Off-Design Reynolds Number Effects for a Supersonic Transport
NASA Technical Reports Server (NTRS)
Owens, Lewis R.; Wahls, Richard A.; Rivers, S. Melissa
2005-01-01
A high Reynolds number wind tunnel test was conducted to assess Reynolds number effects on the aerodynamic performance characteristics of a realistic, second-generation supersonic transport concept. The tests included longitudinal studies at transonic and low-speed, high-lift conditions across a range of chord Reynolds numbers (8 million to 120 million). Results presented focus on Reynolds number and static aeroelastic sensitivities at Mach 0.30 and 0.90 for a configuration without a tail. Static aeroelastic effects, which mask Reynolds number effects, were observed. Reynolds number effects were generally small and the drag data followed established trends of skin friction as a function of Reynolds number. A more nose-down pitching moment was produced as Reynolds number increased because of an outward movement of the inboard leading-edge separation at constant angles of attack. This study extends the existing Reynolds number database for supersonic transports operating at off-design conditions.
The effect of Reynolds number on boattail drag
NASA Technical Reports Server (NTRS)
Reubush, D. E.
1975-01-01
An investigation has been conducted in the Langley pilot transonic cryogenic tunnel to determine the effects of varying Reynolds number on boattail drag at subsonic speeds. Six boattailed cone-cylinder nacelle models were tested with the jet exhaust simulated by a cylindrical sting. Reynolds number was varied from about 2.6 million to 132 million by changing model length and unit Reynolds number. Boattail pressure coefficient distributions show that increasing Reynolds number tends to make the pressure coefficients in the expansion region more negative and the pressure coefficients in the recompression region more positive. These two effects were compensating and as a result there was little or no effect of Reynolds number on the pressure drag of the isolated boattails.
Reynolds Number Effects on the Performance of Lateral Control Devices
NASA Technical Reports Server (NTRS)
Mineck, Raymond E.
2000-01-01
The influence of Reynolds number on the performance of outboard spoilers and ailerons was investigated on a generic subsonic transport configuration in the National Transonic Facility over a chord Reynolds number range 41 from 3x10(exp 6) to 30xl0(exp 6) and a Mach number range from 0.50 to 0.94, Spoiler deflection angles of 0, 10, 15, and 20 deg and aileron deflection angles of -10, 0, and 10 deg were tested. Aeroelastic effects were minimized by testing at constant normalized dynamic pressure conditions over intermediate Reynolds number ranges. Results indicated that the increment in rolling moment due to spoiler deflection generally becomes more negative as the Reynolds number increases from 3x10(exp 6) to 22x10(exp 6) with only small changes between Reynolds numbers of 22x10(exp 6) and 30x10(exp 6). The change in the increment in rolling moment coefficient with Reynolds number for the aileron deflected configuration is generally small with a general trend of increasing magnitude with increasing Reynolds number.
Reynolds Number Effects on a Supersonic Transport at Transonic Conditions
NASA Technical Reports Server (NTRS)
Wahls, R. N.; Owens, L. R.; Rivers, S. M. B.
2001-01-01
A High Speed Civil Transport configuration was tested in the National Transonic Facility at the NASA Langley Research Center as part of NASA's High Speed Research Program. The primary purposes of the tests were to assess Reynolds number scale effects and the high Reynolds number aerodynamic characteristics of a realistic, second generation supersonic transport while providing data for the assessment of computational methods. The tests included longitudinal and lateral/directional studies at low speed high-lift and transonic conditions across a range of Reynolds numbers from that available in conventional wind tunnels to near flight conditions. Results are presented which focus on both the Reynolds number and static aeroelastic sensitivities of longitudinal characteristics at Mach 0.90 for a configuration without an empennage.
Small scale turbulence and the finite Reynolds number effect
NASA Astrophysics Data System (ADS)
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.
Subsonic Reynolds Number Effects on a Diamond Wing Configuration
NASA Technical Reports Server (NTRS)
Luckring, J. M.; Ghee, T. A.
2001-01-01
An advanced diamond-wing configuration was tested at low speeds in the National Transonic Facility (NTF) in air at chord Reynolds numbers from 4.4 million (typical wind-tunnel conditions) to 24 million (nominal flight value). Extensive variations on high-lift rigging were explored as part of a broad multinational program. The analysis for this study is focused on the cruise and landing settings of the wing high-lift systems. Three flow domains were identified from the data and provide a context for the ensuing data analysis. Reynolds number effects were examined in incremental form based upon attached-flow theory. A similar approach showed very little effect of low-speed compressibility.
Reynolds number effects on mixing due to topological chaos
Smith, Spencer A.; Warrier, Sangeeta
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.
Reynolds number effects on mixing due to topological chaos.
Smith, Spencer A; Warrier, Sangeeta
2016-03-01
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-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.
Reynolds number and geometry effects in laminar axisymmetric isothermal counterflows
NASA Astrophysics Data System (ADS)
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.
The effect of collision, Stokes and Reynolds numbers on turbophoresis
NASA Astrophysics Data System (ADS)
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.
NASA Technical Reports Server (NTRS)
Baals, D. D. (Editor)
1977-01-01
Fundamental aerodynamic questions for which high Reynolds number experimental capability is required are discussed. The operational characteristics and design features of the National Transonic Facility are reviewed.
Reynolds Number Effects at High Angles of Attack
NASA Technical Reports Server (NTRS)
Fisher, David F.; Cobleigh, Brent R.; Banks, Daniel W.; Hall, Robert M.; Wahls, Richard A.
1998-01-01
Lessons learned from comparisons between ground-based tests and flight measurements for the high-angle-of-attack programs on the F-18 High Alpha Research Vehicle (HARV), the X-29 forward-swept wing aircraft, and the X-31 enhanced fighter maneuverability aircraft are presented. On all three vehicles, Reynolds number effects were evident on the forebodies at high angles of attack. The correlation between flight and wind tunnel forebody pressure distributions for the F-18 HARV were improved by using twin longitudinal grit strips on the forebody of the wind-tunnel model. Pressure distributions obtained on the X-29 wind-tunnel model at flight Reynolds numbers showed excellent correlation with the flight data up to alpha = 50 deg. Above (alpha = 50 deg. the pressure distributions for both flight and wind tunnel became asymmetric and showed poorer agreement, possibly because of the different surface finish of the model and aircraft. The detrimental effect of a very sharp nose apex was demonstrated on the X-31 aircraft. Grit strips on the forebody of the X-31 reduced the randomness but increased the magnitude of the asymmetry. Nose strakes were required to reduce the forebody yawing moment asymmetries and the grit strips on the flight test noseboom improved the aircraft handling qualities.
Reynolds Number Effects on the Richtmyer-Meshkov Instability
NASA Technical Reports Server (NTRS)
Niederhaus, Charles; Vitaliy, Krivets; Collins, Brett; Jacobs, Jeffrey
2002-01-01
This presentation compares the results of two very different experimental studies of Richtmyer-Meshkov instability: shock tube experiments in which an air/SF6 interface is accelerated by a weak shock wave; and incompressible experiments in which a box containing two different density miscible liquids is impulsively accelerated by bouncing it off of a fixed coil spring. Both experiments are initiated with sinusoidal initial perturbations. The interface perturbation initially remains sinusoidal as it grows in amplitude, but eventually the interfacial vorticity concentrates into points, forming a row of line vortices of alternating sign. The Reynolds number based on vortex circulation ranges from 1,000 to 45,000 in these experiments. It is found that viscous effects have a large, quantifiable effect on the evolution of the individual vortices. The effects of viscosity on the overall perturbation amplitude, however, are small and will be compared to theory.
Effect of Reynolds Number and Mach Number on flow angularity probe sensitivity
NASA Technical Reports Server (NTRS)
Smith, L. A.; Adcock, J. B.
1986-01-01
Preliminary calibrations were performed on nine flow angularity probes in the Langley 7- by 10-Foot High-Speed Tunnel (7 x 10 HST) and the Langley 0.3-Meter Transonic Cryogenic Tunnel (0.3-m TCT). These probes will be used in surveying the test section flows of the National Transonic Facility (NTF). The probes used in this study have a pyramid head with five pressure orifices. The calibrations consisted of both isolated probe measurements and rake-mounted multiprobe measurements that covered a range of subsonic Mach numbers up to 0.90 and Reynolds numbers per foot up to 40 X 10 to the 6th power. The preliminary calibration in the 7 x 10 HST included testing the probes both individually and in a rake. The 0.3-m TCT calibration tested two probes singly at varying Reynolds numbers. The results from these tests include Mach number, Reynolds number, and rake-mounting effects. The results of these tests showed probe sensitivity to be slightly affected by Mach number. At Reynolds numbers per foot above 10 x 10 to the 6th power, the probe did not exhibit a Reynolds number sensitivity.
1985-02-01
numbers. At high altitudes aircraft gas turbine engine fan, compressor, and turbine blades with their small chords encounter Reynolds numhers...light man-carrying/man-powered aircraft , minl-RPVs at low altitude, and wind turbines . Since the airfoil section forms the basic element in the...Wind turbine blades also require high aerodynamic efficiency and all-weather capabilities. The need for efficient low Reynolds number airfoils which
High Reynolds Number Turbulence
2007-03-27
wall relation of McKeon et al. (2005), and the results for the smallest sandgrain roughness used by Nikuradse (1933). 3 57xI03 "eI : uhp - 2 8 1 6 8 x l 0...Reynolds Number Turbulent Pipe Flow," ASME International Mechanical Engineering Conference and Exposition, Washington, D.C., November 16-21, 2003... Engineering Sciences, Vol. 365 (1852) pp. 699-714, 2007. 14 ’Pipe flow roughness Allen, J.J., Shockling, M.A. and Smits, A.J. "Effects of a machined rough
High Reynolds number effects on a localized stratified turbulent flow
NASA Astrophysics Data System (ADS)
Zhou, Qi; Diamessis, Peter
2015-11-01
We report large-eddy simulations (LES) of the turbulent flow behind a sphere of diameter D translating at speed U in a linearly stratified Boussinesq fluid with buoyancy frequency N. These simulations are performed using a spectral-multidomain-penalty incompressible Navier-Stokes solver, at Reynolds numbers Re ≡ UD / ν ∈ { 5 ×103 , 105 , 4 ×105 } and Froude numbers Fr ≡ 2 U / (ND) ∈ { 4 , 16 , 64 } . An increasingly richer turbulent fine-structure is observed within the larger-scale quasi-horizontal vortices at later times. Turbulent transport of momentum is examined during the non-equilibrium (NEQ) regime of the turbulent life cycle, with an emphasis on the vertical transport that occurs after the establishment of local buoyancy control. The turbulent viscosities in both horizontal and vertical directions are estimated through the LES data; possible parameterization of the vertical turbulent viscosity with the buoyancy Reynolds number Reb = ɛ / (νN2) (or its easy-to-obtain surrogates) is discussed. The dynamical role of the buoyancy Reynolds number in choosing the vertical turbulence length scales is also investigated. ONR grant N00014-13-1-0665 (managed by Dr. R. Joslin); HPCMP Frontier Project FP-CFD-FY14-007 (P.I.: Dr. S. de Bruyn Kops).
Reynolds and Atwood Numbers Effects on Homogeneous Rayleigh Taylor Instability
NASA Astrophysics Data System (ADS)
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.
Effects of Reynolds Number on Mixing and Dispersion
NASA Astrophysics Data System (ADS)
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.
Numerical investigation of the bowed stator effects in a transonic fan at low Reynolds number
NASA Astrophysics Data System (ADS)
Huang, Enliang; Zhao, Shengfeng; Gong, Jianbo; Lu, Xingen; Zhu, Junqiang
2017-02-01
The performance of fan stage in a small turbofan engines is significantly affected at high-altitude low Reynolds number. In order to examine the effect of low Reynolds number on the fan stage, 3D numerical simulation method was employed to analyse the performance variations and the underlying flow structure in the fan stage. For the sake of decreasing the influence of low Reynolds number, the different bowed stator airfoils were redesigned and the effect of the modified design was evaluated.
NASA Technical Reports Server (NTRS)
Yamauchi, G. K.; Johnson, W.
1983-01-01
The primary effects of Reynolds number on two dimensional airfoil characteristics are discussed. Results from an extensive literature search reveal the manner in which the minimum drag and maximum lift are affected by the Reynolds number. C sub d sub min and C sub l sub max are plotted versus Reynolds number for airfoils of various thickness and camber. From the trends observed in the airfoil data, universal scaling laws and easily implemented methods are developed to account for Reynolds number effects in helicopter rotor analyses.
NASA Technical Reports Server (NTRS)
Dorney, Daniel J.
1996-01-01
Experimental data from jet-engine tests have indicated that unsteady blade-row interaction effects can have a significant impact on the efficiency of low-pressure turbine stages. Measured turbine efficiencies at takeoff can be as much as two points higher than those at cruise conditions. Preliminary studies indicate that Reynolds number effects may contribute to the lower efficiencies at cruise conditions. In the current study, numerical experiments have been performed to quantify the Reynolds number dependence of unsteady wake/separation bubble interaction on the performance of a low-pressure turbine.
Comments on Reynolds number effects in wall-bounded shear layers
NASA Technical Reports Server (NTRS)
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.
High Reynolds number and turbulence effects on aerodynamics and heat transfer in a turbine cascade
NASA Technical Reports Server (NTRS)
Yeh, Frederick C.; Hippensteele, Steven A.; Vanfossen, G. James; Poinsatte, Philip E.; Ameri, Ali
1993-01-01
Experimental data on pressure distribution and heat transfer on a turbine airfoil were obtained over a range of Reynolds numbers from 0.75 to 7.5 x 10 exp 6 and a range of turbulence intensities from 1.8 to about 15 percent. The purpose of this study was to obtain fundamental heat transfer and pressure distribution data over a wide range of high Reynolds numbers and to extend the heat transfer data base to include the range of Reynolds numbers encountered in the Space Shuttle main engine (SSME) turbopump turbines. Specifically, the study aimed to determine (1) the effect of Reynolds number on heat transfer, (2) the effect of upstream turbulence on heat transfer and pressure distribution, and (3) the relationship between heat transfer at high Reynolds numbers and the current data base. The results of this study indicated that Reynolds number and turbulence intensity have a large effect on both the transition from laminar to turbulent flow and the resulting heat transfer. For a given turbulence intensity, heat transfer for all Reynolds numbers at the leading edge can be correlated with the Frossling number developed for lower Reynolds numbers. For a given turbulence intensity, heat transfer for the airfoil surfaces downstream of the leading edge can be approximately correlated with a dimensionless parameter. Comparison of the experimental results were also made with a numerical solution from a two-dimensional Navier-Stokes code.
Comments on Reynolds number effects and the role of NTF in the development of air vehicles
NASA Technical Reports Server (NTRS)
Nagel, A. L.
1981-01-01
The National Transonic Facility (NTF) capability to match the full scale Reynolds numbers of all but the largest airplanes is discussed. Conversion factors to enable calculation of Sl-unit equivalents for all U.S. units are listed. Using data from several facilities, analytic methods, and flight test data, a competetive aircraft in the relatively low Reynolds number was developed. The NTF offers the capability to obtain data at full scale Reynolds numbers in the cruise condition for most of the products, and will be much closer than previous tunnels to full scale Reynolds number for the operating envelopes. It is primarily on the operating envelope that Reynolds number effects are most important and least predictable.
Reynolds Number Effects on the Stability and Control Characteristics of a Supersonic Transport
NASA Technical Reports Server (NTRS)
Owens, L. R.; Wahls, R. A.; Elzey, M. B.; Hamner, M. P.
2002-01-01
A High Speed Civil Transport (HSCT) configuration was tested in the National Transonic Facility at the NASA Langley Research Center as part of NASA's High Speed Research Program. A series of tests included longitudinal and lateral/directional studies at transonic and low speed, high-lift conditions across a range of Reynolds numbers from that available in conventional wind tunnels to near flight conditions. Results presented focus on Reynolds number sensitivities of the stability and control characteristics at Mach 0.30 and 0.95 for a complete HSCT aircraft configuration including empennage. The angle of attack where the pitching-moment departure occurred increased with higher Reynolds numbers for both the landing and transonic configurations. The stabilizer effectiveness increased with Reynolds number for both configurations. The directional stability also increased with Reynolds number for both configurations. The landing configuration without forebody chines exhibited a large yawing-moment departure at high angles of attack and zero sideslip that varied with increasing Reynolds numbers. This departure characteristic nearly disappeared when forebody chines were added. The landing configuration's rudder effectiveness also exhibited sensitivities to changes in Reynolds number.
Reynolds Number Effects on the Performance of Ailerons and Spoilers (Invited)
NASA Technical Reports Server (NTRS)
Mineck, R. E.
2001-01-01
The influence of Reynolds number on the performance of outboard spoilers and ailerons was investigated on a generic subsonic transport configuration in the National Transonic Facility over a chord Reynolds number range from 3 to 30 million and a Mach number range from 0.70 to 0.94. Spoiler deflection angles of 0, 10, and 20 degrees and aileron deflection angles of -10, 0, and 10 degrees were tested. Aeroelastic effects were minimized by testing at constant normalized dynamic pressure conditions over intermediate Reynolds number ranges. Results indicated that the increment in rolling moment due to spoiler deflection generally becomes more negative as the Reynolds number increases from 3 x 10(exp 6) to 22 x 10 (exp 6) with only small changes between Reynolds numbers of 22 x 10(exp 6) and 30 x 10(exp 6). The change in the increment in rolling moment coefficient with Reynolds number for the aileron deflected configuration is generally small with a general trend of increasing magnitude with increasing Reynolds number.
Study of Low Reynolds Number Effects on the Losses in Low-Pressure Turbine Blade Rows
NASA Technical Reports Server (NTRS)
Ashpis, David E.; Dorney, Daniel J.
1998-01-01
Experimental data from jet-engine tests have indicated that unsteady blade row interactions and separation can have a significant impact on the efficiency of low-pressure turbine stages. Measured turbine efficiencies at takeoff can be as much as two points higher than those at cruise conditions. Several recent studies have revealed that Reynolds number effects may contribute to the lower efficiencies at cruise conditions. In the current study numerical experiments have been performed to study the models available for low Reynolds number flows, and to quantify the Reynolds number dependence of low-pressure turbine cascades and stages. The predicted aerodynamic results exhibit good agreement with design data.
Reynolds Number Effects on a Supersonic Transport at Subsonic High-Lift Conditions (Invited)
NASA Technical Reports Server (NTRS)
Owens, L.R.; Wahls, R. A.
2001-01-01
A High Speed Civil Transport configuration was tested in the National Transonic Facility at the NASA Langley Research Center as part of NASA's High Speed Research Program. The primary purposes of the tests were to assess Reynolds number scale effects and high Reynolds number aerodynamic characteristics of a realistic, second generation supersonic transport while providing data for the assessment of computational methods. The tests included longitudinal and lateral/directional studies at transonic and low-speed, high-lift conditions across a range of Reynolds numbers from that available in conventional wind tunnels to near flight conditions. Results are presented which focus on Reynolds number and static aeroelastic sensitivities of longitudinal characteristics at Mach 0.30 for a configuration without an empennage. A fundamental change in flow-state occurred between Reynolds numbers of 30 to 40 million, which is characterized by significantly earlier inboard leading-edge separation at the high Reynolds numbers. Force and moment levels change but Reynolds number trends are consistent between the two states.
Roughness and Reynolds number effects in turbulent flows over forward facing step
NASA Astrophysics Data System (ADS)
Thacher, Eric W.; Essel, Ebenezer E.; Tachie, Mark F.
2013-11-01
Experiments were performed to study the effects of Reynolds number and surface roughness on turbulent flows over a forward facing step (FFS). The test conditions include an upstream smooth surface and a smooth FFS (SM-SM), an upstream surface roughened with sand grains and a smooth FFS (SG-SM) and upstream rough surface roughened with sand grains and a FFS roughened with sand grains (SG-SG). In each of the above three test conditions, measurements were made at 9 different Reynolds numbers that varied from 2140 to 9130. The average size of the sand grains was 1.8 mm. A particle image velocimetry technique was used to conduct detailed velocity measurements upstream of the FFS and in the separating and reattachment region over the step. Preliminary results show that the reattachment length increases with Reynolds number but becomes independent of Reynolds number at moderate Reynolds numbers. The reattachment length is independent of upstream roughness at very low Reynolds numbers, however, upstream roughness was observed to decrease the reattachment by 41% at higher Reynolds numbers. The authors acknowledge financial support from NSERC DG.
The effect of Reynolds number on the boattail drag of two wing-body configurations
NASA Technical Reports Server (NTRS)
Reubush, D. E.
1975-01-01
An investigation has been conducted in the Langley 1/3-meter transonic cryogenic tunnel to determine the effects of varying Reynolds number on the boattail drag of wing-body configurations at subsonic speeds. Two boattailed cone-cylinder nacelle models were tested with a 60 deg delta wing at an angle of attack of 0 deg. Reynolds number, based on model length, was varied from about 2.5 million to 67 million. Even though the presence of the wing had large effects on the boattail pressure coefficients, the results of this investigation were similar to those previously found for a series of isolated boattails. Boattail pressure coefficients in the expansion region became more negative with increasing Reynolds number, while those in the recompression region became more positive. These two effects were compensating, and as a result, there was virtually no effect of Reynolds number on boattail pressure drag.
NASA Technical Reports Server (NTRS)
Tomek, W. G.; Hall, R. M.; Wahls, R. A.; Luckring, J. M.; Owens, L. R.
2002-01-01
A wind tunnel test of a generic fighter configuration was tested in the National Transonic Facility through a cooperative agreement between NASA Langley Research Center and McDonnell Douglas. The primary purpose of the test was to assess Reynolds number scale effects on a thin-wing, fighter-type configuration up to full-scale flight conditions (that is, Reynolds numbers of the order of 60 million). The test included longitudinal and lateral/directional studies at subsonic and transonic conditions across a range of Reynolds numbers from that available in conventional wind tunnels to flight conditions. Results are presented for three Mach numbers (0.6, 0.8, and 0.9) and three configurations: (1) Fuselage/Wing; (2) Fuselage/Wing/Centerline Vertical Tail/Horizontal Tail; and (3) Fuselage/Wing/Trailing-Edge Extension/Twin Vertical Tails. Reynolds number effects on the longitudinal aerodynamic characteristics are presented herein.
Effect of Reynolds number on the subsonic boattail drag of several wing-body configurations
NASA Technical Reports Server (NTRS)
Reubush, D. E.
1976-01-01
An investigation was conducted in a transonic cryogenic tunnel to determine the effect of varying Reynolds number on the boattail drag of several wing-body configurations. This study was made at 0 deg angle of attack at Mach numbers from 0.6 to 0.9 for Reynolds numbers up to 67 x 1 million (based on distance from the nose to the start of the boattail). Results indicate that as the Reynolds number was increased the boattail static pressure coefficients in the expansion region of the boattail became more negative while those in the recompression region became more positive. Results show that there was only a small effect of Reynolds number of boattail pressure drag.
NASA Technical Reports Server (NTRS)
Rivers, Melissa B.; Wahls, Richard A.
1999-01-01
This paper gives the results of a grid study, a turbulence model study, and a Reynolds number effect study for transonic flows over a high-speed aircraft using the thin-layer, upwind, Navier-Stokes CFL3D code. The four turbulence models evaluated are the algebraic Baldwin-Lomax model with the Degani-Schiff modifications, the one-equation Baldwin-Barth model, the one-equation Spalart-Allmaras model, and Menter's two-equation Shear-Stress-Transport (SST) model. The flow conditions, which correspond to tests performed in the NASA Langley National Transonic Facility (NTF), are a Mach number of 0.90 and a Reynolds number of 30 million based on chord for a range of angle-of-attacks (1 degree to 10 degrees). For the Reynolds number effect study, Reynolds numbers of 10 and 80 million based on chord were also evaluated. Computed forces and surface pressures compare reasonably well with the experimental data for all four of the turbulence models. The Baldwin-Lomax model with the Degani-Schiff modifications and the one-equation Baldwin-Barth model show the best agreement with experiment overall. The Reynolds number effects are evaluated using the Baldwin-Lomax with the Degani-Schiff modifications and the Baldwin-Barth turbulence models. Five angles-of-attack were evaluated for the Reynolds number effect study at three different Reynolds numbers. More work is needed to determine the ability of CFL3D to accurately predict Reynolds number effects.
NASA Technical Reports Server (NTRS)
Rivers, S. M. B.; Wahls, R. A.; Owens, L. R.
2001-01-01
A computational study focused on leading-edge radius effects and associated Reynolds number sensitivity for a High Speed Civil Transport configuration at transonic conditions was conducted as part of NASA's High Speed Research Program. The primary purposes were to assess the capabilities of computational fluid dynamics to predict Reynolds number effects for a range of leading-edge radius distributions on a second-generation supersonic transport configuration, and to evaluate the potential performance benefits of each at the transonic cruise condition. Five leading-edge radius distributions are described, and the potential performance benefit including the Reynolds number sensitivity for each is presented. Computational results for two leading-edge radius distributions are compared with experimental results acquired in the National Transonic Facility over a broad Reynolds number range.
A review of some Reynolds number effects related to bodies at high angles of attack
NASA Technical Reports Server (NTRS)
Polhamus, E. C.
1984-01-01
A review of some effects of Reynolds number on selected aerodynamic characteristics of two- and three-dimensional bodies of various cross sections in relation to fuselages at high angles of attack at subsonic and transonic speeds is presented. Emphasis is placed on the Reynolds number ranges above the subcritical and angles of attack where lee side vortex flow or unsteady wake type flows predominate. Lists of references, arranged in subject categories, are presented with emphasis on those which include data over a reasonable Reynolds number range. Selected Reynolds number data representative of various aerodynamic flows around bodies are presented and analyzed and some effects of these flows on fuselage aerodynamic parameters are discussed.
NASA Technical Reports Server (NTRS)
Stanewsky, E.; Freimuth, P.
1989-01-01
A comparison of results from conventional and adaptive wall wind tunnels with regard to Reynolds number effects was carried out. The special objective of this comparison was to confirm or reject earlier conclusions, soley based on conventional wind tunnel results, concerning the influence of viscous effects on the characteristics of partially open wind tunnel walls, hence wall interference. The following postulations could be confirmed: (1) certain classes of supercritical airfoils exhibit a non-linear increase in lift which is, at least in part, related to viscous-inviscid interactions on the airfoil. This non-linear lift characteristic can erroneously be suppressed by sidewall interference effects in addition to being affected by changes in Reynolds number. Adaptive walls seem to relieve the influence of sidewall interference; (2) the degree of (horizontal) wall interference effects can be significantly affected by changes in Reynolds number, thus appearing as true Reynolds number effects; (3) perforated wall characteristics seem much more susceptible to viscous changes than the characteristics of slotted walls; here, blockage interference may be most severely influenced by viscous changes; and (4) real Reynolds number effects are present on the CAST 10-2/DOA 2 airfoil; they were shown to be appreciable also by the adaptive wall wind tunnel tests.
Reynolds and froude number effect on the flow past an interface-piercing circular cylinder
NASA Astrophysics Data System (ADS)
Koo, Bonguk; Yang, Jianming; Yeon, Seong Mo; Stern, Frederick
2014-09-01
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 the performance and near-wake of a cross-flow turbine
NASA Astrophysics Data System (ADS)
Bachant, Peter; Wosnik, Martin
2013-11-01
To design wind or marine hydrokinetic (MHK) turbine farms with high efficiency, interactions between turbine wakes must be accurately predicted. However, to date numerical models predicting detailed wake properties of cross-flow (or vertical-axis) turbines have been validated with experimental data taken at Reynolds numbers significantly lower than those of full scale devices, casting doubt on the models' accuracy. To address this uncertainty, we investigated the effects of Reynolds number on the performance and near-wake characteristics of a 3-bladed cross-flow turbine, both experimentally and numerically. Mechanical power output and overall streamwise drag were measured in a towing tank at turbine diameter Reynolds numbers ReD = 0 . 5 ×105 - 2 . 0 ×106 . A detailed map of the near-wake one turbine diameter downstream was acquired via acoustic Doppler velocimetry for each Reynolds number case, from which differences in the mean velocity, turbulence intensity, and Reynolds stresses are highlighted. Finally, Reynolds-averaged Navier-Stokes (RANS) numerical simulations were performed, the results from which are compared with the experimental data. Work supported by NSF-CBET grant 1150797.
Reynolds Number, Compressibility, and Leading-Edge Bluntness Effects on Delta-Wing Aerodynamics
NASA Technical Reports Server (NTRS)
Luckring, James M.
2004-01-01
An overview of Reynolds number, compressibility, and leading edge bluntness effects is presented for a 65 degree delta wing. The results of this study address both attached and vortex-flow aerodynamics and are based upon a unique data set obtained in the NASA-Langley National Transonic Facility (NTF) for i) Reynolds numbers ranging from conventional wind-tunnel to flight values, ii) Mach numbers ranging from subsonic to transonic speeds, and iii) leading-edge bluntness values that span practical slender wing applications. The data were obtained so as to isolate the subject effects and they present many challenges for Computational Fluid Dynamics (CFD) studies.
Low Reynolds Number Effects on Hypersonic Blunt Body Shock Standoff
NASA Astrophysics Data System (ADS)
Inger, George R.
2003-05-01
An approximate analytical theory of the onset of viscous effects on hypersonic body nose shock standoff distance is developed. Boundary layers displacement, shock layer vorticity and longitudinal curvature effects are all included, as are the influence of both shock layer density ratio and arbitrary body surface temperature. Validating comparisons with both CFD results and experimental data are also presented.
Negative Magnus Effect on a Rotating Sphere at around the Critical Reynolds Number
NASA Astrophysics Data System (ADS)
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.
Effective swimming strategies in low Reynolds number flows
NASA Astrophysics Data System (ADS)
Olla, P.
2011-03-01
The optimal strategy for a microscopic swimmer to migrate across a linear shear flow is discussed. The two cases, in which the swimmer is located at large distance, and in the proximity of a solid wall, are taken into account. It is shown that migration can be achieved by means of a combination of sailing through the flow and swimming, where the swimming strokes are induced by the external flow without need of internal energy sources or external drives. The structural dynamics required for the swimmer to move in the desired direction is discussed and two simple models, based respectively on the presence of an elastic structure, and on an orientation dependent friction, to control the deformations induced by the external flow, are analyzed. In all cases, the deformation sequence is a generalization of the tank-treading motion regimes observed in vesicles in shear flows. Analytic expressions for the migration velocity as a function of the deformation pattern and amplitude are provided. The effects of thermal fluctuations on propulsion have been discussed and the possibility that noise be exploited to overcome the limitations imposed on the microswimmer by the scallop theorem have been discussed.
Effect of Reynolds number on stability characteristics of a cruciform wing-body
NASA Technical Reports Server (NTRS)
Stallings, R. L., Jr.; Lamb, M.; Watson, C. B.
1980-01-01
An experimental investigation was conducted to determine the effect of Reynolds number on the stability characteristics of a body with cruciform wings at large angles of attack. Pressure distributions and force and moment data (axial force not measured) are presented for Mach 1.60 and 2.70, Reynolds numbers based on body diameter from approximately 130,000 to 2,800,000, and angles of attack from 0 deg to 50 deg. In general, the data show only small effects of Reynolds number throughout the range of test condition. Also discussed are force balance and pressure data that suggest a direct relationship between wind choking and the onset of a nonlinear stability variaton with angle of attack.
Reynolds Number and Leading-Edge Bluntness Effects on a 65 Deg Delta Wing
NASA Technical Reports Server (NTRS)
Luckring, J. M.
2002-01-01
A 65 deg delta wing has been tested in the National Transonic Facility (NTF) at mean aerodynamic chord Reynolds numbers from 6 million to 120 million at subsonic and transonic speeds. The configuration incorporated systematic variation of the leading edge bluntness. The analysis for this paper is focused on the Reynolds number and bluntness effects at subsonic speeds (M = 0.4) from this data set. The results show significant effects of both these parameters on the onset and progression of leading-edge vortex separation.
Transonic Reynolds Number and Leading-Edge Bluntness Effects on a 65 deg Delta Wing
NASA Technical Reports Server (NTRS)
Luckring, J. M.
2003-01-01
A 65 deg delta wing has been tested in the National Transonic Facility (NTF) at mean aerodynamic chord Reynolds numbers from 6 million to 120 million at subsonic and transonic speeds. The configuration incorporated a systematic variation of the leading edge bluntness. The analysis for this paper is focused on the Reynolds number and bluntness effects at transonic speeds (M = 0.85) from this data set. The results show significant effects of both these parameters on the onset and progression of leading- edge vortex separation.
Transonic Reynolds Number and Leading-Edge Bluntness Effects on a 65 deg Delta Wing
NASA Technical Reports Server (NTRS)
Luckring, J. M.
2003-01-01
A 65 deg delta wing has been tested in the National Transonic Facility (NTF) at mean aerodynamic chord Reynolds numbers from 6 million to 120 million at subsonic and transonic speeds. The configuration incorporated a systematic variation of the leading edge bluntness. The analysis for this paper is focused on the Reynolds number and bluntness effects at transonic speeds (M=0.85) from this data set. The results show significant effects of both these parameters on the onset and progression of leading-edge vortex separation.
Transonic Reynolds Number and Leading-Edge Bluntness Effects on a 65 deg Delta Wing
NASA Technical Reports Server (NTRS)
Luckring, J. M.
2003-01-01
A 65 degree delta wing has been tested in the National Transonic Facility (NTF) at mean aerodynamic chord Reynolds numbers from 6 million to 120 million at subsonic and transonic speeds. The configuration incorporated a systematic variation of the leading edge bluntness. The analysis for this paper is focused on the Reynolds number and bluntness effects at transonic speeds (M = 0.85) from this data set. The results show significant effects of both these parameters on the onset and progression of leading edge vortex separation.
Transonic Reynolds Number and Leading-Edge Bluntness Effects on a 65 deg Delta Wing
NASA Technical Reports Server (NTRS)
Luckring, J. M.
2003-01-01
A 65 deg delta wing has been tested in the National Transonic Facility (NTF) at mean aerodynamic chord Reynolds numbers from 6 million to 120 million at subsonic and transonic speeds. The configuration incorporated a systematic variation of the leading edge bluntness. The analysis for this paper is focused on the Reynolds number and bluntness effects at transonic speeds (M = 0.85) from this data set. The results show significant effects of both these parameters on the onset and progression of leading- edge vortex separation.
Transonic Reynolds Number and Leading-Edge Bluntness Effects on a 65 deg Delta Wing
NASA Technical Reports Server (NTRS)
Luckring, J. M.
2003-01-01
A 65 deg delta wing has been tested in the National Transonic Facility (NTF) at mean aerodynamic chord Reynolds numbers from 6 million to 120 million at subsonic and transonic speeds. The configuration incorporated a systematic variation of the leading edge bluntness. The analysis for this paper is focused on the Reynolds number and bluntness effects at transonic speeds (M=0.85) from this data set. The results show significant effects of both these parameters on the onset and progression of leading-edge vortex separation.
Reynolds Number and Leading-Edge Bluntness Effects on a 65 deg Delta Wing
NASA Technical Reports Server (NTRS)
Luckring, J. M.
2002-01-01
A 65 degree delta wing has been tested in the National Transonic Facility (NTF) at mean aerodynamic chord Reynolds numbers from 6 million to 120 million at subsonic and transonic speeds. The configuration incorporated systematic variation of the leading edge bluntness. The analysis for this paper is focused on the Reynolds number and bluntness effects at subsonic speeds (M = 0.4) from this data set. The results show significant effects of both these parameters on the onset and progression of leading-edge vortex separation.
Reynolds number and pressure gradient effects on compressible turbulent boundary layers
NASA Technical Reports Server (NTRS)
Acharya, M.; Kussoy, M. I.; Horstman, C. C.
1978-01-01
A detailed investigation of attached supersonic turbulent boundary layers over an extensive range of Reynolds numbers (12 x 10 to the 6th to 314 x 10 to the 6th) is presented. Experimental measurements were obtained for adverse pressure gradients ranging in magnitude from those of previous investigations to those approaching separation. The measurements include mean values of surface pressure and skin-friction, mean-flow profiles, and profiles of the three turbulent velocity fluctuation components and turbulent shear stress. Numerical solutions, employing three turbulence models of various degrees of complexity have been compared with the details of the measured flow fields. Generally, it was found that the more sophisticated turbulence models are superior to a mixing length model for predicting the Reynolds number and pressure gradient effects. However, some details of the turbulent fluctuations as well as the exact Reynolds number trends indicated by the data were not accurately predicted with any of the turbulence models considered.
Reynolds Number Effects on Thrust Coefficients and PIV for Flapping Wing Micro Air Vehicles
2012-03-09
16 Figure 6. AeroVironment Nano Hummingbird [Keennon 2012] ................................................ 17 Figure 7. Berkeley Micro ...Keennon, M., Klingebiel, K., Won, H., Andriukov, A., “Development of the Nano Hummingbird: A Tailless Flapping Wing Micro Air Vehicle,” AIAA 2012...REYNOLDS NUMBER EFFECTS ON THRUST COEFFICIENTS AND PIV FOR FLAPPING WING MICRO AIR VEHICLES
Effects of droplet interactions on droplet transport at intermediate Reynolds numbers
NASA Technical Reports Server (NTRS)
Shuen, J. S.
1986-01-01
Effects of droplet interactions on drag, evaporation, and combustion of a planar droplet array, oriented perpendicular to the approaching flow, are studied numerically. The three-dimensional Navier-Stokes equations, with variable thermophysical properties, are solved using finite-difference techniques. Parameters investigated include the droplet spacing, droplet Reynolds number, approaching stream oxygen concentration, and fuel type. Results are obtained for the Reynolds number range of 5 to 100, droplet spacing from 2 to 24 diameters, oxygen concentrations of 0.1 and 0.2, and methanol and n-butanol fuels. The calculations show that the gasification rates of interacting droplets decrease as the droplet spacings decrease. The reduction in gasification rates is significant only at small spacings and low Reynolds numbers. For the present array orientation, the effects of interactions on the gasification rates diminish rapidly for Reynolds numbers greater than 10 and spacings greater than 6 droplet diameters. The effects of adjacent droplets on drag are shown to be small.
Turbulence effect on crossflow around a circular cylinder at subcritical Reynolds numbers
NASA Technical Reports Server (NTRS)
Sadeh, W. Z.; Saharon, D. B.
1982-01-01
An investigation of the effect of freestream turbulence on the flow around a smooth circular cylinder at subcritical Reynolds numbers from 5.2 x 10 to the 4th power to 2.09 x 10 to the 5th power was conducted. Measurements show that the interaction of incident turbulence with the initial laminar boundary layer: (1) modifies the characteristics of the mean surface pressure distribution; (2) induces an aft shift in the separation point ranging from 5 to 50 beyond the laminar separation angle of 80 degrees; and, (3) reduces the mean drag coefficient to values between 97 and 46% of its nearly constant laminar counterpart. The extent of these changes depends on the particular Reynolds number background turbulence combination. These results demonstrate that a boundary-layer flow similar to that found in critical, supercritical and/or transcritical flow regimes is induced by turbulence at subcritical Reynolds numbers and, hence, the effect of turbulence is equivalent to an effective increase in the Reynolds number. The change in the nature and properties of the boundary layer in the subcritical regime, consequent upon the penetration of turbulence into it, is in agreement with the model proposed by the vorticity-amplification theory.
Reynolds number influences in aeronautics
NASA Technical Reports Server (NTRS)
Bushnell, Dennis M.; Yip, Long P.; Yao, Chung-Sheng; Lin, John C.; Lawing, Pierce L.; Batina, John T.; Hardin, Jay C.; Horvath, Thomas J.; Fenbert, James W.; Domack, Christopher S.
1993-01-01
Reynolds number, a measure of the ratio of inertia to viscous forces, is a fundamental similarity parameter for fluid flows and therefore, would be expected to have a major influence in aerodynamics and aeronautics. Reynolds number influences are generally large, but monatomic, for attached laminar (continuum) flow; however, laminar flows are easily separated, inducing even stronger, non-monatomic, Reynolds number sensitivities. Probably the strongest Reynolds number influences occur in connection with transitional flow behavior. Transition can take place over a tremendous Reynolds number range, from the order of 20 x 10(exp 3) for 2-D free shear layers up to the order of 100 x 10(exp 6) for hypersonic boundary layers. This variability in transition behavior is especially important for complex configurations where various vehicle and flow field elements can undergo transition at various Reynolds numbers, causing often surprising changes in aerodynamics characteristics over wide ranges in Reynolds number. This is further compounded by the vast parameterization associated with transition, in that any parameter which influences mean viscous flow development (e.g., pressure gradient, flow curvature, wall temperature, Mach number, sweep, roughness, flow chemistry, shock interactions, etc.), and incident disturbance fields (acoustics, vorticity, particulates, temperature spottiness, even electro static discharges) can alter transition locations to first order. The usual method of dealing with the transition problem is to trip the flow in the generally lower Reynolds number wind tunnel to simulate the flight turbulent behavior. However, this is not wholly satisfactory as it results in incorrectly scaled viscous region thicknesses and cannot be utilized at all for applications such as turbine blades and helicopter rotors, nacelles, leading edge and nose regions, and High Altitude Long Endurance and hypersonic airbreathers where the transitional flow is an innately critical
Reynolds number effects on the single-mode Richtmyer-Meshkov instability
NASA Astrophysics Data System (ADS)
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 Re<256 . Within this range there is a sharp change in instability properties. The bubble and spike amplitudes move towards equal size at lower Reynolds numbers and the bubble velocities decay faster than predicted by Sohn's model [S.-I. Sohn, Phys. 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.
Wall Cooling Effects on Hypersonic Transitional/Turbulent Boundary Layers at High Reynolds Numbers
NASA Technical Reports Server (NTRS)
Watson, Ralph D.
1975-01-01
A 4 degree wedge was used to produce a thick turbulent boundary layer with an edge Mach number of 11. By using a two-dimensional model, the boundary layer was nearly free from upstream history effects associated with nozzle wall turbulent boundary layers. Heat-transfer distributions were used to define regions of laminar, transitional, and turbulent flow at several values of T(sub w)/T(sub t) for an edge unit Reynolds number of 0.47 x lot per cm. Pitot and total temperature profiles and skin-friction measurements were obtained at selected stations along the model. Turbulence parameters (mixing length/sigma and epsilon) were derived from the fully turbulent profiles and used to more completely define the "low Reynolds number" effect. Turbulent Prandtl number distributions are also presented.
NASA Technical Reports Server (NTRS)
Sweberg, Harold H.; Lange, Roy H.
1947-01-01
The available foreign and American data relating to Reynolds number effects on the maximum lift coefficients of swept-back wings are summarized and discussed. The data show that at low Reynolds numbers (below about 2.0 x 10(exp 6)) higher maximum lift coefficients were measured in most cases for moderately swept-back wings than for unswept wings of similar plan form; at high Reynolds numbers, however, increasing sweepback resulted in decreasing maximum lift coefficients. A smaller rate of increase of the maximum lift coefficient with Reynolds number was measured for the swept-back wings than for similar unswept wings in the critical range of Reynolds number. Increasing the Reynolds number resulted in decreases in the maximum lift coefficients of the two wings of approximately triangular plan form that were investigated.
Effect of Reynolds Number in Turbulent-Flow Range on Flame Speeds of Bunsen Burner Flames
NASA Technical Reports Server (NTRS)
Bollinger, Lowell M; Williams, David T
1949-01-01
The effect of flow conditions on the geometry of the turbulent Bunsen flame was investigated. Turbulent flame speed is defined in terms of flame geometry and data are presented showing the effect of Reynolds number of flow in the range of 3000 to 35,000 on flame speed for burner diameters from 1/4 to 1 1/8 inches and three fuels -- acetylene, ethylene, and propane. The normal flame speed of an explosive mixture was shown to be an important factor in determining its turbulent flame speed, and it was deduced from the data that turbulent flame speed is a function of both the Reynolds number of the turbulent flow in the burner tube and of the tube diameter.
Biotechnology at low Reynolds numbers.
Brody, J P; Yager, P; Goldstein, R E; Austin, R H
1996-01-01
The shrinking of liquid handling systems to the micron and submicron size range entails moving into the area of small Reynolds numbers. The fluid dynamics in this regime are very different from the macroscale. We present an intuitive explanation of how the different physics of small Reynolds numbers flow, along with microscopic sizes, can influence device design, and give examples from our own work using fluid flow in microfabricated devices designed for biological processing. Images FIGURE 1 FIGURE 9 FIGURE 10 FIGURE 11 FIGURE 12 FIGURE 13 PMID:8968612
Nevmerzhitskiy, N V; Sotskov, E A; Sen'kovskiy, E D; Krivonos, O L; Polovnikov, A A; Levkina, E V; Frolov, S V; Abakumov, S A; Marmyshev, V V
2014-09-01
The results of the experimental study of the Reynolds number effect on the process of the Rayleigh-Taylor (R-T) instability transition into the turbulent stage are presented. The experimental liquid layer was accelerated by compressed gas. Solid particles were scattered on the layer free surface to specify the initial perturbations in some experiments. The process was recorded with the use of a high-speed motion picture camera. The following results were obtained in experiments: (1) Long-wave perturbation is developed at the interface at the Reynolds numbers Re < 10(4). If such perturbation growth is limited by a hard wall, the jet directed in gas is developed. If there is no such limitation, this perturbation is resolved into the short-wave ones with time, and their growth results in gas-liquid mixing. (2) Short-wave perturbations specified at the interface significantly reduce the Reynolds number Re for instability to pass into the turbulent mixing stage.
High Reynolds number research - 1980
NASA Technical Reports Server (NTRS)
Mckinney, L. W. (Editor); Baals, D. D. (Editor)
1981-01-01
The fundamental aerodynamic questions for which high Reynolds number experimental capability is required were examined. Potential experiments which maximize the research returns from the use of the National Transonic Facility (NTF) were outlined. Calibration plans were reviewed and the following topics were discussed: fluid dynamics; high lit; configuration aerodynamics; aeroelasticity and unsteady aerodynamics; wind tunnel/flight correlation; space vehicles; and theoretical aerodynamics
Squirming At Finite Reynolds Number
NASA Astrophysics Data System (ADS)
Chisholm, Nicholas; Zhu, Ziyi; Khair, Aditya
2013-11-01
The dynamics of swimming microorganisms at zero Reynolds number (Re = 0) has been the subject of extensive theoretical and experimental investigation over the past decade, and the study of locomotion at high Reynolds number (Re >> 1), where inertial forces are dominant, has a venerable history. In this talk, we consider swimming between these limits, i.e. at finite Reynolds numbers, using the popular ``squirmer'' model of self-propulsion, wherein locomotion is achieved through surface distortions. We first utilize matched asymptotic expansions to derive an analytical expression for the swimming velocity of a squirmer through O (Re2) , which highlights that inertia affects so-called ``pusher'' and ``puller'' swimmers in fundamentally different manners. (The equivalent of Whitehead's paradox for a self-propelled object is elucidated in the process.) Next, we employ numerical methods to compute the swimming velocity of pushers and pullers for higher Reynolds numbers. Finally, we demonstrate that inertia causes squirmers (which are non-chiral) to drift across the streamlines of an imposed shear flow.
NASA Astrophysics Data System (ADS)
Hammer, Patrick R.
