Large eddy simulation of the wind turbine wake characteristics in the numerical wind tunnel model

Abstract

Large Eddy Simulation of NREL Phase VI wind turbine was performed in a virtual wind tunnel (24.4m×36.6m) in order to achieve a better understanding of the turbine wake characteristics. For this purpose, ANSYS-Fluent package was used to run the simulation using the dynamic Smagorinsky-Lilly model. For the purpose of validation, the pressure distribution at different span-wise sections along the turbine blade and the power produced by the wind turbine were compared with the published experimental results for the NREL phase VI rotor tested in the NASA wind tunnel with the same dimensions as in the model and a good agreement was found between the two. The airflow immediately behind the wind turbine was observed to be a system of intense and stable rotating helical vortices, which determined the dynamics of the far-wake. The system of vortices in the near-wake became unstable and broke down due to wake instability at a distance of five rotor diameters downstream of the wind turbine. This was defined as the boundary between the near- and far-wake regions. The collapsed spiral wake was found to spread in all directions in the far-wake resulting in the formation of the two pairs of counter-rotating vortices which caused the gradual increase of turbulence in these regions. The turbulence intensity in the wake was observed to increase immediately behind the turbine with a maximum of 12.12% at a distance of three rotor diameters downstream of the turbine, after which a gradual decrease in the turbulence intensity was observed in the near-wake regions due to wake instability. However, in the far-wake regions, due to counter-rotating vortices formed by the wake instability, the turbulence intensity showed a tendency to increase intensity. Finally the time-averaged wake velocities from the LES, with and without the blockage corrections, were compared with WAsP and a comparatively good agreement for the axial velocity predictions was observed in the far-wake.