Turbulent flow over large-amplitude wavy surfaces

1984 ◽  
Vol 140 ◽  
pp. 27-44 ◽  
Author(s):  
Jeffrey Buckles ◽  
Thomas J. Hanratty ◽  
Ronald J. Adrian
Keyword(s):  
Boundary Layer ◽  
Turbulent Flow ◽  
Velocity Fields ◽  
Laser Doppler Velocimeter ◽  
Free Shear Layer ◽  
Wave Surface ◽  
Velocity Measurements ◽  
Mean Velocity ◽  
Wavy Surfaces ◽  
The Mean

The laser-Doppler velocimeter is used to measure the mean and the fluctuating velocity for turbulent flow over a solid sinusoidal wave surface having a wavelength λ of 50.8 mm and a wave amplitude of 5.08 mm. For this flow, a large separated region exists, extending from x/λ = 0.14 to 0.69. From the mean velocity measurements, the time-averaged streamlines and therefore the extent of the separated region are calculated. Three flow elements are identified: the separated region, an attached boundary layer, and a free shear layer formed by the detachment of the boundary layer from the wave surface. The characteristics of these flow elements are discussed in terms of the properties of the mean and fluctuating velocity fields.

Turbulent Boundary Layers on Surfaces Covered With Filamentous Algae

2000 ◽  
Vol 122 (2) ◽  
pp. 357-363 ◽  
Author(s):  
Michael P. Schultz
Keyword(s):  
Boundary Layer ◽  
Shear Stress ◽  
Turbulence Intensity ◽  
Reynolds Shear Stress ◽  
Skin Friction Coefficient ◽  
Laser Doppler Velocimeter ◽  
Mean Velocity ◽  
Rough Walls ◽  
The Mean ◽  
Stress Profiles

Turbulent boundary layer measurements have been made on surfaces covered with filamentous marine algae. These experiments were conducted in a closed return water tunnel using a two-component, laser Doppler velocimeter (LDV). The mean velocity profiles and parameters, as well as the axial and wall-normal turbulence intensities and Reynolds shear stress, are compared with flows over smooth and sandgrain rough walls. Significant increases in the skin friction coefficient for the algae-covered surfaces were measured. The boundary layer and integral thickness length scales were also increased. The results indicate that profiles of the turbulence quantities for the smooth and sandgrain rough walls collapse when friction velocity and boundary layer thickness are used as normalizing parameters. The algae-covered surfaces, however, exhibited a significant increase in the wall-normal turbulence intensity and the Reynolds shear stress, with only a modest increase in the axial turbulence intensity. The peak in the Reynolds shear stress profiles for the algae surfaces corresponded to the maximum extent of outward movement of the algae filaments. [S0098-2202(00)01902-7]

Turbulent Boundary Layers Over Surfaces Smoothed by Sanding

2003 ◽  
Vol 125 (5) ◽  
pp. 863-870 ◽  
Author(s):  
Michael P. Schultz ◽  
Karen A. Flack
Keyword(s):  
Boundary Layer ◽  
Skin Friction Coefficient ◽  
Laser Doppler Velocimeter ◽  
Reynolds Stresses ◽  
Smooth Wall ◽  
Mean Velocity ◽  
Towing Tank ◽  
The Mean ◽  
Stress Profiles ◽  
Similarity Law

Flat-plate turbulent boundary layer measurements have been made on painted surfaces, smoothed by sanding. The measurements were conducted in a closed return water tunnel, over a momentum thickness Reynolds number Reθ range of 3000 to 16,000, using a two-component laser Doppler velocimeter (LDV). The mean velocity and Reynolds stress profiles are compared with those for smooth and sandgrain rough walls. The results indicate an increase in the boundary layer thickness (δ) and the integral length scales for the unsanded, painted surface compared to a smooth wall. More significant increases in these parameters, as well as the skin-friction coefficient Cf were observed for the sandgrain surfaces. The sanded surfaces behave similarly to the smooth wall for these boundary layer parameters. The roughness functions ΔU+ for the sanded surfaces measured in this study agree within their uncertainty with previous results obtained using towing tank tests and similarity law analysis. The present results indicate that the mean profiles for all of the surfaces collapse well in velocity defect form. The Reynolds stresses also show good collapse in the overlap and outer regions of the boundary layer when normalized with the wall shear stress.

