Nowadays, two of the more important challenges for the humanity are the climate change and the management of the energy. For this reason the performance of a machine for the conversion of renewable energy has been studied in this thesis. Specifically, the GAMM Francis turbine runner working at its best efficiency point under cavitation conditions has been analysed using CFX (Ansys®). Firstly, a slice of the 3D fluid domain has been made, using SolidWorks®, benefiting of the runner axisymmetry and thus decreasing the computational power needed. In order to take into account the runner rotation relative to the incoming flow field, three different domains have been defined. Two of the domains have been considered to be still and the other one has been considered to be in rotation around the turbine axis ensuring a model closer to the reality, which has permitted to study the relative and absolute velocity fields. Then, a uniform incoming flow has been simulated through the runner blades under different levels of static pressure at the draft tube outlet boundary condition. The pressure has been gradually decreased, and consequently the sigma coefficient of cavitation, until the vapour pressure has been reached inside the runner. Thus, the onset of blade cavitation and its growth has been studied while keeping the remaining parameters unchanged. Based on the numerical results, the value of the torque, the pressure coefficient and the shape, localization and size of the cavity have been analysed according to the sigma coefficient. To finish a sensitivity study of the empirical parameters that the cavitation model takes into account has been carried out based on their impact on the cavitation results.