Unsteady aerodynamic performance of a tandem flapping–fixed airfoil configuration at low Reynolds number

Abstract

In nature, insects with their forewings and hindwings undergoing small-gap flapping motion experience strong aerodynamic interaction. Conventional studies mainly focus on the propulsion performance of tandem flapping wings, while the interaction between a flapping wing and a fixed wing in the tandem configuration at low Reynolds numbers ( Re) is unclear. In this paper, we numerically studied the aerodynamic performance and vortex structure of this tandem flapping–fixed airfoil configuration. The effects of horizontal distance ( LX), vertical distance ( LY), and geometric angle of attack ( α) of the fixed wing on the thrust and lift performance are investigated. The results show that LX dominates the propulsion performance, while LY and α control the lift performance. The thrust enhancement of the flapping airfoil is effective only within a small range of LX, and the thrust is mainly determined by the changing rate of the impulse of the vortices directly connected to the airfoils. The lift reaches its peak when LY approaches the plunging amplitude. Compared with a fixed airfoil, the flapping–fixed configuration shows a larger lift-to-drag ratio, indicating a lift enhancement led by the interaction with the upstream flapping airfoil. Moreover, increasing LY and α simultaneously can lead to additional advantages in lift generation. Further analysis shows that changes of LY and α both manifest in a variation of the effective angle of attack of the fixed airfoil, thereby manipulating its lift generation. This paper provides an aerodynamic database and guidance for the design of micro air vehicles using tandem flapping–fixed wings.