COMSOL 6.4 - The Three-Phase Flow, Phase Field Coupling Feature

) multiphysics coupling to simulate the flow of a three immiscible fluids while explicitly tracking the interface separating each pair of the fluids.

The Three-Phase Flow interface solves Navier–Stokes equations for the conservation of momentum, and a continuity equation for the conservation of mass. The positions of the interfaces separating the fluid phases are tracked by solving four additional transport equations: two equations governing phase field variables and two equations for the corresponding generalized chemical potentials. The movement of the fluid-fluid interfaces is determined by minimization of free energy.

The is the default multiphysics coupling feature name.

The is used primarily as a scope prefix for variables defined by the coupling node. Refer to such variables in expressions using the pattern <name>.<variable_name>. In order to distinguish between variables belonging to different coupling nodes or physics interfaces, the name string must be unique. Only letters, numbers, and underscores (_) are permitted in the field. The first character must be a letter.

The default (for the first multiphysics coupling feature in the model) is tfpf1.

Fluid properties of each phase, such as density and viscosity, can be defined through user inputs, variables, or by selecting a material. For the latter option, additional inputs, for example temperature or pressure, may be required to define these properties.

By default, the model input is set to , and the temperature is controlled from Default Model Inputs under or by a locally defined Model Input. If a Heat Transfer interface is included in the component, it controls the temperature . Alternatively, the temperature field can be selected from another physics interface. All physics interfaces have their own tags (). For example, if a Heat Transfer in Fluids interface is included in the component, the option is available for T.

You can also select from the model input in order to manually prescribe T.

This input appears when a material requires the absolute pressure as a model input. The default pA is p + pref, where p is the dependent pressure variable from the Navier–Stokes equations, and pref is from the user input defined at the fluid flow physics interface level.

The field can be edited by clicking (

) and entering the desired value in the input field.

	Model Inputs and Multiphysics Couplings in the COMSOL Multiphysics Reference Manual

The effective density and dynamic viscosity can either be or picked up from a Multiphase Material. When the properties are defined locally, provide the properties for each phase in the respective fluid properties section, and the averaging method in the advanced settings section. If a multiphase material is used, these settings are controlled from the material. Using a multiphase material is advantageous for models coupling two-phase flow with other physics interfaces, such as Heat Transfer in Fluids or Electrostatics. In such cases, the multiphase material can ensure that the effective material properties for these physics interfaces are averaged using the volume fraction from the Ternary Phase Field physics interface.

Click the button (

) to move to the selected material node. Click the button (

) to add a multiphase material with three phases. The added material then becomes the one selected in the list.

	Multiphase Material in the COMSOL Multiphysics Reference Manual

A section will be available per phase if the are set to . Use the corresponding section to specify the properties of all three fluids. The fluids are denoted , , and , respectively.

To specify the properties of from a material, select the appropriate material in the list. Also make sure that the ρA and μA are both set to .

The density in a material can depend on temperature and/or pressure and these dependencies are automatically replaced by pref and Tref for incompressible flows (as specified by the setting of the laminar flow interface).

The non-Newtonian Power law, Carreau, Bingham–Papanastasiou, Herschel–Bulkley–Papanastasiou, and Casson–Papanastasiou models can alternatively be used to specify the dynamic viscosities of the three fluids.

To instead apply a variable or expression for the density or dynamic viscosity for Fluid A, select in the ρA or the μA list and enter the expression in the corresponding text field.

This section controls which individual interfaces are coupled by the current coupling feature. If a physics interface is deleted and then added to the model again, then in order to reestablish the coupling, you need to choose the correct physics interface again from the or lists.