for different types of grains can be universally presented as functions of the flow rate and the granular-bed thickness when the two quantities are properly renormalized. We further reveal that the meandering shapes are self-similar as was shown for meandering rivers. The experimental findings are explained by theory, with elucidating the physics. The theory is based on force balance, a minimum-dissipation principle,...
ON INSTABILITY OF DILUTE FIBER SUSPENSIONS IN AN AXISYMMETRIC TAYLOR-COUETTE FLOW
Institute of Scientific and Technical Information of China (English)
WAN Zhan-hong; LIN Jian-zhong; LI Bing-ru
2005-01-01
A stability analysis of flow between two coaxial rotating cylinders in the presence of fibers was performed. Based on the axisymmetric normal mode for the disturbances, a modified stability equation was derived. Then the numerical solution of the eigenvalue problem about the equation was obtained using the fourth order finite difference scheme. The marginal curves that determine the instability property were drawn. The results show that the flow instability of the dilute fiber suspensions is governed by the aspect ratio of fiber H, the increment of H causes an increase of critical inner Reynolds number. The fiber additives effectively attenuate the instability of the flow, which is in agreement with those of related experiments.
Computational and Experimental Investigation of Transverse Combustion Instabilities
2013-07-01
5113. 4. Pomeroy, B., Lamont, W., Anderson, W., “Subscale Tool for Determining Transverse Combustion Response,” 45th JPC , AIAA 2009-5490. 5. Li, D...Combustor”, 46th JPC , AIAA 2010-7146. 10. Ducruix, S., Rey, C., Candel, S., “A Method for the transverse modulation of reactive flows with application
Dynamics of Zonal FLow Instability and Saturation in Drift Wave Turbulence
Katt, S. T.; Kim, E.; Diamond, P. H.
2001-10-01
We study generalized Kelvin-Helmholtz (GKH) instability as a saturation mechanism for a collisionless zonal flow in the background of drift waves. By treating drift waves as adiabatically modified by GKH, we investigate the modulation instability of drift waves due to GKH modes as well as the linear inflection-type instability of zonal flow. In the case where zonal flows evolve on the time scale much larger than GKH mode, GKH mode is shown to become destabilized not only by the linear instability of zonal flow but also by coupling to drift waves, with a growth rate which is enhanced over the linear value. Furthermore, the nonlinear (modulational) generation of a zonal flow is estimated to dominate over that of GKH. Our results indicate that GKH may not play an important role in a collisionless saturation of zonal flow, in contrast to [1] and [2]. The effect of temperature fluctuation will be discussed. [1] B.N. Rogers, W. Dorland, and M. Kotschenreuther, PRL, 85, 5336, (2000). [2] Y. Idomura, M. Wakatani, and S. Tokuda, PoP, 7, 3551, (2000).
The stratorotational instability of Taylor-Couette flows of moderate Reynolds numbers
Rüdiger, G; Schultz, M; Gellert, M; Harlander, U; Egbers, Chr
2016-01-01
The instability against nonaxisymmetric perturbations of a Taylor-Couette flow with an axial density stratification is considered. The potential flow (driven by cylinders rotating according to the Rayleigh limit) becomes unstable if the Froude number Fr (= rotation frequency/buoyancy frequency) fulfills ${\\rm Fr}_{\\rm min} 1$ so that measurements for too high Reynolds numbers are excluded for axially bounded containers. The instability pattern migrates azimuthally with $\\dot{\\phi} / \\Omega_{\\rm out} \\simeq 1$ so that the SRI pattern always drifts (slightly) faster than the outer cylinder rotates. The measurements confirm this prediction with high accuracy.
The Instability of Void Fraction Waves in Vertical Gas—Liquid Two—Phase Flow
Institute of Scientific and Technical Information of China (English)
BaojiangSUN; DachunYAN; 等
1999-01-01
The measuring and analyzing results of void fraction waves in different flow regimes show that the propagating velocity of void fraction waves depends on flow regimes and mean void fraction.The disturbance at some frequencies can enhance the void fraction wave velocity.Non-linear analysis show that the instability process of bubble flow is a chaotic process.Before the bubbly flow transits to cap-bubbly flow the growth rate of void fraction waves becomes the maximum value when the disturbance frequency is around the main frequency of void fraction waves.
Dynamics and Instabilities of Free Surface and Vortex Flows
DEFF Research Database (Denmark)
Tophøj, Laust Emil Hjerrild
2012-01-01
This PhD thesis consists of two main parts. The first part describes the dynamics of an ideal fluid on a stationary free surface of a given shape. It turns out that one can formulate a set of self-contained equations of momentum conservation for the tangential flow, with no reference to the flow...... of the fluid bulk. With these equations, one can in principle predict the surface flow on a given free surface, once its shape has been measured. The equations are expressed for a general surface using Riemannian geometry and their solutions are discussed, including some difficulties that may arise...
Study of flow instability in a centrifugal fan based on energy gradient theory
Energy Technology Data Exchange (ETDEWEB)
Xiao, Meina; Dou, Hua-Shu; Ma, Xiaoyang [Zhejiang Sci-Tech University, Hangzhou (China); Xiao, Qing [University of Strathclyde, Glasgow (United Kingdom); Chen, Yongning; He, Haijiang; Ye, Xinxue [Zhejiang Yilida Ventilator Co. Ltd, Taizhou (China)
2016-02-15
Flow instability in a centrifugal fan was studied using energy gradient theory. Numerical simulation was performed for the three dimensional turbulent flow field in a centrifugal fan. The flow is governed by the three-dimensional incompressible Navier-Stokes equations coupled with the RNG k-ε turbulent model. The finite volume method was used to discretize the governing equations and the Semiimplicit method for pressure linked equation (SIMPLE) algorithm is employed to iterate the system of the equations. The interior flow field in the centrifugal fan and the distribution of the energy gradient function K are obtained at different flow rates. According to the energy gradient method, the area with larger value of K is the place where the flow loses stability easier. The results show that instability is easier to generate in the regions of impeller outlet and volute tongue. The air flow near the hub is more stable than that near the shroud. That is due to the influences of variations of the velocity and the inlet angle along the axial direction. With the decrease of the flow rate, instability zone in a blade channel moves to the impeller inlet from the outlet and the unstable regions in different channels develop in opposite direction to the rotation of impeller.
Mikhailenko, V. V.; Mikhailenko, V. S.; Lee, Hae June
2016-11-01
The stability of the magnetic field aligned sheared flow with anisotropic ion temperatures, which have the anisotropic spatial inhomogeneities across the magnetic field and are comparable with or are above the electron temperature, is investigated numerically and analytically. The ion temperatures gradients across the magnetic field affect the instability development only when the inhomogeneous is the ion temperature along the magnetic field irrespective the inhomogeneity of the ion temperature across the magnetic field. In this case, the instability is developed due to the combined effect of the ion Landau damping, velocity shear, ion temperature anisotropy, and anisotropy of the ion temperature gradients. In the case when the ion temperature along the magnetic field is homogeneous, but the ion temperature across the magnetic field is inhomogeneous, the short wavelength instability develops with the wave length less than the thermal ion Larmor radius. This instability excites due to the coupled effect of the ion Landau damping, velocity shear and ion temperature anisotropy.
Experimental growth of inertial forced Richtmyer-Meshkov instabilities for different Atwood numbers
Redondo, J. M.; Castilla, R.
2009-04-01
Richtmyer-Meshkov instability occurs when a shock wave impinges on an interface separating two fluids having different densities [1,2]. The instability causes perturbations on the interface to grow, bubbles and spikes, producing vortical structures which potentially result in a turbulent mixing layer. In addition to shock tube experiments, the incompressible Richtmyer-Meshkov instability has also been studied by impulsively accelerating containers of incompressible fluids. Castilla and Redondo (1994) [3] first exploited this technique by dropping tanks containing a liquid and air or two liquids onto a cushioned surface. This technique was improved upon by Niederhaus and Jacobs (2003)[4] by mounting the tank onto a rail system and then allowing it to bounce off of a fixed spring. A range of both miscible and inmiscible liquids were used, giving a wide range of Atwood numbers using the combinations of air, water, alcohol, oil and mercury. Experimental results show the different pattern selection of both the bubbles and spikes for the different Atwood numbers. Visual analysis of the marked interfaces allows to distinguish the regions of strong mixing and compare self-similarity growth of the mixing region. [1] Meshkov, E. E. 1969 Instability of the interface of two gases accelerated by a shock wave. Fluid Dynamics 4, 101-104. [2] Brouillette, M. & Sturtevant, B. 1994 Experiments on the Richtmyer-Meshkov instability: single-scale perturbations on a continuous interface. Journal of Fluid Mechanics 263, 271-292. [3] Castilla, R. & Redondo, J. M. 1994 Mixing Front Growth in RT and RM Instabilities. Proceedings of the Fourth International Workshop on the Physics of Compressible Turbulent Mixing, Cambridge, United Kingdom, edited by P. F. Linden, D. L. Youngs, and S. B. Dalziel, 11-31. [4] Niederhaus, C. E. & Jacobs, J. W. 2003 Experimental study of the Richtmyer-Meshkov instability of incompressible fluids. Journal of Fluid Mechanics 485, 243-277.
Energy Technology Data Exchange (ETDEWEB)
Benvenuto, G.; Troilo, M. (L' Aquila Univ. (Italy); Genoa Univ. (Italy))
1988-06-01
A mathematical model is developed for fluid dynamic auto-excitated vibrations in turbine blades. In particular, with reference to theoretical aspects, the paper deals with the numerical analytical methods developed for the prediction of flutter instability in turbine cascades. With reference to experimental aspects, a description is given of recently installed instrumentation (at the test facility of the University of Genoa, Italy) for flow visualization and detection of unsteady flows by means of interferometry.
CSIR Research Space (South Africa)
Napier, JAL
2002-03-01
Full Text Available Final Report Experimental investigation of fundamental processes in mining induced fracturing and rock instability J.A.L. Napier, K. Drescher, M.W. Hildyard, M.O. Kataka, D.F. Malan, E.J. Sellers Research Agency : CSIR Miningtek Project No : GAP...
A pure hydrodynamic instability in shear flows and its application to astrophysical accretion disks
Nath, Sujit Kumar
2016-01-01
We provide the possible resolution for the century old problem of hydrodynamic shear flows, which are apparently stable in linear analysis but shown to be turbulent in astrophysically observed data and experiments. This mismatch is noticed in a variety of systems, from laboratory to astrophysical flows. There are so many uncountable attempts made so far to resolve this mismatch, beginning with the early work of Kelvin, Rayleigh, and Reynolds towards the end of the nineteenth century. Here we show that the presence of stochastic noise, whose inevitable presence should not be neglected in the stability analysis of shear flows, leads to pure hydrodynamic linear instability therein. This explains the origin of turbulence, which has been observed/interpreted in astrophysical accretion disks, laboratory experiments and direct numerical simulations. This is, to the best of our knowledge, the first solution to the long standing problem of hydrodynamic instability of Rayleigh stable flows.
Nasibullayev, I S; Krekhov, A P; Kramer, L
2005-01-01
We study the homogeneous and the spatially periodic instabilities in a nematic liquid crystal layer subjected to steady plane {\\em Couette} or {\\em Poiseuille} flow. The initial director orientation is perpendicular to the flow plane. Weak anchoring at the confining plates and the influence of the external {\\em electric} and/or {\\em magnetic} field are taken into account. Approximate expressions for the critical shear rate are presented and compared with semi-analytical solutions in case of Couette flow and numerical solutions of the full set of nematodynamic equations for Poiseuille flow. In particular the dependence of the type of instability and the threshold on the azimuthal and the polar anchoring strength and external fields is analysed.
Absolute and convective instabilities in a one-dimensional Brusselator flow model
DEFF Research Database (Denmark)
Kuznetsov, S.P.; Mosekilde, Erik; Dewel, G.
1997-01-01
The paper considers a one-dimensional Brusselator model with a uniform flow of the mixture of reaction components. An absolute as well as a convective instability can arise for both the Hopf and the Turing modes. The corresponding linear stability analysis is presented and supported by the results...
Simulation techniques for spatially evolving instabilities in compressible flow over a flat plate
Wasistho, B.; Geurts, B.J.; Kuerten, J.G.M.
1997-01-01
In this paper we present numerical techniques suitable for a direct numerical simulation in the spatial setting. We demonstrate the application to the simulation of compressible flat plate flow instabilities. We compare second and fourth order accurate spatial discretization schemes in combination w
Stationary instability of an axiosymmetric fluid flow in a rotating magnetic field
Energy Technology Data Exchange (ETDEWEB)
Kapusta, A.B.; Zibol' d, A.F.
1977-07-01
A study is made in a noninduction approximation of the effect that the profile deformation of a primary velocity and the interactions between secondary flows and a primary magnetic field have on the stationary instability of an axiosymmetric fluid flow in a rotating magnetic field. The critical state was shown to be determined by two or three independent criteria. Two regions of absolute primary flow stability were identified, and the critical values for the Reynolds number for these regions were calculated. Profiles of velocity perturbances and secondary flow lines were constructed for various sets of values. 6 references, 3 figures, 2 tables.
Landscape stability and instability in an experimental mountain
Reinhardt, L.; Ellis, M.
2008-12-01
We have designed a series of physical experiments to explore the interrelationships between drainage network organization and topographic development in high-relief landscapes. Our experiments reveal striking internal variability in catchment-scale sediment efflux and drainage network evolution under constant rainfall and rates of base-level fall. We capture this internal dynamic using a unique set of measurement systems that allow us to relate the 3D evolution of topography to sediment flux from the model-orogen. We also observed remarkable stability off the main catchment drainage divides, suggesting that catchment structure is more robust than recent studies propose. We infer that a transition in the mix of dominant processes at the scale of a catchment generates the observed scale dependant autogenic dynamics. Our experimental apparatus is an erosion box in which two opposing panels slide downwards, so simulating base-level fall across emerging topography. Rainfall is generated by an ultra-fine misting apparatus.
Yoneda, Tsuyoshi
2016-01-01
The dynamics along the particle trajectories for the 3D axisymmetric Euler equations in an infinite cylinder are considered. It is shown that if the inflow-outflow is rapidly increasing in time, the corresponding laminar profile of the Euler flow is not (in some sense) stable provided that the swirling component is not small. This exhibits an instability mechanism of pulsatile flow. In the proof, Frenet-Serret formulas and orthonormal moving frame are essentially used.
Meandering instability of air flow in a granular bed: self-similarity and fluid-solid duality
Yoshimura, Yuki; Yagisawa, Yui; Okumura, Ko
2016-12-01
Meandering instability is familiar to everyone through river meandering or small rivulets of rain flowing down a windshield. However, its physical understanding is still premature, although it could inspire researchers in various fields, such as nonlinear science, fluid mechanics and geophysics, to resolve their long-standing problems. Here, we perform a small-scale experiment in which air flow is created in a thin granular bed to successfully find a meandering regime, together with other remarkable fluidized regimes, such as a turbulent regime. We discover that phase diagrams of the flow regimes for different types of grains can be universally presented as functions of the flow rate and the granular-bed thickness when the two quantities are properly renormalized. We further reveal that the meandering shapes are self-similar as was shown for meandering rivers. The experimental findings are explained by theory, with elucidating the physics. The theory is based on force balance, a minimum-dissipation principle, and a linear-instability analysis of a continuum equation that takes into account the fluid-solid duality, i.e., the existence of fluidized and solidified regions of grains along the meandering path. The present results provide fruitful links to related issues in various fields, including fluidized bed reactors in industry.
Leclercq, Colin; Kerswell, Rich R
2016-01-01
The `Rayleigh line' mu=eta^2, where mu=Omega_o/Omega_i and eta=r_i/r_o are respectively the rotation and radius ratios between inner (subscript `i') and outer (subscript `o') cylinders, is regarded as marking the limit of centrifugal instability (CI) in unstratified inviscid Taylor--Couette flow, for both axisymmetric [1] and non-axisymmetric [2] modes. Non-axisymmetric stratorotational instability (SRI) is known to set in for anticyclonic rotation ratios beyond that line, i.e. eta^2<\\mu<1 for axially stably-stratified Taylor--Couette flow [3,4], but the competition between CI and SRI in the range mu
Instabilities of a liquid film flowing down an inclined porous plane
Liu, Rong; Liu, Qiusheng
2009-09-01
The problem of a film flowing down an inclined porous layer is considered. The fully developed basic flow is driven by gravitation. A careful linear instability analysis is carried out. We use Darcy’s law to describe the porous layer and solve the coupling equations of the fluid and the porous medium rather than the decoupled equations of the one-sided model used in previous works. The eigenvalue problem is solved by means of a Chebyshev collocation method. We compare the instability of the two-sided model with the results of the one-sided model. The result reveals a porous mode instability which is completely neglected in previous works. For a falling film on an inclined porous plane there are three instability modes, i.e., the surface mode, the shear mode, and the porous mode. We also study the influences of the depth ratio d̂ , the Darcy number δ , and the Beavers-Joseph coefficient αBJ on the instability of the system.
Mironov, A. K.; Krasheninnikov, S. Yu.; Maslov, V. P.; Zakharov, D. E.
2016-07-01
An experimental study was conducted on the specific features of instability wave propagation in the mixing layer of a turbulent jet when the jet is excited by an external acoustic wave. We used the technique of conditional phase averaging of data obtained by particle image velocimetry using the reference signal of a microphone placed near the jet. The influence of the excitation frequency on the characteristics of large-scale structures in the mixing layer was investigated. It is shown that the propagation patterns of the instability waves agree well with previously obtained data on the localization of acoustic sources in turbulent jets.
Kerner, Boris S
2015-12-01
We have revealed a growing local speed wave of increase in speed that can randomly occur in synchronized flow (S) at a highway bottleneck. The development of such a traffic flow instability leads to free flow (F) at the bottleneck; therefore, we call this instability an S→F instability. Whereas the S→F instability leads to a local increase in speed (growing acceleration wave), in contrast, the classical traffic flow instability introduced in the 1950s-1960s and incorporated later in a huge number of traffic flow models leads to a growing wave of a local decrease in speed (growing deceleration wave). We have found that the S→F instability can occur only if there is a finite time delay in driver overacceleration. The initial speed disturbance of increase in speed (called "speed peak") that initiates the S→F instability occurs usually at the downstream front of synchronized flow at the bottleneck. There can be many speed peaks with random amplitudes that occur randomly over time. It has been found that the S→F instability exhibits a nucleation nature: Only when a speed peak amplitude is large enough can the S→F instability occur; in contrast, speed peaks of smaller amplitudes cause dissolving speed waves of a local increase in speed (dissolving acceleration waves) in synchronized flow. We have found that the S→F instability governs traffic breakdown-a phase transition from free flow to synchronized flow (F→S transition) at the bottleneck: The nucleation nature of the S→F instability explains the metastability of free flow with respect to an F→S transition at the bottleneck.
Kerner, Boris S.
2015-12-01
We have revealed a growing local speed wave of increase in speed that can randomly occur in synchronized flow (S) at a highway bottleneck. The development of such a traffic flow instability leads to free flow (F) at the bottleneck; therefore, we call this instability an S →F instability. Whereas the S →F instability leads to a local increase in speed (growing acceleration wave), in contrast, the classical traffic flow instability introduced in the 1950s-1960s and incorporated later in a huge number of traffic flow models leads to a growing wave of a local decrease in speed (growing deceleration wave). We have found that the S →F instability can occur only if there is a finite time delay in driver overacceleration. The initial speed disturbance of increase in speed (called "speed peak") that initiates the S →F instability occurs usually at the downstream front of synchronized flow at the bottleneck. There can be many speed peaks with random amplitudes that occur randomly over time. It has been found that the S →F instability exhibits a nucleation nature: Only when a speed peak amplitude is large enough can the S →F instability occur; in contrast, speed peaks of smaller amplitudes cause dissolving speed waves of a local increase in speed (dissolving acceleration waves) in synchronized flow. We have found that the S →F instability governs traffic breakdown—a phase transition from free flow to synchronized flow (F →S transition) at the bottleneck: The nucleation nature of the S →F instability explains the metastability of free flow with respect to an F →S transition at the bottleneck.
Directory of Open Access Journals (Sweden)
Mukesh Kumar Awasthi
2013-01-01
Full Text Available The instability of a thin sheet of viscous and dielectric liquid moving in the same direction as an air stream in the presence of a uniform horizontal electric field has been carried out using viscous potential flow theory. It is observed that aerodynamic-enhanced instability occurs if the Weber number is much less than a critical value related to the ratio of the air and liquid stream velocities, viscosity ratio of two fluids, the electric field, and the dielectric constant values. Liquid viscosity has stabilizing effect in the stability analysis, while air viscosity has destabilizing effect.
Wave Instabilities and Unidirectional Light Flow in a Cavity with Rotating Walls
Lannebère, Sylvain
2016-01-01
We investigate the conditions for the emergence of wave instabilities in a vacuum cavity delimited by cylindrical metallic walls in relative rotation. It is shown that for a small vacuum gap and for a rotation velocity exceeding a certain threshold, the interactions between the surface plasmon polaritons supported by each wall give rise to an unstable behavior of the electromagnetic field manifested in an exponential growth with time. The instabilities occur only for certain modes of oscillation and are due to the transformation of kinetic energy into electromagnetic energy. We also study the possibility of having asymmetric light flows and optical isolation relying on the relative motion of the cavity walls.
Institute of Scientific and Technical Information of China (English)
无
2003-01-01
A hybrid model of MHD and kinetic theory is proposed to investigate the synergetic stabilizing effects of sheared axial flow and finite Larmor radius on the Rayleigh-Taylor instability in Z-pinch implosions.In our model the MHD plasma is considered to respond to a perturbation with exp[i(k*x-ωt)] at frequency ω+ik2⊥ρ2iΩi instead of frequency ω,where k2⊥ρ2i is the finite Larmor radius effects given from the general kinetic theory of magnetized plasma.Therefore linearized continuity and momentum equations include automatically the finite Larmor radius effects.Dispersion relation is derived,which includes the effects of a density discontinuity and the finite Larmor radius as well as a sheared flow that produces the Kelvin-Helmholtz instability.The dispersion equation is examined in three cases.The results indicate that the synergetic effect of sheared axial flow and the finite Larmor radius can mitigate both the Rayleigh-Taylor instability and the hybrid Rayleigh-Taylor/Kelvin-Helmholtz instability.Moreover,the synergetic mitigation effect is stronger than either of them acting separately.
Investigation of Flow Instabilities in the Inlet Ducts of DP-1C VTOL Aircraft
Lepicovsky, Jan
2008-01-01
An investigation of flow instabilities in the inlet ducts of a two-engine vertical takeoff and landing aircraft DP-1C is described in this report. Recent tests revealed that the engines stall during run ups while the aircraft is operating on the ground. These pop stalls occurred at relatively low power levels, sometimes as low as 60 percent of the engine full speed. Inability to run the engines up to the full speed level is attributed to in-ground effects associated with hot gas ingestion. Such pop stalls were never experienced when the aircraft was tested on a elevated grid platform, which ensured that the aircraft was operating in out-of-the-ground-effect conditions. Based on available information on problems experienced with other vertical takeoff and landing aircraft designs, it was assumed that the engine stalls were caused by partial ingestion of hot gases streaming forward from the main exit nozzle under the aircraft inlets, which are very close to the ground. It was also suggested that the nose wheel undercarriage, located between the inlets, may generate vortices or an unstable wake causing intense mixing of hot exit gases with incoming inlet flow, which would enhance the hot gas ingestion. After running a short three-day series of tests with fully instrumented engine inlets, it is now believed the most probable reason for engine pop stalls are random ingestions of a vortex generated between the two streams moving in opposite directions: outbound hot gas stream from the main nozzle close to the ground and inbound inlet flow above. Originally, the vortex is in a horizontal plane. However, at a certain velocity ratio of these two streams, the vortex attaches either to the ground or the aircraft surface at one end and the other end is swallowed by one of the aircraft inlets. Once the vortex enters the inlet duct, a puff of hot air can be sucked through the vortex core into the engine, which causes a serious inlet flow field distortion followed by an engine
New instability modes for bounded, free shear flows
Macaraeg, Michele G.; Streett, Craig L.
1989-01-01
A class of highly amplified supersonic disturbances are found for high-speed, bounded mixing layers at high values of streamwise wavenumber. Their amplification is an order of magnitude greater than the most amplified modes, which occur at 60-65 deg at low streamwise wavenumber. These disturbances are stabilized by increasing Mach number, viscosity, and sweep; however, the effect of sweep on the most amplified mode is not significant until the wave propagation angle reaches 30 deg. The maximum growth rate of the unstable disturbances decreases as the temperature of the higher Mach number stream is increased. The structure of these disturbances is such that the phase speed with respect to the mean flow is subsonic in a small region in the center of the shear layer, and supersonic on either side of this region.
Scollo, Simona; Bonadonna, Costanza; Manzella, Irene
2016-04-01
Gravitational instabilities are often observed at the bottom of volcanic plumes and clouds generating fingers that propagate downward enhancing sedimentation of fine ash. Regardless of their potential influence on tephra dispersal and deposition, their dynamics is not completely understood, undermining the accuracy of volcanic ash transport and dispersal models. Here we present new laboratory experiments that investigate the effects of particle size, composition and concentration on finger dynamics and generation. The experimental set-up consists of a Plexiglas tank of 50 x 30.3 x 7.5 cm equipped with a removable banner for the partition of two separate layers. The lower partition is a solution of water and sugar and is therefore characterized by a higher density than the upper partition which is filled with water and particles. The upper layer is quiescent (unmixed experiments), or continually mixed using a rotary stirrer (mixed experiments). After removing the horizontal barrier that separates the two fluids, particles are illuminated with a 2W Nd-YAG laser named RayPower 2000 and filmed with a HD camera (1920x1080 pixels). Images are analysed by the Dynamic Studio Software (DANTEC) that is a tool for the acquisition and analysis of velocity and related properties of particles inside the fluids. Each particle that follows the flow and scatters light captured by the camera is analysed based on velocity vectors. Experiments are carried out in order to evaluate the main features of fingers (number, width and speed) as a function of particle type, size and initial concentration. Particles include Glass Beads (GB) with diameter 180 μm. Three initial particle concentrations in the upper layer were employed: 3 g/l, 4 g/l and 5 g/l. Results show that the number and the speed of fingers increases with particle concentration and the speed increases with particles size while it is independent on particle types. Finally, experiments point out that development of instability
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.
Directory of Open Access Journals (Sweden)
Xu Zhang
2016-01-01
Full Text Available To study the effects of increasing and decreasing flow velocities on the fluid-elastic instability of tube bundles, the responses of an elastically mounted tube in a rigid parallel triangular tube bundle with a pitch-to-diameter ratio of 1.67 were tested in a water tunnel subjected to crossflow. Aluminum and stainless steel tubes were tested, respectively. In the in-line and transverse directions, the amplitudes, power spectrum density functions, response frequencies, added mass coefficients, and other results were obtained and compared. Results show that the nonlinear hysteresis phenomenon occurred in both tube bundle vibrations. When the flow velocity is decreasing, the tubes which have been in the state of fluid-elastic instability can keep on this state for a certain flow velocity range. During this process, the response frequencies of the tubes will decrease. Furthermore, the response frequencies of the aluminum tube can decrease much more than those of the stainless steel tube. The fluid-elastic instability constants fitted for these experiments were obtained from experimental data. A deeper insight into the fluid-elastic instability of tube bundles was also obtained by synthesizing the results. This study is beneficial for designing and operating equipment with tube bundles inside, as well as for further research on the fluid-elastic instability of tube bundles.
Linear and nonlinear instability in vertical counter-current laminar gas-liquid flows
Schmidt, Patrick; Lucquiaud, Mathieu; Valluri, Prashant
2015-01-01
We consider the genesis and dynamics of interfacial instability in gas-liquid flows, using as a model the two-dimensional channel flow of a thin falling film sheared by counter-current gas. The methodology is linear stability theory (Orr-Sommerfeld analysis) together with direct numerical simulation of the two-phase flow in the case of nonlinear disturbances. We investigate the influence of three main flow parameters (density contrast between liquid and gas, film thickness, pressure drop applied to drive the gas stream) on the interfacial dynamics. Energy budget analyses based on the Orr-Sommerfeld theory reveal various coexisting unstable modes (interfacial, shear, internal) in the case of high density contrasts, which results in mode coalescence and mode competition, but only one dynamically relevant unstable internal mode for low density contrast. The same linear stability approach provides a quantitative prediction for the onset of (partial) liquid flow reversal in terms of the gas and liquid flow rates. ...
On secondary instabilities generating footbridges between spiral vortex flow
Altmeyer, Sebastian A.; Hoffman, Ch
2014-04-01
This work investigates the transition between different traveling helical waves (spirals, SPIs) in the setup of differentially independent rotating cylinders. We use direct numerical simulations to consider an infinite long and periodic Taylor-Couette apparatus with fixed axial periodicity length. We find so-called mixed-cross-spirals (MCSs), that can be seen as nonlinear superpositions of SPIs, to establish stable footbridges connecting SPI states. While bridging the bifurcation branches of SPIs, the corresponding contributions within the MCS vary continuously with the control parameters. Here discussed MCSs presenting footbridge solutions start and end in different SPI branches. Therefore they differ significantly from the already known MCSs that present bypass solutions (Altmeyer and Hoffmann 2010 New J. Phys. 12 113035). The latter start and end in the same SPI branch, while they always bifurcate out of those SPI branches with the larger mode amplitude. Meanwhile, these only appear within the coexisting region of both SPIs. In contrast, the footbridge solutions can also bifurcate out of the minor SPI contribution. We also find they exist in regions where only one of the SPIs contributions exists. In addition, MCS as footbridge solution can appear either stable or unstable. The latter detected transient solutions offer similar spatio-temporal characteristics to the flow establishing stable footbridges. Such transition processes are interesting for pattern-forming systems in general because they accomplish transitions between traveling waves of different azimuthal wave numbers and have not been described in the literature yet.
On secondary instabilities generating footbridges between spiral vortex flow
Energy Technology Data Exchange (ETDEWEB)
Altmeyer, Sebastian A; Hoffman, Ch, E-mail: sebastian_altmeyer@t-online.de [Institute of Science and Technology Austria (IST Austria), A-3400 Klosterneuburg, Austria Max-Planck-Institute for Dynamic and Self-Organization, D-37073 Göttingen (Germany)
2014-04-01
This work investigates the transition between different traveling helical waves (spirals, SPIs) in the setup of differentially independent rotating cylinders. We use direct numerical simulations to consider an infinite long and periodic Taylor–Couette apparatus with fixed axial periodicity length. We find so-called mixed-cross-spirals (MCSs), that can be seen as nonlinear superpositions of SPIs, to establish stable footbridges connecting SPI states. While bridging the bifurcation branches of SPIs, the corresponding contributions within the MCS vary continuously with the control parameters. Here discussed MCSs presenting footbridge solutions start and end in different SPI branches. Therefore they differ significantly from the already known MCSs that present bypass solutions (Altmeyer and Hoffmann 2010 New J. Phys. 12 113035). The latter start and end in the same SPI branch, while they always bifurcate out of those SPI branches with the larger mode amplitude. Meanwhile, these only appear within the coexisting region of both SPIs. In contrast, the footbridge solutions can also bifurcate out of the minor SPI contribution. We also find they exist in regions where only one of the SPIs contributions exists. In addition, MCS as footbridge solution can appear either stable or unstable. The latter detected transient solutions offer similar spatio-temporal characteristics to the flow establishing stable footbridges. Such transition processes are interesting for pattern-forming systems in general because they accomplish transitions between traveling waves of different azimuthal wave numbers and have not been described in the literature yet. (paper)
Bourouaine, Sofiane; Chandran, Benjamin D G; Maruca, Bennett A; Kasper, Justin C
2013-01-01
Previous studies have shown that the observed temperature anisotropies of protons and alpha particles in the solar wind are constrained by theoretical thresholds for pressure-anisotropy-driven instabilities such as the Alfv\\'en/ion-cyclotron (A/IC) and fast-magnetosonic/whistler (FM/W) instabilities. In this letter, we use a long period of in-situ measurements provided by the {\\em Wind} spacecraft's Faraday cups to investigate the combined constraint on the alpha-proton differential flow velocity and the alpha-particle temperature anisotropy due to A/IC and FM/W instabilities. We show that the majority of the data are constrained to lie within the region of parameter space in which A/IC and FM/W waves are either stable or have extremely low growth rates. In the minority of observed cases in which the growth rate of the A/IC (FM/W) instability is comparatively large, we find relatively higher values of $T_{\\perp\\alpha}/T_{\\perp p}$ ($T_{\\parallel\\alpha}/T_{\\parallel p}$) when alpha-proton differential flow vel...
Energy Technology Data Exchange (ETDEWEB)
Potocki, Mark L [Los Alamos National Laboratory; Hull, Lawrence M [Los Alamos National Laboratory
2010-01-01
The detonation of explosives with thin shells can cause the shells to expand to over 200% strain at strain rates on the order of 10{sup 4} s{sup -1} before failure. Experimental data indicate the development and growth of multiple plastic instabilities lead to the formation of failure and fragmentation in the near periodic pattern. Presented are comparisons of the onset of instabilities from simulations and experimental data. At Los Alamos National Laboratory material models have been evolving for several years to simulate high strain-rate behavior. Our models include the effects of shock heating and damage evolutions as well as failure. The current edition of one of our models uses a tabular EOS, the PTW strength model, a modified Gurson yield surface to compute damage evolution, and a Johnson-Cook failure model. Presented are some of the details of these models. An experiment confirmed the temperature discontinuities.
Olson, J; Egedal, J; Greess, S; Myers, R; Clark, M; Endrizzi, D; Flanagan, K; Milhone, J; Peterson, E; Wallace, J; Weisberg, D; Forest, C B
2016-06-24
The spontaneous formation of magnetic islands is observed in driven, antiparallel magnetic reconnection on the Terrestrial Reconnection Experiment. We here provide direct experimental evidence that the plasmoid instability is active at the electron scale inside the ion diffusion region in a low collisional regime. The experiments show the island formation occurs at a smaller system size than predicted by extended magnetohydrodynamics or fully collisionless simulations. This more effective seeding of magnetic islands emphasizes their importance to reconnection in naturally occurring 3D plasmas.
An Experimental Study of Nonstationary Instabilities of Planar Shock Waves in Ionizing Argon
1980-08-01
amplification of acoustic disturbances in an electrically -heated two-temperature (Te >> Ta) plasma. Since 4the thermally-heated ionizing argon plasma...the schlieren system. In the present study it was felt that any sensitibity ’ 3st by using a diffused slit-source instead of an undiff’used point...parameters such as XE, it can help stimulate discussion on the instability phenomenon. Experimental and analytical research into acoustic stability
Olson, J.; Egedal, J.; Greess, S.; Myers, R.; Clark, M.; Endrizzi, D.; Flanagan, K.; Milhone, J.; Peterson, E.; Wallace, J.; Weisberg, D.; Forest, C. B.
2016-06-01
The spontaneous formation of magnetic islands is observed in driven, antiparallel magnetic reconnection on the Terrestrial Reconnection Experiment. We here provide direct experimental evidence that the plasmoid instability is active at the electron scale inside the ion diffusion region in a low collisional regime. The experiments show the island formation occurs at a smaller system size than predicted by extended magnetohydrodynamics or fully collisionless simulations. This more effective seeding of magnetic islands emphasizes their importance to reconnection in naturally occurring 3D plasmas.
Brunetti, J.; Massi, F.; Saulot, A.; Renouf, M.; D`Ambrogio, W.
2015-06-01
Mechanical systems present several contact surfaces between deformable bodies. The contact interface can be either static (joints) or in sliding (active interfaces). The sliding interfaces can have several roles and according to their application they can be developed either for maximizing the friction coefficient and the energy dissipation (e.g. brakes) or rather to allow the relative displacement at joints with a maximum efficiency. In both cases the coupling between system and local contact dynamics can bring to system dynamics instabilities (e.g. brake squeal or squeaking of hip prostheses). This results in unstable vibrations of the system, induced by the oscillation of the contact forces. In the literature, a large number of works deal with such kind of instabilities and are mainly focused on applied problems such as brake squeal noise. This paper shows a more general numerical analysis of a simple system constituted by two bodies in sliding contact: a rigid cylinder rotating inside a deformable one. The parametrical Complex Eigenvalue Analysis and the transient numerical simulations show how the friction forces can give rise to in-plane dynamic instabilities due to the interaction between two system modes, even for such a simple system characterized by one deformable body. Results from transient simulations highlight the key role of realistic values of the material damping to have convergence of the model and, consequently, reliable physical results. To this aim an experimental estimation of the material damping has been carried out. Moreover, the simplicity of the system allows for a deeper analysis of the contact instability and a balance of the energy flux among friction, system vibrations and damping. The numerical results have been validated by comparison with experimental ones, obtained by a specific test bench developed to reproduce and analyze the contact friction instabilities.
Surfactant and gravity dependent instability of two-layer Couette flows and its nonlinear saturation
Frenkel, Alexander L
2016-01-01
A horizontal flow of two immiscible fluid layers with different densities, viscosities and thicknesses, subject to vertical gravitational forces and with an insoluble surfactant present at the interface, is investigated. The base Couette flow is driven by the horizontal motion of the channel walls. Linear and nonlinear stages of the (inertialess) surfactant and gravity dependent long-wave instability are studied using the lubrication approximation, which leads to a system of coupled nonlinear evolution equations for the interface and surfactant disturbances. The linear stability is determined by an eigenvalue problem for the normal modes. The growth rates and the amplitudes of disturbances of the interface, surfactant, velocities, and pressures are found analytically. For each wavenumber, there are two active normal modes. For each mode, the instability threshold conditions in terms of the system parameters are determined. In particular, it transpires that for certain parametric ranges, even arbitrarily stron...
Kelvin-Helmholtz instability for a bounded plasma flow in a longitudinal magnetic field
Energy Technology Data Exchange (ETDEWEB)
Burinskaya, T. M.; Shevelev, M. M. [Russian Academy of Sciences, Space Research Institute (Russian Federation); Rauch, J.-L. [Centre National de la Recherche Scientifique, Laboratoire de Physique et Chimie de l' Environnement et de l' Espace (France)
2011-01-15
Kelvin-Helmholtz MHD instability in a plane three-layer plasma is investigated. A general dispersion relation for the case of arbitrarily orientated magnetic fields and flow velocities in the layers is derived, and its solutions for a bounded plasma flow in a longitudinal magnetic field are studied numerically. Analysis of Kelvin-Helmholtz instability for different ion acoustic velocities shows that perturbations with wavelengths on the order of or longer than the flow thickness can grow in an arbitrary direction even at a zero temperature. Oscillations excited at small angles with respect to the magnetic field exist in a limited range of wavenumbers even without allowance for the finite width of the transition region between the flow and the ambient plasma. It is shown that, in a low-temperature plasma, solutions resulting in kink-like deformations of the plasma flow grow at a higher rate than those resulting in quasi-symmetric (sausage-like) deformations. The transverse structure of oscillatory-damped eigenmodes in a low-temperature plasma is analyzed. The results obtained are used to explain mechanisms for the excitation of ultra-low-frequency long-wavelength oscillations propagating along the magnetic field in the plasma sheet boundary layer of the Earth's magnetotail penetrated by fast plasma flows.
Savenkov, I. V.
2015-02-01
For the pressure-driven flow in an annular channel with a wall moving in the axial direction, its linear instability with respect to axisymmetric perturbations at high Reynolds numbers is investigated within the framework of the triple-deck theory. When the gap between the cylinders is sufficiently small (as compared to the radii of the cylinders), it is shown that the perturbations can split into two wave packets, the first of which grows faster and moves at a higher velocity.
Split Stream Flow Past a Blunt Trailing Edge with Application to Combustion Instabilities
2012-08-01
woven cotton cheesecloth, a slightly denser muslin cheesecloth, and polyester filter felt. To vary the velocity ratio, honeycomb, mesh, and cloth...good characterization of hydrodynamic instabilities for the case of non -reacting flows is the first step to understand how combustion and heat...properties of water at room temperature and the average velocity of the two streams. The shedding frequency was non -dimensionalized using a Strouhal number
The Kelvin-Helmholtz instability in photospheric flows - Effects of coronal heating and structure
Karpen, Judith T.; Antiochos, Spiro K.; Dahlburg, Russell B.; Spicer, Daniel S.
1993-01-01
A series of hydrodynamic numerical simulations has been used to investigate the nonlinear evolution of driven, subsonic velocity shears under a range of typical photospheric conditions. These calculations show that typical photospheric flows are susceptible to the Kelvin-Helmholtz instability (KHI), with rapid nonlinear growth times that are approximately half of a typical granule lifetime. The KHI produces vortical structures in intergranule lanes comparable to a typical fluxule radius; this is precisely the correct scale for maximum power transfer to the corona.
Experimental characterization of vegetation uprooting by flow
Edmaier, K.; Crouzy, B.; Perona, P.
