Leading-edge vortex enhancement of a flexible flapping wing with the clap-and-fling mechanism

The clap-and-fling mechanism, initially discovered in insect flight, has been widely adopted in Flapping-Wing Micro Air Vehicle (FWMAV) designs to enhance their lift generation. Unlike rigid wings, artificial FWMAV wings exhibit distinct deformation characteristics due to their unique material properties and structural features. These wings rely on deformation rather than flipping to achieve appropriate angles of attack and facilitate the clap-and-fling motion. While such flexibility is inherent in FWMAV wings, the impact of its resulting clap-and-fling motion on aerodynamics is still underexplored, especially lacking a quantitative survey of leading-edge vortex (LEV) enhancement. This study proposes a refined deformation model and employs the lattice Boltzmann method to investigate the clap-and-fling mechanism of flexible flapping wings. Results demonstrate that a small wing spacing and rapid clap-and-fling motion can boost the lift enhancement, in that the LEV growth in the fling phase is accelerated. This is because the vortex ring generated by the clap motion promotes the roll-up and subsequent downstream stretching of the trailing-edge vortex. Quantitative analysis also reveals that the transient lift reaches its peaks slightly before the LEV strength is maximized, which is more prominent at a small wing spacing. These findings provide valuable insights for FWMAV designs that attempt to take advantage of the clap-and-fling mechanisms.