Turbulent flow over spanwise-varying roughness in a minimal streamwise channel

Abstract

We report direct numerical simulations in a minimal streamwise domain of turbulent channel flow over spanwise-alternating patches of rough and smooth walls. Despite the minimal streamwise domain overpredicting streamwise-velocity fluctuations and inhibiting the meandering of long turbulent structures, it captures the rotational behaviour of mean secondary flows also observed in other studies with spanwise-varying roughness. To extend the study of spanwise-varying roughness, we prescribe a lateral velocity to the wall roughness to mimic flow over oblique patches of roughness. Far from the wall, long-lived turbulent structures are convected in the direction of the moving roughness, but their speeds are only weakly perturbed from a preferential value of around 40% of the friction velocity. The turbulence-driven secondary flows laterally convect at comparable speeds, but depend on the roughness patch width.