T URBULENT F LOWS OVER R OUGH W ALLS

▪ Abstract We review the experimental evidence on turbulent flows over rough walls. Two parameters are important: the roughness Reynolds number [Formula: see text], which measures the effect of the roughness on the buffer layer, and the ratio of the boundary layer thickness to the roughness height, which determines whether a logarithmic layer survives. The behavior of transitionally rough surfaces with low [Formula: see text] depends a lot on their geometry. Riblets and other drag-reducing cases belong to this regime. In flows with δ/k ≲ 50, the effect of the roughness extends across the boundary layer, and is also variable. There is little left of the original wall-flow dynamics in these flows, which can…