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Cavitation modelling

An important project objective was to develop a multidimensional model for cavitation erosion. It was therefore necessary to develop statistical description of the bubble impact loads responsible for cavitation erosion. The procedure involved modelling of the material response taking into account its microstructure, computation of the erosion rate for the materials tested in WP1 and comparison with the measured mass-loss in order to validate the model. Finally, a CFD model was derived to simulate transient flows of bubbles that are generated at low-pressure regions and eventually collapse near the solid surface using the multi-fluid model. Predicting the probability for cavitation erosion was the main goal.

In order to quantify the bubble collapses accurately the cavitation model implemented in the FIRE code had to be improved. Eulerian Multifluid Method was adopted to model the flow. In the scope of multi-fluid concept for multiphase flows, each phase is viewed as a continuous phase coexisting, in a statistical sense, with other fluid phases in time and space. Further it is assumed that the transport equations, derived from the conservation laws of mass, momentum and energy, are valid for each phase as they are for singlephase flows.

However, it is necessary to apply an averaging procedure to smooth out otherwise intractable interfaces between the phases and to formulate the equations on the macroscopic basis which is solvable numerically. The averaging process introduces
inevitably new variables and terms associated with the status of the multiphase mixture and the interactions between the phases. In addition, a bubble population balance equation is added to the family of conservation equations to account for bubble population evolutionary characteristics.

These new terms have their respective physical implications and must be modelled, or closed, on the physical basis. Therefore, solution for multiphase flow problems ultimately entails the multiplication of the number of variables and governing equations and sub-models for phase-interactions.

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