Vortices around Dragonfly Wings

TL;DR

This study investigates how different phase differences between dragonfly wings affect vortex formation and aerodynamic performance during hovering, revealing that wing phase significantly influences lift and vortex interactions.

Contribution

It provides detailed numerical analysis of vortex structures and aerodynamic forces for various wing phase differences in hovering dragonflies, highlighting the impact of wing coordination.

Findings

Counter-stroke vortex connection reduces lift in downstroke

Leading hind-wing minimizes vortex interference, enhancing lift

Wing phase difference critically affects vortex dynamics

Abstract

Dragonfly beats its wings independently, resulting in its superior maneuverability. Depending on the magnitude of phase difference between the fore- and hind-wings of dragonfly, the vortical structures and their interaction with wings become significantly changed, and so does the aerodynamic performance. In this study, we consider hovering flights of modelled dragonfly with three different phase differences (phi=-90, 90, 180 degrees). The three-dimensional wing shape is based on that of Aeschna juncea (Norberg, 1972), and the Reynolds number is 1,000 based on the maximum translational velocity and mean chord length. The numerical method is based on an immersed boundary method (Kim et al., 2001). In counter-stroke (phi=180 degree), the wing-tip vortices from both wings are connected in the wake, generating an entangled wing-tip vortex (e-WTV). A strong downward motion induced by this vortex decreases the lift force in the following downstroke (Kweon and Choi, 2008). When the fore-wing leads the hind-wing (phi=90 degree), the hind-wing is submerged in the vortices generated by the fore-wing and suffers from their induced downwash flow throughout the downstroke, resulting in a significant reduction of lift force. On the other hand, when the hind-wing leads the fore-wing (phi=-90 degree), the e-WTV is found only near the start of hind-wing upstroke. In the following downstroke of hind-wing, most of the e-WTV disappears and the hind-wing is little affected by this vortex, which produces relatively large lift force.