More Than One Force of Nature

more than one force of nature

With multiphysics, engineers can examine many influences in one simulation.

By Jean Thilmany, Associate Editor

Mechanical problems often require the engineer to run more than one type of analysis. Few parts or systems have only one physical force acting on them. But a relatively new technological development is the increasing number of software packages that allow users to couple more than one type of analysis—by running them at the same time or by running them separately and then combining results to get an immediate and clearer prediction of how something will function in the real world.

In other words, the myriad analyses acting on the whole create a virtual prototype that accounts for all stresses and forces working on the part or system at once. That way, engineers simulate the model more realistically rather than see the result of one analysis and then the result of another as an unrelated case.

Engineers can simulate, say, the combined electrical and mechanical behavior of an overall system as part of one virtual prototype.

Multiphysics software blends several analyses. This MSC.Software image shows temperatures of a microchip and its cooling system.

Multiphysics, then, can be looked at as a series of finite element and computational fluid flow analyses layered on top of each other to describe the whole and real-life working conditions of the part. FEA solves simultaneous algebraic equations and lets engineers simulate a wide variety of physical phenomena, including laminar flow, turbulent flow, impact, and nonlinear geometric or material simulations. CFD describes how a fluid will flow through a system.

To understand multiphysics technologies or how to carry out multiphysical applications, it can be useful to look first at an object that's always close at hand. The human body is an ultimate multiphysical system, according to Reza Sadeghi, director of development for nonlinear products at MSC.Software in Los Angeles. As such, it can be understood as a number of physical forces and events working in tandem to produce—miracle of miracles—you.

Take your heart, for example.

"If you want to understand how the heart valve works, you have to look at blood flow and the blood flow against tissue," Sadeghi said. "That's a fluid-flow problem that also has to be analyzed in terms of the blood's resistance against the valve."

It's not enough to merely look at how the blood flows through the heart. To get a total picture of organ functioning, you also have to look at resistance against the valve as it opens and shuts with the beating of the heart.

Everyday Complexities

Many software tools that perform multiphysical analyses are now on the market. A product from MSC.Software called MSC.Marc lets the user perform a wide variety of structural, fluid, and coupled analyses using the finite element method to gain this overall view of complete functioning. With Femlab, from Comsol of Burlington, Mass., users can model combinations of physical phenomena (such as fluid flow, friction, deformation, and strain), view the results, and then change the solid model immediately with built-in computer-aided design tools.

A manufacturer of large forestry machines, Timberjack, headquartered in Alpharetta, Ga., and owned by Deere and Co., has started using multiphysics software to carry out what Juha Kanto, manager of product modeling and simulation applications, called real-time simulation. Kanto works in the Timberjack office in Tampere, Finland. Before implementing the software, from Lumeo Software of Toronto, the heavy-equipment manufacturer did analyses and simulations with other technology, then studied the separate results to determine product behavior.

"Usually, the simulation is done and the calculation is done, and you wait to look at the results afterward," Kanto said. "We wanted to be able to do those things in real time and see the results right away."

For Timberjack, seeing a simulation of the product running with all forces applied and operating as it would in real time was of great importance.

"Before, when we simulated larger dynamic structures, we saw that motion wasn't really in real time. We're talking about the motion only being off by seconds, but that still wasn't as it would operate in real time," he said.

In manufacturing, combining simulations of different physical disciplines into one whole helps engineers of varying disciplines work together in the very early stages of product creation, said Jari Strandman, Lumeo Software's president.

"Previously, and traditionally, the simulations of mechanisms, hydraulics, actuators, and electrical or other physical domains were separate," he said. "And engineers tended to understand only their field. A hydraulics engineer only did hydraulics, a mechanical engineer only did mechanics. And they weren't really connected with each other; the first time they came together was quite late in product development. That's the first time you could test overall product performance."

Multiphysics software allows the testing of the overall system much earlier, even when the product is being initially defined, he added.

Traditionally, high-end computer-aided engineering applications required an extensive knowledge of physics to enable engineers to perform analyses, Strandman said. "That would rule out CAD users and target analysis experts," he said. But that's changing.

Timberjack, a maker of large forestry machines based in Alpharetta, Ga., recently began using multiphysics software for product modeling and simulation.

Strandman said his software company is bringing multiphysical functionality to the level of the CAD user. His software is integrated with Pro/Engineer from PTC of Waltham, Mass., which means that users can test a CAD model immediately by running Lumeo software to perform analyses.

Although Timberjack is currently using one trained analyst to carry out all multiphysics applications, Kanto said the company is considering expanding the user base after an initial trial period by training CAD users in the multiphysics application.

