Prediction of turbulent rough-wall skin friction using a discrete element approach

A discrete element model for turbulent flow over rough surfaces has been derived from basic principles. This formulation includes surface roughness form drag and blockage effects as a constituent part of the partial differential equations and does not rely on a single-length-scale concept such as equivalent sandgrain roughness. The roughness model includes the necessary empirical information on the interaction between three-dimensional roughness elements and the flow in a general way which does not require experimental data on each specific surface. This empirical input was determined using data from well-accepted experiments. Predictions using the model are compared with additional data for fully-developed and boundary layer flows. The predictions are shown to compare equally well with both transitionally rough and fully rough turbulent flows without modification of the roughness model.