Does Fluid Flow Affect Your Product?

	Bob Williams Product Manager ALGOR, Inc. Pittsburgh, PA

Multiphysics software helps you find out by simulating fluid-structure interaction.

This article was published in Machine Design, "Solid Ideas for Fluid-Structure Simulations", July 13, 2006.

When a fluid flows around or through a product, it can cause significant displacement and stress. Understanding and accounting for this type of fluid-structure interaction (FSI) is often an important consideration during the design process.

Typical applications where engineers study FSI include:

• A wind load on the face of a building.
• A "water hammer" pressure surge or shock wave that occurs inside piping when moving water is forced to stop or suddenly changes direction.
• A fan assembly to ensure that tolerances between the blades and housing are maintained.

Forces exerted by fluid flow on a fan were calculated during a steady fluid flow analysis (left). Results from the fluid flow analysis were then applied as loading in a linear static stress analysis (center) to determine the structural effects of fluid flow on the fan blades (right).

In the past, engineers largely relied on estimated force loads to represent fluid flow effects in structural analysis. Now, modern multiphysics software automates the process of simulating FSI – removing the guess-work, speeding up the analysis preparation and increasing accuracy.

How to Simulate FSI

The process of simulating FSI involves the following general steps:

1. Create a finite element model of both the fluid and structure with the contacting surfaces matched exactly to allow for transfer of analysis results.
2. Perform a fluid flow analysis on the fluid to calculate the force exerted by the fluid on the solid boundaries.
3. Perform a structural analysis on the product with force loads applied from the fluid flow analysis results.
4. Evaluate the structural analysis results – displacements, stresses and more – to see the effects of the fluid flow force loads on the structure.

Whether the fluid flow and structural analyses are performed independently (where first the fluid flow analysis runs to completion and then the force results are input to a subsequent structural analysis) or interactively (where both analyses are run simultaneously and results are transferred as each load step is solved), the general concept is essentially the same.

Importance of Meshing

As with any fluid flow analysis, the quality of the finite element mesh can greatly affect the speed and accuracy of the solution. Meshing is particularly important for FSI though because the fluid flow results of interest only occur where the fluid contacts the structure; the results in the rest of the fluid domain are relatively unimportant.

Multiphysics software packages provide tools that make it fast and easy to create a suitable model for FSI simulation. For example, ALGOR's multiphysics software provides a suite of modeling and meshing capabilities for fluid flow analysis including:

• Automatic modeling of the fluid medium based on a CAD solid model – The user simply specifies the surfaces that bound the region of the fluid medium using a built-in dialog and then ALGOR software automatically creates new parts where fluid flow analysis will be performed.

ALGOR's multiphysics software allows the automatic modeling of the fluid medium based on a CAD solid model. The user simply specifies the surfaces that bound the region of the fluid medium using a built-in dialog and the software automatically creates new geometry where the fluid flow analysis will be performed. This makes modeling fluid flow systems easier and faster.

• Boundary layer meshing – The mesh engines automatically generate a finer mesh (with smaller, more numerous elements) near the fluid boundary while keeping a coarser mesh throughout the rest of the fluid domain.

ALGOR software automatically generated a finer mesh near solid surfaces while keeping a coarser mesh throughout the rest of the fluid domain. The boundary layer meshing capability quickly and easily created a mesh that was optimized for accurately simulating the detailed behavior of fluid flow along its boundary.

With these automatic modeling and meshing tools, users can easily create a finite element mesh that is suitable for FSI simulation.

Multiphysics Made More Practical

In addition to modeling and meshing tools, other recent technological developments have come together to make multiphysics analysis – such as FSI – more practical. These developments include:

• Support for more powerful operating systems – With the availability of 32- and 64-bit operating systems, affordable CAE packages now allow users to analyze larger, more complex models faster than ever. For example, ALGOR now supports 32- and 64-bit Windows, 32- and 64-bit Linux and 64-bit HP-UX operating systems for all analysis types including full multiphysics.
• Faster solvers – ALGOR recently added a new equal-order segregate solver for transient fluid flow analysis for faster runtimes with less memory. The new solver's segregated solution method breaks the global matrix into smaller sub-matrices, which are then solved quickly while using less computer memory.

Future CAE developments, including support for distributed processing, will continue to make FSI and other multiphysics applications part of your everyday design process.