The effect of porous walls on very large scale motions in turbulent boundary layer*

Two aspects of porous materials that substantially influence the turbulent boundary layer convecting over them are surface roughness and permeability, and although their individual effects is well documented, their combined action and non-linear interactions are poorly understood. Therefore, novel experiments were performed, where both permeability and roughness were altered systematically over the wall. Hot-wire anemometry and microphone measurements were performed above these surfaces in the boundary layer wind tunnel. The measurements performed at same permeability but different roughness based Reynolds number reveal the impact of surface roughness on the outer layer. Furthermore, the impact of wall-permeability to outer layer based scale demonstrates the relative importance of roughness and permeability on the outer layer statistics. In particular, for Very-Large-Scale-Motions (VLSMs) a demarcation based on permeability based Reynolds number is achieved, which sets the maximum levels of permeability permissible for the existence of VLSMs over porous walls. Increase in permeability based Reynolds number leads to spectral shrinkage, which reduces the wavelengths associated with the outer-layer peak.