The Variation of Slat Noise with Mach and Reynolds Numbers
Lockhard, David P.; Choudhari, Meelan M.
2011-01-01
The slat noise from the 30P30N high-lift system has been computed using a computational fluid dynamics code in conjunction with a Ffowcs Williams-Hawkings solver. By varying the Mach number from 0.13 to 0.25, the noise was found to vary roughly with the 5th power of the speed. Slight changes in the behavior with directivity angle could easily account for the different speed dependencies reported in the literature. Varying the Reynolds number from 1.4 to 2.4 million resulted in almost no differences, and primarily served to demonstrate the repeatability of the results. However, changing the underlying hybrid Reynolds-averaged-Navier-Stokes/Large-Eddy-Simulation turbulence model significantly altered the mean flow because of changes in the flap separation. However, the general trends observed in both the acoustics and near-field fluctuations were similar for both models.
Seiff, Alvin; Wilkins, Max E.
1961-01-01
The aerodynamic characteristics of a hypersonic glider configuration, consisting of a slender ogive cylinder with three highly swept wings, spaced 120 apart, with the wing chord equal to the body length, were investigated experimentally at a Mach number of 6 and at Reynolds numbers from 6 to 16 million. The objectives were to evaluate the theoretical procedures which had been used to estimate the performance of the glider, and also to evaluate the characteristics of the glider itself. A principal question concerned the viscous drag at full-scale Reynolds number, there being a large difference between the total drags for laminar and turbulent boundary layers. It was found that the procedures which had been applied for estimating minimum drag, drag due to lift, lift curve slope, and center of pressure were generally accurate within 10 percent. An important exception was the non-linear contribution to the lift coefficient which had been represented by a Newtonian term. Experimentally, the lift curve was nearly linear within the angle-of-attack range up to 10 deg. This error affected the estimated lift-drag ratio. The minimum drag measurements indicated that substantial amounts of turbulent boundary layer were present on all models tested, over a range of surface roughness from 5 microinches maximum to 200 microinches maximum. In fact, the minimum drag coefficients were nearly independent of the surface smoothness and fell between the estimated values for turbulent and laminar boundary layers, but closer to the turbulent value. At the highest test Reynolds numbers and at large angles of attack, there was some indication that the skin friction of the rough models was being increased by the surface roughness. At full-scale Reynolds number, the maximum lift-drag ratio with a leading edge of practical diameter (from the standpoint of leading-edge heating) was 4.0. The configuration was statically and dynamically stable in pitch and yaw, and the center of pressure was less
A Reynolds Number Study of Wing Leading-Edge Effects on a Supersonic Transport Model at Mach 0.3
Williams, M. Susan; Owens, Lewis R., Jr.; Chu, Julio
1999-01-01
A representative supersonic transport design was tested in the National Transonic Facility (NTF) in its original configuration with small-radius leading-edge flaps and also with modified large-radius inboard leading-edge flaps. Aerodynamic data were obtained over a range of Reynolds numbers at a Mach number of 0.3 and angles of attack up to 16 deg. Increasing the radius of the inboard leading-edge flap delayed nose-up pitching moment to a higher lift coefficient. Deflecting the large-radius leading-edge flap produced an overall decrease in lift coefficient and delayed nose-up pitching moment to even higher angles of attack as compared with the undeflected large- radius leading-edge flap. At angles of attack corresponding to the maximum untrimmed lift-to-drag ratio, lift and drag coefficients decreased while lift-to-drag ratio increased with increasing Reynolds number. At an angle of attack of 13.5 deg., the pitching-moment coefficient was nearly constant with increasing Reynolds number for both the small-radius leading-edge flap and the deflected large-radius leading-edge flap. However, the pitching moment coefficient increased with increasing Reynolds number for the undeflected large-radius leading-edge flap above a chord Reynolds number of about 35 x 10 (exp 6).
Slot, H.J.; Moore, P.; Delfos, R.; Boersma, B.J.
2009-01-01
In this paper we present the experimental results of a detailed investigation of the flow and acoustic properties of a turbulent jet with Mach number 0·75 and Reynolds number 3·5 103. We describe the methods and experimental procedures followed during the measurements, and subsequently present the f
Powers, Sheryll Goecke; Huffman, Jarrett K.; Fox, Charles H., Jr.
1986-01-01
The effectiveness of a trailing disk, or trapped vortex concept, in reducing the base drag of a large body of revolution was studied from measurements made both in flight and in a wind tunnel. Pressure data obtained for the flight experiment, and both pressure and force balance data were obtained for the wind tunnel experiment. The flight test also included data obtained from a hemispherical base. The experiment demonstrated the significant base drag reduction capability of the trailing disk to Mach 0.93 and to Reynolds numbers up to 80 times greater than for earlier studies. For the trailing disk data from the flight experiment, the maximum decrease in base drag ranged form 0.08 to 0.07 as Mach number increased from 0.70 to 0.93. Aircraft angles of attack ranged from 3.9 to 6.6 deg for the flight data. For the trailing disk data from the wind tunnel experiment, the maximum decrease in base and total drag ranged from 0.08 to 0.05 for the approximately 0 deg angle of attack data as Mach number increased from 0.30 to 0.82.
Holland, Scott D.; Murphy, Kelly J.
1993-01-01
Since mission profiles for airbreathing hypersonic vehicles such as the National Aero-Space Plane include single-stage-to-orbit requirements, real gas effects may become important with respect to engine performance. The effects of the decrease in the ratio of specific heats have been investigated in generic three-dimensional sidewall compression scramjet inlets with leading-edge sweep angles of 30 and 70 degrees. The effects of a decrease in ratio of specific heats were seen by comparing data from two facilities in two test gases: in the Langley Mach 6 CF4 Tunnel in tetrafluoromethane (where gamma=1.22) and in the Langley 15-Inch Mach 6 Air Tunnel in perfect gas air (where gamma=1.4). In addition to the simulated real gas effects, the parametric effects of cowl position, contraction ratio, leading-edge sweep, and Reynolds number were investigated in the 15-Inch Mach 6 Air Tunnel. The models were instrumented with a total of 45 static pressure orifices distributed on the sidewalls and baseplate. Surface streamline patterns were examined via oil flow, and schlieren videos were made of the external flow field. The results of these tests have significant implications to ground based testing of inlets in facilities which do not operate at flight enthalpies.
Quasiperpendicular high Mach number Shocks
Sulaiman, A H; Dougherty, M K; Burgess, D; Fujimoto, M; Hospodarsky, G B
2015-01-01
Shock waves exist throughout the universe and are fundamental to understanding the nature of collisionless plasmas. Reformation is a process, driven by microphysics, which typically occurs at high Mach number supercritical shocks. While ongoing studies have investigated this process extensively both theoretically and via simulations, their observations remain few and far between. In this letter we present a study of very high Mach number shocks in a parameter space that has been poorly explored and we identify reformation using in situ magnetic field observations from the Cassini spacecraft at 10 AU. This has given us an insight into quasi-perpendicular shocks across two orders of magnitude in Alfven Mach number (MA) which could potentially bridge the gap between modest terrestrial shocks and more exotic astrophysical shocks. For the first time, we show evidence for cyclic reformation controlled by specular ion reflection occurring at the predicted timescale of ~0.3 {\\tau}c, where {\\tau}c is the ion gyroperio...
Quasiperpendicular High Mach Number Shocks
Sulaiman, A. H.; Masters, A.; Dougherty, M. K.; Burgess, D.; Fujimoto, M.; Hospodarsky, G. B.
2015-09-01
Shock waves exist throughout the Universe and are fundamental to understanding the nature of collisionless plasmas. Reformation is a process, driven by microphysics, which typically occurs at high Mach number supercritical shocks. While ongoing studies have investigated this process extensively both theoretically and via simulations, their observations remain few and far between. In this Letter we present a study of very high Mach number shocks in a parameter space that has been poorly explored and we identify reformation using in situ magnetic field observations from the Cassini spacecraft at 10 AU. This has given us an insight into quasiperpendicular shocks across 2 orders of magnitude in Alfvén Mach number (MA ) which could potentially bridge the gap between modest terrestrial shocks and more exotic astrophysical shocks. For the first time, we show evidence for cyclic reformation controlled by specular ion reflection occurring at the predicted time scale of ˜0.3 τc , where τc is the ion gyroperiod. In addition, we experimentally reveal the relationship between reformation and MA and focus on the magnetic structure of such shocks to further show that for the same MA , a reforming shock exhibits stronger magnetic field amplification than a shock that is not reforming.
Propulsion at low Reynolds number
Energy Technology Data Exchange (ETDEWEB)
Najafi, Ali [Institute for Advanced Studies in Basic Sciences, Zanjan 45195-159 (Iran, Islamic Republic of); Faculty of Science, Zanjan University, Zanjan 313 (Iran, Islamic Republic of); Golestanian, Ramin [Institute for Advanced Studies in Basic Sciences, Zanjan 45195-159 (Iran, Islamic Republic of)
2005-04-13
We study the propulsion of two model swimmers at low Reynolds number. Inspired by Purcell's model, we propose a very simple one-dimensional swimmer consisting of three spheres that are connected by two arms whose lengths can change between two values. The proposed swimmer can swim with a special type of motion, which breaks the time-reversal symmetry. We also show that an ellipsoidal membrane with tangential travelling wave on it can also propel itself in the direction preferred by the travelling wave. This system resembles the realistic biological animals like Paramecium.
Numerical simulation of LBGK model for high Reynolds number flow
Institute of Scientific and Technical Information of China (English)
Zhou Xiao-Yang; Shi Bao-Chang; Wang Neng-Chao
2004-01-01
A principle of selecting relaxation parameter was proposed to observe the limit computational capability of the incompressible LBGK models developed by Guo ZL (Guo model) and He SY (He model) for high Reynolds number flow.To the two-dimensional driven cavity flow problem, the highest Reynolds numbers covered by Guo and He models are in the range 58000-52900 and 28000-29000, respectively, at 0.3 Mach number and 1/256 lattice space. The simulation results also show that the Guo model has stronger robustness due to its higher accuracy.
Ebrahimian, Mehran; Yekehzare, Mohammad; Ejtehadi, Mohammad Reza
2015-12-01
To generalize simple bead-linker model of swimmers to higher dimensions and to demonstrate the chemotaxis ability of such swimmers, here we introduce a low-Reynolds predator, using a two-dimensional triangular bead-spring model. Two-state linkers as mechanochemical enzymes expand as a result of interaction with particular activator substances in the environment, causing the whole body to translate and rotate. The concentration of the chemical stimulator controls expansion versus the contraction rate of each arm and so affects the ability of the body for diffusive movements; also the variation of activator substance's concentration in the environment breaks the symmetry of linkers' preferred state, resulting in the drift of the random walker along the gradient of the density of activators. External food or danger sources may attract or repel the body by producing or consuming the chemical activators of the organism's enzymes, inducing chemotaxis behavior. Generalization of the model to three dimensions is straightforward.
Chaotic behaviour of high Mach number flows
Varvoglis, H.; Ghosh, S.
1985-01-01
The stability of the super-Alfvenic flow of a two-fluid plasma model with respect to the Mach number and the angle between the flow direction and the magnetic field is investigated. It is found that, in general, a large scale chaotic region develops around the initial equilibrium of the laminar flow when the Mach number exceeds a certain threshold value. After reaching a maximum the size of this region begins shrinking and goes to zero as the Mach number tends to infinity. As a result high Mach number flows in time independent astrophysical plasmas may lead to the formation of 'quasi-shocks' in the presence of little or no dissipation.
Resolving high Reynolds numbers in SPH simulations of subsonic turbulence
Price, Daniel J
2011-01-01
Accounting for the Reynolds number is critical in numerical simulations of turbulence, particularly for subsonic flow. For Smoothed Particle Hydrodynamics (SPH) with constant artificial viscosity coefficient alpha, it is shown that the effective Reynolds number in the absence of explicit physical viscosity terms scales linearly with the Mach number - compared to mesh schemes, where the effective Reynolds number is largely independent of the flow velocity. As a result, SPH simulations with alpha=1 will have low Reynolds numbers in the subsonic regime compared to mesh codes, which may be insufficient to resolve turbulent flow. This explains the failure of Bauer and Springel (2011, arXiv:1109.4413v1) to find agreement between the moving-mesh code AREPO and the GADGET SPH code on simulations of driven, subsonic (v ~ 0.3 c_s) turbulence appropriate to the intergalactic/intracluster medium, where it was alleged that SPH is somehow fundamentally incapable of producing a Kolmogorov-like turbulent cascade. We show tha...
National transonic facility Mach number system
Kern, F. A.; Knight, C. W.; Zasimowich, R. F.
1985-01-01
The Mach number system for the Langley Research Center's National Transonic Facility was designed to measure pressures to determine Mach number to within + or - 0.002. Nine calibration laboratory type fused quartz gages, four different range gages for the total pressure measurement, and five different range gages for the static pressure measurement were used to satisfy the accuracy requirement over the 103,000-890,000 Pa total pressure range of the tunnel. The system which has been in operation for over 1 year is controlled by a programmable data process controller to select, through the operation of solenoid valves, the proper range fused quartz gage to maximize the measurement accuracy. The pressure gage's analog outputs are digitized by the process controller and transmitted to the main computer for Mach number computation. An automatic two-point on-line calibration of the nine quartz gages is provided using a high accuracy mercury manometer.
Calibration of the 7—Equation Transition Model for High Reynolds Flows at Low Mach
Colonia, S.; Leble, V.; Steijl, R.; Barakos, G.
2016-09-01
The numerical simulation of flows over large-scale wind turbine blades without considering the transition from laminar to fully turbulent flow may result in incorrect estimates of the blade loads and performance. Thanks to its relative simplicity and promising results, the Local-Correlation based Transition Modelling concept represents a valid way to include transitional effects into practical CFD simulations. However, the model involves coefficients that need tuning. In this paper, the γ—equation transition model is assessed and calibrated, for a wide range of Reynolds numbers at low Mach, as needed for wind turbine applications. An aerofoil is used to evaluate the original model and calibrate it; while a large scale wind turbine blade is employed to show that the calibrated model can lead to reliable solutions for complex three-dimensional flows. The calibrated model shows promising results for both two-dimensional and three-dimensional flows, even if cross-flow instabilities are neglected.
Boundary induced nonlinearities at small Reynolds numbers
Sbragaglia, M.; Sugiyama, K.
2007-01-01
We investigate the importance of boundary slip at finite Reynolds numbers for mixed boundary conditions. Nonlinear effects are induced by the non-homogeneity of the boundary condition and change the symmetry properties of the flow with an overall mean flow reduction. To explain the observed drag
Room Airflows with Low Reynolds Number Effects
DEFF Research Database (Denmark)
Topp, Claus; Nielsen, Peter V.; Davidson, Lars
The behaviour of room airflows under fully turbulent conditions is well known both in terms of experiments and, numerical calculations by computational fluid dynamics (CFD). For room airflows where turbulence is not fully developed though, i.e. flows at low Reynolds numbers, the existing knowledge...
Pires, O.; Munduate, X.; Ceyhan, O.; Jacobs, M.; Madsen, J.; Schepers, J. G.
2016-09-01
2D wind tunnel tests at high Reynolds numbers have been done within the EU FP7 AVATAR project (Advanced Aerodynamic Tools of lArge Rotors) on the DU00-W-212 airfoil and at two different test facilities: the DNW High Pressure Wind Tunnel in Gottingen (HDG) and the LM Wind Power in-house wind tunnel. Two conditions of Reynolds numbers have been performed in both tests: 3 and 6 million. The Mach number and turbulence intensity values are similar in both wind tunnels at the 3 million Reynolds number test, while they are significantly different at 6 million Reynolds number. The paper presents a comparison of the data obtained from the two wind tunnels, showing good repeatability at 3 million Reynolds number and differences at 6 million Reynolds number that are consistent with the different Mach number and turbulence intensity values.
Time-dependent measurement of base pressure in a blowdown tunnel with varying unit Reynolds number
Kangovi, S.; Rao, D. M.
1978-01-01
An operational characteristic of blowdown-type of wind tunnels is the drop in the stagnation temperature with time and the accompanying change in the test-section unit Reynolds number at constant stagnation pressure and Mach number. This apparent disadvantage can be turned to advantage in some cases where a Reynolds number scan is desired in order to study the effect of unit Reynolds number variation on a particular viscous flow phenomenon. This note presents such an instance arising from recent investigations on base pressure at transonic speeds conducted in the NAL 1-ft tunnel.
3 TUNNELS IN THE ENGINE RESEARCH BUILDING ERB - IN CELL CE-26 VARIABLE REYNOLDS NUMBER SUPERSONIC NO
1956-01-01
3 TUNNELS IN THE ENGINE RESEARCH BUILDING ERB - IN CELL CE-26 VARIABLE REYNOLDS NUMBER SUPERSONIC NOZZLE - CELL CE-4 6X6 INCH MACH NUMBER 2.96 SUPERSONIC AIRPLANE - CELL 1-NW 1X1 FOOT MACH 3.12 SUPERSONIC TUNNEL
Analysis of compressible light dynamic stall flow at transitional Reynolds numbers
DEFF Research Database (Denmark)
Dyken, R.D. Van; Ekaterinaris, John A.; Chandrasekhara, M.S.;
1996-01-01
Numerical and experimental results of steady and light dynamic stall flow over an oscillating NACA 0012 airfoil at a freestream Mach number of 0.3 and Reynolds number of 0.54 x 10(6) are compared, The experimental observation that dynamic stall is induced from the bursting of a laminar separation...
Low Mach Number Fluctuating Hydrodynamics for Electrolytes
Péraud, Jean-Philippe; Chaudhri, Anuj; Bell, John B; Donev, Aleksandar; Garcia, Alejandro L
2016-01-01
We formulate and study computationally the low Mach number fluctuating hydrodynamic equations for electrolyte solutions. We are interested in studying transport in mixtures of charged species at the mesoscale, down to scales below the Debye length, where thermal fluctuations have a significant impact on the dynamics. Continuing our previous work on fluctuating hydrodynamics of multicomponent mixtures of incompressible isothermal miscible liquids (A. Donev, et al., Physics of Fluids, 27, 3, 2015), we now include the effect of charged species using a quasielectrostatic approximation. Localized charges create an electric field, which in turn provides additional forcing in the mass and momentum equations. Our low Mach number formulation eliminates sound waves from the fully compressible formulation and leads to a more computationally efficient quasi-incompressible formulation. We demonstrate our ability to model saltwater (NaCl) solutions in both equilibrium and nonequilibrium settings. We show that our algorithm...
Flapping hydrofoil performance at low Reynolds numbers
Energy Technology Data Exchange (ETDEWEB)
Pedro, G.; Suleman, A.; Djilali, N. [Univ. of Victoria, Dept. of Mechanical Engineering, Victoria, British Columbia (Canada)]. E-mail: gpedro@uvic.ca; suleman@uvic.ca; ndjilali@uvic.ca
2003-07-01
This paper relates the study of unsteady flow past oscillating hydrofoils at low Reynolds numbers using a computational fluid dynamics research code based on structured grids. The solver utilizes an explicit, time-stepping algorithm with an Arbitrary Lagrangian-Eulerian formulation to account for mesh movement. The viscous flow past a NACA0012 hydrofoil at various pitching and heaving frequencies and other design parameters is simulated. The effect of these parameters on thrust, power and efficiency is studied along with flow field visualisations to account for these variations. (author)
Three-dimensional vortex organization in a high-Reynolds-number supersonic turbulent boundary layer
Elsinga, G.E.; Adrian, R.J.; Van Oudheusden, B.W.; Scarano, F.
2010-01-01
Tomographic particle image velocimetry was used to quantitatively visualize the three-dimensional coherent structures in a supersonic (Mach 2) turbulent boundary layer in the region between y/δ = 0.15 and 0.89. The Reynolds number based on momentum thickness Reθ = 34000. The instantaneous velocity f
Low Mach number fluctuating hydrodynamics for electrolytes
Péraud, Jean-Philippe; Nonaka, Andy; Chaudhri, Anuj; Bell, John B.; Donev, Aleksandar; Garcia, Alejandro L.
2016-11-01
We formulate and study computationally the low Mach number fluctuating hydrodynamic equations for electrolyte solutions. We are interested in studying transport in mixtures of charged species at the mesoscale, down to scales below the Debye length, where thermal fluctuations have a significant impact on the dynamics. Continuing our previous work on fluctuating hydrodynamics of multicomponent mixtures of incompressible isothermal miscible liquids [A. Donev et al., Phys. Fluids 27, 037103 (2015), 10.1063/1.4913571], we now include the effect of charged species using a quasielectrostatic approximation. Localized charges create an electric field, which in turn provides additional forcing in the mass and momentum equations. Our low Mach number formulation eliminates sound waves from the fully compressible formulation and leads to a more computationally efficient quasi-incompressible formulation. We demonstrate our ability to model saltwater (NaCl) solutions in both equilibrium and nonequilibrium settings. We show that our algorithm is second order in the deterministic setting and for length scales much greater than the Debye length gives results consistent with an electroneutral approximation. In the stochastic setting, our model captures the predicted dynamics of equilibrium and nonequilibrium fluctuations. We also identify and model an instability that appears when diffusive mixing occurs in the presence of an applied electric field.
Revolutionary Performance For Ultra Low Reynolds Number Vehicles Project
National Aeronautics and Space Administration — A novel technique for controlling transition from laminar to turbulent flow in very low Reynolds number conditions has been developed. Normally flows with Reynolds...
Pair separation in high Reynolds number turbulence
Bourgoin, M O; Xu, H; Joergensen, J B; Bodenschatz, E; Bourgoin, Mickael; Ouellette, Nicholas T.; Xu, Haitao; Joergensen, Jacob B.; Bodenschatz, Eberhard
2005-01-01
The separation of two nearby particles in a turbulent flow is fundamental in our everyday lives. Turbulent mixing is important everywhere from mundane applications like stirring milk into a cup of tea to technological processes such as the mixing of chemicals in reactors, combustion engines, or jet turbines. Environmental problems such as the spread of pollutants or bioagents in the atmosphere and oceans are fundamentally turbulent mixing processes. Even biological organisms use it to survive in marine ecosystems. Despite intense scientific inquiry, however, no convincing agreement has been found with the Richardson and Batchelor two-particle dispersion predictions over a wide range of timescales. Here we report measurements in a laboratory water flow at very high turbulence intensities (Taylor microscale Reynolds numbers of R_lambda = 690 and 815) that show excellent agreement with a refinement of Batchelor's prediction. We find that even for large initial spatial separations Batchelor scaling is fulfilled. ...
Holography of the QGP Reynolds number
McInnes, Brett
2017-08-01
The viscosity of the Quark-Gluon Plasma (QGP) is usually described holographically by the entropy-normalized dynamic viscosity η / s. However, other measures of viscosity, such as the kinematic viscosity ν and the Reynolds number Re, are often useful, and they too should be investigated from a holographic point of view. We show that a simple model of this kind puts an upper bound on Re for nearly central collisions at a given temperature; this upper bound is in very good agreement with the observational lower bound (from the RHIC facility). Furthermore, in a holographic approach using only Einstein gravity, η / s does not respond to variations of other physical parameters, while ν and Re can do so. In particular, it is known that the magnetic fields arising in peripheral heavy-ion collisions vary strongly with the impact parameter b, and we find that the holographic model predicts that ν and Re can also be expected to vary substantially with the magnetic field and therefore with b.
Holography of the QGP Reynolds number
Directory of Open Access Journals (Sweden)
Brett McInnes
2017-08-01
Full Text Available The viscosity of the Quark–Gluon Plasma (QGP is usually described holographically by the entropy-normalized dynamic viscosity η/s. However, other measures of viscosity, such as the kinematic viscosity ν and the Reynolds number Re, are often useful, and they too should be investigated from a holographic point of view. We show that a simple model of this kind puts an upper bound on Re for nearly central collisions at a given temperature; this upper bound is in very good agreement with the observational lower bound (from the RHIC facility. Furthermore, in a holographic approach using only Einstein gravity, η/s does not respond to variations of other physical parameters, while ν and Re can do so. In particular, it is known that the magnetic fields arising in peripheral heavy-ion collisions vary strongly with the impact parameter b, and we find that the holographic model predicts that ν and Re can also be expected to vary substantially with the magnetic field and therefore with b.
Experimental studies of Reynolds number dependence of turbulent mixing & transport
Energy Technology Data Exchange (ETDEWEB)
Warhaft, Z. [Cornell Univ., Ithaca, NY (United States)
1996-12-31
An overview of recent experiments, in which the author generated high Reynolds number homogeneous grid turbulence, is provided. The author shows that in a small wind tunnel, Reynolds numbers that are sufficiently high (R{sub {lambda}} {approximately} 800, R{sub {ell}} {approximately} 36, 000) such that many of the aspects of turbulence that hitherto have only been observed in large scale anisotropic shear flows, are obtained. In particular the author studied the evolution of the spectrum with Reynolds number, the Kolmogorov constant and the internal intermittency, showing the way they tend to their high Reynolds number asymptotes. Thus the author links previous low Reynolds number laboratory experiments with large scale environmental measurements.
Design of a continuously variable Mach-number nozzle
Institute of Scientific and Technical Information of China (English)
郭善广; 王振国; 赵玉新
2015-01-01
A design method was developed to specify the profile of the continuously variable Mach-number nozzle for the supersonic wind tunnel. The controllable contour design technique was applied to obtaining the original nozzle profile, while other Mach- numbers were derived from the transformation of the original profile. A design scheme, covering a Mach-number range of 3.0
Crossover from High to Low Reynolds Number Turbulence
Lohse, Detlef
1994-01-01
The Taylor-Reynolds and Reynolds number (Re lambda and Re) dependence of the dimensionless energy dissipation rate c epsilon = epsilon L / u31,rms is derived for statistically stationary isotropic turbulence, employing the results of a variable range mean field theory. Here epsilon is the energy di
A new numerical solver for flows at various Mach numbers
Miczek, F; Edelmann, P V F
2014-01-01
Many problems in stellar astrophysics feature low Mach number flows. However, conventional compressible hydrodynamics schemes frequently used in the field have been developed for the transonic regime and exhibit excessive numerical dissipation for these flows. While schemes were proposed that solve hydrodynamics strictly in the low Mach regime and thus restrict their applicability, we aim at developing a scheme that correctly operates in a wide range of Mach numbers. Based on an analysis of the asymptotic behavior of the Euler equations in the low Mach limit we propose a novel scheme that is able to maintain a low Mach number flow setup while retaining all effects of compressibility. This is achieved by a suitable modification of the well-known Roe solver. Numerical tests demonstrate the capability of this new scheme to reproduce slow flow structures even in moderate numerical resolution. Our scheme provides a promising approach to a consistent multidimensional hydrodynamical treatment of astrophysical low Ma...
Numerical investigation of transition critical Reynolds number of channel flow.
Zhang, Yongming
2015-11-01
Two critical Reynolds numbers are mentioned in investigation of laminar-turbulent transition. One is instability critical Reynolds number from linear stability theory (LST). The other is transition critical Reynolds number at which transition occurs in reality, which is significantly lower than the former in general. The determination of transition critical Reynolds number is of important practical significance in some engineering problems. Theoretical method has not been proposed for its determination, so it has to depend on experiments. However, for some flows with important practical significance, such as hypersonic boundary layer, transition critical Reynolds number cannot be determined by experiments in current situation. In this paper, transition critical Reynolds number of incompressible channel flow is determined by direct numerical simulations (DNS). It is found as Re =1114, which agrees with experimental data. In subsequent paper, transition critical Reynolds number of boundary layer will be investigation by the similar method. Project supported by the National Natural Science Foundation of China (Nos. 11202147, 11332007, 11172203, and 91216111) and the Specialized Research Fund (New Teacher Class) for the Doctoral Program of Higher Education (No. 20120032120007).
Reynolds number effects on near-wall turbulence
Metzger, Meredith; Klewicki, Joseph
2001-11-01
Reynolds number effects in the zero pressure gradient turbulent boundary layer are presented in the context of near-wall axial velocity data. Complementary experiments were conducted in a boundary layer wind tunnel and in the atmospheric surface layer over Utah's western desert yielding a total Reynolds number range over three orders of magnitude (2 × 10^3 hot-wires spanning 1 convection velocities. Event detection analyses are used to examine Reynolds number differences in the nature of sweeps related to these observations.
ON THE EFFECT OF REYNOLDS NUMBER ON VON KARMAN'S CONSTANT
Institute of Scientific and Technical Information of China (English)
赵金印; 解茂昭; F.Durst
2002-01-01
Fully developed turbulence measurements in pipe flow were made in the Reynolds number ranging from 10× 103 to 350 × 103 with a hot-wire anemometer and a Pitot tube. Comparisons were made with the experimental results of previous work. The mean velocity profile and the turbulent intensity in the experiments indicate that for the mean velocity profile, in the fully developed turbulent pipe flow,with the Reynolds number. The empirical relationships could not be considered to be accurate enough to describe the fully developed turbulence over the whole Reynolds number range in pipe flow.
Theoretical models in low-Reynolds-number locomotion
Pak, On Shun
2014-01-01
The locomotion of microorganisms in fluids is ubiquitous and plays an important role in numerous biological processes. In this chapter we present an overview of theoretical modeling for low-Reynolds-number locomotion.
The Influence of Realistic Reynolds Numbers on Slat Noise Simulations
Lockard, David P.; Choudhari, Meelan M.
2012-01-01
The slat noise from the 30P/30N high-lift system has been computed using a computational fluid dynamics code in conjunction with a Ffowcs Williams-Hawkings solver. Varying the Reynolds number from 1.71 to 12.0 million based on the stowed chord resulted in slight changes in the radiated noise. Tonal features in the spectra were robust and evident for all Reynolds numbers and even when a spanwise flow was imposed. The general trends observed in near-field fluctuations were also similar for all the different Reynolds numbers. Experiments on simplified, subscale high-lift systems have exhibited noticeable dependencies on the Reynolds number and tripping, although primarily for tonal features rather than the broadband portion of the spectra. Either the 30P/30N model behaves differently, or the computational model is unable to capture these effects. Hence, the results underscore the need for more detailed measurements of the slat cove flow.
Vortex Shedding from Tapered Cylinders at high Reynolds Numbers
DEFF Research Database (Denmark)
Johansson, Jens; Andersen, Michael Styrk; Christensen, Silas Sverre
2015-01-01
percent for strakes of circular cross section. The present paper argues that this height can be reduced for structures where the critical wind velocity for vortex shedding is in the Supercritical Reynolds number regime. The present investigations are aimed for suppressing VIV on offshore wind turbine......^5 (Supercritical). Results indicate that circular strakes with a diameter corresponding to 3 percent of the structures mean diameter can be used to efficiently reduce VIV in the Supercritical Reynolds number regime....
Edge, cavity and aperture tones at very low Mach numbers
Howe, M. S.
1997-01-01
This paper discusses self-sustaining oscillations of high-Reynolds-number shear layers and jets incident on edges and corners at infinitesimal Mach number. These oscillations are frequently sources of narrow-band sound, and are usually attributed to the formation of discrete vortices whose interactions with the edge or corner produce impulsive pressures that lead to the formation of new vorticity and complete a feedback cycle of operation. Linearized analyses of these interactions are presented in which free shear layers are modelled by vortex sheets. Detailed results are given for shear flows over rectangular wall apertures and shallow cavities, and for the classical jet edge interaction. The operating stages of self-sustained oscillations are identified with poles in the upper half of the complex frequency plane of a certain impulse response function. It is argued that the real parts of these poles determine the Strouhal numbers of the operating stages observed experimentally for the real, nonlinear system. The response function coincides with the Rayleigh conductivity of the ‘window’ spanned by the shear flow for wall apertures and jet edge interactions, and to a frequency dependent drag coefficient for shallow wall cavities. When the interaction occurs in the neighbourhood of an acoustic resonator, exemplified by the flue organ pipe, the poles are augmented by a sequence of poles whose real parts are close to the resonance frequencies of the resonator, and the resonator can ‘speak’ at one of these frequencies (by extracting energy from the mean flow) provided the corresponding pole has positive imaginary part.
The wake of falling disks at low Reynolds numbers
Institute of Scientific and Technical Information of China (English)
Hong-Jie Zhong; Cun-Biao Lee
2012-01-01
We visualized the wake structure of circular disks falling vertically in quiescent water.The evolution of the wake was shown to be similar to the flow patterns behind a fixed disk.The Reynolds number,Re =Ud/v,is in the range of 40- 200.With the ascension of Reynolds numbers,a regular bifurcation occurred at the first critical Reynolds number Rec1,leading to a transition from an axisymmetric wake structure to a plane symmetric one; A Hopf bifurcation took place at the second critical Reynolds number Rec2,as the wake structure became unsteady.Plane symmetry of the wake structure was first lost as periodic voaex shedding appeared,but recovered at higher Reynolds number.The difference between the two critical Reynolds numbers was found to be shape-dependent,as we compared our results for thin discs with those for other falling bodies,such as spheres and cones.This observation could be understood in terms of the instability mechanism of the vortical structure.
DRE-Enhanced Swept-Wing Natural Laminar Flow at High Reynolds Numbers
Malik, Mujeeb; Liao, Wei; Li, Fe; Choudhari, Meelan
2013-01-01
Nonlinear parabolized stability equations and secondary instability analyses are used to provide a computational assessment of the potential use of the discrete roughness elements (DRE) technology for extending swept-wing natural laminar flow at chord Reynolds numbers relevant to transport aircraft. Computations performed for the boundary layer on a natural laminar flow airfoil with a leading-edge sweep angle of 34.6deg, free-stream Mach number of 0.75 and chord Reynolds numbers of 17 x 10(exp 6), 24 x 10(exp 6) and 30 x 10(exp 6) suggest that DRE could delay laminar-turbulent transition by about 20% when transition is caused by stationary crossflow disturbances. Computations show that the introduction of small wavelength stationary crossflow disturbances (i.e., DRE) also suppresses the growth of most amplified traveling crossflow disturbances.
Discrete-Roughness-Element-Enhanced Swept-Wing Natural Laminar Flow at High Reynolds Numbers
Malik, Mujeeb; Liao, Wei; Li, Fei; Choudhari, Meelan
2015-01-01
Nonlinear parabolized stability equations and secondary-instability analyses are used to provide a computational assessment of the potential use of the discrete-roughness-element technology for extending swept-wing natural laminar flow at chord Reynolds numbers relevant to transport aircraft. Computations performed for the boundary layer on a natural-laminar-flow airfoil with a leading-edge sweep angle of 34.6 deg, freestream Mach number of 0.75, and chord Reynolds numbers of 17 × 10(exp 6), 24 × 10(exp 6), and 30 × 10(exp 6) suggest that discrete roughness elements could delay laminar-turbulent transition by about 20% when transition is caused by stationary crossflow disturbances. Computations show that the introduction of small-wavelength stationary crossflow disturbances (i.e., discrete roughness element) also suppresses the growth of most amplified traveling crossflow disturbances.
Numerical Investigation of the Influence of Reynolds Number on Probe Measurements
Institute of Scientific and Technical Information of China (English)
无
2000-01-01
The influence of Reynolds number (Re) on probe measurements was investigated numerically, including the effects of the pressure holes and their geometry to obtain accurate hole-pressures. The results indicate that Re influences the probe measurements and cannot be neglected for Re larger than 105 and that the influence increases with Mach number (Ma). The calculations show that the pressures in the downwind holes are influenced more by Re than those of the upwind and central holes when the probe is at an angle. Thus, 7-hole probes may be more suitable for measurements at different Re than 5-hole probes.
Large scale dynamics in a turbulent compressible rotor/stator cavity flow at high Reynolds number
Lachize, C.; Verhille, G.; Le Gal, P.
2016-08-01
This paper reports an experimental investigation of a turbulent flow confined within a rotor/stator cavity of aspect ratio close to unity at high Reynolds number. The experiments have been driven by changing both the rotation rate of the disk and the thermodynamical properties of the working fluid. This fluid is sulfur hexafluoride (SF6) whose physical properties are adjusted by imposing the operating temperature and the absolute pressure in a pressurized vessel, especially near the critical point of SF6 reached for T c = 45.58 ◦C, P c = 37.55 bar. This original set-up allows to obtain Reynolds numbers as high as 2 × 107 together with compressibility effects as the Mach number can reach 0.5. Pressure measurements reveal that the resulting fully turbulent flow shows both a direct and an inverse cascade as observed in rotating turbulence and in accordance with Kraichnan conjecture for 2D-turbulence. The spectra are however dominated by low-frequency peaks, which are subharmonics of the rotating disk frequency, involving large scale structures at small azimuthal wavenumbers. These modes appear for a Reynolds number around 105 and experience a transition at a critical Reynolds number Re c ≈ 106. Moreover they show an unexpected nonlinear behavior that we understand with the help of a low dimensional amplitude equations.
Vortex shedding from slender cones at low Reynolds numbers
Papangelou, A.
1992-09-01
Wind-tunnel experiments on the flows created by a number of slightly tapered models of circular cross-section have shown the presence of spanwise cells (regions of constant shedding frequency) at Reynolds numbers of the order of 100. The experiments have also shown a number of other interesting features of these flows: the cellular flow configuration is dependent on the base Reynolds number and independent of the tip Reynolds number, the frequency jump between adjacent cells is a function of flow speed, taper angle and kinematic viscosity, but is constant along a cone's span, and the unsteady hot-wire anemometer signal is both amplitude and phase modulated. A mathematical model is proposed based on the complex Landau-Stuart equation with a spanwise diffusive coupling term. Numerical solutions of this equation have shown many of the qualitative features observed in the experiments.
Small scale turbulence and the finite Reynolds number effect
Antonia, R. A.; Djenidi, L.; Danaila, L.; Tang, S. L.
2017-02-01
Failure to recognize the importance of the finite Reynolds number effect on small scale turbulence has, by and large, resulted in misguided assessments of the first two hypotheses of Kolmogorov ["Local structure of turbulence in an incompressible fluid for very large Reynolds numbers," Dokl. Akad. Nauk SSSR 30, 299-303 (1941)] or K41 as well as his third hypothesis [A. N. Kolmogorov, "A refinement of previous hypotheses concerning the local structure of turbulence in a viscous incompressible fluid at high Reynolds number," J. Fluid Mech. 13, 82-85 (1962)] or K62. As formulated by Kolmogorov, all three hypotheses require local isotropy to be valid and the Reynolds number to be very large. In the context of the first hypothesis, there is now strong evidence to suggest that this requirement can be significantly relaxed, at least for dissipative scales and relatively low order moments of the velocity structure function. As the scale increases, the effect of the large scale motion on these moments becomes more prominent and higher Reynolds numbers are needed before K41 and K62 can be tested unambiguously.
Analysis of low Reynolds number separation bubbles using semiempirical methods
Schmidt, Gordon S.; Mueller, Thomas J.
1989-01-01
The formation and growth of transitional separation bubbles can significantly affect boundary-layer development on airfoils operating at low chord Reynolds numbers. Of primary concern is the change in boundary-layer thickness between laminar separation and turbulent reattachment. This can be estimated using semiempirical methods, such as the one devised by Horton (1968), which are based on solutions to the integral forms of the boundary-layer equations. The applicability of these methods at low Reynolds numbers was investigated using hot-wire measurements of bubbles formed on an NACA 66(3)-018 airfoil at chord Reynolds numbers of 50,000-200,000. The momentum thickness growth between separation and transition was found to be similar to that predicted for a laminar half-jet and appears to be influenced by the momentum thickness Reynolds number at separation. This parameter also was found to have a noticeable effect on the Reynolds number based on the length of a bubble's laminar portion.
Simulating High Reynolds Number Flow by Lattice Boltzmann Method
Institute of Scientific and Technical Information of China (English)
KANG Xiu-Ying; LIU Da-He; ZHOU Jing; JIN Yong-Juan
2005-01-01
@@ A two-dimensional channel flow with different Reynolds numbers is tested by using the lattice Boltzmann method under different pressure and velocity boundary conditions. The results show that the simulation error increases,and the pressure and the flow rate become unstable under a high Reynolds number. To improve the simulation precision under a high Reynolds number, the number of fluid nodes should be enlarged. For a higher Reynoldsnumber flow, the velocity boundary with an approximately parabolic velocity profile is found to be more adaptive.Blood flow in an artery with cosine shape symmetrical narrowing is then simulated under a velocity boundary condition. Its velocity, pressure and wall shear stress distributions are consistent with previous studies.
Tanaka, Kento; Watanabe, Tomoaki; Nagata, Koji; Sasoh, Akihiro; Sakai, Yasuhiko; Hayase, Toshiyuki; Nagoya Univ Collaboration
2016-11-01
The interaction between homogeneous isotropic turbulence and normal shock wave is investigated by direct numerical simulations (DNSs). In the DNSs, a normal shock wave with a shock Mach number 1.1 passes through homogeneous isotropic turbulence with a low turbulent Mach number and a moderate turbulent Reynolds number. The statistics are calculated conditioned on the distance from the shock wave. The results showed that the shock wave makes length scales related to turbulence small. This effect is significant for the Taylor microscale defined with the velocity derivative orthogonal to the shock wave. The decrease in the Kolmogorov scale is also found. Statistics of velocity derivative are found to be changed by the shock wave propagation. The shock wave causes enstrophy amplification due to the dilatation/vorticity interaction. By this interaction, the vorticity components parallel to the shock wave is more amplified than the normal component. The strain rate is also amplified by the shock wave.
High Reynolds number test of a NACA 651-213, a equals 0.5 airfoil at transonic speeds
Burdges, K. P.; Blackwell, J. A., Jr.; Pounds, G. A.
1975-01-01
Wind-Tunnel tests were conducted in the Lockheed-Georgia Company's compressible flow facility to determine the transonic two-dimensional aerodynamic characteristics of a NACA 65 sub 1-213 a = 0.50 airfoil. The results are correlated with data obtained in the NASA-Langley 8-foot transonic pressure tunnel and the NAE high Reynolds number 15x60-inch two-dimensional test facility. The tests were conducted over a Mach number range from 0.60 to 0.80 and an angle of attack range from -1 deg to 8 deg. Reynolds numbers, based on the airfoil chord, were varied.
The Performance of Discrete Models of Low Reynolds Number Swimmers
Wang, Qixuan
2015-01-01
Swimming by shape changes at low Reynolds number is widely used in biology and understanding how the efficiency of movement depends on the geometric pattern of shape changes is important to understand swimming of microorganisms and in designing low Reynolds number swimming models. The simplest models of shape changes are those that comprise a series of linked spheres that can change their separation and/or their size. Herein we compare the efficiency of three models in which these modes are used in different ways.
Low Mach Number Fluctuating Hydrodynamics of Diffusively Mixing Fluids
Donev, A; Sun, Y; Fai, T; Garcia, A L; Bell, J B
2012-01-01
We formulate low Mach number fluctuating hydrodynamic equations appropriate for modeling diffusive mixing in isothermal mixtures of fluids with different density and transport coefficients. These equations eliminate the fast isentropic fluctuations in pressure associated with the propagation of sound waves by replacing the equation of state with a local thermodynamic constraint. We demonstrate that the low Mach number model preserves the spatio-temporal spectrum of the slower diffusive fluctuations. We develop a strictly conservative finite-volume spatial discretization of the low Mach number fluctuating equations in both two and three dimensions. We construct several explicit Runge-Kutta temporal integrators that strictly maintain the equation of state constraint. The resulting spatio-temporal discretization is second-order accurate deterministically and maintains fluctuation-dissipation balance in the linearized stochastic equations. We apply our algorithms to model the development of giant concentration fl...
Reynolds number influences on turbulent boundary layer momentum transport
Priyadarshana, Paththage A.
There are many engineering applications at Reynolds numbers orders of magnitude higher than existing turbulent boundary layer studies. Currently, the mechanisms for turbulent transport and the Reynolds number dependence of these mechanisms are not well understood. This dissertation presents Reynolds number influences on velocity and vorticity statistics, Reynolds shear stress, and velocity-vorticity correlations for turbulent boundary layers. Well resolved hot-wire data for this study were acquired in the atmospheric surface layer at the SLTEST facility in western Utah. It is shown that during near neutral thermal stability, the flow behaves as a canonical zero pressure gradient turbulent boundary layer, in which the Reynolds number based on momentum thickness, Rtheta, is approximately 2 x 106. The present study also provides information regarding the effects of wall roughness over a limited range of roughness. It is observed that with increasing Rtheta, the inner normalized streamwise intensity increases. This statistic is less sensitive to wall roughness away from the roughness sublayer. In contrast, the inner normalized wall normal intensity is less sensitive to the variation of Rtheta, and it is significantly sensitive to wall roughness. Outside the viscous sublayer, the inner normalized vorticity intensity is less sensitive to both Rtheta and roughness. A primary observation of the Reynolds stress study is that the predominant motions underlying the Reynolds shear stress undergo a significant shift from large to intermediate scales as Rtheta becomes large, irrespective of surface roughness. Quadrant analysis shows that types of motions contributing to the Reynolds stress change significantly at comparable wall normal locations with increasing Rtheta. The mean wall normal gradients of the Reynolds shear stress and the turbulent kinetic energy have direct connections to the transport mechanisms of the turbulent boundary layer. These gradients can be expressed in
Mathematical and numerical aspects of low mach number flows
Energy Technology Data Exchange (ETDEWEB)
Schochet, St.; Bresch, D.; Grenier, E.; Alazard, T.; Gordner, A.; Sankaran, V.; Massot, M.; Sery, R.; Pebay, P.; Lunch, O.; Mazhorova, O.; Turkel, O.E.; Faille, I.; Danchin, R.; Allain, O.; Birken, P.; Lafitte, O.; Kloczko, T.; Frick, W.; Bui, T.; Dellacherie, S.; Klein, R.; Roe, Ph.; Accary, G.; Braack, M.; Picano, F.; Cadiou, A.; Dinescu, C.; Lesage, A.C.; Wesseling, P.; Heuveline, V.; Jobelin, M.; Weisman, C.; Merkle, C.
2004-07-01
Low Mach number flows represent a significant part of the various flows encountered in geophysics, industry or every day life. Paradoxically, the mathematical analysis of the equations governing these flows is difficult and on the practical side, the research of numerical algorithms valid for all flow speeds is continuing to be a challenge. However, in the last decade, both from the theoretical and the numerical sides, significant progresses were made in the understanding and analysis of the equations governing these flows. This conference intends to provide an up-to-date inventory of recent mathematical and numerical results in the analysis of these flows by bringing together both mathematicians and numericists active in this area. In the framework of the conference, a numerical workshop is organized which proposes to compute several challenging low Mach number flows: liquid flow around non-cavitating and cavitating NACA0015 hydrofoil, natural convection with large temperature differences, free convection, free surface flow, vessel pressurization. This document brings together the descriptions of the test cases of the numerical workshop and the abstracts of the conference papers: A 3D high order finite volume method for the prediction of near-critical fluid flows (G. ACCARY, I. RASPO, P. BONTOUX, B. ZAPPOLI); low Mach number limit of the non-isentropic Navier-Stokes equations (T. ALAZARD); simulation of cavitation rolls past a forward step with a bubble model (O. ALLAIN, N. BLASKA, C. LECA); flux preconditioning methods and fire events (P. BIRKEN, A. MEISTER); an adaptive finite element solver for compressible flows: application to heat-driven cavity benchmarks in 2D and 3D (M. BRAACK); comparison of various implicit, explicit, centered and upwind schemes for the simulation of compressed flows on moving mesh (A. CADIOU, M. BUFFAT, L. Le PENVEN, C. Le RIBAULT); low Mach number limit for viscous compressible flows (R. DANCHIN); some Properties of the low Mach number
Mathematical and numerical aspects of low mach number flows
Energy Technology Data Exchange (ETDEWEB)
Schochet, St.; Bresch, D.; Grenier, E.; Alazard, T.; Gordner, A.; Sankaran, V.; Massot, M.; Sery, R.; Pebay, P.; Lunch, O.; Mazhorova, O.; Turkel, O.E.; Faille, I.; Danchin, R.; Allain, O.; Birken, P.; Lafitte, O.; Kloczko, T.; Frick, W.; Bui, T.; Dellacherie, S.; Klein, R.; Roe, Ph.; Accary, G.; Braack, M.; Picano, F.; Cadiou, A.; Dinescu, C.; Lesage, A.C.; Wesseling, P.; Heuveline, V.; Jobelin, M.; Weisman, C.; Merkle, C.
2004-07-01
Low Mach number flows represent a significant part of the various flows encountered in geophysics, industry or every day life. Paradoxically, the mathematical analysis of the equations governing these flows is difficult and on the practical side, the research of numerical algorithms valid for all flow speeds is continuing to be a challenge. However, in the last decade, both from the theoretical and the numerical sides, significant progresses were made in the understanding and analysis of the equations governing these flows. This conference intends to provide an up-to-date inventory of recent mathematical and numerical results in the analysis of these flows by bringing together both mathematicians and numericists active in this area. In the framework of the conference, a numerical workshop is organized which proposes to compute several challenging low Mach number flows: liquid flow around non-cavitating and cavitating NACA0015 hydrofoil, natural convection with large temperature differences, free convection, free surface flow, vessel pressurization. This document brings together the descriptions of the test cases of the numerical workshop and the abstracts of the conference papers: A 3D high order finite volume method for the prediction of near-critical fluid flows (G. ACCARY, I. RASPO, P. BONTOUX, B. ZAPPOLI); low Mach number limit of the non-isentropic Navier-Stokes equations (T. ALAZARD); simulation of cavitation rolls past a forward step with a bubble model (O. ALLAIN, N. BLASKA, C. LECA); flux preconditioning methods and fire events (P. BIRKEN, A. MEISTER); an adaptive finite element solver for compressible flows: application to heat-driven cavity benchmarks in 2D and 3D (M. BRAACK); comparison of various implicit, explicit, centered and upwind schemes for the simulation of compressed flows on moving mesh (A. CADIOU, M. BUFFAT, L. Le PENVEN, C. Le RIBAULT); low Mach number limit for viscous compressible flows (R. DANCHIN); some Properties of the low Mach number
Stokesian swimming of a sphere at low Reynolds number
Felderhof, B U
2016-01-01
Explicit expressions are derived for the matrices determining the mean translational and rotational swimming velocities and the mean rate of dissipation for Stokesian swimming at low Reynolds number of a distorting sphere in a viscous incompressible fluid. As an application an efficient helical propeller-type stroke is found and its properties are calculated.
High-Reynolds Number Taylor-Couette Turbulence
Grossmann, Siegfried; Lohse, Detlef; Sun, Chao
2016-01-01
Taylor-Couette flow, the flow between two coaxial co- or counter-rotating cylinders, is one of the paradigmatic systems in the physics of fluids. The (dimensionless) control parameters are the Reynolds numbers of the inner and outer cylinders, the ratio of the cylinder radii, and the aspect ratio. O
Bifurcation to forward flapping flight at intermediate Reynolds number.
Vandenberghe, Nicolas; Zhang, Jun; Childress, Stephen
2003-11-01
The locomotion of most fish and birds is realized by flapping wings or fins transverse to the direction of travel. According to early theoretical studies, a flapping wing translating at finite speed in an inviscid fluid experiences a propulsive force. In steady forward flight this thrust is balanced by drag. Such "lift-based mechanisms" of thrust production are characteristic of the Eulerian realm, where discrete vortical structures are shed. But, when the Reynolds number is small, viscous forces dominate and reciprocal flapping motions are ineffective. A flapping wing experiences a net drag and cannot be used to propel an organism. We have devised an experiment to bridge the two regimes, and to examine the transition to forward flight at intermediate Reynolds numbers. We study the dynamics of an horizontal wing that is flapped up and down and is free to move either forwards or backwards. This very simple kinematics emphasizes the demarcation between low and high Reynolds number because it is effective in the Eulerian realm but has no effect in the Stokesian realm. We show that flapping flight occurs abruptly as a symmetry breaking bifurcation at a critical flapping frequency. Beyond the bifurcation the forward speed increases linearly with the flapping frequency. The experiment establishes a clear demarcation between the different strategies of locomotion at large and small Reynolds number.
Swimming of a circular disk at low Reynolds number
Felderhof, B U
2014-01-01
The swimming of a circular disk at low Reynolds number is studied for distortion waves along its two planar surfaces with wavelength much smaller than the size of the disk. The calculation is based on an extension of Taylor's work for a planar sheet. It is shown that in general the disk performs both translational and rotational swimming, resulting in a circular orbit.
Aeroacoustic Properties of Moderate Reynolds Number Elliptic and Rectangular Supersonic Jets.
Kinzie, Kevin Wayne
1995-01-01
The aerodynamic and acoustic properties of supersonic elliptic, rectangular, and circular jets are experimentally investigated. All three jets are perfectly expanded with an exit Mach number of approximately 1.5 and are operated in the Reynolds number range of 25,000 to 50,000. The reduced Reynolds number facilitates the use of conventional hot-wire anemometry and a glow discharge excitation technique which preferentially excites the varicose or flapping modes in the jets. In order to simulate the high velocity and low density effects of heated jets, helium is mixed with the air jets. This allows the large-scale structures in the jet shear layer to achieve high enough convective velocity to radiate noise through the Mach wave emission process. Experiments in the present work focus on comparisons between the cold and simulated heated jet conditions and on the beneficial aeroacoustic properties of non-circular jets. Comparisons are also made between the elliptic and rectangular jets. When helium is added to the jets, the instability wave phase velocity is found to approach or exceed the ambient sound speed. The radiated noise is also louder and directed at a higher angle from the jet axis. In addition, near field hot-wire spectra are found to match the far-field acoustic spectra only for the helium/air mixture case. These results demonstrate that there are significant differences between unheated and heated asymmetric jets in the Mach 1.5 speed range, many of which have been found previously for circular jets. The asymmetric jets were also found to radiate less noise than the round jet at comparable operating conditions. Strong similarities were also found between the aerodynamic and acoustic properties of the elliptic and rectangular jets.
Low Mach Number Fluctuating Hydrodynamics of Multispecies Liquid Mixtures
Donev, A; Bhattacharjee, A K; Garcia, A L; Bell, J B
2014-01-01
We develop a low Mach number formulation of the hydrodynamic equations describing transport of mass and momentum in a multispecies mixture of incompressible miscible liquids at specified temperature and pressure that generalizes our prior work on ideal mixtures of ideal gases and binary liquid mixtures. In this formulation we combine and extend a number of existing descriptions of multispecies transport available in the literature. The formulation applies to non-ideal mixtures of arbitrary number of species, without the need to single out a 'solvent' species, and includes contributions to the diffusive mass flux due to gradients of composition, temperature and pressure. Momentum transport and advective mass transport are handled using a low Mach number approach that eliminates fast sound waves (pressure fluctuations) from the full compressible system of equations and leads to a quasi-incompressible formulation. Thermal fluctuations are included in our fluctuating hydrodynamics description following the princi...
Pedrosa, A. C. F.; Nagamatsu, H. T.; Hinckel, J. A.
1984-01-01
Heat transfer measurements were determined for a flat plate with and without pressure gradient for various free stream temperatures, wall temperature ratios, and Reynolds numbers for an inlet flow Mach number of 0.45, which is a representative inlet Mach number for gas turbine rotor blades. A shock tube generated the high temperature and pressure air flow, and a variable geometry test section was used to produce inlet flow Mach number of 0.45 and accelerate the flow over the plate to sonic velocity. Thin-film platinum heat gages recorded the local heat flux for laminar, transition, and turbulent boundary layers. The free stream temperatures varied from 611 R (339 K) to 3840 R (2133 K) for a T(w)/T(r,g) temperature ratio of 0.87 to 0.14. The Reynolds number over the heat gages varied from 3000 to 690,000. The experimental heat transfer data were correlated with laminar and turbulent boundary layer theories for the range of temperatures and Reynolds numbers and the transition phenomenon was examined.
Statistical error in particle simulations of low mach number flows
Energy Technology Data Exchange (ETDEWEB)
Hadjiconstantinou, N G; Garcia, A L
2000-11-13
We present predictions for the statistical error due to finite sampling in the presence of thermal fluctuations in molecular simulation algorithms. The expressions are derived using equilibrium statistical mechanics. The results show that the number of samples needed to adequately resolve the flowfield scales as the inverse square of the Mach number. Agreement of the theory with direct Monte Carlo simulations shows that the use of equilibrium theory is justified.
Local Reynolds number and thresholds of transition in shear flows
Tao, JianJun; Chen, ShiYi; Su, WeiDong
2013-02-01
Recent experimental and numerical investigations reveal that the onset of turbulence in plane-Poiseuille flow and plane-Couette flow has some similar stages separated with different threshold Reynolds numbers. Based on these observations and the energy equation of a disturbed fluid element, a local Reynolds number Re L is derived to represent the maximum ratio of the energy supplement to the energy dissipation in a cross section. It is shown that along the sequence of transition stages, which include transient localized turbulence, "equilibrium" localized turbulence, spatially intermittent but temporally persistent turbulence and uniform turbulence, the corresponding thresholds of Re L for plane-Couette flow, Hagen-Poiseuille flow and plane-Poiseuille flow are consistent, indicating that the critical (threshold) states during the laminar-turbulent transition are determined by the local properties of the base flow and are independent of global features, such as flow geometries (pipe or channel) and types of driving forces (shear driving or pressure driving).
Turbulence measurements in high Reynolds number boundary layers
Vallikivi, Margit; Smits, Alexander
2013-11-01
Measurements are conducted in zero pressure gradient turbulent boundary layers for Reynolds numbers from Reθ = 9,000 to 225,000. The experiments were performed in the High Reynolds number Test Facility (HRTF) at Princeton University, which uses compressed air as the working fluid. Nano-Scale Thermal Anemometry Probes (NSTAPs) are used to acquire data with very high spatial and temporal precision. These new data are used to study the scaling behavior of the streamwise velocity fluctuations in the boundary layer and make comparisons with the scaling of other wall-bounded turbulent flows. Supported under ONR Grant N00014-09-1-0263 (program manager Ron Joslin) and NSF Grant CBET-1064257 (program manager Henning Winter).
Numerical simulations of undulatory swimming at moderate Reynolds number.
Eldredge, Jeff D
2006-12-01
We perform numerical simulations of the swimming of a three-linkage articulated system in a moderately viscous regime. The computational methodology focuses on the creation, diffusion and transport of vorticity from the surface of the bodies into the fluid. The simulations are dynamically coupled, in that the motion of the three-linkage swimmer is computed simultaneously with the dynamics of the fluid. The novel coupling scheme presented in this work is the first to exploit the relationship between vorticity creation and body dynamics. The locomotion of the system, when subject to undulatory inputs of the hinges, is computed at Reynolds numbers of 200 and 1000. It is found that the forward swimming speed increases with the Reynolds number, and that in both cases the swimming is slower than in an inviscid medium. The vortex shedding is examined, and found to exhibit behavior consistent with experimental flow visualizations of fish.
Vortex Tubes in Turbulence Velocity Fields at High Reynolds Numbers
Mouri, H
2008-01-01
The elementary structures of turbulence, i.e., vortex tubes, are studied using velocity data obtained in laboratory experiments for boundary layers and duct flows at microscale Reynolds numbers 332-1934. While past experimental studies focused on intense vortex tubes, the present study focuses on all vortex tubes with various intensities. We obtain the mean velocity profile. The radius scales with the Kolmogorov length. The circulation velocity scales with the Kolmogorov velocity, in contrast to the case of intense vortex tubes alone where the circulation velocity scales with the rms velocity fluctuation. Since these scaling laws are independent of the configuration for turbulence production, they appear to be universal at high Reynolds numbers.
Reynolds number scaling of velocity increments in isotropic turbulence
Iyer, Kartik P.; Sreenivasan, Katepalli R.; Yeung, P. K.
2017-02-01
Using the largest database of isotropic turbulence available to date, generated by the direct numerical simulation (DNS) of the Navier-Stokes equations on an 81923 periodic box, we show that the longitudinal and transverse velocity increments scale identically in the inertial range. By examining the DNS data at several Reynolds numbers, we infer that the contradictory results of the past on the inertial-range universality are artifacts of low Reynolds number and residual anisotropy. We further show that both longitudinal and transverse velocity increments scale on locally averaged dissipation rate, just as postulated by Kolmogorov's refined similarity hypothesis, and that, in isotropic turbulence, a single independent scaling adequately describes fluid turbulence in the inertial range.
Reynolds number and geometry effects in laminar axisymmetric isothermal counterflows
Scribano, Gianfranco
2016-12-29
The counterflow configuration is a canonical stagnation flow, featuring two opposed impinging round jets and a mixing layer across the stagnation plane. Although counterflows are used extensively in the study of reactive mixtures and other applications where mixing of two streams is required, quantitative data on the scaling properties of the flow field are lacking. The aim of this work is to characterize the velocity and mixing fields in isothermal counterflows over a wide range of conditions. The study features both experimental data from particle image velocimetry and results from detailed axisymmetric simulations. The scaling laws for the nondimensional velocity and mixture fraction are obtained as a function of an appropriate Reynolds number and the ratio of the separation distance of the nozzles to their diameter. In the range of flow configurations investigated, the nondimensional fields are found to depend primarily on the separation ratio and, to a lesser extent, the Reynolds number. The marked dependence of the velocity field with respect to the separation ratio is linked to a high pressure region at the stagnation point. On the other hand, Reynolds number effects highlight the role played by the wall boundary layer on the interior of the nozzles, which becomes less important as the separation ratio decreases. The normalized strain rate and scalar dissipation rate at the stagnation plane are found to attain limiting values only for high values of the Reynolds number. These asymptotic values depend markedly on the separation ratio and differ significantly from the values produced by analytical models. The scaling of the mixing field does not show a limiting behavior as the separation ratio decreases to the smallest practical value considered.
Reynolds number and geometry effects in laminar axisymmetric isothermal counterflows
Scribano, Gianfranco; Bisetti, Fabrizio
2016-12-01
The counterflow configuration is a canonical stagnation flow, featuring two opposed impinging round jets and a mixing layer across the stagnation plane. Although counterflows are used extensively in the study of reactive mixtures and other applications where mixing of two streams is required, quantitative data on the scaling properties of the flow field are lacking. The aim of this work is to characterize the velocity and mixing fields in isothermal counterflows over a wide range of conditions. The study features both experimental data from particle image velocimetry and results from detailed axisymmetric simulations. The scaling laws for the nondimensional velocity and mixture fraction are obtained as a function of an appropriate Reynolds number and the ratio of the separation distance of the nozzles to their diameter. In the range of flow configurations investigated, the nondimensional fields are found to depend primarily on the separation ratio and, to a lesser extent, the Reynolds number. The marked dependence of the velocity field with respect to the separation ratio is linked to a high pressure region at the stagnation point. On the other hand, Reynolds number effects highlight the role played by the wall boundary layer on the interior of the nozzles, which becomes less important as the separation ratio decreases. The normalized strain rate and scalar dissipation rate at the stagnation plane are found to attain limiting values only for high values of the Reynolds number. These asymptotic values depend markedly on the separation ratio and differ significantly from the values produced by analytical models. The scaling of the mixing field does not show a limiting behavior as the separation ratio decreases to the smallest practical value considered.
Lagrangian coherent structures in low Reynolds number swimming
Energy Technology Data Exchange (ETDEWEB)
Wilson, Megan M; Eldredge, Jeff D [Mechanical and Aerospace Engineering Department, University of California Los Angeles, Los Angeles, CA 90095 (United States); Peng Jifeng; Dabiri, John O [Department of Bioengineering, California Institute of Technology, Pasadena, CA 91125 (United States)
2009-05-20
This work explores the utility of the finite-time Lyapunov exponent (FTLE) field for revealing flow structures in low Reynolds number biological locomotion. Previous studies of high Reynolds number unsteady flows have demonstrated that ridges of the FTLE field coincide with transport barriers within the flow, which are not shown by a more classical quantity such as vorticity. In low Reynolds number locomotion (O(1)-O(100)), in which viscous diffusion rapidly smears the vorticity in the wake, the FTLE field has the potential to add new insight to locomotion mechanics. The target of study is an articulated two-dimensional model for jellyfish-like locomotion, with swimming Reynolds number of order 1. The self-propulsion of the model is numerically simulated with a viscous vortex particle method, using kinematics adapted from previous experimental measurements on a live medusan swimmer. The roles of the ridges of the computed forward- and backward-time FTLE fields are clarified by tracking clusters of particles both backward and forward in time. It is shown that a series of ridges in front of the jellyfish in the forward-time FTLE field transport slender fingers of fluid toward the lip of the bell orifice, which are pulled once per contraction cycle into the wake of the jellyfish, where the fluid remains partitioned. A strong ridge in the backward-time FTLE field reveals a persistent barrier between fluid inside and outside the subumbrellar cavity. The system is also analyzed in a body-fixed frame subject to a steady free stream, and the FTLE field is used to highlight differences in these frames of reference.
