The turbulent boundary layer over transverse square cavities

1999 ◽  
Vol 395 ◽  
pp. 271-294 ◽  
Author(s):  
L. DJENIDI ◽  
R. ELAVARASAN ◽  
R. A. ANTONIA
Keyword(s):  
Boundary Layer ◽  
Turbulent Boundary Layer ◽  
Flow Direction ◽  
Reynolds Shear Stress ◽  
Local Maximum ◽  
Turbulent Energy ◽  
Skin Friction Coefficient ◽  
Reynolds Stresses ◽  
Smooth Wall ◽  
Local Skin

Laser-induced uorescence (LIF) and laser Doppler velocimetry (LDV) are used to explore the structure of a turbulent boundary layer over a wall made up of two-dimensional square cavities placed transversely to the flow direction. There is strong evidence of occurrence of outflows of fluid from the cavities as well as inflows into the cavities. These events occur in a pseudo-random manner and are closely associated with the passage of near-wall quasi-streamwise vortices. These vortices and the associated low-speed streaks are similar to those found in a turbulent boundary layer over a smooth wall. It is conjectured that outflows play an important role in maintaining the level of turbulent energy in the layer and enhancing the approach towards self-preservation. Relative to a smooth wall layer, there is a discernible increase in the magnitudes of all the Reynolds stresses and a smaller streamwise variation of the local skin friction coefficient. A local maximum in the Reynolds shear stress is observed in the shear layers over the cavities.

The Effect of Biofilms on Turbulent Boundary Layers

1999 ◽  
Vol 121 (1) ◽  
pp. 44-51 ◽  
Author(s):  
M. P. Schultz ◽  
G. W. Swain
Keyword(s):  
Boundary Layer ◽  
Friction Coefficient ◽  
Turbulent Boundary Layer ◽  
Skin Friction ◽  
Layer Structure ◽  
Reynolds Shear Stress ◽  
Skin Friction Coefficient ◽  
Laser Doppler Velocimeter ◽  
Reynolds Stresses ◽  
The Mean

Materials exposed in the marine environment, including those protected by antifouling paints, may rapidly become colonized by microfouling. This may affect frictional resistance and turbulent boundary layer structure. This study compares the mean and turbulent boundary layer velocity characteristics of surfaces covered with a marine biofilm with those of a smooth surface. Measurements were made in a nominally zero pressure gradient, boundary layer flow with a two-component laser Doppler velocimeter at momentum thickness Reynolds numbers of 5600 to 19,000 in a recirculating water tunnel. Profiles of the mean and turbulence velocity components, including the Reynolds shear stress, were measured. An average increase in the skin friction coefficient of 33 to 187 percent was measured on the fouled specimens. The skin friction coefficient was found to be dependent on both biofilm thickness and morphology. The biofilms tested showed varying effect on the Reynolds stresses when those quantities were normalized with the friction velocity.

The accelerated fully rough turbulent boundary layer

1977 ◽  
Vol 82 (3) ◽  
pp. 507-528 ◽  
Author(s):  
Hugh W. Coleman ◽  
Robert J. Moffat ◽  
William M. Kays
Keyword(s):  
Boundary Layer ◽  
Shear Stress ◽  
Turbulent Boundary Layer ◽  
Pressure Gradient ◽  
Stress Tensor ◽  
Skin Friction ◽  
Shape Factor ◽  
Reynolds Shear Stress ◽  
Smooth Wall ◽  
Mean Velocity

