Due to intermittentcy in electrical grids introduced with ever larger share of renewable power sources, hydroelectric plant operation saw an increase in load variations and overall off-design operation at prolonged time as they are expected to deliver regulating power for the electric grid. Frequent load variations increased the dynamic loading of turbine parts, particularly runner, affecting the operating life and increasing maintenance-related cost. Detailed studies of transient operating regimes are therefore necessary. In present work, methods of computational fluid mechanics are applied on Francis 99 high head turbine using the open source finite volume code foam-extend. Simulations of both steady state and transient turbine operation were performed. Best efficiency operating point is simulated with both steady-state solver with frozen rotor approach, as well as a transient dynamic mesh solver implementing sliding grid approach. The steady operation simulations were conducted first to test the created mesh and computational modeling choices. Load variation simulations are implemented for load reduction phase, where discharge is regulated by closing of guide vanes from best efficiency point to part load. Two approaches with custom boundary conditions were tested for the domain inlet, one with prescribed head (total pressure) and another with prescribed flow rate (velocity). The simulation with prescribed head showed good agreement with pressure measurements and realistic trend of other integral values. Two well established turbulence models were used in present work, namely the "standard" k − ε and k − ω SST . All parameters in setup of presented simulations were adjusted to the corresponding conditions during experiments on the Tokke turbine model, results of which were made available by the Francis 99 workshop.