## Chapter 3: GGI and MFR Theory

This chapter provides an overview of the numerical methods used for GGI (general grid interface) in CFX.

A control surface approach is used to perform the connection across a GGI attachment or periodic condition. A physically based intersection algorithm is employed to provide the complete freedom to change the grid topology and physical distribution across the interface.

A general intersection algorithm permits connections to be successfully made, even when the resultant surfaces on either side of an interface do not physically “fit” together to form a well defined physical connection. In addition, an automatic surface trimming function is performed by the GGI algorithm, to account for mismatched surface extent. This means that a GGI attachment or periodic condition can be successfully defined where the surface on one side of the interface is larger (in extent) than the surface on the other side of the interface. The interface is constructed between the overlapping regions of the two sides of the interface.

Multiple Frames of Reference (MFR) allows the analysis of situations involving domains that are rotating relative to one another. For CFX, this feature focuses on the investigation of rotor/stator interaction for rotating machinery. Because MFR is based on the GGI technology, the most appropriate meshing style may be used for each component in the analysis.

When using a Reynolds stress turbulence model (see Reynolds Stress Model in the CFX-Solver Manager User's Guide) the contribution of the Reynolds stresses to the momentum equations at GGI interfaces is approximated by an eddy viscosity approach. This approximation influences only the discretization of the control volumes along the interface and is expected to be justifiable compared to the other approximations at the interface.

This chapter describes:

- Interface Characteristics
- Numerics

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