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The MSc thesis concerns the study of the aerodynamics of a horizontal axis wind turbine. Nowadays the design of wind turbines is based on design codes that originate from the Blade Element Momentum Theory (BEMT). This design tools use also correction models which are trying to correct the shortcomings of the above theory. The significant increase of the size of wind turbines stimulates the importance of accurate design. Aim of this study is to model and investigate the physical characteristics of the flow field around a wind turbine. In order to achieve that Computational Fluid Dynamics (CFD) design tools are used. First, a two-dimensional approach was executed using CFD-Reynolds Average Navier-Stokes (CFD-RANS) calculations for NACA4415 airfoil in order to validate the capability of the selected turbulence models, k-ω SST and RSM. The results were compared with experimental data taken from literature as well as with those produced by the airfoil design and analysis tool XFOIL. All CFD calculations were executed using the commercial package ANSYS CFX. Following the twodimensional approach, a design of a blade based on BEMT was performed. The airfoil NACA4415 that was examined in the two-dimensional approach was used for the design of the blade and the open source turbine calculation tool QBLADE was selected for this purpose. The three-dimensional approach in CFD was carried out using a moving reference frame. The calculations were performed for six degrees angle of attack, since for this angle the selected airfoil displayed the highest lift to drag ratio. The rotational effects on the blade were studied using four cross sections along the blade and studying the pressure and velocity distribution. The k-ω SST turbulence model vi presented the better performance. Finally the performance of the wind turbine was investigated and a comparison with the results of the BEMT design and analysis tool was made.