Effects of gust structures on aerodynamics of flapping wings*

Birds and insects can stabilize themselves in gusty environments with their flapping wings, surpassing what fixed-wing aerial vehicles can do in similar conditions. Here, we test the hypothesis that the plunging movements of a wing can indeed suppress the effects of incoming gust structures by studying both the fluid flow surrounding the wing and the wing's aerodynamic loading. The wing plunged at frequencies determined by Strouhal numbers (St) from 0 (fixed wing) to 0.5, covering the range commonly found in nature. Inside a closed-circuit water channel, a gust generator with sinusoidally pitching vanes created vortex gusts that interacted with the wing. Particle Image Velocimetry (PIV) was used to observe the flow around the wing, and a force/torque transducer was used to measure the loads acting on it. Flow quantities such as circulation, turbulence intensity, and forces were calculated. Because the interactions between the wing and gusty flow was unpredicted, statistics were used instead of the time-series data to quantify the gusts' effects. With increasing St, the circulation and the forces increased in magnitude, but the increase in the spread of both quantities, due to the gusts, lowered. This suggests better predictability of the flow and forces for plunging wings at higher St, which could be exploited for better stabilization against gusts.