## 14.14. Bidirectional (Two-Way) Couplings with ANSYS Multi-field

Enabling an external solver coupling with ANSYS Multi-field introduces a coupling (or stagger) iteration layer in addition to the time step and coefficient loop iteration structure used for simulations involving CFX only. As outlined in Bidirectional (Two-Way) FSI, time steps are executed using a sequence of coupling iterations, which involve one or more coefficient loop iterations within either the CFX or ANSYS field solver.

Additional memory is not required when external solver couplings are used. However, additional CPU time is required because of the CFX solver coefficient loops performed per coupling iteration. In general, the CFX solver CPU usage will increase by a factor that is slightly smaller than the number of coupling iterations required per time step. A summary of expected CPU time increases is tabulated below, according to the degree of coupling between the fluid and solid physical problems.

Degree of Coupling | CPU Increase
--- | ---
Weak | 2× to 5×
Typical | 5× to 10×
Strong | > 10×

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