The behaviour of a fully rough turbulent boundary layer subjected to favourable pressure gradients both with and without blowing was investigated experimentally using a porous test surface composed of densely packed spheres of uniform size. Measurements of profiles of mean velocity and the components of the Reynolds-stress tensor are reported for both unblown and blown layers. Skin-friction coefficients were determined from measurements of the Reynolds shear stress and mean velocity.An appropriate acceleration parameterKrfor fully rough layers is defined which is dependent on a characteristic roughness dimension but independent of molecular viscosity. For a constant blowing fractionFgreater than or equal to zero, the fully rough turbulent boundary layer reaches an equilibrium state whenKris held constant. Profiles of the mean velocity and the components of the Reynolds-stress tensor are then similar in the flow direction and the skin-friction coefficient, momentum thickness, boundary-layer shape factor and the Clauser shape factor and pressure-gradient parameter all become constant.Acceleration of a fully rough layer decreases the normalized turbulent kinetic energy and makes the turbulence field much less isotropic in the inner region (forFequal to zero) compared with zero-pressure-gradient fully rough layers. The values of the Reynolds-shear-stress correlation coefficients, however, are unaffected by acceleration or blowing and are identical with values previously reported for smooth-wall and zero-pressure-gradient rough-wall flows. Increasing values of the roughness Reynolds number with acceleration indicate that the fully rough layer does not tend towards the transitionally rough or smooth-wall state when accelerated.

Modification of Near-Wall Structure in a Shear-Driven 3-D Turbulent Boundary Layer

2001 ◽  
Vol 124 (1) ◽  
pp. 118-126 ◽  
Author(s):  
Robert O. Kiesow ◽  
Michael W. Plesniak
Keyword(s):  
Boundary Layer ◽  
Turbulent Boundary Layer ◽  
Reynolds Shear Stress ◽  
Power Spectra ◽  
Streamwise Velocity ◽  
Wall Structure ◽  
Reynolds Stresses ◽  
Planar Shear ◽  
Inner Region ◽  
Near Wall

The near-wall physics of a planar, shear-driven, 3-D turbulent boundary layer with varying strengths of crossflow are examined. Flow visualization data reveals a reduction of mean streak length by as much as 50% with increasing spanwise shear. Power spectra of velocity confirm this shift towards higher temporal frequencies, corresponding to decreased streamwise length scales. PIV measurements indicate a significant modification of the inner region of the boundary layer with increasing spanwise shear. Streamwise velocity profiles exhibit an increasing velocity deficit with increased crossflow. Increased levels of the normal Reynolds stresses u′2¯ and v′2¯ and an increase in the −u′v′¯ Reynolds shear stress are also observed. Modifications in the spanwise and transverse vorticity were also observed at higher shear rates.

scholarly journals Large-eddy simulation of the zero-pressure-gradient turbulent boundary layer up to Reθ = O(1012)

2011 ◽  
Vol 686 ◽  
pp. 507-533 ◽  
Author(s):  
M. Inoue ◽  
D. I. Pullin
Keyword(s):  
Boundary Layer ◽  
Friction Coefficient ◽  
Turbulent Boundary Layer ◽  
Large Eddy Simulation ◽  
Pressure Gradient ◽  
Skin Friction Coefficient ◽  
Smooth Wall ◽  
Eddy Simulation ◽  
Large Eddy ◽  
Near Wall

AbstractA near-wall subgrid-scale (SGS) model is used to perform large-eddy simulation (LES) of the developing, smooth-wall, zero-pressure-gradient flat-plate turbulent boundary layer. In this model, the stretched-vortex, SGS closure is utilized in conjunction with a tailored, near-wall model designed to incorporate anisotropic vorticity scales in the presence of the wall. Large-eddy simulations of the turbulent boundary layer are reported at Reynolds numbers ${\mathit{Re}}_{\theta } $ based on the free-stream velocity and the momentum thickness in the range ${\mathit{Re}}_{\theta } = 1{0}^{3} \text{{\ndash}} 1{0}^{12} $. Results include the inverse square-root skin-friction coefficient, $ \sqrt{2/ {C}_{f} } $, velocity profiles, the shape factor $H$, the von Kármán ‘constant’ and the Coles wake factor as functions of ${\mathit{Re}}_{\theta } $. Comparisons with some direct numerical simulation (DNS) and experiment are made including turbulent intensity data from atmospheric-layer measurements at ${\mathit{Re}}_{\theta } = O(1{0}^{6} )$. At extremely large ${\mathit{Re}}_{\theta } $, the empirical Coles–Fernholz relation for skin-friction coefficient provides a reasonable representation of the LES predictions. While the present LES methodology cannot probe the structure of the near-wall region, the present results show turbulence intensities that scale on the wall-friction velocity and on the Clauser length scale over almost all of the outer boundary layer. It is argued that LES is suggestive of the asymptotic, infinite Reynolds number limit for the smooth-wall turbulent boundary layer and different ways in which this limit can be approached are discussed. The maximum ${\mathit{Re}}_{\theta } $ of the present simulations appears to be limited by machine precision and it is speculated, but not demonstrated, that even larger ${\mathit{Re}}_{\theta } $ could be achieved with quad- or higher-precision arithmetic.

