The Response of a Turbulent Boundary Layer to an Upstanding Step Change in Surface Roughness

1971 ◽  
Vol 93 (1) ◽  
pp. 22-32 ◽  
Author(s):  
R. A. Antonia ◽  
R. E. Luxton
Keyword(s):  
Boundary Layer ◽  
Surface Roughness ◽  
Turbulent Boundary Layer ◽  
Linear Trend ◽  
Roughness Element ◽  
Step Change ◽  
Internal Layer ◽  
Smooth Wall ◽  
Mean Velocity ◽  
Straight Lines

Measurements of the flow field downstream of an upstanding step change in surface roughness are presented. The roughness has the form of two-dimensional square section ribs placed transversely across the floor of the wind tunnel with the first element upstanding from the surface. The surface upstream of the roughness is smooth and is of sufficient length to allow a fully developed smooth wall turbulent boundary layer to be established. The roughness height is approximately 6 percent of the boundary layer thickness on the smooth wall just upstream of the first roughness element. It is observed that downstream of the start of the roughness, the mean velocity profiles inside the internal layer (i.e., that part of the boundary layer which has been affected by the new inner boundary condition) exhibit a linear trend when plotted in the form U versus y1/2. Remarkably, it is also found that a linear trend is exhibited by points in the “undisturbed” boundary layer outside the internal layer when plotted in the above manner, and that the slope in the undisturbed layer differs from that in the internal layer. The undisturbed layer slope appears to depend on conditions upstream of the roughness. It is suggested that the point of inter section of the two straight lines (the “knee” point) on the U versus y1/2 plot may be used to define the edge of the internal layer. Turbulence intensity distributions and spectra are presented from which it is deduced that the internal and external layer structures are largely independent and that stream-wise length scales in the internal layer over the rough wall are reduced significantly below those at the equivalent station over a smooth wall.

An Investigation of Turbulent Boundary Layer on a Flat Plate With a Small Step Change in Surface Roughness

1980 ◽  
Vol 31 (4) ◽  
pp. 221-237
Author(s):  
P.A. Aswatha Narayana
Keyword(s):  
Boundary Layer ◽  
Surface Roughness ◽  
Turbulent Boundary Layer ◽  
Flat Plate ◽  
Sudden Change ◽  
Outer Region ◽  
Adverse Pressure Gradient ◽  
Step Change ◽  
Small Step ◽  
Mean Velocity

SummaryThe response of turbulent boundary layer to sudden change in surface roughness have been studied experimentally. Mean velocity measurements have been made in the boundary layer on a flat plate, downstream of a small step change in surface roughness under 3 different pressure gradients. The surface upstream of the step consisted of ‘k* type ‘large roughness’ wall (or ‘small roughness’ wall) and downstream of the step consisted of smooth surface (or ‘small roughness’ wall). Velocity profiles after the step change have been analysed on the basis of the two layer model. The inner region responds very quickly to the new boundary condition while the outer region takes more time to attain equilibrium or a state of local self-preservation. The skin-friction coefficient initially increased after the step change and gradually reached towards a constant value except for a particular roughness combination under adverse pressure gradient wherein the change in the roughness function is gradual over the transition.

Calculation of a Turbulent Boundary Layer Downstream of a Small Step Change in Surface Roughness

1975 ◽  
Vol 26 (3) ◽  
pp. 202-210 ◽  
Author(s):  
R A Antonia ◽  
D H Wood
Keyword(s):  
Boundary Layer ◽  
Surface Roughness ◽  
Shear Stress ◽  
Turbulent Boundary Layer ◽  
Reynolds Shear Stress ◽  
Step Change ◽  
Rough Wall ◽  
Small Step ◽  
Mean Velocity ◽  
Good Agreement

SummaryMeasurements of mean velocity and Reynolds shear stress have been made in a turbulent boundary layer downstream of a small step change in surface roughness. Upstream of the step the surface is smooth, while downstream it consists of a d-type rough wall made up by a series of two-dimensional elements of square cross section placed transversely across the flow and spaced one element width apart in the direction of the flow. The calculated mean velocity and Reynolds shear stress profiles obtained using the method of Bradshaw, Ferriss and Atwell are in good agreement with the measurements throughout the relaxation region of the layer. Well downstream the calculation method adequately reproduces the self-preserving features of a d-type rough wall.

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.

Measurements in a Turbulent Boundary Layer Over a d-Type Surface Roughness

1975 ◽  
Vol 42 (3) ◽  
pp. 591-597 ◽  
Author(s):  
D. H. Wood ◽  
R. A. Antonia
Keyword(s):  
Boundary Layer ◽  
Surface Roughness ◽  
Turbulent Boundary Layer ◽  
Turbulent Energy ◽  
Shear Stresses ◽  
Normal Velocity ◽  
Mean Velocity ◽  
Outer Flow ◽  
Intensity Measurements ◽  
Normal And Shear Stresses

Mean velocity and turbulence intensity measurements have been made in a fully developed turbulent boundary layer over a d-type surface roughness. This roughness is characterised by regular two-dimensional elements of square cross section placed one element width apart, with the cavity flow between elements being essentially isolated from the outer flow. The measurements show that this boundary layer closely satisfies the requirement of exact self-preservation. Distribution across the layer of Reynolds normal and shear stresses are closely similar to those found over a smooth surface except for the region immediately above the grooves. This similarity extends to distributions of third and fourth-order moments of longitudinal and normal velocity fluctuations and also to the distribution of turbulent energy dissipation. The present results are compared with those obtained for a k-type or sand grained roughness.

