Numerical study of three-dimensional flapping wings hovering in ultra-low-density atmosphere

This paper presents a numerical study on the aerodynamic performance of three-dimensional flapping wings hovering in ultra-low-density fluid by using an immersed boundary method with a focus on the effects of compressibility on force production and flapping efficiency. Simulations are conducted by varying Mach number, aspect ratio, stroke amplitude, and flexibility of the wing. It is found that the lift coefficient and efficiency of rigid wings are reduced by up to 10.6% and 10.7%, respectively, when the Mach number is increased from 0.2 (weakly compressible) to 0.9 (highly compressible). To achieve sufficient lift force in the ultra-low-density atmosphere, three main strategies including varying the aspect ratio, stroke amplitude, and flexibility of wings are explored. It is found that a wing with high aspect ratio, small and fast stroke motion, and moderate flexibility is able to generate a high lift. An optimized flexible wing according to the aforementioned analysis is further proposed and simulated, which shows 38.3% and 20.8% enhancements of the mean lift coefficient and efficiency, respectively. The present study shows that the flapping aerial vehicle in ultra-low-density atmosphere is highly feasible from the aerodynamic point of view.