The Free and Porous Media Flow (fp) interface () is found under the Porous Media and Subsurface Flow branch () when adding a physics interface. It is used to compute fluid velocity and pressure fields of single-phase flow where free flow is connected to porous media. The Free and Porous Media Flow interface is used over at least two different domains: a free channel and a porous medium. The physics interface is well suited for transitions between slow flow in porous media, governed by the Brinkman equations, and fast flow in channels described by the Navier–Stokes equations. Fluids with varying density can be included at Mach numbers below 0.3. Also the viscosity of a fluid can vary, for example, to describe non-Newtonian fluids. The physics interface can be used for stationary and time-dependent analyses.

When this physics interface is added, the following default nodes are also added in the Model Builder — Fluid Properties, Wall, and Initial Values. Then, from the Physics toolbar, add a Porous Medium node to be used on the domain selection corresponding to the porous media, or add other nodes that implement, for example, boundary conditions and volume forces. You can also right-click Free and Porous Media Flow 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 fp.

By default the physics interface uses the Incompressible flow formulation of the Navier–Stokes and Brinkman equations to model constant density flow. If required, select Weakly compressible flow from the Compressibility list, to account for small variations in the density, which are only dependent on the temperature (nonisothermal flow) or Compressible flow (Ma<0.3) for fully compressible flow. However, for the flow modeled with this physics interface, the Mach number must be below 0.3.

For 2D axisymmetric components, select the Swirl flow check box to include the swirl velocity component, that is the velocity component in the azimuthal direction. While can be nonzero, there can be no gradients in the direction. Note that this feature is only available for specific modules. Visit https://www.comsol.com/products/specifications/ for a detailed overview.

Select the Neglect inertial term (Stokes flow) check box if the inertial forces are small compared to the viscous forces. This is typical for creeping flow, where .

The Enable porous media domains check box is selected by default to solve the Brinkman equations in porous domains. If it is unchecked, the Porous Medium node is no longer available in this interface.

Gravity is not included by default. Select the Include gravity check box to activate the acceleration of gravity. This automatically adds a global Gravity feature node to the interface model tree, and the buoyancy force is included in the Equations.

Also, when the Include gravity check box is selected, the Use reduced pressure option changes the pressure formulation from using the total pressure (default) to using the reduced pressure. This option is suitable for configurations where the density changes are very small; otherwise, the default formulation can be used. For more information, see Gravity.

Choose how the No Slip condition on internal boundaries should be treated. The options are Standard no slip formulation (default) and Porous slip. The latter option provides a unified treatment when the porous matrix is fully resolved as well as when it is under-resolved ensuring a smooth transition between regions with different resolutions; see No Slip under Wall in the The Creeping Flow, Laminar Flow, Turbulent Flow, Large Eddy Simulation, and Detached Eddy Simulation Interfaces section.

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

Enter a Reference temperature Tref (SI unit: K). The default value is 293.15[K].

If Include gravity is selected, a Reference position rref (SI unit: m) can be specified which is then used for the calculation of the hydrostatic pressure.

Turbulent flow can be simulated by changing the Turbulence model type to RANS (Reynolds-Averaged Navier–Stokes) or Large Eddy Simulation (which is only available in 3D). If turbulent flow is activated, you can choose from different Turbulence models and options for Wall treatment. For a description of the different turbulence models, wall treatment options, and turbulence model parameters see Theory for the Turbulent Flow Interfaces in the CFD Module User’s Guide.

The following dependent variables (fields) are defined for this physics interface — the Velocity field u (SI unit: m/s) and its components, and the Pressure p (SI unit: Pa).