The Porous Media and Subsurface Flow branch () has the physics interfaces to model flow in porous media. The flow can be modeled by Darcy’s Law or the Brinkman Equations interfaces. Additionally the Brinkman equations can be combined with laminar flow in the Free and Porous Media Flow interface.

The Darcy’s Law Interface () is used to model fluid movement through interstices in a porous medium in the case where the fluid viscosity dominates over its inertia. Together with the continuity equation and the equation of state for the pore fluid (or gas) Darcy’s law is used to model low velocity flows, for which the pressure gradient is the dominant driving force (typically when the Reynolds number of the flow is less than one). The penetration of reacting gases in a tight catalytic layer, such as a washcoat or membrane, is an example in which Darcy’s law applies. Often Darcy’s law is applicable for microfluidic applications, as it is appropriate for the limit of small pore size.

The Brinkman Equations Interface () is used in situations where the fluid inertia, or other effects such as body forces, can no longer be neglected. Fluid penetration of filters and packed beds are applications for this physics interface. The Brinkman equations are similar in form to the Navier Stokes equations. Figure 2-2 shows the Settings window for the Brinkman Equations interface.

For low Reynolds number flows in which other terms in the Brinkman equations are necessary the Neglect Inertial Term (Stokes–Brinkman Flow) feature can be selected to neglect the inertial term in the equations (this is selected by default). The flow is treated as incompressible by default, but compressible flow can be enabled by selecting Compressible Flow (Ma<0.3). When using the compressible flow feature the fluid density must be defined as a function of the local pressure.

The Free and Porous Media Flow Interface () has predefined couplings between the Brinkman Equations interface and the Laminar Flow interface. Porous regions of the models are included by adding a Porous Medium node with the appropriate selections to the model. The default Fluid Properties node can then be used to define regions of free laminar flow. This physics interface has all the options from both The Laminar Flow Interface and The Brinkman Equations Interface. An example application area for this physics interface would be a catalytic converter.