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 centered 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 as well as 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.