Unsteady Flow Structures and Aerodynamic Loads over a Pitching-and-Plunging Flexible Membrane Wing

Structural flexibility offers considerable potential for improving wing performance; however, its influence on the dynamic stall process remains insufficiently understood. In this study, synchronous force and particle image velocimetry measurements are conducted for a flexible membrane wing undergoing prescribed pitching-and-plunging motions. The results show that wing flexibility contributes to lift enhancement and drag reduction during dynamic stall. These aerodynamic benefits are mainly attributed to the favourable camber variations induced by membrane deformation. Furthermore, the lift curve exhibits a distinct double-peak pattern, which is governed by two different mechanisms. Specifically, the first lift peak occurs before the formation of the leading-edge vortex (LEV) and is primarily associated with the increase in circulation lift caused by rapid growth in wing camber. Alternatively, the second lift peak arises from the interaction between the LEV and the flexible membrane, which helps sustain a large positive camber during the downstream development of the LEV. These findings deepen the understanding of fluid-structure interaction during dynamic stall and highlight the important role of flexibility-induced camber variation in modulating unsteady aerodynamic loads.