Effects of spanwise spacing on large-scale secondary flows in rough-wall turbulent boundary layers

Large-scale secondary flows can sometimes appear in turbulent boundary layers formed over rough surfaces, creating low-and high-momentum pathways along the surface (Barros & Christensen, J. Fluid Mech., vol. 748, 2014, R1). We investigate experimentally the dependence of these secondary flows on surface/flow conditions by measuring the flows over streamwise strips of roughness with systematically varied spanwise spacing. We find that the large-scale secondary flows are accentuated when the spacing of the roughness elements is roughly proportional to the boundary layer thickness delta, and do not appear for cases with finer spacing. Cases with coarser spacing also generate delta-scale secondary flows with tertiary flows in the spaces in between. These results show that the ratio of the spanwise length scale of roughness heterogeneity to the boundary layer thickness is a critical parameter for the occurrence of these secondary motions in turbulent boundary layers over rough walls.