Turbulent Boundary Layer Subjected to a Sudden Change in Surface Roughness and Temperature

1999 ◽  
Author(s):  
João Henrique D. Guimarães ◽  
Sergio J. F. dos Santos ◽  
Jian Su ◽  
Atila P. Silva Freire
Keyword(s):  
Boundary Layer ◽  
Surface Roughness ◽  
Turbulent Boundary Layer ◽  
Skin Friction ◽  
Sudden Change ◽  
Stanton Number ◽  
Temperature Profiles ◽  
Internal Layers ◽  
Mean Velocity ◽  
Wall Boundary Conditions

Abstract In present work, the dynamic and thermal behaviour of flows that develop over surfaces that simultaneously present a sudden change in surface roughness and temperature are discussed. In particular, the work is concerned with the physical validation of a newly proposed formulation for the near wall temperature profile. The theory uses the concept of the displacement in origin, together with some asymptotic arguments, to propose a new expression for the logarithmic region of the turbulent boundary layer. The new expressions are, therefore, of universal applicability, being independent of the type of rough surface considered. The present formulation may be used to give wall boundary conditions for two-equation differential models. The theoretical results are validated with experimental data obtained for flows that develop over flat surfaces with sudden changes in surface roughness and in temperature conditions. Measurements of mean velocity and of mean temperature are presented. A reduction of the data provides an estimate of the skin-friction coefficient, the Stanton number, the displacement in origin for both the velocity and the temperature profiles, and the thickness of the internal layers for the velocity and temperature profiles. The skin-friction co-efficient was calculated based on the chart method of Perry and Joubert (J.F.M., 17, 193–211, 1963) and on a balance of the integral momentum equation. The same chart method was used for the evaluation of the Stanton number and the displacement in origin.

Measurements in adverse-pressure-gradient turbulent boundary layers with a step change in surface roughness

1975 ◽  
Vol 70 (3) ◽  
pp. 573-593 ◽  
Author(s):  
W. H. Schofield
Keyword(s):  
Surface Roughness ◽  
Pressure Gradient ◽  
Boundary Layers ◽  
Leading Edge ◽  
Adverse Pressure Gradient ◽  
Turbulent Boundary Layers ◽  
Step Change ◽  
Internal Layer ◽  
Mean Velocity ◽  
Law Of The Wall

The response of turbulent boundary layers to sudden changes in surface roughness under adverse-pressure-gradient conditions has been studied experimentally. The roughness used was in the ‘d’ type array of Perry, Schofield & Joubert (1969). Two cases of a rough-to-smooth change in surface roughness were considered in the same arbitrary adverse pressure gradient. The two cases differed in the distance of the surface discontinuity from the leading edge and gave two sets of flow conditions for the establishment and growth of the internal layer which develops downstream from a change in surface roughness. These conditions were in turn different from those in the zero-pressure-gradient experiments of Antonia & Luxton. The results suggest that the growth of the new internal layer depends solely on the new conditions at the wall and scales with the local roughness length of that wall. Mean velocity profiles in the region after the step change in roughness were accurately described by Coles’ law of the wall-law of the wake combination, which contrasts with the zero-pressure-gradient results of Antonia & Luxton. The skin-friction coefficient after the step change in roughness did not overshoot the equilibrium distribution but made a slow adjustment downstream of the step. Comparisons of mean profiles indicate that similar defect profile shapes are produced in layers with arbitrary adverse pressure gradients at positions where the values of Clauser's equilibrium parameter β (= δ*τ−10dp/dx) are similar, provided that the pressure-gradient history and local values of the pressure gradient are also similar.

The Effects of Surface Roughness on the Mean Velocity Profile in a Turbulent Boundary Layer

2002 ◽  
Vol 124 (3) ◽  
pp. 664-670 ◽  
Author(s):  
Donald J. Bergstrom ◽  
Nathan A. Kotey ◽  
Mark F. Tachie
Keyword(s):  
Boundary Layer ◽  
Surface Roughness ◽  
Turbulent Boundary Layer ◽  
Velocity Profile ◽  
Friction Velocity ◽  
Outer Region ◽  
Stream Velocity ◽  
Mean Velocity ◽  
Outer Flow ◽  
The Mean

Experimental measurements of the mean velocity profile in a canonical turbulent boundary layer are obtained for four different surface roughness conditions, as well as a smooth wall, at moderate Reynolds numbers in a wind tunnel. The mean streamwise velocity component is fitted to a correlation which allows both the strength of the wake, Π, and friction velocity, Uτ, to vary. The results show that the type of surface roughness affects the mean defect profile in the outer region of the turbulent boundary layer, as well as determining the value of the skin friction. The defect profiles normalized by the friction velocity were approximately independent of Reynolds number, while those normalized using the free stream velocity were not. The fact that the outer flow is significantly affected by the specific roughness characteristics at the wall implies that rough wall boundary layers are more complex than the wall similarity hypothesis would allow.

An Investigation of two Turbulent Flows over Smooth and Rough Surfaces

1974 ◽  
Vol 16 (2) ◽  
pp. 71-78 ◽  
Author(s):  
W. K. Allan ◽  
V. Sharma
Keyword(s):  
Experimental Data ◽  
Boundary Layer ◽  
Surface Roughness ◽  
Turbulent Boundary Layer ◽  
Turbulent Flows ◽  
Smooth Surface ◽  
Two Dimensional ◽  
Mean Velocity ◽  
Turbulent Boundary Layer Flow ◽  
Layer Flow

Experimental data for two-dimensional, low-speed, turbulent boundary layer flow has been used to verify the description of mean-velocity distributions proposed by Allan and to re-evaluate the entrainment function. The independence of pressure gradient and surface roughness as regards their effects on velocity profiles has been demonstrated. Boundary layer predictions agree with experimental data for a smooth surface, but further investigation is required for flow over a rough surface.

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