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American Institute of Physics

 

 

Multiphysics analysis

In the absence of a superphysicist, all members of the team can help solve these multiphysics problems

by Paul Lethbridge

pdf version of this article

Finite element analysis (FEA) is a powerful analysis technology that has been commercially available for more than 30 years for determining individual product characteristics. FEA is typically used to predict how a part or system responds to the effects of a single physical phenomenon, such as structural deformation due to a mechanical load, or temperature profile due to heat transfer (see The Industrial Physicist, February 2001, pp. 22–23).

Figure 1.  In the multiphysics analysis of a turbine blade, pressure and temperature data from computational fluid dynamics results (left) are merged into the structural analysis (right) to determine stress, displacements, and temperature.

Commercial analysis software has evolved to address many other physical phenomena as well, such as fluid flow, acoustics, and electromagnetics. In the past, the effects of multiple physical phenomena were investigated by separately analyzing each phenomenon individually—for example, thermal, electric, or mechanical—without regard to any physics interaction. But advances in computer hardware and software now make it practical for analyses to account for the effects of two or more interacting physical phenomena (Figure 1).

The applications for multiphysics (or coupled- physics) analysis cover most industries, including automotive, aerospace, electronics, semiconductor, telecommunications, pharmaceutical, and biomedical. Many such applications include sensors, transducers, and actuators (see table), where one physical phenomenon is converted into another: electricity into motion, for example, or fluid pressure into electricity. Researchers also can study the effects of temperature on electromagnetic properties that might affect the performance of electric motors in extremely hot or cold environments.

There are many applications of multiphysics in sensors, transducers, and actuators, where, for example, electricity is converted into motion and fluid pressure into electricity.

Coupled-field analysis

The essence of multiphysics analysis is coupled-field analysis, which allows users to determine the combined effects of multiple physical phenomena (fields) on a design. Implemented properly, multiphysics software can significantly lower the cost of ownership by replacing multiple analyses with a single tool that is easier to learn and maintain.

The more common and mature analyses of coupled physics include fluid–structure (technically, fluid–solid), thermal–mechanical, and electric–thermal interactions. In fluid–structure interaction, fluid flow exerts pressure on a solid structure, which causes it to deform such that it perturbs the initial flow. This type of interaction results in the deformation of an aircraft wing during flight, for example, or the wind-blown sail of a boat. Thermal– mechanical coupling is omnipresent, and many structures change their shapes and material properties as a result of a temperature increase or decrease. Sections of bridges and highways expand on hot days, and many plastics become extremely brittle at low temperatures. In an electric– thermal interaction, current flowing in a conductor generates resistive heating. This effect is critical in the electronics industry, where high-density microchip circuits often create surprisingly large heat loads that need to be managed with heat-transfer techniques.

In the development of microsystems such as microelectromechanical systems (MEMS), multiphysics effects are inherent and absolutely must be addressed. Microsystems have components with micrometer dimensions, and they are in widespread use in the automotive, aerospace, and biomedical industries.

Common applications include inertial air-bag sensors, ink-jet printer nozzles, and field-emission display devices. These microminiature systems (some even thinner than a human hair) derive their function from the interrelated effects of stress, temperature, electrostatic, piezoelectric, and electromagnetic effects.

In the past, solving such coupled-physics interactions required many manual file transfers, data exchanges, and problem setups to perform each physics analysis. Such cumbersome, error-prone, and time-consuming analysis often took days or weeks to perform.

Today, multiphysics software packages automatically combine the effects of two or more interrelated physical phenomena. These tools automatically manage the exchange of data between the different kinds of physics and perform information transfers. As a result, coupledphysics analyses can be performed in a fraction of the time otherwise required, and many users report that problem setup and the time-to-solution take about onetenth the time of manual methods. Because multiphysics analyses can be performed quickly, users can complete many more iterations in a given time, and, thus, they can explore a broader range of engineering parameters and obtain more accurate solutions.

To gain these advantages of multiphysics, some sophisticated users develop their own codes to cobble together different analysis programs, which generally results in cumbersome and idiosyncratic command structures. However, many software providers now offer a variety of general-purpose multiphysics packages, some more capable than others and most limited to niche applications in industry.