The response of a turbulent boundary layer to a step change in surface roughness Part 1. Smooth to rough

1971 ◽  
Vol 48 (4) ◽  
pp. 721-761 ◽  
Author(s):  
R. A. Antonia ◽  
R. E. Luxton
Keyword(s):  
Boundary Layer ◽  
Turbulent Boundary Layer ◽  
Pressure Gradient ◽  
Rough Surface ◽  
Sudden Change ◽  
Turbulent Energy ◽  
Rough Wall ◽  
Internal Layer ◽  
Internal Boundary ◽  
Smooth Wall

The structure and growth of the internal boundary layer which forms downstream of a sudden change from a smooth to a rough surface under zero pressure gradient conditions has been studied experimentally. To keep pressure disturbances due to the roughness change small, the level of the rough surface was depressed, so that the crest of the roughness was aligned with the level of the smooth surface. It has been found that, in the region near the change, the structure of the internal layer is largely independent of that in the almost undisturbed outer layer, whilst both the zero time delay and the moving axis integral length scales in the internal layer are significantly reduced below those on the smooth wall. The growth-rate of the internal layer is similar to that of the zero pressure gradient boundary layer, whilst the level of turbulence inside the internal layer is high because of the large turbulent energy production near the rough wall. From the mixing length results, and an analysis of the turbulent energy equation, it is deduced that the internal layer flow near the wall is not in energy equilibrium, and hence the concept of inner layer similarity breaks down. From an initially self-preserving state on the smooth wall, the turbulent boundary layer approaches a second self-preserving state on the rough wall well downstream of the roughness step.

Transport processes in a combustible turbulent boundary layer

1962 ◽  
Vol 12 (3) ◽  
pp. 417-437 ◽  
Author(s):  
Norman G. Kulgein
Keyword(s):  
Boundary Layer ◽  
Turbulent Boundary Layer ◽  
Flow Direction ◽  
Skin Friction Coefficient ◽  
Transport Processes ◽  
Viscous Layer ◽  
Transfer Processes ◽  
Mass Injection ◽  
Low Speed Wind Tunnel ◽  
Transfer Numbers

Coexistent processes of heat, mass and momentum transfer operative within a combustible turbulent boundary layer have been experimentally investigated. The boundary layer was established on a porous cylinder mounted in a low-speed wind tunnel with its long axis in the flow direction. Methane was transpired into the boundary layer and ignited. Results indicate that the dimensionless transfer numbers corresponding to the three transfer processes can be correlated by the formula 0.038Re−0.2 to within ± 30% of measured values so that a rough numerical analogy exists among all three processes. The effect of mass injection on the skin friction coefficient is reasonably well accounted for by available theory. No effect of mass injection was found on the values of heat and mass transfer parameters. Finally, there was a lack of evidence indicating any sort of reaction-generated turbulence or that the experimentally demonstrated disturbance of the viscous layer by mass injection substantially affected the transport phenomena.

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.

