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MEAN AND TURBULENT VELOCITY PROFILES FOR SANDGRAIN ROUGH SURFACES

Karen A. Flack
Department of Mechanical Engineering United States Naval Academy Annapolis, MD 21402 USA

Michael P. Schultz
Department of Naval Architecture and Ocean Engineering United States Naval Academy Annapolis, Maryland 21402 USA

Resumo

Flat plate turbulent boundary layer measurements have been made on surfaces covered with fine and coarse sandpaper. The measurements were conducted in a closed return water tunnel, over a momentum thickness Reynolds number (Reθ) range of 3000 to 16000, using a two-component, laser Doppler velocimeter (LDV). The results indicate an increase in the boundary layer thickness (δ), the integral length scales, and the skin-friction coefficient (Cf) for the rough surfaces compared to the smooth wall. The roughness functions (ΔU+) for the sandgrain surfaces agree within their uncertainty with previous results obtained using towing tank tests and similarity law analysis. For sandpaper, it appears that a single roughness length scale (k = 0.75 Rt) is sufficient to characterize the physical surface. The present results indicate that the mean profiles for both the smooth and rough surfaces collapse well in velocity defect form. The Reynolds stresses also show good collapse outside the roughness sublayer when normalized with the wall shear stress.