The boundary condition parameters for the Multiphase module apply to boundaries in the Flow Analysis Tree. The options also apply to interfaces for which the Multiphase module is blanked on one side of the interface, creating a Boundary.

The boundary conditions appear in the Properties panel when you select a boundary in the Flow Analysis Tree under General Boundaries.

The Zero Flux boundary condition corresponds to a solid boundary in the model. Zero Flux for the Multiphase module means that there is no flux or gradient of components normal to the boundary. When you set the mixture boundary condition to Zero Flux, then the boundary conditions for the components are also fixed to Zero Flux. This is the default boundary condition for Multiphase module when the boundary condition for Flow module is Wall or Rotating Wall.

Contact Model: The contact angle allows you to set the wetting angle for a Component at a Wall. The contact model is set to Yes for a component under the Flow module and the Angle is specified.

Use Specified Value boundary condition to set volume fractions of the components at an opening. When you set the mixture boundary condition to Specified Value, then boundary conditions for the components are also fixed to Specified Value. The volume fraction is specified for Value under each component. Specified Value is the default boundary condition for Multiphase module when you set the boundary condition for Flow module to Specified Velocity, Specified Volumetric Flux, Specified Total Pressure, or Specified Pressure Inlet. It is not recommended to prescribe a Specified Value condition for a boundary that is not an opening.

Use Specified Volumetric Flux boundary condition to set the volumetric flux of each component at an opening. This corresponds to the volumetric flow boundary condition for the Flow module. For the Specified Volumetric Flux boundary condition, the Value of the volume fraction for each component is set under Component > Specified Value.

Use Outlet boundary condition to set the volume fractions of the components at an opening. The integrated quantities available as output from the Flow module for the Boundaries appear in output variables. When you set the mixture boundary condition to Outlet, then the boundary conditions are fixed to Outlet for the components. The volume fraction is specified for Value under each component. Outlet is the default boundary condition for Multiphase module when you set the boundary condition under the Flow module to Specified Pressure Outlet or Resistor Capacitor. Setting an Outlet boundary condition for a boundary that is not an opening is not recommended.

Symmetry means that there is no shear, no normal component of velocity, and no normal gradient of pressure at the boundary. A Symmetry boundary condition usually corresponds to a physical symmetry in the model. When you set the mixture boundary condition to Outlet, the boundary conditions are fixed to Symmetry for the components. This is the default boundary condition for Multiphase module when the boundary condition under Flow module is Symmetry.

The interface conditions for the Multiphase module are similar to those for the boundaries, only if the state of the volume on one side of the interface is Blanked for flow. If Volume is set to Blanked or Active Multi-component Flow on both sides of an interface, then it can only be assigned as a Default Interface.