On the Mean Flow Structure of a d-Type Rough Wall Turbulent Boundary Layer : Low Reynolds Number Effect

The Low Reynolds number effect on the mean flow structure of a d-type rough wall boundary layer were investigated experimentally under the zero pressure gradient. The local skin-friction coefficient and mean velocity profile were obtained in the Reynolds number range, R_θ=700-5140. The scaling laws for the mean flow structure were studied with the friction velocity obtained by the direct skin-friction measurement. It is found that the logarithmic layer with the same value of the Karman constant as the smooth wall flow, κ=0.41,does exist in the present low Reynolds number range. A detailed analysis of the outerlayer velocity profile with the law of the wake shows that both the development of wake parameter and the profile of wake function differ from those of the smooth wall boundary layer.