Moving Up to Multiphysics Analysis

When the job calls for more than structural mechanics, it's time to explore thepower of single-environment, coupled-physics analysis.

When the job calls for more than structural mechanics, it's time to explore thepower of single-environment, coupled-physics analysis.

By Pamela J. Waterman



Sometimes the most commonplace technologies have the most esoteric physics. That'sthe assessment of Paul Lethbridge, ANSYS vice president for product development.And while we're not talking rocket science, they're involved in some rather intriguingdown-to-earth projects. Lethbridge says the most interesting technology he's workedwith recently involves the CTS Cadillac, which is fitted with “shock absorbersusing damper technology based on ferrofluidic devices: basically a fluid thatchanges viscosity based on magnetic fields.”

GM's magnetic selective ride control uses electromagnetically charged particlessuspended in shock fluid. When a magnetic field surrounding the shock absorberis activated, the fluid properties change to a near-solid state. Each wheel hasposition sensors that send real-time road condition information to a controllerand the system responds by hardening or liquifying the fluid; you can even chooseTour or Sport responsiveness. Thus, the designers need to analyze fluids, solids,and electromagnetics as a single system.

From catheters to circuit breakers and from microphones to MEMS (microelectromechanicalsystems), many of today's design challenges require evaluating not only more thanone aspect of physics, but also coupled, and often interdependent, phenomena.For users accustomed to doing just structural FEA (finite element analysis), thiscan mean stepping up to incorporate thermal, fluidic, and various electromagneticbehaviors in their design analyses. (See “Multiphysics Marvels,” below.)



Multiphysics Marvels
What types of physics problems can be combined within a coupled analysis? Accordingto Paul Lethbridge at ANSYS, the possibilities for
multiphysics analyses are seemingly endless:

Problem Type

Electromagnetic-charged particle Magnetic-Structural
Fluid-Solid Electromagnetic-Solid-Fluid Thermal-CFD

Example Application

Gas turbine
Acoustics, sonar, SAW
Induction heating, RF heating
Ion optics, field emission display technology, analytical instruments
Solenoids, electromagnetic machines
Aerospace, automotive fuel, hydraulic systems, fluid-bearing, drug delivery pumps,heart valves
Fluid handling systems, EFI, hydraulic systems Electronics cooling Microphones,sensors
Pressure sensors, accelerometers

Multiphysics analysis is a powerful approach that is becoming easier to applydue to improvements in offering a single, improved-GUI software environment withoutthe need for neutral file data exchange. Where it used to take almost handcraftingto format the results from one simulation as input for another, today's productsare tightly integrated. To find out just where multiphysics currently stands,we asked a number of developers to tell us what to look for, what to expect, andwhy analysts should buy into the technology.

ABAQUS heat-transfer capabilities, contact, and user subroutines facilitate complexsimulations, such as this application from Honeywell involving coupled carbonresistance heating in a vacuum between contacting bodies for electronics
packaging soldering.

Fundamental Physics at Work

One of the more common tasks asked of multiphysics software involves fluid-solidinteraction (FSI), the analysis of which takes place in many aspects of automotiveand aerospace design. But biotechnology engineers are using it too in productssuch as hydraulically steered intravenous catheters. And developing the newestMEMS devices can require even more combinations, integrating thermal, structural,fluidic, and electrostatic computations. Add transient issues such as changesin temperature, airflow, and electrostatics as optical or electronic parts heatup, and you have some complicated physics at work.

ABAQUS heat-transfer capabilities, contact, and user subroutines facilitate complexsimulations, such as this application from Honeywell involving coupled carbonresistance heating in a vacuum between contacting bodies for electronics packingsoldering.

The key to getting the most out of such analyses is tight coupling. “You wantto simultaneously study the effects, instead of taking the results from ModelA and then bringing them into Model B,” says MSC.Software's Reza Sadeghi, vicepresident of product development.

Sadeghi cites a case where a material with temperature-sensitive mechanical propertiesis analyzed. “The benefit is that the impact of, say, structural mechanics, istruly coupled with the thermal mechanics of the system. At the same time thatthe temperature is rising, we're modifying the material properties; at the sametime the material properties are modified, it impacts the way temperature maychange.”

