In the paper, a novel flapping mode is presented that can generate high lift force by a dragonfly wing in hover. The new mode, named partial advanced mode (PAM), starts pitching earlier than symmetric rotation during the downstroke cycle of the hovering motion. As a result, high lift force can be generated due to rapid pitching coupling with high flapping velocity in the stroke plane. Aerodynamic performance of the new mode is investigated thoroughly using numerical simulation. The results obtained show that the period-averaged lift coefficient, C_L, increases up to 16% compared with that of the traditional symmetrical mode when an earlier pitching time is set to 8% of the flapping period. The reason for the high lift force generation mechanism is explained in detail using not only force investigation, but also by analyzing vortices produced around the wing. The proposed PAM is believed to lengthen the dynamic stall mechanism and enhance the LEV generated during the downstroke. The improvement of lift force could be considered as a result of a combination of the dynamic stall mechanism and rapid pitch mechanism. Finally, the energy expenditure of the new mode is also analyzed.

In the paper, a novel numerical method is present to numerically investigate life force generation mechanisms of 3D dragonfly wing during hovering motions. The dynamic mesh which descripts the motion of 3D wing and gird change in inner computational domain is generated by an ALE model we developed. Then the mesh is transferred at each time step into ANSYS CFX using Junction Box Routine. The simulation results show that a high lift force generated by implementing one specified advanced flapping action during hovering motion in which the translation time period of downstroke is shortened.