Thrust Generation by Shark Denticles

Increasing interest has emerged in studying the potential of shark dermal scales - known as denticles - to reduce pressure drag by controlling flow separation. Direct numerical simulations of turbulent channel flow over a bump on the bottom wall were performed, comparing a smooth bump with one where the lee side featured an array of shark denticle replicas. Complete shark denticles, including the neck and crown, were employed. As the flow over the bump was subjected to an adverse pressure gradient (APG), the flow over the denticle array was injected through the slits between the crowns, forming a reverse pore flow (RPF) in the cavity region created by the denticle necks underneath. The staggered necks in this region accelerate the RPF in the gaps between them, forming arrays of wall-parallel jets that produce thrust through the augmented shear and pressure forces. This resulted in more than a twofold increase in thrust on the lee side and a net 4\% total drag reduction compared with the smooth bump. The DNS provided detailed pore flow statistics and revealed the geometrical features responsible for the generation and maintenance of the RPF and thrust generation. In particular, the RPF is activated by the APG before the onset of massive flow separation and does not appear when the APG is absent. These mechanisms indicate a flow-activated, on-demand passive drag-reduction mechanism that leverages and transforms an APG into a favorable outcome.