Effects of asymmetric stroke deviation on the aerodynamic performance of flapping wing

The kinematics of insect flapping flight are complex and asymmetric, which are contributed to their superior flying capabilities, and the design of novel flapping micro air vehicles can draw inspiration from relevant researches. Previous studies usually focus on the wing with asymmetric stroke or pitch motions. A trajectory with asymmetric deviation motion, named as "pear-shaped" pattern, is proposed in current work. The hovering aerodynamics and vortex dynamics of a rigid flapping wing have been numerically investigated by comparing with that of "line-shaped" pattern with no deviation. In order to have a better insight into the influences of the asymmetric deviation, we change the kinematic parameters, that is, stroke amplitude, pitching amplitude, deviation amplitude, and phase lag between stroke and pitch angles. The results show that the wing with asymmetric deviation exhibits superior capability in lift enhancement for most of the cases analyzed, which is accompanied by the extra power cost and slight reduction in efficiency. The asymmetric deviation in cases with high stroke amplitude or low pitching amplitude may be considered as a cost-saving strategy, subject to slight damage on lift generation (if acceptable). Additionally, the asymmetric deviation brings a strong asymmetry into the instantaneous forces during one flapping cycle. The underlying lift-enhancing mechanism is explored by examining the dominant vortex structures in the adjacent flow field of the wing, which is mainly attributed to the changes in the effective angle of attack, increasing with downward deviation and decreasing with upward deviation.