Secondary Flows in Boundary Layers over Streamwise-aligned Wall-roughness

etc15 Tracking Number 237

The cause of recently observed spanwise-alternating low- and high-momentum pathways [1,2,3] to appear across the entire turbulent boundary layer formed over rough surfaces is experimentally investigated by measuring the flows over roughness elements with systematically varied spanwise spacing. We found that the secondary flows associated with these low- and high-momentum pathways only appeared when the spanwise spacing of roughness elements was roughly proportional to the boundary layer thickness, and did not appear for cases with spacing much less than the boundary layer height. This suggests that the ratio of the spanwise spacing to the boundary layer thickness, as opposed to the roughness height (or width), is the most important parameter to predict the occurrence of these secondary motions in turbulent boundary layers over rough walls.