V O L U M E F O R C E

Enter the components of the Volume force F (SI unit: N/m3).

Initial Values

The Initial Values feature adds initial values for the velocity field u and the pressure p that can serve as an initial condition for a transient simulation or as an initial guess for a nonlinear solver.

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

From the Selection list, choose the domains to define initial values.

I N I T I A L V A L U E S

Enter values or expressions for the initial value of the velocity field u (SI unit: m/s) in the Velocity field fields and for the pressure p (SI unit: Pa) in the Pressure field. The default values are 0.

Boundary Conditions for the Free and Porous Media Flow Interface

The boundary conditions for The Free and Porous Media Flow Interface are the same as for the following:

• The Single-Phase Flow, Laminar Flow and Creeping Flow Interfaces

Note

• The Brinkman Equations Interface

Microfluidic Wall Conditions

Use the Microfluidic Wall Conditions feature to add boundary conditions to the moving wall and set whether to use viscous slip or thermal creep.

B O U N D A R Y S E L E C T I O N

From the Selection list, choose the boundaries that represent solid walls.

P A I R S E L E C T I O N

If Microfluidic Wall Conditions is selected from the Pairs menu, choose the pair to define. An identity pair has to be created first. Ctrl-click to deselect.

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B O U N D A R Y C O N D I T I O N

The default Boundary condition for the wall is Slip velocity. Enter values or expressions for the components of the Velocity of moving wall uw (SI unit: m/s).

Select the Use viscous slip check box and define the slip length:

• The default is User defined. Enter a value for Ls (SI unit: m).

• Select Maxwell’s Model to calculate it from:

2 – v

L = ---------------

s v

Enter values or expressions for the Tangential momentum accommodation coefficient

(TMAC) av and the Mean free path (SI unit: m).

Select the Use thermal creep check box to activate the thermal creep component of the boundary condition. Enter the fluid’s Temperature T (SI unit: K) and the Thermal slip coefficient T. The default temperature is 293.15 K and the default thermal slip coefficient is 0.75.

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

The Two-Phase Darcy’s Law interface (), found under the Porous Media and Subsurface Flow branch () in the Model Wizard, has the equations and boundary conditions for modeling two-phase fluid movement through interstices in a porous medium using Darcy’s law. The main feature is the Fluids and Matrix Properties which provides an interface for defining the two immiscible fluids properties along with the porous medium properties. The interface is available in 2D, 2D axisymmetric, and 3D.

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

Values.

Right-click the Two-Phase 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 tpdl.

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 variables (field variables) are for the Pressure and Fluid content 1. 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

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

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• Model Builder Options for Physics Feature Node Settings Windows

• Domain, Boundary, and Pair Conditions for the Two-Phase Darcy’s

Law Interface

See Also

• Theory for the Two-Phase Darcy’s Law Interface

Domain, Boundary, and Pair Conditions for the Two-Phase Darcy’s Law Interface

The Two-Phase Darcy’s Law Interface domain conditions are described in this section:

•Fluid and Matrix Properties

•Initial Values

These boundary conditions are available and described in this section:

•No Flux (the default boundary condition)

•Pressure and Saturation

•Mass Flux

•Inflow Boundary

•Outflow

For axisymmetric models, COMSOL Multiphysics takes the axial symmetry boundaries (at r = 0) into account and automatically adds an Axial Symmetry feature to the model that is valid on the axial symmetry

2D Axi

boundaries only.

These conditions are available from the Pairs menu:

• Pressure and Saturation

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