Vorticity dynamics of fully developed leading-edge vortices on revolving wings undergoing pitch-up maneuvers

Abstract

In nature, birds and insects usually execute pitch-up maneuvers, which is either an active perching or a passive response against gusts. During such maneuvers, their wings flap with a concomitant nose-up rotation around an axis, and thus, both vortex structures and aerodynamic forces of the wings are influenced. This research focuses on the impact of pitch-up maneuvers on the evolution and underlying vorticity dynamics of a fully developed leading-edge vortex (LEV), which has received limited interest in previous research. Based on data obtained from numerical simulations, an analysis of vortex dynamics and vorticity transport is conducted at different pitch rates and pitch axis locations. Our findings show that an increase in pitch rate and a shift of pitch axis toward the trailing edge can both terminate the growth of LEV and then initiate its movement toward the trailing edge via strong downward convection. However, the contributions of spanwise convection and vortex stretching (or compression) are distinct in these two scenarios, leading to different lift generations. Other vortex-tilting-based mechanisms, e.g., the planetary vorticity tilting and the dual-stage radial-tangential vortex tilting, are reduced during pitch-up maneuvers. Moreover, a rapid pitch-up around the leading edge is encouraged to maximize the lift during the maneuver, although this should be accompanied by constraints in flight height and kinetic energy when being applied to guide the perching of bio-inspired flapping wing micro air vehicles.