Control of Turbulent Boundary Layer Separation by a 3D Printed Shark Skin Model with Passive Bristling

Turbulent boundary layer separation can be problematic in many engineering applications. However, nature may have a solution in the form of flexible shark scales found on the shortfin mako which have been proven to passively bristle under reversing flow conditions and control flow separation in past experiments. 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. Enlarging the geometry and constructing 3D printed models of shark skin is the best route forward to developing a bio-inspired surface for real applications. Using a rotating cylinder above a flat plate in a water tunnel setup, an adverse pressure gradient was induced creating a separated region over a tripped turbulent boundary layer with approximate Reynolds numbers up to 8 x 10⁵. 3D printed shark scales were mounted into a plate using wire to replicate low-resistance passive bristling angles of 50 degrees. Rigid scale models were also constructed to observe how the motion of the scales during separation is important to passive control. The model scales were constructed with crown lengths of 3 mm, fifteen times greater than those observed on a real shark. This low-speed flow study makes the boundary layer dynamics and shark scale motions more measurable while allowing for actuation heights of the scales to be within the bottom 10% of the boundary layer. Baseline studies document flow separation and reversing flow development in the presence of an adverse pressure gradient over a smooth plate. The same experiments are then repeated with the flexible and rigid shark skin models to document control of flow separation and observe how reversing flow induces scale bristling.