Aerodynamic characteristics and vortex ring features of propeller in vertical descent

Propellers are susceptible to entering the perilous Vortex Ring State (VRS) during vertical descent, a phenomenon that results in complex aerodynamic characteristics that remain incompletely understood. Utilizing the Viscous Vortex Particle Method (VVPM), a thorough investigation is conducted on the aerodynamic characteristics and vortex ring features of the APC thin electric 10X7 propeller at different vertical descent rates. Results show that the propeller's aerodynamic characteristics are highly susceptible to the change in the descent rate. Specifically, as the vertical descent rate increases, the thrust initially experiences a slight increase, followed by a decrease, and ultimately a subsequent increase. Concurrently, the wake compression effect becomes progressively more pronounced, and the vortex ring center gradually shifts upwards. During this process, the vortex ring state initially hovers, evolves gradually to the fully developed state, and ultimately transforms into a windmill-like flow state. The wake fields of the propeller gradually change from tubular helicoidal downwash vortical structures to the coexistence of tubular and toroidal vortical structures, and then to the fully-toroidal ones, which finally move completely to the top of the propeller disk. Such a topological evolution of the vortex field has a direct impact on the propeller's aerodynamic characteristics. Tubular helicoidal vortical structures exert negligible influence, while toroidal vortical structures have a significant impact as the descent rate increases, evident in a sudden thrust decrease, the enhancement of the wake compression effect, the encirclement of propeller discs by backflows, and the increased aerodynamic force.