Coherent structures in the inner part of a rough-wall channel flow resolved using holographic PIV

Microscopic holographic PIV performed in an optically index-matched facility resolves the three-dimensional flow in the inner part of a turbulent channel flow over a rough wall at Reynolds number Re-tau = 3520. The roughness consists of uniformly distributed pyramids with normalized height of k(s)(+) = 1.5k(+) = 97. Distributions of mean flow and Reynolds stresses agree with two-dimensional PIV data except very close to the wall (<0.7k) owing to the higher resolution of holography. Instantaneous realizations reveal that the roughness sublayer is flooded by low-lying spanwise and groove-parallel vortical structures, as well as quasi-streamwise vortices, some quite powerful, that rise at sharp angles. Conditional sampling and linear stochastic estimation (LSE) reveal that the prevalent flow phenomenon in the roughness sublayer consists of interacting U-shaped vortices, conjectured in Hong et al. (J. Fluid M e c h., 2012, doi:10.1017/jfm.2012.403). Their low-lying base with primarily spanwise vorticity is located above the pyramid ridgeline, and their inclined quasi-streamwise legs extend between ridgelines. These structures form as spanwise vorticity rolls up in a low-speed region above the pyramid's forward face, and is stretched axially by the higher-speed flow between ridgelines. Ejection induced by interactions among legs of vortices generated by neighbouring pyramids appears to be the mechanism that lifts the quasi-streamwise vortex legs and aligns them preferentially at angles of 54 degrees-63 degrees to the streamwise direction.