Aerodynamic Studies of Vertical Axis Wind Turbines using the Actuator Line Model

Sammanfattning: This thesis addresses the unsteady aerodynamics involved in the operation of vertical axis wind turbines (VAWTs). The main focus is to represent and understand the most relevant phenomena within the resulting flow pattern as the wake structure, loads on the different turbine components and the performance of the rotor. An actuator line model has been used for this purpose.This model has been validated against experimental measurements from diverse cases with different operating conditions in both confined wind tunnels and open site locations. Numerical works were carried out considering a wide range of tip speed ratios (TSRs), and therefore covering from the no stall to the deep stall regime. The latter requires the implementation of a dynamic stall model for the proper representation of the unsteady forces on the blades. Also, different inlet conditions such as a uniform flow, a logarithmic wind shear and an atmospheric boundary layer (ABL) have been tested. The so-called recycling method technique was used to produce the fully developed ABL flow. Additionally, the resulting wake and performance of interacting turbines has been studied.Once the model was validated, two numerical study cases for large scale turbines were carried out. First, the performance and resulting flow field from both a horizontal axis wind turbine (HAWT) and VAWT were investigated when the turbines were operating at their optimal TSR and within the same ABL inflow boundary conditions. The influence of the variation on the atmospheric turbulence levels was also studied, as well as the differences and similarities on the obtained results for both type of turbines. Later, the performance improvement of two interacting VAWTs was investigated through the deflected wake produced by the pitched struts of the upstream turbine. This is presented as a novel mechanism to mitigate losses on interacting turbine arrangements (i.e. wind farms).In general, there is a reasonable good agreement between numerical results and experimental measurements, and therefore, the applied ALM can be considered as a potential tool for VAWTs simulations, characterized by relatively low computational cost showing accuracy and numerical stability.