Studies of drag reducing d-type roughness - Including flow visualization image enhancement, and quantitative measurements

The mechanisms responsible for drag reduction are investigated such as cavity spacing in order to determine which scaling factors and parameters are significant for the optimization of surface geometries. Flows are visualized in a low-speed wind tunnel over d-type surfaces built from perspex blocks. The flows are visualized with horizontal light-plane illumination recorded on video images and photographic stills while measurements of the pressure and velocity profiles are generated. The concurrent experimental and visualized data confirm the notion that the drag reduction related to d-type roughness is related to the modification of the near-wall turbulence structure. The establishment of stable vortices also plays a minor role, but the key parameters for establishing stable flows are d super + and the relaxation length of the wake parameter.