### 13.1.3. System Coupling Related Settings in CFX

- When using System Coupling, you should set the Solution cell property “Keep Latest Solution Data Only” to “False” (which is not the default) to retain old solution data. ”Keep Latest Solution Data Only” is described in Properties View in the CFX Introduction.

- Run Calculation

  - When running as part of a transient coupled analysis, the step size for the analysis and duration of the analysis are controlled by the System Coupling service.

    - The time step size(s) specified in the CFX setup is ignored. The CFX solution will be advanced using the time step size specified as part of the System Coupling setup.

    - The number of time steps specified in the CFX setup is also ignored.

  - The specified Maximum Number of Iterations corresponds to the maximum number of nonlinear solver iterations performed per coupling iteration.

- Wall boundary participating in System Coupling

  - The System Coupling option must be selected for the Mesh Motion.

- Heat Transfer

  - Heat transfer is only available on walls. It is not available on inlets/outlets/openings.

  - The heat transfer option for a wall boundary must be set to `System Coupling` (Boundary Details > Heat Transfer > Option set to `System Coupling`) in order to obtain wall temperature from other participants taking part in the coupled analysis.

  - A proper value for the htc reference temperature promotes fast solution convergence.

    By default, the htc reference temperature is set to the near wall temperature. This default behavior is usually suitable when the y+ value (near wall element thickness) is small, particularly when there is also a large variation in wall temperature.

    In cases where the y+ value is large, the reference temperature might not be accurate, in which case you can override the default behavior by manually specifying a value for the htc reference temperature.

    To do this, select Expert Parameters > Physical Models > Miscellaneous > tbulk for htc and then enter a (constant) temperature value.

    A specified near wall reference temperature applies to the entire wall. If there is a large temperature variation over the wall, you should specify an average expected wall temperature.

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