### 13.1.2. Domain Interface: Additional Interface Models Tab

The Additional Interface Models tab contains settings as described in the following sections:

- Mass And Momentum
- Heat Transfer
- Electric Field
- Additional Variable
- Conditional Connection Control

#### 13.1.2.1. Mass And Momentum

Allows the specification of additional models at the interface.

The Mass And Momentum options are described in the following sections:

- Conservative Interface Flux
- No Slip Wall
- Free Slip Wall
- Side Dependent

##### 13.1.2.1.1. Conservative Interface Flux

The `Conservative Interface Flux` Mass And Momentum option enables you to define the physics across a thin surface.

The Interface Model options are:

- `None`

  The default behavior (conservative interface flux) is applied to the interface. No additional models are applied.

- `Pressure Change`

  Enter a numerical quantity or CEL expression that specifies the pressure change across the interface from side 1 to side 2. If there is a pressure drop, the specified value should be negative.

- `Mass Flow Rate`

  Enter a numerical quantity or CEL expression that specifies the value of the mass flow rate from side 1 to side 2 of the interface.

  Note: When imposing a mass flow rate at a domain interface, the CFX-Solver updates the pressure change to drive the mass flow rate toward the specified value. The update is based on an internally-estimated coefficient, which may not be optimal.

  The Pressure Update Multiplier enables you to tune convergence behavior. Enter a numerical quantity or CEL expression that specifies the pressure update multiplier. The default value is 0.25. If convergence is slow (as may occur for low Reynolds number flows), consider increasing the value. If convergence is unstable, consider decreasing the value. Note that values above 1 are permissible.

##### 13.1.2.1.2. No Slip Wall

For a description of the options that influence flow on a wall boundary, see Mass and Momentum in the CFX-Solver Modeling Guide.

###### 13.1.2.1.2.1. No Slip Wall: Wall Velocity

When set, this option enables you to specify the wall velocity. Various options are available for specifying the wall velocity.

##### 13.1.2.1.3. Free Slip Wall

Free Slip Wall has no sub-options.

##### 13.1.2.1.4. Side Dependent

Selecting this option allows a different boundary condition to be applied on each side of the interface. Each boundary condition is specified on the related interface boundary (which appears in the Outline tree view under the associated domain).

#### 13.1.2.2. Heat Transfer

Allows the specification of additional models at the interface.

The Heat Transfer options are described in the following sections:

- Conservative Interface Flux
- Side Dependent

##### 13.1.2.2.1. Conservative Interface Flux

This option enables you to define the Thermal Contact Resistance or Thin Material, which are two ways of defining the same characteristics. That is, if you do not know the contact resistance, you can define the thin material and its thickness and have the solver derive the resistance.

The Interface Model options are:

- `None`

  The default behavior (conservative interface flux) is applied to the interface. No additional models are applied.

- `Thermal Contact Resistance`

  Enter a numerical quantity or CEL expression that specifies the value of the thermal contact resistance between side 1 and side 2 of the interface.

- `Thin Material`

  Select a material and enter a numerical quantity or CEL expression that specifies the value of the thickness of the material spanning from side 1 to side 2 of the interface.

##### 13.1.2.2.2. Side Dependent

Selecting this option allows a different boundary condition to be applied on each side of the interface. Each boundary condition is specified on the related interface boundary (which appears in the Outline tree view under the associated domain).

#### 13.1.2.3. Electric Field

Allows the specification of additional models at the interface.

The Electric Field options are:

- Conservative Interface Flux
- Side Dependent

##### 13.1.2.3.1. Conservative Interface Flux

The Interface Model options are:

- `None`

  The default behavior (conservative interface flux) is applied to the interface. No additional models are applied.

- `Electrical Contact Resistance`

  Enter a numerical quantity or CEL expression that specifies the value of the electrical contact resistance between side 1 and side 2 of the interface.

##### 13.1.2.3.2. Side Dependent

Selecting this option allows a different boundary condition to be applied on each side of the interface. Each boundary condition is specified on the related interface boundary (which appears in the Outline tree view under the associated domain).

#### 13.1.2.4. Additional Variable

Allows the specification of additional models at the interface.

The Additional Variable options are described in the following sections:

- Conservative Interface Flux
- Side Dependent

##### 13.1.2.4.1. Conservative Interface Flux

The Interface Model options are:

- `None`

  The default behavior (conservative interface flux) is applied to the interface. No additional models are applied.

- `Additional Variable Contact Resistance`

  Enter a numerical quantity or CEL expression that specifies the value of the Additional Variable contact resistance between side 1 and side 2 of the interface.

##### 13.1.2.4.2. Side Dependent

Selecting this option allows a different boundary condition to be applied on each side of the interface. Each boundary condition is specified on the related interface boundary (which appears in the Outline tree view under the associated domain).

#### 13.1.2.5. Conditional Connection Control

Conditional Connection Control is an optional group of settings. These settings enable a CEL expression to control whether an interface is open (connected) or closed (not connected; a wall boundary is applied).

When Conditional Connection Control is selected, you have the following options:

- `Specified Open State`

  Provide a CEL expression. The connection is open when the expression evaluates to true and closed when the expression evaluates to false.

- `Irreversible State Change`

  Provide a CEL expression and an initial state (`Open` or `Closed`).

  The state switches once the expression evaluates to true but then remains in that opposite state (that is, opposite to the initial condition) regardless of what happens to the expression after that point.

For details on conditional connection control, see Conditional Connections in the CFX-Solver Modeling Guide.

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