Modelling the nacelle wake of a horizontal-axis wind turbine under different yaw conditions

Recently, actuator line model become popular in studying wind-turbine wakes. However, existing models ignore or inaccurately describe nacelle effects, which have been shown to pose significantly impact on wakes. To address the physics underlying here, we develop the actuator line model with large-eddy simulation by introducing a new anisotropic body-force projection model. We validate the new model against a field experiment and the validation indicates that the new anisotropic model can predict the wake more precise than the existing isotropic model. Furthermore, we extend the study to wake characteristics under various yaw conditions. The results show that the thrust component normal to the flow direction creates a skewed wake behind the turbine, which in turn promotes the wake transition from the two-peak profile to the one-peak profile. The wake skew exacerbates the instability of the tip vortex and causes the wake region to narrow. At small yaw angles, the nacelle vortex radially diffuses and blends with the tip vortex in the far wake. At large yaw angles, the nacelle vortex intercepts the tip vortex in the near wake due to the different spatial distribution of thrust. It is concluded that the nacelle significantly affects wind-turbine wakes especially during yaw condition.