## 13.2. Coupling CFX to an External Solver: ANSYS Multi-field Simulations

Some simulations require the coupling of CFX-Solver to other solvers in order to take account of physics that cannot be readily simulated within CFX. For example, bidirectional (that is, two-way) fluid-structure interaction (FSI) requires CFX to couple to an FEA (structural) solver when the structural physics is more complex than CFX can simulate with its CHT capability. Examples of when this is appropriate are described in Reference Guide for Mesh Deformation and Fluid-Structure Interaction in the CFX Reference Guide.

Coupled simulations follow a timestep/iteration structure that is similar to the one within the CFX-Solver. During coupled simulations, the ANSYS CFX and any external field solvers (such as the Mechanical application solver) run the simulation through a sequence of multi-field timesteps, each of which consists of one or more "stagger" (or coupling) iterations. During every stagger iteration, each field solver gathers the data it requires from the other solver, and solves its field equations for the current multi-field timestep. Stagger iterations are repeated until a maximum number of stagger iterations is reached or until the data transferred between solvers and all field equations have converged. The latter guarantees an implicit solution of all solution fields for each multi-field timestep.

The only coupling capability that is currently available enables CFX to couple to the Mechanical application solver within an ANSYS Multi-field simulation. The MFX solver is a version of the ANSYS Multi-field solver that enables the coupling of CFX with ANSYS Mechanical or ANSYS Multiphysics. It allows simulations where it is necessary to couple the fluid physics and structural physics throughout the solution process, rather than just by passing data from one solver to the other at the end of the simulation. Some examples of such simulations are given in Using CFX and the Mechanical Application in the CFX Reference Guide.

Data is communicated between the CFX and Mechanical application field solvers through standard internet sockets using a custom client-server communication protocol. This implies that no third-party coupling software is required, and maximizes execution robustness.

The ANSYS Multi-field requires the setup of the fluid physics in CFX, the setup of the structural physics for the Mechanical application, and the specification of the MFX coupling settings. This is described in Preprocessing.

The following topics are discussed:

- Preprocessing
- Running an MFX Simulation
- Postprocessing
- General Information

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