Effect of heterogeneous roughness patch arrangement in turbulent channel flow

The effects of different heterogeneous rough surfaces are investigated by analyzing three canonical configurations combining smooth and rough areas, namely streamwise and spanwise roughness strips as well as a checkerboard pattern. All surfaces share the same smooth-to-rough area ratio but differ in their spatial arrangement. In previous experiments with similar surface patterns, clear differences in the global friction drag were reported. Using direct numerical simulations, we identify flow features of the particular surface patterns, e.g., meandering high-velocity regions in the checkerboard-type roughness and the repeated formation of internal boundary layers. For the checkerboard-case, spatial heterogeneities are observed in the dispersive velocity, and the repeated formation of stress bores is visible downstream of surface condition transitions.