Journal articles: 'Freestream velocity'

1

Thole, K. A., and D. G. Bogard. "High Freestream Turbulence Effects on Turbulent Boundary Layers." Journal of Fluids Engineering 118, no. 2 (June 1, 1996): 276–84. http://dx.doi.org/10.1115/1.2817374.

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High freestream turbulence levels significantly alter the characteristics of turbulent boundary layers. Numerous studies have been conducted with freestreams having turbulence levels of 7 percent or less, but studies using turbulence levels greater than 10 percent have been essentially limited to the effects on wall shear stress and heat transfer. This paper presents measurements of the boundary layer statistics for the interaction between a turbulent boundary layer and a freestream with turbulence levels ranging from 10 to 20 percent. The boundary layer statistics reported in this paper include mean and rms velocities, velocity correlation coefficients, length scales, and power spectra. Although the freestream turbulent eddies penetrate into the boundary layer at high freestream turbulence levels, as shown through spectra and length scale measurements, the mean velocity profile still exhibits a log-linear region. Direct measurements of total shear stress (turbulent shear stress and viscous shear stress) confirm the validity of the log-law at high freestream turbulence levels. Velocity defects in the outer region of the boundary layer were significantly decreased resulting in negative wake parameters. Fluctuating rms velocities were only affected when the freestream turbulence levels exceeded the levels of the boundary layer generated rms velocities. Length scales and power spectra measurements showed large scale turbulent eddies penetrate to within y+ = 15 of the wall.

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Fraser, C. J., and J. S. Milne. "Integral Calculations of Transitional Boundary Layers." Proceedings of the Institution of Mechanical Engineers, Part C: Journal of Mechanical Engineering Science 200, no. 3 (May 1986): 179–87. http://dx.doi.org/10.1243/pime_proc_1986_200_113_02.

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A method is presented for the calculation of two-dimensional, incompressible, transitional boundary layers under small pressure gradient and moderate freestream turbulence conditions. Established integral techniques are used in conjunction with an intermittency weighted model of the transitional boundary layer, and empirical correlations are used to predict the onset and length of the transition region. The only input data required to compute the entire unseparated boundary layer are the ambient pressure and temperature, the freestream turbulence level and the freestream velocity distribution in a power law, or a polynomial form. Alternatively, the freestream velocity can be input in tabular form as a function of x. The computed integral parameters and mean velocity profiles are seen to compare favourably with present and other published experimental data.

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Matsunuma, Takayuki, and Takehiko Segawa. "Effects of Input Voltage and Freestream Velocity on Active Flow Control of Passage Vortex in a Linear Turbine Cascade Using Dielectric Barrier Discharge Plasma Actuator." Energies 13, no. 3 (February 9, 2020): 764. http://dx.doi.org/10.3390/en13030764.

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Passage vortex exists as one of the typical secondary flows in turbomachines and generates a significant total pressure loss and degrades the aerodynamic performance. Herein, a dielectric barrier discharge (DBD) plasma actuator was utilized for an active flow control of the passage vortex in a linear turbine cascade. The plasma actuator was installed on the endwall, 10 mm upstream from the leading edge of the turbine cascade. The freestream velocity at the outlet of the linear turbine cascade was set to range from UFS,out = 2.4 m/s to 25.2 m/s, which corresponded to the Reynolds number ranging from Reout = 1.0 × 104 to 9.9 × 104. The two-dimensional velocity field at the outlet of the linear turbine cascade was experimentally analyzed by particle image velocimetry (PIV). At lower freestream velocity conditions, the passage vortex was almost negligible as a result of the plasma actuator operation (UPA,max/UFS,out = 1.17). Although the effect of the jet induced by the plasma actuator weakened as the freestream velocity increased, the magnitude of the peak vorticity was reduced under all freestream velocity conditions. Even at the highest freestream velocity condition of UFS,out = 25.2 m/s, the peak value of the vorticity was reduced approximately 17% by the plasma actuator operation at VAC = 15 kVp-p (UPA,max/UFS,out = 0.18).

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4

Radomsky, R. W., and K. A. Thole. "Detailed Boundary Layer Measurements on a Turbine Stator Vane at Elevated Freestream Turbulence Levels." Journal of Turbomachinery 124, no. 1 (February 1, 2001): 107–18. http://dx.doi.org/10.1115/1.1424891.

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High freestream turbulence levels have been shown to greatly augment the heat transfer on a gas turbine airfoil. To better understand these effects, this study has examined the effects elevated freestream turbulence levels have on the boundary layer development along a stator vane airfoil. Low freestream turbulence measurements (0.6 percent) were performed as a baseline for comparison to measurements at combustor simulated turbulence levels (19.5 percent). A two-component LDV system was used for detailed boundary layer measurements of both the mean and fluctuating velocities on the pressure and suction surfaces. Although the mean velocity profiles appeared to be more consistent with laminar profiles, large velocity fluctuations were measured in the boundary layer along the pressure side at the high freestream turbulence conditions. Along the suction side, transition occurred further upstream due to freestream turbulence.

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Yaras, M. I. "An Experimental Study of Artificially-Generated Turbulent Spots Under Strong Favorable Pressure Gradients and Freestream Turbulence." Journal of Fluids Engineering 129, no. 5 (September 13, 2006): 563–72. http://dx.doi.org/10.1115/1.2717608.

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This paper presents experimental results on the internal flow structure of turbulent spots, and examines the sensitivity of this structure to streamwise acceleration rate and freestream turbulence. Measurements were performed on a flat plate, with two levels of freestream acceleration rate and three levels of freestream turbulence. The turbulent spots were generated artificially at a fixed distance from the test-surface leading edge, and the development of the spot was documented through hotwire measurements at three streamwise locations. The measurements were performed at multiple spanwise locations to allow observation of the three-dimensional spatial structure of the turbulent spot and the temporal evolution of this structure. Analysis of the perturbation velocity and rms velocity fluctuations provides insight into the variations of the streaky streamwise-velocity structure within the turbulent spot, with a focus on the effects of freestream acceleration rate and turbulence level.