Reynolds number effects on mixing due to topological chaos
Energy Technology Data Exchange (ETDEWEB)
Smith, Spencer A.; Warrier, Sangeeta [Department of Physics, Mount Holyoke College, South Hadley, Massachusetts 01075 (United States)
2016-03-15
Topological chaos has emerged as a powerful tool to investigate fluid mixing. While this theory can guarantee a lower bound on the stretching rate of certain material lines, it does not indicate what fraction of the fluid actually participates in this minimally mandated mixing. Indeed, the area in which effective mixing takes place depends on physical parameters such as the Reynolds number. To help clarify this dependency, we numerically simulate the effects of a batch stirring device on a 2D incompressible Newtonian fluid in the laminar regime. In particular, we calculate the finite time Lyapunov exponent (FTLE) field for three different stirring protocols, one topologically complex (pseudo-Anosov) and two simple (finite-order), over a range of viscosities. After extracting appropriate measures indicative of both the amount of mixing and the area of effective mixing from the FTLE field, we see a clearly defined Reynolds number range in which the relative efficacy of the pseudo-Anosov protocol over the finite-order protocols justifies the application of topological chaos. More unexpectedly, we see that while the measures of effective mixing area increase with increasing Reynolds number for the finite-order protocols, they actually exhibit non-monotonic behavior for the pseudo-Anosov protocol.
Turbulent Flow Physics and Noise in High Reynolds Number Compressible Jets
Glauser, Mark
2016-11-01
In this talk I will present a snapshot of our ongoing research in high Reynolds number turbulent compressible jets. The high speed axisymmetric jet work (Mach 0.6 - 1.1) has been jointly performed with Spectral Energies LLC through AFRL support and involves 10 kHz and large window PIV data extracted from the near field jet plume, simultaneously sampled with near field pressure and far field noise. We have learned from the simultaneously sampled 10 kHz PIV near field plume and far field noise data, using POD/OID and Wavelet filtering, that there are certain "loud" velocity modes that have low averaged turbulent kinetic energy content but strongly correlate with the far field noise. From the large window PIV data obtained at Mach 1.0 and 1.1, specific POD modes were found to contain important physics of the problem. For example, the large-scale structure of the jet, shock-related fluctuations, and turbulent mixing regions of the flow were isolated through POD. By computing cross correlations, particular POD modes were found to be related to particular noise spectra. I will conclude with a description of our complex nozzle work which uses the multi-stream supersonic single expansion rectangular nozzle (SERN) recently installed in our large anechoic chamber at SU. This work is funded from both AFOSR (joint with OSU with a primary focus on flow physics) and Spectral Energies LLC (via AFRL funds with a focus on noise). Particular emphasis will be on insight gained into this complex 3D flow field (and its relationship to the far field noise) from applications of POD, Wavelet filtering and DMD to various numerical (LES) and experimental (PIV, high speed schlieren, near and far field pressure) data sets, at a core nozzle Mach number of 1.6 and a second stream Mach number of 1.0.
Low Mach Number Fluctuating Hydrodynamics of Binary Liquid Mixtures
Nonaka, A J; Bell, J B; Donev, A
2014-01-01
Continuing on our previous work [ArXiv:1212.2644], we develop semi-implicit numerical methods for solving low Mach number fluctuating hydrodynamic equations appropriate for modeling diffusive mixing in isothermal mixtures of fluids with different densities and transport coefficients. We treat viscous dissipation implicitly using a recently-developed variable-coefficient Stokes solver [ArXiv:1308.4605]. This allows us to increase the time step size significantly compared to the earlier explicit temporal integrator. For viscous-dominated flows, such as flows at small scales, we develop a scheme for integrating the overdamped limit of the low Mach equations, in which inertia vanishes and the fluid motion can be described by a steady Stokes equation. We also describe how to incorporate advanced higher-order Godunov advection schemes in the numerical method, allowing for the treatment of fluids with high Schmidt number including the vanishing mass diffusion coefficient limit. We incorporate thermal fluctuations in...
Courant Number and Mach Number Insensitive CE/SE Euler Solvers
Chang, Sin-Chung
2005-01-01
It has been known that the space-time CE/SE method can be used to obtain ID, 2D, and 3D steady and unsteady flow solutions with Mach numbers ranging from 0.0028 to 10. However, it is also known that a CE/SE solution may become overly dissipative when the Mach number is very small. As an initial attempt to remedy this weakness, new 1D Courant number and Mach number insensitive CE/SE Euler solvers are developed using several key concepts underlying the recent successful development of Courant number insensitive CE/SE schemes. Numerical results indicate that the new solvers are capable of resolving crisply a contact discontinuity embedded in a flow with the maximum Mach number = 0.01.
Effects of nonuniform Mach-number entrance on scramjet nozzle flowfield and performance
Zhang, Pu; Xu, Jinglei; Quan, Zhibin; Mo, Jianwei
2016-12-01
Considering the non-uniformities of nozzle entrance influenced by the upstream, the effects of nonuniform Mach-number coupled with shock and expansion-wave on the flowfield and performances of single expansion ramp nozzle (SERN) are numerically studied using Reynolds-Averaged Navier-Stokes equations. The adopted Reynolds-averaged Navier-Stokes methodology is validated by comparing the numerical results with the cold experimental data, and the average method used in this paper is discussed. Uniform and nonuniform facility nozzles are designed to generate different Mach-number profile for the inlet of SERN, which is direct-connected with different facility nozzle, and the whole flowfield is simulated. Because of the coupling of shock and expansion-wave, flow direction of nonuniform SERN entrance is distorted. Compared with Mach contour of uniform case, the line is more curved for coupling shock-wave entrance (SWE) case, and flatter for the coupling expansion-wave entrance (EWE) case. Wall pressure distribution of SWE case appears rising region, whereas decreases like stairs of EWE case. The numerical results reveal that the coupled shock and expansion-wave play significant roles on nozzle performances. Compared with the SERN performances of uniform entrance case at the same work conditions, the thrust of nonuniform entrance cases reduces by 3-6%, pitch moment decreases by 2.5-7%. The negative lift presents an incremental trend with EWE while the situation is the opposite with SWE. These results confirm that considering the entrance flow parameter nonuniformities of a scramjet nozzle coupled with shock or expansion-wave from the upstream is necessary.
Hysteresis phenomenon of hypersonic inlet at high Mach number
Jiao, Xiaoliang; Chang, Juntao; Wang, Zhongqi; Yu, Daren
2016-11-01
When the hypersonic inlet works at a Mach number higher than the design value, the hypersonic inlet is started with a regular reflection of the external compression shock at the cowl, whereas a Mach reflection will result in the shock propagating forwards to cause a shock detachment at the cowl lip, which is called "local unstart of inlet". As there are two operation modes of hypersonic inlet at high Mach number, the mode transition may occur with the operation condition of hypersonic inlet changing. A cowl-angle-variation-induced hysteresis and a downstream-pressure-variation-induced hysteresis in the hypersonic inlet start↔local unstart transition are obtained by viscous numerical simulations in this paper. The interaction of the external compression shock and boundary layer on the cowl plays a key role in the hysteresis phenomenon. Affected by the transition of external compression shock reflection at the cowl and the transition between separated and attached flow on the cowl, a hysteresis exists in the hypersonic inlet start↔local unstart transition. The hysteresis makes the operation of a hypersonic inlet very difficult to control. In order to avoid hysteresis phenomenon and keep the hypersonic inlet operating in a started mode, the control route should never pass through the local unstarted boundary.
Optimal translational swimming of a sphere at low Reynolds number
Felderhof, B U
2015-01-01
Swimming velocity and rate of dissipation of a sphere with surface distortions are discussed on the basis of the Stokes equations of low Reynolds number hydrodynamics. At first the surface distortions are assumed to cause an irrotational axisymmetric flow pattern. The efficiency of swimming is optimized within this class of flows. Subsequently more general axisymmetric polar flows with vorticity are considered. This leads to a considerably higher maximum efficiency. An additional measure of swimming performance is proposed based on the energy consumption for given amplitude of stroke.
Collinear swimmer propelling a cargo sphere at low Reynolds number
Felderhof, B U
2014-01-01
The swimming velocity and rate of dissipation of a linear chain consisting of two or three little spheres and a big sphere is studied on the basis of low Reynolds number hydrodynamics. The big sphere is treated as a passive cargo, driven by the tail of little spheres via hydrodynamic and direct elastic interaction. The fundamental solution of Stokes' equations in the presence of a sphere with no-slip boundary condition, as derived by Oseen, is used to model the hydrodynamic interactions between the big sphere and the little spheres.
High Reynolds number liquid layer flow with flexible walls
Indian Academy of Sciences (India)
J S B Gajjar
2015-05-01
The stability of liquid layer flow over an inclined flexible wall is studied using asymptotic methods based on the assumption that the Reynolds number is large. The flexible wall behaviour is described by a spring-plate model, and parameters chosen so that the wall flexibility affects the governing boundary layer problem. For the case of a rigid wall, the problem reverts to one studied by Gajjar. Asymptotic analysis of the governing equations leads to the triple-deck equations governing the interaction between the wall layer and the free-surface. The linearised and other solution properties of these set of equations are discussed.
The effect of collision, Stokes and Reynolds numbers on turbophoresis
Esmaily-Moghadam, Mahdi; Mani, Ali
2016-11-01
Migration of inertial particles toward solid boundaries in turbulent flows is known as turbophoresis. In this study, we investigate the effect of various parameters on turbophoresis through direct numerical simulations of turbulent flow laden with Lagrangian point-particles. We consider a flow of air in a square duct at a bulk Reynolds number of 5,000 to 20,000 dispersed with nickel particles ranging in size from 4 to 16 micron in diameter. We examine the effect of the Stokes and Reynolds numbers on the near-wall particle concentration and its relationship to the turbophoretic velocity. Our results are consistent with the previously published results pertaining to the saturation of the turbophoretic velocity for Stokes numbers larger than 10. Adopting a hard sphere collision model, we examine the role of collisions on the near wall concentration and demonstrate the sensitivity of the results to the restitution coefficient. Our findings show that while reducing the restitution coefficient leads to a higher degree of turbophoresis; collision can decrease the near wall concentration by orders of magnitude for a global particle volume fraction of O (10-5) . This work was supported by the United States Department of Energy under the Predictive Science Academic Alliance Program 2 (PSAAP2) at Stanford University.
Low Mach number fluctuating hydrodynamics of multispecies liquid mixtures
Energy Technology Data Exchange (ETDEWEB)
Donev, Aleksandar, E-mail: donev@courant.nyu.edu; Bhattacharjee, Amit Kumar [Courant Institute of Mathematical Sciences, New York University, New York, New York 10012 (United States); Nonaka, Andy; Bell, John B. [Center for Computational Science and Engineering, Lawrence Berkeley National Laboratory, Berkeley, California 94720 (United States); Garcia, Alejandro L. [Department of Physics and Astronomy, San Jose State University, San Jose, California 95192 (United States)
2015-03-15
We develop a low Mach number formulation of the hydrodynamic equations describing transport of mass and momentum in a multispecies mixture of incompressible miscible liquids at specified temperature and pressure, which generalizes our prior work on ideal mixtures of ideal gases [Balakrishnan et al., “Fluctuating hydrodynamics of multispecies nonreactive mixtures,” Phys. Rev. E 89 013017 (2014)] and binary liquid mixtures [Donev et al., “Low mach number fluctuating hydrodynamics of diffusively mixing fluids,” Commun. Appl. Math. Comput. Sci. 9(1), 47-105 (2014)]. In this formulation, we combine and extend a number of existing descriptions of multispecies transport available in the literature. The formulation applies to non-ideal mixtures of arbitrary number of species, without the need to single out a “solvent” species, and includes contributions to the diffusive mass flux due to gradients of composition, temperature, and pressure. Momentum transport and advective mass transport are handled using a low Mach number approach that eliminates fast sound waves (pressure fluctuations) from the full compressible system of equations and leads to a quasi-incompressible formulation. Thermal fluctuations are included in our fluctuating hydrodynamics description following the principles of nonequilibrium thermodynamics. We extend the semi-implicit staggered-grid finite-volume numerical method developed in our prior work on binary liquid mixtures [Nonaka et al., “Low mach number fluctuating hydrodynamics of binary liquid mixtures,” http://arxiv.org/abs/1410.2300 (2015)] and use it to study the development of giant nonequilibrium concentration fluctuations in a ternary mixture subjected to a steady concentration gradient. We also numerically study the development of diffusion-driven gravitational instabilities in a ternary mixture and compare our numerical results to recent experimental measurements [Carballido-Landeira et al., “Mixed-mode instability of a
Chan, David T.; Brauckmann, Gregory J.
2011-01-01
A 6%-scale unpowered model of the Orion Launch Abort Vehicle (LAV) ALAS-11-rev3c configuration was tested in the NASA Langley National Transonic Facility to obtain static aerodynamic data at flight Reynolds numbers. Subsonic and transonic data were obtained for Mach numbers between 0.3 and 0.95 for angles of attack from -4 to +22 degrees and angles of sideslip from -10 to +10 degrees. Data were also obtained at various intermediate Reynolds numbers between 2.5 million and 45 million depending on Mach number in order to examine the effects of Reynolds number on the vehicle. Force and moment data were obtained using a 6-component strain gauge balance that operated both at warm temperatures (+120 . F) and cryogenic temperatures (-250 . F). Surface pressure data were obtained with electronically scanned pressure units housed in heated enclosures designed to survive cryogenic temperatures. Data obtained during the 3-week test entry were used to support development of the LAV aerodynamic database and to support computational fluid dynamics code validation. Furthermore, one of the outcomes of the test was the reduction of database uncertainty on axial force coefficient for the static unpowered LAV. This was accomplished as a result of good data repeatability throughout the test and because of decreased uncertainty on scaling wind tunnel data to flight.
Driving large magnetic Reynolds number flow in highly ionized, unmagnetized plasmas
Weisberg, D. B.; Peterson, E.; Milhone, J.; Endrizzi, D.; Cooper, C.; Désangles, V.; Khalzov, I.; Siller, R.; Forest, C. B.
2017-05-01
Electrically driven, unmagnetized plasma flows have been generated in the Madison plasma dynamo experiment with magnetic Reynolds numbers exceeding the predicted Rmcrit = 200 threshold for flow-driven MHD instability excitation. The plasma flow is driven using ten thermally emissive lanthanum hexaboride cathodes which generate a J ×B torque in helium and argon plasmas. Detailed Mach probe measurements of plasma velocity for two flow topologies are presented: edge-localized drive using the multi-cusp boundary field and volumetric drive using an axial Helmholtz field. Radial velocity profiles show that the edge-driven flow is established via ion viscosity but is limited by a volumetric neutral drag force, and measurements of velocity shear compare favorably to the Braginskii transport theory. Volumetric flow drive is shown to produce larger velocity shear and has the correct flow profile for studying the magnetorotational instability.
Fabrication and control of simple low Reynolds number microswimmers
Cheang, U. Kei; Kim, Min Jun
2016-07-01
The development of miniaturized robotic swimmers is hindered by technical limitations in micro- and nanofabrication. To circumvent these limitations, we investigated the minimal geometrical requirements for swimming in low Reynolds number. Micro- and nanofabrication of complex shapes, such as helices, on a massive scale requires sophisticated state of the art technologies and has size limitations. In contrast, simple shaped structures, such as spherical particles, can be fabricated massively using chemical synthesis with relative ease. Here, simple microswimmers were fabricated using two microparticles with debris attached to their surface. The debris on the microswimmer's surface creates a geometry with two or more planes of symmetry, allowing the microswimmer to swim in bulk fluid at low Reynolds number. The microswimmers are magnetically actuated and controlled via a uniform rotating magnetic field generated by an approximate Helmholtz electromagnetic coil system. We characterized the microswimmer's velocity profile with respect to rotating frequency and analyzed the motion of the microswimmer using image processing. Finally, we demonstrated the controllability of the microswimmers by freely steering them in any desired directions.
High Reynolds number rough-wall turbulent boundary layers
Squire, Dougal; Morrill-Winter, Caleb; Schultz, Michael; Hutchins, Nicholas; Klewicki, Joseph; Marusic, Ivan
2015-11-01
In his review of turbulent flows over rough-walls, Jimenez (2004) concludes that there are gaps in the current database of relevant experiments. The author calls for measurements in which δ / k and k+ are both large--low blockage, fully-rough flow--and where δ / k is large and k+ is small--low blockage, transitionally-rough flow--to help clarify ongoing questions regarding the physics of rough-wall-bounded flows. The present contribution details results from a large set of measurements carried out above sandpaper in the Melbourne Wind Tunnel. The campaign spans 45 rough-wall measurements using single and multiple-wire hot-wire anemometry sensors and particle image velocimetry. A floating element drag balance is employed to obtain the rough-wall skin friction force. The data span 20
Reynolds number effects on mixing due to topological chaos
Smith, Spencer A
2016-01-01
Topological chaos has emerged as a powerful tool to investigate fluid mixing. While this theory can guarantee a lower bound on the stretching rate of certain material lines, it does not indicate what fraction of the fluid actually participates in this minimally mandated mixing. Indeed, the area in which effective mixing takes place depends on physical parameters such as the Reynolds number. To help clarify this dependency, we numerically simulate the effects of a batch stirring device on a 2D incompressible Newtonian fluid in the laminar regime. In particular, we calculate the finite time Lyapunov exponent (FTLE) field for three different stirring protocols, one topologically complex (pseudo-Anosov) and two simple (finite-order), over a range of viscosities. After extracting appropriate measures indicative of both the amount of mixing and the area of effective mixing from the FTLE field, we see a clearly defined Reynolds number range in which the relative efficacy of the pseudo-Anosov protocol over the finite...
The random walk of a low-Reynolds-number swimmer
Rafaï, Salima; Garcia, Michaël; Berti, Stefano; Peyla, Philippe
2010-11-01
Swimming at a micrometer scale demands particular strategies. Indeed when inertia is negligible as compared to viscous forces (i.e. Reynolds number Re is lower than unity), hydrodynamics equations are reversible in time. To achieve propulsion a low Reynolds number, swimmers must then deform in a way that is not invariant under time reversal. Here we investigate the dispersal properties of self propelled organisms by means of microscopy and cell tracking. Our system of interest is the microalga Chlamydomonas Reinhardtii, a motile single celled green alga about 10 micrometers in diameter that swims with two flagellae. In the case of dilute suspensions, we show that tracked trajectories are well modelled by a correlated random walk. This process is based on short time correlations in the direction of movement called persistence. At longer times, correlations are lost and a standard random walk caracterizes the trajectories. Moreover, high speed imaging enables us to show how speed fluctuations at very short times affect the statistical description of the dynamics. Finally we show how drag forces modify the characteristics of this particular random walk.
Design of a High-Reynolds Number Recirculating Water Tunnel
Daniel, Libin; Elbing, Brian
2014-11-01
An experimental fluid mechanics laboratory focused on turbulent boundary layers, drag reduction techniques, multiphase flows and fluid-structure interactions has recently been established at Oklahoma State University. This laboratory has three primary components; (1) a recirculating water tunnel, (2) a multiphase pipe flow loop, and (3) a multi-scale flow visualization system. The design of the water tunnel is the focus of this talk. The criteria used for the water tunnel design was that it had to produce a momentum-thickness based Reynolds number in excess of 104, negligible flow acceleration due to boundary layer growth, maximize optical access for use of the flow visualization system, and minimize inlet flow non-uniformity. This Reynolds number was targeted to bridge the gap between typical university/commercial water tunnels (103) and the world's largest water tunnel facilities (105) . These objectives were achieved with a 152 mm (6-inch) square test section that is 1 m long and has a maximum flow speed of 10 m/s. The flow non-uniformity was mitigated with the use of a tandem honeycomb configuration, a settling chamber and an 8.5:1 contraction. The design process that produced this final design will be presented along with its current status.
Hydrodynamic synchronization of nonlinear oscillators at low Reynolds number.
Leoni, M; Liverpool, T B
2012-04-01
We introduce a generic model of a weakly nonlinear self-sustained oscillator as a simplified tool to study synchronization in a fluid at low Reynolds number. By averaging over the fast degrees of freedom, we examine the effect of hydrodynamic interactions on the slow dynamics of two oscillators and show that they can lead to synchronization. Furthermore, we find that synchronization is strongly enhanced when the oscillators are nonisochronous, which on the limit cycle means the oscillations have an amplitude-dependent frequency. Nonisochronity is determined by a nonlinear coupling α being nonzero. We find that its (α) sign determines if they synchronize in phase or antiphase. We then study an infinite array of oscillators in the long-wavelength limit, in the presence of noise. For α>0, hydrodynamic interactions can lead to a homogeneous synchronized state. Numerical simulations for a finite number of oscillators confirm this and, when α<0, show the propagation of waves, reminiscent of metachronal coordination.
DSMC Simulation of High Mach Number Taylor-Couette Flow
Pradhan, Sahadev, , Dr.
2017-01-01
The main focus of this work is to characterise the Taylor-Couette flow of an ideal gas between two coaxial cylinders at Mach number Ma = (U_w /√{ kbT_w / m }) in the range 0.01 Boltzmann constant. The cylindrical surfaces are specified as being diffusely reflecting with the thermal accommodation coefficient equal to one. In the present analysis of high Mach number compressible Taylor-Couette flow using DSMC method, wall slip in the temperature and the velocities are found to be significant. Slip occurs because the temperature/velocity of the molecules incident on the wall could be very different from that of the wall, even though the temperature/velocity of the reflected molecules is equal to that of the wall. Due to the high surface speed of the inner cylinder, significant heating of the gas is taking place. The gas temperature increases until the heat transfer to the surface equals the work done in moving the surface. The highest temperature is obtained near the moving surface of the inner cylinder at a radius of about (1.26 r_1).
Electrohydrodynamic deformation of drops and bubbles at large Reynolds numbers
Schnitzer, Ory
2015-11-01
In Taylor's theory of electrohydrodynamic drop deformation by a uniform electric field, inertia is neglected at the outset, resulting in fluid velocities that scale with E2, E being the applied-field magnitude. When considering strong fields and low viscosity fluids, the Reynolds number predicted by this scaling may actually become large, suggesting the need for a complementary large-Reynolds-number analysis. Balancing viscous and electrical stresses reveals that the velocity scales with E 4 / 3. Considering a gas bubble, the external flow is essentially confined to two boundary layers propagating from the poles to the equator, where they collide to form a radial jet. Remarkably, at leading order in the Capillary number the unique scaling allows through application of integral mass and momentum balances to obtain a closed-form expression for the O (E2) bubble deformation. Owing to a concentrated pressure load at the vicinity of the collision region, the deformed profile features an equatorial dimple which is non-smooth on the bubble scale. The dynamical importance of internal circulation in the case of a liquid drop leads to an essentially different deformation mechanism. This is because the external boundary layer velocity attenuates at a short distance from the interface, while the internal boundary-layer matches with a Prandtl-Batchelor (PB) rotational core. The dynamic pressure associated with the internal circulation dominates the interfacial stress profile, leading to an O (E 8 / 3) deformation. The leading-order deformation can be readily determined, up to the PB constant, without solving the circulating boundary-layer problem. To encourage attempts to verify this new scaling, we shall suggest a favourable experimental setup in which inertia is dominant, while finite-deformation, surface-charge advection, and gravity effects are negligible.
Nagata, T.; Nonomura, T.; Takahashi, S.; Mizuno, Y.; Fukuda, K.
2016-05-01
In this study, analysis of flow properties around a sphere and its aerodynamic coefficients in the high-Mach-and-low-Reynolds-numbers conditions is carried out by direct numerical simulations solving the three-dimensional compressible Navier-Stokes equations. The calculation is performed on a boundary-fitted coordinate system with a high-order scheme of sufficient accuracy. The analysis is conducted by assuming a rigid sphere with a Reynolds number of between 50 and 300, based on the diameter of the sphere and the freestream velocity and a freestream Mach number of between 0.3 and 2.0, together with the adiabatic wall boundary condition. The calculation shows the following yields: (1) unsteady fluctuation of hydrodynamic forces become smaller as the Mach number increases under the same Reynolds number condition, (2) the drag coefficient increases with the Mach number due to an increase in the pressure drag by the shock wave, and (3) an accurate prediction of the drag coefficient in the supersonic regime using traditional models might be difficult.
Low Reynolds number flow near tiny leaves, stems, and trichomes
Strickland, Christopher; Pasour, Virginia; Miller, Laura
2016-11-01
In terrestrial and aquatic environments such as forest canopies, grass fields, and seagrass beds, the density and shape of trunks, branches, stems, leaves and trichomes (the hairs or fine outgrowths on plants) can drastically alter both the average wind speed and profile through these environments and near each plant. While many studies of flow in these environments have focused on bulk properties of the flow at scales on the order of meters, the low Reynolds number flow close to vegetative structures is especially complex and relevant to nutrient exchange. Using three-dimensional immersed boundary simulations, we resolve the flow around trichomes and small leaves and quantify velocities, shear stresses, and mixing while varying the height and density of idealized structures. National Science Foundation Grant DMS-1127914 to the Statistical and Applied Mathematical Sciences Institute, and the Army Research Office.
Stokesian swimming of a prolate spheroid at low Reynolds number
Felderhof, B U
2016-01-01
The swimming of a spheroid immersed in a viscous fluid and performing surface deformations periodically in time is studied on the basis of Stokes equations of low Reynolds number hydrodynamics. The average over a period of time of the swimming velocity and the rate of dissipation are given by integral expressions of second order in the amplitude of surface deformations. The first order flow velocity and pressure, as functions of spheroidal coordinates, are expressed as sums of basic solutions of Stokes equations. Sets of superposition coefficients of these solutions which optimize the mean swimming speed for given power are derived from an eigenvalue problem. The maximum eigenvalue is a measure of the efficiency of the optimal stroke within the chosen class of motions. The maximum eigenvalue for sets of low order is found to be a strongly increasing function of the aspect ratio of the spheroid.
Swimming with a friend at low Reynolds number
Pooley, C M; Yeomans, J M
2007-01-01
We investigate the hydrodynamic interactions between microorganisms swimming at low Reynolds number. By considering simple model swimmers, and combining analytic and numerical approaches, we investigate the time-averaged flow field around a swimmer. At short distances the swimmer behaves like a pump. At large distances the velocity field depends on whether the swimming stroke is invariant under a combined time-reversal and parity transformation. We then consider two swimmers and find that the interaction between them consists of two parts; a dead term, independent of the motion of the second swimmer, which takes the expected dipolar form and a live term resulting from the simultaneous swimming action of both swimmers which does not. We argue that, in general, the latter dominates. The swimmer--swimmer interaction is a complicated function of their relative displacement, orientation and phase, leading to motion that can be attractive, repulsive or oscillatory.
Shrimp theorem: paddle swimming at low Reynolds number
Takagi, Daisuke
2014-11-01
A large variety of aquatic organisms, such as small planktonic crustaceans, use multiple legs as paddles; however the resultant dynamics and efficiency of locomotion are not yet clear. I will present a simple model of swimming with multiple pairs of stiff legs. The legs are assumed to oscillate in a metachronal pattern in a model based on slender-body theory for Stokes flow. The model predicts locomotion in the direction of the metachronal wave, as frequently observed in nature. Unlike scallops undergoing reciprocal motion, shrimp can swim at low Reynolds number. This study offers a possible explanation why crustaceans thrive in aquatic environments, and could inspire a new generation of powerful biomimetic robots.
Effects of viscoelasticity in the high Reynolds number cylinder wake
Richter, David
2012-01-16
At Re = 3900, Newtonian flow past a circular cylinder exhibits a wake and detached shear layers which have transitioned to turbulence. It is the goal of the present study to investigate the effects which viscoelasticity has on this state and to identify the mechanisms responsible for wake stabilization. It is found through numerical simulations (employing the FENE-P rheological model) that viscoelasticity greatly reduces the amount of turbulence in the wake, reverting it back to a state which qualitatively appears similar to the Newtonian mode B instability which occurs at lower Re. By focusing on the separated shear layers, it is found that viscoelasticity suppresses the formation of the Kelvin-Helmholtz instability which dominates for Newtonian flows, consistent with previous studies of viscoelastic free shear layers. Through this shear layer stabilization, the viscoelastic far wake is then subject to the same instability mechanisms which dominate for Newtonian flows, but at far lower Reynolds numbers. © Copyright Cambridge University Press 2012.
There can be turbulence in microfluidics at low Reynolds number.
Wang, G R; Yang, Fang; Zhao, Wei
2014-04-21
Turbulence is commonly viewed as a type of macroflow, where the Reynolds number (Re) has to be sufficiently high. In microfluidics, when Re is below or on the order of 1 and fast mixing is required, so far only chaotic flow has been reported to enhance mixing based on previous publications since turbulence is believed not to be possible to generate in such a low Re microflow. There is even a lack of velocimeter that can measure turbulence in microchannels. In this work, we report a direct observation of the existence of turbulence in microfluidics with Re on the order of 1 in a pressure driven flow under electrokinetic forcing using a novel velocimeter having ultrahigh spatiotemporal resolution. The work could provide a new method to control flow and transport phenomena in lab-on-a-chip and a new perspective on turbulence.
The Aerodynamics of Deforming Wings at Low Reynolds Number
Medina, Albert
responsive to flexibility satisfying an inverse proportionality to stiffness. In hover, an effective pitch angle can be defined in a flexible wing that accounts for deflection which shifts results toward trend lines of rigid wings. Three-dimensional simulations examining the effects of two distinct deformation modes undergoing prescribed deformation associated with root and tip deflection demonstrated a greater aerodynamic response to tip deflection in hover. Efficiency gains in flexion wings over rigid wing counterpart were shown to be dependent on Reynolds number with efficiency in both modes increasing with increased Reynolds number. Additionally, while the leading-edge vortex axis proved insensitive to deformation, the shape and orientation of the LEV core is modified. Experiments on three-dimensional dynamically-scaled fruit fly wings with passive deformation operating in the bursting limit Reynolds number regime revealed enhanced leading-edge vortex bursting with tip deflection promoting greater LEV core flow deceleration in stroke. Experimental studies on rotary wings highlights a universal formation time of the leading-edge vortex independent of Reynolds number, acceleration profile and aspect ratio. Efforts to replicate LEV bursting phenomena of higher aspect ratio wings in a unity aspect ratio wing such that LEV growth is no limited by span but by the LEV traversing the chord revealed a flow regime of oscillatory lift generation reminiscent of behavior exhibited in translating wings that also maintains magnitude peak to peak.
Quasi-static magnetohydrodynamic turbulence at high Reynolds number
Favier, B F N; Cambon, C; Delache, A; Bos, W J T
2011-01-01
We analyse the anisotropy of homogeneous turbulence in an electrically conducting fluid submitted to a uniform magnetic field, for low magnetic Reynolds number, in the quasi- static approximation. We interpret disagreeing previous predictions between linearized theory and simulations: in the linear limit, the kinetic energy of transverse velocity components, normal to the magnetic field, decays faster than the kinetic energy of the axial component, along the magnetic field (Moffatt (1967)); whereas many numerical studies predict a final state characterised by dominant energy of transverse velocity components. We investigate the corresponding nonlinear phenomenon using Direct Numerical Simulations of freely-decaying turbulence, and a two-point statistical spectral closure based on the Eddy Damped Quasi-Normal Markovian model. The transition from the three-dimensional turbulent flow to a "two-and-a-half-dimensional" flow (Montgomery & Turner (1982)) is a result of the combined effects of short-time linear J...
Magnetic propulsion of robotic sperms at low-Reynolds number
Khalil, Islam S. M.; Fatih Tabak, Ahmet; Klingner, Anke; Sitti, Metin
2016-07-01
We investigate the microswimming behaviour of robotic sperms in viscous fluids. These robotic sperms are fabricated from polystyrene dissolved in dimethyl formamide and iron-oxide nanoparticles. This composition allows the nanoparticles to be concentrated within the bead of the robotic sperm and provide magnetic dipole, whereas the flexibility of the ultra-thin tail enables flagellated locomotion using magnetic fields in millitesla range. We show that these robotic sperms have similar morphology and swimming behaviour to those of sperm cells. Moreover, we show experimentally that our robotic sperms swim controllably at an average speed of approximately one body length per second (around 125 μm s-1), and they are relatively faster than the microswimmers that depend on planar wave propulsion in low-Reynolds number fluids.
Quadroar: a versatile low-Reynolds-number swimmer
Jalali, Mir Abbas; Mousavi, SeyyedHossein
2014-01-01
We design and simulate the motion of a new swimmer, the Quadroar, with three dimensional translation and reorientation capabilities in low Reynolds number conditions. The Quadroar is composed of an $\\texttt{I}$-shaped frame whose body link is a simple linear actuator, and four disks that can rotate about the axes of flange links. The time symmetry is broken by a combination of disk rotations and the one-dimensional expansion/contraction of the body link. The Quadroar propels on forward and transverse straight lines and performs full three dimensional reorientation maneuvers, which enable it to swim along arbitrary trajectories. We find continuous operation modes that propel the swimmer on planar and three dimensional rosette orbits, which can be periodic or quasi-periodic. Precessing rosette orbits consist of slow lingering phases with cardioid or multiloop turns followed by directional propulsive phases. Quasi-periodic orbits allow the swimmer to access large parts of its neighboring space without using comp...
Elastic Turbulence in Channel Flows at Low Reynolds number
Qin, Boyang
2016-01-01
We experimentally demonstrate the existence of elastic turbulence in straight channel flow at low Reynolds numbers. Velocimetry measurements show non-periodic fluctuations in the wake of curved cylinders as well as in a parallel shear flow region. The flow in these two locations of the channel is excited over a broad range of frequencies and wavelengths, consistent with the main features of elastic turbulence. However, the decay of the initial elastic turbulence around the cylinders is followed by a growth downstream in the straight region. The emergence of distinct flow characteristics both in time and space suggests a new type of elastic turbulence, markedly different from that near the curved cylinders. We propose a self-sustaining mechanism to explain the sustained fluctuations in the parallel shear region.
Numerical simulation of high Reynolds number bubble motion
Energy Technology Data Exchange (ETDEWEB)
McLaughlin, J.B. [Clarkson Univ., Potsdam, NY (United States)
1995-12-31
This paper presents the results of numerical simulations of bubble motion. All the results are for single bubbles in unbounded fluids. The liquid phase is quiescent except for the motion created by the bubble, which is axisymmetric. The main focus of the paper is on bubbles that are of order 1 mm in diameter in water. Of particular interest is the effect of surfactant molecules on bubble motion. Results for the {open_quotes}insoluble surfactant{close_quotes} model will be presented. These results extend research by other investigators to finite Reynolds numbers. The results indicate that, by assuming complete coverage of the bubble surface, one obtains good agreement with experimental observations of bubble motion in tap water. The effect of surfactant concentration on the separation angle is discussed.
Lumley decomposition of turbulent boundary layer at high Reynolds numbers
Tutkun, Murat; George, William K.
2017-02-01
The decomposition proposed by Lumley in 1966 is applied to a high Reynolds number turbulent boundary layer. The experimental database was created by a hot-wire rake of 143 probes in the Laboratoire de Mécanique de Lille wind tunnel. The Reynolds numbers based on momentum thickness (Reθ) are 9800 and 19 100. Three-dimensional decomposition is performed, namely, proper orthogonal decomposition (POD) in the inhomogeneous and bounded wall-normal direction, Fourier decomposition in the homogeneous spanwise direction, and Fourier decomposition in time. The first POD modes in both cases carry nearly 50% of turbulence kinetic energy when the energy is integrated over Fourier dimensions. The eigenspectra always peak near zero frequency and most of the large scale, energy carrying features are found at the low end of the spectra. The spanwise Fourier mode which has the largest amount of energy is the first spanwise mode and its symmetrical pair. Pre-multiplied eigenspectra have only one distinct peak and it matches the secondary peak observed in the log-layer of pre-multiplied velocity spectra. Energy carrying modes obtained from the POD scale with outer scaling parameters. Full or partial reconstruction of turbulent velocity signal based only on energetic modes or non-energetic modes revealed the behaviour of urms in distinct regions across the boundary layer. When urms is based on energetic reconstruction, there exists (a) an exponential decay from near wall to log-layer, (b) a constant layer through the log-layer, and (c) another exponential decay in the outer region. The non-energetic reconstruction reveals that urms has (a) an exponential decay from the near-wall to the end of log-layer and (b) a constant layer in the outer region. Scaling of urms using the outer parameters is best when both energetic and non-energetic profiles are combined.
Muraoka, Masahiro; Yatagawa, Yuta; Kumagai, Yuki
2016-07-01
The coalescence of droplets in flow through a tube at low Reynolds number is potentially useful for different purposes including the handling of fluids, control of chemical reaction, and in drug delivery systems. The phenomenon is also the basis for analyzing the flow of multiphase fluids through porous media such as in enhanced oil recovery and the breaking of emulsions in porous coalescers. With regard to examples of studies on the creeping motion of droplets in a flow through a tube, Hetsroni G. et al.[1] theoretically examined the motion of a spherical droplet or bubble with small d/D, where d is the undeformed diameter of the droplet or bubble, and D is the tube diameter. Higdon J.J.L. and Muldowney G.P. [2] numerically obtained the resistance functions for a spherical particle, droplet, and bubble. Olbricht, W.L. and Kung D.M.[3] and Aul R.W. and Olbricht, W.L.[4] mainly investigated the coalescence time of droplets. Aul R.W. and Olbricht W.L. proposed a semi-theoretical formula of the coalescence time. Based on the formula by them, Muraoka, M. et al.[5] proposed other semi-theoretical formulas of the coalescence time in terms of the resistance experienced by the liquid droplet in creeping flow through a tube. The latter formulas take the eccentricity of the following droplets into consideration. In the present study, a glass tube of inner diameter 2.0mm, outer diameter 7.0mm, and length 1500 mm was used as the test tube. Silicon oil with a kinematic viscosity of 3000cSt was employed as the test fluid of the droplet. A mixture of glycerol and pure water was used as the surrounding fluid of the creeping flow through a tube. A large volumetric syringe pump was used to maintain steady flow through the tube at a designated average velocity. The test tube was immersed in temperature-controlled water contained in a tank to maintain constant temperature of the system. The droplets were injected into the test tube. The behaviors of the droplets were monitored by a
Low Mach number theory of freely cooling granular gases
Meerson, Baruch; Vilenkin, Arkady
2007-01-01
We use hydrodynamic equations to investigate the dynamics of a freely cooling dilute granular gas with nearly elastic particle collisions. We assume a narrow channel geometry and focus on the regime where the sound travel time through the system is much shorter than the typical cooling time of the gas. As a result, the pressure rapidly becomes almost homogeneous, while the Mach number is small. Eliminating the sound waves and employing Lagrangian coordinates, we reduce the full hydrodynamics to a single nonlinear/nonlocal equation of a reaction-diffusion type. This equation describes a broad class of flows and, in particular, can follow the development of strongly nonlinear states during clustering instability. Without heat diffusion, the reduced equation is exactly soluble and develops a finite-time density blowup with the same local features as those exhibited by the recently found family of exact solutions of the full set of ideal hydrodynamic equations (Fouxon et al. 2007). The heat diffusion, however, ar...
Turbomachinery for Low-to-High Mach Number Flight
Tan, Choon S.; Shah, Parthiv N.
2004-01-01
The thrust capability of turbojet cycles is reduced at high flight Mach number (3+) by the increase in inlet stagnation temperature. The 'hot section' temperature limit imposed by materials technology sets the maximum heat addition and, hence, sets the maximum flight Mach number of the operating envelope. Compressor pre-cooling, either via a heat exchanger or mass-injection, has been suggested as a means to reduce compressor inlet temperature and increase mass flow capability, thereby increasing thrust. To date, however, no research has looked at compressor cooling (i.e., using a compressor both to perform work on the gas path air and extract heat from it simultaneously). We wish to assess the feasibility of this novel concept for use in low-to-high Mach number flight. The results to-date show that an axial compressor with cooling: (1) relieves choking in rear stages (hence opening up operability), (2) yields higher-pressure ratio and (3) yields higher efficiency for a given corrected speed and mass flow. The performance benefit is driven: (i) at the blade passage level, by a decrease in the total pressure reduction coefficient and an increase in the flow turning; and (ii) by the reduction in temperature that results in less work required for a given pressure ratio. The latter is a thermodynamic effect. As an example, calculations were performed for an eight-stage compressor with an adiabatic design pressure ratio of 5. By defining non-dimensional cooling as the percentage of compressor inlet stagnation enthalpy removed by a heat sink, the model shows that a non-dimensional cooling of percent in each blade row of the first two stages can increase the compressor pressure ratio by as much as 10-20 percent. Maximum corrected mass flow at a given corrected speed may increase by as much as 5 percent. In addition, efficiency may increase by as much as 5 points. A framework for characterizing and generating the performance map for a cooled compressor has been developed
Reynolds and Atwood Numbers Effects on Homogeneous Rayleigh Taylor Instability
Aslangil, Denis; Livescu, Daniel; Banerjee, Arindam
2015-11-01
The effects of Reynolds and Atwood numbers on turbulent mixing of a heterogeneous mixture of two incompressible, miscible fluids with different densities are investigated by using high-resolution Direct Numerical Simulations (DNS). The flow occurs in a triply periodic 3D domain, with the two fluids initially segregated in random patches, and turbulence is generated in response to buoyancy. In turn, stirring produced by turbulence breaks down the scalar structures, accelerating the molecular mixing. Statistically homogeneous variable-density (VD) mixing, with density variations due to compositional changes, is a basic mixing problem and aims to mimic the core of the mixing layer of acceleration driven Rayleigh Taylor Instability (RTI). We present results covering a large range of kinematic viscosity values for density contrasts including small (A =0.04), moderate (A =0.5), and high (A =0.75 and 0.9) Atwood numbers. Particular interest will be given to the structure of the turbulence and mixing process, including the alignment between various turbulence and scalar quantities, as well as providing fidelity data for verification and validation of mix models. Arindam Banerjee acknowledges support from NSF CAREER award # 1453056.
Flow characteristics over NACA4412 airfoil at low Reynolds number
Directory of Open Access Journals (Sweden)
Genç Mustafa Serdar
2016-01-01
Full Text Available In this study, the flow phenomena over NACA4412 were experimentally observed at various angle of attack and Reynolds number of 25000, 50000 and 75000, respectively. NACA4412 airfoil was manufactured at 3D printer and each tips of the wing were closed by using plexiglas to obtain two-dimensional airfoil. The experiments were conducted at low speed wind tunnel. The force measurement and hot-wire experiments were conducted to obtain data so that the flow phenomenon at the both top and bottom of the airfoil such as the flow separation and vortex shedding were observed. Also, smoke-wire experiment was carried out to visualize the surface flow pattern. After obtaining graphics from both force measurement experiment and hot-wire experiment compared with smoke wire experiment, it was noticed that there is a good coherence among the experiments. It was concluded that as Re number increased, the stall angle increased. And the separation bubble moved towards leading edge over the airfoil as the angle of attack increased.
High Reynolds number magnetohydrodynamic turbulence using a Lagrangian model.
Graham, J Pietarila; Mininni, P D; Pouquet, A
2011-07-01
With the help of a model of magnetohydrodynamic (MHD) turbulence tested previously, we explore high Reynolds number regimes up to equivalent resolutions of 6000(3) grid points in the absence of forcing and with no imposed uniform magnetic field. For the given initial condition chosen here, with equal kinetic and magnetic energy, the flow ends up being dominated by the magnetic field, and the dynamics leads to an isotropic Iroshnikov-Kraichnan energy spectrum. However, the locally anisotropic magnetic field fluctuations perpendicular to the local mean field follow a Kolmogorov law. We find that the ratio of the eddy turnover time to the Alfvén time increases with wave number, contrary to the so-called critical balance hypothesis. Residual energy and helicity spectra are also considered; the role played by the conservation of magnetic helicity is studied, and scaling laws are found for the magnetic helicity and residual helicity spectra. We put these results in the context of the dynamics of a globally isotropic MHD flow that is locally anisotropic because of the influence of the strong large-scale magnetic field, leading to a partial equilibration between kinetic and magnetic modes for the energy and the helicity.
Low-Reynolds number modelling of flows with transpiration
Hwang, C. B.; Lin, C. A.
2000-03-01
An improved low-Reynolds number model was adopted to predict the dynamic and thermal fields in flows with transpiration. The performance of the adopted model was first contrasted with the direct numerical simulation (DNS) data of channel flow with uniform wall injection and suction. The validity of the present model applied to flows with a high level of transpiration was further examined. To explore the model's performance in complex environments, the model was applied to simulate a transpired developing channel flow. By contrasting the predictions with DNS data and measurements, the results indicated that the present model reproduced correctly the deceleration and acceleration of the flow caused by the injection and suction from the permeable part of the wall. The turbulence structure of transpired flows was also well captured and the superior performance of the adopted model was reflected by the predicted correct level of with the maximum being located at both the injection and the suction walls. The predicted thermal field by the present model also compared favourably with the DNS data and measurements. Copyright
Unsteady flow over flexible wings at different low Reynolds numbers
Directory of Open Access Journals (Sweden)
Genç Mustafa Serdar
2016-01-01
Full Text Available In this study, unsteady flow around flexible membrane wing which had aspect ratio of 1 (AR=1 was investigated experimentally at various Reynolds numbers (Re = 25000 and Re = 50000. Smoke-wire technique for flow visualization over the flexible membrane wing was utilized in the experiments. Digital Image Correlation system (DIC was used for measuring deformation of AR = 1 flexible membrane wing. Instantaneous deformation measurements of membrane wing were combined with the flow field measurements. In low aspect ratio flexible membrane wings, unsteadiness includes tip vortices and vortex shedding, and the combination of tip vortices. In these types of wings, complex unsteady deformations occurred due to vortex shedding. The results showed that the increasing angle of attack results in increase of membrane deformation. Moreover, it was concluded that analysis of the instantaneous deformation revealed chordwise and spanwise, modes which were due to the shedding of leading-edge vortices as well as tip vortices. Consequently, vibrational mode decreased and maximum standard deviation location approached to the trailing edge by reason of increasing angle of attack.
Effects of Reynolds Number on Mixing and Dispersion
Stockman, H. W.
2001-12-01
The lattice Boltzmann (LB) method was used to estimate the effects of Reynolds number (Re), and sidewall boundaries, on dispersion of gases in fractured and porous media. The systems studied ranged from idealized channels with parallel grooves and honeycomb structures, to casts of natural fractures and aggregates of sedimented, quasi-spherical particles. For specific configurations of rough, intersecting fractures, Re variation from 0 to 100 causes only a factor ~2 variation in the mixing ratio C2/(C1+C2), where C1 and C2 are the concentrations of solute in the outlet legs of the fracture intersection. However, slight changes in the intersection alignment yield up to factor 5 range in the mixing ratio, for the geometries studied. For both individual fractures and fracture intersections, sidewall boundary effects tend to be overwhelmed by velocity variations within the fracture planes. LB simulations for porous aggregates give good agreement with experimental studies. However, in random aggregates at high Re, it becomes impractical to obtain dispersion coefficients by LB and the method of moments. Alternative LB methods are discussed.
Interaction of two high Reynolds number axisymmetric turbulent wakes
Obligado, M.; Klein, S.; Vassilicos, J. C.
2015-11-01
With the recent discovery of non-equilibrium high Reynolds number scalings in the wake of axisymmetric plates (Nedic et al., PRL, 2013), it has become of importance to develop an experimental technique that permits to easily discriminate between different wake scalings. We propose an experimental setup that tests the presence of non-equilibrium turbulence using the streamwise variation of velocity fluctuations between two bluff bodies facing a flow. We have studied two different sets of plates (one with regular and another with irregular peripheries) with Hot-Wire Anemometry in a wind tunnel. By acquiring streamwise profiles for different plate separations and identifying the wake interaction length for each separation it is possible to estimate the streamwise evolution of the single wake width. From this evolution it is also possible to deduce the turbulence dissipation scalings. This work generalizes previous studies on the interaction of plane wakes (see Gomes-Fernandes et al., JFM, 2012) to include axisymmetric wakes. We find that the wake interaction length proposed in this cited work and a constant anisotropy assumption can be used to collapse the streamwise developments of the first three moments.
A coin vibrational motor swimming at low Reynolds number
Quillen, Alice C; Kelley, Douglas H; Friedmann, Tamar; Oakes, Patrick W
2016-01-01
Low-cost coin vibrational motors, used in haptic feedback, exhibit rotational internal motion inside a rigid case. Because the motor case motion exhibits rotational symmetry, when placed into a fluid such as glycerin, the motor does not swim even though its vibrations induce steady streaming in the fluid. However, a piece of rubber foam stuck to the curved case and giving the motor neutral buoyancy also breaks the rotational symmetry allowing it to swim. We measured a 1 cm diameter coin vibrational motor swimming in glycerin at a speed of a body length in 3 seconds or at 3 mm/s. The swim speed puts the vibrational motor in a low Reynolds number regime similar to bacterial motility, but because of the vibration it is not analogous to biological organisms. Rather the swimming vibrational motor may inspire small inexpensive robotic swimmers that are robust as they contain no external moving parts. A time dependent Stokes equation planar sheet model suggests that the swim speed depends on a steady streaming veloc...
Unsteady flow over flexible wings at different low Reynolds numbers
Genç, Mustafa Serdar; Özden, Mustafa; Hakan Açikel, Halil; Demir, Hacımurat; Isabekov, Iliasbek
2016-03-01
In this study, unsteady flow around flexible membrane wing which had aspect ratio of 1 (AR=1) was investigated experimentally at various Reynolds numbers (Re = 25000 and Re = 50000). Smoke-wire technique for flow visualization over the flexible membrane wing was utilized in the experiments. Digital Image Correlation system (DIC) was used for measuring deformation of AR = 1 flexible membrane wing. Instantaneous deformation measurements of membrane wing were combined with the flow field measurements. In low aspect ratio flexible membrane wings, unsteadiness includes tip vortices and vortex shedding, and the combination of tip vortices. In these types of wings, complex unsteady deformations occurred due to vortex shedding. The results showed that the increasing angle of attack results in increase of membrane deformation. Moreover, it was concluded that analysis of the instantaneous deformation revealed chordwise and spanwise, modes which were due to the shedding of leading-edge vortices as well as tip vortices. Consequently, vibrational mode decreased and maximum standard deviation location approached to the trailing edge by reason of increasing angle of attack.
Universal decay of high Reynolds number Taylor-Couette turbulence
Verschoof, Ruben A; van der Veen, Roeland C A; Sun, Chao; Lohse, Detlef
2015-01-01
We study the decay of high-Reynolds number Taylor-Couette turbulence, i.e. the turbulent flow between two coaxial rotating cylinders. To do so, the rotation of the inner cylinder ($Re_i = 2 \\cdot 10^6$, the outer cylinder is at rest) was suddenly stopped. Using a combination of laser Doppler anemometry and particle image velocimetry measurements, six decay decades of the kinetic energy could be captured. First, in the absence of cylinder rotation, the flow-velocity during the decay does not develop any height dependence in contrast to the well-known Taylor vortex state. Next, the radial profile of the azimuthal velocity is found to be self-similar, i.e. when normalizing it with the mean velocity, it is universal. Nonetheless, the decay of this wall-bounded inhomogeneous turbulent flow does not follow a strict power law as for decaying turbulent homogeneous isotropic flows, but it is faster, due to the strong viscous drag applied by the bounding walls. We theoretically describe the decay in a quantitative way ...
Shear-driven dynamo waves at high magnetic Reynolds number.
Tobias, S M; Cattaneo, F
2013-05-23
Astrophysical magnetic fields often display remarkable organization, despite being generated by dynamo action driven by turbulent flows at high conductivity. An example is the eleven-year solar cycle, which shows spatial coherence over the entire solar surface. The difficulty in understanding the emergence of this large-scale organization is that whereas at low conductivity (measured by the magnetic Reynolds number, Rm) dynamo fields are well organized, at high Rm their structure is dominated by rapidly varying small-scale fluctuations. This arises because the smallest scales have the highest rate of strain, and can amplify magnetic field most efficiently. Therefore most of the effort to find flows whose large-scale dynamo properties persist at high Rm has been frustrated. Here we report high-resolution simulations of a dynamo that can generate organized fields at high Rm; indeed, the generation mechanism, which involves the interaction between helical flows and shear, only becomes effective at large Rm. The shear does not enhance generation at large scales, as is commonly thought; instead it reduces generation at small scales. The solution consists of propagating dynamo waves, whose existence was postulated more than 60 years ago and which have since been used to model the solar cycle.
Propulsion at low Reynolds number via beam extrusion
Gosselin, Frederick; Neetzow, Paul
2014-03-01
We present experimental and theoretical results on the extrusion of a slender beam in a viscous fluid. We are particularly interested in the force necessary to extrude the beam as it buckles with large amplitude due to viscous friction. The problem is inspired by the propulsion of Paramecium via trichocyst extrusion. Self-propulsion in micro-organisms is mostly achieved through the beating of flagella or cilia. However, to avoid a severe aggression, unicellular Paramecium has been observed to extrude trichocysts in the direction of the aggression to burst away. These trichocysts are rod-like organelles which, upon activation, grow to about 40 μm in length in 3 milliseconds before detaching from the animal. The drag force created by these extruding rods pushing against the viscous fluid generates thrust in the opposite direction. We developed an experimental setup to measure the force required to push a steel piano wire into an aquarium filled with corn syrup. This setup offers a near-zero Reynolds number, and allows studying deployments for a range of constant extrusion speeds. The experimental results are reproduced with a numerical model coupling a large amplitude Euler-Bernoulli beam theory with a fluid load model proportional to the local beam velocity. This study was funded in part by the The Natural Sciences and Engineering Research Council of Canada.
Reynolds number dependence of thermal diffusion from a line source in decaying grid turbulence
Johnson, Erika; Warhaft, Zellman
2008-11-01
Existing experiments on line source dispersion in isotropic turbulence are for low Reynolds numbers (Taylor scale Reynolds numbers of less than 100) and there has been no attempt to systematically vary the Reynolds number. Here we present new results of passive temperature fluctuations produced by a fine heated wire in decaying grid turbulence. The Taylor Reynolds number is varied from approximately 50 to 500 by means of active and passive grids. We study the dependence of the mean and r.m.s. temperature profiles on the Reynolds number. The effects of source size are also investigated. The results are compared with the recent modeling work of Viswanathan and Pope (Physics of Fluids, to be published) who find significant Reynolds number dependence but small effects when varying the source size. The peak centerline ratio of the r.m.s. to the mean of the scalar is also examined and compared with predictions. This work is funded by the US National Science Foundation.
Comments on Reynolds number effects in wall-bounded shear layers
Bandyopadhyay, Promode R.
1991-01-01
The effect of Reynolds number on the structure of turbulent boundary layers and channel flows is discussed. Published data are reexamined in light of the following questions: (1) does the boundary layer turbulence structure change after the well known Reynolds number limit viz, when Re(theta) is greater than 6000?; (2) is it possible to disturb a high Reynolds number flat plate turbulent boundary layer near the wall such that the recovery length is O(100 delta)?; and (3) how close is the numerically simulated low Reynolds number flat plate turbulence structure to that observed experimentally? The turbulence structure appears to change continuously with Reynolds number virtually throughout the bounday layer and sometimes in unexpected manners at high Reynolds numbers.
Pulsatility role in cylinder flow dynamics at low Reynolds number
Qamar, Adnan
2012-01-01
We present dynamics of pulsatile flow past a stationary cylinder characterized by three non-dimensional parameters: the Reynolds number (Re), non-dimensional amplitude (A) of the pulsatile flow velocity, and Keulegan-Carpenter number (KC = Uo/Dωc). This work is motivated by the development of total artificial lungs (TAL) device, which is envisioned to provide ambulatory support to patients. Results are presented for 0.2 ≤ A ≤ 0.6 and 0.57 ≤ KC ≤ 2 at Re = 5 and 10, which correspond to the operating range of TAL. Two distinct fluid regimes are identified. In both regimes, the size of the separated zone is much greater than the uniform flow case, the onset of separation is function of KC, and the separation vortex collapses rapidly during the last fraction of the pulsatile cycle. The vortex size is independent of KC, but with an exponential dependency on A. In regime I, the separation point remains attached to the cylinder surface. In regime II, the separation point migrates upstream of the cylinder. Two distinct vortex collapse mechanisms are observed. For A < 0.4 and all KC and Re values, collapse occurs on the cylinder surface, whereas for A > 0.4 the separation vortex detaches from the cylinder surface and collapses at a certain distance downstream of the cylinder. The average drag coefficient is found to be independent of A and KC, and depends only on Re. However, for A > 0.4, for a fraction of the pulsatile cycle, the instantaneous drag coefficient is negative indicating a thrust production. © 2012 American Institute of Physics.
Does the flatness of the velocity derivative blow up at a finite Reynolds number?
Indian Academy of Sciences (India)
K R Sreenivasan; A Bershadskii
2005-06-01
A tentative suggestion is made that the flatness of the velocity derivative could reach an infinite value at finite (though very large) Reynolds number, with possible implications for the singularities of the Navier–Stokes equations. A direct test of this suggestion requires measurements at Reynolds numbers presently outside the experimental capacity, so an alternative suggestion that can be tested at accessible Reynolds numbers is also made.
Irrecoverable pressure loss coefficients for two out-of-plane piping elbows at high Reynolds number
Energy Technology Data Exchange (ETDEWEB)
Coffield, R.D.; Hammond, R.B.; McKeown, P.T.
1999-02-08
Pressure drops of multiple piping elbows were experimentally determined for high Reynolds number flows. The testing described has been performed in order to reduce uncertainties in the currently used methods for predicting irrecoverable pressure losses and also to provide a qualification database for computational fluid dynamics (CFD) computer codes. The earlier high Reynolds number correlations had been based on extrapolations over several orders of magnitude in Reynolds number from where the original database existed. Recent single elbow test data shows about a factor of two lower elbow pressure loss coefficient (at 40x 106 Reynolds number) than those from current correlations. This single piping elbow data has been extended in this study to a multiple elbow configuration of two elbows that are 90o out-of-plane relative to each other. The effects of separation distance and Reynolds number have been correlated and presented in a form that can be used for design application. Contrary to earlier extrapolations from low Reynolds numbers (Re c 1.0x 106), a strong Reynolds number dependence was found to exist. The combination of the high Reynolds number single elbow data with the multiple elbow interaction effects measured in this study shows that earlier design correlations are conservative by significant margins at high Reynolds numbers. Qualification of CFD predictions with this new high Reynolds number database will help guide the need for additional high Reynolds number testing of other piping configurations. The study also included velocity measurements at several positions downstream of the first and second test elbows using an ultrasonic flowmeter. Reasonable agreement after the first test elbow was found relative to flow fields that are known to exist from low Reynolds number visual tests and also from CFD predictions. This data should help to qualify CFD predictions of the three-dimensional flow stream downstream of the second test elbow.
Hybrid RANS/LES method for high Reynolds numbers, applied to atmospheric flow over complex terrain
DEFF Research Database (Denmark)
Bechmann, Andreas; Sørensen, Niels N.; Johansen, Jeppe
2007-01-01
The use of Large-Eddy Simulation (LES) to predict wall-bounded flows has presently been limited to low Reynolds number flows. Since the number of computational grid points required to resolve the near-wall turbulent structures increase rapidly with Reynolds number, LES has been unattainable for...
2016-06-23
AFRL-AFOSR-VA-TR-2016-0277 Experimental Investigation of Turbulence- Chemistry Interaction in High-Reynolds-Number Turbulent Partially Premixed...4. TITLE AND SUBTITLE [U] Experimental investigation of turbulence- chemistry interaction in high-Reynolds-number 5a. CONTRACT NUMBER turbulent...flames. Mixture fraction is an important variable in understanding and modeling turbulent mixing and turbulence- chemistry interaction, two key
Lee, Donghwi; Kawai, Soshi; Nonomura, Taku; Anyoji, Masayuki; Aono, Hikaru; Oyama, Akira; Asai, Keisuke; Fujii, Kozo
2015-02-01
Mechanisms behind the pressure distribution and skin friction within a laminar separation bubble (LSB) are investigated by large-eddy simulations around a 5% thickness blunt flat plate at the chord length based Reynolds number 5.0 × 103, 6.1 × 103, 1.1 × 104, and 2.0 × 104. The characteristics inside the LSB change with the Reynolds number; a steady laminar separation bubble (LSB_S) at the Reynolds number 5.0 × 103 and 6.1 × 103, and a steady-fluctuating laminar separation bubble (LSB_SF) at the Reynolds number 1.1 × 104, and 2.0 × 104. Different characteristics of pressure and skin friction distributions are observed by increasing the Reynolds number, such that a gradual monotonous pressure recovery in the LSB_S and a plateau pressure distribution followed by a rapid pressure recovery region in the LSB_SF. The reasons behind the different characteristics of pressure distributions at different Reynolds numbers are discussed by deriving the Reynolds averaged pressure gradient equation. It is confirmed that the viscous stress distributions near the surface play an important role in determining the formation of different pressure distributions. Depending on the Reynolds numbers, the viscous stress distributions near the surface are affected by the development of a separated laminar shear layer or the Reynolds shear stress. In addition, we show that the same analyses can be applied to the flows around a NACA0012 airfoil.
Eaves, R. H.; Buchanan, T. D.; Warmbrod, J. D.; Johnson, C. B.
1972-01-01
Heat transfer tests for two delta wing configurations were conducted in the hypervelocity wind tunnel. The 24-inch long models were tested at a Mach number of approximately 10.5 and at angles of attack of 20, 40, and 60 degrees over a length Reynolds number range from 5 million to 23 million on 4 May to 4 June 1971. Heat transfer results were obtained from model surface heat gage measurements and thermographic phosphor paint.
DSMC simulations of leading edge flat-plate boundary layer flows at high Mach number
Pradhan, Sahadev, , Dr.
2017-01-01
The flow over a 2D leading-edge flat plate is studied at Mach number Ma = (Uinf /√{kBTinf / m }) in the range Boltzmann constant. The variation of streamwise velocity, temperature, number-density, and mean free path along the wall normal direction away from the plate surface is studied. The qualitative nature of the streamwise velocity at high Mach number is similar to those in the incompressible limit (parabolic profile). However, there are important differences. The amplitudes of the streamwise velocity increase as the Mach number increases and turned into a more flatter profile near the wall. There is significant velocity and temperature slip at the surface of the plate, and the slip increases as the Mach number is increased. It is interesting to note that for the highest Mach numbers considered here, the streamwise velocity at the wall exceeds the sound speed, and the flow is supersonic throughout the flow domain.
Aerodynamics Investigation of Faceted Airfoils at Low Reynolds Number
Napolillo, Zachary G.
