Abstract:
Shark skin is investigated as a means of passive flow separation control due to its preferential flow direction and the potential for its scales to obstruct low-momentum backflow resulting from an adverse pressure gradient. In this study, the effect of the scales on flow reversal is observed in a tripped turbulent boundary layer by comparing the flow over a NACA 4412 hydrofoil with a smooth, painted surface to that over the same hydrofoil with samples of mako shark skin affixed to its upper surface. These samples were taken from the shark's flank region because the scales at that location have been shown to have the greatest angle of bristling, and thus the best potential for separation control. All sets of flow data in this study were obtained using Time-Resolved Digital Particle Image Velocimetry and recorded at multiple angles of attack (between 8 and 16 degrees) and two Reynolds numbers. The flow was primarily analyzed by means of the backflow coefficient (a value based on the percentage of time that flow in a region over the hydrofoil is reversed), average backflow magnitude, and the time history of instantaneous flow velocity values at specific points in the boundary layer over the hydrofoil models. Results showed that at angles of attack of 12° and below, the shark skin generated a slightly larger, higher magnitude region of reversed flow than was seen over the painted surface. At an angle of attack of 16°, the backflow region of the shark skin surface was significantly reduced in size and magnitude compared to that of the painted surface. These results support the hypothesis that in order for the scales to be an effective means of flow control, sufficient shear must be present in the backflow to cause them to bristle fully.
