Passive Flow-actuated Control of Turbulent Boundary Layer Separation by Shortfin Mako Shark Skin Specimens*

The speedy shortfin mako has flexible scales located on key locations, most specifically the flank region from which specimens were obtained. The scales in this region can reach angles of 50° and their bristling capability by reversing flow has been documented. It is hypothesized that the impedance of reversing flow close to the skin by the scales is the primary mechanism by which flow separation is controlled by this passive, flow-actuated mechanism. Experiments were conducted in a water tunnel facility and the flow field of a separating turbulent boundary layer is measured using DPIV.  A controllable adverse pressure gradient (APG) was induced by a rotating cylinder over a flat plate (range of Re = 5.2 xto  8.8 x ). Shark skin specimens (SSS) were placed in two different locations (at the beginning and inside) of the separated region to quantify the separation control capability using backflow coefficient. It is surmised that the shark scales are sized according to the low speed streak (LSS) formation occurring for real swimming turbulent boundary layer conditions.  Measurement of the LSS before separation matches the predicted size based on viscous length scale calculations with and without the SSS.