It is well established that natural flyers flap their wings to sustain flight due to poor performance of steady wing aerodynamics at low Reynolds number. Natural flyers also benefit from the propulsive force generated by flapping. Unsteady airfoils allow for simplified study of flapping wing aerodynamics. Limited previous work has suggested that both the Reynolds number and motion trajectory asymmetry play a non-negligible role in the resulting forces and wake structure of an oscillating airfoil. In this work, computations are performed to on this topic for a NACA 0012 airfoil purely pitching about its quarter-chord point. Two-dimensional computations are undertaken using the high-order, extensively validated FDL3DI Navier-Strokes solver developed at Wright-Patterson Air Force Base. The Reynolds number range of this study is 2,000-22,000, reduced frequencies as high as 16 are considered, and the pitching amplitude varies from 2° to 10°. In order to simulate the incompressible limit with the current compressible solver, freestream Mach numbers as low as 0.005 are used. The wake structure is accurately resolved using an overset grid approach. The results show that the streamwise force depends on Reynolds number such that the drag-to-thrust crossover reduced frequency decreases with increasing Reynolds number at a given amplitude. As the amplitude increases, the crossover reduced frequency decreases at a given Reynolds number. The crossover frequency data show good collapse for all pitching amplitudes considered when expressed as the Strouhal number based on trailing edge-amplitude for different Reynolds numbers. Appropriate scaling causes the thrust data to become nearly independent of Reynolds number and amplitude. An increase in propulsive efficiency is observed as the Reynolds number increases while less dependence is seen in the peak-to-peak lift and drag amplitudes. Reynolds number dependence is also seen for the wake structure. The crossover reduced frequency
Zhou, Ye; Thornber, Ben
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.
Aerodynamic effects of wing corrugation at gliding flight at low Reynolds numbers
NASA Astrophysics Data System (ADS)
Meng, Xue Guang; Sun, Mao
2013-07-01
Corrugation gives an insect-wing the advantages of low mass, high stiffness, and low membrane stress. Researchers are interested to know if it is also advantageous aerodynamically. Previous works reported that corrugation enhanced the aerodynamic performance of wings at gliding flight. However, Reynolds numbers considered in these studies were higher than that of gliding insects. The present study showed that in the Reynolds number range of gliding insects, corrugation had negative aerodynamic effects. We studied aerodynamic effects of corrugation at gliding motion using the method of computational fluid dynamics, in the Reynolds number range of Re = 200-2400. Different corrugation patterns were considered. The effect of corrugation on aerodynamic performance was identified by comparing the aerodynamic forces between the corrugated and flat-plate wings, and the underlying flow mechanisms of the corrugation effects were revealed by analyzing the flow fields and surface pressure distributions. The findings are as follows: (1) the effect of corrugation is to decrease the lift, and change the drag only slightly (at 15°-25° angles of attack, lift is decreased by about 16%; at smaller angles of attack, the percentage of lift reduction is even larger because the lift is small). (2) Two mechanisms are responsible for the lift reduction. One is that the pleats at the lower surface of the corrugated wing produce relatively strong vortices, resulting in local low-pressure regions on the lower surface of the wing. The other is that corrugation near the leading edge pushes the leading-edge-separation layer slightly upwards and increases the size of the separation bubble above the upper surface, reducing the "suction pressure," or increasing the pressure, on the upper surface.
CFD-Aided Evaluation of Reynolds Number Scaling Effect Accounting for Static Model Deformation
NASA Astrophysics Data System (ADS)
Yasue, Kanako; Sawada, Keisuke
A static aeroelasticity analysis is accomplished for an ONERA-M5 wind tunnel calibration model. The Reynolds-averaged Navier-Stokes (RANS) solution obtained using the cell-wise relaxation implicit discontinuous Galerkin (DG) computational fluid dynamics (CFD) solver is fed into the structural analysis method to iteratively determine the aerodynamic equilibrium configuration of the wind tunnel model. For the freestream conditions of M=0.84, α=-1.0°, Re=4 × 106, P0=220 kPa and T0=274 K, the aerodynamic equilibrium shape is successfully obtained within three iterations. The maximum deformation of 3.11 mm appears at the wing tip of the wind tunnel model, and the resulting change in aerodynamic force produces a nose-down effect. A detailed examination reveals that the deformation mostly causes pure bending which reduces the effective angle of attack for the present swept wing. Moreover, we attempt to split the change in aerodynamic coefficients into that due to the model deformation effect and that due to the Reynolds (Re) number effect. By comparing the computed results for Re=1 × 106 and Re=4 × 106, it is indicated that an increase in lift coefficient due to the Re number effect is totally offset by the model deformation effect. It is also shown that the amount of drag reduction can be overestimated due to the model deformation effect. In addition, a CFD-aided data correction method utilizing the wind tunnel data is discussed.
Roughness Effects on Compressor Blade Performance in Cascade at High Reynolds Number.
1983-11-01
practically impossible to duplicate test conditions from run to run because of ’changes in the weather that affected the air supply’s humidity and...RD-fl36 896 ROUGHNESS EFFECTS ON COMPRESSOR BLADE PERFORMANCE IN 1/2 CASCADE AT HIGH REY ..(U) AIR FORCE INST OF TECH WJRIGHT-PATTERSON APR OH...CASCADE AT HIGH REYNOLDS NUMBER THESIS FIT/GAE/AA/83D-23 Frederick J. Tanis Jr. 2Lt. U.S. Air Force 0.. iiC5- DEPARTMENT OF THE AIR FORCE AIR FORCE
Simple low Reynolds number microswimmers
NASA Astrophysics Data System (ADS)
Cheang, U. Kei; Kim, Min Jun
2016-11-01
An extremely simple low Reynolds number microswimmer had been observed to swim in bulk fluid. The development of microscopic swimmers had been hindered by technical limitations in micro- and nanofabrication. To address this practical problem, the minimal geometrical requirements for swimming in low Reynolds number has been investigated. Micro- and nanofabrication of complex shapes with specialized materials, such as helices or flexible bodies, on a massive scale requires sophisticated state of the art technologies which have size limitations. In contrast, simple shaped structures, such as spherical particles, can be synthesized massively using chemical methods with relative ease at low costs. In this work, simple microswimmers were fabricated by conjugating two microbeads with debris attached to their surface. The debris allow the 2-bead structures to have two or more planes of symmetry, thus, allowing them to swim in bulk fluid at low Reynolds number. The microswimmers are magnetically actuated and controlled via a rotating magnetic field generated by an electromagnetic coil system. The microswimmers' velocity profiles had been characterized with respect to increasing rotating frequency. Furthermore, the motion of the microswimmers were analyzed using image processing. Finally, their swimming capability had been shown through experiments by steering the microswimmers in any desired direction.
A study of the spray injection Reynolds number effects on gasoline yields of an FCC riser reactor
Bowman, B. J.; Zhou, C. Q.; Chang, S. L.; Lottes, S. A.
2000-04-03
A computational analysis of the combined effects of feed oil injection parameters in a commercial-scale fluidized catalytic cracking riser reactor was performed using a three-phase, multiple species kinetic cracking computer code. The analysis showed that the injection operating parameters (droplet diameter and injection velocity) had strong impacts on the gasoline yields of the FCC unit. A spray injection Reynolds number combining the two parameters was defined. A correlation between the spray injection Reynolds number and the gasoline product yields for various feed injection conditions was developed. A range of spray injection Reynolds number for the maximum gasoline yield was identified.
Sweep and Compressibility Effects on Active Separation Control at High Reynolds Numbers
NASA Technical Reports Server (NTRS)
Seifert, Avi; Pack, LaTunia G.
2000-01-01
This paper explores the effects of compressibility, sweep and excitation location on active separation control at high Reynolds numbers. The model, which was tested in a cryogenic pressurized wind tunnel, simulates the upper surface of a 20% thick GlauertGoldschmied type airfoil at zero angle of attack. The flow is fully turbulent since the tunnel sidewall boundary layer flows over the model. Without control, the flow separates at the highly convex area and a large turbulent separation bubble is formed. Periodic excitation is applied to gradually eliminate the separation bubble. Two alternative blowing slot locations as well as the effect of compressibility, sweep and steady suction or blowing were studied. During the test the Reynolds numbers ranged from 2 to 40 million and Mach numbers ranged from 0.2 to 0.7. Sweep angles were 0 and 30 deg. It was found that excitation must be introduced slightly upstream of the separation region regardless of the sweep angle at low Mach number. Introduction of excitation upstream of the shock wave is more effective than at its foot. Compressibility reduces the ability of steady mass transfer and periodic excitation to control the separation bubble but excitation has an effect on the integral parameters, which is similar to that observed in low Mach numbers. The conventional swept flow scaling is valid for fully and even partially attached flow, but different scaling is required for the separated 3D flow. The effectiveness of the active control is not reduced by sweep. Detailed flow field dynamics are described in the accompanying paper.
Sweep and Compressibility Effects on Active Separation Control at High Reynolds Numbers
NASA Technical Reports Server (NTRS)
Seifert, Avi; Pack, LaTunia G.
2000-01-01
This paper explores the effects of compressibility, sweep and excitation location on active separation control at high Reynolds numbers. The model, which was tested in a cryogenic pressurized wind tunnel, simulates the upper surface of a 20% thick Glauert Goldschmied type airfoil at zero angle of attack. The flow is fully turbulent since the tunnel sidewall boundary layer flows over the model. Without control, the flow separates at the highly convex area and a large turbulent separation bubble is formed. Periodic excitation is applied to gradually eliminate the separation bubble. Two alternative blowing slot locations as well as the effect of compressibility, sweep and steady suction or blowing were studied. During the test the Reynolds numbers ranged from 2 to 40 million and Mach numbers ranged from 0.2 to 0.7. Sweep angles were 0 and 30 deg. It was found that excitation must be introduced slightly upstream of the separation region regardless of the sweep angle at low Mach number. Introduction of excitation upstream of the shock wave is more effective than at its foot. Compressibility reduces the ability of steady mass transfer and periodic excitation to control the separation bubble but excitation has an effect on the integral parameters, which is similar to that observed in low Mach numbers. The conventional swept flow scaling is valid for fully and even partially attached flow, but different scaling is required for the separated 3D flow. The effectiveness of the active control is not reduced by sweep. Detailed flow field dynamics are described in the accompanying paper.
Three-dimensional effects on airfoil measurements at high Reynolds numbers
NASA Astrophysics Data System (ADS)
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).
Reynolds number and roughness effects on turbulent stresses in sandpaper roughness boundary layers
NASA Astrophysics Data System (ADS)
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
NASA Astrophysics Data System (ADS)
Shields, Matthew; Mohseni, Kamran
2011-11-01
The innovation of micro aerial vehicles (MAVs) has brought to attention the unique flow regime associated with low aspect ratio (LAR), low Reynolds number fliers. The dominant effects of developing tip vortices and leading edge vortices create a fundamentally different flow regime than that of conventional aircraft. An improved knowledge of low aspect ratio, low Reynolds number aerodynamics can be greatly beneficial for future MAV design. A little investigated but vital aspect of LAR aerodynamics is the behavior of the fluid as the wing yaws. Flow visualization experiments undertaken in the group for the canonical case of varying AR flat plates indicate that the propagation of the tip vortex keeps the flow attached over the upstream portion of the wing, while the downstream vortex is convected away from the wing. This induces asymmetric, destabilizing loading on the wing which has been observed to adversely affect MAV flight. In addition, experimental load measurements indicate significant nonlinearities in forces and moments which can be attributed to the development and propagation of these vortical structures. A non-dimensional analysis of the rigid body equations of motion indicates that these nonlinearities create dependencies which dramatically change the conventional linearization process. These flow phenomena are investigated with intent to apply to future MAV design.
The effects of Reynolds number, tip speed ratio, and solidity in VAWTs
NASA Astrophysics Data System (ADS)
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.
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
Hydrodynamics of Fishlike Swimming: Effects of swimming kinematics and Reynolds number
NASA Astrophysics Data System (ADS)
Gilmanov, Anvar; Posada, Nicolas; Sotiropoulos, Fotis
2003-11-01
We carry out a series of numerical simulations to investigate the effects of swimming kinematics and Reynolds number on the flow past a three-dimensional fishlike body undergoing undulatory motion. The simulated body shape is that of a real mackerel fish. The mackerel was frozen and subsequently sliced in several thin fillets whose dimensions were carefully measured and used to construct the fishlike body shape used in the simulations. The flow induced by the undulating body is simulated by solving the 3D, unsteady, incompressible Navier-Stokes equations with the second-order accurate, hybrid Cartesian/Immersed Boundary formulation of Gilmanov and Sotiropoulos (J. Comp. Physics, under review, 2003). We consider in-line swimming at constant speed and carry out simulations for various types of swimming kinematics, varying the tailbeat amplitude, frequency, and Reynolds number (300
NASA Technical Reports Server (NTRS)
Pauley, H.; Ralston, J.; Dickes, E.
1995-01-01
The National Aeronautics and Space Administration and the Defense Research Agency (United Kingdom) have ongoing experimental research programs in rotary-flow aerodynamics. A cooperative effort between the two agencies is currently underway to collect an extensive database for the development of high angle of attack computational methods to predict the effects of Reynolds number on the forebody flowfield at dynamic conditions, as well as to study the use of low Reynolds number data for the evaluation of high Reynolds number characteristics. Rotary balance experiments, including force and moment and surface pressure measurements, were conducted on circular and rectangular aftbodies with hemispherical and ogive noses at the Bedford and Farnborough wind tunnel facilities in the United Kingdom. The bodies were tested at 60 and 90 deg angle of attack for a wide range of Reynolds numbers in order to observe the effects of laminar, transitional, and turbulent flow separation on the forebody characteristics when rolling about the velocity vector.
Propulsion at low Reynolds number
NASA Astrophysics Data System (ADS)
Najafi, Ali; Golestanian, Ramin
2005-04-01
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.
Effect of Reynolds number on flow and mass transfer characteristics of a 90 degree elbow
NASA Astrophysics Data System (ADS)
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.
NASA Astrophysics Data System (ADS)
Johansson, Peter B. V.; George, William K.; Gourlay, Michael J.
2003-03-01
Equilibrium similarity considerations are applied to the axisymmetric turbulent wake, without the arbitrary assumptions of earlier theoretical studies. Two solutions for the turbulent flow are found: one for infinite local Reynolds number which grows spatially as x1/3; and another for small local Reynolds number which grows as x1/2. Both solutions can be dependent on the upstream conditions. Also, the local Reynolds number diminishes with increasing downstream distance, so that even when the initial Reynolds number is large, the flow evolves downstream from one state to the other. Most of the available experimental data are at too low an initial Reynolds number and/or are measured too near the wake generator to provide evidence for the x1/3 solution. New results, however, from a laboratory experiment on a disk wake and direct numerical simulations (DNS) are in excellent agreement with this solution, once the flow has had large enough downstream distance to evolve. Beyond this the ratio of turbulence intensity to centerline velocity deficit is constant, until the flow unlocks itself from this behavior when the local Reynolds number goes below about 500 and the viscous terms become important. When this happens the turbulence intensity ratio falls slowly until the x1/2 region is reached. No experimental data are available far enough downstream to provide unambiguous evidence for the x1/2 solution. The prediction that the flow should evolve into such a state, however, is confirmed by recent DNS results which reach the x1/2 solution at about 200 000 momentum thicknesses downstream. After this the turbulence intensity ratio is again constant, until box-size affects the calculation and the energy decays exponentially.
Turbulence at Finite Reynolds Number
NASA Astrophysics Data System (ADS)
George, William K.
2003-11-01
Some of the unique features and challenges of turbulence at modest Reynolds number will be discussed and illustrated by example. Almost all laboratory and DNS turbulent flows fall into this class of flows, as do all turbulent wall-bounded flows within at least a few hundred viscous lengths from the surface. Particularly important is absence of an inertial subrange in the energy spectrum, which precludes the applicability of most turbulence theories to-date. Also interesting is the apparent tendency of at least free (no boundary) turbulent flows (e.g., decaying homogeneous turbulence, jets, wakes, etc.) to preserve asymptotically remnants of their initial (or upstream) conditions. These dependencies create great difficulties for the turbulence modellers and theoreticians alike. Equilibrium similarity and near-asymptotics provide useful theoretical tools for analyzing these flows, but troublesome questions remain. These anomalies challenge both the new ideas and the classical high Reynolds number ones as well. Will they vanish as our experiments, computations and theories improve, or are they harbingers of a paradigm shift? Regardless, we must acknowledge the problems and learn to ask the right questions to find out.
Inertial effects at moderate Reynolds number in thin-film rimming flows driven by surface shear
NASA Astrophysics Data System (ADS)
Kay, E. D.; Hibberd, S.; Power, H.
2013-10-01
In this paper, we study two-dimensional thin-film flow inside a stationary circular cylinder driven by an imposed surface shear stress. Modelling is motivated by a need to understand the cooling and film dynamics provided by oil films in an aero-engine bearing chamber characterised by conditions of very high surface shear and additional film mass flux from oil droplets entering the film through the surface. In typical high-speed operation, film inertial effects can provide a significant leading-order mechanism neglected in existing lubrication theory models. Inertia at leading-order is included within a depth-averaged formulation where wall friction is evaluated similar to hydraulic models. This allows key nonlinear inertial effects to be included while retaining the ability to analyse the problem in a mathematically tractable formulation and compare with other approaches. In constructing this model, a set of simplified mass and momentum equations are integrated through the depth of the film yielding a spatially one-dimensional depth-averaged formulation of the problem. An a priori assumed form of velocity profile is needed to complete the system. In a local Stokes flow analysis, a quadratic profile is the exact solution for the velocity field though it must be modified when inertial effects become important. Extension of the velocity profile to a cubic profile is selected enabling specification of a wall friction model to include the roughness of the cylinder wall. A modelling advantage of including the inertia term, relevant to the applications considered, is that a smooth progression in solution can be obtained between cases of low Reynolds number corresponding to lubrication theory, and high Reynolds number corresponding to uniform rimming-flow. Importantly, we also investigate the effect of inertia on some typical solutions from other studies and present a greater insight to existing and new film solutions which arise from including inertia effects.
Transition of effective hydraulic properties from low to high Reynolds number flow in porous media
NASA Astrophysics Data System (ADS)
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.
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.
Magnus effects at high angles of attack and critical Reynolds numbers
NASA Technical Reports Server (NTRS)
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.
Magnus effects at high angles of attack and critical Reynolds numbers
NASA Technical Reports Server (NTRS)
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.
NASA Technical Reports Server (NTRS)
Gentry, L., Jr.; Gentry, C. L., Jr.
1983-01-01
The aerodynamic characteristics of pressure loss and turbulence on four tube-bundle configurations representing heat-exchanger geometries with nominally the same heat capacity were measured as a function of Reynolds numbers from about 4000 to 400,000 based on tube hydraulic diameter. Two configurations had elliptical tubes, the other two had round tubes, and all four had plate fins. The elliptical-tube configurations had lower pressure loss and turbulence characteristics than the round-tube configurations over the entire Reynolds number range.
NASA Astrophysics Data System (ADS)
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.
High Reynolds Number Effects on Multi-Hole Probes and Hot Wire Anemometers
NASA Technical Reports Server (NTRS)
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.
Effect of spanwise shear on flow past a square cylinder at intermediate Reynolds numbers
NASA Astrophysics Data System (ADS)
Visakh, M. G.; Saha, Arun K.; Muralidhar, K.
2016-03-01
Three-dimensional numerical simulation of flow past a square cylinder with linear spanwise shear has been performed in the Reynolds numbers range of 165-250. Navier-Stokes equations are discretized using second order central differencing for both advection and diffusion terms, explicitly marching in time using the second order Adams-Bashforth scheme. The solution methodology used is the Simplified Marker and Cell algorithm. Spanwise shear is simulated by providing a linear variation in the inlet velocity profile along the spanwise direction and the resulting wake characteristics are compared with uniform inflow. Presence of mode-A, mode-B, and large scale vortical irregularities of two-sided symmetrical vortex dislocations is detected when the inflow is uniform. With shear, oblique and cellular vortex shedding are identified for the square cylinder. Shedding frequency of the vortices varies in a stepwise manner along the span, giving rise to local cells of constant frequency. Constant frequency cell sizes are found to be longer for a square cylinder in shear flow as compared to the circular one. Spanwise shear leads to vortex splitting and one-sided vortex dislocation, contrary to the naturally occurring two-sided vortex dislocation for uniform flow. Reynolds number dependence of spanwise shear flow is studied by comparing wake dynamics at three mid-span Reynolds numbers of 165, 200, and 250 with a shear parameter of 0.025. With an increase in the average Reynolds number, cell sizes decrease and dislocations become irregular in space and time. Simulations with three different shear parameters of 0.0125, 0.025, and 0.05 have been carried out for an average Reynolds number of 250. With an increase in the shear parameter, the frequency of occurrence of vortex dislocations increases in the wake. In addition, obliqueness of the primary vortices and the number of cells also show an increase.
Turbulent Skin Friction at High Mach Numbers and Reynolds Numbers
NASA Technical Reports Server (NTRS)
Matting, Fred W.; Chapman, Dean R.
1958-01-01
For a number of years now, experimenters have been making measurements of skin friction. Formerly, the main interest was at low Mach numbers; later, measurements were made at supersonic Mach numbers. However, almost all of these measurements were over a limited range of Reynolds numbers. On the other hand, these measurements fairly well determined the effects of Mach number and heat transfer on skin friction. The purpose of this paper is to give the results of skin-friction measurements in turbulent boundary layers at high Mach numbers and high Reynolds numbers where data have not previously existed. The equipment used was expressly designed to provide these conditions. As is well known, it is difficult to obtain high Mach numbers and high Reynolds numbers simultaneously with air in a wind tunnel. In order to avoid condensation, it is necessary to heat the air, with a resulting loss in density and Reynolds number. It is desirable, then, to use a gas that does not condense at high Mach numbers. This suggested helium, which was used as a working fluid in some of the tests. At high Mach numbers in a given wind tunnel, higher Reynolds numbers can be obtained with helium than with air, principally because no heating of the helium is required. The different ratios of specific heats also contribute to the increase. In using helium as a working fluid, it is, of course, necessary to determine the equivalence of air and helium in the turbulent boundary layer.
Reynolds number effects on the fluctuating velocity distribution in wall-bounded shear layers
NASA Astrophysics Data System (ADS)
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.
Reynolds number effects on flow over twisted cylinder with drag reduction and vortex suppression
NASA Astrophysics Data System (ADS)
Jung, Jae Hwan; Yoon, Hyun Sik
2015-11-01
We investigated the Reynolds number effects on the flow over a twisted cylinder in the range of 3×103 <= Re <=1×104. To analyze the effect of the twisted cylinder, a large eddy simulation (LES) with a dynamic subgrid model was employed. A simulation of the smooth cylinder was also carried out to compare the results with those of the twisted cylinder. As Re increased, the mean drag and lift coefficient of the twisted cylinder increased with the same tendency as those of the smooth cylinder. However, the increases in the mean drag and lift coefficient of the twisted cylinder were much smaller than those of the smooth. Furthermore, elongated shear layer and suppressed vortex shedding from the twisted cylinder occurred compared to those of the smooth cylinder, resulting in a drag reduction and suppression of the vortex-induced vibration (VIV). In particular, the twisted cylinder achieved a significant reduction of over 96% in VIV compared with that of the smooth cylinder, regardless of increasing Re. As a result, we concluded that the twisted cylinder effectively controlled the flow structures with reductions in the drag and VIV compared with the smooth cylinder, irrespective of increasing Re. This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIP) through GCRC-SOP (No. 2011-0030013) and (NRF-2015R1D1A3A01020867).
Aerodynamic effects of leading-edge tape on aerofoils at low Reynolds numbers
NASA Astrophysics Data System (ADS)
Giguère, Philippe; Selig, Michael S.
1999-07-01
A systematic wind tunnel study was conducted to gain an understanding of the aerodynamic effects of leading-edge tape, which is typically used on small wind turbines as a protection from blade erosion. The wind tunnel tests included lift and drag measurements over the Reynolds number range from 150,000 to 500,000. In addition, flow visualization experiments were carried out. Various tape configurations were tested on five aerofoils, namely the BW-3, FX 63-137, S822, SG6042 and SG6051. Although the magnitude of the aerodynamic effects of the tape was aerofoil-dependent, it was found that extending the tape beyond 5% chord and staggering multiple tape layers were most beneficial in minimizing the loss in aerofoil performance. The practical significance of the results on wind turbine performance is discussed. In particular, the data for the SG6042 aerofoil were used to quantify the effects of the tape on the power coefficient of small variable-speed wind turbines. Overall, the different tape configurations tested reduced the power coefficient by no more than 2·1%. From the trends shown, however, larger reductions in power coefficient should be expected for larger wind turbines than those considered, particularly if two layers of tape are used. In light of this study, guidelines for optimum application are suggested.
Reynolds number effects on Rayleigh-Taylor Instability with Implications for Type Ia Supernovae
Cabot, W H; Cook, A W
2006-03-22
Spontaneous mixing of materials at unstably stratified interfaces occurs in a wide variety of atmospheric, oceanic, geophysical and astrophysical flows. The Rayleigh-Taylor instability, in particular, plays key roles in the death of stars, planet formation and the quest for controlled thermonuclear fusion. Despite its ubiquity, fundamental questions regarding Rayleigh-Taylor instability persist. Among such questions are: Does the flow forget its initial conditions? Is the flow self-similar? What is the value of the scaling constant? How does mixing influence the growth rate? Here we show results from a 3072{sup 3} grid-point Direct Numerical Simulation in an attempt to answer these and other questions. The data indicate that the scaling constant cannot be found by fitting a curve to the width of the mixing region (as is common practice) but can only be accurately obtained by recourse to the similarity equation for the growth rate. The data further establish that the ratio of kinetic energy to released potential energy is not constant, as has heretofore been assumed. The simulated flow reaches a Reynolds number of 32,000, far exceeding that of all previous simulations. The latter stages of the simulation reveal a weak Reynolds number dependence, which may have profound consequences for modeling Type Ia supernovae as well as other high Reynolds number flows.
Roughness Effects on Compressor Outlet Guide Vanes at High Reynolds Number and High Turning Angle.
1982-11-01
controlled environment was necessary to reduce the effects of ambient temperture fluctuations on the resistance of the bridoe circuit of the anemormeter...RD-Ai24 688 ROUGHNESS EFFECTS ON COMPRESSOR OUTLET GUIDE YANES AT I HIGH REYNOLDS NUMBE..(U) AR FORCE INST OF TECH IRIGHT-PRTTERSONRRFB OH SCHOOL OF...RESOLUTION TEST CHART ~.~~-NAONAL BUREAU OF STAMDOARDS-1963-A AF....ED , . - . . . .Genovese ili t~t.V ROUGHESS EFFECTS ON COMP RROTT .AND HIGH
Modelling low-Reynolds-number effects in the turbulent air flow over water waves
NASA Astrophysics Data System (ADS)
Meirink, Jan F.; Makin, Vladimir K.
2000-07-01
In studies of the turbulent air flow over water waves it is usually assumed that the effect of viscosity near the water surface is negligible, i.e. the Reynolds number, Re = u[low asterisk][lambda]/v, is considered to be high. However, for short waves or low wind speeds this assumption is not valid. Therefore, a second-order turbulence closure that takes into account viscous effects is used to simulate the air flow. The model shows reasonable agreement with laboratory measurements of wave-induced velocity profiles. Next, the dependence of the dimensionless energy flux from wind to waves, or growth rate, on Re is investigated. The growth rate of waves that are slow compared to the wind is found to increase strongly when Re decreases below 104, with a maximum around Re = 800. The numerical model predictions are in good agreement with analytical theories and laboratory observations. Results of the study are useful in field conditions for the short waves in the spectrum, which are particularly important for remote sensing applications.
Effect of low Reynolds number flow on the quorum sensing behavior of sessile bacteria
NASA Astrophysics Data System (ADS)
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.
NASA Astrophysics Data System (ADS)
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,
NASA Technical Reports Server (NTRS)
Broeren, Andy; Woodard, Brian; Diebold, Jeff; Moens, Frederic
2017-01-01
This paper presents the results of an experimental and computational study of low-Reynolds number swept wing aerodynamics. This work has been conducted in preparation for icing effects on a swept wing. A complete abstract will be written for the final paper.
Effect of radius of gyration on a wing rotating at low Reynolds number: A computational study
NASA Astrophysics Data System (ADS)
Tudball Smith, Daniel; Rockwell, Donald; Sheridan, John; Thompson, Mark
2017-06-01
This computational study analyzes the effect of variation of the radius of gyration (rg), expressed as the Rossby number Ro=rg/C , with C the chord, on the aerodynamics of a rotating wing at a Reynolds number of 1400. The wing is represented as an aspect-ratio-unity rectangular flat plate aligned at 45 ∘ . This plate is accelerated near impulsively to a constant rotational velocity and the flow is allowed to develop. Flow structures are analyzed and force coefficients evaluated. Trends in velocity field degradation with increasing Ro are consistent with previous experimental studies. At low Ro the flow structure generated initially is mostly retained with a strong laminar leading-edge vortex (LEV) and tip vortex (TV). As both Ro and travel distance increase, the flow structure degrades such that at high Ro it begins to resemble that of a translating wing. Additionally, the present study has shown the following. (i) At low Ro the LEV and TV structure is laminar and steady; as Ro increases this structure breaks down, and the location at which it breaks down shifts closer to the wing root. (ii) For moderate Ro of 1.4 and higher, the LEV is no longer steady but enters a shedding regime fed by the leading-edge shear layer. (iii) At the lowest Ro of 0.7 the lift force rises during start-up and then stabilizes, consistent with the flow structure being retained, while for higher Ro a force peak occurs after the initial acceleration is complete, followed by a reduction in lift which appears to correspond to shedding of excess leading-edge vorticity generated during start-up. (iv) All rotating wings produced greater lift than a translating wing, this increase varied from ˜65 % at the lowest Ro=0.7 down to ˜5 % for the highest Ro examined of 9.1.
Effect of Reynolds number variation on aerodynamics of a hydrogen-fueled transport concept at Mach 6
NASA Technical Reports Server (NTRS)
Penland, Jim A.; Marcum, Don C., Jr.
1987-01-01
Two separate tests have been made on the same blended wing-body hydrogen-fueled transport model at a Mach number of about 6 and a range of Reynolds number (based on theoretical body length) of 1.577 to 55.36 X 10 to the 6th power. The results of these tests, made in a conventional hypersonic blowdown tunnel and a hypersonic shock tunnel, are presented through a range of angle of attack from -1 to 8 deg, with an extended study at a constant angle of attack of 3 deg. The model boundary layer flow appeared to be predominately turbulent except for the low Reynolds number shock tunnel tests. Model wall temperatures varied considerably; the blowdown tunnel varied from about 255 F to 340 F, whereas the shock tunnel had a constant 70 F model wall temperature. The experimental normal-force coefficients were essentially independent of Reynolds number. A current theoretical computer program was used to study the effect of Reynolds number. Theoretical predictions of normal-force coefficients were good, particularly at anticipated cruise angles of attack, that is 2 to 5 deg. Axial-force coefficients were generally underestimated for the turbulent skin friction conditions, and pitching-moment coefficients could not be predicted reliably.
NASA Astrophysics Data System (ADS)
Ma, Rong; Ma, Hongwei; Zhang, Zhenyang; Zhang, Jinghui
2016-10-01
Pipes are widely used to transport gas, oil and water in industries. Drag reduction in pipes is an increasingly concerned problem to save energy. Some researches have indicated that the non-smooth surface with special structures can reduce flow loss. In this paper, an experimental investigation has been performed on the effects of a kind of surface groove on the drag in both rectangular and circular duct at different Reynolds numbers. In the experiment of the rectangular duct, total pressure at both inlet and outlet were measured. Static pressure on the wall was measured on the surface with smooth and grooved film respectively. In the circular duct, a boundary layer pressure probe was used to measure the total pressure distribution at both inlet and outlet. Four taps at inlet and outlet were used to measure static pressure. The loss coefficient is used to evaluate the effects of the surface groove on drag reduction. The experiment was conducted with the Reynolds number range from 1.28×104 to 2.57×104. The result shows a maximum drag loss reduction of approximately 2.4% in rectangular duct at Reynolds number of 2.4×104. A 10% reduction of pipe pressure loss by grooved surface is measured in circular duct at a Reynolds number of 3.0×105.
NASA Technical Reports Server (NTRS)
Polhamus, Edward C.
1996-01-01
This paper presents a survey of the effects of Reynolds number on the low- speed lift characteristics of wings encountering separated flows at their leading and side edges, with emphasis on the region near the stall. The influence of leading-edge profile and Reynolds number on the stall characteristics of two- dimensional airfoils are reviewed first to provide a basis for evaluating three- dimensional effects associated with various wing planforms. This is followed by examples of the effects of Reynolds number and geometry on the lift characteristics near the stall for a series of three-dimensional wings typical of those suitable for high-speed aircraft and missiles. Included are examples of the effects of wing geometry on the onset and spanwise progression of turbulent reseparation near the leading edge and illustrations of the degree to which simplified theoretical approaches can be useful in defining the influence of the various geometric parameters. Also illustrated is the manner in which the Reynolds number and wing geometry parameters influence whether the turbulent reseparation near the leading edge results in a sudden loss of lift, as in the two-dimensional case, or the formation of a leading-edge vortex with Rs increase in lift followed by a gentle stall as in the highly swept wing case. Particular emphasis is placed on the strong influence of 'induced camber' on the development of turbulent reseparation. R is believed that the examples selected for this report may be useful in evaluating viscous flow solutions by the new computational methods based on the Navier-Stokes equations as well as defining fruitful research areas for the high-Reynolds-number wind tunnels.
Low Reynolds number airfoil survey, volume 1
NASA Technical Reports Server (NTRS)
Carmichael, B. H.
1981-01-01
The differences in flow behavior two dimensional airfoils in the critical chordlength Reynolds number compared with lower and higher Reynolds number are discussed. The large laminar separation bubble is discussed in view of its important influence on critical Reynolds number airfoil behavior. The shortcomings of application of theoretical boundary layer computations which are successful at higher Reynolds numbers to the critical regime are discussed. The large variation in experimental aerodynamic characteristic measurement due to small changes in ambient turbulence, vibration, and sound level is illustrated. The difficulties in obtaining accurate detailed measurements in free flight and dramatic performance improvements at critical Reynolds number, achieved with various types of boundary layer tripping devices are discussed.
Reynolds number effects on leading edge vortex development on a waving wing
NASA Astrophysics Data System (ADS)
Jones, A. R.; Babinsky, H.
2011-07-01
The waving wing experiment is a fully three-dimensional simplification of the flapping wing motion observed in nature. The spanwise velocity gradient and wing starting and stopping acceleration that exist on an insect-like flapping wing are generated by rotational motion of a finite span wing. The flow development around a waving wing at Reynolds number between 10,000 and 60,000 has been studied using flow visualization and high-speed PIV to capture the unsteady velocity field. Lift and drag forces have been measured over a range of angles of attack, and the lift curve shape was similar in all cases. A transient high-lift peak approximately 1.5 times the quasi-steady value occurred in the first chord length of travel, caused by the formation of a strong attached leading edge vortex. This vortex appears to develop and shed more quickly at lower Reynolds numbers. The circulation of the leading edge vortex has been measured and agrees well with force data.
Large Eddy Simulation of an SD7003 Airfoil: Effects of Reynolds number and Subgrid-scale modeling
NASA Astrophysics Data System (ADS)
Sarlak, Hamid
2017-05-01
This paper presents results of a series of numerical simulations in order to study aerodynamic characteristics of the low Reynolds number Selig-Donovan airfoil, SD7003. Large Eddy Simulation (LES) technique is used for all computations at chord-based Reynolds numbers 10,000, 24,000 and 60,000 and simulations have been performed to primarily investigate the role of sub-grid scale (SGS) modeling on the dynamics of flow generated over the airfoil, which has not been dealt with in great detail in the past. It is seen that simulations are increasingly getting influenced by SGS modeling with increasing the Reynolds number, and the effect is visible even at a relatively low chord-Reynolds number of 60,000. Among the tested models, the dynamic Smagorinsky gives the poorest predictions of the flow, with overprediction of lift and a larger separation on airfoils suction side. Among various models, the implicit LES offers closest pressure distribution predictions compared with literature.
The effect of tip speed ratio on a vertical axis wind turbine at high Reynolds numbers
NASA Astrophysics Data System (ADS)
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.
NASA Technical Reports Server (NTRS)
Perkins, Edward W; Jorgensen, Leland H
1956-01-01
Effects of Reynolds number and angle of attack on the pressure distribution and normal-force characteristics of a body of revolution consisting of a fineness ratio 3 ogival nose tangent to a cylindrical afterbody 7 diameters long have been determined. The test Mach number was 1.98 and the angle-of-attack range from 0 degree to 20 degrees. The Reynolds numbers, based on body diameter, were 0.15 x 10(6) and 0.45 x 10(6). The experimental results are compared with theory.
Dissipative Effects on Inertial-Range Statistics at High Reynolds Numbers
NASA Astrophysics Data System (ADS)
Sinhuber, Michael; Bewley, Gregory; Bodenschatz, Eberhard
2015-11-01
Using the unique capabilities of the Variable Density Turbulence Tunnel at the Max Planck Institute for Dynamics and Self-Organization, we were able to measure extremely long time series of up to 1010 samples of the turbulent fluctuating velocity in a well-controlled environment at a wide range of high Reynolds numbers up to Rλ = 1600 . These classical grid measurements were conducted using both classical hot-wire probes as well as NSTAP probes developed at Princeton University. With these long datasets, we were able to uncover fine details of the structure functions and their scaling behavior. We find that deviations from ideal scaling is anchored to the small scales and that dissipation influences the inertial-range statistics even up to r / η = 1000 .
NASA Astrophysics Data System (ADS)
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.
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.
Suslov, D; Schulz, A; Wittig, S
2001-05-01
The development of effective cooling methods is of major importance for the design of new gas turbines blades. The conception of optimal cooling schemes requires a detailed knowledge of the heat transfer processes on the blade's surfaces. The thermal load of turbine blades is predominantly determined by convective heat transfer which is described by the local heat transfer coefficient. Heat transfer is closely related to the boundary layer development along the blade surface and hence depends on various flow conditions and geometrical parameters. Particularly Reynolds number, pressures gradient and turbulence level have great impact on the boundary layer development and the according heat transfer. Therefore, in the present study, the influence of Reynolds number, turbulence intensity, and periodic unsteady inflow on the local heat transfer of a typical low pressure turbine airfoil is experimentally examined in a plane cascade.
NASA Astrophysics Data System (ADS)
Rodrigues, Neil S.; Kulkarni, Varun; Sojka, Paul E.
2014-11-01
While like-on-like doublet impinging jet atomization has been extensively studied in the literature, there is poor agreement between experimentally observed spray characteristics and theoretical predictions (Ryan et al. 1995, Anderson et al. 2006). Recent works (Bremond and Villermaux 2006, Choo and Kang 2007) have introduced a non-uniform jet velocity profile, which lead to a deviation from the standard assumptions for the sheet velocity and the sheet thickness parameter. These works have assumed a parabolic profile to serve as another limit to the traditional uniform jet velocity profile assumption. Incorporating a non-uniform jet velocity profile results in the sheet velocity and the sheet thickness parameter depending on the sheet azimuthal angle. In this work, the 1/7th power-law turbulent velocity profile is assumed to provide a closer match to the flow behavior of jets at high Reynolds and Weber numbers, which correspond to the impact wave regime. Predictions for the maximum wavelength, sheet breakup length, ligament diameter, and drop diameter are compared with experimental observations. The results demonstrate better agreement between experimentally measured values and predictions, compared to previous models. U.S. Army Research Office under the Multi-University Research Initiative Grant Number W911NF-08-1-0171.
Low-Reynolds-number swimming at pycnoclines.
Doostmohammadi, Amin; Stocker, Roman; Ardekani, Arezoo M
2012-03-06
Microorganisms play pivotal functions in the trophic dynamics and biogeochemistry of aquatic ecosystems. Their concentrations and activities often peak at localized hotspots, an important example of which are pycnoclines, where water density increases sharply with depth due to gradients in temperature or salinity. At pycnoclines organisms are exposed to different environmental conditions compared to the bulk water column, including reduced turbulence, slow mass transfer, and high particle and predator concentrations. Here we show that, at an even more fundamental level, the density stratification itself can affect microbial ecology at pycnoclines, by quenching the flow signature, increasing the energetic expenditure, and stifling the nutrient uptake of motile organisms. We demonstrate this through numerical simulations of an archetypal low-Reynolds-number swimmer, the "squirmer." We identify the Richardson number--the ratio of buoyancy forces to viscous forces--as the fundamental parameter that quantifies the effects of stratification. These results demonstrate an unexpected effect of buoyancy on low-Reynolds-number swimming, potentially affecting a broad range of abundant organisms living at pycnoclines in oceans and lakes.
Low-Reynolds-number swimming at pycnoclines
Doostmohammadi, Amin; Stocker, Roman; Ardekani, Arezoo M.
2012-01-01
Microorganisms play pivotal functions in the trophic dynamics and biogeochemistry of aquatic ecosystems. Their concentrations and activities often peak at localized hotspots, an important example of which are pycnoclines, where water density increases sharply with depth due to gradients in temperature or salinity. At pycnoclines organisms are exposed to different environmental conditions compared to the bulk water column, including reduced turbulence, slow mass transfer, and high particle and predator concentrations. Here we show that, at an even more fundamental level, the density stratification itself can affect microbial ecology at pycnoclines, by quenching the flow signature, increasing the energetic expenditure, and stifling the nutrient uptake of motile organisms. We demonstrate this through numerical simulations of an archetypal low-Reynolds-number swimmer, the “squirmer.” We identify the Richardson number—the ratio of buoyancy forces to viscous forces—as the fundamental parameter that quantifies the effects of stratification. These results demonstrate an unexpected effect of buoyancy on low-Reynolds-number swimming, potentially affecting a broad range of abundant organisms living at pycnoclines in oceans and lakes. PMID:22355147
Low Reynolds-number turbulent boundary layers
NASA Astrophysics Data System (ADS)
Wang, Ting
1993-06-01
An experimental investigation was performed to determine the Reynolds analogy factor and turbulent Prandtl number in the transitional and low-Reynolds-number turbulent boundary layer. A miniature three-wire probe was specially designed to measure the mean velocity and temperature profiles and the Reynolds stresses and heat fluxes. Tests were conducted over a heated flat wall with zero pressure gradient and three levels of streamwise acceleration: K = 0.07, 0.16, and 0.25 x 10(exp -6). Mean temperature profiles lagged in development compared to the mean velocity profiles and the values of the Reynolds analogy factor, 2St/Cf, in the late-transition and early-turbulent regions were lower than the values known to apply to the high-Reynolds-number turbulent flow. The profiles of Reynolds cross-stream heat flux showed negative values in the near wall region. The region of negative vt narrowed as the flow proceeded downstream. These negative values of vt in a flow with a negative mean temperature gradient result in negative eddy thermal diffusivity and negative Pr(sub t). It is speculated that the negative values might be caused by the size of the sensor and the three-dimensional behavior of transition. A conditional sampling technique was utilized to separate the flow into turbulent and non-turbulent portions. Detailed flow and thermal structures were investigated.
NASA Technical Reports Server (NTRS)
Foley, J. E.
1971-01-01
An analysis was made to determine the effects of Mach number and Reynolds number on the local and total crossflow drag characteristics of ogive-cylinders and ogive-cylinder-frustum-cylinders at angles of the MSFC 14 in TWT and the LTV 4 ft HSWT, and pressure data obtained in the TWT, at Mach numbers 0.14, 0.8, 1.2, and 2.0, and a wide range of Reynolds numbers. Results indicate that the streamwise Reynolds number, VD/nusin alpha, is an important correlation parameter in the subcritical Reynolds number range at imcompressible speeds and that the crossflow Mach number correlates compressibility effects.
NASA Technical Reports Server (NTRS)
Bertin, J. J.; Senalug, G.; Mcbride, M.; Willman, R. B.
1975-01-01
The effect of windward surface temperature on the heat transfer to the leeward surface of the shuttle orbiter was investigated. Heat-transfer distributions, surface-pressure distributions, and schlieren photographs were obtained for an 0.01-scale model of the 37-0 shuttle orbiter at angles-of-attack of 30 deg and of 40 deg. Similar data were obtained for a fuselage-only configuration at angles-of-attack of 30 deg and of 90 deg. Data were obtained for various Mach numbers, Reynolds numbers, and surface temperatures.
Experimental study of low Reynolds number nozzles
NASA Technical Reports Server (NTRS)
Grisnik, Stanley P.; Smith, Tamara A.; Saltz, Larry E.
1987-01-01
High-performance electrothermal thrusters operate in a low nozzle-throat Reynolds number regime. Under these conditions, the flow boundary layer occupies a large volume inside the nozzle, contributing to large viscous losses. Four nozzles (conical, bell, trumpet, and modified trumpet) and a sharp-edged orifice were evaluated over a Reynolds number range of 500 to 9000 with unheated nitrogen and hydrogen. The nozzles showed significant decreases in specific impulse efficiency with decreasing Reynolds number. At Reynolds numbers less than 1000, all four nozzles were probably filled with a large boundary layer. The discharge coefficient decreased with Reynolds number in the same manner as the specific impulse efficiency. The bell and modified trumpet nozzles had discharge coefficients 4 to 8 percent higher than those of the cone or trumpet nozzles. The Two-Dimensional Kinetics (TDK) nozzle analysis computer program was used to predict nozzle performance. The results were then compared to the experimental results in order to determine the accuracy of the program within this flow regime.
NASA Astrophysics Data System (ADS)
Tsuji, Yoshiyuki
2009-04-01
Why do we need high Reynolds number experiments? This is a question I sometimes ask myself. You may have your own answer to this question, but those people who are doing numerical simulation, theorists and experimentalists should each have their own answer. In this special issue, the leading experts present their new ideas or original experiments in response to this question. Personally, I think that high Reynolds number experiments are necessary to seek novel physics in turbulence. For instance, we do not have much information about the Lagrangian quantities. You can understand this point by reading the article 'Why we need experiments at high Reynolds numbers' by Warhaft. High Reynolds number experiments are also indispensable to reveal the universality of turbulence. One famous example is Kolmogorov's similarity hypothesis; another is the logarithmic velocity profile derived by von Kármán. They become clearly satisfied as Reynolds number increases. But there have been many arguments over these problems even in this century, thus we still have to make an effort to reveal the nature of turbulence. Kolmogorov's idea is based on small scale physics; in this sense, Mouri and Hori's paper 'Vortex tubes in turbulence velocity fields at high Reynolds numbers' is a contribution to understanding how eddy size is defined and scaled. In contrast to the universality in the small scale limit, the large scale anisotropy effect is a key factor in considering the local isotropic condition even in grid turbulence. This point is discussed by Kurian and Fransson in 'Grid generated turbulence revisited'. The mean velocity profile over a flat plate in a zero-pressure gradient boundary layer is discussed with the help of a composite profile in 'Criteria for assessing experiments in zero pressure gradient boundary layers' by Chauhan et al. Related important physical quantities are computed, and how they are scaled against Reynolds number is discussed, analyzing the vast experimental
NASA Astrophysics Data System (ADS)
Simoni, Daniele; Lengani, Davide; Ubaldi, Marina; Zunino, Pietro; Dellacasagrande, Matteo
2017-06-01
The effects of free-stream turbulence intensity (FSTI) on the transition process of a pressure-induced laminar separation bubble have been studied for different Reynolds numbers (Re) by means of time-resolved (TR) PIV. Measurements have been performed along a flat plate installed within a double-contoured test section, designed to produce an adverse pressure gradient typical of ultra-high-lift turbine blade profiles. A test matrix spanning 3 FSTI levels and 3 Reynolds numbers has been considered allowing estimation of cross effects of these parameters on the instability mechanisms driving the separated flow transition process. Boundary layer integral parameters, spatial growth rate and saturation level of velocity fluctuations are discussed for the different cases in order to characterize the base flow response as well as the time-mean properties of the Kelvin-Helmholtz instability. The inspection of the instantaneous velocity vector maps highlights the dynamics of the large-scale structures shed near the bubble maximum displacement, as well as the low-frequency motion of the fore part of the separated shear layer. Proper Orthogonal Decomposition (POD) has been implemented to reduce the large amount of data for each condition allowing a rapid evaluation of the group velocity, spatial wavelength and dominant frequency of the vortex shedding process. The dimensionless shedding wave number parameter makes evident that the modification of the shear layer thickness at separation due to Reynolds number variation mainly drives the length scale of the rollup vortices, while higher FSTI levels force the onset of the shedding phenomenon to occur upstream due to the higher velocity fluctuations penetrating into the separating boundary layer.
