modeling cavitation in open channel in fluent
 

Hi every body.I want to model an open channel in which cavitation occurs. Should I have to use VOF (multiphase), or in fact, I should use Mixture (multiphase), and what should be the primary and secondary phases in my model (the materials include water, water-vapor and The air exists above the surface of water in open channel). there is one inlet and two outlets (one free surface and other outlet is under the main surface channel that flow could exit as submerged, through a pipe), velocity in entrance of channel and depth is specified, and outlet conditions aren't specified, that could be pressure-outlet or outflow. what should I define for boundary condition for inlet and two outlets.
thanks very much.