The aim of this research is to understand the mechanism(s) of RVR formation duringthe changes in operating condition from the Best Efficiency Point (BEP) to Part Load (PL). AComputational Fluid Dynamic (CFD) methodology by the means of ANSYS-CFX is applied ona reduced high head Francis turbine model. The reduced model consists of one stay vane, twoguide vanes, one runner blade, one splitter and a full draft tube. Numerical simulation is firstperformed at BEP as well as PL to ensure the appropriate employment of turbulence models andboundary conditions. In the second step, the inlet boundary conditions are changed linearly fromBEP to PL in order to achieve the transient conditions inside the draft tube. The initial conditionof the second step is the converged BEP result. The transient simulation is continued until theRVR is fully developed in the draft tube at part load condition. The numerical results for BEP,PL and BEP to PL are in a good agreement with the experimental data. The effect of the RVR isconsidered from two aspects. The first one is the frequency, and the amplitude of the pressurepulsations induced by the RVR in the draft tube. The second one is the velocity field in the drafttube which is investigated over time during load rejection. Moreover, the flow structure isvisualized using the λ2 criterion. The mechanism(s) of RVR formation and damping is accuratelyinvestigated by the presented approach. Furthermore, the results provide a better understandingof the physics behind the RVR formation. The obtained results aim to design an effective RVRcontrolling approach.