Three-dimensional numerical study on the interaction of contralateral insect wings in asymmetric stroke

Asymmetric flight of insects and birds is often seen in nature which is a conventional way for them to obtain the flexibility of maneuver in turning, hovering, and gliding. A numerical study on the interaction between contralateral wings of a dragonfly in asymmetric forward flight is carried out using the finite volume method. Various asymmetric flights with different advance ratios are considered where the aerodynamic forces, torques of each wing, pressure distributions, vorticity, and velocity fields are analyzed. A number of symmetric flights corresponding to the asymmetric flights are also studied. The results indicate that the interaction between the contralateral wings of a dragonfly is very small even when the dragonfly is in an asymmetric flight no matter how the advance ratios vary in the range concerned. With a typical example of asymmetric forward flights the difference in the mean value of vertical force coefficient is generally less than 5% compared with that for its corresponding symmetric flight. It is found that a small lateral flow region (LFR) is formed near the body, and there is a small lateral flow across the symmetric plane of the body. But this flow is very weak and resulted interaction between contralateral wings is very small. The result has confirmed in a way that dragonflies take a quite different way from fruit flies to obtain the lift with the contralateral wings. That is, fruit flies employ clap-and-fling mechanism which needs contralateral wings to be close enough to extrude the flow and generate lift, which means the contralateral two wings are so close that strong interaction happens, while dragonflies flap their contralateral wings on two sides of the vertical central plane with a relatively far distance between the wings where the interaction of contralateral wings is negligibly weak.