Effects of Aspect Ratio on Vortex Dynamics of a Rotating Wing

Abstract

The effects of aspect ratio on the leading-edge vortex, tip vortex, and their interactions under sinusoidal rotating kinematics, simulating a half-stroke of a flapping wing, are investigated using phase-locked particle image velocimetry. Measurements are conducted on rectangular wings with aspect ratios of 1, 2, and 4, while holding a constant Reynolds number and a fixed 45 deg angle of attack. In the inner span region, the center of the leading-edge vortex is found moving along a dimensionless trajectory when normalized by the wing chord. In the outer span region, the trajectory is influenced by the tip vortex and, in turn, by the aspect ratio. The tip vortex is found to have benign effects by suppressing the leading-edge vortex to the wing surface in the inboard region via its downwash and by making the Kutta condition satisfied in the wingtip region. At the late stage, the leading-edge vortex breakdown on the high-aspect-ratio wing yields the breakdown of the tip vortex, followed by liftoff of the leading-edge vortex in the inboard region. Force measurement shows larger added-mass effect on lower-aspect-ratio wings, whereas wings with higher aspect ratio show higher mean lift coefficient and lift-to-drag ratio due to the larger vortex lift in the extended outboard region. An optimal aspect ratio is estimated to be around 2.8 in view of leading-edge vortex coherency and attachment, consistent with flies and bees.