Two approaches

Two numerical techniques are available for combining physics fields: direct-coupled and sequential-coupled fields. Many commercial FEA codes provide either direct or sequential approaches.

Direct-coupled analysis assembles all the physics fields as finite-element equations in one matrix and solves the matrix as a whole. In sequential coupling (often referred to as load-vector or staggered coupling), the equation for one field is partially solved and the results passed as loads (the results of one physics field interacting with another) to the next physics field to drive its partial solution. The analysis software then passes this iteration to the next physics field, and so on, down to the last field. Then the sequential iteration process begins all over and continues until a final solution is achieved.

One example of a direct-coupled field is the combination of thermal and electrical effects to study Joule heating produced by electromagnetic energy passing through a resistive or dielectric material. In certain transducers, piezoelectric direct coupling of electrical and mechanical physics is useful in determining both the amount of deformation resulting from an applied voltage and vice versa, as in piezoelectric ignition devices. In these types of analyses, all physics fields can be accounted for in a single solution.

An example of sequential-coupled physics is an electrostatic– structural analysis required for many MEMS actuators. At the microsystem scale, a voltage applied between electrodes creates an electrostatic force large enough to cause the part to deform and/or move. In a sequential-coupled field analysis, the initial electrostatic force is computed and then passed to the structural analysis. The deformation of the structure changes the electrode shape, and a description of the new configuration is passed to the electrostatic analysis for the next numerical computation iteration. Multiple passes are typically required until the electrostatic force and structural deformation reach equilibrium (convergence).

A comprehensive multiphysics solution created by ANSYS, Inc. (Canonsburg, PA), provides both direct-coupled and sequential-coupled analyses for a broad range of physics interactions, including structural, thermal, fluid, and electromagnetic. The software allows for optimized design based on geometry, materials, physics, and boundary conditions, such as the temperature range at which a product must operate. The package includes a solid modeler, which enables users to build a model without the need to import elements from an external computer-aided design (CAD) system. The software allows passing results between the different models that represent each of the physics fields during a sequential-coupled analysis. This innocuous- sounding capability has far-reaching implications and changes the manner in which companies can implement multiphysics analysis.

Typically, with multiphysics analysis, the exchange of data between the physics fields requires careful coordination, and the different mesh requirements for the various fields, loads, and boundary conditions must be correlated. For all this to function correctly requires a complex feedback loop between the various fields so that the coupled analysis converges to an accurate solution. This level of analysis typically requires the skill of a superphysicist, one intimately familiar with each of the physics fields being studied and the programming/scripting language needed to perform the coupling iterations. ANSYS Multiphysics software eliminates the need for a superphysicist because it enables independent users to work on different parts of a problem. Although everyone involved in the analysis uses the same CAD solid model, individuals define their own FEA model—such as loads, boundary conditions, material properties, and mesh requirements—according to their own particular physics discipline. One researcher can set up the structural portion of the multifield analysis, another can simultaneously work on the electromagnetic analysis, and yet another can handle the fluid-flow aspects of the analysis.

This team approach to multiphysics analysis, in which different disciplines set up their problems (Figure 2), has the potential to revolutionize the role of advanced analysis technology in product development. Companies can now realistically perform multiphysics analysis as a routine part of all stages of development, from up-front conceptual studies to design verification and failure-mode analysis. In this way, multiphysics becomes a strategic tool for system-engineering methodologies that account for all relevant physical phenomena that influence design. The predictive FEA simulations more closely represent the real world than do simplified assumptions that often neglect to account for things that turn out to be critical. Performing multiphysics analysis early in product development enables companies to easily and inexpensively spot and fix problems that can become costly and timeconsuming to resolve later. Some studies have shown that the time and cost of fixing such problems increases 10-fold at each successive stage of the product life cycle. Factoring in the effect of more physics yields moreaccurate analysis, fewer physical prototypes, a shorter product-development cycle, lower development costs, and a faster time to market and response time to market changes.

