DRAG REDUCTION BY MEANS OF AN ARRAY OF STAGGERED CIRCULAR CAVITIES

Further insight into the physics of boundary layer developing over perforated plates at moderate Reynolds numbers is gained from a combined experimental and numerical investigation. The findings are consistent with literature as to the possibility of reducing drag with circular perforations. Indeed, experimental results evidence a modification of the mean velocity profile, a reduction of the burst intensity and a thickening of the viscous sublayer. In contrast to other studies, the skin friction reduction is more pronounced when increasing the open area ratio, which is achieved by increasing the cavity diameter or equivalently by decreasing the inter-cavity spacing. The numerical investigations reveal spanwise patterns promoted by the perforations, which resemble those generated by spanwise wall oscillations and could explain the reduction in turbulent activity observed experimentally.