Symmetric Versus Asymmetric Pitching of a Cycloidal Rotor Blade at Ultra-Low Reynolds Numbers

This paper provides a fundamental understanding of the unsteady aerodynamic phenomena on a cycloidal rotor blade operating with symmetric as well as asymmetric pitching at ultralow Reynolds numbers (Re approximate to 18,000) by using a combination of experimental (force and flowfield measurements) and computational fluid dynamics (CFD) studies. The instantaneous blade fluid dynamic forces on a rotating cycloidal rotor blade were measured, which, along with particle image velocimetry (PIV) based flowfield measurements revealed the key fluid dynamic mechanisms acting on the blade. A two-dimensional CFD analysis of the cycloidal rotor was developed and systematically validated using both force and flowfield measurements. The CFD flow solution and PIV-measured flowfield correlated well, especially in the upper half of the circular blade trajectory, and both showed the formation and shedding of strong dynamic stall or leading-edge vortices at high pitch amplitudes, which is the reason for the stall delay and force enhancement. The dynamic virtual camber induced by the flow curvature decreased the blade lift in the upper half due to negative induced camber; however, the lift increased in the lower half due to positive camber. Therefore, introducing an asymmetry in pitch kinematics with higher pitch in the upper half and lower pitch in the bottom half counteracted this inherent lift asymmetry and improved efficiency through a more uniform lift production.