Outer flow scaling of smooth and rough wall turbulent boundary layers

Mean velocity profiles in a zero pressure gradient turbulent boundary layer were measured on a hydraulically smooth surface and three different rough surfaces created from sand paper, perforated plate, and woven wire mesh. The physical size and geometry of the roughness elements were chosen to encompass both transitionally and fully rough flow regimes. The mean velocity profiles were measured using a Pitot tube in a subsonic wind tunnel, for Reynolds numbers (based on momentum thickness) ranging from 3,730 to 12,260. Three different outer velocity scales were used to analyze the defect profile. The results show that application of a so called mixed outer scale causes the velocity profile in the outer region to collapse onto the same curve for different Reynolds numbers and roughness conditions. Although the mixed scale collapses defect profiles on different surfaces, the effect of surface roughness is still observed in the outer region.