2015-09-01
We investigate vegetation uprooting by flow for Avena sativa seedlings with stem-to-sediment size ratio close to unity and vanishing obstacle-induced scouring. By inducing parallel riverbed erosion within an experimental flume, we measure the time-to-uprooting in relation to root anchoring and flow drag forces. We link the erosion rate to the uprooting timescales for seedlings with varying mean root length. We show that the process of continuous erosion leading to uprooting resembles that of mechanical fatigue where system collapsing occurs after a given exposure time. By this analogy, we also highlight the nonlinear role of the residual root anchoring versus the flow drag acting on the canopy when uprooting occurs. As a generalization, we propose a framework to extend our results to time-dependent erosion rates, which typically occur for real river hydrographs. Finally, we discuss how the characteristic timescale of plant uprooting by flow erosion suggests that vegetation survival is conditioned by multiple erosion events and their interarrival time.
The Experimental Study of Rayleigh-Taylor Instability using a Linear Induction Motor Accelerator
Yamashita, Nicholas; Jacobs, Jeffrey
2009-11-01
The experiments to be presented utilize an incompressible system of two stratified miscible liquids of different densities that are accelerated in order to produce the Rayleigh-Taylor instability. Three liquid combinations are used: isopropyl alcohol with water, a calcium nitrate solution or a lithium polytungstate solution, giving Atwood numbers of 0.11, 0.22 and 0.57, respectively. The acceleration required to drive the instability is produced by two high-speed linear induction motors mounted to an 8 m tall drop tower. The motors are mounted in parallel and have an effective acceleration length of 1.7 m and are each capable of producing 15 kN of thrust. The liquid system is contained within a square acrylic tank with inside dimensions 76 x76x184 mm. The tank is mounted to an aluminum plate, which is driven by the motors to create constant accelerations in the range of 1-20 g's, though the potential exists for higher accelerations. Also attached to the plate are a high-speed camera and an LED backlight to provide continuous video of the instability. In addition, an accelerometer is used to provide acceleration measurements during each experiment. Experimental image sequences will be presented which show the development of a random three-dimensional instability from an unforced initial perturbation. Measurements of the mixing zone width will be compared with traditional growth models.
Directory of Open Access Journals (Sweden)
P. Hasal
2000-01-01
Full Text Available Velocity data obtained by laser Doppler velocimetry (LDV in a flat-bottomed cylindrical stirred vessel (diameter: 300 mm, filling height: 300 mm, working liquids: water and aqueous glycerine, impeller Reynolds number values (ReM: 750, 1200 and 75000 equipped with four radial baffles and stirred with a pitched blade impeller are analyzed by methods of non-linear analysis. The macro-instability of the flow pattern (MI was extracted from the experimental data by a combination of the proper orthogonal decomposition (POD technique and spectral analysis. The relative magnitude of the MI (the fraction of flow total kinetic energy captured by MI was evaluated and its spatial distribution was determined. The temporal evolution of the MI was constructed from the POD eigenmodes. The chaotic attractors of the macro-instabilities were reconstructed by the method of delays. The embedding dimension was determined by the false nearest neighbor analysis (FNN method, and the time delay from the first min imum of mutual information. Correlation dimension de and the largest Lyapunov exponents λmax of the reconstructed attractorswere evaluated. The correlation dimension slightly increases with the increasing ReM value. The spatial distribution of dc is quite uniform at all ReM values. The maximum Lyapunov exponent is clearly positive for all analyzed at tractors. Spatial distribution of λmax is markedly non-uniform and exhibits irregular variations. Possible applications of nonlinear analysis of local velocity data in mixing processes are mentioned.
Alfvén wave coupled with flow-driven fluid instability in interpenetrating plasmas
Energy Technology Data Exchange (ETDEWEB)
Vranjes, J. [Instituto de Astrofisica de Canarias, 38205 La Laguna, Tenerife, Spain and Departamento de Astrofisica, Universidad de La Laguna, 38205 La Laguna, Tenerife (Spain)
2015-05-15
The Alfvén wave is analyzed in case of one quasineutral plasma propagating with some constant speed v{sub 0} through another static quasineutral plasma. A dispersion equation is derived describing the Alfvén wave coupled with the flow driven mode ω=kv{sub 0} and solutions are discussed analytically and numerically. The usual solutions for two oppositely propagating Alfvén waves are substantially modified due to the flowing plasma. More profound is modification of the solution propagating in the negative direction with respect to the magnetic field and the plasma flow. For a large enough flow speed (exceeding the Alfvén speed in the static plasma), this negative solution may become non-propagating, with frequency equal to zero. In this case, it represents a spatial variation of the electromagnetic field. For greater flow speed it becomes a forward mode, and it may merge with the positive one. This merging of the two modes represents the starting point for a flow-driven instability, with two complex-conjugate solutions. The Alfvén wave in interpenetrating plasmas is thus modified and coupled with the flow-driven mode and this coupled mode is shown to be growing when the flow speed is large enough. The energy for the instability is macroscopic kinetic energy of the flowing plasma. The dynamics of plasma particles caused by such a coupled wave still remains similar to the ordinary Alfvén wave. This means that well-known stochastic heating by the Alfvén wave may work, and this should additionally support the potential role of the Alfvén wave in the coronal heating.
An instability of hyperbolic space under the Yang-Mills flow
Energy Technology Data Exchange (ETDEWEB)
Gegenberg, Jack; Day, Andrew C.; Liu, Haitao; Seahra, Sanjeev S. [Department of Mathematics and Statistics, University of New Brunswick Fredericton, New Brunswick, E3B 5A3 (Canada)
2014-04-15
We consider the Yang-Mills flow on hyperbolic 3-space. The gauge connection is constructed from the frame-field and (not necessarily compatible) spin connection components. The fixed points of this flow include zero Yang-Mills curvature configurations, for which the spin connection has zero torsion and the associated Riemannian geometry is one of constant curvature. We analytically solve the linearized flow equations for a large class of perturbations to the fixed point corresponding to hyperbolic 3-space. These can be expressed as a linear superposition of distinct modes, some of which are exponentially growing along the flow. The growing modes imply the divergence of the (gauge invariant) perturbative torsion for a wide class of initial data, indicating an instability of the background geometry that we confirm with numeric simulations in the partially compactified case. There are stable modes with zero torsion, but all the unstable modes are torsion-full. This leads us to speculate that the instability is induced by the torsion degrees of freedom present in the Yang-Mills flow.
Martinez, David
2015-11-01
We investigate on the National Ignition Facility (NIF) the ablative Rayleigh-Taylor (RT) instability in the transition from linear to highly nonlinear regimes. This work is part of the Discovery Science Program on NIF and of particular importance to indirect-drive inertial confinement fusion (ICF) where careful attention to the form of the rise to final peak drive is calculated to prevent the RT instability from shredding the ablator in-flight and leading to ablator mixing into the cold fuel. The growth of the ablative RT instability was investigated using a planar plastic foil with pre-imposed two-dimensional broadband modulations and diagnosed using x-ray radiography. The foil was accelerated for 12ns by the x-ray drive created in a gas-filled Au radiation cavity with a radiative temperature plateau at 175 eV. The dependence on initial conditions was investigated by systematically changing the modulation amplitude, ablator material and the modulation pattern. For each of these cases bubble mergers were observed and the nonlinear evolution of the RT instability showed insensitivity to the initial conditions. This experiment provides critical data needed to validate current theories on the ablative RT instability for indirect drive that relies on the ablative stabilization of short-scale modulations for ICF ignition. This paper will compare the experimental data to the current nonlinear theories. This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under contract DE-AC52-07NA27344. Lawrence Livermore National Security, LLC.
Numerical instability in a 2D gyrokinetic code caused by divergent E X B flow
Energy Technology Data Exchange (ETDEWEB)
Byers, J.A.; Dimits, Y.M.; Langdon, A.B. [Lawrence Livermore National Lab., CA (United States)
1994-12-01
In this paper, a numerical instability first observed in an 2D electrostatic gyrokinetic code is described. The instability should also be present in some form in many versons of particle-in-cell simulation codes that employ guiding center drifts. A perturbation analysis of the instability is given and its results agree quantitatively with the observations from the gyrokinetic code in all respects. The basic mechanism is a false divergence of the E X B flow caused by the interpolation between the grid and the particles as coupled with the specific numerical method for calculating E = -{del} {phi}. Stability or instability depends in detail on the specific choice of particle interpolation method and field method. One common interpolation method, subtracted dipole, is stable. Other commonly used interpolation methods, linear and quadratic, are unstable when combined with a finite difference for the electric field. Linear and quadratic interpolation can be rendered stable if combined with another method for the electric field, the analytic differential of the interpolated potential.
Numerical Instability in a 2D Gyrokinetic Code Caused by Divergent E × B Flow
Byers, J. A.; Dimits, A. M.; Matsuda, Y.; Langdon, A. B.
1994-12-01
In this paper, a numerical instability first observed in a 2D electrostatic gyrokinetic code is described. The instability should also be present in some form in many versons of particle-in-cell simulation codes that employ guiding center drifts. A perturbation analysis of the instability is given and its results agree quantitatively with the observations from the gyrokinetic code in all respects. The basic mechanism is a false divergence of the E × B flow caused by the interpolation between the grid and the particles as coupled with the specific numerical method for calculating E - ∇φ. Stability or instability depends in detail on the specific choice of particle interpolation method and field method. One common interpolation method, subtracted dipole, is stable. Other commonly used interpolation methods, linear and quadratic, are unstable when combined with a finite difference for the electric field. Linear and quadratic interpolation can be rendered stable if combined with another method for the electric field, the analytic differential of the interpolated potential.
Kerner, Boris S
2015-01-01
We have revealed a growing local speed wave of increase in speed that can randomly occur in synchronized flow (S) at a highway bottleneck. The development of such a traffic flow instability leads to free flow (F) at the bottleneck; therefore, we call this instability as an S$\\rightarrow$F instability. Whereas the S$\\rightarrow$F instability leads to a local {\\it increase in speed} (growing acceleration wave), in contrast, the classical traffic flow instability introduced in 50s--60s and incorporated later in a huge number of traffic flow models leads to a growing wave of a local {\\it decrease in speed} (growing deceleration wave). We have found that the S$\\rightarrow$F instability can occur only, if there is a finite time delay in driver over-acceleration. The initial speed disturbance of increase in speed (called "speed peak") that initiates the S$\\rightarrow$F instability occurs usually at the downstream front of synchronized flow at the bottleneck. There can be many speed peaks with random amplitudes that ...
Dynamical Instability of Laminar Axisymmetric Flow of Perfect Fluid with Stratification
Zhuravlev, V V
2007-01-01
The instability of non-homoentropic axisymmetric flow of perfect fluid with respect to non-axisymmetric infinitesimal perturbations was investigated by numerical integration of hydrodynamical differential equations in two-dimensional approximation. The non-trivial influence of entropy gradient on unstable sound and surface gravity waves was revealed. In particular, both decrease and growth of entropy against the direction of effective gravitational acceleration $g_{eff}$ give rise to growing surface gravity modes which are stable with the same parameters in the case of homoentropic flow. At the same time increment of sound modes either grows monotonically while the rate of entropy decrease against $g_{eff}$ gets higher or vanishes at some values of positive and negative entropy gradient in the basic flow. The calculations have showed also that growing internal gravity modes appear only in the flow unstable to axisymmetric perturbations. At last, the analysis of boundary problem with free boundaries uncovered ...
Subramanian, C. S.; Ligrani, P. M.; Tuzzolo, M. F.
1992-01-01
The paper presents and compares fluid-flow and heat transfer properties from artificially induced vortices in a flat-plate turbulent boundary layer and naturally occurring vortices due to centrifugal instabilities in a curved-channel laminar flow. Pairs and arrays of vortices are artificially induced by placing half-delta wings on the plate surface. With both arrays and pairs of vortices, streamwise velocities and total pressures are high, and surface heat transfer is locally augmented in vortex downwash regions. In contrast to vortices in the arrays vortices in the pairs tend to move in the streamwise direction with significant divergence (when the common flow between pair is toward the wall) or convergence (when the common flow between pair is away from the wall). The vortices in the arrays cause maximum peak-to-peak heat transfer variations of up to 12 percent of local spanwise-averaged values for initial vortex spacings between 1 to 2.5 generator heights.
Xie, Beibei; Yang, Dong; Xie, Haiyan; Nie, Xin; Liu, Wanyu
2016-08-01
In order to expand the study on flow instability of supercritical circulating fluidized bed (CFB) boiler, a new numerical computational model considering the heat storage of the tube wall metal was presented in this paper. The lumped parameter method was proposed for wall temperature calculation and the single channel model was adopted for the analysis of flow instability. Based on the time-domain method, a new numerical computational program suitable for the analysis of flow instability in the water wall of supercritical CFB boiler with annular furnace was established. To verify the code, calculation results were respectively compared with data of commercial software. According to the comparisons, the new code was proved to be reasonable and accurate for practical engineering application in analysis of flow instability. Based on the new program, the flow instability of supercritical CFB boiler with annular furnace was simulated by time-domain method. When 1.2 times heat load disturbance was applied on the loop, results showed that the inlet flow rate, outlet flow rate and wall temperature fluctuated with time eventually remained at constant values, suggesting that the hydrodynamic flow was stable. The results also showed that in the case of considering the heat storage, the flow in the water wall is easier to return to stable state than without considering heat storage.
Berréhar, J.; Caroli, C.; Lapersonne-Meyer, C.; Schott, M.
1992-11-01
We study the stress relaxation in single-crystal films of polymerized polydiacetylene, in epitaxy with their monomer substrate. Polymerization induces a uniaxial stress. Two types of surface patterns are observed and studied by atomic force microscopy: films thicker than 175 nm exhibit quasiperiodic cracks perpendicular to the polymer chains; thinner ones exhibit regular wrinkles with the same orientation. The wrinkle surface deformation is stress relaxing and plastic. We show that all experimental results, in particular, the order of magnitude of the pattern spacings, are compatible with the following interpretation: as polymerization proceeds, the uniaxial stress generates a Grinfeld instability (Dok. Akad. Nauk SSSR 290, 1358 (1986) [Sov. Phys. Dokl. 31, 831 (1986)]) fed by surface diffusion. The crack pattern is a secondary instability, initiated at the sites of stress concentration provided by the wrinkles.
Karjanto, N
2016-01-01
A number of qualitative comparisons of experimental results on unidirectional freak wave generation in a hydrodynamic laboratory are presented in this paper. A nonlinear dispersive type of wave equation, the nonlinear Schr\\"{o}dinger equation, is chosen as the theoretical model. A family of exact solutions of this equation the so-called Soliton on Finite Background describing modulational instability phenomenon is implemented in the experiments. It is observed that all experimental results show an amplitude increase according to the phenomenon. Both the carrier wave frequency and the modulation period are preserved during the wave propagation. As predicted by the theoretical model, a phase singularity is also observed in the experiments. Due to frequency downshift phenomenon, the experimental signal and spectrum lose their symmetric property. Another qualitative comparison indicates that the Wessel curves for the experimental results are the perturbed version of the theoretical ones.
Roll/streak Structure Instability Induced by Free-stream Turbulence in Couette Flow
Farrell, Brian; Ioannou, Petros; Nikolaidis, Marios
2016-11-01
Statistical state dynamics (SSD) provides a new perspective for studying mechanisms underlying turbulence in shear flow including instabilities which arise intrinsically from interaction between coherent and incoherent components of the turbulence. Implementations of SSD in the form of a closure at second order is used in this work to analyze the instability emergent from the statistical interaction between coherent perturbations of roll/streak form and the incoherent free-stream turbulence in a minimal channel configuration of Couette flow. By perturbing the nonlinear SSD dynamics a new manifold of stable modes with roll/streak structure is shown to exist in the presence of small amplitude free-stream turbulence. With increase in a parameter controlling the free-stream turbulence energy, a member of this set of stable roll/streak structures is destabilized at a bifurcation and the associated roll/streak eigenmode is found to equilibrate at finite amplitude. The bifurcation structure predicted by the SSD roll/streak instability is reflected in both a closely related quasi-linear dynamical system, referred to as the restricted non-linear (RNL) system, and in DNS. This correspondence is further verified using ensemble implementations of the RNL and DNS systems.
Institute of Scientific and Technical Information of China (English)
Jiebin Liu; Jifu Zhou
2016-01-01
The Kelvin–Helmholtz instability is believed to be the dominant instability mechanism for free shear flows at large Reynolds numbers. At small Reynolds numbers, a new instability mode is identified when the temporal instability of parallel viscous two fluid mixing layers is extended to current-fluid mud systems by considering a composite error function velocity profile. The new mode is caused by the large viscosity difference between the two fluids. This interfacial mode exists when the fluid mud boundary layer is sufficiently thin. Its performance is different from that of the Kelvin–Helmholtz mode. This mode has not yet been reported for interface instability problems with large viscosity contrasts. These results are essential for further stability analysis of flows relevant to the breaking up of this type of interface.
Energy Technology Data Exchange (ETDEWEB)
Andronov, V.A.; Zhidov, I.G.; Meskov, E.E.; Nevmerzhitskii, N.V.; Nikiforov, V.V.; Razin, A.N.; Rogatchev, V.G.; Tolshmyakov, A.I.; Yanilkin, Y.V. [Russian Federal Nuclear Center (Russian Federation)
1994-12-31
The report presents the basic results of some calculations, theoretical and experimental efforts in the study of Rayleigh-Taylor, Kelvin-Helmholtz, Richtmyer-Meshkov instabilities and the turbulent mixing which is caused by their evolution. Since the late forties the VNIIEF has been conducting these investigations. This report is based on the data which were published in different times in Russian and foreign journals. The first part of the report deals with calculations an theoretical techniques for the description of hydrodynamic instabilities applied currently, as well as with the results of several individual problems and their comparison with the experiment. These methods can be divided into two types: direct numerical simulation methods and phenomenological methods. The first type includes the regular 2D and 3D gasdynamical techniques as well as the techniques based on small perturbation approximation and on incompressible liquid approximation. The second type comprises the techniques based on various phenomenological turbulence models. The second part of the report describes the experimental methods and cites the experimental results of Rayleigh-Taylor and Richtmyer-Meskov instability studies as well as of turbulent mixing. The applied methods were based on thin-film gaseous models, on jelly models and liquid layer models. The research was done for plane and cylindrical geometries. As drivers, the shock tubes of different designs were used as well as gaseous explosive mixtures, compressed air and electric wire explosions. The experimental results were applied in calculational-theoretical technique calibrations. The authors did not aim at covering all VNIIEF research done in this field of science. To a great extent the choice of the material depended on the personal contribution of the author in these studies.
Current-driven flow instabilities in large-scale liquid metal batteries, and how to tame them
Weber, Norbert; Stefani, Frank; Weier, Tom
2013-01-01
The use of liquid metal batteries is considered as one promising option for electric grid stabilisation. While large versions of such batteries are preferred in view of the economies of scale, they are susceptible to various magnetohydrodynamic instabilities which imply a risk of short-circuiting the battery due to the triggered fluid flow. Here we focus on the current driven Tayler instability and give critical electrical currents for its onset as well as numerical estimates for the appearing flow structures and speeds. Scaling laws for different materials, battery sizes and geometries are found. We further discuss and compare various means for preventing the instability.
Green, Adam; Marshall, Jeffrey S.; Ma, Dong; Wu, Junru
2016-10-01
A vertically orientated ultrasonic transducer contained within a closed cylindrical Pyrex tube was used to study the acoustic streaming flow within a cylindrical container. A particle-image velocimetry (PIV) system incorporating fluorescent 1.5 μm seeding particles suspended in a mixture of diethyl-phthalate and ethanol, whose optical index was matched to that of Pyrex, was used to allow for undistorted PIV imaging within the Pyrex tube. Temperature on the end-wall surface and acoustic pressure within the cylinder were measured for different end-wall materials. Variables considered included acoustic absorption and reflection coefficients, ultrasound intensity, container height, and thermal properties of the end-wall material. It was observed that a quasi-steady flow field driven by acoustic streaming is rapidly established within the container, which is typically dominated by a stationary vortex ring with downward flow along the ring axis. After sufficient time this quasi-stationary flow exhibits a thermal instability causing it to transform into a secondary flow state. Different types of secondary flow states were observed, including cases where the flow along the cylinder axis is oriented upward toward the ultrasound transducer and cases where the axial flow changes directions along the cylinder axis.
Instability of the Superfluid Flow as Black-Hole Lasing Effect
Finazzi, S.; Piazza, F.; Abad, M.; Smerzi, A.; Recati, A.
2015-06-01
We show that the critical velocity of a superfluid flow through a penetrable barrier coincides with the onset of the analog black-hole lasing effect. This dynamical instability is triggered by modes resonating in an effective cavity formed by two horizons enclosing the barrier. The location of the horizons is set by v (x )=c (x ) , with v (x ),c (x ) being the local fluid velocity and sound speed, respectively. We compute the critical velocity analytically and show that it is univocally determined by the configuration of the horizons. In the limit of broad barriers, the continuous spectrum at the origin of the Hawking-like radiation and of the Landau energetic instability is recovered.
Ilin, Konstantin
2015-01-01
We study the stability of two-dimensional flows in an annulus between two permeable cylinders with respect to three-dimensional perturbations. The basic flow is irrotational, and both radial and azimuthal components of the velocity are non-zero. The direction of the radial flow can be from the inner cylinder to the outer one (the diverging flow) or from the outer cylinder to the inner one (the converging flow). It is shown that, independent of the direction of the radial flow, the basic flow is unstable to small two-dimensional perturbations provided that the ratio of the azimuthal component of the velocity to the radial one is sufficiently large. The instability is oscillatory, and the unstable modes represent travelling azimuthal waves. Neutral curves in the space of parameters of the problem are computed. It turns out that for any geometry of the problem, the most unstable modes (corresponding to the smallest ratio of the azimuthal velocity to the radial one) are two-dimensional ones studied earlier in \\ci...
Experimental investigation of absolute instability of a rotating-disk boundary layer
Othman, H.; Corke, T. C.
2006-10-01
A series of experiments were performed to study the absolute instability of Type I travelling crossflow modes in the boundary layer on a smooth disk rotating at constant speed. The basic flow agreed with analytic theory, and the growth of natural disturbances matched linear theory predictions. Controlled temporal disturbances were introduced by a short-duration air pulse from a hypodermic tube located above the disk and outside the boundary layer. The air pulse was positioned just outboard of the linear-theory critical radius for Type I crossflow modes. A hot-wire sensor primarily sensitive to the azimuthal velocity component, was positioned at different spatial (r,theta) locations on the disk to document the growth of disturbances produced by the air pulses. Ensemble averages conditioned on the air pulses revealed wave packets that evolved in time and space. Two amplitudes of air pulses were used. The lower amplitude was verified to produced wave packets with linear amplitude characteristics. The space time evolution of the leading and trailing edges of the wave packets were followed past the critical radius for the absolute instability, r_{c_{A}}. With the lower amplitudes, the spreading of the disturbance wave packets did not continue to grow in time as r_{c_{A}} was approached. Rather, the spreading of the trailing edge of the wave packet decelerated and asymptotically approached a constant. This result supports previous linear DNS simulations where it was concluded that the absolute instability does not produce a global mode and that linear disturbance wave packets are dominated by the convective instability. The larger-amplitude disturbances were found to produce larger temporal spreading of the wave packets. This was accompanied by a sharp growth in the wave packet amplitude past r_{c_{A}}. Explanations for this are discussed.
Nasibullayev, I Sh; Tarasov, O S; Krekhov, A P; Kramer, L
2005-11-01
We study the homogeneous and the spatially periodic instabilities in a nematic liquid crystal layer subjected to steady plane Couette or Poiseuille flow. The initial director orientation is perpendicular to the flow plane. Weak anchoring at the confining plates and the influence of the external electric and/or magnetic field are taken into account. Approximate expressions for the critical shear rate are presented and compared with semianalytical solutions in case of Couette flow and numerical solutions of the full set of nematodynamic equations for Poiseuille flow. In particular the dependence of the type of instability and the threshold on the azimuthal and the polar anchoring strength and external fields is analyzed.
Convective and absolute instabilities in counter-rotating spiral Poiseuille flow
Energy Technology Data Exchange (ETDEWEB)
Langenberg, J.; Heise, M.; Pfister, G. [University of Kiel, Institute of Experimental and Applied Physics, Kiel (Germany); Abshagen, J. [University of Kiel, Leibniz-Institute of Marine Science, Kiel (Germany)
2004-11-01
We present results of an experimental study on the stability of Taylor-Couette flow in case of counter-rotating cylinders and an imposed axial through flow. We are able to confirm results form recent numerical investigations done by Pinter et al. [24] by measuring the absolute and convective stability boundaries of both propagating Taylor vortices (PTV) and spiral vortices (SPI). Thus our work shows that these theoretical concepts from hydrodynamic stability in open flows apply to experimental counter-rotating Taylor-Couette systems with an imposed axial through flow. (orig.)
Inflectional instabilities in the wall region of bounded turbulent shear flows
Swearingen, Jerry D.; Blackwelder, Ron F.; Spalart, Philippe R.
1987-01-01
The primary thrust of this research was to identify one or more mechanisms responsible for strong turbulence production events in the wall region of bounded turbulent shear flows. Based upon previous work in a transitional boundary layer, it seemed highly probable that the production events were preceded by an inflectional velocity profile which formed on the interface between the low-speed streak and the surrounding fluid. In bounded transitional flows, this unstable profile developed velocity fluctuations in the streamwise direction and in the direction perpendicular to the sheared surface. The rapid growth of these instabilities leads to a breakdown and production of turbulence. Since bounded turbulent flows have many of the same characteristics, they may also experience a similar type of breakdown and turbulence production mechanism.
The influence of surfactants on thermocapillary flow instabilities in low Prandtl melting pools
Kidess, Anton; Kenjereš, Saša; Kleijn, Chris R.
2016-06-01
Flows in low Prandtl number liquid pools are relevant for various technical applications and have so far only been investigated for the case of pure fluids, i.e., with a constant, negative surface tension temperature coefficient ∂γ/∂T. Real-world fluids containing surfactants have a temperature dependent ∂γ/∂T > 0, which may change sign to ∂γ/∂T forces are the main driving force, this can have a tremendous effect on the resulting flow patterns and the associated heat transfer. Here we investigate the stability of such flows for five Marangoni numbers in the range of 2.1 × 106 ≤ Ma ≤ 3.4 × 107 using dynamic large eddy simulations, which we validate against a high resolution direct numerical simulation. We find that the five cases span all flow regimes, i.e., stable laminar flow at Ma ≤ 2.1 × 106, transitional flow with rotational instabilities at Ma = 2.8 × 106 and Ma = 4.6 × 106, and turbulent flow at Ma = 1.8 × 107 and Ma = 3.4 × 107.
Jourdan, G.; Houas, L.
1996-06-01
Results of an experimental investigation on the Richtmyer-Meshkov instability of a He-CO2 interface are reported. A simultaneous three-directional laser absorption technique is used to follow, at a single abscissa, the evolution of the created mixing zone before and after the interaction with the reflected shock, during the same run. CO2 density profiles have been determined within both the incident and the compressed mixing zones. However, near the pure CO2, the wall boundary layer reflected shock interaction perturbs measurements and does not allow objective conclusions.
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...
Experimental Studies of the Stimulated Brillouin Scattering Instability in the Saturated Regime
Energy Technology Data Exchange (ETDEWEB)
Froula, D
2002-10-29
An experimental study of the stimulated Brillouin scattering (SBS) instability has investigated the effects of velocity gradients and kinetic effects on the saturation of ion-acoustic waves in a plasma. For intensities less than I < 1.5 x 10{sup 15} W cm{sup -2}, the SBS instability is moderated primarily by velocity gradients, and for intensities above this threshold, nonlinear trapping is invoked to saturate the instability. We report direct evidence of detuning of SBS by a velocity gradient which was achieved by directly measuring the frequency shift of the SBS driven acoustic wave relative to the local resonant acoustic frequency. Furthermore, a novel use of Thomson scattering has allowed us to gather direct evidence of kinetic effects associated with the SBS process. Specifically, a measured two-fold increase of the ion temperature has been linked with laser beam excitation of ion-acoustic waves to large amplitudes by the SBS instability. Ion-acoustic waves were excited to large amplitude with a 2{omega} 1.2-ns long interaction beam with intensities up to 5 x 10{sup 15} W cm{sup -2}. The local frequency, amplitude, and spatial range of these waves were measured with a 3{omega} 200ps Thomson-scattering probe beam. These detailed and accurate measurements in well-characterized plasma conditions allow for the first time a direct test of non-linear models of the saturation of SBS. The measured two-fold increase of the ion temperature and its correlation with SBS reactivity measurements is the first quantitative evidence of hot ions created by ion trapping in laser plasmas.
Role of interfacial friction for flow instabilities in a thin polar-ordered active fluid layer
Sarkar, Niladri; Basu, Abhik
2015-11-01
We construct a generic coarse-grained dynamics of a thin inflexible planar layer of polar-ordered suspension of active particles that is frictionally coupled to an embedding isotropic passive fluid medium with a friction coefficient Γ . Being controlled by Γ , our model provides a unified framework to describe the long-wavelength behavior of a variety of thin polar-ordered systems, ranging from wet to dry active matter and free-standing active films. Investigations of the linear instabilities around a chosen orientationally ordered uniform reference state reveal generic moving and static instabilities in the system that can depend sensitively on Γ . Based on our results, we discuss estimation of bounds on Γ in experimentally accessible systems.
Magnetohydrodynamic instabilities in rotating and precessing sheared flows: an asymptotic analysis.
Salhi, A; Lehner, T; Cambon, C
2010-07-01
Linear magnetohydrodynamic instabilities are studied analytically in the case of unbounded inviscid and electrically conducting flows that are submitted to both rotation and precession with shear in an external magnetic field. For given rotation and precession the possible configurations of the shear and of the magnetic field and their interplay are imposed by the "admissibility" condition (i.e., the base flow must be a solution of the magnetohydrodynamic Euler equations): we show that an "admissible" basic magnetic field must align with the basic absolute vorticity. For these flows with elliptical streamlines due to precession we undertake an analytical stability analysis for the corresponding Floquet system, by using an asymptotic expansion into the small parameter ε (ratio of precession to rotation frequencies) by a method first developed in the magnetoelliptical instabilities study by Lebovitz and Zweibel [Astrophys. J. 609, 301 (2004)]10.1086/420972. The present stability analysis is performed into a suitable frame that is obtained by a systematic change of variables guided by symmetry and the existence of invariants of motion. The obtained Floquet system depends on three parameters: ε , η (ratio of the cyclotron frequency to the rotation frequency) and χ=cos α, with α being a characteristic angle which, for circular streamlines, ε=0, identifies with the angle between the wave vector and the axis of the solid body rotation. We look at the various (centrifugal or precessional) resonant couplings between the three present modes: hydrodynamical (inertial), magnetic (Alfvén), and mixed (magnetoinertial) modes by computing analytically to leading order in ε the instabilities by estimating their threshold, growth rate, and maximum growth rate and their bandwidths as functions of ε, η, and χ. We show that the subharmonic "magnetic" mode appears only for η>square root of 5/2 and at large η (>1) the maximal growth rate of both the "hydrodynamic" and
Energy Technology Data Exchange (ETDEWEB)
Neise, W.; Maerz, J.; Neuhaus, L.
2000-12-01
Blowers have a radial gap between the rotor tips and casing. Owing to the pressure difference between the suction and pressure side of a blower, flow aground the blade tips in the gap is caused. In case of wide gaps, this gap flow will affect the aerodynamic and acoustic performance. Noise is caused by an interaction between a rotary instability in the blower and the blades of the blower rotor. Although flow visualization did not provide further insight into the transient processes, the flow conditions on the blades of the rotor could be characterized. As expected in view of the low Reynolds numbers, long laminar flow was found on the blades in the transcritical region as well as the track of roll-up of the leakage vortex on the suction side. In order to investigate the source mechanism of the rotary instability, its azimuthal mode structure was measured in different points of the characteristic and with different numbers of blades. The azimuthal modes were calculated from the wall pressure variations measured across the circumference according to a method described by Holste. [German] Ventilatoren haben zwischen den Schaufelspitzen des Laufrades und dem Gehaeuse einen radialen Spalt. Wegen der Druckdifferenz zwischen Saug- und Druckseite des Ventilators kommt es zu einer Ueberstroemung der Schaufelspitzen im Spalt. Bei grossen Spaltweiten fuehrt diese Spaltstroemung zu einer Verschlechterung des aerodynamischen und des akustischen Verhaltens. Es entsteht ein Spaltgeraeusch, auch Blattspitzen-Wirbellaerm genannt. Als Quellmechanismus des Spaltgeraeusches wurde von Kameier, Nawrot, Neise, Kameier und Kameier, Neise, eine Wechselwirkung zwischen einer rotierenden Instabilitaet im Ventilator und den Schaufeln des Ventilatorlaufrades festgestellt. Durch Stroemungsvisualisierung konnte zwar kein weiterer Aufschluss ueber das instationaere Verhalten gewonnen werden, dennoch lassen sich mit der verwendeten Methode die Stroemungsverhaeltnisse auf den Schaufeln des
Experimental and numerical analysis of unsteady behaviour of high efficiency mixed-flow pump
Directory of Open Access Journals (Sweden)
Sedlář Milan
2014-03-01
Full Text Available This work deals with the experimental and numerical investigation of cavitating and noncavitating flow inside a mixed-flow pump and its influence on performance curves of this pump. The experimental research has been carried out in the closed horizontal loop with the main tank capacity of 35 m3. The loop is equipped with both the compressor and the vacuum pump capable of creating different pressure levels while maintaining constant volume flow rate. Pump investigated in this project has been equipped with transparent windows, which enabled the visualization of flow and cavitation phenomena for a wide range of flow conditions. A comprehensive CFD analysis of tested pump has been done both in the cavitating and noncavitating regimes. The ANSYS CFX commercial CFD package has been used to solve URANS equations together with the Rayleigh-Plesset model and the SST-SAS turbulence model. Both the experimental research and the CFD analysis have provided a good illustration of the flow structures inside the pump and their dynamics for a wide range of flow rates and NPSH values. Flow and cavitation instabilities have been detected at suboptimal flow rates which correspond to increased values of noise and vibrations. The calculated results agree well with the measurements.
Experimental and numerical analysis of unsteady behaviour of high efficiency mixed-flow pump
Sedlář, Milan; Komárek, Martin; Vyroubal, Michal; Doubrava, Vít; Varchola, Michal; Hlbočan, Peter
2014-03-01
This work deals with the experimental and numerical investigation of cavitating and noncavitating flow inside a mixed-flow pump and its influence on performance curves of this pump. The experimental research has been carried out in the closed horizontal loop with the main tank capacity of 35 m3. The loop is equipped with both the compressor and the vacuum pump capable of creating different pressure levels while maintaining constant volume flow rate. Pump investigated in this project has been equipped with transparent windows, which enabled the visualization of flow and cavitation phenomena for a wide range of flow conditions. A comprehensive CFD analysis of tested pump has been done both in the cavitating and noncavitating regimes. The ANSYS CFX commercial CFD package has been used to solve URANS equations together with the Rayleigh-Plesset model and the SST-SAS turbulence model. Both the experimental research and the CFD analysis have provided a good illustration of the flow structures inside the pump and their dynamics for a wide range of flow rates and NPSH values. Flow and cavitation instabilities have been detected at suboptimal flow rates which correspond to increased values of noise and vibrations. The calculated results agree well with the measurements.
Tests of a numerical algorithm for the linear instability study of flows on a sphere
Energy Technology Data Exchange (ETDEWEB)
Perez Garcia, Ismael; Skiba, Yuri N [Univerisidad Nacional Autonoma de Mexico, Mexico, D.F. (Mexico)
2001-04-01
A numerical algorithm for the normal mode instability of a steady nondivergent flow on a rotating sphere is developed. The algorithm accuracy is tested with zonal solutions of the nonlinear barotropic vorticity equation (Legendre polynomials, zonal Rossby-Harwitz waves and monopole modons). [Spanish] Ha sido desarrollado un algoritmo numerico para estudiar la inestabilidad lineal de un flujo estacionario no divergente en una esfera en rotacion. La precision del algoritmo se prueba con soluciones zonales de la ecuacion no lineal de vorticidad barotropica (polinomios de Legendre, ondas zonales Rossby-Harwitz y modones monopolares).
Pinning effects on hot-electron vortex flow instability in superconducting films
Shklovskij, Valerij A.
2017-07-01
The hot-electron vortex flow instability in superconducting films in magnetic field B at substrate temperature T0 ≪ Tc is theoretically considered in the presence of pinning. The magnetic field dependences of the instability critical parameters (electric field E*, current density j*, resistivity ρ*, power density P* and vortex velocity v*) are derived for a cosine and a saw-tooth washboard pinning potential and compared with the results obtained earlier by M. Kunchur [Phys. Rev. Lett. 89 (2002) 137005] in absence of pinning. It is shown that the B-behavior of E*, j* and ρ* is monotonic, whereas the B-dependence of v* is quite different, namely dv*/dB may change its sign twice, as sometimes observed in experiments. The simplest heat balance equation for electrons in low-Tc superconducting films is considered within the framework of the two-fluid model. A theoretical analysis reveals that the instability critical temperature T* ≈ 5Tc/6 at T0 < T*/2 with T* being independent of B.
Grassi, A.; Grech, M.; Amiranoff, F.; Pegoraro, F.; Macchi, A.; Riconda, C.
2017-02-01
The Weibel instability driven by two symmetric counterstreaming relativistic electron plasmas, also referred to as current-filamentation instability, is studied in a constant and uniform external magnetic field aligned with the plasma flows. Both the linear and nonlinear stages of the instability are investigated using analytical modeling and particle-in-cell simulations. While previous studies have already described the stabilizing effect of the magnetic field, we show here that the saturation stage is only weakly affected. The different mechanisms responsible for the saturation are discussed in detail in the relativistic cold fluid framework considering a single unstable mode. The application of an external field leads to a slight increase of the saturation level for large wavelengths, while it does not affect the small wavelengths. Multimode and temperature effects are then investigated. While at high temperature the saturation level is independent of the external magnetic field, at low but finite temperature the competition between different modes in the presence of an external magnetic field leads to a saturation level lower with respect to the unmagnetized case.
Linear and nonlinear instability in vertical counter-current laminar gas-liquid flows
Schmidt, Patrick; Ó Náraigh, Lennon; Lucquiaud, Mathieu; Valluri, Prashant
2016-04-01
We consider the genesis and dynamics of interfacial instability in vertical gas-liquid flows, using as a model the two-dimensional channel flow of a thin falling film sheared by counter-current gas. The methodology is linear stability theory (Orr-Sommerfeld analysis) together with direct numerical simulation of the two-phase flow in the case of nonlinear disturbances. We investigate the influence of two main flow parameters on the interfacial dynamics, namely the film thickness and pressure drop applied to drive the gas stream. To make contact with existing studies in the literature, the effect of various density contrasts is also examined. Energy budget analyses based on the Orr-Sommerfeld theory reveal various coexisting unstable modes (interfacial, shear, internal) in the case of high density contrasts, which results in mode coalescence and mode competition, but only one dynamically relevant unstable interfacial mode for low density contrast. A study of absolute and convective instability for low density contrast shows that the system is absolutely unstable for all but two narrow regions of the investigated parameter space. Direct numerical simulations of the same system (low density contrast) show that linear theory holds up remarkably well upon the onset of large-amplitude waves as well as the existence of weakly nonlinear waves. For high density contrasts, corresponding more closely to an air-water-type system, linear stability theory is also successful at determining the most-dominant features in the interfacial wave dynamics at early-to-intermediate times. Nevertheless, the short waves selected by the linear theory undergo secondary instability and the wave train is no longer regular but rather exhibits chaotic motion. The same linear stability theory predicts when the direction of travel of the waves changes — from downwards to upwards. We outline the practical implications of this change in terms of loading and flooding. The change in direction of the
Alfven wave coupled with flow-driven fluid instability in interpenetrating plasmas
Vranjes, J
2015-01-01
The Alfven wave is analyzed in case of one quasineutral plasma propagating with some constant speed $v_0$ through another static quasineutral plasma. A dispersion equation is derived describing the Alfven wave coupled with the flow driven mode $\\omega= k v_0$ and solutions are discussed analytically and numerically. The usual solutions for two oppositely propagating Alfv\\'en waves are substantially modified due to the flowing plasma. More profound is modification of the solution propagating in the negative direction with respect to the magnetic field and the plasma flow. For a large enough flow speed (exceeding the Alfven speed in the static plasma), this negative solution may become non-propagating, with frequency equal to zero. In this case it represents a spatial variation of the electromagnetic field. For greater flow speed it becomes a forward mode, and it may merge with the positive one. This merging of the two modes represents the starting point for a flow-driven instability, with two complex-conjugate...
