Large-eddy simulation of turbulent flow in a channel with rib roughness

Turbulent flow in a channel with transverse rib roughness on one wall is investigated by large-eddy simulation (LES). The spacing of the roughness elements is varied to reproduce the so-called d- and k-type roughness, and an intermediate roughness between the two. The time–mean and instantaneous flows are analyzed. The LES results agree with the rather limited laboratory observations in flows with rib roughness, and provide new insights into the effects of roughness on the mean flow as well as the turbulence structure. In d-type roughness, the outer flow almost rides over the roughness layer, in the mean, with the separation eddies confined to the gaps between the ribs. For intermediate roughness, between the d-type and k-type, the mean separation region is about the same size as the cavity between the ribs, but the outer flow is affected by large turbulent eddies emanating from the roughness layer. For k-type roughness, separation and reattachment occur between two adjoining ribs, much larger and more frequent eddies are thrown into the outer flow, resulting in strong interaction between the roughness layer and the outer flow. Time and space averaged velocity profiles show the well-known downward shift of the semi-logarithmic portion of the law of the wall. This result is quite surprising in view of the grossly varying spatial structure of the instantaneous flow.