Experimental investigation of blowing effects on turbulent flow over a rough surface

A high Reynolds number turbulent channel flow facility was used to study the combined effects of roughness and flow injection on the mean flow and turbulence characteristics. 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. However, for the turbulence statistics, this analogy broke down in that the addition of blowing resulted in behavior which did not follow that predicted by Townsend's hypothesis. This was observed as a reduction of outer-scaled Reynolds stress well into the outer layer. The reduction in Reynolds stress was accomplished primarily by a suppression of the kinetic energy content associated with large-scale motions, which are believed to be formed at the surface but extend into the outer layer.