Cycloidal Rotor-Blade Tip-Vortex Analysis at Low Reynolds Number

Abstract

This study provides the first in-depth analysis of the formation, strength, and convection of cycloidal rotor tip vortices. The measurements of blade force and tip vortex using particle image velocimetry were conducted for different blade aspect ratios and pitch kinematics at a chord-based Reynolds number of 18,000. Particle image velocimetry was employed to investigate vortex development at increasing vortex ages and the early development for varying azimuthal locations. The instantaneous blade force measurements on the cycloidal rotor showed a decrease in nondimensional lift with decreasing blade aspect ratio. The aspect ratio of the blade did not affect the shape of the vortex convection trajectory. However, the rate of downward convection increased with increasing aspect ratio due to the higher thrust produced. The tip vortices showed self-similarity in the velocity and the circulation profiles. The measurements indicate that the vortex core experiences a logarithmic growth and the swirl velocity experiences a logarithmic decay due to viscous diffusion. The tip-vortex strength varied cyclically with blade azimuthal location due to the cyclic variation of blade pitch angle and the dynamic virtual camber effects. This periodic variation in the tip-vortex strength leads to a periodic variation in the induced flow velocity on the blade.