The gust-mitigating potential of flapping wings

Nature's flapping-wing flyers are adept at negotiating highly turbulent flows across a wide range of scales. This is in part due to their ability to quickly detect and counterract disturbances to their flight path, but may also be assisted by an inherent aerodynamic property of flapping wings. In this study, we subject a mechanical flapping wing to replicated atmospheric turbulence across a range of flapping frequencies and turbulence intensities. By means of flow visualization and surface pressure measurements, we determine the salient effects of large-scale freestream turbulence on the flow field, and on the phase-average and fluctuating components of pressure and lift. It is shown that at lower flapping frequencies, turbulence dominates the instantaneous flow field, and the random fluctuating component of lift contributes significantly to the total lift. At higher flapping frequencies, kinematic forcing begins to dominate and the flow field becomes more consistent from cycle to cycle. Turbulence still modulates the flapping-induced flow field, as evidenced in particular by a variation in the timing and extent of leading edge vortex formation during the early downstroke. The random fluctuating component of lift contributes less to the total lift at these frequencies, providing evidence that flapping wings do indeed provide some inherent gust mitigation.