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Wang, Qing, and Qijun Zhao. "Unsteady Aerodynamic Characteristics Simulations of Rotor Airfoil under Oscillating Freestream Velocity." Applied Sciences 10, no. 5 (March 6, 2020): 1822. http://dx.doi.org/10.3390/app10051822.

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The dynamic stall characteristics of rotor airfoil are researched by employing unsteady Reynolds-Averaged Navier-Stokes (RANS) method under oscillating freestream velocity conditions. In order to simulate the oscillating freestream velocity of airfoil under dynamic stall conditions, the moving-embedded grid method is employed to simulate the oscillating velocity. By comparing the simulated dynamic stall characteristics of two-dimensional airfoil and three-dimensional rotor, it is indicated that the dynamic stall characteristics of airfoil under oscillating freestream velocity reflect the actual dynamic stall characteristics of rotor airfoil in forward flight more accurately. By comparing the simulated results of OA209 airfoil under coupled freestream velocity/pitching oscillation conditions, it is indicated that the dynamic stall characteristics of airfoil associate with the critical value of Cp peaks (i.e., the dynamic stall characteristics of OA209 airfoil would be enhanced when the maximum negative pressure is larger than −1.08, and suppressed when this value is smaller than −1.08). By comparing the characteristics of vortices under different oscillating velocities, it indicates that the dissipation rate of leading edge vortex presents as exponent characteristics, and it is not sensitive to different oscillating velocities.

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Dechant, Lawrence J. "Effect of Freestream Velocity Disturbances on Hypersonic Vehicles." Journal of Spacecraft and Rockets 49, no. 4 (July 2012): 751–56. http://dx.doi.org/10.2514/1.a32113.

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8

Heathcote, S., D. Martin, and I. Gursul. "Flexible Flapping Airfoil Propulsion at Zero Freestream Velocity." AIAA Journal 42, no. 11 (November 2004): 2196–204. http://dx.doi.org/10.2514/1.5299.

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9

Carlson, Bailey, Al Habib Ullah, and Jordi Estevadeordal. "Experimental Investigation of Vortex-Tube Streamwise-Vorticity Characteristics and Interaction Effects with a Finite-Aspect-Ratio Wing." Fluids 5, no. 3 (July 24, 2020): 122. http://dx.doi.org/10.3390/fluids5030122.

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An experimental study is conducted to analyze a streamwise-oriented vortex and investigate the unsteady interaction with a finite-aspect-ratio wing. A pressurized vortex tube is used to generate streamwise vortices in a wind tunnel and the resulting flow behavior is analyzed. The vortex tube, operated at various pressures, yields flows that evolve downstream under several freestream wind tunnel speeds. Flow measurements are performed using two- and three- dimensional (2D and 3D) particle image velocimetry to observe vortices and their freestream interactions from which velocity and vorticity data are comparatively analyzed. Results indicate that vortex velocity greater than freestream flow velocity is a primary factor in maintaining vortex structures further downstream, while increased supply pressure and reduced freestream velocity also reduce vortex dissipation rate. The generated streamwise-oriented vortex is also impinged on a finite-aspect-ratio airfoil wing with a cross-section of standard NACA0012 airfoil. The wingtip-aligned vortex is shown to investigate the interaction of the streamwise vortex and the wingtip vortex region. The results indicate that the vorticity at the high vortex-tube pressure has a significant effect on the boundary layer of airfoil.

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10

Hoffmann, J. A., and K. Mohammadi. "Velocity Profiles for Turbulent Boundary Layers Under Freestream Turbulence." Journal of Fluids Engineering 113, no. 3 (September 1, 1991): 399–404. http://dx.doi.org/10.1115/1.2909509.

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Correlations for changes of skin friction coefficients (Δcf) and wake parameters (ΔΠ), relative to the low freestream turbulence condition, are presented for the case of turbulent boundary layers under freestream turbulence with zero and adverse pressure gradients. The turbulent boundary layers were evaluated on a plate in a wind tunnel using a monoplane rod set turbulence generator; comparisons were also made using the data of several other investigators. The results, which define the velocity profiles within the boundary layers, were found to collapse for a large range of the pressure gradient parameter.

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11

Rixon, Gregory S., and Hamid Johari. "Development of a Steady Vortex Generator Jet in a Turbulent Boundary Layer." Journal of Fluids Engineering 125, no. 6 (November 1, 2003): 1006–15. http://dx.doi.org/10.1115/1.1627833.

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The development of a vortex generator jet within a turbulent boundary layer was studied by the particle image velocimetry method. Jet velocities ranging from one to three times greater than the freestream velocity were examined. The jet was pitched 45 deg and skewed 90 deg with respect to the surface and flow direction, respectively. The velocity field in planes normal to the freestream was measured at four stations downstream of the jet exit. The jet created a pair of streamwise vortices, one of which was stronger and dominated the flow field. The circulation, peak vorticity, and wall-normal position of the primary vortex increased linearly with the jet velocity. The circulation and peak vorticity decreased exponentially with the distance from the jet source for the jet-to-freestream velocity ratios of 2 and 3. The wandering of the streamwise vortex can be as much as ±30% of the local boundary layer thickness at the farthest measurement station.

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12

Murawski, C. G., and K. Vafai. "An Experimental Investigation of the Effect of Freestream Turbulence on the Wake of a Separated Low-Pressure Turbine Blade at Low Reynolds Numbers." Journal of Fluids Engineering 122, no. 2 (December 20, 1999): 431–33. http://dx.doi.org/10.1115/1.483281.

