The Darcy’s Law (dl) interface (), found under the Porous Media and Subsurface Flow branch () when adding a physics interface, is used to simulate fluid flow through interstices in a porous medium. It can be used to model low-velocity flows or media where the permeability and porosity are very small, and for which the pressure gradient is the major driving force and the flow is mostly influenced by the frictional resistance within the pores. Set up multiple Darcy's Law interfaces to model multiphase flows involving more than one mobile phase. The Darcy’s Law interface can be used for stationary and time-dependent analyses.

The main feature is the Porous Medium Properties node, which provides interfaces for defining the fluid material and the porous matrix properties.

When this physics interface is added, the following default nodes are also added in the Model Builder — Porous Medium, No Flow (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 Darcy's Law 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 dl.

Enter a Reference pressure level pref (SI unit: Pa). The default value is 1[atm].

By default there are no gravity effects added to Darcy’s Law interface. Select the check box Include gravity to activate the acceleration of gravity. When this check box is selected, a global Gravity feature node is shown in the interface model tree.

Enter a value for the acceleration of gravity. The default value g_const is the predefined standard acceleration of gravity on earth.

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.

You can choose the order of the shape functions used for the pressure variable solved by the Darcy’s Law interface. The default shape functions are Quadratic Lagrange.

The Compute boundary fluxes check box is not activated by default. When this option is selected, the solver computes variables storing accurate boundary fluxes from each boundary into the adjacent domain.

Also, the Apply smoothing to boundary fluxes check box is available if the previous check box is checked. The smoothing can provide a better behaved flux value close to singularities.

For details about the boundary fluxes settings, see Computing Accurate Fluxes in the COMSOL Multiphysics Reference Manual.

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.

The dependent variable (field variable) is the Pressure. The name can be changed but the names of fields and dependent variables must be unique within a model.