### 5.4.3. GGI (General Grid Interface) Connections

General Grid Interface (GGI) connections refer to the class of grid connections where the grid on either side of the two connected surfaces does not match. In general, GGI connections permit non-matching of node location, element type, surface extent, surface shape and even non-matching of the flow physics across the connection.

All model options within CFX (for example, turbulence models, multiphase models, particle tracking, mixture models, reaction, and so on) support the use of GGI connections. Any number of GGI connection conditions are possible within a computational domain.

The GGI connection will be made in a conservative and implicit fashion, even if the nodes on the two sides of the connection are not aligned. Likewise, the element types on each side do not have to match. In addition, if the size of the connection region on one side is different than that on the other side, the connection will be made automatically between the mutually-overlapping surfaces. Finally, it is possible to perform a connection where there is a slight gap or interference between the two sides of the GGI connection. Here, “slight” means an interference or gap on the order of ½ the average depth of the elements touching the connection surface. This permits connections where the surfaces do not fit together perfectly (for example, curved connection with different grid densities on each side), or where the components are not perfectly aligned with each other. Even in these situations, conservation is guaranteed and strictly enforced; for details, see GGI and MFR Theory in the CFX-Solver Theory Guide.

The regions that are on each side of a GGI connection are allowed to be of different size and are allowed to have non-overlap regions (see Non-overlap Boundary Conditions). The extent of the non-overlap region is reported in the solver output file (see CFX-Solver Output File (GGI Runs) in the CFX-Solver Manager User's Guide). If the extent of the non-overlap area is larger than expected, confirm that the correct regions have been selected for each side of the interface. There are Intersection Control settings that can be used to increase the accuracy of the overlap calculation if necessary (see Intersection Control: Option in the CFX-Pre User's Guide).

#### 5.4.3.1. Non-overlap Boundary Conditions

The portions of the connecting surfaces in a GGI connection that do not overlap each other behave by default in the same way as the default wall boundary condition that is made by CFX-Pre in a fluid domain. This default condition is a no-slip adiabatic wall with a zero-flux condition applied to all other transport equations. Particles will bounce off these walls with restitution coefficients of 1, and radiation will see an adiabatic wall.

The default behavior can be changed after the domain interface has been created by visiting the boundary condition object for the side of the interface where you want the boundary condition changed. For example, you may choose to set a wall roughness, or set a wall velocity, or change to a free slip wall. You can also change the zero-flux condition for any transport equation. For example, you can enable heat transfer or the transport of Additional Variables.

CFX-Solver will output a variable, `Nonoverlap Fraction`, for each side of the interface to indicate which nodes are in the overlap portion (`Nonoverlap Fraction=0`) and which are in the non-overlap portion (`Nonoverlap Fraction=1`) of the interface. This variable is available to CFD-Post. For example, you could multiply a quantity in an expression by `Nonoverlap Fraction` or (`1-Nonoverlap Fraction`) to ignore contributions from nodes on a GGI interface that are in the overlap portion, or non-overlap portion, respectively.

#### 5.4.3.2. Conditional Connections

Conditional Connection Control enables you to use an expression to control whether an interface is open (connected) or closed (not connected; a wall boundary is applied). This feature can be used to avoid having to use a moving mesh to suddenly snap interface surfaces apart at a given time (in order to force a non-overlap condition to exist), or to use multiple configurations to load different meshes during a simulation.

To set up a conditional connection control, configure the Conditional Connection Control settings on the Additional Interface Models tab for the domain interface. These settings are described in Conditional Connection Control in the CFX-Pre User's Guide.

Note: When conditional connection control is used, the Mesh Connection option (on the Mesh Connection tab) is forced to be `GGI`.

The CFX-Solver will write diagnostics information to the .out file whenever the state of an interface with a specified Conditional Connection Control is changing.

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