Wing-wake interaction: comparison of 2D and 3D flapping wings in hover flight.

The wing-wake interaction of flapping wings while hovering has been investigated, with the focus on the difference in wing-wake interaction between 2D and 3D flapping wings. Numerical simulations are conducted at a Reynolds number of 100, and the flapping configurations are divided into the 2D, quasi-3D and 3D categories. Variations of the aspect ratio and Rossby number allow the flapping configuration to morph gradually between categories. The wing-wake interaction mechanisms are identified and the effect of three-dimensionality on these mechanisms is discussed. Three-dimensionality affects wing-wake interaction through four primary aerodynamic mechanisms, namely, induced jet, downwash/upwash, leading-edge vortex (LEV) shedding due to vortex pairing, and the formation of a closely attached LEV. The first two mechanisms are well-established in the literature. With regard to the LEV shedding mechanism, it is revealed that the interaction between the LEV and the residue vortex from the previous stroke plays an important role in the early vortex shedding of 2D flapping wings. This effect diminishes with increasing three-dimensionality. With regard to the mechanism of the closely attached LEV, the wake encourages the formation of an LEV that is closely attached to the wing's top surface, which is beneficial to lift generation. This closely attached LEV mechanism accounts for most of the lift enhancement that arises from wake effects. Three-dimensionality alters the efficacy of the different aerodynamic mechanisms. Consequently, the dual peak lift coefficient pattern typically seen on 2D flapping wings transforms into the single peak lift coefficient pattern of the 3D flapping wing. It is also demonstrated that the mean lift enhancement due to wing-wake interaction diminishes rapidly when three-dimensionality is introduced. Results suggest that, for wings with parameters close to those of natural flyers, wing-wake interaction yields marginal lift enhancement and a small increase in energy consumption.