The influence of surfactants on thermocapillary flow instabilities in low Prandtl melting pools
Kidess, Anton; Kleijn, Chris R
2016-01-01
Flows in low Prandtl number liquid pools are relevant for various technical applications, and have so far only been investigated for the case of pure fluids, i.e. with a constant, negative surface tension temperature coefficient $\\partial\\gamma/\\partial T$. Real-world fluids containing surfactants have a temperature dependent $\\partial\\gamma/\\partial T > 0$, which may change sign to $\\partial\\gamma/\\partial T < 0$ at a critical temperature $T_c$. Where thermocapillary forces are the main driving force, this can have a tremendous effect on the resulting flow patterns and the associated heat transfer. Here we investigate the stability of such flows for five Marangoni numbers in the range of $2.1\\times 10^6 \\leq Ma \\leq 3.4\\times 10^7$ using dynamic large eddy simulations (LES), which we validate against a high resolution direct numerical simulation (DNS). We find that the five cases span all flow regimes, i.e. stable laminar flow at $Ma \\leq 2.1\\times 10^6$, transitional flow with rotational instabilities at...
Analytical and experimental studies of flow-induced vibration of SSME components
Chen, S. S.; Jendrzejczyk, J. A.; Wambsganss, M. W.
1987-01-01
Components of the Space Shuttle Main Engines (SSMEs) are subjected to a severe environment that includes high-temperature, high-velocity flows. Such flows represent a source of energy that can induce and sustain large-amplitude vibratory stresses and/or result in fluidelastic instabilities. Three components are already known to have experienced failures in evaluation tests as a result of flow-induced structural motion. These components include the liquid-oxygen (LOX) posts, the fuel turbine bellows shield, and the internal inlet tee splitter vane. Researchers considered the dynamic behavior of each of these components with varying degrees of effort: (1) a theoretical and experimental study of LOX post vibration excited by a fluid flow; (2) an assessment of the internal inlet tee splitter vane vibration (referred to as the 4000-Hz vibration problem); and (3) a preliminary consideration of the bellows shield problem. Efforts to resolve flow-induced vibration problems associated with the SSMEs are summarized.
Experimental study of crossflow instability on a Mach 6 yawed cone
Craig, Stuart; Saric, William
2014-11-01
Boundary-layer stability and transition represents a key challenge for the designer of hypersonic vehicles, which typically feature highly-swept and conical features inclined to the free stream. The transition process on each of these geometries is typically dominated by the three-dimensional crossflow instability. In order to advance the goal of a physics-based transition prediction method, crossflow experiments were undertaken in the Mach 6 Quiet Tunnel at Texas A&M University. Detailed boundary-layer measurements were performed on a 7-degree cone at a 6-degree angle of incidence using constant-temperature hot-wire anemometry (CTA) to produce boundary-layer contours at constant axial location. These contours illustrate the characteristic streamwise vortex pattern and mean-flow distortion characteristic of crossflow-dominated flows. Additionally, the high frequency response of the CTA system allows for analysis of the spectral content of the flow. These measurements show a high degree of qualitative agreement with analogous studies performed in low-speed flows.
Institute of Scientific and Technical Information of China (English)
Jian Guangde; Huang Lin; Qiu Xiaoming
2005-01-01
The assembling stabilizing effect of the finite Larmor radius (FLR) and the sheared axial flow (SAF) on the Rayleigh-Taylor instability in Z-pinch implosions is studied by means of the incompressible finite Larmor radius magnetohydrodynamic (MHD) equations. The finite Larmor radius effects are introduced in the momentum equation with the sheared axial flow through an anisotropic ion stress tensor. In this paper a linear mode equation is derived that is valid for arbitrary kL, where k is the wave number and L is the plasma shell thickness. Numerical solutions are presented. The results indicate that the short-wavelength modes of the RayleighTaylor instability are easily stabilized by the individual effect of the finite Larmor radius or the sheared axial flow. The assembling effects of the finite Larmor radius and sheared axial flow can heavily mitigate the Rayleigh-Taylor instability, and the unstable region can be compressed considerably.
Energy Technology Data Exchange (ETDEWEB)
Yu, Yu; Lv, Xuefeng; Wang, Shengfei; Niu, Fenglei; Tian, Li [North China Electric Power Univ., Beijing (Switzerland)
2012-03-15
The steam generator composed of multi-helical tubes is used in high temperature gas cooled reactors and two-phase flow instability should be avoided in design. And density-wave oscillation which is mainly due to flow, density and the relationship between the pressure drop delays and feedback effects is one of the two-phase flow instability phenomena easily to occur. Here drift-flux model is used to simulate the performance of the fluid in the secondary side and frequency domain and time domain methods are used to evaluate whether the density-wave oscillation will happen or not. Several operating conditions with nominal power from 15% to 30% are calculated in this paper. The results of the two methods are in accordance, flow instability will occur when power is less than 20% nominal power, which is also according with the result of the experiments well.
Farrell, Brian F.; Ioannou, Petros J.; Nikolaidis, Marios-Andreas
2017-03-01
Although the roll-streak structure is ubiquitous in both observations and simulations of pretransitional wall-bounded shear flow, this structure is linearly stable if the idealization of laminar flow is made. Lacking an instability, the large transient growth of the roll-streak structure has been invoked to explain its appearance as resulting from chance occurrence in the background turbulence of perturbations configured to optimally excite it. However, there is an alternative interpretation for the role of free-stream turbulence in the genesis of the roll-streak structure, which is that the background turbulence interacts with the roll-streak structure to destabilize it. Statistical state dynamics (SSD) provides analysis methods for studying instabilities of this type that arise from interaction between the coherent and incoherent components of turbulence. SSD in the form of a closure at second order is used in this work to analyze the cooperative eigenmodes arising from interaction between the coherent streamwise invariant component and the incoherent background component of turbulence. In pretransitional Couette flow a manifold of stable modes with roll-streak form is found to exist in the presence of low-intensity background turbulence. The least stable mode of this manifold is destabilized at a critical value of a parameter controlling the background turbulence intensity and a finite-amplitude roll-streak structure arises from this instability through a bifurcation in this parameter. Although this bifurcation has analytical expression only in the infinite ensemble formulation of second order SSD, referred in this work as the S3T system, it is closely reflected in numerical simulations of both the dynamically similar quasilinear system, referred to as the restricted nonlinear (RNL) system, as well as in the full Navier-Stokes equations. This correspondence is verified using ensemble implementations of the RNL system and the Navier-Stokes equations. The S3T
Experimental Investigation of Stator Flow in Diagonal Flow Fan
Institute of Scientific and Technical Information of China (English)
Jie Wang; Yoichi Kinoue; Norimasa Shiomi; Toshiaki Setoguchi; Kenji Kaneko; Yingzi Jin
2008-01-01
perimental investigations were conducted for the internal flow of the stator of the diagonal flow fan. Comer separation near the hub surface and the suction surface of the stator blade are focused on. At the design flow rate, the values of the axial velocity and the total pressure at stator outlet decrease near the suction surface at around the hub surface by the influence of the comer wall. At low flow rate of 80-90 % of the design flow rate, the comer separation between the suction surface and the hub surface can be found, which become widely spread at 80 % of the design flow rate.
Sheared Flow Driven Drift Instability and Vortices in Dusty Plasmas with Opposite Polarity
Mushtaq, A.; Shah, AttaUllah; Ikram, M.; Clark, R. E. H.
2016-02-01
Low-frequency electrostatic drift waves are studied in an inhomogeneous dust magnetoplasma containing dust with components of opposite polarity. The drift waves are driven by the magnetic-field-aligned (parallel) sheared flows in the presence of electrons and ions. Due to sheared flow in the linear regime, the electrostatic dust drift waves become unstable. The conditions of mode instability, with the effects of dust streaming and opposite polarity, are studied. These are excited modes which gain large amplitudes and exhibit interactions among themselves. The interaction is governed by the Hasegawa-Mima (HM) nonlinear equation with vector nonlinearity. The stationary solutions of the HM equation in the form of a vortex chain and a dipolar vortex, including effects of dust polarity and electron (ion) temperatures, are studied. The relevance of the present work to space and laboratory four component dusty plasmas is noted.
Overheat Instability in an Ascending Moist Air Flow as a Mechanism of Hurricane Formation
Nechayev, Andrei
2011-01-01
The universal instability mechanism in an ascending moist air flow is theoretically proposed and analyzed. Its origin comes to the conflict between two processes: the increasing of pressure forcing applied to the boundary layer and the decelerating of the updraft flow due to air heating. It is shown that the intensification of tropical storm with the redistribution of wind velocities, pressure and temperature can result from the reorganization of the dissipative structure which key parameters are the moist air lifting velocity and the temperature of surrounding atmosphere. This reorganization can lead to formation of hurricane eye and inner ring of convection. A transition of the dissipative structure in a new state can occur when the temperature lapse rate in a zone of air lifting reaches certain critical value. The accordance of observational data with the proposed theoretical description is shown.
Instability of plane-parallel flow of incompressible liquid over a saturated porous medium
Lyubimova, T. P.; Lyubimov, D. V.; Baydina, D. T.; Kolchanova, E. A.; Tsiberkin, K. B.
2016-07-01
The linear stability of plane-parallel flow of an incompressible viscous fluid over a saturated porous layer is studied to model the instability of water flow in a river over aquatic plants. The saturated porous layer is bounded from below by a rigid plate and the pure fluid layer has a free, undeformable upper boundary. A small inclination of the layers is imposed to simulate the riverbed slope. The layers are inclined at a small angle to the horizon. The problem is studied within two models: the Brinkman model with the boundary conditions by Ochoa-Tapia and Whitaker at the interface, and the Darcy-Forchheimer model with the conditions by Beavers and Joseph. The neutral curves and critical Reynolds numbers are calculated for various porous layer permeabilities and relative thicknesses of the porous layer. The results obtained within the two models are compared and analyzed.
Toroidal field instability and eddy viscosity in Taylor-Couette flows
Gellert, M
2008-01-01
Toroidal magnetic fields subject to the Tayler instability can transport angular momentum. We show that the Maxwell and Reynolds stress of the nonaxisymmetric field pattern depend linearly on the shear in the cylindrical gap geometry. Resulting angular momentum transport also scales linear with shear. It is directed outwards for astrophysical relevant flows and directed inwards for superrotating flows with dOmega/dR>0. We define an eddy viscosity based on the linear relation between shear and angular momentum transport and show that its maximum for given Prandtl and Hartmann number depends linear on the magnetic Reynolds number Rm. For Rm=1000 the eddy viscosity is of the size of 30 in units of the microscopic value.
Fingering instability in the flow of a power-law fluid on a rotating disc
Arora, Akash; Doshi, Pankaj
2016-01-01
A computational study of the flow of a non-Newtonian power law fluid on a spinning disc is considered here. The main goal of this work is to examine the effect of non-Newtonian nature of the fluid on the flow development and associated contact line instability. The governing mass and momentum balance equations are simplified using the lubrication theory. The resulting model equation is a fourth order non-linear PDE which describes the spatial and temporal evolutions of film thickness. The movement of the contact line is modeled using a constant angle slip model. To solve this moving boundary problem, a numerical method is developed using a Galerkin/finite element method based approach. The numerical results show that the spreading rate of the fluid strongly depends on power law exponent n. It increases with the increase in the shear thinning character of the fluid (n 1). It is also observed that the capillary ridge becomes sharper with the value of n. In order to examine the stability of these ridges, a linear stability theory is also developed for these power law fluids. The dispersion relationship depicting the growth rate for a given wave number has been reported and compared for different power-law fluids. It is found that the growth rate of the instability decreases as the fluid becomes more shear thinning in nature, whereas it increases for more shear thickening fluids.
Instability modes on a solid-body-rotation flow in a finite-length pipe
Feng, Chunjuan; Liu, Feng; Rusak, Zvi; Wang, Shixiao
2017-09-01
Numerical solutions of the incompressible Navier-Stokes equations are obtained to study the time evolution of both axisymmetric and three-dimensional perturbations to a base solid-body-rotation flow in a finite-length pipe with non-periodic boundary conditions imposed at the pipe inlet and outlet. It is found that for a given Reynolds number there exists a critical swirl number beyond which the initial perturbations grow, in contrast to the solid-body rotation flow in an infinitely-long pipe or a finite-length pipe with periodic inlet and exit boundary conditions for which the classical Kelvin analysis and Rayleigh stability criterion affirm neutrally stable for all levels of swirl. This paper uncovers for the first time the detailed evolution of the perturbations in both the axisymmetric and three-dimensional situations. The computations reveal a linear growth stage of the perturbations with a constant growth rate after a brief initial period of decay of the imposed initial perturbations. The fastest growing axisymmetric and three-dimensional instability modes and the associated growth rates are identified numerically for the first time. The computations show that the critical swirl number increases and the growth rate of instability decreases at the same swirl number with decreasing Reynolds number. The growth rate of the axisymmetric mode at high Reynolds number agrees well with previous stability theory for inviscid flow. More importantly, three-dimensional simulations uncover that the most unstable mode is the spiral type m = 1 mode, which appears at a lower critical swirl number than that for the onset of the axisymmetric mode. This spiral mode grows faster than the unstable axisymmetric mode at the same swirl. Moreover, the computations reveal that after the linear growing stage of the perturbation the flow continues to evolve nonlinearly to a saturated axisymmetric vortex breakdown state.
Spanwise effects on instabilities of compressible flow over a long rectangular cavity
Sun, Y.; Taira, K.; Cattafesta, L. N.; Ukeiley, L. S.
2016-11-01
The stability properties of two-dimensional (2D) and three-dimensional (3D) compressible flows over a rectangular cavity with length-to-depth ratio of L/D=6 are analyzed at a free-stream Mach number of M_∞ =0.6 and depth-based Reynolds number of Re_D=502 . In this study, we closely examine the influence of three-dimensionality on the wake mode that has been reported to exhibit high-amplitude fluctuations from the formation and ejection of large-scale spanwise vortices. Direct numerical simulation (DNS) and bi-global stability analysis are utilized to study the stability characteristics of the wake mode. Using the bi-global stability analysis with the time-averaged flow as the base state, we capture the global stability properties of the wake mode at a spanwise wavenumber of β =0 . To uncover spanwise effects on the 2D wake mode, 3D DNS are performed with cavity width-to-depth ratio of W/D=1 and 2. We find that the 2D wake mode is not present in the 3D cavity flow with W/D=2 , in which spanwise structures are observed near the rear region of the cavity. These 3D instabilities are further investigated via bi-global stability analysis for spanwise wavelengths of λ /D=0.5{-}2.0 to reveal the eigenspectra of the 3D eigenmodes. Based on the findings of 2D and 3D global stability analysis, we conclude that the absence of the wake mode in 3D rectangular cavity flows is due to the release of kinetic energy from the spanwise vortices to the streamwise vortical structures that develops from the spanwise instabilities.
Flow Instabilities in Feather Seals due to Upstream Harmonic Pressure Fluctuations
Deng, D.; Braun, M. J.; Henricks, Robert C.
2008-01-01
Feather seals (also called slot seals) typically found in turbine stators limit leakage from the platform into the core cavities and from the shroud to the case. They are of various geometric shapes, yet all are contoured to fit the aerodynamic shape of the stator and placed as close as thermomechanically reasonable the powerstream flow passage. Oscillations engendered in the compressor or combustor alter the steady leakage characteristics of these sealing elements and in some instances generate flow instabilities downstream of the seal interface. In this study, a generic feather seal geometry was studied numerically by imposing an upstream harmonic pressure disturbance on the simulated stator-blade gap. The flow and thermal characteristics were determined; it was found that for high pressure drops, large fluctuations in flows in the downstream blade-stator gap can occur. These leakages and pulsations in themselves are not all that significant, yet if coupled with cavity parameters, they could set up resonance events. Computationally generated time-dependent flow fields are captured in sequence video streaming.
Institute of Scientific and Technical Information of China (English)
甘才俊; 吴子牛
2003-01-01
The slipflow model is usually used to study microflows when the Knudsen number lies between 0.01 and 0.1. The instability due to microscale effect seems to have never been studied before.In this paper we present preliminary results for the instability (not physical instability) of this model when applied to microchannel flow with a vanishing Reynolds number. The present paper is restricted to symmetrical mode. Both first-order and second-order slip boundary conditions will be considered.
Experimental study of morphological and convective instabilities: The MEPHISTO space program
Favier, J. J.; Rouzaud, A.
It is now well established that the morphological instability, i.e. the transition during solidification from a planar L/S interface to a cellular one, is strongly influenced by convection. The most recent theories on this topic, which are very advanced, suffer from the lack of experimental tests because uncontrolled convective effects cannot be avoid on the ground. Moreover the check of all the pertinent solidification parameters are not controlled in the same time or measured in real time. After a review of the main 1g experiments and their own limitations, we describe a new apparatus (MEPHISTO) which allows : in-situ measurements of the main solidification parameters and in particular the undercooling at the solidification front in real time by a non perturbative method. real time supervision of convective motions influence on crystal growth. First results obtained under 1g condition are presented : hydrodynamic scaling laws testing, instabilities detection, transient phenomena. Main space results are then anticipated including the role of g-jitters.
Si, Ting; Zhang, Leilei; Li, Guangbin; Roberts, Cynthia J; Yin, Xiezhen; Xu, Ronald
2013-07-01
Recent developments in multimodal imaging and image-guided therapy requires multilayered microparticles that encapsulate several imaging and therapeutic agents in the same carrier. However, commonly used microencapsulation processes have multiple limitations such as low encapsulation efficiency and loss of bioactivity for the encapsulated biological cargos. To overcome these limitations, we have carried out both experimental and theoretical studies on coaxial electrospray of multilayered microparticles. On the experimental side, an improved coaxial electrospray setup has been developed. A customized coaxial needle assembly combined with two ring electrodes has been used to enhance the stability of the cone and widen the process parameter range of the stable cone-jet mode. With this assembly, we have obtained poly(lactide-co-glycolide) microparticles with fine morphology and uniform size distribution. On the theoretical side, an instability analysis of the coaxial electrified jet has been performed based on the experimental parameters. The effects of process parameters on the formation of different unstable modes have been studied. The reported experimental and theoretical research represents a significant step toward quantitative control and optimization of the coaxial electrospray process for microencapsulation of multiple drugs and imaging agents in multimodal imaging and image-guided therapy.
The onset of thermal instability of a two-dimensional hydromagnetic stagnation point flow
Energy Technology Data Exchange (ETDEWEB)
Amaouche, Mustapha; Bouda, Faical Nait [Laboratoire de physique theorique, Universite de Bejaia, Route de Targua Ouzemour Bejaia (Algeria); Sadat, Hamou [Laboratoire d' Etudes Thermiques, Universite de Poitiers, 40 Avenue du Recteur Pineau, 86022 Poitiers (France)
2005-10-01
The aim of the present paper is to examine the effects of a constant magnetic field on the thermal instability of a two-dimensional stagnation point flow. First, it is shown that a basic flow, described by an exact solution of the full Navier-Stokes equations exists under some conditions relating the orientation of the magnetic field in the plane of motion to the obliqueness of free stream. The stability of the basic flow is then investigated in the usual fashion by making use of the normal mode decomposition. The resulting eigenvalue problem is solved numerically by means of a pseudo spectral collocation method based upon Laguerre's functions. The use of this procedure is warranted by the exponential damping of disturbances far from the boundary layer and the appropriate distribution of the roots of Laguerre's polynomials to treat boundary layer problems. It is found through the calculation of neutral stability curves that magnetic field acts to increase the stability of the basic flow. (author)
Numerical simulation on macro-instability of coupling flow field structure in jet-stirred tank
Luan, D. Y.; Lu, J. P.; Bu, Q. X.; Zhang, S. F.; Zheng, S. X.
2016-05-01
The velocity field macro-instability (MI) can help to improve the mixing efficiency. In this work, the MI features of flow field induced by jet-stirred coupling action is studied by using computational fluid dynamics (CFD) simulations. The numerical simulation method of jet-stirred model was established based on standard turbulent equations, and the impeller rotation was modeled by means of the Sliding Mesh (SM) technology. The numerical results of test fluid (water) power consumption were compared with the data obtained by power test experiments. The effects of jet flow velocity and impeller speed on MI frequency were analyzed thoroughly. The results show that the calculated values of power consumption agree well with the experiment measured data, which validates the turbulent model, and the flow structure and MI frequency distribution are affected by both impeller speed and jet flow rate. The amplitude of MI frequency increases obviously with the increasing rotation speed of impeller and the eccentric jet rate, and it can be enhanced observably by eccentric jet rate, in condition of comparatively high impeller speed. At this time, the MI phenomenon disappears with the overall chaotic mixing.
Experimental study of libration-driven zonal flows in non-axisymmetric containers
Noir, Jerome; Bars, Michael Le; Sauret, Alban; Aurnou, J M; 10.1016/j.pepi.2012.05.005
2013-01-01
Orbital dynamics that lead to longitudinal libration of celestial bodies also result in an elliptically deformed equatorial core-mantle boundary. The non-axisymmetry of the boundary leads to a topographic coupling between the assumed rigidmantle and the underlying low viscosity fluid.The present experimental study investigates theeffect of non axisymmetric boundaries on the zonal flow driven by longitudinal libration. For large enough equatorial ellipticity, we report intermittent space-filling turbulence in particular bands of resonant frequency correlated with larger amplitude zonal flow. The mechanism underlying the intermittent turbulence has yet to be unambiguously determined. Nevertheless, recent numerical simulations in triaxial and biaxial ellipsoids suggest that it may be associated with the growth and collapse of an elliptical instability (Cebron et al., 2012). Outside of the band of resonance, we find that the background flow is laminar and the zonal flow becomes independent of the geometry at firs...
Directory of Open Access Journals (Sweden)
Mathieu Zellhuber
2014-03-01
Full Text Available Flame dynamics related to high-frequency instabilities in gas turbine combustors are investigated using experimental observations and numerical simulations. Two different combustor types are studied, a premix swirl combustor (experiment and a generic reheat combustor (simulation. In both cases, a very similar dynamic behaviour of the reaction zone is observed, with the appearance of transverse displacement and coherent flame wrinkling. From these observations, a model for the thermoacoustic feedback linked to transverse modes is proposed. The model splits heat release rate fluctuations into distinct contributions that are related to flame displacement and variations of the mass burning rate. The decomposition procedure is applied on the numerical data and successfully verified by comparing a reconstructed Rayleigh index with the directly computed value. It thus allows to quantify the relative importance of various feedback mechanisms for a given setup.
Evaporative instabilities in climbing films
Hosoi, A. E.; Bush, John W. M.
2001-09-01
We consider flow in a thin film generated by partially submerging an inclined rigid plate in a reservoir of ethanol or methanol water solution and wetting its surface. Evaporation leads to concentration and surface tension gradients that drive flow up the plate. An experimental study indicates that the climbing film is subject to two distinct instabilities. The first is a convective instability characterized by flattened convection rolls aligned in the direction of flow and accompanied by free-surface deformations; in the meniscus region, this instability gives rise to pronounced ridge structures aligned with the mean flow. The second instability, evident when the plate is nearly vertical, takes the form of transverse surface waves propagating up the plate.
Instability of the middle cerebral artery blood flow in response to CO2.
Directory of Open Access Journals (Sweden)
Rosemary E Regan
Full Text Available BACKGROUND: The middle cerebral artery supplies long end-artery branches to perfuse the deep white matter and shorter peripheral branches to perfuse cortical and subcortical tissues. A generalized vasodilatory stimulus such as carbon dioxide not only results in an increase in flow to these various tissue beds but also redistribution among them. We employed a fast step increase in carbon dioxide to detect the dynamics of the cerebral blood flow response. METHODOLOGY/PRINCIPAL FINDINGS: The study was approved by the Research Ethics Board of the University Health Network at the University of Toronto. We used transcranial ultrasound to measure the time course of middle cerebral artery blood flow velocity in 28 healthy adults. Normoxic, isoxic step increases in arterial carbon dioxide tension of 10 mmHg from both hypocapnic and normocapnic baselines were produced using a new prospective targeting system that enabled a more rapid step change than has been previously achievable. In most of the 28 subjects the responses at both carbon dioxide ranges were characterised by more complex responses than a single exponential rise. Most responses were characterised by a fast initial response which then declined rapidly to a nadir, followed by a slower secondary response, with some showing oscillations before stabilising. CONCLUSIONS/SIGNIFICANCE: A rapid step increase in carbon dioxide tension is capable of inducing instability in the cerebral blood flow control system. These dynamic aspects of the cerebral blood flow responses to rapid changes in carbon dioxide must be taken into account when using transcranial blood flow velocity in a single artery segment to measure cerebrovascular reactivity.
Instability due to interfacial tension in parallel liquid-liquid flow
Rodriguez, Oscar M. H.
2016-06-01
The frequent occurrence of multiphase flows in pipes has motivated a great research interest over the last decades. The particular case of liquid-liquid flow is commonly encountered in the petroleum industry, where a number of applications involve oil-water flow such as crude oil production in directional wells. However, it has not received the same attention when compared to gas-liquid flow. In addition, most of the available information has to do with flow in pipes. When it comes to flows in annular ducts the data are scanty. A general transition criterion has been recently proposed in order to obtain the stratified and core-annular flow-pattern transition boundaries in viscous oil-water flow. The proposed criterion was based on an one-dimensional two-fluid model of liquid-liquid two-phase flow. A stability analysis was carried out and interfacial tension is considered. A new destabilizing term arises, which is a function of the cross-section curvature of the interface. It is well accepted that interfacial tension favors the stable condition. However, the analysis of the new interfacial-tension term shows that it can actually destabilize the basic flow pattern, playing an important role in regions of extreme volumetric fractions. Such an interesting effect seems to be more pronounced in flows of viscous fluids and in annular-duct flow. The effect of interfacial tension is explored and the advantages of using a more complete model are discussed and illustrated through comparisons with experimental data from the literature. The evaluation of the effects of fluid viscosity and interfacial tension allows the correction and enhancement of transition models based essentially on data of pipe flow of low viscosity fluids.
Energy Technology Data Exchange (ETDEWEB)
Kim, Eui Kwang; Kim, Dehee; Kim, Jong Bum; Lee, Tae-Ho [Korea Atomic Energy Research Institute, Daejeon (Korea, Republic of)
2015-10-15
A SG selected for PGSFR is of a once-through integrated type. It is a vertical counter flow shell and tube heat exchanger with sodium on the shell side and water-steam in the tubes. The phenomenon of two-phase flow instability has been observed in many industrial domains such as boiling systems and steam generators. In this paper, a computer program developed for predicting two-phase flow instability in a steam generator under axial non-uniform heat flux is presented, and analysis results for verification are presented. A computer code was developed for investigating the two-phase flow stability under sodium-heated conditions in the shell-side of a SG. A solution algorithm for the sodium flow field and tube conduction has been developed for application to sodium-heated SG.
Two-Phase Flow Instability in Water-side Tube of SG under Axially Uniform Heat Flux Conditions
Energy Technology Data Exchange (ETDEWEB)
Kim, Eui Kwang; Kim, Dehee; Jung, Yoon; Lee, Tae-Ho [Korea Atomic Energy Research Institute, Daejeon (Korea, Republic of)
2015-05-15
The SG of PGSFR is of once through integrated type, and is a vertical counter flow shell and tube heat exchanger with sodium on shell side and water-steam in tubes. The phenomenon of two-phase flow instability has been observed in many industrial domains like boiling systems, steam generators. In this paper the numerical methods were studied, and a computer code was developed for two-phase flow instability in steam generator, and representative results of the model calculations are presented. A computer code is developed for investigating two-phase flow stability under constant heat flux conditions. Solution algorithm for the sodium flow field and tube conduction will be developed for the application to sodium-heated SG.
Capponi, A.; James, M. R.; Lane, S. J.
2016-02-01
The canonical Strombolian paradigm of a gas slug ascending and bursting in a homogeneous low-viscosity magma cannot explain the complex details in eruptive dynamics recently revealed by field measurements and textural and geochemical analyses. Evidence points to the existence of high-viscosity magma at the top of the conduit of Strombolian-type volcanoes, acting as a plug. Here, new experiments detail the range of flow configurations that develop during the ascent and burst of a slug through rheologically stratified magma within a conduit. End-member scenarios of a tube fully filled with either high- or low-viscosity liquid bracket three main flow configurations: (1) a plug sufficiently large to fully accommodate an ascending gas slug; (2) A plug that can accommodate the intrusion of low-viscosity liquid driven by the gas expansion, but not all the slug volume, so the slug bursts with the nose in the plug whilst the base is still in the low-viscosity liquid; (3) Gas expansion is sufficient to drive the intrusion of low-viscosity liquid through the plug, with the slug bursting in the low-viscosity layer emplaced dynamically above the plug. We show that the same flow configurations are viable at volcanic-scale through a new experimentally-validated 1D model and 3D computational fluid dynamic simulations. Applied to Stromboli, our results demonstrate that the key parameters controlling the transition between each configuration are gas volume, plug thickness and plug viscosity. The flow processes identified include effective dynamic narrowing and widening of the conduit, instabilities within the falling magma film, transient partial and complete blockage of the conduit, and slug disruption. These complexities influence eruption dynamics and vigour, promoting magma mingling and resulting in pulsatory release of gas.
Comparison of analytic models of instability of rarefied gas flow in a channel
Energy Technology Data Exchange (ETDEWEB)
Aksenova, Olga A. [St.-Petersburg State University, Department of Mathematics and Mechanics, 198504, Universitetskiy pr., 28, Peterhof, St.-Petersburg (Russian Federation); Khalidov, Iskander A. [St.-Petersburg State Polytechnic University, Department of Mathematics and Mechanics, 195251, Polytechnicheskaya ul., 29, St.-Petersburg (Russian Federation)
2014-12-09
Numerical and analytical results are compared concerning the limit properties of the trajectories, attractors and bifurcations of rarefied gas flows in channels. The cascade of bifurcations obtained in our previous analytical and numerical investigations is simulated numerically for different scattering functions V generalizing the ray-diffuse reflection of gas particles from the surface. The main purpose of numerical simulation by Monte Carlo method is the investigation of the properties of different analytic nonlinear dynamic systems corresponding to rarefied gas flow in a channel. The results are compared as well for the models suggested originally by R. N. Miroshin, as for the approximations considered for the first time or for studied in our subsequent papers. Analytical solutions we obtained earlier for the ray reflection which means only one determined velocity of scattered from the walls gas atoms, generally different from the specular reflection. The nonlinear iterative equation describing a rarefied gas flow in a long channel becomes unstable in some regions of corresponding parameters of V (it means the sensitivity to boundary conditions). The values of the parameters are found from analytical approximations in these regions. Numerical results show that the chaotic behavior of the nonlinear dynamic system corresponds to strange attractors and distinguishes clearly from Maxwellian distribution and from the equilibrium on the whole. In the regions of instability (as the dimension of the attractor increases) the search for a corresponding state requires a lot more computation time and a lot of data (the amount of data required increases exponentially with embedding dimension). Therefore the main complication in the computation is reducing as well the computing time as the amount of data to find a suitably close solution. To reduce the computing time our analytical results are applied. Flow conditions satisfying the requirements to the experiment are
Reese, Daniel; Ames, Alex; Noble, Chris; Oakley, Jason; Rothamer, Dave; Bonazza, Riccardo
2016-11-01
The present work investigates the evolution of the Richtmyer-Meshkov instability through simultaneous measurements of concentration and velocity. In the Wisconsin Shock Tube Laboratory at the University of Wisconsin, a broadband, shear-layer initial condition is created at the interface between helium and argon (Atwood number A = 0.7). The helium is seeded with acetone vapor for use in planar laser-induced fluorescence (PLIF), while each gas in the shear layer cross flow is seeded with particulate TiO2, which is used to track the flow and allow for the Mie scattering of light. Once impulsively accelerated by a M = 1.57 shock wave, the interface is imaged twice in close succession using a planar laser sheet containing both the second and fourth harmonic output (532 nm and 266 nm, respectively) of a dual-cavity Nd:YAG laser. Particle image pairs are captured on a dual-frame CCD camera, for use in particle image velocimetry (PIV), while PLIF images are corrected to show concentration. Velocity fields are obtained from particle images using the Insight 4G software package by TSI, and velocity field structure is investigated and compared against concentration images. Probability density functions (PDFs) and planar energy spectra (of both velocity fluctuations and concentration) are then calculated and results are discussed.
Experimental investigation on operating instability of a dual compensation chamber loop heat pipe
Institute of Scientific and Technical Information of China (English)
无
2009-01-01
The operating instability of a dual compensation chamber loop heat pipe (DCC-LHP) including temperature hysteresis, reverse flow and temperature oscillation is described and explained in this paper. Test results indicate that the steady state operating temperature under the variable conductance mode is not the same during the power cycle tests with the same heat load, and it is lower during the power reduction cycle than that during the power increase cycle. Orientation has an effect on the heat load range when temperature hysteresis occurs, and the effect of power variation amplitude can be ignored. Reverse flow tends to occur in some of the startups at low heat loads, even if vapor existed in the vapor grooves initially, which is caused by a higher pressure inside the wick due to evaporation in the evaporator core or vapor penetration into it. Temperature oscillation tends to occur in some of the startups at low head loads or some steady-state operations at high heat loads. Especially when the compensation chamber with the bayonet through is above the evaporator, the incidence rate of temperature oscillation is high.
Experimental Investigation of the Free Surface Effect on the Conical Taylor-Couette Flow System
Directory of Open Access Journals (Sweden)
F. Yahi
2016-01-01
Full Text Available The aim of this work is to highlight the critical thresholds corresponding to the onset of different instabilities considered in the flow between two vertical coaxial cones with and without free surface. The inner cone is rotating and the outer one is maintained at rest. Both cones have the same apex angle Φ =12° giving a constant annular gap δ =d/R1max. The height of the fluid column is H=155mm and It can be progressively decreased for each studied case of the flow system. Two kinds of configurations are studied, small and large gap. The working fluid is assumed as Newtonian and having constant properties like density and viscosity within the range of the required experimental conditions. By means of visualization technique of the flow we have been able to show the different transition modes occurring in the conical flow system according to the aspect ratio and then the induced action of the free surface which introduces a delay in the onset of different instability modes. The obtained results in term of features and stability of the flow are compared to those of Wimmer and Noui-Mehidi.
Jain, Neeraj
2016-01-01
The dissipation mechanism by which the magnetic field reconnects in the presence of an external (guide) magnetic field in the direction of the main current is not well understood. In thin electron current sheets (ECS) (thickness ~ an electron inertial length) formed in collisionless magnetic reconnection, electron shear flow instabilities (ESFI) are potential candidates for providing an anomalous dissipation mechanism which can break the frozen-in condition of the magnetic field affecting the structure and rate of reconnection. We investigate the evolution of ESFI in guide field magnetic reconnection. The properties of the resulting plasma turbulence and their dependence on the strength of the guide field are studied. Utilizing 3-D electron-magnetohydrodynamic simulations of ECS we show that, unlike the case of ECS self-consistently embedded in anti-parallel magnetic fields, the evolution of thin ECS in the presence of a guide field (equal to the asymptotic value of the reconnecting magnetic field or larger) ...
Nonequilibrium thermohydrodynamic effects on the Rayleigh-Taylor instability in compressible flows
Lai, Huilin; Xu, Aiguo; Zhang, Guangcai; Gan, Yanbiao; Ying, Yangjun; Succi, Sauro
2016-08-01
The effects of compressibility on Rayleigh-Taylor instability (RTI) are investigated by inspecting the interplay between thermodynamic and hydrodynamic nonequilibrium phenomena (TNE, HNE, respectively) via a discrete Boltzmann model. Two effective approaches are presented, one tracking the evolution of the local TNE effects and the other focusing on the evolution of the mean temperature of the fluid, to track the complex interfaces separating the bubble and the spike regions of the flow. It is found that both the compressibility effects and the global TNE intensity show opposite trends in the initial and the later stages of the RTI. Compressibility delays the initial stage of RTI and accelerates the later stage. Meanwhile, the TNE characteristics are generally enhanced by the compressibility, especially in the later stage. The global or mean thermodynamic nonequilibrium indicators provide physical criteria to discriminate between the two stages of the RTI.
Large eddy simulations of flow instabilities in a stirred tank generate by a Rushton turbine
DEFF Research Database (Denmark)
Fan, Jianhua; Wang, Yundong; Fei, Weiyang
2007-01-01
The aim of this paper is to investigate the flow instabilities in a baffled, stirred tank generated by a single Rushton turbine by means of large eddy simulation (LES) and simulation using the k-ε turbulent model. A sliding mesh method was used for the coupling between the rotating...... that CFD simulations using k-ε model and LES approach agreed well with the DPIV measurement. Fluctuations of the radial and axial velocity were well predicted at different frequencies by the LES simulation. Velocity fluctuations of high frequencies were observed in the impeller region, while low...... computational time and computer memories. The results of the present work give better understanding to the mixing mechanisms in the mechanically agitated tank....
Kinet, Maxime; Knaepen, Bernard; Molokov, Sergei
2009-10-01
This Letter presents a numerical study of a magnetohydrodynamic flow in a square duct with electrically conducting walls subject to a uniform, transverse magnetic field. Two regimes of instability and transition of Hunt's jets at the walls parallel to the magnetic field have been identified. The first one occurs for relatively low values of the Reynolds number Re and is associated with weak, periodic, counterrotating vortices discovered previously in linear stability studies. The second is a new regime taking place for higher values of Re. It is associated with trains of small-scale vortices enveloped into larger structures, and involves partial detachment of jets from parallel walls. Once this regime sets in, the kinetic energy of perturbations increases by 2 orders of magnitude.
The weakly nonlinear magnetorotational instability in a thin-gap Taylor-Couette flow
Clark, S E
2016-01-01
The magnetorotational instability (MRI) is a fundamental process of accretion disk physics, but its saturation mechanism remains poorly understood despite considerable theoretical and computational effort. We present a multiple scales analysis of the non-ideal MRI in the weakly nonlinear regime -- that is, when the most unstable MRI mode has a growth rate asymptotically approaching zero from above. Here, we develop our theory in a thin-gap, Cartesian channel. Our results confirm the finding by Umurhan et al. (2007) that the perturbation amplitude follows a Ginzburg-Landau equation. We extend these results by performing a detailed force balance for the saturated azimuthal velocity and vertical magnetic field, demonstrating that even when diffusive effects are important, the bulk flow saturates via the combined processes of reducing the background shear and rearranging and strengthening the background vertical magnetic field. We directly simulate the Ginzburg-Landau amplitude evolution for our system and demons...
The weakly nonlinear magnetorotational instability in a global, cylindrical Taylor-Couette flow
Clark, S E
2016-01-01
We conduct a global, weakly nonlinear analysis of the magnetorotational instability (MRI) in a Taylor-Couette flow. This is a multiscale perturbative treatment of the nonideal, axisymmetric MRI near threshold, subject to realistic radial boundary conditions and cylindrical geometry. We analyze both the standard MRI, initialized by a constant vertical background magnetic field, and the helical MRI, with an azimuthal background field component. This is the first weakly nonlinear analysis of the MRI in a global Taylor-Couette geometry, as well as the first weakly nonlinear analysis of the helical MRI. We find that the evolution of the amplitude of the standard MRI is described by a real Ginzburg-Landau equation (GLE), while the amplitude of the helical MRI takes the form of a complex GLE. This suggests that the saturated state of the helical MRI may itself be unstable on long spatial and temporal scales.
Suppression mechanism of Kelvin-Helmholtz instability in compressible fluid flows.
Karimi, Mona; Girimaji, Sharath S
2016-04-01
The transformative influence of compressibility on the Kelvin-Helmholtz instability (KHI) at the interface between two fluid streams of different velocities is explicated. When the velocity difference is small (subsonic), shear effects dominate the interface flow dynamics causing monotonic roll-up of vorticity and mixing between the two streams leading to the KHI. We find that at supersonic speed differentials, compressibility forces the dominance of dilatational (acoustic) rather than shear dynamics at the interface. Within this dilatational interface layer, traveling pressure waves cause the velocity perturbations to become oscillatory. We demonstrate that the oscillatory fluid motion reverses vortex roll-up and segregates the two streams leading to KHI suppression. Analysis and illustrations of the compressibility-induced suppression mechanism are presented.
Energy Technology Data Exchange (ETDEWEB)
Bartosiewicz, Yann [Universite Catholique de Louvain (UCL), Faculty of Applied Sciences, Mechanical Engineering Department, TERM Division, Place du Levant 2, 1348 Louvain-la-Neuve (Belgium)], E-mail: yann.bartosiewicz@uclouvain.be; Lavieville, Jerome [Universite Catholique de Louvain (UCL), Faculty of Applied Sciences, Mechanical Engineering Department, TERM Division, Place du Levant 2, 1348 Louvain-la-Neuve (Belgium); Seynhaeve, Jean-Marie [Universite Catholique de Louvain (UCL), Faculty of Applied Sciences, Mechanical Engineering Department, TERM Division, Place du Levant 2, 1348 Louvain-la-Neuve (Belgium)], E-mail: jm.seynhaeve@uclouvain.be
2008-04-15
This paper presents some results concerning a first benchmark for the new European research code for thermal hydraulics computations: NEPTUNE{sub C}FD. This benchmark relies on the Thorpe experiment to model the occurrence of instabilities in a stratified two-phase flow. The first part of this work is to create a numerical trial case with the VOF approach. The results, in terms of time of onset of the instability, critical wave-number or wave phase speed, are rather good compared to linear inviscid theory and experimental data. Additional numerical tests showed the effect of the surface tension and density ratio on the growing dynamics of the instability and the structure of the waves. In the second part, a code to code (VOF/multi-field) comparison is performed for a case with zero surface tension. The results showed some discrepancies in terms of wave amplitudes, growing rates and a time shifting in the global dynamics. Afterward, two surface tension formulations are proposed in the multi-field approach. Both formulations provided similar results. The time for onset of the instability, the most amplified wave-number and its amplitude were in rather good agreement with the linear analysis and VOF results. However, the time-shifted dynamics was still observed.
