Experimental and Computational Studies to Understand Unsteady Aerodynamics of Cycloidal Rotors in Hover at Ultra-low Reynolds Numbers

This paper provides a fundamental understanding of the unsteady aerodynamic phenomena on a cycloidal rotor blade operating at ultra-low Reynolds numbers (Re∼18,000) by utilizing a combination of experimental (force and flowfield measurements) and computational (CFD) studies. For the first time ever, the instantaneous blade fluid dynamic forces on a rotating cyclorotor blade were measured, which, along with PIV-based flowfield measurements revealed the key fluid dynamic mechanisms acting on the blade. A 2D CFD analysis of the cycloidal rotor was developed and systematically validated using both force and flowfield measurements. Studies were performed with both static and dynamic blade pitching. Direct comparison of the static and dynamic pitch experimental results helped isolate the unsteady phenomena (such as dynamic stall, unsteady virtual camber, etc.) from the steady effects. The dynamic blade force coefficients were almost double the static ones clearly indicating the role of unsteady mechanisms on force production on cyclorotor blades. For the dynamic case, the blade lift monotonically increased even up to ±45° pitch amplitude due to dynamic stall phenomenon; however, as expected, for the static case, the flow separated from the leading edge after around 15° with large laminar separation bubble (LSB) and eventually completely separating at higher pitch angles. For both static and dynamic pitching cases, there was significant asymmetry in the lift and drag coefficients between positive and negative pitch angles due to the flow curvature effects (virtual camber). CFD flow solution and PIV measured flowfield correlated well and both showed the formation and shedding of strong dynamic stall or leading edge vortices, especially at higher pitch amplitudes, which is the reason for the stall delay and force enhancement. Also, the dynamic stall process during the upper half of the trajectory was significantly different from the lower half even with symmetric blade pitch kinematics because of the reversal of dynamic virtual camber from the upper to the lower half. Even at such low Reynolds numbers the pressure forces, as opposed to viscous forces, were found to be dominant on the cyclorotor blade. The power required for rotation (rather than pitching power) was the domineering component of the total blade power for the dynamic pitching case.