NASA Astrophysics Data System (ADS)
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.
High Reynolds number oscillating contact lines
NASA Astrophysics Data System (ADS)
Liu, Ziyuan; Schultz, William W.; Perlin, Marc
1999-11-01
Stainless steel, instead of Ting and Perlin's (1995) glass, is used in vertically oscillating plate experiments for a large range of Reynolds numbers. We used the non-wetting stainless steel to minimize the static meniscus that we ignore in our analysis. Except for the different static contact angle serving as an initial condition, the dynamic features in both cases are similar. In low Reynolds number oscillation, a pinned-edge condition can appropriately describe the contact line motion. In high Reynolds number oscillation, contact-line behavior becomes nonlinear and very complicated. The periodic, non-sinusoidal motion exhibits three types of motion: stick (associated with contact angle hysteresis), partial stick, and total slip. Increasing the Reynolds number, reduces the hysteresis phenomenon that still cannot be ignored. An edge condition allowing both the static range and dynamic interface behavior uses a slip coefficient mode that varies with time, stroke amplitude and frequency by introducing additional harmonic modes. Using this edge condition, we calculate the dynamic contact angle and the contact-line position for both stick and slip motion and compare them to our experimental data. Results show that the inviscid, linearized boundary-value problem combined with our slip coefficient model provides an improved prediction of the contact-line behavior.
The aerodynamics of small Reynolds numbers
NASA Technical Reports Server (NTRS)
Schmitz, F. W.
1980-01-01
Aerodynamic characteristics of wing model gliders and bird wings in particular are discussed. Wind tunnel measurements and aerodynamics of small Reynolds numbers are enumerated. Airfoil behavior in the critical transition from laminar to turbulent boundary layer, which is more important to bird wing models than to large airplanes, was observed. Experimental results are provided, and an artificial bird wing is described.
Transition Reynolds Numbers of Separated Flows at Supersonic Speeds
NASA Technical Reports Server (NTRS)
Larson, Howard K.; Keating, Stephen J., Jr.
1960-01-01
Experimental research has been conducted on the effects of wall cooling, Mach number, and unit Reynolds number on the transition Reynolds number of cylindrical separated boundary layers on an ogive-cylinder model. Results were obtained from pressure and temperature measurements and shadowgraph observations. The maximum scope of measurements encompassed Mach numbers between 2.06 and 4.24, Reynolds numbers (based on length of separation) between 60,000 and 400,000, and ratios of wall temperature to adiabatic wall temperature between 0.35 and 1.0. Within the range of tile present tests, the transition Reynolds number was observed to decrease with increasing wall cooling, increase with increasing Mach number, and increase with increasing unit Reynolds number. The wall cooling effect was found to be four times as great when the attached boundary layer upstream of separation was cooled in conjunction with cooling of the separated boundary layer as when only the separated boundary layer was cooled. Wall cooling of both the attached and separated flow regions also caused, in some cases, reattachment in the otherwise separated region. Cavity resonance present in the separated region for some model configurations was accompanied by a large decrease in transition Reynolds number at the lower test Mach numbers.
NASA Technical Reports Server (NTRS)
Rao, Dhanvada M.; Hoffler, Keith D.
1988-01-01
A low-speed wind tunnel test was performed to investigate Reynolds number effects on the aerodynamic characteristics of a supersonic cruise wing concept model with a 60-deg swept wing incorporating leading-edge and trailing-edge flap deflections. The Reynolds number ranged from 0.3 to 1.6 x 10 to the 6th, and corresponding Mach numbers from .05 to 0.3. The objective was to define a threshold Reynolds number above which the flap aerodynamics basically remained unchanged, and also to generate a data base useful for validating theoretical predictions for the Reynolds number effects on flap performance. This report documents the test procedures used and the basic data acquired in the investigation.
NASA Technical Reports Server (NTRS)
Reubush, D. E.; Putnam, L. E.
1976-01-01
An investigation was conducted to determine whether large Reynolds number effects occur on isolated boattails, including an analytical study and tests in a 1/3-meter transonic cryogenic tunnel. This investigation was conducted at an angle of attack of 0 deg at Mach numbers from 0.6 to 0.9 for Reynolds numbers up to 130 million. Results indicate that as the Reynolds number was increased, the static pressure coefficients in the expansion region of the boattail became more negative whereas those in the recompression region became more positive. These two trends were compensating and, as a result, there was only a small effect (if any) of Reynolds numbers on boattail pressure drag.
NASA Astrophysics Data System (ADS)
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.
A Reynolds Number Study of Wing Leading-Edge Effects on a Supersonic Transport Model at Mach 0.3
NASA Technical Reports Server (NTRS)
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).
Ryu, Sangjin; Matsudaira, Paul
2010-01-01
Abstract Contraction of Vorticella convallaria, a sessile ciliated protozoan, is completed within a few milliseconds and results in a retraction of its cell body toward the substratum by coiling its stalk. Previous studies have modeled the cell body as a sphere and assumed a drag force that satisfies Stokes' law. However, the contraction-induced flow of the medium is transient and bounded by the substrate, and the maximum Reynolds number is larger than unity. Thus, calculations of contractile force from the drag force are incomplete. In this study, we analyzed fluid flow during contraction by the particle tracking velocimetry and computational fluid dynamics simulations to estimate the contractile force. Particle paths show that the induced flow is limited by the substrate. Simulation-based force estimates suggest that the combined effect of the flow unsteadiness, the finite Reynolds number, and the substrate comprises 35% of the total force. The work done in the early stage of contraction and the maximum power output are similar regardless of the medium viscosity. These results suggest that, during the initial development of force, V. convallaria uses a common mechanism for performing mechanical work irrespective of viscous loading conditions. PMID:20513401
Ryu, Sangjin; Matsudaira, Paul
2010-06-02
Contraction of Vorticella convallaria, a sessile ciliated protozoan, is completed within a few milliseconds and results in a retraction of its cell body toward the substratum by coiling its stalk. Previous studies have modeled the cell body as a sphere and assumed a drag force that satisfies Stokes' law. However, the contraction-induced flow of the medium is transient and bounded by the substrate, and the maximum Reynolds number is larger than unity. Thus, calculations of contractile force from the drag force are incomplete. In this study, we analyzed fluid flow during contraction by the particle tracking velocimetry and computational fluid dynamics simulations to estimate the contractile force. Particle paths show that the induced flow is limited by the substrate. Simulation-based force estimates suggest that the combined effect of the flow unsteadiness, the finite Reynolds number, and the substrate comprises 35% of the total force. The work done in the early stage of contraction and the maximum power output are similar regardless of the medium viscosity. These results suggest that, during the initial development of force, V. convallaria uses a common mechanism for performing mechanical work irrespective of viscous loading conditions.
NASA Technical Reports Server (NTRS)
Blair, M. F.; Dring, R. P.; Joslyn, H. D.
1988-01-01
Part I of this paper presents airfoil heat transfer data obtained in a rotating turbine model at its design rotor incidence. This portion of the paper presents heat transfer data obtained in the same model for various combinations of Reynolds number and inlet turbulence and for a very wide range of rotor incidence. On the suction surfaces of the first stage airfoils the locations and lengths of transition were influenced by both the inlet turbulence level and the Reynolds number. In addition it was demonstrated that on the first stage pressure surfaces combinations of high Reynolds number and high turbulence can produce heat transfer rates well in excess of two-dimensional turbulent flow. Rotor heat transfer distributions indicate that for relatively small deviations from the design incidence, local changes to the heat transfer distributions were produced on both pressure and suction sides near the stagnation region. For extremely large negative incidence the flow was completely separated from the rotor pressure surface producing very high local heat transfer.
High Reynolds Number Thermal Stability Experiments
NASA Technical Reports Server (NTRS)
Emens, Jessica M.; Brown, Sarah P.; Frederick Robert A., Jr.; Wood, A. John
2004-01-01
This work represents preliminary thermal stability results for liquid hydrocarbon fuels. High Reynolds Number Thermal Stability experiments with Jet A and RP-1 resulted in a quantitative measurement of the thermal stability. Each fuel flowed through a heated capillary tube that held the outlet temperature at 290 C. An optical pyrometer measured the surface temperature of the tube at 12 locations as a function of time. The High Reynolds Number Thermal Stability number was then determined using standards published by the American Society for Testing and Materials. The results for Jet A showed lower thermal stability than similar tests conducted at another facility. The RP-1 results are the first reported using this technique. Because the temperature rise on the capillary tube during testing for the RP-1 fuels was not significant, a new standard for the testing conditions should be developed for these types of fuels.
The Influence of Realistic Reynolds Numbers on Slat Noise Simulations
NASA Technical Reports Server (NTRS)
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.
Analysis of high Reynolds numbers effects on a wind turbine airfoil using 2D wind tunnel test data
NASA Astrophysics Data System (ADS)
Pires, O.; Munduate, X.; Ceyhan, O.; Jacobs, M.; Snel, H.
2016-09-01
The aerodynamic behaviour of a wind turbine airfoil has been measured in a dedicated 2D wind tunnel test at the DNW High Pressure Wind Tunnel in Gottingen (HDG), Germany. The tests have been performed on the DU00W212 airfoil at different Reynolds numbers: 3, 6, 9, 12 and 15 million, and at low Mach numbers (below 0.1). Both clean and tripped conditions of the airfoil have been measured. An analysis of the impact of a wide Reynolds number variation over the aerodynamic characteristics of this airfoil has been performed.
Reynolds Number Effects in the Flow of a Vočadlo Electrorheological Fluid in a Curved Gap
NASA Astrophysics Data System (ADS)
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.
MHD Turbulence at Moderate Magnetic Reynolds Number
NASA Technical Reports Server (NTRS)
Knaepen, B.; Kassinos, S.; Carati, D.
2003-01-01
In the present article we will consider the decay of MHD turbulence under the influence of a strong external magnetic field at moderate magnetic Reynolds numbers. Typical values of R(sub m) that are considered here range from R(sub m) approx. 0.1 to R(sub m) approx. 20. As a comparison, the initial kinetic Reynolds number common to all our simulations is Re(sub L) = 199. This means that the range of Prandtl numbers explored is 5 x 10(exp -4) to 10(exp -1). Our motivation is mainly to exhibit how the transition from the QS approximation to FMHD occurs. At the lowest values of R(sub m) studied here, the QS approximation is shown to model the flow faithfully. However, for the higher values of R(sub m) considered, it is clearly inadequate but can be replaced by another approximation which will be referred to as the Quasi-Linear (QL) approximation. Another objective of the present study is to describe how variations in the magnetic Reynolds number (while maintaining all other parameters constant) affect the dynamics of the flow. This complements past studies where variations in either the strength of the external magnetic field or the kinetic Reynolds number were considered. This article is organized as follows. In section 2 we recall the definition of the quasi-static approximation. Section 3 is devoted to the description of the numerical experiments performed using the quasi-static approximation and full MHD. In section 4 we describe the quasi-linear approximation and test it numerically against full MHD. A concluding summary is given in section 5.
The effects of an algal biofilm on the turbulent boundary layer at high Reynolds number
NASA Astrophysics Data System (ADS)
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.
Low Reynolds number, long endurance aircraft design
Foch, R.J.; Ailinger, K.G. )
1992-02-01
Airplanes are typically designed to maximize range at the highest practical cruising speed. However, several missions require extended duration rather than range, and favor the slowest possible cruise speed. Such missions include surveillance, radio relay, and ship's electronic decoy. These missions are ideally suited for advanced technology unmanned aircraft, either remotely piloted or autonomous. Feasibility studies have been conducted and flight demonstrator prototypes of such unique aircraft have been under steady research and development at the Naval Research Laboratory since 1978. This paper discusses the design aspects and tradeoffs unique to small, slow speed long endurance unmanned aircraft operating at wing chord Reynolds numbers between 150,000 and 500,000. Additionally, many of these low Reynolds number-driven design features have applicability to high altitude, long endurance aircraft. 6 refs.
Nichols, J Tyler; Krueger, Paul S
2012-09-01
Recent results have demonstrated that pulsed-jet propulsion can achieve propulsive efficiency greater than that for steady jets when short, high frequency pulses are used, and the pulsed-jet advantage increases as Reynolds number decreases into the intermediate range (∼50). An important aspect of propulsive performance, however, is the vehicle configuration. The nozzle configuration influences the jet speed and, in the case of pulsed-jets, the formation of the vortex rings with each jet pulse, which have important effects on thrust. Likewise, the hull configuration influences the vehicle speed through its effect on drag. To investigate these effects, several flow inlet, nozzle, and hull tail configurations were tested on a submersible, self-propelled pulsed-jet vehicle ('Robosquid' for short) for jet pulse length-to-diameter ratios (L/D) in the range 0.5-6 and pulsing duty cycles (St(L)) of 0.2 and 0.5. For the configurations tested, the vehicle Reynolds number (Re(υ)) ranged from 25 to 110. In terms of propulsive efficiency, changing between forward and aft-facing inlets had little effect for the conditions considered, but changing from a smoothly tapered aft hull section to a blunt tail increased propulsive efficiency slightly due to reduced drag for the blunt tail at intermediate Re(υ). Sharp edged orifices also showed increased vehicle velocity and propulsive efficiency in comparison to smooth nozzles, which was associated with stronger vortex rings being produced by the flow contraction through the orifice. Larger diameter orifices showed additional gains in propulsive efficiency over smaller orifices if the rate of mass flow was matched with the smaller diameter cases, but using the same maximum jet velocity with the larger diameter decreased the propulsive efficiency relative to the smaller diameter cases.
NASA Technical Reports Server (NTRS)
Moss, J. E., Jr.; Braithwaite, W. M.
1977-01-01
A tip-treated J85-13 engine was tested at Reynolds number indices (RNI) of 0.3 and 0.7 with a clean inlet for 80- and 100-percent corrected engine speeds. This engine was equipped with a compressor case which allowed changes to the case wall over the rotor tips of six of its eight stages. For all tests, the principal effects were (1) with tip treatment, a stall pressure ratio loss of 6 percent at 100 percent corrected engine speeds for both the 0.3 and 0.7 RNI, and (2) with and without tip treatment, decreasing the RNI from 0.7 to 0.3 decreased the stall pressure ratio 3.8 percent for 100 percent corrected engine speed and 2.8 percent for 80-percent corrected engine speed.
Preston Probe Calibrations at High Reynolds Number
NASA Technical Reports Server (NTRS)
Smits, Alexander J.
1998-01-01
The overall goal of the research effort is to study the performance of two Preston probes designed by NASA Langley Research Center across an unprecedented range of Reynolds number (based on friction velocity and probe diameter), and perform an accurate calibration over the same Reynolds number range. Using the Superpipe facility in Princeton, two rounds of experiments were performed. In each round of experiments for each Reynolds number, the pressure gradient, static pressure from the Preston probes and the total pressure from the Preston probes were measured. In the first round, 3 Preston probes having outer diameters of 0.058 inches, 0.083 inches and 0.203 inches were tested over a large range of pipe Reynolds numbers. Two data reduction methods were employed: first, the static pressure measured on the Preston probe was used to calculate P (modified Preston probe configuration), and secondly, the static pressure measured at the reference pressure tap was used to calculate P (un-modified Preston probe configuration). For both methods, the static pressure was adjusted to correspond with the static pressure at the Preston probe tip using the pressure gradient. The measurements for Preston probes with diameters of 0.058 inches, and 0.083 inches respectively were performed in the test pipe before it was polished a second time. Therefore, the measurements at high pipe Reynolds numbers may have been affected by roughness. In the second round of experiments the 0.058 inches and 0.083 inches diameter, un-modified probes were tested after the pipe was polished and prepared to ensure that the surface was smooth. The average velocity was estimated by assuming that the connection between the centerline velocity and the average velocity was known, and by using a Pitot tube to measure the centerline velocity. A preliminary error estimate suggests that it is possible to introduce a 1% to 2% error in estimating the average velocity using this approach. The evidence on the errors
NASA Technical Reports Server (NTRS)
Ladson, Charles L.
1988-01-01
A comprehensive data base is given for the low speed aerodynamic characteristics of the NACA 0012 airfoil section. The Langley low-turbulence pressure tunnel is the facility used to obtain the data. Included in the report are the effects of Mach number and Reynolds number and transition fixing on the aerodynamic characteristics. Presented are also comparisons of some of the results with previously published data and with theoretical estimates. The Mach number varied from 0.05 to 0.36. The Reynolds number, based on model chord, varied from 3 x 10 to the 6th to 12 x 10 to the 6th power.
NASA Technical Reports Server (NTRS)
Broeren, Andy P.; Woodard, Brian S.; Diebold, Jeffrey M.; Moens, Frederic
2017-01-01
Aerodynamic assessment of icing effects on swept wings is an important component of a larger effort to improve three-dimensional icing simulation capabilities. An understanding of ice-shape geometric fidelity and Reynolds and Mach number effects on the iced-wing aerodynamics is needed to guide the development and validation of ice-accretion simulation tools. To this end, wind-tunnel testing and computational flow simulations were carried out for an 8.9 percent-scale semispan wing based upon the Common Research Model airplane configuration. The wind-tunnel testing was conducted at the Wichita State University 7 by 10 ft Beech wind tunnel from Reynolds numbers of 0.8×10(exp 6) to 2.4×10(exp 6) and corresponding Mach numbers of 0.09 to 0.27. This paper presents the results of initial studies investigating the model mounting configuration, clean-wing aerodynamics and effects of artificial ice roughness. Four different model mounting configurations were considered and a circular splitter plate combined with a streamlined shroud was selected as the baseline geometry for the remainder of the experiments and computational simulations. A detailed study of the clean-wing aerodynamics and stall characteristics was made. In all cases, the flow over the outboard sections of the wing separated as the wing stalled with the inboard sections near the root maintaining attached flow. Computational flow simulations were carried out with the ONERA elsA software that solves the compressible, threedimensional RANS equations. The computations were carried out in either fully turbulent mode or with natural transition. Better agreement between the experimental and computational results was obtained when considering computations with free transition compared to turbulent solutions. These results indicate that experimental evolution of the clean wing performance coefficients were due to the effect of three-dimensional transition location and that this must be taken into account for future
NASA Technical Reports Server (NTRS)
Broeren, Andy P.; Woodard, Brian S.; Diebold, Jeffrey M.; Moens, Frederic
2017-01-01
Aerodynamic assessment of icing effects on swept wings is an important component of a larger effort to improve three-dimensional icing simulation capabilities. An understanding of ice-shape geometric fidelity and Reynolds and Mach number effects on the iced-wing aerodynamics is needed to guide the development and validation of ice-accretion simulation tools. To this end, wind-tunnel testing and computational flow simulations were carried out for an 8.9%-scale semispan wing based upon the Common Research Model airplane configuration. The wind-tunnel testing was conducted at the Wichita State University 7 ft x 10 ft Beech wind tunnel from Reynolds numbers of 0.8×10(exp 6) to 2.4×10(exp 6) and corresponding Mach numbers of 0.09 to 0.27. This paper presents the results of initial studies investigating the model mounting configuration, clean-wing aerodynamics and effects of artificial ice roughness. Four different model mounting configurations were considered and a circular splitter plate combined with a streamlined shroud was selected as the baseline geometry for the remainder of the experiments and computational simulations. A detailed study of the clean-wing aerodynamics and stall characteristics was made. In all cases, the flow over the outboard sections of the wing separated as the wing stalled with the inboard sections near the root maintaining attached flow. Computational flow simulations were carried out with the ONERA elsA software that solves the compressible, three-dimensional RANS equations. The computations were carried out in either fully turbulent mode or with natural transition. Better agreement between the experimental and computational results was obtained when considering computations with free transition compared to turbulent solutions. These results indicate that experimental evolution of the clean wing performance coefficients were due to the effect of three-dimensional transition location and that this must be taken into account for future data
Waving Wing Aerodynamics at Low Reynolds Numbers
2010-07-01
canonical pitch - up , pitch -down wing maneuver, in 39th AIAA Fluid Dynamics Conference, AIAA 2009-3687, San Antonio, TX, 22-25 June 2009. [7] C. P. Ellington...unsteady lift generation on three-dimensional flapping wings in the MAV flight regime and, if a leading edge vortex develops at MAV-like Reynolds numbers... wing rotates in a propeller-like motion through a wing stroke angle up to 90 degrees. Unsteady lift and drag force data was acquired throughout the
Generating high Reynolds-number flows.
NASA Technical Reports Server (NTRS)
Russell, D. A.
1972-01-01
Present test facilities are seriously limited regarding investigations involving high Reynolds numbers due to financial considerations. Quasi-steady testing facilities offer a practical immediate solution to the problem of high-Re testing. A familiar example is the blowdown wind tunnel, but even more flexibility and economy may be provided by using shock-tube devices. The Ludwieg tube is the shock-tube device most often proposed as a means of generating high-Re flows. Two-stage nozzles may be used with a Ludwieg tube. Quasi-steady facilities will be useful only if the available test time exceeds that required to establish steady flow.
Hysteresis and wall-effects in low Reynolds number propulsion by driven elastic filaments
NASA Astrophysics Data System (ADS)
Clark, Sarah; Ranganathan, Prabhakar; Friend, James
2009-11-01
There is currently intense interest in developing micron-sized robots for uses such as minimally invasive surgery. Although progress has been made in miniaturizing the motor, the hydrodynamic behavior of associated propellers is far from being fully understood. An example is an elastic filament driven by a torque at one end where the shape assumed by the filament is strongly coupled to the hydrodynamics forces. Investigation of these dynamics has only recently commenced, for instance Manghi et al. [PRL 96, 068101 (2006)] uncovered an intriguing shape transition in an elastic filament spun in a bulk fluid. Since such transitions can be expected to have a crucial bearing on the operation of microbot swimmers we examine this behavior in detail with simulations. We also study the effect of planar no-slip walls on the propulsion characteristics. The slender filament is represented as a bead-spring chain and inter-bead hydrodynamic-interactions are described using the appropriate Greens functions. We study the origin of the shape transition and hysteresis in detail and show the relationship to sedimenting filaments. We show that the presence of a boundary either perpendicular or parallel to the axis of the applied torque has a significant effect on the overall motion. We also point out the possible detrimental consequences of these effects on operation of microbots in the vicinity of conduit walls.
Effect of superhydrophobic surfaces on the flow over a hydrofoil at low Reynolds number
NASA Astrophysics Data System (ADS)
Kim, Hyunseok; Kim, Nayoung; Park, Hyungmin
2014-11-01
In the present study, we experimentally investigate the effect of superhydrophobic surface on the flow over a hydrofoil at low Rec <104 , where c is the chord length of a hydrofoil. As a hydrofoil, we consider the cross-sections typically used for airfoils like NACA0012, NACA0024, and NACA4412, which stand for thin, thick and cambered hydrofoils, respectively. Spray-coating of hydrophobic nanoparticles are applied onto the hydrofoil surface and subsequent velocity fields are measured in a water tunnel using two-dimensional particle image velocimetry at different angles of attack, α =0° -20° . At small α's (for example, less than 10°), it is found that the surface slip tends to affect the flow separation slightly and also modify the size of recirculation region in the wake. Since a massive separation occurs at the leading edge at larger α's, however, the effect of superhydrophobic surface becomes diminished. In the talk, the dependence of the hydrodynamic role of surface slip on the hydrofoil shape and Rec will be presented. Supported by the NRF Programs (NRF-2012M2A8A4055647, NRF-2013R1A1A1008373) of Korean government.
NASA Astrophysics Data System (ADS)
Kapral, Raymond; Mikhailov, Alexander S.
2016-04-01
Most of the proteins in the cell, including not only molecular motors and machines, but also enzymes, are active. When ATP or other substrates are supplied, these macromolecules cyclically change their conformations. Therefore, they mechanically stir the cytoplasm and nucleoplasm, so that non-thermal fluctuating flows are produced. As we have recently shown (Mikhailov and Kapral, 2015), stochastic advection by such flows might lead to substantial diffusion enhancement of particles inside a living cell. Additionally, when gradients in the concentrations of active particles or in the ATP/substrate supply are present, chemotaxis-like drift should take place. Here, the motion of passive tracers with various sizes in a mixture of different kinds of active proteins is analyzed. Moreover, effects of hydrodynamic interactions on the motion of active proteins are explored. Theoretical results are compared with available experimental data for ATP-dependent diffusion of natural and microinjected particles in biological cells.
Analysis of low Reynolds number separation bubbles using semiempirical methods
NASA Technical Reports Server (NTRS)
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.
NASA Technical Reports Server (NTRS)
Bernot, P. T.
1974-01-01
Longitudinal and lateral-directional stability characteristics of the 139-B orbiter (model 32-0) were obtained in a continuous flow hypersonic tunnel at Mach 10.3. Tests were made at Reynolds numbers of 1.04 million and 2.17 million (based on body length) over an angle of attack range of 12 deg to 36 deg at sideslip angles of 0 deg and minus 5 deg. Data were obtained at three elevon/body flap settings.
NASA Technical Reports Server (NTRS)
Furlong, G. Chester; Fitzpatrick, James E.
1947-01-01
Wing was tested with full-span, partial-span, or split flaps deflected 60 Degrees and without flaps. Chordwise pressure-distribution measurements were made for all flap configurations.. Peak values of maximum lift coefficient were obtained at relatively low free-stream Mach numbers and, before critical Mach number was reached, were almost entirely dependent on Reynolds Number. Lift coefficient increased by increasing Mach number or deflecting flaps while critical pressure coefficient was reached at lower free-stream Mach numbers.
NASA Astrophysics Data System (ADS)
Yucel, S. Banu; Sahin, Mehmet; Unal, M. Fevzi
2015-12-01
The flow pattern around a NACA0012 airfoil undergoing harmonic plunging motion corresponding to the deflected wake phenomenon reported by Jones and Platzer (Exp Fluids 46:799-810, 2009) is investigated in detail using direct numerical simulations. An arbitrary Lagrangian-Eulerian formulation based on an unstructured side-centered finite volume method is utilized in order to solve the incompressible unsteady Navier-Stokes equations. The Reynolds number is chosen to be 252, and the reduced frequency of plunging motion ( k = 2π fc/ U ∞) and the plunge amplitude non-dimensionalized with respect to chord are set to be 12.3 and 0.12, respectively, as in the experimental study of Jones and Platzer (2009). The present numerical simulations reveal a highly persistent transient effect, and it takes two orders of magnitude larger duration than the heave period to reach the time-periodic state. In addition, the three-dimensional simulation reveals that the flow field is three-dimensional for the parameters used herein. The calculation reproduces the deflected wake and shows a good agreement with the experimental wake pattern. The instantaneous vorticity contours, finite-time Lyapunov exponent fields and particle traces are presented along with the aerodynamic parameters including the lift and thrust coefficients.
Low Reynolds number hydrodynamics and mesoscale simulations
NASA Astrophysics Data System (ADS)
Winkler, Roland G.
2016-11-01
Hydrodynamics and hydrodynamic interactions are fundamental for the motility of microswimmers. This includes the propulsion mechanism itself, the synchronized motion of flagella in flagellar bundles and beating cilia of cilia arrays, and even extends to collective behaviors. The general importance of hydrodynamics has stimulated the development of mesoscale simulation approaches to efficiently study dynamical properties of objects embedded in a fluid. In this minireview, the properties of flows at low Reynolds numbers are discussed, thereby the unsteady acceleration term is typically taken into account (Landau-Lifshitz Navier-Stokes equations). Specifically, the synchronization of microrotors by time-dependent hydrodynamic interactions is discussed and the propulsion of a rotating helix. Moreover, the multiparticle collisions dynamics method (MPC), a mesoscale simulation approach for fluids, is outlined. Simulation results for the flow field of a model E. Coli bacterium and its swimming behavior next to a surface are presented.
NASA Technical Reports Server (NTRS)
Kruse, R. L.; Lovette, G. H.; Spencer, B., Jr.
1977-01-01
The subsonic aerodynamic characteristics of a series of irregular planform wings were studied in wind tunnel tests conducted at M = 0.3 over a range of Reynolds numbers from 1.6 million to 26 million/m. The five basic wing planforms varied from a trapezoidal to a delta shape. Leading edge extensions, added to the basic shape, varied in approximately 5 deg increments from the wing leading edge sweep-back angle to a maximum 80 deg. Most of the tests were conducted using an NACA 0008 airfoil section with grit boundary layer trips. Tests were also conducted using an NACA 0012 airfoil section and an 8% thick wedge. In addition, the effect of free transition (no grit) was investigated. A body was used on all models.
Reynolds number influences on turbulent boundary layer momentum transport
NASA Astrophysics Data System (ADS)
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
NASA Technical Reports Server (NTRS)
Horstman, C. C.; Settles, G. S.; Vas, I. E.; Bogdonoff, S. M.; Hung, C. M.
1977-01-01
An experiment is described that tests and guides computations of a shock-wave turbulent boundary-layer interaction flow over a 20-deg compression corner at Mach 2.85. Numerical solutions of the time-averaged Navier-Stokes equations for the entire flow field, employing various turbulence models, are compared with the data. Each model is critically evaluated by comparisons with the details of the experimental data. Experimental results for the extent of upstream pressure influence and separation location are compared with numerical predictions for a wide range of Reynolds numbers and shock-wave strengths.
Finite-span rotating flat-plate wings at low reynolds number and the effects of aspect ratio
NASA Astrophysics Data System (ADS)
Carr, Zakery R.
In the complex and dangerous environments of the modern warrior and emergency professional, the small size, maneuverability, and stealth of flapping-wing micro air vehicles (MAVs), scaled to the size of large insects or hummingbirds, has the potential to provide previously inaccessible levels of situational awareness, reconnaissance capability, and flexibility directly to the front lines. Although development of such an efficient, autonomous, and capable MAV is years away, there are immediate contributions that can be made to the fundamental science of the flapping-wing-type propulsion that makes MAVs so attractive. This investigation contributes to those fundamentals by considering the unsteady vortex dynamics problem of a rigid, rectangular flat plate at a fixed angle of attack rotating from rest---a simplified hovering half-stroke. Parameters are chosen to be biologically-relevant and relevant to MAVs operating at Reynolds numbers of O (103), and experiments are performed in a 50% by mass glycerin-water mixture. These experiments use novel application of methodologies verified by rigorous uncertainty analysis. The overall objective is to understand the vortex formation and forces as well as aspect ratio ( AR) effects. Of interest is the overall, time-varying, three-dimensional vortex structure obtained qualitatively from dye visualization and quantitatively from volumes reconstructed using planar stereoscopic digital particle image velocimetry (S-DPIV) measurements. The velocity information from S-DPIV also allows statements to be made on leading-edge vortex (LEV) stability, spanwise flow, LEV and tip-vortex (TV) circulation, and numerous circulation scalings. Force measurements are made and the lift coefficient is discussed in the context of the flow structure, the dimensional lift and the ability to relate velocity and force measurements going forward. AR effects is a topic of continued interest to those performing MAV-related research and also a primary
High Reynolds number turbulent pipe flow
NASA Astrophysics Data System (ADS)
Zhao, Rongrong
Fully developed turbulent pipe is studied in this thesis. Streamwise and wall-normal turbulence components are measured using a crossed hot-wire probe. In the process, a new calibration method for the crossed hot-wire probe is developed, and the binormal cooling error for hot-wire measurement, which is caused by cooling in the direction normal to the hot-wire measurement plane, is studied and found to be the major error contributor for both mean velocity and turbulence intensity measurements using a crossed-wire probe. The new calibration scheme utilizes the fact that the total stress in a fully developed turbulent pipe flow is defined by the streamwise pressure gradient, so directional sensibility calibration could be done by recording the crossed hot-wire signals against a known shear stress distribution. This information, when combined with mean velocity calibration against a Pitot tube measurement, provide a full calibration for crossed hot-wire probes. The new calibration method is especially convenient for pipe and channel flow measurements. For other measurements, the calibration could be done by using a simple pipe apparatus as the calibration device. Streamwise and wall-normal turbulence components are measured over a Reynolds number range from 1.1 x 105 to 9.8 x 10 6. Similarity arguments are studied for turbulence intensity and spectra. The most relevant physical assumption for the 'similarity' is Townsend's distinction between 'active' and 'inactive' motions. Perry's attached eddy hypothesis, which is based on Townsend's work, offers a more detailed physical model and provides extensive quantitative prediction, is also reviewed and discussed in the context of these new measurements. For the wall-normal turbulence intensity, a constant region in u'rms is found for the region 200 ≤ y+ ≤ 0.1R+ in inner and outer scaling for Reynolds numbers up to 1.0 x 106. An increase in u'rms is observed closer to the wall at about y + ˜ 100, and is suggestive of
NASA Astrophysics Data System (ADS)
Moran-Lopez, Tiberius; Schilling, Oleg
2014-11-01
Reshocked Richtmyer-Meshkov turbulent mixing for various gas pairs and large shock Mach numbers is simulated using a third-order weighted essentially nonoscillatory (WENO) implementation of a new K- ɛ multicomponent Reynolds-averaged Navier-Stokes model. Experiments previously performed at the University of Provence with gas pairs CO2 /He, CO2 /Ar, and CO2 /Kr (with At = - 0 . 73 , - 0 . 05 , and 0 . 3 , respectively) and incident shock Mach numbers Ma = 2 . 4 , 3 . 1 , 3 . 7 , 4 . 2 , and 4 . 5 are considered. The evolution of the mixing layer widths is shown to be in good agreement with the experimental data. Budgets of the turbulent transport equations are used to elucidate the mechanisms contributing to turbulent mixing in large Mach number reshocked Richtmyer-Meshkov instability. These results are contrasted with those from previous modeling of smaller Mach number experiments to identify the physical effects which require accurate modeling, including mean and turbulent enthalpy diffusion, pressure-dilatation, and dilatation dissipation. This work was performed under the auspices of the US Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344.
NASA Astrophysics Data System (ADS)
Thomareis, Nikitas; Papadakis, George
2017-01-01
Direct numerical simulations of the flow field around a NACA 0012 airfoil at Reynolds number 50 000 and angle of attack 5° with 3 different trailing edge shapes (straight, blunt, and serrated) have been performed. Both time-averaged flow characteristics and the most dominant flow structures and their frequencies are investigated using the dynamic mode decomposition method. It is shown that for the straight trailing edge airfoil, this method can capture the fundamental as well as the subharmonic of the Kelvin-Helmholtz instability that develops naturally in the separating shear layer. The fundamental frequency matches well with relevant data in the literature. The blunt trailing edge results in periodic vortex shedding, with frequency close to the subharmonic of the natural shear layer frequency. The shedding, resulting from a global instability, has an upstream effect and forces the separating shear layer. Due to forcing, the shear layer frequency locks onto the shedding frequency while the natural frequency (and its subharmonic) is suppressed. The presence of serrations in the trailing edge creates a spanwise pressure gradient, which is responsible for the development of a secondary flow pattern in the spanwise direction. This pattern affects the mean flow in the near wake. It can explain an unexpected observation, namely, that the velocity deficit downstream of a trough is smaller than the deficit after a protrusion. Furthermore, the insertion of serrations attenuates the energy of vortex shedding by de-correlating the spanwise coherence of the vortices. This results in weaker forcing of the separating shear layer, and both the subharmonics of the natural frequency and the shedding frequency appear in the spectra.
Investigation of Transonic Reynolds Number Scaling on a Twin-Engine Transport
NASA Technical Reports Server (NTRS)
Curtin, M. M.; Bogue, D. R.; Om, D.; Rivers, S. M. B.; Pendergraft, O. C., Jr.; Wahls, R. A.
2002-01-01
This paper discusses Reynolds number scaling for aerodynamic parameters including force and wing pressure measurements. A full-span model of the Boeing 777 configuration was tested at transonic conditions in the National Transonic Facility (NTF) at Reynolds numbers (based on mean aerodynamic chord) from 3.0 to 40.0 million. Data was obtained for a tail-off configuration both with and without wing vortex generators and flap support fairings. The effects of aeroelastics were separated from Reynolds number effects by varying total pressure and temperature independently. Data from the NTF at flight Reynolds number are compared with flight data to establish the wind tunnel/flight correlation. The importance of high Reynolds number testing and the need for developing a process for transonic Reynolds number scaling is discussed. This paper also identifies issues that need to be worked for Boeing Commercial to continue to conduct future high Reynolds number testing in the NTF.
Reynolds-number dependence of turbulence enhancement on collision growth
NASA Astrophysics Data System (ADS)
Onishi, Ryo; Seifert, Axel
2016-10-01
This study investigates the Reynolds-number dependence of turbulence enhancement on the collision growth of cloud droplets. The Onishi turbulent coagulation kernel proposed in Onishi et al. (2015) is updated by using the direct numerical simulation (DNS) results for the Taylor-microscale-based Reynolds number (Reλ) up to 1140. The DNS results for particles with a small Stokes number (St) show a consistent Reynolds-number dependence of the so-called clustering effect with the locality theory proposed by Onishi et al. (2015). It is confirmed that the present Onishi kernel is more robust for a wider St range and has better agreement with the Reynolds-number dependence shown by the DNS results. The present Onishi kernel is then compared with the Ayala-Wang kernel (Ayala et al., 2008a; Wang et al., 2008). At low and moderate Reynolds numbers, both kernels show similar values except for r2 ˜ r1, for which the Ayala-Wang kernel shows much larger values due to its large turbulence enhancement on collision efficiency. A large difference is observed for the Reynolds-number dependences between the two kernels. The Ayala-Wang kernel increases for the autoconversion region (r1, r2 < 40 µm) and for the accretion region (r1 < 40 and r2 > 40 µm; r1 > 40 and r2 < 40 µm) as Reλ increases. In contrast, the Onishi kernel decreases for the autoconversion region and increases for the rain-rain self-collection region (r1, r2 > 40 µm). Stochastic collision-coalescence equation (SCE) simulations are also conducted to investigate the turbulence enhancement on particle size evolutions. The SCE with the Ayala-Wang kernel (SCE-Ayala) and that with the present Onishi kernel (SCE-Onishi) are compared with results from the Lagrangian Cloud Simulator (LCS; Onishi et al., 2015), which tracks individual particle motions and size evolutions in homogeneous isotropic turbulence. The SCE-Ayala and SCE-Onishi kernels show consistent results with the LCS results for small Reλ. The two SCE
Holography of the QGP Reynolds number
NASA Astrophysics Data System (ADS)
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.
Noisy swimming at low Reynolds numbers
NASA Astrophysics Data System (ADS)
Dunkel, Jörn; Zaid, Irwin M.
2009-08-01
Small organisms (e.g., bacteria) and artificial microswimmers move due to a combination of active swimming and passive Brownian motion. Considering a simplified linear three-sphere swimmer, we study how the swimmer size regulates the interplay between self-driven and diffusive behavior at low Reynolds number. Starting from the Kirkwood-Smoluchowski equation and its corresponding Langevin equation, we derive formulas for the orientation correlation time, the mean velocity and the mean-square displacement in three space dimensions. The validity of the analytical results is illustrated through numerical simulations. Tuning the swimmer parameters to values that are typical of bacteria, we find three characteristic regimes: (i) Brownian motion at small times, (ii) quasiballistic behavior at intermediate time scales, and (iii) quasidiffusive behavior at large times due to noise-induced rotation. Our analytical results can be useful for a better quantitative understanding of optimal foraging strategies in bacterial systems, and they can help to construct more efficient artificial microswimmers in fluctuating fluids.
Wave phenomena in a high Reynolds number compressible boundary layer
NASA Technical Reports Server (NTRS)
Bayliss, A.; Maestrello, L.; Parikh, P.; Turkel, E.
1985-01-01
Growth of unstable disturbances in a high Reynolds number compressible boundary layer is numerically simulated. Localized periodic surface heating and cooling as a means of active control of these disturbances is studied. It is shown that compressibility in itself stabilizes the flow but at a lower Mach number, significant nonlinear distortions are produced. Phase cancellation is shown to be an effective mechanism for active boundary layer control.
NASA Technical Reports Server (NTRS)
Schobeiri, M. T.; Ozturk, B.; Ashpis, David E.
2007-01-01
The paper experimentally studies the effects of periodic unsteady wake flow and different Reynolds numbers on boundary layer development, separation and re-attachment along the suction surface of a low pressure turbine blade. The experimental investigations were performed on a large scale, subsonic unsteady turbine cascade research facility at Turbomachinery Performance and Flow Research Laboratory (TPFL) of Texas A&M University. The experiments were carried out at Reynolds numbers of 110,000 and 150,000 (based on suction surface length and exit velocity). One steady and two different unsteady inlet flow conditions with the corresponding passing frequencies, wake velocities, and turbulence intensities were investigated. The reduced frequencies chosen cover the operating range of LP turbines. In addition to the unsteady boundary layer measurements, surface pressure measurements were performed. The inception, onset, and the extent of the separation bubble information collected from the pressure measurements were compared with the hot wire measurements. The results presented in ensemble-averaged, and the contour plot forms help to understand the physics of the separation phenomenon under periodic unsteady wake flow and different Reynolds number. It was found that the suction surface displayed a strong separation bubble for these three different reduced frequencies. For each condition, the locations defining the separation bubble were determined carefully analyzing and examining the pressure and mean velocity profile data. The location of the boundary layer separation was dependent of the Reynolds number. It is observed that starting point of the separation bubble and the re-attachment point move further downstream by increasing Reynolds number from 110,000 to 150,000. Also, the size of the separation bubble is smaller when compared to that for Re=110,000.
Low Reynolds number suspension gravity currents.
Saha, Sandeep; Salin, Dominique; Talon, Laurent
2013-08-01
The extension of a gravity current in a lock-exchange problem, proceeds as square root of time in the viscous-buoyancy phase, where there is a balance between gravitational and viscous forces. In the presence of particles however, this scenario is drastically altered, because sedimentation reduces the motive gravitational force and introduces a finite distance and time at which the gravity current halts. We investigate the spreading of low Reynolds number suspension gravity currents using a novel approach based on the Lattice-Boltzmann (LB) method. The suspension is modeled as a continuous medium with a concentration-dependent viscosity. The settling of particles is simulated using a drift flux function approach that enables us to capture sudden discontinuities in particle concentration that travel as kinematic shock waves. Thereafter a numerical investigation of lock-exchange flows between pure fluids of unequal viscosity, reveals the existence of wall layers which reduce the spreading rate substantially compared to the lubrication theory prediction. In suspension gravity currents, we observe that the settling of particles leads to the formation of two additional fronts: a horizontal front near the top that descends vertically and a sediment layer at the bottom which aggrandises due to deposition of particles. Three phases are identified in the spreading process: the final corresponding to the mutual approach of the two horizontal fronts while the laterally advancing front halts indicating that the suspension current stops even before all the particles have settled. The first two regimes represent a constant and a decreasing spreading rate respectively. Finally we conduct experiments to substantiate the conclusions of our numerical and theoretical investigation.
NASA Astrophysics Data System (ADS)
Frederiksen, Richard D.; Dahm, Werner J. A.; Dowling, David R.
1998-12-01
Experimental results are presented for the influence of Reynolds number on multifractal scale similarity in turbulent flows. These are obtained from single-point measurements of a dynamically passive Sc[approximate]1 conserved scalar quantity [zeta](t) in a turbulent shear flow at outer-scale Reynolds numbers of 14000[less-than-or-eq, slant]Re[delta][less-than-or-eq, slant]110000. Statistical criteria based on the maximum allowable scale-to-scale variation L1([epsilon]) in multiplier distributions P(M[epsilon]) from multifractal gauge sets allow accurate discrimination between multifractal and non-multifractal scaling. Results show that the surrogate scalar energy dissipation rate [chi]s(t)[identical with](d[zeta]/dt)2is found to display a scale-invariant similarity consistent with a random multiplicative cascade characterized by a bilinear multiplier distribution P(M[epsilon]) over a range of scales extending downward from the outer scaleT[delta]. For a range of scales extending upward from the inner (diffusive) scale TD, the dissipation rate displays a different scale-invariant similarity characterized by a uniform multiplier distribution. The former scale-invariance becomes evident in the present Sc[approximate]1 data only when Re[delta] is sufficiently large. Comparisons with results from Sc 1 data indicate that this scale-invariant similarity applies when the outer-to-inner scale ratio T[delta]/TD[approximate]0.09 Re3/4[delta]Sc1/2 is greater than about 400. In contrast to the scalar dissipation rate field, the scalar field is found to lack any multifractal scale similarity.
NASA Technical Reports Server (NTRS)
Henderson, W. P.
1976-01-01
An investigation was conducted in the Langley low turbulence pressure tunnel to determine the effects of wing leading edge radius and Reynolds number on the longitudinal aerodynamic characteristics of a series of highly swept wing-body configurations. The tests were conducted at Mach numbers below 0.30, angles of attack up to 16 deg, and Reynolds numbers per meter from 6.57 million to 43.27 million. The wings under study in this investigation had leading edge sweep angles of 61.7 deg, 64.61 deg, and 67.01 deg in combination with trailing edge sweep angles of 0 deg and 40.6 deg. The leading edge radii of each wing planform could be varied from sharp to nearly round.
NASA Technical Reports Server (NTRS)
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
NASA Technical Reports Server (NTRS)
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
Experimental studies of Reynolds number dependence of turbulent mixing & transport
Warhaft, Z.
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.
NASA Technical Reports Server (NTRS)
Cunningham, Atlee M., Jr.; Spragle, Gregory S.
1987-01-01
The influence of Mach and Reynolds numbers as well as airfoil and planform geometry on the phenomenon of constant shock jump pressure coefficient for conditions of shock induced trailing edge separation (SITES) was studied. It was demonstrated that the phenomenon does exist for a wide variety of two and three dimensional flow cases and that the influence of free stream Mach number was not significant. The influence of Reynolds number was found to be important but was not strong. Airfoil and planform geometric characteristics were found to be very important where the pressure coefficient jump was shown to vary with the sum of: (1) airfoil curvature at the upper surface crest, and (2) camber surface slope at the trailing edge. It was also determined that the onset of SITES could be defined as a function of airfoil geometric parameters and Mach number normal to the leading edge. This onset prediction was shown to predict the angle of onset to within + or - 1 deg accuracy or better for about 90% of the cases studied.
NASA Technical Reports Server (NTRS)
Chitsomboon, Tawit
1994-01-01
Wall functions, as used in the typical high Reynolds number k-epsilon turbulence model, can be implemented in various ways. A least disruptive method (to the flow solver) is to directly solve for the flow variables at the grid point next to the wall while prescribing the values of k and epsilon. For the centrally-differenced finite-difference scheme employing artificial viscocity (AV) as a stabilizing mechanism, this methodology proved to be totally useless. This is because the AV gives rise to a large error at the wall due to too steep a velocity gradient resulting from the use of a coarse grid as required by the wall function methodology. This error can be eliminated simply by extrapolating velocities at the wall, instead of using the physical values of the no-slip velocities (i.e. the zero value). The applicability of the technique used in this paper is demonstrated by solving a flow over a flat plate and comparing the results with those of experiments. It was also observed that AV gives rise to a velocity overshoot (about 1 percent) near the edge of the boundary layer. This small velocity error, however, can yield as much as 10 percent error in the momentum thickness. A method which integrates the boundary layer up to only the edge of the boundary (instead of infinity) was proposed and demonstrated to give better results than the standard method.
On the modeling of low-Reynolds-number turbulence
NASA Technical Reports Server (NTRS)
So, R. M. C.; Yoo, G. J.
1986-01-01
A full Reynolds-stress closure that is capable of describing the flow all the way to the wall was formulated for turbulent flow through circular pipe. Since viscosity does not appear explicitly in the pressure redistribution terms, conventional high-number models for these terms are found to be applicable. However, the models for turbulent diffusion and viscous dissipation have to be modified to account for viscous diffusion near a wall. Two redistribution and two diffusion models are investigated for their effects on the model calculations. Wall correction to pressure redistribution modeling is also examined. Diffusion effects on calculated turbulent properties are further investigated by simplifying the transport equations to algebraic equations for Reynolds stress. Two approximations are explored. These are the equilibrium and nonequilibrium turbulence assumptions. Finally, the two-equation closure is also used to calculate the flow in question and the results compared with all the other model calculations. Fully developed pipe flows at two moderate Reynolds numbers are used to validate these model calculations.
