Vorticity dynamics and stability of the leading-edge vortex on revolving wings

The leading-edge vortex (LEV) is well known for its contribution to the high-lift generation in a wide variety of biological organisms, such as flying insects, auto-rotating samaras, and gliding snakes. Based on revolving wings, the temporal–spatial evolution of the LEV, including the fundamental vorticity dynamics and stabilizing mechanisms, is reviewed here, considering the effects of Reynolds number (Re), Rossby number (Ro), and aspect ratio (AR). The literature agrees that the saturation of LEV intensity at the steady state can be predicted by the chord length of travel at the radius of gyration, which falls between 2 and 4 within a large variety of wing geometries and kinematics. In contrast, the lift almost arrives at a constant value by the end of acceleration. These findings indicate distinct mechanisms for the steady-state LEV vorticity and constant lift. For the stabilizing mechanisms of LEV, four existing hypotheses are reviewed, followed by the introduction of a novel vorticity transport-based perspective. Two vortex-tilting-based mechanisms, named planetary vorticity tilting and dual-stage radial-tangential vortex tilting, were recently proposed to expand our understanding of LEV stability. It is concluded that the vorticity transport inside the LEV is strongly correlated with the local Ro as well as Re and AR. This review presents a comprehensive summary of existing work on LEV dynamics, stabilizing mechanisms, and high-lift generation.