Experimental Investigation of Turbulent Boundary Layer Separation Control by 3D Printed Shark Skin Models

Turbulent boundary layer separation can be problematic in many engineering applications. However, nature may have a solution in the form of passively actuated micro-flaps inspired by shortfin mako shark scales, which have been proven to passively bristle under reversing flow conditions and control flow separation in DPIV experiments using real skin samples. An investigation of how these shark scales interact with reversing flow in the near-wall regions of the boundary layer is of interest to better understand the fluid-shark scale interactions. Previous studies investigated the passive bristling motion of the scales and determined that the responsive motion of the scales was vitally important to flow control. Moreover, the bristling height into the boundary layer appeared to be an important parameter for control, with scales reaching too far beyond the buffer layer having little to no effect on separation control compared to larger boundary layer thicknesses. Using a rotating cylinder above a flat plate in a water tunnel setup, an adverse pressure gradient was induced creating a separated region within a tripped turbulent boundary layer with approximate Reynolds numbers up to 8 x 10⁵. Three different 3D printed micro arrays for kinetic optimization (MAKO) were mounted into a plate to replicate low-resistance passive bristling angles of 50 degrees. The model scales were constructed with maximum bristling heights of 1.4 mm, 2 mm, and 2.8 mm (a y⁺ range of 11 – 31) to investigate how bristle height into the buffer layer affects separation control. This dynamically scaled low-speed flow study makes the boundary layer dynamics and shark scale motions more measurable while allowing for actuations heights of the scales to be within the buffer layer. Baseline studies document flow separation and reversing flow development in the presence of an adverse pressure gradient over a smooth plate for direct comparison. Results quantify the separation control and observe how bristle height affects flow control.