Possibility of drag reduction using d-type roughness

An experiment was carried out in a low-speed wind tunnel to study the turbulence structure of the boundary layer over a two-dimensional square cavity on a flat plate. The main purpose of this investigation is to examine the way a square cavity modifies the near-wall structure of the turbulent boundary layer leading to a possible drag reduction overd-type roughness. The experimental results on pressure coefficient and friction coefficient indicated a small reduction in total drag in this configuration. This seems to be due to the stable vortex flow observed within the cavity which absorbs and reorganizes the incoming turbulence in the cavity, thereby modifying the near-wall turbulence structure of the boundary layer. The resultant turbulence structure was very similar to that over drag-reducing riblets surface.