The Effect of Flow Sampling on the Robustness of the Actuator Line Method

ABSTRACT The actuator line method (ALM) is a widely used tool for the modelling of horizontal axis turbines and wind and tidal farm flows. The method uses a virtual blade representation to simulate the dynamics of wind turbines without the computational expense associated with resolving the blade geometry. Within the ALM, the flow is first sampled at each blade section, allowing for the calculation of the sectional lift and drag forces that are in turn imposed on the flow domain using a smearing kernel. In this work, the effect of the flow sampling method on the robustness of the ALM is discussed. Implementations of two widely used types of methods for horizontal axis wind turbines are tested: point sampling, where the flow is sampled at or near the collocation point, and volume average sampling where flow field information from multiple points centred at the collocation point is averaged into a representative blade‐local velocity vector. A third method, line average sampling, that was initially proposed for sampling flows from blade‐resolved simulations is adapted for the ALM framework. This method samples the flow symmetrically around a control element in order to eliminate the interference of the bound circulation, thus allowing the inflow velocity at the aerofoil to be determined. When evaluating power and thrust coefficients and the flow field at the rotor plane predicted by the ALM, the line average sampling approach is demonstrated to be more robust and converges faster with time step for two different rotors: a small wind rotor and a high‐solidity tidal rotor. This suggests that carefully selecting the sampling method could be key in alleviating the very strict time‐step restriction imposed by the ALM, a widely acknowledged limitation of the method. Such an advancement could contribute towards improving the computational efficiency and tractability of the method.