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An experimental study was conducted in a two-dimensional linear cascade, focusing on the suction surface of a low pressure turbine blade. Flow Reynolds numbers, based on exit velocity and suction length, have been varied from 50,000 to 300,000. The freestream turbulence intensity was varied from 1.1 to 8.1 percent. Separation was observed at all test Reynolds numbers. Increasing the flow Reynolds number, without changing freestream turbulence, resulted in a rearward movement of the onset of separation and shrinkage of the separation zone. Increasing the freestream turbulence intensity, without changing Reynolds number, resulted in shrinkage of the separation region on the suction surface. The influences on the blade’s wake from altering freestream turbulence and Reynolds number are also documented. It is shown that width of the wake and velocity defect rise with a decrease in either turbulence level or chord Reynolds number. [S0098-2202(00)00202-9]

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13

Zhang, Qiang, and Phillip M. Ligrani. "Wake Turbulence Structure Downstream of a Cambered Airfoil in Transonic Flow: Effects of Surface Roughness and Freestream Turbulence Intensity." International Journal of Rotating Machinery 2006 (2006): 1–12. http://dx.doi.org/10.1155/ijrm/2006/60234.

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The wake turbulence structure of a cambered airfoil is studied experimentally, including the effects of surface roughness, at different freestream turbulence levels in a transonic flow. As the level of surface roughness increases, all wake profile quantities broaden significantly and nondimensional vortex shedding frequencies decrease. Freestream turbulence has little effect on the wake velocity profiles, turbulence structure, and vortex shedding frequency, especially downstream of airfoils with rough surfaces. Compared with data from a symmetric airfoil, wake profiles produced by the cambered airfoils also have significant dependence on surface roughness, but are less sensitive to variations of freestream turbulence intensity. The cambered airfoil also produces larger streamwise velocity deficits, and broader wakes compared to the symmetric airfoil.

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14

Hong, Je Won, and Phillip J. Ansell. "Influence of Freestream Velocity on Tilt-Rotor Fountain Effect." Journal of Aircraft 55, no. 4 (July 2018): 1742–45. http://dx.doi.org/10.2514/1.c034599.

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15

Lai, Joseph C. S., and Max F. Platzer. "Characteristics of a Plunging Airfoil at Zero Freestream Velocity." AIAA Journal 39, no. 3 (March 2001): 531–34. http://dx.doi.org/10.2514/2.1340.

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16

Lai, Joseph C. S., and Max F. Platzer. "Characteristics of a plunging airfoil at zero freestream velocity." AIAA Journal 39 (January 2001): 531–34. http://dx.doi.org/10.2514/3.14764.

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17

Brandner, P. A., B. W. Pearce, and K. L. de Graaf. "Cavitation about a jet in crossflow." Journal of Fluid Mechanics 768 (March 4, 2015): 141–74. http://dx.doi.org/10.1017/jfm.2015.73.

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Cavitation occurrence about a jet in crossflow is investigated experimentally in a variable-pressure water tunnel using still and high-speed photography. The 0.012 m diameter jet is injected on the centreplane of a 0.6 m square test section at jet to freestream velocity ratios ranging from 0.2 to 1.6, corresponding to jet-velocity-based Reynolds numbers of $25\times 10^{3}$ to $160\times 10^{3}$ respectively. Measurements were made at a fixed freestream-based Reynolds number, for which the ratio of the undisturbed boundary layer thickness to jet diameter is 1.18. The cavitation number was varied from inception (up to about 10) down to 0.1. Inception is investigated acoustically for bounding cases of high and low susceptibility to phase change. The influence of velocity ratio and cavitation number on cavity topology and geometry are quantified from the photography. High-speed photographic recordings made at 6 kHz provide insight into cavity dynamics, and derived time series of spatially averaged pixel intensities enable frequency analysis of coherent phenomena. Cavitation inception was found to occur in the high-shear regions either side of the exiting jet and to be of an intermittent nature, increasing in occurrence and duration from 0 to 100 % probability with decreasing cavitation number or increasing jet to freestream velocity ratio. The frequency and duration of individual events strongly depends on the cavitation nuclei supply within the approaching boundary layer. Macroscopic cavitation develops downstream of the jet with reduction of the cavitation number beyond inception, the length of which has a power-law dependence on the cavitation number and a linear dependence on the jet to freestream velocity ratio. The cavity closure develops a re-entrant jet with increase in length forming a standing wave within the cavity. For sufficiently low cavitation numbers the projection of the re-entrant jet fluid no longer reaches the cavity leading edge, analogous to supercavitation forming about solid cavitators. Hairpin-shaped vortices are coherently shed from the cavity closure via mechanisms of shear-layer roll-up similar to those shed from protuberances and jets in crossflow in single-phase flows. These vortices are shed at an apparently constant frequency, independent of the jet to freestream velocity ratio but decreasing in frequency with reducing cavitation number and cavity volume growth. Highly coherent cavitating vortices form along the leading part of the cavity due to instability of the jet upstream shear layer for jet to freestream velocity ratios greater than about 0.8. These vortices are cancelled and condense as they approach the trailing edge in the shear layer of opposing vorticity associated with the cavity closure and the hairpin vortex formation. For lower velocity ratios, where there is decreased jet penetration, the jet upstream shear velocity gradient reverses and vortices of the opposite sense form, randomly modulated by boundary layer turbulence.

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18

Dris, Antonis, and Mark W. Johnson. "Transition on Concave Surfaces." Journal of Turbomachinery 127, no. 3 (March 1, 2004): 507–11. http://dx.doi.org/10.1115/1.1861914.

