Aerodynamic performance of flapping wing with alula under different kinematics of complex flapping motion

The flight of birds is a remarkable feat, and their remarkable ability to fly derives from complex multi-degree-of-freedom flapping motions and small-scale feather structures that have evolved over millions of years. One of these feather structures is the alula, which can enhance the birds' flight performance at low speeds and large angles of attack. Previous studies on the alula have focused on the steady state. This undoubtedly ignores the unsteady effect caused by complex flapping motion, which is also the most important characteristic of avian flight. Therefore, this paper carries out a study on the effect of different motion modes and motion parameters on the aerodynamic mechanism of the alula. Previous studies found the dominate effect in the lift enhancement is influenced by Reynolds number, stall condition and geometric parameters. After coupling complex flapping motion, aerodynamic characteristics of the flapping wing are greatly influenced by different motion patterns and parameters. For pure plunge motion, both the slot effect and the vortex generator effect of the alula dominate the lift enhancement; while for plunge-twist and plunge-sweep motion, the vortex generator dominates more. At a low plunge amplitude, a low twist amplitude and a low sweep amplitude, the deflection of the alula has a good lift enhancement compared with the baseline wing. Increasing these amplitudes attenuates both the slot effect and the vortex generator effect. The alula can enhance the lift by 10.4% at the plunge amplitude of 25 deg (for pure plunge motion), by 7.9% at the plunge amplitude of 25 deg and twist amplitude of 10 deg (for plunge-twist motion), by 3.3% at the plunge amplitude of 25 deg and sweep amplitude of 15 deg (for plunge-sweep motion). Meanwhile, at a large sweep phase angle, the alula has a better lift enhancement. Increasing the phase angle enhances the vortex generator effect of the alula, and it has an optimal lift enhancement effect of 11% at the phase angle of 180 deg.