The desire and demand to fly farther and faster has progressively integrated the concept of optimization with airfoil design, resulting in increasingly complex numerical tools pursuing efficiency often at diminishing returns; while the costs and difficulty associated with fabrication increases with design complexity. Such efficiencies may often be necessary due to the power density limitations of certain aircraft such as small unmanned aerial vehicles (UAVs) and micro air vehicles (MAVs). This research, however, focuses on reducing the complexity of airfoils for applications where aerodynamic performance is less important than the efficiency of manufacturing; in this case a Hybrid Projectile. By employing faceted sections to approximate traditional contoured wing sections it may be possible to expedite manufacturing and reduce costs. We applied this method to the development of a low Reynolds number, disposable Hybrid Projectile requiring a 4.5:1 glide ratio, resulting in a series of airfoils which are geometric approximations to highly contoured cross-sections called ShopFoils. This series of airfoils both numerically and experimentally perform within a 10% margin of the SD6060 airfoil at low Re. Additionally, flow visualization has been conducted to qualitatively determine what mechanisms, if any, are responsible for the similarity in performance between the faceted ShopFoil sections and the SD6060. The data obtained by these experiments did not conclusively reveal how the faceted surfaces may influence low Re flow but did indicate that the ShopFoil s did not maintain flow attachment at higher angles of attack than the SD6060. Two reasons are provided for the unexpected performance of the ShopFoil: one is related to downwash effects, which are suspected of placing the outer portion of the span at an effective angle of attack where the ShopFoils outperform the SD6060; the other is the influence of the tip vortex on separation near the wing tips, which possibly
Variation with Mach Number of Static and Total Pressures Through Various Screens
Adler, Alfred A
1946-01-01
Tests were conducted in the Langley 24-inch highspeed tunnel to ascertain the static-pressure and total-pressure losses through screens ranging in mesh from 3 to 12 wires per inch and in wire diameter from 0.023 to 0.041 inch. Data were obtained from a Mach number of approximately 0.20 up to the maximum (choking) Mach number obtainable for each screen. The results of this investigation indicate that the pressure losses increase with increasing Mach number until the choking Mach number, which can be computed, is reached. Since choking imposes a restriction on the mass rate of flow and maximum losses are incurred at this condition, great care must be taken in selecting the screen mesh and wire dimmeter for an installation so that the choking Mach number is
Unsteady behavior of a confined jet in a cavity at moderate Reynolds numbers
Energy Technology Data Exchange (ETDEWEB)
Bouchet, G [Laboratoire IUSTI, UMR 7343 CNRS, Aix Marseille Universite, 5 rue Enrico Fermi, 13453 Marseille Cedex 13 (France); Climent, E, E-mail: Gilles.Bouchet@univ-amu.fr, E-mail: Gilles.Bouchet@univ-provence.fr, E-mail: climent@imft.fr [Institut de Mecanique des Fluides de Toulouse, UMR 5502 Universite de Toulouse-CNRS-INPT-UPS, 1 allee du Professeur Camille Soula, 31400 Toulouse (France)
2012-04-01
Self-sustained oscillations in the sinuous mode are observed when a jet impinges on a rigid surface. Confined jet instability is experimentally and numerically investigated here at moderate Reynolds numbers. When the Reynolds number is varied, the dynamic response of the jet is unusual in comparison with that of similar configurations (hole-tone, jet edge, etc). Modal transitions are clearly detected when the Reynolds number is varied. However, these transitions result in a reduction of the frequency, which means that the wavelength grows with Reynolds number. Moreover, the instability that sets in at low Reynolds number, as a subcritical Hopf bifurcation, disappears only 25% above the threshold. Then, the flow becomes steady again and symmetric. This atypical behavior is compared with our previous study on a submerged fountain (Bouchet et al 2002 Europhys. Lett. 59 826). (paper)
Laminar-Turbulent Transition: The change of the flow state temperature with the Reynolds number
Chekmarev, Sergei F
2014-01-01
Using the previously developed model to describe laminar/turbulent states of a viscous fluid flow, which treats the flow as a collection of coherent structures of various size (Chekmarev, Chaos, 2013, 013144), the statistical temperature of the flow state is determined as a function of the Reynolds number. It is shown that at small Reynolds numbers, associated with laminar states, the temperature is positive, while at large Reynolds numbers, associated with turbulent states, it is negative. At intermediate Reynolds numbers, the temperature changes from positive to negative as the size of the coherent structures increases, similar to what was predicted by Onsager for a system of parallel point-vortices in an inviscid fluid. It is also shown that in the range of intermediate Reynolds numbers the temperature exhibits a power-law divergence characteristic of second-order phase transitions.
Background-oriented schlieren imaging of flow around a circular cylinder at low Mach numbers
Stadler, Hannes; Bauknecht, André; Siegrist, Silvan; Flesch, Robert; Wolf, C. Christian; van Hinsberg, Nils; Jacobs, Markus
2017-09-01
The background-oriented schlieren (BOS) imaging method has, for the first time, been applied in the investigation of the flow around a circular cylinder at low Mach numbers (Mnumbers of 0.1× 10^6 ≤ Re ≤ 6.0× 10^6. Even at ambient pressure and the lowest Reynolds number investigated, density gradients associated with the flow around the cylinder were recorded. The signal-to-noise ratio of the evaluated gradient field improved with increasing stagnation pressure. The separation point could easily be identified with this non-intrusive measurement technique and corresponds well to simultaneous surface pressure measurements. The resulting displacement field is in principle of qualitative nature as the observation angle was parallel to the cylinder axis only in a single point of the recorded images. However, it has been possible to integrate the density field along the surface of the cylinder by successive imaging at incremental angular positions around the cylinder. This density distribution has been found to agree well with the pressure measurements and with potential theory where appropriate.
A Device for Measuring Sonic Velocity and Compressor Mach Number
1948-07-01
resonator (the only 4 NACA TN No. 1664 accurate measurement required) is measured, as shomn in figure 1, by means of a mercury manometer . The compressor Mach...tube vs not connected to the ccmpressor inlet until after calibration. The pressure in the device was measured by means of the mercury manometer . Fram
Choi, H.; You, D.; Choi, M.-R.; Kang, S.-H.
1996-11-01
Laminar vortex sheddings behind a circular cylinder with and without splitter plates attached to the cylinder at low Reynolds numbers are simulated by solving the unsteady incompressible Navier-Stokes equations. The Strouhal number, lift and drag rapidly change with the length of the splitter plate. Far-field noise from the vortex shedding behind the cylinder is computed using the Curle's formulation of the Lighthill acoustic analogy. The acoustic source functions are obtained from the computed near-field velocity and pressure. Numerical results show that the volume quadrupole noise is small at a low Mach number, compared to the surface dipole noise from the cylinder. Variations of the far-field noise characteristics with respect to the splitter plate are being investigated and will be shown in the final presentation. ^* Supported by KOSEF under Contract No. 961-1009-075-2
Numerical Simulation of Low Mach Number Fluid - Phenomena.
Reitsma, Scott H.
A method for the numerical simulation of low Mach number (M) fluid-acoustic phenomena is developed. This computational fluid-acoustic (CFA) methodology is based upon a set of conservation equations, termed finite-compressible, derived from the unsteady Navier-Stokes equations. The finite-compressible and more familiar pseudo-compressible equations are compared. The impact of derivation assumptions are examined theoretically and through numerical experimentation. The error associated with these simplifications is shown to be of O(M) and proportional to the amplitude of unsteady phenomena. A computer code for the solution of the finite -compressible equations is developed from an existing pseudo -compressible code. Spatial and temporal discretization issues relevant in the context of near field fluid-acoustic simulations are discussed. The finite volume code employs a MUSCL based third order upwind biased flux difference splitting algorithm for the convective terms. An explicit, three stage, second order Runge-Kutta temporal integration is employed for time accurate simulations while an implicit, approximately factored time quadrature is available for steady state convergence acceleration. The CFA methodology is tested in a series of problems which examine the appropriateness of the governing equations, the exacerbation of spatial truncation errors and the degree of temporal accuracy. Characteristic based boundary conditions employing a spatial formulation are developed. An original non-reflective boundary condition based upon the generalization and extension of existing methods is derived and tested in a series of multi-dimensional problems including those involving viscous shear flows and propagating waves. The final numerical experiment is the simulation of boundary layer receptivity to acoustic disturbances. This represents the first simulation of receptivity at a surface inhomogeneity in which the acoustic phenomena is modeled using physically appropriate
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.
Lift Production on Flapping and Rotary Wings at Low Reynolds Numbers
2016-02-26
AFRL-AFOSR-VA-TR-2016-0098 Flapping and Rotary Wing Lift at Low Reynolds Number Anya Jones MARYLAND UNIV COLLEGE PARK Final Report 02/26/2016... Lift Production on Flapping and Rotary Wings at Low Reynolds Numbers (YIP) 5a. CONTRACT NUMBER 5b. GRANT NUMBER FA9550-12-1-0251 5c. PROGRAM...The objective of this research was to identify the mechanisms of lift production on models of an entomological flapping wing stroke by evaluating
Multiobjective Design Optimization of Supersonic Jet Engine in Different Cruise Mach Numbers
Ogawa, Masamichi; Sato, Tetsuya; Kobayashi, Hiroaki; Taguchi, Hideyuki
The aim of this paper is to apply a multi-objective optimization generic algorithm (MOGA) to the conceptual design of the hypersonic/supersonic vehicles with different cruise Mach number. The pre-cooled turbojet engine is employed as a propulsion system and some engine parameters such as the precooler size, compressor size, compression ratio and fuel type are varied in the analysis. The result shows that the optimum cruise Mach number is about 4 if hydrogen fuel is used. Methane fuel instead of hydrogen reduces the vehicle gross weight by 33% in case of the Mach 2 vehicle.
Turbulent mixing of a slightly supercritical Van der Waals fluid at Low-Mach number
Battista, Francesco; Casciola, Carlo Massimo
2014-01-01
Supercritical fluids near the critical point are characterized by liquid-like densities and gas-like transport properties. These features are purposely exploited in different contexts ranging from natural products extraction/fractionation to aerospace propulsion. Large part of studies concerns this last context, focusing on the dynamics of supercritical fluids at high Mach number where compressibility and thermodynamics strictly interact. Despite the widespread use also at low Mach number, the turbulent mixing properties of slightly supercritical fluids have still not investigated in detail in this regime. This topic is addressed here by dealing with Direct Numerical Simulations (DNS) of a coaxial jet of a slightly supercritical Van der Waals fluid. Since acoustic effects are irrelevant in the Low Mach number conditions found in many industrial applications, the numerical model is based on a suitable low-Mach number expansion of the governing equation. According to experimental observations, the weakly superc...
Mydlarski, Laurent Bernard
1998-10-01
Turbulence theories are generally posed for isotropic turbulence in the limit of infinite turbulent Reynolds and Peclet numbers. Until now, it has been impossible to satisfy these constraints simultaneously in either experiments or simulations. By use of an active grid, devised by Makita, nearly isotropic turbulence with large turbulent Reynolds and Peclet numbers is generated. Turbulent Reynolds numbers based on the Taylor microscale, Rλ, in excess of 700 are achieved. The evolution of the velocity and passive scalar fields from low to high Reynolds and Peclet numbers is studied by generating turbulent fields in wind tunnels. The measurements are made by hot-wire anemometry and cold- wire thermometry. The passive scalar (generated by a mean scalar gradient) is temperature in air. The velocity field shows significant variation with Reynolds number. The slope of the inertial subrange is a function of Reynolds number and is noticeably below the Kolmogorov value of 5/3 for Rλconvective scaling range for the scalar (with slope close to 5/3) is observed for all Peclet numbers. The effects of the internal intermittency of the scalar are present at all Peclet numbers. The scalar field exhibits some (Peclet-number- independent) violations of local isotropy in the direction of the imposed gradient for odd-ordered statistics. The understanding of the 'ramp-cliff' structures (to which this anisotropy is attributed) is extended by describing it in terms of three-point statistics-the most fundamental order at which the odd- ordered statistics can be examined.
Is Ultra-High Reynolds Number Necessary for Comprehensive Log Scaling in a Turbulent Boundary Layer?
Dixit, Shivsai Ajit
2015-01-01
Experiments in an extraordinary turbulent boundary layer called the sink flow, displaying a perfect streamwise invariance, show a wide extent of logarithmic scaling for moments of streamwise velocity up to order 12, even at moderate Reynolds numbers. This is in striking contrast to canonical constant-pressure turbulent boundary layers that show such comprehensive log scaling only at ultra-high Reynolds numbers. This suggests that for comprehensive log scaling, ultra-high-Reynolds-number is not a necessary condition; while specific details of the sink flow are special, the relevance to general turbulent boundary layers is that the sink flow underscores the importance of the streamwise invariance condition that needs to be met in a general flow for obtaining log scaling. Indeed, a simple theory shows that, for log scaling in the inertial sublayer, the invariance of dimensionless mean velocity and higher-order moments along a mean streamline is a necessary and sufficient condition. Ultra-high Reynolds number pri...
Model Experiments with Low Reynolds Number Effects in a Ventilated Room
DEFF Research Database (Denmark)
Nielsen, Peter V.; Filholm, Claus; Topp, Claus;
The flow in a ventilated room will not always be a fully developed turbulent flow . Reduced air change rates owing to energy considerations and the application of natural ventilation with openings in the outer wall will give room air movements with low turbulence effects. This paper discusses...... the isothermal low Reynolds number flow from a slot inlet in the end wall of the room. The experiments are made on the scale of 1 to 5. Measurements indicate a low Reynolds number effect in the wall jet flow. The virtual origin of the wall jet moves forward in front of the opening at a small Reynolds number......, an effect that is also known from measurements on free jets. The growth rate of the jet, or the length scale, increases and the velocity decay factor decreases at small Reynolds numbers....
High Reynolds Number Studies in the Wake of a Submarine Model
Jimenez, Juan; Reynolds, Ryan; Smits, Alexander
2005-11-01
Results are presented from submarine wake studies conducted in Princeton University's High Reynolds Number Test Facility (HRTF). Compressed air is used as a working fluid enabling Reynolds numbers based on length of up to 10^8, about 1/5 of full scale. Measurements at Reynolds numbers up to 3 x10^6 have been completed, and show that, for the model condition without fins, the wake mean velocity was self-similar at locations 6 and 9 diameters downstream. Also, PIV at Reynolds numbers near 10^4 showed that when the yaw angle was varied the sail-tip and sail-hull junction vortices increased in magnitude emphasizing the importance of fully understanding the flow characteristics of a maneuvering submarine.
Model-based control of vortex shedding at low Reynolds numbers
Illingworth, Simon J.
2016-10-01
Model-based feedback control of vortex shedding at low Reynolds numbers is considered. The feedback signal is provided by velocity measurements in the wake, and actuation is achieved using blowing and suction on the cylinder's surface. Using two-dimensional direct numerical simulations and reduced-order modelling techniques, linear models of the wake are formed at Reynolds numbers between 45 and 110. These models are used to design feedback controllers using {H}_∞ loop-shaping. Complete suppression of shedding is demonstrated up to Re = 110—both for a single-sensor arrangement and for a three-sensor arrangement. The robustness of the feedback controllers is also investigated by applying them over a range of off-design Reynolds numbers, and good robustness properties are seen. It is also observed that it becomes increasingly difficult to achieve acceptable control performance—measured in a suitable way—as Reynolds number increases.
Le, G.; Russell, C. T.; Gosling, J. T.
1994-12-01
We use International Sun-Earth Explorer (ISEE) magnetic field and plasma data to examine dayside magnetopause crossing under conditions of low Mach number and strongly northward interplanetary magnetic field (IMF). When the solar wind Mach number is low, the IMF stregth and magnetoseath field stregth are large, and we expect the effects of magnetic reconection to be the strongest. When the IMF is strongly northward, we find that the location of the magnetopause boundary layer is very stationary in the space, and we observe many features that are common for both typical and low Mach numbers. However, under low Mach number conditions, we have observed some features that would be expected for cusp reconnection. The boundary layer near the subsolar region contains heated magnetosheath plasma with little hot magnetospheric component that has clearly entered the magnetosphere elsewhere. At least some of the structures present in the boundary layer are impulsive. Inside the boundary layer there is also clear evidence of acceleratedflow from the cusp region for strongly northward IMF at low Mach number. Reconnection beyond the cusp can explain the observed field, plasma, and flow signatures. Therefore at low Mach number, reconection is important in the formation of the boundary layer for northward IMF.
Effects of relative thickness on aerodynamic characteristics of airfoil at a low Reynolds number
Directory of Open Access Journals (Sweden)
Ma Dongli
2015-08-01
Full Text Available This study focuses on the characteristics of low Reynolds number flow around airfoil of high-altitude unmanned aerial vehicles (HAUAVs cruising at low speed. Numerical simulation on the flows around several representative airfoils is carried out to investigate the low Reynolds number flow. The water tunnel model tests further validate the accuracy and effectiveness of the numerical method. Then the effects of the relative thickness of airfoil on aerodynamic performance are explored, using the above numerical method, by simulating flows around airfoils of different relative thicknesses (12%, 14%, 16%, 18%, as well as different locations of the maximum relative thickness (x/c = 22%, 26%, 30%, 34%, at a low Reynolds number of 5 × 105. Results show that performance of airfoils at low Reynolds number is mainly affected by the laminar separation bubble. On the premise of good stall characteristics, the value of maximum relative thickness should be as small as possible, and the location of the maximum relative thickness ought to be closer to the trailing edge to obtain fine airfoil performance. The numerical method is feasible for the simulation of low Reynolds number flow. The study can help to provide a basis for the design of low Reynolds number airfoil.
Influence of Reynolds Number on Multi-Objective Aerodynamic Design of a Wind Turbine Blade.
Ge, Mingwei; Fang, Le; Tian, De
2015-01-01
At present, the radius of wind turbine rotors ranges from several meters to one hundred meters, or even more, which extends Reynolds number of the airfoil profile from the order of 105 to 107. Taking the blade for 3MW wind turbines as an example, the influence of Reynolds number on the aerodynamic design of a wind turbine blade is studied. To make the study more general, two kinds of multi-objective optimization are involved: one is based on the maximum power coefficient (CPopt) and the ultimate load, and the other is based on the ultimate load and the annual energy production (AEP). It is found that under the same configuration, the optimal design has a larger CPopt or AEP (CPopt//AEP) for the same ultimate load, or a smaller load for the same CPopt//AEP at higher Reynolds number. At a certain tip-speed ratio or ultimate load, the blade operating at higher Reynolds number should have a larger chord length and twist angle for the maximum Cpopt//AEP. If a wind turbine blade is designed by using an airfoil database with a mismatched Reynolds number from the actual one, both the load and Cpopt//AEP will be incorrectly estimated to some extent. In some cases, the assessment error attributed to Reynolds number is quite significant, which may bring unexpected risks to the earnings and safety of a wind power project.
Energy Technology Data Exchange (ETDEWEB)
Wang, L.; Tang, X. [Univ. of Central Lancashire. Engineering and Physical Sciences, Preston (United Kingdom); Liu, X. [Univ. of Cumbria. Sustainable Engineering, Workington (United Kingdom)
2012-07-01
The aerodynamic performance of a wind turbine depends very much on its blade geometric design, typically based on the blade element momentum (BEM) theory, which divides the blade into several blade elements. In current blade design practices based on Schmitz rotor design theory, the blade geometric parameters including chord and twist angle distributions are determined based on airfoil aerodynamic data at a specific Reynolds number. However, rotating wind turbine blade elements operate at different Reynolds numbers due to variable wind speed and different blade span locations. Therefore, the blade design through Schmitz rotor theory at a specific Reynolds number does not necessarily provide the best power performance under operational conditions. This paper aims to provide an optimal blade design strategy for horizontal-axis wind turbines operating at different Reynolds numbers. A fixed-pitch variable-speed (FPVS) wind turbine with S809 airfoil is chosen as a case study and a Matlab program which considers Reynolds number effects is developed to determine the optimized chord and twist angle distributions of the blade. The performance of the optimized blade is compared with that of the preliminary blade which is designed based on Schmitz rotor design theory at a specific Reynolds number. The results demonstrate that the proposed blade design optimization strategy can improve the power performance of the wind turbine. This approach can be further developed for any practice of horizontal axis wind turbine blade design. (Author)
Finite Reynolds number properties of a turbulent channel flow similarity solution
Klewicki, Joseph; Oberlack, Martin
2015-11-01
Finite Reynolds number behaviors of the asymptotically logarithmic mean velocity profile in fully developed turbulent channel flow are investigated. This is accomplished by exploiting invariance properties admitted by the appropriately simplified form of the mean momentum equation. These properties underlie the existence of a similarity solution over an interior inertial domain. This similarity solution, which was originally demonstrated by numerically integrating the relevant nonlinear equation, is consistent with the emergence of a logarithmic mean velocity profile as the Reynolds number becomes large. It is now shown that the governing nonlinear equation has an analytical solution that contains both linear and logarithmic terms, but with the coefficient on the linear term decaying to zero with Reynolds number. Existing DNS are used to elucidate Reynolds number dependent properties of this finite Reynolds number form of the similarity solution. Correspondences between these properties and those indicated by finite Reynolds number corrections to the classical overlap layer formulation for the mean velocity profile are described and discussed. Support of the 2014 Mathematics of Turbulence program at the Institute for Pure and Applied Mathematics, UCLA, is gratefully acknowledged.
Klewicki, J. C.; Chini, G. P.; Gibson, J. F.
2017-01-01
Recent and on-going advances in mathematical methods and analysis techniques, coupled with the experimental and computational capacity to capture detailed flow structure at increasingly large Reynolds numbers, afford an unprecedented opportunity to develop realistic models of high Reynolds number turbulent wall-flow dynamics. A distinctive attribute of this new generation of models is their grounding in the Navier–Stokes equations. By adhering to this challenging constraint, high-fidelity models ultimately can be developed that not only predict flow properties at high Reynolds numbers, but that possess a mathematical structure that faithfully captures the underlying flow physics. These first-principles models are needed, for example, to reliably manipulate flow behaviours at extreme Reynolds numbers. This theme issue of Philosophical Transactions of the Royal Society A provides a selection of contributions from the community of researchers who are working towards the development of such models. Broadly speaking, the research topics represented herein report on dynamical structure, mechanisms and transport; scale interactions and self-similarity; model reductions that restrict nonlinear interactions; and modern asymptotic theories. In this prospectus, the challenges associated with modelling turbulent wall-flows at large Reynolds numbers are briefly outlined, and the connections between the contributing papers are highlighted. This article is part of the themed issue ‘Toward the development of high-fidelity models of wall turbulence at large Reynolds number’. PMID:28167585
Note: A high Mach number arc-driven shock tube for turbulence studies.
Titus, J B; Alexander, A B; Johnson, J A
2013-04-01
A high Mach arc-driven shock tube has been built at the Center for Plasma Science and Technology of Florida A&M University to study shock waves. A larger apparatus with higher voltage was built to study more stable shock waves and subsequent plasmas. Initial measurements of the apparatus conclude that the desired Mach numbers can be reached using only two-thirds the maximum possible energy that the circuit can provide.
Reynolds and froude number effect on the flow past an interface-piercing circular cylinder
Directory of Open Access Journals (Sweden)
Koo Bonguk
2014-09-01
Full Text Available The two-phase turbulent flow past an interface-piercing circular cylinder is studied using a high-fidelity orthogonal curvilinear grid solver with a Lagrangian dynamic subgrid-scale model for large-eddy simulation and a coupled level set and volume of fluid method for air-water interface tracking. The simulations cover the sub-critical and critical and post critical regimes of the Reynolds and sub and super-critical Froude numbers in order to investigate the effect of both dimensionless parameters on the flow. Significant changes in flow features near the air-water interface were observed as the Reynolds number was increased from the sub-critical to the critical regime. The interface makes the separation point near the interface much delayed for all Reynolds numbers. The separation region at intermediate depths is remarkably reduced for the critical Reynolds number regime. The deep flow resembles the single-phase turbulent flow past a circular cylinder, but includes the effect of the free-surface and the limited span length for sub-critical Reynolds numbers. At different Froude numbers, the air-water interface exhibits significantly changed structures, including breaking bow waves with splashes and bubbles at high Froude numbers. Instantaneous and mean flow features such as interface structures, vortex shedding, Reynolds stresses, and vorticity transport are also analyzed. The results are compared with reference experimental data available in the literature. The deep flow is also compared with the single-phase turbulent flow past a circular cylinder in the similar ranges of Reynolds numbers. Discussion is provided concerning the limitations of the current simulations and available experimental data along with future research
Reynolds Number Effects on Turbulent Characteristics of Taylor-Couette Flow
Park, Joonhwi; Fukushima, Naoya; Shimura, Masayasu; Tanahashi, Mamoru; Miyauchi, Toshio
2012-11-01
Laminar and turbulent Taylor-Couette flow is of great importance in a wide range of engineering applications, such as viscosity measurement devices, rotating machineries and reactors. In this study, we focus on turbulent Taylor-Couette flow with a fixed outer cylinder and a rotating inner cylinder. Direct numerical simulation (DNS) of turbulent Taylor-Couette flow has been conducted to investigate turbulent characteristics including Reynolds stress budget at Reynolds number from 8000 to 20000. Reynolds number, Re, is defined by gap width and rotating speed of inner cylinder. In this range of Re, turbulent characteristics are expected to change around Re=10000, referring to Wendt's empirical formula. Averaged torque from DNS agrees well with Wendt's empirical formula and torque transition is confirmed around Re=10000. Averaged azimuthal velocity normalized by friction velocity on inner/outer wall increases in logarithmic region with increase in Re. All components of Reynolds stress tensor also increase in all domain. The minute movement of center of Taylor vortices is observed spatially and temporally when Re is over 12000. Finally, Reynolds stress budgets are investigated to figure out Reynolds number effects on turbulent statistics in detail.
Energy Technology Data Exchange (ETDEWEB)
Sigg, K. C.; Coffield, R. D.
2002-09-01
High Reynolds number test data has recently been reported for both single and multiple piping elbow design configurations at earlier ASME Fluid Engineering Division conferences. The data of these studies ranged up to a Reynolds number of 42 x 10[sup]6 which is significantly greater than that used to establish design correlations before the data was available. Many of the accepted design correlations, based on the lower Reynolds number data, date back as much as fifty years. The new data shows that these earlier correlations are extremely conservative for high Reynolds number applications. Based on the recent high Reynolds number information a new recommended method has been developed for calculating irrecoverable pressure loses in piping systems for design considerations such as establishing pump sizing requirements. This paper describes the recommended design approach and additional testing that has been performed as part of the qualification of the method. This qualification testing determined the irrecoverable pressure loss of a piping configuration that would typify a limiting piping section in a complicated piping network, i.e., multiple, tightly coupled, out-of-plane elbows in series under high Reynolds number flow conditions. The overall pressure loss measurements were then compared to predictions, which used the new methodology to assure that conservative estimates for the pressure loss (of the type used for pump sizing) were obtained. The recommended design methodology, the qualification testing and the comparison between the predictions and the test data are presented. A major conclusion of this study is that the recommended method for calculating irrecoverable pressure loss in piping systems is conservative yet significantly lower than predicted by early design correlations that were based on the extrapolation of low Reynolds number test data.
Multiscale structures of resistive magnetic reconnection at high magnetic Reynolds numbers
Miyoshi, Takahiro; Kusano, Kanya
Magnetic reconnection is the most important process of explosive phenomena in space plasmas. The magnetic Reynolds number for the space plasmas are extremely high in general since those plasmas are thought to be collisionless or semi-collisional. However, magnetic reconnection rate becomes low as magnetic Reynolds number increases within the framework of a stationary resistive MHD model. Thus, modern models of magnetic reconnection often include kinetic effects such as the Hall effect to explain realistic explosive magnetic reconnection. It is thought, on the other hand, that the MHD approximation is valid for the plasmas within a very wide range of scales since the scale gap between the macro-and micro-scale is quite large, e.g., in the solar corona, the ratio of the macro to micro will be more than 107 . Such multiscale structures of MHD with wide range of scales, however, have not been clarified so far. Therefore, in this paper, resistive magnetic reconnection at high magnetic Reynolds numbers are investigated using very high-resolution MHD simulations. Simulation results show that the magnetic energy at high magnetic Reynolds numbers is explosively released, while that at not-so-high magnetic Reynolds numbers is steadily dissipated. In the case of high magnetic Reynolds numbers, multiple small scale plasmoids are intermittently created and ejected via secondary tearing modes in a nonlinearly developed thin current sheet. It is revealed that a secondary plasmoid is not only accelerated up to a local magnetosonic speed toward the down-stream region but also perturbs the up-stream region. Thus, complicated multiscale structures appear around the magnetic field reversal layer. Perspective for the high-resolution simulation of extremely high magnetic Reynolds numbers will be also discussed.
Control of Low Reynolds Number Flows with Fluid Structure Interactions
2014-02-02
public release; distribution is unlimited. 27 the direct numerical simulations of Andro and Jacquin [37] for a plunging NACA 0012 airfoil at Re...34Bifurcating Flows of Plunging Airfoils at High Strouhal Numbers," Journal of Fluid Mechanics, Vol. 708, 2012, pp. 349-376. [37] Andro , J.Y
Reynolds number and roughness effects on turbulent stresses in sandpaper roughness boundary layers
Morrill-Winter, C.; Squire, D. T.; Klewicki, J. C.; Hutchins, N.; Schultz, M. P.; Marusic, I.
2017-05-01
Multicomponent turbulence measurements in rough-wall boundary layers are presented and compared to smooth-wall data over a large friction Reynolds number range (δ+). The rough-wall experiments used the same continuous sandpaper sheet as in the study of Squire et al. [J. Fluid Mech. 795, 210 (2016), 10.1017/jfm.2016.196]. To the authors' knowledge, the present measurements are unique in that they cover nearly an order of magnitude in Reynolds number (δ+≃2800 -17 400 ), while spanning the transitionally to fully rough regimes (equivalent sand-grain-roughness range, ks+≃37 -98 ), and in doing so also maintain very good spatial resolution. Distinct from previous studies, the inner-normalized wall-normal velocity variances, w2¯, exhibit clear dependencies on both ks+ and δ+ well into the wake region of the boundary layer, and only for fully rough flows does the outer portion of the profile agree with that in a comparable δ+ smooth-wall flow. Consistent with the mean dynamical constraints, the inner-normalized Reynolds shear stress profiles in the rough-wall flows are qualitatively similar to their smooth-wall counterparts. Quantitatively, however, at matched Reynolds numbers the peaks in the rough-wall Reynolds shear stress profiles are uniformly located at greater inner-normalized wall-normal positions. The Reynolds stress correlation coefficient, Ru w, is also greater in rough-wall flows at a matched Reynolds number. As in smooth-wall flows, Ru w decreases with Reynolds number, but at different rates depending on the roughness condition. Despite the clear variations in the Ru w profiles with roughness, inertial layer u , w cospectra evidence invariance with ks+ when normalized with the distance from the wall. Comparison of the normalized contributions to the Reynolds stress from the second quadrant (Q2) and fourth quadrant (Q4) exhibit noticeable differences between the smooth- and rough-wall flows. The overall time fraction spent in each quadrant is, however
The small-scale dynamo: Breaking universality at high Mach numbers
Schleicher, Dominik R G; Federrath, Christoph; Bovino, Stefano; Schmidt, Wolfram
2013-01-01
(Abridged) The small-scale dynamo may play a substantial role in magnetizing the Universe under a large range of conditions, including subsonic turbulence at low Mach numbers, highly supersonic turbulence at high Mach numbers and a large range of magnetic Prandtl numbers Pm, i.e. the ratio of kinetic viscosity to magnetic resistivity. Low Mach numbers may in particular lead to the well-known, incompressible Kolmogorov turbulence, while for high Mach numbers, we are in the highly compressible regime, thus close to Burgers turbulence. In this study, we explore whether in this large range of conditions, a universal behavior can be expected. Our starting point are previous investigations in the kinematic regime. Here, analytic studies based on the Kazantsev model have shown that the behavior of the dynamo depends significantly on Pm and the type of turbulence, and numerical simulations indicate a strong dependence of the growth rate on the Mach number of the flow. Once the magnetic field saturates on the current ...
The influence of Reynolds numbers on resistance properties of jet pumps
Geng, Q.; Zhou, G.; Li, Q.
2014-01-01
Jet pumps are widely used in thermoacoustic Stirling heat engines and pulse tube cryocoolers to eliminate the effect of Gedeon streaming. The resistance properties of jet pumps are principally influenced by their structures and flow regimes which are always characterized by Reynolds numbers. In this paper, the jet pump of which cross section contracts abruptly is selected as our research subject. Based on linear thermoacoustic theory, a CFD model is built and the oscillating flow of the working gas is simulated and analyzed with different Reynolds numbers in the jet pump. According to the calculations, the influence of different structures and Reynolds numbers on the resistance properties of the jet pump are analyzed and presented. The results show that Reynolds numbers have a great influence on the resistance properties of jet pumps and some empirical formulas which are widely used are unsuitable for oscillating flow with small Reynolds numbers. This paper provides a more comprehensive understanding on resistance properties of jet pumps with oscillating flow and is significant for the design of jet pumps in practical thermoacoustic engines and refrigerators.
Numerical Simulations of Subscale Wind Turbine Rotor Inboard Airfoils at Low Reynolds Number
Energy Technology Data Exchange (ETDEWEB)
Blaylock, Myra L. [Sandia National Lab. (SNL-CA), Livermore, CA (United States). Thermal/ Fluid Sciences & Engineering Dept.; Maniaci, David Charles [Sandia National Lab. (SNL-NM), Albuquerque, NM (United States). Wind Energy Technologies Dept.; Resor, Brian R. [Sandia National Lab. (SNL-NM), Albuquerque, NM (United States). Wind Energy Technologies Dept.
2015-04-01
New blade designs are planned to support future research campaigns at the SWiFT facility in Lubbock, Texas. The sub-scale blades will reproduce specific aerodynamic characteristics of utility-scale rotors. Reynolds numbers for megawatt-, utility-scale rotors are generally above 2-8 million. The thickness of inboard airfoils for these large rotors are typically as high as 35-40%. The thickness and the proximity to three-dimensional flow of these airfoils present design and analysis challenges, even at the full scale. However, more than a decade of experience with the airfoils in numerical simulation, in the wind tunnel, and in the field has generated confidence in their performance. Reynolds number regimes for the sub-scale rotor are significantly lower for the inboard blade, ranging from 0.7 to 1 million. Performance of the thick airfoils in this regime is uncertain because of the lack of wind tunnel data and the inherent challenge associated with numerical simulations. This report documents efforts to determine the most capable analysis tools to support these simulations in an effort to improve understanding of the aerodynamic properties of thick airfoils in this Reynolds number regime. Numerical results from various codes of four airfoils are verified against previously published wind tunnel results where data at those Reynolds numbers are available. Results are then computed for other Reynolds numbers of interest.
Negative Magnus Effect on a Rotating Sphere at around the Critical Reynolds Number
Muto, Masaya; Watanabe, Hiroaki; Tsubokura, Makoto; Oshima, Nobuyuki
2011-12-01
Negative Magnus lift acting on a sphere rotating about the axis perpendicular to an incoming flow is investigated using large-eddy simulation at three Reynolds numbers of 1.0× 104, 2.0 × 105, and 1.14 × 106. The numerical methods adopted are first validated on a non-rotating sphere and the spatial resolution around the sphere is determined so as to reproduce the laminar separation, reattachment, and turbulent transition of the boundary layer observed at around the critical Reynolds number. In the rotating sphere, positive or negative Magnus effect is observed depending on the Reynolds number and the rotating speed imposed. At the Reynolds number in the subcritical or supercritical region, the direction of the lift force follows the Magnus effect to be independent of the rotational speed tested here. In contrast, negative lift is observed at the Reynolds number at the critical region when particular rotating speeds are imposed. The negative Magnus effect is discussed in the context of the suppression or promotion of boundary layer transition around the separation point.
Klewicki, J. C.; Chini, G. P.; Gibson, J. F.
2017-03-01
Recent and on-going advances in mathematical methods and analysis techniques, coupled with the experimental and computational capacity to capture detailed flow structure at increasingly large Reynolds numbers, afford an unprecedented opportunity to develop realistic models of high Reynolds number turbulent wall-flow dynamics. A distinctive attribute of this new generation of models is their grounding in the Navier-Stokes equations. By adhering to this challenging constraint, high-fidelity models ultimately can be developed that not only predict flow properties at high Reynolds numbers, but that possess a mathematical structure that faithfully captures the underlying flow physics. These first-principles models are needed, for example, to reliably manipulate flow behaviours at extreme Reynolds numbers. This theme issue of Philosophical Transactions of the Royal Society A provides a selection of contributions from the community of researchers who are working towards the development of such models. Broadly speaking, the research topics represented herein report on dynamical structure, mechanisms and transport; scale interactions and self-similarity; model reductions that restrict nonlinear interactions; and modern asymptotic theories. In this prospectus, the challenges associated with modelling turbulent wall-flows at large Reynolds numbers are briefly outlined, and the connections between the contributing papers are highlighted.
Thermocapillary migration of a droplet with a thermal source at large Reynolds and Marangoni numbers
Wu, Zuo-Bing
2014-01-01
The {\\it unsteady} process for thermocapillary droplet migration at large Reynolds and Marangoni numbers has been previously reported by identifying a nonconservative integral thermal flux across the surface in the {\\it steady} thermocapillary droplet migration, [Wu and Hu, J. Math. Phys. {\\bf 54} 023102, (2013)]. Here we add a thermal source in the droplet to preserve the integral thermal flux across the surface as conservative, so that thermocapillary droplet migration at large Reynolds and Marangoni numbers can reach a {\\it quasi-steady} process. Under assumptions of {\\it quasi-steady} state and non-deformation of the droplet, we make an analytical result for the {\\it steady} thermocapillary migration of droplet with the thermal source at large Reynolds and Marangoni numbers. The result shows that the thermocapillary droplet migration speed slowly increases with the increase of Marangoni number.
Li, Pu; Weng, Linlu; Niu, Haibo; Robinson, Brian; King, Thomas; Conmy, Robyn; Lee, Kenneth; Liu, Lei
2016-12-15
This study was aimed at testing the applicability of modified Weber number scaling with Alaska North Slope (ANS) crude oil, and developing a Reynolds number scaling approach for oil droplet size prediction for high viscosity oils. Dispersant to oil ratio and empirical coefficients were also quantified. Finally, a two-step Rosin-Rammler scheme was introduced for the determination of droplet size distribution. This new approach appeared more advantageous in avoiding the inconsistency in interfacial tension measurements, and consequently delivered concise droplet size prediction. Calculated and observed data correlated well based on Reynolds number scaling. The relation indicated that chemical dispersant played an important role in reducing the droplet size of ANS under different seasonal conditions. The proposed Reynolds number scaling and two-step Rosin-Rammler approaches provide a concise, reliable way to predict droplet size distribution, supporting decision making in chemical dispersant application during an offshore oil spill.
Mack, R. J.
1974-01-01
Wing models were tested in the high-speed section of the Langley Unitary Plan wind tunnel to study the effects of the leading-edge sweep angle and the design lift coefficient on aerodynamic performance and efficiency. The models had leading-edge sweep angles of 69.44 deg, 72.65 deg, and 75.96 deg which correspond to values of the design Mach-number-sweep-angle parameter (beta cotangent A) sub DES of 0.6, 0.75, and 0.9, respectively. For each sweep angle, camber surfaces having design lift coefficients of 0,0.08, and 0.12 at a design Mach number of 2.6 were generated. The wind-tunnel tests were conducted at Mach numbers of 2.3, 2.6, and 2.96 with a stagnation temperature of 338.7 K (150 F) and a Reynolds number per meter of 9.843 times 10 to the 6th power. The results of the tests showed that only a moderate sweeping of the wing leading edge aft of the Mach line along with a small-to-moderate amount of camber and twist was needed to significantly improve the zero-lift (flat camber surface) wing performance and efficiency.
Mahto, Navin Kumar; Choubey, Gautam; Suneetha, Lakka; Pandey, K. M.
2016-11-01
The two equation standard k-ɛ turbulence model and the two-dimensional compressible Reynolds-Averaged Navier-Stokes (RANS) equations have been used to computationally simulate the double cavity scramjet combustor. Here all the simulations are performed by using ANSYS 14-FLUENT code. At the same time, the validation of the present numerical simulation for double cavity has been performed by comparing its result with the available experimental data which is in accordance with the literature. The results are in good agreement with the schlieren image and the pressure distribution curve obtained experimentally. However, the pressure distribution curve obtained numerically is under-predicted in 5 locations by numerical calculation. Further, investigations on the variations of the effects of the length-to-depth ratio of cavity and Mach number on the combustion characteristics has been carried out. The present results show that there is an optimal length-to-depth ratio for the cavity for which the performance of combustor significantly improves and also efficient combustion takes place within the combustor region. Also, the shifting of the location of incident oblique shock took place in the downstream of the H2 inlet when the Mach number value increases. But after achieving a critical Mach number range of 2-2.5, the further increase in Mach number results in lower combustion efficiency which may deteriorate the performance of combustor.
Reynolds number influence on preferential concentration of heavy particles in turbulent flows
Energy Technology Data Exchange (ETDEWEB)
Obligado, Martin; Missaoui, Mahrane; Cartellier, Alain; Bourgoin, Mickaeel [Laboratoire des Ecoulements Geophysiques et Industriels, CNRS/UJF/G-INP UMR5519, BP53, 38041 Grenoble cedex 9 (France); Monchaux, Romain, E-mail: mickael.bourgoin@hmg.inpg.fr [Unite de mecanique, Ecole Nationale Superieure de Techniques Avancees, ParisTech, Chemin de la Huniere, 91761, Palaiseau Cedex (France)
2011-12-22
We present a study of the preferential concentration and clustering in homogeneous and isotropic turbulence. Using Voronoie diagrams, we have formerly quantified preferential concentration as a function of the Stokes number in moderate turbulence conditions up to Reynolds number based on Taylor microscale of the order of R{sub {lambda}} {approx} 120. Using an active grid recently implemented in our windtunnel, we investigate in the present study, the effect of Reynolds number on particles clustering, in the range R{sub {lambda}} {approx} 200 - 400.
Binary tree models of high-Reynolds-number turbulence
Aurell, Erik; Dormy, Emmanuel; Frick, Peter
1997-08-01
We consider hierarchical models for turbulence, that are simple generalizations of the standard Gledzer-Ohkitani-Yamada shell models (E. B. Gledzer, Dokl, Akad. Nauk SSSR 209, 5 (1973) [Sov. Phys. Dokl. 18, 216 (1973)]; M. Yamada and K. Ohkitani, J. Phys. Soc. Jpn. 56, 4210 (1987)). The density of degrees of freedom is constant in wave-number space. Looking only at this behavior and at the quadratic invariants in the inviscid unforced limit, the models can be thought of as systems living naturally in one spatial dimension, but being qualitatively similar to hydrodynamics in two (2D) and three dimensions. We investigated cascade phenomena and intermittency in the different cases. We observed and studied a forward cascade of enstrophy in the 2D case.
Shrout, B. L.; Corlett, W. A.; Collins, I. K.
1979-01-01
The tabulated results of surface pressure tests conducted on the wing and fuselage of an airplane model in the Langley Unitary Plan wind tunnel are presented without analysis. The model tested was that of a supersonic-cruise airplane with a highly swept arrow-wing planform, two engine nacelles mounted beneath the wing, and outboard vertical tails. Data were obtained at Mach numbers of 2.30, 2.96, and 3.30 for angles of attack from -4 deg to 12 deg. The Reynolds number for these tests was 6,560,000 per meter.
PIV measurements of isothermal plane turbulent impinging jets at moderate Reynolds numbers
Khayrullina, A.; van Hooff, T.; Blocken, B.; van Heijst, G. J. F.
2017-04-01
This paper contains a detailed experimental analysis of an isothermal plane turbulent impinging jet (PTIJ) for two jet widths at moderate Reynolds numbers (7200-13,500) issued on a horizontal plane at fixed relative distances equal to 22.5 and 45 jet widths. The available literature on such flows is scarce. Previous studies on plane turbulent jets mainly focused on free jets, while most studies on impinging jets focused on the heat transfer between the jet and an impingement plane, disregarding jet development. The present study focuses on isothermal PTIJs at moderate Reynolds numbers characteristic of air curtains. Flow visualisations with fluorescent dye and 2D particle image velocimetry (PIV) measurements have been performed. A comparison is made with previous studies of isothermal free turbulent jets at moderate Reynolds numbers. Mean and instantaneous velocity and vorticity, turbulence intensity, and Reynolds shear stress are analysed. The jet issued from the nozzle with higher aspect ratio shows more intensive entrainment and a faster decay of the centreline velocity compared to the jet of lower aspect ratio for the same value of jet Reynolds number. The profiles of centreline and cross-jet velocity and turbulence intensity show that the PTIJs behave as a free plane turbulent jet until 70-75% of the total jet height. Alongside the information obtained on the jet dynamics, the data will be useful for the validation of numerical simulations.
Unsteady Numerical Simulation of Flow around 2-D Circular Cylinder for High Reynolds Numbers
Institute of Scientific and Technical Information of China (English)
Yanhui Ai; Dakui Feng; Hengkui Ye; Lin Li
2013-01-01
In this paper,2-D computational analyses were conducted for unsteady high Reynolds number flows around a smooth circular cylinder in the supercritical and upper-transition flow regimes,i.e.8.21×104＜Re＜l.54×106.The calculations were performed by means of solving the 2-D Unsteady Reynolds-Averaged Navier-Stokes (URANS) equations with a k-ε turbulence model.The calculated results,produced flow structure drag and lift coefficients,as well as Strouhal numbers.The findings were in good agreement with previous published data,which also supplied us with a good understanding of the flow across cylinders of different high Reynolds numbers.Meanwhile,an effective measure was presented to control the lift force on a cylinder,which points the way to decrease the vortex induced vibration of marine structure in future.
Effect of Reynolds number on flow and mass transfer characteristics of a 90 degree elbow
Fujisawa, Nobuyuki; Ikarashi, Yuya; Yamagata, Takayuki; Taguchi, Syoichi
2016-11-01
The flow and mass transfer characteristics of a 90 degree elbow was studied experimentally by using the mass transfer measurement by plaster dissolution method, the surface flow visualization by oil film method and stereo PIV measurement. The experiments are carried out in a water tunnel of a circular pipe of 56mm in diameter with a working fluid of water. The Reynolds number was varied from 30000 to 200000. The experimental result indicated the change of the mass transfer coefficient distribution in the elbow with increasing the Reynolds number. This phenomenon is further examined by the surface flow visualization and measurement of secondary flow pattern in the elbow, and the results showed the suggested change of the secondary flow pattern in the elbow with increasing the Reynolds numbers.
Reynolds number effects on the single-mode Richtmyer-Meshkov instability
Walchli, B.; Thornber, B.
2017-01-01
The Reynolds number effects on the nonlinear growth rates of the Richtmyer-Meshkov instability are investigated using two-dimensional numerical simulations. A decrease in Reynolds number gives an increased time to reach nonlinear saturation, with Reynolds number effects only significant in the range RePhys. Rev. E 80, 055302 (2009), 10.1103/PhysRevE.80.055302]. Predicted amplitudes show reasonable agreement with the existing theory of Carles and Popinet [P. Carles and S. Popinet, Phys. Fluids Lett. 13, 1833 (2001), 10.1063/1.1377863; Eur. J. Mech. B 21, 511 (2002), 10.1016/S0997-7546(02)01199-8] and Mikaelian [K. O. Mikaelian, Phys. Rev. E 47, 375 (1993), 10.1103/PhysRevE.47.375; K. O. Mikaelian, Phys. Rev. E 87, 031003 (2013), 10.1103/PhysRevE.87.031003], with the former being the closest match to the current computations.
Reynolds number invariance of the structure inclination angle in wall turbulence.
Marusic, Ivan; Heuer, Weston D C
2007-09-14
Cross correlations of the fluctuating wall-shear stress and the streamwise velocity in the logarithmic region of turbulent boundary layers are reported over 3 orders of magnitude change in Reynolds number. These results are obtained using hot-film and hot-wire anemometry in a wind tunnel facility, and sonic anemometers and a purpose-built wall-shear stress sensor in the near-neutral atmospheric surface layer on the salt flats of Utah's western desert. The direct measurement of fluctuating wall-shear stress in the atmospheric surface layer has not been available before. Structure inclination angles are inferred from the cross correlation results and are found to be invariant over the large range of Reynolds number. The findings justify the prior use of low Reynolds number experiments for obtaining structure angles for near-wall models in the large-eddy simulation of atmospheric surface layer flows.
On the motion of non-spherical particles at high Reynolds number
DEFF Research Database (Denmark)
Mandø, Matthias; Rosendahl, Lasse
2010-01-01
This paper contains a critical review of available methodology for dealing with the motion of non-spherical particles at higher Reynolds numbers in the Eulerian- Lagrangian methodology for dispersed flow. First, an account of the various attempts to classify the various shapes and the efforts...... motion it is necessary to account for the non-coincidence between the center of pressure and center of gravity which is a direct consequence of the inertial pressure forces associated with particles at high Reynolds number flow. Extensions for non-spherical particles at higher Reynolds numbers are far...... in between and usually based on semi-heuristic approaches utilizing concepts from airfoil theory such as profile lift. Even for regular particles there seems to be a long way before a complete theory can be formulated. For irregular particles with small aspect ratio, where the secondary motion...
The shock tube as a device for testing transonic airfoils at high Reynolds numbers
Cook, W. J.; Presley, L. L.; Chapman, G. T.
1978-01-01
A performance analysis of gas-driven shock tubes shows that transonic airfoil flows with chord Reynolds numbers in the range of 100 million can be generated behind the primary shock in a large shock tube. A study of flow over simple airfoils has been carried out at low and intermediate Reynolds numbers to assess the testing technique. Results obtained from schlieren photos and airfoil pressure measurements show that steady transonic flows similar to those observed for the airfoils in wind tunnels can be generated within the available testing time in a shock tube with either properly-contoured test section walls or a properly-designed slotted-wall test section. The study indicates that the shock tube is a useful facility for studying two-dimensional high Reynolds number transonic airfoil flows.
Effect of Mach number on the efficiency of microwave energy deposition in supersonic flow
Lashkov, V. A.; Karpenko, A. G.; Khoronzhuk, R. S.; Mashek, I. Ch.
2016-05-01
The article is devoted to experimental and numerical studies of the efficiency of microwave energy deposition into a supersonic flow around the blunt cylinder at different Mach numbers. Identical conditions for energy deposition have been kept in the experiments, thus allowing to evaluate the pure effect of varying Mach number on the pressure drop. Euler equations are solved numerically to model the corresponding unsteady flow compressed gas. The results of numerical simulations are compared to the data obtained from the physical experiments. It is shown that the momentum, which the body receives during interaction of the gas domain modified by microwave discharge with a shock layer before the body, increases almost linearly with rising of Mach number and the efficiency of energy deposition also rises.
Energy Technology Data Exchange (ETDEWEB)
Dellacherie, St
2004-07-01
This work deals with the derivation of a diphasic low Mach number model obtained through a Mach number asymptotic expansion applied to the compressible diphasic Navier Stokes system, expansion which filters out the acoustic waves. This approach is inspired from the work of Andrew Majda giving the equations of low Mach number combustion for thin flame and for perfect gases. When the equations of state verify some thermodynamic hypothesis, we show that the low Mach number diphasic system predicts in a good way the dilatation or the compression of a bubble and has equilibrium convergence properties. Then, we propose an entropic and convergent Lagrangian scheme in mono-dimensional geometry when the fluids are perfect gases and we propose a first approach in Eulerian variables where the interface between the two fluids is captured with a level set technique. (author)
LIFT FORCE ON ROTATING SPHERE AT LOW REYNOLDS NUMBERS AND HIGH ROTATIONAL SPEEDS
Institute of Scientific and Technical Information of China (English)
由长福; 祁海鹰; 徐旭常
2003-01-01
The lift force on an isolated rotating sphere in a uniform flow was investigated by means of a three-dimensional numerical simulation for low Reynolds numbers (based on the sphere diameter) (Re ＜ 68.4) and high dimensionless rotational speeds (Γ＜ 5). The Navier-Stokes equations in Cartesian coordinate system were solved using a finite volume formulation based on SIMPLE procedure. The accuracy of the numerical simulation was tested through a comparison with available theoretical, numerical and experimental results at low Reynolds numbers, and it was found that they were in close agreement under the above mentioned ranges of the Reynolds number and rotational speed. From a detailed computation of the flow field around a rotational sphere in extended ranges of the Reynolds number and rotational speed, the results show that, with increasing the rotational speed or decreasing the Reynolds number, the lift coefficient increases. An empirical equation more accurate than those obtained by previous studies was obtained to describe both effects of the rotational speed and Reynolds number on the lift force on a sphere. It was found in calculations that the drag coefficient is not significantly affected by the rotation of the sphere. The ratio of the lift force to the drag force, both of which act on a sphere in a uniform flow at the same time, was investigated. For a small spherical particle such as one of about 100μm in diameter, even if the rotational speed reaches about 106 revolutions per minute, the lift force can be neglected as compared with the drag force.
A comparative study of near-wall turbulence in high and low Reynolds number boundary layers
Metzger, M. M.; Klewicki, J. C.
2001-03-01
The present study explores the effects of Reynolds number, over three orders of magnitude, in the viscous wall region of a turbulent boundary layer. Complementary experiments were conducted both in the boundary layer wind tunnel at the University of Utah and in the atmospheric surface layer which flows over the salt flats of the Great Salt Lake Desert in western Utah. The Reynolds numbers, based on momentum deficit thickness, of the two flows were Rθ=2×103 and Rθ≈5×106, respectively. High-resolution velocity measurements were obtained from a five-element vertical rake of hot-wires spanning the buffer region. In both the low and high Rθ flows, the length of the hot-wires measured less than 6 viscous units. To facilitate reliable comparisons, both the laboratory and field experiments employed the same instrumentation and procedures. Data indicate that, even in the immediate vicinity of the surface, strong influences from low-frequency motions at high Rθ produce noticeable Reynolds number differences in the streamwise velocity and velocity gradient statistics. In particular, the peak value in the root mean square streamwise velocity profile, when normalized by viscous scales, was found to exhibit a logarithmic dependence on Reynolds number. The mean streamwise velocity profile, on the other hand, appears to be essentially independent of Reynolds number. Spectra and spatial correlation data suggest that low-frequency motions at high Reynolds number engender intensified local convection velocities which affect the structure of both the velocity and velocity gradient fields. Implications for turbulent production mechanisms and coherent motions in the buffer layer are discussed.
Dynamic unified RANS-LES simulations of high Reynolds number separated flows
Mokhtarpoor, Reza; Heinz, Stefan; Stoellinger, Michael
2016-09-01
The development of hybrid RANS-LES methods is seen to be a very promising approach to enable efficient simulations of high Reynolds number turbulent flows involving flow separation. To contribute to further advances, we present a new, theoretically well based, dynamic hybrid RANS-LES method, referred to as DLUM. It is applied to a high Reynolds number flow involving both attached and separated flow regimes: a periodic hill flow is simulated at a Reynolds number of 37 000. Its performance is compared to pure LES, pure RANS, other hybrid RANS-LES (given by DLUM modifications), and experimental observations. It is shown that the use of this computational method offers huge cost reductions (which scale with Re/200, Re refers to the Reynolds number) of very high Reynolds number flow simulations compared to LES, it is much more accurate than RANS, and more accurate than LES, which is not fully resolved. In particular, this conclusion does also apply to the comparison of DLUM and pure LES simulations on rather coarse grids, which are often simply required to deal with simulations of very high Reynolds number flows: the DLUM provides mean velocity fields which are hardly affected by the grid, whereas LES velocity fields reveal significant shortcomings. We identified the reason for the superior performance of our new dynamic hybrid RANS-LES method compared to LES: it is the model's ability to respond to a changing resolution with adequate turbulent viscosity changes by ensuring simultaneously a physically correct turbulence length scale specification under the presence of interacting RANS and LES modes.
Simplified physical models of the flow around flexible insect wings at low Reynolds numbers
Harenberg, Steve; Reis, Johnny; Miller, Laura
2011-11-01
Some of the smallest insects fly at Reynolds numbers in the range of 5-100. We built a dynamically scaled physical model of a flexible insect wing and measured the resulting wing deformations and flow fields. The wing models were submerged in diluted corn syrup and rotated about the root of the wing for Reynolds numbers ranging from 1-100. Spatially resolved flow fields were obtained using particle image velocimetry (PIV). Deformations of the wing were tracked using DLTdv software to determine the motion and induced curvature of the wing.
The effects of nozzle geometry on waterjet breakup at high Reynolds numbers
Energy Technology Data Exchange (ETDEWEB)
Vahedi Tafreshi, H.; Pourdeyhimi, B. [Nonwovens Cooperative Research Center, North Carolina State University, NC 27695-8301, Raleigh (United States)
2003-10-01
Waterjet breakup is traditionally considered to follow the Ohnesorge classification. In this classification, high Reynolds number waterjets are considered to atomize quickly after discharge. By generating a constricted waterjet where the water flow stays detached all the way through the nozzle, we have observed the first wind-induced breakup mode at high Reynolds numbers. Such a peculiar behavior, however, was not observed in non-constricted waterjets. Our results indicate that, constricted jets do not follow the Ohnesorge classification, in contrast to the non-constricted waterjets. We discuss the impact of nozzle geometry on the characteristics of waterjets and support our discussion by numerical simulations. (orig.)
Magnus effects at high angles of attack and critical Reynolds numbers
Seginer, A.; Ringel, M.
1983-01-01
The Magnus force and moment experienced by a yawed, spinning cylinder were studied experimentally in low speed and subsonic flows at high angles of attack and critical Reynolds numbers. Flow-field visualization aided in describing a flow model that divides the Magnus phenomenon into a subcritical region, where reverse Magnus loads are experienced, and a supercritical region where these loads are not encountered. The roles of the spin rate, angle of attack, and crossflow Reynolds number in determining the boundaries of the subcritical region and the variations of the Magnus loads were studied.
Sundkvist, David; Krasnoselskikh, V; Bale, S D; Schwartz, S J; Soucek, J; Mozer, F
2012-01-13
Whistler wave trains are observed in the foot region of high Mach number quasiperpendicular shocks. The waves are oblique with respect to the ambient magnetic field as well as the shock normal. The Poynting flux of the waves is directed upstream in the shock normal frame starting from the ramp of the shock. This suggests that the waves are an integral part of the shock structure with the dispersive shock as the source of the waves. These observations lead to the conclusion that the shock ramp structure of supercritical high Mach number shocks is formed as a balance of dispersion and nonlinearity.
Kawata, Takuya; Alfredsson, P. Henrik
2016-07-01
Plane Couette flow under spanwise, anticyclonic system rotation [rotating plane Couette flow (RPCF)] is studied experimentally using stereoscopic particle image velocimetry for different Reynolds and rotation numbers in the fully turbulent regime. Similar to the laminar regime, the turbulent flow in RPCF is characterized by roll cells, however both instantaneous snapshots of the velocity field and space correlations show that the roll cell structure varies with the rotation number. All three velocity components are measured and both the mean flow and all four nonzero Reynolds stresses are obtained across the central parts of the channel. This also allows us to determine the wall shear stress from the viscous stress and the Reynolds stress in the center of the channel, and for low rotation rates the wall shear stress increases with increasing rotation rate as expected. The results show that zero absolute vorticity is established in the central parts of the channel of turbulent RPCF for high enough rotation rates, but also that the mean velocity profile for certain parameter ranges shows an S shape giving rise to a negative velocity gradient in the center of the channel. We find that from an analysis of the Reynolds stress transport equation using the present data there is a transport of the Reynolds shear stress towards the center of the channel, which may then result in a negative mean velocity gradient there.
Increased Mach Number Capability for the NASA Glenn 10x10 Supersonic Wind Tunnel
Slater, J. W.; Saunders, J. D.
2015-01-01
Computational simulations and wind tunnel testing were conducted to explore the operation of the Abe Silverstein Supersonic Wind Tunnel at the NASA Glenn Research Center at test section Mach numbers above the current limit of Mach 3.5. An increased Mach number would enhance the capability for testing of supersonic and hypersonic propulsion systems. The focus of the explorations was on understanding the flow within the second throat of the tunnel, which is downstream of the test section and is where the supersonic flow decelerates to subsonic flow. Methods of computational fluid dynamics (CFD) were applied to provide details of the shock boundary layer structure and to estimate losses in total pressure. The CFD simulations indicated that the tunnel could be operated up to Mach 4.0 if the minimum width of the second throat was made smaller than that used for previous operation of the tunnel. Wind tunnel testing was able to confirm such operation of the tunnel at Mach 3.6 and 3.7 before a hydraulic failure caused a stop to the testing. CFD simulations performed after the wind tunnel testing showed good agreement with test data consisting of static pressures along the ceiling of the second throat. The CFD analyses showed increased shockwave boundary layer interactions, which was also observed as increased unsteadiness of dynamic pressures collected in the wind tunnel testing.
Moualeu, Leolein Patrick Gouemeni
Runway-independent aircraft are expected to be the future for short-haul flights by improving air transportation and reducing area congestion encountered in airports. The Vehicle Systems Program of NASA identified a Large Civil Tilt-Rotor, equipped with variable-speed power-turbine engines, as the best concept. At cruise altitude, the engine rotor-speed will be reduced by as much as the 50% of take-off speed. The large incidence variation in the low pressure turbine associated with the change in speed can be detrimental to the engine performance. Low pressure turbine blades in cruise altitude are more predisposed to develop regions of boundary layer separation. Typical phenomenon such as impinging wakes on downstream blades and mainstream turbulences enhance the complexity of the flow in low pressure turbines. It is therefore important to be able to understand the flow behavior to accurately predict the losses. Research facilities are seldom able to experimentally reproduce low Reynolds numbers at relevant engine Mach number. Having large incidence swing as an additional parameter in the investigation of the boundary layer development, on a low pressure turbine blade, makes this topic unique and as a consequence requires a unique facility to conduct the experimental research. The compressible flow wind tunnel facility at the University of North Dakota had been updated to perform steady state experiments on a modular-cascade, designed to replicate a large variation of the incidence angles. The high speed and low Reynolds number facility maintained a sealed and closed loop configuration for each incidence angle. The updated facility is capable to produce experimental Reynolds numbers as low as 45,000 and as high as 570,000 at an exit Mach number of 0.72. Pressure and surface temperature measurements were performed at these low pressure turbine conditions. The present thesis investigates the boundary layer development on the surface of an Incidence-tolerant blade. The
Numerical simulation of 3D backward facing step flows at various Reynolds numbers
Directory of Open Access Journals (Sweden)
Louda Petr
2015-01-01
Full Text Available The work deals with the numerical simulation of 3D turbulent flow over backward facing step in a narrow channel. The mathematical model is based on the RANS equations with an explicit algebraic Reynolds stress model (EARSM. The numerical method uses implicit finite volume upwind discretization. While the eddy viscosity models fail in predicting complex 3D flows, the EARSM model is shown to provide results which agree well with experimental PIV data. The reference experimental data provide the 3D flow field. The simulations are compared with experiment for 3 values of Reynolds number.
Estimation of the Reynolds number in a Poiseuille flow using artificial neural networks
Carrillo, M.; Gónzalez, J. A.; Que, U.
2017-01-01
In this work the estimation of Reynolds number in a 2-dimensional Poiseuille flow is explored employing artificial neural networks (ANNs). The velocity fields of the fluids were generated evaluating the Hage-Poiseuille equation for different Reynolds (Re) from 20 to 2000. The velocity profile obtained for each case is used as input data for the ANNs, which is then trained to predict the Re. The results show an accuracy of at least of 99.5% in all prediction cases. This analysis is the first step towards the construction of a Machine Learning algorithm capable of computing physical parameters in more general scenarios.
Guilloud, G.; Schram, C.; Golliard, J.
2009-01-01
Despite the aeroacoustic expertise reached nowadays in air and ground transportation, energy sector or domestic appliances, reaching a decibel accuracy of an acoustic prediction for industrial cases is still challenging. Strong investments are made nowadays by oil and gas companies to determine and
Large velocity fluctuations in small-Reynolds-number pipe flow of polymer solutions
Bonn, D.; Ingremeau, F.; Amarouchene, Y.; Kellay, H.
2011-01-01
The flow of polymer solutions is examined in a flow geometry where a jet is used to inject the viscoelastic solution into a cylindrical tube. We show that this geometry allows for the generation of a "turbulentlike" flow at very low Reynolds numbers with a fluctuation level which can be as high as 3
Self-similar decay of high Reynolds number Taylor-Couette turbulence
Verschoof, R.A.; Huisman, S.G.; Veen, van der R.C.A.; Sun, C.; Lohse, D.
