Meeting Abstract
The largest contributing factor of drag during swimming is generally that due to flow separation. It is hypothesized that the flexible denticles found on key body locations of the fast-swimming shortfin mako (Isurus oxyrinchus) shark aid in controlling flow separation. Previous work has documented that flank scales, located downstream of the gills, are capable of being actuated to angles of 40 degrees or more. Skin samples were affixed to a flat plate and placed in a water tunnel. A boundary layer was generated over a long plate, passed over the skin sample, and a region of flow separation was induced by the presence of a rotating cylinder located above the test area. The flow was measured by time-averaging thousands of velocity fields acquired using Digital Particle Image Velocimetry (DPIV). Flow separation was quantified using backflow coefficient, or the percentage of time the flow was reversed. Results show control of flow separation under both laminar and tripped turbulent boundary layer conditions. It should be noted that testing took place at speeds on the order of 0.5 m/s while burst swimming speeds can exceed 10 m/s. However, in spite of this lack in flow similarity separation control was demonstrated. This shows the main mechanism is Re independent, in that controlling the flow involves the scales reaching into the bottom 5% of the boundary layer to inhibit flow reversal occurring near the wall that leads to global flow separation and pressure drag. Large scale 3D printed models of shark denticles have also been fabricated (from a characteristic length of 0.2 mm on real skin to 1 cm for the models) to match flow similarity to the lower testing speeds. Scale actuation and flow control have also been observed using the 3D printed scale models.
