Experimental Evidence of Passive Separation Control by Shortfin Mako Shark Scale Bristling

The shortfin mako has scales (on the order of 0.2 mm in size) flexible to angles in excess of 40 degrees, but only in the direction of reversing flow and strategically placed on the body, such as flank and fins, for controlling flow separation to reduce pressure drag. Various experiments have been carried out to document the separation control capability of mako skin samples, including high-speed video evidence of passive, flow-actuated scale bristling. In water tunnel studies, a flat plate boundary layer was grown to Re > 10⁵ and passed over flank skin samples whereon flow separation was induced by a controllable adverse pressure gradient produced by a rotating cylinder located above the test area. Velocity measurements, both instantaneous and time-averaged, were captured using Digital Particle Image Velocimetry (DPIV). Results confirm separation control was achieved under both laminar and tripped turbulent boundary layer conditions as quantified by backflow coefficient, or the percentage of time the flow was reversed. We hypothesize that the width of a single shark scale corresponds to the sizing of the reversing flow, which induces scale actuation, documented within the turbulent boundary layer case as occurring within a low speed streak.