Outer layer similarity in fully rough turbulent boundary layers

Abstract

Turbulent boundary layer measurements were made on a flat plate covered with uniform spheres and also on the same surface with the addition of a finer-scale grit roughness. The measurements were carried out in a closed return water tunnel, over a momentum thickness Reynolds number (Re θ ) range of 3,000–15,000, using a two-component, laser Doppler velocimeter (LDV). The results show that the mean profiles for all the surfaces collapse well in velocity defect form. Using the maximum peak to trough height (Rt) as the roughness length scale (k), the roughness functions (ΔU+) for both surfaces collapse, indicating that roughness texture has no effect on ΔU+. The Reynolds stresses for the two rough surfaces also show good agreement throughout the entire boundary layer and collapse with smooth wall results outside of the roughness sublayer. Quadrant analysis and the velocity triple products show changes in the rough wall boundary layers that are confined to y<8ks, where ks is the equivalent sand roughness height. The present results provide support for Townsend’s wall similarity hypothesis for uniform three-dimensional roughness. However, departures from wall similarity may be observed for rough surfaces where 5ks is large compared to the thickness of the inner layer.