Near-wake behavior of an asymmetric wind turbine rotor

With symmetric rotors, tip vortex helices develop regularly before experiencing the leapfrogging instability. This instability can occur earlier when the consecutive helices are radially offset, which is the case for a rotor with non-identical blade lengths. Inspired by this, the current study investigates the spatiotemporal development of near-wake behavior for a rotor with significant blade length differences. Large-eddy simulations with an actuator line model are performed on a two-bladed wind turbine rotor under laminar and turbulent inflow conditions to evaluate the impacts of blade length differences ranging from 0 % to 30 % of its radius. The study analyzes the formation and development of helical tip vortices, the onset of leapfrogging, and the growth rate of this instability. The results show that the relative distance where leapfrogging takes place and the growth rate of the leapfrogging instability both decrease with increasing blade length difference, which agrees fairly well with the prediction of the two-dimensional point vortex model. The results also reveal that the effects of inflow turbulence on the leapfrogging instability are minimal in the context of the growth rate. While the considered rotor asymmetries accelerate the leapfrogging, the outcomes demonstrate that the leapfrogging does not necessarily induce large-scale breakdowns of the helical vortex system and has little impact on the wake recovery rate. Particularly, this work discovers that the inflow turbulence plays a dominating role in wake recovery, promoting the breakdown of helical tip vortices regardless of rotor asymmetry.