## 14.1. Default Boundary Condition

You should be familiar with the concept of primitive and composite regions before reading this section. If you are not, see Mesh Topology in CFX-Pre for details.

When a domain is created, all of the bounding 2D regions that are not used elsewhere are assigned to a default boundary condition that is created automatically. These regions can be considered to be the boundary between the current domain and the rest of the "world". The boundary that is generated is given the name `<Domain name> Default`. When 2D primitives (or composites that reference them) are assigned to other boundary conditions and domain interfaces, they are removed from the `<Domain name> Default` boundary condition. The default boundary condition is a no-slip adiabatic wall, but this can be edited like any other boundary condition. Solid-world 2D primitives behave in a similar way.

### Removing Regions from the Default Domain

Fluid-solid regions are initially contained in the `<Domain Name> Default` boundary condition. When a CFX-Solver input file is written, or a user-defined domain interface is created, any fluid-solid regions referenced by this interface are removed from the default boundary.

If every region is assigned to another boundary condition, the `<Domain Name> Default` boundary object will cease to exist. In such a case, if a boundary condition is subsequently deleted, the `<Domain name> Default` wall boundary will be recreated for the unspecified region. Because the `<Domain name> Default` wall boundary condition is controlled automatically, you should never need to explicitly edit its Location list.

### Internal 2D Regions

Any 2D regions that lie within a domain are ignored unless a boundary condition is explicitly assigned (these are treated as thin surfaces). Each side of a fluid-fluid 2D primitive can have a different boundary condition, but most often both sides will be a wall. Thin surfaces are created by assigning a wall boundary condition to each side of a fluid-fluid 2D region. You can specify physics (such as thermal conduction) across thin surfaces in CFX-Pre by defining a domain interface. For details, see Defining Domain Interfaces as Thin Surfaces in the CFX-Solver Modeling Guide.

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