In coupled multiphysics, one solves the relevant equations as a single mathematicallycoupled system (matrix coupling) that reflects real-world behavior. One matrixshould embody all of the degrees-of-freedom parameters that drive both the physicsand the fields of that physics, and converge upon a single solution. Paul Lethbridge at ANSYS cautions that a successful process must particularlymonitor the convergence at the interface where loads are transferred.

Above: ABAQUS offers a comprehensive library of connector elements to model differenttypes of connections or joints. These connectors—together with other capabilitiesin ABAQUS such as rigid bodies, display bodies, and finite rotation substructures—extendthe finite element analysis capabilities of ABAQUS to include flexible multibodysimulations.

UGS Director of Marketing Don Tolle agrees that bidirectional capabilities areimportant. “This enables the user to make design changes without having to manuallyset up boundary conditions when moving from one analysis domain to another (e.g.,NX TMG thermal results mapped as a load case onto an NX Nastran structural analysis).

This comparative analysis inside from PlassoTech looks at the thermalstress and dynamic response of an assembly structure with weldings. Accuracy ofthe mesh is helped by using automated adaptive techniques, a key when differentanalyses are concerned.

However, there are scenarios where it's difficult to efficiently write the combineddifferential equations that have all coefficients in one matrix. In these cases,you solve two matrices (performing sequential multiphysics) and “add” the results.This is the traditional staggered or cosimulated approach, where you pass theresults of one analysis as a load vector to the next.

Ever-shrinking gas-tank spaces in new vehicles must still handle effects likepremature gas-pump switch-off or backsplashing. MSC.Marc uses Multiple AdaptiveEuler Domains for Multiple Material when simulating and analyzing a fuel tankwith a filling pipe and a vent pipe, accommodating the multiphysics aspects ofair and fuel. Image courtesy MSC.Software.
When asked how and when the modeling might change as you go from one physicstype to another, Ramesh Ramalingam, COSMOS product manager at Structural Researchand Analysis Corp., a Dassault Systemes company, says, “It depends on the typeof coupling. In case of thermal stress, the model remains the same, and most materiallibraries contain all the required material properties. In the case of structuraland electrostatic coupling, electrostatic analysis meshes the air, and we excludethe air for structural analysis.”
Dave Weinberg, president of Noran Engineering, describes how starting with twodifferent models is no problem in its NeiNastran Modeler environment. “The modelsdo not have to be the same to directly interface, as we have a unique interpolationfeature, which maps data from one model to another. This allows sharing resultsfrom one analysis type as direct input for another regardless of mesh alignment,element type, or model features and size.”
A case where the mesh may change involves accuracy. PlassoTech CEO Tomi Mossessianexplains, “A mesh good enough for thermal may not be good enough for stress analysis,therefore automated adaptive techniques are desirable. This way the required accuracyis met for each type of analysis.”
A fluid-solid coupled analysis of a gas turbine blade performed using CFX-5.7and ANSYS Multiphysics. Pressure and temperature CFD results (left) are passedto and incorporated into the structural analysis (right), where resultant displacements,stress (far right), and structural temperature are computed.

What Customers Want

ESI Group knows its customers want analyses that run in one environment, areeasy to use, and can incorporate new robust capabilities. Louis Doggett, Jr.,ESI's director of worldwide CFD sales and marketing, notes that ESI software alreadyhandles multidimensional (0D-3D) problems in steady or transient form in suchapplications as multiphase flow, FSI, and plasmas.

ESI Group analysis of a planar solid oxide fuel cell (SOFC) shows temperaturecontours.

Other customer questions are: Can the software do linear as well as nonlinearanalysis within a single run? Can it handle higher physics, such as acoustic-structureinteractions or piezoelectric behavior? Do the analyses attain equilibrium atthe same time? Is there a limit to problem size? Is the material property databasecommon to all analysis types? What are the hardware (CPU, memory) requirements?How steep is the learning curve?

ALGOR developers suggest two more questions: Can the package directly exchangedata with all the solid modeling tools you might be using? Are there adequatefeatures for evaluating and presenting analysis results?


Bidirectional Coupling—
Do You Need It?

