Free-vortex models for wind turbine wakes under yaw misalignment – a validation study on far-wake effects

Abstract. Near-wake effects of wind turbine models using the free-vortex wake have been extensively studied, but there is a lack of validation for such predictions in the mid to far wake. This paper presents a novel validation study using three free-vortex wake models of increasing complexity: an actuator disc, an actuator disc with rotation, and an actuator-line model. We emphasise the application for dynamic wind farm flow control optimisation with a focus on wake redirection using yaw misalignment. For that purpose, surrogate wake models should provide sufficiently accurate power predictions at low computational expense. Three sets of wind tunnel data are used for validation: flow measurements under steady yaw misalignment, time-resolved flow measurements for a step change in yaw, and turbine output measurements with yaw control and simulated wind direction variation. Results indicate that the actuator-disc model provides the best balance of computational cost and accuracy in power predictions for the mid to far wake, which is not significantly improved upon by the addition of rotation. In the near wake, the added complexity of the actuator-line model may provide value as it models blade loading and individual tip vortices. Altogether, this study conclusively demonstrates that the actuator-disc model is suitable for yaw control optimisation and provides important validation for further studies into optimisation of wake steering under time-varying conditions.