Influence of multiscale roughness height in turbulent boundary layers

Wind tunnel experiments were conducted under zero pressure gradient conditions using hot-wire anemometry to investigate the influence of the multiscale roughness height effect on turbulent boundary layers at friction Reynolds numbers ranging from 1100 to 4000. The study analysed various rough-wall turbulent statistics to evaluate the effects of uniform and multiscale roughness element size distributions and area coverage. The rough surfaces were designed with an average roughness height of 0.9 mm, constructed from sand grains with three variations of height distribution: (1) uniform size of 0.9 mm, (2) 0.6 mm and 1.2 mm with a 1:1 weight ratio, and (3) 0.6 mm, 0.9 mm, and 1.2 mm with a 1:2:1 weight ratio. These surfaces covered 12%, 25%, 38%, 55%, and 75% of the test section wall area. The study examined the effects of multiscale roughness height on the Hama roughness function and turbulence characteristics. The findings reveal that multiscale roughness height reduces the Hama roughness function, suggesting a mutual sheltering effect among roughness elements. Additionally, the multiscale sand composition with range 0% to 50% of the average roughness height yielded the minimum Hama roughness function at the corresponding roughness area coverage. These results indicate that the application of the Hama roughness function based on equivalent sand-grain roughness might overestimate its value in multiscale roughness cases. • The Hama roughness function peaked at 38% RAC across different compositions and free-stream velocities. • The mutual sheltering effect is affected by roughness height difference or roughness height ratio and the roughness spacing distance. • Multiscale roughness height promotes more active turbulence mixing, resulting in a more homogeneous distribution of lower turbulence energy throughout the boundary layer than in the uniform-size case. • The sand compositions have a more prominent impact on the turbulence intensities, as the sand composition cases tend to have lower turbulence intensities compared to the uniform size cases.