Influence of Roughness Height Distribution on Drag in Turbulent Boundary Layers

Abstract: Wind tunnel experiments using hot-wire anemometry at zero pressure gradient conditions are performed to study the influence of roughness height distribution in turbulent boundary layers at friction Reynolds number range 1500 to 3700. Various rough-wall turbulent statistics are analysed to compare the influence of uniform and mixed-size roughness element distribution. The rough surface elements are intentionally designed (constrained) to have an average roughness dimension/height of 0.9 mm. They are made from sand grains with three variations of roughness height distributions: (1) uniform size of 0.9 mm, (2) 0.6 mm and 1.2 mm with a weight ratio of 1:1, and (3) 0.6 mm, 0.9 mm, and 1.2 mm with a weight ratio of 1:2:1. For each of these cases, they are covering 10%, 30%, 50%, 70%, and 100% of the wall area of the wind tunnel's test section. This work aims to study the effect of roughness height distribution on the Hama roughness function and turbulence characteristics. Initial results suggest that the roughness height ratio reduces the Hama roughness function as the roughness area coverage (RAC) increases. Therefore, the roughness height ratio is an important factor in the rough-wall turbulent boundary layer. Keywords: Turbulent boundary layer, Roughness, Wind tunnel experiment, Hama Roughness function