Base flow and exhaust plume interaction. Part 1: Experimental study
Schoones, M.M.J.; Bannink, W.J.
1998-01-01
An experimental study of the flow field along an axi-symmetric body with a single operating exhaust nozzle has been performed in the scope of an investigation on base flow-jet plume interactions. The structure of under-expanded jets in a co-flowing supersonic free stream was described using analytic
Subcritical transition in plane Poiseuille flow as a linear instability process
Roizner, Federico; Karp, Michael; Cohen, Jacob
2016-05-01
In this work, a transition scenario is demonstrated, in which most of the stages are followed analytically. The transition is initiated by the linear transient growth mechanism in plane Poiseuille flow subjected to an infinitesimally small secondary disturbance. A novel analytical approximation of the linear transient growth mechanism enables us to perform a secondary linear stability analysis of the modified base-flow. Two possible routes to transition are highlighted here, both correspond to a small secondary disturbance superimposed on a linear transient growth. The first scenario is initiated by four decaying odd normal modes which form a counter-rotating vortex pair; the second is initiated by five even decaying modes which form a pair of counter-rotating pairs. The approximation of the linear transient growth stage by a combination of minimal number of modes allows us to follow the transition stages analytically by employing the multiple time scale method. In particular, the secondary instability stage is followed analytically using linear tools, and is verified by obtaining transition in a direct numerical simulation initiated by conditions dictated by the transient growth analytical expressions. Very good agreement is observed, verifying the theoretical model. The similarities between the two transition routes are discussed and the results are compared with similar results obtained for plane Couette flow.
An experimental description of the flow in a centrifugal compressor from alternate stall to surge
Moënne-Loccoz, V.; Trébinjac, I.; Benichou, E.; Goguey, S.; Paoletti, B.; Laucher, P.
2017-08-01
The present paper gives the experimental results obtained in a centrifugal compressor stage designed and built by SAFRAN Helicopter Engines. The compressor is composed of inlet guide vanes, a backswept splittered unshrouded impeller, a splittered vaned radial diffuser and axial outlet guide vanes. Previous numerical simulations revealed a particular S-shape pressure rise characteristic at partial rotation speed and predicted an alternate flow pattern in the vaned radial diffuser at low mass flow rate. This alternate flow pattern involves two adjacent vane passages. One passage exhibits very low momentum and a low pressure recovery, whereas the adjacent passage has very high momentum in the passage inlet and diffuses efficiently. Experimental measurements confirm the S-shape of the pressure rise characteristic even if the stability limit experimentally occurs at higher mass flow than numerically predicted. At low mass flow the alternate stall pattern is confirmed thanks to the data obtained by high-frequency pressure sensors. As the compressor is throttled the path to instability has been registered and a first scenario of the surge inception is given. The compressor first experiences a steady alternate stall in the diffuser. As the mass flow decreases, the alternate stall amplifies and triggers the mild surge in the vaned diffuser. An unsteady behavior results from the interaction of the alternate stall and the mild surge. Finally, when the pressure gradient becomes too strong, the alternate stall blows away and the compressor enters into deep surge.
Hub vortex instability and wake dynamics in axial flow wind turbines
Foti, Daniel; Howard, Kevin; Yang, Xiaolei; Guala, Michele; Sotiropoulos, Fotis
2014-11-01
The near wake region of an axial flow wind turbine has two distinct shear layers: an outer tip vortex shear layer, which rotates in the same direction as the rotor, and an inner counter-rotating hub vortex shear layer. Recent simulations (Kang et al., J. Fluid Mech. 744, 376 (2014)), corroborated with experiments (Chamorro et al., J. Fluid Mech. 716, 658 (2013)), showed that the hub vortex can undergo spiral vortex breakdown immediately downstream of the turbine. The precessing hub vortex core intercepts and interacts with the tip vortex shear layer causing the large-scale wake meandering motions in the far wake to intensify. These results were obtained for an axial flow hydrokinetic turbine in a turbulent open channel flow. Here we integrate high-resolution LES with experiments to show that a hub vortex instability also occurs in the near wake of a wind turbine in a wind tunnel. We show that the interactions of the hub vortex with the outer flow have significant effects on the wake meandering amplitude and frequency. Our results reinforce the conclusions of Kang et al. (2014) that the hub vortex must be included in wake models to simulate wake interactions at the power plant scale and optimize turbine siting for realistic terrain and wind conditions. This work was supported by DOE (DE-EE0002980, DE-EE0005482 and DE-AC04-94AL85000), the NSF (IIP-1318201), the IREE early career award (UMN) and NSF CAREER: Geophysical Flow Control (CBET-1351303). Computational resources were provided by MSI.
EXPERIMENTAL AND NUMERICAL INVESTIGATIONS ON HORIZONTAL OIL-GAS FLOW
Institute of Scientific and Technical Information of China (English)
无
2007-01-01
Experiments were carried out to investigate the characteristics of oil-gas flow in a horizontal pipe on a large scale (with the inner diameter D = 125 mm). With the experimental data, the flow patterns were presented. Through the analyses for the flow regime transition, it was found that there was a critical superficial velocity of liquid phase for the flow regime transiting from stratified flow to slug flow. The slug flow could not occur until the superficial velocity of liquid phase was higher than the critical velocity. For the flow pattern transiting from stratified to slug flow, the transmitting velocity of gas phase decreases with the augmentation of superficial velocity of liquid phase. On the basis of the experiments, numerical simulations of different flow patterns and their transitions were performed with the use of the Volume Of Fluid (VOF) technique. The results of the computations are shown to match well with the measured data in the experiments.
Institute of Scientific and Technical Information of China (English)
无
2010-01-01
The instability of forced flow in a rotating cylindrical pool with a differentially rotating disk on the free surface is investigated through a series of unsteady three-dimensional numerical simulations.The results show that the basic flow state of this system is axisymmetric and steady,but has rich structures at the meridian plane.However,when the rotation Reynolds number exceeds a critical value,the flow will undergo a transition to three-dimensional oscillatory flow,characterized by the velocity fluctuation waves traveling in the azimuthal direction.The main characteristics of the flow patterns are presented,including the propagating direction,velocity,amplitude and wave number,which depend on the rotation rates and directions of the disk and the cylindrical pool,and the critical conditions for the onset of oscillatory flow are also determined.For the case of disk-only rotation,the centrifugal instability is responsible for the flow transition,and when the disk isoand counter-rotates with the cylindrical pool,the mechanisms for the transition are elliptic and of circular shear instabilities,respectively.
Energy Technology Data Exchange (ETDEWEB)
Smalyuk, V A
2012-06-07
Rayleigh-Taylor (RT) instability is one of the major concerns in inertial confinement fusion (ICF) because it amplifies target modulations in both acceleration and deceleration phases of implosion, which leads to shell disruption and performance degradation of imploding targets. This article reviews experimental results of the RT growth experiments performed on OMEGA laser system, where targets were driven directly with laser light. RT instability was studied in the linear and nonlinear regimes. The experiments were performed in acceleration phase, using planar and spherical targets, and in deceleration phase of spherical implosions, using spherical shells. Initial target modulations consisted of 2-D pre-imposed modulations, and 2-D and 3-D modulations imprinted on targets by the non-uniformities in laser drive. In planar geometry, the nonlinear regime was studied using 3-D modulations with broadband spectra near nonlinear saturation levels. In acceleration-phase, the measured modulation Fourier spectra and nonlinear growth velocities are in good agreement with those predicted by Haan's model [Haan S W 1989 Phys. Rev. A 39 5812]. In a real-space analysis, the bubble merger was quantified by a self-similar evolution of bubble size distributions [Oron D et al 2001 Phys. Plasmas 8, 2883]. The 3-D, inner-surface modulations were measured to grow throughout the deceleration phase of spherical implosions. RT growth rates are very sensitive to the drive conditions, therefore they can be used to test and validate drive physics in hydrodynamic codes used to design ICF implosions. Measured growth rates of pre-imposed 2-D target modulations below nonlinear saturation levels were used to validate non-local thermal electron transport model in laser-driven experiments.
Investigation on the liquid water droplet instability in a simulated flow channel of PEM fuel cell
Energy Technology Data Exchange (ETDEWEB)
Ha, Tae Hun; Kim, Bok Yung; Kim, Han Sang; Min, Kyoung Doug [Seoul National University, Seoul (Korea, Republic of)
2008-05-15
To investigate the characteristics of water droplets on the gas diffusion layer from both top-view and side-view of the flow channel, a rig test apparatus was designed and fabricated with prism attached plate. This experimental device was used to simulate the growth of a single liquid water droplet and its transport process with various air flow velocity and channel height. Not only dry condition but also fully humidified condition was also simulated by using a water absorbing sponge. The detachment height of the water droplet with dry and wet conditions was measured and analyzed. It was found that the droplet tends towards becoming unstable by decreased channel height, increased flow velocity or making a gas diffusion layer (GDL) dryer. Also, peculiar behavior of the water droplet in the channel was presented like attachment to hydrophilic wall or sudden breaking of droplet in case of fully hydrated condition. The simplified force balance model matches with experimental data as well
Foglizzo, Thierry; Masset, Frédéric; Guilet, Jérôme; Durand, Gilles
2012-02-03
Despite the sphericity of the collapsing stellar core, the birth conditions of neutron stars can be highly nonspherical due to a hydrodynamical instability of the shocked accretion flow. Here we report the first laboratory experiment of a shallow water analogue, based on the physics of hydraulic jumps. Both the experiment and its shallow water modeling demonstrate a robust linear instability and nonlinear properties of symmetry breaking, in a system which is one million times smaller and about one hundred times slower than its astrophysical analogue.
2007-03-28
LST 035 0 o2 0O3 o2o ,0) 7 02 0! lO0 .0, 2 20,O 161) 20 200 110 110 22, 240 150 100 220 frequen•/, kHz ftequtnCy, kHr Fig. 2 Experimental, simulation...validated in simulations of supersonic and hypersonic steady base flows over a flat plate by Ma and Zhong [26], and Wang and Zhong [4]. 0 00’ 0.’ o2o
DEFF Research Database (Denmark)
Javier Alvarez, Nicolas; Uguz, A. Kerem
2013-01-01
-examines the Marangoni-Bénard instability for a three-layer system with deformable interfaces undergoing Poiseuille flow. Taking into account the deformability of the interface reveals new physics. Linear stability analysis shows that at small wave numbers a deformable interface is of the orders of magnitude less...
Hall, Philip; Bennett, James
1986-01-01
The Taylor-Goertler vortex instability equations are formulated for steady and unsteady interacting boundary-layer flows. The effective Goertler number is shown to be a function of the wall shape in the boundary layer and the possibility of both steady and unsteady Taylor-Goertler modes exists. As an example the steady flow in a symmetrically constricted channel is considered and it is shown that unstable Goertler vortices exist before the boundary layers at the wall develop the Goldstein singularity discussed by Smith and Daniels (1981). As an example of an unsteady spatially varying basic state, it is considered the instability of high-frequency large-amplitude two- and three-dimensional Tollmien-Schlichting waves in a curved channel. It is shown that they are unstable in the first 'Stokes-layer stage' of the hierarchy of nonlinear states discussed by Smith and Burggraf (1985). This instability of Tollmien-Schlichting waves in an internal flow can occur in the presence of either convex or concave curvature. Some discussion of this instability in external flows is given.
Experimental Tests of Particle Flow Calorimetry
Sefkow, Felix; Kawagoe, Kiyotomo; Pöschl, Roman; Repond, José
2015-01-01
Precision physics at future colliders requires highly granular calorimeters to support the Particle Flow Approach for event reconstruction. This article presents a review of about 10 - 15 years of R\\&D, mainly conducted within the CALICE collaboration, for this novel type of detector. The performance of large scale prototypes in beam tests validate the technical concept of particle flow calorimeters. The comparison of test beam data with simulation, of e.g.\\ hadronic showers, supports full detector studies and gives deeper insight into the structure of hadronic cascades than was possible previously.
A new stereolithography experimental porous flow device.
Crandall, Dustin; Ahmadi, Goodarz; Leonard, Douglas; Ferer, Martin; Smith, Duane H
2008-04-01
A new method for constructing laboratory-scale porous media with increased pore-level variabilities for two-phase flow experiments is presented here. These devices have been created with stereolithography directly on glass, thus improving the stability of the model created with this precision rapid construction technique. The method of construction and improved parameters are discussed in detail, followed by a brief comparison of two-phase drainage results for air invasion into the water-saturated porous medium. Flow through the model porous medium is shown to substantiate theoretical fractal predictions.
Yagi, Takanobu; Sato, Ayaka; Shinke, Manabu; Takahashi, Sara; Tobe, Yasutaka; Takao, Hiroyuki; Murayama, Yuichi; Umezu, Mitsuo
2013-05-01
This study experimentally investigated the instability of flow impingement in a cerebral aneurysm, which was speculated to promote the degradation of aneurysmal wall. A patient-specific, full-scale and elastic-wall replica of cerebral artery was fabricated from transparent silicone rubber. The geometry of the aneurysm corresponded to that found at 9 days before rupture. The flow in a replica was analysed by quantitative flow visualization (stereoscopic particle image velocimetry) in a three-dimensional, high-resolution and time-resolved manner. The mid-systolic and late-diastolic flows with a Reynolds number of 450 and 230 were compared. The temporal and spatial variations of near-wall velocity at flow impingement delineated its inherent instability at a low Reynolds number. Wall shear stress (WSS) at that site exhibited a combination of temporal fluctuation and spatial divergence. The frequency range of fluctuation was found to exceed significantly that of the heart rate. The high-frequency-fluctuating WSS appeared only during mid-systole and disappeared during late diastole. These results suggested that the flow impingement induced a transition from a laminar regime. This study demonstrated that the hydrodynamic instability of shear layer could not be neglected even at a low Reynolds number. No assumption was found to justify treating the aneurysmal haemodynamics as a fully viscous laminar flow.
Experimental study of the characteristics of the flow in the first rows of tube banks
Energy Technology Data Exchange (ETDEWEB)
Olinto, Claudio R.; Indrusiak, Maria Luiza S. [Programa de Pos-Graduacao em Engenharia Mecanica - PROMEC, Universidade Federal do Rio Grande do Sul - UFRGS, Rua Sarmento Leite, 425, CEP 90050-170, Porto Alegre, RS (Brazil); Endres, Luiz Augusto M. [Instituto de Pesquisas Hidraulicas - IPH, Universidade Federal do Rio Grande do Sul - UFRGS, Av. Bento Goncalves, 950, CEP 91501-970, Porto Alegre, RS (Brazil); Moeller, Sergio V. [Programa de Pos-Graduacao em Engenharia Mecanica - PROMEC, Universidade Federal do Rio Grande do Sul - UFRGS, Rua Sarmento Leite, 425, CEP 90050-170, Porto Alegre, RS (Brazil)], E-mail: svmoller@ufrgs.br
2009-10-15
This paper presents the experimental study of the flow instabilities in the first rows of tube banks. The study is performed using hot wire anemometry technique in an aerodynamic channel as well as flow visualizations in a water channel. In the wind channel three tube banks with square arrangement and pitch to diameter ratios P/D = 1.26, 1.4 and 1.6 were studied. The Reynolds number range for the velocities measurements, computed with the tube diameter and the flow velocity in the narrow gap between tubes was 7 x 10{sup 4}-8 x 10{sup 4}. Continuous and discrete wavelets were applied to decompose the velocity results, thus allowing the analysis of phenomena in time-frequency domain. Visualizations in a water channel complemented the analysis of the hot wire results. For this purpose, dye was injected in the flow in the water channel with a tube bank with P/D = 1.26. The range of the Reynolds number of the experiments was 3 x 10{sup 4}-4 x 10{sup 4}. The main results show the presence of instabilities, generated after the second row of the tube bank, which propagates to the interior of the bank. In the resulting flow, the three orthogonal components are equally significant. The three-dimensional behavior of the flow is responsible for a mass redistribution inside the bank that leads to velocity values not expected for the studied geometry, according to the known literature. The resulting flow process can be interpreted as a secondary flow which is characteristic of tube banks.
Study of the instability of the Poiseuille flow using a thermodynamic formalism.
Wang, Jianchun; Li, Qianxiao; E, Weinan
2015-08-04
The stability of the plane Poiseuille flow is analyzed using a thermodynamic formalism by considering the deterministic Navier-Stokes equation with Gaussian random initial data. A unique critical Reynolds number, Rec ≈ 2,332, at which the probability of observing puffs in the solution changes from 0 to 1, is numerically demonstrated to exist in the thermodynamic limit and is found to be independent of the noise amplitude. Using the puff density as the macrostate variable, the free energy of such a system is computed and analyzed. The puff density approaches zero as the critical Reynolds number is approached from above, signaling a continuous transition despite the fact that the bifurcation is subcritical for a finite-sized system. An action function is found for the probability of observing puffs in a small subregion of the flow, and this action function depends only on the Reynolds number. The strategy used here should be applicable to a wide range of other problems exhibiting subcritical instabilities.
Zhang, Meng; Maxworthy, Tony
1999-01-01
It has long been recognized that flow in the melt can have a profound influence on the dynamics of a solidifying interface and hence the quality of the solid material. In particular, flow affects the heat and mass transfer, and causes spatial and temporal variations in the flow and melt composition. This results in a crystal with nonuniform physical properties. Flow can be generated by buoyancy, expansion or contraction upon phase change, and thermo-soluto capillary effects. In general, these flows can not be avoided and can have an adverse effect on the stability of the crystal structures. This motivates crystal growth experiments in a microgravity environment, where buoyancy-driven convection is significantly suppressed. However, transient accelerations (g-jitter) caused by the acceleration of the spacecraft can affect the melt, while convection generated from the effects other than buoyancy remain important. Rather than bemoan the presence of convection as a source of interfacial instability, Hurle in the 1960s suggested that flow in the melt, either forced or natural convection, might be used to stabilize the interface. Delves considered the imposition of both a parabolic velocity profile and a Blasius boundary layer flow over the interface. He concluded that fast stirring could stabilize the interface to perturbations whose wave vector is in the direction of the fluid velocity. Forth and Wheeler considered the effect of the asymptotic suction boundary layer profile. They showed that the effect of the shear flow was to generate travelling waves parallel to the flow with a speed proportional to the Reynolds number. There have been few quantitative, experimental works reporting on the coupling effect of fluid flow and morphological instabilities. Huang studied plane Couette flow over cells and dendrites. It was found that this flow could greatly enhance the planar stability and even induce the cell-planar transition. A rotating impeller was buried inside the
Wan, Xiaoliang; Yu, Haijun; Weinan, E.
2015-05-01
In this work, we study the nonlinear instability of two-dimensional (2D) wall-bounded shear flows from the large deviation point of view. The main idea is to consider the Navier-Stokes equations perturbed by small noise in force and then examine the noise-induced transitions between the two coexisting stable solutions due to the subcritical bifurcation. When the amplitude of the noise goes to zero, the Freidlin-Wentzell (F-W) theory of large deviations defines the most probable transition path in the phase space, which is the minimizer of the F-W action functional and characterizes the development of the nonlinear instability subject to small random perturbations. Based on such a transition path we can define a critical Reynolds number for the nonlinear instability in the probabilistic sense. Then the action-based stability theory is applied to study the 2D Poiseuille flow in a short channel.
Experimental evaluations of the microchannel flow model.
Parker, K J
2015-06-07
Recent advances have enabled a new wave of biomechanics measurements, and have renewed interest in selecting appropriate rheological models for soft tissues such as the liver, thyroid, and prostate. The microchannel flow model was recently introduced to describe the linear response of tissue to stimuli such as stress relaxation or shear wave propagation. This model postulates a power law relaxation spectrum that results from a branching distribution of vessels and channels in normal soft tissue such as liver. In this work, the derivation is extended to determine the explicit link between the distribution of vessels and the relaxation spectrum. In addition, liver tissue is modified by temperature or salinity, and the resulting changes in tissue responses (by factors of 1.5 or greater) are reasonably predicted from the microchannel flow model, simply by considering the changes in fluid flow through the modified samples. The 2 and 4 parameter versions of the model are considered, and it is shown that in some cases the maximum time constant (corresponding to the minimum vessel diameters), could be altered in a way that has major impact on the observed tissue response. This could explain why an inflamed region is palpated as a harder bump compared to surrounding normal tissue.
Growth of Bubble layer and Onset of Flow Instability in a vertical Narrow rectangular channel
Energy Technology Data Exchange (ETDEWEB)
Lee, Juhyung; Chang, Soon Heung; Jeong, Yong Hoon [Korea Advanced Institute of Science and Technology, Daejeon (Korea, Republic of)
2015-05-15
Even numerous studies have been constantly conducted to date, however the prediction of OFI is still questionable for wide range of conditions especially for low mass flux condition in narrow rectangular channel as reported in the previous works. In addition, the understanding of subcooled flow boiling structures at OFI is not sufficient due to lack of studies with visualization. In this regards, OFI experiment for downward and upward flow boiling in a narrow rectangular channel are newly conducted while visualizing boiling structure. Image processing method is adopted to quantify bubble layer thickness, which is turned out to be important factor to understand the OFI. Experimental studies on OFI in a narrow rectangular channel having gap size of 2.35 mm was conducted not only for downward flow but also upward flow condition. Flow boiling structures are visualized using HSV method and also quantized bubble boundary layers are obtained by using image processing method. Based on observation and analysis, the merging of facing vapor layers on opposite boiling surfaces is the key phenomena triggering OFI for both upward and downward flow.
Experimental observation of crystalline particle flows in toroidal dust clouds
Energy Technology Data Exchange (ETDEWEB)
Wilms, Jochen, E-mail: wilms@physik.uni-kiel.de; Piel, Alexander [IEAP, Christian-Albrechts-Universität, D-24098 Kiel (Germany); Reichstein, Torben [IEAP, Christian-Albrechts-Universität, D-24098 Kiel (Germany); DME, Kiel University of Applied Sciences, Grenzstr. 3, D-24147 Kiel (Germany)
2015-06-15
The dust flow in a toroidal dust trap is studied experimentally. The flow is driven by the Hall component of the ion drag force in a magnetized plasma. Dust density waves are found in a torus with a large minor radius a, which allows for several wavelength, 2a>5λ, in the (mostly) radial direction of the ion flow. Beyond an intermediate state with radial sloshing oscillations, a crystalline dust flow with suppressed wave activity could be realized for 2a<2λ. The particles arrange themselves in distinct layers with hexagonal-like local order. Smooth transitions between states with different numbers of layers are found in the inhomogeneous flow.
An experimental study of the elastic theory for granular flows
Guo, Tongtong; Campbell, Charles S.
2016-08-01
This paper reports annular shear cell measurements granular flows with an eye towards experimentally confirming the flow regimes laid out in the elastic theory of granular flow. Tests were carried out on four different kinds of plastic spherical particles under both constant volume flows and constant applied stress flows. In particular, observations were made of the new regime in that model, the elastic-inertial regime, and the predicted transitions between the elastic-inertial and both the elastic-quasistatic and pure inertial regimes.
Institute of Scientific and Technical Information of China (English)
QIN Xiao-hong; JIA Lin; LI Ni
2010-01-01
In this paper, the theoretical analysis proves that the relationship between radius r of jet and the axial distance z from the onset of whipping instability follows an allometric law in the form r ∝ z-1/4 whatever surface charge parameter a is. Polyvinylalcohol (PVA) was used to study the effect of surface charge on the variation of jet diameter with axial coordinate after the onset of whipping instability during electrospinning by adding LiCI. The experiment shows that the relationship between radius r of jet and the axial distance z from the onset of whipping instability also follows the law in the form r∝ z-1/4 whenthe content of LiCl is from 0.2 wt% to 4 wt%. That is, the law does not depend upon the content of salt, and the theoretical prediction agrees quite well with the experimental data.
Khair, Aditya
2011-11-01
We revisit and build upon on the prototypical problem of ion transport across a flat ideal ion-selective surface. Specifically, we examine the influence of imposed fluid flows on concentration polarization (CP) and electrokinetic instability at over-limiting currents. We consider an ion-selective surface, or membrane, that admits a uniform flow across itself. The membrane contacts an electrolyte, whose concentration is uniform in a well-mixed region at a prescribed distance from the membrane. A voltage across the system drives an ionic current, leading to CP in the ``unstirred layer'' between the membrane and well-mixed bulk. The CP reflects a balance between advection of ions with the ``normal flow'' and diffusion. A Peclet number, Pe, parameterizes their relative importance; note, Pe is signed, as the flow can be toward or away from the membrane. An asymptotic analysis for thin Debye layers reveals a nonlinear CP profile, in contrast to the familiar linear profile at Pe=0. Next, we consider over-limiting currents, wherein a non-equilibrium space-charge layer emerges near the membrane surface. Finally, we examine the instability of the quiescent concentration polarization due to second-kind electro-osmosis in the space-charge layer. A stability analysis shows that the imposed normal flow can enhance or retard the instability, depending on its direction.
Saleem, H.; Ali Shan, S.; Haque, Q.
2016-11-01
It is shown that the inhomogeneous field-aligned flow of heavier ions into the stationary plasma of the upper ionosphere produces very low frequency (of the order of a few Hz) electrostatic unstable ion acoustic waves (IAWs). This instability is an oscillatory instability unlike D'Angelo's purely growing mode. The growth rate of the ion acoustic wave (IAW) corresponding to heavier ions is due to shear flow and is larger than the ion Landau damping. However, the ion acoustic waves corresponding to non-flowing lighter ions are Landau damped. It is found that even if D'Angelo's instability condition is satisfied, the unstable mode develops its real frequency in this coupled system. Hence, the shear flow of one type of ions in a bi-ion plasma system produces ion acoustic wave activity. If the density non-uniformity is taken into account, then the drift wave becomes unstable. The coupled nonlinear equations for stationary ions "a," flowing ions "b," and inertialess electrons are also solved using the small amplitude limit. The solutions predict the existence of the order of a few kilometers electric field structures in the form of solitons and vortices, which is in agreement with the satellite observations.
Institute of Scientific and Technical Information of China (English)
XUZenyu; KANGWeishan; PANChuanjie; DENHongyin; ZHANGYanxu
2003-01-01
In lost paper of magneto-hydrodynamic instability analysis of liquid metal free surface jet flow (Part Ⅰ), the magneto-hydrodynamic instability was analyzed for the jet flow in a transverse non-uniform magnetic field, BT. But, as all known, for the real conditions of liquid metal limiter-divertor plasma-facing components are in toroidal and poloidal fields. So, we try to analyze the magneto-hydrodynamic instability of the jet flow (hereby only for circular shape cross section jet flow) in the zone of non-uniform magnetic fields,
Spin-Up Instability of a Levitated Molten Drop in MHD-Flow Transition to Turbulence
Abedian, B.; Hyers, R. W.; Curreri, Peter A. (Technical Monitor)
2002-01-01
When an alternating magnetic field interacts with induced eddy currents in a conducting body, there will be a repulsive force between the body and the driving coil system generating the field. This repulsive force is the basis of electromagnetic levitation, which allows containerless processing of different materials. The eddy currents in the conducting body also generate Joule heating. Axial rotation of electromagnetically levitated objects is a common observation in levitation systems and often an undesirable side effect of such experiments on 1-g and -g. There have been recent efforts to use magnetic damping and suppress this tendency of body rotation. The first report of rotation in EML drops was attributed to a slight asymmetry of the shape and location of the levitation coils could change the axis and speed of rotation. Other theories of sample rotation include a frequency difference in the traveling electromagnetic waves and a phase difference in two different applied fields of the same frequency. All of these different mechanisms share the following characteristics: the torque is small, constant for constant field strength, and very weakly dependent on the sample's temperature and phase (solid or liquid). During experiments on the MSL-1 (First Microgravity Science Laboratory) mission of the Space Shuttle (STS-83 and STS-94, April and July 1997), a droplet of palladium-silicon alloy was electromagnetically levitated for viscosity measurements. For the non-deforming droplet, the resultant MHD flow inside the drop is inferred from motion of impurities on the surface. These observations indicate formation of a pair of co-rotating toroidal flow structures inside the spheroidal levitated drop that undergo secondary flow instabilities. As rise in the fluid temperature rises, the viscosity falls and the internal flow accelerates and becomes oscillatory; and beyond a point in the experiments, the surface impurities exhibit non-coherent chaotic motion signifying
Parametric and experimental analysis using a power flow approach
Cuschieri, J. M.
1990-01-01
A structural power flow approach for the analysis of structure-borne transmission of vibrations is used to analyze the influence of structural parameters on transmitted power. The parametric analysis is also performed using the Statistical Energy Analysis approach and the results are compared with those obtained using the power flow approach. The advantages of structural power flow analysis are demonstrated by comparing the type of results that are obtained by the two analytical methods. Also, to demonstrate that the power flow results represent a direct physical parameter that can be measured on a typical structure, an experimental study of structural power flow is presented. This experimental study presents results for an L shaped beam for which an available solution was already obtained. Various methods to measure vibrational power flow are compared to study their advantages and disadvantages.
Experimental study of rotating Hagen-Poiseuille flow discharging into a 1:8 sudden expansion
Miranda-Barea, A.; Martínez-Arias, B.; Parras, L.; Burgos, M. A.; del Pino, C.
2015-03-01
In this paper, we present experimental evidence for the five different states that result from rotating Hagen-Poiseuille flow when it discharges into a 1:8 sudden expansion, namely: stable, convectively unstable, unstable shear layer, stable and unstable vortex breakdowns. Sanmiguel-Rojas et al. ["Three-dimensional structure of confined swirling jets at moderately large Reynolds numbers," Phys. Fluids 20, 044104 (2008)] numerically predicted four of these five states and mapped the transition from one state to another. Our main objective is to study the onset of instabilities and vortex breakdown in these states experimentally. For this purpose, we visualize the flow at the inlet of the expansion for several values of moderately large Reynolds numbers, Re, and of swirl parameters, S. We analyze the inner region of the state that corresponds to the unstable shear layer in the sudden expansion and find two different states that share the same character, although they have different non-dimensional frequencies. The first relates to an oscillating structure near the axis, which arises at a small value of the swirl parameter, as well as to a generation of vortices that move downstream. The second shows, for greater values of the swirl parameter, vortices interacting with the boundary layer located on the wall of the static container that is perpendicular to the flow direction. In addition, we find a transition from stable to unstable vortex breakdown when perturbations become absolutely unstable inside the rotating pipe flow. Therefore, the most remarkable experimental finding is a new state, namely, unstable or transient vortex breakdown that takes place for the same pair of values (Rea, Sa) at which the onset of the absolute instability curve appears and intersects the region of stable vortex breakdown.
Directory of Open Access Journals (Sweden)
Bogdanović-Jovanović Jasmina B.
2012-01-01
Full Text Available Flow over a sphere is a typical bluff-body flow with many engineering applications. However, it has not been studied in depth, as compared to flow over a circular cylinder, because of the difficulties in the experimental set-up as well as in the computational approach for studying flow over a sphere. The main challenges are to understand the flow hydrodynamics and to clarify the flow pattern around a dimpled sphere because the flow pattern complying with the dimple structure on its surface is very complicated. In this paper experimental and numerical investigations of the fluid flow around a sphere with dimples, are represented. The sphere with dimples is placed in a quadratic cross section duct (measuring section and numerical simulation results are obtained by solving RANS equations. Furthermore, experimental measurements are carried out using a Laser-Doppler Anemometer (LDA. Experimental and numerical results of flow velocity fields were compared for three different flow regimes (Re=8×103, 2×104 and 4×104. Numerical investigation was performed for wide range of Reynolds numbers (Re=270%106. The final purpose of this paper is experimental and numerical determination of velocity field, separation point, pressure and drag coefficient, the length of reverse flow region in the wake and RANS turbulent model which gives the best results for engineering practice.
Experimental and numerical results on the fluid flow driven by a traveling magnetic field
Lantzsch, R.; Galindo, V.; Grants, I.; Zhang, C.; Pätzold, O.; Gerbeth, G.; Stelter, M.
2007-07-01
A traveling magnetic field (TMF) driven flow and its transition from a laminar to a time-dependent flow is studied by means of ultrasonic Doppler velocimetry and numerical simulations. The experimental setup comprises a cylindrical cavity containing the electrically conducting model fluid GaInSn and a system of six equidistant coils, which are fed by an out-of-phase current to create an up- or downward directed TMF. Hence, a Lorentz force is induced in the melt which leads to meridional flow patterns. For numerical simulations commercial codes (Opera/Fidap) and a spectral code are used. The characteristic parameters of the magnetohydrodynamic model system are chosen close to the conditions used for vertical gradient freeze (VGF) crystal growth. The axisymmetric basic flow and its dependence on the dimensionless shielding parameter S are examined. It is shown that, for S>10, the flow velocity decreases significantly, whereas almost no influence is found for a smaller shielding parameter. The critical Reynolds number for the onset of instability is found in the range of 300-450. Good agreement between experimental results and the numerical simulations is achieved.
Energy Technology Data Exchange (ETDEWEB)
Ham, J. van; Beer, R.J. van; Builtjes, P.J.H.; Roemer, M.G.M. [TNO Inst. of Environmental Sciences, Delft (Netherlands); Koennen, G.P. [KNMI, Royal Netherlands Meteorological Inst., de Bilt (Netherlands); Oerlemans, J. [Utrecht Univ. (Netherlands). Inst. for Meteorological and Atmospheric Research
1995-12-31
In this presentation part of an investigation is described into risks for climate change which are presently not adequately covered in General Circulation Models. In the concept of climate change as a result of the enhanced greenhouse effect it is generally assumed that the radiative forcings from increased concentrations of greenhouse gases (GHG) will result in a proportional or quasilinear global warming. Though correlations of this kind are known from palaeoclimate research, the variability of the climate seems to prevent the direct proof of a causal relation between recent greenhouse gas concentrations and temperature observations. In order to resolve the issue the use of General Circulation Models (GCMs), though still inadequate at present, is indispensable. Around the world some 10 leading GCMs exist which have been the subject of evaluation and intercomparison in a number of studies. Their results are regularly assessed in the IPCC process. A discussion on their performance in simulating present or past climates and the causes of their weak points shows that the depiction of clouds is a major weakness of GCMs. A second element which is virtually absent in GCMs are the feedbacks from natural biogeochemical cycles. These cycles are influenced by man in a number of ways. GCMs have a limited performance in simulating regional effects on climate. Moreover, albedo instability, in part due to its interaction with cloudiness, is only roughly represented. Apparently, not all relevant processes have been included in the GCMs. That situation constitutes a risk, since it cannot be ruled out that a missing process could cause or trigger a non-linear climate change. In the study non-linear climate change is connected with those processes which could provide feedbacks with a risk for non-monotonous or discontinuous behaviour of the climate system, or which are unpredictable or could cause rapid transitions
Numerical and Experimental Study of Pump Sump Flows
Directory of Open Access Journals (Sweden)
Wei-Liang Chuang
2014-01-01
Full Text Available The present study analyzes pump sump flows with various discharges and gate submergence. Investigations using a three-dimensional large eddy simulation model and an acoustic Doppler velocimeter are performed. Flow patterns and velocity profiles in the approaching flow are shown to describe the flow features caused by various discharges and gate submergence. The variation of a large-scale spanwise vortex behind a sluice gate is examined and discussed. The suction effect on approaching flow near the pipe column is examined using numerical modeling. To gain more understanding of the vortices variation, a comparison between time-averaged and instantaneous flow patterns is numerically conducted. Additionally, swirl angle, a widely used index for evaluating pump efficiency, is experimentally and numerically examined under various flow conditions. The results indicate that the pump becomes less efficient with increasing discharge and gate submergence. The fluctuation of the free surface over the pump sump is also discussed.
Mikhailenko, V. V.; Mikhailenko, V. S.; Lee, Hae June
2016-06-01
The temporal evolution of the kinetic ion temperature gradient driven instability and of the related anomalous transport of the ion thermal energy of plasma shear flow across the magnetic field is investigated analytically. This instability develops in a steady plasma due to the inverse ion Landau damping and has the growth rate of the order of the frequency when the ion temperature is equal to or above the electron temperature. The investigation is performed employing the non-modal methodology of the shearing modes which are the waves that have a static spatial structure in the frame of the background flow. The solution of the governing linear integral equation for the perturbed potential displays that the instability experiences the non-modal temporal evolution in the shearing flow during which the unstable perturbation becomes very different from a canonical modal form. It transforms into the non-modal structure with vanishing frequency and growth rate with time. The obtained solution of the nonlinear integral equation, which accounts for the random scattering of the angle of the ion gyro-motion due to the interaction of ions with ensemble of shearing waves, reveals similar but accelerated process of the transformations of the perturbations into the zero frequency structures. It was obtained that in the shear flow the anomalous ion thermal conductivity decays with time. It is a strictly non-modal effect, which originates from the temporal evolution of the shearing modes turbulence.
Experimental Studies on Turbulence Kinetic Energy in Confined Vortex Flows
Institute of Scientific and Technical Information of China (English)
L.Yan; G.H.Vatistas; 等
2000-01-01
Turbulence kinetic energies in confined vortex flows have been studied.The studies were based on the experiments performed in a vortex chamber,In the experiments,a Laser Doppler Anemometry(LDA) was used to perform flow measurements inside the vortex chamber,which provided the data for the kinetic energy analysis.The studies concentrated on the influences of the contraction ratio and the inlet air flow rate on the kinetic energy,and analyzed the characteristics of the kinetic energy in the confined vortex flows,including the distributions of the tangential component,radial component and total turbulence kinetic energy,In the paper,both the experimental techniques and the experimental results were presented.Based on a similarity analyis and the experimental data,an empirical scaling formula was proposed so that the tangential component of the turbulence kinetic energy was dependent only on the parameter of the contraction ratio.
Experimental measurements of the cavitating flow after horizontal water entry
Tat Nguyen, Thang; Hai, Duong Ngoc; Quang Thai, Nguyen; Phuong, Truong Thi
2017-10-01
Water-entry cavitating flow is of considerable importance in underwater high-speed applications. That is because of the drag-reduction effect that concerns the presence of a cavity around moving objects. Though the study of the flow has long been carried out, little data are documented in literature so far. Besides, currently, in the case of unsteady flow, experimental measurements of some flow parameters such as the cavity pressure still encounter difficulties. Hence continuing research efforts are of important significance. The objective of this study is to investigate experimentally the unsteady cavitating flow after the horizontal water entry of projectiles. An experimental apparatus has been developed. Qualitative and quantitative optical visualizations of the flow have been carried out by using high-speed videography. Digital image processing has been applied to analyzing the recorded flow images. Based on the known correlations between the ellipsoidal super-cavity’s size and the corresponding cavitation number, the cavity pressure has been measured by utilizing the data of image processing. A comparison between the partial- and super-cavitating flow regimes is reported. The received results can be useful for the design of high-speed underwater projectiles.
Experimental investigation on flow characteristics of deionized water in microtubes
Institute of Scientific and Technical Information of China (English)
XU ShaoLiang; YUE XiangAn; HOU JiRui
2007-01-01
The flow characteristics of deionized water in microtubes with diameters ranging from 2 to 30 μm are investigated. The experimental results show that the flow characteristics in microtubes with diameters of 16 μm and larger ones are in agreement with the classical theory. However, as the diameters are decreased to 5 and 2 μm, the nonlinear flow characteristics prevail and the results indicate significant departure of flow characteristics from the predictions of the conventional theory, and the smaller the diameters, the larger the departure. As the Reynolds number increases, the degree of nonlinear flow characteristics decrease gradually and the experimental results are approximately equal to the theoretical expectation. The minimum Reynolds number in this study is only 2.46×10-5.
