Effects of aspect ratio and angle of attack on tip vortex structures and aerodynamic performance for rotating flat plates

Numerical simulations are used to investigate the effects of aspect ratio and angle of attack on the vortex structure, circulation and aerodynamic performance of revolving wings at a low Reynolds number regime (200 ~ 1600). Rectangular wings with various aspect ratio (1 ~ 8) are considered and rotating from rest at angle of attack ranging from 15 to 90 degrees. Simulations are carried out using an in-house immersed-boundary-method-based direct numerical simulation (DNS) solver. A detailed analysis of the vortex formation shows that the general wake pattern near the wingtip shift from a single vortex loop to a pair of counter-rotating vortex loops. This tip vortex evolution was found related to the strength of LEV. In general, the stronger LEV will enhance the counter-pair trailing-edge vorticity. As the TEV increased, a secondary tip vortex will be formed at the bottom corner of wingtip and generate a pair of counter-rotating tip vortex loops. With the increment of the Reynolds number, these vortex loops will interact with each other and form hairpin-like vortical structures in the tail of the arc-shaped vortex loops. In addition to the vortex structures, the aerodynamic loading and power consumption were also examined. The findings from this work could be used to further understand the vortex dynamics of finite-aspect-ratio revolving wings at high angle of attack.