Cluster Computation of Flight Reynolds Number Flows
NASA Technical Reports Server (NTRS)
Atwood, Christopher A.; Smith, Merritt H.; Kutler, Paul (Technical Monitor)
1994-01-01
The performance of a workstation cluster used for the solution of the Reynolds-averaged Navier-Stokes equations is compared with a conventional vector supercomputer architecture. The application simulation of the steady flowfield about a transonic transport was computed using an implicit diagonal scheme in an overset mesh framework. Static load balancing was used, while coarse grain decomposition was achieved by solution of a grid zone per processor. Price/performance ratios are estimated for several scenarios in which such clusters may be utilized.
Support-sting interference on boattail pressure drag for Reynolds numbers up to 70 x 10 to the 6th
NASA Technical Reports Server (NTRS)
Gloss, B. B.; Sewall, W. G.
1983-01-01
A model was tested in the Langley 0.3-Meter Transonic Cryogenic Tunnel to investigate the effects of Reynolds number on boattail pressure drag for a variety of sting shapes. The boattail pressure drag for constant Mach number increased linearly with Reynolds number over the Reynolds number range tested. The data indicated that, as the disturbance produced by the sting on the boattail increased, the boattail pressure drag became less sensitive to Reynolds number change. Also, it was found that the model base pressure versus Reynolds number curve reached a plateau within the Reynolds number range examined.
Numerical investigation of transition critical Reynolds number of channel flow.
NASA Astrophysics Data System (ADS)
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).
Design and Calibration of the ARL Mach 3 High Reynolds Number Facility
1975-01-01
polytropically during the depressurization process (a function of mass flow, run time and tank volume), and by the cooling (Joule- Thomson effect ) associated...Reynolds numbers, where the mass flows are largest. Joule- Thomson effects will be more pronounced at the lower Reynolds numbers, due to the greater
NASA Technical Reports Server (NTRS)
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
Direct numerical simulation of moderate-Reynolds-number flow past arrays of rotating spheres
NASA Astrophysics Data System (ADS)
Zhou, Qiang; Fan, Liang-Shih
2015-07-01
Direct numerical simulations with an immersed boundary-lattice Boltzmann method are used to investigate the effects of particle rotation on flows past random arrays of mono-disperse spheres at moderate particle Reynolds numbers. This study is an extension of a previous study of the authors [Q. Zhou and L.-S. Fan, "Direct numerical simulation of low-Reynolds-number flow past arrays of rotating spheres," J. Fluid Mech. 765, 396-423 (2015)] that explored the effects of particle rotation at low particle Reynolds numbers. The results of this study indicate that as the particle Reynolds number increases, the normalized Magnus lift force decreases rapidly when the particle Reynolds number is in the range lower than 50. For the particle Reynolds number greater than 50, the normalized Magnus lift force approaches a constant value that is invariant with solid volume fractions. The proportional dependence of the Magnus lift force on the rotational Reynolds number (based on the angular velocity and the diameter of the spheres) observed at low particle Reynolds numbers does not change in the present study, making the Magnus lift force another possible factor that can significantly affect the overall dynamics of fluid-particle flows other than the drag force. Moreover, it is found that both the normalized drag force and the normalized torque increase with the increase of the particle Reynolds number and the solid volume fraction. Finally, correlations for the drag force, the Magnus lift force, and the torque in random arrays of rotating spheres at arbitrary solids volume fractions, rotational Reynolds numbers, and particle Reynolds numbers are formulated.
NASA Astrophysics Data System (ADS)
Guda, Venkata Subba Sai Satish
There have been several advancements in the aerospace industry in areas of design such as aerodynamics, designs, controls and propulsion; all aimed at one common goal i.e. increasing efficiency --range and scope of operation with lesser fuel consumption. Several methods of flow control have been tried. Some were successful, some failed and many were termed as impractical. The low Reynolds number regime of 104 - 105 is a very interesting range. Flow physics in this range are quite different than those of higher Reynolds number range. Mid and high altitude UAV's, MAV's, sailplanes, jet engine fan blades, inboard helicopter rotor blades and wind turbine rotors are some of the aerodynamic applications that fall in this range. The current study deals with using dynamic roughness as a means of flow control over a NACA 0012 airfoil at low Reynolds numbers. Dynamic 3-D surface roughness elements on an airfoil placed near the leading edge aim at increasing the efficiency by suppressing the effects of leading edge separation like leading edge stall by delaying or totally eliminating flow separation. A numerical study of the above method has been carried out by means of a Large Eddy Simulation, a mathematical model for turbulence in Computational Fluid Dynamics, owing to the highly unsteady nature of the flow. A user defined function has been developed for the 3-D dynamic roughness element motion. Results from simulations have been compared to those from experimental PIV data. Large eddy simulations have relatively well captured the leading edge stall. For the clean cases, i.e. with the DR not actuated, the LES was able to reproduce experimental results in a reasonable fashion. However DR simulation results show that it fails to reattach the flow and suppress flow separation compared to experiments. Several novel techniques of grid design and hump creation are introduced through this study.
NASA Astrophysics Data System (ADS)
Yu, Zhaosheng; Wang, Yu; Shao, Xueming
2012-11-01
A direct-forcing fictitious domain method was employed to perform fully-resolved numerical simulations of turbulent channel flow laden with large neutrally buoyant particles at constant pressure gradients. The effects of the particles on the turbulence (including the fluid-phase average velocity, the root-mean-square (rms) of the velocity fluctuation, the probability density function of the velocity and the vortex structures) at the friction Reynolds number of 180 and 395 were investigated. The results show that the drag-reduction effect caused by the spherical particle at low particle volumes is very small. The presence of particles decreases the maximum rms of streamwise velocity fluctuation near wall via weakening the large-scale streamwise vortices, and on the other hand increases the rms of transverse and spanwise fluctuating velocities in vicinity of the wall via inducing smaller-scale vortices. The effects of the particles on the fluid velocity PDF (probability density function) normalized with the rms velocity are small, irrespective of the particle size, particle volume fraction and Reynolds number. The work was supported by the National Natural Science Foundation of China (Nos. 11072217 and 11132008), the Fundamental Research Funds for the Central Universities, and the Program for New Century Excellent Talents in University.
Vortex shedding from slender cones at low Reynolds numbers
NASA Astrophysics Data System (ADS)
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.
NASA Technical Reports Server (NTRS)
Choudhari, Meelan; Street, Craig L.
1991-01-01
Previous theoretical work on the boundary layer receptivity problem has utilized large Reynolds number asymptotic theories, thus being limited to a narrow part of the frequency - Reynolds number domain. An alternative approach is presented for the prediction of localized instability generation which has a general applicability, and also accounts for finite Reynolds number effects. This approach is illustrated for the case of Tollmien-Schlichting wave generation in a Blasius boundary layer due to the interaction of a free stream acoustic wave with a region of short scale variation in the surface boundary condition. The specific types of wall inhomogeneities studied are: regions of short scale variations in wall suction, wall admittance, and wall geometry (roughness). Extensive comparison is made between the results of the finite Reynolds number approach and previous asymptotic predictions, which also suggests an alternative way of using the latter at Reynolds numbers of interest in practice.
Kumar, Anoop; Gupta, S K; Kale, S R
2007-04-01
Cross-flow gravity towers are particle scrubbing devices in which water is sprayed from the top into particle-laden flow moving horizontally. Models for predicting particle capture assume drops traveling at terminal velocity and potential flow (ReD > 1000) around it, however, Reynolds numbers in the intermediate range of 1 to 1000 are common in gravity towers. Drops are usually injected at velocities greater than their terminal velocities (as in nozzles) or from near rest (perforated tray) and they accelerate/decelerate to their terminal velocity in the tower. Also, the effects of intermediate drop Reynolds number on capture efficiency have been simulated for (a) drops at their terminal velocity and (b) drops accelerating/decelerating to their terminal velocity. Tower efficiency based on potential flow about the drop is 40%-50% greater than for 200 mm drops traveling at their terminal velocity. The corresponding values for 500 mm drops are about 10%-20%. The drop injection velocity is important operating parameter. Increase in tower efficiency by about 40% for particles smaller than 5 mm is observed for increase in injection velocity from 0 to 20 m/s for 200 and 500mm drops.
Asymptotic exponents from low-Reynolds-number flows
NASA Astrophysics Data System (ADS)
Schumacher, Jörg; Sreenivasan, Katepalli R.; Yakhot, Victor
2007-04-01
The high-order statistics of fluctuations in velocity gradients in the crossover range from the inertial to the Kolmogorov and sub-Kolmogorov scales are studied by direct numerical simulations (DNS) of homogeneous isotropic turbulence with vastly improved resolution. The derivative moments for orders 0 <= n <= 8 are represented well as powers of the Reynolds number, Re, in the range 380 <= Re <= 5275, where Re is based on the periodic box length Lx. These low-Reynolds-number flows give no hint of scaling in the inertial range even when extended self-similarity is applied. Yet, the DNS scaling exponents of velocity gradients agree well with those deduced, using a recent theory of anomalous scaling, from the scaling exponents of the longitudinal structure functions at infinitely high Reynolds numbers. This suggests that the asymptotic state of turbulence is attained for the velocity gradients at far lower Reynolds numbers than those required for the inertial range to appear. We discuss these findings in the light of multifractal formalism. Our numerical studies also resolve the crossover of the velocity gradient statistics from Gaussian to non-Gaussian behaviour that occurs as the Reynolds number is increased.
The cryogenic wind tunnel concept for high Reynolds number testing
NASA Technical Reports Server (NTRS)
Kilgore, R. A.; Goodyer, M. J.; Adcock, J. B.; Davenport, E. E.
1974-01-01
Theoretical considerations indicate that cooling the wind-tunnel test gas to cryogenic temperatures will provide a large increase in Reynolds number with no increase in dynamic pressure while reducing the tunnel drive-power requirements. Studies were made to determine the expected variations of Reynolds number and other parameters over wide ranges of Mach number, pressure, and temperature, with due regard to avoiding liquefaction. Practical operational procedures were developed in a low-speed cryogenic tunnel. Aerodynamic experiments in the facility demonstrated the theoretically predicted variations in Reynolds number and drive power. The continuous-flow-fan-driven tunnel is shown to be particularly well suited to take full advantage of operating at cryogenic temperatures.
NASA Technical Reports Server (NTRS)
Loftin, Laurence K, Jr; Bursnall, William J
1950-01-01
Results are presented of an investigation made to determine the two-dimensional lift and drag characteristics of nine NACA 6-series airfoil section at Reynolds numbers of 15.0 x 10sub6, 20.0 x 10sub6, and 25.0 x 10sub6. Also presented are data from NACA Technical Report 824 for the same airfoils at Reynolds numbers of 3.0 x 10sub6, 6.0 x 10sub6, and 9.0 x 10sub6. The airfoils selected represent sections having variations in the airfoil thickness, thickness form, and camber. The characteristics of an airfoil with a split flap were determined in one instance, as was the effect of surface roughness. Qualitative explanations in terms of flow behavior are advanced for the observed types of scale effect.
Complex magnetohydrodynamic low-Reynolds-number flows.
Xiang, Yu; Bau, Haim H
2003-07-01
The interaction between electric currents and a magnetic field is used to produce body (Lorentz) forces in electrolyte solutions. By appropriate patterning of the electrodes, one can conveniently control the direction and magnitude of the electric currents and induce spatially and temporally complicated flow patterns. This capability is useful, not only for fundamental flow studies, but also for inducing fluid flow and stirring in minute devices in which the incorporation of moving components may be difficult. This paper focuses on a theoretical and experimental study of magnetohydrodynamic flows in a conduit with a rectangular cross section. The conduit is equipped with individually controlled electrodes uniformly spaced at a pitch L. The electrodes are aligned transversely to the conduit's axis. The entire device is subjected to a uniform magnetic field. The electrodes are divided into two groups A and C in such a way that there is an electrode of group C between any two electrodes of group A. We denote the various A and C electrodes with subscripts, i.e., A(i) and C(i), where i=0,+/-1,+/-2, .... When positive and negative potentials are, respectively, applied to the even and odd numbered A electrodes, opposing electric currents are induced on the right and left hand sides of each A electrode. These currents generate transverse forces that drive cellular convection in the conduit. We refer to the resulting flow pattern as A. When electrodes of group C are activated, a similar flow pattern results, albeit shifted in space. We refer to this flow pattern as C. By alternating periodically between patterns A and C, one induces Lagrangian chaos. Such chaotic advection may be beneficial for stirring fluids, particularly in microfluidic devices. Since the flow patterns A and C are shifted in space, they also provide a mechanism for Lagrangian drift that allows net migration of passive tracers along the conduit's length.
Compensatory escape mechanism at low Reynolds number
Gemmell, Brad J.; Sheng, Jian; Buskey, Edward J.
2013-01-01
Despite high predation pressure, planktonic copepods remain one of the most abundant groups on the planet. Their escape response provides one of most effective mechanisms to maximize evolutionary fitness. Owing to their small size (100 µm) compared with their predators (>1 mm), increasing viscosity is believed to have detrimental effects on copepods’ fitness at lower temperature. Using high-speed digital holography we acquire 3D kinematics of the nauplius escape including both location and detailed appendage motion. By independently varying temperature and viscosity we demonstrate that at natural thermal extremes, contrary to conventional views, nauplii achieve equivalent escape distance while maintaining optimal velocity. Using experimental results and kinematic simulations from a resistive force theory propulsion model, we demonstrate that a shift in appendage timing creates an increase in power stroke duration relative to recovery stroke duration. This change allows the nauplius to limit losses in velocity and maintain distance during escapes at the lower bound of its natural thermal range. The shift in power stroke duration relative to recovery stroke duration is found to be regulated by the temperature dependence of swimming appendage muscle groups, not a dynamic response to viscosity change. These results show that copepod nauplii have natural adaptive mechanisms to compensate for viscosity variations with temperature but not in situations in which viscosity varies independent of temperature, such as in some phytoplankton blooms. Understanding the robustness of escapes in the wake of environmental changes such as temperature and viscosity has implications in assessing the future health of performance compensation. PMID:23487740
Electrified film flows at moderate Reynolds number
NASA Astrophysics Data System (ADS)
Craster, Richard; Wray, Alex; Papageorgiou, Demetrios; Matar, Omar
2014-11-01
We examine the flow of a thin, inclined film sandwiched between two parallel electrodes. We follow the Weighted Residual Integral Boundary Layer method, which has been shown via comparison with both direct numerical simulations and experiments to give good results in both the drag-gravity and drag-inertia regimes. We extend existing models to give an accurate model of electrostatic effects via a similar separation of variables approach. A disparity in material properties between the liquid and gas regions induces a Maxwell stress at the interface, which affords a significant degree of control over the behaviour of the film. In one dimension, linear stability comparisons are made with a full Orr-Sommerfeld calculation, and nonlinear comparisons are made with direct numerical simulations, both showing excellent agreement in large parts of parameter space. The model is also extended to fully two-dimensional simulations. EPSRC Programme Grant, MEMPHIS, EP/K0039761/1, EPSRC DTG Studentship (AWW).
Study of mixing at low Reynolds number
NASA Astrophysics Data System (ADS)
Arco, R. M.; Zenit, R.; Lauga, E.
2011-11-01
In many mixing applications is often impossible to operate the mixer in turbulent regime due to the high viscosity of the liquid used (polymers solutions). In this study we propose a technique to induce mixing in laminar flows. A flapper is oscillated at certain frequency and the flow field around it is studied with a PIV technique. The rigidity of the flapper is varied using different materials (acrylic, neoprene). The time-reversibility of the flow was broken when the flapper became flexible. Also we used a Newtonian, and non-Newtonian fluids with elastic effects and nearly constant viscosity. All test were conducted at Re <0.02. Velocity profiles were measured having a impeller oscillation with +/- 60 degrees from the vertical, measurement were taken in the vertical position varying the angular velocities for each impeller. Both the increase of flexibility of the impeller and the increase of the angular velocity was seen to modify the pumping capacity in both fluids. In this talk the nature of this behavior and its implications in mixing processes will be discussed.
Time-dependent measurement of base pressure in a blowdown tunnel with varying unit Reynolds number
NASA Technical Reports Server (NTRS)
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.
Numerical simulation of high Reynolds number bubble motion
McLaughlin, J.B.
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.
NASA Astrophysics Data System (ADS)
Radl, Stefan; Municchi, Federico; Goniva, Christoph
2016-11-01
Understanding transport phenomena in fluid-particle systems is of primary importance for the design of large-scale equipment, e.g., in the chemical industry. Typically, the analysis of such systems is performed by numerically solving a set of partial differential equations modeling the particle phase and the fluid phase as interpenetrating continua. Such models require a number of closure models that are often constructed via spatial filtering of data obtained from particle-resolved direct numerical simulations (PR-DNS). In the present work we make use of PR-DNS to evaluate corrections to existing closure models. Specifically, we aim on accounting for wall effects on the fluid-particle drag force and the particle-individual Nusselt number. We then propose an improved closure model to be used in particle-unresolved Euler-Lagrange (PU-EL) simulations. We demonstrate that such an advanced closure should account for a dimensionless filter size, as well as a normalized distance from the wall. In addition, we make an attempt to model the filtered fluid velocity profile in wall-bounded suspension flows. The authors acknowledge funding from the European Commission through FP7 Grant Agreement No. 604656, as well as VSC-3 and dcluster.tugraz.at.
Electrohydrodynamic deformation of drops and bubbles at large Reynolds numbers
NASA Astrophysics Data System (ADS)
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.
Experimental investigations of He II flows at high Reynolds number
NASA Technical Reports Server (NTRS)
Van Sciver, Steve W.
1991-01-01
Fluid dynamics studies of He II at high Reynolds number (Re(d) greater than 10 exp 6) reveal characteristics which are best interpreted in terms of classical scaling relationships. In particular, the smooth tube friction factor is seen to correlate with the Von Karman-Nikuradse formulation. Also, the performance of a centrifugal pump is unchanged whether being used with He I or He II. These effects are expected to result provided the He II possesses a viscous sublayer and that the drag is determined by laminar flow within this layer. On the other hand, heat transfer in He II is substantially different from that of He I because of the unique internal convection mechanism present in this quantum fluid. These experiments are performed in the University of Wisconsin liquid helium flow facility which has unique capabilities of He II temperature, pressure and flow.
NASA Technical Reports Server (NTRS)
Weilmuenster, K. J.
1974-01-01
Experimental measurements of boundary-layer transition in an expansion-tube test-gas flow are presented along with radial distributions of pitot pressure. An integral method for calculating constant Reynolds number lines for an expansion-tube flow is introduced. Comparison of experimental data and constant Reynolds number calculations has shown that for given conditions, wall boundary-layer transition occurs at a constant Reynolds number in an expansion-tube flow. Operating conditions in the expansion tube were chosen so that the effects of test-gas nonequilibrium on boundary-layer transition could be studied.
Flow in glass models of arterial bifurcations and berry aneurysms at low Reynolds numbers.
Stehbens, W E
1975-07-01
Using the dye injection technique, flow at low Reynolds numbers was studied in glass models which simulated arterial bifurcations of varying angles and had varying bluntness of the apex or crotch. Forks bearing small saccular evaginations simulating berry aneurysms were investigated similarly.. At quite low Reynolds numbers in all the models small wave motion was observed at the forks. At higher Reynolds numbers and below the critical values for turbulence, larger Vortices akin to a Kármán vortex street were shed from most of the forks. The disturbances were attributed to boundary layer separation and a jet-edge effect.
The Aerodynamics of Deforming Wings at Low Reynolds Number
NASA Astrophysics Data System (ADS)
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.
Strouhal-Reynolds number relationship for vortex streets.
Ponta, Fernando L; Aref, Hassan
2004-08-20
A rationale for the empirically observed Strouhal-Reynolds number relation for vortex shedding in the wake of a cylinder is provided. This rationale derives from a mechanism of vortex formation observed in numerical simulations of two-dimensional vortex shedding coupled with an order of magnitude estimate of the terms in the vorticity transport equation based on this mechanism.
Probability density distribution of velocity differences at high Reynolds numbers
NASA Technical Reports Server (NTRS)
Praskovsky, Alexander A.
1993-01-01
Recent understanding of fine-scale turbulence structure in high Reynolds number flows is mostly based on Kolmogorov's original and revised models. The main finding of these models is that intrinsic characteristics of fine-scale fluctuations are universal ones at high Reynolds numbers, i.e., the functional behavior of any small-scale parameter is the same in all flows if the Reynolds number is high enough. The only large-scale quantity that directly affects small-scale fluctuations is the energy flux through a cascade. In dynamical equilibrium between large- and small-scale motions, this flux is equal to the mean rate of energy dissipation epsilon. The pdd of velocity difference is a very important characteristic for both the basic understanding of fully developed turbulence and engineering problems. Hence, it is important to test the findings: (1) the functional behavior of the tails of the probability density distribution (pdd) represented by P(delta(u)) is proportional to exp(-b(r) absolute value of delta(u)/sigma(sub delta(u))) and (2) the logarithmic decrement b(r) scales as b(r) is proportional to r(sup 0.15) when separation r lies in the inertial subrange in high Reynolds number laboratory shear flows.
Prediction of Very High Reynolds Number Compressible Skin Friction
NASA Technical Reports Server (NTRS)
Carlson, John R.
1998-01-01
Flat plate skin friction calculations over a range of Mach numbers from 0.4 to 3.5 at Reynolds numbers from 16 million to 492 million using a Navier Stokes method with advanced turbulence modeling are compared with incompressible skin friction coefficient correlations. The semi-empirical correlation theories of van Driest; Cope; Winkler and Cha; and Sommer and Short T' are used to transform the predicted skin friction coefficients of solutions using two algebraic Reynolds stress turbulence models in the Navier-Stokes method PAB3D. In general, the predicted skin friction coefficients scaled well with each reference temperature theory though, overall the theory by Sommer and Short appeared to best collapse the predicted coefficients. At the lower Reynolds number 3 to 30 million, both the Girimaji and Shih, Zhu and Lumley turbulence models predicted skin-friction coefficients within 2% of the semi-empirical correlation skin friction coefficients. At the higher Reynolds numbers of 100 to 500 million, the turbulence models by Shih, Zhu and Lumley and Girimaji predicted coefficients that were 6% less and 10% greater, respectively, than the semi-empirical coefficients.
Bubble shapes in steady axisymmetric flows at intermediate Reynolds number
NASA Technical Reports Server (NTRS)
Ryskin, G.; Leal, L. G.
1982-01-01
The shape of a gas bubble which rises through a quiescent incompressible, Newtonian fluid at intermediate Reynolds numbers is considered. Exact numerical solutions for the velocity and pressure fields, as well as the bubble shape, are obtained using finite difference techniques and a numerically generated transformation to an orthogonal, boundary-fitted coordinate system. No restriction is placed on the allowable magnitude of deformation.
NASA Technical Reports Server (NTRS)
Mueller, Thomas J.
1989-01-01
As a result of the continued interest in designing efficient low Reynolds number systems, the University of Notre Dame decided to organize a Conference on Low Reynolds Number Aerodynamics in June 1989. This Conference followed the 1986 International Conference in London by about three years and the first Conference on Low Reynolds Number Airfoil Aerodynamics at Notre Dame in 1985 by four years. The emphasis of the 1989 Conference was to assess the state-of-the-art in the chord Reynolds number range from about 10,000 to about 700,000. Applications of current interest include high altitude remotely or robotically piloted vehicles, ultra-light, and human powered vehicles as well as mini-RPVs at low altitudes. Other examples include small axial-flow fans used to cool electronic equipment in the unpressurized sections of high-altitude aircraft and gas turbine blades. High Reynolds number airfoil design strategies attempt to control the onset and development of turbulent boundary layers. This is difficult at low Reynolds numbers because of the increased stability of attached laminar boundary layers. Therefore, laminar separation is common even at small angles of attack at low Reynolds numbers. Under these conditions, the development of a turbulent boundary layer usually depends on the formation of a transitional separation bubble. The purpose of this Conference on Low Reynolds Number Aerodynamics was to bring together those researchers who have been active in areas closely related to this subject. It is clear from the papers presented that a great deal of progress has been made in understanding the occurrence and behavior of laminar separation and transition as well as their overall effect on the performance of airfoils at low chord Reynolds numbers. This progress has brought us closer to our goal of improving analytical methods for the design and evaluation of a variety of practical applications.
Reynolds Number Scaling and Parameterization of Stratified Turbulent Wakes
2017-04-17
predictor models linking surface- observed wavelengths to body-generating depth and passage time have been constructed . High Reynolds/Froude number...Froude numbers, the patterns of subsurface motion observed above a stratified turbulent wake with the flow patterns inside the wake core to infer...simulations into three broader categories, based on the observed particle orbits within the reflection region (Figures 2 to 4). Type-A trajectories
Method for producing a uniform, low Reynolds number jet
NASA Technical Reports Server (NTRS)
Greene, G. C.; Brink, D. F.
1974-01-01
In one attempt to produce a simple inexpensive nozzle, a 2-in. diam plate with 37 holes was investigated (Stadler, 1960), anticipating that the small jets emanating from the plate would combine to form a uniform stream. This experiment was unsuccessful because a uniform flow was not established until the flow had progressed many nozzle diameters downstream. However, an extension of this concept to a much larger number of very small jets, viz., a porous plate, did provide a method for producing a uniform, low Reynolds number jet almost immediately downstream of the nozzle (Greene, 1973). The method is described and some typical jet velocity profiles for nozzle Reynolds numbers from 50 to 1000 are given.
NASA Technical Reports Server (NTRS)
Ventrice, M.
1979-01-01
The amplification of a Reynolds number dependent process by wave distortion and the possibility of applying the results to other similar Reynolds number dependent processes were investigated. The process investigated was that associated with the operation of a constant-temperature hot-wire anemometer. The application of vaporization limited combustion, the type of combustion typically associated with liquid propellant rocket engines, was studied. A series of experiments were carried out to determine the effect of wave distortion on a Reynolds number dependent process and to establish the analogy between the anemometer process and the combustion process. Parametric trends, behavior common to different chamber geometries, and stability boundaries were identified. The results indicate a high degree of similarity between the two processes and the possibility of using the anemometer system to investigate combustion instability. The nonlinear aspects of a Reynolds number dependent process appear to be the dominant mechanisms controlling instability.
1990-01-01
pressure gradient and a strong (relative to the mild case) pressure gradient. The objeetive-of the study was to determine the effect of pressure... determine the various boundary layer integral parameters and to create velocity records needed to calculate the period of the bursting cycle. Over the range...role in the production of turbulence and in momentum transport. For this reason, this study undertakes to determine the effect of pressure gradient on
Influence of Reynolds number on performance modeling of horizontal axis wind rotors
Musial, W.D.; Cromack, D.E.
1988-05-01
This paper investigates the influence of Reynolds number on performance modeling of horizontal axis wind rotors. A procedure for accounting for Reynolds number effects on airfoil section models was developed and implemented for NACA 0012 and NACA 4415 profiles; both of these models is valid through angles of attack up to 90 deg and for Reynolds numbers ranging from 4 x 10/sup 4/ to 3 x 10/sup 6/. These models were incorporated into both a lifting line computer code, LL200R, adapted for this report. This enabled greater uncertainty to be obtained in evaluating theoretical performance codes with respect to actual data, as well as providing a means by which a parametric analysis of the relative effects of Re changes on rotor performance to be performed. The use of low Reynolds number section data was found to significantly lower the predicted values of power coefficient, particularly at off-design tip speed-ratios. For symmetrical airfoils, this effect on performance was only significant for low tip-speed- ratios, while cambered airfoils were affected more uniformly at all operating conditions. Changes in performance were induced by parametric variations of wind speed, rotor scale, and rotor generating mode using the Reynolds number dependent section models. Results show that wind speed variations are more significant for smaller rotors at lower wind-speeds, and section models represented at only a single Reynolds number are more suitable for the analysis of constant RPM rotors.
NASA Astrophysics Data System (ADS)
Wallen, G.
The Reynolds number influence of a five-hole sonde with spherical head was investigated. The sonde shaft was designed with a narrow cross section like that occurring in turbines between the impellers. The sonde was calibrated for Mach numbers 0.6 and 0.8 with 25 different initial directions of flow inside a closed wind tunnel. This was done at three Reynolds numbers. The static pressure showed a departure from the wall pressure amounting to six percent of the kinetic pressure. Near the wall the flow direction deviated by 9.5 degrees from the axial direction. The turbulence and flow speed profile were also studied. A gradient correction was made taking into account the exact positions of the measurement intrusions into the flow.
Wall-bounded turbulence at high Reynolds numbers
NASA Astrophysics Data System (ADS)
Vallikivi, Margit
Measurements are reported that give new insight into the behavior of turbulent wall-bounded flows at high Reynolds number. Turbulent pipe and boundary layer flows are examined experimentally over a wide range of Reynolds numbers -- up to Retau=100,000 (Re D=6x106) in pipe flow, and up to Re tau=73,000 (ReD=235x103) in a flat plate zero pressure gradient boundary layer. A Nano-Scale Thermal Anemometry Probe (NSTAP) was developed for very high spatial and temporal resolution measurements. Sensors with wire lengths 30 and 60 mum were fabricated, tested and validated in known flows, and then used to obtain single-point measurements at high Reynolds numbers in pipe and boundary layers. The mean velocity data together with data from previous studies and extensive error analysis showed that the von Karman's constant in the log-law is kappa=0.40+/-0.02. It was shown that the streamwise Reynolds stress exhibits a logarithmic behavior in the inertial sublayer for Retau≥20,000, in both pipes and boundary layers. Variances as well as higher order even moments were compared for pipes and boundary layers and it was shown that all even moments have a logarithmic behavior in the inertial sublayer, suggesting a true scale separation. Streamwise turbulent spectra showed a clear k --5/3 region for up to two decades in wavenumber. No k--1 region was found to be present in any of the cases in the pipe or the boundary layer. The location of the outer spectral peak, associated with very large scale motions, was found to have only a weak dependence on Reynolds number. The loci of these peak occur at the same wall-normal distance where the streamwise stresses establish a logarithmic behavior and where the amplitude modulation coefficient has a zero value. This suggests that with Reynolds number increasing to infinity most of the energy is contained within a diminishing wall-layer in physical coordinates.
1988-03-01
The Theory And Design Of Gas Turbines And Jet Engines . New York McGraw-Hill Book Company Incorporated, 1950. 14. Evans, R. L. The Effects Of Free...Fulfillment of the Requirement for the degree of Master of Science in Aeronautical Engineering SQN.LDR (MAJ.) Salman Absar Pakistan Air Force March 1988...center blade . Variation of the above parameters was studied for a cascade with NACA 65-A506 aerofoils . Three different categories of surface roughness
NASA Technical Reports Server (NTRS)
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.
The Variation of Slat Noise with Mach and Reynolds Numbers
NASA Technical Reports Server (NTRS)
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.
Reynolds number scaling of velocity increments in isotropic turbulence
NASA Astrophysics Data System (ADS)
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.
Turbulence Model Selection for Low Reynolds Number Flows
2016-01-01
One of the major flow phenomena associated with low Reynolds number flow is the formation of separation bubbles on an airfoil’s surface. NACA4415 airfoil is commonly used in wind turbines and UAV applications. The stall characteristics are gradual compared to thin airfoils. The primary criterion set for this work is the capture of laminar separation bubble. Flow is simulated for a Reynolds number of 120,000. The numerical analysis carried out shows the advantages and disadvantages of a few turbulence models. The turbulence models tested were: one equation Spallart Allmars (S-A), two equation SST K-ω, three equation Intermittency (γ) SST, k-kl-ω and finally, the four equation transition γ-Reθ SST. However, the variation in flow physics differs between these turbulence models. Procedure to establish the accuracy of the simulation, in accord with previous experimental results, has been discussed in detail. PMID:27104354
Quasi-static magnetohydrodynamic turbulence at high Reynolds number
NASA Astrophysics Data System (ADS)
Delache, A.; Favier, B.; Godeferd, F. S.; Cambon, C.; Bos, W. J. T.
2011-12-01
We analyse the anisotropy of turbulence in an electrically conducting fluid submitted to a uniform magnetic field, for low magnetic Reynolds number, using the quasi-static approximation. In the linear limit, the kinetic energy of velocity components normal to the magnetic field decays faster than the kinetic energy of the component along the magnetic field (Moffatt, 1967). However, numerous numerical studies predict a different behaviour, wherein the final state is characterised by dominant horizontal energy. We investigate the corresponding nonlinear phenomenon using Direct Numerical Simulations (DNS) and spectral closures based on Eddy Damping Quasi-Normal Markovian (EDQNM) models. The initial temporal evolution of the decaying flow indicates that the turbulence is very similar to the so-called "two-and-a-half-dimensional" flow (Montgomery & Turner, 1982) which explains the observations in numerical studies. EDQNM models confirm this statement at higher Reynolds number.
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.
NASA Technical Reports Server (NTRS)
Holland, Scott D.; Murphy, Kelly J.
1993-01-01
The effects of the decrease in the ratio of specific heats have been investigated in generic 3D 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 and in the Langley 15-Inch Mach 6 Air Tunnel in perfect gas air. 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.
Local anisotropy in strained turbulence at high Reynolds numbers
NASA Technical Reports Server (NTRS)
Durbin, P. A.; Speziale, C. G.
1991-01-01
It is shown that the hypothesis of local isotropy is implausible in the presence of significant mean rates of strain. In fact, it appears that in uniform shear flow near equilibrium, local isotropy can never constitute a systematic approximation, even in the limit of infinite Reynolds number. An estimate of the level of mean strain rate for which local isotropy is formally a good approximation is provided.
Optical diagnostic investigation of low Reynolds number nozzle flows
NASA Technical Reports Server (NTRS)
Micci, Michael M.
1991-01-01
The objectives are to obtain temperature, density and velocity profile measurements in the expansion region of low Reynolds number nozzles through the use of optical diagnostics. An LIF system will be used to probe the expansion of a microwave-heated expansion in the Center vacuum facility. The experimental measurements made in this program will be compared to numerical predictions obtained by Drs. Charles Merkle and Lyle Long.
High Reynolds Number Transition Experiments in ETW (TELFONA project)
NASA Astrophysics Data System (ADS)
Perraud, J.; Archambaud, J.-P.; Schrauf, G.; Donelli, R. S.; Hanifi, A.; Quest, J.; Hein, S.; Streit, T.; Fey, U.; Egami, Y.
A wind-tunnel experiment on laminar-turbulent transition has been performed in ETW (the European Transonic Wind Tunnel in Koln) at high Reynolds number and cryogenic conditions. The studied geometry is a sting mounted full model in swept-wing configuration. The transition location was determined by means of Temperature Sensitive Paint (CryoTSP). The experimental observations were further analysed using different transition prediction tools, based on linear stability theory.
Aerodynamic Performances of Corrugated Dragonfly Wings at Low Reynolds Numbers
NASA Astrophysics Data System (ADS)
Tamai, Masatoshi; He, Guowei; Hu, Hui
2006-11-01
The cross-sections of dragonfly wings have well-defined corrugated configurations, which seem to be not very suitable for flight according to traditional airfoil design principles. However, previous studies have led to surprising conclusions of that corrugated dragonfly wings would have better aerodynamic performances compared with traditional technical airfoils in the low Reynolds number regime where dragonflies usually fly. Unlike most of the previous studies of either measuring total aerodynamics forces (lift and drag) or conducting qualitative flow visualization, a series of wind tunnel experiments will be conducted in the present study to investigate the aerodynamic performances of corrugated dragonfly wings at low Reynolds numbers quantitatively. In addition to aerodynamics force measurements, detailed Particle Image Velocimetry (PIV) measurements will be conducted to quantify of the flow field around a two-dimensional corrugated dragonfly wing model to elucidate the fundamental physics associated with the flight features and aerodynamic performances of corrugated dragonfly wings. The aerodynamic performances of the dragonfly wing model will be compared with those of a simple flat plate and a NASA low-speed airfoil at low Reynolds numbers.
NASA Technical Reports Server (NTRS)
Foley, J. E.
1972-01-01
An experimental program was conducted to survey the lee side vortex flow field about an ogive-cylinder-frustum-cylinder at angles of attack to 25 degrees for two Reynolds numbers at Mach number 0.8, and one Reynolds number at Mach number 1.96. The data were obtained using miniature 5-port conical pressure probes calibrated for angle of attack and roll angle over a Mach number range of 0.6 to 3.0. The results are presented here as local flow field properties and circulation strengths for various body stations.
Vortex wake of tip loaded rotors at low Reynolds numbers
NASA Astrophysics Data System (ADS)
Savas, Omer; Bilgi, Onur
2016-11-01
The effect of tip tabs on the flow characteristics of a three bladed rotor is investigated using strain gauge thrust measurements, flow visualization and particle image velocimetry at chord Reynolds numbers of 0 . 4 - 2 . 9 ×105 . The tab angles of attack of 0 , -/+3° & -/+5° with respect to the rotation of the rotor are used to vary the tip loading. The rotor wakes and thrust characteristics at positive angles of attack, when the tip loading is outward, are qualitatively similar to those with no-tabs. In contrast, when the tip loading is inward at zero and negative angles of attack, the vortex wake is radically altered; the thrust nearly vanishes, even reverses with increasing inward loading. The key factors influencing the behavior of the wake are the vortex system off the tabs and their associated downwash, which is inward for the outward tab loading and causes increased volume and momentum flux and outward for the inward tab loading and causes expansion of the wake and nearly complete loss of thrust. At negative angles of attack, the flow fields exhibit a quasi-steady bound ring vortex system around at the edge of the rotor disk and the flow direction on the pressure side of the rotor disk reverses: it flows toward the rotor disk.
Stability Characteristics of Low Reynolds Number, Low Aspect Ratio Wings
NASA Astrophysics Data System (ADS)
Shields, Matthew; Mohseni, Kamran
2010-11-01
The recent interest in Micro Aerial Vehicles (MAVs) has led to the development of many different aircraft; however, little progress has been made in understanding the physics of MAV flow. MAVs aerodynamics is affected by low Reynolds number flow and low aspect ratios. As a result nonlinear effects due to tip vortices are quite important. We have developed a new experimental setup for measuring stability derivatives in a small wind tunnel. Using a four degree of freedom actuation system, a model can be placed in the test section and maneuvered in such a way to isolate the flow components responsible for creating stability derivatives. Accurate measurements of the aerodynamic loading can then be used to compute these values. Initial testing was conducted primarily on a series of flat plates of different aspect ratios. In addition, the CU MAV was tested as a specific case study. Test results indicate that some of the cross coupled stability derivatives, ignored for larger aircrafts, are on the same order of magnitude as standard derivatives and thus can not be ignored in the derivation of the linear equations of motion for a micro aerial vehicle. As a result, a more general set of equations of motion are derived based upon experimentally obtained stability derivatives.
Low Reynolds Number Drag Alteration Inspired by Butterfly Scales
NASA Astrophysics Data System (ADS)
Laforte, Brent; Kronenberger, Courtney; Lang, Amy
2012-11-01
Biomimetics is the process of looking towards nature's adaptations for answers to today's engineering obstacles. An age-old engineering dilemma is trying to find new methods to reduce the amount of drag over a body. This research finds inspiration from butterfly scales which are hypothesized to alter surface friction over the wings. Drop testing was performed on axisymmetric, streamlined, teardrop models which were rapid-prototyped such that the surface was covered with either streamwise or transverse cavities modeled after the Monarch butterfly. The drop tank contained silicone oil with a viscosity two hundred times that of water insuring flow similarity between the model cavities (2.5 mm cavity depth) and the butterfly scale structures (about 30 microns cavity depth). A variation in Reynolds number was achieved by altering the model weight such that terminal speeds ranged from 5 to 70 cm/s. Results showed a reduction in surface friction for the transverse cavity configurations based on the roller-bearing effect. These findings suggest that the cavity shape and ratio is directly correlated to the amount of drag alteration. Funded by NSF REU grant 1062611.
Linearized pipe flow to Reynolds number 10 7
NASA Astrophysics Data System (ADS)
Meseguer, Á.; Trefethen, L. N.
2003-03-01
A Fourier-Chebyshev Petrov-Galerkin spectral method is described for high-accuracy computation of linearized dynamics for flow in an infinite circular pipe. Our code is unusual in being based on solenoidal velocity variables and in being written in MATLAB. Systematic studies are presented of the dependence of eigenvalues, transient growth factors, and other quantities on the axial and azimuthal wave numbers and the Reynolds number R for R ranging from 10 2 to the idealized (physically unrealizable) value 10 7. Implications for transition to turbulence are considered in the light of recent theoretical results of S.J. Chapman.
Dynamics of Active Separation Control at High Reynolds Numbers
NASA Technical Reports Server (NTRS)
Pack, LaTunia G.; Seifert, Avi
2000-01-01
A series of active flow control experiments were recently conducted at high Reynolds numbers on a generic separated configuration. The model simulates the upper surface of a 20% thick Glauert-Goldschmied type airfoil at zero angle of attack. The flow is fully turbulent since the tunnel sidewall boundary layer flows over the model. The main motivation for the experiments is to generate a comprehensive data base for validation of unsteady numerical simulation as a first step in the development of a CFD design tool, without which it would not be possible to effectively utilize the great potential of unsteady flow control. This paper focuses on the dynamics of several key features of the baseline as well as the controlled flow. It was found that the thickness of the upstream boundary layer has a negligible effect on the flow dynamics. It is speculated that separation is caused mainly by the highly convex surface while viscous effects are less important. The two-dimensional separated flow contains unsteady waves centered on a reduced frequency of 0.8, while in the three dimensional separated flow, frequencies around a reduced frequency of 0.3 and 1 are active. Several scenarios of resonant wave interaction take place at the separated shear-layer and in the pressure recovery region. The unstable reduced frequency bands for periodic excitation are centered on 1.5 and 5, but these reduced frequencies are based on the length of the baseline bubble that shortens due to the excitation. The conventional swept wing-scaling works well for the coherent wave features. Reproduction of these dynamic effects by a numerical simulation would provide benchmark validation.
Multiscale structures of resistive magnetic reconnection at high magnetic Reynolds numbers
NASA Astrophysics Data System (ADS)
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.
Numerical Study of Unsteady Low-Reynolds Number Wing Performance
2008-02-29
Number," AIAA Paper 2007-4233, June 2007. 2 Krist, S . L., Biedron , R. T., and Rumsey, C. L., "CFL3D User’s Manual," NASA TM 208444, 1998. 3 McGowan, G ...1 (d) Exp, € -- 4 (e) CFL3D, ¢ = 4 (f) 1B, 0 -+ ( g ) E x p o v 2o ( h ) F Lr o e 1 2 t i ) , a i me a p x m ey 0 c r la s . e g e in t FT1 Flt T...Low-Reynolds Number Wing Performance 5b. GRANT NUMBER FA9550-06-1-0265 Sc. PROGRAM ELEMENT NUMBER 6. AUTHOR( S ) Sd. PROJECT NUMBER Gopalarathnam, Ashok
Vortex shedding from a hydrofoil at high Reynolds number
NASA Astrophysics Data System (ADS)
Bourgoyne, Dwayne A.; Ceccio, Steven L.; Dowling, David R.
2005-05-01
High Reynolds number (Re) wall-bounded turbulent flows occur in many hydro- and aerodynamic applications. However, the limited amount of high-Re experimental data has hampered the development and validation of scaling laws and modelling techniques applicable to such flows. This paper presents measurements of the turbulent flow near the trailing edge of a two-dimensional lifting surface at chord-based Reynolds numbers, Re_{C}, typical of heavy-lift aircraft wings and full-scale ship propellers. The experiments were conducted in the William B. Morgan Large Cavitation Channel at flow speeds from 0.50 to 18.3ms(-1) with a cambered hydrofoil having a 3.05m span and a 2.13m chord that generated 60 metric tons of lift at the highest flow speed, Re_{C}{≈}50{×}10(6) . Flow-field measurements concentrated on the foil's near wake and include results from trailing edges having terminating bevel angles of 44(°) and 56(°) . Although generic turbulent boundary layer and wake characteristics were found at any fixed Re_{C} in the trailing-edge region, the variable strength of near-wake vortex shedding caused the flow-field fluctuations to be Reynolds-number and trailing-edge-geometry dependent. In the current experiments, vortex-shedding strength peaked at Re_{C}{=}4{×}10(6) with the 56(°) bevel-angle trailing edge. A dimensionless scaling for this phenomenon constructed from the free-stream speed, the wake thickness, and an average suction-side shear-layer vorticity at the trailing edge collapses the vortex-shedding strength measurements for 1.4{×}10(6}{≤) Re_{C}{≤}50{×}10(6) from both trailing edges and from prior measurements on two-dimensional struts at Re_{C}{˜}2{×}10(6) with asymmetrical trailing edges.
Axisymmetric bubble pinch-off at high Reynolds numbers.
Gordillo, J M; Sevilla, A; Rodríguez-Rodríguez, J; Martínez-Bazán, C
2005-11-04
Analytical considerations and potential-flow numerical simulations of the pinch-off of bubbles at high Reynolds numbers reveal that the bubble minimum radius, rn, decreases as tau proportional to r2n sqrt[1lnr2n], where tau is the time to break up, when the local shape of the bubble near the singularity is symmetric. However, if the gas convective terms in the momentum equation become of the order of those of the liquid, the bubble shape is no longer symmetric and the evolution of the neck changes to a rn proportional to tau1/3 power law. These findings are verified experimentally.
Characterizing elastic turbulence in channel flows at low Reynolds number
NASA Astrophysics Data System (ADS)
Qin, Boyang; Arratia, Paulo E.
2017-08-01
We experimentally investigate the flow of a viscoelastic fluid in a parallel shear geometry at low Reynolds number. As the flow becomes unstable via a nonlinear subcritical instability, velocimetry measurements show nonperiodic fluctuations over a broad range of frequencies and wavelengths, consistent with the main features of elastic turbulence. Using the same experimental setup, we compare these features to those in the flow around cylinders, which is upstream of the parallel shear region; we find significant differences in power spectrum scaling, intermittency statistics, and flow structures. We propose a simple mechanism to explain the growth of velocity fluctuations in parallel shear flows based on polymer stretching due to fluctuations in streamwise velocity gradients.
Anisotropy of MHD Turbulence at Low Magnetic Reynolds Number
NASA Technical Reports Server (NTRS)
Zikanov, O.; Vorobev, A.; Thess, A.; Davidson, P. A.; Knaepen, B.
2004-01-01
Turbulent fluctuations in MHD flows are known to become dimensionally anisotropic under the action of a sufficiently strong magnetic field. We consider the technologically relevant case of low magnetic Reynolds number and apply the method of DNS of forced flow in a periodic box to generate velocity fields. The analysis based on different anisotropy characteristics shows that the dimensional anisotropy is virtually scale-independent. We also find that, except for the case of very strong magnetic field, the flow is componentally isotropic. Its kinetic energy is practically uniformly distributed among the velocity components.
Computation of high Reynolds number internal/external flows
NASA Technical Reports Server (NTRS)
Cline, M. C.; Wilmoth, R. G.
1981-01-01
A general, user oriented computer program, called VNAP2, was developed to calculate high Reynolds number, internal/ external flows. The VNAP2 program solves the two dimensional, time dependent Navier-Stokes equations. The turbulence is modeled with either a mixing-length, a one transport equation, or a two transport equation model. Interior grid points are computed using the explicit MacCormack Scheme with special procedures to speed up the calculation in the fine grid. All boundary conditions are calculated using a reference plane characteristic scheme with the viscous terms treated as source terms. Several internal, external, and internal/external flow calculations are presented.
Wave phenomena in a high Reynolds number compressible boundary layer
NASA Technical Reports Server (NTRS)
Bayliss, A.; Maestrello, L.; Parikh, P.; Turkel, E.