Experimental analysis of the flow pattern of a pump turbine model in pump mode
Guggenberger, Mark; Senn, Florian; Jaberg, Helmut; Gehrer, Arno; Sallaberger, Manfred; Widmer, Christian
2016-11-01
Reversible pump turbines are the only means to store primary energy in an highly efficient way. Within a short time their operation can be switched between the different operational regimes thus enhancing the stabilization of the electric grid. These qualities in combination with the operation even at off-design conditions offer a high flexibility to the energy market. However, pump turbines pass through operational regimes where their behaviour becomes unstable. One of these effects occurs when the flowrate is decreased continuously down to a minimum. This point is the physical limitation of the pump operation and is very difficult to predict properly by numerical design without a model test. The purpose of the present study is to identify the fluid mechanical phenomena leading to the occurrence of instabilities of pump turbines in pump mode. A reduced scale model of a ANDRITZ pump turbine was installed on a 4-quadrant test rig for the experimental investigation of unstable conditions in pump mode. The performed measurements are based on the IEC60193-standard. Characteristic measurements at a single guide vane opening were carried out to get a detailed insight into the instabilities in pump mode. The interaction between runner and guide vane was analysed by Particle Image Velocimetry. Furthermore, high-speed visualizations of the suction side part load flow and the suction recirculation were performed. Like never before the flow pattern in the draft tube cone became visible with the help of a high-speed camera by intentionally caused cavitation effects which allow a qualitative view on the flow pattern in the draft tube cone. Suction recirculation is observed in form of single vortices separating from each runner blade and stretching into the draft tube against the main flow direction. To find an explanation for the flow phenomena responsible for the appearance of the unstable head curve also characteristic velocity distributions on the pressure side were combined
Monami as an oscillatory hydrodynamic instability in a submerged sea grass bed
Singh, Ravi; Bandi, M M; Mahadevan, Amala
2014-01-01
The onset of monami, the synchronous waving of sea grass beds driven by a steady flow, is modeled as a linear instability of the flow. Our model treats the drag exerted by the grass in establishing the steady flow profile, and in damping out perturbations to it. This damping leads to a finite threshold flow for the instability, which agrees with experimental observations. This role of vegetation drag differentiates our mechanism from the previous hypothesis that the Kelvin-Helmholtz instability underlies monami.
Energy Technology Data Exchange (ETDEWEB)
Gilmore, Mark Allen [Univ. of New Mexico, Albuquerque, NM (United States)
2017-02-05
Turbulence, and turbulence-driven transport are ubiquitous in magnetically confined plasmas, where there is an intimate relationship between turbulence, transport, instability driving mechanisms (such as gradients), plasma flows, and flow shear. Though many of the detailed physics of the interrelationship between turbulence, transport, drive mechanisms, and flow remain unclear, there have been many demonstrations that transport and/or turbulence can be suppressed or reduced via manipulations of plasma flow profiles. This is well known in magnetic fusion plasmas [e.g., high confinement mode (H-mode) and internal transport barriers (ITB’s)], and has also been demonstrated in laboratory plasmas. However, it may be that the levels of particle transport obtained in such cases [e.g. H-mode, ITB’s] are actually lower than is desirable for a practical fusion device. Ideally, one would be able to actively feedback control the turbulent transport, via manipulation of the flow profiles. The purpose of this research was to investigate the feasibility of using both advanced model-based control algorithms, as well as non-model-based algorithms, to control cross-field turbulence-driven particle transport through appropriate manipulation of radial plasma flow profiles. The University of New Mexico was responsible for the experimental portion of the project, while our collaborators at the University of Montana provided plasma transport modeling, and collaborators at Lehigh University developed and explored control methods.
Hall, P.
1985-01-01
The Taylor-Gortler vortex instability equations are formulated for steady and unsteady interacting boundary layer flows of the type which arise in triple-deck theory. The effective Gortler number is shown to be a function of the all shape in the boundary layer and the possibility of both steady and unsteady Taylor-Gortler modes exists. As an example the steady flow in a symmetrically constricted channel is considered and it is shown that unstable Gortler vortices exist before the boundary layers at the wall develop the Goldstein singularity. As an example of an unsteady spatially varying basic state the instability of high frequency large amplitude Tollmien-Schlichting waves in a curved channel were considered. It is shown that they are unstable in the first Stokes layer stage of the hierarchy of nonlinear states. The Tollmien-Schlichting waves are shown to be unstable in the presence of both convex and concave curvature.
Farrell, Brian F; Nikolaidis, Marios-Andreas
2016-01-01
Although the roll/streak structure is ubiquitous in pre-transitional wall-bounded shear flow, this structure is linearly stable if the idealization of laminar flow is made. Lacking an instability, the large transient growth of the roll/streak structure has been invoked to explain its appearance as resulting from chance occurrence in the free-stream turbulence (FST) of perturbations configured to optimally excite it. However, there is an alternative interpretation which is that FST interacts with the roll/streak structure to destabilize it. Statistical state dynamics (SSD) provides analysis methods for studying instabilities of this type which arise from interaction between the coherent and incoherent components of turbulence. Stochastic structural stability theory (S3T), which implements SSD in the form of a closure at second order, is used to analyze the SSD modes arising from interaction between the coherent streamwise invariant component and the incoherent FST component of turbulence. The least stable S3T ...
Numerical and experimental characterizations of automotive catalytic converter internal flows
Lai, M.-C.; Lee, T.; Kim, J.-Y.; Cheng, C.-Y.; Li, P.; Chui, G.
1992-07-01
The three-dimensional non-reacting flow field inside a typical dual-monolith automotive catalytic converter subject to different flow and structural conditions is studied numerically and experimentally. In the numerical analysis, the monolith brick resistance is formulated by using the pressure gradient of a fully developed laminar duct-flow and is corrected for the entrance effect. This correlation is found to agree with experimental pressure drop data and is introduced as an additional source term into the governing nondimensional momentum equation within the monolith brick. Simulation results show that the level of gas flow maldistribution in the monolith depends on the inlet flow Reynolds number, the brick resistance, and the inlet pipe length and its bending angles. The flow distribution is found to be more uniform inside a monolith brick with a lower inlet flow Reynolds number, a larger brick resistance, a shorter inlet pipe, and a straight inlet pipe instead of a bent one. Point-velocity measurements using laser Doppler velocimetry and smoke-flow visualization techniques at selected flow sections are also conducted to verify the simulation results.
The competition of convective and absolute instabilities in rotating-disk flow transition
Imayama, Shintaro; Alfredsson, P. Henrik; Lingwood, R. J.
2014-11-01
The main objective of this experimental study is to investigate laminar-turbulent transition mechanisms in the rotating-disk boundary-layer flow. Lingwood (1995) found that the flow becomes locally absolutely unstable above a critical Reynolds number and suggested that absolutely unstable travelling waves triggered nonlinearity leading to transition. However, the growth of convectively unstable stationary vortices is also a possible alternative route if the surface roughness of the disk is sufficiently large. The convectively unstable stationary vortices are attributed to an inviscid crossflow mechanism. Flow-visualization studies and hot-wire measurements of the rotating-disk boundary layer typically capture 28-32 stationary vortices in the transition regime (e.g. Imayama et al. 2014). The hot-wire measurements presented here were performed on a smooth glass disk with a diameter of 474 mm. To excite stationary vortices disk-shaped roughness elements with a diameter of 2 mm and a height of 5 micron were put on the disk at a radial position of 110 mm. In the presentation, the details of the convectively unstable stationary vortices in the rotating-disk boundary layer are shown and compared with travelling waves and similarities/differences in the turbulent transition discussed. This work is supported by the Swedish Research Council (VR) and the Linné FLOW Centre.
Hsu, S. C.; Bellan, P. M.
2003-01-01
The magnetohydrodynamic kink instability is observed and identified experimentally as a poloidal flux amplification mechanism for coaxial gun spheromak formation. Plasmas in this experiment fall into three distinct regimes which depend on the peak gun current to magnetic flux ratio, with (I) low values resulting in a straight plasma column with helical magnetic field, (II) intermediate values leading to kinking of the column axis, and (III) high values leading immediately to a detached plasma...
Hamlin, Nathaniel D; Newman, William I
2013-04-01
We explore, via analytical and numerical methods, the Kelvin-Helmholtz (KH) instability in relativistic magnetized plasmas, with applications to astrophysical jets. We solve the single-fluid relativistic magnetohydrodynamic (RMHD) equations in conservative form using a scheme which is fourth order in space and time. To recover the primitive RMHD variables, we use a highly accurate, rapidly convergent algorithm which improves upon such schemes as the Newton-Raphson method. Although the exact RMHD equations are marginally stable, numerical discretization renders them unstable. We include numerical viscosity to restore numerical stability. In relativistic flows, diffusion can lead to a mathematical anomaly associated with frame transformations. However, in our KH studies, we remain in the rest frame of the system, and therefore do not encounter this anomaly. We use a two-dimensional slab geometry with periodic boundary conditions in both directions. The initial unperturbed velocity peaks along the central axis and vanishes asymptotically at the transverse boundaries. Remaining unperturbed quantities are uniform, with a flow-aligned unperturbed magnetic field. The early evolution in the nonlinear regime corresponds to the formation of counter-rotating vortices, connected by filaments, which persist in the absence of a magnetic field. A magnetic field inhibits the vortices through a series of stages, namely, field amplification, vortex disruption, turbulent breakdown, and an approach to a flow-aligned equilibrium configuration. Similar stages have been discussed in MHD literature. We examine how and to what extent these stages manifest in RMHD for a set of representative field strengths. To characterize field strength, we define a relativistic extension of the Alfvénic Mach number M(A). We observe close complementarity between flow and magnetic field behavior. Weaker fields exhibit more vortex rotation, magnetic reconnection, jet broadening, and intermediate turbulence
Experimental Investigation of Flow Boiling in Parallel Mini-channels
Wu, Wan.; Zhang, M. T.; Zhang, X. B.; Xia, J. J.; Wen, S.-Z.; Wang, Z.-R.; He, Z.-H.; Huang, Z.-C.
2015-07-01
Flow boiling in micro-channels and mini-channels has received significant attention due to its capability for dissipating highflux heat, especially in the thermal management of high precision electronics. A heat sink with narrow rectangular mini-channels is designed to investigate flow boiling in the mini-channels, including the effect of gravity. It contains 14 parallel channels with a cross section, of 1×4mm 2, of which the hydraulic diameter is 1.6mm. The cooling capability, the temperature uniformity, and the temperature stability of the flow boiling in minichannels are investigated with R22, with total mass flow flux ranges from 35 to 70kg/m 2s. The results show that the cooling capability of the heat- sink is up to 340W(˜ 3.0W/cm 2), and the temperature difference is below 4 ∘C(even down to 2 ∘C) on the heat sink. The temperature uniformity isn't quite sensitive to heat flux. The instability has not been observed in the present researches.
Suslov, Sergey A; Bozhko, Alexandra A; Sidorov, Alexander S; Putin, Gennady F
2012-07-01
Flow patterns arising in a vertical differentially heated layer of nonconducting ferromagnetic fluid placed in an external uniform transverse magnetic field are studied experimentally and discussed from the point of view of the perturbation energy balance. A quantitative criterion for detecting the parametric point where the dominant role in generating a flow instability is transferred between the thermogravitational and thermomagnetic mechanisms is suggested, based on the disturbance energy balance analysis. A comprehensive experimental study of various flow patterns is undertaken, and the existence is demonstrated of oblique thermomagnetic waves theoretically predicted by Suslov [Phys. Fluids 20, 084101 (2008)] and superposed onto the stationary magnetoconvective pattern known previously. It is found that the wave number of the detected convection patterns depends sensitively on the temperature difference across the layer and on the applied magnetic field. In unsteady regimes its value varies periodically by a factor of almost 2, indicating the appearance of two different competing wave modes. The wave numbers and spatial orientation of the observed dominant flow patterns are found to be in good agreement with theoretical predictions.
Laminar flow in radial flow cell with small aspect ratios: Numerical and experimental study
DEFF Research Database (Denmark)
Detry, J. G.; Deroanne, C.; Sindic, M.
2009-01-01
distance from the center. The simulations provided a thorough description of the complex flow pattern encountered close to the inlet section, which were validated for the laminar regime by dye injection. A total of up to four recirculation zones were identified in both numerical and experimental...... investigations. The experimental positions of these recirculation zones corresponded well to the numerical predictions. Based on this work, a map of the flow for the different aspect ratios was developed, which can be particularly interesting for the design of experimental devices involving axisymmetrical flow....
Modeling of two-phase flow instabilities during startup transients utilizing RAMONA-4B methodology
Energy Technology Data Exchange (ETDEWEB)
Paniagua, J.; Rohatgi, U.S.; Prasad, V.
1996-10-01
RAMONA-4B code is currently under development for simulating thermal hydraulic instabilities that can occur in Boiling Water Reactors (BWRs) and the Simplified Boiling Water Reactor (SBWR). As one of the missions of RAMONA-4B is to simulate SBWR startup transients, where geysering or condensation-induced instability may be encountered, the code needs to be assessed for this application. This paper outlines the results of the assessments of the current version of RAMONA-4B and the modifications necessary for simulating the geysering or condensation-induced instability. The test selected for assessment are the geysering tests performed by Prof Aritomi (1993).
Experimental Study of Sediment Incipience Under Complex Flows
Institute of Scientific and Technical Information of China (English)
LIU Chunrong; DENG Liying; HUANG Zhenhua; HUHE Aode
2008-01-01
Sediment incipience under flows passing a backward-facing step was studied. A series of experiments were conducted to measure scouring depth, probability of sediment incipience, and instantaneous flow velocity field downstream of a backward-facing step. Instantaneous flow velocity fields were measured by using Particle Image Velocimetry (PIV), and an image processing method for determining probability of sediment incipience was employed to analyze the experimental data.The experimental results showed that the probability of sediment incipience was the highest near the reattachment point, even though the near-wall instantaneous flow velocity and the Reynolds stress were both much higher further downstream of the backward-facing step. The possible mechanisms are discussed for the sediment incipience near the reattachment point.
Experimental studies of occupation times in turbulent flows
DEFF Research Database (Denmark)
Mann, J.; Ott, Søren; Pécseli, H.L.;
2003-01-01
The motion of passively convected particles in turbulent flows is studied experimentally in approximately homogeneous and isotropic turbulent flows, generated in water by two moving grids. The simultaneous trajectories of many small passively convected, neutrally buoyant, polystyrene particles ar....... In the present formulation, the results of the analysis are relevant for understanding details in the feeding rate of micro-organisms in turbulent waters, for instance....
Poludnenko, Alexei
2016-11-01
Turbulent reacting flows are pervasive both in our daily lives on Earth and in the Universe. They power modern society being at the heart of many energy generation and propulsion systems, such as gas turbines, internal combustion and jet engines. On astronomical scales, thermonuclear turbulent flames are the driver of some of the most powerful explosions in the Universe, knows as Type Ia supernovae. Despite this ubiquity in Nature, turbulent reacting flows still pose a number of fundamental questions often exhibiting surprising and unexpected behavior. In this talk, we will discuss several such phenomena observed in direct numerical simulations of high-speed, premixed, turbulent flames. We show that turbulent flames in certain regimes are intrinsically unstable even in the absence of the surrounding combustor walls or obstacles, which can support the thermoacoustic feedback. Such instability can fundamentally change the structure and dynamics of the turbulent cascade, resulting in a significant (and anisotropic) redistribution of kinetic energy from small to large scales. In particular, three effects are observed. 1) The turbulent burning velocity can develop pulsations with significant peak-to-peak amplitudes. 2) Unstable burning can result in pressure build-up and the formation of pressure waves or shocks when the flame speed approaches or exceeds the speed of a Chapman-Jouguet deflagration. 3) Coupling of pressure and density gradients across the flame can lead to the anisotropic generation of turbulence inside the flame volume and flame acceleration. We extend our earlier analysis, which relied on a simplified single-step reaction model, by demonstrating existence of these effects in realistic chemical flames (hydrogen and methane) and in thermonuclear flames in degenerate, relativistic plasmas found in stellar interiors. Finally, we discuss the implications of these results for subgrid-scale LES combustion models. This work was supported by the Air Force
Experimental Investigation on Cavitating Flow Shedding over an Axisymmetric Blunt Body
Institute of Scientific and Technical Information of China (English)
HU Changli; WANG Guoyu; HUANG Biao
2015-01-01
Nowadays, most researchers focus on the cavity shedding mechanisms of unsteady cavitating flows over different objects, such as 2D/3D hydrofoils, venturi-type section, axisymmetric bodies with different headforms, and so on. But few of them pay attention to the differences of cavity shedding modality under different cavitation numbers in unsteady cavitating flows over the same object. In the present study, two kinds of shedding patterns are investigated experimentally. A high speed camera system is used to observe the cavitating flows over an axisymmetric blunt body and the velocity fields are measured by a particle image velocimetry (PIV) technique in a water tunnel for different cavitation conditions. The U-type cavitating vortex shedding is observed in unsteady cavitating flows. When the cavitation number is 0.7, there is a large scale cavity rolling up and shedding, which cause the instability and dramatic fluctuation of the flows, while at cavitation number of 0.6, the detached cavities can be conjunct with the attached part to induce the break-off behavior again at the tail of the attached cavity, as a result, the final shedding is in the form of small scale cavity and keeps a relatively steady flow field. It is also found that the interaction between the re-entrant flow and the attached cavity plays an important role in the unsteady cavity shedding modality. When the attached cavity scale is insufficient to overcome the re-entrant flow, it deserves the large cavity rolling up and shedding just as that at cavitation number of 0.7. Otherwise, the re-entrant flow is defeated by large enough cavity to induce the cavity-combined process and small scale cavity vortexes shedding just as that of the cavitation number of 0.6. This research shows the details of two different cavity shedding modalities which is worthful and meaningful for the further study of unsteady cavitation.
Experimental study on fluid flow in arciform clearance
Institute of Scientific and Technical Information of China (English)
邵俊鹏; 汤卉; 贾慧娟
2002-01-01
The system damping and dynamic characteristics can be further improved by properly increasing thedamping coefficient ξh. For a special hydraulic damping structure, an arciform damping clearance often used inFCS, a mathematical model has been established for fluid flow using the theory of laminar flow in the clearanceof parallel plates. Analytical calculations are made for fluid flow in the arciform clearance and relational expres-sion is deduced for flow rate along the arciform cleaance height, pressure difference, maximum arciform clear-ance height, the flow rate for the fluid flow in arciform clearance as well, and its simplified formula is obtainedby using the theory of hydrodynamics and the curve - fitting method. This paper consists of two sections: the firstsection focuses on the theoretical analysis by using the simplified mathematical model and the second sectionmainly describes experimental analysis. The simplified formula is corrected with experimental results by consid-erig various boundary conditions of the damping clearance. Experimental results show that this study of arciformdamping clearance is reliable and practical.
Experimental evaluation of numerical simulation of cavitating flow around hydrofoil
Energy Technology Data Exchange (ETDEWEB)
Dular, M.; Bachert, R.; Stoffel, B. [Darmstadt Univ. of Technology, Lab. for Turbomachinery and Fluid Power (Germany); Sirok, B. [Ljubljana Univ., Lab. for Water and Turbine Machines (Slovenia)
2005-08-01
Cavitation in hydraulic machines causes different problems that can be related to its unsteady nature. An experimental and numerical study of developed cavitating flow was performed. Until now simulations of cavitating flow were limited to the self developed 'in house' CFD codes. The goal of the work was to experimentally evaluate the capabilities of a commercial CFD code (Fluent) for simulation of a developed cavitating flow. Two simple hydrofoils that feature some 3D effects of cavitation were used for the experiments. A relatively new technique where PIV method combined with LIF technique was used to experimentally determine the instantaneous and average velocity and void ratio fields (cavity shapes) around the hydrofoils. Distribution of static pressure on the hydrofoil surface was determined. For the numerical simulation of cavitating flow a bubble dynamics cavitation model was used to describe the generation and evaporation of vapour phase. An unsteady RANS 3D simulation was performed. Comparison between numerical and experimental results shows good correlation. The distribution and size of vapour structures and the velocity fields agree well. The distribution of pressure on the hydrofoil surface is correctly predicted. The numerically predicted shedding frequencies are in fair agreement with the experimental data. (authors)
Machado, Anaïs; Bodiguel, Hugues; Beaumont, Julien; Clisson, Gérald; Colin, Annie
2016-01-01
We study flows of hydrolized polyacrylamide solutions in two dimensional porous media made using microfluidics, for which elastic effects are dominant. We focus on semi-dilute solutions (0.1%–0.4%) which exhibit a strong shear thinning behavior. We systematically measure the pressure drop and find that the effective permeability is dramatically higher than predicted when the Weissenberg number is greater than about 10. Observations of the streamlines of the flow reveal that this effect coincides with the onset of elastic instabilities. Moreover, and importantly for applications, we show using local measurements that the mean flow is modified: it appears to be more uniform at high Weissenberg number than for Newtonian fluids. These observations are compared and discussed using pore network simulations, which account for the effect of disorder and shear thinning on the flow properties. PMID:27478522
Experimental overview on flow observables in heavy ion collisions
Mohapatra, Soumya
2016-01-01
This paper summarizes the experimental results on flow phenomena that were presented at Quark matter 2015, with a focus on new flow observables and correlations in small systems. The results presented include event-shape selected pT spectra and vn measurements, correlations between flow harmonics of different orders, study of factorization breakdown in two-particle correlations, and principal component analysis of two-particle correlations. Recent developments in investigation of collective effects in small collisions systems, namely, p+A, d+A and 3He + A as well as in pp collisions are also presented.
Experimental investigation on a high subsonic compressor cascade flow
Directory of Open Access Journals (Sweden)
Zhang Haideng
2015-08-01
Full Text Available With the aim of deepening the understanding of high-speed compressor cascade flow, this paper reports an experimental study on NACA-65 K48 compressor cascade with high subsonic inlet flow. With the increase of passage pressurizing ability, endwall boundary layer behavior is deteriorated, and the transition zone is extended from suction surface to the endwall as the adverse pressure gradient increases. Cross flow from endwall to midspan, mixing of corner boundary layer and the main stream, and reversal flow on the suction surface are caused by corner separation vortex structures. Passage vortex is the main corner separation vortex. During its movement downstream, the size grows bigger while the rotating direction changes, forming a limiting circle. With higher incidence, corner separation is further deteriorated, leading to higher flow loss. Meanwhile, corner separation structure, flow mixing characteristics and flow loss distribution vary a lot with the change of incidence. Compared with low aspect-ratio model, corner separation of high aspect-ratio model moves away from the endwall and is more sufficiently developed downstream the cascade. Results obtained present details of high-speed compressor cascade flow, which is rare in the relating research fields and is beneficial to mechanism analysis, aerodynamic optimization and flow control design.
Experimental study on visualization of the flow field in microtube
Institute of Scientific and Technical Information of China (English)
LIU Zhigang; ZHAO Yaohua
2005-01-01
An experimental study was conducted to visualize the flow field and confirm the transitional Reynolds number from laminar to turbulent flow, as distilled water flows through quartz glass microtubes with inner diameter 315 and 520 μm. With gentian violet as colorant, the flow field pictures in the microtube, and therefore, is shot by a CCD camera with a microscope at different Reynolds numbers. Pressure drop data were also used to characterize the friction factor for those microtubes in the Reynolds number range of 200―2300. The experimental results clearly showed that the flow in the microtube was the laminar state and the friction factors agreed well with the Poiseuille equations when the Reynolds number was low. As the Reynolds number was larger than 1200 and 1500 for the microtube with inner diameter 315 and 520 μm, respectively, the friction factor departed from the classical laminar solution due to the earlier transition from laminar to turbulent flow. The flow turned into full turbulent when the Reynolds number reached 1500―1800.
Continuous, pulsed and stopped flow in a u-flow injection system (numerical vs experimental)
van Akker, E.B.; Bos, M.; van der Linden, W.E.
1999-01-01
The effects of continuous, pulsed and stopped flow on the dispersion of a sample injected into a μ-flow injection system were studied. A channel with a volume of 1 μl was used to compare experimental results with numerical results. The injection was 0.067 μl of bromocresolgreen solution into a borax
Continuous, pulsed and stopped flow in a μ-flow injection system (numerical vs experimental)
Akker, van E.B.; Bos, M.; Linden, van der W.E.
1999-01-01
The effects of continuous, pulsed and stopped flow on the dispersion of a sample injected into a μ-flow injection system were studied. A channel with a volume of 1 μl was used to compare experimental results with numerical results. The injection was 0.067 μl of bromocresolgreen solution into a borax
Flow Field and Performance of Cross Flow Fans--Experimental and Theoretical Investigations
Institute of Scientific and Technical Information of China (English)
Martin Gabi; Simon Dornstetter; Toni Klemm
2003-01-01
Due to the construction and the operating principle the prediction of performance of Cross Flow Fans (CFF) is difficult and the knowledge about the internal flow regime is limited. To investigate the impact of geometrical parameters on the performance of CFF, experimental investigations, using Particle Imaging Velocimetry (PIV),and CFD calculations were carried out. Some results of PIV measurements and CFD calculations are presented,which give an impression of the internal flow and confirm the numerical calculations.
Particle image velocimetry measurement of an instability wave over a porous wall in a duct with flow
Alomar, Antoni; Aurégan, Yves
2017-01-01
The flow in a rectangular channel lined with a porous material and acoustically excited with an upstream loudspeaker has been investigated using particle image velocimetry. The measurements are phase-locked to the loudspeaker signal so that the phase-averaged velocity in the lined section is obtained during an excitation period. Most features of the phase-averaged velocity field in the lined section are found to be well described from the sum of three single duct modes: the hydrodynamic instability wave, a standing wave and an acoustic wave. The hydrodynamic instability wave travels at half the mean flow velocity, and its structure shows differences to the case of a locally reacting liner. The relative phase lag between the hydrodynamic and acoustic waves at the liner end dictates the interference between both waves, giving rise to the oscillations of the acoustical transmission coefficient as a function of the frequency. A detachment of the instability wave from the porous wall is observed in the vicinity of the liner downstream edge, together with the separation of the mean vorticity core.
Review of two-phase instabilities
Energy Technology Data Exchange (ETDEWEB)
Kang, Han Ok; Seo, Han Ok; Kang, Hyung Suk; Cho, Bong Hyun; Lee, Doo Jeong
1997-06-01
KAERI is carrying out a development of the design for a new type of integral reactors. The once-through helical steam generator is important design features. The study on designs and operating conditions which prevent flow instability should precede the introduction of one-through steam generator. Experiments are currently scheduled to understand two-phase instability, evaluate the effect of each design parameter on the critical point, and determine proper inlet throttling for the prevention of instability. This report covers general two-phase instability with review of existing studies on this topics. The general classification of two phase flow instability and the characteristics of each type of instability are first described. Special attention is paid to BWR core flow instability and once-through steam generator instability. The reactivity feedback and the effect of system parameters are treated mainly for BWR. With relation to once-through steam generators, the characteristics of convective heating and dryout point oscillation are first investigated and then the existing experimental studies are summarized. Finally chapter summarized the proposed correlations for instability boundary conditions. (author). 231 refs., 5 tabs., 47 figs
Experimental and mathematical modeling of flow in headboxes
Shariati, Mohammad Reza
The fluid flow patterns in a paper-machine headbox have a strong influence on the quality of the paper produced by the machine. Due to increasing demand for high quality paper there is a need to investigate the details of the fluid flow in the paper machine headbox. The objective of this thesis is to use experimental and computational methods of modeling the flow inside a typical headbox in order to evaluate and understand the mean flow patterns and turbulence created there. In particular, spatial variations of the mean flow and of the turbulence quantities and the turbulence generated secondary flows are studied. In addition to the flow inside the headbox, the flow leaving the slice is also modeled both experimentally and computationally. Comparison of the experimental and numerical results indicated that streamwise mean components of the velocities in the headbox are predicted well by all the turbulence models considered in this study. However, the standard k-epsilon model and the algebraic turbulence models fail to predict the turbulence quantities accurately. Standard k-epsilon-model also fails to predict the direction and magnitude of the secondary flows. Significant improvements in the k-epsilon model predictions were achieved when the turbulence production term was artificially set to zero. This is justified by observations of the turbulent velocities from the experiments and by a consideration of the form of the kinetic energy equation. A better estimation of the Reynolds normal stress distribution and the degree of anisotropy of turbulence was achieved using the Reynolds stress turbulence model. Careful examination of the measured turbulence velocity results shows that after the initial decay of the turbulence in the headbox, there is a short region close to the exit, but inside the headbox, where the turbulent kinetic energy actually increases as a result of the distortion imposed by the contraction. The turbulence energy quickly resumes its decay in the
Sironi, Lorenzo
2014-01-01
In systems accreting well below the Eddington rate, the plasma in the innermost regions of the disk is collisionless and two-temperature, with the ions hotter than the electrons. Yet, whether a collisionless faster-than-Coulomb energy transfer mechanism exists in two-temperature accretion flows is still an open question. We study the physics of electron heating during the growth of ion velocity-space instabilities, by means of multi-dimensional particle-in-cell (PIC) simulations. A large-scale compression - embedded in a novel form of the PIC equations - continuously amplifies the field. This constantly drives a pressure anisotropy P_perp > P_parallel, due to the adiabatic invariance of the particle magnetic moments. We find that, for ion plasma beta values beta_i ~ 5-30 appropriate for the midplane of low-luminosity accretion flows, mirror modes dominate if the electron-to-proton temperature ratio is > 0.2, whereas if it is m_e/m_i - governed by the conservation of the magnetic moment in the growing fields ...
Numerical and Experimental Study of Electromagnetically Driven Vortical Flows
Kenjeres, S.; Verdoold, J.; Tummers, M.J.; Hanjalic, K.; Kleijn, C.R.
2009-01-01
The paper reports on numerical and experimental investigations of electromagnetically driven vortical flows of an electrically conductive fluid in a generic setup. Two different configurations of permanent magnets are considered: a 3-magnet configuration in which the resulting Lorentz force is focus
Renal blood flow in experimental septic acute renal failure
Langenberg, C.; Wan, L.; Egi, M.; May, C. N.; Bellomo, R.
2006-01-01
Reduced renal blood flow (RBF) is considered central to the pathogenesis of septic acute renal failure (ARF). However, no controlled experimental studies have continuously assessed RBF during the development of severe septic ARF. We conducted a sequential animal study in seven female Merino sheep. F
Experimental Investigation of the Wind Turbine Blade Root Flow
Akay, B.; Ferreira, C.S.; Van Bussel, G.J.W.
2010-01-01
Several methods from experimental to analytical are used to investigate the aerodynamics of a horizontal axis wind turbine. To understand 3D and rotational effects at the root region of a wind turbine blade, correct modeling of the flow field is essential. Aerodynamic models need to be validated by
Lehe, Remi; Kirchen, Manuel; Godfrey, Brendan B.; Maier, Andreas R.; Vay, Jean-Luc
2016-11-01
Particle-in-cell (PIC) simulations of relativistic flowing plasmas are of key interest to several fields of physics (including, e.g., laser-wakefield acceleration, when viewed in a Lorentz-boosted frame) but remain sometimes infeasible due to the well-known numerical Cherenkov instability (NCI). In this article, we show that, for a plasma drifting at a uniform relativistic velocity, the NCI can be eliminated by simply integrating the PIC equations in Galilean coordinates that follow the plasma (also sometimes known as comoving coordinates) within a spectral analytical framework. The elimination of the NCI is verified empirically and confirmed by a theoretical analysis of the instability. Moreover, it is shown that this method is applicable both to Cartesian geometry and to cylindrical geometry with azimuthal Fourier decomposition.
Lehe, Remi; Godfrey, Brendan B; Maier, Andreas R; Vay, Jean-Luc
2016-01-01
Particle-In-Cell (PIC) simulations of relativistic flowing plasmas are of key interest to several fields of physics (including e.g. laser-wakefield acceleration, when viewed in a Lorentz-boosted frame), but remain sometimes infeasible due to the well-known numerical Cherenkov instability (NCI). In this article, we show that, for a plasma drifting at a uniform relativistic velocity, the NCI can be eliminated by simply integrating the PIC equations in Galilean coordinates that follow the plasma (also sometimes known as comoving coordinates) within a spectral analytical framework. The elimination of the NCI is verified empirically and confirmed by a theoretical analysis of the instability. Moreover, it is shown that this method is applicable both to Cartesian geometry and to cylindrical geometry with azimuthal Fourier decomposition.
Experimental and theoretical study of metal combustion in oxygen flows
El-Rabii, Hazem; Muller, Maryse
2016-01-01
The effects of oxygen flow speed and pressure on the iron and mild steel combustion are investigated experimentally and theoretically. The studied specimens are vertical cylindrical rods subjected to an axial oxygen flow and ignited at the upper end by laser irradiation. Three main stages of the combustion process have been identified experimentally: (1) Induction period, during which the rod is heated until an intensive metal oxidation begins at its upper end; (2) Static combustion, during which a laminar liquid "cap" slowly grows on the upper rod end; and, after the liquid cap detachment from the sample, (3) Dynamic combustion, which is characterized by a rapid metal consumption and turbulent liquid motions. An analytical description of these stages is given. In particular, a model of the dynamic combustion is constructed based on the turbulent oxygen transport through the liquid metal-oxide flow. This model yields a simple expression for the fraction of metal burned in the process, and allows one to calcul...
Horsthemke, W.; Hannon, L.
1984-11-01
We present a stochastic model for stirred chemical reactors. In the limiting case of practical interest, i.e., fast stirring, we solve for the characteristic function in steady state and derive expressions for the stationary moments through a perturbation expansion. Moments are explicitly calculated for a generic model of bistable behavior. We find that stirring decreases the area of the bistable region essentially by changing the point of transition from the high reaction rate state to the low reaction rate state. This is in remarkable agreement with the experimental findings of Roux, et al. Our results indicate that stirring should not be considered simply as an ``enhanced diffusion'' process and that nucleation plays only a minor role in transitions between multiple steady states in a continuous flow stirred tank reactor (CSTR).
Sironi, Lorenzo; Narayan, Ramesh
2015-02-01
In systems accreting well below the Eddington rate, such as the central black hole in the Milky Way (Sgr A*), the plasma in the innermost regions of the disk is believed to be collisionless and have two temperatures, with the ions substantially hotter than the electrons. However, whether a collisionless faster-than-Coulomb energy transfer mechanism exists in two-temperature accretion flows is still an open question. We study the physics of electron heating during the growth of ion velocity-space instabilities by means of multidimensional, fully kinetic, particle-in-cell (PIC) simulations. A background large-scale compression—embedded in a novel form of the PIC equations—continuously amplifies the field. This constantly drives a pressure anisotropy P > P ∥ because of the adiabatic invariance of the particle magnetic moments. We find that, for ion plasma beta values β0i ~ 5-30 appropriate for the midplane of low-luminosity accretion flows (here, β0i is the ratio of ion thermal pressure to magnetic pressure), mirror modes dominate if the electron-to-proton temperature ratio is T 0e /T 0i >~ 0.2, whereas for T 0e /T 0i ~ 2 me /mi —governed by the conservation of the particle magnetic moment in the growing fields of the instability—is proportional to the initial electron temperature, and it scales with the magnetic energy of ion cyclotron waves. Our results have implications for two-temperature accretion flows as well as for solar wind and intracluster plasmas.
Experimental investigation of axially aligned flow past spinning cylinders
Carlucci, Pasquale; Buckley, Liam; Mehmedagic, Igbal; Carlucci, Donald; Thangam, Siva
2016-11-01
Experimental and numerical results of ongoing subsonic investigations of the flow field about axially aligned spinning cylinders with variable inter-cylinder spacing are presented. The experimental design is capable of investigating wake dynamics of the modeled system up to a Reynolds Number of 300,000 and rotation numbers up to 2. The experimental results are used to validate and confirm numerical simulations with and without the effects of swirl. The focus of the overall effort is an understanding of the dynamics of multi-body problems in a flow field, as such we relate the ongoing effort to previous studies by both the authors and the community at large and our ongoing work in developing accurate plant models for use in engineering analysis and design. Funded in part by U. S. Army ARDEC, Picatinny Arsenal, NJ.
Experimental study of periodic flow effects on spanwise vortex
Garcia Molina, Cruz Daniel; Lopez Sanchez, Erick Javier; Ruiz Chavarria, Gerardo; Medina Ovando, Abraham
2014-11-01
We present an experimental study about the spanwise vortex produced in a flow going out of a channel in shallow waters. This vortex travels in front of the dipole. The velocity field measurement was done using the PIV technique, and DPIVsoft (https://www.irphe.fr/ ~meunier/) was used for data processing. In this case the flow has a periodic forcing to simulate ocean tides. The experiment was conducted in a channel with variable width and the measurements were made using three different values of the aspect ratio width-depth. We present results of the position, circulation of this spanwise vortex and the flow inversion effect. The change of flow direction modify the intensity of the vortex, but it does not destroy it. The vertical components of the velocity field contributes particle transport. G. Ruiz Chavarria, E. J. Lopez Sanchez and C. D. Garcia Molina acknowledge DGAPA-UNAM by support under project IN 116312 (Vorticidad y ondas no lineales en fluidos).
Experimental and Numerical Analysis of the Bulk Flow Parameters Within an Arteriovenous Fistula.
Browne, Leonard D; Walsh, Michael T; Griffin, Philip
2015-12-01
The creation of an arteriovenous fistula for hemodialysis has been reported to generate unstable to turbulent flow behaviour. On the other hand, the vast majority of computational fluid dynamic studies of an arteriovenous fistula use low spatial and temporal resolutions resolution in conjunction with laminar assumptions to investigate bulk flow and near wall parameters. The objective of the present study is to investigate if adequately resolved CFD can capture instabilities within an arteriovenous fistula. An experimental model of a representative fistula was created and the pressure distribution within the model was analysed for steady inlet conditions. Temporal CFD simulations with steady inflow conditions were computed for comparison. Following this verification a pulsatile simulation was employed to assess the role of pulsatility on bulk flow parameters. High frequency fluctuations beyond 100 Hz were found to occupy the venous segment of the arteriovenous fistula under pulsatile conditions and the flow within the venous segment exhibited unstable behaviour under both steady and pulsatile inlet conditions. The presence of high frequency fluctuations may be overlooked unless adequate spatial and temporal resolutions are employed. These fluctuations may impact endothelial cell function and contribute to the cascade of events leading to aggressive intimal hyperplasia and the loss of functionality of the vascular access.
Cybersickness without the wobble: Experimental results speak against postural instability theory.
Dennison, Mark Stephen; D'Zmura, Michael
2017-01-01
It has been suggested that postural instability is necessary for cybersickness to occur. Seated and standing subjects used a head-mounted display to view a virtual tunnel that rotated about their line of sight. We found that the offset direction of perceived vertical settings matched the direction of the tunnel's rotation, so replicating earlier findings. Increasing rotation speed caused cybersickness to increase, but had no significant impact on perceived vertical settings. Postural sway during rotation was similar to postural sway during rest. While a minority of subjects exhibited postural sway in response to the onset of tunnel rotation, the majority did not. Furthermore, cybersickness increased with rotation speed similarly for the seated and standing conditions. Finally, subjects with greater levels of cybersickness exhibited less variation in postural sway. These results lead us to conclude that the link between postural instability and cybersickness is a weak one in the present experiment.
Pettigrew, M. J.; Taylor, C. E.
2003-11-01
Design guidelines were developed to prevent tube failures due to excessive flow-induced vibration in shell-and-tube heat exchangers. An overview of vibration analysis procedures and recommended design guidelines is presented in this paper. This paper pertains to liquid, gas and two-phase heat exchangers such as nuclear steam generators, reboilers, coolers, service water heat exchangers, condensers, and moisture-separator-reheaters. Generally, a heat exchanger vibration analysis consists of the following steps: (i) flow distribution calculations, (ii) dynamic parameter evaluation (i.e. damping, effective tube mass, and dynamic stiffness), (iii) formulation of vibration excitation mechanisms, (iv) vibration response prediction, and (v) resulting damage assessment (i.e., comparison against allowables). The requirements applicable to each step are outlined in this paper. Part 1 of this paper covers flow calculations, dynamic parameters and fluidelastic instability.
Sironi, Lorenzo
2014-01-01
In the innermost regions of low-luminosity accretion flows, including Sgr A* at the center of our Galaxy, the frequency of Coulomb collisions is so low that the plasma is two-temperature, with the ions substantially hotter than the electrons. This paradigm assumes that Coulomb collisions are the only channel for transferring the ion energy to the electrons. In this work, the second of a series, we assess the efficiency of electron heating by ion velocity-space instabilities in collisionless accretion flows. The instabilities are seeded by the pressure anisotropy induced by magnetic field amplification, coupled to the adiabatic invariance of the particle magnetic moments. Using two-dimensional (2D) particle-in-cell (PIC) simulations, we showed in Paper I that if the electron-to-ion temperature ratio is < 0.2, the ion cyclotron instability is the dominant mode for values of ion beta_i ~ 5-30 (here, beta_i is the ratio of ion thermal pressure to magnetic pressure), as appropriate for the midplane of low-lumin...
Analysis of the flow instability among channels of the OTSG in the naval craft NPP
Energy Technology Data Exchange (ETDEWEB)
Hou, Su-xia; Luo, Ji-jun; Xu, Jun; Liu, Jie-yu [Xi' an Hi-Tech Institute, Shaanxi (China)
2014-11-15
The instability occurring of the OTSG (Once-Through Steam Generator) in naval craft nuclear power plants is presented by the multivariable frequency domain theory. As concerning coupling interactions of the OTSG tubing, it is more accurate for analyzing the instability of OTSG compared to the common single variable method. A mathematical model for the system is derived from the fundamental equations by using the perturbation, Laplace-transform and the nodalization techniques. The stable boundary and parameters which influence the stability of the system are evaluated through computer simulation.
