Unsteady aerodynamics of dragonfly using a simple wing–wing model from the perspective of a force decomposition

Abstract

Insects perform their multitude of flight skills at frequencies of tens to hundreds of Hertz, and the aerodynamics of these skills are fundamentally unsteady. Intuitively, unsteadiness may come from unsteady wing motion, unsteady surface vorticity or vorticity being shed into the rear and front wakes. In this study, we propose to investigate the aerodynamics of dragonfly using a simplified wing–wing model from the perspective of many-body force decomposition and the associated force elements. Insect flight usually operates at Reynolds numbers of the order of several hundreds, at which the surface vorticity is shown to play a substantial role. There are important cases where the added mass effect is non-negligible. Nevertheless, the major contribution to the forces comes from the vorticity within the flow. This study focused on the effects of mutual interactions due to phase differences between the fore- and hindwings in the translational as well as rotational motions. It is well known that the dynamic stall vortex is an important mechanism for an unsteady wing to gain lift. In analysing the life cycles of lift and thrust elements, we also associate some high lift and thrust with the mechanisms identified as ‘riding on’ lift elements, ‘driven by’ thrust elements and ‘sucked by’ thrust elements, by which a wing makes use of a shed or fused vortex below, in front of, and behind it, respectively. In addition, a shear layer attaching to each wing may also provide significant thrust elements.