### 5.2.2. Bidirectional (Two-Way) FSI

In some simulations, there is a strong and potentially nonlinear relationship between the fields that are coupled in the Fluid Structure Interaction. Under these conditions, the ability to reach a converged solution will likely require the use of bidirectional FSI. As for unidirectional interaction, examples are provided below that demonstrate the variety of strategies to execute such simulations.

#### 5.2.2.1. Using CFX Only

Conjugate heat transfer is an example of bidirectional interaction that can be solved using the CFX-Solver only.

#### 5.2.2.2. Using CFX and the Mechanical Application

Communicating data between CFX and the Mechanical application is automated by the MFX branch of the ANSYS Multi-field solver. In this branch of the ANSYS Multi-field solver, data is communicated between the CFX and the Mechanical application field solvers through standard internet sockets using a custom client-server communication protocol. This custom solution maximizes execution efficiency and robustness, and greatly facilitates future extensibility.

Setup requires creation of the fluid and solid domain/physical models in the CFX-Pre and the Mechanical application user interfaces, respectively, and the specification of coupling data transfers and controls in the CFX-Pre user interface. Execution and run-time monitoring of the coupled simulation is performed from the CFX-Solver Manager. Note that a dedicated MFX-ANSYS/CFX tab is also provided in the ANSYS Product Launcher to begin execution of the coupled simulation (see General Procedure in the CFX-Solver Manager User's Guide).

Refer to the following sections for more information:

- Coupling CFX to an External Solver: ANSYS Multi-field Simulations in the CFX-Solver Modeling Guide

- Multi-field Analysis Using Code Coupling in the Mechanical APDL Coupled-Field Analysis Guide.

Coupled simulations begin with the execution of the Mechanical application and CFX field solvers. The Mechanical application solver acts as a coupling master process to which the CFX-Solver connects. Once that connection is established, the solvers advance through a sequence of six pre-defined synchronization points (SPs), as illustrated in Figure 5.4: Sequence of Synchronization Points. At each of these SPs, each field solver gathers the data it requires from the other solver in order to advance to the next point.

The first three SPs are used to prepare the solvers for the calculation intensive solution process, which takes place during the final three SPs. These final SPs define a sequence of coupling steps, each of which consists of one or more stagger/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 coupling step. 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 fields for each coupling step.

Figure 5.4: Sequence of Synchronization Points

#### 5.2.2.3. Using CFX and Other CAE Software

Third-party code-coupling software or proprietary interfaces provided by the CAE software vendors can also be used in conjunction with CFX. Contact those software providers and your CFX service representative for more information.

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