Keywords: CFD, CFD-FEM code coupling, fluid-structure interaction, data interface, coupling library, moving grids, multi-physics, parallel computing.

A challenge in engineering computational technology is the simulation of multi- physics applications, involving different physical phenomena. An important example is the simulation of flows with a strong mechanical or thermal interaction with the surrounding solid structure, which is of great relevance in industry. This type of problem can be found in external aerodynamics (wing fluttering), mechanical engineering (hydraulic valves), rotating machinery (blade fluttering), biomedical applications (artery flows, heart valves), etc. for a correct simulation of these cases, the fluid dynamics must be solved simultaneously with the structural analysis. In practice sophisticated commercial software packages exist to solve both problems, fluid dynamics and stress analysis individually. A strategy to analyse such cases is to use existing software packages in order to benefit from their advanced features and to develop a powerful coupling technique.

CFX is a leading and established CFD code for fluid dynamics [1,2], which has been used for a number of projects where coupling with other software packages was required. Originally file based data exchange was realized in order to couple CFX with ANSYS for the analysis of heart valves [3]. A shortcoming of this coupling method is that it is based on a direct coupling interface between two different codes, which is very specific and requires continuous maintenance with every release of each software package. As a consequence, the goal is to have a standardized interface, which allows coupling of different CFD and FEM (structural analysis) codes without further maintenance. The coupling library with unified interfaces MpCCI, which is being developed by Pallas GmbH and Frauenhofer Gesellschaft SCAI provides such a solution [4]. In addition, MpCCI offers a solution to couple software packages running in parallel mode, without a sequential bottleneck. This is essential for large and computer intensive applications in CFD and fluid-structure interaction applications.

The MpCCI concept was developed and tested as part of the EU project DEBUT (Multi-Disciplinary Engineering by Using Coupling Technology). In this particular project an interface between CFX-5 and MSC.Marc [5] via the MpCCI interface software was implemented. The coupling interface in CFX-5 is implemented in order to have a larger flexibility for the import and export of data. CFX-5 has an implicit scheme solving the non-linear Navier-Stokes equations. The solution procedure contains a time step loop for proceeding in physical time and a coefficient loop for continuously updating the non-linear coefficients of the equations. Two coupling strategies are realized. For a weak coupling interaction between the two codes is only required during the time step loop. Stronger coupling is enabled by data exchange during the coefficient loop. The overall interface strategy, the required moving mesh algorithms, and the data transfer method is described. The advantage of the coupling technique is demonstrated for a number of applications such as a bending flap, a hinged flap and thermal coupling. Furthermore, the parallel performance of the software is presented, which reduces the overall turnaround time and memory requirements per processor. The interface has no negative impact the good parallel performance of the underlying software package.

1

Raw, M.J., "Robustness of coupled algebraic multigrid for the Navier-Stokes equations", AIAA Paper 96-0297, 1996.

2

Menter, F.R. and Esch, T. "Elements of Industrial Heat Transfer Predictions; presented at the 16th Brazilian Congress of Mechanical Engineering (COBEM), Nov. 2001, Uberlandia, Brazil, 2001.

3

Penrose, J.M.T., Hose, D.R., Staples, C.J., Hamill, I.S., Jones, I.P. and Sweeney, D., "Fluid Structure Interactions: Coupling of CFD and FE", 18. CAD-FEM Users' Meeting, Internationale FEM-Technologietage, Sep. 20- 22, 2000.

4

MpCCI Specification, Release 1.2, November 2000; MpCCI Homepage http://www.mpcci.org

5

MSC.Marc Documentation, MSC.Marc 2001, MSC Software.

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