Passive separation control of shortfin mako shark skin in a turbulent boundary layer

Abstract

This experimental investigation discusses the effectiveness of shortfin mako shark skin to passively control turbulent boundary layer separation. Experiments were conducted in a water tunnel facility with the shark skin specimens mounted on a smooth flat plate subjected to an adverse pressure gradient. The two skin specimens have shark scales with different sizes, shapes, and bristling angles. The flank region scales (B2) are slender, tall, and can bristle at 50°, while the scales in the region between flank and dorsal fin (B1) are wide, short, and can bristle at 30°. The adverse pressure gradient on the plate was generated by a rotating cylinder. DPIV measurements indicate that unpainted scales B2 can fully control or delay boundary layer separation and reduce the fraction of time the flow is reversed. The bristling of the scales caused fluctuation in the skin friction curve and the single spike intensity of the normal stress very close to the wall. These capabilities are directly related to the scale bristling angle. In contrast, the low bristling angle of the scales from region B1 did not impede the reversing flow, and therefore separation was enhanced. The increase in Reynold’s stress fluctuations away from the wall indicated that the separation region increased in size due to vortex structures further from the wall. Once the bristling mechanism of the B2 scales was eliminated by painting over the skin, the scales functioned as a rough surface but still promoted separation delay. In addition, the streamwise Reynolds stress profile for the flow over the painted scales measured after separation occurred, along with the high intensity of the normal stress near the surface are both characteristics of a flow over a rough surface. In summary, the shark skin surface showed an ability to both delay and increase flow separation depending on the capabilities of the scale bristling to inhibit reversing flow.