Flow Structures Around a Flapping-Wing Micro Air Vehicle Performing a Clap-and-Peel Motion

The vortical flow structures generated by the flapping wings of the DelFly II micro air vehicle in hovering flight configuration are investigated using particle image velocimetry. Synchronous force measurements are carried out to establish the relation between the unsteady forces and force generation mechanisms: particularly, the leading-edge vortex and the clap-and-peel motion. The formation of conical leading-edge vortices on both wings is revealed, which occurs rapidly at the start of the outstroke as a result of the wing-wing interaction. The leading-edge vortices of the outstroke interact with those of the instroke, which are shed and, by mutual induction, advect upstream as a vortex pair at the end of previous instroke. The leading-edge vortex pairs induce a strong inflow into the region formed between the upper and lower wings during the peeling phase, resulting in the formation of a low-pressure region. This, together with the leading-edge vortices and a momentum increase formed by the clap, accounts for the generation of relatively higher forces during the outstroke. The cycle-averaged forces are estimated with reasonable accuracy by means of a momentum-based approach using wake velocity information with an average error of 15%