The role of wing morphology in the aerodynamics of insect flight

Abstract

Recent interest in developing micro air vehicles (MAVs) for a variety of both civil and military uses has driven significant research into the aerodynamics of natural flyers, such as insects, birds and bats. Insects, in particular, exhibit desirable flight characteristics that MAV designers wish to incorporate into their designs, however our understanding of how these animals fly is still limited. Research into insect flight has shown that they employ a number of unsteady mechanisms, the most prevalent of these being the formation of a leading-edge vortex (LEV) which provides the wing with enhanced lift. While this research has greatly improved our understanding of insect flight, the effect of the wing's shape on these unsteady mechanisms is not well understood. This thesis describes an investigation into the effect of two wing morphological parameters, aspect ratio and camber, on the flow structures around flapping and rotating wings in an insect-like flight regime. The effect of wing aspect ratio is first explored at different Reynolds numbers using a numerical model of an altered fruit fly wing revolving at a constant angular velocity. Increasing the Reynolds number for an aspect ratio of 2.91 resulted in the development of a dual LEV structure, however increasing aspect ratio at a fixed Reynolds number generated the same flow structures. This result shows that the effects of Reynolds number and aspect ratio are linked. An alternate flow scaling method, using the wing span as the characteristic length, is presented to decouple the effects of Reynolds number from those of aspect ratio. This resulted in a span-based Reynolds number, which can be used to independently describe the development of the LEV. Indeed, universal behaviour was found for various parameters using this scaling. The effect of aspect ratio on the vortex structures was then assessed at different span-based Reynolds numbers and it was found the wing aspect ratio had the effect of shortening the wing's chord length relative to a fixed LEV size. Scaling the flow using the wing span was found to apply for revolving wings at large angles of attack, such that the flow separates from the leading-edge and a strong spanwise velocity is generated on the leeward side of the wing. These conditions are typical of those seen in nature, and hence this scaling could be applied to similar investigations involving insects and birds as well as nature-mimicking MAVs. The effect of wing aspect ratio was then explored at different advance ratios using a numerical model that mimicked a flapping insect wing. It was demonstrated that increasing the advance ratio enhances vorticity production at the leading-edge during the downstroke, and this results in more rapid growth of the LEV for non-zero advance ratios. This effect, combined with that of aspect ratio, determines whether the LEV remains stably attached to the wing or if it is shed. For high advance ratios and large aspect ratios the LEV was observed to quickly grow to envelop the entire wing during the early stages of the downstroke. Continued rotation of the wing resulted in the LEV being eventually shed as part of a vortex loop that peels away from the wing's tip. It is shown that the shedding of the LEV for high aspect ratio wings at non-zero advance ratios leads to reduced aerodynamic performance of these wings. This helps to explain why a number of insect species have evolved to have low aspect ratio wings, as they outperform high aspect ratios across a wide range of flight speeds. Finally, wing deformation is observed during the flight of some insect species, which results in the wing becoming cambered throughout each half stroke. In this study, the effect of wing camber on the flow structures and aerodynamic forces for insect-like wings is investigated using the rotating-wing numerical model. Both positive and negative camber was investigated at Reynolds numbers of 120 and 1500, along with the chordwise location of maximum camber. It was found that negatively cambered wings produce similar LEV structures to non-cambered wings at both Reynolds numbers, but high positive camber resulted in the formation of multiple streamwise vortices at the higher Reynolds number, which disrupt the development of the main LEV. Despite this, positively cambered wings were found to produce higher lift on drag ratios than flat or negatively cambered wings. It was determined that a region of low pressure near the wing's leading edge, combined with the curvature of the wing's upper surface in this region, resulted in a vertical tilting of the net force vector for positively cambered wings, which explains how insects can benefit from wing camber.