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Boundary layer measurements have been made on the concave surfaces of two constant curvature blades using hot wire anemometry. All the current experiments were performed with negligible streamwise pressure gradient. Grids were used to produce a range of freestream turbulence levels between 1% and 4%. The freestream velocity increases with distance from a concave wall according to the free vortex condition making the determination of the boundary layer edge difficult. A flat plate equivalent boundary layer procedure was adopted, therefore, to overcome this problem. The Taylor–Goertler (TG) vortices resulting from the concave curvature were found to make the laminar and turbulent boundary layer profiles fuller and to increase the skin friction coeffiicent by up to 40% compared with flat plate values. This leads to a more rapid growth in boundary layer thickness. The evolution in the intermittency through transition is very similar to that for a flat plate, however, the shape factors are depressed slightly throughout the flow due to the fuller velocity profiles. For all the current experiments, curvature promoted transition. This was very marked at low freestream turbulence level but remained significant even at the highest levels. It appears that the velocity fluctuations associated with the TG vortices enhance the freestream turbulence resulting in a higher effective turbulence level. A new empirical correlation for start of transition based on this premise is presented. The ratio of end to start of transition momentum thickness Reynolds numbers was found to be approximately constant.

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19

Giguere, Philippe, and Michael S. Selig. "Freestream Velocity Corrections for Two-Dimensional Testing with Splitter Plates." AIAA Journal 35, no. 7 (July 1997): 1195–200. http://dx.doi.org/10.2514/2.213.

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Giguere, Philippe, and Michael S. Selig. "Freestream velocity corrections for two-dimensional testing with splitter plates." AIAA Journal 35 (January 1997): 1195–200. http://dx.doi.org/10.2514/3.13645.

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21

Nix, A. C., T. E. Diller, and W. F. Ng. "Experimental Measurements and Modeling of the Effects of Large-Scale Freestream Turbulence on Heat Transfer." Journal of Turbomachinery 129, no. 3 (October 5, 2006): 542–50. http://dx.doi.org/10.1115/1.2515555.

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The influence of freestream turbulence representative of the flow downstream of a modern gas turbine combustor and first stage vane on turbine blade heat transfer has been measured and analytically modeled in a linear, transonic turbine cascade. High-intensity, large length-scale freestream turbulence was generated using a passive turbulence-generating grid to simulate the turbulence generated in modern combustors after passing through the first stage vane row. The grid produced freestream turbulence with intensity of approximately 10–12% and an integral length scale of 2cm(Λx∕c=0.15) near the entrance of the cascade passages. Mean heat transfer results with high turbulence showed an increase in heat transfer coefficient over the baseline low turbulence case of approximately 8% on the suction surface of the blade, with increases on the pressure surface of approximately 17%. Time-resolved surface heat transfer and passage velocity measurements demonstrate strong coherence in velocity and heat flux at a frequency correlating with the most energetic eddies in the turbulence flow field (the integral length scale). An analytical model was developed to predict increases in surface heat transfer due to freestream turbulence based on local measurements of turbulent velocity fluctuations and length scale. The model was shown to predict measured increases in heat flux on both blade surfaces in the current data. The model also successfully predicted the increases in heat transfer measured in other work in the literature, encompassing different geometries (flat plate, cylinder, turbine vane, and turbine blade) and boundary layer conditions.

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22

Eckerle, W. A., and J. K. Awad. "Effect of Freestream Velocity on the Three-Dimensional Separated Flow Region in Front of a Cylinder." Journal of Fluids Engineering 113, no. 1 (March 1, 1991): 37–44. http://dx.doi.org/10.1115/1.2926493.

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Details of the horseshoe vortex formation around a cylinder were studied to determine the flow parameters that affect the flow separation in front of the cylinder. An experimental setup consisting of a circular cylinder vertically mounted on the floor of the wind tunnel test section was assembled. The approaching turbulent boundary layer was four centimeters thick. Pressures were measured on the cylinder surface and the tunnel floor with surface static pressure taps. Surface flow visualizations were accomplished to locate singlar points and the size of separation region on the endwall surface. Interior mean and fluctuating velocity data and Reynolds stresses in front of the cylinder were nonintrusively measured with a two-component Laser Doppler Anemometer system. Freestream stagnation at the endwall/cylinder surface occurred in all cases, but two types of separation were identified in this investigation. The flow pattern in the separation region depends on the freestream momentum and the boundary layer displacement thickness. A large-scale fully developed vortex was formed in the plane of symmetry for low approaching freestream velocities. A fully developed vortex was not present at higher approach velocities. Maximum production of turbulent kinetic energy was measured around the core of the vortex when fully formed.

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23

Barrett, Michael J., and D. Keith Hollingsworth. "Correlating Friction Velocity in Turbulent Boundary Layers Subjected to Freestream Turbulence." AIAA Journal 41, no. 8 (August 2003): 1444–51. http://dx.doi.org/10.2514/2.2127.

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24

Mustafa, M. A., N. J. Parziale, M. S. Smith, and E. C. Marineau. "Nonintrusive Freestream Velocity Measurement in a Large-Scale Hypersonic Wind Tunnel." AIAA Journal 55, no. 10 (October 2017): 3611–16. http://dx.doi.org/10.2514/1.j056177.

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Gharali, Kobra, and David A. Johnson. "Dynamic stall simulation of a pitching airfoil under unsteady freestream velocity." Journal of Fluids and Structures 42 (October 2013): 228–44. http://dx.doi.org/10.1016/j.jfluidstructs.2013.05.005.

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26

Wang, Qing, and Qijun Zhao. "Unsteady aerodynamic characteristics investigation of rotor airfoil under variational freestream velocity." Aerospace Science and Technology 58 (November 2016): 82–91. http://dx.doi.org/10.1016/j.ast.2016.08.001.

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27

McAuliffe, Brian R., and Steen A. Sjolander. "Active Flow Control Using Steady Blowing for a Low-Pressure Turbine Cascade." Journal of Turbomachinery 126, no. 4 (October 1, 2004): 560–69. http://dx.doi.org/10.1115/1.1791291.

