Friction correlations for rectangular roughness

A relation for the roughness parameter R of the velocity profile in a fully rough flow is established as a function of the geometrical parameters of the roughness elements. The evaluation is based on a detailed analysis of experimental results described in the literature. Numerous data of friction factors and velocity profiles in channels with artificial rectangular wall roughness were taken into account. The geometrical parameters are the ratio of distance to height, the ratio of height to width of the roughness, and the ratio of the height of the roughness to the length of the velocity profile from the rough wall to the position of zero shear stress. Application of the relation determined in this way, which holds over a very broad range of geometrical parameters, allows the pressure drop to be calculated with a rather good accuracy in channels with rectangular wall roughness. (author)