1987-01-01
The behavior of spatially unstable waves in a high Reynolds number compressible laminar boundary layer is investigated by solution of the laminar two-dimensional compressible Navier-Stokes equations (solved to fourth-order accuracy) over a flat plate with a fluctuating disturbance generated at the inflow. A significant nonlinear distortion is produced, in qualitative agreement with experimental data. It is shown that increasing compressibility can significantly stabilize the flow over a flat plate, and that the mechanism of phase cancellation is a viable mechanism for the control of growing disturbances.
Computation of high Reynolds number internal/external flows
NASA Technical Reports Server (NTRS)
Cline, M. C.; Wilmoth, R. G.
1981-01-01
A general, user oriented computer program, called VNAF2, developed to calculate high Reynolds number internal/external flows is described. The program solves the two dimensional, time dependent Navier-Stokes equations. Turbulence is modeled with either a mixing length, a one transport equation, or a two transport equation model. Interior grid points are computed using the explicit MacCormack scheme with special procedures to speed up the calculation in the fine grid. All boundary conditions are calculated using a reference plane characteristic scheme with the viscous terms treated as source terms. Several internal, external, and internal/external flow calculations are presented.
High Reynolds number pump facility for cavitation research
NASA Astrophysics Data System (ADS)
Farrell, K. J.; McBride, M. W.; Billet, M. L.
1987-09-01
A High Reynolds Number Pump Facility (HIREP) designed for cavitation studies in the bladetip/endwall region of an axial flow pump is described. The facility consists of a 1.07-m diameter pump state driven by a 1.22-m diameter downstream turbine. An incompressible Reynolds Number of 6,000,000 at the rotor tip is achievable. The two units rotate on a common shaft and operate in the 1.22-m diameter test section of the Garfield Thomas Water Tunnel of the Applied Research Laboratory at Penn State. The facility was designed to accommodate laser velocimeter (LV) measurements in the pump stage, radially traversing five-hole probes in every stage, and a number of transducers in the rotating frame of reference: steady and unsteady pressure transducers force and torque cells, and accelerometers. The latter capability is provided by a slip-ring unit and hollow blade passage ways for conductors from the instrumentation in rotor-tip region. An optical quality window for LV measurements and other windows and ports are available for visual observation and instrumentation access.
Relevancy of the buoyancy Reynolds number in stably stratified turbulence
NASA Astrophysics Data System (ADS)
Mater, Benjamin; Venayagamoorthy, Subhas Karan
2013-11-01
The buoyancy Reynolds number, Reb = ɛ / (νN2) , has become a widely popular parameter with which to describe turbulent mixing in the stratified environment of the open ocean. This popularity has arisen largely on the practical grounds that the constituent quantities are available through common measurement techniques: estimates of turbulent kinetic energy dissipation (ɛ) are available from observations of fine-scale shear, and the buoyancy frequency (N) can be determined from profiles of density. Despite practical appeal, however, Reb is ambiguous in that it fails to distinguish between regimes of weak stratification and strong turbulence. This becomes obvious in the formulation Reb = ReL(Frk) 2 , where ReL =k2 / (ɛν) is a turbulent Reynolds number, Frk = ɛ / (Nk) is a turbulent Froude number, and k is the turbulent kinetic energy. In considering both ReL and Frk independently, the time scale of the turbulence, TL = k / ɛ , is made explicit. We explore the duality of Reb in describing mixing efficiency using a ReL - Frk parameter space and argue the importance of TL in parameterization of flow behavior. Data from direct numerical simulations, laboratory experiments, and field observations are considered. This work funded by the Office of Naval Research.
NASA Astrophysics Data System (ADS)
Revil, A.; Linde, N.
2006-12-01
We determine the macroscopic transport properties of isotropic microporous media by volume-averaging the local Nernst-Planck and Navier-Stokes equations in non-isothermal conditions incorporating streaming, diffusional, and thermal effects. In porous media, the excess of charge, that counterbalance the charge deficiency of the surface of the minerals, is partitioned between the Gouy-Chapman and the Stern layer. Rather than using Poisson-Boltzmann distributions to describe the ionic concentrations in the pore space of the medium, we rely on Donnan distributions obtained by equating the chemical potentials of the water molecules and ions between a reservoir of ions and the pore space of the medium. The macroscopic Maxwell equations and the macroscopic linear constitutive transport equations are derived in the vicinity of equilibrium assuming that the porous material is deformable. In the vicinity of thermodynamic equilibrium, the cross- coupling phenomena of the macroscopic constitutive equations of transport follow Onsager reciprocity. In addition, all the material properties entering the constitutive equations only depend on two textural properties, the permeability and the electrical formation factor. Extension of this model is then performed incorporating three distinct additional effects (a) extension of the electrokinetic equations at high Reynolds numbers, in the inertial laminar flow regime ; (b) extension of the model to multi-phase flow conditions (under the assumptions that the two fluid phases are continuous), and (3) introduction of electro-redox theory and the development of a fundamental model of bio-battery with application to contaminant plumes. Applications will be discussed shortly regarding (1) fracturing, (2) ground water flow, (c) leakage in embankment dams, and (d) contaminant plumes rich in organic matter (http://www.andre-revil.com).
Unsteady aerodynamic models for agile flight at low Reynolds numbers
NASA Astrophysics Data System (ADS)
Brunton, Steven L.
This work develops low-order models for the unsteady aerodynamic forces on a wing in response to agile maneuvers at low Reynolds number. Model performance is assessed on the basis of accuracy across a range of parameters and frequencies as well as of computational efficiency and compatibility with existing control techniques and flight dynamic models. The result is a flexible modeling procedure that yields accurate, low-dimensional, state-space models. The modeling procedures are developed and tested on direct numerical simulations of a two-dimensional flat plate airfoil in motion at low Reynolds number, Re=100, and in a wind tunnel experiment at the Illinois Institute of Technology involving a NACA 0006 airfoil pitching and plunging at Reynolds number Re=65,000. In both instances, low-order models are obtained that accurately capture the unsteady aerodynamic forces at all frequencies. These cases demonstrate the utility of the modeling procedure developed in this thesis for obtaining accurate models for different geometries and Reynolds numbers. Linear reduced-order models are constructed from either the indicial response (step response) or realistic input/output maneuvers using a flexible modeling procedure. The method is based on identifying stability derivatives and modeling the remaining dynamics with the eigensystem realization algorithm. A hierarchy of models is developed, based on linearizing the flow at various operating conditions. These models are shown to be accurate and efficient for plunging, pitching about various points, and combined pitch and plunge maneuvers, at various angle of attack and Reynolds number. Models are compared against the classical unsteady aerodynamic models of Wagner and Theodorsen over a large range of Strouhal number and reduced frequency for a baseline comparison. Additionally, state-space representations are developed for Wagner's and Theodorsen's models, making them compatible with modern control-system analysis. A number of
Reynolds number dependency of an insect-based flapping wing.
Han, Jong-Seob; Chang, Jo-Won; Kim, Sun-Tae
2014-01-01
Aerodynamic characteristics depending on Reynolds number (Re) ranges were studied to investigate the suitable design parameters of an insect-based micro air vehicle (MAV). The tests centered on the wing rotation timing and Re ranges, and were conducted to understand the lift augmentations and unsteady effects. A dynamically scaled-up flapping wing controlled by a pair of servos was installed underwater with a micro force/torque sensor. A high-speed camera and a laser sheet were also put in front of the water tank for the time-resolved digital particle image velocimetry (DPIV). The lift augmentations clearly appeared at low Re and were well reflected on the insect's flight range. In the case of the high Re, however, the peak standing for the wing–wake interaction was delayed, and the pitching-up rotation was not able to lead to another lift enhancement, i.e., rotational lift. In such Re, the mean CL and the L/D of the advanced rotation were substantially decreased from those of the other rotations. The DPIV results at high Re well described turbulent characteristics such as the irregular, unstable, and high-intensity vortex structures with a short temporal delay. In the advanced rotation, the LEV in the rotational phase could not maintain the attachment. Thus, the rotational lift was not able to work. On the contrary, the temporal response delay benefitted the wing in the delayed rotation. Therefore, the wing in the delayed rotation had both a similar level of the mean CL and a higher marked L/D than those of the advanced rotation. Such results indicate that the high Re could interrupt lift augmentation mechanisms, and these augmentations would not be suitable for a heavier MAV. In conclusion, using adequate wing kinematics to acquire estimations of the weight and range of the Re is highly recommended at the aerodynamic design step.
High-Reynolds Number Taylor-Couette Turbulence
NASA Astrophysics Data System (ADS)
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. One key response of the system is the torque required to retain constant angular velocities, which can be connected to the angular velocity transport through the gap. Whereas the low-Reynolds number regime was well explored in the 1980s and 1990s of the past century, in the fully turbulent regime major research activity developed only in the past decade. In this article, we review this recent progress in our understanding of fully developed Taylor-Couette turbulence from the experimental, numerical, and theoretical points of view. We focus on the parameter dependence of the global torque and on the local flow organization, including velocity profiles and boundary layers. Next, we discuss transitions between different (turbulent) flow states. We also elaborate on the relevance of this system for astrophysical disks (quasi-Keplerian flows). The review ends with a list of challenges for future research on turbulent Taylor-Couette flow.
Design of a High-Reynolds Number Recirculating Water Tunnel
NASA Astrophysics Data System (ADS)
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.
The random walk of a low-Reynolds-number swimmer
NASA Astrophysics Data System (ADS)
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.
Fabrication and control of simple low Reynolds number microswimmers
NASA Astrophysics Data System (ADS)
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
NASA Astrophysics Data System (ADS)
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
A coin vibrational motor swimming at low Reynolds number
NASA Astrophysics Data System (ADS)
Quillen, Alice C.; Askari, Hesam; Kelley, Douglas H.; Friedmann, Tamar; Oakes, Patrick W.
2016-12-01
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 oscillatory motions 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 oscillations of the motor 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 velocity V stream Re s 1/2 U 0 where U 0 is the velocity of surface oscillations, and streaming Reynolds number Re s = U 0 2 /( ων) for motor angular frequency ω and fluid kinematic viscosity ν.
Simulations of Three-dimensional Droplet Deformation in a Square-Duct at Moderate Reynolds numbers
NASA Astrophysics Data System (ADS)
Horwitz, Jeremy; Kumar, Purushotam; Vanka, Pratap
2013-11-01
We present results of numerical simulations of deformation of a confined droplet in a three-dimensional square-duct flow using a multiphase Lattice Boltzmann Method. We have studied the effects of capillary number, Reynolds number, and viscosity ratio on the droplet deformation characteristics. Unlike in the Stokes' limit where deformation is governed by a competition between viscous shear and interfacial tension, at higher Reynolds numbers, inertial effects play an increasingly important role. We observe that the deformation history is non-monotonic and contains an overshoot before relaxing to a steady deformed state. In contrast, the capillary number is seen to affect the magnitude of the deformation history and the time at which the peak deformation occurs. The viscosity ratio has a relatively modest effect on the magnitude of the deformation compared with the effects of Reynolds and capillary numbers. However, compared with the Reynolds number, the viscosity ratio and capillary number have a significant effect on the time to reach a steady state. University of Illinois at Urbana-Champaign, Air Conditioning and Refrigeration Center.
A survey of the three-dimensional high Reynolds number transonic wind tunnel
NASA Technical Reports Server (NTRS)
Takashima, K.; Sawada, H.; Aoki, T.
1982-01-01
The facilities for aerodynamic testing of airplane models at transonic speeds and high Reynolds numbers are surveyed. The need for high Reynolds number testing is reviewed, using some experimental results. Some approaches to high Reynolds number testing such as the cryogenic wind tunnel, the induction driven wind tunnel, the Ludwieg tube, the Evans clean tunnel and the hydraulic driven wind tunnel are described. The level of development of high Reynolds number testing facilities in Japan is discussed.
The influence of Reynolds numbers on resistance properties of jet pumps
Geng, Q.; Zhou, G.; Li, Q.
2014-01-29
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.
The influence of Reynolds numbers on resistance properties of jet pumps
NASA Astrophysics Data System (ADS)
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.
Swimming with stiff legs at low Reynolds number
NASA Astrophysics Data System (ADS)
Takagi, Daisuke
2015-08-01
Locomotion at low Reynolds number is not possible with cycles of reciprocal motion, an example being the oscillation of a single pair of rigid paddles or legs. Here, I demonstrate the possibility of swimming with two or more pairs of legs. They are assumed to oscillate collectively in a metachronal wave pattern in a minimal model based on slender-body theory for Stokes flow. The model predicts locomotion in the direction of the traveling wave, as commonly observed along the body of free-swimming crustaceans. The displacement of the body and the swimming efficiency depend on the number of legs, the amplitude, and the phase of oscillations. This study shows that paddling legs with distinct orientations and phases offers a simple mechanism for driving flow.
Vorticity spectra in high Reynolds number anisotropic turbulence
NASA Astrophysics Data System (ADS)
Morris, Scott C.; Foss, John F.
2005-08-01
Assuming a turbulent flow to be homogeneous and isotropic allows for significant theoretical simplification in the description of its motions. The validity of these assumptions, first put forth by Kolmogorov [A. N. Kolmogorov, "The local structure of turbulence in incompressible viscous fluids for very large Reynolds numbers," C. R. Acad. Sci. URSS 30, 301 (1941)], has been the subject of considerable analytical development and extensive research as they are applied to actual flows. The present investigation describes the one-dimensional vorticity spectra of two flow fields: a single stream shear layer and the near surface region of an atmospheric boundary layer. Both flow fields exhibit a power-law region with a slope of -1 in the one-dimensional spectra of the spanwise component of vorticity in the same wave-number range for which the velocity spectra indicated the isotropic behavior. This is in strong disagreement with the isotropic prediction, which does not exhibit a power-law behavior.
Fluid Dynamic Constraints on Morphology and Propulsion of Medusae at Low Reynolds Numbers
NASA Astrophysics Data System (ADS)
Dabiri, J. O.; Colin, S. P.; Costello, J. H.
2007-12-01
A recently developed mathematical model for physical constraints on the size and morphology of medusae was extended to include viscous effects that dominate at low Reynolds numbers. This fluid dynamic regime is experienced by all medusae during development and also by some adults. The lack of inertia in the flow field generated by medusae at low Reynolds numbers limits the vortex formation that is ubiquitous at higher Reynolds numbers. This consequently leads to the need for large oblate rowing medusae to use different propulsive solutions during their juvenile stages of development. Specifically, empirical observations of scyphomedusae (e.g., Aurelia sp.) and hydromedusae (e.g., Aequorea victoria and Obelia sp.) indicate distinct wake structures and swimming kinematics for each body type that exists during different stages of development. These differences can be explained by the extended mathematical model. In addition, ontogenetic changes can be visualized as trajectories within a plot of the new model.
NASA Astrophysics Data System (ADS)
Jaman, Md. Shah; Islam, Showmic; Saha, Sumon; Hasan, Mohammad Nasim; Islam, Md. Quamrul
2016-07-01
A numerical analysis is carried out to study the performance of steady laminar mixed convection flow inside a square lid-driven cavity filled with water-Al2O3 nanofluid. The top wall of the cavity is moving at a constant velocity and is heated by an isothermal heat source. Two-dimensional Navier-stokes equations along with the energy equations are solved using Galerkin finite element method. Results are obtained for a range of Reynolds and Grashof numbers by considering with and without the presence of nanoparticles. The parametric studies for a wide range of governing parameters in case of pure mixed convective flow show significant features of the present problem in terms of streamline and isotherm contours, average Nusselt number and average temperature profiles. The computational results indicate that the heat transfer coeffcient is strongly influenced by the above governing parameters at the pure mixed convection regime.
NASA Astrophysics Data System (ADS)
West, G. S.; Apelt, C. J.
1982-01-01
In the present investigation, particular attention was given to aspects of measurement accuracy and the control of secondary parameters in order to avoid masking the small changes associated with blockage. The data, obtained principally from surface pressure measurements on the cylinder but also relating to wake frequencies and tunnel-wall pressures, are presented, generally in graphical form. The data give comprehensive information on the flow parameters for flow past a circular cylinder within the range of Reynolds numbers from 10,000 to 100,000. If the blockage is less than 6%, the shape of the pressure distribution around the circular cylinder varies only slightly with blockage and the Strouhal number is independent of both the blockage ratio and the aspect ratio. For blockage ratios in the range from 6 to 16%, there is considerable distortion of the flow compared with that of the unblocked state.
NASA Technical Reports Server (NTRS)
Choudhari, Meelan; Streett, Craig L.
1991-01-01
A new approach to the prediction of instability generation due to the interaction of free-stream disturbances with regions of short-scale variations in surface boundary conditions is presented which accounts for the finite Reynolds number effects while providing a framework for studying receptivity in a variety of flows. The approach is demonstrated for the case of Tollmien-Schlichting wave generation in a Blasius boundary layer due to the interaction of a free-stream acoustic wave with a localized wall inhomogeneity. The results obtained by the finite Reynolds number approach are compared with predictions of the asymptotic approach.
Turbulent bands in plane-Poiseuille flow at moderate Reynolds numbers
NASA Astrophysics Data System (ADS)
Xiong, Xiangming; Tao, Jianjun; Chen, Shiyi; Brandt, Luca
2015-04-01
In this letter, we show via numerical simulations that the typical flow structures appearing in transitional channel flows at moderate Reynolds numbers are not spots but isolated turbulent bands, which have much longer lifetimes than the spots. Localized perturbations can evolve into isolated turbulent bands by continuously growing obliquely when the Reynolds number is larger than 660. However, interactions with other bands and local perturbations cause band breaking and decay. The competition between the band extension and breaking does not lead to a sustained turbulence until Re becomes larger than about 1000. Above this critical value, the bands split, providing an effective mechanism for turbulence spreading.
NASA Technical Reports Server (NTRS)
Ozturk, B.; Schobeiri, M. T.; Ashpis, David E.
2005-01-01
The paper experimentally studies the effects of periodic unsteady wake flow and different Reynolds numbers on boundary layer development, separation and re-attachment along the suction surface of a low pressure turbine blade. The experimental investigations were performed on a large scale, subsonic unsteady turbine cascade research facility at Turbomachinery Performance and Flow Research Laboratory (TPFL) of Texas A&M University. The experiments were carried out at Reynolds numbers of 110,000 and 150,000 (based on suction surface length and exit velocity). One steady and two different unsteady inlet flow conditions with the corresponding passing frequencies, wake velocities, and turbulence intensities were investigated. The reduced frequencies chosen cover the operating range of LP turbines. In addition to the unsteady boundary layer measurements, surface pressure measurements were performed. The inception, onset, and the extent of the separation bubble information collected from the pressure measurements were compared with the hot wire measurements. The results presented in ensemble-averaged, and the contour plot forms help to understand the physics of the separation phenomenon under periodic unsteady wake flow and different Reynolds number. It was found that the suction surface displayed a strong separation bubble for these three different reduced frequencies. For each condition, the locations defining the separation bubble were determined carefully analyzing and examining the pressure and mean velocity profile data. The location of the boundary layer separation was dependent of the Reynolds number. It is observed that starting point of the separation bubble and the re-attachment point move further downstream by increasing Reynolds number from 110,000 to 150,000. Also, the size of the separation bubble is smaller when compared to that for Re=110,000.
NASA Technical Reports Server (NTRS)
Blair, Michael F.; Anderson, Olof L.
1989-01-01
A combined experimental and computational program was conducted to examine the heat transfer distribution in a turbine rotor passage geometrically similiar to the Space Shuttle Main Engine (SSME) High Pressure Fuel Turbopump (HPFTP). Heat transfer was measured and computed for both the full-span suction and pressure surfaces of the rotor airfoil as well as for the hub endwall surface. The primary objective of the program was to provide a benchmark-quality data base for the assessment of rotor passage heat transfer computational procedures. The experimental portion of the study was conducted in a large-scale, ambient temperature, rotating turbine model. Heat transfer data were obtained using thermocouple and liquid-crystal techniques to measure temperature distributions on the thin, electrically-heated skin of the rotor passage model. Test data were obtained for various combinations of Reynolds number, rotor incidence angle and model surface roughness. The data are reported in the form of contour maps of Stanton number. These heat distribution maps revealed numerous local effects produced by the three-dimensional flows within the rotor passage. Of particular importance were regions of local enhancement produced on the airfoil suction surface by the main-passage and tip-leakage vortices and on the hub endwall by the leading-edge horseshoe vortex system. The computational portion consisted of the application of a well-posed parabolized Navier-Stokes analysis to the calculation of the three-dimensional viscous flow through ducts simulating the a gas turbine passage. These cases include a 90 deg turning duct, a gas turbine cascade simulating a stator passage, and a gas turbine rotor passage including Coriolis forces. The calculated results were evaluated using experimental data of the three-dimensional velocity fields, wall static pressures, and wall heat transfer on the suction surface of the turbine airfoil and on the end wall. Particular attention was paid to an
Oscillatory Excitation of Unsteady Compressible Flows over Airfoils at Flight Reynolds Numbers
NASA Technical Reports Server (NTRS)
Seifert, Avi; Pack, LaTunia G.
1999-01-01
An experimental investigation, aimed at delaying flow separation due to the occurrence of a shock-wave-boundary-layer interaction, is reported. The experiment was performed using a NACA 0012 airfoil and a NACA 0015 airfoil at high Reynolds number incompressible and compressible flow conditions. The effects of Mach and Reynolds numbers were identified, using the capabilities of the cryogenic-pressurized facility to maintain one parameter fixed and change the other. Significant Reynolds number effects were identified in the baseline compressible flow conditions even at Reynolds number of 10 and 20 million. The main objectives of the experiment were to study the effects of periodic excitation on airfoil drag-divergence and to alleviate the severe unsteadiness associated with shock-induced separation (known as "buffeting"). Zero-mass-flux oscillatory blowing was introduced through a downstream directed slot located at 10% chord on the upper surface of the NACA 0015 airfoil. The effective frequencies generated 2-4 vortices over the separated region, regardless of the Mach number. Even though the excitation was introduced upstream of the shock-wave, due to experimental limitations, it had pronounced effects downstream of it. Wake deficit (associated with drag) and unsteadiness (associated with buffeting) were significantly reduced. The spectral content of the wake pressure fluctuations indicates of steadier flow throughout the frequency range when excitation was applied. This is especially important at low frequencies which are more likely to interact with the airframe.
Aerodynamics Investigation of Faceted Airfoils at Low Reynolds Number
NASA Astrophysics Data System (ADS)
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
Microelectrokinetic turbulence in microfluidics at low Reynolds number.
Wang, Guiren; Yang, Fang; Zhao, Wei
2016-01-01
There is commonly no turbulence in microfluidics, and the flows are believed to be either laminar or chaotic, since Reynolds number (Re) in microflows is usually on the order of unity or lower. However, we recently demonstrated that it is possible to achieve turbulence with low Re (based on the measured flow velocity and the width of the channel entrance) when a pressure-driven flow is electrokinetically forced in a quasi T-microchannel. To be able to measure high frequency velocity fluctuations in microchannels, a velocimeter with submicrometer spatial resolution and microsecond temporal resolution, called a laser-induced fluorescence photobleaching anemometer, is developed. Here we characterize the microelectrokinetic turbulence and observe some typical and important features of high Re flows, such as Kolmogorov -5/3 spectrum of velocity fluctuation, which usually can be realized only at very high Re in macroturbulent flows.
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.
Shrimp theorem: paddle swimming at low Reynolds number
NASA Astrophysics Data System (ADS)
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.
Magnetic propulsion of robotic sperms at low-Reynolds number
NASA Astrophysics Data System (ADS)
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.
Optimal Strokes for Low Reynolds Number Swimmers: An Example
NASA Astrophysics Data System (ADS)
Alouges, François; Desimone, Antonio; Lefebvre, Aline
2008-06-01
Swimming, i.e., being able to advance in the absence of external forces by performing cyclic shape changes, is particularly demanding at low Reynolds numbers. This is the regime of interest for micro-organisms and micro- or nano-robots. We focus in this paper on a simple yet representative example: the three-sphere swimmer of Najafi and Golestanian ( Phys. Rev. E, 69, 062901-062904, 2004). For this system, we show how to cast the problem of swimming in the language of control theory, prove global controllability (which implies that the three-sphere swimmer can indeed swim), and propose a numerical algorithm to compute optimal strokes (which turn out to be suitably defined sub-Riemannian geodesics).
Feedback control of flow vorticity at low Reynolds numbers.
Zeitz, Maria; Gurevich, Pavel; Stark, Holger
2015-03-01
Our aim is to explore strategies of feedback control to design and stabilize novel dynamic flow patterns in model systems of complex fluids. To introduce the control strategies, we investigate the simple Newtonian fluid at low Reynolds number in a circular geometry. Then, the fluid vorticity satisfies a diffusion equation. We determine the mean vorticity in the sensing area and use two control strategies to feed it back into the system by controlling the angular velocity of the circular boundary. Hysteretic feedback control generates self-regulated stable oscillations in time, the frequency of which can be adjusted over several orders of magnitude by tuning the relevant feedback parameters. Time-delayed feedback control initiates unstable vorticity modes for sufficiently large feedback strength. For increasing delay time, we first observe oscillations with beats and then regular trains of narrow pulses. Close to the transition line between the resting fluid and the unstable modes, these patterns are relatively stable over long times.
Stokesian swimming of a prolate spheroid at low Reynolds number
NASA Astrophysics Data System (ADS)
Felderhof, B. U.
2016-11-01
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.
Resistance of plates and pipes at high Reynolds numbers
NASA Technical Reports Server (NTRS)
Schiller, L; Hermann, R
1931-01-01
It was learned that the law of resistance for high R values does not follow the simple powers, and that the powers, which can be obtained approximately for the velocity distribution, gradually change. Since, moreover, very important investigations have recently been made on the resistance of plates at very high R values, it seemed of interest to apply the above line of reasoning to the new general law of resistance. For this purpose, the resistance and velocity distribution along the plate must always be equal to the values of the pipe flow at the corresponding Reynolds number. We made two kinds of calculations, of which the one given here is the simpler and more practical and also agrees better with the experimental results.
Experiments with He II at Reynolds Number 107
NASA Astrophysics Data System (ADS)
Fuzier, Silvie
2000-11-01
We report the first measurements of pressure drop and heat transport in He II at Reynolds numbers exceeding 10^7 in a 10 mm ID tube. Pressure drop measurements confirm the previously reported(P.L. Walstrom, J.G. Weisend II, J.R. Maddocks, and S.W. Van Sciver, Turbulent Flow Pressure Drop in Various He II Transfer System Components , Cryogenics 28, 101 (1988).) classical form for the friction factor. These are compared to the von Karman-Nikuradse correlation developed for classical fluids. Frictional heating due to Joule Thomson expansion is also observed and compared with analysis. Steady state and transient heat transport measurements further confirm the applicability of the He II energy equation for forced flow He II processes. We will also describe the experimental apparatus used to perform the measurements.
Low Reynolds number Couette flow facility for drag measurements.
Johnson, Tyler J; Lang, Amy W; Wheelus, Jennifer N; Westcott, Matthew
2010-09-01
For this study a new low Reynolds number Couette facility was constructed to investigate surface drag. In this facility, mineral oil was used as the working fluid to increase the shear stress across the surface of the experimental models. A mounted conveyor inside a tank creates a flow above which an experimental model of a flat plate was suspended. The experimental plate was attached to linear bearings on a slide system that connects to a force gauge used to measure the drag. Within the gap between the model and moving belt a Couette flow with a linear velocity profile was created. Digital particle image velocimetry was used to confirm the velocity profile. The drag measurements agreed within 5% of the theoretically predicted Couette flow value.
Filament actuation by an active colloid at low Reynolds number
NASA Astrophysics Data System (ADS)
Laskar, Abhrajit; Adhikari, R.
2017-03-01
Active colloids and externally actuated semi-flexible filaments provide basic building blocks for designing autonomously motile micro-machines. Here, we show that a passive semi-flexible filament can be actuated and transported by attaching an active colloid to its terminus. We study the dynamics of this assembly when it is free, tethered, or clamped using overdamped equations of motion that explicitly account for active fluid flow and the forces it mediates. Linear states are destabilised by buckling instabilities to produce stable states of non-zero curvature and writhe. We demarcate boundaries of these states in the two-dimensional parameter space representing dimensionless measures of polar and apolar activity. Our proposed assembly can be used as a novel component in the design of micro-machines at low Reynolds number and to study elastic instabilities driven by ‘follower’ forces.
Low Reynolds number flow near tiny leaves, stems, and trichomes
NASA Astrophysics Data System (ADS)
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.
Low Reynolds number Couette flow facility for drag measurements
NASA Astrophysics Data System (ADS)
Johnson, Tyler J.; Lang, Amy W.; Wheelus, Jennifer N.; Westcott, Matthew
2010-09-01
For this study a new low Reynolds number Couette facility was constructed to investigate surface drag. In this facility, mineral oil was used as the working fluid to increase the shear stress across the surface of the experimental models. A mounted conveyor inside a tank creates a flow above which an experimental model of a flat plate was suspended. The experimental plate was attached to linear bearings on a slide system that connects to a force gauge used to measure the drag. Within the gap between the model and moving belt a Couette flow with a linear velocity profile was created. Digital particle image velocimetry was used to confirm the velocity profile. The drag measurements agreed within 5% of the theoretically predicted Couette flow value.
Prandtl and Rayleigh number dependence of the Reynolds number in turbulent thermal convection.
Grossmann, Siegfried; Lohse, Detlef
2002-07-01
The Prandtl and Rayleigh number dependences of the Reynolds number in turbulent thermal convection following from the unifying theory by Grossmann and Lohse [J. Fluid Mech. 407, 27 (2000); Phys. Rev. Lett. 86, 3316 (2001)] are presented and compared with various recent experimental findings. This dependence Re(Ra,Pr) is more complicated than a simple global power law. For Pr=5.5 and 10(8)
Lumley decomposition of turbulent boundary layer at high Reynolds numbers
NASA Astrophysics Data System (ADS)
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.
NASA Technical Reports Server (NTRS)
Choudhari, Meelan; Streett, Craig L.
1992-01-01
Previous theoretical work on the boundary layer receptivity problem has utilized large Reynolds number asymptotic theories, thus being limited to a narrow part of the frequency-Reynolds number domain. An alternative approach is presented for the prediction of localized instability generation which has a general applicability, and also accounts for finite Reynolds number effects. This approach is illustrated for the case of Tollmien-Schlichting wave generation in a Blasius boundary layer due to the interaction of a free stream acoustic wave with a region of short scale variation in the surface boundary condition. The specific types of wall inhomogeneities studied are: regions of short scale variations in wall suction, wall admittance, and wall geometry (roughness). Extensive comparison is made between the results of the finite Reynolds number approach and previous asymptotic predictions, which also suggests an alternative way of using the latter at Reynolds numbers of interest in practice.
NASA Technical Reports Server (NTRS)
Choudhari, Meelan; Streett, Craig L.
1992-01-01
Previous theoretical work on the boundary layer receptivity problem has utilized large Reynolds number asymptotic theories, thus being limited to a narrow part of the frequency-Reynolds number domain. An alternative approach is presented for the prediction of localized instability generation which has a general applicability, and also accounts for finite Reynolds number effects. This approach is illustrated for the case of Tollmien-Schlichting wave generation in a Blasius boundary layer due to the interaction of a free stream acoustic wave with a region of short scale variation in the surface boundary condition. The specific types of wall inhomogeneities studied are: regions of short scale variations in wall suction, wall admittance, and wall geometry (roughness). Extensive comparison is made between the results of the finite Reynolds number approach and previous asymptotic predictions, which also suggests an alternative way of using the latter at Reynolds numbers of interest in practice.
Stationary shapes of deformable particles moving at low Reynolds numbers
NASA Astrophysics Data System (ADS)
Boltz, Horst-Holger; Kierfeld, Jan
2016-11-01
We introduce an iterative solution scheme in order to calculate stationary shapes of deformable elastic capsules which are steadily moving through a viscous fluid at low Reynolds numbers. The iterative solution scheme couples hydrodynamic boundary integral methods and elastic shape equations to find the stationary axisymmetric shape and the velocity of an elastic capsule moving in a viscous fluid governed by the Stokes equation. We use this approach to systematically study dynamical shape transitions of capsules with Hookean stretching and bending energies and spherical resting shape sedimenting under the influence of gravity or centrifugal forces. We find three types of possible axisymmetric stationary shapes for sedimenting capsules with fixed volume: a pseudospherical state, a pear-shaped state, and buckled shapes. Capsule shapes are controlled by two dimensionless parameters, the Föppl-von-Kármán number characterizing the elastic properties and a Bond number characterizing the driving force. For increasing gravitational force the spherical shape transforms into a pear shape. For very large bending rigidity (very small Föppl-von-Kármán number) this transition is discontinuous with shape hysteresis. The corresponding transition line terminates, however, in a critical point, such that the discontinuous transition is not present at typical Föppl-von-Kármán numbers of synthetic capsules. In an additional bifurcation, buckled shapes occur upon increasing the gravitational force.
NASA Astrophysics Data System (ADS)
Krause, E.; Schweitzer, W.-B.
1990-05-01
Results of an experimental investigation of the flow around a NACA-4412 profile in an oscillating freestream are presented. The experiment took place in an Eiffel-type windtunnel at a chord Reynolds-number of Re = 2 · 105. Measurements of unsteady pressure distributions and boundary-layer profiles as well as flow photographs reveal that even at moderate reduced frequencies k = π f/overline u _infty significant changes of the flow field may occur, provided that the relative amplitude of the freestream A = Δ u_infty /overline u _infty is sufficiently large. So a periodical separation and reattachment of the flow could be observed while in another case the periodical relaminarization of the boundary-layer could be found.
NASA Technical Reports Server (NTRS)
Carlson, John R.
1996-01-01
The ability of the three-dimensional Navier-Stokes method, PAB3D, to simulate the effect of Reynolds number variation using non-linear explicit algebraic Reynolds stress turbulence modeling was assessed. Subsonic flat plate boundary-layer flow parameters such as normalized velocity distributions, local and average skin friction, and shape factor were compared with DNS calculations and classical theory at various local Reynolds numbers up to 180 million. Additionally, surface pressure coefficient distributions and integrated drag predictions on an axisymmetric nozzle afterbody were compared with experimental data from 10 to 130 million Reynolds number. The high Reynolds data was obtained from the NASA Langley 0.3m Transonic Cryogenic Tunnel. There was generally good agreement of surface static pressure coefficients between the CFD and measurement. The change in pressure coefficient distributions with varying Reynolds number was similar to the experimental data trends, though slightly over-predicting the effect. The computational sensitivity of viscous modeling and turbulence modeling are shown. Integrated afterbody pressure drag was typically slightly lower than the experimental data. The change in afterbody pressure drag with Reynolds number was small both experimentally and computationally, even though the shape of the distribution was somewhat modified with Reynolds number.
NASA Astrophysics Data System (ADS)
Hirata, Katsuya; Kawakita, Masatoshi; Iijima, Takayoshi; Koga, Mitsuhiro; Kihira, Mitsuhiko; Funaki, Jiro
The aerodynamic characteristics of airfoils have been researched in higher Reynolds-number ranges more than 106, in a historic context closely related with the developments of airplanes and fluid machineries in the last century. However, our knowledge is not enough at low and middle Reynolds-number ranges. So, in the present study, we investigate such basic airfoils as a NACA0015, a flat plate and the flat plates with modified fore-face and after-face geometries at Reynolds number Re < 1.0×105, using two- and three-dimensional computations together with wind-tunnel and water-tank experiments. As a result, we have revealed the effect of the Reynolds number Re upon the minimum drag coefficient CDmin. Besides, we have shown the effects of attack angle α upon various aerodynamic characteristics such as the lift coefficient CL, the drag coefficient CD and the lift-to-drag ratio CL/CD at Re = 1.0×102, discussing those effects on the basis of both near-flow-field information and surface-pressure profiles. Such results suggest the importance of sharp leading edges, which implies the possibility of an inversed NACA0015. Furthermore, concerning the flat-plate airfoil, we investigate the influences of fore-face and after-face geometries upon such effects.
NASA Astrophysics Data System (ADS)
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
Anisotropic Structure of Rotating Homogeneous Turbulence at High Reynolds Numbers
NASA Technical Reports Server (NTRS)
Cambon, Claude; Mansour, Nagi N.; Squires, Kyle D.; Rai, Man Mohan (Technical Monitor)
1995-01-01
Large eddy simulation is used to investigate the development of anisotropies and the evolution towards a quasi two-dimensional state in rotating homogeneous turbulence at high Reynolds number. The present study demonstrates the existence of two transitions in the development of anisotropy. The first transition marks the onset of anisotropy and occurs when a macro-Rossby number (based on a longitudinal integral lengthscale) has decreased to near unity while the second transition occurs when a micro-Rossby number (defined in this work as the ratio of the rms fluctuating vorticity to background vorticity) has decreased to unity. The anisotropy marked by the first transition corresponds to a reduction in dimensionality while the second transition corresponds to a polarization of the flow, i.e., relative dominance of the velocity components in the plane normal to the rotation axis. Polarization is reflected by emergence of anisotropy measures based on the two-dimensional component of the turbulence. Investigation of the vorticity structure shows that the second transition is also characterized by an increasing tendency for alignment between the fluctuating vorticity vector and the background angular velocity vector with a preference for corrotative vorticity.
Anisotropic Structure of Rotating Homogeneous Turbulence at High Reynolds Numbers
NASA Technical Reports Server (NTRS)
Cambon, Claude; Mansour, Nagi N.; Squires, Kyle D.; Rai, Man Mohan (Technical Monitor)
1995-01-01
Large eddy simulation is used to investigate the development of anisotropies and the evolution towards a quasi two-dimensional state in rotating homogeneous turbulence at high Reynolds number. The present study demonstrates the existence of two transitions in the development of anisotropy. The first transition marks the onset of anisotropy and occurs when a macro-Rossby number (based on a longitudinal integral lengthscale) has decreased to near unity while the second transition occurs when a micro-Rossby number (defined in this work as the ratio of the rms fluctuating vorticity to background vorticity) has decreased to unity. The anisotropy marked by the first transition corresponds to a reduction in dimensionality while the second transition corresponds to a polarization of the flow, i.e., relative dominance of the velocity components in the plane normal to the rotation axis. Polarization is reflected by emergence of anisotropy measures based on the two-dimensional component of the turbulence. Investigation of the vorticity structure shows that the second transition is also characterized by an increasing tendency for alignment between the fluctuating vorticity vector and the background angular velocity vector with a preference for corrotative vorticity.
Low-Reynolds number compressible flow around a triangular airfoil
NASA Astrophysics Data System (ADS)
Munday, Phillip; Taira, Kunihiko; Suwa, Tetsuya; Numata, Daiju; Asai, Keisuke
2013-11-01
We report on the combined numerical and experimental effort to analyze the nonlinear aerodynamics of a triangular airfoil in low-Reynolds number compressible flow that is representative of wings on future Martian air vehicles. The flow field around this airfoil is examined for a wide range of angles of attack and Mach numbers with three-dimensional direct numerical simulations at Re = 3000 . Companion experiments are conducted in a unique Martian wind tunnel that is placed in a vacuum chamber to simulate the Martian atmosphere. Computational findings are compared with pressure sensitive paint and direct force measurements and are found to be in agreement. The separated flow from the leading edge is found to form a large leading-edge vortex that sits directly above the apex of the airfoil and provides enhanced lift at post stall angles of attack. For higher subsonic flows, the vortical structures elongate in the streamwise direction resulting in reduced lift enhancement. We also observe that the onset of spanwise instability for higher angles of attack is delayed at lower Mach numbers. Currently at Mitsubishi Heavy Industries, Ltd., Nagasaki.
Negative Magnus lift on a rotating sphere at around the critical Reynolds number
NASA Astrophysics Data System (ADS)
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.
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.
Performance and slipstream characteristics of small-scale propellers at low Reynolds numbers
NASA Astrophysics Data System (ADS)
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
Onset of turbulence in accelerated high-Reynolds-number flow.
Zhou, Ye; Robey, Harry F; Buckingham, Alfred C
2003-05-01
A new criterion, flow drive time, is identified here as a necessary condition for transition to turbulence in accelerated, unsteady flows. Compressible, high-Reynolds-number flows initiated, for example, in shock tubes, supersonic wind tunnels with practical limitations on dimensions or reservoir capacity, and high energy density pulsed laser target vaporization experimental facilities may not provide flow duration adequate for turbulence development. In addition, for critical periods of the overall flow development, the driving background flow is often unsteady in the experiments as well as in the physical flow situations they are designed to mimic. In these situations transition to fully developed turbulence may not be realized despite achievement of flow Reynolds numbers associated with or exceeding stationary flow transitional criteria. Basically our transitional criterion and prediction procedure extends to accelerated, unsteady background flow situations the remarkably universal mixing transition criterion proposed by Dimotakis [P. E. Dimotakis, J. Fluid Mech. 409, 69 (2000)] for stationary flows. This provides a basis for the requisite space and time scaling. The emphasis here is placed on variable density flow instabilities initiated by constant acceleration Rayleigh-Taylor instability (RTI) or impulsive (shock) acceleration Richtmyer-Meshkov instability (RMI) or combinations of both. The significant influences of compressibility on these developing transitional flows are discussed with their implications on the procedural model development. A fresh perspective for predictive modeling and design of experiments for the instability growth and turbulent mixing transitional interval is provided using an analogy between the well-established buoyancy-drag model with applications of a hierarchy of single point turbulent transport closure models. Experimental comparisons with the procedural results are presented where use is made of three distinctly different types
Flow field behavior with Reynolds number variance around a spiked body
NASA Astrophysics Data System (ADS)
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.
Mechanisms Of Pressure Distributions Within Laminar Separation Bubble At Different Reynolds Numbers
NASA Astrophysics Data System (ADS)
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.
Electrokinetic turbulence in a microchannel at low Reynolds number
NASA Astrophysics Data System (ADS)
Zhao, Wei; Yang, Fang; Wang, Guiren
2015-11-01
Turbulence is commonly viewed as a type of macroflow phenomenon under a sufficiently high Reynolds number (Re). On the other hand, it has been widely perceived in science, engineering and medicine that there is never any turbulence in low Re flow for Newtonian fluids. There is even difficulty to characterize turbulence in microchannels with current available velocimeters, due to the requirement of simultaneously high spatial and temporal resolution. Recently, we generated micro-electrokinetic (EK) turbulence in a microchannel when a pressure driven flow at low Re on the order of unity is electrokinetically forced. We also developed a novel velocimeter, i.e. laser induced fluorescence photobleaching anemometer (LIFPA) that enables us to measure the velocity fluctuations with simultaneously high spatial and temporal resolution. Here we surprisingly observed with LIFPA that the corresponding micro EK turbulence can also have some features of high Re flows, such as Kolmogorov -5/3 spectrum and the exponential tail of probability density function of velocity fluctuation, and the scaling behavior of velocity structure function. This work could provide a new perspective on turbulence. The work was supported by NSF under grant no. CAREER CBET-0954977, MRI CBET-1040227.
Material properties of Caenorhabditis elegans swimming at low Reynolds number.
Sznitman, J; Purohit, Prashant K; Krajacic, P; Lamitina, T; Arratia, P E
2010-02-17
Undulatory locomotion, as seen in the nematode Caenorhabditis elegans, is a common swimming gait of organisms in the low Reynolds number regime, where viscous forces are dominant. Although the nematode's motility is expected to be a strong function of its material properties, measurements remain scarce. Here, the swimming behavior of C. elegans is investigated in experiments and in a simple model. Experiments reveal that nematodes swim in a periodic fashion and generate traveling waves that 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 C. elegans, we find E approximately 3.77 kPa and eta approximately -860 Pa.s; values of eta for live 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 quantitative tools with which to more accurately describe new and existing muscle mutants. Copyright 2010 Biophysical Society. Published by Elsevier Inc. All rights reserved.
High amplitude surging and plunging motions at low Reynolds number
NASA Astrophysics Data System (ADS)
Choi, Jeesoon; Colonius, Tim; Williams, David; Caltech Collaboration; IIT Collaboration
2014-11-01
Aerodynamic forces and flow structures associated with high amplitude oscillations of an airfoil in the streamwise (surging) and transverse (plunging) direction are investigated in two-dimensional simulations at low Reynolds number (Re = 102 ~ 103). While the unsteady aerodynamic forces for low-amplitude motions were mainly affected by the leading-edge vortex (LEV) acting in- or out-of phase with the quasi-component of velocity, large-amplitude motions involve complex vortex interactions of LEVs and trailing-edge vortices (TEVs) with the moving body. For high-amplitude surging, the TEV, instead of the LEV, induces low-pressure regions above the airfoil during the retreating portion of the cycle near the reduced frequency, k = 0.5, and enhances the time-average forces. The time required for the LEV to convect along the chord becomes an intrinsic time scale, and for plunging motions, there is a sudden change of flow structure when the period of the motion is not long enough for the LEV to convect through the whole chord.
Low-Reynolds number modelling of flows with transpiration
NASA Astrophysics Data System (ADS)
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
Free and confined jets at low Reynolds numbers
NASA Astrophysics Data System (ADS)
Koller-Milojevie, Dušica; Schneider, Wilhelm
1993-12-01
Free jets, and jets with tubular confinements, are investigated in the jet Reynolds number regime 80 ⩽ Re j ⩽ 1000 being of interest for micro-jet pumps, among other applications. For issuing the jets, conventional (single-hole) nozzles as well as dual-hole nozzles of a particular design are used. Both flow visualization and LDA measurement indicate that, in agreement with previous findings, the jets issuing from conventional nozzles remain laminar up to large distances from the orifice. Thus there is but little entrainment of ambient fluid, and the performance of conventional nozzles in micro-jet pumps is rather poor. The dual-hole nozzles, on the other hand, are found to enforce transition to turbulent flow near the orifices. As a result, the entrainment rate is considerably increased, and the performance of jet pumps is improved when the dual-hole nozzles are applied. The experimental data are found to be in fair agreement with predictions based on mass and momentum balances.
Swimming by reciprocal motion at low Reynolds number
NASA Astrophysics Data System (ADS)
Qiu, Tian; Lee, Tung-Chun; Mark, Andrew G.; Morozov, Konstantin I.; Münster, Raphael; Mierka, Otto; Turek, Stefan; Leshansky, Alexander M.; Fischer, Peer
2014-11-01
Biological microorganisms swim with flagella and cilia that execute nonreciprocal motions for low Reynolds number (Re) propulsion in viscous fluids. This symmetry requirement is a consequence of Purcell’s scallop theorem, which complicates the actuation scheme needed by microswimmers. However, most biomedically important fluids are non-Newtonian where the scallop theorem no longer holds. It should therefore be possible to realize a microswimmer that moves with reciprocal periodic body-shape changes in non-Newtonian fluids. Here we report a symmetric ‘micro-scallop’, a single-hinge microswimmer that can propel in shear thickening and shear thinning (non-Newtonian) fluids by reciprocal motion at low Re. Excellent agreement between our measurements and both numerical and analytical theoretical predictions indicates that the net propulsion is caused by modulation of the fluid viscosity upon varying the shear rate. This reciprocal swimming mechanism opens new possibilities in designing biomedical microdevices that can propel by a simple actuation scheme in non-Newtonian biological fluids.
Swimming by reciprocal motion at low Reynolds number.
Qiu, Tian; Lee, Tung-Chun; Mark, Andrew G; Morozov, Konstantin I; Münster, Raphael; Mierka, Otto; Turek, Stefan; Leshansky, Alexander M; Fischer, Peer
2014-11-04
Biological microorganisms swim with flagella and cilia that execute nonreciprocal motions for low Reynolds number (Re) propulsion in viscous fluids. This symmetry requirement is a consequence of Purcell's scallop theorem, which complicates the actuation scheme needed by microswimmers. However, most biomedically important fluids are non-Newtonian where the scallop theorem no longer holds. It should therefore be possible to realize a microswimmer that moves with reciprocal periodic body-shape changes in non-Newtonian fluids. Here we report a symmetric 'micro-scallop', a single-hinge microswimmer that can propel in shear thickening and shear thinning (non-Newtonian) fluids by reciprocal motion at low Re. Excellent agreement between our measurements and both numerical and analytical theoretical predictions indicates that the net propulsion is caused by modulation of the fluid viscosity upon varying the shear rate. This reciprocal swimming mechanism opens new possibilities in designing biomedical microdevices that can propel by a simple actuation scheme in non-Newtonian biological fluids.