Multiphysics applications are a linkage of varied analyses performed quickly. They fit what Bruce Jenkins, executive vice president of Daratech, has defined as a growing trend among providers of analysis tools: the marketing of analysis software integrated with CAD packages and accessible to engineers without special training.

In the past, engineers specifically trained in analysis ran studies on prototypes of parts or products, according to Jenkins, whose firm, based in Cambridge, Mass., researches the computer-aided engineering market. With the relatively recent advent of easy-to-use software packages, mechanical engineers who perhaps haven't been specifically trained in FEA handbook techniques can still perform fluid-flow, finite element, and thermal analysis, or many other kinds of calculations with the software's help.

The problem, Jenkins said, is that even with the new software, the accuracy of the analysis depends on how well the engineer has modeled the part and has applied conditions, such as stress or load, during analysis. In other words, analysis results are only as good as the decisions of the engineer who designed the part and who ran the analysis.

Whether CAD users will be performing more and more multiphysics analyses in the future remains to be seen. Timberjack won't be giving its CAD users access to Lumeo quite yet.

"I can see us pushing it to CAD users," Kanto said. "That's one way to expand the user base. But we'll still have the high-end experts for analysis. We need them to verify results."

Don't Forget the Stress

For his part, Keith Orgeron, a certified professional engineer who is president of a Houston consulting company, Integra Engineering Inc., has long used coupled analysis programs from Algor of Pittsburgh. He uses steady-state, and transient heat-transfer analysis programs combined with either structural analysis or Algor's Mechanical Event Simulation software to simulate motion in mechanical systems. In this way, Orgeron—who began combining the analysis software 12 years ago before the advent of the mechanical simulation software—models all the physical phenomena necessary for many of the engineering jobs he does.

"After I've analyzed heat transfer, I prepare that model for stress analysis," he said. "Generally, you need to analyze for stress nearly all the time you analyze for heat transfer. There are only a few times you don't need to figure out stress."
Orgeron began coupling analysis technologies when he was a weld engineer in order to find the residual stresses that occur when parts or pieces of steel are welded together. For example, he's analyzed continued weld-connection cracking at a Mississippi power station's 12-story boiler. According to Orgeron, analyzing residual stresses from a weld is one of the most challenging scenarios you can carry out with a desktop multiphysics application.

"It's such a complicated multitude of events occurring all at once," Orgeron said. "First, you have to apply the thermal shock analysis of the welding torch to a cold set of materials. You're usually welding two pieces to each other and generally it's two metals—usually steel, though it could be aluminum or brass.

"Generating the thermal shock loading is no small task," he added. "You have to apply it in the three-dimensional elliptical shape of the welding torch, and applying it in that shape is not easy. To avoid problems, organize the model's mesh or grid."

Using multiphysics technology from Lumeo Software of Toronto, Timberjack engineers saw a simulation of the product running with all forces applied and operating in real time.

Even then, the task is far from completed. The engineer must create an analysis model in which the weld isn't there and then suddenly is there. And when the weld appears, the metal turns from heated liquid to a more solid matter in seconds. That needs to be shown. To make matters worse, the crystalline structure of the material changes as it begins to solidify, Orgeron said. That event also needs to be accounted for in the analysis model.

"The change is due to a new crystalline structure for the metal, occurring around 1,400°F," he said. "This phase transformation causes some pulling and stretching that is not easy to duplicate, but it is also not as significant as the effects due to the shrinkage of the weld through the course of its solidification. Also, the material's thermal and physical properties change with temperature, and this significantly affects the accuracy of a multiphysics simulation."

But that's not all. The engineer who has chosen to do this task still must find a way to include an account of the new surface-to-surface contact at the end of the model run.

"Up until a few years ago, only supercomputers with dedicated, not general-purpose, software could perform a simulation of a weld," Orgeron said. "But given a limited set of weld parameters, we can simulate it with general-purpose software and still match the output results by skipping over some of the physics that occur."

He's currently developing a software procedure that will allow users to do just that, using the Algor software and the techniques he's put into practice over the years, Orgeron said. Welding two metals is a common occurrence that's little modeled by engineers, often because modeling a weld is so complicated, he maintains.

"What is modeled is often the fact that the pieces are already joined," Orgeron said. "But because it's already joined, there's no account of the residual stresses from the joining process and therein lies a huge amount of error. The stresses can be so high that just during cooling you can crack and explode a vessel."

Orgeron witnessed the explosion of a half-ton gas compression piston that, as he put it, zippered and then split in two. "There was no load on it. That's just how huge the buildup of energy and stress is from cooling after a weld," he said. "And this crazy phenomenon is often completely ignored in analysis."

Such are the stakes when thorough analysis isn't performed before production, Orgeron and others maintain. With the development of increasingly easier-to-use multiphysics programs, it is likely that more engineering firms will be turning toward these full-scale analyses packages in the near future.