## 5.5. Defining Domain Interfaces as Thin Surfaces

In Release 12.0, thin surfaces have been implemented to enable the modeling of physics (such as heat transfer, mass transfer, and momentum losses) across thin-surfaces. In previous releases, the Create Thin Partner option created two independent walls, which did not enable physics to be implemented. Starting with Release 12.0, you can model a thin surface either as:

- A boundary condition (for details see Default Boundary Condition in the CFX-Pre User's Guide)

- A domain interface with the physics modeled across the interface.

The types of domain interfaces you can define as thin surfaces depends on the domains involved:

Domains | Allowed Domain Interface
--- | ---
Fluid-Fluid | Solid
Fluid-Porous
Porous-Porous
Solid-Fluid | Solid or Constant-Property Substance[a]
Solid-Porous
Solid-Solid
[a] A Constant-Property Substance could be a fluid that has no motion and that is not pressure-dependent.

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