Drag partition revisited at low packing densities

It is a direct consequence of the conventional drag partition theory that the ground skin friction and the drag coefficient of a single roughness element are non-increasing functions of the roughness packing density. This talk will present empirical evidence from DNS and LES that challenges the above conclusion. We perform direct numerical simulations (DNS) of flow over a cube at a moderate Reynolds number. Three different aspect ratios (H/W=0.5, 1, 2) are considered. An increase in the ground skin friction is found behind the cube for all three aspect ratios. Detailed analysis shows that the increase in the ground skin friction is due to a strong downwash motion behind the cube. Due to this downwash motion, the drag coefficient on a single cubic roughness element is an increasing function of the packing density if the roughness elements are sparsely packed. This is shown by large-eddy simulations of flow over aligned and staggered arranged cube arrays. In the light of the above findings, the conventional drag partition theory may need to be revised.