Numerical evaluation of suppression mechanism of shark-inspired riblet on tip leakage vortex of a NACA0009 hydrofoil with tip clearance

The tip leakage vortex (TLV) formed by tip clearance has adverse effects on the operation stability of bulb turbines in tidal power stations. In this paper, the NACA0009 hydrofoil is selected as the research object, a method of suppressing TLV by shark-inspired riblet is proposed, and the suppression mechanism is numerically studied. The results show that the shark-inspired riblet effectively suppresses the TLV, decreases the high-vorticity region generated by TLV, and increases the pressure in the centre of the vortex core. The area of the low-pressure region on hydrofoil tip decreases, with a decreasing trend moving downstream and to the suction surface. The tip leakage flow is controlled by reverse rotating vortex inside the riblet. The leakage velocity decreases, which leads to a reduction in the driving force of the tip leakage flow. In addition, the leakage rate declines 5.9%. Moreover, the TLV trajectory is close to the suction surface, the vertical distance of TLV movement is reduced, and the maximum TLV intensity decreases by 7%. With increasing riblet height, the suppression effect of shark-inspired riblet on TLV is more obvious, but the lift-to-drag ratio of hydrofoil decreases rapidly when the riblet height is greater than a critical value.