Abstract
Wakes behind rotors such as wind turbines, propellers, and helicopters can have detrimental effects on downstream structures including increased structural loading. These wakes are characterized by helical vortices shed from the blade tips, which are subject to instabilities that cause the vortices to break down, reducing the strength of the wake. One type of instability, vortex pairing, can be triggered by introducing a slight asymmetry to the rotor. The current study investigates the effectiveness of different types of rotor asymmetries at triggering the pairing instability through an experimental campaign involving 14 rotor configurations. The leapfrogging distance, or the point where adjacent vortex loops swap positions, is used as a metric to compare the different types of asymmetries. For all cases tested, leapfrogging occurs between 4.1 and 1.8 rotor radii downstream of the rotor for the baseline and most perturbed configurations, respectively. Even a small perturbation of 6% of the vortex spacing reduces the leapfrogging distance by 25% relative to the baseline configuration. Some of the most interesting similarities and differences between cases are discussed in more detail, including the analogy between azimuthal and axial displacements, the importance of perturbation direction, and the effects of blade devices such as winglets and fins on the vortex system. The findings of the current study can be used to design rotors that passively accelerate wake recovery, mitigating detrimental effects on downstream structures.