Fluid-structure interaction characteristics of butterfly flexible flapping wings in the forward flight

The stiffness of flapping wings significantly influences the aerodynamic characteristics and structural response of insects in forward flight. While numerous studies have focused on the fluid-structure interaction of small insect wings, research on butterflies, characterized by large wing areas and low aspect ratios, remains relatively scarce. To investigate the effects of different flexibility distributions on the fluid-structure interaction characteristics of butterfly-like flapping wings in forward flight, a geometric model and kinematic equations were established based on the morphology and motion patterns of Chilasa clytia. A fluid-structure interaction solver combining the lattice Boltzmann method (LBM) and the finite element method (FEM) was developed and validated with benchmark cases. The developed method was then employed to systematically compare the aerodynamic performance and structural responses among isotropic flexible, anisotropic flexible, and rigid butterfly wings during flapping. The results indicate that the isotropic flexible wing exhibits significantly enhanced flight performance compared to the anisotropic and rigid wings. The flapping of the isotropic flexible wing generates more complex vortex structures, including the wingtip secondary flow vortex (WSFV) and the hindwing tip vortex (HTV). These vortices create an additional low-pressure region, thereby improving the aerodynamic performance. This study provides new insights into the vortex dynamics associated with lift enhancement mechanisms in flapping flight and offers valuable references for the design of novel bio-inspired micro flapping-wing aerial vehicles.