Investigation of Turbulent Boundary Layer Separation Control Using both Flexible and Rigid 3D Printed Shark Skin Models

The separation of a turbulent boundary layer can cause reversing flow in the region aft of the separation point and close to the wall, leading to an increase in drag. However, nature may offer a way to reduce this drag in the form of passively actuated scales from the shortfin mako shark. Previous experiments have shown that both real shark skin and flexible shark scale models exhibit passive control of the flow separation. This experiment used 3D printed rigid shark skin models to study the effect on flow separation and confirm that the flexible nature of the scales is key to the separation control mechanism. The measurements were collected in a water tunnel using a DPIV system, and a rotating cylinder was placed just before the test section to induce an adverse pressure gradient and flow separation of a tripped turbulent boundary layer. Baseline experiments used a smooth flat plate in the test section with a maximum Re of 8.5 x 10⁵. The second set of data used a Micro-actuated Array for Kinematic Optimization (MAKO) model that is geometrically similar to the scales found on the flank of the shortfin mako where scales are capable of bristling to an angle of 50 degrees. A third set of experiments used unactuated, rigid 3D printed shark scales, and a fourth experiment tested the same scales at a fixed actuation angle of 50 degrees. Results show that the separation control occurs due to the flexibility of the scales while the rigid models lose the ability to control flow separation.