### 5.4.4. Mesh Connection Recommendations

Domain interfaces that use GGI connections are a very powerful and flexible mesh connection method, but they do require some additional computational effort and memory, and may introduce numerical inaccuracy compared to an equivalent computation that does not use GGI connections. For these reasons, you should use GGI connections wisely and sparingly.

For periodic interfaces, GGI connections always introduce some error into your simulation compared with direct connections. This is because direct connections solve the equations as if the mesh were actually replicated, so there is no additional discretization error due to the presence of the interface.

For simulations involving conjugate heat transfer, on the other hand, direct connections are less desirable than GGI connections because in direct connections the discretization of heat flow through fluid-solid, porous-solid, and solid-solid interfaces is nonsymmetric (that is, direct connections use more information from the more conductive side of the interface, while GGI connections sample both regions equally). The direct connection’s nonsymmetric treatment may lead to accuracy and/or convergence difficulties in the following situations:

- The domain interface is fluid-solid or porous-solid.

- The domain interface is solid-solid and there is a large conductivity ratio across the interface.

Note:

- In laminar domains, fluid-solid interfaces should be GGI to avoid the possibility of a Class 3 error.

- To avoid spurious overshoots and undershoots of the solution in the vicinity of the interface, the Automatic setting employs GGI for fluid-solid, porous-solid, and solid-solid interfaces.

In these situations, you should consider using a GGI connection rather than a direct connection, even if a direct connection is possible.

If you believe that an interface should be a one-to-one connection, but a GGI connection is made instead, then you should first check that you have set the correct rotation axis for a rotational periodic interface. Next, you should load the CFX-Solver input file into CFD-Post to check that the mesh appears to match. You can plot the mesh on each side of the interface using a different color to check that the two sides match. In the case of periodic interfaces, you can use an instancing transform to manually perform the transformation and then check that the meshes match. Before increasing the Mesh Match Tolerance, you should try to reproduce the mesh such that it matches within the default tolerance. If the meshes appear to match but a GGI connection is still created, you can increase the Mesh Match Tolerance. Increasing the tolerance above 0.02 (2%) is not recommended as this will result in a loss of accuracy.

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