In this paper, a numerical investigation into the aerodynamic performances of a vertical axis wind turbine was conducted, focusing on the semi-circular Savonius rotor. The airflow through the wind turbine is studied within a two dimensional computation domain. The unsteady state regime of the Reynolds-Averaged Navier-Stokes equations, along with the k-ω SST turbulence model, was employed for the numerical simulation. The numerical calculations have been performed using the open source solver: Open Field Operation and Manipulation OpenFOAM. A hybrid grid has been considered to fit the complex geometry of the Savonius rotor. A comparison between the numerical results and experimental findings revealed a satisfactory agreement, with an average relative error of 16%. It was observed that the aerodynamic performance of the Savonius turbine is primarily influenced by the negative torque generated by the returning blade. Nevertheless, further research is needed to mitigate the adverse effects of the returning blade on the turbine’s performance.