Numerical Investigation On High-lift Generation Mechanism Of Dragonfly Hovering

Fliers in nature has long been a source of inspiration for human to design aircraft.With the development of Micro Air Vehicles(MAV),people are more and more interested in flapping-wing flight of insects.Studying the aerodynamic characteristics of insects is of great significance for revealing the biological evolution process and designing aircraft.In this paper,the aerodynamic performance and energy consumption of the three-dimensional rigid wing during hovering flight are analyzed by using CFD numerical simulation method which combines the ALE program developed by our research group and commercial software CFX.The results show that in a complete flapping period,the lift to maintain hovering flight is larger in the downstroke and smaller in the upstroke.During the downstroke,a new vortex ring completes the whole process from formation,development to shedding.However during the upstroke,almost no new vortices are generated.Therefore,it is believed that the formation and development of the new vortex ring during the downstroke is the main reason for large lift.During the upstroke,the shedding vortex ring stays under the wing and has little influence on the wing.The difference between two-dimensional wing and three-dimensional wing is that the flow field of three-dimensional is constantly changing along the spanwise direction.Along the spanwise direction,the vortex ring,pressure and flow rate increase gradually,and the leading edge vortex(LEV)shed earlier and earlier.Through the study of the start time of the stroke reversal,a flapping mode that may generate large lift is obtained.Under the same translational motion mode,the start time of stroke reversal is advanced or delayed by 2%,4%,6% and 8% of a period,respectively.By calculating the vertical force coefficient,thrust coefficient,power coefficient and analyzing the change of the flow field structure,the influence of the start time of the stroke reversal on the lift generation mechanism and energy consumption is explained.It shows that delaying start time of the stroke reversal will reduce the lift of the model wing,but advancing start time of the stroke reversal is beneficial to improve the lift of the model wing.When the start time of the stroke reversal is advanced by 8% of a period,the lift increases by 15.56% of the symmetrical mode.By analyzing the flow field structure,it is considered that the rapid increase of the angle of attack at a large translational velocity leads to the continuous increaseof lift after the stroke reversal.This is a new way to generate lift,or it may be the result of the interaction of delayed stall mechanism and rapid reversal mechanism.