### 5.4.2. Direct (One-to-One) Connections

The direct (one-to-one) connection is available if, after any applicable rotational or translational transformation (such as for a periodic interface), all of the nodes on one side of the interface correspond in location with all of the nodes on the other side of the interface, to within a spatial tolerance governed by the Mesh Match Tolerance parameter. The latter can be set in CFX-Pre by changing Edit > Options > CFX-Pre > Mesh > Mesh Match Tolerance setting. For details, see Mesh in the CFX-Pre User's Guide. You can verify the type of interface that CFX-Pre will apply by right-clicking `Simulation` in the Outline tree view, and selecting Report Interface Summary from the shortcut menu.

In addition to requiring a precise node correspondence from one side to the other, a direct (one-to-one) connection requires that:

- The number of regions in the list for one side is equal to the number of regions in the list for the other side

- For each region in the region list for one side, there is exactly one region in the list for the other side that corresponds entirely in terms of the number of nodes and their positions

- Both sides of the interface are in the same domain for `Fluid Fluid` and `Solid Solid` interfaces

- The interface type is not `Fluid Porous` or `Porous Porous`

- A pressure change or mass flow condition is not set at the interface.

In addition, for periodic interfaces, CFX-Solver will change direct connections to GGI connections as necessary to ensure that every node that participates in multiple direct connections has only one unique required transformation (translation or rotation). For example, in Figure 5.9: Nodes Shared by Interfaces with Different Transformations, the corner nodes are involved in two interfaces that have different transformations. In this case, at least one of the interfaces will be changed to use a GGI connection.

Figure 5.9: Nodes Shared by Interfaces with Different Transformations

A direct (one-to-one) connection cannot be made when conditional connection control is used for the interface. For details on conditional connection control, see Conditional Connection Control in the CFX-Pre User's Guide and Conditional Connections.

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