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The paper presents mid-span measurements for a turbine cascade with active flow control. Steady blowing through an inclined plane wall jet has been used to control the separation characteristics of a high-lift low-pressure turbine airfoil at low Reynolds numbers. Measurements were made at design incidence for blowing ratios from approximately 0.25 to 2.0 (ratio of jet-to-local freestream velocity), for Reynolds numbers of 25,000 and 50,000 (based on axial chord and inlet velocity), and for freestream turbulence intensities of 0.4% and 4%. Detailed flow field measurements were made downstream of the cascade using a three-hole pressure probe, static pressure distributions were measured on the airfoil suction surface, and hot-wire measurements were made to characterize the interaction between the wall jet and boundary layer. The primary focus of the study is on the low-Reynolds number and low-freestream turbulence intensity cases, where the baseline airfoil stalls and high profile losses result. For low freestream turbulence (0.4%), the examined method of flow control was effective at preventing stall and reducing the profile losses. At a Reynolds number of 25,000, a blowing ratio greater than 1.0 was required to suppress stall. At a Reynolds number of 50,000, a closed separation bubble formed at a very low blowing ratio (0.25) resulting in a significant reduction in the profile loss. For high freestream turbulence intensity (4%), where the baseline airfoil has a closed separation bubble and low profile losses, blowing ratios below 1.0 resulted in a larger separation bubble and higher losses. The mechanism by which the wall jet affects the separation characteristics of the airfoil is examined through hot-wire traverse measurements in the vicinity of the slot.

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Kumar, Rajeev, Justin T. King, and Melissa A. Green. "Momentum Distribution in the Wake of a Trapezoidal Pitching Panel." Marine Technology Society Journal 50, no. 5 (September 1, 2016): 9–23. http://dx.doi.org/10.4031/mtsj.50.5.2.

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AbstractThe oscillation of bioinspired fin-like panels in a uniform freestream flow creates chains of vortex rings, including streamwise segments that induce significant three-dimensional effects. With increasing Strouhal number, this wake structure induces flow with increasing nondimensional momentum, defined relative to the freestream velocity, in the downstream direction. This increase in relative momentum with increasing Strouhal number is consistent with greater nondimensional thrust production, which has been shown previously in the literature. These results were obtained via stereoscopic particle image velocimetry water tunnel experiments at Strouhal numbers ranging from 0.17 to 0.56 downstream of a continuously pitching trapezoidal panel. Features of the wake dynamics including spanwise compression, transverse expansion, transverse wake splitting or bifurcation, and wake breakdown are elucidated through analyses of phase-averaged as well as time-averaged velocity fields, in addition to common vortex identification methods.

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Willmarth, William W., and Timothy Wei. "Static Pressure Distribution on Long Cylinders as a Function of the Yaw Angle and Reynolds Number." Fluids 6, no. 5 (April 22, 2021): 169. http://dx.doi.org/10.3390/fluids6050169.

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This paper addresses the challenges of pressure-based sensing using axisymmetric probes whose axes are at small angles to the mean flow. Mean pressure measurements around three yawed circular cylinders with aspect ratios of 28, 64, and 100 were made to determine the effect of changes in the yaw angle, γ, and freestream velocity on the average pressure coefficient, C¯pN, and drag coefficient, CDN. The existence of four distinct types of circumferential pressure distributions—subcritical, transitional, supercritical, and asymmetric—were confirmed, along with the appropriateness of scaling C¯pN and CDN on a streamwise Reynolds number, Resw, based on the freestream velocity and the fluid path length along the cylinder in the streamwise direction. It was found that there was a distinct difference in the values of CDN and C¯pN at identical Resw values for cylinders yawed between 5° and 30°, and for cylinders at greater than a 30° yaw. For γ < 5°, there did not appear to be any large-scale vortices in the near wake, and CDN and C¯pN appeared to become independent of Resw. Over the range of 5° ≤ γ ≤ 30°, there was a complex interplay of freestream speed, yaw angle, and aspect ratio that affected the formation and shedding of Kármán-like vortices.

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Matsunuma, Takayuki. "Effects of Reynolds Number and Freestream Turbulence on Turbine Tip Clearance Flow." Journal of Turbomachinery 128, no. 1 (February 1, 2005): 166–77. http://dx.doi.org/10.1115/1.2103091.

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Tip clearance losses represent a major efficiency penalty of turbine blades. This paper describes the effect of tip clearance on the aerodynamic characteristics of an unshrouded axial-flow turbine cascade under very low Reynolds number conditions. The Reynolds number based on the true chord length and exit velocity of the turbine cascade was varied from 4.4×104 to 26.6×104 by changing the velocity of fluid flow. The freestream turbulence intensity was varied between 0.5% and 4.1% by modifying turbulence generation sheet settings. Three-dimensional flow fields at the exit of the turbine cascade were measured both with and without tip clearance using a five-hole pressure probe. Tip leakage flow generated a large high total pressure loss region. Variations in the Reynolds number and freestream turbulence intensity changed the distributions of three-dimensional flow, but had no effect on the mass-averaged tip clearance loss of the turbine cascade.

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Abid, R., and C. G. Speziale. "The Freestream Matching Condition for Stagnation Point Turbulent Flows: An Alternative Formulation." Journal of Applied Mechanics 63, no. 1 (March 1, 1996): 95–100. http://dx.doi.org/10.1115/1.2787215.

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The problem of plane stagnation point flow with freestream turbulence is examined from a basic theoretical standpoint. It is argued that the singularity which arises in the standard K–ε model results from the use of an inconsistent freestream boundary condition. The inconsistency lies in the implementation of a production-equals-dissipation equilibrium hypothesis in conjunction with a freestream mean velocity field that corresponds to homogeneous plane strain—a turbulent flow for which the standard K–ε model does not predict such a simple equilibrium. The ad hoc adjustment that has been made in the constants of the ε-transport equation to eliminate this singularity is shown to be inconsistent for homogeneous plane-strain turbulence as well as other benchmark turbulent flows. An alternative means to eliminate this singularity—without compromising model predictions in more basic turbulent flows—is proposed based on the incorporation of nonequilibrium vortex stretching effects in the turbulent dissipation rate equation.

