Turbulence over porous walls with structural roughness of k- and d-types

Abstract

Planar particle image velocimetry (PIV) measurements are performed for turbulent channel flows over porous media with structural surface roughness. The roughness elements are mounted on the porous media and their spacing is controlled to reproduce so-called k- and d-type roughness. For comparison, flows over solid impermeable rough walls of both types are also measured. It is found that, in the k-type roughness case, time and space averaged turbulent intensities over the solid wall is higher than those over the porous wall, while an opposite tend occurs in the d-type roughness case. By the discussion on logarithmic mean velocity profiles, it is confirmed that the parameters show different trends between solid and porous cases regardless of the roughness types, and there is a characteristic roughness scale which is larger than the porous scales.