Shark Inspired Separation Flow Control in Unsteady Laminar Boundary Layer Conditions

Previous work suggests that the flexible scales on the shortfin mako shark exhibit flow control, specifically in both turbulent and laminar boundary layers to mitigate separation. As the flow reverses close to the skin, the scales bristle and impede further flow reversal. 3D printed flexible scale surfaces, designated as a Micro-adaptive Array for Kinematic Optimization or MAKO surface, that are geometrically similar to shortfin mako scales have been manufactured to determine if these flow control results can be recreated for a man-made surface. This study specifically documents when the scales are actuated for unsteady, laminar conditions in a boundary layer subjected to an increasing adverse pressure gradient (APG). Two MAKO models, with scale crown lengths of 2.5 mm and 3.6 mm, have been tested in a water tunnel with a boundary layer grown over a flat plate for Re up to 4x10⁵. The APG was induced to create a separation bubble using a spinning cylinder located just upstream from the MAKO surface. The unsteady APG was created by moving this spinning cylinder vertically toward the plate from a distance of 14 cm to 4 cm. Using DPIV, the formation of the separated region can be visualized, for both a smooth surface and over the MAKO models so this passive flow control mechanism can be studied. Instantaneous flow fields capture the bristling motion of the scales, and results will show if bristling of the scales impedes the formation of reversing flow.