Near wake behavior of an asymmetric wind turbine rotor

With symmetric rotors, tip vortex helices develop regularly before interacting, following the leapfrogging instability. This instability can occur earlier when the helices are radially offset by using blades of different lengths. This study investigates the spatio-temporal development of near-wake behavior for rotors with a significant blade length difference. Large eddy simulations with an actuator line model were conducted on a modified NREL 5MW wind turbine under both laminar and turbulent inflow conditions, to evaluate the impact of blade length differences ranging from 5 to 30 %. The study analyzed the development of tip vortex helices, the onset of leapfrogging, vortex merging, and, ultimately, their three-dimensional breakdown. The analysis is corroborated using a simplified two-dimensional point vortex model. The results show that the leapfrogging process begins immediately downstream of the vortex release when blades of different lengths are considered. The instability growth rate obtained from the 2D vortex model agrees with the LES results. Although the rotor asymmetry accelerates the leapfrogging and, in some conditions, also the vortex merging process, it proves insufficient to cause a large-scale breakdown of the helix system and, therefore, enhance wake recovery. Inflow turbulence, however, plays a larger role in wake recovery, promoting the breakdown of tip helical vortices regardless of rotor symmetry.