Turbulent wake study of the NREL phase VI wind turbine in a virtual wind tunnel using Large Eddy Simulation

Abstract

The objective of the current investigation is to achieve a better understanding of the turbulent wake characteristics behind a wind turbine for the purpose of the improvement of the overall wind farm efficiency and a better prediction of the fatigue loads on tandem wind turbines. Large Eddy Simulation was performed at a low wind speed of 7m/s in a numerical wind tunnel model with dimensions similar to the NASA Ames Wind Tunnel (24.4 m x 36.6 m). An ANSYS CFD Fluent solver based on the dynamic Smagorinsky-Lilly model was employed to simulate the flow surrounding the wind turbine placed inside the numerical wind tunnel. For validation purposes, the aerodynamic characteristics of the blade were compared with the published experimental data of the NREL Phase VI. A good agreement was found between the experimental and the numerical results for the surface pressure distributions along the blade. It was observed that in the near-wake the turbulence intensity was a maximum of 12.12% at a distance of three rotor diameters downstream, after which a gradual decrease in the turbulence intensity was observed due to the wake instability. In the far-wake region, the turbulence intensity showed a tendency to increase, due to the presence of counter-rotating helical vortices determining the dynamics of far-wake. The system of vortices in the near-wake becomes unstable and breaks down due to the wake instability at a distance of approximately five rotor diameters downstream of the wind turbine. The vortex breakdown was used to define 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 counter-rotating vortices which caused the gradual increase of turbulent intensity. The results and related discussion of the turbulent wake characteristics are presented in the current article.