Effects of Uniform Vertical Inflow Perturbations on the Performance of Flapping Wings

Flapping wings have attracted significant interest for use in miniature unmanned flying vehicles. Although numerous studies have investigated the performance of flapping wings under quiescent conditions, effects of  freestream disturbances on their performance remain under-explored. In this study, we experimentally investigated the effects of uniform vertical inflows on flapping wings using a Reynolds-scaled apparatus operating in water at the typical Reynolds number of large insects (Re ≈ 3600). The overall lift and drag produced by flapping wings were measured by varying the magnitude of inflow perturbation from J_Vert =-1 (downward inflow) to J_Vert =1 (upward inflow), where J_Vert  is the ratio of the inflow velocity to the wing's velocity. The interaction between flapping wings and downward oriented inflows resulted in a steady linear reduction in mean lift and drag coefficients with increasing inflow magnitude. While a steady linear increase was noted for upwards oriented inflows until J_Vert= 0.3 and after J_Vert = 0.7, a significant unsteady wing-wake interaction occurred when 0.3 ≤ J_Vert< 0.7, which caused large variations in instantaneous forces over the wing and led to a reduction in mean performance. These findings highlight asymmetrical effects of vertically oriented perturbations on the performance of flapping wings and pave the way for development of suitable control strategies.