2016-01-01
We study the decay of high-Reynolds-number Taylor-Couette turbulence, i.e., the turbulent flow between two coaxial rotating cylinders. To do so, the rotation of the inner cylinder (Re i =2×10 6 , the outer cylinder is at rest) is stopped within 12 s, thus fully removing the energy input to the syst
Mechanisms Of Pressure Distributions Within Laminar Separation Bubble At Different Reynolds Numbers
Lee, Donghwi; Kawai, Soshi; Nonomura, Taku; Oyama, Akira; Fujii, Kozo
2014-11-01
Large-eddy simulation around 5 % thickness flat plate at Re = 5 , 000 , 6 , 100 , 11 , 000 and 20 , 000 are performed and the physical mechanisms of the pressure distributions (Cp) in laminar separation bubbles are analyzed. Depending on the Reynolds number, a gradual pressure recovery and plateau pressure distribution are observed as experiments by Anyoji et al. [AIAA paper 2011-0852]. The causes of the pressure distributions are quantitatively shown by deriving the pressure gradient (momentum budget) equation from the steady momentum equation. From the results, we identify that the viscous diffusion term near the surface has a major contribution to the pressure gradients, and a different growth of the separated shear layer relying on the Reynolds numbers affects the viscous stress near the surface. The gradual pressure recovery at the lower Reynolds numbers is caused by the progressive development of separated shear layer due to the viscous stress which makes a non-negligible viscous stress. On the other hand, a thin laminar separated shear layer is created at the higher Reynolds numbers because of the relatively small viscous diffusion effects, which results in a negligible shear stress distribution. It makes dp / dx ~ 0 and the plateau pressure distribution is generated. Asahi Glass Scholarship.
Flow field behavior with Reynolds number variance around a spiked body
Khurana, Shashank; Suzuki, Kojiro; Rathakrishnan, Ethirajan
2016-10-01
An experimental visualization study was performed to investigate the dependence of the pressure hill height and the influence zone expanse, for flow past a spiked body with different nose configurations, over a Reynolds number range from 2278 to 4405 to establish the vortex shedding process, and applicability in low speed flow regime for effective pressure reduction. It is found that the spike reduces the radius of curvature of the approaching streamline, leading to the deflection of the streamlines towards the shoulder of the basic body, resulting in a narrow zone of the positive pressure hill at the body nose. It is also observed that the pressure hill length and the influence zone expanse decrease with the introduction of spike over the present range of Reynolds numbers. For Reynolds numbers less than 2700, spike with conical nose is found to be more efficient than the spikes with other nose shapes of the present study in reducing the positive pressure at the nose of the blunt body. For higher Reynolds numbers, greater than 2700, the size of the vortex at the junction of the spike and basic body is the largest for the spike with hemispherical nose, and emerges as a potential candidate for application in possible wind-design resistant structures.
Two-dimensional energy spectra in a high Reynolds number turbulent boundary layer
Chandran, Dileep; Baidya, Rio; Monty, Jason; Marusic, Ivan
2016-11-01
The current study measures the two-dimensional (2D) spectra of streamwise velocity component (u) in a high Reynolds number turbulent boundary layer for the first time. A 2D spectra shows the contribution of streamwise (λx) and spanwise (λy) length scales to the streamwise variance at a given wall height (z). 2D spectra could be a better tool to analyse spectral scaling laws as it is devoid of energy aliasing errors that could be present in one-dimensional spectra. A novel method is used to calculate the 2D spectra from the 2D correlation of u which is obtained by measuring velocity time series at various spanwise locations using hot-wire anemometry. At low Reynolds number, the shape of the 2D spectra at a constant energy level shows λy √{ zλx } behaviour at larger scales which is in agreement with the literature. However, at high Reynolds number, it is observed that the square-root relationship gradually transforms into a linear relationship (λy λx) which could be caused by the large packets of eddies whose length grows proportionately to the growth of its width. Additionally, we will show that this linear relationship observed at high Reynolds number is consistent with attached eddy predictions. The authors gratefully acknowledge the support from the Australian Research Council.
Multigrid solution of the convection-diffusion equation with high-Reynolds number
Energy Technology Data Exchange (ETDEWEB)
Zhang, Jun [George Washington Univ., Washington, DC (United States)
1996-12-31
A fourth-order compact finite difference scheme is employed with the multigrid technique to solve the variable coefficient convection-diffusion equation with high-Reynolds number. Scaled inter-grid transfer operators and potential on vectorization and parallelization are discussed. The high-order multigrid method is unconditionally stable and produces solution of 4th-order accuracy. Numerical experiments are included.
Experiments on low Reynolds number turbulent flow through a square duct
Owolabi, Bayode; Poole, Robert; Dennis, David
2015-11-01
Previous experimental studies on square duct turbulent flow have focused mainly on high Reynolds numbers for which a turbulence induced eight-vortex secondary flow pattern exists in the cross sectional plane. More recently, Direct Numerical Simulations (DNS) have revealed that the flow field at Reynolds numbers close to transition can be very different; the flow in this marginally turbulent regime alternating between two states characterised by four vortices. In this study, we experimentally investigate the onset criteria for transition to turbulence in square ducts. We also present experimental data on the mean flow properties and turbulence statistics in both marginally and fully turbulent flow at relatively low Reynolds numbers using laser Doppler velocimetry. Results for both flow categories show good agreement with DNS. The switching of the flow field between two flow states at marginally turbulent Reynolds numbers is confirmed by bimodal probability density functions of streamwise velocity at certain distances from the wall as well as joint probability density functions of streamwise and wall normal velocities which feature two peaks.
Maccormack, R. W.
1978-01-01
The calculation of flow fields past aircraft configuration at flight Reynolds numbers is considered. Progress in devising accurate and efficient numerical methods, in understanding and modeling the physics of turbulence, and in developing reliable and powerful computer hardware is discussed. Emphasis is placed on efficient solutions to the Navier-Stokes equations.
High and Low Reynolds number Measurements in a Room with an Impinging Isothermal Jet
DEFF Research Database (Denmark)
Skovgaard, M.; Hyldgaard, C. E.; Nielsen, Peter V.
The present paper, which is within the work of the lEA - annex 20, presents a series of full-scale velocity measurements in a room with isothermal mixing ventilation. The measurements are in the Reynolds number range 1000 - 7000 based on inlet dimensions. This means that a transition from laminar...
Negative Magnus lift on a rotating sphere at around the critical Reynolds number
Muto, Masaya; Tsubokura, Makoto; Oshima, Nobuyuki
2012-01-01
Negative Magnus lift acting on a sphere rotating about the axis perpendicular to an incoming flow was investigated using large-eddy simulation at three Reynolds numbers of 1.0 × 104, 2.0 × 105, and 1.14 × 106. The numerical methods used were first validated on a non-rotating sphere, and the spatial resolution around the sphere was determined so as to reproduce the laminar separation, reattachment, and turbulent transition of the boundary layer observed in the vicinity of the critical Reynolds number. The rotating sphere exhibited a positive or negative Magnus effect depending on the Reynolds number and the imposed rotating speed. At Reynolds numbers in the subcritical or supercritical regimes, the direction of the Magnus lift force was independent of the rotational speed. In contrast, the lift force was negative in the critical regime when particular rotating speeds were imposed. This negative Magnus effect was investigated in the context of suppression or promotion of boundary layer transition around the separation point.
Stokesian swimming of a sphere at low Reynolds number by helical surface distortion
Felderhof, B. U.; Jones, R. B.
2016-07-01
Explicit expressions are derived for the matrices determining the mean translational and rotational swimming velocities and the mean rate of dissipation for Stokesian swimming at low Reynolds number of a distorting sphere in a viscous incompressible fluid. As an application, an efficient helical propeller-type stroke is found and its properties are calculated.
Performance and slipstream characteristics of small-scale propellers at low Reynolds numbers
Deters, Robert W.
The low Reynolds number effects of small-scale propellers were investigated. At the Reynolds numbers of interest (below 100,000), a decrease in lift and an increase in drag is common making it difficult to predict propeller performance characteristics. A propeller testing apparatus was built to test small scale propellers in static conditions and in an advancing flow. Twenty-seven off-the-shelf propellers, with diameters ranging from 2.25 in to 9 in, were tested in order to determine the general effects of low Reynolds numbers on small propellers. From these tests, increasing the Reynolds number for a propeller increases its efficiency by either increasing the thrust produced or decreasing the power. By doubling the Reynolds number of a propeller, it is not uncommon to increase the efficiency by more the 10%. Using off-the-shelf propellers limits the geometry available and finding propellers of the same geometry but of different scale is very difficult. To solve this problem, four propellers were design and built using a 3D printer. Two of the propellers were simple rectangular twisted blades of different chords. Another propeller was modeled after a full-scale propeller. The fourth propeller was created using inverse design to minimize power loss. Each propeller was built in a 5-in and 9-in diameter version in order to test a larger range of Reynolds numbers. A separate propeller blade and hub system was created to allow each propeller to be tested with different pitch angles and to test each propeller in a 2-, 3-, and 4-blade version. From the performance results of the 3D printed propellers, it was shown that propellers of different scale, but tested at the same Reynolds number, had about the same performance results. Finally, the slipstreams of different propellers were measured using a 7-hole probe. Propeller slipstreams can have a large effect on the aerodynamics of lifting surfaces downstream of the propeller. Small UAVs and MAVs flying in close proximity
Design and installation of a high Reynolds number recirculating water tunnel
Daniel, Libin
The High-Reynolds Number Fluid Mechanics Laboratory has recently been established at Oklahoma State University (OSU). The three primary components of the laboratory are 1) a recirculating water tunnel, 2) a multiphase pipe flow facility, and 3) a multi-scale flow visualization system. This thesis focuses on the design and fabrication of the water tunnel, which will be used for high-Reynolds number turbulent boundary layer research. Two main design criteria for the water tunnel were to achieve a momentum thickness based Reynolds number in excess of 104 and to have high optical access to the flow surfaces in the test section. This is being achieved with a 1 m. long test section and a maximum flow speed of 10 m/s. This Reynolds number was targeted to bridge the gap between typical university water tunnels (103) and the world's largest water tunnel facilities (105). The water tunnel is powered by a 150 hp motor and a 4500 gpm capacity centrifugal pump. The water tunnel is designed for a maximum operating pressure of 40 psi. This will make the facility a low cost option to perform high-Reynolds number aerodynamic and hydrodynamic tests. Improved flow imaging capability is a major advantage to liquid based fluid facilities because of the increased density for seeding and reduced field-of-view for equivalent Reynolds number. The laboratory's state-of-the-art flow visualization system can be used for time-resolved and phase averaged stereo- particle-image-velocimetry (sPIV), laser-induced-fluorescence, and high-speed imaging. Design provisions are also made to allow a multi-phase loop to share the pump and motor configuration of this water tunnel facility. The major design decisions that went into the design of the water tunnel facility are discussed. The design considerations that were taken into account for the test section, flow conditioning sections and the entire flow loop are discussed in greater detail. The final configuration and the technical drawings of the water
Mach number study of supersonic turbulence: The properties of the density field
Konstandin, Lukas; Girichidis, Philipp; Peters, Thomas; Shetty, Rahul; Klessen, Ralf S
2015-01-01
We model driven, compressible, isothermal, turbulence with Mach numbers ranging from the subsonic ($\\mathcal{M} \\approx 0.65$) to the highly supersonic regime ($\\mathcal{M}\\approx 16 $). The forcing scheme consists both solenoidal (transverse) and compressive (longitudinal) modes in equal parts. We find a relation $\\sigma_{s}^2 = \\mathrm{b}\\log{(1+\\mathrm{b}^2\\mathcal{M}^2)}$ between the Mach number and the standard deviation of the logarithmic density with $\\mathrm{b} = 0.457 \\pm 0.007$. The density spectra follow $\\mathcal{D}(k,\\,\\mathcal{M}) \\propto k^{\\zeta(\\mathcal{M})}$ with scaling exponents depending on the Mach number. We find $\\zeta(\\mathcal{M}) = \\alpha \\mathcal{M}^{\\beta}$ with a coefficient $\\alpha$ that varies slightly with resolution, whereas $\\beta$ changes systematically. We extrapolate to the limit of infinite resolution and find $\\alpha = -1.91 \\pm 0.01,\\, \\beta =-0.30\\pm 0.03$. The dependence of the scaling exponent on the Mach number implies a fractal dimension $D=2+0.96 \\mathcal{M}^{-0.3...
Comparison of turbulent channel and pipe flows with varying Reynolds number
Energy Technology Data Exchange (ETDEWEB)
Ng, H.C.H.; Monty, J.P.; Hutchins, N.; Chong, M.S.; Marusic, I. [University of Melbourne, Department Mechanical Engineering, Melbourne, VIC (Australia)
2011-11-15
Single normal hot-wire measurements of the streamwise component of velocity were taken in fully developed turbulent channel and pipe flows for matched friction Reynolds numbers ranging from 1,000 {<=} Re{sub {tau}}{<=} 3,000. A total of 27 velocity profile measurements were taken with a systematic variation in the inner-scaled hot-wire sensor length l {sup +} and the hot-wire length-to-diameter ratio (l/d). It was observed that for constant l {sup +} = 22 and l/d >or similar 200, the near-wall peak in turbulence intensity rises with Reynolds number in both channels and pipes. This is in contrast to Hultmark et al. in J Fluid Mech 649:103-113, (2010), who report no growth in the near-wall peak turbulence intensity for pipe flow with l {sup +} = 20. Further, it was found that channel and pipe flows have very similar streamwise velocity statistics and energy spectra over this range of Reynolds numbers, with the only difference observed in the outer region of the mean velocity profile. Measurements where l {sup +} and l/d were systematically varied reveal that l {sup +} effects are akin to spatial filtering and that increasing sensor size will lead to attenuation of an increasingly large range of small scales. In contrast, when l/d was insufficient, the measured energy is attenuated over a very broad range of scales. These findings are in agreement with similar studies in boundary layer flows and highlight the need to carefully consider sensor and anemometry parameters when comparing flows across different geometries and when drawing conclusions regarding the Reynolds number dependency of measured turbulence statistics. With an emphasis on accuracy, measurement resolution and wall proximity, these measurements are taken at comparable Reynolds numbers to currently available DNS data sets of turbulent channel/pipe flows and are intended to serve as a database for comparison between physical and numerical experiments. (orig.)
Wissocq, Gauthier; Gourdain, Nicolas; Malaspinas, Orestis; Eyssartier, Alexandre
2017-02-01
This paper reports the investigations done to adapt the Characteristic Boundary Conditions (CBC) to the Lattice-Boltzmann formalism for high Reynolds number applications. Three CBC formalisms are implemented and tested in an open source LBM code: the baseline local one-dimension inviscid (BL-LODI) approach, its extension including the effects of the transverse terms (CBC-2D) and a local streamline approach in which the problem is reformulated in the incident wave framework (LS-LODI). Then all implementations of the CBC methods are tested for a variety of test cases, ranging from canonical problems (such as 2D plane and spherical waves and 2D vortices) to a 2D NACA profile at high Reynolds number (Re =105), representative of aeronautic applications. The LS-LODI approach provides the best results for pure acoustics waves (plane and spherical waves). However, it is not well suited to the outflow of a convected vortex for which the CBC-2D associated with a relaxation on density and transverse waves provides the best results. As regards numerical stability, a regularized adaptation is necessary to simulate high Reynolds number flows. The so-called regularized FD (Finite Difference) adaptation, a modified regularized approach where the off-equilibrium part of the stress tensor is computed thanks to a finite difference scheme, is the only tested adaptation that can handle the high Reynolds computation.
Introducing a nano-scale crossed hot-wire for high Reynolds number measurements
Fan, Yuyang; Fu, Matthew; Hultmark, Marcus
2016-11-01
Hot-wire anemometry is commonly used for high Reynolds number flow measurements, mainly because of its continuous signal and high bandwidth. However, measuring two components of velocity in high Reynolds number wall-bounded flows has proven to be quite challenging with conventional crossed hot-wires, especially close to the wall, due to insufficient resolution and obstruction from the probe. The Nano-Scale Thermal Anemometry Probe (NSTAP) is a miniature hot-wire that drastically increased the spatial and temporal resolutions for single-component measurements by using a nano-scale platinum wire. Applying a novel combining method and reconfiguration of the NSTAP design, we created a sensor (x-NSTAP) that is capable of two-component velocity measurements with a sensing volume of approximately 50 × 50 × 50 μ m, providing spatial and temporal resolutions similar to the single component NSTAP. The x-NSTAP is deployed in the Superpipe facility for accurate measurements of the Reynolds stresses at very high Reynolds numbers. Supported under NSF Grant CBET-1510100 (program manager Dimitrios Papavassiliou).
DNS/LES Simulations of Separated Flows at High Reynolds Numbers
Balakumar, P.
2015-01-01
Direct numerical simulations (DNS) and large-eddy simulations (LES) simulations of flow through a periodic channel with a constriction are performed using the dynamic Smagorinsky model at two Reynolds numbers of 2800 and 10595. The LES equations are solved using higher order compact schemes. DNS are performed for the lower Reynolds number case using a fine grid and the data are used to validate the LES results obtained with a coarse and a medium size grid. LES simulations are also performed for the higher Reynolds number case using a coarse and a medium size grid. The results are compared with an existing reference data set. The DNS and LES results agreed well with the reference data. Reynolds stresses, sub-grid eddy viscosity, and the budgets for the turbulent kinetic energy are also presented. It is found that the turbulent fluctuations in the normal and spanwise directions have the same magnitude. The turbulent kinetic energy budget shows that the production peaks near the separation point region and the production to dissipation ratio is very high on the order of five in this region. It is also observed that the production is balanced by the advection, diffusion, and dissipation in the shear layer region. The dominant term is the turbulent diffusion that is about two times the molecular dissipation.
Flegel, Ashlie B.; Giel, Paul W.; Welch, Gerard E.
2014-01-01
The effects of high inlet turbulence intensity on the aerodynamic performance of a variable speed power turbine blade are examined over large incidence and Reynolds number ranges. These results are compared to previous measurements made in a low turbulence environment. Both high and low turbulence studies were conducted in the NASA Glenn Research Center Transonic Turbine Blade Cascade Facility. The purpose of the low inlet turbulence study was to examine the transitional flow effects that are anticipated at cruise Reynolds numbers. The current study extends this to LPT-relevant turbulence levels while perhaps sacrificing transitional flow effects. Assessing the effects of turbulence at these large incidence and Reynolds number variations complements the existing database. Downstream total pressure and exit angle data were acquired for 10 incidence angles ranging from +15.8deg to -51.0deg. For each incidence angle, data were obtained at five flow conditions with the exit Reynolds number ranging from 2.12×10(exp 5) to 2.12×10(exp 6) and at a design exit Mach number of 0.72. In order to achieve the lowest Reynolds number, the exit Mach number was reduced to 0.35 due to facility constraints. The inlet turbulence intensity, Tu, was measured using a single-wire hotwire located 0.415 axial-chord upstream of the blade row. The inlet turbulence levels ranged from 8 to 15 percent for the current study. Tu measurements were also made farther upstream so that turbulence decay rates could be calculated as needed for computational inlet boundary conditions. Downstream flow field measurements were obtained using a pneumatic five-hole pitch/yaw probe located in a survey plane 7 percent axial chord aft of the blade trailing edge and covering three blade passages. Blade and endwall static pressures were acquired for each flow condition as well. The blade loading data show that the suction surface separation that was evident at many of the low Tu conditions has been eliminated. At
Gapcynski, John P; Carlson, Harry W
1955-01-01
The changes in the aerodynamic characteristics of a body of revolution with a fineness ratio of 8 have been determined at Mach numbers of 1.41 and 2.01, a Reynolds number, based on body length, of 4.54 x 10 to the 6th power, and angles of incidence of 0 degrees and plus or minus 3 degrees as the position of the body is varied with respect to a reflection plane. The data are compared with theoretical results.
Aerodynamics of wings at low Reynolds numbers: Boundary layer separation and reattachment
McArthur, John
Due to advances in electronics technology, it is now possible to build small scale flying and swimming vehicles. These vehicles will have size and velocity scales similar to small birds and fish, and their characteristic Reynolds number will be between 104 and 105. Currently, these flying and swimming vehicles do not perform well, and very little research has been done to characterize them, or to explain why they perform so poorly. This dissertation documents three basic investigations into the performance of small scale lifting surfaces, with Reynolds numbers near 104. Part I. Low Reynolds number aerodynamics. Three airfoil shapes were studied at Reynolds numbers of 1 and 2x104: a flat plate airfoil, a circular arc cambered airfoil, and the Eppler 387 airfoil. Lift and drag force measurements were made on both 2D and 3D conditions, with the 3D wings having an aspect ratio of 6, and the 2D condition being approximated by placing end plates at the wing tips. Comparisons to the limited number of previous measurements show adequate agreement. Previous studies have been inconclusive on whether lifting line theory can be applied to this range of Re, but this study shows that lifting line theory can be applied when there are no sudden changes in the slope of the force curves. This is highly dependent on the airfoil shape of the wing, and explains why previous studies have been inconclusive. Part II. The laminar separation bubble. The Eppler 387 airfoil was studied at two higher Reynolds numbers: 3 and 6x10 4. Previous studies at a Reynolds number of 6x104 had shown this airfoil experiences a drag increase at moderate lift, and a subsequent drag decrease at high lift. Previous studies suggested that the drag increase is caused by a laminar separation bubble, but the experiments used to show this were conducted at higher Reynolds numbers and extrapolated down. Force measurements were combined with flow field measurements at Reynolds numbers 3 and 6x104 to determine whether
Effects of Low Reynolds Number on Wake-Generated Unsteady Flow of an Axial-Flow Turbine Rotor
Directory of Open Access Journals (Sweden)
Matsunuma Takayuki
2005-01-01
Full Text Available The unsteady flow field downstream of axial-flow turbine rotors at low Reynolds numbers was investigated experimentally using hot-wire probes. Reynolds number, based on rotor exit velocity and rotor chord length Re out,RT , was varied from 3.2× 10 4 to 12.8× 10 4 at intervals of 1.0× 10 4 by changing the flow velocity of the wind tunnel. The time-averaged and time-dependent distributions of velocity and turbulence intensity were analyzed to determine the effect of Reynolds number. The reduction of Reynolds number had a marked influence on the turbine flow field. The regions of high turbulence intensity due to the wake and the secondary vortices were increased dramatically with the decreasing Reynolds number. The periodic fluctuation of the flow due to rotor-stator interaction also increased with the decreasing Reynolds number. The energy-dissipation thickness of the rotor midspan wake at the low Reynolds number Re out,RT =3.2× 10 4 was 1.5 times larger than that at the high Reynolds number Re out,RT =12.8× 10 4 . The curve of the −0.2 power of the Reynolds number agreed with the measured energy-dissipation thickness at higher Reynolds numbers. However, the curve of the −0.4 power law fitted more closely than the curve of the −0.2 power law at lower Reynolds numbers below 6.4× 10 4 .
Practical computational aeroacoustics for compact surfaces in low mach number flows
DEFF Research Database (Denmark)
Pradera-Mallabiabarrena, Ainara; Keith, Graeme; Jacobsen, Finn
2011-01-01
compared to the wavelength of interest. This makes it possible to focus on the surface source term of the Ffowcs Williams-Hawkings equation. In this paper, in order to illustrate the basic method for storing and utilizing data from the CFD analysis, the flow past a circular cylinder at a Reynolds number...
Exact two-dimensionalization of rapidly rotating large-Reynolds-number flows
Gallet, Basile
2015-01-01
We consider the flow of a Newtonian fluid in a three-dimensional domain, rotating about a vertical axis and driven by a vertically invariant horizontal body-force. This system admits vertically invariant solutions that satisfy the 2D Navier-Stokes equation. At high Reynolds number and without global rotation, such solutions are usually unstable to three-dimensional perturbations. By contrast, for strong enough global rotation, we prove rigorously that the 2D (and possibly turbulent) solutions are stable to vertically dependent perturbations: the flow becomes 2D in the long-time limit. These results shed some light on several fundamental questions of rotating turbulence: for arbitrary Reynolds number and small enough Rossby number, the system is attracted towards purely 2D flow solutions, which display no energy dissipation anomaly and no cyclone-anticyclone asymmetry. Finally, these results challenge the applicability of wave turbulence theory to describe stationary rotating turbulence in bounded domains.
The 3D MHD code GOEMHD3 for large-Reynolds-number astrophysical plasmas
Skála, J; Büchner, J; Rampp, M
2014-01-01
The numerical simulation of turbulence and flows in almost ideal, large-Reynolds-number astrophysical plasmas motivates the implementation of almost conservative MHD computer codes. They should efficiently calculate, use highly parallelized schemes scaling well with large numbers of CPU cores, allows to obtain a high grid resolution over large simulation domains and which can easily be adapted to new computer architectures as well as to new initial and boundary conditions, allow modular extensions. The new massively parallel simulation code GOEMHD3 enables efficient and fast simulations of almost ideal, large-Reynolds-number astrophysical plasma flows, well resolved and on huge grids covering large domains. Its abilities are validated by major tests of ideal and weakly dissipative plasma phenomena. The high resolution ($2048^3$ grid points) simulation of a large part of the solar corona above an observed active region proved the excellent parallel scalability of the code using more than 30.000 processor cores...
Failure of energy stability in Oldroyd-B fluids at arbitrarily low Reynolds numbers
Döring, C; Schumacher, J
2004-01-01
Energy theory for incompressible Newtonian fluids is, in many cases, capable of producing strong absolute stability criteria for steady flows. In those fluids the kinetic energy naturally defines a norm in which perturbations decay monotonically in time at sufficiently low (but non-zero) Reynolds numbers. There are, however, at least two obstructions to the generalization of such methods to Oldroyd-B fluids. One previously recognized problem is the fact that the natural energy does not correspond to a proper functional norm on perturbations. Another problem, original to this work, is the fact that fluctuations in Oldroyd-B fluids may be subject to non-normal amplification at arbitrarily low Reynolds numbers (albeit at sufficiently large Weissenberg numbers). Such transient growth, occuring even when the base flow is linearly stable, precludes the uniform monotonic decay of any reasonable measure of the disturbance's amplitude.
Generalizing the Reynolds number from turbulence to Self Organized Criticality and ecosystems
Chapman, S C; Watkins, N W
2007-01-01
In fluid turbulence a single control parameter, the Reynolds number R_E, which is a function of macroscopic system variables is sufficient to quantify the transition from ordered (laminar) to disordered (turbulent) flow. We suggest that a wider class of systems has this property, including Self Organized Criticality (SOC) and ecosystem models for species abundance. These systems can all be driven into a state with defining characteristics: they have many degrees of freedom (d.o.f.); are driven, dissipating and out of equilibrium; are on average in a steady state; and show scaling over a large dynamic range. The Reynolds number expresses the number of d.o.f., or energy carrying modes in the system. For avalanche models exhibiting SOC, d.o.f. refer to avalanche sizes and the Reynolds number R_A that we identify is simply the well known ratio of the driving rate to system dissipation rate. The SOC slowly driven interaction dominated limit is reached by taking R_A to zero; we show this maximizes the number of d.o...
Long-range lPIV to resolve the small scales in a jet at high Reynolds number
Fiscaletti, D.; Westerweel, J.; Elsinga, G.E.
2014-01-01
The investigation of flows at high Reynolds number is of great interest for the theory of turbulence, in that the large and the small scales of turbulence show a clear separation. But, as the Reynolds number of the flow increases, the size of the Kolmogorov length scale ( η ) drops almost proportion
Experiment on smooth, circular cylinders in cross-flow in the critical Reynolds number regime
Energy Technology Data Exchange (ETDEWEB)
Miau, J.J.; Tsai, H.W.; Lin, Y.J.; Tu, J.K.; Fang, C.H.; Chen, M.C. [National Cheng Kung University, Department of Aeronautics and Astronautics, Tainan (China)
2011-10-15
Experiments were conducted for 2D circular cylinders at Reynolds numbers in the range of 1.73 x 10{sup 5}-5.86 x 10{sup 5}. In the experiment, two circular cylinder models made of acrylic and stainless steel, respectively, were employed, which have similar dimensions but different surface roughness. Particular attention was paid to the unsteady flow behaviors inferred by the signals obtained from the pressure taps on the cylinder models and by a hot-wire probe in the near-wake region. At Reynolds numbers pertaining to the initial transition from the subcritical to the critical regimes, pronounced pressure fluctuations were measured on the surfaces of both cylinder models, which were attributed to the excursion of unsteady flow separation over a large circumferential region. At the Reynolds numbers almost reaching the one-bubble state, it was noted that the development of separation bubble might switch from one side to the other with time. Wavelet analysis of the pressure signals measured simultaneously at {theta} = {+-}90 further revealed that when no separation bubble was developed, the instantaneous vortex-shedding frequencies could be clearly resolved, about 0.2, in terms of the Strouhal number. The results of oil-film flow visualization on the stainless steel cylinder of the one-bubble and two-bubble states showed that the flow reattachment region downstream of a separation bubble appeared not uniform along the span of the model. Thus, the three dimensionality was quite evident. (orig.)
Reynolds number effects on the fluctuating velocity distribution in wall-bounded shear layers
Li, Wenfeng; Roggenkamp, Dorothee; Jessen, Wilhelm; Klaas, Michael; Schröder, Wolfgang
2017-01-01
The streamwise turbulence intensity and wall-shear stress fluctuations of zero pressure gradient (ZPG) turbulent boundary layers are investigated for seven Reynolds numbers based on the momentum thickness in the range of 1009 ⩽ Re θ ⩽ 4070 by particle-image velocimetry (PIV) and micro-particle tracking velocimetry (µ-PTV) at a spatial resolution up to 0.06-0.23 wall units such that the viscous sublayer is well resolved. The statistics evidence good agreement with direct numerical simulations (DNS) and experimental results from the literature. The experimental results show the streamwise turbulence intensity and wall-shear stress fluctuation to grow at increasing Reynolds numbers.
Influence of Reynolds number and forcing type in a turbulent von K\\'arm\\'an flow
Saint-Michel, Brice; Marié, Louis; Ravelet, Florent; Daviaud, François
2014-01-01
We present a detailed study of of a global bifurcation occuring in a turbulent von K\\'arm\\'an swirling flow. In this system, the statistically steady states progressively display hysteretic behaviour when the Reynolds number is increased above the transition to turbulence. We examine in detail this hysteresis using asymmetric forcing conditions --- rotating the impellers at different speeds. For very high Reynolds numbers, we study the sensitivity of the hysteresis cycle --- using complementary Particle Image Velocimetry (PIV) and global mechanical measurements --- to the forcing nature, imposing either the torque or the speed of the impellers. New mean states, displaying multiple quasi-steady states and negative differential responses, are experimentally observed in torque control. A simple analogy with electrical circuits is performed to understand the link between multi-stability and negative responses. The system is compared to other, similar "bulk" systems, to understand some relevant ingredients of nega...
Reynolds number limits for jet propulsion: A numerical study of simplified jellyfish
Herschlag, Gregory
2010-01-01
The Scallop Theorem states that reciprocal methods of locomotion, such as jet propulsion or paddling, will not work in Stokes flow (Reynolds number = 0). In nature the effective limit of jet propulsion is still in the range where inertial forces are significant. It appears that almost all animals that use jet propulsion swim at Reynolds numbers (Re) of about 5 or more. Juvenile squid and octopods hatch from the egg already swimming in this inertial regime. The limitations of jet propulsion at intermediate Re is explored here using the immersed boundary method to solve the two-dimensional Navier Stokes equations coupled to the motion of a simplified jellyfish. The contraction and expansion kinematics are prescribed, but the forward and backward swimming motions of the idealized jellyfish are emergent properties determined by the resulting fluid dynamics. Simulations are performed for both an oblate bell shape using a paddling mode of swimming and a prolate bell shape using jet propulsion. Average forward veloc...
Characteristics of a hyperboloid-flare configuration at high Reynolds numbers
Zvegintsev, V. I.; Kharitonov, A. M.; Chirkashenko, V. F.; Chibisov, S. V.; Fletcher, D.; Paris, S.
2006-12-01
Results on a hyperboloid-flare model tested in a new hypersonic wind tunnel with adiabatic compression AT-303 based at ITAM SB RAS at M∞ = 10 and 15 and in a wide range of Reynolds numbers are presented. Pressure and heat-flux distributions along the model are compared with data obtained previously in various European hypersonic wind tunnels (Longshot — Belgium, HEG — Germany) and with results of numerical computations. Pressure and heat-flux coefficients measured in the attached flow region are demonstrated to be in good qualitative agreement. Reasons for the differences in results measured in regions of flow separation and reattachment are discussed. Significant viscous effects on characteristics of the flow around the model are demonstrated; a particularly strong effect is exerted on the heat-flux distribution. This fact confirms that it is important to model real Reynolds numbers in wind-tunnel testing of aerospace plane models.
Okamoto, Naoya; Yoshimatsu, Katsunori; Schneider, Kai; Farge, Marie
2014-03-01
Small-scale anisotropic intermittency is examined in three-dimensional incompressible magnetohydrodynamic turbulence subjected to a uniformly imposed magnetic field. Orthonormal wavelet analyses are applied to direct numerical simulation data at moderate Reynolds number and for different interaction parameters. The magnetic Reynolds number is sufficiently low such that the quasistatic approximation can be applied. Scale-dependent statistical measures are introduced to quantify anisotropy in terms of the flow components, either parallel or perpendicular to the imposed magnetic field, and in terms of the different directions. Moreover, the flow intermittency is shown to increase with increasing values of the interaction parameter, which is reflected in strongly growing flatness values when the scale decreases. The scale-dependent anisotropy of energy is found to be independent of scale for all considered values of the interaction parameter. The strength of the imposed magnetic field does amplify the anisotropy of the flow.
Application of shock tubes to transonic airfoil testing at high Reynolds numbers
Cook, W. J.; Chaney, M. J.; Presley, L. L.; Chapman, G. T.
1978-01-01
Performance analysis of a gas-driven shock tube shows that transonic airfoil flows with chord Reynolds numbers of the order of 100 million can be produced, with limitations being imposed by the structural integrity of the facility or the model. A study of flow development over a simple circular arc airfoil at zero angle of attack was carried out in a shock tube at low and intermediate Reynolds numbers to assess the testing technique. Results obtained from schlieren photography and airfoil pressure measurements show that steady transonic flows similar to those produced for the same airfoil in a wind tunnel can be generated within the available testing time in a shock tube with properly contoured test section walls.
Institute of Scientific and Technical Information of China (English)
HAN Shan-ling; ZHU Ping; LIN Zhong-qin
2005-01-01
The fractional volumetric lattice Boltzmann method with much better stability was used to simulate two dimensional cavity flows. Because the effective viscosity was reduced by the fraction factor, it is very effective forsimulating high Reynolds number flows. Simulations were carried out on a uniform grids system. The stream lines and the velocity profiles obtained from the simulations agree well with the standard lattice Boltzmann method simulations. Comparisons of detailed flow patterns with other studies via location of vortex centers are also satisfactory.
Reynolds number trend of hierarchies and scale interactions in turbulent boundary layers
Baars, W. J.; Hutchins, N.; Marusic, I.
2017-03-01
Small-scale velocity fluctuations in turbulent boundary layers are often coupled with the larger-scale motions. Studying the nature and extent of this scale interaction allows for a statistically representative description of the small scales over a time scale of the larger, coherent scales. In this study, we consider temporal data from hot-wire anemometry at Reynolds numbers ranging from Reτ≈2800 to 22 800, in order to reveal how the scale interaction varies with Reynolds number. Large-scale conditional views of the representative amplitude and frequency of the small-scale turbulence, relative to the large-scale features, complement the existing consensus on large-scale modulation of the small-scale dynamics in the near-wall region. Modulation is a type of scale interaction, where the amplitude of the small-scale fluctuations is continuously proportional to the near-wall footprint of the large-scale velocity fluctuations. Aside from this amplitude modulation phenomenon, we reveal the influence of the large-scale motions on the characteristic frequency of the small scales, known as frequency modulation. From the wall-normal trends in the conditional averages of the small-scale properties, it is revealed how the near-wall modulation transitions to an intermittent-type scale arrangement in the log-region. On average, the amplitude of the small-scale velocity fluctuations only deviates from its mean value in a confined temporal domain, the duration of which is fixed in terms of the local Taylor time scale. These concentrated temporal regions are centred on the internal shear layers of the large-scale uniform momentum zones, which exhibit regions of positive and negative streamwise velocity fluctuations. With an increasing scale separation at high Reynolds numbers, this interaction pattern encompasses the features found in studies on internal shear layers and concentrated vorticity fluctuations in high-Reynolds-number wall turbulence.
High-Reynolds Number Viscous Flow Simulations on Embedded-Boundary CartesianGrids
2016-05-05
term goal of this research is to develop algorithms to simulate high Reynolds number turbulent flow in complicated geometries using embedded boundary...Spalding’s formula of matching the pro- files actually computed in the flow field by the Spalart-Allmaras turbulence model. In particular the profiles ...turbu- lent viscosity to be computed, see e.g. the profiles in the bottom row of Fig. 4. The streamwise velocity and especially the turbulent viscosity
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.
Wake flow pattern modified by small control cylinders at low Reynolds number
Kuo, C.-H.; Chiou, L.-C.; Chen, C.-C.
2007-08-01
Passive wake control behind a circular cylinder in uniform flow is studied by numerical simulation for ReD ranging from 80 to 300. Two small control cylinders, with diameter d/D=1/8, are placed at x/D=0.5 and y/D=±0.6. Unlike the 1990 results of Strykowski and Sreenivasan, in the present study, the vortex street behind the main cylinder still exists but the fluctuating lift and the form drag on the main cylinder reduces significantly and monotonously as the Reynolds number increases from 80 to 300. Obstruction of the control cylinders to the incoming flow deflects part of the fluid to pass through the gap between the main and control cylinders, forming two symmetric streams. These streams not only eliminate the flow separation along the rear surface of the main cylinder, they also merge toward the wake centerline to create an advancing momentum in the immediate near-wake region. These two effects significantly reduce the wake width behind the main cylinder and lead to monotonous decrease of the form drag as the Reynolds number increases. As the Reynolds number gets higher, a large amount of the downstream advancing momentum significantly delays the vortex formation farther downstream, leading to a more symmetric flow structure in the near-wake region of the main cylinder. As the Reynolds number increases from 80 to 300, both increasing symmetry of the flow structure in the near-wake and significant delay of the vortex formation are the main reasons for the fluctuating lift to decrease monotonously.
Space experimental device on Marangoni drop migrations of large Reynolds numbers
Institute of Scientific and Technical Information of China (English)
张璞; 胡良; 刘方; 姚永龙; 解京昌; 林海; 胡文瑞
2001-01-01
The space experimental device for testing the Marangoni drop migrations has been discussed in the present paper. The experiment is one of the spaceship projects of China. In comparison with similar devices, it has the ability of completing all the scientific experiments by both auto controlling and telescience methods. It not only can perform drop migration experiments of large Reynolds numbers but also has an equi-thick interferential system.
2014-05-26
parametric subharmonic instability. 15. SUBJECT TERMS Stratified turbulent wakes, high Reynolds numbers, internal waves, nonlinear effects, harmonics, mean...beam and the potential for parametric subharmonic instability. In all these efforts, a uniform linear stratification was considered. A subset of our...found for all simulated waves. c) For sufficiently high-amplitude beams, a parametric subharmonic instability is observed after a long enough time
The hydrodynamics of swimming at intermediate Reynolds numbers in the water boatman (Corixidae).
Ngo, Victoria; McHenry, Matthew James
2014-08-01
The fluid forces that govern propulsion determine the speed and energetic cost of swimming. These hydrodynamics are scale dependent and it is unclear what forces matter to the tremendous diversity of aquatic animals that are between a millimeter and a centimeter in length. Animals at this scale generally operate within the regime of intermediate Reynolds numbers, where both viscous and inertial fluid forces have the potential to play a role in propulsion. The present study aimed to resolve which forces create thrust and drag in the paddling of the water boatman (Corixidae), an animal that spans much of the intermediate regime (10acceleration reaction force. Based on these findings, we developed a forward-dynamic model of propulsion in free swimming that accurately predicted changes in the body's center of mass over time. For both tethered and free swimming, we used non-linear optimization algorithms to determine the force coefficients that best matched our measurements. With this approach, the drag coefficients on the body and paddle were found to be up to three times greater than on static structures in fully developed flow at the same Reynolds numbers. This is likely a partial consequence of unsteady interactions between the paddles or between the paddles and the body. In addition, the maximum values for these coefficients were inversely related to the Reynolds number, which suggests that viscous forces additionally play an important role in the hydrodynamics of small water boatmen. This understanding for the major forces that operate at intermediate Reynolds numbers offers a basis for interpreting the mechanics, energetics and functional morphology of swimming in many small aquatic animals.
Turbulent boundary layer separation control using plasma actuator at Reynolds number 2000000
Institute of Scientific and Technical Information of China (English)
Zhang Xin; Huang Yong; Wang Xunnian; Wang Wanbo; Tang Kun; Li Huaxing
2016-01-01
An experimental investigation was conducted to evaluate the effect of symmetrical plasma actuators on turbulent boundary layer separation control at high Reynolds number. Com-pared with the traditional control method of plasma actuator, the whole test model was made of aluminum and acted as a covered electrode of the symmetrical plasma actuator. The experimental study of plasma actuators’ effect on surrounding air, a canonical zero-pressure gradient turbulent boundary, was carried out using particle image velocimetry (PIV) and laser Doppler velocimetry (LDV) in the 0.75 m ? 0.75 m low speed wind tunnel to reveal the symmetrical plasma actuator characterization in an external flow. A half model of wing-body configuration was experimentally investigated in the £ 3.2 m low speed wind tunnel with a six-component strain gauge balance and PIV. The results show that the turbulent boundary layer separation of wing can be obviously sup-pressed and the maximum lift coefficient is improved at high Reynolds number with the symmetri-cal plasma actuator. It turns out that the maximum lift coefficient increased by approximately 8.98% and the stall angle of attack was delayed by approximately 2? at Reynolds number 2 ? 106. The effective mechanism for the turbulent separation control by the symmetrical plasma actuators is to induce the vortex near the wing surface which could create the relatively large-scale disturbance and promote momentum mixing between low speed flow and main flow regions.
Control of wing-tip vortex using winglets at low Reynolds number
Cho, Seunghyun; Choi, Haecheon
2014-11-01
Winglets are considered as one of the effective devices for reducing induced drag, and thus many studies have been conducted, but mainly at high Reynolds numbers (Re ~106 ~107) for commercial airplanes. However, small-size unmanned air vehicles (UAV), operating at low Reynolds numbers (Re aerodynamic performance of an UAV by varying the cant angle. The WASP UAV model is used and the Reynolds numbers considered are 110,000 ~ 140,000 based on the free stream velocity and mean chord length of the WASP wing. The lift and drag forces on UAV are measured, and PIV measurements are conducted at several cross-flow planes for a few different angles of attack (α) . At high angles of attack (7° ~13°) , the winglets with the cant angle of 70° increase the aerodynamic performance, whereas at low angles of attack (2° ~6°) , the wing-tip extension (cant angle of 0°) shows better performances. The velocity fields measured from PIV indicate that, with the winglet, the wing-tip vortex moves away from the wing surface at α =12° , and the downwash motion in the wake behind the trailing edge is decreased, reducing the magnitude of the induced drag. A concept of changing the cant angle during flight is also suggested at this talk. Supported by 2011-0028032.
Simulation of three-dimensional nonideal MHD flow at low magnetic Reynolds number
Institute of Scientific and Technical Information of China (English)
LU HaoYu; LEE ChunHian
2009-01-01
A numerical procedure based on a five-wave model associated with non-ideal,low magnetic Reynolds number magnetohydrodynamic(MHD)flows was developed.It is composed of an entropy conditioned scheme for solving the non-homogeneous Navier-Stokes equations,in conjunction with an SOR method for solving the elliptic equation governing the electrical potential of flow field.To validate the developed procedure,two different test cases were used which included MHD Rayleigh problem and MHD Hartmann problem.The simulations were performed under the assumption of low magnetic Reynolds number.The simulated results were found to be in good agreement with the closed form analytical solutions deduced in the present study,showing that the present algorithm could simulate engineering MHD flow at low magnetic Reynolds number effectively.In the end,a flow field between a pair of segmented electrodes in a three dimensional MHD channel was simulated using the present algorithm with and without including Hall effects.Without the introduction of Hall effects,no distortion was observed in the current and potential lines.By taking the Hall effects into account,the potential lines distorted and clustered at the upstream and downstream edges of the cathode and anode,respectively.
Large scale Direct Numerical Simulation of premixed turbulent jet flames at high Reynolds number
Attili, Antonio; Luca, Stefano; Lo Schiavo, Ermanno; Bisetti, Fabrizio; Creta, Francesco
2016-11-01
A set of direct numerical simulations of turbulent premixed jet flames at different Reynolds and Karlovitz numbers is presented. The simulations feature finite rate chemistry with 16 species and 73 reactions and up to 22 Billion grid points. The jet consists of a methane/air mixture with equivalence ratio ϕ = 0 . 7 and temperature varying between 500 and 800 K. The temperature and species concentrations in the coflow correspond to the equilibrium state of the burnt mixture. All the simulations are performed at 4 atm. The flame length, normalized by the jet width, decreases significantly as the Reynolds number increases. This is consistent with an increase of the turbulent flame speed due to the increased integral scale of turbulence. This behavior is typical of flames in the thin-reaction zone regime, which are affected by turbulent transport in the preheat layer. Fractal dimension and topology of the flame surface, statistics of temperature gradients, and flame structure are investigated and the dependence of these quantities on the Reynolds number is assessed.
Mach number scaling of helicopter rotor blade/vortex interaction noise
Leighton, Kenneth P.; Harris, Wesley L.
1985-01-01
A parametric study of model helicopter rotor blade slap due to blade vortex interaction (BVI) was conducted in a 5 by 7.5-foot anechoic wind tunnel using model helicopter rotors with two, three, and four blades. The results were compared with a previously developed Mach number scaling theory. Three- and four-bladed rotor configurations were found to show very good agreement with the Mach number to the sixth power law for all conditions tested. A reduction of conditions for which BVI blade slap is detected was observed for three-bladed rotors when compared to the two-bladed baseline. The advance ratio boundaries of the four-bladed rotor exhibited an angular dependence not present for the two-bladed configuration. The upper limits for the advance ratio boundaries of the four-bladed rotors increased with increasing rotational speed.
Two-dimensional lattice Boltzmann model for compressible flows with high Mach number
Gan, Yanbiao; Xu, Aiguo; Zhang, Guangcai; Yu, Xijun; Li, Yingjun
2008-03-01
In this paper we present an improved lattice Boltzmann model for compressible Navier-Stokes system with high Mach number. The model is composed of three components: (i) the discrete-velocity-model by M. Watari and M. Tsutahara [Phys. Rev. E 67 (2003) 036306], (ii) a modified Lax-Wendroff finite difference scheme where reasonable dissipation and dispersion are naturally included, (iii) artificial viscosity. The improved model is convenient to compromise the high accuracy and stability. The included dispersion term can effectively reduce the numerical oscillation at discontinuity. The added artificial viscosity helps the scheme to satisfy the von Neumann stability condition. Shock tubes and shock reflections are used to validate the new scheme. In our numerical tests the Mach numbers are successfully increased up to 20 or higher. The flexibility of the new model makes it suitable for tracking shock waves with high accuracy and for investigating nonlinear nonequilibrium complex systems.
Flow-induced cylinder noise formulated as a diffraction problem for low Mach numbers
Gloerfelt, X.; Pérot, F.; Bailly, C.; Juvé, D.
2005-10-01
The role of surfaces in the mechanism of sound generation by low Mach number flows interacting with solid nonvibrating surfaces is well established by the classical aeroacoustic papers by Powell, Doak, Ffowcs Williams, Crighton, or Howe. It can be formulated as a problem of diffraction of the flow sources by the rigid body. The present study illustrates this statement in the case of flow-induced cylinder noise. Curle's formulation is analytically and numerically compared to a formulation based on an exact Green's function tailored to a cylindrical geometry. The surface integral of Curle's formulation represents exactly the diffraction effects by the rigid body. The direct and scattered parts of the sound field are studied. In this low Mach number configuration, the cylinder is compact, and the scattered (dipole) field dominates the direct (quadrupole) field. The classical properties of the scattering by a cylinder are retrieved by considering a point quadripole source near the cylinder surface.
Extension of the pressure correction method to zero-Mach number compressible flows
Institute of Scientific and Technical Information of China (English)
无
2009-01-01
In the present paper,the classical pressure correction method was extended into low Mach number compressible flow regime by integrating equation of state into SIMPLE algorithm.The self-developed code based on this algorithm was applied to predicting the lid-driven cavity flow and shock tube prob-lems,and the results showed good agreement with benchmark solutions and the Mach number can reach the magnitude of as low as 10-5.The attenuation of sound waves in viscous medium was then simulated.The results agree well with the analytical solutions given by theoretical acoustics.This demonstrated that the present method could also be implemented in acoustics field simulation,which is crucial for thermoacoustic simulation.
The Density Variance--Mach Number Relation in Supersonic Turbulence: I. Isothermal, magnetised gas
Molina, F Z; Federrath, C; Klessen, R S
2012-01-01
It is widely accepted that supersonic, magnetised turbulence plays a fundamental role for star formation in molecular clouds. It produces the initial dense gas seeds out of which new stars can form. However, the exact relation between gas compression, turbulent Mach number, and magnetic field strength is still poorly understood. Here, we introduce and test an analytical prediction for the relation between the density variance and the root-mean-square Mach number in supersonic, isothermal, magnetised turbulent flows. We approximate the density and velocity structure of the interstellar medium as a superposition of shock waves. We obtain the density contrast considering the momentum continuity equation for a single magnetised shock and extrapolate this result to the entire cloud. Depending on the field geometry, we then make three different assumptions based on observational and theoretical constraints: B independent of density, B proportional to the root square of the density and B proportional to the density....
Extension of the pressure correction method to zero-Mach number compressible flows
Institute of Scientific and Technical Information of China (English)
HE YaLing; HUANG Jing; TAO YuBing; TAO WenQuan
2009-01-01
In the present paper, the classical pressure correction method was extended into low Mach number compressible flow regime by integrating equation of state into SIMPLE algorithm. The self-developed code based on this algorithm was applied to predicting the lid-driven cavity flow and shock tube prob-lems, and the results showed good agreement with benchmark solutions and the Mach number can reach the magnitude of as low as 10-5. The attenuation of sound waves in viscous medium was then simulated. The results agree well with the analytical solutions given by theoretical acoustics. This demonstrated that the present method could also be implemented in acoustics field simulation, which is crucial for thermoacoustic simulation.
Axisymmetric vortex method for low-Mach number, diffusion-controlled combustion
Lakkis, I
2003-01-01
A grid-free, Lagrangian method for the accurate simulation of low-Mach number, variable-density, diffusion-controlled reacting flow is presented. A fast-chemistry model in which the conversion rate of reactants to products is limited by the local mixing rate is assumed in order to reduce the combustion problem to the solution of a convection-diffusion-generation equation with volumetric expansion and vorticity generation at the reaction fronts. The solutions of the continuity and vorticity equations, and the equations governing the transport of species and energy, are obtained using a formulation in which particles transport conserved quantities by convection and diffusion. The dynamic impact of exothermic combustion is captured through accurate integration of source terms in the vorticity transport equations at the location of the particles, and the extra velocity field associated with volumetric expansion at low Mach number computed to enforced mass conservation. The formulation is obtained for an axisymmet...
Low Mach and Peclet number limit for a model of stellar tachocline and upper radiative zones
Directory of Open Access Journals (Sweden)
Donatella Donatelli
2016-09-01
Full Text Available We study a hydrodynamical model describing the motion of internal stellar layers based on compressible Navier-Stokes-Fourier-Poisson system. We suppose that the medium is electrically charged, we include energy exchanges through radiative transfer and we assume that the system is rotating. We analyze the singular limit of this system when the Mach number, the Alfven number, the Peclet number and the Froude number approache zero in a certain way and prove convergence to a 3D incompressible MHD system with a stationary linear transport equation for transport of radiation intensity. Finally, we show that the energy equation reduces to a steady equation for the temperature corrector.
Nearfield Unsteady Pressures at Cruise Mach Numbers for a Model Scale Counter-Rotation Open Rotor
Stephens, David B.
2012-01-01
An open rotor experiment was conducted at cruise Mach numbers and the unsteady pressure in the nearfield was measured. The system included extensive performance measurements, which can help provide insight into the noise generating mechanisms in the absence of flow measurements. A set of data acquired at a constant blade pitch angle but various rotor speeds was examined. The tone levels generated by the front and rear rotor were found to be nearly equal when the thrust was evenly balanced between rotors.
Bispen, Georgij; Lukáčová-Medvid'ová, Mária; Yelash, Leonid
2017-04-01
In this paper we will present and analyze a new class of the IMEX finite volume schemes for the Euler equations with a gravity source term. We will in particular concentrate on a singular limit of weakly compressible flows when the Mach number M ≪ 1. In order to efficiently resolve slow dynamics we split the whole nonlinear system in a stiff linear part governing the acoustic and gravity waves and a non-stiff nonlinear part that models nonlinear advection effects. For time discretization we use a special class of the so-called globally stiffly accurate IMEX schemes and approximate the stiff linear operator implicitly and the non-stiff nonlinear operator explicitly. For spatial discretization the finite volume approximation is used with the central and Rusanov/Lax-Friedrichs numerical fluxes for the linear and nonlinear subsystem, respectively. In the case of a constant background potential temperature we prove theoretically that the method is asymptotically consistent and asymptotically stable uniformly with respect to small Mach number. We also analyze experimentally convergence rates in the singular limit when the Mach number tends to zero.
Particle-in-cell simulations of particle energization from low Mach number fast mode shocks
Park, Jaehong; Blackman, Eric G; Ren, Chuang; Siller, Robert
2012-01-01
Astrophysical shocks are often studied in the high Mach number limit but weakly compressive fast shocks can occur in magnetic reconnection outflows and are considered to be a site of particle energization in solar flares. Here we study the microphysics of such perpendicular, low Mach number collisionless shocks using two-dimensional particle-in-cell (PIC) simulations with a reduced ion/electron mass ratio and employ a moving wall boundary method for initial generation of the shock. This moving wall method allows for more control of the shock speed, smaller simulation box sizes, and longer simulation times than the commonly used fixed wall, reflection method of shock formation. Our results, which are independent of the shock formation method, reveal the prevalence shock drift acceleration (SDA) of both electron and ions in a purely perpendicular shock with Alfv\\'en Mach number $M_A=6.8$ and ratio of thermal to magnetic pressure $\\beta=8$. We determine the respective minimum energies required for electrons and ...
A half-explicit, non-split projection method for low Mach number flows.
Energy Technology Data Exchange (ETDEWEB)
Pousin, Jerome G. (National Institute for Applied Sciences, France); Najm, Habib N.; Pebay, Philippe Pierre
2004-02-01
In the context of the direct numerical simulation of low MACH number reacting flows, the aim of this article is to propose a new approach based on the integration of the original differential algebraic (DAE) system of governing equations, without further differentiation. In order to do so, while preserving a possibility of easy parallelization, it is proposed to use a one-step index 2 DAE time-integrator, the Half Explicit Method (HEM). In this context, we recall why the low MACH number approximation belongs to the class of index 2 DAEs and discuss why the pressure can be associated with the constraint. We then focus on a fourth-order HEM scheme, and provide a formulation that makes its implementation more convenient. Practical details about the consistency of initial conditions are discussed, prior to focusing on the implicit solve involved in the method. The method is then evaluated using the Modified KAPS Problem, since it has some of the features of the low MACH number approximation. Numerical results are presented, confirming the above expectations. A brief summary of ongoing efforts is finally provided.
The Dynamics of Very High Alfvén Mach Number Shocks in Space Plasmas
Sundberg, Torbjörn; Burgess, David; Scholer, Manfred; Masters, Adam; Sulaiman, Ali H.
2017-02-01
Astrophysical shocks, such as planetary bow shocks or supernova remnant shocks, are often in the high or very-high Mach number regime, and the structure of such shocks is crucial for understanding particle acceleration and plasma heating, as well inherently interesting. Recent magnetic field observations at Saturn’s bow shock, for Alfvén Mach numbers greater than about 25, have provided evidence for periodic non-stationarity, although the details of the ion- and electron-scale processes remain unclear due to limited plasma data. High-resolution, multi-spacecraft data are available for the terrestrial bow shock, but here the very high Mach number regime is only attained on extremely rare occasions. Here we present magnetic field and particle data from three such quasi-perpendicular shock crossings observed by the four-spacecraft Cluster mission. Although both ion reflection and the shock profile are modulated at the upstream ion gyroperiod timescale, the dominant wave growth in the foot takes place at sub-proton length scales and is consistent with being driven by the ion Weibel instability. The observed large-scale behavior depends strongly on cross-scale coupling between ion and electron processes, with ion reflection never fully suppressed, and this suggests a model of the shock dynamics that is in conflict with previous models of non-stationarity. Thus, the observations offer insight into the conditions prevalent in many inaccessible astrophysical environments, and provide important constraints for acceleration processes at such shocks.
Reynolds number trend of hierarchies and scale interactions in turbulent boundary layers.
Baars, W J; Hutchins, N; Marusic, I
2017-03-13
Small-scale velocity fluctuations in turbulent boundary layers are often coupled with the larger-scale motions. Studying the nature and extent of this scale interaction allows for a statistically representative description of the small scales over a time scale of the larger, coherent scales. In this study, we consider temporal data from hot-wire anemometry at Reynolds numbers ranging from Reτ≈2800 to 22 800, in order to reveal how the scale interaction varies with Reynolds number. Large-scale conditional views of the representative amplitude and frequency of the small-scale turbulence, relative to the large-scale features, complement the existing consensus on large-scale modulation of the small-scale dynamics in the near-wall region. Modulation is a type of scale interaction, where the amplitude of the small-scale fluctuations is continuously proportional to the near-wall footprint of the large-scale velocity fluctuations. Aside from this amplitude modulation phenomenon, we reveal the influence of the large-scale motions on the characteristic frequency of the small scales, known as frequency modulation. From the wall-normal trends in the conditional averages of the small-scale properties, it is revealed how the near-wall modulation transitions to an intermittent-type scale arrangement in the log-region. On average, the amplitude of the small-scale velocity fluctuations only deviates from its mean value in a confined temporal domain, the duration of which is fixed in terms of the local Taylor time scale. These concentrated temporal regions are centred on the internal shear layers of the large-scale uniform momentum zones, which exhibit regions of positive and negative streamwise velocity fluctuations. With an increasing scale separation at high Reynolds numbers, this interaction pattern encompasses the features found in studies on internal shear layers and concentrated vorticity fluctuations in high-Reynolds-number wall turbulence.This article is part of the
Keinan, Eliezer; Nahmias, Yaakov
2015-01-01
Inertial focusing is the migration of particles in fluid toward equilibrium, where current theory predicts that shear-induced and wall-induced lift forces are balanced. First reported in 1961, this Segre-Silberberg effect is particularly useful for microfluidic isolation of cells and particles. Interestingly, recent work demonstrated particle focusing at high Reynolds numbers that cannot be explained by current theory. In this work, we show that non-monotonous velocity profiles, such as those developed in curved channels, create peripheral velocity maxima around which opposing shear-induced forces dominate over wall effects. Similarly, entry effects amplified in high Reynolds flow produce an equivalent trapping mechanism in short, straight channels. This new focusing mechanism in the developing flow regime enables a 10-fold miniaturization of inertial focusing devices, while our model corrects long-standing misconceptions about the nature of mechanical forces governing inertial focusing in curved channels.
Ameri, A. A.; Rigby, D. L.; Steinthorsson, E.; Gaugler, Raymond (Technical Monitor)
2002-01-01
The Low Reynolds number version of the Stress-omega model and the two equation k-omega model of Wilcox were used for the calculation of turbulent heat transfer in a 180 degree turn simulating an internal coolant passage. The Stress-omega model was chosen for its robustness. The turbulent thermal fluxes were calculated by modifying and using the Generalized Gradient Diffusion Hypothesis. The results showed that using this Reynolds Stress model allowed better prediction of heat transfer compared to the k-omega two equation model. This improvement however required a finer grid and commensurately more CPU time.
Flegel, Ashlie Brynn; Giel, Paul W.; Welch, Gerard E.
2014-01-01
The effects of inlet turbulence intensity on the aerodynamic performance of a variable speed power turbine blade are examined over large incidence and Reynolds number ranges. Both high and low turbulence studies were conducted in the NASA Glenn Research Center Transonic Turbine Blade Cascade Facility. The purpose of the low inlet turbulence study was to examine the transitional flow effects that are anticipated at cruise Reynolds numbers. The high turbulence study extends this to LPT-relevant turbulence levels while perhaps sacrificing transitional flow effects. Downstream total pressure and exit angle data were acquired for ten incidence angles ranging from +15.8 to 51.0. For each incidence angle, data were obtained at five flow conditions with the exit Reynolds number ranging from 2.12105 to 2.12106 and at a design exit Mach number of 0.72. In order to achieve the lowest Reynolds number, the exit Mach number was reduced to 0.35 due to facility constraints. The inlet turbulence intensity, Tu, was measured using a single-wire hotwire located 0.415 axial-chord upstream of the blade row. The inlet turbulence levels ranged from 0.25 - 0.4 for the low Tu tests and 8- 15 for the high Tu study. Tu measurements were also made farther upstream so that turbulence decay rates could be calculated as needed for computational inlet boundary conditions. Downstream flow field measurements were obtained using a pneumatic five-hole pitchyaw probe located in a survey plane 7 axial chord aft of the blade trailing edge and covering three blade passages. Blade and endwall static pressures were acquired for each flow condition as well. The blade loading data show that the suction surface separation that was evident at many of the low Tu conditions has been eliminated. At the extreme positive and negative incidence angles, the data show substantial differences in the exit flow field. These differences are attributable to both the higher inlet Tu directly and to the thinner inlet endwall
Agarwal, Shankar
2013-01-01
We calculate the cosmic Mach number M - the ratio of the bulk flow of the velocity field on scale R to the velocity dispersion within regions of scale R. M is effectively a measure of the ratio of large-scale to small-scale power and can be a useful tool to constrain the cosmological parameter space. Using a compilation of existing peculiar velocity surveys, we calculate M and compare it to that estimated from mock catalogues extracted from the LasDamas (a LCDM cosmology) numerical simulations. We find agreement with expectations for the LasDamas cosmology at ~ 1.5 sigma CL. We also show that our Mach estimates for the mocks are not biased by selection function effects. To achieve this, we extract dense and nearly-isotropic distributions using Gaussian selection functions with the same width as the characteristic depth of the real surveys, and show that the Mach numbers estimated from the mocks are very similar to the values based on Gaussian profiles of the corresponding widths. We discuss the importance of ...
High Reynolds number flows about bodies of revolution with application to submarines and torpedoes
Jimenez, Juan M.
The work presented here is an investigation of the wake flow field over a DARPA SUBOFF submarine model at a large range of Reynolds numbers based on model length, 1.1x106 ≤ ReL ≤ 25 x 106, on the centerline of the wake for locations 3, 6, 9, 12, and 15 diameters downstream from the tail. The model is an axisymmetric body without appendages (fins) supported by a streamlined support. The support models the flow of a semi-infinite sail. The wake experimental results, obtained using Particle Image Velocimetry (PIV) and crossed hot-wires, provide qualitative and quantitative insight into the flow field created by a submarine. In addition, the pressure was measured at 45 different locations along the submarine model for three different Reynolds numbers, ReL = 1.1 x 10 6, 12 x 106, and 25 x 106. Also, PIV measurements were conducted in the wake of the sail attached to a DARPA SUBOFF submarine model at ReL = 93.6 x 10 3. Four different yaw angles, 6 ≤ alpha ≤ 17, were investigated yielding insights into the behavior of the junction/hull and sail tip vortices. For all Reynolds numbers studied, the mean velocity distribution attains self-similarity at distances between 3 and 6 diameters downstream for the side where the support is not located, and follows an exponential function as expected from similarity arguments. In contrast, the mean velocity distribution for the support side does not attain self similarity, and displays significant effects of the support wake and support/body junction flows. In addition, none of the Reynolds stress distributions of the flow attain self similarity. For the higher Reynolds numbers studied the presence of the support introduces an asymmetry into the wake which results in the overall decrease of radial and axial turbulence intensities for the support side. Also, the coefficient of pressure, CP, distribution along the top meridian line of the model, r/D > 0, is generally lower for ReL = 1.1 x 106 than that for ReL = 12 x 10 6 and 25
Directory of Open Access Journals (Sweden)
Yacine Khelili
2017-03-01
Full Text Available Numerical investigation of heat transfer phenomena of low Reynolds number nano-fluid flow over an isothermal cylinder is presented in this paper. Steady state governing equations (continuity, N–S and energy equations have been solved using finite volume method. Stationary heat transfer, and flow characteristics over the cylinder have been studied for water based copper nanofluid with different solid fraction values. The effect of volume fraction of nano- particles on the fluid flow and heat transfer were investigated numerically. It was found that at a given Nusselt number, drag coefficient, re-circulation length, and pressure coefficient increase by increasing the volume fraction of nano-particles.
