The Effect of Unsteady Flapping Motion on the Aerodynamic Performance of Bio-Inspired Alula

Avian flight has inspired scientists and engineers for a long time. Flapping-wing micro air vehicle (FMAV) is a kind of air vehicle mimicking the avian flapping flight that has achieved great success in this century. However, in some complex flight conditions, its flight performance is not as good as that of birds. Birds have evolved many unique features to adapt to different and complex situations. The leading-edge alula is considered a typical structure that can enhance birds' flight capabilities in slow-speed flight. In this paper, we numerically investigate the effects of bionic alula on the aerodynamic performance of flapping airfoil under different flapping kinematic parameters. The influence of different pitch amplitudes, plunge amplitudes, and mean angles of attack (AoAs) of freestream are considered. Results show that the deflection of the alula will suppress the flow separation and restrain the formation and development of the leading-edge vortex (LEV). This will cause a significant discrepancy in aerodynamic performances between the airfoil with and without bionic alula. Observing their different flow structures, the time-averaged lift coefficient of the single flapping airfoil without alula first decreases then increases with the increasing pitch amplitude, but almost increases continuously with the rising plunge amplitude. However, as for the airfoil with alula, the time-averaged lift coefficient first rises then drops as the pitch amplitude increases, and also first rises then falls with the increasing plunge amplitude. The maximum enhancement of the alula on the time-averaged lift coefficient can reach 82.2% under the optimum kinematic parameters. The bionic alula can also delay stall, and the stall angle postpones by 7° in the scope of our research.