Properties of d- and k-type roughness in a turbulent channel flow

Roughness is classified by the so-called roughness function, which represents the downward shift of the velocity profile relative to a smooth wall. The dependence of the roughness function on the Reynolds number is discussed with the aim of clarifying the difference between d-type and k-type behaviors. This difference has been traditionally associated with the stability of the flow within the roughness elements. The present direct numerical simulation results indicate that the difference more correctly reflects the different contributions from the frictional drag and pressure drag to the total stress.