The position of wind turbines relative to each other is important in terms of the performance of the turbines. The experiments and CFD studies in the literature have shown that Vertical Axis Wind Turbines (VAWT's) have a higher energy production per unit of land used than Horizontal Axis Wind Turbines (HAWT's) and it is found that there is a performance improvement of the VAWT's when they are operated in pairs. In this thesis, CFD simulations of the H-type VAWT's working in pairs have been perpormed to investigate the energy interaction of the turbines. A standalone one-bladed VAWT was modelled based on the previous studies in the literature for the validation of the CFD methodology. Simulation parameters and simulation settings are compared with the reference study in the mesh independency analysis for four different mesh settings resulting in a deviation of up to %14, and in the time step sensitivity analysis for two different time steps corresponding to 0.25 and 0.5 degrees of azimuthal angle increments resulting in a deviation of up to %15. 1, 2, and 3 bladed stand-alone turbines are investigated to reveal the effect of the inter-turbine blade interaction on the energy output. A pair of co-rotating turbines configuration is analyzed at various Tip Speed Ratios (TSR) (1.7, 2.2, 3.3, 4.4) and at compared with the standalone VAWT for each configuration. The results of CFD simulations show that adding blades to the standalone VAWT results in a more stable moment coefficient, but it also leads to a decrease in the power coefficient at high TSRs. The co-located turbines cause flow disruption for the VAWTs working in pairs operating at unstable TSRs (<2), resulting in a performance reduction of up 13.5%. Increasing the distance between turbines minimize the negative effect of disruption and improves turbine performance. As the TSR increases to a stable operation, the existence of the second turbine affects the energy output of both turbines positively, with the highest performance increase of 46% observed at TSR 3.3 when the turbines were placed closest to each other at 3D. The positive effect of the neighbouring turbine decreases as the distance between the turbines increase and the impact of distance between turbines on performance vanishes for the dewnstream turbine at 8D.