Influence of Wall Proximity on the Wake Dynamics Behind a Square Cylinder

2021 ◽  
Author(s):  
Samuel Addai ◽  
Afua A. Mante ◽  
Sedem Kumahor ◽  
Xingjun Fang ◽  
Mark F. Tachie
Keyword(s):  
Boundary Layer ◽  
Turbulent Boundary Layer ◽  
Normal Stress ◽  
Reynolds Shear Stress ◽  
Stream Velocity ◽  
Reynolds Stresses ◽  
Square Cylinder ◽  
Mean Flow ◽  
The Mean ◽  
Wall Proximity

Abstract In the present study, the effects of wall proximity on the wake dynamics behind a square cylinder subjected to a thick upstream turbulent boundary layer were experimentally investigated using particle image velocimetry. The Reynolds number based on the free-stream velocity and the cylinder height (h) was 12750 while the ratio of the turbulent boundary layer thickness to the cylinder height was 3.6. The gap distance (G) between the bottom face of the cylinder and the wall was varied, resulting in gap ratios (G/h) of 0, 0.3, 0.5, 1.0, 2.0 and 8.0. The flow topological differences among the various gap ratios were analyzed in terms of the mean flow and Reynolds stresses. The results show that as the cylinder approaches the wall, the mean flow becomes increasingly asymmetric about the horizontal centerline of the cylinder and the size of the mean separation bubbles in the cylinder wake increases. Also, the magnitudes of the Reynolds stresses decrease with decreasing gap ratio. For G/h > 0, the distributions of the streamwise Reynolds normal stress and Reynolds shear stress are concentrated along the upper and lower separated shear layers, resulting in characteristic double peaks. The distributions of the vertical Reynolds normal stress, however, are concentrated in the wake about the horizontal centerline of the cylinder and reveal only single peaks.

Statistical structure of turbulent-boundary-layer velocity–vorticity products at high and low Reynolds numbers

2007 ◽  
Vol 570 ◽  
pp. 307-346 ◽  
Author(s):  
P. J. A. PRIYADARSHANA ◽  
J. C. KLEWICKI ◽  
S. TREAT ◽  
J. F. FOSS
Keyword(s):  
Boundary Layer ◽  
Reynolds Number ◽  
Shear Stress ◽  
Turbulent Boundary Layer ◽  
Large Scale ◽  
Reynolds Shear Stress ◽  
Rough Wall ◽  
Reynolds Stresses ◽  
Wall Boundary Layer ◽  
Layer Flow

The mean wall-normal gradients of the Reynolds shear stress and the turbulent kinetic energy have direct connections to the transport mechanisms of turbulent-boundary-layer flow. According to the Stokes–Helmholtz decomposition, these gradients can be expressed in terms of velocity–vorticity products. Physical experiments were conducted to explore the statistical properties of some of the relevant velocity–vorticity products. The high-Reynolds-number data (Rθ≃O(106), where θ is the momentum thickness) were acquired in the near neutrally stable atmospheric-surface-layer flow over a salt playa under both smooth- and rough-wall conditions. The low-Rθdata were from a database acquired in a large-scale laboratory facility at 1000 >Rθ> 5000. Corresponding to a companion study of the Reynolds stresses (Priyadarshana & Klewicki,Phys. Fluids, vol. 16, 2004, p. 4586), comparisons of low- and high-Rθas well as smooth- and rough-wall boundary-layer results were made at the approximate wall-normal locationsyp/2 and 2yp, whereypis the wall-normal location of the peak of the Reynolds shear stress, at each Reynolds number. In this paper, the properties of thevωz,wωyanduωzproducts are analysed through their statistics and cospectra over a three-decade variation in Reynolds number. Hereu,vandware the fluctuating streamwise, wall-normal and spanwise velocity components and ωyand ωzare the fluctuating wall-normal and spanwise vorticity components. It is observed thatv–ωzstatistics and spectral behaviours exhibit considerable sensitivity to Reynolds number as well as to wall roughness. More broadly, the correlations between thevand ω fields are seen to arise from a ‘scale selection’ near the peak in the associated vorticity spectra and, in some cases, near the peak in the associated velocity spectra as well.

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