Tailoring the actuator line theory to the simulation of Vertical-Axis Wind Turbines

Analysis tools with a fidelity higher than the ubiquitous Blade Element Momentum (BEM) method are needed by now in wind energy; in particular, different research groups have recently proposed the application of the Actuator Line Method (ALM) to wind turbines, to exploit the benefits of an accurate discretization of the wake through Computational Fluid Dynamics and the computational cost saving associated to the lumped parameter modeling of the blade. When applied to Vertical-Axis Darrieus rotors, however, several shortcomings of present models are known to the scientific community, especially regarding the spreading of aerodynamic forces in the domain and the implementation of robust aerodynamic polars and dynamic stall models. Moving from this background, an ALM method numerical model for the simulation of VAWTs has been here developed within the commercial solver ANSYS® FLUENT®. Then, in the effort of tailoring the ALM to this type of machines, different features have been implemented and discussed in the present study, including a novel strategy for the sampling of the angle of attack from the resolved flow field, a sensitivity analysis on the force spreading within the domain and numerous sub-models to account for secondary aerodynamics effects. Attention has been given at ensuring robustness to the implementation of the pivotal modeling of dynamic stall. To prove the effectiveness of proposed solutions, an extensive validation has been carried out on selected test cases, for which both high-fidelity CFD and experimental data were available: a real 2-blade H-Darrieus rotor and a fictitious 1-blade machine. The developed solutions have increased the accuracy of the predicted torque up to 16% with respect to the ALM standard formulation.