Drag control on a fully rough oscillating flat plate

The dynamics of a turbulent channel flow over spanwise square bars on the wall with sinusoidal spanwise oscillations are studied by direct numerical simulations. For smooth wall channels, spanwise wall oscillations have been shown to reduce skin friction drag by as much as 30%. In a practical flow, wall roughness needs to be considered where form drag is non-negligible. Here, we consider a configuration where 93% of the drag comes from form drag; the ratio of pitch (λ) to the height of bars (k) is λ/k=7.5, where the height of the bars in wall units is k⁺=50, which is considered a “fully rough” wall. A 25% form drag reduction is obtained with the wall oscillations considered. Inspection of the mean streamlines shows that the secondary recirculation region downstream from the bars is completely suppressed while the recirculation upstream slightly increases in size. Surprisingly, the optimal amplitude and frequency for drag reduction in a smooth wall used here as a starting point also give significant form drag reduction, despite the fact that skin-friction drag is negligible in this scenario.