Comparison Between a Slot Jet and Round Jets Impinging a Curved Wall

2003 ◽  
Author(s):  
V. Gilard ◽  
L.-E. Brizzi
Keyword(s):  
Data Processing ◽  
Flow Structure ◽  
Statistical Data ◽  
Velocity Fields ◽  
Reynolds Stresses ◽  
Velocity Measurements ◽  
Mean Velocity ◽  
Statistical Data Processing ◽  
Round Jets ◽  
The Mean

Velocity measurements by PIV are realized in order to compare a slot jet and round jets impinging a curved surface. A statistical data processing allows us to obtain the mean velocity fields and the Reynolds stresses. For the two jet geometries, the flow structure is described. Some velocity distributions according to different axis are extracted of the mean velocity fields and are also described.

Velocity, Temperature, and Heat-Transfer Measurements in a Turbulent Boundary Layer Downstream of a Stepwise Discontinuity in Wall Temperature

1957 ◽  
Vol 24 (1) ◽  
pp. 2-8
Author(s):  
D. S. Johnson
Keyword(s):  
Heat Transfer ◽  
Boundary Layer ◽  
Turbulent Boundary Layer ◽  
Free Stream ◽  
Local Heat Transfer ◽  
Local Heat ◽  
Velocity Fields ◽  
Temperature Fields ◽  
Mean Velocity ◽  
The Mean

Abstract Results are presented of an experimental investigation of the concomitant thermal and velocity fields occurring when there is a small stepwise discontinuity in the temperature of the wall on which a zero-pressure-gradient, low-speed, turbulent boundary layer has formed. The mean velocity and temperature fields have been measured and local heat-transfer-coefficient values in the stream-wise direction have been obtained in the region where the thermal boundary layer has not yet reached the free stream. No over-all similarity between the thermal and velocity fields was found.

Prediction of Turbulent Flow Over a Flat Plate With a Step Change From a Smooth to a Rough Surface Using a Near-Wall RANS Model

2019 ◽  
Vol 142 (1) ◽  
Author(s):  
Minghan Chu ◽  
Donald J. Bergstrom
Keyword(s):  
Boundary Layer ◽  
Surface Roughness ◽  
Kinetic Energy ◽  
Turbulent Flow ◽  
Flat Plate ◽  
Rough Surface ◽  
Step Change ◽  
Turbulence Kinetic Energy ◽  
Mean Velocity ◽  
The Mean

Abstract The present paper reports a numerical study of fully developed turbulent flow over a flat plate with a step change from a smooth to a rough surface. The Reynolds number based on momentum thickness for the smooth flow was Reθ=5950. The focus of the study was to investigate the capability of the Reynolds-averaged Navier–Stokes (RANS) equations to predict the internal boundary layer (IBL) created by the flow configuration. The numerical solution used a two-layer k−ε model to implement the effects of surface roughness on the turbulence and mean flow fields via the use of a hydrodynamic roughness length y0. The prediction for the mean velocity field revealed a development zone immediately downstream of the step in which the mean velocity profile included a lower region affected by the surface roughness below and an upper region with the characteristics of the smooth-wall boundary layer above. In this zone, both the turbulence kinetic energy and Reynolds shear stress profiles were characterized by a significant reduction in magnitude in the outer region of the flow that is unaffected by the rough surface. The turbulence kinetic energy profile was used to estimate the thickness of the IBL, and the resulting growth rate closely matched the experimental results. As such, the IBL is a promising test case for assessing the ability of RANS models to predict the discrete roughness configurations often encountered in industrial and environmental applications.