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Shyne, Rickey J., Ki-Hyeon Sohn, and Kenneth J. De Witt. "Experimental Investigation of Boundary Layer Behavior in a Simulated Low Pressure Turbine." Journal of Fluids Engineering 122, no. 1 (August 30, 1999): 84–89. http://dx.doi.org/10.1115/1.483229.

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A detailed investigation of the flow physics occurring on the suction side of a simulated low pressure turbine (LPT) blade was performed. A contoured upper wall was designed to simulate the pressure distribution of an actual LPT blade onto a flat plate. The experiments were carried out at Reynolds numbers of 100,000 and 250,000 with three levels of freestream turbulence. The main emphasis in this paper is placed on flow field surveys performed at a Reynolds number of 100,000 with levels of freestream turbulence ranging from 0.8 percent to 3 percent. Smoke-wire flow visualization data were used to confirm that the boundary layer was separated and formed a bubble. The transition process over the separated flow region is observed to be similar to a laminar free shear layer flow with the formation of a large coherent eddy structure. For each condition, the locations defining the separation bubble were determined by careful examination of pressure and mean velocity profile data. Transition onset location and length determined from intermittency profiles decrease as freestream turbulence levels increase. Additionally, the length and height of the laminar separation bubbles were observed to be inversely proportional to the levels of freestream turbulence. [S0098-2202(00)00701-X]

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Volino, Ralph J., Michael P. Schultz, and Christopher M. Pratt. "Conditional Sampling in a Transitional Boundary Layer Under High Freestream Turbulence Conditions." Journal of Fluids Engineering 125, no. 1 (January 1, 2003): 28–37. http://dx.doi.org/10.1115/1.1521957.

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Conditional sampling has been performed on data from a transitional boundary layer subject to high (initially 9%) freestream turbulence and strong (K=ν/U∞2dU∞/dx as high as 9×10−6) acceleration. Methods for separating the turbulent and nonturbulent zone data based on the instantaneous streamwise velocity and the turbulent shear stress were tested and found to agree. Mean velocity profiles were clearly different in the turbulent and nonturbulent zones, and skin friction coefficients were as much as 70% higher in the turbulent zone. The streamwise fluctuating velocity, in contrast, was only about 10% higher in the turbulent zone. Turbulent shear stress differed by an order of magnitude, and eddy viscosity was three to four times higher in the turbulent zone. Eddy transport in the nonturbulent zone was still significant, however, and the nonturbulent zone did not behave like a laminar boundary layer. Within each of the two zones there was considerable self-similarity from the beginning to the end of transition. This may prove useful for future modeling efforts.

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O¨lc¸men, M. S., and R. L. Simpson. "Perspective: On the Near Wall Similarity of Three-Dimensional Turbulent Boundary Layers (Data Bank Contribution)." Journal of Fluids Engineering 114, no. 4 (December 1, 1992): 487–95. http://dx.doi.org/10.1115/1.2910059.

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The possible existence of a “law-of-the-wall” similarity velocity profile for 3-D boundary layers was investigated using nine different proposed relations with the data from nine experiments carried out in 3-D turbulent boundary layers. Both for pressure driven and shear-driven flows, the “law-of-the-wall” relation of Johnston for the local freestream velocity direction component best applies. Although not well described by any relation, the crosswise velocity component of pressure-driven flows and shear-driven flows is best represented by Mager’s relation and Chandrashekhar and Swamy equation, respectively.

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35

Liao, James C., and Otar Akanyeti. "Fish Swimming in a Kármán Vortex Street: Kinematics, Sensory Biology and Energetics." Marine Technology Society Journal 51, no. 5 (September 1, 2017): 48–55. http://dx.doi.org/10.4031/mtsj.51.5.8.

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AbstractFishes often live in environments characterized by complex flows. To study the mechanisms of how fishes interact with unsteady flows, the periodic shedding of vortices behind cylinders has been employed to great effect. In particular, fishes that hold station in a vortex street (i.e., Kármán gaiting) show swimming kinematics that are distinct from their patterns of motion during freestream swimming in uniform flows, although both behaviors can be modeled as an undulatory body wave. Kármán gait kinematics are largely preserved across flow velocities. Larger fish have a shorter body wavelength and slower body wave speed than smaller fish, in contrast to freestream swimming where body wavelength and wave speed increases with size. The opportunity for Kármán gaiting only occurs under specific conditions of flow velocity and depends on the length of the fish; this is reflected in the highest probability of Kármán gaiting at intermediate flow velocities. Fish typically Kármán gait in a region of the cylinder wake where the velocity deficit is about 40% of the nominal flow. The lateral line plays a role in tuning the kinematics of the Kármán gait, since blocking it leads to aberrant kinematics. Vision allows fish to maintain a consistent position relative to the cylinder. In the dark, fish do not show the same preference to hold station behind a cylinder though Kármán gait kinematics are the same. When oxygen consumption level is measured, it reveals that Kármán gaiting represents about half of the cost of swimming in the freestream.

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Reed, H. L., N. Lin, and W. S. Saric. "Boundary Layer Receptivity to Sound: Navier-Stokes Computations." Applied Mechanics Reviews 43, no. 5S (May 1, 1990): S175—S180. http://dx.doi.org/10.1115/1.3120798.

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Here we discuss the computational modeling of the receptivity of the laminar boundary layer on a semi-infinite flat plate with an elliptic leading edge. The incompressible flow is computed in a spatial simulation using the full Navier-Stokes equations in general curvilinear coordinates. Finite differences are used in both space directions and in time. First, the steady basic state is obtained in a transient approach using spatially varying time steps. Then, small-amplitude acoustic disturbances of the freestream velocity are applied as unsteady boundary conditions, and the governing equations are solved time-accurately to evaluate the spatial and temporal developments of instability waves (Tollmien-Schlichting waves) in the boundary layer. A sharper leading edge is found to be less receptive to freestream disturbances.

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Ma, Penglei, Yong Wang, Yudong Xie, and Zhipu Huo. "Effects of time-varying freestream velocity on energy harvesting using an oscillating foil." Ocean Engineering 153 (April 2018): 353–62. http://dx.doi.org/10.1016/j.oceaneng.2018.01.115.

