Effect of leading-edge protuberances on unsteady airfoil performance at low Reynolds number

Humpback whales have higher hydrodynamic maneuverability due to the unique protuberance structures on the leading edge of their flippers. The leading-edge protuberance, therefore, is regarded as an attractive flow control method. Although some results have shown the effect of the leading-edge protuberance on a static airfoil, its effect on the aerodynamic characteristics of dynamic airfoils is still not clear. In present study, direct force measurements are conducted on dynamic NACA 0012 wings with sinusoidal leading-edge protuberances as well as a baseline configuration at a low Reynolds number of 2 × 10⁴. Three typical motions of the wing, including pitching, plunging, and combined pitching-plunging, are employed to examine the effect of motion styles on aerodynamic characteristics of airfoils. Under the static case, it is found that leading-edge protuberances delay the stall angle of attack of airfoils by about 67%, and increase the maximum lift coefficient, which are different from previous studies of thick straight wings, such as NACA 63₄–021, NACA 0020, and NACA 0021. Under dynamic cases, motion styles have a significant effect on the aerodynamic characteristics of airfoils. The lift coefficient of the modified airfoil becomes larger than the baseline airfoil mainly at upstroke phases for pitching motion, downstroke phases for plunging motion, and all phases for combined pitching-plunging motion, considering positive angles of attack. Compared to the static condition, the range of angles of attack where the lift of the modified airfoil is larger than the baseline airfoil extends; however, the lift difference between both airfoils decreases. Under pitching motion, phase-locked two-dimensional particle image velocimetry is performed to investigate the mechanism behind different aerodynamic characteristics between two dynamic airfoils. It is revealed that leading-edge protuberances inhibit formation of the secondary vortex, and weaken coherency of the leading-edge vortex (LEV). The flow acceleration effect of trough planes, different evolution of the LEV and the spanwise flow on the upper surface, are the main reasons that affect the aerodynamic characteristics of the modified airfoil.