Wake Structure Diagnostics of a Flapping Wing MAV 2005-01-3198
Experiments were performed to better understand the aerodynamic flow field of a flapping-wing micro air vehicle. High-resolution laser sheet flow visualization and particle image velocimetry (PIV) analyses have shown the presence of folded vortex filaments that are trailed from the tip and root of the wing, which are combined with a shed dynamic stall vortex with a strong spanwise flow toward the wing tip. This leading-edge vortex gains strength as the translational motion of the wing accelerates through mid-stroke. There is a subsequent shedding of this vortex, but with the simultaneous formation of another leading-edge vortex. The generation of the second vortex occurs before the first vortex reaches mid-chord, enhancing overall lift. This second vortex moves along the chord during supination, before finally being shed from the trailing-edge of the wing. A starting vortex forms near the trailing-edge as the wing starts to accelerate during the downstroke/upstroke of the flapping cycle. This starting vortex grows larger in size, gaining energy from further shed vortices, until the wing reaches the mid-point of the cycle. The folded root and tip vortices that trail from the flapping wing have been found to be relatively strong, and move inward and axially downward as the wing moves through its flapping cycle. The close proximity of the starting vortex, as well as the trailed root and tip vortices, has a large influence on the downwash over the wing. This suggests that any modeling techniques used to predict the lift on flapping wings must fundamentally take into account the three-dimensional, unsteady effects associated with its complex vortex wake structure.