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ISO, Takuro, Yu NISHIO, Seiichiro IZAWA, and Yu FUKUNISHI. "127 Development of an Apparatus Generating Disturbance in Freestream without a Velocity Defect." Proceedings of Conference of Tohoku Branch 2016.51 (2016): 51–52. http://dx.doi.org/10.1299/jsmeth.2016.51.51.

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Qi, Zhanfeng, Lishuang Jia, Yufeng Qin, Jian Shi, and Jingsheng Zhai. "Propulsion Performance of the Full-Active Flapping Foil in Time-Varying Freestream." Applied Sciences 10, no. 18 (September 8, 2020): 6226. http://dx.doi.org/10.3390/app10186226.

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A numerical investigation of the propulsion performance and hydrodynamic characters of the full-active flapping foil under time-varying freestream is conducted. The finite volume method is used to calculate the unsteady Reynolds averaged Navier–Stokes by commercial Computational Fluid Dynamics (CFD) software Fluent. A mesh of two-dimensional (2D) NACA0012 foil with the Reynolds number Re = 42,000 is used in all simulations. We first investigate the propulsion performance of the flapping foil in the parameter space of reduced frequency and pitching amplitude at a uniform flow velocity. We define the time-varying freestream as a superposition of steady flow and sinusoidal pulsating flow. Then, we study the influence of time-varying flow velocity on the propulsion performance of flapping foil and note that the influence of the time-varying flow is time dependent. For one period, we find that the oscillating amplitude and the oscillating frequency coefficient of the time-varying flow have a significant influence on the propulsion performance of the flapping foil. The influence of the time-varying flow is related to the motion parameters (reduced frequency and pitching amplitude) of the flapping foil. The larger the motion parameters, the more significant the impact of propulsion performance of the flapping foil. For multiple periods, we note that the time-varying freestream has little effect on the propulsion performance of the full-active flapping foil at different pitching amplitudes and reduced frequency. In summary, we conclude that the time-varying incoming flow has little effect on the flapping propulsion performance for multiple periods. We can simplify the time-varying flow to a steady flow field to a certain extent for numerical simulation.

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Liu, W., and A. Plotkin. "Application of the Cosserat Spectrum Theory to Stokes Flow." Journal of Applied Mechanics 66, no. 3 (September 1, 1999): 811–14. http://dx.doi.org/10.1115/1.2791761.

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This paper presents an application of the Cosserat spectrum theory in elasticity to the solution of low Reynolds number (Stokes flow) problems. The velocity field is divided into two components: a solution to the vector Laplace equation and a solution associated with the discrete Cosserat eigenvectors. Analytical solutions are presented for the Stokes flow past a sphere with uniform, extensional, and linear shear freestream profiles.

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Stewart, Robert L., and James F. Fox. "Outer region scaling using the freestream velocity for nonuniform open channel flow over gravel." Advances in Water Resources 104 (June 2017): 271–83. http://dx.doi.org/10.1016/j.advwatres.2017.04.004.

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Balachandar, R., L. Robillard, and A. S. Ramamurthy. "Some Characteristics of Counter Flowing Wall Jets." Journal of Fluids Engineering 114, no. 4 (December 1, 1992): 554–58. http://dx.doi.org/10.1115/1.2910067.

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Experimental results related to the interaction between a uniform flow and a two-dimensional counter flowing wall jet are presented for various ratios of the jet velocity to the freestream velocity. Both visual observations and wall pressure surveys were made in the jet penetration zone. Attempts were made to choose the proper scaling variables to suitably nondimensionalize the wall pressure distributions. The geometrical characteristics of the dividing streamline were determined for a range of test conditions. Limited tests were also carried out to check the influence of the size of the jet injection device on the flow characteristics.

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Arnal, M. P., D. J. Goering, and J. A. C. Humphrey. "Vortex Shedding From a Bluff Body Adjacent to a Plane Sliding Wall." Journal of Fluids Engineering 113, no. 3 (September 1, 1991): 384–98. http://dx.doi.org/10.1115/1.2909508.

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The characteristics of the flow around a bluff body of square cross-section in contact with a solid-wall boundary are investigated numerically using a finite difference procedure. Previous studies (Taneda, 1965; Kamemoto et al., 1984) have shown qualitatively the strong influence of solid-wall boundaries on the vortex-shedding process and the formation of the vortex street downstream. In the present study three cases are investigated which correspond to flow past a square rib in a freestream, flow past a rib on a fixed wall and flow past a rib on a sliding wall. Values of the Reynolds number studied ranged from 100 to 2000, where the Reynolds number is based on the rib height, H, and bulk stream velocity, Ub. Comparisons between the sliding-wall and fixed-wall cases show that the sliding wall has a significant destabilizing effect on the recirculation region behind the rib. Results show the onset of unsteadiness at a lower Reynolds number for the sliding-wall case (50 ≤ Recrit ≤100) than for the fixed-wall case (Recrit≥100). A careful examination of the vortex-shedding process reveals similarities between the sliding-wall case and both the freestream and fixed-wall cases. At moderate Reynolds numbers (Re≥250) the sliding-wall results show that the rib periodically sheds vortices of alternating circulation in much the same manner as the rib in a freestream; as in, for example, Davis and Moore [1982]. The vortices are distributed asymmetrically downstream of the rib and are not of equal strength as in the freestream case. However, the sliding-wall case shows no tendency to develop cycle-to-cycle variations at higher Reynolds numbers, as observed in the freestream and fixed-wall cases. Thus, while the moving wall causes the flow past the rib to become unsteady at a lower Reynolds number than in the fixed-wall case, it also acts to stabilize or “lock-in” the vortex-shedding frequency. This is attributed to the additional source of positive vorticity immediately downstream of the rib on the sliding wall.

