Vertical axis wind turbines : a 2D unsteady free wake panel method based analysis

In wind engineering, one of the most important goals is to transform the energy available in the wind into a usable form of energy, such as electricity, as efficiently as possible. In a wind turbine, this purpose is accomplished by extracting the kinetic energy of the wind and converting it into a usable torque. There exists a wide variety of concepts to achieve this conversion, each with its own strengths and weaknesses. Among these, there is the Vertical Axis Wind Turbine (VAWT), whose complex aerodynamics pose many challenges for researchers and designers and are the subject of this thesis. Therefore, the aim of this research work is to predict the aerodynamic performances of VAWT using the 2D unsteady Panel Method based on potential flow in its ’free wake’ formulation. The present method is a conventional 2D panel code, in which a combination of sources and doublets is distributed onto the airfoil, and the near-wake is represented with doublets, while the mid and far wake are modelled with vortex points. The geometry of the wake is not known a priori from the assumptions based on experimental evidence, but is created by considerations of self-induced velocity. After an accurate analysis of the equations upon which the panel method is based, the computation of the pressure coefficient has been modified taking into account the velocity component due to wake induction. Moreover, the integration of the incoming flow condition to the panel has been done by modifying the ’wind script’ in order to obtain the correct value of the wind speed on each control point. In the analysis, several useful values such as the tangential and normal forces, the attack angle and the relative velocity have been calculated using the new added functions. The performances of the model have been tested by comparing it to two numerical models: Double Multiple Stream Tube (BEM) and Actuator Line (CFD). Under specific conditions (low solidity value), the models present the same trend of the main physics parameters, as blade forces and relative velocity. Nevertheless, no model is able, for different reasons, to capture correctly all the aerodynamic aspects of the airflow. Finally, it has been proved that the panel code is a suitable tool to investigate the aerodynamic behaviour of VAWT, when the assumption of potential flow is reasonable.