Aerodynamics of 2D and 3D flapping wings in water treading motion

Flying insects demonstrate extraordinary flight performance and have inspired the design of flapping wing micro air vehicles (FWMAVs). However, FWMAVs are not confined to undergoing the same wing kinematics as those observed on natural flyers. Rather than undergoing the ubiquitous normal hovering motion typically observed on flying insects, FWMAVs may instead opt to undergo the water treading motion which originates from aquatic propulsion. In this study, the aerodynamic performance of normal hovering and water treading motions are compared for 2D and 3D rigid flapping wings in hover. Numerical simulations are conducted at varying mid-stroke angles of attack (αM). The results show that for both 2D and 3D, water treading can achieve higher maximum mean lift coefficient compared to normal hovering. Additionally, water treading is more efficient than normal hovering at any target mean lift coefficient within the parameter range considered. Visualisation of the flow structures indicate that the performance augmentation of water treading motion can be attributed to three mechanisms. Firstly, compared to normal hovering, water treading motion delays the shedding of the leading-edge vortex. Secondly, water treading tends to yield more beneficial wing-wake interaction. Thirdly, normal hovering enters a high angle of attack (α), high drag phase near stroke reversal, which incurs high aerodynamic power. This high α phase is absent in water treading, resulting in higher efficiency. For 3D cases, the leading-edge vortex is more stable and hence the first and the second mechanisms become less significant. At α_M=45°, water treading outperforms normal hovering in terms of hovering efficiency by up to 54% in 2D and 29% in 3D. Hence, the water treading motion is a promising alternative for FWMAV.