An experimental investigation of the aerodynamic effects of roughness consisting of arrays of depressions in surfaces

Wind tunnel tests were carried out on flat plates containing regular and random arrays of depression surface roughness. Arrays of cylindrical depressions and hemispherical depressions were used as the regular roughness, while small dimples in vinyl floor coverings were used to investigate flow over a randomly roughened surface. Skin friction coefficients were determined using the velocity defect-law. The skin friction coefficients found using the defect-law were found to be in good agreement with skin friction coefficients calculated using the momentum integral method. The skin friction coefficients for the depression roughness were found to be less than skin friction coefficients of protrusion roughness of the corresponding geometry and spacing.; Computational work was also performed using a discrete element roughness model. Boundary layer development computations were done to identify an equivalent protrusion roughness height that would produce the same boundary layer behaviour as the actual depression roughness geometries. Skin friction coefficients generated by the computer code were matched to the experimental skin friction coefficients found from the defect-law method to identify this 'equivalent height'. Equivalent protrusion heights were found to be about one order of magnitude less than the actual depression depths.