Slot Jet Impinging a Curved Wall

2002 ◽  
Author(s):  
V. Gilard ◽  
L.-E. Brizzi
Keyword(s):  
Data Processing ◽  
Statistical Data ◽  
Velocity Fields ◽  
Pressure Measurements ◽  
Reynolds Stresses ◽  
Velocity Measurements ◽  
Mean Velocity ◽  
Concave Wall ◽  
Wall Flow ◽  
The Mean

In order to study the aerodynamics of a slot jet impinging a concave wall, flow visualisations, velocity measurements by PIV and pressure measurements are carried out. A statistical data processing allows us to obtain the mean velocity fields and the Reynolds stresses. Among the studied parameters, the effect of the relative curvature of the wall is studied in particular because of a jet beating phenomenon observed for a low relative curvature. Three flow modes are then described.

Experimental Investigation of Boundary Layer Relaminarization in Accelerated Flow

2018 ◽  
Vol 140 (8) ◽  
Author(s):  
Pascal Bader ◽  
Manuel Pschernig ◽  
Wolfgang Sanz ◽  
Jakob Woisetschläger ◽  
Franz Heitmeir ◽  
...  
Keyword(s):  
Boundary Layer ◽  
Turbulent Flow ◽  
Skin Friction ◽  
Laser Doppler Anemometry ◽  
Plate Boundary ◽  
Mean Velocity ◽  
Local Skin ◽  
Velocity Fluctuations ◽  
Mean Values ◽  
The Mean

Flow in turbomachines is generally highly turbulent. Nonetheless, boundary layers may exhibit laminar-to-turbulent transition, and relaminarization of the turbulent flow may also occur. The state of flow of the boundary layer is important since it influences transport phenomena like skin friction and heat transfer. In this paper, relaminarization in accelerated flat-plate boundary-layer flows is experimentally investigated, measuring flow velocities with laser Doppler anemometry (LDA). Besides the mean values, statistical properties of the velocity fluctuations are discussed in order to understand the processes in relaminarization. It is shown that strong acceleration leads to a suppression of turbulence production. The velocity fluctuations in the accelerated boundary layer flow “freeze,” while the mean velocity increases, thus reducing the turbulence intensity. This leads to a laminar-like velocity profile close to the wall, resulting in a decrease of the local skin friction coefficient. Downstream from the section with enforced relaminarization, a rapid retransition to turbulent flow is observed. The findings of this work also describe the mechanism of retransition.

On the turbulent flow over a wavy boundary

1970 ◽  
Vol 42 (4) ◽  
pp. 721-731 ◽  
Author(s):  
Russ E. Davis
Keyword(s):  
Turbulent Flow ◽  
Numerical Solutions ◽  
Approximate Solutions ◽  
Travelling Wave ◽  
Experimental Results ◽  
Reynolds Stresses ◽  
Wave Surface ◽  
Asymptotic Results ◽  
Mean Velocity ◽  
The Mean

Two hypotheses concerning the turbulent flow over an infinitesimal-amplitude travelling wave are investigated. One hypothesis, originally made by Miles, is that the wave does not affect the turbulence and therefore the turbulent Reynolds stresses are dependent only on height above the mean wave surface. Alternatively, the proposal that turbulent stresses are primarily dependent on height above the instantaneous wave surface is examined. Numerical solutions of the appropriate equations are compared with Stewart's recent experimental results and with the approximate solutions employed by Miles and others. No definite conclusion can be reached from comparison with experimental results since the predicted flows are quite sensitive to details of the mean velocity profile near the wave surface where no data was taken. It is found that the asymptotic results do not apply for the conditions investigated.

The structure of a highly decelerated axisymmetric turbulent boundary layer

2021 ◽  
Vol 929 ◽  
Author(s):  
N. Agastya Balantrapu ◽  
Christopher Hickling ◽  
W. Nathan Alexander ◽  
William Devenport
Keyword(s):  
Boundary Layer ◽  
Pressure Gradient ◽  
Shear Layer ◽  
Layer Thickness ◽  
Boundary Layer Thickness ◽  
Large Scale ◽  
Convection Velocity ◽  
Mean Flow ◽  
Mean Velocity ◽  
The Mean

