Observations of the mitigation of reversing flow within a separating turbulent boundary layer through flexible shortfin mako shark scales*

A turbulent boundary layer in the presence of an adverse pressure gradient sees flow reversal first occurring within the low-speed streaks close to the wall.  Previous research has demonstrated that the flexible scales found on the skin of the shortfin mako can respond to this reversing flow, be passively actuated and mitigate this flow reversal as a primary mechanism to control flow separation.  To further test this bio-inspired flow control mechanism, both real shark skin samples and 3D printed models of these flexible scales (designated MAKO models) have been tested in in a water tunnel using DPIV for a tripped turbulent boundary layer subjected to an adverse pressure gradient induced by the presence of a rotating cylinder.  The real shark skin scales measure approximately 200 microns in crown length, while the MAKO models were manufactured at both 15 and 20 times the size of real shark scales.  The flow has been analyzed in planes parallel to the wall to visualize the formation of the low-speed streaks and study the formation of flow reversal within these streaks for both a smooth wall and over the MAKO models.  Specific examples of the mitigation of flow reversal by both the MAKO models and real shark skin will be discussed.