Meeting Abstract
6.4 Saturday, Jan. 4 08:45 A Passive Flow Control Mechanism to Decrease Drag LANG, A. L.*; BRADSHAW, M. T.; SMITH, J. A.; MOTTA, P. J.; HABEGGER, M. L.; HUETER, R. E.; Univ. of Alabama; Univ. of Alabama; Univ. of Alabama; Univ. of South Florida; Univ. of South Florida; Mote Marine Lab alang@eng.ua.edu
Flow separation is a major source of pressure drag on mobile aquatic organisms and separation control can lead to increased swimming speeds and maneuverability. Our recent results from a collaboration involving biologists and engineers have demonstrated the separation control capability of shark skin. With a specific focus on the shortfin mako (Isurus oxyrinchus) known for its high speed and agility, we investigated the bristling angle of placoid scales across the body. Our observations indicate larger bristling angles are linked to scale morphology, more specifically an increase in crown length and decrease in size of the base, to promote flexibility primarily in the streamwise direction. Results also show key regions of high bristling capability correspond with those most prone to flow separation, including the tail, flank regions aft of the gills, and pectoral fins. Shark skin samples also were tested in a water tunnel facility using Digital Particle Image Velocimetry (DPIV) and evidence of flow separation control was observed under laminar and tripped boundary layer conditions. Experiments conducted in the Re = 105 range resulted in sufficiently strong backflow induced close to the surface such that the shear threshold to induce bristling on the real skin sample was achieved. Flow control at lower Re was not as evident. We hypothesize the presence of an adverse pressure gradient induces flow reversal in a region very close to the skin. This reversed flow induces passive, localized scale bristling, thereby interrupting the subsequent flow development that leads to global flow separation from the body. Thus, scale bristling on sharks decreases pressure drag.
