Flow Field and Performance Measurements of a Flapping-Wing Device Using Particle Image Velocimetry
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A flexible flapping wing was tested using various flow interrogation techniques including particle image velocimetry (PIV) to further the understanding of its complex, unsteady, three-dimensional flow field. The flow field was characterized using high-speed flow visualization (FV) in chordwise and spanwise planes to observe and characterize the evolution of flow structures produced. The formation, growth, convection, and shedding of a leading-edge vortex (LEV) was observed on the upper surface of the wing, which was found to mimic the classical process of dynamic stall. A motion-tracking system was used to characterize the complex wing kinematics and aeroelastic deformations of the flexible wing. These measurements were then used to estimate the noncirculatory forces and moments acting on the wing. Two-dimensional velocity fields around the wing contour and in its wake were obtained using PIV. These velocity fields were used to calculate the circulatory lift as well as the drag produced on the wing. It was found that the process of LEV formation, growth, and convection significantly increased the lift production on the flapping wing. The noncirculatory and circulatory lift measurements were then combined in amplitude and phase to calculate the total lift on the wing. It was shown that the noncirculatory contributions to the airloads were small except near pronation and supination. The flow field results were also used to calculate the lift-to-drag ratio during the wing stroke, where surprisingly it was found that the lift-to-drag ratio increased during the process of LEV formation and shedding. This observation perhaps suggests a reason why flapping-wing flyers intentionally produce LEVs during their wing stroke.