The Two-Phase Flow, Phase Field Coupling Feature

)and(

) interfaces contain a multiphysics coupling feature, , which is added automatically.

The multiphysics coupling feature defines the density and dynamic viscosity of the fluid used in the and interfaces, and it defines the surface tension on the interface in form of a volume force used in the momentum equation. It also enables the interface to use the velocity field calculated from the or interface to transport the interface.

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 tpf1.

When nodes are added from the context menu, you can select (the default) or select from the list to choose specific domains.

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.

Fluid properties of each phase, such as density, viscosity, or the surface tension coefficient, 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 or RANS 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 Phase Field physics interface.

Click the button (

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

) to add a multiphase material with two 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 the two 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 ρ1 and μ1 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 fluid 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 two fluids.

To instead apply a variable or expression for the density or dynamic viscosity for Fluid A, select in the ρ1 or the μ1 list and enter the expression in the corresponding text field.

Similarly, the properties of can be specified. The default material is set to .

	Care should be taken when using the Domain Material setting for the material properties for Fluid 1 and Fluid 2. The material properties are obtained from the domain irrespective of the location of the interface. If two different materials are selected in domains 1 and 2, with the phase boundary initially coincident with the domain boundary, the model has convergence issues once the phase boundary moves away from the domain boundary. This is because a density discontinuity and a viscosity discontinuity occurs at the boundary separating the two fluids. For this reason, selecting the material directly is recommended when setting the material properties for Fluid 1 and Fluid 2.

The fluid defined as Fluid 1 affects the wetting characteristics on wetted walls. See the Wetted Wall node for details.

f the are set to , you can specify the averaging method for density and dynamic viscosity in this section. Select the method used for and . The default method for both settings is . In addition to the default method, can be set to or while can be set to , , or . When a is used, enter a value for the corresponding mixing parameter, lρ or lμ.

When the surface tension force is included in the momentum equation, you can select the check box. This can prevent significant spurious oscillations in the velocity field for the lighter phase in cases with a large difference in density between the two phases. The amount of shifting is controlled by the ds,Fst (default 0.1). The surface tension force is then multiplied by

works better when combined with for the averaging of density and viscosity.

Select the check box to include the surface tension force in the momentum equation. If the surface tension force is included, select to account for the Marangoni effect due to gradients in the surface tension coefficient.

Select a σ (SI unit: N/m):

•

To use a predefined expression, select or . Then select an option from the list that displays below (for example, or ). The predefined correlations are based on the data in Ref. 7, Ref. 8, and Ref. 9.