### 13.1.2. Variables Available for System Coupling

The following variables are available on all boundary wall regions.

Table 13.1: Variables On Boundary Wall Regions

Display Name / Internal Name | Transfer Direction | Data Type | Physical Type
force / `force` | Output | VectorXYZ* | Force
displacement / `displacement` | Input | VectorXYZ* | Length
temperature / `temperature` | Input | Scalar | Temperature
heat flow / `heatflow` | Output | Scalar | Heat Rate
heat transfer coefficient / `heat-transfer-coefficient` | Output | Scalar | Heat Transfer Coefficient
HTC Reference Temperature / `HTC Reference Temperature` | Output | Scalar | Temperature

#### 13.1.2.1. Wall Force Data transferred from CFX System to System Coupling System

Force transferred from a CFX wall boundary in a System Coupling case is, by default, based on the solved pressure, which is relative to the specified reference pressure. You can change the force calculation to be based on absolute pressure by setting the expert parameter `include pref in forces = t`.

Viscous forces are included in the force transferred.

#### 13.1.2.2. Displacement transferred from System Coupling System to CFX System

The following applies in a coupled analysis where displacement is received by CFX.

- The displacement data received by CFX represents the incremental displacement for the current time step (the incremental displacement since the end of the previous time step).

- When the System Coupling analysis type is “general” (as opposed to “transient”), and when CFX solves before or simultaneously to the solver sending the displacement (such as Mechanical), then during the first coupling iteration of each coupling step the displacement received is 0 [m]. This behavior can be changed by using `GeneralAnalysis_IncrDisp_InitIterationValue_Zero` in System Coupling’s Expert Settings .

#### 13.1.2.3. Thermal Data exchange between CFX System and System Coupling System

A CFX system provides EITHER

- heat flow, OR

- (for cases not involving radiation) heat transfer coefficient reference temperature plus heat transfer coefficient

to a System Coupling system.

A CFX system receives wall temperature from the System Coupling system.

For multiphase flows:

- If the CFX system is configured to send heat flow to the System Coupling system, the heat flow is a bulk quantity.

  If instead the CFX system is configured to send the heat transfer coefficient reference temperature plus heat transfer coefficient to the System Coupling system, the heat transfer coefficient reference temperature and heat transfer coefficient are bulk quantities.

- The received temperature is applied as a bulk temperature.

Release 18.2 - © ANSYS, Inc. All rights reserved.