Numerical investigation of rotor asymmetry to promote wake recovery

Abstract

Tip vortices in the wakes of wind turbines are known to have detrimental effects on downstream turbines such as reduced performance and increased fatigue loading. Rotor asymmetry is investigated as a passive method for mitigating these effects by triggering the helical vortex pairing instability. The study is conducted using MIRAS, a multi-fidelity vortex solver, to compare the wakes of the standard NREL 5MW turbine and a modified asymmetric version where one blade is extended radially relative to the other two. The asymmetric rotor is shown to successfully trigger the vortex instability, increasing the wake average velocity by a maximum of 3.5% and the power available to a downstream turbine by up to 11%. The turbulence in the wake of the asymmetric rotor is also modified, exhibiting enhanced mixing. Using the available power gains from the simulations and operational data from the Lillgrund wind farm, the total impact of rotor asymmetry on wind farm efficiency is estimated, showing increases > 2% under certain wind conditions. The findings of this study indicate that rotor asymmetry has strong potential as a wake control method and would benefit from further investigation to understand the effects of inflow turbulence and the impacts on rotor loading.