Aerodynamic performance of dragonfly wing model that starts impulsively: how vortex motion works

Abstract

The cross-section of dragonfly wings has corrugated structures. In particular, the leading-edge side of the wing consists of V-shaped structures. According to previous studies, a corrugated wing may exhibit high aerodynamic performance at low Reynolds numbers (Re = O(10 3 )), and vortex dynamics associated with wing structure are expected to play an important role. We study the relationship between the wing structure and vortex dynamics by direct numerical simulations. It is known that, when a two-dimensional flat wing impulsively starts from a rest state, a coherent vortex called lambda vortex, which has the opposite sign to a leading-edge vortex (LEV), is generated and remains for a time interval. Our previous study suggests that, in the case of the corrugated wing, the lambda vortex collapses and is stuck inside V-shaped structures near the leading edge. The collapse of the lambda vortex was proposed as a key factor of high performance for the particular shape of the corrugated wing. In this paper, we investigate the relationship between vortex dynamics and high performance in a wider parameter range than in our previous studies. We analyse two corrugated models with different Reynolds numbers. It is revealed that the collapse of the lambda vortex is also the key to high performance for these cases.