Structural alterations of the bladder induced by detrusor instability: experimental study in rabbits
Directory of Open Access Journals (Sweden)
Joao L. Amaro
2005-12-01
Full Text Available OBJECTIVES: The aim of this study was to evaluate the histopathological and immunohistochemical alterations induced by detrusor instability in the bladder of rabbits submitted to partial bladder outlet obstruction. MATERIALS AND METHODS: Thirty male Norfolk rabbits were divided into 2 groups, a clinical control and a group with detrusor instability. Urine culture, cystometric study, histopathological and immunohistochemical analysis were performed in all animals prior to surgery (M1 and 4 weeks after-surgery (M2. RESULTS: Partial obstruction (G2 resulted in a 2.5 fold increment (p < 0.05 in bladder weight when compared to control (G1. Four weeks after surgery, 93% of animals in G2 developed cystitis. Partial obstruction resulted in detrusor instability at M2 and bladder capacity was significantly increased (p < 0.05 from M1 to M2. The incidence of mild to moderate mucosal and adventitious fibrosis at M2 was higher in G2 (p < 0.05 when compared to G1. Inflammatory reaction at M2 was statistically higher (p < 0.05 in G2. There was no difference in muscular hypertrophy between M1 and M2 in G1. However, 67% of G2 bladders showed a moderate to intense muscular hypertrophy at M2. Hyperplasia of the epithelium was also increased in G2 when M1 and M2 were compared (p < 0.05. CONCLUSION: Detrusor instability induced by partial bladder outlet obstruction caused significant histopathological and immunohistochemical alterations in the bladder of rabbits.
Takala, E; Bremer, J; Balle, C; Bottura, L; Rossi, L
2012-01-01
Magneto-thermal instability may affect high critical current density Nb3Sn superconducting strands that can quench even though the transport current is low compared to the critical current with important implications in the design of next generation superconducting magnets. The instability is initiated by a small perturbation energy which is considerably lower than the Minimum Quench Energy (MQE). At CERN, a new experimental setup was developed to measure the smallest perturbation energy (Minimum Trigger Energy, MTE) which is able to trigger the magneto-thermal instability in superconducting Nb3Sn-strands. The setup is based on Q-switched laser technology which is able to provide a localized perturbation in nano-second time scale. Using this technique the energy deposition into the strand is well defined and reliable. The laser is located outside the cryostat at room temperature. The beam is guided from room temperature on to the superconducting strand by using a UV-enhanced fused silica fibre. The strand is ...
Steiner, Adam; Yager-Elorriaga, David; Patel, Sonal; Jordan, Nicholas; Gilgenbach, Ronald; Lau, Y. Y.
2015-11-01
The electrothermal instability (ETI) and magneto-Rayleigh Taylor instability (MRT) are important in the implosion of metallic liners, such as magnetized liner implosion fusion (MagLIF). The MAIZE linear transformer driver (LTD) at the University of Michigan generates 200 ns risetime-current pulses of 500 to 600 kA into Al foil liners to study plasma instabilities and implosion dynamics, most recently MRT growth on imploding cylindrical liners. A full circuit model of MAIZE, along with I-V measurements, yields time-resolved load inductance. This has enabled measurements of an effective current-carrying radius to determine implosion velocity and plasma-vacuum interface acceleration. Measurements are also compared to implosion data from 4-time-frame laser shadowgraphy. Improved resolution measurements on the laser shadowgraph system have been used to examine the liner interface early in the shot to examine surface perturbations resulting from ETI for various seeding conditions. Fourier analysis examines the growth rates of wavelength bands of these structures to examine the transition from ETI to MRT. This work was supported by the U.S. DoE through award DE-SC0012328. S.G. Patel is supported by Sandia National Labs. D.A. Yager is supported by NSF fellowship grant DGE 1256260.
EXPERIMENTAL STUDY ON EQUILIBRIUM CONCENTRATION OF DEBRIS FLOWS
Institute of Scientific and Technical Information of China (English)
Bin YU
2001-01-01
The paper presents experimental study of debris flows. The equilibrium concentration of solid particle in the flow is a function of the energy slope, density of solid particle and kinetic friction angle of particles. The kinetic friction angle is a function of internal friction angle, the concentration of solid particles and the maximum possible concentration. To determine the function between the kinetic friction angle and internal friction angle is the aim of this research. Flume experiments of equilibrium concentration about particles in water and slurry were conducted. The large density slurry made the coarse particles be able to move in small slope. The function between the kinetic friction angle and internal friction angle was found from these experiments. The coarse particles and fine particles are well mixed. D50 demarcation line was suggested in this paper to demarcate the coarse particle and fine particle of debris flows. The equilibrium concentration of debris flows was calculated by using Ds0 demarcation for the debris flows in field. The equilibrium concentration of debris flows calculated by the function between the kinetic friction angle and internal friction angle was close to the equilibrium concentration data of debris flows in field.
Experimental study of two phase flow in inclined channel
Energy Technology Data Exchange (ETDEWEB)
Park, Goon Cherl; Lee, Tae Ho; Lee, Sang Won [Seoul National University, Seoul (Korea, Republic of)
1997-07-01
Local two-phase flow parameters were measured to investigate the internal flow structures of steam-water boiling flow in an inclined channel. The vapor phase local flow parameters, such as void fraction, bubble frequency, vapor velocity, interfacial area concentration and chord length, were measured, using two conductivity probe method, and local liquid phase velocity was measured by pitot tube. In order to investigate the effects of channel inclination on two phase flow structure, the experiments were conducted for three angles of inclination; 0 degree(vertical), 30 degree and 60 degree. The experimental flow conditions were confined to the liquid superficial velocities less than 1.4 m/sec and nearly atmospheric pressure, and the flow regime was limited to the subcooled boiling. Using the measured distributions of the local phasic parameters, correlations for the drift-flux parameters such as distribution parameter and drift velocity were proposed. Those correlations were compared with the available correlation applicable to the inclined channel by the calculation of average void fraction using the present data. 44 refs., 4 tabs., 88 figs. (author)
Guedra, Matthieu; Penelet, Guillaume; Lotton, Pierrick; Dalmont, Jean-Pierre
2011-07-01
The aim of this paper is to propose a method to predict the onset conditions of the thermoacoustic instability for various thermoacoustic engines. As an accurate modeling of the heat exchangers and the stack submitted to a temperature gradient is a difficult task, an experimental approach for the characterization of the amplifying properties of the thermoacoustic core is proposed. An experimental apparatus is presented which allows to measure the transfer matrix of a thermoacoustic core under various heating conditions by means of a four-microphone method. An analytical model for the prediction of the onset conditions from this measured transfer matrix is developed. The experimental data are introduced in the model and theoretical predictions of the onset conditions are compared with those actually observed in standing-wave and traveling-wave engines. The results show good agreement between predictions from the model and experiments.
Experimental investigation on SPS casing treatment with bias flow
Institute of Scientific and Technical Information of China (English)
Dong Xu; Sun Dakuna; Liu Xiaohua ba; Sun Xiaofeng
2014-01-01
Generally, casing treatment (CT) is a passivity method to enhance the stall margin of fan/compressor. A novel casing treatment based on the small disturbance theory and vortex and wave interaction suggestion is a method combining passive control and active control, which has been proved effective at enhancing the stall margin of fan/compressor in experiment. In order to investigate the mechanism of this kind of casing treatment, an experimental investigation of a stall precursor-suppressed (SPS) casing treatment with air suction or blowing air is conducted in the present paper. The SPS casing treatment is designed to suppressing stall precursors to realize stall margin enhancement in turbomachinery. The experimental results show that the casing treatment with blowing air of small quantity can improve the stall margin by about 8%with about 1%effi-ciency loss. By contrast, the SPS casing treatment with micro-bias flow does not improve the stall margin much more than that without bias flow, even worse. Meanwhile, the present investigation has also attempted to reveal the mechanism of stall margin improvement with the casing treatment. It is found that the stall margin improvements vary with the modification of the unsteady shedding flow and the unsteady wall boundary impedance. The experimental results agree fairly well with the theoretical prediction using a flow stability model of rotating stall.
Experimental investigation on SPS casing treatment with bias flow
Directory of Open Access Journals (Sweden)
Dong Xu
2014-12-01
Full Text Available Generally, casing treatment (CT is a passivity method to enhance the stall margin of fan/compressor. A novel casing treatment based on the small disturbance theory and vortex and wave interaction suggestion is a method combining passive control and active control, which has been proved effective at enhancing the stall margin of fan/compressor in experiment. In order to investigate the mechanism of this kind of casing treatment, an experimental investigation of a stall precursor-suppressed (SPS casing treatment with air suction or blowing air is conducted in the present paper. The SPS casing treatment is designed to suppressing stall precursors to realize stall margin enhancement in turbomachinery. The experimental results show that the casing treatment with blowing air of small quantity can improve the stall margin by about 8% with about 1% efficiency loss. By contrast, the SPS casing treatment with micro-bias flow does not improve the stall margin much more than that without bias flow, even worse. Meanwhile, the present investigation has also attempted to reveal the mechanism of stall margin improvement with the casing treatment. It is found that the stall margin improvements vary with the modification of the unsteady shedding flow and the unsteady wall boundary impedance. The experimental results agree fairly well with the theoretical prediction using a flow stability model of rotating stall.
Numerical Study of the Instability and Flow Transition in a Vortex-Ring/Wall Interaction
Directory of Open Access Journals (Sweden)
Heng Ren
2016-01-01
Full Text Available Instability and fl w transition of a vortex ring impinging on a wall were investig ated by means of large-eddy simulation for two vortex core thicknesses corresponding to thin and thick vortex rings. Various fundamental mechanisms dictating the fl w behaviours, such as evolution of vortical structures, instability and breakdown of vortex rings, development of modal energies, and transition from laminar to turbulent state, have been studied systematically . Analysis of the enstrophy of wrapping vortices and turbulent kinetic energy (TKE in fl w fiel indicates that the formation and evolution of wrapping vortices are closely associated with the fl w transition to turbulent state. It is found that the temporal development of wrapping vortices and the growth rate of axial fl w generated around the circumference of the core region for the thin ring are faster than those for the thick ring. The azimuthal instabilities of primary and secondary vortex rings are analysed and the development of modal energies reveals the fl w transition to turbulent state. The law of energy decay follows a characteristic k 5=3 law, indicating that the vortical fl w has become turbulent. The results obtained in this study provide physical insight into the understanding of the instability mechanisms relevant to the vortical fl w evolution.
New numerical tools to study waves and instabilities of flowing plasmas
Beliën, A.J.C.; Botchev, M.A.; Goedbloed, J.P.; Holst, van der B.; Keppens, R.
2002-01-01
Studying plasma waves and instabilities is an indispensable part of present thermonuclear fusion and astrophysical magnetohydrodynamics (MHD). Up till recently, spectral analysis was mostly restricted to static plasmas. However, the assumption of a static plasma is unrealistic not only for astrophys
Bridgeman, Devon; Tsow, Francis; Xian, Xiaojun; Forzani, Erica
2016-03-01
The development and performance characterization of a new differential pressure-based flow meter for human breath measurements is presented in this article. The device, called a "Confined Pitot Tube," is comprised of a pipe with an elliptically shaped expansion cavity located in the pipe center, and an elliptical disk inside the expansion cavity. The elliptical disk, named Pitot Tube, is exchangeable, and has different diameters, which are smaller than the diameter of the elliptical cavity. The gap between the disk and the cavity allows the flow of human breath to pass through. The disk causes an obstruction in the flow inside the pipe, but the elliptical cavity provides an expansion for the flow to circulate around the disk, decreasing the overall flow resistance. We characterize the new sensor flow experimentally and theoretically, using Comsol Multiphysics(®) software with laminar and turbulent models. We also validate the sensor, using inhalation and exhalation tests and a reference method.
Preliminary experimental investigation of boundary layer in decelerating flow
Directory of Open Access Journals (Sweden)
Příhoda J.
2013-04-01
Full Text Available Investigations of characteristics of turbulence inside boundary layer under decelerating flow were studied by means of constant temperature anemometer. The decelerating flow was simulated in the closed circuit wind tunnel 0.9 m × 0.5 m at IT AS CR. The free stream turbulence was either natural o risen up by square mesh plane grid. The details of experimental settings and measurement procedures of the instantaneous longitudinal velocity component are described and the distributions of intensity, skewness and kurtosis of turbulent fluctuations are discussed in the contribution.
DEFF Research Database (Denmark)
Friberg, L; Olsen, T S; Roland, P E
1987-01-01
During the course of hemiplegic migraine in 3 patients, changes in regional cerebral blood flow (rCBF) were recorded by the intracarotid 133Xe method and a 254 multidetector camera covering one hemisphere. The rCBF measurements were performed in conjunction with cerebral angiography. During...... was probably the cause of the neurological deficits. On the basis of these observations and previous work from our laboratory we conclude that instability of cerebrovascular tone may cause focal ischaemia during the course of attacks of classical and hemiplegic migraine....
Experimental study on the rheological behaviour of debris flow
Directory of Open Access Journals (Sweden)
A. Scotto di Santolo
2010-12-01
Full Text Available A model able to describe all the processes involved in a debris flow can be very complex owing to the sudden changing of the material that turns from solid into liquid state. The two phases of the phenomenon are analysed separately referring to soil mechanics procedures with regard to the trigger phase, and to an equivalent fluid for the post-failure phase. The present paper is devoted to show the experimental results carried out to evaluate the behaviour assumed by a pyroclastic-derived soil during the flow. A traditional fluid tool has been utilized: a standard rotational rheometer equipped with two different geometries. The soils tested belong to deposits that cover the slopes of the Campania region, Italy, often affected by debris flows. The influence of solid concentration C_{v} and grain size distribution was tested: the soils were destructurated, sieved and mixed with water starting from the in situ porosity. All material mixtures showed a non-Newtonian fluid behaviour with a yield stress τ_{y} that increases with a solid volumetric concentration and decreases for an increase of sand fraction. The experimental data were fitted with standard model for fluids. A simple relation between C_{v} and τ_{y} was obtained. The yield stress seems to be a key parameter for describing and predicting the post-failure behaviour of debris flows. These results suggest that in the field a small change in solid fraction, due to rainfall, will cause a slight decrease of the static yield stress, readily inducing a rapid flow which will stop only when the dynamic yield stress is reached, namely on a much smoother slope. This can explain the in situ observed post-failure behaviour of debris flows, which are able to flow over very long distances even on smooth slopes.
Sadeghi-Goughari, Moslem; Jeon, Soo; Kwon, Hyock-Ju
2017-09-01
In drug delivery systems, carbon nanotubes (CNTs) can be used to deliver anticancer drugs into target site to kill metastatic cancer cells under the magnetic field guidance. Deep understanding of dynamic behavior of CNTs in drug delivery systems may enable more efficient use of the drugs while reducing systemic side effects. In this paper, we study the effect of magnetic-fluid flow on the structural instability of a CNT conveying nanoflow under a longitudinal magnetic field. The Navier-Stokes equation of magnetic-fluid flow is coupled with Euler-Bernoulli beam theory for modeling fluid structure interaction (FSI). Size effects of the magnetic fluid and the CNT are addressed through small-scale parameters including the Knudsen number (Kn) and the nonlocal parameter. Results show the positive role of magnetic properties of fluid flow on the structural stability of CNT. Specifically, magnetic force applied to the fluid flow has an effect of decreasing the structural stiffness of system while increasing the critical flow velocity. Furthermore, we discover that the nanoscale effects of CNT and fluid flow tend to amplify the influence of magnetic field on the vibrational behavior of the system.
Impact of Ion Acoustic Wave Instabilities in the Flow Field of a Hypersonic Vehicle on EM Signals
Mudaliar, Saba; Sotnikov, Vladimir
2016-10-01
Flow associated with a high speed air vehicle (HSAV) can get partially ionized. In the absence of external magnetic field the flow field turbulence is due to ion acoustic wave (IAW) instabilities. Our interest is in studying the impact of this turbulence on the radiation characteristics of EM signals from the HSAV. We decompose the radiated signal into coherent and diffuse parts. We find that the coherent part has the same spectrum as that of the source signal, but it is distorted because of dispersive coherent attenuation. The diffuse part is expressed as a convolution (in wavenumber and frequency) of the source signal with the spectrum of electron density fluctuations. This is a constrained convolution in the sense that the spectrum has to satisfy the IAW dispersion relation. A quantity that characterizes the flow is the mean free path (MFP). When the MFP is large compared to the thickness of the flow the coherent part is significant. If the MFP is larger than the thickness of the flow the diffuse part is the dominant part of the received signal. In the special case when the source signal frequency is close the electron plasma frequency, there can exist in the flow region Langmuir modes in addition to the EM modes. The radiation characteristics of EM source signals from the HSAV in this case are quite different.
Experimental study of controlled tip disturbance effect on flow asymmetry
Degani, David; Tobak, Murray
1992-01-01
The effect on the asymmetric mean flow observed on pointed bodies of revolution at incidence of changing the size and location of a controlled disturbance as well as changes in angle of attack and flow conditions are evaluated experimentally. Flow visualization and side-force measurements are carried out for a generic ogive-cylinder body inclined at high angle of attack in a low-speed wind tunnel. For all angles of attack tested (30-60 deg), minute changes in the size or location of the controlled disturbance result in finite changes in the asymmetric flow field, even to the extent of reversing the sign of the side force or becoming almost symmetric. The process is reversible; returning the wire to an original position likewise restores the corresponding flow field and mean side force. The variation of side force with continuous variation of a perturbation's size or location remains continuous and single valued, even in the incidence range of 50 to 60 deg, where 'bistable' behavior of the asymmetric flow field is observed.
Confined granular flow in silos experimental and numerical investigations
Tejchman, Jacek
2013-01-01
During confined flow of bulk solids in silos some characteristic phenomena can be created, such as: — sudden and significant increase of wall stresses, — different flow patterns, — formation and propagation of wall and interior shear zones, — fluctuation of pressures and, — strong autogenous dynamic effects. These phenomena have not been described or explained in detail yet. The main intention of the experimental and theoretical research presented in this book is to explain the above mentioned phenomena in granular bulk solids and to describe them with numerical FE models verified by experimental results.
Experimental study of effect of stenosis geometry on flow parameters
Directory of Open Access Journals (Sweden)
Veselý Ondřej
2015-01-01
Full Text Available A stenosis is a narrowing in a tubular organ or structure. In medicine, vessel stenosis poses health risks for people. In this work, experimental investigations of pressure loss coefficient for varying stenosis eccentricity and shape were performed. Five models of different geometry were studied; all models were stenosis of 75 % area reduction. The flow conditions approximate physiological flow. The measuring range of Reynolds number was from 130 to 2730, measured values of pressure loss coefficient were from 12 to 20. The steady experimental results indicated that static pressure loss coefficient is affected by the shape of stenosis, but it was affected more significantly by the eccentricity. Visualization experiments have been performed in Polycarbonate models.
Experimental comparison of mammalian and avian blood flow in microchannels
Fink, Kathryn; Liepmann, Dorian
2015-11-01
The non-Newtonian, shear rate dependent behavior of blood in microchannel fluid dynamics has been studied for nearly a century, with a significant focus on the characteristics of human blood. However, for over 200 years biologists have noted significant differences in red blood cell characteristics across vertebrate species, with particularly drastic differences in cell size and shape between mammals and non-mammalian classes. We present an experimental analysis of flow in long microchannels for several varieties of mammalian and avian blood, across a range of hematocrits, channel diameters, and flow rates. Correlation of shear rate and viscosity is compared to existing constitutive equations for human blood to further quantify the importance of red blood cell characteristics. Ongoing experimental results are made available in an online database for reference or collaboration. K.F. acknowledges funding from the ARCS Foundation and an NSF Graduate Research Fellowship through NSF Grant DGE 1106400.
Experimental study of critical flow of water at supercritical pressure
Institute of Scientific and Technical Information of China (English)
Yuzhou CHEN; Chunsheng YANG; Shuming ZHANG; Minfu ZHAO; Kaiwen DU; Xu CHENG
2009-01-01
Experimental studies of the critical flow of water were conducted under steady-state conditions with a nozzle 1.41mm in diameter and 4.35 mm in length, covering the inlet pressure range of 22.1-26.8 MPa and inlet temperature range of 38^74°C. The parametric trend of the flow rate was investigated, and the experimental data were compared with the predictions of the homogeneous equilibrium model, the Bernoulli correlation, and the models used in the reactor safety analysis code RELAP5/ MOD3.3. It is concluded that in the near or beyond pseudo-critical region, thermal-dynamic equilibrium is dominant, and at a lower temperature, choking does not occur. The onset of the choking condition is not predicted reasonably by the RELAP5 code.
Experimental investigation and simulation of flow boiling of nanofluids in different flow directions
Afrand, Masoud; Abedini, Ehsan; Teimouri, Hamid
2017-03-01
In this work, the flow boiling of TiO2/water and Al2O3/water nanofluids was investigated experimentally and simulated with two phases. Experimental results were obtained in two directions and compared together. The volume fraction and heat transfer coefficient obtained from the vertical tube were compared with those obtained from the horizontal tube. The results showed that the contours of vapor volume fraction in horizontal tube are completely different from the vertical tube, which is due to the buoyancy effect. Moreover, the effect of nanoparticles on both flow directions was almost the same, while heat transfer coefficient was not the same in these flow directions. Based on the experimental result, presence of nanoparticles in the base fluid cannot increase the heat transfer coefficient.
Jeans instability in superfluids
Energy Technology Data Exchange (ETDEWEB)
Hason, Itamar; Oz, Yaron [Tel-Aviv University, Raymond and Beverly Sackler School of Physics and Astronomy, Tel Aviv (Israel)
2014-11-15
We analyze the effect of a gravitational field on the sound modes of superfluids. We derive an instability condition that generalizes the well-known Jeans instability of the sound mode in normal fluids. We discuss potential experimental implications. (orig.)
Experimental Study on the Unified Power Flow Controller
Matsuki, Junya; Hayashi, Yasuhiro; Kitajima, Shunsuke; Takahashi, Masahiro; Murata, Kenji
This paper presents the results of experimental study on the performance of a Unified Power Flow Controller (UPFC), one of the FACTS (Flexible AC Transmission Systems) controllers. A laboratory-scale UPFC was manufactured and installed on a laboratory electric power system to investigate its multifunctional capabilities as a power flow controller. The UPFC consists of two 4.5kVA, 200V back-to-back voltage-sourced converters, labeled “Converter 1" and “Converter 2", operated from a common DC link provided by a DC storage capacitor of 380V. It can provide independent control of both the real and reactive power flow in the line. Tests were performed to examine the capabilities of UPFC, under one-machine connected to an infinite-bus system. Steady-state responses under various kinds of operating conditions were measured and analyzed.
Simulation and experimental study of resin flow in fibre fabrics
Yan, Fei; Yan, Shilin; Li, Yongjing
2017-06-01
Liquid Composite Moulding (LCM) is gradually becoming the most competitive manufacturing technology for producing large composite parts with complex geometry with high quality and low cost. These parts include those for airplanes, wind turbine blades and automobile components. Fibre fabrics in liquid composite moulding can be considered as dual-scale porous media. In different gap scales, an unsaturated flow is produced during the mould filling process. This particular flow behaviour deviates from the traditional Darcy’s law, which is used to calculate the filling pressure and will cause errors. According to sink theory, the unsaturated flow characteristics of this dual-scale porous media were studied in this paper, and a FEM solution program was developed. The results showed that the pressure curves against the position which simulated by sink functions were departure from the position of traditional theory. In addition, the simulation results of partially-saturated region were consistent with the experimental data.
Directory of Open Access Journals (Sweden)
Fabre J.
2006-11-01
Full Text Available Les instabilités hydrodynamiques de l'écoulement dans les systèmes pipeline/riser ont été étudiées sur une boucle d'essais en tubes transparents de diamètre intérieur 0,053 m, longueur 25 m pour la partie pipeline et 13,5 m pour la partie riser. Les expériences effectuées sur de l'eau et de l'air, à différentes pentes du pipeline et pour plusieurs valeurs du débit d'entrée, ont permis l'observation d'instabilités à grande échelle à faible débit de liquide et de gaz. Les cartes de configuration d'écoulement ont été établies pour trois pentes différentes du pipeline. Le modèle numérique développé à partir de ces données résout les équations locales instantanées de bilan de masse et de quantité de mouvement de ces écoulements par la méthode des caractéristiques. L'évolution des pressions, fraction de vide, débit de gaz et de liquide au cours du temps, prévue par le modèle, est généralement en bon accord avec les données expérimentales. Hydrodynamic instabilities of flow in pipeline/riser systems were studied on a test loop made of transparent tubes with an ID of 0. 053 m and 25 m long for the pipeline part and 13. 5 m long for the riser. Experiments performed with water and air, with different slopes of the pipeline and with several inlet flow rates, reveal large-scale instabilities at low liquid and gas flow rates. Flow configuration maps were compiled for three different pipeline slopes. The numerical model developed from these data solves instantaneous local equations for the mass balance and amount of movement of such flows by the method of characteristics. Variations in pressures, in the void fraction and in gas and liquid flow in the time forecast by the model are generally in good agreement with experimental data.
Numerical and experimental investigation of vortical flow-flame interaction
Energy Technology Data Exchange (ETDEWEB)
Najm, H.N.; Schefer, R.W.; Milne, R.B.; Mueller, C.J. [Sandia National Labs., Livermore, CA (United States); Devine, K.D.; Kempka, S.N. [Sandia National Labs., Albuquerque, NM (United States)
1998-02-01
A massively parallel coupled Eulerian-Lagrangian low Mach number reacting flow code is developed and used to study the structure and dynamics of a forced planar buoyant jet flame in two dimensions. The numerical construction uses a finite difference scheme with adaptive mesh refinement for solving the scalar conservation equations, and the vortex method for the momentum equations, with the necessary coupling terms. The numerical model construction is presented, along with computational issues regarding the parallel implementation. An experimental acoustically forced planar jet burner apparatus is also developed and used to study the velocity and scalar fields in this flow, and to provide useful data for validation of the computed jet. Burner design and laser diagnostic details are discussed, along with the measured laboratory jet flame dynamics. The computed reacting jet flow is also presented, with focus on both large-scale outer buoyant structures and the lifted flame stabilization dynamics. A triple flame structure is observed at the flame base in the computed flow, as is theoretically expected, but was not observable with present diagnostic techniques in the laboratory flame. Computed and experimental results are compared, along with implications for model improvements.
Experimental stand for investigation of fluid flow in heat exchangers with cross-flow arrangement
Directory of Open Access Journals (Sweden)
Łopata Stanisław
2017-01-01
Full Text Available The operation analysis of high-performance heat exchanger with tubes elliptical indicated that the heat exchangers can be subject to damage. The reason for this is probably improper distribution of working fluid in tubular space of heat exchanger. Therefore, a part of the tubes may be improperly cooled and subject to compressible stresses. The paper presents an experimental stand allowing to confirm the given assumption. The experimental investigation enables to examine the mass flow rate in heat exchanger tubes. Also, it is possible to assess the impact of the construction of inlet, intermediate and outlet chambers on the flow distribution within the heat exchanger tubes.
EXPERIMENTAL STUDY ON THE MECHANISM OF THE RICHTMYER-MESHKOV INSTABILITY AT A GAS-LIQUID INTERFACE
Institute of Scientific and Technical Information of China (English)
SHI Hong-hui; ZHANG Ga; DU Kai; JIA Hui-xia
2009-01-01
The mechanism of the Richtmyer-Meshkov instability was experimentally studied in a vertical rectangular shock tube. The velocity of the interface driven by the shock wave was measured by a self-designed measurement system, which consists of semiconductor lasers, signal amplification circuits, digital oscilloscope and computer. Tests were carried out at several shock wave Mach numbers. In addition, the movement of the interface and the variation of the mixed zone width with time were recorded by high-speed photography. The experimental results show that the interface velocity increases with the increase of the Mach number, and the distance of the interface's movement and the width of the mixed zone vary with time in a linear relationship.
On phase change in Marangoni-driven flows and its effects on the hydrothermal-wave instabilities
Sáenz, P. J.; Valluri, P.; Sefiane, K.; Karapetsas, G.; Matar, O. K.
2014-02-01
This paper investigates the effects of phase change on the stability of a laterally heated liquid layer for the first time. The interface is open to the atmosphere and vapor diffusion is the rate-limiting mechanism for evaporation. In this configuration, the planar layer is naturally vulnerable to the formation of travelling thermal instabilities, i.e., hydrothermal waves (HTWs), due to the presence of temperature gradients along the gas-liquid interface. Recent work carried out for deformable interfaces and negligible evaporation indicates that the HTWs additionally give rise to interface deformations of similar features, i.e., physical waves. The study presented here reveals that phase change plays a dual role through its effect on these instabilities: the latent energy required during the evaporation process tends to inhibit the HTWs while the accompanying level reduction enhances the physical waves by minimizing the role of gravity. The dynamics of the gas phase are also discussed. The HTW-induced convective patterns in the gas along with the travelling nature of the instabilities have a significant impact on the local evaporation flux and the vapor distribution above the interface. Interestingly, high (low) concentrations of vapor are found above cold (hot) spots. The phase-change mechanism for stable layers is also investigated. The Marangoni effect plays a major role in the vapor distribution generating a vacuum effect in the warm region and vapor accumulations at the cold boundary capable of inverting the phase change, i.e., the capillary flow can lead to local condensation. This work also demonstrates the inefficiencies of the traditional phase change models based on pure vapor diffusion to capture the dynamics of thermocapillary flows.
Experimental techniques for turbulent Taylor–Couette flow and Rayleigh–Bénard convection
Sun, Chao; Zhou, Quan
2014-01-01
Taylor–Couette (TC) flow and Rayleigh–B´enard (RB) convection are two systems in hydrodynamics, which have been widely used to investigate the primary instabilities, pattern formation, and transitions from laminar to turbulent flow. These two systems are known to have an elegant mathematical similar
Papaloizou, J C B
2004-01-01
We carry out a general study of the stability of astrophysical flows that appear steady in a uniformly rotating frame. Such a flow might correspond to a stellar pulsation mode or an accretion disk with a free global distortion giving it finite eccentricity. We consider perturbations arbitrarily localized in the neighbourhood of unperturbed fluid streamlines.When conditions do not vary around them, perturbations take the form of oscillatory inertial or gravity modes. However, when conditions do vary so that a circulating fluid element is subject to periodic variations, parametric instability may occur. For nearly circular streamlines, the dense spectra associated with inertial or gravity modes ensure that resonance conditions can always be satisfied when twice the period of circulation round a streamline falls within. We apply our formalism to a differentially rotating disk for which the streamlines are Keplerian ellipses, with free eccentricity up to 0.7, which do not precess in an inertial frame. We show tha...
Identifying the active flow regions that drive linear and nonlinear instabilities
Marquet, Olivier
2015-01-01
A new framework for the analysis of unstable oscillator flows is explored. In linear settings, temporally growing perturbations in a non-parallel flow represent unstable eigenmodes of the linear flow operator. In nonlinear settings, self-sustained periodic oscillations of finite amplitude are commonly described as nonlinear global modes. In both cases the flow dynamics may be qualified as being endogenous, as opposed to the exogenous behaviour of amplifier flows driven by external forcing. This paper introduces the endogeneity concept, a specific definition of the sensitivity of the global frequency and growth rate with respect to variations of the flow operator. The endogeneity, defined both in linear and nonlinear settings, characterizes the contribution of localized flow regions to the global eigendynamics. It is calculated in a simple manner as the local point-wise inner product between the time derivative of the direct flow state and an adjoint mode. This study demonstrates for two canonical examples, th...
Numerical and Experimental Approaches Toward Understanding Lava Flow Heat Transfer
Rumpf, M.; Fagents, S. A.; Hamilton, C.; Crawford, I. A.
2013-12-01
We have performed numerical modeling and experimental studies to quantify the heat transfer from a lava flow into an underlying particulate substrate. This project was initially motivated by a desire to understand the transfer of heat from a lava flow into the lunar regolith. Ancient regolith deposits that have been protected by a lava flow may contain ancient solar wind, solar flare, and galactic cosmic ray products that can give insight into the history of our solar system, provided the records were not heated and destroyed by the overlying lava flow. In addition, lava-substrate interaction is an important aspect of lava fluid dynamics that requires consideration in lava emplacement models Our numerical model determines the depth to which the heat pulse will penetrate beneath a lava flow into the underlying substrate. Rigorous treatment of the temperature dependence of lava and substrate thermal conductivity and specific heat capacity, density, and latent heat release are imperative to an accurate model. Experiments were conducted to verify the numerical model. Experimental containers with interior dimensions of 20 x 20 x 25 cm were constructed from 1 inch thick calcium silicate sheeting. For initial experiments, boxes were packed with lunar regolith simulant (GSC-1) to a depth of 15 cm with thermocouples embedded at regular intervals. Basalt collected at Kilauea Volcano, HI, was melted in a gas forge and poured directly onto the simulant. Initial lava temperatures ranged from ~1200 to 1300 °C. The system was allowed to cool while internal temperatures were monitored by a thermocouple array and external temperatures were monitored by a Forward Looking Infrared (FLIR) video camera. Numerical simulations of the experiments elucidate the details of lava latent heat release and constrain the temperature-dependence of the thermal conductivity of the particulate substrate. The temperature-dependence of thermal conductivity of particulate material is not well known
Sun, Yiyang; Taira, Kunihiko; Cattafesta, Louis; Ukeiley, Lawrence
2016-11-01
Direct numerical simulation (DNS) and biglobal stability analysis are performed to examine the spanwise effects on the appearance of the so-called wake mode in the flow over long rectangular cavities. The wake mode has been reported to exhibit high-amplitude fluctuations and eject large spanwise vortices in numerical studies, despite its lack of observation in experiments, leaving its existence an open question. The present study focuses on a rectangular cavity flow with aspect ratio of L / D = 6 , free stream Mach number of M∞ = 0 . 6 and ReD = 502 . The properties of the wake mode are revealed via 2D DNS. From the biglobal stability analysis, the wake mode can be captured with a zero spanwise wavenumber. Furthermore, 3D eigenmodes are calculated with spanwise wavelength λ / D ∈ [ 0 . 5 , 2 ] . With the knowledge of the features of the wake mode and the 3D eigenmodes, 3D DNS are performed with width-to-depth ratio of W / D = 1 and 2. We find the flow exhibits the wake mode with W / D = 1 but presents a moderate shear-layer mode with W / D = 2 . Based on the findings, we argue that the spanwise instabilities in flows over wide cavities redistribute energy from spanwise vortices to streamwise vortical structures, which suppresses the emergence of the wake mode in the 3D cavity flows. This work was supported by the US Air Force Office of Scientific Research (Grant FA9550-13-1-0091).
Direct Experimental Evidence of Hole Trapping in Negative Bias Temperature Instability
Institute of Scientific and Technical Information of China (English)
JI Xiao-Li; LIAO Yi-Ming; YAN Feng; SHI Yi; ZHANG Guan; GUO Qiang
2011-01-01
Negative bias temperature instability (NBTI) in ultrathin-plasma-nitrided-oxide (PNO) based p-type metal-oxide-semiconductor Reid effect transistors (pMOSFETs) is investigated at temperatures ranging from 220 K to 470 K. It is found that the threshold voltage VT degradation below 290 K is dominated by the hole trapping process. Further studies unambiguously show that this process is unnecessarily related to nitrogen but the incorporation of nitrogen in the gate dielectric increases the probability of hole trapping in the NBTI process as it introduces extra trap states located in the upper half of the Si band gap. The possible hole trapping mechanism in NBTI stressed PNO pMOSFETs is suggested by taking account of oxygen and nitrogen related trap centers.%Negative bias temperature instability (NBTI) in ultrathin-plasma-nitrided-oxide (PNO) based p-type metal-oxidesemiconductor field effect transistors (pMOSFETs) is investigated at temperatures ranging from 220 K to 470 K.It is found that the threshold voltage VT degradation below 290 K is dominated by the hole trapping process.Further studies unambiguously show that this process is unnecessarily related to nitrogen but the incorporation of nitrogen in the gate dielectric increases the probability of hole trapping in the NBTI process as it introduces extra trap states located in the upper half of the Si band gap.The possible hole trapping mechanism in NBTI stressed PNO pMOSFETs is suggested by taking account of oxygen and nitrogen related trap centers.Negative bias temperature instability (NBTI) is one of the most important reliability research topics in modern CMOS technologies.Although it has been known for forty years,the degradation mechanism is still under debate and is of great interest.The negative bias temperature (NBT) degradation has been largely attributed to two different traps.One is the interface states due to Pb centers,resulting from the dissociation of Si-H bonds.[1,2
Gilmore, M.; Desjardins, T. R.; Fisher, D. M.
2016-10-01
Ongoing experiments and numerical modeling on the effects of flow shear on electrostatic turbulence in the presence of electrode biasing are being conducted in helicon plasmas in the linear HelCat (Helicon-Cathode) device. It is found that changes in flow shear, affected by electrode biasing through Er x Bz rotation, can strongly affect fluctuation dynamics, including fully suppressing the fluctuations or inducing chaos. The fundamental underlying instability, at least in the case of low magnetic field, is identified as a hybrid resistive drift-Kelvin-Helmholtz mode. At higher magnetic fields, multiple modes (resistive drift, rotation-driven interchange and/or Kelvin-Helmholtz) are present, and interact nonlinearly. At high positive electrode bias (V >10Te), a large amplitude, global instability, identified as the potential relaxation instability is observed. Numerical modeling is also being conducted, using a 3 fluid global Braginskii solver for no or moderate bias cases, and a 1D PIC code for high bias cases. Recent experimental and numerical results will be presented. Supported by U.S. National Science Foundation Award 1500423.
Experimental Investigation on the Mechanical Instability of Superelastic NiTi Shape Memory Alloy
Xiao, Yao; Zeng, Pan; Lei, Liping
2016-09-01
In this paper, primary attention is paid to the mechanical instability of superelastic NiTi shape memory alloy (SMA) during localized forward transformation at different temperatures. By inhibiting the localized phase transformation, we can obtain the up-down-up mechanical response of NiTi SMA, which is closely related to the intrinsic material softening during localized martensitic transformation. Furthermore, the material parameters of the up-down-up stress-strain curve are extracted, in such a way that this database can be utilized for simulation and validation of the theoretical analysis. It is found that during forward transformation, the upper yield stress, lower yield stress, Maxwell stress, and nucleation stress of NiTi SMA exhibit linear dependence on temperature. The relation between nucleation stress and temperature can be explained by the famous Clausius-Clapeyron equation, while the relation between upper/lower yield stress and temperature lacks theoretical study, which needs further investigation.
Direct Experimental Evidence of Hole Trapping in Negative Bias Temperature Instability
Ji, Xiao-Li; Liao, Yi-Ming; Yan, Feng; Shi, Yi; Zhang, Guan; Guo, Qiang
2011-10-01
Negative bias temperature instability (NBTI) in ultrathin-plasma-nitrided-oxide (PNO) based p-type metal-oxide-semiconductor field effect transistors (pMOSFETs) is investigated at temperatures ranging from 220K to 470K. It is found that the threshold voltage VT degradation below 290 K is dominated by the hole trapping process. Further studies unambiguously show that this process is unnecessarily related to nitrogen but the incorporation of nitrogen in the gate dielectric increases the probability of hole trapping in the NBTI process as it introduces extra trap states located in the upper half of the Si band gap. The possible hole trapping mechanism in NBTI stressed PNO pMOSFETs is suggested by taking account of oxygen and nitrogen related trap centers.
Rüdiger, G.; Gellert, M.; Schultz, M.; Hollerbach, R.; Stefani, F.
2014-02-01
The interaction of differential rotation and toroidal fields that are current-free in the gap between two corotating axially unbounded cylinders is considered. It is shown that non-axisymmetric perturbations are unstable if the rotation rate and Alfvén frequency of the field are of the same order, almost independent of the magnetic Prandtl number Pm. For the very steep rotation law Ω ∝ R-2 (the Rayleigh limit) and for small Pm, the threshold values of rotation and field for this azimuthal magnetorotational instability (AMRI) scale with the ordinary Reynolds number and the Hartmann number, respectively. A laboratory experiment with liquid metals like sodium or gallium in a Taylor-Couette container has been designed on the basis of this finding. For fluids with more flat rotation laws, the Reynolds number and the Hartmann number are no longer typical quantities for the instability. For the weakly non-linear system, the numerical values of the kinetic energy and the magnetic energy are derived for magnetic Prandtl numbers ≤ 1. We find that the magnetic energy grows monotonically with the magnetic Reynolds number Rm, while the kinetic energy grows with Rm/√Pm. The resulting turbulent Schmidt number, as the ratio of the `eddy' viscosity and the diffusion coefficient of a passive scalar (such as lithium), is of the order of 20 for Pm = 1, but for small Pm it drops to the order of unity. Hence, in a stellar core with fossil fields and steep rotation law, the transport of angular momentum by AMRI is always accompanied by an intense mixing of the plasma, until the rotation becomes rigid.
