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Understanding and accounting for this fluidstructure interaction (FSI) is often important to the design. "Typical applications for FSI include wind loads on the face of buildings and water hammer, the pressure surges or shock waves generated inside piping when water suddenly stops or changes direction," says Algor's Bob Williams. "Even a fan is an application. FSI software can predict how airflow will deform the blades and whether or not they'll hit the housing as a result. With that information, you can better set tolerances on the design."
Past FSI efforts relied largely on estimated force loads to represent fluid flow in structural analysis. "Multiphysics software now simplifies the process of simulating FSI. It eliminates guesswork, speed analysis, and increases accuracy," says Williams.
When using specialized multiphysics software, Williams suggests this approach to simulating fluid and structures:
 Create a finiteelement model of both the fluid and structure with contacting surfaces exactly matched so analysis results can transfer from one body to another.
 Perform a fluidflow analysis to calculate forces exerted by the fluid on the solid boundaries.
 Perform a structural analysis using the force loads predicted by the fluidflow analysis.
 Evaluate the structuralanalysis results, such as displacements and stresses, and gauge how fluidflow loads affect the structure.
Generally, the quality of the finiteelement mesh greatly affects speed and accuracy of the solution. Meshing is particularly important for FSI because the fluidflow results of interest are only where the fluid contacts the structure. Results in the rest of the fluid domain are relatively unimportant.
Multiphysics software has features that make it fast and easy to create suitable models for FSI simulation. Williams says more advanced software often has a suite of modeling and meshing features for fluidflow analysis. These include:
 Modeling the fluid based on CAD solid models. "Users specify surfaces that bound the fluid using a builtin dialog," says Williams. The software then automatically creates the new geometry (often the inverse of the solid) where fluidflow analysis will be performed.
 Boundary layer meshing. The mesher should generate a finer mesh near fluid boundaries and build a coarser mesh throughout the rest of the fluid.
In addition to modeling and meshing tools, Williams says other technological developments have converged to make multiphysics analysis more practical. For instance, support for more powerful operating systems. "With 64bit operating systems, affordable CAE packages let users analyze larger, morecomplex models faster than ever," he adds.
In addition, faster solvers also cut time off previous simulations. "A recent addition to our software is an equalorder segregate solver to handle fluid flow and coupled fluidflow and heattransfer analyses for faster runtimes with less memory. The new solver's segregated solution method breaks the global matrix into smaller submatrices, which are then solved quickly with less computer memory.
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