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.
Energy Technology Data Exchange (ETDEWEB)
Zhou, Ye [Lawrence Livermore National Lab. (LLNL), Livermore, CA (United States); Thornber, Ben [The Univ. of Sydney, Sydney, NSW (Australia)
2016-04-12
Here, the implicit large-eddy simulation (ILES) has been utilized as an effective approach for calculating many complex flows at high Reynolds number flows. Richtmyer–Meshkov instability (RMI) induced flow can be viewed as a homogeneous decaying turbulence (HDT) after the passage of the shock. In this article, a critical evaluation of three methods for estimating the effective Reynolds number and the effective kinematic viscosity is undertaken utilizing high-resolution ILES data. Effective Reynolds numbers based on the vorticity and dissipation rate, or the integral and inner-viscous length scales, are found to be the most self-consistent when compared to the expected phenomenology and wind tunnel experiments.
Turbulent boundary layer separation control using plasma actuator at Reynolds number 2000000
Directory of Open Access Journals (Sweden)
Zhang Xin
2016-10-01
Full Text Available An experimental investigation was conducted to evaluate the effect of symmetrical plasma actuators on turbulent boundary layer separation control at high Reynolds number. Compared with the traditional control method of plasma actuator, the whole test model was made of aluminum and acted as a covered electrode of the symmetrical plasma actuator. The experimental study of plasma actuators’ effect on surrounding air, a canonical zero-pressure gradient turbulent boundary, was carried out using particle image velocimetry (PIV and laser Doppler velocimetry (LDV in the 0.75 m × 0.75 m low speed wind tunnel to reveal the symmetrical plasma actuator characterization in an external flow. A half model of wing-body configuration was experimentally investigated in the ∅ 3.2 m low speed wind tunnel with a six-component strain gauge balance and PIV. The results show that the turbulent boundary layer separation of wing can be obviously suppressed and the maximum lift coefficient is improved at high Reynolds number with the symmetrical plasma actuator. It turns out that the maximum lift coefficient increased by approximately 8.98% and the stall angle of attack was delayed by approximately 2° at Reynolds number 2 × 106. The effective mechanism for the turbulent separation control by the symmetrical plasma actuators is to induce the vortex near the wing surface which could create the relatively large-scale disturbance and promote momentum mixing between low speed flow and main flow regions.
Parameter study of simplified dragonfly airfoil geometry at Reynolds number of 6000.
Levy, David-Elie; Seifert, Avraham
2010-10-21
Aerodynamic study of a simplified Dragonfly airfoil in gliding flight at Reynolds numbers below 10,000 is motivated by both pure scientific interest and technological applications. At these Reynolds numbers, the natural insect flight could provide inspiration for technology development of Micro UAV's and more. Insect wings are typically characterized by corrugated airfoils. The present study follows a fundamental flow physics study (Levy and Seifert, 2009), that revealed the importance of flow separation from the first corrugation, the roll-up of the separated shear layer to discrete vortices and their role in promoting flow reattachment to the aft arc, as the leading mechanism enabling high-lift, low drag performance of the Dragonfly gliding flight. This paper describes the effect of systematic airfoil geometry variations on the aerodynamic properties of a simplified Dragonfly airfoil at Reynolds number of 6000. The parameter study includes a detailed analysis of small variations of the nominal geometry, such as corrugation placement or height, rear arc and trailing edge shape. Numerical simulations using the 2D laminar Navier-Stokes equations revealed that the flow accelerating over the first corrugation slope is followed by an unsteady pressure recovery, combined with vortex shedding. The latter allows the reattachment of the flow over the rear arc. Also, the drag values are directly linked to the vortices' magnitude. This parametric study shows that geometric variations which reduce the vortices' amplitude, as reduction of the rear cavity depth or the reduction of the rear arc and trailing edge curvature, will reduce the drag values. Other changes will extend the flow reattachment over the rear arc for a larger mean lift coefficients range; such as the negative deflection of the forward flat plate. These changes consequently reduce the drag values at higher mean lift coefficients. The detailed geometry study enabled the definition of a corrugated airfoil
Nagib, Hassan; Monkewitz, Peter; Österlund, Jens; Christensen, Kenneth; Adrian, Ronald
2001-11-01
Tony Perry, et al. (J. Fluid Mech., v. 439, 2001) have recently contributed to the discussion concerning the reasons for systematic deviations with Re’s (Reynolds numbers) in the Princeton “Super-Pipe” data. Perry et al. demonstrate that the deviation of the constant within the “log-law” is compatible with the “Colebrook formula” for transitionally rough pipes. Since the experiments were completed, Lex Smits and the Princeton Group have argued that the pipe is smooth for at least the majority of the Re range. Here we show that the observed deviations are equally compatible with the finite Re effects obtained from a methodology based on matched asymptotic expansion techniques proposed by us (see abstract at this meeting), in which the infinite-Re limit of the “log-law”, as well as its correction for large but finite Re’s, are derived in a systematic manner. As argued by Perry et al., in these cases one cannot rely on the variation of the centerline velocity with Re to extract the log-law coefficients. The values of the “Karman constant” extracted using either interpretation is significantly lower than the 0.436 value originally proposed and it is closer to the value of 0.38 based on our recent work on boundary layers; see two publications by Österlund et al. (Phys. of Fluids, v. 12 no. 1 and no. 9, 2001). *Supported by NSF, AFOSR & ERCOFTAC.
Kim, Ildoo; Wu, X. L.
2015-10-01
A structure-based Strouhal-Reynolds number relationship, St =1 /(A +B /Re ) , has been recently proposed based on observations of laminar vortex shedding from circular cylinders in a flowing soap film. Since the new St -Re relation was derived from a general physical consideration, it raises the possibility that it may be applicable to vortex shedding from bodies other than circular ones. The work presented herein provides experimental evidence that this is the case. Our measurements also show that, in the asymptotic limit (Re →∞ ), St∞=1 /A ≃0.21 is constant independent of rod shapes, leaving B the only parameter that is shape dependent.
Stratlets: Low Reynolds Number Point-Force Solutions in a Stratified Fluid
Ardekani, A. M.; Stocker, R.
2010-08-01
We present fundamental solutions of low Reynolds number flows in a stratified fluid, including the case of a point force (Stokeslet) and a doublet. Stratification dramatically alters the flow by creating toroidal eddies, and velocity decays much faster than in a homogeneous fluid. The fundamental length scale is set by the competition of buoyancy, diffusion and viscosity, and is O(100μm-1mm) in aquatic environments. Stratification can therefore affect the swimming of small organisms and the sinking of marine snow particles, and diminish the effectiveness of mechanosensing in the ocean.
Turbulent Transport at High Reynolds Numbers in an Inertial Confinement Fusion Context
2014-09-01
of Turbulent Mixing ,” Phys. Scr ., T142, p. 014014. Fig. 4 Turbulent transport as a fraction of total transport plotted versus Re for each of four...Diffusion in Turbulent Mixing ,” Phys. Scr ., T142, p. 014062. [9] George, E., Glimm, J., Grove, J. W., Li, X.-L., Liu, Y.-J., Xu, Z.-L., and Zhao, N., 2003...ABSTRACT Turbulent Transport at High Reynolds Numbers in an Inertial Confinement Fusion Context Report Title Mix is a critical input to hydro
Reynolds number dependence of large-scale friction control in turbulent channel flow
Canton, Jacopo; Örlü, Ramis; Chin, Cheng; Schlatter, Philipp
2016-12-01
The present work investigates the effectiveness of the control strategy introduced by Schoppa and Hussain [Phys. Fluids 10, 1049 (1998), 10.1063/1.869789] as a function of Reynolds number (Re). The skin-friction drag reduction method proposed by these authors, consisting of streamwise-invariant, counter-rotating vortices, was analyzed by Canton et al. [Flow, Turbul. Combust. 97, 811 (2016), 10.1007/s10494-016-9723-8] in turbulent channel flows for friction Reynolds numbers (Reτ) corresponding to the value of the original study (i.e., 104) and 180. For these Re, a slightly modified version of the method proved to be successful and was capable of providing a drag reduction of up to 18%. The present study analyzes the Reynolds number dependence of this drag-reducing strategy by performing two sets of direct numerical simulations (DNS) for Reτ=360 and 550. A detailed analysis of the method as a function of the control parameters (amplitude and wavelength) and Re confirms, on the one hand, the effectiveness of the large-scale vortices at low Re and, on the other hand, the decreasing and finally vanishing effectiveness of this method for higher Re. In particular, no drag reduction can be achieved for Reτ=550 for any combination of the parameters controlling the vortices. For low Reynolds numbers, the large-scale vortices are able to affect the near-wall cycle and alter the wall-shear-stress distribution to cause an overall drag reduction effect, in accordance with most control strategies. For higher Re, instead, the present method fails to penetrate the near-wall region and cannot induce the spanwise velocity variation observed in other more established control strategies, which focus on the near-wall cycle. Despite the negative outcome, the present results demonstrate the shortcomings of the control strategy and show that future focus should be on methods that directly target the near-wall region or other suitable alternatives.
Dixon, G. V.; Barringer, S. R.; Gray, C. E.; Leatherman, A. D.
1975-01-01
Computer programs and resulting tabulations are presented of pipeline length-to-diameter ratios as a function of Mach number and pressure ratios for compressible flow. The tabulations are applicable to air, nitrogen, oxygen, and hydrogen for compressible isothermal flow with friction and compressible adiabatic flow with friction. Also included are equations for the determination of weight flow. The tabulations presented cover a wider range of Mach numbers for choked, adiabatic flow than available from commonly used engineering literature. Additional information presented, but which is not available from this literature, is unchoked, adiabatic flow over a wide range of Mach numbers, and choked and unchoked, isothermal flow for a wide range of Mach numbers.
Modeling the Aerodynamic Lift Produced by Oscillating Airfoils at Low Reynolds Number
Khalid, Muhammad Saif Ullah
2015-01-01
For present study, setting Strouhal Number (St) as control parameter, numerical simulations for flow past oscillating NACA-0012 airfoil at 1,000 Reynolds Numbers (Re) are performed. Temporal profiles of unsteady forces; lift and thrust, and their spectral analysis clearly indicate the solution to be a period-1 attractor for low Strouhal numbers. This study reveals that aerodynamic forces produced by plunging airfoil are independent of initial kinematic conditions of airfoil that proves the existence of limit cycle. Frequencies present in the oscillating lift force are composed of fundamental (fs), even and odd harmonics (3fs) at higher Strouhal numbers. Using numerical simulations, shedding frequencies (f_s) were observed to be nearly equal to the excitation frequencies in all the cases. Unsteady lift force generated due to the plunging airfoil is modeled by modified van der Pol oscillator. Using method of multiple scales and spectral analysis of steady-state CFD solutions, frequencies and damping terms in th...
Tomo-PIV Measurement of High Reynolds Number Dissipation Scale Structures
Worth, Nicholas; Nickels, Timothy
2008-11-01
Understanding the sources of dissipative intermittency in high Reynolds number turbulence is of significant interest, especially given the increasing affordability of LES. Coherent dissipation scale structures have been identified in numerous numerical and experiment investigations, although the latter are typically restricted by the need for accurate resolution of extremely small fast motions. These investigations are therefore often limited to one-dimensional measurements, making the study of these 3D structures and their relationship to the dissipation field difficult. The current investigation employs a very large water mixing tank (2m in diameter), which uses counter-rotating impellors to generate high Reynolds number turbulence (Rλ 1000) that is close to isotropic and homogeneous. The large scale of the tank brings the smallest scales within the resolution of Tomo-PIV, allowing full 3D realization of these structures. This unique experimental setup presents a number of challenges, which include: seeding density limitations imposed by optical attenuation through the tank; demanding light sheet intensity requirements; and the extremely high computational cost of Tomographic reconstruction for the thousands of velocity fields required for statistical analysis. Initial results will be presented along with future plans for measurement refinement.
Gap-flow patterns behind twin-cylinders at low Reynolds number
Energy Technology Data Exchange (ETDEWEB)
Yen, Shun Chang; Liu, Chien Ting [National Taiwan Ocean University, Keelung (China)
2011-11-15
The flow structures, drag coefficients (C{sub d}) and vortex shedding characteristics around a single square cylinder and twin side-by-side square cylinders were experimentally investigated with various Reynolds numbers (Re) and gap ratios (g{sup *}) in a vertical water tunnel. The Reynolds number (Re) and gap ratio (g{sup *}) were 178 < Re < 892 and 0 {<=} g{sup *} {<=} 2.5, respectively. The flow patterns and vortex shedding frequency were determined using the particle tracking flow visualization (PTFV). The flow structures, velocity properties, and drag coefficients were calculated using the particle image velocimetry (PIV). The topological flow patterns of vortex evolution processes were plotted and analyzed based on critical point theory. Furthermore, the flow structures behind twin side-by-side square cylinders were classified into three modes - single vortex-street mode, gap-flow mode and couple vortex-streets mode. The maximum C{sub d} occurred in the single vortex-street mode, and the minimum C{sub d} occurred in the gap-flow mode. The highest Strouhal number (St) occurred in the single vortex-street mode, and the lowest St occurred in the gap-flow mode.
Phenomenology of a flow around a circular cylinder at sub-critical and critical Reynolds numbers
Capone, Alessandro; Klein, Christian; Di Felice, Fabio; Miozzi, Massimo
2016-07-01
In this work, the flow around a circular cylinder is investigated at Reynolds numbers ranging from 79 000 up to 238 000 by means of a combined acquisition system based on Temperature Sensitive Paint (TSP) and particle velocimetry. The proposed setup allows simultaneous and time-resolved measurement of absolute temperature and relative skin friction fields onto the cylinder surface and near-wake velocity field. Combination of time-resolved surface measurements and planar near-field velocity data allows the investigation of the profound modifications undergone by the wall shear stress topology and its connections to the near-field structure as the flow regime travels from the sub-critical to the critical regime. Laminar boundary-layer separation, transition, and re-attachment are analyzed in the light of temperature, relative skin friction maps, and Reynolds stress fields bringing about a new perspective on the relationship between boundary layer development and shear layer evolution. The fast-responding TSP employed allows high acquisition frequency and calculation of power spectral density from surface data. Correlation maps of surface and near-wake data provide insight into the relationship between boundary-layer evolution and vortex shedding. We find that as the Reynolds number approaches the critical state, the separation line oscillations feature an increasingly weaker spectrum peak compared to the near-wake velocity spectrum. In the critical regime, separation line oscillations are strongly reduced and the correlation to the local vorticity undergoes an overall decrease giving evidence of modifications in the vortex shedding mechanism.
Experiments on a low aspect ratio wing at low Reynolds numbers
Morse, Daniel R.
At the start of the 21st century much of the focus of aircraft design has been turned to unmanned aerial vehicles (UAVs) which generally operate at much lower speeds in higher risk areas than manned aircraft. One subset of UAVs are Micro Air Vehicles (MAVs) which usually are no larger than 20cm and rely on non-traditional shapes to generate lift at very low velocities. This purpose of this work is to describe, in detail with experimental methods, the flow field around a low aspect ratio wing operating at low Reynolds numbers and at high angles of attack. Quantitative measurements are obtained by Three Component Time Resolved Particle Image Velocimetry (3C TR PIV) which describe the mean and turbulent flow field. This research focuses on the leading edge separation zone and the vortex shedding process which occurs at the leading edge. Streamwise wing tip vortices which dominate the lift characteristics are described with flow visualization and 3C TR PIV measurements. Turbulent Kinetic Energy (TKE) is described at the leading edge over several angles of attack. Turbulent Reynolds stresses in all three directions are described over the wing span and several Reynolds numbers. Two primary cyclic processes are observed within the flow field; one low frequency oscillation in the separated region and one high frequency event associated with leading edge vortex formation and convection. Two length scales are proposed and are shown to match well with each other, one based on leading edge vortex shedding frequency and convective velocity and the other based on mean vortex separation distance. A new method of rendering velocity frequency content over large data sets is proposed and used to illustrate the different frequencies observed at the leading edge.
Federrath, Christoph; Schober, Jennifer; Banerjee, Robi; Klessen, Ralf S; Schleicher, Dominik R G; 10.1103/PhysRevLett.107.114504
2011-01-01
We study the growth rate and saturation level of the turbulent dynamo in magnetohydrodynamical simulations of turbulence, driven with solenoidal (divergence-free) or compressive (curl-free) forcing. For models with Mach numbers ranging from 0.02 to 20, we find significantly different magnetic field geometries, amplification rates, and saturation levels, decreasing strongly at the transition from subsonic to supersonic flows, due to the development of shocks. Both extreme types of turbulent forcing drive the dynamo, but solenoidal forcing is more efficient, because it produces more vorticity.
The Experimental Measurement of Aerodynamic Heating About Complex Shapes at Supersonic Mach Numbers
Neumann, Richard D.; Freeman, Delma C.
2011-01-01
In 2008 a wind tunnel test program was implemented to update the experimental data available for predicting protuberance heating at supersonic Mach numbers. For this test the Langley Unitary Wind Tunnel was also used. The significant differences for this current test were the advances in the state-of-the-art in model design, fabrication techniques, instrumentation and data acquisition capabilities. This current paper provides a focused discussion of the results of an in depth analysis of unique measurements of recovery temperature obtained during the test.
Experimental Study of High Moisture Content Gas Flow Across a Cylinder at Moderate Reynolds Numbers
Institute of Scientific and Technical Information of China (English)
D. M. Christopher; GUO Liang(郭亮)
2003-01-01
The Nusselt number for cross flow of a mixture of air and vapor over a cylinder was measured at moderate Reynolds numbers (3000-7000) for temperatures from 300℃ to 700℃ and for vapor mass fractions of 0.18-0.35. Results are also presented for a set of three cylinders aligned perpendicular to the flow for the same range of conditions. The effect of the vapor concentration and temperature on the convection coefficients was investigated to develop a modified Zhukauskas correlation. The results show that the Nusselt number increases as the moisture content increases and that the increase is more than could be accounted for by typical models for the property variations of mixtures. The exponent of the vapor concentration term in the modified correlation is 0.145 for the entire data set indicating the importance of the property variation due to the moisture content.
DEFF Research Database (Denmark)
Bhattacharyya, S.; Singh, Ashok
2010-01-01
vorticity on the wake formation is addressed in the present study. The variation of Strouhal number and Nusselt number with the 'effective Reynolds number', is analyzed for different values of cylinder to free stream temperature ratio. Both Strouhal number and the rate of heat transfer increases...... the effective Reynolds number and the computed data for Strouhal number and Nusselt number do not collapse for the range of temperature ratio considered here. The flow field is found to be asymmetric and the cylinder experiences a negative lift. The drag coefficient increases steadily with the rise of surface...... temperature. © 2010 Elsevier Ltd. All rights reserved....
Institute of Scientific and Technical Information of China (English)
Xiao-bin ZHANG; Wei ZHANG; Xue-jun ZHANG
2012-01-01
The volume of fluid (VOF) formulation is applied to model the combustion process of a single droplet in a hightemperature convective air free stream environment.The calculations solve the flow field for both phases,and consider the droplet deformation based on an axisymmetrical model.The chemical reaction is modeled with one-step finite-rate mechanism and the thcrmo-physica1 properties for the gas mixture are species and temperature dependence.A mass transfer model applicable to the VOF calculations due to vaporization of the liquid phases is developed in consideration with the fluctuation of the liquid surface.The model is validated by examining the burning rate constants at different convective air temperatures,which accord well with experimental data of previous studies.Other phenomena from the simulations,such as the transient history of droplet deformation and flame structure,are also qualitatively accordant with the descriptions of other numerical results.However,a different droplet deformation mechanism for the low Reynolds number is explained compared with that for the high Reynolds number.The calculations verified the feasibility of the VOF computational fluid dynamics (CFD) formulation as well as the mass transfer model due to vaporization.
The high Reynolds number flow through an axial-flow pump
Zierke, W. C.; Straka, W. A.; Taylor, P. D.
1993-11-01
The high Reynolds number pump (HIREP) facility at ARL Penn State has been used to perform a low-speed, large-scale experiment of the incompressible flow of water through a two-blade-row turbomachine. HIREP can involve blade chord Reynolds numbers as high as 6,000,000 and can accommodate a variety of instrumentation in both a stationary and a rotating frame of reference. The objectives of this experiment were as follows: to provide a database for comparison with three-dimensional, viscous (turbulent) flow computations; to evaluate the engineering models; and to improve our physical understanding of many of the phenomena involved in this complex flow field. The experimental results include a large quantity of data acquired throughout HIREP. A five-hole probe survey of the inlet flow 37.0 percent chord upstream of the inlet guide vane (IGV) leading edge is sufficient to give information for the inflow boundary conditions, while some static-pressure information is available to help establish an outflow boundary condition.
Sectional lift coefficient of a rotating wing at low Reynolds numbers
Energy Technology Data Exchange (ETDEWEB)
Kim, Ji Eun [Hyundai Motor Company, Hwaseong (Korea, Republic of); Kweon, Ji Hoon [Asan Medical Center, Seoul (Korea, Republic of); Choi, Hae Cheon [Seoul National University, Seoul (Korea, Republic of)
2015-11-15
We study the characteristics of the sectional lift coefficient (C{sub L,S}) of low-aspect-ratio wings in rotating motion at low Reynolds number (Re = 136), by conducting three-dimensional numerical simulations. Three different shapes of thin-plate wings (fruit-fly, rectangular, and triangular wings) are considered but keeping their aspect ratio (wing span/wing chord) the same at 3.74. Each wing rotates at a constant angular velocity and the angle of attack (α) is fixed during rotation. During rotation, the wing is exposed to the downward flows generated from the previous rotation, and thus C{sub L,S} is overall reduced due to the decrease in the effective angle of attack. At low α's, C{sub L,S} becomes almost constant on the whole span. At high α's, C{sub L,S} on the wing mid-section is inversely proportional to the radial position. The radial distribution of the sectional lift coefficient is less affected by the wing planform, while the lift coefficient significantly depends on the wing planforms. Finally, we show that the effect of the Reynolds number on the sectional lift coefficient is insignificant at low angle of attack but becomes important at high angle of attack.
Curling dynamics of naturally curved ribbons from high to low Reynolds numbers
Albarran Arriagada, Octavio; Massiera, Gladys; Abkarian, Manouk
2012-11-01
Curling deformation of thin elastic sheets appears in numerous structures in nature, such as membranes of red blood cells, epithelial tissues or green algae colonies to cite just a few examples. However, despite its ubiquity, the dynamics of curling propagation in a naturally curved material remains still poorly investigated. Here, we present a coupled experimental and theoretical study of the dynamical curling deformation of naturally curved ribbons. Using thermoplastic and metallic ribbons molded on cylinders of different radii, we tune separately the natural curvature and the geometry to study curling dynamics in air, water and in viscous oils, thus spanning a wide range of Reynolds numbers. Our theoretical and experimental approaches separate the role of elasticity, gravity and hydrodynamic dissipation from inertia and emphasize the fundamental differences between the curling of a naturally curved ribbon and a rod described by the classical Elastica. Our work shows evidence for the propagation of a single instability front, selected by a local buckling condition. We show that depending on gravity, and both the Reynolds and the Cauchy numbers, the curling speed and shape are modified by the large scale drag and the local lubrication forces. This work was supported by the French Ministry of Research, the CNRS Physics-Chemistry-Biology Interdisciplinary Pro- gram, the University Montpellier 2 Interdisciplinary Program and the Region Languedoc-Roussillon.
Low Reynolds number turbulence modeling of blood flow in arterial stenoses.
Ghalichi, F; Deng, X; De Champlain, A; Douville, Y; King, M; Guidoin, R
1998-01-01
Moderate and severe arterial stenoses can produce highly disturbed flow regions with transitional and or turbulent flow characteristics. Neither laminar flow modeling nor standard two-equation models such as the kappa-epsilon turbulence ones are suitable for this kind of blood flow. In order to analyze the transitional or turbulent flow distal to an arterial stenosis, authors of this study have used the Wilcox low-Re turbulence model. Flow simulations were carried out on stenoses with 50, 75 and 86% reductions in cross-sectional area over a range of physiologically relevant Reynolds numbers. The results obtained with this low-Re turbulence model were compared with experimental measurements and with the results obtained by the standard kappa-epsilon model in terms of velocity profile, vortex length, wall shear stress, wall static pressure, and turbulence intensity. The comparisons show that results predicted by the low-Re model are in good agreement with the experimental measurements. This model accurately predicts the critical Reynolds number at which blood flow becomes transitional or turbulent distal an arterial stenosis. Most interestingly, over the Re range of laminar flow, the vortex length calculated with the low-Re model also closely matches the vortex length predicted by laminar flow modeling. In conclusion, the study strongly suggests that the proposed model is suitable for blood flow studies in certain areas of the arterial tree where both laminar and transitional/turbulent flows coexist.
Finite volume simulation of 2-D steady square lid driven cavity flow at high reynolds numbers
Directory of Open Access Journals (Sweden)
K. Yapici
2013-12-01
Full Text Available In this work, computer simulation results of steady incompressible flow in a 2-D square lid-driven cavity up to Reynolds number (Re 65000 are presented and compared with those of earlier studies. The governing flow equations are solved by using the finite volume approach. Quadratic upstream interpolation for convective kinematics (QUICK is used for the approximation of the convective terms in the flow equations. In the implementation of QUICK, the deferred correction technique is adopted. A non-uniform staggered grid arrangement of 768x768 is employed to discretize the flow geometry. Algebraic forms of the coupled flow equations are then solved through the iterative SIMPLE (Semi-Implicit Method for Pressure-Linked Equation algorithm. The outlined computational methodology allows one to meet the main objective of this work, which is to address the computational convergence and wiggled flow problems encountered at high Reynolds and Peclet (Pe numbers. Furthermore, after Re > 25000 additional vortexes appear at the bottom left and right corners that have not been observed in earlier studies.
Jet Impingement Heat Transfer at High Reynolds Numbers and Large Density Variations
DEFF Research Database (Denmark)
Jensen, Michael Vincent; Walther, Jens Honore
2010-01-01
Jet impingement heat transfer from a round gas jet to a flat wall has been investigated numerically in a configuration with H/D=2, where H is the distance from the jet inlet to the wall and D is the jet diameter. The jet Reynolds number was 361000 and the density ratio across the wall boundary la...... density from the ideal gas law versus real gas data. In both cases the effect was found to be negligible.......Jet impingement heat transfer from a round gas jet to a flat wall has been investigated numerically in a configuration with H/D=2, where H is the distance from the jet inlet to the wall and D is the jet diameter. The jet Reynolds number was 361000 and the density ratio across the wall boundary....... The results also show a noticeable difference in the heat transfer predictions when applying different turbulence models. Furthermore calculations were performed to study the effect of applying temperature dependent thermophysical properties versus constant properties and the effect of calculating the gas...
The effects of Reynolds number, tip speed ratio, and solidity in VAWTs
Parker, Colin; Schult, Allen; Leftwich, Megan C.
2015-11-01
The wakes of several scale models of vertical axis wind turbines (VAWTs) are investigated in a wind tunnel using particle imaging velocimetry (PIV). The tip speed ratio, Reynolds number, and solidity (chord to diameter ratio) is varied to see effect each parameter. The solidity is changed by varying the chord length of a three blade turbine of constant diameter. The range of parameters (Reynolds number and tip-speed ratio) investigated, closely matches those of full size turbines. Time averaging behind the turbines shows the asymmetry in wake. A more complete picture of the wake is seen using phase averaging by syncing the imaging to the position of the turbine. These results show a cycle of structures developing on the blades and then being shed into the wake. Imaging is done at the midplane of the turbine from upstream of the turbine back into the wake. Additionally a vertical plane behind the center of the turbine is used to measure the horizontal components in the wake.
Vortex Clusters and Their Time Evolution in High- Reynolds-Number Turbulence
Ishihara, Takashi; Uno, Atsuya; Morishita, Koji; Yokokawa, Mitsuo; Kaneda, Yukio
2016-11-01
Time series data (with a time interval of 4τη) obtained by high-resolution direct numerical simulations (DNSs) of forced incompressible turbulence in a periodic box, with a maximum of 122883 grid points and Taylor micro-scale Reynolds numbers Rλ up to 2300, are used to study the vortex dynamics in high Reynolds number (Re) turbulent flows. Here τη is the Kolmogorov time scale. A visualization method to handle such large-scale data was developed for this study. In the high Re turbulence generated by the DNS, we observed the dynamics of tube-like vortex clusters of various sizes, which are constructed by strong micro vortices. For example, we observed the generation of the tube-like clusters of various sizes and the processes of their merging and breakdown. We also observed layer-like vortex clusters of the order of the integral length scale forming shear layers in the high Re turbulence. This research used computational resources of the K computer and other computers of the HPCI system provided by the AICS and the ITC of Nagoya University through the HPCI System Research Project (Project ID:hp150174, hp160102).
Improvements of a nano-scale crossed hot-wire for high Reynolds number measurements
Fan, Yuyang; Hultmark, Marcus
2015-11-01
Hot-wire anemometry, despite its limited spatial and temporal resolution, is still the preferred tool for high Reynolds number flow measurements, mainly due to the continuous signal. To address the resolution issues, the Nano-Scale Thermal Anemometry Probe (NSTAP) was developed at Princeton University. The NSTAP has a sensing volume more than one order of magnitude smaller than conventional hot-wires, and it has displayed superior performance. However, the NSTAP can only measure a single component of the velocity. Using a novel combining method, a probe that enables two-component velocity measurements has been created (the x-NSTAP). The measurement volume is approximately 50 × 50 × 50 μ m, more than one order of magnitude smaller in all directions compared to conventional crossed hot-wires. The x-NSTAP has been further improved to allow more accurate measurements with the help of flow visualization using a scaled model but matching Reynolds number. Results from turbulent flow measurements with the new x-NSTAP are also presented. Supported under NSF grant CBET-1510100 (program manager Dimitrios Papavassiliou).
Effect of ambient flow inhomogeneity on drag forces on a sphere at finite Reynolds numbers
Kim, Jungwoo; Balachandar, S.; Lee, Hyungoo
2013-11-01
For studies on particle-laden flows involving particle transport and dispersion, the prediction capability of hydrodynamic forces on the particle in a non-uniform flow is one of the central issues. However, existing analytical expressions and empirical correlations are mainly made based on the homogeneous flow conditions such as uniform or uniform shear flows. Therefore, the objective of this study is to investigate the effect of flow inhomogeneity on drag forces on a sphere at finite Reynolds numbers. To do so, we perform direct numerical simulations of flow over a sphere in an inhomogeneous flow. In this study, we consider three different kinds of the inhomogeneous flows: cosine, hyperbolic cosine and hyperbolic secant profiles. The Reynolds number of the sphere based on the freestream velocity and sphere diameter is 100. The present simulations show that the quasi-steady drag forces in inhomogeneous flows are reasonably estimated by standard drag law based on the relative velocity if the fluid velocity seen by the particle is evaluated by surface average. The results support Loth and Dorgan (2009)'s proposed formula. In the final presentation, the effect of ambient flow inhomogeneity on drag forces would be presented in more detail.
Characteristics of low reynolds number shear-free turbulence at an impermeable base.
Wan Mohtar, W H M; ElShafie, A
2014-01-01
Shear-free turbulence generated from an oscillating grid in a water tank impinging on an impermeable surface at varying Reynolds number 74 ≤ Re(l) ≤ 570 was studied experimentally, where the Reynolds number is defined based on the root-mean-square (r.m.s) horizontal velocity and the integral length scale. A particular focus was paid to the turbulence characteristics for low Re(l) < 150 to investigate the minimum limit of Re l obeying the profiles of rapid distortion theory. The measurements taken at near base included the r.m.s turbulent velocities, evolution of isotropy, integral length scales, and energy spectra. Statistical analysis of the velocity data showed that the anisotropic turbulence structure follows the theory for flows with Re(l) ≥ 117. At low Re(l) < 117, however, the turbulence profile deviated from the prediction where no amplification of horizontal velocity components was observed and the vertical velocity components were seen to be constant towards the tank base. Both velocity components sharply decreased towards zero at a distance of ≈ 1/3 of the integral length scale above the base due to viscous damping. The lower limit where Re(l) obeys the standard profile was found to be within the range 114 ≤ Re(l) ≤ 116.
Jet Impingement Heat Transfer at High Reynolds Numbers and Large Density Variations
DEFF Research Database (Denmark)
Jensen, Michael Vincent; Walther, Jens Honore
2010-01-01
Jet impingement heat transfer from a round gas jet to a flat wall has been investigated numerically in a configuration with H/D=2, where H is the distance from the jet inlet to the wall and D is the jet diameter. The jet Reynolds number was 361000 and the density ratio across the wall boundary la...... density from the ideal gas law versus real gas data. In both cases the effect was found to be negligible........ The results also show a noticeable difference in the heat transfer predictions when applying different turbulence models. Furthermore calculations were performed to study the effect of applying temperature dependent thermophysical properties versus constant properties and the effect of calculating the gas......Jet impingement heat transfer from a round gas jet to a flat wall has been investigated numerically in a configuration with H/D=2, where H is the distance from the jet inlet to the wall and D is the jet diameter. The jet Reynolds number was 361000 and the density ratio across the wall boundary...
Transport coefficients for the shear dynamo problem at small Reynolds numbers.
Singh, Nishant K; Sridhar, S
2011-05-01
We build on the formulation developed in S. Sridhar and N. K. Singh [J. Fluid Mech. 664, 265 (2010)] and present a theory of the shear dynamo problem for small magnetic and fluid Reynolds numbers, but for arbitrary values of the shear parameter. Specializing to the case of a mean magnetic field that is slowly varying in time, explicit expressions for the transport coefficients α(il) and η(il) are derived. We prove that when the velocity field is nonhelical, the transport coefficient α(il) vanishes. We then consider forced, stochastic dynamics for the incompressible velocity field at low Reynolds number. An exact, explicit solution for the velocity field is derived, and the velocity spectrum tensor is calculated in terms of the Galilean-invariant forcing statistics. We consider forcing statistics that are nonhelical, isotropic, and delta correlated in time, and specialize to the case when the mean field is a function only of the spatial coordinate X(3) and time τ; this reduction is necessary for comparison with the numerical experiments of A. Brandenburg, K. H. Rädler, M. Rheinhardt, and P. J. Käpylä [Astrophys. J. 676, 740 (2008)]. Explicit expressions are derived for all four components of the magnetic diffusivity tensor η(il)(τ). These are used to prove that the shear-current effect cannot be responsible for dynamo action at small Re and Rm, but for all values of the shear parameter.
Spectrum of a passive scalar in stretched grid turbulence at low Reynolds numbers
Lee, S. K.; Djenidi, L.; Antonia, R. A.; Rajagopalan, S.
2011-12-01
Approximately homogeneous isotropic turbulence is obtained by stretching a wind-tunnel grid flow with a 1.36:1 contraction. The flow is mildly heated so that temperature serves as a passive scalar. For three different grids, the dissipation rates and spectra of velocity and temperature fluctuations are obtained from simultaneous hot-wire and cold-wire measurements. The dissipation rates follow a power-law decay. Comparison with an unstretched grid flow shows that the contraction improves the isotropy and reduces the effect of grid shape on the decay exponents. At low Reynolds numbers, there is a significant scaling range for the temperature spectrum but not for the velocity spectrum. With stretching, the temperature spectrum shows a wider scaling range, and that the scaling range exponent is closer to 5/3. The scaling exponent for the temperature spectrum (mθ) is represented by a power-law function of Reynolds number, and it approaches 5/3 faster than that for the velocity spectrum (mu). Results show that the ratio between the velocity and temperature scaling range exponents, (5/3+mu)/mθ, is about 1.98.
A passive planar micromixer with obstructions for mixing at low Reynolds numbers
Bhagat, Ali Asgar S.; Peterson, Erik T. K.; Papautsky, Ian
2007-05-01
Passive mixers rely on the channel geometry to mix fluids. However, many previously reported designs either work efficiently only at moderate to high Reynolds numbers (Re), or require a complex 3D channel geometry that is often difficult to fabricate. In this paper, we report design, simulation, fabrication and characterization of a planar passive microfluidic mixer capable of mixing at low Reynolds numbers. The design incorporates diamond-shaped obstructions within the microchannel to break-up and recombine the flow. Simulation and experimental results of the developed micromixer show excellent mixing performance over a wide range of flow conditions (numerically: 0.01 < Re < 100, experimentally: 0.02 < Re < 10). The micromixer is also characterized by low pressure drop, an important characteristic for integration into complex, cascading microfluidic systems. Due to the simple planar structure of the micromixer, it can be easily realized and integrated with on-chip microfluidic systems, such as micro total analysis systems (μTAS) or lab on a chip (LOC).
Time resolved, near wall PIV measurements in a high Reynolds number turbulent pipe flow
Willert, C.; Soria, J.; Stanislas, M.; Amili, O.; Bellani, G.; Cuvier, C.; Eisfelder, M.; Fiorini, T.; Graf, N.; Klinner, J.
2016-11-01
We report on near wall measurements of a turbulent pipe flow at shear Reynolds numbers up to Reτ = 40000 acquired in the CICLoPE facility near Bologna, Italy. With 900 mm diameter and 110 m length the facility offers a well-established turbulent flow with viscous length scales ranging from y+ = 85 μ m at Reτ = 5000 to y+ = 11 μ m at Reτ = 40000 . These length scales can be resolved with a high-speed PIV camera at image magnification near unity. For the measurement the light of a high-speed, double-pulse laser is focused into a 300 μ m thin light sheet that is introduced radially into the pipe. The light scattered by 1 μ m water-glycerol droplet seeding is observed from the side by the camera via a thin high-aspect ratio mirror with a field of view covering 20mm in wall-normal and 5mm in stream-wise direction. Statistically converged velocity profiles could be achieved using 70000 samples per sequence acquired at low laser repetition rates (100Hz). Higher sampling rates of 10 kHz provide temporally coherent data from which frequency spectra can be derived. Preliminary analysis of the data shows a well resolved inner peak that grows with increasing Reynolds number. (Project funding through EuHIT - www.euhit.org)
Institute of Scientific and Technical Information of China (English)
ZHANG Ling; ZHOU Jun-li; CHEN Xiao-chun; LAN Li; ZHANG Nan
2008-01-01
ABE-KONDOH-NAGANO, ABID, YANG-SHIH and LAUNDER-SHARMA low-Reynolds number turbulence models were applied to simulating unsteady turbulence flow around a square cylinder in different phases flow field and time-averaged unsteady flow field. Meanwhile, drag and lift coefficients of the four different low-Reynolds number turbulence models were analyzed. The simulated results of YANG-SHIH model are close to the large eddy simulation results and experimental results, and they are significantly better than those of ABE-KONDOH-NAGANO, ABID and LAUNDER-SHARMR models. The modification of the generation of turbulence kinetic energy is the key factor to a successful simulation for YANG-SHIH model, while the correction of the turbulence near the wall has minor influence on the simulation results. For ABE-KONDOH-NAGANO, ABID and LAUNDER-SHARMA models satisfactory simulation results cannot be obtained due to lack of the modification of the generation of turbulence kinetic energy. With the joint force of wall function and the turbulence models with the adoption of corrected swirl stream,flow around a square cylinder can be fully simulated with less grids by the near-wall.
Effects of Reynolds and Womersley Numbers on the Hemodynamics of Intracranial Aneurysms
Asgharzadeh, Hafez
2016-01-01
The effects of Reynolds and Womersley numbers on the hemodynamics of two simplified intracranial aneurysms (IAs), that is, sidewall and bifurcation IAs, and a patient-specific IA are investigated using computational fluid dynamics. For this purpose, we carried out three numerical experiments for each IA with various Reynolds (Re = 145.45 to 378.79) and Womersley (Wo = 7.4 to 9.96) numbers. Although the dominant flow feature, which is the vortex ring formation, is similar for all test cases here, the propagation of the vortex ring is controlled by both Re and Wo in both simplified IAs (bifurcation and sidewall) and the patient-specific IA. The location of the vortex ring in all tested IAs is shown to be proportional to Re/Wo2 which is in agreement with empirical formulations for the location of a vortex ring in a tank. In sidewall IAs, the oscillatory shear index is shown to increase with Wo and 1/Re because the vortex reached the distal wall later in the cycle (higher resident time). However, this trend was not observed in the bifurcation IA because the stresses were dominated by particle trapping structures, which were absent at low Re = 151.51 in contrast to higher Re = 378.79. PMID:27847544
Reyt, Ida; Bailliet, Hélène; Valière, Jean-Christophe
2014-01-01
Measurements of streaming velocity are performed by means of Laser Doppler Velocimetry and Particle Image Velociimetry in an experimental apparatus consisting of a cylindrical waveguide having one loudspeaker at each end for high intensity sound levels. The case of high nonlinear Reynolds number ReNL is particularly investigated. The variation of axial streaming velocity with respect to the axial and to the transverse coordinates are compared to available Rayleigh streaming theory. As expected, the measured streaming velocity agrees well with the Rayleigh streaming theory for small ReNL but deviates significantly from such predictions for high ReNL. When the nonlinear Reynolds number is increased, the outer centerline axial streaming velocity gets distorted towards the acoustic velocity nodes until counter-rotating additional vortices are generated near the acoustic velocity antinodes. This kind of behavior is followed by outer streaming cells only and measurements in the near wall region show that inner streaming vortices are less affected by this substantial evolution of fast streaming pattern. Measurements of the transient evolution of streaming velocity provide an additional insight into the evolution of fast streaming.
Hogg, Charlie A. R.; Dalziel, Stuart B.; Huppert, Herbert E.; Imberger, Jörg
2015-09-01
In many important natural and industrial systems, gravity currents of dense fluid feed basins. Examples include lakes fed by dense rivers and auditoria supplied with cooled air by ventilation systems. As we will show, the entrainment into such buoyancy driven currents can be influenced by viscous forces. Little work, however, has examined this viscous influence and how entrainment varies with the Reynolds number, Re. Using the idea of an entrainment coefficient, E, we derive a mathematical expression for the rise of the front at the top of the dense fluid ponding in a basin, where the horizontal cross-sectional area of the basin varies linearly with depth. We compare this expression to experiments on gravity currents with source Reynolds numbers, Res, covering the broad range 100 < Res < 1500. The form of the observed frontal rises was well approximated by our theory. By fitting the observed frontal rises to the theoretical form with E as the free parameter, we find a linear trend for E(Res) over the range 350 < Res < 1100, which is in the transition to turbulent flow. In the experiments, the entrainment coefficient, E, varied from 4 × 10-5 to 7 × 10-2. These observations show that viscous damping can be a dominant influence on gravity current entrainment in the laboratory and in geophysical flows in this transitional regime.
Three-dimensional effects on airfoil measurements at high Reynolds numbers
Kiefer, Janik; Miller, Mark; Hultmark, Marcus; Hansen, Martin
2016-11-01
Blade Element Momentum codes (BEM) are widely used in the wind turbine industry to determine a turbine's operational range and its limits. Empirical two-dimensional airfoil data serve as the primary and fundamental input to the BEM code. Consequently, the results of BEM simulations are strongly dependent on the accuracy of these data. In this presentation, an experimental study is described in which airfoils of different aspect ratios were tested at identical Reynolds numbers. A high-pressure wind tunnel facility is used to achieve large Reynolds numbers of Rec = 3 ×106 , even with small chord lengths. This methodology enables testing of very high aspect ratio airfoils to characterize 3-D effects on the lift and drag data. The tests were performed over a large range of angles of attack, which is especially important for wind turbines. The effect of varying aspect ratio on the aerodynamic characteristics of the airfoil is discussed with emphasis on the outcome of a BEM simulation. The project was partially funded by NSF CBET-1435254 (program manager Dr. Gregory Rorrer).
Convective heat transport in stratified atmospheres at low and high Mach number
Anders, Evan H
2016-01-01
Convection in astrophysical systems is stratified and often occurs at high Rayleigh number (Ra) and low Mach number (Ma). Here we study stratified convection in the context of plane-parallel, polytropically stratified atmospheres. We hold the density stratification ($n_{\\rho}$) and Prandtl number (Pr) constant while varying Ma and Ra to determine the behavior of the Nusselt number (Nu), which quantifies the efficiency of convective heat transport. As Ra increases and $\\text{Ma} \\rightarrow 1$, a scaling of Nu $\\propto$ Ra$^{0.45}$ is observed. As Ra increases to a regime where Ma $\\geq 1$, this scaling gives way to a weaker Nu $\\propto$ Ra$^{0.19}$. In the regime of Ma $\\ll 1$, a consistent Nu $\\propto$ Ra$^{0.31}$ is retrieved, reminiscent of the Nu $\\propto$ Ra$^{2/7}$ seen in Rayleigh-B\\'{e}nard convection.
Allan Brian G.; Owens, Lewis, R.
2006-01-01
This paper will investigate the validation of a NASA developed, Reynolds-averaged Navier-Stokes (RANS) flow solver, OVERFLOW, for a boundary-layer-ingesting (BLI) offset (S-shaped) inlet in transonic flow with passive and active flow control devices as well as the baseline case. Numerical simulations are compared to wind tunnel results of a BLI inlet conducted at the NASA Langley 0.3-Meter Transonic Cryogenic Tunnel. Comparisons of inlet flow distortion, pressure recovery, and inlet wall pressures are performed. The numerical simulations are compared to the BLI inlet data at a freestream Mach number of 0.85 and a Reynolds number of approximately 2 million based on the length of the fan-face diameter. The numerical simulations with and without wind tunnel walls are performed, quantifying effects of the tunnel walls on the BLI inlet flow measurements. The wind tunnel test evaluated several different combinations of jet locations and mass flow rates as well as a vortex generator (VG) vane case. The numerical simulations will be performed on a single jet configuration for varying actuator mass flow rates at a fix inlet mass flow condition. Validation of the numerical simulations for the VG vane case will also be performed for varying inlet mass flow rates. Overall, the numerical simulations were able to predict the baseline circumferential flow distortion, DPCPavg, very well for comparisons made within the designed operating range of the BLI inlet. However the CFD simulations did predict a total pressure recovery that was 0.01 lower than the experiment. Numerical simulations of the baseline inlet flow also showed good agreement with the experimental inlet centerline surface pressures. The vane case showed that the CFD predicted the correct trends in the circumferential distortion for varying inlet mass flow but had a distortion level that was nearly twice as large as the experiment. Comparison to circumferential distortion measurements for a 15 deg clocked 40 probe
Specularly reflected He sup 2+ at high Mach number quasi-parallel shocks
Energy Technology Data Exchange (ETDEWEB)
Fuselier, S.A.; Lennartsson, O.W. (Lockheed Palo Alto Research Lab., CA (United States)); Thomsen, M.F. (Los Alamos National Lab., NM (United States)); Russell, C.T. (Univ. of California, Los Angeles (United States))
1990-04-01
Upstream from the Earth's quasi-parallel bow shock, the Lockheed Plasma Composition Experiment on ISEE 1 often observes two types of suprathermal He{sup 2+} distributions. Always present to some degree is an energetic (several keV/eto 17.4 keV/e, the maximum energy of the detector) diffuse He{sup 2+} distribution. Sometimes, apparently when the Alfven Mach number, M{sub A}, is high enough and the spacecraft is near the shock (within a few minutes of a crossing), a second type of suprathermal He{sup 2+} distribution is also observed. This nongyrotropic, gyrating He{sup 2+} distribution has velocity components parallel and perpendicular to the magnetic field that are consistent with near-specular reflection of a portion of the incident solar wind He{sup 2+} distribution off the shock. Specularly reflected and diffuse proton distributions are associated with these gyrating He{sup 2+} distributions. The presence of these gyrating He{sup 2+} distributions suggests that specular reflection is controlled primarily by magnetic forces in high Mach number quasi-parallel shocks and that these distributions may be a seed population for more energetic diffuse He{sup 2+} distributions.
The influence of incident shock Mach number on radial incident shock wave focusing
Directory of Open Access Journals (Sweden)
Xin Chen
2016-04-01
Full Text Available Experiments and numerical simulations were carried out to investigate radial incident shock focusing on a test section where the planar incident shock wave was divided into two identical ones. A conventional shock tube was used to generate the planar shock. Incident shock Mach number of 1.51, 1.84 and 2.18 were tested. CCD camera was used to obtain the schlieren photos of the flow field. Third-order, three step strong-stability-preserving (SSP Runge-Kutta method, third-order weighed essential non-oscillation (WENO scheme and adaptive mesh refinement (AMR algorithm were adopted to simulate the complicated flow fields characterized by shock wave interaction. Good agreement between experimental and numerical results was observed. Complex shock wave configurations and interactions (such as shock reflection, shock-vortex interaction and shock focusing were observed in both the experiments and numerical results. Some new features were observed and discussed. The differences of structure of flow field and the variation trends of pressure were compared and analyzed under the condition of different Mach numbers while shock wave focusing.
The density variance - Mach number relation in isothermal and non-isothermal adiabatic turbulence
Nolan, Chris A; Sutherland, Ralph S
2015-01-01
The density variance - Mach number relation of the turbulent interstellar medium is relevant for theoretical models of the star formation rate, efficiency, and the initial mass function of stars. Here we use high-resolution hydrodynamical simulations with grid resolutions of up to 1024^3 cells to model compressible turbulence in a regime similar to the observed interstellar medium. We use Fyris Alpha, a shock-capturing code employing a high-order Godunov scheme to track large density variations induced by shocks. We investigate the robustness of the standard relation between the logarithmic density variance (sigma_s^2) and the sonic Mach number (M) of isothermal interstellar turbulence, in the non-isothermal regime. Specifically, we test ideal gases with diatomic molecular (gamma = 7/5) and monatomic (gamma = 5/3) adiabatic indices. A periodic cube of gas is stirred with purely solenoidal forcing at low wavenumbers, leading to a fully-developed turbulent medium. We find that as the gas heats in adiabatic comp...
A NOVEL SLIGHTLY COMPRESSIBLE MODEL FOR LOW MACH NUMBER PERFECT GAS FLOW CALCULATION
Institute of Scientific and Technical Information of China (English)
邓小刚; 庄逢甘
2002-01-01
By analyzing the characteristics of low Mach number perfect gas flows, a novel Slightly Compressible Model (SCM) for low Mach number perfect gas flows is derived. In view of numerical calculations, this model is proved very efficient,for it is kept within the p-v frame but does not have to satisfy the time consuming divergence-free condition in order to get the incompressible Navier-Stokes equation solutions. Writing the equations in the form of conservation laws, we have derived the characteristic systems which are necessary for numerical calculations. A cellcentered finite-volume method with flux difference upwind-biased schemes is used for the equation solutions and a new Exact Newton Relaxation (ENR) implicit method is developed. Various computed results are presented to validate the present model.Laminar flow solutions over a circular cylinder with wake developing and vortex shedding are presented. Results for inviscid flow over a sphere are compared in excellent agreement with the exact analytic incompressible solution. Three-dimensional viscous flow solutions over sphere and prolate spheroid are also calculated and compared well with experiments and other incompressible solutions. Finally, good convergent performaces are shown for sphere viscous flows.
Airfoil Aeroelastic Flutter Analysis Based on Modified Leishman-Beddoes Model at Low Mach Number
Institute of Scientific and Technical Information of China (English)
SHAO Song; ZHU Qinghua; ZHANG Chenglin; NI Xianping
2011-01-01
Based on modified Leishman-Beddoes(L-B)state space model at low Mach number(lower than 0.3),the airfoil aeroelastic system is presented in this paper.The main modifications for L-B model include a new dynamic stall criterion and revisions of normal force and pitching moment coefficient.The bifurcation diagrams,the limit cycle oscillation (LCO)phase plane plots and the time domain response figures are applied to investigating the stall flutter bifurcation behavior of airfoil aeroelastic systems with symmetry or asymmetry.It is shown that the symmetric periodical oscillation happens after subcritical bifurcation caused by dynamic stall,and the asymmetric periodical oscillation,which is caused by the interaction of dynamic stall and static divergence,only happens in the airfoil aeroelastic system with asymmetry.Validations of the modified L-B model and the airfoil aeroelastic system are presented with the experimental airload data of NACA0012 and OA207 and experimental stall flutter data of NACA0012 respectively.Results demonstrate that the airfoil aeroelastic system presented in this paper is effective and accurate,which can be applied to the investigation of airfoil stall flutter at low Mach number.
Large eddy simulation of the FDA benchmark nozzle for a Reynolds number of 6500.
Janiga, Gábor
2014-04-01
This work investigates the flow in a benchmark nozzle model of an idealized medical device proposed by the FDA using computational fluid dynamics (CFD). It was in particular shown that a proper modeling of the transitional flow features is particularly challenging, leading to large discrepancies and inaccurate predictions from the different research groups using Reynolds-averaged Navier-Stokes (RANS) modeling. In spite of the relatively simple, axisymmetric computational geometry, the resulting turbulent flow is fairly complex and non-axisymmetric, in particular due to the sudden expansion. The resulting flow cannot be well predicted with simple modeling approaches. Due to the varying diameters and flow velocities encountered in the nozzle, different typical flow regions and regimes can be distinguished, from laminar to transitional and to weakly turbulent. The purpose of the present work is to re-examine the FDA-CFD benchmark nozzle model at a Reynolds number of 6500 using large eddy simulation (LES). The LES results are compared with published experimental data obtained by Particle Image Velocimetry (PIV) and an excellent agreement can be observed considering the temporally averaged flow velocities. Different flow regimes are characterized by computing the temporal energy spectra at different locations along the main axis.
Structure Functions in Wall-bounded Flows at High Reynolds Number
Yang, Xiang; Marusic, Ivan; Johnson, Perry; Meneveau, Charles
2016-11-01
The scaling of the structure function Dij = (where i = 1,2,3 and r is the two-point displacement, ui is the velocity fluctuation in the xi direction), is studied in wall-bounded flows at high Reynolds number within the framework of the Townsend attached eddy model. While the scaling of Dij has been the subject of several studies, previous work focused on the scaling of D11 for r = (Δx ,0,0) (for streamwise velocity component and displacements only in the streamwise direction). Using the Hierarchical-Random-Additive formalism, a recently developed attached-eddy formalism, we propose closed-form formulae for the structure functionDij with two-point displacements in arbitrary directions, focusing on the log region . The work highlights new scalings that have received little attention, e.g. the scaling of Dij for r =(0, Δy, Δz) and for i ≠ j . As the knowledge on Dij leads directly to that of the Reynolds stress, statistics of the filtered flow field, etc., an analytical formula of Dij for arbitrary r can be quite useful for developing physics-based models for wall-bounded flows and validating existing LES and reduced order models.
The effect of tip speed ratio on a vertical axis wind turbine at high Reynolds numbers
Parker, Colin M.; Leftwich, Megan C.
2016-05-01
This work visualizes the flow surrounding a scaled model vertical axis wind turbine at realistic operating conditions. The model closely matches geometric and dynamic properties—tip speed ratio and Reynolds number—of a full-size turbine. The flow is visualized using particle imaging velocimetry (PIV) in the midplane upstream, around, and after (up to 4 turbine diameters downstream) the turbine, as well as a vertical plane behind the turbine. Time-averaged results show an asymmetric wake behind the turbine, regardless of tip speed ratio, with a larger velocity deficit for a higher tip speed ratio. For the higher tip speed ratio, an area of averaged flow reversal is present with a maximum reverse flow of -0.04U_∞. Phase-averaged vorticity fields—achieved by syncing the PIV system with the rotation of the turbine—show distinct structures form from each turbine blade. There were distinct differences in results by tip speed ratios of 0.9, 1.3, and 2.2 of when in the cycle structures are shed into the wake—switching from two pairs to a single pair of vortices being shed—and how they convect into the wake—the middle tip speed ratio vortices convect downstream inside the wake, while the high tip speed ratio pair is shed into the shear layer of the wake. Finally, results show that the wake structure is much more sensitive to changes in tip speed ratio than to changes in Reynolds number.
Mass transfer controlled reactions in packed beds at low Reynolds numbers
Energy Technology Data Exchange (ETDEWEB)
Fedkiw, P.S.
1978-12-01
The a priori prediction and correlation of mass-transfer rates in transport limited, packed-bed reactors at low Reynolds numbers is examined. The solutions to the governing equations for a flow-through porous electrode reactor indicate that these devices must operate at a low space velocity to suppress a large ohmic potential drop. Packed-bed data for the mass-transfer rate at such low Reynolds numbers were examined and found to be sparse, especially in liquid systems. Prior models to simulate the solid-void structure in a bed are reviewed. Here the bed was envisioned as an array of sinusoidal periodically constricted tubes (PCT). Use of this model has not appeared in the literature. The velocity field in such a tube should be a good approximation to the converging-diverging character of the velocity field in an actual bed. The creeping flow velocity profiles were calculated. These results were used in the convective-diffusion equation to find mass transfer rates at high Peclet number for both deep and shallow beds, for low Peclet numbers in a deep bed. All calculations assumed that the reactant concentration at the tube surface is zero. Mass-transfer data were experimentally taken in a transport controlled, flow-through porous electrode to test the theoretical calculations and to provide data resently unavailable for deeper beds. It was found that the sinusoidal PCT model could not fit the data of this work or that available in the literature. However, all data could be adequately described by a model which incorporates a channelingeffect. The bed was successfully modeled as an array of dual sized straight tubes.
Low Reynolds number flow in rectangular cooling channels provided with low aspect ratio pin fins
Energy Technology Data Exchange (ETDEWEB)
Armellini, Alessandro; Casarsa, Luca [Dipartimento di Energetica e Macchine, Universita di Udine, Via delle Scienze 208, 33100 Udine (Italy); Giannattasio, Pietro, E-mail: pietro.giannattasio@uniud.i [Dipartimento di Energetica e Macchine, Universita di Udine, Via delle Scienze 208, 33100 Udine (Italy)
2010-08-15
The flow structures around single heat transfer promoters of different shapes (square, circular, triangular and rhomboidal) have been investigated experimentally by means of a 2-D Particle Image Velocimetry (PIV) technique. The geometrical configuration and flow conditions considered are typical of real liquid cooling channels. They include low aspect ratio pin fins confined at both ends by the walls of a rectangular channel, water flow at low Reynolds numbers (Re = 800, 1800, 2800), high core flow turbulence and undeveloped boundary layers at the position of the obstacle. In front of the pin fins the high turbulence level is found to promote a strong instability of the horseshoe vortex system that forms at the wall/obstacle junction. In particular, frequent events of break-away of the primary vortices and inrush of core fluid, which are known to enhance the wall heat transfer, are observed in the cases of square and circular pins already from Re = 1800. The near wake downstream of the obstacles appears to be influenced by streamwise oriented vortical structures produced at the wall/obstacle junction. They give rise to spanwise velocity components (up-wash flow) that lead to a three-dimensional mass recirculation behind the pins. The combination of up-wash flows, low Reynolds number and high core flow turbulence gives rise to a competition between the classical alternate vortex shedding and an irregular shedding mode characterized by the decoupling of the shear layers and the absence of well organized primary structures. At Re = 800, the irregular shedding prevails and the mean wake topology is almost insensitive to the obstacle shape. As the Reynolds number is increased, the junction flow structures reduce in size and strength, their effect on the wake flow weakens and the recirculation structures behind the obstacles differentiate significantly according to the pin shape. Besides investigating complex flow structures in geometrical and flow configurations of
Fuller, Nathaniel J
2016-01-01
Obtaining a detailed understanding of the physical interactions between a cell and its environment often requires information about the flow of fluid surrounding the cell. Cells must be able to effectively absorb and discard material in order to survive. Strategies for nutrient acquisition and toxin disposal, which have been evolutionarily selected for their efficacy, should reflect knowledge of the physics underlying this mass transport problem. Motivated by these considerations, in this paper we consider a two-dimensional advection-diffusion problem at small Reynolds number and large P\\'eclet number. We discuss the problem of mass transport for a circular cell in a uniform far-field flow. We approach the problem numerically, and also analytically through a rescaling of the concentration boundary layer. A biophysically motivated first-passage problem for the absorption of material by the cell demonstrates quantitative agreement between the numerical and analytical approaches.
Directory of Open Access Journals (Sweden)
Moritz Schulze
2016-10-01
Full Text Available The interaction of a plane acoustic wave and a sheared flow is numerically investigated for simple orifice and perforated plate configurations in an isolated, non-resonant environment for Mach numbers up to choked conditions in the holes. Analytical derivations found in the literature are not valid in this regime due to restrictions to low Mach numbers and incompressible conditions. To allow for a systematic and detailed parameter study, a low-cost hybrid Computational Fluid Dynamic/Computational Aeroacoustic (CFD/CAA methodology is used. For the CFD simulations, a standard k–ϵ Reynolds-Averaged Navier–Stokes (RANS model is employed, while the CAA simulations are based on frequency space transformed linearized Euler equations (LEE, which are discretized in a stabilized Finite Element method. Simulation times in the order of seconds per frequency allow for a detailed parameter study. From the application of the Multi Microphone Method together with the two-source location procedure, acoustic scattering matrices are calculated and compared to experimental findings showing very good agreement. The scattering properties are presented in the form of scattering matrices for a frequency range of 500–1500 Hz.
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Haiqing Si
2015-03-01
Full Text Available Lattice Boltzmann method combined with large eddy simulation is developed in the article to simulate fluid flow at high Reynolds numbers. A subgrid model is used as a large eddy simulation model in the numerical simulation for high Reynolds flow. The idea of subgrid model is based on an assumption to include the physical effects that the unresolved motion has on the resolved fluid motion. It takes a simple form of eddy viscosity models for the Reynolds stress. Lift and drag evaluation in the lattice Boltzmann equation takes momentum-exchange method for curved body surface. First of all, the present numerical method is validated at low Reynolds numbers. Second, the developed lattice Boltzmann method/large eddy simulation method is performed to solve flow problems at high Reynolds numbers. Some detailed quantitative comparisons are implemented to show the effectiveness of the present method. It is demonstrated that lattice Boltzmann method combined with large eddy simulation model can efficiently simulate high Reynolds numbers’ flows.
Energy Technology Data Exchange (ETDEWEB)
Kawamura, T.; Nakao, T.; Takahashi, M.; Hayashi, M.; Goto, N. [Hitachi, Ltd., Tokyo (Japan)
1999-07-25
Vortex-induced vibrations were measured for a circular cylinder subjected to a water cross flow at supercritical Reynolds numbers for a wide range of reduced velocities. Turbulence intensities were changed from 1% to 13% in order to investigate the effect of the Strouhal number on the region of synchronization by symmetrical and Karman vortex shedding. The reduced damping of the test cylinder was about 0.1 in water. The surface roughness of the cylinder was a mirror-polished surface. Strouhal number decreased from about 0.48 to 0.29 with increasing turbulence intensity. Synchronized vibrations were observed even at supercritical Reynolds numbers where fluctuating fluid force was small. Reduced velocities at which drag and lift direction lock-in by Karman vortex shedding were initiated decreased with increasing Strouhal number. When Strouhal number was about 0.29, the self-excited vibration in drag direction by symmetrical vortex shedding began at which the frequency ratio of Karman vortex shedding frequency to the natural frequency of cylinder was 0.32. (author)
Control of mean and fluctuating forces on a circular cylinder at high Reynolds numbers
Institute of Scientific and Technical Information of China (English)
Chuanping Shao; Jianming Wang
2007-01-01
A narrow strip is used to control mean and fluctuating forces on a circular cylinder at Reynolds numbers from 2.0 x 104 to 1.0 x 105. The axes of the strip and cylinder are parallel. The control parameters are strip width ratio and strip position characterized by angle of attack and distance from the cylinder. Wind tunnel tests show that the vortex shedding from both sides of the cylinder can be suppressed, and mean drag and fluctuating lift on the cylinder can be reduced if the strip is installed in an effective zone downstream of the cylinder. A phenomenon of mono-side vortex shedding is found. The strip-induced local changes of velocity profiles in the near wake of the cylinder are measured, and the relation between base suction and peak value in the power spectrum of fluctuating lift is studied. The control mechanism is then discussed from different points of view.
