Numerical Modelling of Cavitation Erosion

The goal of the work is to develop an expert system for monitoring and control of cavitation in hydraulic machines and to research the possibility of cavitation erosion prediction using CFD tools only. The geometry in question is a simple single hydrofoil, which is exposed to the developed cavitating flow at different flow conditions. The work was divided in more parts: numerical simulation of cavitating flow, experimental evaluation of the simulation, measurements of cavitation erosion, development of cavitation erosion model and finally the prediction of cavitation erosion using solely CFD. A study of erosion effects of cavitation on simple single hydrofoil configurations in a cavitation tunnel was made. A thin copper foil, applied to the surface of the hydrofoils, was used as an erosion sensor. A pit-count method was used to evaluate the damage. The cavitation phenomenon on hydrofoils at different flow conditions (system pressure, flow velocity) was observed. The erosion model is based on the physical description of different phenomena (cavitation cloud implosion, pressure wave emission and its attenuation, micro-jet formation and finally pit formation), which are involved in the process of pit formation. The cavitating flow was simulated using an “in house” CFD code which uses barotropic state law. The code was previously tested on numerous experiments. For the present case the predictions of velocity profiles and pressure evolutions in the vicinity of the hydrofoil were compared to experimentally measured data. In all cases a very good correlation was obtained. The erosion model was implemented into the code. It used values of local pressure, local void fraction and flow velocity to determine the magnitude of damage at a certain point. The results of prediction were compared to the experimentally measured damage on the hydrofoil and it was shown that it is possible, for this simple case, to use solely CFD tools to predict cavitation erosion evolution in time, final extent and final magnitude with a very good accuracy.