Energy Technology Data Exchange (ETDEWEB)
Toya, Y. [Nagano National College of Technology, Nagano (Japan); Nakamura, I. [Nagoya University, Nagoya (Japan). Faculty of Engineering
1997-08-25
The present study concerns an experimental investigation of the instability of a Taylor vortex flow with two immiscible fluids. The fluids are used containing aqueous solution of glycerol and silicone oil. The experiments are made in a symmetric system and at a range of the aspect ratio approximately from 4 to 6. As results, a cell configuration is clarified by flow visualization, and through a surprising phenomenon where there are opposite directions of flow on a boundary surface between the fluids. The heights of cells are measured to clarify the instability of the vortices. The critical Reynolds number at which the primary mode is mutually bifurcated to the secondary mode and another critical Reynolds number at which the secondary mode changes to the wavy Taylor vortex flow are investigated. These results are compared with the features of the Taylor vortex flow with one fluid and are similar to those in an asymmetric system. But the phenomenon that there are two flow directions on the boundary surface has never been reported. 15 refs., 9 figs.
Three-dimensional instability of a multipolar vortex in a rotating flow
Le Dizès, Stéphane
2000-11-01
In this paper, the elliptic instability is generalized to account for Coriolis effects and higher order symmetries. We consider, in a frame rotating at the angular frequency Ω, a stationary vortex which is described near its center r=0 by the stream function written in polar coordinates Ψ=-(r2/2)+p(rn/n)cos(nθ), where the integer n is the order of the azimuthal symmetry, and p is a small positive parameter which measures the strength of the nonaxisymmetric field. Based on the Lifschitz and Hameiri [Phys. Fluids A 3, 2644-2651 (1991)] theory, the local stability analysis of the streamline Ψ=-1/2 is performed in the limit of small p. As for the elliptic instability [Bayly, Phys. Rev. Lett. 57, 2160-2163 (1986)], the instability is shown to be due to a parametric resonance of inertial waves when the inclination angle ξ of their wave vector with respect to the rotation axis takes a particular value given by cos ξ=±4/(n(1+Ω)). An explicit formula for the maximum growth rate of the inertial wave is obtained for arbitrary ξ, Ω, and n. As an immediate consequence, it is shown that a vortex core of relative vorticity Wr (assumed positive) is locally unstable if Ω(-1+n/4-p(n-1)/2)Wr/2. The predictive power of the local theory is demonstrated on several vortex examples by comparing the local stability predictions with global stability results. For both the Kirchhoff vortex and Moore and Saffman vortex, it is shown how global stability results can be derived from the local stability analysis using the dispersion relation of normal (Kelvin) modes. These results are compared to those obtained by global methods and a surprisingly good agreement is demonstrated. The local results are also applied to rotating Stuart vortices and compared to available numerical data.
Large Eddy Simulations of Kelvin Helmholtz instabilities at high Reynolds number stratified flows
Brown, Dana; Goodman, Lou; Raessi, Mehdi
2015-11-01
Simulations of Kelvin Helmholtz Instabilities (KHI) at high Reynolds numbers are performed using the Large Eddy Simulation technique. Reynolds numbers up to 100,000 are achieved using our model. The resulting data set is used to examine the effect of Reynolds number on various statistics, including dissipation flux coefficient, turbulent kinetic energy budget, and Thorpe length scale. It is shown that KHI are qualitatively different at high Re, up to and including the onset of vortex pairing and billow collapse and quantitatively different afterward. The effect of Richardson number is also examined. The results are discussed as they apply to ocean experiments.
1 Hz flaring in SAX J1808.4--3658: flow instabilities near the propeller stage
Patruno, A; Klein-Wolt, M; Wijnands, R; Van der Klis, M
2009-01-01
We present a simultaneous periodic and aperiodic timing study of the accreting millisecond X-ray pulsar SAX J1808.4-3658. We analyze five outbursts of the source and for the first time provide a full and systematic investigation of the enigmatic phenomenon of the 1 Hz flares observed during the final stages of some of the outbursts. We show that strong links between pulsations and 1 Hz flares exist, and suggest they are related with hydrodynamic disk instabilities that are triggered close to the disk-magnetosphere boundary layer when the system is entering the propeller regime.
Directory of Open Access Journals (Sweden)
R. Mantovani
2002-01-01
Full Text Available This paper presents the analysis of symmetric circulations of a rotating baroclinic flow, forced by a steady thermal wind and dissipated by Laplacian friction. The analysis is performed with numerical time-integration. Symmetric flows, vertically bound by horizontal walls and subject to either periodic or vertical wall lateral boundary conditions, are investigated in the region of parameter-space where unstable small amplitude modes evolve into stable stationary nonlinear solutions. The distribution of solutions in parameter-space is analysed up to the threshold of chaotic behaviour and the physical nature of the nonlinear interaction operating on the finite amplitude unstable modes is investigated. In particular, analysis of time-dependent energy-conversions allows understanding of the physical mechanisms operating from the initial phase of linear instability to the finite amplitude stable state. Vertical shear of the basic flow is shown to play a direct role in injecting energy into symmetric flow since the stage of linear growth. Dissipation proves essential not only in limiting the energy of linearly unstable modes, but also in selecting their dominant space-scales in the finite amplitude stage.
Experimental study of collisional granular flows down an inclined plane
Azanza, Emmanuel; Chevoir, François; Moucheront, Pascal
1999-12-01
The collisional flow of a slightly inelastic granular material down a rough inclined plane is usually described by kinetic theories. We present an experimental study aimed at analysing the assumptions and the quantitative predictions of such theories. A two-dimensional channel coupled to a model granular material and image analysis allow detailed and complete measurement of the kinematics and structure of the flows. We determine the range of inclination and particle flux for which the flow is stationary and uniform. The characteristic profiles of solid fraction, mean velocity and granular temperature are systematically measured. Both the true collisional and the dilute kinetic regimes are examined. We show that a quasi-hydrodynamic description of these regimes seems relevant, and that the pressure and the viscosity terms are in good qualitative agreement with the prediction of the kinetic theory. The profiles are well described by the kinetic theory near the top of the flow, at low solid fraction. Conversely there are large discrepancies near the rough plane, where the material is structured in layers.
Experimental Observations of Cavitating Flows Around a Hydrofoil
Institute of Scientific and Technical Information of China (English)
ZHANG Min-di; WANG Guo-yu; DONG Zi-qiao; LI Xiang-bin; GAO De-ming
2008-01-01
The cavitation around a hydrofoil is studied experimentally tO shed light on the multiphase fluid dynamies.Different cavitation regimes are studied by using high speed visualization and particle image velocimetry(PIV).As decreasing the cavitation number,four cavitating flow regimes are observed:incipient cavitation,sheet cavitation,cloud cavitation,and supercavitation.From the incipient cavitation to the cloud cavitation,bubbles become more and more.Phenomena with large-scale vortex structure and rear re-entrant jet associated with the cloud cavitation,and subsequent development in the supercavitation are described.The velocity in the cavitation regions in the different cavitation conditions is low compared to that of the free stream.The large velocity gradient is also observed in the cavitating flow region near the suHace of the hydrofoil.
Experimental observation of precursor solitons in a flowing complex plasma
Jaiswal, Surabhi; Bandyopadhyay, P.; Sen, A.
2016-04-01
The excitation of precursor solitons ahead of a rapidly moving object in a fluid, a spectacular phenomenon in hydrodynamics that has often been observed ahead of moving ships, has surprisingly not been investigated in plasmas where the fluid model holds good for low frequency excitations such as ion acoustic waves. In this Rapid Communication we report an experimental observation of precursor solitons in a flowing dusty plasma. The nonlinear solitary dust acoustic waves (DAWs) are excited by a supersonic mass flow of the dust particles over an electrostatic potential hill. In a frame where the fluid is stationary and the hill is moving the solitons propagate in the upstream direction as precursors while wake structures consisting of linear DAWs are seen to propagate in the downstream region. A theoretical explanation of these excitations based on the forced Korteweg-deVries model equation is provided and their practical implications in situations involving a charged object moving in a plasma are discussed.
Numerical and experimental study of rotating jet flows
Shin, Seungwon; Che, Zhizhao; Kahouadji, Lyes; Matar, Omar; Chergui, Jalel; Juric, Damir
2015-11-01
Rotating jets are investigated through experimental measurements and numerical simulations. The experiments are performed on a rotating jet rig and the effects of a range of parameters controlling the liquid jet are investigated, e.g. jet flow rate, rotation speed, jet diameter, etc. Different regimes of the jet morphology are identified, and the dependence on several dimensionless numbers is studied, e.g. Reynolds number, Weber number, etc. The breakup process of droplets is visualized through high speed imaging. Full three-dimensional direct numerical simulations are performed using BLUE, a massively parallel two-phase flow code. The novel interface algorithms in BLUE track the gas-liquid interface through a wide dynamic range including ligament formation, break up and rupture. EPSRC Programme Grant, MEMPHIS, EP/K0039761/1.
Institute of Scientific and Technical Information of China (English)
邱孝明; 黄林; 简广德
2002-01-01
A magnetohydrodynamic (MHD) formulation is derived to investigate and compare the mitigation effects of both the sheared axial flow and finite Larmor radius (FLR) on the Rayleigh-Taylor (RT) instability in Z-pinch implosions. The sheared axial flow is introduced into MHD equations in a conventional way and the FLR effect into the equations via а/аt → -i(ω + ik2⊥ρi2Ωi), as proposed in our previous paper [Chin. Phys. Lett. 2002, 19:217] , where k2⊥ρ2i is referred to FLR effect from the general kinetic theory of magnetized plasma. Therefore the linearized continuity and momentum equations for the perturbed massdensity and velocity include both the sheared axial flow and the FLR effect. It is found that the effect of sheared axial flow with a lower peak velocity can mitigate RT instability in the whole wavenumber region and the effect of sheared axial flow with a higher one can mitigate RT instability only in the large wavenumber region (for normalized wavenumber κ＞ 2.4); The effect of FLR can mitigate RT instability in the whole wavenumber region and the mitigation effect is stronger than that of the sheared axial flow with a lower peak velocity in the almost whole wavenumber region.
Experimental investigation of a rapidly rotating turbulent duct flow
Energy Technology Data Exchange (ETDEWEB)
Maartensson, G.E.; Johansson, A.V. [Department of Mechanics, KTH, 10044 Stockholm (Sweden); Gunnarsson, J. [Bombardier Transportation, Vaesteraas (Sweden); Moberg, H. [Alfa Laval, 14780 Tumba (Sweden)
2002-09-01
Rapidly rotating duct flow is studied experimentally with Rotation numbers in the interval. To achieve this, in combination with relatively high Reynolds numbers (5,000-30,000 based on the hydraulic radius), water was used as the working medium. Square and rectangular duct cross-sections were used and the angle between the rotation vector and the main axis of the duct was varied. The influence of the rotation on the pressure drop in the duct was investigated and suitable scalings of this quantity were studied. (orig.)
Experimental testing of flexible barriers for containment of debris flows
DeNatale, Jay S.; Iverson, Richard M.; Major, Jon J.; LaHusen, Richard G.; Fliegel, Gregg L.; Duffy, John D.
1999-01-01
In June 1996, six experiments conducted at the U.S. Geological Survey Debris Flow Flume demonstrated that flexible, vertical barriers constructed of wire rope netting can stop small debris flows. All experimental debris flows consisted of water-saturated gravelly sand with less than two percent finer sediment by weight. All debris flows had volumes of about 10 cubic meters, masses of about 20 metre tons, and impact velocities of 5 to 9 meters per second. In four experiments, the debris flow impacted pristine, unreformed barriers of varying design; in the other two experiments, the debris flow impacted barriers already loaded with sediment from a previous flow. Differences in barrier design led to differences in barrier performance. Experiments were conducted with barriers constructed of square-mesh wire-rope netting with 30centimeter, 20centimeter, and 15 centimeter mesh openings as well as 30centimeter diameter interlocking steel rings. In all cases, sediment cascading downslope at the leading edge of the debris flows tended to spray through the nets. Nets fitted with finer-mesh chain link or chicken wire liners contained more sediment than did unlined nets, and a ring net fitted with a synthetic silt screen liner contained nearly 100 percent of the sediment. Irreversible net displacements of up to 2 meters and friction brake engagement on the support and anchor cables dissipated some of the impact energy. However, substantial forces developed in the steel support columns and the lateral and tie-back anchor cables attached to these columns. As predicted by elementary mechanics, the anchor cables experienced larger tensile forces when the support columns were hinged at the base rather than bolted rigidly to the foundation. Measured loads in the lateral anchor cables exceeded those in the tie-back anchor cables and the load cell capacity of 45 kilo-Newtons. Measurements also indicated that the peak loads in the tie- back anchors were highly transient and occurred at
Hsu, S C
2003-01-01
The magnetohydrodynamic kink instability is observed and identified experimentally as a poloidal flux amplification mechanism for coaxial gun spheromak formation. Plasmas in this experiment fall into three distinct regimes which depend on the peak gun current to magnetic flux ratio, with (I) low values resulting in a straight plasma column with helical magnetic field, (II) intermediate values leading to kinking of the column axis, and (III) high values leading immediately to a detached plasma. Onset of column kinking agrees quantitatively with the Kruskal-Shafranov limit, and the kink acts as a dynamo which converts toroidal to poloidal flux. Regime~II clearly leads to both poloidal flux amplification and the development of a spheromak configuration.
Hsu, S C; Bellan, P M
2003-05-30
The magnetohydrodynamic kink instability is observed and identified experimentally as a poloidal flux amplification mechanism for coaxial gun spheromak formation. Plasmas in this experiment fall into three distinct regimes which depend on the peak gun current to magnetic flux ratio, with (I) low values resulting in a straight plasma column with helical magnetic field, (II) intermediate values leading to kinking of the column axis, and (III) high values leading immediately to a detached plasma. Onset of column kinking agrees quantitatively with the Kruskal-Shafranov limit, and the kink acts as a dynamo which converts toroidal to poloidal flux. Regime II clearly leads to both poloidal flux amplification and the development of a spheromak configuration.
Vécsei, Miklós; Hardt, Steffen
2016-01-01
The stability of liquid films coating the walls of a parallel-plate channel and sheared by a pressure-driven gas flow along the channel centre is studied. The films are susceptible to a long-wavelength instability, whose dynamic behaviour is found - for sufficiently low Reynolds numbers and thick gas layers - to be described by two coupled non-linear partial differential equations. To the best of our knowledge, such coupled fully non-linear equations for the film thicknesses have not been derived previously. A linear stability analysis conducted under the condition that the material properties and the initial undisturbed liquid film thicknesses are equal can be utilized to determine whether the interfaces are predominantly destabilized by the variations of the shear stress or by the pressure gradient acting upon them. The analysis of the weakly non-linear equations performed for this case shows that instabilities corresponding to a vanishing Reynolds number are absent from the system. Moreover, for this confi...
Experimental study of choking flow of water at supercritical conditions
Muftuoglu, Altan
Future nuclear reactors will operate at a coolant pressure close to 25 MPa and at outlet temperatures ranging from 500°C to 625°C. As a result, the outlet flow enthalpy in future Supercritical Water-Cooled Reactors (SCWR) will be much higher than those of actual ones which can increase overall nuclear plant efficiencies up to 48%. However, under such flow conditions, the thermal-hydraulic behavior of supercritical water is not fully known, e.g., pressure drop, forced convection and heat transfer deterioration, critical and blowdown flow rate, etc. Up to now, only a very limited number of studies have been performed under supercritical conditions. Moreover, these studies are conducted at conditions that are not representative of future SCWRs. In addition, existing choked flow data have been collected from experiments at atmospheric discharge pressure conditions and in most cases by using working fluids different than water which constrain researchers to analyze the data correctly. In particular, the knowledge of critical (choked) discharge of supercritical fluids is mandatory to perform nuclear reactor safety analyses and to design key mechanical components (e.g., control and safety relief valves, etc.). Hence, an experimental supercritical water facility has been built at Ecole Polytechnique de Montreal which allows researchers to perform choking flow experiments under supercritical conditions. The facility can also be used to carry out heat transfer and pressure drop experiments under supercritical conditions. In this thesis, we present the results obtained at this facility using a test section that contains a 1 mm inside diameter, 3.17 mm long orifice plate with sharp edges. Thus, 545 choking flow of water data points are obtained under supercritical conditions for flow pressures ranging from 22.1 MPa to 32.1 MPa, flow temperatures ranging from 50°C to 502°C and for discharge pressures from 0.1 MPa to 3.6 MPa. Obtained data are compared with the data given in
Experimental investigation of plasma flows in open trap with toroidal diverter under ECR discharge
Energy Technology Data Exchange (ETDEWEB)
Berezkin, A. V., E-mail: Berezkin-AV@nrcki.ru; Bragin, E. Yu., E-mail: Bragin-EY@nrcki.ru; Zhil’tsov, V. A., E-mail: Zhiltsov-VA@nrcki.ru; Kulygin, V. M., E-mail: Kulygin-VM@nrcki.ru; Yanchenkov, S. V., E-mail: Yanchenkov-SV@nrcki.ru [National Research Center Kurchatov Institute (Russian Federation)
2015-12-15
The results of experimental investigations of plasma flows from an open trap with a toroidal diverter are presented. Cold plasma is generated when introducing microwave power under conditions of electron cyclotron resonance (ECR). The radiation is introduced by a waveguide through a vacuum-tight ceramic window across the axis of the device. By means of the Langmuir probes, the spatial distributions of plasma parameters are measured. The highest density is limited to a critical value n{sub c} (∼10{sup 12} cm{sup –3}) for the generator frequency under use. It is found that the temperature and density of the plasma in the trap and in escaping flows are almost independent of the radius when the ECR zone is located near the open-trap confinement region and the density is close to n{sub c}. At the density n < n{sub c}, ring plasma structures, which collapse under the action of a low-frequency instability, are observed near the separatrix. The possible mechanisms of the occurrence of plasma structures and the nature of the plasma streams are discussed.
Institute of Scientific and Technical Information of China (English)
LIN Shang-jing; WEI Gang; LIU Li-long; LIU Yu-di
2005-01-01
Beta effects on surface flows in a rotating annulus with a radial temperature gradient and a sloping bottom were studied experimentally. An azimuthal jet was produced by the action of the Coriolis force in the convective region between the two side walls of the annulus. Propagating velocity and patterns of the baroclinic wave on the surface were obtained by using a frequency-meter and a streak photograph respectively. It is shown that there exists the nonlinear interaction between the baroclinic and beta effects. The beta effect exerts little influence on the stratification flows and constrains the baroclinic instability, and it prompts the instability of the weak stratification flows and results in the surface pattern of waves with higher frequency. It is also indicated that the beta effect can reduce the propagating speed of the surface waves in the jet, and increase the thermal Rossby number for those same surface patterns under a given Taylor number.
Vertical Instability in EAST: Comparison of Model Predictions with Experimental Results
Institute of Scientific and Technical Information of China (English)
QIAN Jinping; WAN Baonian; SHEN Biao; XIAO Bingjia; SUN Youwen; SHI Yuejiang; LIN Shiyao; LI Jiangang; GONG Xianzu
2008-01-01
Growth rates of the axisymmetric mode in elongated plasmas in the experimental advanced superconducting tokamak (EAST) are measured with zero feedback gains and then compared with numerically calculated growth rates for the reconstructed shapes. The comparison is made after loss of vertical position control. The open-loop growth rates were scanned with the number of vessel eigenmodes, which up to 20 is enough to make the growth rates settled. The agreement between the growth rates measured experimentally and the growth rates determined numerically is good. The results show that a linear RZIP model is essentially good enough for the vertical position feedback control.
An Experimental Study of the Flowrate Transients in Slug Flow
Institute of Scientific and Technical Information of China (English)
HELimin; CHENZhenyu; 等
2002-01-01
An investigation of the characteristics of flowrate transients within slug flow was conducted in a largescale outdoor testing facility.The test section consisted of a 378m long,7.62cm diameter stainless steel pipe. Air and water were used as the test fluids.The response to a change of flowrate o either phase or two phases was measured using a series of pressure transducers and differential pressure transducers.An increase or decrease in gas flowrate caused a pressure overshoot above the value at new steady state or led to a pressure undershoot to form a temporary stratified flow.Pressure waves existed in the pipeline,spreading from the entrance to the exit.The magnitude of pressure overshoot in “up-gas”transient or of pressure undershoot and period of the temporary stratified flow in “Down-gas” transient are related to the change of gas flowrate and the distance away from the entrance.In contrast,the change in liquid flowrate was accommodated by smooth transitions between the corresponding steady states,and only one obvious change was found in the slug frequency.According to experimental results,the pressure overshoot,pressure undershoot and the pressure wave propagation were analyzed,and the phenomena were explained reasonably.Some correlations for the calculation of the pressure overshoot and undershoot were proposed.
Experimental investigation of a plane wall jet subjected to an external lateral flow
Kaffel, Ahmed; Moureh, Jean; Harion, Jean-Luc; Russeil, Serge
2015-05-01
The present work aims to experimentally investigate the aerodynamic behavior of a wall jet subjected to external lateral stream by means of time-resolved PIV measurement technique. The experiments are performed on a reduced-scale model representing a generic configuration of a refrigerated display cabinet by focusing on the near-field region ( x/ e < 10) where strong interactions are expected between the jet core, wall boundary and external lateral stream. Comparisons of experimental data obtained with and without external perturbation make it possible to quantify the effect of the perturbation on the time-averaged wall jet characteristics such as airflow patterns, velocity profiles, maximum velocity decay, half-width jet growth, jet entrainment, RMS velocities and coherent structures as well as those related to the development of instabilities. Temporal PIV visualizations have allowed gaining insight on the effect of external lateral perturbation at the outer edge of the jet on the topology of Kelvin-Helmholtz vortices that dominate the early stages of wall jet transition process and play a relevant role on the jet entrainment inwards. Special attention was paid to bring new knowledge of the flow physics related to mutual interactions between outer and inner layer of the wall jet after the break-up of vortex filaments in the braid region due to the external perturbation.
Anupindi, Kameswararao; Lai, Weichen; Frankel, Steven
2014-01-01
In the present work, lattice Boltzmann method (LBM) is applied for simulating flow in a three-dimensional lid driven cubic and deep cavities. The developed code is first validated by simulating flow in a cubic lid driven cavity at 1000 and 12000 Reynolds numbers following which we study the effect of cavity depth on the steady-oscillatory transition Reynolds number in cavities with depth aspect ratio equal to 1, 2 and 3. Turbulence modeling is performed through large eddy simulation (LES) using the classical Smagorinsky sub-grid scale model to arrive at an optimum mesh size for all the simulations. The simulation results indicate that the first Hopf bifurcation Reynolds number correlates negatively with the cavity depth which is consistent with the observations from two-dimensional deep cavity flow data available in the literature. Cubic cavity displays a steady flow field up to a Reynolds number of 2100, a delayed anti-symmetry breaking oscillatory field at a Reynolds number of 2300, which further gets restored to a symmetry preserving oscillatory flow field at 2350. Deep cavities on the other hand only attain an anti-symmetry breaking flow field from a steady flow field upon increase of the Reynolds number in the range explored. As the present work involved performing a set of time-dependent calculations for several Reynolds numbers and cavity depths, the parallel performance of the code is evaluated a priori by running the code on up to 4096 cores. The computational time required for these runs shows a close to linear speed up over a wide range of processor counts depending on the problem size, which establishes the feasibility of performing a thorough search process such as the one presently undertaken. PMID:24587561
Analytical and experimental study of instabilities in buoyancy-driven convection in porous media
Energy Technology Data Exchange (ETDEWEB)
Georgiadis, J.G.; Behringer, R.; Johnson, G.A.
1992-04-01
During the second year of support under the DOE grant, significant progress was made in two directions: (1) Visualization of structure and tow field in randomly packed beds via Magnetic Resonance Imaging, and (2) Shadowgraphic visualization of natural convection in porous systems. This report describes the activities in detail, cite publications which resulted from this project, and conclude with plans for the last phase of the experimental investigation.
Grass, A. J.; Stuart, R. J.; Mansour-Tehrani, M.
1991-01-01
The current status of knowledge regarding coherent vortical structures in turbulent boundary layers and their role in turbulence generation are reviewed. The investigations reported in the study concentrate attention on rough-wall flows prevailing in the geophysical environment and include an experiment determining the three-dimensional form of the turbulence structures linked to the ejection and inrush events observed over rough walls and an experiment concerned with measuring the actual spanwise scale of the near-wall structures for boundary conditions ranging from hydrodynamically smooth to fully rough. It is demonstrated that horseshoe vortical structures are present and play an important role in rough-wall flows and they increase in scale with increasing wall distance, while a dominant spanwise wavelength occurs in the instantaneous cross-flow distribution of streamwise velocity close to the rough wall.
Le Corvec, Nicolas
2014-07-01
Mount Etna volcano is subject to transient magmatic intrusions and flank movement. The east flank of the edifice, in particular, is moving eastward and is dissected by the Timpe Fault System. The relationship of this eastward motion with intrusions and tectonic fault motion, however, remains poorly constrained. Here we explore this relationship by using analogue experiments that are designed to simulate magmatic rift intrusion, flank movement, and fault activity before, during, and after a magmatic intrusion episode. Using particle image velocimetry allows for a precise temporal and spatial analysis of the development and activity of fault systems. The results show that the occurrence of rift intrusion episodes has a direct effect on fault activity. In such a situation, fault activity may occur or may be hindered, depending on the interplay of fault displacement and flank acceleration in response to dike intrusion. Our results demonstrate that a complex interplay may exist between an active tectonic fault system and magmatically induced flank instability. Episodes of magmatic intrusion change the intensity pattern of horizontal flank displacements and may hinder or activate associated faults. We further compare our results with the GPS data of the Mount Etna 2001 eruption and intrusion. We find that syneruptive displacement rates at the Timpe Fault System have differed from the preeruptive or posteruptive periods, which shows a good agreement of both the experimental and the GPS data. Therefore, understanding the flank instability and flank stability at Mount Etna requires consideration of both tectonic and magmatic forcing. Key Points Analyzing Mount Etna east flank dynamics during the 2001 eruption Good correlation between analogue models and GPS data Understanding the different behavior of faulting before/during/after an eruption © 2014. American Geophysical Union. All Rights Reserved.
Sano, T; Sano, Takayoshi; Inutsuka, Shu-ichiro
2001-01-01
The nonlinear evolution and the saturation mechanism of the magnetorotational instability (MRI) are investigated using three-dimensional resistive MHD simulations. A local shearing box is used for our numerical analysis and the simulations begin with a purely vertical magnetic field. We find that the magnetic stress in the nonlinear stage of the MRI is strongly fluctuating. The time evolution shows the quasi-periodic recurrence of spike-shape variations typically for a few orbits which correspond to the rapid amplification of the magnetic field by the nonlinear growth of a two-channel solution followed by the decay through magnetic reconnections. The increase rate of the total energy in the shearing box system is analytically related to the volume-averaged torque in the system. We find that at the saturated state this energy gain of the system is balanced with the increase of the thermal energy mostly due to the joule heating. The spike-shape time evolution is a general feature of the nonlinear evolution of t...
Energy Technology Data Exchange (ETDEWEB)
Schilling, O; Latini, M
2010-01-12
The dynamics of the reshocked multi-mode Richtmyer-Meshkov instability is investigated using 513 x 257{sup 2} three-dimensional ninth-order weighted essentially nonoscillatory shock-capturing simulations. A two-mode initial perturbation with superposed random noise is used to model the Mach 1.5 air/SF{sub 6} Vetter-Sturtevant shock tube experiment. The mass fraction and enstrophy isosurfaces, and density cross-sections are utilized to show the detailed flow structure before, during, and after reshock. It is shown that the mixing layer growth agrees well with the experimentally measured growth rate before and after reshock. The post-reshock growth rate is also in good agreement with the prediction of the Mikaelian model. A parametric study of the sensitivity of the layer growth to the choice of amplitudes of the short and long wavelength initial interfacial perturbation is also presented. Finally, the amplification effects of reshock are quantified using the evolution of the turbulent kinetic energy and turbulent enstrophy spectra, as well as the evolution of the baroclinic enstrophy production, buoyancy production, and shear production terms in the enstrophy and turbulent kinetic transport equations.
Segregation-induced fingering instabilities in granular free-surface flows
Woodhouse, M.J.; Thornton, A.R.; Johnson, C.G.; Kokelaar, B.P.; Gray, J.M.N.T.
2012-01-01
Particle-size segregation can have a significant feedback on the bulk motion of granular avalanches when the larger grains experience greater resistance to motion than the fine grains. When such segregation-mobility feedback effects occur the flow may form digitate lobate fingers or spontaneously se
Schwabe, D.; Benz, S.
We investigated thermocapillary flow in an annular gap with outer heater container of radius R 1 = 40 mm and inner cooled cylinder of R 2 = 20 mm and with an adjustable height h, 2.5 ≤ h ≤ 20 mm. The gap was filled flat up to the rim with the 0.65 cSt silicone oil hexamethyldisiloxane (Prandtl number Pr = 6.7). The temperature differences ΔT, 0 K ≤ ΔT ≤ 40 K between outer and inner wall generated thermocapillary flow in the free upper surface and various flow structures have been observed under microgravity. We identified hydrothermal waves for small h and more complicated oscillations for larger h. For small h and small ΔT the multiroll structure was visible via IR-images of the free surface: concentric steady convection rolls with the same sense of rotation, embedded into the main thermocapillary roll. We measured the critical Marangoni number Ma c for the transition to time-dependent flow in the aspect ratio range A = h/(R 1 - R 2), 0.125 ≤ A ≤ 1, where it was virtually constant Ma c ≈ 5 · 10 4. We report and discuss a steady temperature asymmetry, changing its direction from time to time, as recorded by the IR-camera. This symmetry breaking is most probably due to slowly changing residual acceleration in the satellite.
The Clustering Instability in Rapid Granular and Gas-Solid Flows
Fullmer, William D.; Hrenya, Christine M.
2017-01-01
Flows of solid particles are known to exhibit a clustering instability—dynamic microstructures characterized by a dense region of highly concentrated particles surrounded by a dilute region with relatively few particles—that has no counterpart in molecular fluids. Clustering is pervasive in rapid flows. Its presence impacts momentum, heat, and mass transfer, analogous to how turbulence affects single-phase flows. Yet predicting clustering is challenging, again analogous to the prediction of turbulent flows. In this review, we focus on three key areas: (a) state-of-the-art mathematical tools used to study clustering, with an emphasis on kinetic theory–based continuum models, which are critical to the prediction of the larger systems found in nature and industry, (b) mechanisms that give rise to clustering, most of which are explained via linear stability analyses of kinetic theory–based models, and (c) a critical review of validation studies of kinetic theory–based models to highlight the accuracies and limitations of such theories.
An analogy of Taylor's instability criterion in Couette and rotating-magnetic-field-driven flows
Ungarish, Marius
2012-01-01
The classical stability solution of Taylor for the Couette flow between a rotating inner cylinder and a stationary outer cylinder is used to model the "critical magnetic Taylor number," Tacr, in a flow of a liquid metal driven by a rotating magnetic field (RMF) in a cylindrical cavity characterized by the parameter H = height/radius. (The magnetic Taylor number is defined as Ta =σωBo2Ro4/(2ρν2), where σ ,ν, and ρ are the electrical conductivity, kinematic viscosity, and density of the liquid; ω and Bo are the magnetic field frequency and induction; Ro is the radius of the cavity; the cr superscript means "critical") In typical conditions, the RMF flow develops a solid-body-rotating core analogous to the inner rotating cylinder, embedded in a layer in which the swirl decays to zero at the outer wall. Using small-Ekman-number approximations for the core and gap flow, the analogy yields an insightful expression for Tacr. In particular, the model indicates that Tacr depends strongly on the parameter H. Comparisons of the present theoretical results with available realistic data show a good qualitative agreement and plausible quantitative agreement. The model was improved by an empirical adjustment of a coefficient and can be used as simple approximate prediction tool for Tacr in a quite wide range of cylindrical cavity configurations.
Experimental evidence of conformal invariance in soap film turbulent flows
Thalabard, S; Artana, G; Mininni, P D; Pouquet, A
2010-01-01
We present experimental evidence of statistical conformal invariance in isocontours of fluid thickness in experiments of two-dimensional turbulence using soap films. A Schlieren technique is used to visualize regions of the flow with constant film thickness, and association of isocontours with Schramm-L\\"owner evolution (SLE) is used to identify conformal invariance. In experiments where an inverse energy cascade develops, statistical evidence is consistent with such an association. The diffusivity of the associated one-dimensional Brownian process is close to 8/3, a value previously identified in isocontours of vorticity in high-resolution numerical simulations of two-dimensional turbulence (D. Bernard et al., Nature Phys. 2, 124, 2006). In experiments where the inverse energy cascade is not sufficiently developed, no statistical evidence of conformal invariance is found.
Chekanov, V. V.; Kandaurova, N. V.; Chekanov, V. S.
2014-09-01
A variation in the reflection coefficient of an interface of two liquids (water and magnetic liquid) in the presence of an electric field is experimentally studied. An increase in the reflection coefficient of the interface is demonstrated. A surface instability of the water-magnetic liquid interface, the wave motion at the interface, and wave interference are observed.
Institute of Scientific and Technical Information of China (English)
无
2010-01-01
In this work, a large eddy simulation (LES) model, which includes momentum and heat source (or sink) inside the tree planting layer, is proposed for the simulation of flow in a street canyon with tree planting. Vegetation canopy layer simulation shows that this model can be used to simulate the velocity distribution and temperature variation inside the canopy layer. Effects of atmospheric instability on flow and pollutant distribution in a street canyon with tree planting of an aspect ratio of 0.5 are studied. Results show that compared with the canyon with no tree planting (or the exposed street canyon), the canyon with tree planting shows a reduced wind circulation and pollutant exchange rate (PER) at the top layer of the street canyon, which induces the increase in the pollutant concentrations near road surface, leeward wall and windward wall. When street canyon atmosphere is under a strongly unstable condition, wind velocity decreases while pollutant concentration is increased in the areas near the street canyon top, road surface, leeward and windward walls, compared with the wind velocity in the street canyon with the neutral stratification. When street canyon atmosphere is under a weakly unstable condition, wind velocity weakens near the street canyon top and windward wall, but strengthens near the road surface and leeward wall, and pollutant concentration is decreased near the leeward and windward walls and is increased between the two rows of trees. When the street canyon atmosphere is under an unstable condition, PER is lower than that under the neutral stratification.
New criteria to detect singularities in experimental incompressible flows
Kuzzay, Denis; Martins, Fabio J W A; Faranda, Davide; Foucaut, Jean-Marc; Daviaud, François; Dubrulle, Bérengère
2016-01-01
We introduce two new singularity detection criteria based on the work of Duchon-Robert (DR) [J. Duchon and R. Robert, Nonlinearity, 13, 249 (2000)], and Eyink [G.L. Eyink, Phys. Rev. E, 74 (2006)] which allow for the local detection of singularities with scaling exponent $h\\leqslant1/2$ in experimental flows, using PIV measurements. We show that in order to detect such singularities, one does not need to have access to the whole velocity field inside a volume but can instead look for them from stereoscopic particle image velocimetry (SPIV) data on a plane. We discuss the link with the Beale-Kato-Majda (BKM) [J.T. Beale, T. Kato, A. Majda, Commun. Math. Phys., 94, 61 (1984)] criterion, based on the blowup of vorticity, which applies to singularities of Navier-Stokes equations. We illustrate our discussion using tomographic PIV data obtained inside a high Reynolds number flow generated inside the boundary layer of a wind tunnel. In such a case, BKM and DR criteria are well correlated with each other.
Numerical and Experimental Investigation of Flow Structures During Insect Flight
Badrya, Camli; Baeder, James D.
2015-11-01
Insect flight kinematics involves complex interplay between aerodynamics structural response and insect body control. Features such as cross-coupling kinematics, high flapping frequencies and geometrical small-scales, result in experiments being challenging to perform. In this study OVERTURNS, an in-house 3D compressible Navier-Stokes solver is utilized to simulate the simplified kinematics of an insect wing in hover and forward flight. The flapping wings simulate the full cycle of wing motion, i.e., the upstroke, downstroke, pronation and supination.The numerical results show good agreement against experimental data in predicting the lift and drag over the flapping cycle. The flow structures around the flapping wing are found to be highly unsteady and vortical. Aside from the tip vortex on the wings, the formation of a prominent leading edge vortex (LEV) during the up/down stroke portions, and the shedding of a trailing edge vortex (TEV) at end of each stroke were observed. Differences in the insect dynamics and the flow features of the LEV are observed between hover and forward flight. In hover the up and downstroke cycles are symmetric, whereas in forward flight, these up and downstroke are asymmetric and LEV strength varies as a function of the kinematics and advance ratio. This work was supported by the Micro Autonomous Systems and Technology (MAST) CTA at the Univer- sity of Maryland.
Directory of Open Access Journals (Sweden)
Ravi Borana
2016-09-01
Full Text Available In the petroleum reservoir at an early stage the oil is recovered due to existing natural pressure and such type of oil recovery is referred as primary oil recovery. It ends when pressure equilibrium occurs and still large amount of oil remains in the reservoir. Consequently, secondary oil recovery process is employed by injection water into some injection wells to push oil towards the production well. The instability phenomenon arises during secondary oil recovery process. When water is injected into the oil filled region, due to the force of injecting water and difference in viscosities of water and native oil, protuberances occur at the common interface. It gives rise to the shape of fingers (protuberances at common interface. The injected water shoots through inter connected capillaries at very high speed. It appears in the form of irregular trembling fingers, filled with injected water in the native oil field; this is due to the immiscibility of water and oil. The homogeneous porous medium is considered with a small inclination with the horizontal, the basic parameters porosity and permeability remain uniform throughout the porous medium. Based on the mass conservation principle and important Darcy's law under the specific standard relationships and basic assumptions considered, the governing equation yields a non-linear partial differential equation. The Crank–Nicolson finite difference scheme is developed and on implementing the boundary conditions the resulting finite difference scheme is implemented to obtain the numerical results. The numerical results are obtained by generating a MATLAB code for the saturation of water which decreases with the space variable and increases with time. The obtained numerical solution is efficient, accurate, and reliable, matches well with the physical phenomenon.
Temporal and Spatio-Temporal Dynamic Instabilities: Novel Computational and Experimental approaches
Doedel, Eusebius J.; Panayotaros, Panayotis; Lambruschini, Carlos L. Pando
2016-11-01
This special issue contains a concise account of significant research results presented at the international workshop on Advanced Computational and Experimental Techniques in Nonlinear Dynamics, which was held in Cusco, Peru in August 2015. The meeting gathered leading experts, as well as new researchers, who have contributed to different aspects of Nonlinear Dynamics. Particularly significant was the presence of many active scientists from Latin America. The topics covered in this special issue range from advanced numerical techniques to novel physical experiments, and reflect the present state of the art in several areas of Nonlinear Dynamics. It contains seven review articles, followed by twenty-one regular papers that are organized in five categories, namely (1) Nonlinear Evolution Equations and Applications, (2) Numerical Continuation in Self-sustained Oscillators, (3) Synchronization, Control and Data Analysis, (4) Hamiltonian Systems, and (5) Scaling Properties in Maps.
Experimental and numerical studies of two-phase microfluidic flows
CSIR Research Space (South Africa)
Mbanjwa, MB
2010-09-01
Full Text Available Flow of immiscible fluids is important in microfluidics for applications such as generation of emulsions and vesicles, drug delivery capsules, cell encapsulation and chemical reactions. The behaviour of these flows differs from large scale flows...
Jo, Young Hyun; Lee, Hae June; Mikhailenko, Vladimir V.; Mikhailenko, Vladimir S.
2016-01-01
It was derived that the drift-Alfven instabilities with the shear flow parallel to the magnetic field have significant difference from the drift-Alfven instabilities of a shearless plasma when the ion temperature is comparable with electron temperature for a finite plasma beta. The velocity shear not only modifies the frequency and the growth rate of the known drift-Alfven instability, which develops due to the inverse electron Landau damping, but also triggers a combined effect of the velocity shear and the inverse ion Landau damping, which manifests the development of the ion kinetic shear-flow-driven drift-Alfven instability. The excited unstable waves have the phase velocities along the magnetic field comparable with the ion thermal velocity, and the growth rate is comparable with the frequency. The development of this instability may be the efficient mechanism of the ion energization in shear flows. The levels of the drift--Alfven turbulence, resulted from the development of both instabilities, are determined from the renormalized nonlinear dispersion equation, which accounts for the nonlinear effect of the scattering of ions by the electromagnetic turbulence. The renormalized quasilinear equation for the ion distribution function, which accounts for the same effect of the scattering of ions by electromagnetic turbulence, is derived and employed for the analysis of the ion viscosity and ions heating, resulted from the interactions of ions with drift-Alfven turbulence. In the same way, the phenomena of the ion cyclotron turbulence and anomalous anisotropic heating of ions by ion cyclotron plasma turbulence has numerous practical applications in physics of the near-Earth space plasmas. Using the methodology of the shearing modes, the kinetic theory of the ion cyclotron turbulence of the plasma with transverse current with strong velocity shear has been developed.
On the Goertler instability in hypersonic flows: Sutherland law fluids and real gas effects
Fu, Yibin B.; Hall, Philip; Blackaby, Nicholas D.
