## 3.1. Interface Characteristics

The numerical algorithms employed, as well as the control surface treatment of the numerical fluxes across the interface, are designed and implemented in such a way as to provide for maximum robustness and accuracy. The treatment of the interface fluxes is fully implicit and fully conservative in mass, momentum, energy, scalars, and so on. This means that the multigrid solver can be applied directly, without any penalty in terms of robustness or convergence rate, to problems involving GGI conditions. Each of the different types of GGI interfaces have the following attributes:

1. Strict conservation is maintained across the interface, for all fluxes of all equations (after accounting for changes in pitch).

2. The interface treatment is fully implicit, so that the presence of an interface does not adversely affect overall solution convergence.

3. The interface is applicable to incompressible, subsonic, transonic and supersonic flow conditions, and all model options within CFX (for example, turbulence models, multiphase models, mixture models, CHT, reaction, and so on).

4. The interface accounts internally for pitch change by scaling up or down (as required) the local flows as they cross the interface, for the case of frame change interfaces.

5. Any number of GGI connection conditions are possible within a computational domain.

The surface fluxes along each side of the interface are discretized in terms of nodal dependent variables, and in terms of control surface equations and control surface variables.

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