Wind energy is a clean, renewable, and cost-effective alternative to conventional power sources. It does not require fuel, does not emit pollution, and can be installed near the places where electricity is needed, reducing transmission losses. Vertical Axis Wind Turbines (VAWTs) are a type of wind energy technology that have some advantages over Horizontal Axis Wind Turbines (HAWTs), such as being able to able to harvest wind from all directions, generate less noise, and offering a simpler structure. However, VAWTs also have lower efficiency than HAWTs, and their performance is affected by the wake, which is a lower-velocity turbulent flow behind the rotating blades. 
The wake can interfere with the operation of other wind turbines downstream, but it can also enhance the performance of smaller VAWTs. The aim of this research is to use Computational Fluid Dynamics (CFD) to study the effect of wake on different sizes of VAWTs, and to understand how the turbulence generated by the wake influences the power output of these turbines.  This thesis presents a CFD methodology and identifies the strengths and weaknesses of CFD for the simulation of the interaction of the wake with downstream VAWTs. The contributions are threefold. First, the understanding of how turbulence intensity affects the VAWT’s performance. Second, calculating the performance of a wind turbine that is in the wake of another turbine and the study of some particular VAWTs placement configurations. Third, quantifying the limitations of CFD and identifying when it is appropriate to use two-dimensional models for flow simulations of multiple VAWTs. This research has documented an increased performance of about 20% for small turbines in high turbulence intensity flows. Furthermore, a 20% increase in power output from an optimization array of VAWTs was identified. Finally, this work suggests that two-dimensional CFD simulations are adequate for simulation pairs of upstream and downstream turbines if the turbines are low-solidity and low aspect ratio turbine types.