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 sand grain 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 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. (A)