The Phase Transport in Porous Media interface (), found under the Fluid Flow > Porous Media and Subsurface Flow branch (), when adding a physics interface, is used to simulate the transport of multiple immiscible phases either in flow through a porous medium or in free flow. This interface solves for the averaged volume fractions (also called saturations in a porous medium) of the phases, and does not track the interfaces between the different phases, although microscopic interfacial effects are taken into account in the macroscopic equations for phase transport in porous media through the relative permeabilities and capillary pressure functions. The Phase Transport in Porous Media interface can be used for stationary and time-dependent analyses.

The main feature is the Phase and Porous Media Transport Properties node, which provides an interface for defining the phase materials along with the transporting porous medium flow properties.

When this physics interface is added, the following default nodes are also added in the Model Builder — Phase and Porous Media Transport Properties, No Flux (the default boundary condition), and Initial Values. Then, from the Physics toolbar, add other nodes that implement, for example, boundary conditions and mass sources. You can also right-click the Phase Transport in Porous Media node to select physics features from the context menu.

The Label is the default physics interface name.

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

The default Name (for the first physics interface in the model) is phtr.

Select the volume fraction of the phase that this physics interface solves for using the volume constraint in equation Equation 8-13 (that is, its value comes from the fact that the sum of all volume fractions must equal 1). In the From volume constraint list, select the volume fraction of the preferred phase. By default, the first volume fraction is used:

To display this section, click the Show More Options button () and select Stabilization in the Show More Options dialog box.

There are two consistent stabilization methods: Streamline diffusion and Crosswind diffusion. Both check boxes for these methods are selected by default and should remain selected for optimal performance. Consistent stabilization methods do not perturb the original transport equation.

There is one inconsistent stabilization method: Isotropic diffusion. This method is equivalent to adding a diffusion term to the equations in order to dampen the effect of oscillations by making the system somewhat less dominated by convection. By default, the Isotropic diffusion check box is not selected because this type of stabilization adds artificial diffusion and affects the accuracy of the original problem. If required, select the Isotropic diffusion check box and enter a Tuning parameter as a scalar positive value. The default value is 0.25. A higher value adds more isotropic diffusion.

When the Include gravity check box is selected, a global Gravity feature is shown in the interface model tree, and the gravitational force is added to the phase transport equations in porous domains. For more information, see Gravity.

To display this section, click the Show More Options button () and select Advanced Physics Options in the Show More Options dialog box.

These settings affect the numerical integration, and you do not normally need to change them. The Use automatic quadrature settings check box is selected by default, meaning that the settings are taken from the main equation in the interface.

The Integration order specifies the desired accuracy of integration during discretization. Polynomials of at most the given integration order are integrated without systematic errors. For smooth constraints, a sufficient integration order is typically twice the order of the shape function. For example, the default integration order for linear elements is 2. The integration order is a positive integer.

The settings in this section currently only affect the integration order in the Phase and Transport Properties and Phase and Porous Media Transport Properties domain features.

You can choose the order of the shape functions used for the volume fraction variables. The default shape functions are Linear Lagrange.

To display all settings available in this section, click the Show More Options button () and select Advanced Physics Options in the Show More Options dialog box.

The Value type when using splitting of complex variables setting should in most pure mass transport problems be set to Real which is the default. It makes sure that the dependent variable does not get affected by small imaginary contributions, which can occur, for example, when combining a Time Dependent or Stationary study with a frequency-domain study. For more information, see Splitting Complex-Valued Variables in the COMSOL Multiphysics Reference Manual.

Specify the Number of phases. There must be at least two phases. To add a single phase, click the Add volume fraction button () under the table. To remove a volume fraction, select it in the list and click the Remove volume fraction button () under the table. Edit the names of the phases directly in the table.