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Yang, Chin Tien, and J. S. T’ien. "Numerical Simulation of Combustion and Extinction of a Solid Cylinder in Low-Speed Cross Flow." Journal of Heat Transfer 120, no. 4 (November 1, 1998): 1055–63. http://dx.doi.org/10.1115/1.2825890.

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The combustion and extinction behavior of a diffusion flame around a solid fuel cylinder (PMMA) in low-speed forced flow in zero gravity was studied numerically using a quasi-steady gas phase model. This model includes two-dimensional continuity, full Navier Stokes’ momentum, energy, and species equations with a one-step overall chemical reaction and second-order finite-rate Arrhenius kinetics. Surface radiation and Arrhenius pyrolysis kinetics are included on the solid fuel surface description and a parameter Φ, representing the percentage of gas-phase conductive heat flux going into the solid, is introduced into the interfacial energy balance boundary condition to complete the description for the quasi-steady gas-phase system. The model was solved numerically using a body-fitted coordinate transformation and the SIMPLE algorithm. The effects of varying freestream velocity and Φ were studied. These parameters have a significant effect on the flame structure and extinction limits. Two flame modes were identified: envelope flame and wake flame. Two kinds of flammability limits were found: quenching at low-flow speeds due to radiative loss and blow-off at high flow speeds due to insufficient gas residence time. A flammability map was constructed showing the existence of maximum Φ above which the solid is not flammable at any freestream velocity.

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Heitzig, DNWM, BW van Oudheusden, D. Olejnik, and M. Karásek. "Effects of asymmetrical inflow in forward flight on the deformation of interacting flapping wings." International Journal of Micro Air Vehicles 12 (January 2020): 175682932094100. http://dx.doi.org/10.1177/1756829320941002.

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This study investigates the wing deformation of the DelFly II in forward flight conditions. A measurement setup was developed that maintains adequate viewing axes of the flapping wings for all pitch angles. Recordings of a high-speed camera pair were processed using a point tracking algorithm, allowing 136 points per wing to be measured simultaneously with an estimated accuracy of 0.25 mm. The measurements of forward flight show little change in the typical clap-and-peel motion, suggesting similar effectiveness in all cases. It was found that an air-buffer remains at all times during this phase. The wing rotation and camber reduction during the upstroke suggests low loading during the upstroke in fast forward flight. In slow cases a torsional wave and recoil is found. A study of the isolated effects showed asymmetric deformations even in symmetric freestream conditions. Furthermore, it shows a dominant role of the flapping frequency on the clap-and-peel, while the freestream velocity reduces wing loading outside this phase.

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Bae, S., S. K. Lele, and H. J. Sung. "Influence of Inflow Disturbances on Stagnation-Region Heat Transfer." Journal of Heat Transfer 122, no. 2 (November 29, 1999): 258–65. http://dx.doi.org/10.1115/1.521486.

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Numerical simulations of laminar stagnation-region heat transfer in the presence of freestream disturbances are performed. The sensitivity of heat transfer in stagnation-region to freestream vorticity is scrutinized by varying the length scale, amplitude, and Reynolds number. As an organized inflow disturbance, a spanwise sinusoidal variation is superimposed on the velocity component normal to the wall. An accurate numerical scheme is employed to integrate the compressible Navier-Stokes equations and energy equation. The main emphasis is placed on the length scale of laminar inflow disturbances, which maximizes the heat transfer enhancement. Computational results are presented to disclose the detailed behavior of streamwise vortices. Three regimes of the behavior are found depending on the length scale: these are the “damping,” “attached amplifying,” and “detached amplifying” regimes, respectively. The simulation data are analyzed with an experimental correlation. It is found that the present laminar results follow a general trend of the correlation. [S0022-1481(00)01102-6]

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Yang, Guang Jun, and Jing Sun. "Numerical Simulation of Non-Equal Diameter Cylinder." Applied Mechanics and Materials 249-250 (December 2012): 527–32. http://dx.doi.org/10.4028/www.scientific.net/amm.249-250.527.

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Numerical simulation has been carried out on the wake flow structure of some simplified antenna model which is shaped as a non-equal diameter slender cylinder. The unsteady flow parameter-Strouhal number is confirmed to be a constant approximately in the subcritical state. The results show that at levels of different diameters, when the length-diameter ratio is large enough, each level can still maintain stable periodical vortex shedding phenomenon, the vortex shedding frequency of each level, the flow velocity and the equivalent diameter still meet the Strouhal relationship. The effect of connect transition area on vortex shedding stabilize region are close to the freestream velocity.

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Sobczyk, Jacek. "Experimental study of the flow field disturbance in the vicinity of single sensor hot-wire anemometer." EPJ Web of Conferences 180 (2018): 02094. http://dx.doi.org/10.1051/epjconf/201818002094.

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Preliminary experimental study of the flow field disturbance in the vicinity of single sensor normal hot-wire anemometer (SN) probe was carried out. Regular 2D particle image velocimetry (PIV) setup equipped with micro lens and distance rings was applied to measurements of macroscopic flow around microscopic elements. Experimental results revealed complexity of the flow around the wire and its strong dependence on both – the velocity magnitude and the probe orientation in relation to freestream direction. Examination of the velocity fields in the vicinity of SN probe suggests that it may not be such a “point” measurement method as it is commonly assumed to be.

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Sreenath, Avula, Gurumurthy Ramachandran, and James Vincent. "Effect of orientation and freestream velocity on the sampling efficiency of a blunt body sampler." Journal of Aerosol Science 29 (September 1998): S331—S332. http://dx.doi.org/10.1016/s0021-8502(98)00492-3.

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Scholten, J. W., and D. B. Murray. "Unsteady heat transfer and velocity of a cylinder in cross flow—I. Low freestream turbulence." International Journal of Heat and Mass Transfer 41, no. 10 (May 1998): 1139–48. http://dx.doi.org/10.1016/s0017-9310(97)00250-0.

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Journal articles on the topic 'Freestream velocity'

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