This paper defines and describes hydraulic machines, main types of hydraulic machines and the basic characteristics for each type. Water turbines with double regulation, which are known as Kaplan turbines, as well as the way these turbines work is explained in further details. Furthermore, this paper describes the basis of fluid dynamics with the main types of fluid flow. It also presents the ways the computer software for fluid flow simulations (eng. Computer Fluid Dynamics - CFD) work and the procedure that must be followed to obtain correct and successful computer simulation. A computer model of real Kaplan turbine plant that is going to be installed in the city of Otočac is shown. For this model, a sequence of CFD simulations was conducted. Through these simulations, it was investigated how different flow rates, different number of turbine runner revolutions and different angles of turbine wicket gates influenced the utility and power on the water turbine. Additionally, for one fixed flow rate and angel of wicket gates the row of changes of runner revolution was made. The impact on turbine utility was monitored for each different number of revolutions. Those results of utility for different runner revolutions were used to make a graph that shows dependency of runner revolution and utility. That kind of graph allows us to find the optimal working conditions of turbine for fixed flow rate and angle of wicket gate. In the end, the analysis of obtained results was conducted.