1990-01-01
The Goertler vortex instability mechanism in a hypersonic boundary layer on a curved wall is investigated. The precise roles of the effects of boundary layer growth, wall cooling, and gas dissociation is clarified in the determination of stability properties. It is first assumed that the fluid is an ideal gas with viscosity given by Sutherland's law. It is shown that when the free stream Mach number M is large, the boundary layer divides into two sublayers: a wall layer of O(M sup 3/2) thickness over which the basic state temperature is O(M squared) and a temperature adjustment layer of O(1) thickness over which the basic state temperature decreases monotonically to its free stream value. Goertler vortices which have wavelengths comparable with the boundary layer thickness are referred to as wall modes. It is shown that their downstream evolution is governed by a set of parabolic partial differential equations and that they have the usual features of Goertler vortices in incompressible boundary layers. As the local wavenumber increases, the neutral Goertler number decreases and the center of vortex activity moves towards the temperature adjustment layer. Goertler vortices with wavenumbers of order one or larger must necessarily be trapped in the temperature adjustment layer and it is this mode which is most dangerous. For this mode, it was found that the leading order term in the Goertler number expansion is independent of the wavenumber and is due to the curvature of the basic state. This term is also the asymptotic limit of the neutral Goertler numbers of the wall mode. To determine the higher order corrections terms in the Goertler number expansion, two wall curvature cases are distinguished. Real gas effects were investigated by assuming that the fluid is an ideal dissociating gas. It was found that both gas dissociation and wall cooling are destabilizing for the mode trapped in the temperature adjustment layer, but for the wall mode trapped near the wall the
Hydrodynamic Instabilities in Rotating Fluids
Institute of Scientific and Technical Information of China (English)
KarlBuehler
2000-01-01
Rotating flow systems are often used to study stability phenomena and structure developments.The closed spherical gap prblem is generalized into an open flow system by superimposing a mass flux in meridional direction.The basic solutions at low Reynolds numbers are described by analytical methods.The nonlinear supercritical solutions are simulated numerically and realized in experiments.Novel steady and time-dependent modes of flows are obtained.The extensive results concern the stability behaviour.non-uniqueness of supercritical solutions,symmetry behaviour and transitions between steady and time-dependent solutions.The experimental investigations concern the visualization of the various instabilities and the quatitative description of the flow structures including the laminar-turbulent transition.A Comparison between theoretical and experimental results shows good agreement within the limit of rotational symmetric solutions from the theory.
1990-07-01
Il est toutefois peu probable gu’une dynamique aussi sixupliste qu’est 1quation de Landau (temporelle) puisse gouverner le ddveloppement des...pris en considdration. Une mdthode d’dvaluation de v) a dt entreprise . Le nomibre de Reynolds critique est 65500. En faisant la moyenne des taux de
Experimental investigation of the flow-induced vibration of hydrofoils in cavitating flows
Wang, Guoyu; Wu, Qin; Huang, Biao; Gao, Yuan
2015-12-01
The objective of this paper is to investigate the correlation between fluid induced vibration and unsteady cavitation behaviours. Experimental results are presented for a modified NACA66 hydrofoil, which is fixed at α=8°. The high-speed camera is synchronized with a single point Laser Doppler Vibrometer to analyze the transient cavitating flow structures and the corresponding structural vibration characteristics. The results showed that, with the decreasing of the cavitation number, the cavitating flows in a water tunnel display several types of cavitation patterns, such as incipient cavitation, sheet cavitation and cloud cavitation. The cavity shedding frequency reduces with the decrease of the cavitation number. As for the cloud cavitation regime, the trend of the vibration velocity goes up with the growth of the attached cavity, accompanied with small amplitude fluctuations. Then the collapse and shedding of the large-scale cloud cavities leads to substantial increase of the vibration velocity fluctuations.
An experimental study of the edge effect on transition of the rotating-disk boundary-layer flow
Imayama, Shintaro; Lingwood, R. J.; Alfredsson, P. Henrik
2011-11-01
Lingwood [J. Fluid Mech., 299, 17 (1995)] showed that the flow instability in the rotating-disk boundary layer is not only of convective nature but also that the flow becomes absolutely unstable. Furthermore, in the absence of bypass mechanisms, the absolute instability triggers nonlinearity and transition to turbulence at a fixed Reynolds number (Re). Healey [J. Fluid Mech., 663, 148 (2010)] suggested that the observed spread (albeit small) in transition Re in different experiments is an effect of the Re at the disk edge and provided a nonlinear model to take this effect into account. Here, we further investigate this problem experimentally with hot-wire measurements on a rotating polished glass disk with a diameter of 474 mm and a total imbalance and surface roughness less than 10 μm. To investigate the influence of the disk edge, we vary Re at the disk edge by changing the rotational speed and map the development of the disturbance velocity in the radial direction. Furthermore, the effect of a stationary annular plate around the edge of the rotating disk is also investigated. Our experiments show no effect of the disk edge Re on the stability and transition, however there was a shift of both the growth curve and the transition Re by about 10 units with and without the outer stationary plate, with the lower Re observed with the plate.
Energy Technology Data Exchange (ETDEWEB)
Ahmadi, Goodarz
2006-09-30
Semi-analytical computational models for natural gas flow in hydrate reservoirs were developed and the effects of variations in porosity and permeability on pressure and temperature profiles and the movement of a dissociation front were studied. Experimental data for variations of gas pressure and temperature during propane hydrate formation and dissociation for crushed ice and mixture of crushed ice and glass beads under laboratory environment were obtained. A thermodynamically consistent model for multiphase liquid-gas flows trough porous media was developed. Numerical models for hydrate dissociation process in one dimensional and axisymmetric reservoir were performed. The computational model solved the general governing equations without the need for linearization. A detail module for multidimensional analysis of hydrate dissociation which make use of the FLUENT code was developed. The new model accounts for gas and liquid water flow and uses the Kim-Boshnoi model for hydrate dissociation.
Numerical and experimental studies of droplet-gas flow
Energy Technology Data Exchange (ETDEWEB)
Joesang, Aage Ingebret
2002-07-01
This thesis considers droplet-gas flow by the use of numerical methods and experimental verification. A commercial vane separator was studied both numerical and by experiment. In addition some efforts are put into the numerical analysis of cyclones. The experimental part contains detailed measurements of the flow field between a pair of vanes in a vane separator and droplet size measurements. LDA (Laser Doppler Anemometry) was used to measure the velocity in two dimensions and corresponding turbulence quantities. The results from the LDA measurements are considered to be of high quality and are compared to numerical results obtained from a CFD (Computational Fluid Dynamics) analysis. The simulation showed good agreement between the numerical and experimental results. Combinations of different turbulence models; the standard k-epsilon model and the Reynold Stress Mode, different schemes; first order and higher order scheme and different near wall treatment of the turbulence; the Law of the wall and the Two-Layer Zonal model were used in the simulations. The Reynold Stress Model together with a higher order scheme performed rather poorly. The recirculation in parts of the separator was overpredicted in this case. For the other cases the overall predictions are satisfactory. PDA (Phase Doppler Anemometry) measurements were used to study the changes in the droplet size distribution through the vane separator. The PDA measurements show that smaller droplets are found at the outlet than present at the inlet. In the literature there exists different mechanisms for explaining the re-entrainment and generation of new droplets. The re-entrainments mechanisms are divided into four groups where droplet-droplet interaction, droplet break-up, splashing of impinging droplet and re-entrainment from the film are defined as the groups of re-entrainment mechanisms. Models for these groups are found in the literature and these models are tested for re-entrainment using the operational
A classical flow instability and its connection to gaseous galactic disk hydrodynamics
Vatistas, G H
2007-01-01
Water vortices are known to develop polygon structures inside their cores. The apexes of the polygonal manifestations are the result of satellite vortices attached to the parent vortex. Exploiting the analogy between the shallow water hydraulics and the two-dimensional compressible gas flows we arrive to the following line of reasoning concerning the spiral galactic structure. The backward free surface fore ripples of the advancing pressure disturbance in the hydraulic system should appear as high-density spirals in the gaseous galactic disk. Since the number of galactic arms is equal to the number of satellite eddies in the parent vortex, spiral galaxies with say two arms will possess two nuclei. The last appears to be in complete agreement with the latest observations regarding M 31 and Mrk 315 galaxies. If the present hypothesis is accurate then a closer look at the central regions of multi-tentacle spiral galaxies should also reveal that these have multiple nuclei.
EXPERIMENTAL INVESTIGATION OF TIP CLEARANCE FLOW FOR AN AXIAL FLOW FAN ROTOR
Institute of Scientific and Technical Information of China (English)
GUO Qiang; ZHU Xiaocheng; DU Zhaohui
2006-01-01
The flow field in the tip region of an axial ventilation fan is investigated with a particle image velocimeter (PIV) system at the design condition. Flow fields with three different tip clearances are surveyed on three different circumferential planes, respectively. The phase-locked average method is used to investigate the generation and the development of a tip leakage vortex. The result from PIV system is compared with that from a particle dynamics anemometer(PDA) system. Both data are in good agreement and the structure of the tip leakage vortex for the rotor is illustrated. The characteristic of a leakage vortex is described in both velocity vectors and vortical contours. The unsteadiness of the leakage vortex and the position of the vortex are surveyed in detail, which interprets the discrepancy between the numerical simulation and PDA experimental results to a certain extent. The center loci of tip leakage vortex at different times and the mean center loci of the leakage vortex are displayed particularly. Finally, the trajectories of the tip leakage vortex by the experimental measurement are compared with predictions from the existing models for high speed and high-pressure compressors and turbines when appropriately interpreted. A good agreement is obtained.
Energy Technology Data Exchange (ETDEWEB)
Jayashri, R. [Department of Electrical and Electronics Engineering, Sri Venkateswara College of Engineering (Affiliated to Anna University), Pennalur, Sriperumbudur, Tamilnadu 602105 (India); Kumudini Devi, R.P. [Department of Electrical and Electronics Engineering, College of Engineering, Anna University, Chennai (India)
2009-03-15
In this paper, the dynamic performance of grid connected Wind Energy Conversion System (WECS) is analysed in terms of the newly defined concept of rotor speed stability. The WECS is considered as a fixed-speed system that is equipped with a squirrel-cage induction generator. The drive-train is represented as a two-mass model. Results show that for a particular fault simulated the voltage at the Point of Common Coupling (PCC) drops below 80% immediately after fault application and settles at a low value. The rotor speed of induction generators becomes unstable. In order to improve the low voltage ride-through of WECS under fault conditions and to damp the rotor speed oscillations of induction generator, an Unified Power Flow Controller (UPFC) is employed. The gains of this FACTS controller are tuned with a simple Genetic Algorithm (GA). It is observed that UPFC helps not only in regulating the voltage, but also in mitigating the rotor speed instability. (author)
Experimental study of flow inversion in MTR upward flow research reactors
Energy Technology Data Exchange (ETDEWEB)
Abdel-Hadi, Ead A. [Benha Univ., Cairo (Egypt). Shobra Faculty of Engineering; Khedr, Ahmed; Talha, Kamal Eldin Aly; Abdel-Latif, Salwa Helmy
2014-06-15
The core cooling of upward flow MTR pool type Research Reactor (RR) at the later stage of pump coast down is experimentally handled to clarify the effect of some operating parameters on RR core cooling. Therefore, a test rig is designed and built to simulate the core cooling loop at this stage. The core is simulated as two vertical channels, electrically heated, and extended between upper and lower plenums. Two elevated tanks filled with water are connected to the two plenums. The first one constitutes a left branch, connected to the lower plenum, and is electrically heated to simulate the core return pipe. The second one constitutes the right branch, connected to the upper plenum, and is cooled by refrigerant circuit to simulate the reactor pool. Channel coolant and wall temperatures at different power and branch temperatures are measured, registered and analyzed. The results show that at this stage of core cooling two cooling loops are established; an internal circulation loop between the channels dominated by the difference in channel's power and an external circulation loop between the branches dominated by the temperature difference between branches. Also, there is a double inversion in core flow, upward-downward-upward flow. This double inversion increases largely the channel's wall temperature. Complementary safety analysis to evaluate this phenomenon must be performed. (orig.)
Jain, Neeraj
2014-01-01
We examine the effect of an external guide field and current sheet thickness on the growth rates and nature of three dimensional unstable modes of an electron current sheet driven by electron shear flow. The growth rate of the fastest growing mode drops rapidly with current sheet thickness but increases slowly with the strength of the guide field. The fastest growing mode is tearing type only for thin current sheets (half thickness $\\approx d_e$, where $d_e=c/\\omega_{pe}$ is electron inertial length) and zero guide field. For finite guide field or thicker current sheets, fastest growing mode is non-tearing type. However growth rates of the fastest 2-D tearing mode and 3-D non-tearing mode are comparable for thin current sheets ($d_e < $half thickness $ < 2\\,d_e$) and small guide field (of the order of the asymptotic value of the component of magnetic field supporting electron current sheet). It is shown that the general mode resonance conditions for electron-magnetohydrodynamic (EMHD) and magnetohydrody...
Institute of Scientific and Technical Information of China (English)
樊建华; 王运东; 费维扬
2007-01-01
The aim of this work is to investigate the flow instabilities in a baffled,stirred tank generated by a single Rushton turbine by means of large eddy simulation (LES).The sliding mesh method was used for the coupling between the rotating and the stationary frame of references.The calculations were carried out on the "Shengcao-21C" supercomputer using a computational fluid dynamics (CFD) code CFX5.The flow fields predicted by the LES simulation and the simulation using standard k-ε model were compared to the results from particle image velocimetry (PIV) measurements.It is shown that the CFD simulations using the LES approach and the standard k-ε model agree well with the PIV measurements.Fluctuations of the radial and axial velocity are predicted at different frequencies by the LES simulation.Velocity fluctuations of high frequencies are seen in the impeller region,while low frequencies velocity fluctuations are observed in the bulk flow.A low frequency velocity fluctuation with a nondimensional frequency of 0.027Hz is predicted by the LES simulation,which agrees with experimental investigations in the literature.Flow circulation patterns predicted by the LES simulation are asymmetric,stochastic and complex,spanning a large portion of the tanks and varying with time,While circulation patterns calculated by the simulation using the standard k-ε model are symmetric.The results of the present work give better understanding to the flow instabilities in the mechanically agitated tank.However,further analysis of the LES calculated velocity Series by means of fast Fourier transform (FFT) and/or spectra analysis are recommended in future work in order to gain more knowledge of the complicated flow phenomena.
Modelling Fluidelastic Instability Forces in Tube Arrays
Anderson, J. Burns
Historically, heat exchangers have been among the most failure prone components in nuclear power plants. Most of these failures are due to tube failures as a result of corrosion, fatigue and fretting wear. Fatigue and fretting wear are a result of flow induced vibration through turbulent buffeting and fluidelastic instability mechanisms. Fluidelastic instability is by far the most important and complex mechanism. This research deals with modelling fluidelastic instability and the resulting tube response. The proposed time domain model uses the concept of a flow cell (Hassan & Hayder [16]) to represent the complex flow field inside a shell and tube heat exchanger and accounts for temporal variations in the flow separation points as a result of tube motion. The fluidelastic forces are determined by predicting the attachment lengths. The predicted forces are used to simulate the response of a single flexible tube inside a shell and tube heat exchanger. It was found that accounting for temporal variations in the separation points predicted lower critical flow velocities, than that of fixed attachment and separation points. Once unstable a phase lag is predicted between the fluidelastic forces and tube response. It was determined that the predicted critical flow velocities agreed well with available experimental data. The developed model represents an important step towards a realistic fluidelastic instability model which can be used to design the new generation nuclear steam generators.
Experimental Research on Flow Separation Control using Synthetic Jet Actuators
Koopmans, E.; Hoeijmakers, H.W.M.
2014-01-01
Airplane wings can suffer from flow separation, which greatly decreases their aerodynamic per-formance. The flow separates due to the bound-ary layer possessing insufficient momentum to engage the adverse pressure gradient along the airfoil surface. Flow separation control actively influences the fl
Experimental research on flow separation control using synthetic jet actuators
Koopmans, E.; Hoeijmakers, H.W.M.
2014-01-01
Airplane wings can suffer from flow separation, which greatly decreases their aerodynamic per-formance. The flow separates due to the bound-ary layer possessing insufficient momentum to engage the adverse pressure gradient along the airfoil surface. Flow separation control actively influences the fl
Cavitation Instabilities in Inducers
2006-11-01
gas handling turbomachines . The fluctuation of the cavity length is plotted in Fig.8 under the surge mode oscillation vi . The major differences...Cavitation Instabilities of Turbomachines .” AIAA Journal of Propulsion and Power, Vol.17, No.3, 636-643. [5] Tsujimoto, Y., (2006), “Flow Instabilities in
The cycle of instability: stress release and fissure flow as controls on gully head retreat
Collison, A. J. C.
2001-01-01
Gully head and wall retreat has commonly been attributed to fluvial scour and head collapse as a result of soil saturation, sapping or piping. The empirical evidence to substantiate these conceptual models is sparse, however, and often contradictory. This paper explores the hydrological and mechanical controls on gully head and wall stability by modelling the hydrology, stability and elastic deformation of a marl gully complex in Granada Province, south-east Spain. The hydrological and slope-stability simulations show that saturated conditions can be reached only where preferential fissure flow channels water from tension cracks into the base of the gully head, and that vertical or subvertical heads will be stable unless saturation is achieved. Owing to the high unsaturated strengths of marl measured in this research, failure in unsaturated conditions is possible only where the gully head wall is significantly undercut. Head retreat thus requires the formation of either a tension crack or an undercut hollow. Finite-element stress analysis of eroding slopes reveals a build up of shear stress at the gully head base, and a second stress anomaly just upslope of the head wall. Although tension cracks on gully heads have often been attributed to slope unloading, this research provides strong evidence that the so called sapping hollow commonly found in the gully headwall base is also a function of stress release. Although further research is needed, it seems possible that pop out failures in river channels may be caused by the same process. The hydrological analysis shows that, once a tension crack has developed, throughflow velocity in the gully headwall will increase by an order of magnitude, promoting piping and enlargement of this weakened area. It is, therefore, possible to envisage a cycle of gully expansion in which erosion, channel incision or human action unloads the slope below a gully head, leading to stress patterns that account for the tension crack and a
Sahu, K. C.; Matar, O. K.
2010-11-01
The three-dimensional linear stability characteristics of pressure-driven two-layer channel flow are considered, wherein a Newtonian fluid layer overlies a layer of a Herschel-Bulkley fluid. We focus on the parameter ranges for which Squire's theorem for the two-layer Newtonian problem does not exist. The modified Orr-Sommerfeld and Squire equations in each layer are derived and solved using an efficient spectral collocation method. Our results demonstrate the presence of three-dimensional instabilities for situations where the square root of the viscosity ratio is larger than the thickness ratio of the two layers; these "interfacial" mode instabilities are also present when density stratification is destabilizing. These results may be of particular interest to researchers studying the transient growth and nonlinear stability of two-fluid non-Newtonian flows. We also show that the "shear" modes, which are present at sufficiently large Reynolds numbers, are most unstable to two-dimensional disturbances.
Scovazzi, G.; Huang, H.; Collis, S. S.; Yin, J.
2013-11-01
We present a new approach to the simulation of viscous fingering instabilities in incompressible, miscible displacement flows in porous media. In the past, high resolution computational simulations of viscous fingering instabilities have always been performed using high-order finite difference or Fourier-spectral methods which do not posses the flexibility to compute very complex subsurface geometries. Our approach, instead, by means of a fully-coupled nonlinear implementation of the discontinuous Galerkin method, possesses a fundamental differentiating feature, in that it maintains high-order accuracy on fully unstructured meshes. In addition, the proposed method shows very low sensitivity to mesh orientation, in contrast with classical finite volume approximation used in porous media flow simulations. The robustness and accuracy of the method are demonstrated in a number of challenging computational problems.
Energy Technology Data Exchange (ETDEWEB)
Herman, C.; Kang, E. [Dept. of Mechanical Engineering, Johns Hopkins Univ., Baltimore, MD (United States)
2001-01-01
An experimental study was conducted of incompressible, moderate Reynolds number flow of air over heated rectangular blocks in a two-dimensional, horizontal channel. Holographic interferometry combined with high-speed cinematography was used to visualize the unsteady temperature fields in self- sustained oscillatory flow. Experiments were conducted in the laminar, transitional and turbulent flow regimes for Reynolds numbers in the range from Re = 520 to Re = 6600. Interferometric measurements were obtained in the thermally and fluiddynamically periodically fully developed flow region on the ninth heated block. Flow oscillations were first observed between Re = 1054 and Re = 1318. The period of oscillations, wavelength and propagation speed of the Tollmien-Schlichting waves in the main channel were measured at two characteristic flow velocities, Re = 1580 and Re = 2370. For these Reynolds numbers it was observed that two to three waves span one geometric periodicity length. At Re = 1580 the dominant oscillation frequency was found to be around 26 Hz and at Re = 2370 the frequency distribution formed a band around 125 Hz. Results regarding heat transfer and pressure drop are presented as a function of the Reynolds number, in terms of the block-average Nusselt number and the local Nusselt number as well as the friction factor. Measurements of the local Nusselt number together with visual observations indicate that the lateral mixing caused by flow instabilities is most pronounced along the upstream vertical wall of the heated block in the groove region, and it is accompanied by high heat transfer coefficients. At Reynolds numbers beyond the onset of oscillations the heat transfer in the grooved channel exceeds the performance of the reference geometry, the asymmetrically heated parallel plate channel. (orig.)
Herman, C.; Kang, E.
An experimental study was conducted of incompressible, moderate Reynolds number flow of air over heated rectangular blocks in a two-dimensional, horizontal channel. Holographic interferometry combined with high-speed cinematography was used to visualize the unsteady temperature fields in self- sustained oscillatory flow. Experiments were conducted in the laminar, transitional and turbulent flow regimes for Reynolds numbers in the range from Re=520 to Re=6600. Interferometric measurements were obtained in the thermally and fluiddynamically periodically fully developed flow region on the ninth heated block. Flow oscillations were first observed between Re=1054 and Re=1318. The period of oscillations, wavelength and propagation speed of the Tollmien-Schlichting waves in the main channel were measured at two characteristic flow velocities, Re=1580 and Re=2370. For these Reynolds numbers it was observed that two to three waves span one geometric periodicity length. At Re=1580 the dominant oscillation frequency was found to be around 26Hz and at Re=2370 the frequency distribution formed a band around 125Hz. Results regarding heat transfer and pressure drop are presented as a function of the Reynolds number, in terms of the block-average Nusselt number and the local Nusselt number as well as the friction factor. Measurements of the local Nusselt number together with visual observations indicate that the lateral mixing caused by flow instabilities is most pronounced along the upstream vertical wall of the heated block in the groove region, and it is accompanied by high heat transfer coefficients. At Reynolds numbers beyond the onset of oscillations the heat transfer in the grooved channel exceeds the performance of the reference geometry, the asymmetrically heated parallel plate channel.
Xiong, Renqiang; Chung, J. N.
2007-03-01
Adiabatic gas-liquid flow patterns and void fractions in microchannels were experimentally investigated. Using nitrogen and water, experiments were conducted in rectangular microchannels with hydraulic diameters of 0.209mm, 0.412mm and 0.622mm, respectively. Gas and liquid superficial velocities were varied from 0.06-72.3m/s and 0.02-7.13m/s, respectively. The main objective is focused on the effects of microscale channel sizes on the flow regime map and void fraction. The instability of flow patterns was observed. Four groups of flow patterns including bubbly slug flow, slug-ring flow, dispersed-churn flow, and annular flow were observed in microchannels of 0.412mm and, 0.622mm. In the microchannel of 0.209mm, the bubbly slug flow became the slug flow and the dispersed-churn flow disappeared. The current flow regime maps showed the transition lines shifted to higher gas superficial velocity due to a dominant surface tension effect as the channel size was reduced. The regime maps presented by other authors for minichannels were found to not be applicable for microchannels. Time-averaged void fractions were measured by analyzing 8000 high speed video images for each flow condition. The void fractions hold a nonlinear relationship with the homogeneous void fraction as opposed to the relatively linear trend for the minichannels. A new correlation was developed to predict the nonlinear relationship that fits most of the current experimental data and those of the 0.1mm diameter tube reported by Kawahara et al. [Int. J. Multiphase Flow 28, 1411 (2002)] within ±15%.
An Experimental Investigation of the Flow Over the Rear End of a Notchback Automobile Configuration
Jenkins, Luther N.
2000-01-01
An experimental investigation of the flow over the rear end of a 0.16 scale notchback automobile configuration has been conducted in the NASA Langley Basic Aerodynamics Research Tunnel (BART). The objective of this work was to investigate the flow separation that occurs behind the backlight and obtain experimental data that can be used to understand the physics and time-averaged structure of the flow field. A three-component laser velocimeter was used to make non-intrusive, velocity measurements in the center plane and in a single cross-flow plane over the decklid. In addition to off-body measurements, flow conditions on the car surface were documented via surface flow visualization, boundary layer measurements, and surface pressures. The experimental data show several features previously identified by other researchers, but also reveal differences between the flow field associated with this particular configuration and the generally accepted models for the flow over a notchback rear end.
Askari, Omid
This dissertation investigates the combustion and injection fundamental characteristics of different alternative fuels both experimentally and theoretically. The subjects such as lean partially premixed combustion of methane/hydrogen/air/diluent, methane high pressure direct-injection, thermal plasma formation, thermodynamic properties of hydrocarbon/air mixtures at high temperatures, laminar flames and flame morphology of synthetic gas (syngas) and Gas-to-Liquid (GTL) fuels were extensively studied in this work. These subjects will be summarized in three following paragraphs. The fundamentals of spray and partially premixed combustion characteristics of directly injected methane in a constant volume combustion chamber have been experimentally studied. The injected fuel jet generates turbulence in the vessel and forms a turbulent heterogeneous fuel-air mixture in the vessel, similar to that in a Compressed Natural Gas (CNG) Direct-Injection (DI) engines. The effect of different characteristics parameters such as spark delay time, stratification ratio, turbulence intensity, fuel injection pressure, chamber pressure, chamber temperature, Exhaust Gas recirculation (EGR) addition, hydrogen addition and equivalence ratio on flame propagation and emission concentrations were analyzed. As a part of this work and for the purpose of control and calibration of high pressure injector, spray development and characteristics including spray tip penetration, spray cone angle and overall equivalence ratio were evaluated under a wide range of fuel injection pressures of 30 to 90 atm and different chamber pressures of 1 to 5 atm. Thermodynamic properties of hydrocarbon/air plasma mixtures at ultra-high temperatures must be precisely calculated due to important influence on the flame kernel formation and propagation in combusting flows and spark discharge applications. A new algorithm based on the statistical thermodynamics was developed to calculate the ultra-high temperature plasma
Erosion by shallow concentrated flow - experimental model deconstruction
Seeger, M.; Wirtz, S.; Ali, M.
2012-04-01
The force of the flowing water is considered to be the main determinant factor for soil particle detachment and transport. The flow of water is described with flow velocity and discharge, and is often summarised in different composite parameters such as shear stress, stream power etc. The entrainment and transport of soil particles is then expressed as a threshold problem, where a soil specific critical value of shear stress, stream power etc. has to be trespassed. Thereafter, the increase of erosion is considered to be lineal. Despite considerable efforts, the process based model concepts have not been able to produce more reliable and accurate reproduction and forecast of soil erosion than "simple" empirical models such as the USLE and its derivates. Therefore, there still remain some unanswered fundamental questions about soil erosion modelling: 1. What are the main parameters of soils and flowing water influencing soil erosion? 2. What relationship do these parameters have with the intensity and different types of soil erosion? 3. Are the present concepts suitable to describe and quantify soil erosion accurately? For approaching these questions, laboratory flume and field experiments were set up. The aim of the laboratory experiments was to elucidate the influence of basic parameters as grain size, slope, flow and flow velocity on sediment transport by shallow flowing water. Therefore, variable flow was applied under different slopes on moveable beds of non-coherent sands of different grain sizes. The field experiments were designed to quantify the hydraulic and erosive functionality of small rills in the field. Here, small existing rills were flushed with defined flows, and flow velocity, flow depth, discharge at the end of the rill as well as transported sediments were quantified. The laboratory flume experiments clearly show a strong influence of flow velocity on sediment transport, depending this at the same time on the size of the transported grains, and
An Experimental Study of Oil / Water Flow in Horizontal Pipes
Energy Technology Data Exchange (ETDEWEB)
Elseth, Geir
2001-07-01
The purpose of this thesis is to study the behaviour of the simultaneous flow of oil and water in horizontal pipes. In this connection, two test facilities are used. Both facilities have horizontal test sections with inner pipe diameters equal to 2 inches. The largest facility, called the model oil facility, has reservoirs of 1 m{sub 3} of each medium enabling flow rates as high as 30 m{sub 3}/h, which corresponds to mixture velocities as high as 3.35 m/s. The flow rates of oil and water can be varied individually producing different flow patterns according to variations in mixture velocity and input water cut. Two main classes of flows are seen, stratified and dispersed. In this facility, the main focus has been on stratified flows. Pressure drops and local phase fractions are measured for a large number of flow conditions. Among the instruments used are differential pressure transmitters and a traversing gamma densitometer, respectively. The flow patterns that appear are classified in flow pattern maps as functions of either mixture velocity and water cut or superficial velocities. From these experiments a smaller number of stratified flows are selected for studies of velocity and turbulence. A laser Doppler anemometer (LDA) is applied for these measurements in a transparent part of the test section. To be able to produce accurate measurements a partial refractive index matching procedure is used. The other facility, called the matched refractive index facility, has a 0.2 m{sub 3} reservoir enabling mainly dispersed flows. Mixture velocities range from 0.75 m/s to 3 m/s. The fluids in this facility are carefully selected to match the refractive index of the transparent part of the test section. A full refractive index matching procedure is carried out producing excellent optical conditions for velocity and turbulence studies by LDA. In addition, pressure drops and local phase fractions are measured. (author)
Experimental study on the type change of liquid flow in broken coal samples
Institute of Scientific and Technical Information of China (English)
Lu-zhen WANG; Zhan-qing CHEN; Hai-de SHEN
2013-01-01
A test system of the permeability of broken coal samples mainly consists of a CMT5305 electronic universal test machine,crushed rock compaction containing cylinder and a self-designed seepage circuit,which is composed of a gear pump,a reversing valve,a relief valve and other components.By using the steady penetration method,the permeability and non-Darcy flow β factor of broken coal samples under five different porosity levels were measured,the grain diameters of the coal samples were selected as 2.5-5 mm,5-10 mm,10-15 mm,15-20 mm,20-25 mm and 2.5-25 mm,respectively.After measuring the permeability under each porosity,the overfall pressure of the relief valve continuously increased until the coal sample was broken down.In this way,the flow type of liquid inside the broken coal samples changed from seepage to pipe flow.The correlation between breakdown pressure gradient (BPG) and porosity was analyzed,and the BPG was compared with the pressure gradient when seepage instability occurred.The results show that,① the non-Darcy flow β factor was negative before broken coal samples with six kinds of diameters were broken down; ② the BPG of coal samples with a grain size of 2.5-25 mm was lower than that of the others; ③ the BPG of coal samples with a single diameter under the same porosity increased as the grain size increased; ④ the BPG could be fitted by an exponential function with porosity,and the exponent decreased as the grain size increased for coal samples with a single diameter; ⑤ the BPG was slightly less than the seepage instability pressure gradient.The change in liquid flow type from seepage to pipe flow could be regarded as the performance of the seepage instability.
Experimental Evidence of Helical Flow in Porous Media
DEFF Research Database (Denmark)
Ye, Yu; Chiogna, Gabriele; Cirpka, Olaf A.;
2015-01-01
Helical flow leads to deformation of solute plumes and enhances transverse mixing in porous media. We present experiments in which macroscopic helical flow is created by arranging different materials to obtain an anisotropic macroscopic permeability tensor with spatially variable orientation...... mixers, but in porous media....
Experimental observation of fluid flow channels in a single fracture
Brown, Stephen; Caprihan, Arvind; Hardy, Robert
1998-03-01
A method for obtaining precise replicas of real fracture surfaces using transparent epoxy resins was developed, allowing detailed study of fluid flow paths within a fracture plane. A natural rock fracture was collected from the field and prepared for study. Silicon rubber molds of the fracture surfaces were used to make a transparent epoxy replica of the original fracture. Clear and dyed water were injected into the fracture pore space allowing examination of the flow field. Digitized optical images were used to observe wetting, saturated flow, and drying of the specimen. Nuclear magnetic resonance imaging was used for quantitative measurements of flow velocity. Both video imaging and nuclear magnetic resonance imaging techniques show distinct and strong channeling of the flow at the submillimeter to several-centimeter scale. Each phenomenon, including wetting, drying, dye transport, and velocity channeling, has its own distinct geometric structure and scale. We find that fluid velocities measured simultaneously at various locations in the fracture plane during steady state flow range over several orders of magnitude, with the maximum velocity a factor of 5 higher than the mean velocity. This suggests that flow channeling in fractured rock can cause the breakthrough velocity of contaminants to far exceed the mean flow.
Directory of Open Access Journals (Sweden)
B. Cariteau
2006-01-01
Full Text Available We investigate the elliptical instability of a strongly asymmetric vortex pair in a stratified fluid, generated by the acceleration and deceleration of the rotation of a single flap. The dominant parameter is the Froude number, Fr=U/(NR, based on the maximum azimuthal velocity, U, and corresponding radius, R, of the strongest vortex, i.e. the principal vortex, and buoyancy frequency N. For Fr>1, both vortices are elliptically unstable while the instability is suppressed for Fr<1. In an asymmetric vortex pair, the principal vortex is less – and the secondary vortex more – elliptical than the vortices in an equivalent symmetric dipolar vortex. The far more unstable secondary vortex interacts with the principal vortex and increases the strain on the latter, thus increasing its ellipticity and its instability growth rate. The nonlinear interactions render the elliptical instability more relevant. An asymmetric dipole can be more unstable than an equivalent symmetric dipole. Further, the wavelength of the instability is shown to be a function of the Froude number for strong stratifications corresponding to small Froude numbers, whereas it remains constant in the limit of a homogenous fluid.
Energy Technology Data Exchange (ETDEWEB)
Vassilaros, M.G.; Gudas, J.P.; Joyce, J.A.
1982-08-01
The objective of this investigation was to extend the range of tearing-instability validation experiments utilizing the compact specimen to include high-toughness alloys. J-Integral tests of ASTM A106; ASTM A516, Grade 70; ASTM A533B; HY-80; and HY-130 steels were performed in a variably compliant screw-driven test machine. Results were analyzed with respect to the materials J/sub I/-R curves and various models of T/sub applied/ for the compact specimen. Tearing instability theory was validated for these high-toughess materials. For the cases of highly curved J/sub I/-R curves, it was shown that the actual value of T/sub material/ at the point of instability should be employed rather than the average T/sub material/ value. The T/sub applied/ analysis of Paris and coworkers applied to the compact specimen appears to be nonconservative in predicting the point of instability; whereas, the T/sub applied/ analysis of Ernst and coworkers appears to be accurate, but requires precision beyond that displayed in this program. The generalized Paris analysis applied to the compact specimen and evaluated at maximum load was most consistent in predicting instability. 16 figures, 3 tables.
Rubinato, Matteo; Martins, Ricardo; Kesserwani, Georges; Leandro, Jorge; Djordjević, Slobodan; Shucksmith, James
2017-09-01
The linkage between sewer pipe flow and floodplain flow is recognised to induce an important source of uncertainty within two-dimensional (2D) urban flood models. This uncertainty is often attributed to the use of empirical hydraulic formulae (the one-dimensional (1D) weir and orifice steady flow equations) to achieve data-connectivity at the linking interface, which require the determination of discharge coefficients. Because of the paucity of high resolution localised data for this type of flows, the current understanding and quantification of a suitable range for those discharge coefficients is somewhat lacking. To fulfil this gap, this work presents the results acquired from an instrumented physical model designed to study the interaction between a pipe network flow and a floodplain flow. The full range of sewer-to-surface and surface-to-sewer flow conditions at the exchange zone are experimentally analysed in both steady and unsteady flow regimes. Steady state measured discharges are first analysed considering the relationship between the energy heads from the sewer flow and the floodplain flow; these results show that existing weir and orifice formulae are valid for describing the flow exchange for the present physical model, and yield new calibrated discharge coefficients for each of the flow conditions. The measured exchange discharges are also integrated (as a source term) within a 2D numerical flood model (a finite volume solver to the 2D Shallow Water Equations (SWE)), which is shown to reproduce the observed coefficients. This calibrated numerical model is then used to simulate a series of unsteady flow tests reproduced within the experimental facility. Results show that the numerical model overestimated the values of mean surcharge flow rate. This suggests the occurrence of additional head losses in unsteady conditions which are not currently accounted for within flood models calibrated in steady flow conditions.
Experimentally observed flows inside inkjet-printed aqueous rivulets
Bromberg, Vadim; Singler, Timothy
2013-11-01
Understanding the flow inside sessile liquid masses of different shapes is important in a variety of solution-based material deposition and patterning processes. We investigated the shape evolution and internal flow of inkjet-printed aqueous rivulets of finite length using optical microscopy. Rivulets were formed by printing a pre-determined number of drops at controlled frequency and spatial overlap. Capillary-driven rivulet breakup into individual drops was inhibited by chemical modification of substrates that resulted in controlled contact angle hysteresis with zero static receding contact angle. A variety of novel capillary- and evaporatively-driven flows were identified using fluorescent particles as flow tracers. Flow regimes were investigated as a function of advancing contact angle, deposition parameters, and fluid properties.
Experimental investigation of flow through planar double divergent nozzles
Arora, Rajat; Vaidyanathan, Aravind
2015-07-01
Dual bell nozzle is one of the feasible and cost effective techniques for altitude adaptation. Planar double divergent nozzle with a rectangular cross section was designed for two different NPR's to simulate and investigate the flow regimes similar to those inside the dual bell nozzle. Measurements involved flow visualization using Schlieren technique and wall static pressure measurements. The flow transition between the two nozzles at the respective inflection points and the formation of recirculation region due to flow separation was analyzed in detail. Cold flow tests were performed on the double divergent nozzle in the over-expanded conditions to study the shock wave characteristics. The results obtained from the two independent double divergent nozzles were compared with those obtained from a single divergent nozzle of the same area ratio. From the experiments it was observed that inflection angle played a key role in defining the type of shock structures existing inside the double divergent nozzles.
Experimental investigation of flow pattern and sediment deposition in rectangular shallow reservoirs
Institute of Scientific and Technical Information of China (English)
Matthieu DUFRESNE; Benjamin J.DEWALS; Sébastien ERPICUM; Pierre ARCHAMBEAU; Michel PIROTTON
2010-01-01
This paper reports the experimental investigation of flow pattern, preferential regions of deposition and trap efficiency as a function of the length of rectangular shallow reservoirs. Four flow patterns were identified (from longer to shorter reservoirs): an asymmetric flow with two reattachment points, an asymmetric flow with one reattachment point, an unstable flow, and a symmetric flow without any reattachment point. Using dye visualizations, the median value and the temporal variability of the reattachment lengths were precisely measured for the asymmetric flows. For each stable flow, sediment tests with plastic particles were carried out. The regions of deposition on the bed of the reservoir were clearly a function of the flow pattern. The transition from an asymmetric flow pattern to a symmetric flow pattern was responsible for an abrupt decrease of the trap efficiency; a number of regression laws were discussed to take it into account.
Experimental study of subaqueous, clay-rich, gravity flows
Marr, J.; Pratson, L.
2003-04-01
Recent laboratory experiments suggest a broad spectrum of flow and depositional behavior for compositionally varied subaqueous gravity flows. Dilute turbidity currents and cohesive debris flows are the end members of the spectrum. In this study we used geometrically scaled laboratory experiments to examine the flow dynamics and deposits associated with slurries of varying sediment composition. Slurries were composed of a mixture of tap water, kaolinite clay, 45 micron silt and 120 micron sand and were introduced into a 0.2m wide submerged channel. Slurry sediment concentrations ranged from 1-30% by volume. In all slurries, sediment was added in a ratio of 8:1:1 by volume of clay, silt, sand. A total volume of one cubic meter of slurry was used for each experiment and was introduced through a constant head tank allowing examination of sustained and steady gravity flow events lasting up to 5 minutes in duration. The dynamics of the flows (turbulence, hydroplaning, laminar shearing, etc.) were examined through the use of digital video cameras, dye injection tracking, high frequency sonar and visual observation. Vertical suspended sediment concentration and vertical grain size distributions were measured for each run from samples collected from siphon rakes. Deposit thicknesses and grain size distributions were measured from sediment samples taken from flow deposits. Rheological measurements and Atterberg limits of the slurries were made in an effort to link flow and depositional characteristics to bulk properties of the slurry mixture. The experiments show a clear linkage between the initial compositions of the slurries, their rheological properties, flow dynamics and deposits. Slurries with clay concentrations below 10% by volume appeared to be very turbulent. The silt and sand deposited during these events were transported along the bed as ripples. Flows between 10-20% sediment by volume appeared to be hybrid flows having both turbulent and non-turbulent elements