Scale actuation in a 3D printed model of shortfin mako scales in a separating turbulent boundary layer*

Passive control of turbulent boundary layer separation is important from an engineering perspective because of the potential for separation to induce a variety of problems including increased pressure drag, loss of lift, and stall events. In this study, the process of actuation for different sized 3D printed micro-flaps, with a design inspired by the geometry of shortfin mako shark scales, was documented. An adverse pressure gradient was generated utilizing a rotating cylinder above a flat plate to induce turbulent  boundary layer separation. Quantitative data was acquired using DPIV in a water tunnel and flow characteristics correlating with micro-flap actuation were identified. Analysis reveals a statistical cross-covariance of less than -0.4 between micro-flap angle and the velocity downstream of the micro-flaps for each run. This indicates that a negative downstream velocity correlates with a positive micro-flap angle. The cross-covariance was normalized so that the auto-covariances at zero lag were 1 for both the scale angle and the downstream velocity. Results revealed a tendency for the micro-flaps to remain in either an almost fully actuated (>90%) or fully lowered (~30%) state.