CAE-Companion-2018-2019

Theory WISSEN CAE

Simulation of Fluid Structure Interaction

other part of the coupled system. This iteration process is continued until convergence is reached in the solution of the coupled equations.

Since the invention of the computer, several CAE methods for structure and fluid dynamics analysis were success- fully developed by universities, research institutions and engineering software vendors and have been established as standard tools in the daily design practice in the automotive, aerospace, energy, manufacturing and other industries. On the other hand and for several reasons, development of CAE methods for flow and structure analysis was done for both, flows and structures, independently from each other in most cases, without really realizing it, a kind of thought had been established during this time which left interactions between both engineering disciplines for several years practically aside. As a consequence, despite the fact that both numerical algorithms for fluid-structure interactions (today referred by most authors as „iterative“ and „direct“ or „monolithic“) were developed by the author already in the mid eighties having highly nonlinear membrane problems like parachutes, sails and hang-gliders in mind, it took at least two additional decades to stimulate the interest of the industry on flu- id-structure interaction simulations. Starting from established integral simulation programs like the famous ADINA, which offers to the user not only the full (i.e. both „direct“ and „iterative“) fluid-structure interaction (FSI) capability but also full thermal fluid-structure interaction analysis (TFSI) in a really seamless development environment, down to highly specialized FSI simulation tools like PARA2G for gliding parachutes, it is very gratifying to see that in recent years more and more researchers and software vendors started to deal with solutions for a continuously increasing number of FSI applications. Fluid Structure Interaction Basics In fluid structure interaction analysis, fluid forces are applied on the solid and the solid deformation changes the fluid domain. The computational domain is divided into the fluid domain and the solid domain, where the fluid and the solid model are defined respectively, through their material data, boundary conditions, etc. The interaction occurs along the interface of the two domains. This is called the fluid-structure interface. Having the two models coupled, simulations and predictions of many physical phenomena can be performed. In general we distinguish between two general algorithms for fluid-structure interaction: „ „ Iterative or two-way coupling: This algorithm is sometimes also called the partitioned method. In this iterative solution method, the fluid and solid solution variables are fully coupled. The fluid and the solid equations are solved individually in succession, always using the latest information provided from the

Figure 1: Iterative two-way coupling „ „ Direct or monolithic coupling:

This algorithm is sometimes also called the simultaneous solution method. In this direct solution method, similar to the procedure in the above iterative solution method, the fluid and the solid solution variables are also fully coupled but here the fluid and the solid equations are combined and treated in one single system.

Figure 2: Direct (or monolithic) coupling The direct coupling algorithm requires a code de- veloped for a particular combination of the physical problems while the iterative coupling algorithm preserves software modularity because existing flow and structural solvers can be coupled in order to implement it. In addition the iterative approach facilitates solution of the flow equations and the structural equations with different, possibly more efficient numerical techniques which have been developed specifically for either the flow or the structural part of the problem. Explicit airbag simula- tion is an example for this kind of procedure. On the

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