Boschan, A; Annichini, M; Gauthier, G
2016-01-01
A study on the spatial organization and velocity fluctuations of non Brownian spherical particles settling at low Reynolds number in a vertical Hele-Shaw cell is reported. The particle volume fraction ranged from 0.005 to 0.05, while the distance between cell plates ranged from 5 to 15 times the particle radius. Particle tracking revealed that particles were not uniformly distributed in space but assembled in transient settling clusters. The population distribution of these clusters followed an exponential law. The measured velocity fluctuations are in agreement with that predicted theoretically for spherical clusters, from the balance between the apparent weight and the drag force. This result suggests that particle clustering, more than a spatial distribution of particles derived from random and independent events, is at the origin of the velocity fluctuations.
Estimating the effective Reynolds number in implicit large-eddy simulation.
Zhou, Ye; Grinstein, Fernando F; Wachtor, Adam J; Haines, Brian M
2014-01-01
In implicit large-eddy simulation (ILES), energy-containing large scales are resolved, and physics capturing numerics are used to spatially filter out unresolved scales and to implicitly model subgrid scale effects. From an applied perspective, it is highly desirable to estimate a characteristic Reynolds number (Re)-and therefore a relevant effective viscosity-so that the impact of resolution on predicted flow quantities and their macroscopic convergence can usefully be characterized. We argue in favor of obtaining robust Re estimates away from the smallest scales of the simulated flow-where numerically controlled dissipation takes place and propose a theoretical basis and framework to determine such measures. ILES examples include forced turbulence as a steady flow case, the Taylor-Green vortex to address transition and decaying turbulence, and simulations of a laser-driven reshock experiment illustrating a fairly complex turbulence problem of current practical interest.
Institute of Scientific and Technical Information of China (English)
QU Jian-wu; MURAI Yuichi; YAMAMOTO Fujio
2005-01-01
Bubble-bubble interaction in free rising bubbly flows is experimentally investigated in the present study.The velocity vectors of the bubbles are measured by a stereoscopic bubble-tracking technique and then the relative velocity vectors of two nearest-neighbor bubbles are calculated with high statistical reliability.With the measurement data at Reynolds number ranging from 5 to 75, the vertical attraction and the horizontal repulsion are confirmed for Re＜10 as known by the past study based on Navier-Stokes simulation.The new finding of the present measurement is that the bubbles of Re＞30 have repulsive velocity bothin the horizontal and the vertical directions as those rise closely.Moreover, the three-dimensional structure of the bubble-bubble interaction is discussed with the data analysis of the interaction vector fields.
Numerical simulation of low-Reynolds number flows past two tandem cylinders of different diameters
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Yong-tao WANG
2013-10-01
Full Text Available The flow past two tandem circular cylinders of different diameters was simulated using the ?nite volume method. The diameter of the downstream main cylinder (D was kept constant, and the diameter of the upstream control cylinder (d varied from 0.1D to D. The studied Reynolds numbers based on the diameter of the downstream main cylinder were 100 and 150. The gap between the control cylinder and the main cylinder (G ranged from 0.1D to 4D. It is concluded that the gap-to-diameter ratio (G/D and the diameter ratio between the two cylinders (d/D have important effects on the drag and lift coef?cients, pressure distributions around the cylinders, vortex shedding frequencies from the two cylinders, and ?ow characteristics.
DEFF Research Database (Denmark)
Jensen, Michael Vincent; Walther, Jens Honore
2013-01-01
was investigated at a jet Reynolds number of 1.66 × 105 and a temperature difference between jet inlet and wall of 1600 K. The focus was on the convective heat transfer contribution as thermal radiation was not included in the investigation. A considerable influence of the turbulence intensity at the jet inlet......Jet impingement heat transfer from a round gas jet to a flat wall was investigated numerically for a ratio of 2 between the jet inlet to wall distance and the jet inlet diameter. The influence of turbulence intensity at the jet inlet and choice of turbulence model on the wall heat transfer...... was observed in the stagnation region, where the wall heat flux increased by a factor of almost 3 when increasing the turbulence intensity from 1.5% to 10%. The choice of turbulence model also influenced the heat transfer predictions significantly, especially in the stagnation region, where differences of up...
Efficient swimming of an assembly of rigid spheres at low Reynolds number
Felderhof, B U
2015-01-01
The swimming of an assembly of rigid spheres immersed in a viscous fluid of infinite extent is studied in low Reynolds number hydrodynamics. The instantaneous swimming velocity and rate of dissipation are expressed in terms of the time-dependent displacements of sphere centers about their collective motion. For small amplitude swimming with periodically oscillating displacements, optimization of the mean swimming speed at given mean power leads to an eigenvalue problem involving a velocity matrix and a power matrix. The corresponding optimal stroke permits generalization to large amplitude motion in a model of spheres with harmonic interactions and corresponding actuating forces. The method allows straightforward calculation of the swimming performance of structures modeled as assemblies of interacting rigid spheres. A model of three collinear spheres with motion along the common axis is studied as an example.
PIV and LIF study of slot continuous jet at low Reynolds number
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Broučková Zuzana
2016-01-01
Full Text Available This study deals with a continuous jet issuing from a small narrow slot with a width of 0.36 mm. The experimental arrangement is based on the piezoelectric synthetic jet actuator studied previously for easy comparisons. The working fluid is water at room temperature. The experiments were performed using methods of particle image velocimetry (PIV and flow visualization (laser induced fluorescence, LIF. The time-mean volume flux through the exit nozzle was quantified using precise scales. The mean velocity and the Reynolds number were evaluated as Um = 0.12 m/s and Re = 90, respectively. The results of LIF and PIV techniques revealed the three-dimensional character of the flow field, namely the saddle-shape velocity profiles. This behavior is typical for steady jets from a rectangular nozzle. The obtained results were compared with previous measurements of the synthetic jet issuing from the same cavity and the slot nozzle.
Friedrich, J; Schäfer, T; Grauer, R
2016-01-01
We investigate the scaling behavior of longitudinal and transverse structure functions in homogeneous and isotropic magneto-hydrodynamic (MHD) turbulence by means of an exact hierarchy of structure function equations as well as by direct numerical simulations of two- and three-dimensional MHD turbulence. In particular, rescaling relations between longitudinal and transverse structure functions are derived and utilized in order to compare different scaling behavior in the inertial range. It is found that there are no substantial differences between longitudinal and transverse structure functions in MHD turbulence. This finding stands in contrast to the case of hydrodynamic turbulence which shows persistent differences even at high Reynolds numbers. We propose a physical picture that is based on an effective reduction of pressure contributions due to local regions of same magnitude and alignment of velocity and magnetic field fluctuations. Finally, our findings underline the importance of the pressure term for ...
Influence of Turbulence Model for Wind Turbine Simulation in Low Reynolds Number
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Masami Suzuki
2016-01-01
Full Text Available In designing a wind turbine, the validation of the mathematical model’s result is normally carried out by comparison with wind tunnel experiment data. However, the Reynolds number of the wind tunnel experiment is low, and the flow does not match fully developed turbulence on the leading edge of a wind turbine blade. Therefore, the transition area from laminar to turbulent flow becomes wide under these conditions, and the separation point is difficult to predict using turbulence models. The prediction precision decreases dramatically when working with tip speed ratios less than the maximum power point. This study carries out a steadiness calculation with turbulence model and an unsteadiness calculation with laminar model for a three-blade horizontal axis wind turbine. The validation of the calculations is performed by comparing with experimental results. The power coefficients calculated without turbulence models are in agreement with the experimental data for a tip speed ratio greater than 5.
Large-scale magnetic fields at high Reynolds numbers in magnetohydrodynamic simulations.
Hotta, H; Rempel, M; Yokoyama, T
2016-03-25
The 11-year solar magnetic cycle shows a high degree of coherence in spite of the turbulent nature of the solar convection zone. It has been found in recent high-resolution magnetohydrodynamics simulations that the maintenance of a large-scale coherent magnetic field is difficult with small viscosity and magnetic diffusivity (≲10 (12) square centimenters per second). We reproduced previous findings that indicate a reduction of the energy in the large-scale magnetic field for lower diffusivities and demonstrate the recovery of the global-scale magnetic field using unprecedentedly high resolution. We found an efficient small-scale dynamo that suppresses small-scale flows, which mimics the properties of large diffusivity. As a result, the global-scale magnetic field is maintained even in the regime of small diffusivities-that is, large Reynolds numbers.
High Reynolds Number Effects on Multi-Hole Probes and Hot Wire Anemometers
Ramachandran, N.; Smith, A.; Gerry, G.; Kauffman, W.
1995-01-01
The paper reports on the results from an experimental investigation of the response of multi-hole and hot wire probes at high flow Reynolds numbers (Re approx. 10(exp 6)). The limited results available in literature for 5-hole probes are restricted to Re approx. 10(exp 4). The experiment aims to investigate the probe response (in terms of dimensionless pressure ratios, characterizing pitch, and yaw angles and the total and static pressures) at high Re values and to gauge their effect on the calculated velocity vector. Hot wire calibrations were also undertaken with a parametric variation of the flow pressure, velocity and temperature. Different correction and calibration schemes are sought to be tested against the acquired data set. The data is in the analysis stage at the present time. The test provided good benchmark quality data that can be used to test future calibration and testing methods.
Relaminarization of wall turbulence by high-pressure ramps at low Reynolds numbers
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Song Kwonyul
2016-01-01
Full Text Available Reverse transition from the turbulent towards the laminar flow regime was investigated experimentally by progressively increasing the pressure up to 400 MPa in a fully developed pipe flow operated with silicone oil as the working fluid. Using hot-wire anemometry, it is shown indirectly that at low Reynolds numbers a rapid increase in pressure modifies the turbulence dynamics owing to the processes which induce the effects caused by fluid compressibility in the region very close to the wall. The experimental results confirm that under such circumstances, the traditional mechanism responsible for self-maintenance of turbulence in wall-bounded flows is altered in such a way as to lead towards a state in which turbulence cannot persist any longer.
Double large field stereoscopic PIV in a high Reynolds number turbulent boundary layer
Coudert, S.; Foucaut, J. M.; Kostas, J.; Stanislas, M.; Braud, P.; Fourment, C.; Delville, J.; Tutkun, M.; Mehdi, F.; Johansson, P.; George, W. K.
2011-01-01
An experiment on a flat plate turbulent boundary layer at high Reynolds number has been carried out in the Laboratoire de Mecanique de Lille (LML, UMR CNRS 8107) wind tunnel. This experiment was performed jointly with LEA (UMR CNRS 6609) in Poitiers (France) and Chalmers University of Technology (Sweden), in the frame of the WALLTURB European project. The simultaneous recording of 143 hot wires in one transverse plane and of two perpendicular stereoscopic PIV fields was performed successfully. The first SPIV plane is 1 cm upstream of the hot wire rake and the second is both orthogonal to the first one and to the wall. The first PIV results show a blockage effect which based on both statistical results (i.e. mean, RMS and spatial correlation) and a potential model does not seem to affect the turbulence organization.
Angular velocity of a sphere in a simple shear at small Reynolds number
Meibohm, J; Rosén, T; Einarsson, J; Lundell, F; Mehlig, B
2016-01-01
We analyse the angular dynamics of a small neutrally buoyant sphere in a simple shear. When the effect of fluid inertia is negligible the sphere rotates at half the fluid vorticity. We compute how weak fluid inertia reduces the angular velocity, and find $\\omega_3/s \\sim -{1}/{2} +0.0540\\, {\\rm Re}_{\\rm s}^{3/2}$ where $s$ is the shear rate and ${\\rm Re}_{\\rm s}$ is the shear Reynolds number. This result differs from that derived by Lin et al. [J. Fluid Mech. 44 (1970) 1] who obtained a coefficient roughly three times larger. Our result is in good agreement with those of direct numerical simulations at small but not too small values of ${\\rm Re}_{\\rm }s$.
Steady imperfect bifurcation with generic 3D bluff bodies at large Reynolds numbers
Cadot, Olivier; Pastur, Luc; Evrard, Antoine; Soyer, Guillaume
2014-11-01
The turbulent wake of parallelepiped bodies exhibits a strong bi-modal behavior. The wake randomly undergoes symmetry breaking reversals, between two mirror asymmetric steady modes (RSB modes). The characteristic time for reversals is about 2 or three orders of magnitudes larger than the natural time for vortex shedding. Such a dynamics has been recently observed on real car which points out its importance about industrial applications. Both the viscosity and the proximity of a wall in the vicinity of the parallelepiped body (similarly to the road with a car model), stabilize the RSB modes on a single symmetric mode. It is shown that these stabilizations occur through imperfect fork bifurcations at large Reynolds numbers. The extra drag due to the presence of the RSB modes is evidenced.
Three-dimensional flow past rotating wing at low Reynolds number: a computational study
Energy Technology Data Exchange (ETDEWEB)
Ruifeng, Hu, E-mail: rfhu@xidian.edu.cn [School of Mechano-Electronic Engineering, Xidian University, Xi’an 710071 (China)
2015-08-15
In this work, we performed a computational study on the three-dimensional (3D) flow past a rotating wing at a low Reynolds number (Re = 200). The 3D vortical structures and aerodynamic performances of the rotating wing with different aspect ratios and rotating speeds are computed and analyzed. A quasi-steady model is adopted for prediction of aerodynamic performances of the wing, and its applicability is evaluated by the computation. It is found that there exists a periodic vortex shedding pattern at a low rotating speed, while vortices may cluster near the wing when rotating speed is high enough. The wake vortex topology is also affected by the aspect ratio. The current quasi-steady aerodynamic model could only be used for rotating wing aerodynamics at a low rotating speed when regularly periodic vortex shedding exists. (paper)
Flow analysis of the low-Reynolds number swimmer C. elegans
Montenegro-Johnson, Thomas D; Arratia, Paulo E; Lauga, Eric
2016-01-01
Swimming cells and microorganisms are a critical component of many biological processes. In order to better interpret experimental studies of low Reynolds number swimming, we combine experimental and numerical methods to perform an analysis of the flow-field around the swimming nematode Caenorhabditis elegans. We first use image processing and particle tracking velocimetry to extract the body shape, kinematics, and flow-fields around the nematode. We then construct a three-dimensional model using the experimental swimming kinematics and use a boundary element method to simulate flow-fields, obtaining very good quantitative agreement with experiment. We next apply a simple local-drag theory in order to generate a theoretical correction factor, which allows the approximation of three-dimensional shear rates from two-dimensional flow measurements. This correction is verified against 3D numerical simulations, and then applied to experimental data. Our results show how fundamental fluid mechanics considerations ma...
Felderhof, B U
2016-01-01
Swimming at small Reynolds number of a linear assembly of identical spheres immersed in a viscous fluid is studied on the basis of a set of equations of motion for the individual spheres. The motion of the spheres is caused by actuating forces and forces derived from a direct interaction potential, as well as hydrodynamic forces exerted by the fluid as frictional and added mass hydrodynamic interactions. The swimming velocity is deduced from the momentum balance equation for the assembly of spheres, and the mean power required during a period is calculated from an instantaneous power equation. Expressions are derived for the mean swimming velocity and the mean power, valid to second order in the amplitude of displacements from the relative equilibrium positions. Hence these quantities can be evaluated in terms of prescribed periodic displacements. Explicit calculations are performed for a linear chain of three identical spheres.
Carrillo, Mauricio; Que, Ulices; González, José A.
2016-12-01
The present work investigates the application of artificial neural networks (ANNs) to estimate the Reynolds (Re) number for flows around a cylinder. The data required to train the ANN was generated with our own implementation of a lattice Boltzmann method (LBM) code performing simulations of a two-dimensional flow around a cylinder. As results of the simulations, we obtain the velocity field (v ⃗) and the vorticity (∇ ⃗×v ⃗ ) of the fluid for 120 different values of Re measured at different distances from the obstacle and use them to teach the ANN to predict the Re. The results predicted by the networks show good accuracy with errors of less than 4 % in all the studied cases. One of the possible applications of this method is the development of an efficient tool to characterize a blocked flowing pipe.
Transition of effective hydraulic properties from low to high Reynolds number flow in porous media
Sivanesapillai, R.; Steeb, H.; Hartmaier, A.
2014-07-01
We numerically analyze fluid flow through porous media up to a limiting Reynolds number of O(103). Due to inertial effects, such processes exhibit a gradual transition from laminar to turbulent flow for increasing magnitudes of Re. On the macroscopic scale, inertial transition implies nonlinearities in the relationship between the effective macroscopic pressure gradient and the filter velocity, typically accounted for in terms of the quadratic Forchheimer equation. However, various inertia-based extensions to the linear Darcy equation have been discussed in the literature; most prominently cubic polynomials in velocity. The numerical results presented in this contribution indicate that inertial transition, as observed in the apparent permeability, hydraulic tortuosity, and interfacial drag, is inherently of sigmoidal shape. Based on this observation, we derive a novel filtration law which is consistent with Darcy's law at small Re, reproduces Forchheimer's law at large Re, and exhibits higher-order leading terms in the weak inertia regime.
Exploration of plasma-based control for low-Reynolds number airfoil/gust interaction
Rizzetta, Donald P.; Visbal, Miguel R.
2011-12-01
Large-eddy simulation (LES) is employed to investigate the use of plasma-based actuation for the control of a vortical gust interacting with a wing section at a low Reynolds number. Flow about the SD7003 airfoil section at 4° angle of attack and a chord-based Reynolds number of 60,000 is considered in the simulation, which typifies micro air vehicle (MAV) applications. Solutions are obtained to the Navier-Stokes equations that were augmented by source terms used to represent body forces imparted by the plasma actuator on the fluid. A simple phenomenological model provided these body forces resulting from the electric field generated by the plasma. The numerical method is based upon a high-fidelity time-implicit scheme and an implicit LES approach which are used to obtain solutions on a locally refined overset mesh system. A Taylor-like vortex model is employed to represent a gust impinging upon the wing surface, which causes a substantial disruption to the undisturbed flow. It is shown that the fundamental impact of the gust on unsteady aerodynamic forces is due to an inviscid process, corresponding to variation in the effective angle of attack, which is not easily overcome. Plasma control is utilised to mitigate adverse effects of the interaction and improve aerodynamic performance. Physical characteristics of the interaction are described, and several aspects of the control strategy are explored. Among these are uniform and non-uniform spanwise variations of the control configuration, co-flow and counter-flow orientations of the directed force, pulsed and continuous operations of the actuator and strength of the plasma field. Results of the control situations are compared with regard to their effect upon aerodynamic forces. It was found that disturbances to the moment coefficient produced by the gust can be greatly reduced, which may be significant for stability and handling of MAV operations.
Performance losses of drag-reducing spanwise forcing at moderate values of the Reynolds number
Gatti, Davide; Quadrio, Maurizio
2013-12-01
A fundamental problem in the field of turbulent skin-friction drag reduction is to determine the performance of the available control techniques at high values of the Reynolds number Re. We consider active, predetermined strategies based on spanwise forcing (oscillating wall and streamwise-traveling waves applied to a plane channel flow), and explore via Direct Numerical Simulations (DNS) up to Reτ = 2100 the rate at which their performance deteriorates as Re is increased. To be able to carry out a comprehensive parameter study, we limit the computational cost of the simulations by adjusting the size of the computational domain in the homogeneous directions, compromising between faster computations and the increased need of time-averaging the fluctuating space-mean wall shear-stress. Our results, corroborated by a few full-scale DNS, suggest a scenario where drag reduction degrades with Re at a rate that varies according to the parameters of the wall forcing. In agreement with already available information, keeping them at their low-Re optimal value produces a relatively quick decrease of drag reduction. However, at higher Re the optimal parameters shift towards other regions of the parameter space, and these regions turn out to be much less sensitive to Re. Once this shift is accounted for, drag reduction decreases with Re at a markedly slower rate. If the slightly favorable trend of the energy required to create the forcing is considered, a chance emerges for positive net energy savings also at large values of the Reynolds number.
Directory of Open Access Journals (Sweden)
S. M. Yadav
2011-02-01
Full Text Available The computation of bed load allows for the fact that only part of the shear stress is used for transport of sediments and some of the shear stress is wasted in overcoming the resistance due to bed forms therefore the total shear stress developed in the open channel requires correction in the form of correction factor called ripple factor. Different methods have been followed for correcting the actual shear stress in order to compute the sediment load. Correction factors are based on particular characteristics grain size of particle. In the present paper the ripple factor has been obtained for non uniform bed material considering the various variables like discharge, hydraulic mean depth, flow velocity, bed slope, average diameter of particle etc. by collecting the field data of Tapi river for 15 years for a particular gauging station. The ripple factor is obtained using Meyer Peter and Muller formula, Einstein Formula, Kalinske’s formula, Du Boy’s formula, Shield’s formula, Bagnold’s formula, average of six formulae and multiple regression analysis. The variation of ripple factor with particle Reynolds Number is studied. The ripple factor obtained by different approaches are further analyzed using Origin software and carrying out multiple regression on the 15 years of data with more than 10 parameters, ripple factor by multiple regression has been obtained. These values are further analysed and giving statistical mean to the parameters a relationship of power form has been developed. The ripple factor increases with the increase in the value of Particle Reynolds number. The large deviation is observed in case of Kalinske’s approach when compare with other approaches
Reynolds number limits for jet propulsion: a numerical study of simplified jellyfish.
Herschlag, Gregory; Miller, Laura
2011-09-21
The Scallop theorem states that reciprocal methods of locomotion, such as jet propulsion or paddling, will not work in Stokes flow (Reynolds number=0). In nature the effective limit of jet propulsion is still in the range where inertial forces are significant. It appears that almost all animals that use jet propulsion swim at Reynolds numbers (Re) of about 5 or more. Juvenile squid and octopods hatch from the egg already swimming in this inertial regime. Juvenile jellyfish, or ephyrae, break off from polyps swimming at Re greater than 5. Many other organisms, such as scallops, rarely swim at Re less than 100. The limitations of jet propulsion at intermediate Re is explored here using the immersed boundary method to solve the 2D Navier-Stokes equations coupled to the motion of a simplified jellyfish. The contraction and expansion kinematics are prescribed, but the forward and backward swimming motions of the idealized jellyfish are emergent properties determined by the resulting fluid dynamics. Simulations are performed for both an oblate bell shape using a paddling mode of swimming and a prolate bell shape using jet propulsion. Average forward velocities and work put into the system are calculated for Re between 1 and 320. The results show that forward velocities rapidly decay with decreasing Re for all bell shapes when Rejellyfish after two pulse cycles are comparable to those reported for Aurelia aurita, but discrepancies are observed in the vortex dynamics between when the 2D model oblate jellyfish and the organism. This discrepancy is likely due to a combination of the differences between the 3D reality of the jellyfish and the 2D simplification, as well as the rigidity of the time varying geometry imposed by the idealized model.
Experimental Study of Thin and Thick Airfoils at Low Reynolds Numbers
Durgesh, Vibhav; Garcia, Elifalet; Johari, Hamid
2015-11-01
A recent surge in applications of unmanned air vehicles in various fields has led to increased interest in understanding the characteristics of airfoils at Reynolds number regime ~104. At these low Re numbers, aerodynamics of an airfoil is influenced by laminar separation and its possible reattachment, which is in contrast to airfoil behavior at high Re numbers. This study focused on comparing the load characteristics of symmetric, thin (NACA-0009) and thick (NACA-0021) airfoils at low Re numbers ~2 - 4 × 104, and angles of attack between 2° to 12°, along with simultaneous flow visualization. The experiments were performed in a low speed flow visualization water tunnel facility, and two-component Laser Doppler Velocimetry was used to quantify the inflow conditions and turbulence intensity. A high precision force/torque transducer was used for the load measurements, while hydrogen bubble technique was used for flow visualization on the suction side of the airfoils. The presentation will discuss the correlation between observed flow structures and instantaneous load on the airfoils, as well as the aerodynamic load characteristics of thin and thick airfoils at low Re numbers.
Opacity Broadening of $^{13}$CO Linewidths and its Effect on the Variance-Sonic Mach Number Relation
Correia, Caio; Lazarian, Alex; Ossenkopf, Volker; Stutzki, Jürgen; Kainulainen, Jouni; Kowal, Grzegorz; de Medeiros, José Renan
2014-01-01
We study how the estimation of the sonic Mach number ($M_s$) from $^{13}$CO linewidths relates to the actual 3D sonic Mach number. For this purpose we analyze MHD simulations which include post-processing to take radiative transfer effects into account. As expected, we find very good agreement between the linewidth estimated sonic Mach number and the actual sonic Mach number of the simulations for optically thin tracers. However, we find that opacity broadening causes $M_s$ to be overestimated by a factor of ~ 1.16 - 1.3 when calculated from optically thick $^{13}$CO lines. We also find that there is a dependency on the magnetic field: super-Alfv\\'enic turbulence shows increased line broadening as compared with sub-Alfv\\'enic turbulence for all values of optical depth for supersonic turbulence. Our results have implications for the observationally derived sonic Mach number--density standard deviation ($\\sigma_{\\rho/}$) relationship, $\\sigma^2_{\\rho/}=b^2M_s^2$, and the related column density standard deviatio...
Parametric investigation of single-expansion-ramp nozzles at Mach numbers from 0.60 to 1.20
Capone, Francis J.; Re, Richard J.; Bare, E. Ann
1992-01-01
An investigation was conducted in the Langley 16-Foot Transonic Tunnel to determine the effects of varying six nozzle geometric parameters on the internal and aeropropulsive performance characteristics of single-expansion-ramp nozzles. This investigation was conducted at Mach numbers from 0.60 to 1.20, nozzle pressure ratios from 1.5 to 12, and angles of attack of 0 deg +/- 6 deg. Maximum aeropropulsive performance at a particular Mach number was highly dependent on the operating nozzle pressure ratio. For example, as the nozzle upper ramp length or angle increased, some nozzles had higher performance at a Mach number of 0.90 because of the nozzle design pressure was the same as the operating pressure ratio. Thus, selection of the various nozzle geometric parameters should be based on the mission requirements of the aircraft. A combination of large upper ramp and large lower flap boattail angles produced greater nozzle drag coefficients at Mach number greater than 0.80, primarily from shock-induced separation on the lower flap of the nozzle. A static conditions, the convergent nozzle had high and nearly constant values of resultant thrust ratio over the entire range of nozzle pressure ratios tested. However, these nozzles had much lower aeropropulsive performance than the convergent-divergent nozzle at Mach number greater than 0.60.
Rosén, T.; Einarsson, J.; Nordmark, A.; Aidun, C. K.; Lundell, F.; Mehlig, B.
2015-12-01
We numerically analyze the rotation of a neutrally buoyant spheroid in a shear flow at small shear Reynolds number. Using direct numerical stability analysis of the coupled nonlinear particle-flow problem, we compute the linear stability of the log-rolling orbit at small shear Reynolds number Rea. As Rea→0 and as the box size of the system tends to infinity, we find good agreement between the numerical results and earlier analytical predictions valid to linear order in Rea for the case of an unbounded shear. The numerical stability analysis indicates that there are substantial finite-size corrections to the analytical results obtained for the unbounded system. We also compare the analytical results to results of lattice Boltzmann simulations to analyze the stability of the tumbling orbit at shear Reynolds numbers of order unity. Theory for an unbounded system at infinitesimal shear Reynolds number predicts a bifurcation of the tumbling orbit at aspect ratio λc≈0.137 below which tumbling is stable (as well as log rolling). The simulation results show a bifurcation line in the λ -Rea plane that reaches λ ≈0.1275 at the smallest shear Reynolds number (Rea=1 ) at which we could simulate with the lattice Boltzmann code, in qualitative agreement with the analytical results.
MASS TRANSFER CONTROL OF A BACKWARD-FACING STEP FLOW BY LOCAL FORCING- EFFECT OF REYNOLDS NUMBER
Directory of Open Access Journals (Sweden)
Zouhaier MEHREZ
2011-01-01
Full Text Available The control of fluid mechanics and mass transfer in separated and reattaching flow over a backward-facing step by a local forcing, is studied using Large Eddy Simulation (LES.To control the flow, the local forcing is realized by a sinusoidal oscillating jet at the step edge. The Reynolds number is varied in the range 10000 ≤ Re≤ 50000 and the Schmidt number is fixed at 1.The found results show that the flow structure is modified and the local mass transfer is enhanced by the applied forcing. The observed changes depend on the Reynolds number and vary with the frequency and amplitude of the local forcing. For the all Reynolds numbers, the largest recirculation zone size reduction is obtained at the optimum forcing frequency St = 0.25. At this frequency the local mass transfer enhancement attains the maximum.
Tests of Full-Scale Helicopter Rotors at High Advancing Tip Mach Numbers and Advance Ratios
Biggers, James C.; McCloud, John L., III; Stroub, Robert H.
2015-01-01
As a continuation of the studies of reference 1, three full-scale helicopter rotors have been tested in the Ames Research Center 40- by SO-foot wind tunnel. All three of them were two-bladed, teetering rotors. One of the rotors incorporated the NACA 0012 airfoil section over the entire length of the blade. This rotor was tested at advance ratios up to 1.05. Both of the other rotors were tapered in thickness and incorporated leading-edge camber over the outer 20 percent of the blade radius. The larger of these rotors was tested at advancing tip Mach numbers up to 1.02. Data were obtained for a wide range of lift and propulsive force, and are presented without discussion.
On the proper Mach number and ratio of specific heats for modeling the Venus bow shock
Tatrallyay, M.; Russell, C. T.; Luhmann, J. G.; Barnes, A.; Mihalov, J. D.
1984-01-01
Observational data from the Pioneer Venus Orbiter are used to investigate the physical characteristics of the Venus bow shock, and to explore some general issues in the numerical simulation of collisionless shocks. It is found that since equations from gas-dynamic (GD) models of the Venus shock cannot in general replace MHD equations, it is not immediately obvious what the optimum way is to describe the desired MHD situation with a GD code. Test case analysis shows that for quasi-perpendicular shocks it is safest to use the magnetospheric Mach number as an input to the GD code. It is also shown that when comparing GD predicted temperatures with MHD predicted temperatures total energy should be compared since the magnetic energy density provides a significant fraction of the internal energy of the MHD fluid for typical solar wind parameters. Some conclusions are also offered on the properties of the terrestrial shock.
Relativistic Electron Shock Drift Acceleration in Low Mach Number Galaxy Cluster Shocks
Matsukiyo, Shuichi; Yamazaki, Ryo; Umeda, Takayuki
2011-01-01
An extreme case of electron shock drift acceleration in low Mach number collisionless shocks is investigated as a plausible mechanism of initial acceleration of relativistic electrons in large-scale shocks in galaxy clusters where upstream plasma temperature is of the order of 10 keV and a degree of magnetization is not too small. One-dimensional electromagnetic full particle simulations reveal that, even though a shock is rather moderate, a part of thermal incoming electrons are accelerated and reflected through relativistic shock drift acceleration and form a local nonthermal population just upstream of the shock. The accelerated electrons can self-generate local coherent waves and further be back-scattered toward the shock by those waves. This may be a scenario for the first stage of the electron shock acceleration occurring at the large-scale shocks in galaxy clusters such as CIZA J2242.8+5301 which has well defined radio relics.
Low-Mach-number turbulence in interstellar gas revealed by radio polarization gradients
Gaensler, Bryan M; Burkhart, Blakesley; Newton-McGee, Katherine J; Ekers, Ronald D; Lazarian, Alex; McClure-Griffiths, Naomi M; Robishaw, Timothy; Dickey, John M; Green, Anne J; 10.1038/nature10446
2011-01-01
The interstellar medium of the Milky Way is multi-phase, magnetized and turbulent. Turbulence in the interstellar medium produces a global cascade of random gas motions, spanning scales ranging from 100 parsecs to 1000 kilometres. Fundamental parameters of interstellar turbulence such as the sonic Mach number (the speed of sound) have been difficult to determine because observations have lacked the sensitivity and resolution to directly image the small-scale structure associated with turbulent motion. Observations of linear polarization and Faraday rotation in radio emission from the Milky Way have identified unusual polarized structures that often have no counterparts in the total radiation intensity or at other wavelengths, and whose physical significance has been unclear. Here we report that the gradient of the Stokes vector (Q,U), where Q and U are parameters describing the polarization state of radiation, provides an image of magnetized turbulence in diffuse ionized gas, manifested as a complex filamenta...
Varsakelis, Christos; Papalexandris, Miltiadis V.
2017-01-01
A conundrum in non-equilibrium thermodynamics of heterogeneous mixtures with microstructure concerns the selection of thermodynamic currents and forces in the entropy production rate from the multitude of available options. The objective of this article is to demonstrate that the low-Mach-number approximation can narrow down this ambiguity. More specifically, by postulating that the post-constitutive equations are well behaved with respect to this perturbation analysis we assert that thermal non-equilibrium should be chosen as an independent force even if this requires the explicit manipulation of the entropy inequality. According to our analysis, alternative choices result in post-constitutive equations; the incompressible limit of which gives rise to questionable predictions.
Electron acceleration in a nonrelativistic shock with very high Alfv\\'en Mach number
Matsumoto, Y; Hoshino, M
2013-01-01
Electron acceleration associated with various plasma kinetic instabilities in a nonrelativistic, very-high-Alfv\\'en Mach-number ($M_A \\sim 45$) shock is revealed by means of a two-dimensional fully kinetic PIC simulation. Electromagnetic (ion Weibel) and electrostatic (ion-acoustic and Buneman) instabilities are strongly activated at the same time in different regions of the two-dimensional shock structure. Relativistic electrons are quickly produced predominantly by the shock surfing mechanism with the Buneman instability at the leading edge of the foot. The energy spectrum has a high-energy tail exceeding the upstream ion kinetic energy accompanying the main thermal population. This gives a favorable condition for the ion acoustic instability at the shock front, which in turn results in additional energization. The large-amplitude ion Weibel instability generates current sheets in the foot, implying another dissipation mechanism via magnetic reconnection in a three-dimensional shock structure in the very-hi...
The dependence of Nusselt number on Reynolds number for a hot-wire sensor in supercritical CO2 flow
Vukoslavcevic, Petar; Wallace, James
2005-11-01
An analysis of the heat transfer mechanism around a hot-wire sensor in superctitical CO2 flow has been performed, and the dependence of the Nusselt number (Nu) on the Reynolds number (Re) has been determined. A special, closed flow loop, capable of inducing variable speed flow at different pressures and temperatures in the ranges of 0.15-2 m/s, 15-70^oC and 1-100 bar, has been used to create a supercritical CO2 flow around a hot-wire sensor operated in the constant temperature mode. The Nu and Re numbers were determined based on the known heat convected from the sensor, the flow speed and the sensor temperature and dimensions. The experiment was performed along a line of constant 80 bar pressure in the temperature range of 25-65^oC. It was found that, at a given pressure and temperature, the relation Nu=F(Re) has the classical form Nu=M+NRe^n, with the parameters M and N being functions of pressure and temperature. The dependence of these parameters on temperature was analyzed, and the most convenient reference temperature was chosen. In contrast to the operation of hot-wires in air and water, the dependence of the parameters M and N on the Prandtl number can result in nonunique solutions.
Evaluation of Computational Method of High Reynolds Number Slurry Flow for Caverns Backfilling
Energy Technology Data Exchange (ETDEWEB)
Bettin, Giorgia [Sandia National Lab. (SNL-NM), Albuquerque, NM (United States)
2015-05-01
The abandonment of salt caverns used for brining or product storage poses a significant environmental and economic risk. Risk mitigation can in part be address ed by the process of backfilling which can improve the cavern geomechanical stability and reduce the risk o f fluid loss to the environment. This study evaluate s a currently available computational tool , Barracuda, to simulate such process es as slurry flow at high Reynolds number with high particle loading . Using Barracuda software, a parametric sequence of simu lations evaluated slurry flow at Re ynolds number up to 15000 and loading up to 25%. Li mitations come into the long time required to run these simulation s due in particular to the mesh size requirement at the jet nozzle. This study has found that slurry - jet width and centerline velocities are functions of Re ynold s number and volume fractio n The solid phase was found to spread less than the water - phase with a spreading rate smaller than 1 , dependent on the volume fraction. Particle size distribution does seem to have a large influence on the jet flow development. This study constitutes a first step to understand the behavior of highly loaded slurries and their ultimate application to cavern backfilling.
Peng, Naifu; Yang, Yue
2016-11-01
We investigate the evolution of vortex-surface fields (VSFs) in viscous compressible Taylor-Green flows. The VSF is applied to the direct numerical simulation of the Taylor-Green flows at a range of Mach numbers from Ma = 0 . 6 to Ma = 2 . 2 for characterizing the Mach-number effects on evolving vortical structures. We find that the dilatation and baroclinic force strongly influence the geometry of vortex surfaces and the energy dissipation rate in the transitional stage. The vortex tubes in compressible flows are less curved than those in incompressible flows, and the maximum dissipation rate occurs earlier in high-Mach-number flows perhaps owing to the conversion of kinetic energy into heat. Moreover, the relations between the evolutionary geometry of vortical structures and flow statistics are discussed. This work has been supported in part by the National Natural Science Foundation of China (Grant Nos. 11522215 and 11521091), and the Thousand Young Talents Program of China.
Schaeffer, D. B.; Fox, W.; Haberberger, D.; Fiksel, G.; Bhattacharjee, A.; Barnak, D. H.; Hu, S. X.; Germaschewski, K.
2017-07-01
We present the first laboratory generation of high-Mach-number magnetized collisionless shocks created through the interaction of an expanding laser-driven plasma with a magnetized ambient plasma. Time-resolved, two-dimensional imaging of plasma density and magnetic fields shows the formation and evolution of a supercritical shock propagating at magnetosonic Mach number Mms≈12 . Particle-in-cell simulations constrained by experimental data further detail the shock formation and separate dynamics of the multi-ion-species ambient plasma. The results show that the shocks form on time scales as fast as one gyroperiod, aided by the efficient coupling of energy, and the generation of a magnetic barrier between the piston and ambient ions. The development of this experimental platform complements present remote sensing and spacecraft observations, and opens the way for controlled laboratory investigations of high-Mach number collisionless shocks, including the mechanisms and efficiency of particle acceleration.
Institute of Scientific and Technical Information of China (English)
无
2007-01-01
A flow past two side-by-side identical circular cylinders was numerically investigated with the unstructured spectral element method. From the computational results at various non-dimensional distances between cylinder centers T/D and the Reynolds number Re, a total of nine kinds of wake patterns were observed: four steady wake patterns, including single bluff-body steady pattern, separated double-body steady pattern and transition steady pattern for sub-critical Reynolds numbers and biased steady pattern for super-critical Reynolds numbers, and five unsteady wake patterns, including single bluff-body periodic pattern, biased quasi-steady pattern, quasi-periodic (flip-flopping) pattern, in-phase-synchronized pattern and anti-phase-synchronized pattern. Time evolution of lift and drag coefficients corresponding to each unsteady wake pattern was given.
Henneberry, Hugh M.; Snyder, Christopher A.
1993-01-01
An analysis of gas turbine engines using water and oxygen injection to enhance performance by increasing Mach number capability and by increasing thrust is described. The liquids are injected, either separately or together, into the subsonic diffuser ahead of the engine compressor. A turbojet engine and a mixed-flow turbofan engine (MFTF) are examined, and in pursuit of maximum thrust, both engines are fitted with afterburners. The results indicate that water injection alone can extend the performance envelope of both engine types by one and one-half Mach numbers at which point water-air ratios reach 17 or 18 percent and liquid specific impulse is reduced to some 390 to 470 seconds, a level about equal to the impulse of a high energy rocket engine. The envelope can be further extended, but only with increasing sacrifices in liquid specific impulse. Oxygen-airflow ratios as high as 15 percent were investigated for increasing thrust. Using 15 percent oxygen in combination with water injection at high supersonic Mach numbers resulted in thrust augmentation as high as 76 percent without any significant decrease in liquid specific impulse. The stoichiometric afterburner exit temperature increased with increasing oxygen flow, reaching 4822 deg R in the turbojet engine at a Mach number of 3.5. At the transonic Mach number of 0.95 where no water injection is needed, an oxygen-air ratio of 15 percent increased thrust by some 55 percent in both engines, along with a decrease in liquid specific impulse of 62 percent. Afterburner temperature was approximately 4700 deg R at this high thrust condition. Water and/or oxygen injection are simple and straightforward strategies to improve engine performance and they will add little to engine weight. However, if large Mach number and thrust increases are required, liquid flows become significant, so that operation at these conditions will necessarily be of short duration.
Bonhaus, Daryl L.; Maddalon, Dal V.
1998-01-01
Flight-measured high Reynolds number turbulent-flow pressure distributions on a transport wing in transonic flow are compared to unstructured-grid calculations to assess the predictive ability of a three-dimensional Euler code (USM3D) coupled to an interacting boundary layer module. The two experimental pressure distributions selected for comparative analysis with the calculations are complex and turbulent but typical of an advanced technology laminar flow wing. An advancing front method (VGRID) was used to generate several tetrahedral grids for each test case. Initial calculations left considerable room for improvement in accuracy. Studies were then made of experimental errors, transition location, viscous effects, nacelle flow modeling, number and placement of spanwise boundary layer stations, and grid resolution. The most significant improvements in the accuracy of the calculations were gained by improvement of the nacelle flow model and by refinement of the computational grid. Final calculations yield results in close agreement with the experiment. Indications are that further grid refinement would produce additional improvement but would require more computer memory than is available. The appendix data compare the experimental attachment line location with calculations for different grid sizes. Good agreement is obtained between the experimental and calculated attachment line locations.
Fractal properties of isovelocity surfaces in high Reynolds number laboratory shear flows
Praskovsky, Alexander A.; Foss, John F.; Kleis, Stanley J.; Karyakin, Mikhail Yu.
1993-08-01
The fractal properties of isovelocity surfaces are studied in three high Reynolds number (Rλ≊2.0×102-3.2×103) laboratory shear flows using the standard box-counting method. The fractal dimension D=-d(log Nr)/d(log r) was estimated within the range of box sizes r from several Kolmogorov scales up to several integral scales (Nr is the number of boxes with size r required to cover the line intersection of an isovelocity surface). The inertial subrange was of particular interest in this investigation. Measurements were carried out for external intermittency factors γ≊0.6-1.0. The data were processed using threshold levels U±2.5u' (U and u' denote mean and rms values of longitudinal velocity). Over the parameters studied, no wide range of constant fractal dimension was found. On the other hand, the accuracy of constant fractal dimension approximation with D≊0.4 over the inertial subranges was shown to be similar to that of the Kolmogorov [Dokl. Akad. Nauk SSSR 30, 301 (1941)] ``two-thirds law.''
Steady streaming: A key mixing mechanism in low-Reynolds-number acinar flows
Kumar, Haribalan; Tawhai, Merryn H.; Hoffman, Eric A.; Lin, Ching-Long
2011-01-01
Study of mixing is important in understanding transport of submicron sized particles in the acinar region of the lung. In this article, we investigate transport in view of advective mixing utilizing Lagrangian particle tracking techniques: tracer advection, stretch rate and dispersion analysis. The phenomenon of steady streaming in an oscillatory flow is found to hold the key to the origin of kinematic mixing in the alveolus, the alveolar mouth and the alveolated duct. This mechanism provides the common route to folding of material lines and surfaces in any region of the acinar flow, and has no bearing on whether the geometry is expanding or if flow separates within the cavity or not. All analyses consistently indicate a significant decrease in mixing with decreasing Reynolds number (Re). For a given Re, dispersion is found to increase with degree of alveolation, indicating that geometry effects are important. These effects of Re and geometry can also be explained by the streaming mechanism. Based on flow conditions and resultant convective mixing measures, we conclude that significant convective mixing in the duct and within an alveolus could originate only in the first few generations of the acinar tree as a result of nonzero inertia, flow asymmetry, and large Keulegan–Carpenter (KC) number. PMID:21580803
Experiments on a Steady Low Reynolds Number Airfoil in a Shear Flow
Olson, David; Naguib, Ahmed; Koochesfahani, Manoochehr
2016-11-01
The aerodynamics of steady airfoils in uniform flow have received considerably more attention than that of an airfoil operating in a non-uniform flow. Inviscid theory by Tsien (1943) shows that an airfoil experiences a decrease in the zero lift angle of attack for a shear flow with uniform clockwise vorticity. The current work utilizes a shaped honeycomb technique to create a velocity profile with a large region of uniform shear in a water tunnel. Direct force measurements are implemented and validated using experiments on a circular cylinder and NACA 0012 in a uniform cross-flow. Results for a NACA 0012 airfoil with a chord Reynolds number of 1.2 ×104 in a non-uniform approach flow are compared to concurrent CFD calculations (presented in a companion talk) showing an increase in the zero lift angle of attack; in contradiction with inviscid theory. The effect of shear on the mean lift coefficient over a wide range of angles of attack is also explored. This work was supported by AFOSR Award Number FA9550-15-1-0224.
Lift on a Steady Airfoil in Low Reynolds Number Shear Flow
Hammer, Patrick; Visbal, Miguel; Naguib, Ahmed; Koochesfahani, Manoochehr
2016-11-01
Current understanding of airfoil aerodynamics is primarily based on a uniform freestream velocity approaching the airfoil, without consideration for possible presence of shear in the approach flow. Inviscid theory by Tsien (1943) shows that a symmetric airfoil at zero angle of attack experiences positive lift, i.e. a shift in the zero-lift angle of attack, in the presence of positive mean shear in the approach flow. In the current work, 2D computations are conducted on a steady NACA 0012 airfoil at a chord Reynolds number of Re = 12,000, at zero angle of attack. A uniform shear profile (i.e. a linear velocity variation) is used for the approach flow by modifying the FDL3DI Navier-Stokes solver (Visbal and Gaitonde, 1999). Interestingly, opposite to the inviscid prediction of Tsien (1943), the results for the airfoil at zero angle of attack show that the average lift is negative in the shear flow. The magnitude of this lift grows as the shear rate increases. Additional results are presented regarding the physics underlying the shear effect on lift. A companion experimental study is also given in a separate presentation. This work was supported by AFOSR Award Number FA9550-15-1-0224.
Secondary vortex street in the wake of two tandem circular cylinders at low Reynolds number.
Wang, Si-ying; Tian, Fang-bao; Jia, Lai-bing; Lu, Xi-yun; Yin, Xie-zhen
2010-03-01
The experiments on two tandem circular cylinders were conducted in a horizontal soap film tunnel for the Reynolds number Re=60 , 80, and 100 and the nondimensional center-to-center spacing Gamma ranging in 1 approximately 12. The flow patterns were recorded by a high-speed camera and the vortex shedding frequency was obtained by a spatiotemporal evolution method. The secondary vortex formation (SVF) mode characterized by the formation of a secondary vortex street in the wake of the downstream cylinder was found at large gamma. Moreover, some typical modes predicted by previous investigations, including the single bluff-body, shear layer reattachment, and synchronization of vortex shedding modes, were also revisited in our experiments. Further, numerical simulations were carried out using a space-time finite-element method and the results confirmed the existence of the SVF mode. The mechanism of SVF mode was analyzed in terms of the numerical results. The dependence of the Strouhal number Sr on Gamma was given and the flow characteristics relevant to the critical spacing values and the hysteretic mode transitions were investigated.
Linkmann, Moritz; Berera, Arjun; Goldstraw, Erin E.
2017-01-01
This paper examines the behavior of the dimensionless dissipation rate Cɛ for stationary and nonstationary magnetohydrodynamic (MHD) turbulence in the presence of external forces. By combining with previous studies for freely decaying MHD turbulence, we obtain here both the most general model equation for Cɛ applicable to homogeneous MHD turbulence and a comprehensive numerical study of the Reynolds number dependence of the dimensionless total energy dissipation rate at unity magnetic Prandtl number. We carry out a series of medium to high resolution direct numerical simulations of mechanically forced stationary MHD turbulence in order to verify the predictions of the model equation for the stationary case. Furthermore, questions of nonuniversality are discussed in terms of the effect of external forces as well as the level of cross- and magnetic helicity. The measured values of the asymptote Cɛ ,∞ lie between 0.193 ≤Cɛ ,∞≤0.268 for free decay, where the value depends on the initial level of cross- and magnetic helicities. In the stationary case we measure Cɛ ,∞=0.223 .
Active Control of Flow Separation on a High-Lift System with Slotted Flap at High Reynolds Number
Khodadoust, Abdollah; Washburn, Anthony
2007-01-01
The NASA Energy Efficient Transport (EET) airfoil was tested at NASA Langley's Low- Turbulence Pressure Tunnel (LTPT) to assess the effectiveness of distributed Active Flow Control (AFC) concepts on a high-lift system at flight scale Reynolds numbers for a medium-sized transport. The test results indicate presence of strong Reynolds number effects on the high-lift system with the AFC operational, implying the importance of flight-scale testing for implementation of such systems during design of future flight vehicles with AFC. This paper describes the wind tunnel test results obtained at the LTPT for the EET high-lift system for various AFC concepts examined on this airfoil.
Edlund, E M
2014-01-01
We present measurements of quasi-Keplerian flows in a Taylor-Couette device that identify the boundary conditions required to generate near-ideal flows that exhibit self-similarity under scaling of the Reynolds number. These experiments are contrasted with alternate boundary configurations that result in flows that progressively deviate from ideal Couette rotation as the Reynolds number is increased. These behaviors are quantitatively explained in terms of the tendency to generate global Ekman circulation and the balance of angular momentum fluxes through the axial and radial boundary layers.
Energy Technology Data Exchange (ETDEWEB)
Ozmen, Y.; Baydar, E. [Karadeniz Technical University, Department of Mechanical Engineering, Trabzon (Turkey)
2008-09-15
The flow and heat transfer characteristics of an unconfined air jet that is impinged normally onto a heated flat plate have been experimentally investigated for high Reynolds numbers ranging from 30,000 to 70,000 and a nozzle-to-plate spacing range of 1-10. The mean and turbulence velocities by using hot-wire anemometry and impingement surface pressures with pressure transducer are measured. Surface temperature measurements are made by means of an infrared thermal imaging technique. The effects of Reynolds number and nozzle-to-plate spacing on the flow structure and heat transfer characteristics are described and compared with similar experiments. It was seen that the locations of the second peaks in Nusselt number distributions slightly vary with Reynolds number and nozzle-to-plate spacing. The peaks in distributions of Nusselt numbers and radial turbulence intensity are compatible for spacings up to 3. The stagnation Nusselt number was correlated for the jet Reynolds number and the nozzle-to-plate spacing as Nu{sub st}{proportional_to}Re {sup 0.69}(H/D){sup 0.019}. (orig.)
Dou, Zhongwang; Bragg, Andrew; Hammond, Adam; Liang, Zach; Collins, Lance; Meng, Hui
2016-11-01
Effects of Reynolds number (Rλ) and Stokes number (St) on particle-pair relative velocity (RV) were studied using four-frame particle tracking in an enclosed turbulence chamber. Two tests were performed: varying Rλ between 246 and 357 at six St values, and varying St between 0.02 and 4.63 at five Rλ values. By comparing experimental and DNS results of mean inward particle-pair RV, , we observed excellent agreement for all test conditions across a large range of particle separation distance (r) ; however at r values were higher than simulation. At fixed St , was found to be independent of Rλ in the observable St , r, and Rλ ranges. At fixed Rλ, increased with St at small r and decreased with St at large r. We further compared and variance of RV, , between experiments, DNS and theoretical predictions by Pan and Padoan (2010). At 0 theory-predicted and matched with DNS and experiment in the range of r = 1 - 60 η . As St increased, theoretical predictions were lower than experiment and DNS results. The potential causes of these trends are explored. Additionally, we discuss the observed electrostatic charge effect on particle relative motion in isotropic turbulence and our plans of studying this effect using an integrated experimental, numerical and theoretical approach. This work was supported by NSF CBET-0967407 and CBET-0967349.
Mesh Generation and Adaption for High Reynolds Number RANS Computations Project
National Aeronautics and Space Administration — This proposal offers to provide NASA with an automatic mesh generator for the simulation of aerodynamic flows using Reynolds-Averages Navier-Stokes (RANS) models....
Wall-modeled large-eddy simulation of transonic airfoil buffet at high Reynolds number
Fukushima, Yuma; Kawai, Soshi
2016-11-01
In this study, we conduct the wall-modeled large-eddy simulation (LES) of transonic buffet phenomena over the OAT15A supercritical airfoil at high Reynolds number. The transonic airfoil buffet involves shock-turbulent boundary layer interactions and shock vibration associated with the flow separation downstream of the shock wave. The wall-modeled LES developed by Kawai and Larsson PoF (2012) is tuned on the K supercomputer for high-fidelity simulation. We first show the capability of the present wall-modeled LES on the transonic airfoil buffet phenomena and then investigate the detailed flow physics of unsteadiness of shock waves and separated boundary layer interaction phenomena. We also focus on the sustaining mechanism of the buffet phenomena, including the source of the pressure waves propagated from the trailing edge and the interactions between the shock wave and the generated sound waves. This work was supported in part by MEXT as a social and scientific priority issue to be tackled by using post-K computer. Computer resources of the K computer was provided by the RIKEN Advanced Institute for Computational Science (Project ID: hp150254).
Jones, Gregory S.; Milholen, William E., II; Fell, Jared S.; Webb, Sandy R.; Cagle, C. Mark
2016-01-01
The application of a sweeping jet actuator to a circulation control system was initiated by a risk reduction series of experiments to optimize the authority of a single sweeping jet actuator. The sweeping jet design was integrated into the existing Fundamental Aerodynamic Subsonic Transonic- Modular Active Control (FAST-MAC) model by replacing the steady blowing system with an array of thirty-nine sweeping jet cartridges. A constant slot height to wing chord ratio was similar to the steady blowing configuration resulting in each actuator having a unique in size for the sweeping jet configuration. While this paper will describe the scaling and optimization of the actuators for future high Reynolds number applications, the major focus of this effort was to target the transonic flight regime by increasing the amplitude authority of the actuator. This was accomplished by modifying the diffuser of the sweeping jet actuator, and this paper highlights twelve different diffuser designs. The experimental portion of this work was completed in the NASA Langley National Transonic Facility.
Reconciling the Reynolds number dependence of scalar roughness length and laminar resistance
Li, Dan; Rigden, Angela; Salvucci, Guido; Liu, Heping
2017-04-01
The scalar roughness length and laminar resistance are necessary for computing scalar fluxes in numerical simulations and experimental studies. Their dependence on flow properties such as the Reynolds number remains controversial. In particular, two important power laws ("1/4" and "1/2"), both having strong theoretical foundations, have been widely used in various parameterizations and models. Building on a previously proposed phenomenological model for interactions between the viscous sublayer and the turbulent flow, it is shown here that the two scaling laws can be reconciled. The 1/4 power law corresponds to the situation where the vertical diffusion is balanced by the temporal change or advection due to a constant velocity in the viscous sublayer, while the 1/2 scaling corresponds to the situation where the vertical diffusion is balanced by the advection due to a linear velocity profile in the viscous sublayer. In addition, the recently proposed "1" power law scaling is also recovered, which corresponds to the situation where molecular diffusion dominates the scalar budget in the viscous sublayer. The formulation proposed here provides a unified framework for understanding the onset of these different scaling laws and offers a new perspective on how to evaluate them experimentally.
Aerodynamic performance of an airfoil with a prescribed wall protuberance at low Reynolds numbers
Duque-Daza, Carlos; Mejia, Cristian; Camacho, Diego; Lockerby, Duncan
2016-11-01
Numerical simulations of flow around a modified NACA0012 airfoil, featuring a small surface perturbation on the upper wall, were performed at two low Reynolds numbers. The aerodynamic performance was examined under conditions of incompressible steady state flow. Simulations at different angles of attack (AOA) were performed: 0, 6, 9.25 and 12 degrees for Re =5000, and 6, 9.25 and 12 for Re =50000. The effect of the wall-perturbation was assessed in terms of changes of drag and lift coefficients, and alterations of the upper wall turbulent boundary layer. Examination of mean velocity profiles reveals that the wall perturbation promotes boundary-layer separation near the leading edge and increase of the skin friction drag. An arguably improvement of the effectiveness, i.e. ratio of lift to drag, was observed for the modified profile for Re = 5000, especially at AOA of 6 degrees. This effect seems to be caused by a double effect: boundary layer separation approaching the leading edge and an increase of the lift coefficient caused by the larger pressure drop on the upper surface. The effect of the perturbation was always negative for the airfoil operating at Re =50000, independently of AOA.
Triple-deck analysis of transonic high Reynolds number flow through slender channels.
Kluwick, A; Kornfeld, M
2014-07-28
In this work, laminar transonic weakly three-dimensional flows at high Reynolds numbers in slender channels, as found in microsupersonic nozzles and turbomachines of micro-electro-mechanical systems, are considered. The channel height is taken so small that the viscous wall layers forming at the channel walls start to interact strongly rather than weakly with the inviscid core flow and, therefore, the classical boundary layer approach fails. The resulting viscous-inviscid interaction problem is formulated using matched asymptotic expansions and found to be governed by a triple-deck structure. As a consequence, the properties of the predominantly inviscid core region and the viscous wall layers have to be calculated simultaneously in the interaction region. Weakly three-dimensional effects caused by surface roughness, upstream propagating flow perturbations, boundary layer separation as well as bifurcating solutions are discussed. Representative results for subsonic as well as supersonic conditions are presented, and the importance of these flow phenomena in technical applications as, for example, a means to reduce shock losses through the use of deformed geometry is addressed.
Influences of initial velocity, diameter and Reynolds number on a circular turbulent air/air jet
Institute of Scientific and Technical Information of China (English)
Mi Jian-Chun; Du Cheng
2011-01-01
This paper assesses the suitability of the inflow Reynolds number defined by Reo ≡ UoD/v (here Uo and D are respectively the initial jet velocity and diameter while v is kinematic viscosity) for a round air/air jet.Specifically an experimental investigation is performed for the influences of U(o),D and Re(o) on the mean-velocity decay and spread coefficients (Ku,Kr) in the far field of a circular air jet into air from a smoothly contracting nozzle.Present measurements agree well with those previously obtained under similar inflow conditions.The relations Ku (oc) U(o) and Kr (oc) 1/U(o) for U(o) ＜ 5 m/s appear to work,while each coefficient approaches asymptotically to a constant for U(o) ＞ 6 m/s,regardless of the magnitudes of Reo and D.It is revealed that Reo may not be an appropriate dimensionless parameter to characterize the entire flow of a free air/air jet.This paper is the first paper that has challenged the suitability of Re(o) for turbulent free jets.
Material properties of of Caenorhabditis elegans swimming at low Reynolds number
Sznitman, Josue; Krajacic, Predrag; Lamitina, Todd; Arratia, Paulo E
2009-01-01
Undulatory locomotion, as seen in the nematode \\emph{Caenorhabditis elegans}, is a common swimming gait of organisms in the low Reynolds number regime, where viscous forces are dominant. While the nematode's motility is expected to be a strong function of its material properties, measurements remain scarce. Here, the swimming behavior of \\emph{C.} \\emph{elegans} are investigated in experiments and in a simple model. Experiments reveal that nematodes swim in a periodic fashion and generate traveling waves which decay from head to tail. The model is able to capture the experiments' main features and is used to estimate the nematode's Young's modulus $E$ and tissue viscosity $\\eta$. For wild-type \\emph{C. elegans}, we find $E\\approx 3.77$ kPa and $\\eta \\approx-860$ Pa$\\cdot$s; values of $\\eta$ for live \\emph{C. elegans} are negative because the tissue is generating rather than dissipating energy. Results show that material properties are sensitive to changes in muscle functional properties, and are useful quanti...
Effect of low Reynolds number flow on the quorum sensing behavior of sessile bacteria
Ingremeau, Francois; Minyoung, Kevin Kim; Bassler, Bonnie; Stone, Howard; Mechanical; Aerospace Engineering, Complex fluids Group Team; Molecular Biology Lab Team
2014-11-01
Sessile and planktonic bacteria can be sensitive to the bacteria cell density around them through a chemical mediated communication called quorum sensing. When the quorum sensing molecules reach a certain value, the metabolism of the bacteria changes. Quorum sensing is usually studied in static conditions or in well mixed environments. However, bacteria biofilms can form in porous media or in the circulatory system of an infected body: quorum sensing in such flowing environment at low Reynolds number is not well studied. Using microfluidic devices, we observe how the flow of a pure media affects quorum sensing of bacteria attached to the wall. The biofilm formation is quantified by measuring the optical density in brightfield microscopy and the quorum sensing gene expression is observed through the fluorescence of a green fluorescent protein, which is a reporter for one of the quorum sensing genes. We measured without flow the amount of Staphylococcus aureus biofilm when the quorum sensing gene expression starts. In contrast, when the media is flowing in the microchannel, the quorum sensing expression is delayed. This effect can be understood and modelled by considering the diffusion of the quorum sensing molecules in the biofilm and their convection by the flowing media.
High Reynolds number rough wall turbulent boundary layer experiments using Braille surfaces
Harris, Michael; Monty, Jason; Nova, Todd; Allen, James; Chong, Min
2007-11-01
This paper details smooth, transitional and fully rough turbulent boundary layer experiments in the New Mexico State high Reynolds number rough wall wind tunnel. The initial surface tested was generated with a Braille printer and consisted of an uniform array of Braille points. The average point height being 0.5mm, the spacing between the points in the span was 0.5mm and the surface consisted of span wise rows separated by 4mm. The wavelength to peak ratio was 8:1. The boundary layer thickness at the measurement location was 190mm giving a large separation of roughness height to layer thickness. The maximum friction velocity was uτ=1.5m/s at Rex=3.8 x10^7. Results for the skin friction co-efficient show that this surface follows a Nikuradse type inflectional curve and that Townsends outer layer similarity hypothesis is valid for rough wall flows with a large separation of scales. Mean flow and turbulence statistics will be presented.
Thrust generation and wake structure for flow across a pitching airfoil at low Reynolds number
Indian Academy of Sciences (India)
Intesaaf Ashraf; Amit Agrawal; Majid Hassan Khan; Sooraj P; Atul Srivastava; Atul Sharma
2015-12-01
In this work, we present detailed particle image velocimetry (PIV) based investigation of wake structure of a pitching airfoil. PIV measurements have been carried out for NACA0015 airfoil at Re = 2900 with reduced frequency range of 1.82–10.92 and pitching angle of 5°. Two different wake structures (reverse Kármán shedding and deflected vortex shedding) are observed over this parameter range. The vorticity decreases substantially over a distance of two chord-lengths. The velocity profile indicates a jet-like flow downstream of the airfoil. It is shown that the jet-like flow downstream of the airfoil is however not a sufficient condition for the generation of thrust. The vortex strength is found to be invariant of the pitching frequency. Certain differences from the reported results are noted, which may be because of difference in the airfoil shape. These results can help improve understanding of the flow behavior as the low Reynolds number range is not well studied.
