Turbulence structures in rotating channel flows with rough wall

We performed direct simulations of channel flow subjected to rotation about a spanwise axis, comparing smooth walls and cases with random roughness. Three rotation rates were considered: Ro=0, 0.42 and 1.00; in the last case the flow becomes nearly laminar on the cyclonic, stable, side when the wall is smooth. The destabilizing effect of roughness counteracts the stabilizing effect of rotation, maintaining non-zero Reynolds shear stress at all rotation rates considered. The wake fluctuations result in significant dispersive stresses, which are attenuated by rotation on the cyclonic side. The wake production, however, is small compared to the production term caused by the Coriolis force and the pressure work. The increased pressure fluctuations due to the blockage caused by roughness may be more important, in this flow, than the wake production. The instantaneous fields and spectra highlight the presence of elongated roll cells, which are not sensitive to the roughness. The sweeps and ejections responsible for a significant percentage of the Reynolds stress are due to motion whose coherence, in the wall normal direction, is increased, perhaps due to the effect of the roll cells.