Three-dimensional effects of a tandem flapping-fixed wing configuration at low Reynolds number

The flow interaction between the front and rear wings in tandem configuration can lead to superior aerodynamic performance in insects and micro air vehicles. Previous studies have shown that the interaction of a two-dimensional tandem flapping-fixed airfoil configuration can enhance the lift performance of the fixed airfoil at a low Reynolds number. However, it is unclear whether the lift enhancement is maintained in three dimension and the role of the wingtip vortex is not fully understood. In this paper, we numerically studied the three-dimensional flapping-fixed wing configuration. The effect of the aspect ratio is investigated. Results show that the three-dimensional flow interaction enhances the lift generation of the fixed wing through a quasi-two-dimensional strong upwash flow generated by the flapping wing, which increases the effective incoming velocity and the effective angle of attack and strengthens the leading edge vortex of the fixed wing. The upwash flow drops near the wingtip due to the wingtip vortex. Moreover, extra lift enhancement of a low-aspect-ratio fixed wing is induced by a high-aspect-ratio flapping wing, which generates strong upwash flow fully covering the fixed wing in the spanwise direction. The exposure to the strong upwash flow near the wingtip of the fixed wing prevents the sharp reduction in the effective incoming velocity, the effective angle of attack, and the local circulation of the leading edge vortex. Our findings encourage the combination of a high-aspect-ratio flapping wing and a low-aspect-ratio fixed wing in this configuration for the design of micro air vehicles.