In order to model cavitation, your simulation must have definitions for the liquid and vapor state of a material, as well as the saturation temperature.
Two approaches to modeling cavitation are:
Define an Eulerian multiphase simulation that uses one fluid for the liquid state and one fluid for the vapor state, then use an interphase mass transfer cavitation model (see Cavitation Model in the CFX-Solver Modeling Guide) to transfer mass between the two fluids.
If thermodynamic equilibrium can be assumed, you can model cavitation (also flashing) by defining an equilibrium phase change simulation using a homogeneous binary mixture for both states (liquid and vapor). For details, see Setting up an Equilibrium Phase Change Simulation in the CFX-Solver Modeling Guide.
Recommendations
Initially establish the flow without cavitation, for example, by using a high outlet pressure or by turning off interphase mass transfer, depending on the case.
In an equilibrium phase change simulation, initially maintain a large enough time step to avoid instabilities when cavitation starts.
You may need to reduce the time step, especially if the cavitation boundary moves.