House, Christopher; Armstrong, Jenelle; Burkhardt, John; Firebaugh, Samara
2014-06-01
With the end goal of medical applications such as non-invasive surgery and targeted drug delivery, an acoustically driven resonant structure is proposed for microrobotic propulsion. At the proposed scale, the low Reynolds number environment requires non-reciprocal motion from the robotic structure for propulsion; thus, a "flapper" with multiple, flexible joints, has been designed to produce excitation modes that involve the necessary flagella-like bending for non-reciprocal motion. The key design aspect of the flapper structure involves a very thin joint that allows bending in one (vertical) direction, but not the opposing direction. This allows for the second mass and joint to bend in a manner similar to a dolphin's "kick" at the bottom of their stroke, resulting in forward thrust. A 130 mm x 50 mm x 0.2 mm prototype of a swimming robot that utilizes the flapper was fabricated out of acrylic using a laser cutter. The robot was tested in water and in a water-glycerine solution designed to mimic microscale fluid conditions. The robot exhibited forward propulsion when excited by an underwater speaker at its resonance mode, with velocities up to 2.5 mm/s. The robot also displayed frequency selectivity, leading to the possibility of exploring a steering mechanism with alternatively tuned flappers. Additional tests were conducted with a robot at a reduced size scale.
Dynamic non-equilibrium wall-modeling for large eddy simulation at high Reynolds numbers
Kawai, Soshi; Larsson, Johan
2013-01-01
A dynamic non-equilibrium wall-model for large-eddy simulation at arbitrarily high Reynolds numbers is proposed and validated on equilibrium boundary layers and a non-equilibrium shock/boundary-layer interaction problem. The proposed method builds on the prior non-equilibrium wall-models of Balaras et al. [AIAA J. 34, 1111-1119 (1996)], 10.2514/3.13200 and Wang and Moin [Phys. Fluids 14, 2043-2051 (2002)], 10.1063/1.1476668: the failure of these wall-models to accurately predict the skin friction in equilibrium boundary layers is shown and analyzed, and an improved wall-model that solves this issue is proposed. The improvement stems directly from reasoning about how the turbulence length scale changes with wall distance in the inertial sublayer, the grid resolution, and the resolution-characteristics of numerical methods. The proposed model yields accurate resolved turbulence, both in terms of structure and statistics for both the equilibrium and non-equilibrium flows without the use of ad hoc corrections. Crucially, the model accurately predicts the skin friction, something that existing non-equilibrium wall-models fail to do robustly.
RICHTER, DAVID
2010-03-29
The results from a numerical investigation of inertial viscoelastic flow past a circular cylinder are presented which illustrate the significant effect that dilute concentrations of polymer additives have on complex flows. In particular, effects of polymer extensibility are studied as well as the role of viscoelasticity during three-dimensional cylinder wake transition. Simulations at two distinct Reynolds numbers (Re = 100 and Re = 300) revealed dramatic differences based on the choice of the polymer extensibility (L2 in the FENE-P model), as well as a stabilizing tendency of viscoelasticity. For the Re = 100 case, attention was focused on the effects of increasing polymer extensibility, which included a lengthening of the recirculation region immediately behind the cylinder and a sharp increase in average drag when compared to both the low extensibility and Newtonian cases. For Re = 300, a suppression of the three-dimensional Newtonian mode B instability was observed. This effect is more pronounced for higher polymer extensibilities where all three-dimensional structure is eliminated, and mechanisms for this stabilization are described in the context of roll-up instability inhibition in a viscoelastic shear layer. © 2010 Cambridge University Press.
Linear drag law for high-Reynolds-number flow past an oscillating body
Agre, Natalie; Childress, Stephen; Zhang, Jun; Ristroph, Leif
2016-07-01
An object immersed in a fast flow typically experiences fluid forces that increase with the square of speed. Here we explore how this high-Reynolds-number force-speed relationship is affected by unsteady motions of a body. Experiments on disks that are driven to oscillate while progressing through air reveal two distinct regimes: a conventional quadratic relationship for slow oscillations and an anomalous scaling for fast flapping in which the time-averaged drag increases linearly with flow speed. In the linear regime, flow visualization shows that a pair of counterrotating vortices is shed with each oscillation and a model that views a train of such dipoles as a momentum jet reproduces the linearity. We also show that appropriate scaling variables collapse the experimental data from both regimes and for different oscillatory motions into a single drag-speed relationship. These results could provide insight into the aerodynamic resistance incurred by oscillating wings in flight and they suggest that vibrations can be an effective means to actively control the drag on an object.
Numerical Simulation of Low Reynolds Number Particle-Laden Gas Jet by Vortex Method
Uchiyama, Tomomi; Yagami, Hisanori
An air jet, which remains laminar and axisymmetric in the single-phase flow condition, is simulated numerically in the particle-laden condition. The vortex method for particle-laden gas jet proposed by the authors is employed for the simulation. An air issues with velocity U0 from a round nozzle into the air co-flowing with velocity Ua. The Reynolds number based on U0 and the nozzle diameter is 1333, the velocity ratio Ua/U0 is 0.4. Spherical glass particles with diameter 65μm are loaded at the mass loading ratio 0.025. The particle velocity at the nozzle exit is 0.68U0. The particles impose disturbances on the air and induce the three-dimensional flow, resulting in the transition from the axisymmetric flow to the non-axisymmetric one. As the particles make the air velocity fluctuation increase, the air momentum diffuses more in the radial direction, and accordingly the spread of the jet becomes larger. The abovementioned results agree well with the trend of the existing experiments. The proposed vortex method can successfully capture the flow transition caused by the particles laden on an axisymmetric air jet.
COHERENT NONHELICAL SHEAR DYNAMOS DRIVEN BY MAGNETIC FLUCTUATIONS AT LOW REYNOLDS NUMBERS
Energy Technology Data Exchange (ETDEWEB)
Squire, J.; Bhattacharjee, A., E-mail: jsquire@caltech.edu [Department of Astrophysical Sciences and Princeton Plasma Physics Laboratory, Princeton University, Princeton, NJ 08543 (United States)
2015-11-01
Nonhelical shear dynamos are studied with a particular focus on the possibility of coherent dynamo action. The primary results—serving as a follow up to the results of Squire and Bhattacharjee—pertain to the “magnetic shear-current effect” as a viable mechanism to drive large-scale magnetic field generation. This effect raises the interesting possibility that the saturated state of the small-scale dynamo could drive large-scale dynamo action, and is likely to be important in the unstratified regions of accretion disk turbulence. In this paper, the effect is studied at low Reynolds numbers, removing the complications of small-scale dynamo excitation and aiding analysis by enabling the use of quasi-linear statistical simulation methods. In addition to the magnetically driven dynamo, new results on the kinematic nonhelical shear dynamo are presented. These illustrate the relationship between coherent and incoherent driving in such dynamos, demonstrating the importance of rotation in determining the relative dominance of each mechanism.
Weisberg, David
2016-10-01
Differentially rotating plasma flow has been measured in the Madison Plasma Dynamo Experiment (MPDX). Spherical cusp-confined plasmas have been stirred both from the plasma boundary using electrostatic stirring in the magnetized edge and in the plasma core using weak global fields and cross-field currents to impose a body-force torque. Laminar velocity profiles conducive to shear-driven MHD instabilities like the dynamo and the MRI are now being generated and controlled with magnetic Reynolds numbers of Rm method for plasma heating, but limits on input heating power have been observed (believed to be caused by the formation of double-layers at anodes). These confinement studies have culminated in large (R = 1.4 m), warm (Te 1), steady-state plasmas. Results of the ambipolar transport model are good fits to measurements of pressure gradients and fluid drifts in the cusp, and offer a predictive tool for future cusp-confined devices. Hydrodynamic modeling is shown to be a good description for measured plasma flows, where ion viscosity proves to be an efficient mechanism for transporting momentum from the magnetized edge into the unmagnetized core. In addition, the body-force stirring technique produces velocity profiles conducive to MRI experiments where dΩ / dr research of flow-driven astrophysical MHD instabilities.
Flow past a rotating cylinder at high Reynolds number using PANS method
Kumar, Rajesh
2016-11-01
In the present study, high-Reynolds number flow past a rotating cylinder has been simulated using Partially-Averaged Navier-Stokes (PANS) method. The simulations are performed at Re = 140000. The spin ratio of the cylinder, which is defined by the ratio of the circumferential speed of the cylinder to the free-stream speed, varies from a = 0 to a = 4. The resolved and the modeled physical scales have been compared with the corresponding LES data for better understanding of the efficacy of the PANS method. The comparison of PANS results with the LES results showed good agreement. It has been recognized that the PANS simulation is able to produce fairly acceptable results using even a coarse-mesh. It is recognized that the time-averaged flow statistics obtained using PANS and URANS simulations are approximately same. However the vortex structure is much better captured by the PANS method. With the increase in the spin ratio, decrease in the time-averaged drag and increase in the time-averaged lift force acting on the cylinder have been observed. The vortices in far wake region are displaced and deformed but those in the vicinity of the cylinder are stretched at the bottom and accumulated over the top of the cylinder.
LARGE-EDDY SIMULATION OF FLOW AROUND CYLINDER ARRAYS AT A SUBCRITICAL REYNOLDS NUMBER
Institute of Scientific and Technical Information of China (English)
ZOU Lin; LIN Yu-feng; LAM Kit
2008-01-01
The complex three-dimensional turbulent flows around a cylinder array with four cylinders in an in-line square configuration at a subcritical Reynolds number of 1.5 ×104 with the spacing ratio at and 3.5 were investigated using the Large Eddy Simulation (LES). The full field vorticity and velocity distributions as well as turbulent quantities were calculated in detail and the near wake structures were presented. The results show that the bi-stable flow nature was observed at and distinct vortex shedding of the upstream cylinders occurred at at . The techniques of Laser Doppler Anemometry (LDA) and Digital Particle Image Velocimetry (DPIV) are also employed to validate the present LES method. The results show that the numerical predictions are in excellent agreement with the experimental measurements. Therefore, the full field instantaneous and mean quantities of the flow field, velocity field and vorticity field can be extracted from the LES results for further study of the complex flow characteristics.
Extending the restricted nonlinear model for wall-turbulence to high Reynolds numbers
Bretheim, Joel; Meneveau, Charles; Gayme, Dennice
2016-11-01
The restricted nonlinear (RNL) model for wall-turbulence is motivated by the long-observed streamwise-coherent structures that play an important role in these flows. The RNL equations, derived by restricting the convective term in the Navier-Stokes equations, provide a computationally efficient approach due to fewer degrees of freedom in the underlying dynamics. Recent simulations of the RNL system have been conducted for turbulent channel flows at low Reynolds numbers (Re), yielding insights into the dynamical mechanisms and statistics of wall-turbulence. Despite the computational advantages of the RNL system, simulations at high Re remain out-of-reach. We present a new Large Eddy Simulation (LES) framework for the RNL system, enabling its use in engineering applications at high Re such as turbulent flows through wind farms. Initial results demonstrate that, as observed at moderate Re, restricting the range of streamwise varying structures present in the simulation (i.e., limiting the band of x Fourier components or kx modes) significantly affects the accuracy of the statistics. Our results show that only a few well-chosen kx modes lead to RNL turbulence with accurate statistics, including the mean profile and the well-known inner and outer peaks in energy spectra. This work is supported by NSF (WindInspire OISE-1243482).
McHenry, Matthew J; Azizi, Emanuel; Strother, James A
2003-01-01
Understanding how the shape and motion of an aquatic animal affects the performance of swimming requires knowledge of the fluid forces that generate thrust and drag. These forces are poorly understood for the large diversity of animals that swim at Reynolds numbers (Re) between 10(0) and 10(2). We experimentally tested quasi-steady and unsteady blade-element models of the hydrodynamics of undulatory swimming in the larvae of the ascidian Botrylloides sp. by comparing the forces predicted by these models with measured forces generated by tethered larvae and by comparing the swimming speeds predicted with measurements of the speed of freely swimming larvae. Although both models predicted mean forces that were statistically indistinguishable from measurements, the quasi-steady model predicted the timing of force production and mean swimming speed more accurately than the unsteady model. This suggests that unsteady force (i.e. the acceleration reaction) does not play a role in the dynamics of steady undulatory swimming at Re approximately 10(2). We explored the relative contribution of viscous and inertial force to the generation of thrust and drag at 10(0)10(2)) and low (<10(0)) Re, the fluid forces that generate thrust cannot be assumed to be the same as those that generate drag at intermediate Re.
Institute of Scientific and Technical Information of China (English)
Ming Pingjian; Zhang Wenping
2009-01-01
This article introduces a numerical scheme on the basis of semi-implicit method for pressure-linked equations (SIMPLE) algorithm to simulate incompressible unsteady flows with fluid-structure interaction. The Navier-Stokes equation is discretized spatially with collocated finite volume method and Eulerian implicit method in time domain. The hybrid method that combines immersed boundary method (IBM) and volume of fluid (VOF) method is used to deal with rigid body motion in fluid domain. The details of movement of immersed boundary (IB) and calculation of VOF are also described. This method can be easily applied to any existing finite-volume-based computational fluid dynamics (CFD) solver without complex operation, with which fluid flow interaction of arbitrarily complex geometry can be realized on a fixed mesh. The method is verified by low Reynolds number flows passing both stationary and oscillating cylinders. The drag and lift coefficients acquired by the study well accord with other published results, which indicate the reasonability of the proposed method.
Swimming at low Reynolds number: a beginners guide to undulatory locomotion
Cohen, Netta; Boyle, Jordan H.
2010-03-01
Undulatory locomotion is a means of self-propulsion that relies on the generation and propagation of waves along a body. As a mode of locomotion it is primitive and relatively simple, yet can be remarkably robust. No wonder then, that it is so prevalent across a range of biological scales from motile bacteria to gigantic prehistoric snakes. Key to understanding undulatory locomotion is the body's interplay with the physical environment, which the swimmer or crawler will exploit to generate propulsion, and in some cases, even to generate the underlying undulations. This review focuses by and large on undulators in the low Reynolds number regime, where the physics of the environment can be much more tractable. We review some key concepts and theoretical advances, as well as simulation tools and results applied to selected examples of biological swimmers. In particular, we extend the discussion to some simple cases of locomotion in non-Newtonian media as well as to small animals, in which the nervous system, motor control, body properties and the environment must all be considered to understand how undulations are generated and modulated. To conclude, we review recent progress in microrobotic undulators that may one day become commonplace in applications ranging from toxic waste disposal to minimally invasive surgery.
O'Hern, T. J.; Torczynski, J. R.
1993-06-01
The laminar steady flow downstream of fine-mesh screens is studied. Instead of woven-wire screens, high-uniformity screens are fabricated by photoetching holes into 50.8-micron-thick Inconel sheets. The resulting screens have minimum wire widths of 50.8 microns and inter-wire separations of 254 and 318 microns for the two screens examined. A flow facility has been constructed for experiments with these screens. Air is passed through the screens at upstream velocities yielding wire width Reynolds numbers from 2 to 35. To determine the drag coefficient, pressure drops across the screens are measured using pressure transducers and manometers. Three-dimensional flow simulations are also performed. The computational drag coefficients consistently overpredict the experimental values. However, the computational results exhibit sensitivity to the assumed wire cross section, indicating that detailed knowledge of the wire cross section is essential for unambiguous interpretation of experiments using photoetched screens. Standard semiempirical drag correlations for woven-wire screens do not predict the present experimental results with consistent accuracy.
NUMERICAL SIMULATIONS OF FLOW BEHAVIOR IN DRIVEN CAVITY AT HIGH REYNOLDS NUMBERS
Directory of Open Access Journals (Sweden)
Fudhail Bin Abdul Munir
2012-02-01
Full Text Available In recent years, due to rapidly increasing computational power, computational methods have become the essential tools to conduct researches in various engineering fields. In parallel to the development of ultra high speed digital computers, computational fluid dynamics (CFD has become the new third approach apart from theory and experiment in the philosophical study and development of fluid dynamics. Lattice Boltzmann method (LBM is an alternative method to conventional CFD. LBM is relatively new approach that uses simple microscopic models to simulate complicated microscopic behavior of transport phenomena. In this paper, fluid flow behaviors of steady incompressible flow inside lid driven square cavity are studied. Numerical calculations are conducted for different Reynolds numbers by using Lattice Boltzmann scheme. The objective of the paper is to demonstrate the capability of this lattice Boltzmann scheme for engineering applications particularly in fluid transport phenomena. Keywords-component; lattice Boltzmann method, lid driven cavity, computational fluid dynamics.
Learning from jellyfish: Fluid transport in muscular pumps at intermediate Reynolds numbers
Nawroth, Janna; Dabiri, John
2010-11-01
Biologically inspired hydrodynamic propulsion and maneuvering strategies promise the advancement of medical implants and minimally invasive clinical tools. We have chosen juvenile jellyfish as a model system for investigating fluid dynamics and morphological properties underlying fluid transport by a muscular pump at intermediate Reynolds numbers. Recently we have described how natural variations in viscous forces are balanced by changes in jellyfish body shape (phenotypic plasticity), to the effect of facilitating efficient body-fluid interaction. Complementing these studies in our live model organisms, we are also engaged in engineering an artificial jellyfish, that is, a jellyfish-inspired construct of a flexible plastic sheet actuated by a monolayer of rat cardiomyocytes. The main challenges here are (1) to derive a body shape and deformation suitable for effective fluid transport under physiological conditions, (2) to understand the mechanical properties of the muscular film and derive a design capable of the desired deformation, (3) to master the proper alignment and timely contraction of the muscle component needed to achieve the desired deformation, and (4) to evaluate the performance of the design.
Hydrodynamic interaction on large-Reynolds-number aligned bubbles: Drag effects
Energy Technology Data Exchange (ETDEWEB)
Ramirez-Munoz, J., E-mail: jrm@correo.azc.uam.mx [Departamento de Energia, Universidad Autonoma Metropolitana-Azcapotzalco, Av. San Pablo 180, Col. Reynosa Tamaulipas, 02200 Mexico D.F. (Mexico); Centro de Investigacion en Polimeros, Marcos Achar Lobaton No. 2, Tepexpan, 55885 Acolman, Edo. de Mexico (Mexico); Salinas-Rodriguez, E.; Soria, A. [Departamento de IPH, Universidad Autonoma Metropolitana-Iztapalapa, San Rafael Atlixco 186, Col. Vicentina, Iztapalapa, 09340 Mexico D.F. (Mexico); Gama-Goicochea, A. [Centro de Investigacion en Polimeros, Marcos Achar Lobaton No. 2, Tepexpan, 55885 Acolman, Edo. de Mexico (Mexico)
2011-07-15
Graphical abstract: Display Omitted Highlights: > The hydrodynamic interaction of a pair aligned equal-sized bubbles is analyzed. > The leading bubble wake decreases the drag on the trailing bubble. > A new semi-analytical model for the trailing bubble's drag is presented. > The equilibrium distance between bubbles is predicted. - Abstract: The hydrodynamic interaction of two equal-sized spherical gas bubbles rising along a vertical line with a Reynolds number (Re) between 50 and 200 is analyzed. An approach to estimate the trailing bubble drag based on the search of a proper reference fluid velocity is proposed. Our main result is a new, simple semi-analytical model for the trailing bubble drag. Additionally, the equilibrium separation distance between bubbles is predicted. The proposed models agree quantitatively up to small distances between bubbles, with reported data for 50 {<=} Re {<=} 200. The relative average error for the trailing bubble drag, Er, is found to be in the range 1.1 {<=} Er {<=} 1.7, i.e., it is of the same order of the analytical predictions in the literature.
Long-range μPIV in the turbulent region of a jet, at high Reynolds numbers
Fiscaletti, D.; Elsinga, G.E.; Westerweel, J.
The present work involves the investigation of the fine scale motions in the turbulent region of a high Reynolds number air jet. In the fully developed region of the jets, the small scales of turbulence are assumed to be isotropic, and expected to contain elongated vortices (worms), whose diameter s
Oliveira, J.L.G.; Geld, van der C.W.M.; Kuerten, J.G.M.
2013-01-01
Three-dimensional particle tracking velocimetry (3D-PTV) measurements have provided accurate Eulerian and Lagrangian high-order statistics of velocity and acceleration fluctuations and correlations at Reynolds number 10,300, based on the bulk velocity and the pipe diameter. Spatial resolution requir
DEFF Research Database (Denmark)
Yilmaz, Özlem Ceyhan; Pires, Oscar; Munduate, Xabier;
2017-01-01
This paper summarizes the results of a blind test campaign organized in the AVATAR project to predict the high Reynolds number performance of a wind turbine airfoil for wind turbine applications. The DU00-W-210 airfoil was tested in the DNW-HDG pressurized wind tunnel in order to investigate the ...
Directory of Open Access Journals (Sweden)
Martin Skote
2015-01-01
Full Text Available Spanwise oscillation applied on the wall under a spatially developing turbulent boundary layer flow is investigated using direct numerical simulation. The temporal wall forcing produces a considerable drag reduction over the region where oscillation occurs. Downstream development of drag reduction is investigated from Reynolds number dependency perspective. An alternative to the previously suggested power-law relation between Reynolds number and peak drag reduction values, which is valid for channel flow as well, is proposed. Considerable deviation in the variation of drag reduction with Reynolds number between different previous investigations of channel flow is found. The shift in velocity profile, which has been used in the past for explaining the diminishing drag reduction at higher Reynolds number for riblets, is investigated. A new predictive formula is derived, replacing the ones found in the literature. Furthermore, unlike for the case of riblets, the shift is varying downstream in the case of wall oscillations, which is a manifestation of the fact that the boundary layer has not reached a new equilibrium over the limited downstream distance in the simulations. Taking this into account, the predictive model agrees well with DNS data. On the other hand, the growth of the boundary layer does not influence the drag reduction prediction.
Dogan, Eda; Hearst, R. Jason; Ganapathisubramani, Bharathram
2017-03-01
A turbulent boundary layer subjected to free-stream turbulence is investigated in order to ascertain the scale interactions that dominate the near-wall region. The results are discussed in relation to a canonical high Reynolds number turbulent boundary layer because previous studies have reported considerable similarities between these two flows. Measurements were acquired simultaneously from four hot wires mounted to a rake which was traversed through the boundary layer. Particular focus is given to two main features of both canonical high Reynolds number boundary layers and boundary layers subjected to free-stream turbulence: (i) the footprint of the large scales in the logarithmic region on the near-wall small scales, specifically the modulating interaction between these scales, and (ii) the phase difference in amplitude modulation. The potential for a turbulent boundary layer subjected to free-stream turbulence to `simulate' high Reynolds number wall-turbulence interactions is discussed. The results of this study have encouraging implications for future investigations of the fundamental scale interactions that take place in high Reynolds number flows as it demonstrates that these can be achieved at typical laboratory scales.
Rosen, T; Nordmark, A; Aidun, C K; Lundell, F; Mehlig, B
2015-01-01
We numerically analyse the rotation of a neutrally buoyant spheroid in a shear flow at small shear Reynolds number. Using direct numerical stability analysis of the coupled nonlinear particle-flow problem we compute the linear stability of the log-rolling orbit at small shear Reynolds number, ${\\rm Re}_a$. As ${\\rm Re}_a \\to 0$ and as the box size of the system tends to infinity we find good agreement between the numerical results and earlier analytical predictions valid to linear order in ${\\rm Re}_a$ for the case of an unbounded shear. The numerical stability analysis indicates that there are corrections to the analytical result of order ${\\rm Re}_a^{3/2}$. We also compare the analytical results to results of lattice-Boltzmann simulations to analyse the stability of the tumbling orbit at shear Reynolds numbers of order unity. Theory for an unbounded system at infinitesimal shear Reynolds number predicts a bifurcation of the tumbling orbit at aspect ratio $\\lambda_{\\rm c} \\approx 0.137$ below which tumbling ...
Aerodynamic Characteristics of a Revised Target Drone Vehicle at Mach Numbers from 1.60 to 2.86
Blair, A. B., Jr.; Babb, C. Donald
1968-01-01
An investigation has been conducted in the Langley Unitary Plan wind tunnel to determine the aerodynamic characteristics of a revised target drone vehicle through a Mach number range from 1.60 to 2.86. The vehicle had canard surfaces and a swept clipped-delta wing with twin tip-mounted vertical tails.
Vega, E J; Acero, A J; Montanero, J M; Herrada, M A; Gañán-Calvo, A M
2014-06-01
We analyze both experimentally and numerically the formation of microbubbles in the jetting regime reached when a moderately viscous liquid stream focuses a gaseous meniscus inside a converging micronozzle. If the total (stagnation) pressure of the injected gas current is fixed upstream, then there are certain conditions on which a quasisteady gas meniscus forms. The meniscus tip is sharpened by the liquid stream down to the gas molecular scale. On the other side, monodisperse collections of microbubbles can be steadily produced in the jetting regime if the feeding capillary is appropriately located inside the nozzle. In this case, the microbubble size depends on the feeding capillary position. The numerical simulations for an imposed gas flow rate show that a recirculation cell appears in the gaseous meniscus for low enough values of that parameter. The experiments allow one to conclude that the bubble pinch-off comprises two phases: (i) a stretching motion of the precursor jet where the neck radius versus the time before the pinch essentially follows a potential law, and (ii) a final stage where a very thin and slender gaseous thread forms and eventually breaks apart into a number of micron-sized bubbles. Because of the difference between the free surface and core velocities, the gaseous jet breakage differs substantially from that of liquid capillary jets and gives rise to bubbles with diameters much larger than those expected from the Rayleigh-type capillary instability. The dependency of the bubble diameter upon the flow-rate ratio agrees with the scaling law derived by A. M. Gañán-Calvo [Phys. Rev. E 69, 027301 (2004)], although a slight influence of the Reynolds number can be observed in our experiments.
Rescaling of the Roe scheme in low Mach-number flow regions
Boniface, Jean-Christophe
2017-01-01
A rescaled matrix-valued dissipation is reformulated for the Roe scheme in low Mach-number flow regions from a well known family of local low-speed preconditioners popularized by Turkel. The rescaling is obtained explicitly by suppressing the pre-multiplication of the preconditioner with the time derivative and by deriving the full set of eigenspaces of the Roe-Turkel matrix dissipation. This formulation preserves the time consistency and does not require to reformulate the boundary conditions based on the characteristic theory. The dissipation matrix achieves by construction the proper scaling in low-speed flow regions and returns the original Roe scheme at the sonic line. We find that all eigenvalues are nonnegative in the subsonic regime. However, it becomes necessary to formulate a stringent stability condition to the explicit scheme in the low-speed flow regions based on the spectral radius of the rescaled matrix dissipation. With the large disparity of the eigenvalues in the dissipation matrix, this formulation raises a two-timescale problem for the acoustic waves, which is circumvented for a steady-state iterative procedure by the development of a robust implicit characteristic matrix time-stepping scheme. The behaviour of the modified eigenvalues in the incompressible limit and at the sonic line also suggests applying the entropy correction carefully, especially for complex non-linear flows.
Anomalous flow deflection at planetary bow shocks in the low Alfven Mach number regime
Nishino, Masaki N.; Fujimoto, Masaki; Tai, Phan-Duc; Mukai, Toshifumi; Saito, Yoshifumi; Kuznetsova, Masha M.; Rastaetter, Lutz
A planetary magnetosphere is an obstacle to the super-sonic solar wind and the bow shock is formed in the front-side of it. In ordinary hydro-dynamics, the flow decelerated at the shock is diverted around the obstacle symmetrically about the planet-Sun line, which is indeed observed in the magnetosheath most of the time. Here we show a case under a very low density solar wind in which duskward flow was observed in the dawnside magnetosheath of the Earth's magnetosphere. A Rankine-Hugoniot test across the bow shock shows that the magnetic effect is crucial for this "wrong flow" to appear. A full three-dimensional Magneto- Hydro-Dynamics (MHD) simulation of the situation in this previously unexplored parameter regime is also performed. It is illustrated that in addition to the "wrong flow" feature, various peculiar characteristics appear in the global picture of the MHD flow interaction with the obstacle. The magnetic effect at the bow shock should become more conspicuously around the Mercury's magnetosphere, because stronger interplanetary magnetic field and slower solar wind around the Mercury let the Alfven Mach number low. Resultant strong deformation of the magnetosphere induced by the "wrong flow" will cause more complex interaction between the solar wind and the Mercury.
Ireland, Peter J; Collins, Lance R
2015-01-01
In Part I of this study, we analyzed the motion of inertial particles in isotropic turbulence in the absence of gravity using direct numerical simulation (DNS). Here, in Part II, we introduce gravity and study its effect over a wide range of flow Reynolds numbers, Froude numbers, and particle Stokes numbers. We see that gravity causes particles to sample the flow more uniformly and reduces the time particles can spend interacting with the underlying turbulence. We also find that gravity tends to increase inertial particle accelerations, and we introduce a model to explain that effect. We then analyze the particle relative velocities and radial distribution functions (RDFs), which are generally seen to be independent of Reynolds number for low and moderate Kolmogorov-scale Stokes numbers $St$. We see that gravity causes particle relative velocities to decrease, and that the relative velocities have higher scaling exponents with gravity. We observe that gravity has a non-trivial effect on clustering, acting to ...
Xu, Yang; He, GuoSheng; Kulkarni, Varun; Wang, JinJun
2017-01-01
Time-resolved particle image velocimetry was employed to study the effect of Reynolds number ( Re sj) on synthetic jet vortex rings impinging onto a solid wall. Four Reynolds numbers ranging from 166 to 664 were investigated for comparison while other parameters were kept constant. It is found that the Reynolds number has a significant impact on the spatial evolution of near-wall vortical structures of the impinging synthetic jet. Velocity triple decomposition reveals that periodic Reynolds shear stresses produced by both impinging and secondary vortex rings agree well with a four-quadrant-type distribution rule, and the random velocity fluctuations are strengthened as Re sj increases. For radial wall jet, radial velocity profiles exhibit a self-similar behavior for all Re sj, and this self-similar profile gradually deviates from the laminar solution as Re sj is increased. In particular, the self-similar profile for low Re sj (166) coincides with the laminar solution indicating that periodic velocity fluctuations produced by vortex rings have little effect on the velocity profile of the laminar wall jet. This also provides evidence that the impinging synthetic jet is more effective in mixing than the continuous jet for the laminar flow. For the high Re sj, the mean skin friction coefficient has a slower decay rate after reaching peak, and the radial momentum flux has a higher value at locations far away from the impingement region, both of these can be attributed to the enhanced random fluctuations.
Log law of the wall revisited in Taylor-Couette flows at intermediate Reynolds numbers
Singh, Harminder; Suazo, Claudio Alberto Torres; Liné, Alain
2016-11-01
We provide Reynolds averaged azimuthal velocity profiles, measured in a Taylor-Couette system in turbulent flow, at medium Reynolds (7800 image velocimetry technique. We find that in the wall regions, close to the inner and outer cylinders, the azimuthal velocity profile reveals a significant deviation from classical logarithmic law. In order to propose a new law of the wall, the profile of turbulent mixing length was estimated from data processing; it was shown to behave nonlinearly with the radial wall distance. Based on this turbulent mixing length expression, a law of the wall was proposed for the Reynolds averaged azimuthal velocity, derived from momentum balance and validated by comparison to different data. In addition, the profile of viscous dissipation rate was investigated and compared to the global power needed to maintain the inner cylinder in rotation.
The High Reynolds Number Flow Through an Axial-Flow Pump
1993-11-01
velocity field. After some initial computations of the flow field using a viscous form of the code of Adamczyk, Mulac, and Celestina [1986] at a low Mach...Simulating Flows in Multistage Turbomachinery," ASME Paper 85-GT-220, 1985. Adamczyk, J. J., Mulac, R. A., and Celestina , M. L., "A Model for Closing the
Rudmin, Daniel
Previous research at RMC has cataloged the occurrence of limit cycle oscillations at low-to-moderate Reynolds numbers for an elastically mounted aeroelastic airfoil. These oscillations were attributed to boundary layer separation and the formation of a laminar separation bubble. For this thesis, an instrumented and motor-driven oscillating airfoil rig was designed and fabricated for the purpose of investigating the boundary layer of a NACA-0012 airfoil. The oscillating airfoil was driven by a servo motor to mimic the observed aeroelastic pitching with a sinusoid of matched amplitude and frequency. Hot-wire anemometry was used to investigate the near wake of the new motor-driven airfoil and compare it with the aeroelastic experiment. A chord-wise array of hot-film sensors captured the boundary layer state during the airfoil pitching oscillation. A novel analysis technique is introduced; A sliding window (in time) cross-correlation of adjacent sensors was used to detect dynamic laminar separation. Wind tunnel tests were performed at static angles-of-attack, for quasi-static very low frequency sweeps to verify the technique, and for selected cases of oscillations obtained with the aeroelastic rig. The new detection method was verified against the existing static techniques of phase reversal signature detection and signal cross-correlation by comparing quasi-static and static results. A map of the laminar separation bubble was produced for fixed angles of attack as well as for the pitching airfoil. The presence of a laminar separation was linked to the occurrence and characteristics of the limit cycle oscillations. Keywords: laminar separation, NACA0012, hot-film, hot-wire, anemometry, transitional flow, aeroelasticity.
Vanfossen, G. James; Simoneau, Robert J.; Ching, Chan Y.
1994-01-01
The purpose of the present work was threefold: (1) to determine if a free-stream turbulence length scale existed that would cause the greatest augmentation in stagnation-region heat transfer over laminar levels; (2) to investigate the effect of velocity gradient on stagnation-region heat transfer augmentation by free-stream turbulence; and (3) to develop a prediction tool for stagnation heat transfer in the presence of free-stream turbulence. Heat transfer was measured in the stagnation region of four models with elliptical leading edges that had ratios of major to minor axes of 1:1, 1.5:1, 2.25:1, and 3:1. Five turbulence-generating grids were fabricated; four were square mesh, biplane grids made from square bars. The fifth grid was an array of fine parallel wires that were perpendicular to the model spanwise direction. Heat transfer data were taken at Reynolds numbers ranging from 37 000 to 228 000. Turbulence intensities were in the range of 1.1 to 15.9% while the ratio of integral length scale to leading-edge diameter ranged from 0.05 to 0.30. Stagnation-point velocity gradient was varied by nearly 50%. Stagnation-region heat transfer augmentation was found to increase with decreasing length scale but no optimum length scale was found. Heat transfer augmentation due to turbulence was found to be unaffected by the velocity gradient near the leading edge. A correlation was developed that fit heat transfer data for the square-bar grids to within +/- 4%.
Rotation induced flow suppression around two tandem circular cylinders at low Reynolds number
Chatterjee, Dipankar; Gupta, Krishan; Kumar, Virendra; Varghese, Sachin Abraham
2017-08-01
The rotation to a bluff object is known to have a stabilizing effect on the fluid dynamic transport around the body. An unsteady periodic flow can be degenerated into a steady flow pattern depending on the rate of rotation imparted to the body. On the other hand, multiple bodies placed in tandem arrangement with respect to an incoming flow can cause destabilization to the flow as a result of the complicated wake interaction between the bodies. Accordingly, the spacing between the bodies and the rate of rotation have significant impact on the overall fluid dynamic transport around them. The present work aims to understand how these two competing factors are actually influencing the fluidic transport across a pair of identical rotating circular cylinders kept in tandem arrangement in an unconfined medium. The cylinders are subjected to a uniform free stream flow and the gaps between the cylinders are varied as 0.2, 0.7, 1.5 and 3.0. Both the cylinders are made to rotate in the clockwise sense. The Reynolds number based on the free stream flow is taken as 100. A two-dimensional finite volume based transient computation is performed for a range of dimensionless rotational speeds of the cylinders (0 ≤ Ω ≤ 2.75). The results show that the shedding phenomena can be observed up to a critical rate of rotation (Ωcr) depending on the gap spacing. Beyond Ωcr, the flow becomes stabilized and finally completely steady as Ω increases further. Increasing the gap initially causes a slight decrease in the critical rotational speed, however, it increases at a rapid rate for larger gap spacing.
LARGE AERODYNAMIC FORCES ON A SWEEPING WING AT LOW REYNOLDS NUMBER
Institute of Scientific and Technical Information of China (English)
SUN Mao; WU Jianghao
2004-01-01
The aerodynamic forces and flow structure of a model insect wing is studied by solving the Navier-Stokes equations numerically. After an initial start from rest, the wing is made to execute an azimuthal rotation (sweeping) at a large angle of attack and constant angular velocity. The Reynolds number (Re) considered in the present note is 480 (Re is based on the mean chord length of the wing and the speed at 60% wing length from the wing root). During the constant-speed sweeping motion, the stall is absent and large and approximately constant lift and drag coefficients can be maintained. The mechanism for the absence of the stall or the maintenance of large aerodynamic force coefficients is as follows. Soon after the initial start, a vortex ring, which consists of the leading-edge vortex (LEV), the starting vortex, and the two wing-tip vortices, is formed in the wake of the wing. During the subsequent motion of the wing, a base-to-tip spanwise flow converts the vorticity in the LEV to the wing tip and the LEV keeps an approximately constant strength. This prevents the LEV from shedding. As a result,the size of the vortex ring increases approximately linearly with time, resulting in an approximately constant time rate of the first moment of vorticity, or approximately constant lift and drag coefficients.The variation of the relative velocity along the wing span causes a pressure gradient along the wingspan. The base-to-tip spanwise flow is mainly maintained by the pressure-gradient force.
Entropy Analyses of Droplet Combustion in Convective Environment with Small Reynolds Number
Institute of Scientific and Technical Information of China (English)
ZHANG Xiaobin; ZHANG Wei; ZHANG Xuejun
2013-01-01
This paper analyzes the entropy generation rate of simple pure droplet combustion in a temperature-elevated air convective environment based on the solutions of flow,and heat and mass transfer between the two phases.The flow-field calculations are carried out by solving the respective conservation equations for each phase,accounting for the droplet deformation with the axisymmetric model.The effects of the temperature,velocity and oxygen fraction of the free stream air on the total entropy generation rate in the process of the droplet combustion are investigated.Special attention is given to analyze the quantitative effects of droplet deformation.The results reveal that the entropy generation rate due to chemical reaction occupies a large fraction of the total entropy generated,as a result of the large areas covered by the flame.Although,the magnitude of the entropy generation rate per volume due to heat transfer and combined mass and heat transfer has a magnitude of one order greater than that due to chemical reaction,they cover a very limited area,leading to a small fraction of the total entropy generated.The entropy generation rate due to mass transfer is negligible.High temperature and high velocity of the free stream are advantageous to increase the exergy efficiency in the range of small Reynolds number (＜1) from the viewpoint of the second-law analysis over the droplet lifetime.The effect of droplet deformation on the total entropy generation is the modest.
Arko, Bryan M.
Design trends for the low-pressure turbine (LPT) section of modern gas turbine engines include increasing the loading per airfoil, which promises a decreased airfoil count resulting in reduced manufacturing and operating costs. Accurate Reynolds-Averaged Navier-Stokes predictions of separated boundary layers and transition to turbulence are needed, as the lack of an economical and reliable computational model has contributed to this high-lift concept not reaching its full potential. Presented here for what is believed to be the first time applied to low-Re computations of high-lift linear cascade simulations is the Abe-Kondoh-Nagano (AKN) linear low-Re two-equation turbulence model which utilizes the Kolmogorov velocity scale for improved predictions of separated boundary layers. A second turbulence model investigated is the Kato-Launder modified version of the AKN, denoted MPAKN, which damps turbulent production in highly strained regions of flow. Fully Laminar solutions have also been calculated in an effort to elucidate the transitional quality of the turbulence model solutions. Time accurate simulations of three modern high-lift blades at a Reynolds number of 25,000 are compared to experimental data and higher-order computations in order to judge the accuracy of the results, where it is shown that the RANS simulations with highly refined grids can produce both quantitatively and qualitatively similar separation behavior as found in experiments. In particular, the MPAKN model is shown to predict the correct boundary layer behavior for all three blades, and evidence of transition is found through inspection of the components of the Reynolds Stress Tensor, spectral analysis, and the turbulence production parameter. Unfortunately, definitively stating that transition is occurring becomes an uncertain task, as similar evidence of the transition process is found in the Laminar predictions. This reveals that boundary layer reattachment may be a result of laminar
Milholen, William E., II; Jones, Gregory S.; Chan, David T.; Goodliff, Scott L.; Anders, Scott G.; Melton, Latunia P.; Carter, Melissa B.; Allan, Brian G.; Capone, Francis J.
2013-01-01
A second wind tunnel test of the FAST-MAC circulation control model was recently completed in the National Transonic Facility at the NASA Langley Research Center. The model was equipped with four onboard flow control valves allowing independent control of the circulation control plenums, which were directed over a 15% chord simple-hinged flap. The model was configured for low-speed high-lift testing with flap deflections of 30 and 60 degrees, along with the transonic cruise configuration with zero degree flap deflection. Testing was again conducted over a wide range of Mach numbers up to 0.88, and Reynolds numbers up to 30 million based on the mean chord. The first wind tunnel test had poor transonic force and moment data repeatability at mild cryogenic conditions due to inadequate thermal conditioning of the balance. The second test demonstrated that an improvement to the balance heating system significantly improved the transonic data repeatability, but also indicated further improvements are still needed. The low-speed highlift performance of the model was improved by testing various blowing slot heights, and the circulation control was again demonstrated to be effective in re-attaching the flow over the wing at off-design transonic conditions. A new tailored spanwise blowing technique was also demonstrated to be effective at transonic conditions with the benefit of reduced mass flow requirements.
2011-07-28
have been the angle of attack, the turbulence model, the airfoil, the Reynolds number, and the grid spacing. The Taguchi method also allows the...important to remember that Taguchi Methods do not provide "hard engineering" numbers - only the statistical significance of a particular experimental factor...34good", "bad", or "acceptable". To use a Taguchi Method to aid in evaluating a CFD code would require sound engineering judgement since so very many
Self-sustained oscillations between two tandem cylinders at Reynolds number 1,000
Energy Technology Data Exchange (ETDEWEB)
Kuo, C.H.; Chein, S.M.; Hsieh, H.J. [National Chung Hsing University, Department of Mechanical Engineering, Taichung (China)
2008-04-15
This study focuses on the self-sustained oscillatory flow characteristics between two tandem circular cylinders of equal diameter placed in a uniform inflow. The Reynolds number (Re{sub D}), based on the cylinder diameter, was around 1,000 and all experiments were performed in a recirculating water channel. The streamwise distance between two tandem cylinders ranged within 1.5 {<=} X{sub c}/D {<=} 7.0. Here X{sub c} denotes the center-to-center distance between two tandem cylinders. For all experiments studied herein, quantitative velocity measurements were performed using hot-film anemometer and the LDV system. The laser sheet technique was employed for qualitative flow visualization. The wavelet transform was applied to elucidate the temporal variation and phase difference between two spectral components of the velocity signals detected in the flow field. The remarkable finding was that when two tandem circular cylinders were spaced at a distance within 4.5 {<=} X{sub c}/D {<=} 5.5, two symmetrical unstable shear layers with a certain wavelength were observed to impinge onto the downstream cylinder. The responding frequency (f{sub u}), measured between these two cylinders, was much higher than the natural shedding frequency behind a single isolated cylinder at the same Re{sub D}. This responding frequency decreased as the distance X{sub c}/D increased. Not until X{sub c}/D {>=} 6.0, did it recover to the natural shedding frequency behind a single isolated cylinder. Between two tandem cylinders, the Strouhal numbers (St{sub c} = f{sub u} X{sub c}/U{sub c}) maintained a nearly constant value of 3, indicating the self-sustained oscillating flow characteristics with a wavelength X{sub c}/3. Here U{sub c} is the convection speed of the unstable shear layers between two tandem cylinders. At Re{sub D} = 1,000, the self-sustained oscillating characteristics between two tandem circular cylinders were proven to exhibit a sustained flow pattern, not just a sporadic
Energy Technology Data Exchange (ETDEWEB)
Cardenas, Camilo [Karlsruhe Institute of Technology, Institute for Chemical Technology and Polymer Chemistry, Karlsruhe (Germany); Convenio Andres Bello, Instituto Internacional de Investigaciones Educativas para la Integracion, La Paz (Bolivia); Denev, Jordan A.; Bockhorn, Henning [Karlsruhe Institute of Technology, Engler-Bunte-Institute, Combustion Division, Karlsruhe (Germany); Suntz, Rainer [Karlsruhe Institute of Technology, Institute for Chemical Technology and Polymer Chemistry, Karlsruhe (Germany)
2012-10-15
Investigation of the mixing process is one of the main issues in chemical engineering and combustion and the configuration of a jet into a cross-flow (JCF) is often employed for this purpose. Experimental data are gained for the symmetry plane in a JCF-arrangement of an air flow using a combination of particle image velocimetry (PIV) with laser-induced fluorescence (LIF). The experimental data with thoroughly measured boundary conditions are complemented with direct numerical simulations, which are based on idealized boundary conditions. Two similar cases are studied with a fixed jet-to-cross-flow velocity ratio of 3.5 and variable cross-flow Reynolds numbers equal to 4,120 and 8,240; in both cases the jet issues from the pipe at laminar conditions. This leads to a laminar-to-turbulent transition, which depends on the Reynolds number and occurs quicker for the case with higher Reynolds number in both experiments and simulations as well. It was found that the Reynolds number only slightly affects the jet trajectory, which in the case with the higher Reynolds number is slightly deeper. It is attributed to the changed boundary layer shape of the cross-flow. Leeward streamlines bend toward the jet and are responsible for the strong entrainment of cross-flow fluid into the jet. Velocity components are compared for the two Reynolds numbers at the leeward side at positions where strongest entrainment is present and a pressure minimum near the jet trajectory is found. The numerical simulations showed that entrainment is higher for the case with the higher Reynolds number. The latter is attributed to the earlier transition in this case. Fluid entrainment of the jet in cross-flow is more than twice stronger than for a similar flow of a jet issuing into a co-flowing stream. This comparison is made along the trajectory of the two jets at a distance of 5.5 jet diameters downstream and is based on the results from the direct numerical simulations and recently published
A comparative study of scramjet injection strategies for high Mach numbers flows
Riggins, D. W.; Mcclinton, C. R.; Rogers, R. C.; Bittner, R. D.
1992-01-01
A simple method for predicting the axial distribution of supersonic combustor thrust potential is described. A complementary technique for illustrating the spatial evolution and distribution of thrust potential and loss mechanisms in reacting flows is developed. Wall jet cases and swept ramp injector cases for Mach 17 and Mach 13.5 flight enthalpy inflow conditions are numerically modeled and analyzed using these techniques. The visualization of thrust potential in the combustor for the various cases examined provides a unique tool for increasing understanding of supersonic combustor performance potential.
Enjilela, Vali; Salimi, Davood; Tavasoli, Ali; Lotfi, Mohsen
2016-02-01
In the present work, the meshless local Petrov-Galerkin vorticity-stream function (MLPG-VF) method is extended to solve two-dimensional laminar fluid flow and heat transfer equations for high Reynolds and Rayleigh numbers. The characteristic-based split (CBS) scheme which uses unity test function is employed for discretization, and the moving least square (MLS) method is used for interpolation of the field variables. Four test cases are considered to evaluate the present algorithm, namely lid-driven cavity flow with Reynolds numbers up to and including 104, flow over a backward-facing step at Reynolds number of 800, natural convection in a square cavity for Rayleigh numbers up to and including 108, and natural convection in a concentric square outer cylinder and circular inner cylinder annulus for Rayleigh numbers up to and including 107. In each case, the result obtained using the proposed algorithm is either compared with the results from the literatures or with those obtained using conventional numerical techniques. The present algorithm shows stable results at lower or equal computational cost compared to the other upwinding schemes usually employed in the MLPG method. Close agreements between the compared results as well as higher accuracy of the proposed method show the ability of this stabilized algorithm.
Low, R; Pothérat, A
2015-05-01
We investigate aspects of low-magnetic-Reynolds-number flow between two parallel, perfectly insulating walls in the presence of an imposed magnetic field parallel to the bounding walls. We find a functional basis to describe the flow, well adapted to the problem of finding the attractor dimension and which is also used in subsequent direct numerical simulation of these flows. For given Reynolds and Hartmann numbers, we obtain an upper bound for the dimension of the attractor by means of known bounds on the nonlinear inertial term and this functional basis for the flow. Three distinct flow regimes emerge: a quasi-isotropic three-dimensional (3D) flow, a nonisotropic 3D flow, and a 2D flow. We find the transition curves between these regimes in the space parametrized by Hartmann number Ha and attractor dimension d(att). We find how the attractor dimension scales as a function of Reynolds and Hartmann numbers (Re and Ha) in each regime. We also investigate the thickness of the boundary layer along the bounding wall and find that in all regimes this scales as 1/Re, independently of the value of Ha, unlike Hartmann boundary layers found when the field is normal to the channel. The structure of the set of least dissipative modes is indeed quite different between these two cases but the properties of turbulence far from the walls (smallest scales and number of degrees of freedom) are found to be very similar.
Institute of Scientific and Technical Information of China (English)
AHMED N A
2006-01-01
An attempt has been made to explore whether the power relation can be obtained from theoretical considerations. The classical laminar and turbulent boundary layer concepts have been employed to determine appropriate values of the scaling lengths associated with vortex shedding and shear layer frequencies to predict the power law relationship with Reynolds number. The predicted results are in good agreement with experimental results. The findings will provide a greater insight into the overall phenomenon involved.
Schaeffer, Derek; Haberberger, Dan; Fiksel, Gennady; Bhattacharjee, Amitava; Barnak, Daniel; Hu, Suxing; Germaschewski, Kai
2016-01-01
Shocks act to convert incoming supersonic flows to heat, and in collisionless plasmas the shock layer forms on kinetic plasma scales through collective electromagnetic effects. These collisionless shocks have been observed in many space and astrophysical systems [Smith 1975, Smith 1980, Burlaga 2008, Sulaiman 2015], and are believed to accelerate particles, including cosmic rays, to extremely high energies [Kazanas 1986, Loeb 2000, Bamba 2003, Masters 2013, Ackermann 2013]. Of particular importance are the class of high-Mach number, supercritical shocks [Balogh 2013] ($M_A\\gtrsim4$), which must reflect significant numbers of particles back into the upstream to accommodate entropy production, and in doing so seed proposed particle acceleration mechanisms [Blandford 1978, McClements 2001, Caprioli 2014, Matsumoto 2015]. Here we present the first laboratory generation of high-Mach number magnetized collisionless shocks created through the interaction of an expanding laser-driven plasma with a magnetized ambient ...
McMIllin, S. Naomi; Byrd, James E.; Parmar, Devendra S.; Bezos-O'Connor, Gaudy M.; Forrest, Dana K.; Bowen, Susan
1996-01-01
An experimental investigation of the effect of leading-edge radius, camber, Reynolds number, and boundary-layer state on the incipient separation of a delta wing at supersonic speeds was conducted at the Langley Unitary Plan Wind Tunnel at Mach number of 1.60 over a free-stream Reynolds number range of 1 x 106 to 5 x 106 ft-1. The three delta wing models examined had a 65 deg swept leading edge and varied in cross-sectional shape: a sharp wedge, a 20:1 ellipse, and a 20:1 ellipse with a -9.750 circular camber imposed across the span. The wings were tested with and without transition grit applied. Surface-pressure coefficient data and flow-visualization data are electronically stored on the CD-ROM. The data indicated that by rounding the wing leading edge or cambering the wing in the spanwise direction, the onset of leading-edge separation on a delta wing can be raised to a higher angle of attack than that observed on a sharp-edged delta wing. The data also showed that the onset of leading-edge separation can be raised to a higher angle of attack by forcing boundary-layer transition to occur closer to the wing leading edge by the application of grit or the increase in free-stream Reynolds number.
Guala, Michele; Liberzon, Alexander; Tsinober, Arkady; Kinzelbach, Wolfgang
Lagrangian auto- and cross-correlation functions of the rate of strain s(2) , enstrophy omega (2) , their respective production terms -s_{ij}s_{jk}s_{ki} and omega_{i}omega_{j}s_{ij}, and material derivatives, Ds s(2/Ds) t and Dsomega(2/Ds) t are estimated using experimental results obtained through three-dimensional particle tracking velocimetry (three-dimensional-PTV) in homogeneous turbulence at Re_{lambda} {=} 50. The autocorrelation functions are used to estimate the Lagrangian time scales of different quantities, while the cross-correlation functions are used to clarify some aspects of the interaction mechanisms between vorticity omega and the rate of strain tensor s_{ij}, that are responsible for the statistically stationary, in the Eulerian sense, levels of enstrophy and rate of strain in homogeneous turbulent flow. Results show that at the Reynolds number of the experiment these quantities exhibit different time scales, varying from the relatively long time scale of omega(2) to the relatively shorter time scales of s(2) , omega_{i}omega_{j}s_{ij} and -s_{ij}s_{jk}s_{ki}. Cross-correlation functions suggest that the dynamics of enstrophy and strain, in this flow, is driven by a set of different-time-scale processes that depend on the local magnitudes of s(2) and omega(2) . In particular, there are indications that, in a statistical sense, (i) strain production anticipates enstrophy production in low-strain low-enstrophy regions (ii) strain production and enstrophy production display high correlation in high-strain high-enstrophy regions, (iii) vorticity dampening in high-enstrophy regions is associated with weak correlations between -s_{ij}s_{jk}s_{ki} and s(2) and between -s_{ij}s_{jk}s_{ki} and Ds s(2) /Ds t, in addition to a marked anti-correlation between omega_{i}omega_{j}s_{ij} and Ds s(2) /Ds t. Vorticity dampening in high-enstrophy regions is thus related to the decay of s(2) and its production term, -s_{ij}s_{jk}s_{ki}.
Active Flow Separation Control of a Laminar Airfoil at Low Reynolds Number
Packard, Nathan Owen
Detailed investigation of the NACA 643-618 is obtained at a Reynolds number of 6.4x104 and angle of attack sweep of -5° locked investigation, by way of particle image velocimetry, at ten degrees angle of attack illuminates physical mechanisms responsible for separation control of pulsed actuation at a low frequency and duty cycle. Temporal resolution of large structure formation and wake shedding is obtained, revealing a key mechanism for separation control. The Kelvin-Helmholtz instability is identified as responsible for the formation of smaller structures in the separation region which produce favorable momentum transfer, assisting in further thinning the separation region and then fully attaching the boundary layer. Closed-loop separation control of an oscillating NACA 643-618 airfoil at Re = 6.4x104 is investigated in an effort to autonomously minimize control effort while maximizing aerodynamic performance. High response sensing of unsteady flow with on-surface hot-film sensors placed at zero, twenty, and forty percent chord monitors the airfoil performance and determines the necessity of active flow control. Open-loop characterization identified the use of the forty percent sensor as the actuation trigger. Further, the sensor at twenty percent chord is used to distinguish between pre- and post- leading edge stall; this demarcation enables the utilization of optimal blowing parameters for each circumstance. The range of effectiveness of the employed control algorithm is explored, charting the practicality of the closed-loop control algorithm. To further understand the physical mechanisms inherent in the control process, the transients of the aerodynamic response to flow control are investigated. The on-surface hot-film sensor placed at the leading edge is monitored to understand the time delays and response times associated with the initialization of pulsed normal blowing. The effects of angle of attack and pitch rate on these models are investigated. Black
Characteristics of the Velocity Power Spectrum as a Function of Taylor Reynolds Number
Puga, Alejandro J.
An understanding of the wide range of scales present in a turbulent flow as well as the turbulence kinetic energy associated with those scales can provide significant insight into the modeling of such flows. Since turbulence is a stochastic process, statistical quantities such as mean, root mean square, correlations and spectra are used to identify and understand the evolution of turbulent flows. Time-resolved velocity measurements presented herein are obtained using hot-wire anemometry in nearly homogeneous, isotropic and moderately high Taylor Reynolds number, Rlambda , flow downstream of an active grid. Velocity power spectra presented herein are show that the slope, n, of the inertial subrange, where the inertial subrange is defined as the wavenumber range where the power spectrum scales as kappa--n, varies with R lambda as n = 1.69 -- 5.86 Rlambda--0.645. This variation in the slope of the inertial subrange is consistent with measurements presented by Mydlarski and Warhaft (1996) in an active grid flow and Saddoughi and Veeravalli (1994) in a turbulent boundary layer. The effectiveness of velocity power spectrum normalizations proposed by Kolmogorov (1963), Von Karman and Howarth (1938), and George (1992) are compared qualitatively and quantitatively. The effectiveness of these normalizations suggests how the turbulent scales make specific portions of the velocity spectrum self-similar. It is found that the relation between the large and small scales is also shown by the normalized dissipation rate, which is defined as the dissipation rate normalized by the ratio of the turbulence kinetic energy to the time scale of the large scale structure is shown to be a constant with respect to R lambda for Rlambda ≥ 450. A modified model of the one-dimensional velocity power spectrum is proposed that is based on a model proposed by Pope (2000), which has been demonstrated to model power spectra at high value of Rlambda where the slope of the inertial subrange is very
Coupled dynamics of vortex-induced vibration and stationary wall at low Reynolds number
Li, Zhong; Jaiman, Rajeev K.; Khoo, Boo Cheong
2017-09-01
The flow past an elastically mounted circular cylinder placed in proximity to a plane wall is numerically studied in both two dimensions (2D) and three dimensions (3D). This paper aims to explain the mechanism of the cylinder bottom shear layer roll-up suppression in the context of laminar vortex-induced vibration (VIV) of a cylinder placed in the vicinity of a plane stationary wall. In 2D simulations, VIV of a near-wall cylinder with structure-to-displaced fluid mass ratios of m* = 2 and 10 is investigated at the Reynolds number of Re = 100 at a representative gap ratio of e/D = 0.90, where e denotes the gap distance between the cylinder surface and the plane wall. First, the cylinder is placed at five different upstream distances, LU, to study the effects of the normalized wall boundary layer thickness, δ /D , on the hydrodynamic quantities involved in the VIV of a near-wall cylinder. It is found that the lock-in range shifts towards the direction of the higher reduced velocity Ur as δ /D increases and that the lock-in range widens as m* reduces. Second, via visualization of the vortex shedding patterns, four different modes are classified and the regime maps are provided for both m* = 2 and 10. Third, the proper orthogonal decomposition analysis is employed to assess the cylinder bottom shear layer roll-up suppression mechanism. For 3D simulations at Re = 200, the circular cylinder of a mass ratio of m* = 10 with a spanwise length of 4D is placed at a gap ratio of e/D = 0.90 and an upstream distance of LU = 10D. The 3D vortex patterns are investigated to re-affirm the vortex shedding suppression mechanism. The pressure distributions around the cylinder are identified within one oscillation cycle of VIV. The pressure and the shear stress distributions on the bottom wall are examined to demonstrate the effects of near-wall VIV on the force distributions along the plane wall. It is found that both the suction pressure and the shear stress right below the cylinder
TR-PIV measurement of the wake behind a grooved cylinder at low Reynolds number
Liu, Ying Zheng; Shi, Liu Liu; Yu, Jun
2011-04-01
A comparative study of the wakes behind cylinders with grooved and smooth surfaces was performed with a view to understand the wake characteristics associated with the adult Saguaro cacti. A low-speed recirculation water channel was established for the experiment; the Reynolds number, based on the free-stream velocity and cylinder diameter (D), was kept at ReD=1500. State-of-the-art time-resolved particle image velocimetry (TR-PIV) was employed to measure a total of 20 480 realizations of the wake field at a frame rate of 250 Hz, enabling a comprehensive view of the time- and phase-averaged wake pattern. In comparison to the wake behind the smooth cylinder, the length of the recirculation zone behind the grooved cylinder was extended by nearly 18.2%, yet the longitudinal velocity fluctuation intensity was considerably weakened. A global view of the peaked spectrum of the longitudinal velocity component revealed that the intermediate region for the grooved cylinder, which approximately corresponds to the transition region where the shear layer vortices interact, merge and shed before the formation of the Karman-like vortex street, was much wider than that for the smooth one. The unsteady events near St=0.3-0.4 were detected in the intermediate region behind the grooved cylinder, but no such events were found in the smooth cylinder system. Although the formation of the Karman-like vortex street was delayed by about 0.6D downstream for the grooved cylinder, no prominent difference in the vortex street region was found in the far wake for both cylinders. The Proper Orthogonal Decomposition (POD) method was used extensively to decompose the vector and swirling strength fields, which gave a close-up view of the vortices in the near wake. The first two POD modes of the swirling strength clarified the spatio-temporal characteristics of the shear layer vortices behind the grooved cylinder. The small-scale vortices superimposed on the shear layers behind the grooved cylinder
Bauer, Christopher
1993-11-01
Stirling engine heat exchangers are shell-and-tube type with oscillatory flow (zero-mean velocity) for the inner fluid. This heat transfer process involves laminar-transition turbulent flow motions under oscillatory flow conditions. A low Reynolds number kappa-epsilon model, (Lam-Bremhorst form), was utilized in the present study to simulate fluid flow and heat transfer in a circular tube. An empirical transition model was used to activate the low Reynolds number k-e model at the appropriate time within the cycle for a given axial location within the tube. The computational results were compared with experimental flow and heat transfer data for: (1) velocity profiles, (2) kinetic energy of turbulence, (3) skin friction factor, (4) temperature profiles, and (5) wall heat flux. The experimental data were obtained for flow in a tube (38 mm diameter and 60 diameter long), with the maximum Reynolds number based on velocity being Re(sub max) = 11840, a dimensionless frequency (Valensi number) of Va = 80.2, at three axial locations X/D = 16, 30 and 44. The agreement between the computations and the experiment is excellent in the laminar portion of the cycle and good in the turbulent portion. Moreover, the location of transition was predicted accurately. The Low Reynolds Number kappa-epsilon model, together with an empirical transition model, is proposed herein to generate the wall heat flux values at different operating parameters than the experimental conditions. Those computational data can be used for testing the much simpler and less accurate one dimensional models utilized in 1-D Stirling Engine design codes.
Xu, Jinglei; Li, Meng; Zhang, Yang; Chen, Longfei
2016-12-01
The von Karman length scale is able to reflect the size of the local turbulence structure. However, it is not suitable for the near wall region of wall-bounded flows, for its value is almost infinite there. In the present study, a simple and novel length scale combining the wall distance and the von Karman length scale is proposed by introducing a structural function. The new length scale becomes the von Karman length scale once local unsteady structures are detected. The proposed method is adopted in a series of turbulent channel flows at different Reynolds numbers. The results show that the proposed length scale with the structural function can precisely simulate turbulence at high Reynolds numbers, even with a coarse grid resolution.
Numerical study on flow separation in 90° pipe bend under high Reynolds number by k-ε modelling
Directory of Open Access Journals (Sweden)
Prasun Dutta
2016-06-01
Full Text Available The present paper makes an effort to find the flow separation characteristics under high Reynolds number in pipe bends. Single phase turbulent flow through pipe bends is investigated using k-ε turbulence model. After the validation of present model against existing experimental results, a detailed study has been performed to study the influence of Reynolds number on flow separation and reattachment. The separation region and the velocity field of the primary and the secondary flows in different sections have been illustrated. Numerical results show that flow separation can be clearly visualized for bend with low curvature ratio. Distributions of the velocity vector show the secondary motion clearly induced by the movement of fluid from inner to outer wall of the bend leading to flow separation. This paper provides numerical results to understand the flow characteristics of fluid flow in 90° bend pipe.
Indian Academy of Sciences (India)
PRASENJIT DEY; AJOY K R DAS
2017-07-01
The behaviour of the fluid flowing over a square cylinder with rounded edges subjected to an upstream steady laminar flow was investigated numerically. Here, the commercial CFD software Fluent was used. A two-dimensional steady laminar flow has been investigated numerically at low Reynolds number 5<=Re<=45, different corner radii (r = 0.50, 0.51, 0.54, 0.59, 0.64 and 0.71) and blockage 0.05. The effects of the parameters such as Reynolds number and corner radius on the drag and laminar boundary layer have been studied for the first time. The results are shown in the form of drag coefficient, boundary layer and pressure coefficient on the cylinder surface. It is found that the boundary layer thickness and the displacement thickness decrease with decreasing of the corner radius for a particular Re and also the boundary layer profile shifted downwards on decreasing Re.
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.
Moarref, Rashad; Tropp, Joel A; McKeon, Beverley J
2013-01-01
We study the Reynolds number scaling of a gain-based, low-rank approximation to turbulent channel flows, determined by the resolvent formulation of McKeon & Sharma (2010), in order to obtain a description of the streamwise turbulence intensity from direct consideration of the Navier-Stokes equations. Under this formulation, the velocity field is decomposed into propagating waves (with single streamwise and spanwise wavelengths and wave speed) whose wall-normal shapes are determined from the principal singular function of the corresponding resolvent operator. We establish that the resolvent formulation admits three classes of wave parameters that induce universal behavior with Reynolds number on the low-rank model, and which are consistent with scalings proposed throughout the wall turbulence literature. For the rank-1 model subject to broadband forcing, the integrated streamwise energy density takes a universal form which is consistent with the dominant near-wall turbulent motions. When the shape of the f...