Flow control at low Reynolds numbers using periodic airfoil morphing
NASA Astrophysics Data System (ADS)
Jones, Gareth; Santer, Matthew; Papadakis, George; Bouremel, Yann; Debiasi, Marco; Imperial-NUS Joint PhD Collaboration
2014-11-01
The performance of airfoils operating at low Reynolds numbers is known to suffer from flow separation even at low angles of attack as a result of their boundary layers remaining laminar. The lack of mixing---a characteristic of turbulent boundary layers---leaves laminar boundary layers with insufficient energy to overcome the adverse pressure gradient that occurs in the pressure recovery region. This study looks at periodic surface morphing as an active flow control technique for airfoils in such a flight regime. It was discovered that at sufficiently high frequencies an oscillating surface is capable of not only reducing the size of the separated region---and consequently significantly reducing drag whilst simultaneously increasing lift---but it is also capable of delaying stall and as a result increasing CLmax. Furthermore, by bonding Macro Fiber Composite actuators (MFCs) to the underside of an airfoil skin and driving them with a sinusoidal frequency, it is shown that this control technique can be practically implemented in a lightweight, energy efficient way. Imperial-NUS Joint Ph.D. Programme.
Propulsion at low Reynolds number via beam extrusion
NASA Astrophysics Data System (ADS)
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.
Interaction of two high Reynolds number axisymmetric turbulent wakes
NASA Astrophysics Data System (ADS)
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.
Swimming by reciprocal motion at low Reynolds number
Qiu, Tian; Lee, Tung-Chun; Mark, Andrew G.; Morozov, Konstantin I.; Münster, Raphael; Mierka, Otto; Turek, Stefan; Leshansky, Alexander M.; Fischer, Peer
2014-01-01
Biological microorganisms swim with flagella and cilia that execute nonreciprocal motions for low Reynolds number (Re) propulsion in viscous fluids. This symmetry requirement is a consequence of Purcell’s scallop theorem, which complicates the actuation scheme needed by microswimmers. However, most biomedically important fluids are non-Newtonian where the scallop theorem no longer holds. It should therefore be possible to realize a microswimmer that moves with reciprocal periodic body-shape changes in non-Newtonian fluids. Here we report a symmetric ‘micro-scallop’, a single-hinge microswimmer that can propel in shear thickening and shear thinning (non-Newtonian) fluids by reciprocal motion at low Re. Excellent agreement between our measurements and both numerical and analytical theoretical predictions indicates that the net propulsion is caused by modulation of the fluid viscosity upon varying the shear rate. This reciprocal swimming mechanism opens new possibilities in designing biomedical microdevices that can propel by a simple actuation scheme in non-Newtonian biological fluids. PMID:25369018
Efficient simulation of detached flows at high Reynolds number
NASA Astrophysics Data System (ADS)
Vega, Jose M.; Asensio, Victor; Herrero, Raul; Varas, Fernando
2014-11-01
A method is presented for the computationally efficient simulation of quasi-periodic detached flows in multi-parameter problems at very large Reynolds numbers, keeping in mind a variety of applications, including helicopter flight simulators, control and certification of unmanned aerial vehicles, control of wind turbines, conceptual design in aeronautics, and civil aerodynamics. In many of these applications, the large scale flows (ignoring the smaller turbulent scales) are at most quasi-periodic, namely the Fourier transform exhibits a finite set of concentrated peaks resulting from the nonlinear passive interaction of periodic wakes. The method consists in an offline preprocess and the online operation. In the preprocess, a standard CFD solver (such as URANS) is used in combination with several ingredients such as an iterative combination proper orthogonal decomposition and fast Fourier transform. The online operation is made with a combination of high order singular value decomposition and interpolation. The performance of the method is tested considering the ow over a fairly complex urban topography, for various free stream intensities and orientations, seeking real time online simulations.
Flow analysis of the low Reynolds number swimmer C. elegans
NASA Astrophysics Data System (ADS)
Montenegro-Johnson, Thomas D.; Gagnon, David A.; Arratia, Paulo E.; Lauga, Eric
2016-09-01
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 employ a boundary element method to simulate flow fields, obtaining very good quantitative agreement with experiment. We use this numerical model to show that calculation of flow shear rates using purely planar data results in significant underestimates of the true three-dimensional value. Applying symmetry arguments, validated against numerics, we demonstrate that the out-of-plane contribution can be accounted for via incompressibility and therefore simply calculated from particle tracking velocimetry. Our results show how fundamental fluid mechanics considerations may be used to improve the accuracy of measurements in biofluiddynamics.
A self-propelled biohybrid swimmer at low Reynolds number.
Williams, Brian J; Anand, Sandeep V; Rajagopalan, Jagannathan; Saif, M Taher A
2014-01-01
Many microorganisms, including spermatozoa and forms of bacteria, oscillate or twist a hair-like flagella to swim. At this small scale, where locomotion is challenged by large viscous drag, organisms must generate time-irreversible deformations of their flagella to produce thrust. To date, there is no demonstration of a self propelled, synthetic flagellar swimmer operating at low Reynolds number. Here we report a microscale, biohybrid swimmer enabled by a unique fabrication process and a supporting slender-body hydrodynamics model. The swimmer consists of a polydimethylsiloxane filament with a short, rigid head and a long, slender tail on which cardiomyocytes are selectively cultured. The cardiomyocytes contract and deform the filament to propel the swimmer at 5-10 μm s(-1), consistent with model predictions. We then demonstrate a two-tailed swimmer swimming at 81 μm s(-1). This small-scale, elementary biohybrid swimmer can serve as a platform for more complex biological machines.
NASA Technical Reports Server (NTRS)
Fleming, J. L.; Simpson, R. L.
1997-01-01
Laser Doppler velocimetry (LDV) measurements and hydrogen bubble flow visualization techniques were used to examine the near-wall flow structure of 2D and 3D turbulent boundary layers (TBLs) over a range of low Reynolds numbers. The goals of this research were (1) an increased understanding of the flow physics in the near wall region of turbulent boundary layers,(2) to observe and quantify differences between 2D and 3D TBL flow structures, and (3) to document Reynolds number effects for 3D TBLs. The LDV data have provided results detailing the turbulence structure of the 2D and 3D TBLs. These results include mean Reynolds stress distributions, flow skewing results, and U and V spectra. Effects of Reynolds number for the 3D flow were also examined. Comparison to results with the same 3D flow geometry but at a significantly higher Reynolds number provided unique insight into the structure of 3D TBLs. While the 3D mean and fluctuating velocities were found to be highly dependent on Reynolds number, a previously defined shear stress parameter was discovered to be invariant with Reynolds number. The hydrogen bubble technique was used as a flow visualization tool to examine the near-wall flow structure of 2D and 3D TBLs. Both the quantitative and qualitative results displayed larger turbulent fluctuations with more highly concentrated vorticity regions for the 2D flow.
NASA Astrophysics Data System (ADS)
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.
Large-eddy simulations of impinging jets at high Reynolds numbers
NASA Astrophysics Data System (ADS)
Wu, Wen; Piomelli, Ugo
2013-11-01
We have performed large-eddy simulations of an impinging jet with embedded azimuthal vortices. We used a hybrid approach in which the near-wall layer is modelled using the RANS equations with the Spalart-Allmaras model, while away from the wall Lagrangian-averaged dynamic eddy-viscosity modelled LES is used. This method allowed us to reach Reynolds numbers that would be prohibitively expensive for wall-resolving LES. First, we compared the results of the hybrid calculation with a wall-resolved one at moderate Reynolds number, Re = 66 , 000 (based on jet diameter and velocity). The mean velocity and Reynolds stresses were in good agreement between the simulations, and, in particular, the generation of secondary vorticity at the wall and its liftup were captured well. The simulation cost was reduced by 86%. We then carried out simulations at Re = 266 , 000 and 1.3 million. The effect of Reynolds number on vortex development will be discussed. Canada Research Chair in Computational Turbulence, HPCVL-Sun Microsystems Chair in Computational Science and Engineering.
Initial Results of Reynolds Number Testing at LaRC's NTF Using the 2.2% Reference H Model
NASA Technical Reports Server (NTRS)
Owens, Lewis R., Jr. (Editor); Wahls, Richard A. (Editor); Hamner, Marvine
1999-01-01
To develop full scale flight performance predictions an understanding of Reynolds number effects on HSCT-class configurations is essential. A wind tunnel database utilizing a 2.2% scale Reference H model in NASA Langley Research Centers National Transonic Facility is being developed to assess these Reynolds number effects. In developing this database temperature and aeroelastic corrections to the wind tunnel data have been identified and are being analyzed. Once final corrections have been developed and applied, then pure Reynolds number effects can be determined. In addition, final corrections will yield the data required for CFD validation at q = 0. Presented in this report are the results of seven tests involving the wing/body configuration. This includes summaries of data acquired in these tests, uncorrected Reynolds number effects, and temperature and aeroelastic corrections. The data presented herein illustrates the successes achieved to date as well as the challenges that will be faced in obtaining full scale flight performance predictions.
An investigation of small scales of turbulence in a boundary layer at high Reynolds numbers
NASA Technical Reports Server (NTRS)
Wallace, James M.; Ong, L.; Balint, J.-L.
1993-01-01
The assumption that turbulence at large wave-numbers is isotropic and has universal spectral characteristics which are independent of the flow geometry, at least for high Reynolds numbers, has been a cornerstone of closure theories as well as of the most promising recent development in the effort to predict turbulent flows, viz. large eddy simulations. This hypothesis was first advanced by Kolmogorov based on the supposition that turbulent kinetic energy cascades down the scales (up the wave-numbers) of turbulence and that, if the number of these cascade steps is sufficiently large (i.e. the wave-number range is large), then the effects of anisotropies at the large scales are lost in the energy transfer process. Experimental attempts were repeatedly made to verify this fundamental assumption. However, Van Atta has recently suggested that an examination of the scalar and velocity gradient fields is necessary to definitively verify this hypothesis or prove it to be unfounded. Of course, this must be carried out in a flow with a sufficiently high Reynolds number to provide the necessary separation of scales in order unambiguously to provide the possibility of local isotropy at large wave-numbers. An opportunity to use our 12-sensor hot-wire probe to address this issue directly was made available at the 80'x120' wind tunnel at the NASA Ames Research Center, which is normally used for full-scale aircraft tests. An initial report on this high Reynolds number experiment and progress toward its evaluation is presented.
Pulsatility role in cylinder flow dynamics at low Reynolds number
NASA Astrophysics Data System (ADS)
Qamar, Adnan; Samtaney, Ravi; Bull, Joseph L.
2012-08-01
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.
Confined swirling jet impingement on a flat plate at moderate Reynolds numbers
NASA Astrophysics Data System (ADS)
Herrada, M. A.; Del Pino, C.; Ortega-Casanova, J.
2009-01-01
The behavior of a swirling jet issuing from a pipe and impinging on a flat smooth wall is analyzed numerically by means of axisymmetric simulations. The axial velocity profile at the pipe outlet is assumed flat while the azimuthal velocity profile is a Burger's vortex characterized by two non-dimensional parameters; a swirl number S and a vortex core length δ. We concentrate on the effects of these two parameters on the mechanical characteristics of the flow at moderate Reynolds numbers. Our results for S =0 are in agreement with Phares et al. [J. Fluid Mech. 418, 351 (2000)], who provide a theoretical determination of the wall shear stress under nonswirling impinging jets at high Reynolds numbers. In addition, we show that the swirl number has an important effect on the jet impact process. For a fixed nozzle-to-plate separation, we found that depending on the value of δ and the Reynolds number Re, there is a critical swirl number, S =S∗(δ ,Re), above which recirculating vortex breakdown bubbles are observed in the near axis region. For S >S∗, the presence of these bubbles enhances the transition from a steady to a periodic regime. For S
Aerodynamics of wings at low Reynolds numbers: Boundary layer separation and reattachment
NASA Astrophysics Data System (ADS)
McArthur, John
the drag increase is really caused by the formation of a laminar separation bubble. The results clearly indicate that the reverse is true, and that the subsequent drag decrease is caused by the laminar separation bubble. Part III. The leading edge vortex. Four wings with different sweep angles were studied at Reynolds number 5x104: sweep angles of 0, 20, 40, and 60 degrees. The wings had a simple cambered plate airfoil similar to the cambered airfoil of part I above. Each wing was built to have the same aspect ratio, wing area, and streamwise airfoil shape. Previous studies on bird wings speculate that simply sweeping the wings can cause a leading edge vortex to form, which could cause substantial improvements in performance. However, these studies were not well controlled, and were conducted from a biological perspective. Qualitative and quantitative flow field measurements were combined with force measurements to conduct a well controlled engineering experiment on the formation and effect of a leading edge vortex on simple swept wings. A stable vortex was found to form over the 60 degree swept wing at one particular angle of attack, but it was not similar to the traditional notion of a leading edge vortex. The vortex has a small radius, and extends over little of the span. Force measurements indicate that the vortex has no significant impact on the forces measured. Thus, simply sweeping a wing is not sufficient to form a significant leading edge vortex, and other effects must be considered.
Reynolds number tests of an NPL 9510 airfoil in the Langley 0.3-meter transonic cryogenic tunnel
NASA Technical Reports Server (NTRS)
Jenkins, R. V.
1983-01-01
An investigation of the NPL 9510 airfoil was conducted in the Langley 0.3-Meter Transonic Cryogenic Tunnel over the following ranges of test conditions: Mach number of 0.35 to 0.82, total temperature of 94 K to 300 K, total pressure of 1.20 to 5.81 atm, Reynolds number based on airfoil chord of 1.34 x 10 to the 6th power to 48.23 x 10 to the 6th power, and angle of attack of 0 deg to 6 deg. The drag creep previously reported by the British National Physics Laboratory at low Reynolds numbers was also found to be present at high Reynolds numbers; the section drag coefficient continued to decrease even at the highest Reynolds number tested. Tests made close to free-stream saturation did not produce altered aerodynamic coefficients due to condensation effects.
NASA Astrophysics Data System (ADS)
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.
NASA Technical Reports Server (NTRS)
Chu, Julio; Luckring, James M.
1996-01-01
An experimental wind tunnel test of a 65 deg delta wing model with interchangeable leading edges was conducted in the Langley National Transonic Facility (NTF). The objective was to investigate the effects of Reynolds and Mach numbers on slender-wing leading-edge vortex flows with four values of wing leading-edge bluntness. Experimentally obtained pressure data are presented without analysis in tabulated and graphical formats across a Reynolds number range of 6 x 10(exp 6) to 120 x 10(exp 6) at a Mach number of 0.85 and across a Mach number range of 0.4 to 0.9 at Reynolds numbers of 6 x 10(exp 6) and 60 x 10(exp 6). Normal-force and pitching-moment coefficient plots for these Reynolds number and Mach number ranges are also presented.
NASA Technical Reports Server (NTRS)
Chu, Julio; Luckring, James M.
1996-01-01
An experimental wind tunnel test of a 65 deg delta wing model with interchangeable leading edges was conducted in the Langley National Transonic Facility (NTF). The objective was to investigate the effects of Reynolds and Mach numbers on slender-wing leading-edge vortex flows with four values of wing leading-edge bluntness. Experimentally obtained pressure data are presented without analysis in tabulated and graphical formats across a Reynolds number range of 6 x 10(exp 6) to 120 x 10(exp 6) at a Mach number of 0.85 and across a Mach number range of 0.4 to 0.9 at Reynolds numbers of 6 x 10(exp 6), 60 x 10(exp 6), and 120 x 10(exp 6). Normal-force and pitching-moment coefficient plots for these Reynolds number and Mach number ranges are also presented.
NASA Technical Reports Server (NTRS)
Chu, Julio; Luckring, James M.
1996-01-01
An experimental wind tunnel test of a 65 deg. delta wing model with interchangeable leading edges was conducted in the Langley National Transonic Facility (NTF). The objective was to investigate the effects of Reynolds and Mach numbers on slender-wing leading-edge vortex flows with four values of wing leading-edge bluntness. Experimentally obtained pressure data are presented without analysis in tabulated and graphical formats across a Reynolds number range of 6 x 10(exp 6) to 84 x 10(exp 6) at a Mach number of 0.85 and across a Mach number range of 0.4 to 0.9 at Reynolds numbers of 6 x 10(exp 6) and 60 x 10(exp 6). Normal-force and pitching-moment coefficient plots for these Reynolds number and Mach number ranges are also presented.
Study of Nonlinear MHD Tribological Squeeze Film at Generalized Magnetic Reynolds Numbers Using DTM.
Rashidi, Mohammad Mehdi; Freidoonimehr, Navid; Momoniat, Ebrahim; Rostami, Behnam
2015-01-01
In the current article, a combination of the differential transform method (DTM) and Padé approximation method are implemented to solve a system of nonlinear differential equations modelling the flow of a Newtonian magnetic lubricant squeeze film with magnetic induction effects incorporated. Solutions for the transformed radial and tangential momentum as well as solutions for the radial and tangential induced magnetic field conservation equations are determined. The DTM-Padé combined method is observed to demonstrate excellent convergence, stability and versatility in simulating the magnetic squeeze film problem. The effects of involved parameters, i.e. squeeze Reynolds number (N1), dimensionless axial magnetic force strength parameter (N2), dimensionless tangential magnetic force strength parameter (N3), and magnetic Reynolds number (Rem) are illustrated graphically and discussed in detail. Applications of the study include automotive magneto-rheological shock absorbers, novel aircraft landing gear systems and biological prosthetics.
Geometry and Reynolds-Number Scaling on an Iced Business-Jet Wing
NASA Technical Reports Server (NTRS)
Lee, Sam; Ratvasky, Thomas P.; Thacker, Michael; Barnhart, Billy P.
2005-01-01
A study was conducted to develop a method to scale the effect of ice accretion on a full-scale business jet wing model to a 1/12-scale model at greatly reduced Reynolds number. Full-scale, 5/12-scale, and 1/12-scale models of identical airfoil section were used in this study. Three types of ice accretion were studied: 22.5-minute ice protection system failure shape, 2-minute initial ice roughness, and a runback shape that forms downstream of a thermal anti-ice system. The results showed that the 22.5-minute failure shape could be scaled from full-scale to 1/12-scale through simple geometric scaling. The 2-minute roughness shape could be scaled by choosing an appropriate grit size. The runback ice shape exhibited greater Reynolds number effects and could not be scaled by simple geometric scaling of the ice shape.
Characteristics of a hyperboloid-flare configuration at high Reynolds numbers
NASA Astrophysics Data System (ADS)
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.
Study of Nonlinear MHD Tribological Squeeze Film at Generalized Magnetic Reynolds Numbers Using DTM
Rashidi, Mohammad Mehdi; Freidoonimehr, Navid; Momoniat, Ebrahim; Rostami, Behnam
2015-01-01
In the current article, a combination of the differential transform method (DTM) and Padé approximation method are implemented to solve a system of nonlinear differential equations modelling the flow of a Newtonian magnetic lubricant squeeze film with magnetic induction effects incorporated. Solutions for the transformed radial and tangential momentum as well as solutions for the radial and tangential induced magnetic field conservation equations are determined. The DTM-Padé combined method is observed to demonstrate excellent convergence, stability and versatility in simulating the magnetic squeeze film problem. The effects of involved parameters, i.e. squeeze Reynolds number (N1), dimensionless axial magnetic force strength parameter (N2), dimensionless tangential magnetic force strength parameter (N3), and magnetic Reynolds number (Rem) are illustrated graphically and discussed in detail. Applications of the study include automotive magneto-rheological shock absorbers, novel aircraft landing gear systems and biological prosthetics. PMID:26267247
A Fluid Structure Interaction Strategy with Application to Low Reynolds Number Flapping Flight
2010-01-01
and CN(t) (force normal to stroke plane) with time for FSI calculation of flapping wing model in free longitudinal flight: (blue) RB1, (dashed...kinematics. The effects of wing flexibility are evaluated on the performance of a flapping , flexible airfoil. In three-dimensional studies most of the...scale flapping wing devices [91]. As the size of the vehicle and characteristic Reynolds number (Re) of the flow decrease, friction forces are
Unsteady heat transfer from a sphere at low Reynolds and Strouhal numbers
NASA Technical Reports Server (NTRS)
Bayazitoglu, Y.; Anderson, C. F.; Cohen, R. D.; Shampine, R. W.
1993-01-01
An axisymmetric formulation is employed to examine convective heat transfer from a sphere in a traveling sound wave. Time-dependent thermal displacement and induced transient flows are found to increase the minimum value for transient heat transfer significantly above the pure conduction limit. Transient effects are found to cause significant differences when compared to a quasi-steady analysis in the determination of transient convection. General trends are found for a range of low Strouhal and Reynolds numbers.
Theory of viscous transonic flow over airfoils at high Reynolds number
NASA Technical Reports Server (NTRS)
Melnik, R. E.; Chow, R.; Mead, H. R.
1977-01-01
This paper considers viscous flows with unseparated turbulent boundary layers over two-dimensional airfoils at transonic speeds. Conventional theoretical methods are based on boundary layer formulations which do not account for the effect of the curved wake and static pressure variations across the boundary layer in the trailing edge region. In this investigation an extended viscous theory is developed that accounts for both effects. The theory is based on a rational analysis of the strong turbulent interaction at airfoil trailing edges. The method of matched asymptotic expansions is employed to develop formal series solutions of the full Reynolds equations in the limit of Reynolds numbers tending to infinity. Procedures are developed for combining the local trailing edge solution with numerical methods for solving the full potential flow and boundary layer equations. Theoretical results indicate that conventional boundary layer methods account for only about 50% of the viscous effect on lift, the remaining contribution arising from wake curvature and normal pressure gradient effects.
Toward Immersed Boundary Simulation of High Reynolds Number Flows
NASA Technical Reports Server (NTRS)
Kalitzin, Georgi; Iaccarino, Gianluca
2003-01-01
of high Reynolds number wall bounded flows is particularly challenging as it requires the consideration of thin turbulent boundary layers, i.e. near wall regions with large gradients of the flow field variables. For such flows, the representation of the wall boundary has a large impact on the accuracy of the computation. It is also critical for the robustness and convergence of the flow solver.
Pulsed jet dynamics of squid hatchlings at intermediate Reynolds numbers.
Bartol, Ian K; Krueger, Paul S; Stewart, William J; Thompson, Joseph T
2009-05-01
Squid paralarvae (hatchlings) rely predominantly on a pulsed jet for locomotion, distinguishing them from the majority of aquatic locomotors at low/intermediate Reynolds numbers (Re), which employ oscillatory/undulatory modes of propulsion. Although squid paralarvae may delineate the lower size limit of biological jet propulsion, surprisingly little is known about the hydrodynamics and propulsive efficiency of paralarval jetting within the intermediate Re realm. To better understand paralarval jet dynamics, we used digital particle image velocimetry (DPIV) and high-speed video to measure bulk vortex properties (e.g. circulation, impulse, kinetic energy) and other jet features [e.g. average and peak jet velocity along the jet centerline (U(j) and U(jmax), respectively), jet angle, jet length based on the vorticity and velocity extents (L(omega) and L(V), respectively), jet diameter based on the distance between vorticity peaks (D(omega)), maximum funnel diameter (D(F)), average and maximum swimming speed (U and U(max), respectively)] in free-swimming Doryteuthis pealeii paralarvae (1.8 mm dorsal mantle length) (Re(squid)=25-90). Squid paralarvae spent the majority of their time station holding in the water column, relying predominantly on a frequent, high-volume, vertically directed jet. During station holding, paralarvae produced a range of jet structures from spherical vortex rings (L(omega)/D(omega)=2.1, L(V)/D(F)=13.6) to more elongated vortex ring structures with no distinguishable pinch-off (L(omega)/D(omega)=4.6, L(V)/D(F)=36.0). To swim faster, paralarvae increased pulse duration and L(omega)/D(omega), leading to higher impulse but kept jet velocity relatively constant. Paralarvae produced jets with low slip, i.e. ratio of jet velocity to swimming velocity (U(j)/U or U(jmax)/U(max)), and exhibited propulsive efficiency [eta(pd)=74.9+/-8.83% (+/-s.d.) for deconvolved data] comparable with oscillatory/undulatory swimmers. As slip decreased with speed
NASA Astrophysics Data System (ADS)
Ma, Wenyong; Liu, Qingkuan; Du, Xiaoqing; Liu, Xiaobing
2017-08-01
The mechanism of large-amplitude aeroelastic vibrations of cylindrical bodies in the critical Reynolds number range are still unclear. This study concerns the aerodynamic forces acting on elliptical cylinders and the induced galloping instability resulting from skew flows (i.e., the direction of the flow is angled 0°-45° with respect to the central axis of the cylinder) for Reynolds numbers in the range of 37-235 k. The effects of the critical Reynolds number and the skew angle on the aerodynamic forces and the galloping instability are investigated with pressure wind tunnel tests. In all of the cases investigated in the present study, a sharp decrease in the lift coefficient with increasing angle of attack and a reduction in the drag coefficient at the critical Reynolds number could be responsible for the galloping instability. Variations in the torque coefficient leads to a torsional aerodynamic instability at the critical Reynolds number. Furthermore, the skew flow cause a critical flow state at lower Reynolds numbers. One possible reason for this behavior is that the longer effective cross section allows the flow to reattach.
Optimization of an Advanced Design Three-Element Airfoil at High Reynolds Numbers
NASA Technical Reports Server (NTRS)
Lin, John C.; Dominik, Chet J.
1995-01-01
New high-lift components have been designed for a three-element advanced high-lift research airfoil using a state-of-the-art computational method. The new components were designed with the aim to provide high maximum-lift values while maintaining attached flow on the single-segment flap at approach conditions. This three-element airfoil has been tested in the NASA Langley Low-Turbulence Pressure Tunnel at chord Reynolds number up to 16 million. The performance of the NASA research airfoil is compared to a reference advanced high-lift research airfoil. Effects of Reynolds number on slat and flap rigging have been studied experimentally. The performance trend of this new high-lift design is comparable to that predicted by the computational method over much of the angle of attack range. Nevertheless, the method did not accurately predict the airfoil performance or the configuration-based trends near maximum lift.
An experimental investigation of the low Reynolds number performance of the Lissaman 7769 airfoil
NASA Technical Reports Server (NTRS)
Conigliaro, P. E.
1983-01-01
A Lissaman 7769 airfoil, used on the Gossamer Condor and Gossamer Albatross human-powered aircraft, was tested in a low turbulence subsonic wind tunnel. Lift and drag data were collected at chord Reynolds numbers of 100,000, 150,000, 200,000, 250,000, and 300,000; at angles of attack from -10 to +20 deg by using an external strain gage force balance. Lift curves, drag curves, and drag polars were generated from both uncorrected data and data corrected for wind tunnel blockage effects. A flow visualization study was performed to correlate with the force data. The results of the investigation have shown that the airfoil exhibits a significant degradation in performance for chord Reynolds numbers below 150,000.
An experimental investigation of the low Reynolds number performance of the Lissaman 7769 airfoil
NASA Technical Reports Server (NTRS)
Conigliaro, P. E.
1983-01-01
A Lissaman 7769 airfoil, used on the Gossamer Condor and Gossamer Albatross human-powered aircraft, was tested in a low turbulence subsonic wind tunnel. Lift and drag data were collected at chord Reynolds numbers of 100,000, 150,000, 200,000, 250,000, and 300,000; at angles of attack from -10 to +20 deg by using an external strain gage force balance. Lift curves, drag curves, and drag polars were generated from both uncorrected data and data corrected for wind tunnel blockage effects. A flow visualization study was performed to correlate with the force data. The results of the investigation have shown that the airfoil exhibits a significant degradation in performance for chord Reynolds numbers below 150,000.
Angular velocity of a spheroid log rolling in a simple shear at small Reynolds number
NASA Astrophysics Data System (ADS)
Meibohm, Jan; Candelier, Fabien; Rosen, Tomas; Einarsson, Jonas; Lundell, Fredrik; Mehlig, Bernhard
2016-11-01
We analyse the angular velocity of a small neutrally buoyant spheroid log rolling in a simple shear. When the effect of fluid inertia is negligible the angular velocity ω -> equals half the fluid vorticity. We compute by singular perturbation theory how weak fluid inertia reduces the angular velocity in an unbounded shear, and how this reduction depends upon the shape of the spheroid (on its aspect ratio). In addition we determine the angular velocity by direct numerical simulations. The results are in excellent agreement with the theory at small but not too small values of the shear Reynolds number, for all aspect ratios considered. For the special case of a sphere we find ω / s = - 1 / 2 + 0 . 0540Re 3 / 2 where s is the shear rate and Re is the shear Reynolds number. This result differs from that derived by Lin et al. who obtained a numerical coefficient roughly three times larger.
On the period of the coherent structure in boundary layers at large Reynolds numbers
NASA Technical Reports Server (NTRS)
Narayanan, M. A. B.; Marvin, J. G.
1978-01-01
The period of the large coherent structure in a subsonic, compressible, turbulent boundary layer was determined using the autocorrelation of the velocity and pressure fluctuations for Reynolds numbers between 5,000 and 35,000. In low Reynolds number flows the overall correlation period scaled with the outer variables - namely, the free stream velocity and the boundary layer thickness.
N-231 High Reynolds Number Channel I is a blowdown Facility that utilizes interchangeable test
NASA Technical Reports Server (NTRS)
1980-01-01
N-231 High Reynolds Number Channel I is a blowdown Facility that utilizes interchangeable test sections and nozzles. The facility provides experimental support for the fluid mechanics research, including experimental verification of aerodynamic computer codes and boundary-layer and airfoil studies that require high Reynolds number simulation. (Tunnel 1)
N-231 High Reynolds Number Channel I is a blowdown Facility that utilizes interchangeable test
NASA Technical Reports Server (NTRS)
1980-01-01
N-231 High Reynolds Number Channel I is a blowdown Facility that utilizes interchangeable test sections and nozzles. The facility provides experimental support for the fluid mechanics research, including experimental verification of aerodynamic computer codes and boundary-layer and airfoil studies that require high Reynolds number simulation. (Tunnel 1)
High-Reynolds Number Circulation Control Testing in the National Transonic Facility
NASA Technical Reports Server (NTRS)
Milholen, William E., II; Jones, Gregory S.; Chan, David T.; Goodliff, Scott L.
2012-01-01
A new capability to test active flow control concepts and propulsion simulations at high Reynolds numbers in the National Transonic Facility at the NASA Langley Research Center is being developed. The first active flow control experiment was completed using the new FAST-MAC semi-span model to study Reynolds number scaling effects for several circulation control concepts. Testing was conducted over a wide range of Mach numbers, up to chord Reynolds numbers of 30 million. 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. Preliminary analysis of the uncorrected lift data showed that the circulation control increased the low-speed maximum lift coefficient by 33%. At transonic speeds, the circulation control was capable of positively altering the shockwave pattern on the upper wing surface and reducing flow separation. Furthermore, application of the technique to only the outboard portion of the wing demonstrated the feasibility of a pneumatic based roll control capability.
Numerical Upscaling of Transport Through Obstructed Regions Over a Broad Range of Reynolds Numbers
NASA Astrophysics Data System (ADS)
Sund, N. L.; Bolster, D.; Mattis, S. A.
2014-12-01
While historically flow and transport in porous media has focused on low Reynolds number and Peclet number regimes there are a variety of examples relevant to environmental fluid dynamics where higher Reynolds number flows are important. A common example might include flow and transport through wetlands where plants act as the solid phase of an effective porous medium. In particular, heterogeneity in the flow field due to presence of the solid phases gives rise to complex transport and mixing behaviors that cannot be upscaled at pre-asymptotic times using conventional approaches. We numerically simulate pore-scale flow and transport through obstructed domains over a range of Reynolds numbers from 15 to 280 and then upscale transport. We upscale using a correlated continuous time random walk (correlated CTRW) model, originally introduced in [1]. We then assess the correlated CTRW's ability to predict observables for both asymptotic and pre-asymptotic time scales and compare our results to those of a classical CTRW to determine when velocity correlations must be accounted for. REFERENCES[1] T.L. Borgne, M. Dentz, J. Carrera: Lagrangian statistical model for transport in highly heterogeneous velocity fields, Physical Review Letters 101 (2008) 090601.
High Reynolds numbers scaling of the turbulent/non-turbulent interface
NASA Astrophysics Data System (ADS)
Bettencourt da Silva, Carlos; Silva, Tiago S.; Idmec Team
2016-11-01
The scaling of the turbulent/non-turbulent interface (TNTI) at high Reynolds numbers is assessed using new direct numerical simulations (DNS) of turbulent planar jets (PJET) and shear free turbulence (SFT) with Reynolds numbers ranging from 142 <= Reλ <= 300 . The thickness of the turbulent sublayer (TSL), where the enstrophy production dominates over enstrophy diffusion, is of the order of the Taylor micro-scale, and is roughly one order of magnitude larger than the Kolmogorov micro-scale for these Reynolds numbers, however it clearly scales with the Kolmogorov micro-scale, at sufficiently high Reynolds numbers. It is argued the same scaling should be observed in TNTI from mixing layers, wakes and boundary layers, provided the Reynolds number is sufficiently high.
The influence of laminar separation and transition on low Reynolds number airfoil hysteresis
NASA Technical Reports Server (NTRS)
Mueller, T. J.
1984-01-01
An experimental study of the Lissaman 7769 and Miley MO6-13-128 airfoils at low chord Reynolds numbers is presented. Although both airfoils perform well near their design Reynolds number of about 600,000, they each produce a different type of hysteresis loop in the lift and drag forces when operated below chord Reynolds numbers of 300,000. The type of hysteresis loop was found to depend upon the relative location of laminar separation and transition. The influence of disturbance environment and experimental procedure on the low Reynolds number airfoil boundary layer behavior is also presented. The use of potential flow solutions to help predict how a given airfoil will behave at low Reynolds numbers is also discussed.
Wake flow pattern modified by small control cylinders at low Reynolds number
NASA Astrophysics Data System (ADS)
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.
Analysis and Design of Rotors at Ultra-Low Reynolds Numbers
NASA Technical Reports Server (NTRS)
Kunz, Peter J.; Strawn, Roger C.
2003-01-01
Design tools have been developed for ultra-low Reynolds number rotors, combining enhanced actuator-ring / blade-element theory with airfoil section data based on two-dimensional Navier-Stokes calculations. This performance prediction method is coupled with an optimizer for both design and analysis applications. Performance predictions from these tools have been compared with three-dimensional Navier Stokes analyses and experimental data for a 2.5 cm diameter rotor with chord Reynolds numbers below 10,000. Comparisons among the analyses and experimental data show reasonable agreement both in the global thrust and power required, but the spanwise distributions of these quantities exhibit significant deviations. The study also reveals that three-dimensional and rotational effects significantly change local airfoil section performance. The magnitude of this issue, unique to this operating regime, may limit the applicability of blade-element type methods for detailed rotor design at ultra-low Reynolds numbers, but these methods are still useful for evaluating concept feasibility and rapidly generating initial designs for further analysis and optimization using more advanced tools.
NASA Astrophysics Data System (ADS)
Lu, Lin; Qin, Jian-Min; Teng, Bin; Li, Yu-Cheng
2011-03-01
This article describes a strategy of active flow control for lift force reduction of circular cylinder subjected to uniform flow at low Reynolds numbers. The flow control is realized by rotationally oscillating the circular cylinder about its axis with ω(t )=-λCL(t), where ω(t ) is the dimensionless angular speed of rotation cylinder, λ is the control parameter and CL(t) is the feedback signal of lift coefficient. The study focuses on seeking optimum λ for the low Reynolds numbers of 60, 80, 100, 150, and 200. The effectiveness of the proposed flow control in suppressing lift force is examined comprehensively by a numerical model based on the finite element solution of two-dimensional Navier-Stokes equations. The dependence of lift reduction on the control parameter λ is investigated. The threshold of λ, denoted by λc, is identified for the Reynolds numbers considered in this work. The numerical results show that the present active rotary oscillation of circular cylinder is able to reduce the amplitude of lift force significantly as long as λ ≤λc, at least 50% for the laminar flow regime. Meanwhile, the present active flow control does not result in the undesirable increase in the drag force. The Strouhal number is observed to decrease slightly with the increase of λ. As for a specific Reynolds number, the larger λ gives rise to the larger amount of lift reduction. The lift reduction reaches the maximum at λ =λc. The mechanism behind the present lift reduction method is revealed by comparing the flow patterns and pressure distributions near the active rotationally oscillating circular cylinder and the stationary circular cylinder. It is found that the critical value λc generally increases with Reynolds number. Two types of lift shift are observed in the numerical results for the cases with λ >λc. The first is characterized by the regular fluctuation of lift coefficient but with nonzero mean value, while the second is associated with the
Flapping and fixed wing aerodynamics of low Reynolds number flight vehicles
NASA Astrophysics Data System (ADS)
Viieru, Dragos
of flapping wing kinematics) are tested. Two main hovering modes are investigated and compared with experimental and other computational efforts. The analysis shows that, while delayed-stall and rapid pitch-up mechanisms are responsible for most of the lift generation at a Reynolds numbers of O(100) and stroke amplitudes of O(1 chord), other mechanisms, including wake-capturing, are identified to contribute to the overall lift/drag force generation. The effect of the Reynolds number on hovering airfoil aerodynamics is also probed.
High Reynolds number flows about bodies of revolution with application to submarines and torpedoes
NASA Astrophysics Data System (ADS)
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
NASA Astrophysics Data System (ADS)
Jiang, Fangfang
As the pressure increases, air behaves very differently from a perfect gas. Such real gas behavior can have major effects on a supersonic turbulent boundary layer and on the corresponding heat transfer process near the wall. A 206.9Mpa (30,000psi) blow-down facility was designed and used to conduct a series of experiments at various stagnation pressures and temperatures where real gas effects are substantial. The Mach number, displacement thickness, Stanton number and recovery factor of the supersonic, turbulent boundary layer were measured with initial stagnation pressures from 69Mpa (10,000psi) to 206.9Mpa (30,000psi) and initial stagnation temperatures from 300K to 360K. Strong real gas effects were found. The data became comprehensible when the usual transport of heat for a perfect gas rhoc pnu'T' was recast in terms of the enthalpy flux rhonu'h' and the Stanton number and recovery factor were defined not in terms of temperatures but in terms of enthalpies. Self consistent sets of Stanton numbers and recovery factors have been obtained, notwithstanding the complexity of the equation of state and the substantial real gas effects, the quasi-steady nature of the experiments and the variation in one experiment from positive to negative heat transfer. The data now forms a basis for evaluating various compressible boundary layer turbulence models and parameters including the choice of turbulent Prandtl number and its dependence on Mach number etc. Considerations of the energy equation, independent of the equation of state, lead to an expectation that the wall recovery temperature can be much higher than the plenum temperature as a result of strong real gas effects at high pressure. Nozzle wall survivability becomes a substantial issue for a very high pressure, high Mach number wind tunnel such as the RDHWT proposed by Brown and Miles. A coaxial flow experiment with a co-flowing, cooler, outer layer of nitrogen has been designed by extending the single vessel blow
Turbulent boundary layer control at moderate Reynolds numbers by means of uniform blowing/suction
NASA Astrophysics Data System (ADS)
Kametani, Yukinori; Fukagata, Koji; Orlu, Ramis; Schlatter, Philipp
2015-11-01
The effect of uniform blowing or suction from the wall on a spatially developing turbulent boundary layer has been studied in order to use them ultimately for flow control on the surface of high-speed vehicles. In the present study, a series of large eddy simulations is performed to investigate the effects of uniform blowing/suction on the skin friction drag as well as the scale of turbulent structures at moderate Reynolds numbers up to Reθ = 2500, based on free-stream velocity, U∞, and momentum thickness, θ. The amplitude of blowing or suction is fixed to 0.1% of U∞with different streamwise ranges of the control region. While the Reynolds shear and normal stresses and their spectral energy distributions are increased by blowing and decreased by suction, in particular, in the outer region, the FIK identity reveals that drag reduction (DR) or enhancement (DE) are mainly linked to changes in the spatial development of the mean wall-normal convection term rather than the contribution from the Reynolds shear stress. Despite the weak amplitude of the control, over 10% of DR and DE are achieved by blowing and suction, respectively. In case of blowing, the mean DR rate increases as the blowing region extends because the local reduction rate grows in the streamwise direction. Grant-in-Aid for Scientific Research (C) (No. 25420129), Grant-in-Aid for JSPS Fellow (No. 24-3450), the Knut an Alice Wallenberg Foundation.
Low-Reynolds number aerodynamics research at NASA Langley Research Center
NASA Technical Reports Server (NTRS)
Harvey, William D.
1986-01-01
The present status of various types of low-Reynolds number aerodynamics research being conducted at the Fluid Dynamics Branch of NASA Langley Research Center is reviewed. The facilities, testing techniques, airfoil design, and experimental verification are addressed, and ongoing studies of laminar separation bubbles, boundary layer stability and transition control, and low-Reynolds number juncture flow are discussed. The possibility of improving vehicle characteristics at low Reynolds numbers and the general trends of the most promising research in these areas are examined.
NASA Astrophysics Data System (ADS)
Monkewitz, Peter A.; Chauhan, Kapil A.; Nagib, Hassan M.
2007-11-01
The asymptotic behavior of mean velocity and integral parameters in flat plate turbulent boundary layers under zero pressure gradient are studied for Reynolds numbers approaching infinity. Using the classical two-layer approach of Millikan, Rotta, and Clauser with a logarithmic velocity profile in the overlap region between "inner" and "outer" layers, a fully self-consistent leading-order description of the mean velocity profile and all integral parameters is developed. It is shown that this description fits most high Reynolds number data, and in particular their Reynolds number dependence, exceedingly well; i.e., within experimental errors.
The Stealth Biplane: a Proposal in Response to a Low Reynolds Number Station Keeping Mission
NASA Technical Reports Server (NTRS)
Walsh, Timothy E.; Flynn, Kevin T.; Donovan, Steven; Paul, Chris; Pangilinan, Harold; Padgett, John; Twomey, Daniel
1990-01-01
The Stealth Biplane is conceived and constructed to serve as a remotely piloted vehicle designed to navigate a low-level figure-eight course at a target Reynolds number of 100,000. This flight vehicle will combine the latest in lightweight radio controlled hardware in conjunction with current low Reynolds number aerodynamic research to demonstrate feasible operation in a variety of applications. These potential low Reynolds number applications include high altitude atmospheric sampling, search and rescue, and even law enforcement. Design specs and fabrication technique are discussed.
Viscous decay of nonlinear oscillations of a spherical bubble at large Reynolds number
NASA Astrophysics Data System (ADS)
Smith, W. R.; Wang, Q. X.
2017-08-01
The long-time viscous decay of large-amplitude bubble oscillations is considered in an incompressible Newtonian fluid, based on the Rayleigh-Plesset equation. At large Reynolds numbers, this is a multi-scaled problem with a short time scale associated with inertial oscillation and a long time scale associated with viscous damping. A multi-scaled perturbation method is thus employed to solve the problem. The leading-order analytical solution of the bubble radius history is obtained to the Rayleigh-Plesset equation in a closed form including both viscous and surface tension effects. Some important formulae are derived including the following: the average energy loss rate of the bubble system during each cycle of oscillation, an explicit formula for the dependence of the oscillation frequency on the energy, and an implicit formula for the amplitude envelope of the bubble radius as a function of the energy. Our theory shows that the energy of the bubble system and the frequency of oscillation do not change on the inertial time scale at leading order, the energy loss rate on the long viscous time scale being inversely proportional to the Reynolds number. These asymptotic predictions remain valid during each cycle of oscillation whether or not compressibility effects are significant. A systematic parametric analysis is carried out using the above formula for the energy of the bubble system, frequency of oscillation, and minimum/maximum bubble radii in terms of the Reynolds number, the dimensionless initial pressure of the bubble gases, and the Weber number. Our results show that the frequency and the decay rate have substantial variations over the lifetime of a decaying oscillation. The results also reveal that large-amplitude bubble oscillations are very sensitive to small changes in the initial conditions through large changes in the phase shift.
The Penguin: a Low Reynolds Number Powered Glider for Station Keeping Missions
NASA Technical Reports Server (NTRS)
Costello, J. K.; Greene, D. W.; Lee, T. T.; Matier, P. T.; Mccarthy, T. R.; Mcguire, R. J.; Schuette, M. J.
1990-01-01
The Penguin is a low Reynolds number (approx. 100,000) remotely piloted vehicle (RPV). It was designed to fly three laps indoors around two pylons in a figure-eight course while maximizing loiter time. The Penguin's low Reynolds number mission is an important one currently being studied for possible future flights in the atmospheres of other planets and for specialized military missions. Although the Penguin's mission seemed quite simple at first, the challenges of such low Reynolds number flight have proven to be quite unique. In addition to the constraint of low Reynolds number flight, the aircraft had to be robust in its control, highly durable, and it had to carry a small instrument package. The Penguin's flight plan, concept, performance, aerodynamic design, weight estimation, structural design, propulsion, stability and control, and cost estimate is detailed.
NASA Technical Reports Server (NTRS)
Cole, Gregory M.; Mueller, Thomas J.
1990-01-01
An experimental investigation was conducted to measure the flow velocity in the boundary layer of an Eppler 387 airfoil. In particular, the laminar separation bubble that this airfoil exhibits at low Reynolds numbers was the focus. Single component laser Doppler velocimetry data were obtained at a Reynolds number of 100,000 at an angle of attack of 2.0 degree. Static Pressure and flow visualization data for the Eppler 387 airfoil were also obtained. The difficulty in obtaining accurate experimental measurements at low Reynolds numbers is addressed. Laser Doppler velocimetry boundary layer data for the NACA 663-018 airfoil at a Reynolds number of 160,000 and angle of attack of 12 degree is also presented.
Model-based control of vortex shedding at low Reynolds numbers
NASA Astrophysics Data System (ADS)
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.
High Reynolds Number Studies in the Wake of a Submarine Model
NASA Astrophysics Data System (ADS)
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.
Wind tunnel tests of two airfoils for wind turbines operating at high reynolds numbers
Sommers, D.; Tangler, J.
2000-06-29
The objectives of this study were to verify the predictions of the Eppler Airfoil Design and Analysis Code for Reynolds numbers up to 6 x 106 and to acquire the section characteristics of two airfoils being considered for large, megawatt-size wind turbines. One airfoil, the S825, was designed to achieve a high maximum lift coefficient suitable for variable-speed machines. The other airfoil, the S827, was designed to achieve a low maximum lift coefficient suitable for stall-regulated machines. Both airfoils were tested in the NASA Langley Low-Turbulence Pressure Tunnel (LTPT) for smooth, fixed-transition, and rough surface conditions at Reynolds numbers of 1, 2, 3, 4, and 6 x 106. The results show the maximum lift coefficient of both airfoils is substantially underpredicted for Reynolds numbers over 3 x 106 and emphasized the difficulty of designing low-lift airfoils for high Reynolds numbers.
Swimming at low Reynolds number in fluids with odd, or Hall, viscosity.
Lapa, Matthew F; Hughes, Taylor L
2014-04-01
We apply the geometric theory of swimming at low Reynolds number to the study of nearly circular swimmers in two-dimensional fluids with nonvanishing "odd," or Hall, viscosity. The odd viscosity gives an off-diagonal contribution to the fluid stress tensor, which results in a number of striking effects. In particular, we find that a swimmer whose area is changing will experience a torque proportional to the rate of change of the area, with the constant of proportionality given by the coefficient ηo of odd viscosity. After working out the general theory of swimming in fluids with odd viscosity for a class of simple swimmers, we give a number of example swimming strokes which clearly demonstrate the differences between swimming in a fluid with conventional viscosity and a fluid which also has an odd viscosity. We also include a discussion of the extension of the famous Scallop theorem of low Reynolds number swimming to the case where the fluid has a nonzero odd viscosity. A number of more technical results, including a proof of the torque-area relation for swimmers of more general shape, are explained in a set of Appendixes.
