THESIS
2017
xviii, 126 pages : illustrations ; 30 cm
Abstract
Flapping flight of biological flyers provides great inspiration for the development of micro air vehicles (MAVs). However, the unsteady aerodynamics of flapping flight is to be clarified before MAVs can rival nature. In this dissertation, the effects of wing morphology, including the aspect ratio and the wing flexibility, on the flow development and the aerodynamic performance of flapping wings are elucidated. An experimental setup consisting of a water tank, wing and motion modeling system, particle image velocimetry measurement system, force measurement system and deformation measurement system is built. The vortex structures around the flapping wings, the aerodynamic forces acted on the wings and the wing deformation of the flexible wings are experimentally addressed at the Reynolds...[
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Flapping flight of biological flyers provides great inspiration for the development of micro air vehicles (MAVs). However, the unsteady aerodynamics of flapping flight is to be clarified before MAVs can rival nature. In this dissertation, the effects of wing morphology, including the aspect ratio and the wing flexibility, on the flow development and the aerodynamic performance of flapping wings are elucidated. An experimental setup consisting of a water tank, wing and motion modeling system, particle image velocimetry measurement system, force measurement system and deformation measurement system is built. The vortex structures around the flapping wings, the aerodynamic forces acted on the wings and the wing deformation of the flexible wings are experimentally addressed at the Reynolds number regime of (10
3) and a 45° fixed angle of attack. For all aspect ratios, the attached leading edge vortex provides lift enhancement under the flapping kinematics. With increasing aspect ratio, the leading edge vortex enlarges along the wing span and breaks down when it covers the whole chord of the wing. The structure of the leading edge vortex is found to be dominated by the non-dimensional local radius as a key dimensionless parameter and influenced by the tip vortex. With the parameters considered, an optimal aspect ratio exists in view of vortex coherency and the aerodynamic forces. Furthermore, the effects of wing flexibility on the flapping-wing aerodynamics are investigated. The current study shows that flapping wings
with proper flexibility experience pronounced decrease in drag while similar or higher lift is obtained compared to their rigid counterparts. Very flexible wings experience severe deterioration in lift while the drag further decreases. The regime of proper flexibility that offers improved aerodynamic performance is quantified and it matches the stiffness of real insect wings. The flexibility effect strongly correlates with aspect ratio, with high-aspect-ratio wings benefiting from a much larger flexible regime. This implies a novel mechanism of high efficiency for high-aspect-ratio wings when the wing is not confined to 'rigid'. The stiffness of real insect wings supports the correlation between wing flexibility and aspect ratio. The current results can lead to better understanding on the aerodynamics of flapping wings and the aeroelasticity of flexible wings and shed light on the wing design of practical micro air vehicles.
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