Deciding how to model the physics in a simulation depends on what you're tryingto model in the real world. For some problems, a simple sequence is enough: youconverge the results for one physics, say thermal behavior, then “throw the loaddata over the wall” to another, perhaps a stress analysis. If the material propertiesdepend only on the end temperature, this approach is sufficient.
However, many problems have interdependencies that cannot be ignored. Inductionheating is one example, says Tomi Mossessian, CEO of PlassoTech, the providerof “It involves thermal-electric coupling, including the electricalconductivity, ]which] in turn changes the electrically induced heat sources. Thisthen changes the temperature distribution. To get a converged state, you needto iterate back and forth between electrical and thermal analyses.”—PJW

ANSYS's Lethbridge says these days the software is very much user-driven as opposedto technology-driven. To illustrate, he says the company is migrating the powerof its traditional core physics into its newer, unified Workbench environment,though users can still write macro-scripts to tailor their applications.

Lastly, not everyone wants to or can use the predetermined parts of packages.Tim Niu at COMSOL explains that its software lets you specify physical propertiesas analytical expressions or functions, so that you are not constrained to ready-madecapabilities. Users can input arbitrary partial differential equations to extendthe range of applications, and can define coupling needs, including three or more,in a single model.

Most vendors agree on the best advantages of using multiphysics software:

  • Better accuracy since trying to solve a multiphysics problem iteratively usingseveral single-physics programs may not result in the correct solution.
  • Ease of use because there's only one package to master and remember; and it tracksand manages models and results across multiple physics domains.
  • Time and money savings result because there's no need to develop and test tediousinterfaces between single physics programs, there are fewer programs to purchaseand maintain, and less training time is needed.


Left: Thermal performance of a heat exchanger using a fluid cooling medium canbe optimized using I-deas NX Electronic System Cooling.

Left: 3D coupled thermal/flow simulation capabilities within I-deas NX seriesfrom UGS enables engineers to optimize the cooling air flow required in denselypacked consumer electronics enclosures such as personal computers.

Which Way to Future Multiphysics?

As always, the better you understand your own needs, the better fit you can findin software. Dale Berry, manager of engineering applications at ABAQUS, pointsout that “although tight integration is a benefit, it often comes at the expenseof modeling and solution capability. Often, the tightly integrated solutions aremore limited in each domain than separate packages.”

As for the future, Reza Sadeghi at MSC.Soft- ware muses, “Will a fluids companylead this, or ]will] a structures company? As long as the manufacturing worldhas experts and specialties and departments, they'll continue to start from oneend and demand capabilities to be added to that particular discipline. At somepoint in the future, one can imagine we will be able to do it all—letting fluidsdominate or structures dominate—and ask all the questions from the same analysis,but there's time to get there.”

Contributing editor Pamela J. Waterman is an electrical engineer and a freelancetechnical writer based in Arizona. You can contact her about this article viae-mail c/o [email protected].

Companies mentioned in the article:

Pawtucket, RI

Pittsburgh, PA

Canonsburg, PA

Burlington, MA

ESI-Group, Inc.
Bloomfield Hills, MI

MSC.Software Corp.
Santa Ana, CA

Noran Engineering, Inc.
Westminster, CA

PlassoTech, Inc.
Encino, CA

Structural Research and Analysis Corp.
Los Angeles, CA

UGS Corp.
Plano, TX


FEA Provider Product Listing





ALGOR Design and Analysis Software

DesignCheck bundled with Alibre Design Professionaland Alibre Design Expert





Engineering Mechanics Research Corp.

Engineering Software Research and Development


ELECTRO Integrated Engineering Software

Moldflow Plastics Advisors
Moldflow Plastics Insight 5.0

Moldflow Corp.


NEiNastran NEiWorks
Noran Engineering



PDE Solutions Inc.

PlassoTech Inc.

COSMOSMotion 2005COSMOSFloWorks 2005COSMOSWorks 2005
SRAC/SolidWorks Corp.




SRAC/SolidWorks Corp.

ThermoAnalytics Inc.

NX Digital Simulation
NX Nastran


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Pamela Waterman

Pamela Waterman worked as Digital Engineering’s contributing editor for two decades. Contact her via .(JavaScript must be enabled to view this email address).

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