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Unsteady Lift Generation for MAVs

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Final rept. 1 Aug 2007-1 Aug 2010

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The aim of the current research is to understand unsteady lift generation on three-dimensional flapping wings in the MAV flight regime and, if a leading edge vortex develops at MAV-like Reynolds numbers, characterize the time-scale of the development and stability of the vortex. The waving wing experiment is a model of the translational phase of the insect wing stroke. It was designed to produce a simple flow-field while preserving flow unsteadiness, threedimensionality, and wing rotation. The wing rotates in a propeller-like motion through a wing stroke angle up to 90 degrees. Unsteady lift and drag force data was acquired throughout the wing stroke and particle image velocimetry PIV was used to measure the velocity field and quantify the circulation of the leading edge vortex. The shape of the lift curve was similar for all cases tested. A transient high lift peak approximately 1.5 times the quasi-steady value occurred in the first chord-length of travel, caused by the formation of a leading edge vortex. This vortex then separated from the leading edge resulting in a sharp drop in lift. Subsequent vortices continued to form, shed, and move downstream over the wing as lift values recovered to an intermediate value. The circulation of the LEV has been measured and agrees well with force data. Wing kinematics had only a small effect on the aerodynamic forces produced by the waving wing. In the early stages of the wing stroke, velocity profiles with low accelerations affected the timing and magnitude of the lift peak but at higher accelerations the velocity profile was insignificant. Variations in angle of attack between 5 and 45 degrees, Reynolds number between 10,000 and 60,000, and aspect ratio 2 or 4 did not affect the fundamental structure of the flow. Unlike previous studies at much lower Reynolds numbers, there is no evidence of an attached leading edge vortex over an extended period of time.

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  • Aerodynamics

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