Reynolds-Number Dependence of Gas Dispersion Over a Wavy Wall
NASA Astrophysics Data System (ADS)
Michioka, Takenobu; Takimoto, Hiroshi; Ono, Hiroki; Sato, Ayumu
2017-09-01
Large-eddy simulation is used to investigate the Reynolds-number dependence of gas dispersion over a wavy wall, because the Reynolds-number dependence is important for reproducing normal flow and gas dispersion in a wind tunnel. The ratio of amplitude to wavelength of the wavy surface is set to 0.1, and the Reynolds number based on the bulk velocity and the channel height is varied from 6.67× 103 to 6.67× 104. Two tracer gases are emitted from point sources located at a single crest and trough of the wavy wall. For the lowest Reynolds number, the flow over the wavy wall separates behind the crest and reattaches to the upslope. A recirculation zone is observed near the trough, and the gas emitted from the trough is transported upwind by the recirculating reverse flow. Some gas is discharged from the valley by intermittent velocity bursts that originate in the recirculation zone. As the Reynolds number is increased, the recirculation zone shrinks and the flow increasingly follows the wavy wall. The gas generally disperses in the forward direction and is discharged by the advective flow. As for the gas emitted from the crest, this disperses with the separating flow, while some gas is trapped within the recirculation zone at the lower Reynolds number. As the Reynolds number is increased, the gas advection increasingly follows the wavy wall and the height of the peak concentration approaches the wavy wall. In addition, the accumulated concentration within the valley in both sources depends strongly on the Reynolds number.
Reynolds number dependence of large-scale friction control in turbulent channel flow
NASA Astrophysics Data System (ADS)
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.
Stratlets: low Reynolds number point-force solutions in a stratified fluid.
Ardekani, A M; Stocker, R
2010-08-20
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(100 μm-1 mm) 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.
Stratlets: Low Reynolds Number Point-Force Solutions in a Stratified Fluid
NASA Astrophysics Data System (ADS)
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.
NASA Technical Reports Server (NTRS)
Stanewsky, E.; Demurie, F.; Ray, Edward J.; Johnson, C. B.
1989-01-01
The transonic airfoil CAST 10-2/DOA 2 was investigated in several major transonic wind tunnels at Reynolds numbers ranging from Re=1.3 x 10(exp 6) to 45 x 10(exp 6) at ambient and cryogenic temperature conditions. The main objective was to study the degree and extent of the effects of Reynolds number on both the airfoil aerodynamic characteristics and the interference effects of various model-wind-tunnel systems. The initial analysis of the CAST 10-2 airfoil results revealed appreciable real Reynolds number effects on this airfoil and showed that wall interference can be significantly affected by changes in Reynolds number thus appearing as true Reynolds number effects.
NASA Technical Reports Server (NTRS)
Hess, Robert W.; Seidel, David A.; Igoe, William B.; Lawing, Pierce L.
1987-01-01
Steady and unsteady pressures were measured on a 2-D supercritical airfoil in the Langley Research Center 0.3-m Transonic Cryogenic Tunnel at Reynolds numbers from 6 x 1,000,000 to 35 x 1,000,000. The airfoil was oscillated in pitch at amplitudes from plus or minus .25 degrees to plus or minus 1.0 degrees at frequencies from 5 Hz to 60 Hz. The special requirements of testing an unsteady pressure model in a pressurized cryogenic tunnel are discussed. Selected steady measured data are presented and are compared with GRUMFOIL calculations at Reynolds number of 6 x 1,000,000 and 30 x 1,000,000. Experimental unsteady results at Reynolds numbers of 6 x 1,000,000 and 30 x 1,000,000 are examined for Reynolds number effects. Measured unsteady results at two mean angles of attack at a Reynolds number of 30 x 1,000,000 are also examined.
Flow and Noise Control in High Speed and High Reynolds Number Jets Using Plasma Actuators
NASA Technical Reports Server (NTRS)
Samimy, M.; Kastner, J.; Kim, J.-H.; Utkin, Y.; Adamovich, I.; Brown, C. A.
2006-01-01
The idea of manipulating flow to change its characteristics is over a century old. Manipulating instabilities of a jet to increase its mixing and to reduce its radiated noise started in the 1970s. While the effort has been successful in low-speed and low Reynolds number jets, available actuators capabilities in terms of their amplitude, bandwidth, and phasing have fallen short in control of high-speed and high Reynolds number jets of practical interest. Localized arc filament plasma actuators have recently been developed and extensively used at Gas Dynamics and Turbulence Laboratory (GDTL) for control of highspeed and high Reynolds number jets. While the technique has been quite successful and is very promising, all the work up to this point had been carried out using small high subsonic and low supersonic jets from a 2.54 cm diameter nozzle exit with a Reynolds number of about a million. The preliminary work reported in this paper is a first attempt to evaluate the scalability of the technique. The power supply/plasma generator was designed and built in-house at GDTL to operate 8 actuators simultaneously over a large frequency range (0 to 200 kHz) with independent control over phase and duty cycle of each actuator. This allowed forcing the small jet at GDTL with azimuthal modes m = 0, 1, 2, 3, plus or minus 1, plus or minus 2, and plus or minus 4 over a large range of frequencies. This power supply was taken to and used, with minor modifications, at the NASA Nozzle Acoustic Test Rig (NATR). At NATR, 32 actuators were distributed around the 7.5 in. nozzle (a linear increase with nozzle exit diameter would require 60 actuators). With this arrangement only 8 actuators could operate simultaneously, thus limiting the forcing of the jet at NATR to only three azimuthal modes m = plus or minus 1, 4, and 8. Very preliminary results at NATR indicate that the trends observed in the larger NASA facility in terms of the effects of actuation frequency and azimuthal modes are
Finite Reynolds number properties of a turbulent channel flow similarity solution
NASA Astrophysics Data System (ADS)
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.
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.
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. PMID:26528815
Active Control of Flow Separation on a High-Lift System with Slotted Flap at High Reynolds Number
NASA Technical Reports Server (NTRS)
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.
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
Backward-facing step measurements at low Reynolds number, Re(sub h)=5000
NASA Technical Reports Server (NTRS)
Jovic, Srba; Driver, David M.
1994-01-01
An experimental study of the flow over a backward-facing step at low Reynolds number was performed for the purpose of validating a direct numerical simulation (DNS) which was performed by the Stanford/NASA Center for Turbulence Research. Previous experimental data on back step flows were conducted at Reynolds numbers and/or expansion ratios which were significantly different from that of the DNS. The geometry of the experiment and the simulation were duplicated precisely, in an effort to perform a rigorous validation of the DNS. The Reynolds number used in the DNS was Re(sub h)=5100 based on step height, h. This was the maximum possible Reynolds number that could be economically simulated. The boundary layer thickness, d, was approximately 1.0 h in the simulation and the expansion ratio was 1.2. The Reynolds number based on the momentum thickness, Re(sub theta), upstream of the step was 610. All of these parameters were matched experimentally. Experimental results are presented in the form of tables, graphs and a floppy disk (for easy access to the data). An LDV instrument was used to measure mean velocity components and three Reynolds stresses components. In addition, surface pressure and skin friction coefficients were measured. LDV measurements were acquired in a measuring domain which included the recirculating flow region.
Project Dawdler: a Proposal in Response to a Low Reynolds Number Station Keeping Mission
NASA Technical Reports Server (NTRS)
Bartilotti, Rich; Coakley, Jill; Golla, Warren; Scamman, Glenn; Tran, Hoa T.; Trippel, Chris
1990-01-01
In direct response to Request for Proposals: Flight at very low Reynolds numbers - a station keeping mission, the members of Design Squad E present Project Dawdler: a remotely-piloted airplane supported by an independently controlled take-off cart. A brief introduction to Project Dawdler's overall mission and design, is given. The Dawdler is a remotely-piloted airplane designed to fly in an environmentally-controlled closed course at a Reynolds number of 10(exp 5) and at a cruise velocity of 25 ft/s. The two primary goals were to minimize the flight Reynolds number and to maximize the loiter time. With this in mind, the general design of the airplane was guided by the belief that a relatively light aircraft producing a fairly large amount of lift would be the best approach. For this reason the Dawdler utilizes a canard rather than a conventional tail for longitudinal control, primarily because the canard contributes a positive lift component. The Dawdler also has a single vertical tail mounted behind the wing for lateral stability, half of which is used as a rudder for yaw control. Due to the fact that the power required to take-off and climb to altitude is much greater than that required for cruise flight and simple turning maneuvers, it was decided that a take-off cart be used. Based on the current design, there are two unknowns which could possibly threaten the success of Project Dawdler. First, the effect of the fully-movable canard with its large appropriation of total lift on the performance of the plane, and secondly, the ability of the take-off procedure to go as planned are examined. These are questions which can only be answered by a prototype.
NASA Astrophysics Data System (ADS)
Cho, Young-Chang; Shyy, Wei
2011-10-01
Aerodynamic performance of low-Reynolds number flyers, for a chord-based Reynolds number of 10 5 or below, is sensitive to wind gusts and flow separation. Active flow control offers insight into fluid physics as well as possible improvements in vehicle performance. While facilitating flow control by introducing feedback control and fluidic devices, major challenges of achieving a target aerodynamic performance under unsteady flow conditions lie on the high-dimensional nonlinear dynamics of the flow system. Therefore, a successful flow control framework requires a viable as well as accessible control scheme and understanding of underlying flow dynamics as key information of the flow system. On the other hand, promising devices have been developed recently to facilitate flow control in this flow regime. The dielectric barrier discharge (DBD) actuator is such an example; it does not have moving parts and provides fast impact on the flow field locally. In this paper, recent feedback flow control studies, especially those focusing on unsteady low-Reynolds number aerodynamics, are reviewed. As an example of an effective flow control framework, it is demonstrated that aerodynamic lift of a high angle-of-attack wing under fluctuating free-stream conditions can be stabilized using the DBD actuator and an adaptive algorithm based on general input-output models. System nonlinearities and control challenges are discussed by assessing control performance and the variation of the system parameters under various flow and actuation conditions. Other fundamental issues from the flow dynamics view point, such as the lift stabilization mechanism and the influence on drag fluctuation are also explored. Both potentiality and limitation of the linear modeling approach are discussed. In addition, guidelines on system identification and the controller and actuator setups are suggested.
Compressible Boundary Layer Predictions at High Reynolds Number using Hybrid LES/RANS Methods
NASA Technical Reports Server (NTRS)
Choi, Jung-Il; Edwards, Jack R.; Baurle, Robert A.
2008-01-01
Simulations of compressible boundary layer flow at three different Reynolds numbers (Re(sub delta) = 5.59x10(exp 4), 1.78x10(exp 5), and 1.58x10(exp 6) are performed using a hybrid large-eddy/Reynolds-averaged Navier-Stokes method. Variations in the recycling/rescaling method, the higher-order extension, the choice of primitive variables, the RANS/LES transition parameters, and the mesh resolution are considered in order to assess the model. The results indicate that the present model can provide good predictions of the mean flow properties and second-moment statistics of the boundary layers considered. Normalized Reynolds stresses in the outer layer are found to be independent of Reynolds number, similar to incompressible turbulent boundary layers.
Flow over a hydrofoil with trailing edge vortex shedding at high-Reynolds number
NASA Astrophysics Data System (ADS)
Bourgoyne, Dwayne Anthony
At high Reynolds number, the flow of an incompressible fluid over a lifting surface is a rich blend of fluid dynamic phenomena, and the individual elements of this process have been the subject of much prior work. However, controlled experimental investigations of lifting surfaces at Reynolds numbers typical of heavy-lift aircraft wings or full-size ship propellers (chord-based Reynolds numbers, ReC ˜ 107--10 8) are largely unavailable. This paper presents experimental results from the flow over a two-dimensional hydrofoil at nominal ReC values from near one million (1M) to more than 50 million (50M). The tests were conducted in the U.S. Navy's William B. Morgan Large Cavitation Channel with a solid-bronze hydrofoil (2.1 m chord, 3.0 m span, 17 cm maximum thickness) at flow speeds from 0.25 to 18.3 m/s. The foil section, a modified NACA 0016 with a rounded trailing-edge bevel, approximates the cross section of a generic naval propeller blade. Trailing-edge geometries with bevel angles of 44° and 56° are investigated. Flow field velocities are measured with laser Doppler velocimetry and planar particle imaging velocimetry. Pressure measurements are made with static pressure taps along the foil chord and test section walls and with unsteady pressure sensors near the trailing edge. Results are presented from the time-averaged flow (part I), as well as turbulence statistics, pressure and velocity spectra, and instantaneous velocity fields (part II). Geometry and Reynolds-number dependencies in the mean flow are linked to similar dependencies in the dynamic flow. A correlation is shown between the suction side time-average shear rate near the trailing edge and the strength of the near-wake vortex shedding. Peaks in spectra of vertical velocity fluctuations associated with vortex shedding near the trailing edge are strongest when the suction side shear layer, which separates upstream of the trailing edge, most effectively induces roll-up of the pressure side shear
NASA Astrophysics Data System (ADS)
Katti, Vadiraj V.; Yasaswy, S. Nagesh; Prabhu, S. V.
2011-03-01
An experimental investigation is performed to study the effect of jet to plate spacing and low Reynolds number on the local heat transfer distribution to normally impinging submerged circular air jet on a smooth and flat surface. A single jet from a straight circular nozzle of length-to-diameter ratio ( l/d) of 83 is tested. Reynolds number based on nozzle exit condition is varied between 500 and 8,000 and jet-to-plate spacing between 0.5 and 8 nozzle diameters. The local heat transfer characteristics are obtained using thermal images from infrared thermal imaging technique. It was observed that at lower Reynolds numbers, the effect of jet to plate distances covered during the study on the stagnation point Nusselt numbers is minimal. At all jet to plate distances, the stagnation point Nusselt numbers decrease monotonically with the maximum occurring at a z/d of 0.5 as opposed to the stagnation point Nusselt numbers at high Reynolds numbers which occur around a z/d of 6.
NASA Technical Reports Server (NTRS)
Johnson, Charles B.; Dress, David A.; Hill, Acquilla S.; Wilcox, Peter A.; Bui, Minh H.
1986-01-01
A wind-tunnel investigation of a Douglas advanced-technology airfoil was conducted in the Langley 0.3-Meter Transonic Cryogenic Tunnel (0.3-m TCT). The temperature was varied from 227 K (409 R) to 100 K (180 R) at pressures ranging from about 159 kPa (1.57 atm) to about 514 kPa (5.07 atm). Mach number was varied from 0.50 to 0.78. These variables provided a Reynolds number range (based on airfoil chord) from 6.0 to 30.0 x 10 to the 6th power. This investigation was specifically designed to: (1) test a Douglas airfoil from moderately low to flight-equivalent Reynolds numbers, and (2) evaluate sidewall-boundary-layer effects on transonic airfoil performance characteristics by a systematic variation of Mach number, Reynolds number, and sidewall-boundary-layer removal. Data are included which demonstrate the effects of fixing transition, Mach number, Reynolds number, and sidewall-boundary-layer removal on the aerodynamic characteristics of the airfoil. Also included are remarks on model design and model structural integrity.
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.
Aeroacoustic data for high Reynolds number supersonic axisymmetric jets
NASA Astrophysics Data System (ADS)
Seiner, J. M.; Ponton, M. K.
1985-01-01
Both aerodynamic and near field acoustic behavior of several unheated axisymmetric shock free and shock containing high speed jet plumes are reported. The exit Mach number range for these data is from 0.9 to 2.5. The aerodynamic measurements include both mean and turbulence quantities for a shock free jet plume produced by a convergent divergent nozzle designed to have an exit Mach number of 2. The near field acoustic measurements presented include narrow band spectra, directivity and contour plots of select one third octave band data, and near field microphone correlations from a linear array. Shock noise results are also included as obtained by operating an underexpanded convergent nozzle at the design point of two supersonic exist Mach number convergent divergent nozzles.
Separation Control at Flight Reynolds Numbers: Lessons Learned and Future Directions
NASA Technical Reports Server (NTRS)
Seifert, Avi; Pack, LaTunia G.
2000-01-01
Active separation control, using periodic excitation, was studied experimentally at high Reynolds numbers. The effects of compressibility, mild sweep, location o excitation slot and steady momentum transfer on the efficacy of the method were identified. Tests conducted at chord Reynolds numbers as high as 40 x 10(exp 6) demonstrated that active control using oscillatory flow excitation can effectively delay flow separation from and reattach separated flow to aerodynamic surfaces at flight conditions. The effective frequencies generate one to four vortices over the controlled region at all times, regardless of the Reynolds number. The vortices are initially amplified by the separated shear-layer, and after initiating reattachment, the strength of the vortices decay as they are convected downstream. Large amplitude, low frequency vortices break down to smaller ones upon introduction at the excitation slot. The effects of steady mass transfer were compared to those of periodic excitation. It was found that steady blowing is significantly inferior to periodic excitation in terms o performance benefits and that the response to steady blowing is abrupt, and therefore undesirable from a control point of view. Steady suction and periodic excitation are comparable in effectiveness and both exhibit a gradual response to changes in the magnitude of the control input. The combination of weak steady suction and periodic excitation is extremely effective while the addition of steady blowing could be detrimental. Compressibility effects are weak as long as separation is not caused by a shock-wave/boundary-layer interaction The undesirable effects of the shock-induced separation could be alleviated by the introduction of periodic excitation upstream of the shock wave, inside the region of supersonic flow. The effects of mild sweep were also studied and periodic excitation was found to be very effective in reattaching three-dimensional separated flow. Scaling laws that correlate 2D
NASA Astrophysics Data System (ADS)
Ozmen, Y.; Baydar, E.
2008-09-01
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 st∝ Re 0.69( H/D)0.019.
On the drag of model dendrite fragments at low Reynolds number
NASA Technical Reports Server (NTRS)
Zakhem, R.; Weidman, P. D.; De Groh, H. C., III
1992-01-01
An experimental study of low Reynolds number drag on laboratory models of dendrite fragments has been conducted. The terminal velocities of the dendrites undergoing free fall along their axis of symmetry were measured in a large Stokes flow facility. Corrections for wall interference give nearly linear drag vs Reynolds number curves. Corrections for both wall interference and inertia effects show that the dendrite Stokes settling velocities are always less than that of a sphere of equal mass and volume. In the Stokes limit, the settling speed ratio is found to correlate well with the primary dendrite arm aspect ratio and a second dimensionless shape parameter which serves as a measure of the fractal-like nature of the dendrite models. These results can be used to estimate equiaxed grain velocities and distance of travel in metal castings. The drag measurements may be used in numerical codes to calculate the movement of grains in a convecting melt in an effort to determine macrosegregation patterns caused by the sink/float mechanism.
On the drag of model dendrite fragments at low Reynolds number
NASA Technical Reports Server (NTRS)
Zakhem, R.; Weidman, P. D.; Degroh, H. C., III
1993-01-01
An experimental study of low Reynolds number drag on laboratory models of dendrite fragments has been conducted. The terminal velocities of the dendrites undergoing free fall along their axis of symmetry were measured in a large Stokes flow facility. Corrections for wall interference give nearly linear drag vs Reynolds number curves. Corrections for both wall interference and inertia effects show that the dendrite Stokes settling velocities are always less than that of a sphere of equal mass and volume. In the Stokes limit, the settling speed ratio is found to correlate well with primary dendrite arm aspect ratio and a second dimensionless shape paremeter which serves as a measure of the fractal-like nature of the dendrite models. These results can be used to estimate equiaxed grain velocities and distance of travel in metal castings. The drag measurements may be used in numerical codes to calculate the movement of grains in a convecting melt in an effort to determine macrosegregation patterns caused by the sink/float mechanism.
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.
Control of Vortex Shedding on an Airfoil using Mini Flaps at Low Reynolds Number
NASA Astrophysics Data System (ADS)
Oshiyama, Daisuke; Numata, Daiju; Asai, Keisuke
2015-11-01
In this study, the effects of mini flaps (MFs) on a NACA0012 airfoil were investigated experimentally at low Reynolds number. MFs are small flat plates attached to the trailing edge of an airfoil perpendicularly. All the tests were conducted at the Tohoku-University Basic Aerodynamic Research Tunnel at the chord Reynolds number of 25,000. Aerodynamic forces were measured using a 3-component balance and the surface flow was visualized by luminescent oil film technique. The results of force measurement show that attachment of MFs enhances lift and the enhanced lift increases with MF height. On the other hand, the results of oil flow visualization show that attachment of MFs enlarges the separated region on the airfoil rather than diminishes it. To understand the physical mechanism of MFs for lift enhancement, the flow around the airfoil was visualized by the smoke-wire method and the wake profile behind the airfoil was measured using a hot wire anemometer. It was found that vortices shed periodically from the tip of the MFs and interact with the separated shear layer from the upper surface. This unsteady vortex shedding forms a low-pressure region on the upper surface, generating higher lift. These results suggest that the height of MFs controls the frequency of vortex shedding behind the MF, forcing the separated shear layer on the upper surface flow in unsteady manner.
Holzman, Roi; China, Victor; Yaniv, Sarit; Zilka, Miri
2015-07-01
Larval fishes suffer prodigious mortality rates, eliminating 99% of the cohort within a few days after their first feeding. Hjort (1914) famously attributed this "critical period" of low survival to larval inability to obtain sufficient food. We discuss recent experimental and modeling work, suggesting that the viscous hydrodynamic regime have marked effects on the mechanism of suction feeding in larval fish. As larvae grow, the size of the gape and associated volume of the mouth increase. At the same time, larvae swim faster and can generate faster suction flows, thus transiting to a hydrodynamic regime of higher Reynolds numbers. This hydrodynamic regime further leads to changes in the spatio-temporal patterns of flow in front of the mouth, and an increasing ability in larger larvae to exert suction forces on the prey. Simultaneously, the increase in swimming speed and the distance from which the prey is attacked result in higher rates of encountering prey by larger (older) larvae. In contrast, during the first few days after feeding commence the lower rates of encounter and success in feeding translate to low feeding rates. We conclude that young larvae experience "hydrodynamic starvation," in which low Reynolds numbers mechanically limit their feeding performance even under high densities of prey. © The Author 2015. Published by Oxford University Press on behalf of the Society for Integrative and Comparative Biology. All rights reserved. For permissions please email: journals.permissions@oup.com.
Drag Measurements over Embedded Cavities in a Low Reynolds Number Couette Flow
NASA Astrophysics Data System (ADS)
Gilmer, Caleb; Lang, Amy; Jones, Robert
2010-11-01
Recent research has revealed that thin-walled, embedded cavities in low Reynolds number flow have the potential to reduce the net viscous drag force acting on the surface. This reduction is due to the formation of embedded vortices allowing the outer flow to pass over the surface via a roller bearing effect. It is also hypothesized that the scales found on butterfly wings may act in a similar manner to cause a net increase in flying efficiency. In this experimental study, rectangular embedded cavities were designed as a means of successfully reducing the net drag across surfaces in a low Reynolds number flow. A Couette flow was generated via a rotating conveyor belt immersed in a tank of high viscosity mineral oil above which the plates with embedded cavities were placed. Drag induced on the plate models was measured using a force gauge and compared directly to measurements acquired over a flat plate. Various cavity aspect ratios and gap heights were tested in order to determine the conditions under which the greatest drag reductions occurred.
The FX/90: A proposal in response to a low Reynolds Number station keeping mission
NASA Technical Reports Server (NTRS)
Wirthman, David; Palmer, Julie; Gleixner, Aaron; Russell, Scott; Nevala, Tom; Nosek, Mark
1990-01-01
The FX/90 is a remotely piloted vehicle designed to fly at Reynolds numbers below 2 x 10 to the 5th power. Several applications exist for this type of flight, such as low altitude flight of very small aircraft. The design presented here allows investigation into the unique problems involved in low Reynolds number flight, which will, in turn, further understanding of this flight regime. The aircraft will operate in a steady flight environment, free from significant atmospheric turbulence and weather effects. The F-90 has a 39 in. fuselage which is constructed of balsa and plywood. The landing gear for the aircraft is a detachable carriage on which the aircraft rests. The aerodynamic planform is a rectangular wing (no taper or sweep) with a chord of 9 in., a wingspan of 72 in., and is constructed entirely out of styrofoam. The propulsion system is a puller configuration mounted on the front of the fuselage. It consists of an Astro 05 engine and a 10-6 two bladed propeller. Control of the aircraft is accomplished through the use of two movable control surfaces: elevators for pitch control, and a rudder for yaw control. The aircraft is soundly constructed, highly maneuverable, and adequately powered. Furthermore, the investigation into alternative technologies, most notably the styrofoam wing and the detachable landing gear, holds promise to improve the performance of the aircraft.
Low Reynolds number modeling of turbulent flows with and without wall transpiration
NASA Technical Reports Server (NTRS)
So, Ronald M. C.; Yoo, Geun Jong
1987-01-01
A full Reynolds-stress closure that is capable of describing the flow all the way to the wall is formulated. The closure is based on the conventional high Reynolds number form of the redistribution model, the inclusion of molecular diffusion, and a modified dissipation model to account for viscous effects near a wall. Two dissipation models are investigated along with two gradient diffusion and two redistribution models. Their respective effects on the calculated flow properties are assessed by comparing them with the data of fully developed turbulent flows and a developing pipe flow with wall transpiration. The near-wall behavior is very well predicted; however, the wall correction to the redistribution modeling is found to have little effect on the calculated results. The overall behavior of the fully developed turbulent flows is best described by a nonisotropic gradient diffusion model, a return-to-isotropy redistribution model, and a dissipation model that accounts for viscous behavior near a wall. This same closure also gives the best prediction of the axial pressure drop behavior along a pipe with a uniform wall suction. Furthermore, the near-wall behavior of such a flow is very well predicted by this closure.
Numerical Simulations of Subscale Wind Turbine Rotor Inboard Airfoils at Low Reynolds Number
Blaylock, Myra L.; Maniaci, David Charles; Resor, Brian R.
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.
Low-Reynolds-number k-epsilon model for unsteady turbulent boundary-layer flows
NASA Technical Reports Server (NTRS)
Fan, Sixin; Lakshminarayana, Budugur; Barnett, Mark
1993-01-01
An assessment of the near-wall and low-Reynolds-number functions used in low-Reynolds-number k-epsilon models suggests that they are not suitable for the near-wall region of unsteady turbulent boundary layers, where the flow is characterized by rapid changes in phase. An improved low-Reynolds-number k-epsilon model is developed in this paper. The near-wall and low-Reynolds-number functions in this model are formulated as functions of the local turbulent Reynolds numbers instead of the inner variable y(+). The present model also has the correct asymptotic behavior in the near-wall region. The turbulence model has been incorporated in an unsteady boundary-layer code and validated for unsteady turbulent boundary layers with and without adverse pressure gradients. The predictions agree well with the experimental data and the theoretical analysis. For the cases tested, the present model correctly predicts the unsteady near-wall flow and the unsteady shin friction at various frequencies.
NASA Astrophysics Data System (ADS)
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.
Experimental Study of Thin NACA Symmetric and Cambered Airfoils at Low Reynolds Numbers
NASA Astrophysics Data System (ADS)
Durgesh, Vibhav; Garcia, Elifalet; Johari, Hamid
2016-11-01
The low-Reynolds number performance of airfoils is intriguing due to the complex fluid dynamics phenomena associated with flow at these Reynolds numbers, like laminar separated flow, increased transition susceptibility, and the separated shear layer that undergoes a rapid transition to a turbulent flow. Therefore, the objective of this investigation was to experimentally study the aerodynamic performance of a thin symmetric airfoil (NACA-0012) and a cambered (NACA-6412) airfoil at low Reynolds numbers, and to identify the flow structures responsible for altering the aerodynamic performance. Lift and drag force measurements were performed for both airfoils along with flow visualization measurements for Reynolds numbers of 20,000, 30,000, 40,000, and 50,000 and angles of attack between -8o to 15° with an increment of 1°. All the measurements for this study were performed in the water tunnel facility at California State University Northridge. A significant difference in the aerodynamic performance and flow behavior of the thin cambered airfoil is observed as compared to that of the thin symmetric airfoil. The presentation will discuss the correlation between observed flow structures and aerodynamic performance of both airfoils at low-Reynolds numbers.
On an acoustic field generated by subsonic jet at low Reynolds numbers
NASA Technical Reports Server (NTRS)
Yamamoto, K.; Arndt, R. E. A.
1978-01-01
An acoustic field generated by subsonic jets at low Reynolds numbers was investigated. This work is motivated by the need to increase the fundamental understanding of the jet noise generation mechanism which is essential to the development of further advanced techniques of noise suppression. The scope of this study consists of two major investigation. One is a study of large scale coherent structure in the jet turbulence, and the other is a study of the Reynolds number dependence of jet noise. With this in mind, extensive flow and acoustic measurements in low Reynolds number turbulent jets (8,930 less than or equal to M less than or equal to 220,000) were undertaken using miniature nozzles of the same configuration but different diameters at various exist Mach numbers (0.2 less than or equal to M less than or equal to 0.9).
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.
Separation over a flat plate-wedge configuration at oceanic Reynolds numbers
NASA Technical Reports Server (NTRS)
Campbell, D. R.
1973-01-01
An experimental study of flow over a two-dimensional flat plate-wedge configuration is presented. The investigation encompasses a range of Reynolds numbers characteristics of conditions encountered by deep submersible oceanic vehicles. Flow separation, similar to that found on high speed aircraft control surfaces, is reported and discussed in light of the laminar or transitional nature of the separated shear layer. As discovered in previous high Mach number studies of plate-wedge or ramp configurations, the dependency of the size of the separated region on free stream Reynolds number is reversed for laminar and transitional types of flow separation.
Aeroacoustic Data for a High Reynolds Number Axisymmetric Subsonic Jet
NASA Technical Reports Server (NTRS)
Ponton, Michael K.; Ukeiley, Lawrence S.; Lee, Sang W.
1999-01-01
The near field fluctuating pressure and aerodynamic mean flow characteristics of a cold subsonic jet issuing from a contoured convergent nozzle are presented. The data are presented for nozzle exit Mach numbers of 0.30, 0.60, and 0.85 at a constant jet stagnation temperature of 104 F. The fluctuating pressure measurements were acquired via linear and semi-circular microphone arrays and the presented results include plots of narrowband spectra, contour maps, streamwise/azimuthal spatial correlations for zero time delay, and cross-spectra of the azimuthal correlations. A pitot probe was used to characterize the mean flow velocity by assuming the subsonic flow to be pressure-balanced with the ambient field into which it exhausts. Presented are mean flow profiles and the momentum thickness of the free shear layer as a function of streamwise position.
A High Altitude-Low Reynolds Number Aerodynamic Flight Experiment
NASA Technical Reports Server (NTRS)
Greer, Don; Krake, Keith; Hamory, Phil; Drela, Mark; Lee, Seunghee (Technical Monitor)
1999-01-01
A sailplane is currently being developed at NASA's Dryden Flight Research Center to support a high altitude flight experiment. The purpose of the experiment is to measure the performance characteristics of an airfoil at altitudes between 100,000 and 70,000 feet at Mach numbers between 0.65 and 0.5. The airfoil lift and drag are measured from pilot and static pressures. The location of the separation bubble and vortex shedding are measured from a hot film strip. The details of the flight experiment are presented. A comparison of several estimates of the airfoil performance is also presented. The airfoil, APEX-16, was designed by Drela (MIT) with his MSES code. A two dimensional Navier-Stokes analysis has been performed by Tatineni and Zhong (UCLA) and another at the Dryden Flight Research Center. The role these analysis served to define the experiment is discussed.
Binary tree models of high-Reynolds-number turbulence
NASA Astrophysics Data System (ADS)
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.
Direct Numerical Simulation of Flows over an NACA-0012 Airfoil at Low and Moderate Reynolds Numbers
NASA Technical Reports Server (NTRS)
Balakumar, P.
2017-01-01
Direct numerical simulations (DNS) of flow over an NACA-0012 airfoil are performed at a low and a moderate Reynolds numbers of Re(sub c)=50 times10(exp 3) and 1times 10(exp 6). The angles of attack are 5 and 15 degrees at the low and the moderate Reynolds number cases respectively. The three-dimensional unsteady compressible Navier-Stokes equations are solved using higher order compact schemes. The flow field in the low Reynolds number case consists of a long separation bubble near the leading-edge region and an attached boundary layer on the aft part of the airfoil. The shear layer that formed in the separated region persisted up to the end of the airfoil. The roles of the turbulent diffusion, advection, and dissipation terms in the turbulent kinetic-energy balance equation change as the boundary layer evolves over the airfoil. In the higher Reynolds number case, the leading-edge separation bubble is very small in length and in height. A fully developed turbulent boundary layer is observed in a short distance downstream of the reattachment point. The boundary layer velocity near the wall gradually decreases along the airfoil. Eventually, the boundary layer separates near the trailing edge. The Reynolds stresses peak in the outer part of the boundary layer and the maximum amplitude also gradually increases along the chord.
PIV measurements of isothermal plane turbulent impinging jets at moderate Reynolds numbers
NASA Astrophysics Data System (ADS)
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.
Do magnetic fields enhance turbulence at low magnetic Reynolds number?
NASA Astrophysics Data System (ADS)
Pothérat, Alban; Klein, Rico
2017-06-01
Imposing a magnetic field on a turbulent flow of electrically conducting fluid incurs the Joule effect. A current paradigm is that the corresponding dissipation increases with the intensity of the magnetic field and as a result turbulent fluctuations are all the more damped as the magnetic field is strong. While this idea finds apparent support in the phenomenology of decaying turbulence, measurements of turbulence in duct flows and other, more complex configurations have produced seemingly contradicting results. The root of the controversy is that magnetic fields promote sufficient scale-dependent anisotropy to profoundly reorganize the structure of turbulence, so their net effect cannot be understood in terms of the additional dissipation only. Here we show that when turbulence is forced in a magnetic field that acts on turbulence itself rather than on the mechanisms that generate it, the field promotes large, nearly two-dimensional structures capturing sufficient energy to offset the loss due to Joule dissipation, with the net effect of increasing the intensity of turbulent fluctuations. This change of paradigm potentially carries important consequences for systems as diverse as the liquid cores of planets, accretion disks, and a wide range of metallurgical and nuclear engineering applications.
Airfoil Section Characteristics as Affected by Variations of the Reynolds Number
NASA Technical Reports Server (NTRS)
Jacobs, Eastman N; Sherman, Albert
1937-01-01
Report presents the results of an investigation of a systematically chosen representative group of related airfoils conducted in the NACA variable-density wind tunnel over a wide range of Reynolds number extending well into the flight range. The tests were made to provide information from which the variations of airfoil section characteristics with changes in the Reynolds number could be inferred and methods of allowing for these variations in practice could be determined. This work is one phase of an extensive and general airfoil investigation being conducted in the variable-density tunnel and extends the previously published researches concerning airfoil characteristics as affected by variations in airfoil profile determined at a single value of the Reynolds number.
Energy Spectra of Higher Reynolds Number Turbulence by the DNS with up to 122883 Grid Points
NASA Astrophysics Data System (ADS)
Ishihara, Takashi; Kaneda, Yukio; Morishita, Koji; Yokokawa, Mitsuo; Uno, Atsuya
2014-11-01
Large-scale direct numerical simulations (DNS) of forced incompressible turbulence in a periodic box with up to 122883 grid points have been performed using K computer. The maximum Taylor-microscale Reynolds number Rλ, and the maximum Reynolds number Re based on the integral length scale are over 2000 and 105, respectively. Our previous DNS with Rλ up to 1100 showed that the energy spectrum has a slope steeper than - 5 / 3 (the Kolmogorov scaling law) by factor 0 . 1 at the wavenumber range (kη < 0 . 03). Here η is the Kolmogorov length scale. Our present DNS at higher resolutions show that the energy spectra with different Reynolds numbers (Rλ > 1000) are well normalized not by the integral length-scale but by the Kolmogorov length scale, at the wavenumber range of the steeper slope. This result indicates that the steeper slope is not inherent character in the inertial subrange, and is affected by viscosity.
Finite Reynolds number corrections of the 4/5 law for decaying turbulence
NASA Astrophysics Data System (ADS)
Boschung, J.; Gauding, M.; Hennig, F.; Denker, D.; Pitsch, H.
2016-10-01
We examine finite Reynolds number contributions to the inertial range solution of the third order structure functions D3 ,0 and D1 ,2 stemming from the unsteady and viscous terms. Under the assumption that the second order correlations f and g are self-similar under a coordinate change, we are able to rewrite the exact second order equations as a function of a normalized scale r ˜ only. We close the resulting system of equations using a power law and an eddy-viscosity ansatz. If we further assume K41 scaling, we find the same Reynolds number dependence as previously in the literature. We proceed to extrapolate towards higher Reynolds numbers to examine the unsteady and viscous terms in more detail. We find that the intersection between the two terms, where their contribution to the solution of the structure function equations is relatively small, scales with the Taylor scale λ .
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.
The shock tube as a device for testing transonic airfoils at high Reynolds numbers
NASA Technical Reports Server (NTRS)
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.
The Sensitivity Analysis for the Flow Past Obstacles Problem with Respect to the Reynolds Number
Ito, Kazufumi; Li, Zhilin; Qiao, Zhonghua
2013-01-01
In this paper, numerical sensitivity analysis with respect to the Reynolds number for the flow past obstacle problem is presented. To carry out such analysis, at each time step, we need to solve the incompressible Navier-Stokes equations on irregular domains twice, one for the primary variables; the other is for the sensitivity variables with homogeneous boundary conditions. The Navier-Stokes solver is the augmented immersed interface method for Navier-Stokes equations on irregular domains. One of the most important contribution of this paper is that our analysis can predict the critical Reynolds number at which the vortex shading begins to develop in the wake of the obstacle. Some interesting experiments are shown to illustrate how the critical Reynolds number varies with different geometric settings. PMID:24910780
Low Reynolds number two-equation modeling of turbulent flows
NASA Technical Reports Server (NTRS)
Michelassi, V.; Shih, T.-H.
1991-01-01
A k-epsilon model that accounts for viscous and wall effects is presented. The proposed formulation does not contain the local wall distance thereby making very simple the application to complex geometries. The formulation is based on an existing k-epsilon model that proved to fit very well with the results of direct numerical simulation. The new form is compared with nine different two-equation models and with direct numerical simulation for a fully developed channel flow at Re = 3300. The simple flow configuration allows a comparison free from numerical inaccuracies. The computed results prove that few of the considered forms exhibit a satisfactory agreement with the channel flow data. The model shows an improvement with respect to the existing formulations.
Low Reynolds number separation bubble research at UTRC
NASA Technical Reports Server (NTRS)
Patrick, W. P.; Edwards, D. E.
1985-01-01
The facilities and techniques being applied in investigations of laminar separation bubbles on low Re airfoils at the United Technologies Research Center are described. The research is focused on developing a database and predictive models for use in designing compressor, fan and turbine blades. Flow data are gathered primarily by laser Doppler velocimetry. Experimental procedures are being devised which will permit formation of a short bubble for characterizations of the velocity profiles in and around the bubble and boundary layer. Further tests are directed at defining the size and location of the bubble and the transition in the separated shear layer. The sensitivity of the bubble to alterations in the Re, pressure gradient, turbulence intensity and the turbulence scales will be examined. The effects of surface curvature and roughness on the bubble will also be studied. Details of the wind tunnel facility and the steps being taken to define conditions for reliably producing the short bubble are outlined.
Passive swimming in low-Reynolds-number flows
NASA Astrophysics Data System (ADS)
Olla, Piero
2010-07-01
The possibility of microscopic swimming by extraction of energy from an external flow is discussed, focusing on the migration of a simple trimer across a linear shear flow. The geometric properties of swimming, together with the possible generalization to the case of a vesicle, are analyzed. The mechanism of energy extraction from the flow appears to be the generalization to a discrete swimmer of the tank-treading regime of a vesicle. The swimmer takes advantage of the external flow by both extracting energy for swimming and “sailing” through it. The migration velocity is found to scale linearly in the stroke amplitude, and not quadratically as in a quiescent fluid. This effect turns out to be connected with the nonapplicability of the scallop theorem in the presence of external flow fields.
Large-Eddy Simulation of Conductive Flows at Low Magnetic Reynolds Number
NASA Technical Reports Server (NTRS)
Knaepen, B.; Moin, P.
2003-01-01
In this paper we study the LES method with dynamic procedure in the context of conductive flows subject to an applied external magnetic field at low magnetic Reynolds number R(sub m). These kind of flows are encountered in many industrial applications. For example, in the steel industry, applied magnetic fields can be used to damp turbulence in the casting process. In nuclear fusion devices (Tokamaks), liquid-lithium flows are used as coolant blankets and interact with the surrounding magnetic field that drives and confines the fusion plasma. Also, in experimental facilities investigating the dynamo effect, the flow consists of liquid-sodium for which the Prandtl number and, as a consequence, the magnetic Reynolds number is low. Our attention is focused here on the case of homogeneous (initially isotropic) decaying turbulence. The numerical simulations performed mimic the thought experiment described in Moffatt in which an initially homogeneous isotropic conductive flow is suddenly subjected to an applied magnetic field and freely decays without any forcing. Note that this flow was first studied numerically by Schumann. It is well known that in that case, extra damping of turbulence occurs due to the Joule effect and that the flow tends to become progressively independent of the coordinate along the direction of the magnetic field. Our comparison of filtered direct numerical simulation (DNS) predictions and LES predictions show that the dynamic Smagorinsky model enables one to capture successfully the flow with LES, and that it automatically incorporates the effect of the magnetic field on the turbulence. Our paper is organized as follows. In the next section we summarize the LES approach in the case of MHD turbulence at low R(sub m) and recall the definition of the dynamic Smagorinsky model. In Sec. 3 we describe the parameters of the numerical experiments performed and the code used. Section 4 is devoted to the comparison of filtered DNS results and LES results
High-order numerical simulations of the flow around wings at moderately high Reynolds number
NASA Astrophysics Data System (ADS)
Vinuesa, Ricardo; Negi, Prabal; Hosseini, Seyed M.; Hanifi, Ardeshir; Henningson, Dan S.; Schlatter, Philipp
2016-11-01
The results of a DNS of the flow around a wing section represented by a NACA4412 profile, with Rec = 400 , 000 and 5° angle of attack, are presented in this study. The high-order spectral-element code Nek5000 was used for the computations. The Clauser pressure-gradient parameter β ranges from = 0 to 85 on the suction side, and the maximum Reθ and Reτ values are around 2 , 800 and 373, respectively. The adversre pressure gradient (APG) on the suction side of the wing leads to a progressively increasing value of the inner peak in the tangential velocity fluctuations, as well as the development of an outer peak, which is also observed in the other components of the Reynolds-stress tensor. Close to the trailing edge, i.e., at x / c = 0 . 9 , the outer peak in the inner-scaled tangential velocity profile is larger than the inner peak. These effects are connected to the fact that the large-scale motions of the flow become energized due to the APG, as apparent from spanwise-premultiplied power spectral density plots. Preliminary comparisons between DNS and well-resolved LES data, based on a relaxation-term filtering approach, are also presented with the aim of further extending the Reynolds number to Rec = 1 , 000 , 000 . Funded by the Swedish Research Council (VR) and the Knut and Alice Wallenberg Foundation.
Breaking axi-symmetry in stenotic flow lowers the critical transition Reynolds number
NASA Astrophysics Data System (ADS)
Samuelsson, J.; Tammisola, O.; Juniper, M. P.
2015-10-01
Flow through a sinuous stenosis with varying degrees of non-axisymmetric shape variations and at Reynolds number ranging from 250 to 750 is investigated using direct numerical simulation (DNS) and global linear stability analysis. At low Reynolds numbers (Re < 390), the flow is always steady and symmetric for an axisymmetric geometry. Two steady state solutions are obtained when the Reynolds number is increased: a symmetric steady state and an eccentric, non-axisymmetric steady state. Either one can be obtained in the DNS depending on the initial condition. A linear global stability analysis around the symmetric and non-axisymmetric steady state reveals that both flows are linearly stable for the same Reynolds number, showing that the first bifurcation from symmetry to antisymmetry is subcritical. When the Reynolds number is increased further, the symmetric state becomes linearly unstable to an eigenmode, which drives the flow towards the non-axisymmetric state. The symmetric state remains steady up to Re = 713, while the non-axisymmetric state displays regimes of periodic oscillations for Re ≥ 417 and intermittency for Re ≳ 525. Further, an offset of the stenosis throat is introduced through the eccentricity parameter E. When eccentricity is increased from zero to only 0.3% of the pipe diameter, the bifurcation Reynolds number decreases by more than 50%, showing that it is highly sensitive to non-axisymmetric shape variations. Based on the resulting bifurcation map and its dependency on E, we resolve the discrepancies between previous experimental and computational studies. We also present excellent agreement between our numerical results and previous experimental results.
Cryogenic wind tunnels for high Reynolds number testing
NASA Technical Reports Server (NTRS)
Lawing, P. L.; Kilgore, R. A.; Mcguire, P. D.
1986-01-01
A compilation of lectures presented at various Universities over a span of several years is discussed. A central theme of these lectures has been to present the research facility in terms of the service it provides to, and its potential effect on, the entire community, rather than just the research community. This theme is preserved in this paper which deals with the cryogenic transonic wind tunnels at Langley Research Center. Transonic aerodynamics is a focus both because of its crucial role in determining the success of aeronautical systems and because cryogenic wind tunnels are especially applicable to the transonics problem. The paper also provides historical perspective and technical background for cryogenic tunnels, culminating in a brief review of cryogenic wind tunnel projects around the world. An appendix is included to provide up to date information on testing techniques that have been developed for the cryogenic tunnels at Langley Research Center. In order to be as inclusive and as current as possible, the appendix is less formal than the main body of the paper. It is anticipated that this paper will be of particular value to the technical layman who is inquisitive as to the value of, and need for, cryogneic tunnels.
Vortex visualization in ultra low Reynolds number insect flight.
Koehler, Christopher; Wischgoll, Thomas; Dong, Haibo; Gaston, Zachary
2011-12-01
We present the visual analysis of a biologically inspired CFD simulation of the deformable flapping wings of a dragonfly as it takes off and begins to maneuver, using vortex detection and integration-based flow lines. The additional seed placement and perceptual challenges introduced by having multiple dynamically deforming objects in the highly unsteady 3D flow domain are addressed. A brief overview of the high speed photogrammetry setup used to capture the dragonfly takeoff, parametric surfaces used for wing reconstruction, CFD solver and underlying flapping flight theory is presented to clarify the importance of several unsteady flight mechanisms, such as the leading edge vortex, that are captured visually. A novel interactive seed placement method is used to simplify the generation of seed curves that stay in the vicinity of relevant flow phenomena as they move with the flapping wings. This method allows a user to define and evaluate the quality of a seed's trajectory over time while working with a single time step. The seed curves are then used to place particles, streamlines and generalized streak lines. The novel concept of flowing seeds is also introduced in order to add visual context about the instantaneous vector fields surrounding smoothly animate streak lines. Tests show this method to be particularly effective at visually capturing vortices that move quickly or that exist for a very brief period of time. In addition, an automatic camera animation method is used to address occlusion issues caused when animating the immersed wing boundaries alongside many geometric flow lines. Each visualization method is presented at multiple time steps during the up-stroke and down-stroke to highlight the formation, attachment and shedding of the leading edge vortices in pairs of wings. Also, the visualizations show evidence of wake capture at stroke reversal which suggests the existence of previously unknown unsteady lift generation mechanisms that are unique to quad
NASA Astrophysics Data System (ADS)
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.
Simulation of Reynolds number influence on heat exchange in turbulent flow of medium slurry
NASA Astrophysics Data System (ADS)
Bartosik, A.
