Dragonfly Forewing-Hindwing Interaction at Various Flight Speeds and Wing Phasing

D RAGONFLIES are accomplished fliers. Scientists have always been fascinated by theirflight. Experimental and computational studies on a single airfoil in dragonfly hovering mode were conducted by Freymuth [1] and Wang [2], respectively. They showed that large vertical force was produced during each downstroke. In each downstroke, a vortex pair was created; the large vertical force was explained by the downward two-dimensional jet induced by the vortex pair [2]. Recently, due to the advances in computational and experimental techniques and facilities, researchers are beginning to study dragonfly aerodynamics and forewing–hindwing interactions using three-dimensional model wings [3–5]. Sun and Lan [3] studied the aerodynamics and the forewing–hindwing interaction of a dragonfly in hover flight, using the method of computational fluid dynamics (CFD). Maybury and Lehmann [4] and Yamamoto and Isogai [5] conducted experimental studies on the forewing–hindwing interaction at hovering conditions. Wang and Sun [6] extended the computational study of Sun and Lan [3] to the case of forward flight. Inmost of these studies, only hovering flight was considered. Only Wang and Sun [6] investigated the effects of forward flight speed, but the investigation was limited to a few phase differences ( d 0, 60, 90, and 180 deg; d denotes the difference in phase angle between the forewing and the hindwing stroke cycles, positive when the hindwing leads the forewing and negative when the forewing leads the hindwing). Because the distance of a wing from the wake of another wing depends on the flight speed and the relative motion of the foreand hindwings, it is expected that the forewing–hindwing interaction is strongly influenced by the flight speed and the relative phase difference. Therefore, it is desirable to study the forewing–hindwing interaction by systematically varying the flight speed and the phase angle. Moreover, in the above studies [3–6], attention was mainly paid on whether or not the aerodynamic forces were changed by the forewing–hindwing interaction, while how the interaction occurred was not well understood. It is of interest tomake further investigation on the flow field of the wing wake to reveal how the forewing– hindwing interaction occurs. In the present study, we address the above questions by numerical simulation of the flows of model dragonfly wings. The phasing and the flight speed are systematically varied. Advance ratio (the nondimensional flight speed) ranges form 0 to 0.6. At each advance ratio, eight phase differences, 180, 135, 90, 45, 0, 45, 90, and 135 deg, are considered.