Effect of surface skewness on rough-wall turbulent boundary layer structure*

Previous work by the authors and others has shown that the frictional drag of many rough surfaces correlates well with two primary roughness parameters.  These include some measure of the surface amplitude (i.e. root-mean-square height, mean height, etc.) and the skewness of the surface elevation distribution.  In the present work, the results of an experimental investigation are reported in which turbulent boundary layer measurements were obtained over surfaces in which the root-mean-square height (k_rms) was held fixed but the skewness of the surface elevation distribution (Sk) was systematically varied.  The boundary layer measurements were made at a friction Reynolds number (Re_τ) ~2000 using both LDV and PIV.  Specific effects of Sk on the skin-friction and the structure of the boundary layer are discussed.