The function of the alula with different geometric parameters on the flapping wing

Abstract

Birds in nature have the ability to maintain high aerodynamic efficiency in complex flight conditions. This agility stems from the multi-degree-of-freedom flapping motion and specialized feather systems that evolved over millions of years. The leading-edge alula is considered a typical feather system that can enhance the flight envelope and capabilities of birds at low speed and high incidence. Previous studies usually adopted a static model, ignoring unsteady effects caused by flapping motion. Thus, we numerically investigated the function of the alula with different geometric parameters on the flapping wing in this paper. The alula has both the slot effect and vortex generator effect during the flapping motion, whereas the effect that plays a main role in lift enhancement changes as time varies. At the beginning of the upstroke, the slot effect plays the main role. At mid-time of the upstroke, the vortex generator effect plays the main role. Different geometric parameters have different influences on these two effects. The dimensionless spanwise location affects the strength of both the ATEV (alula trailing edge vortex) and ASV (alula streamwise vortex). The relative angle affects mainly the strength of the ATEV, whereas the deflection angle affects mainly the strength of the ASV. The optimal geometric parameters to obtain maximum lift enhancement are a dimensionless spanwise location of 0.5, a relative angle of 0°, and a deflection angle of 10°, with a lift enhancement of 5.5% compared to the baseline wing.