You should first use the Material details view in CFX-Pre to define the properties of the materials in your simulation. You can then select these materials, in addition to predefined library materials, on the Basic Settings tab of the Domain details view. For details, see Basic Settings Tab in the CFX-Pre User's Guide. Defining more than one fluid implies a multiphase simulation.

Models that apply to all fluids in the simulation are set on the Fluid Models tab. For details, see Fluid Models Tab in the CFX-Pre User's Guide. Fluid specific attributes are defined on the Fluid Specific Models tab in CFX-Pre. For details, see Fluid Specific Models Tab in the CFX-Pre User's Guide. Interactions between the fluids are defined on the Fluid Pairs tab. For details, see Fluid Pair Models Tab in the CFX-Pre User's Guide.

Multiphase flows enable low speed compressibility effects to be modeled and you can mix compressible and incompressible fluids in one simulation. An example would be modeling an air-water system in which the water can be considered incompressible but the air is modeled as compressible due to the hydrostatic water pressure.

You should be aware of significant increases in memory and CPU time when using multiple fluids; these increase rapidly as more fluids are added. For details, see CPU and Memory Requirements in the CFX-Solver Manager User's Guide.