Development of a BEM-CFD tool for Vertical Axis Wind Turbines based on the Actuator Disk Model

The present work focuses on the numerical simulation of Vertical Axis Wind Turbines by means of an in-house BEM-based User Defined Function to be used 39ith the CFD code ANSYS Fluent. Typical VAWT unsteady and 3D phenomena, such as dynamic stall, flow curvature and tip losses, are taken into account by original and literature-based sub-models. The presence of the blades is mimicked by replacing them with suitable momentum sources. For the present work, the Actuator Cylinder Model has been employed. 3D analysis, of a SANDIA rotor, are carried out in order to assess the accuracy of our model against numerical simulations and experimental data. The current User Defined Function is able to give a satisfactory agreement with the reference cases especially from a qualitative point of view, with a significant computational time reduction to a factor of 10 compared to the case with the moving bodies. A typical wake behaviour can be noticed in our simulations even though its recovery is strongly influenced by the lack of turbulence inherent to the chosen actuator model. The torque and the power coefficient of the turbines are also analysed and compared against the reference cases, finding a remarkable agreement. The model has been successfully applied to predict the transient aerodynamic loads of an offshore 5MW troposkein turbine sub- jected to the pitching motion of its platform. The operating conditions have been chosen in order to allow a qualitative comparison with a floating 5MW horizontal axis turbine which performance under pitching motion is available in literature.