Subham Sett
Engineering Specialist
Simulia
Providence, R. I.

Multiphysics is used in industry to simulate airplane-wing flutter, underwater shock effects on naval structures, and so on. Improved software technology and rising hardware capacity now allow the simulation of many such real-world problems.

FSI (fluid-structure interactions) is a class of multiphysics that studies the effects of fluid flow on structures and subsequent interactions. Primary fields that interact across domains are pressures for fluid and displacements for structure. Problems where thermal effects are significant also involve an additional temperature field in both domains. Besides primary fields there are secondary fields, such as piezoelectric effects in the structural domain and cavitation effects in the fluid domain, that indirectly contribute to fluidstructure interaction.

What is called the partitioned approach to FSI provides the most general-purpose technique. In Abaqus FEA software, for example, a partitioned or cosimulation approach can be used to solve complex FSI analyses by coupling the program to external computationfluid- dynamics (CFD) solvers. In this scheme, Abaqus and the external solver run concurrently and solve structural and fluid equations independently while exchanging converged solution quantities at the fluid-structure interface.

Communication is critical
Seamless communication between FEA and CFD is critical to the coupled approach because the programs are likely running on remote systems or different hardware. Communication takes place with either an independent coupling or a direct-coupling interface. The recently introduced Simulia multiphysics program supports both. Deciding which to use depends on how far the analysis needs to go to get needed results — in other words, traditional engineering insight. Each coupling has certain advantages.

For example, an independent coupling such as MpCCI (the Meshbased parallel Code Coupling Interface), a code-coupling interface from the Fraunhofer SCAI in Germany, provides flexibility by letting users link Abaqus to CFD code they may already have. MpCCI also works with Star-CD from CD-adapco and Fluent from Ansys. The independent-coupling approach helps foster collaboration between existing FEA and CFD engineering groups.

Use of the Simulia Direct Coupling Interface (DCI), on the other hand, provides a tighter integration between Abaqus and certain third-party CFD solvers, without requiring additional software components. This approach comes in handy for FEA engineering groups who must solve FSI problems on their own. The direct coupling became available with Abaqus Version 6.7 and works with AcuSolve CFD software from Acusim.

Although this discussion focuses on FSI, it is worth noting that either approach works with other external physics solvers as well.

Another helpful element comes into play here. An FSI module available as a plug-in to Abaqus provides a user-friendly way to manage the FSI workflow. The workflow is standardized regardless of the CFD code used and includes these steps: define the CFD model, interactions, and properties, run the analysis, and postprocess the solution.

Analysis of a peristaltic pump
A good example of an FSI problem comes from the analysis of a peristaltic pump. The pumps are used in a wide array of industries, pumping everything from clean or sterile fluids in biomedical devices to corrosive fluids in chemical-processing applications.

The first step to running an analysis is a single Abaqus/ CAE database with two models: one for the flexible hose and the relevant pump structural parts, the other for the corresponding fluid region. The structural model is fully defined and set up to run as an Abaqus/Explicit analysis. A mesh is also generated for the fluid model. Basic CFD settings for the fluid model will be provided in the FSI module.

The overall FSI model is defined in what is called the Study. Here users select the CFD code, identify the coupling step in the structural model, and define the basic CFD model by specifying material type and boundary conditions. The fluid model data is then written in a format supported by the selected CFD code. The appropriate CFD preprocessor launches and users can then define the rest of the CFD model.

The next step associates the structural and the fluid models. A “Create Interaction” dialogue box lets users select from the list of regions in the structural model (for example, the hose exterior and interior), and in the fluid model (for example, blood flow in, flow out, and wall boundary) that can be coupled. Next comes using a “Create Interaction” property to select solution quantities. Users then define coupling specifics such as time-step values. MpCCI users can specify additional settings for the coupling. The analysis is then executed.

Solution results are postprocessed in the Visualization module of Abaqus. Fluid data is extracted in the Abaqus output database format and viewed in conjunction with structural results. Coupled results illustrate the pumping action of the pump.

Beyond couplings
A partitioned approach as illustrated by the FSI solution above is an excellent tool for solving a wide range of multiphysics problems. But it has limitations. For example, numerical distortions can happen when handling interfaces in the structural domain that include extreme contact, severe deformation, and damage or failure.

An example of such a problem comes from the sloshing of liquid inside a tanker that is hit by an outside force. What is the maximum impact load the tanker can withstand? The liquid is responsible for a significant portion of the container loading, and any severe deformation of the tank can lead to rupture and potential spillage. The event is thus highly dynamic and requires studying progressive damage and failure of the interface material.

To tackle problems like this, a coupled Eulerian-Lagrangian (CEL) method is being developed in Abaqus/Explicit. CEL uses multimaterial finite-element formulations to handle the structure and simple fluids behavior, in a single framework. It thus alleviates requirements for continuity in fluid mesh topology, necessary for a coupled approach. CEL will be suitable for solving many interesting FSI problems in industry including tire hydroplaning, automotive airbag inflation, and liquid-product dispensing.

— Edited by Leslie Gordon