region of the interface, then you can always couple a fluid-flow interface to the Darcy’s Law interface, to make your overall model computationally cheaper.

The Free and Porous Media Flow interface is used over at least two differing domains; a free channel and a porous medium. The interface adds functionality that allows the equations to be optimized according to the definition of the material properties of the relevant domain. For example, you can select the Stokes-Brinkman flow feature to reduce the equations’ dependence on inertial effects in the porous domain, or just the Stoke’s flow feature to reduce the equations’ dependence on inertial effects in the free channel.

Compressible flow is also possible to model in this interface at speeds of less than 0.3 Mach, but you have to maintain control over the density and any of the mass balances that are deployed to help with this. You can also choose to model incompressible flow, and simplify the equations to be solved.

As always, the physics interface gives you direct access to defining, with either constants or expressions, the material properties that describe the porous media flow. This includes the density, dynamic viscosity, permeability, porosity, and matrix properties.

T W O - P H A S E D A R C Y ’ S L A W

The Two-Phase Darcy’s Law Interface has the equations and boundary conditions for modeling two-phase fluid movement through interstices in a porous medium using Darcy’s law. The two fluids are considered immiscible, and in general, have different densities and viscosities.

Similarly to the single phase Darcy’s Law (see The Darcy’s Law Interface), the total velocity field is determined by the total pressure gradient and the structure of the porous medium, but the average viscosity and average density are calculated from the saturation of each immiscible phase and the fluid properties. An extra equation is computed—the fluid content of one phase—in order to calculate the saturation transport.

Coupling to Other Physics Interfaces

Often, you may be simulating applications that couples fluid-flow in porous or subsurface media to another type of phenomenon described in another physics interface. This can include chemical reactions and mass transport, as described in Chemical Species Transport Branch, or energy transport in porous media described in Heat Transfer Branch.

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More extensive descriptions of modeling chemical reactions and mass transport is found in the Chemical Reaction Engineering Module. Furthermore, some applications that involve electrochemical reactions and porous electrodes, particularly in electrochemical power source applications, are supported in the Batteries & Fuel Cells Module.

Fluid flow is an important component for cooling electromagnetic phenomena, such as heat created through induction and microwave heating, which are simulated in the AC/DC and RF Modules, respectively. While many applications involve the effect of fluid-imposed momentum on structural applications; poroelasticity. The Structural Mechanics and Subsurface Flow Modules feature interfaces specifically for these multiphysics applications.

The following sections list all the physics interfaces and the features associated with them under the Porous Media Subsurface Flow branch. The descriptions follow a structured order as defined by the order in the branch. Because many of the interfaces are integrated with each other, some features described also cross reference to other interfaces. At the end of this section is a summary of the theory that goes towards deriving the physics interfaces under the Porous Media Subsurface Flow branch.

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T h e D a r c y ’ s L a w I n t e r f a c e

The Darcy’s Law interface (), found under the Porous Media and Subsurface Flow branch () in the Model Wizard, has the equations, boundary conditions, and mass sources for modeling fluid movement through interstices in a porous medium using Darcy’s law. The main feature is the Fluid and Matrix Properties node, which provides an interface for defining the fluid material along with the porous medium properties.

When this interface is added, these default nodes are also added to the Model Builder— Fluid and Matrix Properties, No Flow (the default boundary condition), and Initial Values.

Right-click the Darcy's Law node to add features that implement, for example, boundary conditions and mass sources. The following sections provide information about all feature nodes in the interface.

I N T E R F A C E I D E N T I F I E R

The interface identifier is a text string that can be used to reference the respective physics interface if appropriate. Such situations could occur when coupling this interface to another physics interface, or when trying to identify and use variables defined by this physics interface, which is used to reach the fields and variables in expressions, for example. It can be changed to any unique string in the Identifier field.

The default identifier (for the first interface in the model) is dl.

D O M A I N S E L E C T I O N

The default setting is to include All domains in the model to define the pressure and the Darcy’s law and continuity equation. To choose specific domains, select Manual from the Selection list.

D E P E N D E N T V A R I A B L E S

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

D I S C R E T I Z A T I O N

To display this section, click the Show button () and select Discretization. Select a

Pressure—Quadratic (the default), Linear, Cubic, or Quintic.

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