## 5.7. Automatic Creation and Treatment of Domain Interfaces

You should generally create all domain interfaces that are needed in your simulation explicitly in CFX-Pre. However, CFX-Pre will create default domain interfaces in certain situations.

Domain interfaces will be automatically created, at the Write Solver File stage, when there are multiple domains within a single assembly and you have not explicitly created all the required domain interfaces. These could be `Fluid Fluid`, `Fluid Solid` or `Solid Solid` domain interfaces depending on the type of domain on either side of the interface.

When a default `Fluid Fluid` interface is created and there is no change in reference frame across the interface, the Frame Change option is set to `None`. When a frame change occurs, the Frozen Rotor model is used. If these models are not appropriate, you should explicitly create all domain interfaces to prevent the automatic creation of interfaces.

When a one-to-one node mapping does not exist, you must manually create the domain interfaces. If multiple domains are created using separate assemblies, domain interfaces are always required.

Fluid-solid 2D primitives are initially contained in the default domain boundary conditions. Any that remain there when a CFX-Solver input file is written are assigned to default `Fluid Solid` domain interfaces.

If you create a MFR case using 3D primitives from a single assembly, then a one-to-one node pairing will exist because an assembly contains a continuous mesh. However, a GGI connection is required for MFR cases. A default domain interface using the `Frozen Rotor` frame change model will be automatically created if no other domain interfaces are created.

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