COMSOL 6.4 - The Free and Porous Media Flow, Brinkman Interface

The interface (

) is found under the 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, Brinkman 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 — , , and . Then, from the toolbar, add a 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 to select physics features from the context menu.

The is the default physics interface name.

The 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 field. The first character must be a letter.

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

By default the physics interface uses the formulation of the Navier–Stokes and Brinkman equations to model constant density flow. If required, select from the list, to account for small variations in the density, which are only dependent on the temperature (nonisothermal flow) or 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 checkbox to include the swirl velocity component, that is the velocity component uφ in the azimuthal direction. While uφ can be nonzero, there can be no gradients in the φ direction. Note that this feature is only available for specific modules. Visit www.comsol.com/products/specifications/ for a detailed overview.

Select the checkbox if the inertial forces are small compared to the viscous forces. This is typical for creeping flow, where Re << 1.

The checkbox is selected by default to solve the Brinkman equations in porous domains. If it is cleared, the node is no longer available in this interface.

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

Also, when the checkbox is selected, the 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.

When the checkbox is selected, a global feature is shown in the interface model tree, and the fictitious Coriolis, centrifugal and Euler forces, stemming from the change to a non-inertial frame of reference, are included in the Navier–Stokes equations. Note that this feature is only available for specific modules. Visit www.comsol.com/products/specifications/ for a detailed overview.

For 2D components, the feature and the feature are mutually exclusive, and thus can not be selected simultaneously. For 2D axisymmetric components, the checkbox and feature are only available when also the checkbox is selected.

When the checkbox is selected, the 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 Rotating Frame

When either the or the checkbox is selected, and a turbulence model that solves for the turbulent kinetic energy, k, is used, the checkbox is available. When selected, the is by default set to . Contributions are only obtained from multiphysics couplings that support buoyancy-induced turbulence, such as Nonisothermal Flow. It the is switched to , a text field for the appears.

When the or the option is selected, the checkbox is available (selected by default). See also Theory for Buoyancy-Induced Turbulence in the CFD Module User’s Guide.

Choose how the condition on interior boundaries should be treated. The options are and (default). 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 Creeping Flow, Laminar Flow, Turbulent Flow, Large Eddy Simulation, and Detached Eddy Simulation Interfaces section.

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

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

If or is selected, a 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 to (Reynolds-Averaged Navier–Stokes, eddy-viscosity model), (Reynolds-Averaged Navier–Stokes, Reynolds stress model), or (which is only available in 3D). If turbulent flow is activated, you can choose the and between different options for . For a description of the 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 u (SI unit: m/s) and its components, and the p (SI unit: Pa).