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. 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 bioinspired surface for aircraft 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. Movable and rigid 3D printed shark scales that replicate passive bristling angles of 50^∘are constructed with crown lengths of 3.6 mm, twenty times greater than those of a real shark. In this experiment, the boundary layer grows to sizes large enough that the scale of the flow increases, making it more measurable to digital particle image velocimetry and allowing models to be sized so that they fit within the bottom 10% of the boundary layer. At low reversing flow velocities, the movable scales were seen to passively flap and mix momentum in the lower boundary layer.