Investigation of the scaling of roughness and blowing effects on turbulent channel flow

A turbulent channel flow facility was used to study the scaling of the combined effects of roughness and flow injection on the mean flow and turbulence characteristics of turbulent plane Poiseuille flow. It was found that the additional momentum injection through the surface enhanced the roughness effects and, for the mean flow, the effect of blowing was indistinguishable from that of increased roughness. This analogy broke down for the turbulence statistics in that the addition of blowing resulted in behavior which did not follow that predicted by Townsend’s hypothesis. Instead, the outer-scaled Reynolds stress was found to deviate from that for the rough-walled boundary condition without blowing well into the outer layer. It was found that this deviation from the expected Reynolds stress scaling behavior was caused by the suppression of kinetic energy content associated with large-scale motions.