Reversing flow development in a separating turbulent boundary layer

Fast swimming sharks have micro-structures on their skin consisting of bristling scales. These scales are hypothesized to~bristle~in response to backflow~generated from the separated turbulent boundary layer (TBL) in regions of adverse pressure gradient (APG) on the shark body. Vortices are trapped in the~cavities~between the scales, which induce momentum exchange between the higher momentum fluid in the outer flow and that in the separated region. This momentum exchange causes reattachment of the separated TBL, causing the scales to return to the unbristled~location, and the cycle continues. The rows of scales have widths that are~comparable~to the spanwise length scale of the intermittent backflow patches~that appear in the region of incipient detachment of TBLs. In this~experimental~investigation, correlations between the shark scale's width and the spanwise size of the low backflow streaks are examined, as well as details of the incipient detachment region. The~experiments~are conducted in a water tunnel facility and the flow field is measured using PIV. Turbulent boundary layers are subjected to an APG via a rotating cylinder. Separated TBLs are investigated on a flat plate.