Effects of non-uniform interfacial tension in small Reynolds number ﬂow past a spherical liquid drop
Indian Academy of Sciences (India)
D P Mason; G M Moremedi
2011-09-01
A singular perturbation solution is given for small Reynolds number ﬂow past a spherical liquid drop. The interfacial tension required to maintain the drop in a spherical shape is calculated. When the interfacial tension gradient exceeds a critical value, a region of reversed ﬂow occurs on the interface at the rear and the interior ﬂow splits into two parts with reversed circulation at the rear. The magnitude of the interior ﬂuid velocity is small, of order the Reynolds number. A thin transition layer attached to the drop at the rear occurs in the exterior ﬂow. The effects could model the stagnant cap which forms as surfactant is added but the results apply however the variability in the interfacial tension might have been induced.
Gupta, Nikhil; Das, Siddhartha; Mitra, Sushanta K.; Kumar, Aloke
2015-01-01
In this paper, we obtain analytical results for shear stress distributions inside an elastic body placed in a low Reynolds number transport. The problem definition is inspired by a recent experimental study (Valiei et al., Lab Chip, 2012, 12, 5133-5137) that reports the flow-triggered deformation of bacterial biofilms, formed on cylindrical rigid microposts, into long filamentous structures known as streamers. In our analysis, we consider an elastic body of finite thickness (forming a rim) pl...
Cerutti, Stefano; Knio, Omar M.; Katz, Joseph
2000-10-01
Cavitation inception in the near field of high Reynolds number axisymmetric jets is analyzed using a simplified computational model. The model combines a vorticity-stream-function finite-difference scheme for the simulation of the unsteady flow field with a simplified representation for microscopic bubbles that are injected at the jet inlet. The motion of the bubbles is tracked in a Lagrangian reference frame by integrating a semiempirical dynamical equation which accounts for pressure, drag, and lift forces. The likelihood of cavitation inception is estimated based on the distributions of pressure and microscopic bubbles. The computations are used to examine the role of jet slenderness ratio, Reynolds number, bubble size, and bubble injection location on the cavitation inception indices. The results indicate that, for all bubble sizes considered, the cavitation inception index increases as the jet slenderness ratio decreases. Larger bubbles entrain more rapidly into the cores of concentrated vortices than smaller bubbles, and the corresponding inception indices are generally higher than those of smaller bubbles. The inception indices for larger bubbles are insensitive to the injection location, while the inception indices of smaller bubbles tend to increase when they are injected inside the shear layer near the nozzle lip. Although it affects the bubble distributions, variation of the Reynolds number leads to insignificant changes in pressure minima and in the inception indices of larger bubbles, having noticeable effect only on the inception indices of smaller bubbles. Computed results are consistent with, and provide plausible explanations for, several trends observed in recent jet cavitation experiments.
Structure of Wall-Eddies at Very Large Reynolds Number--A Large-Scale PIV Study
Hommema, S. E.; Adrian, R. J.
2000-11-01
The results of an experiment performed in the first 5 m of the neutral atmospheric boundary layer are presented. Large-scale PIV measurements (up to 2 m × 2 m field-of-view) were obtained in the streamwise / wall-normal plane of a very-large Reynolds number (Re_θ > 10^6, based on momentum thickness and freestream velocity), flat-plate, zero-pressure-gradient boundary layer. Measurements were obtained at the SLTEST facility in the U.S. Army's Dugway Proving Grounds. Coherent packets of ramp-like structures with downstream inclination are observed and show a remarkable resemblance to those observed in typical laboratory-scale experiments at far lower Reynolds number. The results are interpreted in terms of a vortex packet paradigm(Adrian, R.J., C.D. Meinhart, and C.D. Tomkins, Vortex organization in the outer region of the turbulent boundary layer, to appear in J. Fluid Mech., 2000.) and begin to extend the model to high Reynolds numbers of technological importance. Additional results obtained during periods of non-neutral atmospheric stability are contrasted with those of the canonical neutral boundary layer. Sample smoke visualization images (3 m × 15 m field-of-view) are available online from the author.
Directory of Open Access Journals (Sweden)
Alireza Naderi
2016-01-01
Full Text Available Various applications of ornithopter have led to research interest in oscillation airfoils which affect on low Reynolds number flight, like; pitching oscillation, heaving oscillation and flapping of a wing. The purpose of this study is investigation of aerodynamic characteristics of NACA0012 airfoil with a simple harmonic pitching oscillation at zero and 10 degrees of mean angle of attack. Therefore the effects of unstable parameters, including oscillation amplitude up to 10 degrees, reduced frequency up to 1.0, center of oscillation up to 6/8 chord length, and Reynolds number up to 5000 have been studied numerically. A pressure based algorithm using a finite volume element method has been used to solve Navier-Stokes equations. According to results, variation of each studied parameters at mean angle of attack of 0 degree do not cause significant changes in flow phenomena on airfoil but at mean angle of attack of 10 degrees, changing in reduced frequency and specially Reynolds number cause variations in flow phenomena. These variations are because of “wake capturing” and/or “added mass” phenomena.
Kunkel, Gary J.; Marusic, Ivan
2006-02-01
Data from the near-wall-turbulent region of the high-Reynolds-number atmospheric surface layer are used to analyse the attached-eddy model of wall turbulence. All data were acquired during near-neutral conditions at the Surface Layer Turbulence and Environmental Science Test (SLTEST) facility located in the western Utah Great Salt Lake Desert. Instantaneous streamwise and wall-normal components of velocity were collected with a wall-normal array of two-component hot wires within the first 2 m above the surface of the salt flats. Streamwise and wall-normal turbulence intensities and spectra are directly compared to corresponding laboratory data and similarity formulations hypothesized from the attached-eddy model of wall turbulence. This affords the opportunity to compare results with Reynolds numbers varying over three orders of magnitude. The wall-normal turbulence-intensity similarity formulation is extended. The results show good support for the similarity arguments forwarded by the attached-eddy model as well as Townsend's (1956) Reynolds-number similarity hypothesis and lack of the ‘inactive’ motion influence on the wall-normal velocity component. The effects of wall roughness and the spread in the convection velocity due to this roughness are also discussed.
Verjus, Romuald; Guillou, Sylvain; Ezersky, Alexander; Angilella, Jean-Régis
2016-12-01
The sedimentation of a pair of rigid circular particles in a two-dimensional vertical channel containing a Newtonian fluid is investigated numerically, for terminal particle Reynolds numbers (ReT) ranging from 1 to 10, and for a confinement ratio equal to 4. While it is widely admitted that sufficiently inertial pairs should sediment by performing a regular DKT oscillation (Drafting-Kissing-Tumbling), the present analysis shows in contrast that a chaotic regime can also exist for such particles, leading to a much slower sedimentation velocity. It consists of a nearly horizontal pair, corresponding to a maximum effective blockage ratio, and performing a quasiperiodic transition to chaos while increasing the particle weight. For less inertial regimes, the classical oblique doublet structure and its complex behavior (multiple stable states and hysteresis, period-doubling cascade and chaotic attractor) are recovered, in agreement with previous work [Aidun, C. K. and Ding, E.-J., "Dynamics of particle sedimentation in a vertical channel: Period-doubling bifurcation and chaotic state," Phys. Fluids 15, 1612 (2003)]. As a consequence of these various behaviors, the link between the terminal Reynolds number and the non-dimensional driving force is complex: it contains several branches displaying hysteresis as well as various bifurcations. For the range of Reynolds number considered here, a global bifurcation diagram is given.
Guo, Xinyi; Sironi, Lorenzo; Narayan, Ramesh
2014-10-01
Electron acceleration to non-thermal energies in low Mach number (Ms Diffusive shock acceleration, also known as first-order Fermi acceleration, cannot be directly invoked to explain the acceleration of electrons. Rather, an additional mechanism is required to pre-accelerate the electrons from thermal to supra-thermal energies, so they can then participate in the Fermi process. In this work, we use two- and three-dimensional particle-in-cell plasma simulations to study electron acceleration in low Mach number shocks. We focus on the particle energy spectra and the acceleration mechanism in a reference run with Ms = 3 and a quasi-perpendicular pre-shock magnetic field. We find that about 15% of the electrons can be efficiently accelerated, forming a non-thermal power-law tail in the energy spectrum with a slope of p ~= 2.4. Initially, thermal electrons are energized at the shock front via shock drift acceleration (SDA). The accelerated electrons are then reflected back upstream where their interaction with the incoming flow generates magnetic waves. In turn, the waves scatter the electrons propagating upstream back toward the shock for further energization via SDA. In summary, the self-generated waves allow for repeated cycles of SDA, similarly to a sustained Fermi-like process. This mechanism offers a natural solution to the conflict between the bright radio synchrotron emission observed from the outskirts of galaxy clusters and the low electron acceleration efficiency usually expected in low Mach number shocks.
Meerson, Baruch; Fouxon, Itzhak; Vilenkin, Arkady
2008-02-01
We employ hydrodynamic equations to investigate nonstationary channel flows of freely cooling dilute gases of hard and smooth spheres with nearly elastic particle collisions. This work focuses on the regime where the sound travel time through the channel is much shorter than the characteristic cooling time of the gas. As a result, the gas pressure rapidly becomes almost homogeneous, while the typical Mach number of the flow drops well below unity. Eliminating the acoustic modes and employing Lagrangian coordinates, we reduce the hydrodynamic equations to a single nonlinear and nonlocal equation of a reaction-diffusion type. This equation describes a broad class of channel flows and, in particular, can follow the development of the clustering instability from a weakly perturbed homogeneous cooling state to strongly nonlinear states. If the heat diffusion is neglected, the reduced equation becomes exactly soluble, and the solution develops a finite-time density blowup. The blowup has the same local features at singularity as those exhibited by the recently found family of exact solutions of the full set of ideal hydrodynamic equations [I. Fouxon, Phys. Rev. E 75, 050301(R) (2007); I. Fouxon,Phys. Fluids 19, 093303 (2007)]. The heat diffusion, however, always becomes important near the attempted singularity. It arrests the density blowup and brings about previously unknown inhomogeneous cooling states (ICSs) of the gas, where the pressure continues to decay with time, while the density profile becomes time-independent. The ICSs represent exact solutions of the full set of granular hydrodynamic equations. Both the density profile of an ICS and the characteristic relaxation time toward it are determined by a single dimensionless parameter L that describes the relative role of the inelastic energy loss and heat diffusion. At L>1 the intermediate cooling dynamics proceeds as a competition between "holes": low-density regions of the gas. This competition resembles Ostwald
Measurement and Analysis of the Noise Radiated by Low Mach Number Centrifugal Blowers.
Yeager, David Marvin
An investigation was performed of the broad band, aerodynamically generated noise in low tip-speed Mach number, centrifugal air moving devices. An interdisciplinary experimental approach was taken which involved investigation of the aerodynamic and acoustic fields, and their mutual relationship. The noise generation process was studied using two experimental vehicles: (1) a scale model of a homologous family of centrifugal blowers typical of those used to cool computer and business equipment, and (2) a single blade from a centrifugal blower impeller placed in a known, controllable flow field. The radiation characteristics of the model blower were investigated by measuring the acoustic intensity distribution near the blower inlet and comparing it with the intensity near the inlet to an axial flow fan. Results showed that the centrifugal blower is a distributed, random noise source, unlike an axial fan which exhibited the effects of a coherent, interacting source distribution. Aerodynamic studies of the flow field in the inlet and at the discharge to the rotating impeller were used to assess the mean flow distribution through the impeller blade channels and to identify regions of excessive turbulence near the rotating blade row. Both circumferential and spanwise mean flow nonuniformities were identified along with a region of increased turbulence just downstream of the scroll cutoff. The fluid incidence angle, normally taken as an indicator of blower performance, was estimated from mean flow data as deviating considerably from an ideal impeller design. An investigation of the noise radiated from the single, isolated airfoil was performed using modern correlation and spectral analysis techniques. Radiation from the single blade in flow was characterized using newly developed expressions for the correlation area and the dipole source strength per unit area, and from the relationship between the blade surface pressure and the incident turbulent flow field. Results
Pederzani, Jean-Noel; Haj-Hariri, Hossein
2012-11-01
An embedded-boundary (or cut-cell) method for complex geometry with moving boundaries is used to solve the three dimensional Navier-Stokes equation around a self-propelling manta swimming at moderately high Reynolds numbers. The motion of the ray is prescribed using a kinematic model fitted to actual biological data. The dependence of thrust production mechanism on Strouhal and Reynolds numbers is investigated. The vortex core structures are accurately plotted and a correlation between wake structures and propulsive performance is established. This insight is critical in understanding the key flow features that a bio-inspired autonomous vehicle should reproduce in order to swim efficiently. The solution method is implemented, on a block-structured Cartesian grid using a cut-cell approach enabling the code to correctly evaluate the wall shear-stress, a key feature necessary at higher Reynolds. To enhance computational efficiency, a parallel adaptive mesh refinement technique is used. The present method is validated against published experimental results. Supported by ONR MURI.
Motion of a particle between two parallel plane walls in low-Reynolds-number Poiseuille flow
Staben, Michelle E.; Zinchenko, Alexander Z.; Davis, Robert H.
2003-06-01
A new boundary-integral algorithm for the motion of a particle between two parallel plane walls in Poiseuille flow at low Reynolds number was developed to study the translational and rotational velocities for a broad range of particle sizes and depths in the channel. Instead of the free-space Green's function more commonly employed in boundary-integral equations, we used the Green's function for the domain between two infinite plane walls [Liron and Mochon, J. Eng. Math. 10, 287 (1976)]. This formulation allows us to directly incorporate the effects of the wall interactions into the stress tensor, without discretizing the bounding walls, and use well-established iterative methods. Our results are in good agreement with previous computations [Ganatos et al., J. Fluid Mech. 99, 755 (1980)] and limiting cases, over their range of application, with additional results obtained for very small particle-wall separations of less than 1% of the particle radius. In addition to the boundary-integral solution in the mobility formulation, we used the resistance formulation to derive the near-field asymptotic forms for the translational and rotational velocities, extending the results to even smaller particle-wall separations. The decrease in translational velocity from the unperturbed fluid velocity increases with particle size and proximity of the particle to one or both of the walls. The rotational velocity exhibits a maximum magnitude between the centerline and either wall, due to the competing influences of wall retardation and the greater fluid velocity gradient near the walls. The average particle velocity for a uniform distribution of particles was generally found to exceed the average fluid velocity, due in large part to exclusion of the particle centers from the region of slowest fluid near the walls. The maximum average particle velocity is 18% greater than the average fluid velocity and occurs for particle diameters that are 42% of the channel height; particles with
Zhang, Bin; Li, Jingyin; Guo, Penghua; Lv, Qian
2017-09-01
The impact force of low-speed droplets colliding with a solid surface was recorded with an experimental setup involving a highly sensitive piezoelectric force transducer and a high-speed camera recording the droplet shape. Water, ethanol, pure glycerin and aqueous glycerin solutions were used. Experimental results showed that dimensionless force is independent of the Weber number in the experimental range of 68-858 but varies with the Reynolds number. The impact is categorized into three types of processes according to the data on dimensionless peak force against the Reynolds number. The first type is a viscosity-dominated one, in which the Reynolds number ranges between 2.9 and 20. In the second type, transition process, the Reynolds number is in the range of 20-230. In the inertia-dominated type, the Reynolds number is larger than 230. In the viscosity-dominated impact, dimensionless peak force decreases rapidly with increasing Reynolds number, and the effect of viscosity could not be ignored. In the inertia-dominated impact, dimensionless peak force remains constant with varying the Reynolds number, that is, impact force is directly proportional to the product of liquid density, velocity squared and diameter squared but is unaffected by the changes in viscosity and surface tension. Furthermore, the deformation of droplet shape due to oscillation affects the impact force; a small horizontal-to-vertical ratio results in small impact force and vice versa.
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.
Drake, Hubert M; Mclaughlin, Milton D; Goodman, Harold R
1948-01-01
Results are presented of tests up to a Mach number of 0.92 at altitudes around 30,000 feet. The data obtained show that the airplane can be flown to this Mach number above 30,000 feet. Longitudinal trim changes have been experienced but the forces involved have been small. The elevator effectiveness decreased about one-half with increase of Mach number from 0.70 to 0.87. Buffeting has been experienced in level flight but it has been mild and the associated tail loads have been small. No aileron buzz or other flutter phenomena have been noted.
Chung, C K; Shih, T R; Chen, T C; Wu, B H
2008-10-01
A planar micromixer with rhombic microchannels and a converging-diverging element has been systematically investigated by the Taguchi method, CFD-ACE simulations and experiments. To reduce the footprint and extend the operation range of Reynolds number, Taguchi method was used to numerically study the performance of the micromixer in a L(9) orthogonal array. Mixing efficiency is prominently influenced by geometrical parameters and Reynolds number (Re). The four factors in a L(9) orthogonal array are number of rhombi, turning angle, width of the rhombic channel and width of the throat. The degree of sensitivity by Taguchi method can be ranked as: Number of rhombi > Width of the rhombic channel > Width of the throat > Turning angle of the rhombic channel. Increasing the number of rhombi, reducing the width of the rhombic channel and throat and lowering the turning angle resulted in better fluid mixing efficiency. The optimal design of the micromixer in simulations indicates over 90% mixing efficiency at both Re > or = 80 and Re < or = 0.1. Experimental results in the optimal simulations are consistent with the simulated one. This planar rhombic micromixer has simplified the complex fabrication process of the multi-layer or three-dimensional micromixers and improved the performance of a previous rhombic micromixer at a reduced footprint and lower Re.
Nimmagadda, Rajesh; Venkatasubbaiah, K.
2017-06-01
The present study investigates the laminar forced convection flow of single walled carbon nanotube (SWCNT), gold (Au), aluminum oxide (Al2O3), silver (Ag) and hybrid (Al2O3 + Ag) nanofluids (HyNF) in a wide rectangular micro-channel at low Reynolds numbers. The heat transfer characteristics of de-ionized (DI) water and SWCNT nanofluid with different nanoparticle volume concentrations have been experimental studied. Furthermore, numerical study has also been carried out to investigate the flow and heat transfer characteristics of DI water, SWCNT, Au, Al2O3, Ag and HyNF at different Reynolds numbers with different nanoparticle volume concentrations and particle diameters. The numerical study consider the effects of both inertial and viscous forces by solving the full Navier-Stokes equations at low Reynolds numbers. A two dimensional conjugate heat transfer multiphase mixture model has been developed and used for numerical study. A significant enhancement in the average Nusselt number is observed both experimentally and numerically for nanofluids. The study presents four optimized combinations of nanofluids (1 vol% SWCNT and 1 vol% Au with d_p = 50 nm), (2 vol% SWCNT and 3 vol% Au with d_p = 70 nm), (3 vol% Al2O3 and 2 vol% Au with d_p = 70 nm) as well as (3 vol% HyNF (2.4% Al2O3 + 0.6% Ag) and 3 vol% Au with d_p = 50 nm) that provides a better switching option in choosing efficient working fluid with minimum cost based on cooling requirement. The conduction phenomenon of the solid region at bottom of the micro-channel is considered in the present investigation. This phenomenon shows that the interface temperature between solid and fluid region increases along the length of the channel. The present results has been validated with the experimental and numerical results available in the literature.
Nimmagadda, Rajesh; Venkatasubbaiah, K.
2017-01-01
The present study investigates the laminar forced convection flow of single walled carbon nanotube (SWCNT), gold (Au), aluminum oxide (Al2O3), silver (Ag) and hybrid (Al2O3 + Ag) nanofluids (HyNF) in a wide rectangular micro-channel at low Reynolds numbers. The heat transfer characteristics of de-ionized (DI) water and SWCNT nanofluid with different nanoparticle volume concentrations have been experimental studied. Furthermore, numerical study has also been carried out to investigate the flow and heat transfer characteristics of DI water, SWCNT, Au, Al2O3, Ag and HyNF at different Reynolds numbers with different nanoparticle volume concentrations and particle diameters. The numerical study consider the effects of both inertial and viscous forces by solving the full Navier-Stokes equations at low Reynolds numbers. A two dimensional conjugate heat transfer multiphase mixture model has been developed and used for numerical study. A significant enhancement in the average Nusselt number is observed both experimentally and numerically for nanofluids. The study presents four optimized combinations of nanofluids (1 vol% SWCNT and 1 vol% Au with d_p = 50 nm), (2 vol% SWCNT and 3 vol% Au with d_p = 70 nm), (3 vol% Al2O3 and 2 vol% Au with d_p = 70 nm) as well as (3 vol% HyNF (2.4% Al2O3 + 0.6% Ag) and 3 vol% Au with d_p = 50 nm) that provides a better switching option in choosing efficient working fluid with minimum cost based on cooling requirement. The conduction phenomenon of the solid region at bottom of the micro-channel is considered in the present investigation. This phenomenon shows that the interface temperature between solid and fluid region increases along the length of the channel. The present results has been validated with the experimental and numerical results available in the literature.
Yu, Xiaoli; Sun, Zheng; Huang, Rui; Zhang, Yu; Huang, Yuqi
2015-01-01
Thermal effects such as conduction, convection and viscous dissipation are important to lubrication performance, and they vary with the friction conditions. These variations have caused some inconsistencies in the conclusions of different researchers regarding the relative contributions of these thermal effects. To reveal the relationship between the contributions of the thermal effects and the friction conditions, a steady-state THD analysis model was presented. The results indicate that the contribution of each thermal effect sharply varies with the Reynolds number and temperature. Convective effect could be dominant under certain conditions. Additionally, the accuracy of some simplified methods of thermo-hydrodynamic analysis is further discussed.
Directory of Open Access Journals (Sweden)
Orlik Evgeniy
2016-01-01
Full Text Available Experimental setup has been designed and manufactured to study the evaporation processes of liquid drop under blowing gas in mini-channel. The height of channel can be varied from 3 to 20 mm. Substrates are removable and its surface temperature is kept to constant value. The shadow method is main measurement technique. Series of experiments with 100 μl water drop on polished stainless still substrate are carried out in channel with 9 mm height. Dependences of evaporating rate for different range of temperatures and gas Reynolds numbers are obtained.
Heat transfer of a staggered fining flat-oval tube banks in cross flow at the small Reynolds number
Directory of Open Access Journals (Sweden)
Максим Михайлович Вознюк
2015-05-01
Full Text Available Experimental investigations of heat transfer of staggered bundles of flat-oval tubes with incomplete transversal finning in the range of Reynolds numbers 500 < <20000 are performed. New calculation correlations for determining of heat transfer coefficients for 1<3000 are suggested, the impact of basic geometric and regime parameters on intensity of external heat transfer are determined. The received calculation depending is possible to use in developing of heat transfer surfaces for “dry” cooling towers and air cooling apparatuses
Capone, F. J.
1972-01-01
An exploratory investigation was conducted in the Langley 16-foot transonic tunnel at Mach numbers from 0.20 to 1.30 to determine the induced lift characteristics of a body and swept-wing configuration having a partial-span two-dimensional propulsive nozzle with exhaust exit in the notch of the swept-wing trailing edge. The Reynolds number per meter varied from 4,900,000 to 14,030,000. The effects on wing-body characteristics of deflecting the propulsive jet in the flap mode at nominal exhaust-nozzle deflection angles of 0 deg and 30 deg were studied for two nozzle designs with different geometry and wing spans.
Asada, Kengo; Kawai, Soshi
2016-11-01
Wall-resolved large-eddy simulation (LES) of an airfoil flow involving a turbulent transition and separations near stall condition at a high Reynolds number 2.1 x 106 (based on the freestream velocity and the airfoil chord length) is conducted by using K computer. This study aims to provide the wall-resolved LES database including detailed turbulence statistics for near-wall modeling in LES and also to investigate the flow physics of the high Reynolds number airfoil flow near stall condition. The LES well predicts the laminar separation bubble, turbulent reattachment and turbulent separation. The LES also clarified unsteady flow features associated with shear-layer instabilities: high frequency unsteadiness at St = 130 at the laminar separation bubble near the leading edge and low frequency unsteadiness at St = 1.5 at the separated turbulent shear-layer near the trailing edge. Regarding the near-wall modeling in LES, the database indicates that the pressure term in the mean streamwise-momentum equation is not negligible at the laminar and turbulent separated regions. This fact suggests that widely used equilibrium wall model is not sufficient and the inclusion of the pressure term is necessary for wall modeling in LES of such flow. This research used computational resources of the K computer provided by the RIKEN Advanced Institute for Computational Science through the HPCI System Research project (Project ID: hp140028). This work was supported by KAKENHI (Grant Number: 16K18309).
Schober, Jennifer; Federrath, Christoph; Klessen, Ralf; Banerjee, Robi
2011-01-01
The small-scale dynamo is a process by which turbulent kinetic energy is converted into magnetic energy, and thus is expected to depend crucially on the nature of turbulence. In this work, we present a model for the small-scale dynamo that takes into account the slope of the turbulent velocity spectrum v(l) ~ l^theta, where l and v(l) are the size of a turbulent fluctuation and the typical velocity on that scale. The time evolution of the fluctuation component of the magnetic field, i.e., the small-scale field, is described by the Kazantsev equation. We solve this linear differential equation for its eigenvalues with the quantum-mechanical WKB-approximation. The validity of this method is estimated as a function of the magnetic Prandtl number Pm. We calculate the minimal magnetic Reynolds number for dynamo action, Rm_crit, using our model of the turbulent velocity correlation function. For Kolmogorov turbulence (theta=1/3), we find that the critical magnetic Reynolds number is approximately 110 and for Burger...
Wallskog, Harvey A.
1954-01-01
A 1/5-scale, rocket-propelled model of the Convair F-102 configuration was tested in free flight to determine zero-lift drag at Mach numbers up to 1.34 and at Reynolds numbers comparable to those of the full-scale airplane. This large-scale model corresponded to the prototype airplane and had air flow through the duct. Additional zero-lift drag tests involved a series of small equivalent bodies of revolution which were launched by means of a helium gun. The several small-scale models tested corresponded to: the basic configuration, the 1/5-scale rocket-propelled model configuration, a 2-foot (full-scale) fuselage-extension configuration, and a 7-foot (full-scale) fuselage-extension configuration. Models designed to correspond to the area distribution at a Mach number of 1.0 were flown for each of these 'shapes and, in addition, models designed to correspond to the area distribution at a Mach number of 1.2 were flown for the 1/5-scale rocket-propelled model and the 7-foot-fuselage-extension configuration. The value of external pressure drag coefficient (including base drag) obtained from the large-scale rocket model was 0.0190 at a Mach number of 1..05 and the corresponding values from the equivalent-body tests varied from 0.0183 for the rocket-propelled model shape to 0.0137 for the 7-foot-fuselage-extension configuration. From the results of tests of equivalent bodies designed to correspond to the area distribution at a Mach number of 1.0, it is evident that the small changes in shape incorporated in the basic and 2-foot-fuselage-extension configurations from that of the rocket-propelled model configuration will provide no significant change in pressure drag. On the other hand, the data from the 7-foot-fuselage-extension model indicate a substantial reduction in pressure drag at transonic speeds.
The effects of an algal biofilm on the turbulent boundary layer at high Reynolds number
Murphy, Elizabeth; Barros, Julio; Schultz, Michael; Flack, Karen; Steppe, Cecily; Reidenbach, Matthew
2016-11-01
Algal biofilms are an important fouling community on ship hulls, with severe economic consequences due to increased drag. As with other types of roughness on aquatic surfaces, biofilms increase skin friction and thus induce severe drag penalties. In fact, slime layers appear to induce greater drag than would be predicted by the roughness height alone. Our work indicates that this is likely due to two characteristics of algal biofilms: i) flexible streamers that protrude into the flow, and ii) the compliant nature of a biofilm layer. High resolution PIV was used to measure the turbulent boundary layer flow over diatomaceous biofilm grown under dynamic conditions. Local mean streamwise velocity profiles were used to estimate the local wall shear stresses and to determine the similarity between the inner and outer layers of the boundary layer and those of a smooth wall. Spatially explicit turbulent kinetic energy (TKE), Reynolds shear stress (RSS), swirling strength and quadrant analyses over the biofilm were compared to those over a smooth wall and a rigid mesh roughness. We found that the combination of canopy flow due to streamers coupled with compliant wall-flow interactions result in large wall shear stresses and higher turbulence. Funding provided by the ONR NURP program and the NSF GRIP program.
Directory of Open Access Journals (Sweden)
Islam Tariqul
2015-01-01
Full Text Available Bubble formation dynamics has great value in mineral recovery and the oil industry. In this paper, a single bubble formation process through an orifice in a rectangle domain is modelled to study the bubble formation characteristics using the volume of fluid (VOF with the continuum surface force (CSF method. The effect of gas inlet velocities, Ug ~ 0.1 - 0.3 m/s on bubble formation stages (i.e., expansion, elongation and pinch off, bubble contact angle, dynamics and static pressure, bubble departure diameter etc. was investigated through an orifice diameter of 1 mm. The method was also used to study the effect of Reynolds number, Reμ ~ 1.32 - 120 on bubble formation when all other parameters were kept constant. It is found that a high inlet gas velocity accelerated the reducing of the bubble contact angle from an obtuse angle to an acute angle and the faster development of hemispherical shape of the bubble. It is also found that an increasing of Reynolds number caused speeding up of the bubble pinch-off and formed a smaller bubble neck height due to stronger vortex ring around the bubble neck.
Energy Technology Data Exchange (ETDEWEB)
Monty, J.P.; Lien, K.; Chong, M.S. [University of Melbourne, Department of Mechanical Engineering, Parkville, VIC (Australia); Allen, J.J. [New Mexico State University, Department of Mechanical Engineering, Las Cruces, NM (United States)
2011-12-15
A high Reynolds number boundary-layer wind-tunnel facility at New Mexico State University was fitted with a regularly distributed braille surface. The surface was such that braille dots were closely packed in the streamwise direction and sparsely spaced in the spanwise direction. This novel surface had an unexpected influence on the flow: the energy of the very large-scale features of wall turbulence (approximately six-times the boundary-layer thickness in length) became significantly attenuated, even into the logarithmic region. To the author's knowledge, this is the first experimental study to report a modification of 'superstructures' in a rough-wall turbulent boundary layer. The result gives rise to the possibility that flow control through very small, passive surface roughness may be possible at high Reynolds numbers, without the prohibitive drag penalty anticipated heretofore. Evidence was also found for the uninhibited existence of the near-wall cycle, well known to smooth-wall-turbulence researchers, in the spanwise space between roughness elements. (orig.)
Energy Technology Data Exchange (ETDEWEB)
Lee, Eun J.; Oh, Sang Youp; Kim, Ho Y.; Yoon, Sam S. [Dept. of Mechanical, Korea University Anamdong, 5-Ga, Sungbukgu, 136-713 Seoul (Korea); James, Scott C. [Thermal/Fluid Science and Engineering, Sandia National Labs, PO Box 969, Livermore, CA 94551 (United States)
2010-11-15
Because of thermal fluid-property dependence, atomization stability (or flow regime) can change even at fixed operating conditions when subject to temperature change. Particularly at low temperatures, fuel's high viscosity can prevent a pressure-swirl (or simplex) atomizer from sustaining a centrifugal-driven air core within the fuel injector. During disruption of the air core inside an injector, spray characteristics outside the nozzle reflect a highly unstable, nonlinear mode where air core length, Sauter mean diameter (SMD), cone angle, and discharge coefficient variability. To better understand injector performance, these characteristics of the pressure-swirl atomizer were experimentally investigated and data were correlated to Reynolds numbers (Re). Using a transparent acrylic nozzle, the air core length, SMD, cone angle, and discharge coefficient are observed as a function of Re. The critical Reynolds numbers that distinguish the transition from unstable mode to transitional mode and eventually to a stable mode are reported. The working fluids are diesel and a kerosene-based fuel, referred to as bunker-A. (author)
Che Hsin, Lin; Lung Ming, Fu; 10.1088/0960-1317/15/5/006
2005-01-01
This paper proposes a novel three-dimensional (3D) vortex micromixer for micro-total-analysis-systems ( mu TAS) applications which utilizes self-rotation effects to mix fluids in a circular chamber at low Reynolds numbers (Re). The microfluidic mixer is fabricated in a three-layer glass structure for delivering fluid samples in parallel. The fluids are driven into the circular mixing chamber by means of hydrodynamic pumps from two fluid inlet ports. The two inlet channels divide into eight individual channels tangent to a 3D circular chamber for the purpose of mixing. Numerical simulation of the microfluidic dynamics is employed to predict the self-rotation phenomenon and to estimate the mixing performance under various Reynolds number conditions. Experimental flow visualization by mixing dye samples is performed in order to verify the numerical simulation results. A good agreement is found to exist between the two sets of results. The numerical results indicate that the mixing performance can be as high as 9...
Drost, Kevin; Apte, Sourabh
2010-11-01
Direct numerical simulations are performed to investigate the effect of a movable leading edge on the unsteady flow at high angles of attack over a flat, thin airfoil at Reynolds number of 14700 based on the chord length. The leading edge of the airfoil is hinged at one-third chord length allowing dynamic variations in the effective angle of attack through specified oscillations (or flapping). A fictitious-domain based finite volume approach [(Apte et al. (JCP 2009)] is used to compute the flow over an airfoil with a flapping leading edge on a fixed background mesh. Cases were run at 20 degrees angle of attack to study the drag and lift characteristics with sinusoidal flapping of the leading edge about the hinge over a range of reduced frequencies (k=πf c/U∞ = 0.57- 5.7). It is shown that high-frequency low amplitude actuation of the leading edge significantly alters the leading edge boundary-layer and vortex shedding and increases the mean lift- to-drag ratio. The concept of an actuated leading-edge flap has potential for development of control techniques to stabilize and maneuver low-Reynolds number micro-air vehicles in response to unsteady perturbations.
The effect of the Reynolds number on mass transfer at a free surface in a fully developed turbulence
Nagaosa, Ryuichi
2005-11-01
This study deals with mass transfer mechanism into a turbulent liquid at a free surface in an open channel. Both mass flux and subsurface hydrodynamics measured in laboratory measurements and found that the normalized mass transfer coefficient is proportional to the Reynolds number Rem which is defined by water depth and the bulk mean velocity [S. Komori, R. Nagaosa and Y. Murakami, AIChE J. 36, 957, 1991]. Direct numerical simulations (DNS) of mass transport at the free surface in a fully developed turbulence have been carried out in this study to discuss suitability of the results of the previous laboratory experiments. The results of this study show that the predicted mass transfer velocities by the DNS technique agree well with our previous laboratory measurements. The mass transfer velocities predicted in the present DNS are, however, proportional to 3/4 power of Rem, rather than 1 as found in the laboratory experiments. The difference of the exponent could be a reason of underestimation of mass flux in the numerical predictions in a larger Reynolds number turbulence of about Rem>10,000.
Rostamzadeh, Nikan; Kelso, Richard M.; Dally, Bassam
2016-05-01
Leading-edge modifications based on designs inspired by the protrusions on the pectoral flippers of the humpback whale (tubercles) have been the subject of research for the past decade primarily due to their flow control potential in ameliorating stall characteristics. Previous studies have demonstrated that, in the transitional flow regime, full-span wings with tubercled leading edges outperform unmodified wings at high attack angles. The flow mechanism associated with such enhanced loading traits is, however, still being investigated. Also, the performance of full-span tubercled wings in the turbulent regime is largely unexplored. The present study aims to investigate Reynolds number effects on the flow mechanism induced by a full-span tubercled wing with the NACA-0021 cross-sectional profile in the transitional and near-turbulent regimes using computational fluid dynamics. The analysis of the flow field suggests that, with the exception of a few different flow features, the same underlying flow mechanism, involving the presence of transverse and streamwise vorticity, is at play in both cases. With regard to lift-generation characteristics, the numerical simulation results indicate that in contrast to the transitional flow regime, where the unmodified NACA-0021 undergoes a sudden loss of lift, in the turbulent regime, the baseline foil experiences gradual stall and produces more lift than the tubercled foil. This observation highlights the importance of considerations regarding the Reynolds number effects and the stall characteristics of the baseline foil, in the industrial applications of tubercled lifting bodies.
Rostamzadeh, Nikan; Kelso, Richard M.; Dally, Bassam
2017-02-01
Leading-edge modifications based on designs inspired by the protrusions on the pectoral flippers of the humpback whale (tubercles) have been the subject of research for the past decade primarily due to their flow control potential in ameliorating stall characteristics. Previous studies have demonstrated that, in the transitional flow regime, full-span wings with tubercled leading edges outperform unmodified wings at high attack angles. The flow mechanism associated with such enhanced loading traits is, however, still being investigated. Also, the performance of full-span tubercled wings in the turbulent regime is largely unexplored. The present study aims to investigate Reynolds number effects on the flow mechanism induced by a full-span tubercled wing with the NACA-0021 cross-sectional profile in the transitional and near-turbulent regimes using computational fluid dynamics. The analysis of the flow field suggests that, with the exception of a few different flow features, the same underlying flow mechanism, involving the presence of transverse and streamwise vorticity, is at play in both cases. With regard to lift-generation characteristics, the numerical simulation results indicate that in contrast to the transitional flow regime, where the unmodified NACA-0021 undergoes a sudden loss of lift, in the turbulent regime, the baseline foil experiences gradual stall and produces more lift than the tubercled foil. This observation highlights the importance of considerations regarding the Reynolds number effects and the stall characteristics of the baseline foil, in the industrial applications of tubercled lifting bodies.
Schober, Jennifer; Schleicher, Dominik; Federrath, Christoph; Klessen, Ralf; Banerjee, Robi
2012-02-01
The small-scale dynamo is a process by which turbulent kinetic energy is converted into magnetic energy, and thus it is expected to depend crucially on the nature of the turbulence. In this paper, we present a model for the small-scale dynamo that takes into account the slope of the turbulent velocity spectrum v(ℓ)proportional ℓ([symbol see text])V}, where ℓ and v(ℓ) are the size of a turbulent fluctuation and the typical velocity on that scale. The time evolution of the fluctuation component of the magnetic field, i.e., the small-scale field, is described by the Kazantsev equation. We solve this linear differential equation for its eigenvalues with the quantum-mechanical WKB approximation. The validity of this method is estimated as a function of the magnetic Prandtl number Pm. We calculate the minimal magnetic Reynolds number for dynamo action, Rm_{crit}, using our model of the turbulent velocity correlation function. For Kolmogorov turbulence ([symbol see text] = 1/3), we find that the critical magnetic Reynolds number is Rm(crit) (K) ≈ 110 and for Burgers turbulence ([symbol see text] = 1/2) Rm(crit)(B) ≈ 2700. Furthermore, we derive that the growth rate of the small-scale magnetic field for a general type of turbulence is Γ proportional Re((1-[symbol see text])/(1+[symbol see text])) in the limit of infinite magnetic Prandtl number. For decreasing magnetic Prandtl number (down to Pm >/~ 10), the growth rate of the small-scale dynamo decreases. The details of this drop depend on the WKB approximation, which becomes invalid for a magnetic Prandtl number of about unity.
Numerical simulations of flame dynamics in the near-field of high-Reynolds number jets
Venugopal, Rishikesh
Recent experiments in diesel jet flames show that flame lift-off has a significant influence on pollutant formation. Lift-off occurs in the near-field of the jet, which is characterized by complex interactions between turbulence and chemistry. Commonly employed modeling approaches based on Reynolds-averaged Navier-Stokes (RANS) simulations are limited in their capability to predict transient and steady lift-off phenomena, as they ignore effects due to unsteadiness and curvature that are inherent in the near-field. In the present work, we perform numerical investigations of localized flame dynamics in the near-field (x/d diesel engine applications. The primary focus is on the exploration of unsteady extinction/reignition phenomena. A dual approach involving large-eddy simulation (LES) of a 70,000-Re variable-density isothermal gaseous fuel jet, and studies of flame-vortex interactions and unsteady flamelets, under diesel engine conditions, is employed in this work. Results from flame-vortex interaction studies show that in the near-field (x/d 1.0) in the jet near-field,temporary flame weakening/recovery events are likely to occur. Steady flamelet models provide reasonable estimates of the mean temperature, and mean mass fractions of the major species and unburned hydrocarbons (UHCs), but are inadequate for the prediction of mean NO mass fractions. Extrapolation of the analysis to jets with higher global strain rates shows that unsteady effects on the localized flame dynamics are important for the prediction of transient and steady lift-off behavior.
Spreading of Exhaust Jet from 16 Inch Ream Jet at Mach Number 2.0 / Fred Wilcox, Donald Pennington
Wilcox, Fred; Pennington, Donald
1952-01-01
An investigation of the jet-spreading characteristics of a 16 inch ram-jet engine was conducted in the 8 by 6 foot supersonic tunnel at a Mach number of 2.0; both a converging nozzle having a contraction ratio of 0.71 and a cylindrical extension to the combustion chamber were used. The jet boundaries determined by means of pitot pressure surveys were compared with boundaries calculated from one-dimensional continuity and momentum relations. For the cylindrical nozzle, the jet reaches its maximum diameter, 4 percent greater than calculated, about 0.6 nozzle-exit diameter downstream of the nozzle exit. The maximum diameter for the converging nozzle was 7 percent greater than calculated from one dimensional relations and occurred from 1 to 1.5 nozzle-exit diameters downstream of the exit. Non dimensional maximum jet diameters agreed closely with results of an investigation by Rousso and Baughman; these data were obtained with low-temperature jets exhausting into a stream at a Mach number of 1.91 from nozzles having exit diameters of 0.75 inch.
Guo, Xinyi; Narayan, Ramesh
2014-01-01
Electron acceleration to non-thermal energies in low Mach number (M<5) shocks is revealed by radio and X-ray observations of galaxy clusters and solar flares, but the electron acceleration mechanism remains poorly understood. Diffusive shock acceleration, also known as first-order Fermi acceleration, cannot be directly invoked to explain the acceleration of electrons. Rather, an additional mechanism is required to pre-accelerate the electrons from thermal to supra-thermal energies, so they can then participate in the Fermi process. In this work, we use two- and three-dimensional particle-in-cell plasma simulations to study electron acceleration in low Mach number shocks. We focus on the particle energy spectra and the acceleration mechanism in a reference run with M=3. We find that about 15 percent of the electrons can be efficiently accelerated, forming a non-thermal power-law tail in the energy spectrum with a slope of p~2.4. Initially, thermal electrons are energized at the shock front via shock drift a...
Performance characteristics of two multiaxis thrust-vectoring nozzles at Mach numbers up to 1.28
Wing, David J.; Capone, Francis J.
1993-01-01
The thrust-vectoring axisymmetric (VA) nozzle and a spherical convergent flap (SCF) thrust-vectoring nozzle were tested along with a baseline nonvectoring axisymmetric (NVA) nozzle in the Langley 16-Foot Transonic Tunnel at Mach numbers from 0 to 1.28 and nozzle pressure ratios from 1 to 8. Test parameters included geometric yaw vector angle and unvectored divergent flap length. No pitch vectoring was studied. Nozzle drag, thrust minus drag, yaw thrust vector angle, discharge coefficient, and static thrust performance were measured and analyzed, as well as external static pressure distributions. The NVA nozzle and the VA nozzle displayed higher static thrust performance than the SCF nozzle throughout the nozzle pressure ratio (NPR) range tested. The NVA nozzle had higher overall thrust minus drag than the other nozzles throughout the NPR and Mach number ranges tested. The SCF nozzle had the lowest jet-on nozzle drag of the three nozzles throughout the test conditions. The SCF nozzle provided yaw thrust angles that were equal to the geometric angle and constant with NPR. The VA nozzle achieved yaw thrust vector angles that were significantly higher than the geometric angle but not constant with NPR. Nozzle drag generally increased with increases in thrust vectoring for all the nozzles tested.
Syvertson, Clarence A; Gloria, Hermilo R; Sarabia, Michael F
1958-01-01
A study is made of aerodynamic performance and static stability and control at hypersonic speeds. In a first part of the study, the effect of interference lift is investigated by tests of asymmetric models having conical fuselages and arrow plan-form wings. The fuselage of the asymmetric model is located entirely beneath the wing and has a semicircular cross section. The fuselage of the symmetric model was centrally located and has a circular cross section. Results are obtained for Mach numbers from 3 to 12 in part by application of the hypersonic similarity rule. These results show a maximum effect of interference on lift-drag ratio occurring at Mach number of 5, the Mach number at which the asymmetric model was designed to exploit favorable lift interference. At this Mach number, the asymmetric model is indicated to have a lift-drag ratio 11 percent higher than the symmetric model and 15 percent higher than the asymmetric model when inverted. These differences decrease to a few percent at a Mach number of 12. In the course of this part of the study, the accuracy to the hypersonic similarity rule applied to wing-body combinations is demonstrated with experimental results. These results indicate that the rule may prove useful for determining the aerodynamic characteristics of slender configurations at Mach numbers higher than those for which test equipment is really available. In a second part of the study, the aerodynamic performance and static stability and control characteristics of a hypersonic glider are investigated in somewhat greater detail. Results for Mach numbers from 3 to 18 for performance and 0.6 to 12 for stability and control are obtained by standard text techniques, by application of the hypersonic stability rule, and/or by use of helium as a test medium. Lift-drag ratios of about 5 for Mach numbers up to 18 are shown to be obtainable. The glider studied is shown to have acceptable longitudinal and directional stability characteristics through the
Phillips, W. P.
1984-01-01
Aerodynamic characteristics at M=5.97 for the 140 A/B Space Shuttle Orbiter configuration and for the configuration modified by geometric changes in the wing planform fillet region and the fuselage forebody are presented. The modifications, designed to extend the orbiter's longitudinal trim capability to more forward center of gravity locations, include reshaping the baseline wing fillet, changing the fuselage forebody camber, and adding canards. The Langley 20 inch Mach 6 Tunnel at a Reynolds number of approximately 6 million based on fuselage reference length was used. The angle of attack range of the investigation varied from about 15 deg to 35 deg at 0 deg and -5 deg sideslip angles. Data are obtained with the elevators and body flap deflected at appropriate negative and positive conditions to assess the trim limits.
Gupta, Nikhil; Mitra, Sushanta K; Kumar, Aloke
2015-01-01
In this paper, we obtain analytical results for shear stress distributions inside an elastic body placed in a low Reynolds number transport. The problem definition is inspired by a recent experimental study (Valiei et al., Lab Chip, 2012, 12, 5133-5137) that reports the flow-triggered deformation of bacterial biofilms, formed on cylindrical rigid microposts, into long filamentous structures known as streamers. In our analysis, we consider an elastic body of finite thickness (forming a rim) placed over a rigid cylinder, i.e., we mimic the biofilm structure in the experiment. We consider Oseen flow solution to describe the low Reynolds transport past this cylindrical elastic structure. The stress and strain distributions inside the elastic structure are found to be functions of position, Poisson ratio, initial thickness of the elastic rim and the ratio of the flow-driven shear stress to the shear modulus of the elastic body. More importantly, these analyses, which can be deemed as one of the first formal analys...
Hoefnagels, Paul B. J.; Wei, Ping; Narezo Guzman, Daniela; Sun, Chao; Lohse, Detlef; Ahlers, Guenter
2017-07-01
We report on an experimental study of the large-scale flow (LSF) and Reynolds numbers in turbulent convection in a cylindrical sample with height equal to its diameter and heated locally around the center of its bottom plate (locally heated convection). The sample size and shape are the same as those of Narezo Guzman et al. [D. Narezo Guzman et al., J. Fluid Mech. 787, 331 (2015), 10.1017/jfm.2015.701; D. Narezo Guzman et al., J. Fluid Mech. 795, 60 (2016), 10.1017/jfm.2016.178]. Measurements are made at a nearly constant Rayleigh number as a function of the mean temperature, both in the presence of controlled boiling (two-phase flow) and for the superheated fluid (one-phase flow). Superheat values Tb-To n up to about 11 K (Tb is the bottom-plate temperature and To n is the lowest Tb at which boiling is observed) are used. The LSF is less organized than it is in (uniformly heated) Rayleigh-Bénard convection (RBC), where it takes the form of a single convection roll. Large-scale-flow-induced sinusoidal azimuthal temperature variations (like those found for RBC) could be detected only in the lower portion of the sample, indicating a less organized flow in the upper portions. Reynolds numbers are determined using the elliptic model (EM) of He and Zhang [G.-W. He and J.-B. Zhang, Phys. Rev. E 73, 055303(R) (2006), 10.1103/PhysRevE.73.055303]. We found that for our system the EM is applicable over a wide range of space and time displacements, as long as these displacements are within the inertial range of the temporal and spatial spectrum. At three locations in the sample the results show that the vertical mean-flow velocity component is reduced while the fluctuation velocity is enhanced by the bubbles of the two-phase flow. Enhancements of velocity fluctuations up to about 60% are found at the largest superheat values. Local temperature measurements within the sample reveal temperature oscillations that also used to determine a Reynolds number. These results are
De Vincenzo, Annamaria; Brancati, Francesco; Pannone, Marilena
2016-08-01
Laboratory experiments were performed with nearly uniform fluvial gravel (D50=9 mm, D10=5 mm and D90=13 mm) to analyse the relationship between stream power and bed load transport rate in gravel-bed braided rivers at high grain Reynolds numbers. The values of the unit-width dimensionless bed-load rate qb* and unit-width dimensionless stream power ω* were evaluated in equilibrium conditions based on ten different experimental runs. Then, they were plotted along with values obtained during particularly representative field studies documented in the literature, and a regression law was derived. For comparison, a regression analysis was performed using the data obtained from laboratory experiments characterized by smaller grain sizes and, therefore, referring to relatively low grain Reynolds numbers. A numerical integration of Exner's equation was performed to reconstruct the local and time-dependent functional dependence of qb* and ω*. The results led to the following conclusions: 1) At equilibrium, the reach-averaged bed load transport rate is related to the reach-averaged stream power by different regression laws at high and low grain Reynolds numbers. Additionally, the transition from bed to suspended load transport is accelerated by low Re*, with the corresponding bed load discharge increasing with stream power at a lower, linear rate. 2) When tested against the gravel laboratory measurements, the high Re* power law derived in the present study performs considerably better than do previous formulas. 3) The longitudinal variability of the section-averaged equilibrium stream power is much more pronounced than that characterizing the bed load rate, at least for high Re*. Thus, the stream power and its local-scale heterogeneity seem to be directly responsible for transverse sediment re-distribution and, ultimately, for the determination of the spatial and temporal scales that characterize the gravel bedforms. 4) Finally, the stochastic interpretation of the wetted
Directory of Open Access Journals (Sweden)
Mao Liangjie
Full Text Available A considerable number of studies for VIV under the uniform flow have been performed. However, research on VIV under shear flow is scarce. An experiment for VIV under the shear flow with the same shear parameter at the two different Reynolds numbers was conducted in a deep-water offshore basin. Various measurements were obtained by the fiber bragg grating strain sensors. Experimental data were analyzed by modal analysis method. Results show several valuable features. First, the corresponding maximum order mode of the natural frequency for shedding frequency is the maximum dominant vibration mode and multi-modal phenomenon is appeared in VIV under the shear flow, and multi-modal phenomenon is more apparent at the same shear parameter with an increasing Reynolds number under the shear flow effect. Secondly, the riser vibrates at the natural frequency and the dominant vibration frequency increases for the effect of the real-time tension amplitude under the shear flow and the IL vibration frequency is the similar with the CF vibration frequency at the Reynolds number of 1105 in our experimental condition and the IL dominant frequency is twice the CF dominant frequency with an increasing Reynolds number. In addition, the displacement trajectories at the different locations of the riser appear the same shape and the shape is changed at the same shear parameter with an increasing Reynolds number under the shear flow. The diagonal displacement trajectories are observed at the low Reynolds number and the crescent-shaped displacement trajectories appear with an increasing Reynolds number under shear flow in the experiment.
Rikanati, A; Oron, D; Sadot, O; Shvarts, D
2003-02-01
Effects of high-Mach numbers and high initial amplitudes on the evolution of the single-mode Richtmyer-Meshkov shock-wave induced hydrodynamic instability are studied using theoretical models, experiments, and numerical simulations. Two regimes in which there is a significant deviation from the linear dependence of the initial velocity on the initial perturbation amplitude are defined and characterized. In one, the observed reduction of the initial velocity is primarily due to large initial amplitudes. This effect is accurately modeled by a vorticity deposition model, quantifying both the effect of the initial perturbation amplitude and the exact shape of the interface. In the other, the reduction is dominated by the proximity of the shock wave to the interface. This effect is modeled by a modified incompressible model where the shock wave is mimicked by a moving bounding wall. These results are supplemented with high initial amplitude Mach 1.2 shock-tube experiments, enabling separation of the two effects. It is shown that in most of the previous experiments, the observed reduction is predominantly due to the effect of high initial amplitudes.
Sojka, Paul E.; Rodrigues, Neil S.
2015-11-01
The current study investigates the drop characteristics of three Carboxymethylcellulose (CMC) sprays produced by the impingement of two liquid jets. The three water-based solutions used in this work (0.5 wt.-% CMC-7MF, 0.8 wt.-% CMC-7MF, and 1.4 wt.-% CMC-7MF) exhibited strong shear-thinning, non-Newtonian behavior - characterized by the Bird-Carreau rheological model. A generalized Bird-Carreau jet Reynolds number was used as the primary parameter to characterize the drop size and the drop velocity, which were measured using Phase Doppler Anemometry (PDA). PDA optical configuration enabled a drop size measurement range of approximately 2.3 to 116.2 μm. 50,000 drops were measured at each test condition to ensure statistical significance. The arithmetic mean diameter (D10) , Sauter mean diameter (D32) , and mass median diameter (MMD) were used as representative diameters to characterize drop size. The mean axial drop velocity Uz -mean along with its root-mean square Uz -rms were used to characterize drop velocity. Incredibly, measurements for all three CMC liquids and reference DI water sprays seemed to follow a single curve for D32 and MMD drop diameters in the high generalized Bird-Carreau jet Reynolds number range considered in this work (9.21E +03
Sevilla, A.; Martínez-Bazán, C.
2004-09-01
In the present work we study the large-scale helical vortex shedding regime in the wake of an axisymmetric body with a blunt trailing edge at high Reynolds numbers, both experimentally and by means of local, linear, and spatiotemporal stability analysis. In the instability analysis we take into account the detailed downstream evolution of the basic flow behind the body base. The study confirms the existence of a finite region of absolute instability for the first azimuthal number in the near field of the wake. Such instability is believed to trigger the large-scale helical vortex shedding downstream of the recirculating zone. Inhibition of vortex shedding is examined by blowing a given flow rate of fluid through the base of the slender body. The extent of the locally absolute region of the flow is calculated as a function of the bleed coefficient, Cb=qb/(πR2u∞), where qb is the bleed flow rate, R is the radius of the base, and u∞ is the incident free-stream velocity. It is shown that the basic flow becomes convectively unstable everywhere for a critical value of the bleed coefficient of Cb*˜0.13, such that no self-excited regime is expected for Cb>Cb*. In addition, we report experimental results of flow visualizations and hot-wire measurements for increasing values of the bleed coefficient. When a sufficient amount of base bleed is applied, flow visualizations indicate that vortex shedding is suppressed and that the mean flow becomes axisymmetric. The critical bleed coefficient predicted by linear instability analysis is shown to fall within the experimental values in the range of Reynolds numbers analyzed here.
Lift-optimal aspect ratio of a revolving wing at low Reynolds number
Jardin, Thierry; Colonius, Tim
2016-11-01
Lentink & Dickinson (2009) showed that rotational acceleration stabilized the leading-edge vortex on revolving, low-aspect-ratio wings, and hypothesized that a Rossby number of around 3, which is achieved during each half-stroke for a variety of hovering insects, seeds, and birds, represents a convergent high-lift solution across a range of scales in nature. Subsequent work has verified that, in particular, the Coriolis acceleration is responsible for LEV stabilization. Implicit in these results is that there exists an optimal aspect ratio for wings revolving about their root, because it is otherwise unclear why, apart from possible physiological reasons, the convergent solution would not occur for an even lower Rossby number. We perform direct numerical simulations of the flow past revolving wings where we vary the aspect ratio and Rossby numbers independently by displacing the wing root from the axis of rotation. We show that the optimal lift coefficient represents a compromise between competing trends where the coefficient of lift increases monotonically with aspect ratio, holding Rossby number constant, but decreases monotonically with Rossby number, when holding aspect ratio constant. For wings revolving about their root, this favors wings of aspect ratio between 3 and 4. The authors gratefully acknowledge support from Fondation ISAE-Supaero.
Verjus, Romuald; Ezersky, Alexander; Angilella, Jean-Régis
2016-01-01
The sedimentation of a pair of rigid circular particles in a two-dimensional vertical channel containing a Newtonian fluid is investigated numerically, for terminal particle Reynolds numbers ranging from 1 to 10, and for a confinement ratio equal to 4. While it is widely admitted that sufficiently inertial pairs should sediment by performing a regular DKT oscillation (Drafting-Kissing-Tumbling), the present analysis shows in contrast that a chaotic regime can also exist for such particles, leading to a much slower sedimentation velocity. It consists of a nearly horizontal pair, corresponding to a maximum effective blockage ratio, and performing a quasiperiodic transition to chaos under increasing the particle weight. For less inertial regimes, the classical oblique doublet structure and its complex behavior (multiple stable states and hysteresis, period-doubling cascade and chaotic attractor) are recovered, in agreement with previous work [Aidun & Ding, Physics of Fluids 15(6), 2003]. As a consequence of ...
Directory of Open Access Journals (Sweden)
Y. I. Troitskaya
2006-01-01
Full Text Available The objective of the present paper is to develop a theoretical model describing the evolution of a turbulent wake behind a towed sphere in a stably stratified fluid at large Froude and Reynolds numbers. The wake flow is considered as a quasi two-dimensional (2-D turbulent jet flow whose dynamics is governed by the momentum transfer from the mean flow to a quasi-2-D sinuous mode growing due to hydrodynamic instability. The model employs a quasi-linear approximation to describe this momentum transfer. The model scaling coefficients are defined with the use of available experimental data, and the performance of the model is verified by comparison with the results of a direct numerical simulation of a 2-D turbulent jet flow. The model prediction for the temporal development of the wake axis mean velocity is found to be in good agreement with the experimental data obtained by Spedding (1997.
Mishra, G K; Prakash, O; Biswal, R; Dixit, S K; Nakhe, S V
2014-01-01
This paper presents computational and experimental studies on wavelength/frequency fluctuation characteristics of high pulse repetition rate (PRR: 18 kHz) dye laser pumped by frequency doubled Nd:YAG laser (532 nm). The temperature gradient in the dye solution is found to be responsible for wavelength fluctuations of the dye laser at low flow rates (2800
Energy Technology Data Exchange (ETDEWEB)
Schuchardt, Karen L.; Chase, Jared M.; Daily, Jeffrey A.; Elsethagen, Todd O.; Palmer, Bruce J.; Scheibe, Timothy D.
2009-06-15
The Support Architecture for Large-Scale Subsurface Analysis (SALSSA) provides an extensible framework, sophisticated graphical user interface (GUI), and underlying data management system that simplifies the process of running subsurface models, tracking provenance information, and analyzing the model results. The SALSSA software framework is currently being applied to validating the Smoothed Particle Hydrodynamics (SPH) model. SPH is a three-dimensional model of flow and transport in porous media at the pore scale. Fluid flow in porous media at velocities common in natural porous media occur at low Reynolds numbers and therefore it is important to verify that the SPH model is producing accurate flow solutions in this regime. Validating SPH requires performing a series of simulations and comparing these simulation flow solutions to analytical results or numerical results using other methods. This validation study is being facilitated by the SALLSA framework, which provides capabilities to setup, execute, analyze, and administer these SPH simulations.
Chini, G. P.; Montemuro, B.; White, C. M.; Klewicki, J.
2017-03-01
Field observations and laboratory experiments suggest that at high Reynolds numbers Re the outer region of turbulent boundary layers self-organizes into quasi-uniform momentum zones (UMZs) separated by internal shear layers termed `vortical fissures' (VFs). Motivated by this emergent structure, a conceptual model is proposed with dynamical components that collectively have the potential to generate a self-sustaining interaction between a single VF and adjacent UMZs. A large-Re asymptotic analysis of the governing incompressible Navier-Stokes equation is performed to derive reduced equation sets for the streamwise-averaged and streamwise-fluctuating flow within the VF and UMZs. The simplified equations reveal the dominant physics within-and isolate possible coupling mechanisms among-these different regions of the flow.
Kim, Sang-Wook
1988-01-01
A velocity-pressure integrated, mixed interpolation, Galerkin finite element method for the Navier-Stokes equations is presented. In the method, the velocity variables were interpolated using complete quadratic shape functions and the pressure was interpolated using linear shape functions. For the two dimensional case, the pressure is defined on a triangular element which is contained inside the complete biquadratic element for velocity variables; and for the three dimensional case, the pressure is defined on a tetrahedral element which is again contained inside the complete tri-quadratic element. Thus the pressure is discontinuous across the element boundaries. Example problems considered include: a cavity flow for Reynolds number of 400 through 10,000; a laminar backward facing step flow; and a laminar flow in a square duct of strong curvature. The computational results compared favorable with those of the finite difference methods as well as experimental data available. A finite elememt computer program for incompressible, laminar flows is presented.
Felderhof, B U
2016-01-01
Translational and rotational swimming at small Reynolds number of a planar assembly of identical spheres immersed in an incompressible viscous fluid is studied on the basis of a set of equations of motion for the individual spheres. The motion of the spheres is caused by actuating forces and forces derived from a direct interaction potential, as well as hydrodynamic forces exerted by the fluid as frictional and added mass hydrodynamic interactions. The translational and rotational swimming velocities of the assembly are deduced from momentum and angular momentum balance equations. The mean power required during a period is calculated from an instantaneous power equation. Expressions are derived for the mean swimming velocities and the power, valid to second order in the amplitude of displacements from the relative equilibrium positions. Hence these quantities can be evaluated for prescribed periodic displacements. Explicit calculations are performed for three spheres interacting such that they form an equilat...
Reynolds Number Effects in the Flow of a Vočadlo Electrorheological Fluid in a Curved Gap
Walicka, A.; Falicki, J.
2017-08-01
Many electrorheological fluids (ERFs) as fluids with micro-structure demonstrate a non-Newtonian behaviour. Rheometric measurements indicate that some flows of these fluids may by modelled as the flows of a Vočadlo ER fluid. In this paper, the flow of a Vočadlo fluid - with a fractional index of non-linearity - in a narrow gap between two fixed surfaces of revolution with a common axis of symmetry is considered. The flow is externally pressurized and it is considered with inertia effect. In order to solve this problem the boundary layer equations are used. The Reynolds number effects (the effects of inertia forces) on the pressure distribution are examined by using the method of averaged inertia terms of the momentum equation. Numerical examples of externally pressurized flows in the gap between parallel disks and concentric spherical surfaces are presented.
On the effect of rotation on magnetohydrodynamic turbulence at high magnetic Reynolds number
Favier, Benjamin F N; Cambon, Claude; 10.1080/03091929.2010.544655
2011-01-01
This article is focused on the dynamics of a rotating electrically conducting fluid in a turbulent state. As inside the Earth's core or in various industrial processes, a flow is altered by the presence of both background rotation and a large scale magnetic field. In this context, we present a set of 3D direct numerical simulations of incompressible decaying turbulence. We focus on parameters similar to the ones encountered in geophysical and astrophysical flows, so that the Rossby number is small, the interaction parameter is large, but the Elsasser number, defining the ratio between Coriolis and Lorentz forces, is about unity. These simulations allow to quantify the effect of rotation and thus inertial waves on the growth of magnetic fluctuations due to Alfv\\'en waves. Rotation prevents the occurrence of equipartition between kinetic and magnetic energies, with a reduction of magnetic energy at decreasing Elsasser number {\\Lambda}. It also causes a decrease of energy transfer mediated by cubic correlations....