Experiments were performed over a body of revolution at a length-based Reynolds number of 1.9 million. While the lateral curvature parameters are moderate ( $\delta /r_s < 2, r_s^+>500$ , where $\delta$ is the boundary layer thickness and r s is the radius of curvature), the pressure gradient is increasingly adverse ( $\beta _{C} \in [5 \text {--} 18]$ where $\beta_{C}$ is Clauser’s pressure gradient parameter), representative of vehicle-relevant conditions. The mean flow in the outer regions of this fully attached boundary layer displays some properties of a free-shear layer, with the mean-velocity and turbulence intensity profiles attaining self-similarity with the ‘embedded shear layer’ scaling (Schatzman & Thomas, J. Fluid Mech., vol. 815, 2017, pp. 592–642). Spectral analysis of the streamwise turbulence revealed that, as the mean flow decelerates, the large-scale motions energize across the boundary layer, growing proportionally with the boundary layer thickness. When scaled with the shear layer parameters, the distribution of the energy in the low-frequency region is approximately self-similar, emphasizing the role of the embedded shear layer in the large-scale motions. The correlation structure of the boundary layer is discussed at length to supply information towards the development of turbulence and aeroacoustic models. One major finding is that the estimation of integral turbulence length scales from single-point measurements, via Taylor's hypothesis, requires significant corrections to the convection velocity in the inner 50 % of the boundary layer. The apparent convection velocity (estimated from the ratio of integral length scale to the time scale), is approximately 40 % greater than the local mean velocity, suggesting the turbulence is convected much faster than previously thought. Closer to the wall even higher corrections are required.

A theory of turbulence in stratified fluids

1970 ◽  
Vol 42 (2) ◽  
pp. 349-365 ◽  
Author(s):  
Robert R. Long
Keyword(s):  
Boundary Layer ◽  
Turbulent Boundary Layer ◽  
Density Profile ◽  
Richardson Number ◽  
Mixed Volume ◽  
Volume Of Fluid ◽  
Velocity Shear ◽  
Mean Velocity ◽  
Large Wave ◽  
The Mean

An effort is made to understand turbulence in fluid systems like the oceans and atmosphere in which the Richardson number is generally large. Toward this end, a theory is developed for turbulent flow over a flat plate which is moved and cooled in such a way as to produce constant vertical fluxes of momentum and heat. The theory indicates that in a co-ordinate system fixed in the plate the mean velocity increases linearly with heightzabove a turbulent boundary layer and the mean density decreases asz3, so that the Richardson number is large far from the plate. Near the plate, the results reduce to those of Monin & Obukhov.Thecurvatureof the density profile is essential in the formulation of the theory. When the curvature is negative, a volume of fluid, thoroughly mixed by turbulence, will tend to flatten out at a new level well above the original centre of mass, thereby transporting heat downward. When the curvature is positive a mixed volume of fluid will tend to fall a similar distance, again transporting heat downward. A well-mixed volume of fluid will also tend to rise when the density profile is linear, but this rise is negligible on the basis of the Boussinesq approximation. The interchange of fluid of different, mean horizontal speeds in the formation of the turbulent patch transfers momentum. As the mixing in the patch destroys the mean velocity shear locally, kinetic energy is transferred from mean motion to disturbed motion. The turbulence can arise in spite of the high Richardson number because the precise variations of mean density and mean velocity mentioned above permit wave energy to propagate from the turbulent boundary layer to the whole region above the plate. At the levels of reflexion, where the amplitudes become large, wave-breaking and turbulence will tend to develop.The relationship between the curvature of the density profile and the transfer of heat suggests that the density gradient near the level of a point of inflexion of the density curve (in general cases of stratified, shearing flow) will increase locally as time goes on. There will also be a tendency to increase the shear through the action of local wave stresses. If this results in a progressive reduction in Richardson number, an ultimate outbreak of Kelvin–Helmholtz instability will occur. The resulting sporadic turbulence will transfer heat (and momentum) through the level of the inflexion point. This mechanism for the appearance of regions of low Richardson number is offered as a possible explanation for the formation of the surfaces of strong density and velocity differences observed in the oceans and atmosphere, and for the turbulence that appears on these surfaces.

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