Abstract: Dragonfly has the ability to control the aerodynamic forces for flight by modulating the phase relationship between their forewings and hindwings. In this article, unsteady flows of a dragonfly model in hovering (advance ratio J=0) and in forward flight with mediumspeed (J=0.3) are simulated by solving unsteadyNavier-Stokes (N-S) equations on dynamic overset unstructured grids. At each advance ratio, thirteen phases from 0° to 360° with intervals of 30° each are considered. The variation of aerodynamic force and power with phase as well as the aerodynamic perturbation between the forewing and hindwing are studied. It is found that the period average vertical force and power varies in a “U” shape as a function of the phase. The vertical force generated by the model is enough to balance the weight, and the data for aerodynamic power also agree with the statistical data of real dragonflies. In the wide phase region of 90°270°, aerodynamic interaction between the wings is relatively strong and stable. The vertical force and power is relatively small and stay roughly constant. All these results may be useful for explaining the unusual phase relationships between the wings of dragonflies.

Key words: dragonfly, unsteady flow, aerodynamic interaction, phase relationship, numerical simulation