Numerical investigation on aerodynamic characteristics of dragonfly corrugated airfoils in gliding and flapping flight

The dragonfly is a flapping insect with excellent flying capabilities in nature, and its wings feature prominent corrugated structures. In this study, different types of corrugated airfoils, i.e. corrugation at the leading edge, corrugation at the trailing edge and corrugation along the whole chord are designed and compared with their smooth counterpart to investigate the effect of corrugation distribution on the aerodynamic characteristics of airfoils. Both gliding phase and flapping phase are simulated with the method of computational fluid dynamics (CFD). The chord Reynolds number (Re) based on the incoming velocity is varied from 1000 to 2000. The results show that, in the gliding phase, the recirculation zones formed within the corrugation produce negative frictional drag. The leading-edge corrugated airfoil has lower drag while the trailing-edge corrugated airfoil has higher lift and lift-to-drag ratio. The time-averaged lift coefficient and lift-to-drag ratio of the trailing-edge corrugated airfoil are increased by 23.1% and 9.1% respectively compared with the flat airfoil at $\alpha ={10}^{\circ }$. While at higher angles of attack, the aerodynamic characteristics of all airfoils are similar. Flow analysis shows that the trailing-edge corrugation increases the strength of leading-edge vortex (LEV) and the surface pressure difference upstream. In the flapping phase, the trailing-edge corrugation can decrease the negative lift generated in the upstroke, thereby increasing the cyclic-averaged lift. And the airfoil with corrugation along the whole chord has the highest cyclic-averaged lift because of stronger rotational circulation. This study suggests that it is an effective scheme to improve aerodynamic characteristics of airfoil by adjusting the corrugation distribution.