2016-10-01
The paper deals with the numerical simulation of mass and heat exchange in turbulent flow of solid-liquid mixture in the range of averaged solid particle diameter from 0.10mm to 0.80mm, named further as the medium slurry. Physical model assumes that dispersed phase is fully suspended and a turbulent flow is hydro-dynamically, and thermally developed in a straight horizontal pipeline. Taking into account the aforementioned assumptions the slurry is treated as a single-phase flow with increased density, while viscosity is equals to a carrier liquid viscosity. The mathematical model constitutes time averaged momentum equation in which the turbulent stress tensor was designated using a two-equation turbulence model, which makes use of the Boussinesq eddy-viscosity hypothesis. Turbulence damping function in the turbulence model was especially designed for the medium slurry. In addition, an energy equation has been used in which a convective term was determined from the energy balance acting on a unit pipe length, assuming linear changes of temperature in main flow direction. Finally, the mathematical model of non-isothermal medium slurry flow comprises four partial differential equations, namely momentum and energy equations, equations of kinetic energy of turbulence and its dissipation rate. Four partial differential equations were solved by a finite difference scheme using own computer code. The objective of the paper is to examine the influence of Reynolds number on temperature profiles and Nusselt number in turbulent flow of medium slurry in the range of solids concentration from 0% to 30% by volume. The effect of influential factors on heat transfer between the pipe and slurry is analysed. The paper demonstrates substantial impact of Reynolds number and solids volume fraction on the Nusselt number. The results of numerical simulation are reviewed.
NASA Technical Reports Server (NTRS)
Michelassi, V.; Durbin, P. A.; Mansour, N. N.
1996-01-01
A four-equation model of turbulence is applied to the numerical simulation of flows with massive separation induced by a sudden expansion. The model constants are a function of the flow parameters, and two different formulations for these functions are tested. The results are compared with experimental data for a high Reynolds-number case and with experimental and DNS data for a low Reynolds-number case. The computations prove that the recovery region downstream of the massive separation is properly modeled only for the high Re case. The problems in this case stem from the gradient diffusion hypothesis, which underestimates the turbulent diffusion.
The aerodynamic characteristics of seven frequently used wing sections at full Reynolds number
NASA Technical Reports Server (NTRS)
Munk, Max M; Miller, Elton W
1927-01-01
This report contains the aerodynamic properties of the wing sections U.S.A. 5, U.S.A. 27, U.S.A. 35 A, U.S.A. 35 B, Clark Y, R.A.F. 15, and Gottingen 387, as determined at various Reynolds numbers up to an approximately full scale value in the variable density wind tunnel of the National Advisory Committee for Aeronautics. It is shown that the characteristics of the wings investigated are affected greatly and in a somewhat erratic manner by variation of the Reynolds number. In general there is a small increase in maximum lift and an appreciable decrease in drag at all lifts.
Flow and Acoustic Properties of Low Reynolds Number Underexpanded Supersonic Jets. Ph.D. Thesis
NASA Technical Reports Server (NTRS)
Hu, Tieh-Feng
1981-01-01
Jet noise on underexpanded supersonic jets are studied with emphasis on determining the role played by large scale organized flow fluctuations in the flow and acoustic processes. The experimental conditions of the study were chosen as low Reynolds number (Re=8,000) Mach 1.4 and 2.1, and moderate Reynolds number (Re=68,000) Mach 1.6 underexpanded supersonic jets exhausting from convergent nozzles. At these chosen conditions, detailed experimental measurements were performed to improve the understanding of the flow and acoustic properties of underexpanded supersonic jets.
NASA Technical Reports Server (NTRS)
Igoe, William B.
1991-01-01
Dynamic measurements of fluctuating static pressure levels were made using flush mounted high frequency response pressure transducers at eleven locations in the circuit of the National Transonic Facility (NTF) over the complete operating range of this wind tunnel. Measurements were made at test section Mach numbers from 0.2 to 1.2, at pressure from 1 to 8.6 atmospheres and at temperatures from ambient to -250 F, resulting in dynamic flow disturbance measurements at the highest Reynolds numbers available in a transonic ground test facility. Tests were also made independently at variable Mach number, variable Reynolds number, and variable drivepower, each time keeping the other two variables constant thus allowing for the first time, a distinct separation of these three important variables. A description of the NTF emphasizing its flow quality features, details on the calibration of the instrumentation, results of measurements with the test section slots covered, downstream choke, effects of liquid nitrogen injection and gaseous nitrogen venting, comparisons between air and nitrogen, isolation of the effects of Mach number, Reynolds number, and fan drive power, and identification of the sources of significant flow disturbances is included. The results indicate that primary sources of flow disturbance in the NTF may be edge-tones generated by test section sidewall re-entry flaps and the venting of nitrogen gas from the return leg of the tunnel circuit between turns 3 and 4 in the cryogenic mode of operation. The tests to isolate the effects of Mach number, Reynolds number, and drive power indicate that Mach number effects predominate. A comparison with other transonic wind tunnels shows that the NTF has low levels of test section fluctuating static pressure especially in the high subsonic Mach number range from 0.7 to 0.9.
NASA Astrophysics Data System (ADS)
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.
NASA Astrophysics Data System (ADS)
Thompson, D. S.
1980-05-01
The full Navier-Stokes equations for incompressible turbulent flow must be solved to accurately represent all flow phenomena which occur in a high Reynolds number incompressible flow. A two layer algebraic eddy viscosity turbulence model is used to represent the Reynolds stress in the primitive variable formulation. The development of the boundary-fitted coordinate systems makes the numerical solution of these equations feasible for arbitrarily shaped bodies. The nondimensional time averaged Navier-Stokes equations, including the turbulence mode, are represented by finite difference approximations in the transformed plane. The resulting coupled system of nonlinear algebraic equations is solved using a point successive over relaxation iteration. The test case considered was a NACA 64A010 airfoil section at an angle of attack of two degrees and a Reynolds number of 2,000,000.
NASA Astrophysics Data System (ADS)
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.
NASA Technical Reports Server (NTRS)
Phillips, W. P.
1981-01-01
Subsonic longitudinal andd laternal directional characteristics were obtained for several modified configurations of the 140 A/B orbiter (0.010 scale). These modifications, designed to extend longitudinal trim capability forward of the 65 percent fuselage length station, consisted of modified wing planform fillet and a canard. Tests were performed in the Langley Low Turbulence Pressure Tunnel at Reynolds numbers from about 4.2 million to 14.3 million based on the fuselage reference length.
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
NASA Astrophysics Data System (ADS)
Palmore, John; Desjardins, Olivier
2014-11-01
Low waveshell spectral forcing has been proven to be a simple and effective manner to generate isotropic turbulence in a periodic domain. This simplicity is lost for flow problems with complex boundary conditions such as resolved particle flows, fluid-fluid flows with interfaces, and wall-bounded flows. Lundgren's linear forcing in physical space is a straightforward and easy-to-implement method to tackle these problems; however, the use of this method results in a halving of the large turbulence length scale. The technique that will be presented in this talk applies a low-pass filter to the source term used in linear forcing. It is shown to recover the scale resolution of low waveshell spectral forcing which translates to an approximately 60 percent increase in the attainable Reynolds number for a given computation domain. The characteristics of homogeneous isotropic turbulence generated using filtered linear forcing will be discussed. Finally, extension of this idea to scalar forcing will be presented.
NASA Technical Reports Server (NTRS)
Teague, E. C.; Vorburger, T. V.; Scire, F. E.; Baker, S. M.; Jensen, S. W.; Gloss, B. B.; Trahan, C.
1982-01-01
Current work by the National Bureau of Standards at the NASA National Transonic Facility (NTF) to evaluate the performance of stylus instruments for determining the topography of models under investigation is described along with instrumentation for characterization of the surface microtopography. Potential areas of surface effects are reviewed, and the need for finer surfaced models for the NTF high Reynolds number flows is stressed. Current stylus instruments have a radii as large as 25 microns, and three models with surface finishes of 4-6, 8-10, and 12-15 micro-in. rms surface finishes were fabricated for tests with a stylus with a tip radius of 1 micron and a 50 mg force. Work involving three-dimensional stylus profilometry is discussed in terms of stylus displacement being converted to digital signals, and the design of a light scattering instrument capable of measuring the surface finish on curved objects is presented.
On large-scale dynamo action at high magnetic Reynolds number
Cattaneo, F.; Tobias, S. M.
2014-07-01
We consider the generation of magnetic activity—dynamo waves—in the astrophysical limit of very large magnetic Reynolds number. We consider kinematic dynamo action for a system consisting of helical flow and large-scale shear. We demonstrate that large-scale dynamo waves persist at high Rm if the helical flow is characterized by a narrow band of spatial scales and the shear is large enough. However, for a wide band of scales the dynamo becomes small scale with a further increase of Rm, with dynamo waves re-emerging only if the shear is then increased. We show that at high Rm, the key effect of the shear is to suppress small-scale dynamo action, allowing large-scale dynamo action to be observed. We conjecture that this supports a general 'suppression principle'—large-scale dynamo action can only be observed if there is a mechanism that suppresses the small-scale fluctuations.
Elliptic flow computation by low Reynolds number two-equation turbulence models
NASA Technical Reports Server (NTRS)
Michelassi, V.; Shih, T.-H.
1991-01-01
A detailed comparison of ten low-Reynolds-number k-epsilon models is carried out. The flow solver, based on an implicit approximate factorization method, is designed for incompressible, steady two-dimensional flows. The conservation of mass is enforced by the artificial compressibility approach and the computational domain is discretized using centered finite differences. The turbulence model predictions of the flow past a hill are compared with experiments at Re = 10 exp 6. The effects of the grid spacing together with the numerical efficiency of the various formulations are investigated. The results show that the models provide a satisfactory prediction of the flow field in the presence of a favorable pressure gradient, while the accuracy rapidly deteriorates when a strong adverse pressure gradient is encountered. A newly proposed model form that does not explicitly depend on the wall distance seems promising for application to complex geometries.
Double large field stereoscopic PIV in a high Reynolds number turbulent boundary layer
NASA Astrophysics Data System (ADS)
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.
A time-dependent incompressible viscous BEM for moderate Reynolds numbers
NASA Technical Reports Server (NTRS)
Dargush, G. F.; Banerjee, P. K.
1991-01-01
The boundary element method is applied to transient viscous incompressible flow. The time-domain formulation allows a boundary-only solution for linear Stokes flow. For higher speed flows in which the nonlinear convective effects cannot be ignored, a volume integral must be retained. However, the introduction of reference velocities often limits the nonlinear region to the vicinity of obstacles or boundary surfaces. Additionally, the volume terms are rewritten to eliminate the need for the calculation of velocity gradients. A general purpose numerical implementation of this new formulation then produces a very attractive tool for engineering analysis. This implementation includes a Newton-Raphson algorithm, permitting accurate solutions up to the moderate Reynolds number range. Several numerical examples are provided to validate the present approach.
Implications of turbulence interactions: A path toward addressing very high Reynolds number flows
Zhou, Y
2006-05-15
The classical 'turbulence problem' is narrowed down and redefined for scientific and engineering applications. From an application perspective, accurate computation of large-scale transport of the turbulent flows is needed. In this paper, a scaling analysis that allows for the large-scales of very high Reynolds number turbulent flows - to be handled by the available supercomputers is proposed. Current understanding of turbulence interactions of incompressible turbulence, which forms the foundation of our argument, is reviewed. Furthermore, the data redundancy in the inertial range is demonstrated. Two distinctive interactions, namely, the distance and near-grid interactions, are inspected for large-scale simulations. The distant interactions in the subgrid scales in an inertial range can be effectively modelled by an eddy damping. The near-grid interactions must be carefully incorporated.
Mininni, P D; Alexakis, A; Pouquet, A
2008-03-01
We analyze the data stemming from a forced incompressible hydrodynamic simulation on a grid of 2048(3) regularly spaced points, with a Taylor Reynolds number of R(lambda) ~ 1300. The forcing is given by the Taylor-Green vortex, which shares similarities with the von Kàrmàn flow used in several laboratory experiments; the computation is run for ten turnover times in the turbulent steady state. At this Reynolds number the anisotropic large scale flow pattern, the inertial range, the bottleneck, and the dissipative range are clearly visible, thus providing a good test case for the study of turbulence as it appears in nature. Triadic interactions, the locality of energy fluxes, and longitudinal structure functions of the velocity increments are computed. A comparison with runs at lower Reynolds numbers is performed and shows the emergence of scaling laws for the relative amplitude of local and nonlocal interactions in spectral space. Furthermore, the scaling of the Kolmogorov constant, and of skewness and flatness of velocity increments is consistent with previous experimental results. The accumulation of energy in the small scales associated with the bottleneck seems to occur on a span of wave numbers that is independent of the Reynolds number, possibly ruling out an inertial range explanation for it. Finally, intermittency exponents seem to depart from standard models at high R(lambda), leaving the interpretation of intermittency an open problem.
Thrust production of free-to-pivot plates at low Reynolds number
NASA Astrophysics Data System (ADS)
Granlund, Kenneth; Ol, Michael; Bernal, Luis
2012-11-01
As an abstraction of flapping-wing aerodynamics, rigid flat plates free-to-pivot at the leading edge between incidence angle limits of +/-45 are considered in rectilinear as well as waving motion in a quiescent fluid. Thrust (lift) and resistive-force are measured, forming a hover Figure-of-Merit (FoM). The evolution of spatial retention of a leading edge vortex is tracked throughout the motion cycle, showing vortex formation shortly after the plate completes its rotation, and in some cases shedding of subsequent vortices after the initial leading edge vortex is ejected. Vortex evolution in rectilinear- vs. rotating and steady vs. accelerating motion is visualized with fluorescent dye illuminated by a laser light sheet at several spanwise stations along the leading edge. Experiments in water reveal a Reynolds number indifference in thrust and FoM for 8,000
Experiments on low Reynolds number turbulent flow through a square duct
NASA Astrophysics Data System (ADS)
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.
Multigrid solution of the convection-diffusion equation with high-Reynolds number
Zhang, Jun
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.
NASA Astrophysics Data System (ADS)
Montessori, A.; Prestininzi, P.; La Rocca, M.; Succi, S.
2017-09-01
We present an entropic version of the lattice Boltzmann pseudo-potential approach for the simulation of multiphase flows. The method is shown to correctly simulate the dynamics of impinging droplets on hydrophobic surfaces and head-on and grazing collisions between droplets, at Weber and Reynolds number regimes not accessible to previous pseudo-potential methods at comparable resolution.
NASA Technical Reports Server (NTRS)
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.
Application of Magnetic Suspension and Balance Systems to Ultra-High Reynolds Number Facilities
NASA Technical Reports Server (NTRS)
Britcher, Colin P.
1996-01-01
The current status of wind tunnel magnetic suspension and balance system development is briefly reviewed. Technical work currently underway at NASA Langley Research Center is detailed, where it relates to the ultra-high Reynolds number application. The application itself is addressed, concluded to be quite feasible, and broad design recommendations given.
Two-dimensional energy spectra in a high Reynolds number turbulent boundary layer
NASA Astrophysics Data System (ADS)
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.
Performance of a cascade in an annular vortex-generating tunnel over range of Reynolds numbers
NASA Technical Reports Server (NTRS)
Thurston, Sidney; Brunk, Ralph E
1951-01-01
Total-pressure deficiency for an annular cascade of 65-(12)10 blades was measured at three radial stations over a range of Reynolds numbers from 50,000 to 250,000 and at angles of attack of 15 degrees and 25 degrees. The variation of turning angle and shape of static pressure distribution at these stations is also shown.
Drag-n-fly: a Proposal in Response to a Low Reynolds Number Station Keeping Mission
NASA Technical Reports Server (NTRS)
Foohey, Mark; Niehaus, John; Neumann, Jenny; Deviny, Pat; Zurovchak, Jerry; Brenner, Joey; Gendron, Peter
1990-01-01
The Drag-n-Fly is a remotely piloted, low Reynolds number vehicle. It was designed to maintain level controlled flight and fly a closed course at flight speeds corresponding to Reynolds numbers of less than 2 x 10(exp 5) and as close to 1 x 10(exp 5) as possible. The success of the mission will be associated with achieving the lowest mean chord Reynolds number possible and maximizing loiter time on the course. The flight plan for the Drag-n-Fly calls for the vehicle to ascent to a cruise altitude of 25 ft. The airfoil selected for the Drag-n-Fly is a Spica chosen for its high lift coefficient at low Reynolds number. The propulsion system for the Drag-n-Fly consists of a 10 inch diameter propeller mounted on the front of the vehicle. Structural support for the Drag-n-Fly comes from four box beams running the length of the fuselage. The tail and horizontal stabilizers are located far aft of the lifting surface in order to assure proper static stability. The present design for the Drag-n-Fly will meet the criteria for the present mission.
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 < Re < 18000) number with particle 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.
Log law of the wall revisited in Taylor-Couette flows at intermediate Reynolds numbers
NASA Astrophysics Data System (ADS)
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 < Re < 18000) number with particle 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.
Stability of the shape of a surfactant-laden drop translating at low Reynolds number
NASA Astrophysics Data System (ADS)
Johnson, Robert A.; Borhan, Ali
2000-04-01
We examine the effect of surfactants on the evolution of the shape of an initially nonspherical drop translating in an otherwise quiescent fluid at low Reynolds number. A combination of the boundary-integral method and a finite-difference scheme is used to solve the coupled fluid dynamics and surfactant transport problems, in conjunction with the Frumkin adsorption framework to account for the effects of monolayer saturation and nonideal surfactant interactions. For sufficiently small Bond numbers, the drop achieves a nonspherical steady shape. For large initial deformations or Bond numbers, however, the drop deforms continuously, and eventually breaks up through either the formation of an elongated tail or the development of a re-entrant cavity at the trailing end, similar to the mechanisms of drop breakup reported by Koh and Leal [Phys. Fluids A 1, 8 (1989)], and Pozrikidis [J. Fluid Mech. 210, 1 (1990)], for surfactant-free drops. Surfactants are found to have a destabilizing effect on the shape of translating drops. The destabilizing effect is mitigated by the presence of strongly-cohesive surfactant interactions, and by surfactant transport between the bulk and the interface.
NASA Astrophysics Data System (ADS)
Sheehy, Patrick; Owkes, Mark
2015-11-01
Electrohydrodynamics (EHD) has shown great potential for enhancing the atomization of liquid flows by decreasing droplet size and improving dispersion. For many fluid flows relevant to engineering, such as liquid fuel injection, two important fluid properties are the electric Reynolds and Peclet numbers (Re and Pe), which control how fast electric charges relax to the gas-liquid interface and the thickness of electric charge boundary layers. The effect of the numbers is not well understood due to the difficulty of measuring electric charges experimentally. Additionally, predicting the impact of electric charge distribution on atomization is difficult. For example, a smaller electric Re number causes a weaker electric field, higher charge concentrations at interface, and a non-trivial distribution of the Coulomb force. This work uses a numerical approach to simulate a two-phase EHD jet in order to examine the affect of these two non-dimensional numbers on atomization quality. The approach employed is second-order, conservative, and consistently transports the discontinuous electric charge density, momentum, and phase interface. Several three-dimensional test cases are simulated using this process for a range of Re and Pe numbers and comparisons are made. ME Master's Student
Design and installation of a high Reynolds number recirculating water tunnel
NASA Astrophysics Data System (ADS)
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
NASA Technical Reports Server (NTRS)
Watson, Ralph D.; Hall, Robert M.; Anders, John B.
2000-01-01
This paper reviews flat plate skin friction data from early correlations of drag on plates in water to measurements in the cryogenic environment of The NASA Langley National Transonic Facility (NTF) in late 1996. The flat plate (zero pressure gradient with negligible surface curvature) incompressible skin friction at high Reynolds numbers is emphasized in this paper, due to its importance in assessing the accuracy of measurements, and as being important to the aerodynamics of large scale vehicles. A correlation of zero pressure gradient skin friction data minimizing extraneous effects between tests is often used as the first step in the calculation of skin friction in complex flows. Early data compiled by Schoenherr for a range of momentum thickness Reynolds numbers, R(sub Theta) from 860 to 370,000 contained large scatter, but has proved surprisingly accurate in its correlated form. Subsequent measurements in wind tunnels under more carefully controlled conditions have provided inputs to this database, usually to a maximum R(sub Theta) of about 40,000. Data on a large axisymmetric model in the NASA Langley National Transonic Facility extends the upper limit in incompressible R(sub Theta) to 619,800 using the van Driest transformation. Previous data, test techniques, and error sources ar discussed, and the NTF data will be discussed in detail. The NTF Preston tube and Clauser inferred data accuracy is estimated to be within -2 percent of a power-law curve fit, and falls above the Spalding theory by 1 percent at R(sub Theta) of about 600,000.
Chaotic mixing in boundary- and temperature-driven low- Reynolds-number flows
NASA Astrophysics Data System (ADS)
Stickel, Victor George, Jr.
1999-11-01
This dissertation examines chaotic and laminar mixing for several different bounded domains. Flows considered have a Reynolds number between 0.1 and 100 and fall in a laminar regime. In all cases mixing is quantified via Poincaré section, Lyapunov exponents, concentration deviation or a combination thereof. In the first part of the thesis, chaotic isothermal flow in an eccentric annulus is applied to a unique bioengineering problem concerning the collection of fetal erythrocytes from a maternal blood solution. Specific constraints for a practical collecting device are identified and a mixing protocol is developed based on these constraints and experimental results. The next topic considered is mixing in a two-dimensional rectangular domain. For this problem, the cases of isothermal periodic driven cavity flow and periodic driven cavity flow undergoing Rayleigh- Bénard convection are analyzed. In both cases it is found that unequal displacements of the moving boundary produce the most thorough mixing. For the isothermal case, mixing in the cavity is characterized by a large regular region. It is found that this region is reduced in the temperature dependent problem. Chaotic and regular regions for the optimum stirring procedure identified are mapped and quantified. The final part of the thesis concerns three-dimensional isothermal bounded cavity flow. A parallelepiped domain is considered and results from Chapter 3 are used to develop an efficient stirring procedure. Secondary flow effects are found to greatly enhance mixing behavior as Reynolds number is increased from 10 to 100. Also during this transition regular trajectories in the lateral regions become chaotic. Unequal displacements of the moving boundary in this case are found to increase mixing along the central plane of symmetry of the cavity.
Transport coefficients for the shear dynamo problem at small Reynolds numbers
NASA Astrophysics Data System (ADS)
Singh, Nishant K.; Sridhar, S.
2011-05-01
We build on the formulation developed in S. Sridhar and N. K. Singh [J. Fluid Mech.JFLSA70022-112010.1017/S0022112010003745 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 αil and ηiml are derived. We prove that when the velocity field is nonhelical, the transport coefficient αil 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 X3 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.AJLEEY0004-637X10.1086/527373 676, 740 (2008)]. Explicit expressions are derived for all four components of the magnetic diffusivity tensor ηij(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.
Stability of the Shape of Surfactant-Laden Drops Translating at Low Reynolds Number
NASA Astrophysics Data System (ADS)
Johnson, R. A.; Borhan, A.
1998-11-01
The effect of surfactant impurities on the shape evolution of initially nonspherical drops translating at low Reynolds number in an immiscible, otherwise quiescent fluid is studied numerically. The presence of surface-active impurities can significantly alter the rheological properties of the two-phase interface; surfactant convection and diffusion along the deforming interface, as well as the adsorption/desorption exchange between the interface and the bulk phases, can lead to variations of interfacial tension and a subsequent redistribution of surface pressure. A combination of the boundary-integral method and a finite difference scheme is used to solve the coupled fluid dynamics and surfactant transport problems, while the Frumkin adsorption framework is used to explore the effects of monolayer saturation and nonideal surfactant interactions. For small Bond numbers, it is shown that the drop achieves a spherical shape at steady-state, provided the initial deformation is not too large. For large initial deformations or large bond numbers, however, the drop does not reach a steady shape, and drop breakup occurs through either the formation of an elongated tail, or the development of a re-entrant cavity at the rear. These mechanisms of drop breakup are similar to those reported by Koh & Leal [Phys. Fluids A 1, 8 (1989)] for surfactant-free drops. The effect of surfactant impurities on the critical conditions for drop breakup will be discussed.
DNS/LES Simulations of Separated Flows at High Reynolds Numbers
NASA Technical Reports Server (NTRS)
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.
Introducing a nano-scale crossed hot-wire for high Reynolds number measurements
NASA Astrophysics Data System (ADS)
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).
Turbulence statistics in fully developed channel flow at low Reynolds number
NASA Technical Reports Server (NTRS)
Kim, John; Moin, Parviz; Moser, Robert
1987-01-01
A direct numerical simulation of a turbulent channel flow is performed. The unsteady Navier-Stokes equations are solved numerically at a Reynolds number of 3300, based on the mean centerline velocity and channel half-width, with about 4 million grid points. All essential turbulence scales are resolved on the computational grid and no subgrid model is used. A large number of turbulence statistics are computed and compared with the existing experimental data at comparable Reynolds numbers. Agreements as well as discrepancies are discussed in detail. Particular attention is given to the behavior of turbulence correlations near the wall. A number of statistical correlations which are complementary to the existing experimental data are reported for the first time.
NASA Technical Reports Server (NTRS)
Londenberg, W. K.
1993-01-01
Navier-Stokes solutions about a supercritical airfoil with aileron deflection have been computed using the CFL3D code coupled with the Baldwin-Lomax, Johnson-King, Baldwin-Barth, and Spalart-Allmaras turbulence models. Computations were made at a Mach number of 0.716 and chord Reynolds numbers of 5, 15, and 25 million. The airfoil was analyzed with both 0 deg and 2 deg (TED) aileron deflections. Comparisons over a range of angles-of-attack showed that solutions obtained using the Baldwin-Barth turbulence model presented the best agreement with experimental pressures and sectional lift coefficients. However, Reynolds number trends in sectional lift coefficient and in aileron effectiveness were not predicted consistently.
Fluid-Structure Interaction of Oscillating Low Aspect Ratio Wings at Low Reynolds Numbers
2010-03-01
illustration of the rig is provided in Figure 1. Flow measurements were obtained using a Digital Particle Image Velocimetry (DPIV) system. The TSI...increased frequency, even so, no thrust is produced within the frequency range tested. The phase-averaged vorticity and velocity magnitude, at the...Reynolds numbers tested. The switch from drag to thrust occurs within the operational range of Strouhal numbers, at Stc≈1.1-1.25. Figure 11 shows the
Local swirl chamber heat transfer and flow structure at different Reynolds numbers
Hedlung, C.R.; Ligrani, P.M.
2000-04-01
Local flow behavior and heat transfer results are presented from two swirl chambers, which model passages used to cool the leading edges of turbine blades in gas turbine engines. Flow results are obtained in an isothermal swirl chamber. Surface Nusselt number distributions are measured in a second swirl chamber (with a constant wall heat flux boundary condition) using infrared thermography in conjunction with thermocouples, energy balances, and in situ calibration procedures. In both cases, Reynolds numbers Re based on inlet duct characteristics range from 6,000 to about 20,000. Bulk helical flow is produced in each chamber by two inlets, which are tangent to the swirl chamber circumference. Important changes to local and globally averaged surface Nusselt numbers, instantaneous flow structure from flow visualizations, and distributions of static pressure, total pressure, and circumferential velocity are observed throughout the swirl chambers as the Reynolds number increases. Of particular importance are increases of local surface Nusselt numbers (as well as ones globally averaged over the entire swirl chamber surface) with increasing Reynolds number. These are tied to increased advection, as well as important changes to vortex characteristics near the concave surfaces of the swirl chambers. Higher Re also give larger axial components of velocity, and increased turning of the flow from each inlet, which gives Goertler vortex pair trajectories greater skewness as they are advected downstream of each inlet.
Stability of the shape of a translating viscoelastic drop at low Reynolds number
NASA Astrophysics Data System (ADS)
Wu, H.; Haj-Hariri, H.; Borhan, A.
2012-11-01
We examine the stability of the shape of a viscoelastic drop translating in an immiscible, otherwise quiescent Newtonian fluid at low Reynolds number. Non-Newtonian stresses in the drop phase are characterized by the finitely extensible nonlinear elastic-Chilcott-Rallison model. A boundary integral method is used to numerically examine the time-evolution of initially perturbed drop shapes over a range of dimensionless parameters. For sufficiently small capillary numbers, the drop achieves an oblate spheroidal steady shape. For large initial deformations or capillary numbers, however, the drop deforms continuously and eventually breaks up through either the formation of an elongated tail or the development of a re-entrant cavity at its trailing end. These mechanisms of drop breakup are qualitatively similar to those reported earlier for Newtonian drops [C. J. Koh and L. G. Leal, Phys. Fluids A 1, 1309 (1989), 10.1063/1.857359; C. Pozrikidis, J. Fluid Mech. 210, 1 (1990), 10.1017/S0022112090001203; H. A. Stone, Annu. Rev. Fluid Mech. 26, 65 (1994), 10.1146/annurev.fl.26.010194.000433]. Compared to the case of a Newtonian drop, drop phase elasticity is found to have a stabilizing (destabilizing) effect for initially oblate (prolate) shape perturbations, due to the development of polymeric stresses caused by deformation of polymer chains in alignment with streamlines of the flow. Polymer viscosity has a strong influence on the stability of the shape of drops, whereas polymer relaxation time and extensibility have relatively weak influences.
The Use of Heavy Gas for Increased Reynolds Numbers in Transonic Wind Tunnels
NASA Technical Reports Server (NTRS)
Anders, J. B.; Anderson, W. K.; Murthy, A. V.
1998-01-01
The use of a high molecular weight test gas to increase the Reynolds number range of transonic wind tunnels is explored. Modifications to a small transonic wind tunnel are described and the real gas properties of the example heavy gas (sulfur hexafluoride) are discussed. Sulfur hexafluoride is shown to increase the test Reynolds number by a factor of more than 2 over air at the same Mach number. Experimental and computational pressure distributions on an advanced supercritical airfoil configuration at Mach 0.7 in both sulfur hexafluoride and nitrogen are presented. Transonic similarity theory is shown to be partially successful in transforming the heavy gas results to equivalent nitrogen (air) results, provided the correct definition of gamma is used.
Discrete-Roughness-Element-Enhanced Swept-Wing Natural Laminar Flow at High Reynolds Numbers
NASA Technical Reports Server (NTRS)
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.
DRE-Enhanced Swept-Wing Natural Laminar Flow at High Reynolds Numbers
NASA Technical Reports Server (NTRS)
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.
NASA Astrophysics Data System (ADS)
Barnes, F. H.
2000-12-01
Measurements are presented which show that, at a Reynolds number of 60, the Strouhal number for the shedding of vortices from a rotating cylinder is only weakly dependent on the value of α, the ratio of the cylinder's peripheral speed to its translational speed, up to the α value at which shedding is suppressed. This finding agrees with the theoretical results of Kang et al (1999 Phys. Fluids 11 3312), and strongly disagrees with those of Hu et al (1996 Phys. Fluids 8 1972). In addition, measurements were made to determine the α value at which shedding is suppressed for Reynolds numbers between 50 and 65. The results indicate a flow more stable than that predicted by both Hu et al and Kang et al.
Experiments on the flow and acoustic properties of a moderate-Reynolds-number supersonic jet
NASA Technical Reports Server (NTRS)
Troutt, T. R.; Mclaughlin, D. K.
1982-01-01
Flow and acoustic properties of a jet at Reynolds number of 70,000 were studied at Mach 2.1. Measurements in a free jet test facility were made with pitot tubes and hot-wire anemometry. Center-line Mach number distributions for natural and excited jets were obtained. A slow initial growth rate was in the potential core region of the jet, indicating a transition from laminar to turbulent flow in moderate Reynolds number jets. The transition occurred within the first 2-3 diameters. Spectral components were calculated for the fluctuating flowfield, and sound pressure levels were measured for the overall near-field noise. The centroid of noise was located about 8 nozzle diameters downstream. The growth rates of instabilities were determined to be in agreement with linear stability theory predictions over a broad frequency range.
NASA Technical Reports Server (NTRS)
Johnson, W. G., Jr.; Hill, A. S.; Ray, E. J.; Rozendaal, R. A.; Butler, T. W.
1982-01-01
A wind tunnel investigation of an advanced-technology airfoil was conducted in the Langley 0.3-Meter Transonic Cryogenic Tunnel (TCT). This investigation represents the first in a series of NASA/U.X. industry two dimensional airfoil studies to be completed in the Advanced Technology Airfoil Test program. Test temperature was varied from ambient to about 100 K at pressures ranging from about 1.2 to 6.0 atm. Mach number was varied from about 0.40 to 0.80. These variables provided a Reynolds number (based on airfoil chord) range from about .0000044 to .00005. This investigation was specifically designed to: (1) test a Boeing advanced airfoil from low to flight-equivalent Reynolds numbers; (2) provide the industry participant (Boeing) with experience in cryogenic wind-tunnel model design and testing techniques; and (3) demonstrate the suitability of the 0.3-m TCT as an airfoil test facility. All the objectives of the cooperative test were met. Data are included which demonstrate the effects of fixed transition, Mach number, and Reynolds number on the aerodynamic characteristics of the airfoil. Also included are remarks on the model design, the model structural integrity, and the overall test experience.
NASA Technical Reports Server (NTRS)
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.
1976-06-04
D2 /vT y Specific weight of water 0 2irt/T v Kinematic viscosity of water at a given temperature p Density of water -1- - -- - ~-6 ACKNOWLEDGMENTS...In addition to the methods cited above, the velocity at the test section was directly measured with a calibrated magnetic velocimeter. These...Reynolds number is not the most suitable parameter involving viscosity . The primary reasons for this are that the effect of viscosity is relatively small
Aerodynamic performance of an airfoil with a prescribed wall protuberance at low Reynolds numbers
NASA Astrophysics Data System (ADS)
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.
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
Thermocapillary motion of deformable drops at finite Reynolds and Marangoni numbers
NASA Astrophysics Data System (ADS)
Haj-Hariri, H.; Shi, Q.; Borhan, A.
1997-04-01
We present the results of numerical simulations of the three-dimensional thermocapillary motion of deformable viscous drops under the influence of a constant temperature gradient within a second liquid medium. In particular, we examine the effects of shape deformations and convective transport of momentum and energy on the migration velocity of the drop. A numerical method based on a continuum model for the fluid-fluid interface is used to account for finite drop deformations. An oct-tree adaptive grid refinement scheme is integrated into the numerical method in order to track the interface without the need for interface reconstruction. Interface deformations arising from the convection of energy at small Reynolds numbers are found to be negligible. On the other hand, deformations of the drop shape due to inertial effects, though small in magnitude, are found to retard the motion of the drop. The steady drop shapes are found to resemble oblate or prolate spheroids without fore and aft symmetry, with the direction of elongation of the drop depending on the value of the density ratio between the two phases. As in the case of a gas bubble, convection of energy is shown to retard the thermocapillary motion of a viscous drop, as the isotherms get wrapped around the front surface of the drop and effectively reduce the surface temperature gradient which drives the motion. The effect of inertia on the mobility of viscous drops is found to be weaker than that in the case of gas bubbles.
N-231 High Reynolds Number Channel Facility (An example of a Versatile Wind Tunnel) Tunnel 1 I is a
NASA Technical Reports Server (NTRS)
1980-01-01
N-231 High Reynolds Number Channel Facility (An example of a Versatile Wind Tunnel) Tunnel 1 I is a blowdown Facility that utilizes interchangeable test sections and nozzles. The facility provides experimental support for the fluid mechanics research, including experimental verification of aerodynamic computer codes and boundary-layer and airfoil studies that require high Reynolds number simulation. (Tunnel 1)
Biogenic mixing induced by intermediate Reynolds number swimming in stratified fluids
Wang, Shiyan; Ardekani, Arezoo M.
2015-01-01
We study fully resolved motion of interacting swimmers in density stratified fluids using an archetypal swimming model called “squirmer”. The intermediate Reynolds number regime is particularly important, because the vast majority of organisms in the aphotic ocean (i.e. regions that are 200 m beneath the sea surface) are small (mm-cm) and their motion is governed by the balance of inertial and viscous forces. Our study shows that the mixing efficiency and the diapycnal eddy diffusivity, a measure of vertical mass flux, within a suspension of squirmers increases with Reynolds number. The mixing efficiency is in the range of O(0.0001–0.04) when the swimming Reynolds number is in the range of O(0.1–100). The values of diapycnal eddy diffusivity and Cox number are two orders of magnitude larger for vertically swimming cells compared to horizontally swimming cells. For a suspension of squirmers in a decaying isotropic turbulence, we find that the diapycnal eddy diffusivity enhances due to the strong viscous dissipation generated by squirmers as well as the interaction of squirmers with the background turbulence. PMID:26628288
Numerical simulations of flame dynamics in the near-field of high-Reynolds number jets
NASA Astrophysics Data System (ADS)
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 < 25) of high-Reynolds number (Re) jets encountered in 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 < 25) of the 70,000-Re jet at radial locations close to the centerline (r/d < 1.5), unsteady extinction/reignition phenomena characterize the flame dynamics. Extinction is minimally affected by the vortex-induced curvature, but involves strong unsteady effects leading to extinction limits higher than steady values. On the other hand, reignition is governed by curvature effects, and occurs through the dynamics of edge flames. In the simulated jet near-field, unsteady effects are observed to diminish with increasing axial locations due to relatively weaker vortices, whereas curvature effects increase due to the presence of relatively thicker flames. The unsteady flamelet studies show that in stoichiometric regions at radial locations relatively far from the centerline (r/d > 1.0) in the jet near-field,temporary flame weakening/recovery events are likely to occur. Steady
Low Reynolds number k-epsilon modelling with the aid of direct simulation data
NASA Technical Reports Server (NTRS)
Rodi, W.; Mansour, N. N.
1993-01-01
The constant C sub mu and the near-wall damping function f sub mu in the eddy-viscosity relation of the k-epsilon model are evaluated from direct numerical simulation (DNS) data for developed channel and boundary layer flow at two Reynolds numbers each. Various existing f sub mu model functions are compared with the DNS data, and a new function is fitted to the high-Reynolds-number channel flow data. The epsilon-budget is computed for the fully developed channel flow. The relative magnitude of the terms in the epsilon-equation is analyzed with the aid of scaling arguments, and the parameter governing this magnitude is established. Models for the sum of all source and sink terms in the epsilon-equation are tested against the DNS data, and an improved model is proposed.
Determination of the Profile Drag of an Airplane Wing in Flight at High Reynolds Numbers
NASA Technical Reports Server (NTRS)
Bicknell, Joseph
1939-01-01
Flight tests were made to determine the profile-drag coefficients of a portion of the original wing surface of an all-metal airplane and of a portion of the wing made aerodynamically smooth and more nearly fair than the original section. The wing section was approximately the NACA 2414.5. The tests were carried out over a range of airplane speeds giving a maximum Reynolds number of 15,000,000. Tests were also carried out to locate the point of transition from laminar to turbulent boundary layer and to determine the velocity distribution along the upper surface of the wing. The profile-drag coefficients of the original and of the smooth wing portions at a Reynolds number of 15,000,000 were 0.0102 and 0.0068, respectively; i.e., the surface irregularities on the original wing increased the profile-drag coefficient 50 percent above that of the smooth wing.
The cryogenic wind tunnel for high Reynolds number testing. Ph.D. Thesis
NASA Technical Reports Server (NTRS)
Kilgore, R. A.
1974-01-01
Experiments performed at the NASA Langley Research Center in a cryogenic low-speed continuous-flow tunnel and in a cryogenic transonic continuous-flow pressure tunnel have demonstrated the predicted changes in Reynolds number, drive power, and fan speed with temperature, while operating with nitrogen as the test gas. The experiments have also demonstrated that cooling to cryogenic temperatures by spraying liquid nitrogen directly into the tunnel circuit is practical and that tunnel temperature can be controlled within very close limits. Whereas most types of wind tunnel could operate with advantage at cryogenic temperatures, the continuous-flow fan-driven tunnel is particularly well suited to take full advantage of operating at these temperatures. A continuous-flow fan-driven cryogenic tunnel to satisfy current requirements for test Reynolds number can be constructed and operated using existing techniques. Both capital and operating costs appear acceptable.
Low Reynolds number k-epsilon modelling with the aid of direct simulation data
NASA Technical Reports Server (NTRS)
Rodi, W.; Mansour, N. N.
1993-01-01
The constant C sub mu and the near-wall damping function f sub mu in the eddy-viscosity relation of the k-epsilon model are evaluated from direct numerical simulation (DNS) data for developed channel and boundary layer flow at two Reynolds numbers each. Various existing f sub mu model functions are compared with the DNS data, and a new function is fitted to the high-Reynolds-number channel flow data. The epsilon-budget is computed for the fully developed channel flow. The relative magnitude of the terms in the epsilon-equation is analyzed with the aid of scaling arguments, and the parameter governing this magnitude is established. Models for the sum of all source and sink terms in the epsilon-equation are tested against the DNS data, and an improved model is proposed.
Low Mass-Damping Vortex-Induced Vibrations of a Single Cylinder at Moderate Reynolds Number.
Jus, Y; Longatte, E; Chassaing, J-C; Sagaut, P
2014-10-01
The feasibility and accuracy of large eddy simulation is investigated for the case of three-dimensional unsteady flows past an elastically mounted cylinder at moderate Reynolds number. Although these flow problems are unconfined, complex wake flow patterns may be observed depending on the elastic properties of the structure. An iterative procedure is used to solve the structural dynamic equation to be coupled with the Navier-Stokes system formulated in a pseudo-Eulerian way. A moving mesh method is involved to deform the computational domain according to the motion of the fluid structure interface. Numerical simulations of vortex-induced vibrations are performed for a freely vibrating cylinder at Reynolds number 3900 in the subcritical regime under two low mass-damping conditions. A detailed physical analysis is provided for a wide range of reduced velocities, and the typical three-branch response of the amplitude behavior usually reported in the experiments is exhibited and reproduced by numerical simulation.
Application of shock tubes to transonic airfoil testing at high Reynolds numbers
NASA Technical Reports Server (NTRS)
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.
Flow and acoustic properties of low Reynolds number underexpanded supersonic jets
NASA Technical Reports Server (NTRS)
Hu, Tieh-Feng; Mclaughlin, D. K.
1990-01-01
An experimental program to investigate the flow and acoustic properties of model underexpanded supersonic jets was conducted. In particular, the role played by large-scale organized fluctuations in the flow evolution and acoustic production processes was examined in detail. The experimental conditions were chosen as low-Reynolds-number (Re = 8000) Mach 1.4 and 2.1 underexpanded jets exhausting from convergent nozzles. A consequence of performing the experiments at low Reynolds number is that the broadband shock-associated noise is suppressed. The focus of the present study is on the generation of noise by large-scale instabilities in the presence of strong shock cell structures. It is demonstrated that the production of screech is related to the modulation and decay of large-scale turbulence structures.
Transition and separation control on a low-Reynolds number airfoil
NASA Technical Reports Server (NTRS)
Mangalam, S. M.; Bar-Sever, A.; Zaman, K. B. M. Q.; Harvey, W. D.
1986-01-01
The major problem associated with the aerodynamic performance of airfoils at low Reynolds numbers is the presence of extensive laminar boundary-layer separation resulting in a large increase in presssure drag and a decrease in lift. The rapid deterioration in airfoil characteristics can be largely eliminated by artificially controlling the flow through the introduction of suitable disturbances in the boundary layer such that transition occurs ahead of the anticipated laminar separation. This paper presents the results of wind-tunnel tests conducted on a 10-cm model of LRN (1)-1007 airfoil with passive (roughness trips) and active (acoustic excitation) controls to trigger transition and suppress separation. Significant improvements in the aerodynamic characteristics of the airfoil were observed. Results of this study for a chord Reynolds number range of 40,000 to 250,000 are presented in this paper.
An Implicit Immersed Boundary Method for Low Reynolds Number Incompressible Flows
NASA Astrophysics Data System (ADS)
Park, Hyun Wook; Lee, Changhoon; Choi, Jung-Il
2013-11-01
We develop a new formulation of immersed boundary (IB) method based on direct forcing for incompressible viscous flows. The new algorithm for the present IB method is derived using a block LU decomposition and Taylor series expansion, and the direct forcing for imposing no-slip condition on the IB surface is calculated in an iterative procedure. We perform simulations of two-dimensional flows around a circular cylinder and three-dimensional flows over a sphere for low and moderate Reynolds numbers. The result shows that present method yield a better imposition of no-slip condition on IB surface for low Reynolds number with a fairly larger time step than other IB methods based on direct forcing. Supported by EDISON (2011-0029561) program of NRF.
Numerical Simulation of High Reynolds Number Turbulent Flow in a Vane Passage
NASA Astrophysics Data System (ADS)
Huang, J. F.; Zhang, L. X.; Guo, Y. K.
The numerical simulations of two-dimensional high Reynolds number turbulent flow in a guide vane passage of a Francis hydro turbine are performed successfully by using the unstructured dynamic mesh model for moving body. The standard K - ɛ turbulence model is employed for the simulation of turbulence. The pressure-velocity coupling method is realized with the SIMPLEC algorithm. The temporal distributions of the pressure and turbulent viscosity in a passage were obtained in the closing of wicket gate on a platform of the software ANSYS FLUENT. The results show that the evolution of the flow field is unsteady with decrease of the geometrical opening of the gate. The details of the flow changes are obtained in the moving rigid body domain. The method can be used to simulate the high Reynolds number incompressible turbulent flow with moving boundary. The calculation provides some references for vortex-induced vibration of the structure in a complex turbulent flow.
Detached eddy simulation of high-Reynolds-number turbulent flows using the immersed boundary method
NASA Astrophysics Data System (ADS)
Bernardini, Matteo; Pirozzoli, Sergio; Orlandi, Paolo
2015-11-01
Detached Eddy Simulation based on the Spalart-Allmaras turbulence model is applied in conjunction with the immersed boundary method to simulate high-Reynolds number turbulent flows in complex geometries. A fourth-order, finite-difference solver capable of discretely preserving the kinetic energy in the limit of inviscid flow is adopted to solve the compressible Navier-Stokes equations and model-consistent, adaptive wall functions are employed to provide the proper numerical boundary conditions at the fluid/solid interface. Numerical tests, performed for several configurations involving massively separated flows, demonstrate that computations at high-Reynolds number, as typically occurring in flows of industrial relevance, can be successfully carried out using the immersed boundary strategy, providing predictions whose accuracy is comparable to that of standard, body-fitted, structured or unstructured flow solvers.
NASA Astrophysics Data System (ADS)
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 .
Linkmann, Moritz; Berera, Arjun; Goldstraw, Erin E
2017-01-01
This paper examines the behavior of the dimensionless dissipation rate C_{ɛ} 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 C_{ɛ} 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 C_{ɛ,∞} lie between 0.193≤C_{ɛ,∞}≤0.268 for free decay, where the value depends on the initial level of cross- and magnetic helicities. In the stationary case we measure C_{ɛ,∞}=0.223.
Heat transfer predictions for two turbine nozzle geometries at high Reynolds and Mach numbers
NASA Astrophysics Data System (ADS)
Boyle, R. J.; Jackson, R.
1995-09-01
Predictions of turbine vane and endwall heat transfer and pressure distributions are compared with experimental measurements for two vane geometries. The differences in geometries were due to differences in the hub profile, and both geometries were derived from the design of a high rim speed turbine (HRST). The experiments were conducted in the Isentropic Light Piston Facility (ILPF) at Pyestock at a Reynolds number of 5.3 x 10(exp 6), a Mach number of 1.2, and a wall-to-gas temperature ratio of 0.66. Predictions are given for two different steady-state three-dimensional Navier-Stokes computational analyses. C-type meshes were used, and algebraic models were employed to calculate the turbulent eddy viscosity. The effects of different turbulence modeling assumptions on the predicted results are examined. Comparisons are also given between predicted and measured total pressure distributions behind the vane. The combination of realistic engine geometries and flow conditions proved to be quite demanding in terms of the convergence of the CFD solutions. An appropriate method of grid generation, which resulted in consistently converged CFD solutions, was identified.
Experiments on a Steady Low Reynolds Number Airfoil in a Shear Flow
NASA Astrophysics Data System (ADS)
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.
NASA Technical Reports Server (NTRS)
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.