Experimental Investigation of Flexible Hawkmoth-like Wings on the Wing-wake Interaction in Hovering Flight

摘要

To study wing-wake interaction for various wing flexibilities,force measurements and digital particle image velocimetry were carried out on flapping hawkmoth-like wings in a water tank.Wing thickness was employed as a design variable for the wing flexibility distributions.Abrupt flap-down and phase delay in flexible wings influenced the behaviors of the Leading-Edge Vortex (LEV) and Trailing-Edge Vortex (TEV),generated by the previous stroke.While the rigid wing exhibited a detached LEV at the end of the stroke,wing with specific flexibilities obtained attached LEVs.The attached LEVs induced a relatively rapid flow toward the wing surface as a result of encountering the TEV,and the flow caused a higher lift peak.On the other hand,the wings with larger wing deformations generated distinctive changes in LEV and TEV behaviors.The flap-down helped the TEV form closer to the wing surface,and it thus caused a downwash rather than wing-wake interaction.Furthermore,the most flexible wing had a newly-formed pair of LEVs above the wing during the wing reversal,thereby being not able to generate the wing-wake interaction.These results help to understand the different vortex structures generated by flexible wings during the wing reversal and the corresponding effects of wing-wake interaction.