Machine Design

Solid ideas for fluid-structure simulations

Fluid flowing around or through a component can cause significant displacement and stress.

Senior Editor

Forces exerted by fluid flow on fan blades are calculated during a steady fluidflow analysis (left). Analysis results become loads (center) for a linear staticstress analysis to determine structural effects on the fan blades (right).

Algor's multiphysics software models a fluid based on a CAD solid model. Users specify surfaces that bound the fluid region using a built-in dialog and the software creates geometry in which to analyze the flow.

Algor software generates a finer mesh near solid surfaces while keeping a coarser mesh throughout the rest of the fluid domain. Boundary-layer meshing is optimized for simulating a detailed flow behavior along its boundary.

Understanding and accounting for this fluid-structure 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 finite-element model of both the fluid and structure with contacting surfaces exactly matched so analysis results can transfer from one body to another.
  • Perform a fluid-flow analysis to calculate forces exerted by the fluid on the solid boundaries.
  • Perform a structural analysis using the force loads predicted by the fluid-flow analysis.
  • Evaluate the structural-analysis results, such as displacements and stresses, and gauge how fluid-flow loads affect the structure.

Generally, the quality of the finite-element mesh greatly affects speed and accuracy of the solution. Meshing is particularly important for FSI because the fluid-flow 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 fluid-flow analysis. These include:

  • Modeling the fluid based on CAD solid models. "Users specify surfaces that bound the fluid using a built-in dialog," says Williams. The software then automatically creates the new geometry (often the inverse of the solid) where fluid-flow 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 64-bit operating systems, affordable CAE packages let users analyze larger, more-complex models faster than ever," he adds.

In addition, faster solvers also cut time off previous simulations. "A recent addition to our software is an equal-order segregate solver to handle fluid flow and coupled fluid-flow and heat-transfer 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.

Algor Inc.,

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