As hydraulic turbines utilize water, a renewable energy, to perform, considering various operating conditions of these turbines is of great importance. Due to the variation of electricity demand and also the performance head of the turbine, investigating turbine performance at different conditions would lead to a better understanding of the flow phenomenons inside them. One of the main factors decreasing water turbine's efficiency is the creation of a Rotating Vortex Rope (RVR) inside the draft tube at part load condition. RVR, an unstable and strong vortex, emerges due to the existence of stalled regions inside the draft tube at part load and it decreases the available area of the conical diffuser as well as the efficiency of the turbine. On the other hand, RVR induces severe pressure fluctuations inside the draft tube that threatens the turbines behavior by the danger of cavitation and resonance. Therefore, in this project, using Computational Fluid Dynamic approaches by the means of ANSYS-CFX software, an example of Francis turbine is simulated. The aim of this project is to understand the mechanism(s) of RVR formation during the changes in operating condition from BEP to part load and a reduced model of turbine was used in the simulations. Numerical simulation is first performed at the best efficiency point as well as part load to ensure the appropriate employment of turbulence models and boundary conditions. In the second step, the inlet boundary conditions are changed linearly from BEP to part load in order to achieve the transient conditions inside the draft tube. The initial condition of the second step is the converged BEP result. The transient simulation is continued until the RVR is fully developed in the draft tube at part load condition. The numerical results are in a good agreement with the experimental data. The effect of the RVR is considered from two aspects. The first one is the frequency, and the amplitude of pressure pulsations induced by the RVR in the draft tube. The second one is the velocity field in the draft tube which is investigated over time and over space during load rejection. Moreover, the flow structure is visualized using the λ2 criterion. The mechanism(s) of RVR formation and damping is accurately investigated by the presented approach. Furthermore, the results provide a better understanding of the physics behind the RVR formation. The obtained results aim to design an effective RVR controlling approach.