Applying multiphysics during the early stages of design allows companies to realize these benefits, but it also requires a shift in resources up-front in the product- development cycle, and researchers in different disciplines must collaborate in performing joint simulations. Gaining the full benefits of multiphysics entails three major shifts in the way analysis is performed within an organization—who does the work (individuals vs the superphysicist), when analysis is performed (early in development vs later), and how work gets done (collaboratively vs individually). These shifts entail organizational changes, and addressing corporate cultural issues and how management decisions are made. Having researchers perform their own multiphysics analysis necessitates delegating more responsibility to these individuals, training them to use the software, and allowing them to experiment with software tools in the initial stages of implementation.

Multiphysics in action

Many industries use multiphysics to address complex FEA challenges—those in which extremely large problems involve the coupling of multiple physics fields. Many of these applications have high consumer-safety issues, such as those associated with the biomedical and automotive industries, where accurate analysis is critical to understanding and ensuring product reliability. The following examples illustrate the growing number of applications utilizing ANSYS Multiphysics for analysis. Automotive applications range from designing shock absorbers and hydraulic and fuel systems to electromagnetic– structural clutches and braking assemblies. Multiphysics analysis was used to predict the action and improve the reliability of a pressure-limiting ball valve used in an antilock braking system (Figure 3). A steel ball in the valve moves during vehicle braking and changes the brake-fluid pressure delivered. Antilock brake systems carry huge liability ramifications, and it was found that tiny geometric design changes caused wide variations in the valve’s performance. The analysis provided an understanding of how the ball moves with time during braking. The fluid–solid interaction analysis allowed the customer to accurately determine the displacement history of the ball during braking for many designs, and to subsequently develop an optimized design with improved reliability.

 
  A fluid-structure multiphysics analysis was used to detemine the displacement history of the ball valve in an automotive antilock braking system. The stell bal moves during braking and changes the fluid pressure that is delivered.

Xerox Corp. used multiphysics to study the movement and positioning of paper through a photocopier paperhandling assembly. The analysis incorporated the effects of vacuum and mechanical guides on the paper and resulted in an improved photocopier with fewer paper jams, increased paper throughput, and greater reliability. A heavy-equipment manufacturer simulated the action of a diesel fuel injector, whose fluid pressure deformed the cylindrical piston into more of a conical shape, which resulted in fuel leaking past the piston. Multiphysics showed a leakage rate 12 times as high as that determined using an uncoupled analysis, and this finding enabled researchers to better understand the leakage problem and redesign the piston.

Multiphysics provides a valuable tool in biomedical applications, where product performance is often critical in life-or-death situations. Many intravenous drug-delivery applications require fluid–solid interaction-coupled physics. Moreover, an increasing number of biotissue-imaging and tumor-destruction processes in development require coupled electromagnetic–thermal capabilities. For example, Microsulis Medical Ltd. (Denmead, England) uses multiphysics in its R&D of microwave tissue-ablation devices. These instruments have novel microwave antennae that are designed to transfer microwave energy into targeted tissue regions. There, dielectric losses in the tissue convert the microwave energy into heat used to therapeutically destroy malignant tumors. Edwards Lifesciences Corp. (Irvine, CA) accurately predicted flow and deformation of its Advanced Venous Access (AVA) devices, whose tubes deliver drugs, nutrients, and other fluids intravenously to hospital patients. The analysis results showed how the device deformed as the fluid flow in the tube changed, which allowed many more design iterations of the tube’s cross section than would be practical using physical testing, and enabled researchers to develop a more efficient tube.

Some of the most sophisticated multiphysics analyses occur in the development of MEMS microsystems. These tiny devices are usually prohibitively expensive to prototype and, therefore, they need to be simulated prior to manufacture to ensure that they perform as designed. For example, Daimler Chrysler used coupled electrostatic– structural–fluid analysis to accurately predict the critical response time of a MEMS radio-frequency switch consisting of an electrostatic-actuated beam. The fluid-damping response of the beam is controlled largely by its surface area. Multiphysics analysis allowed the company to optimize the device’s performance by changing the diameter and number of fluid-damping holes in the beam.

SilMach (Besançon, France) uses multiphysics to develop advanced microsystem sensor and actuator arrays. Building such MEMS arrays requires an understanding of the different physics acting in each section of the device. The company reports that multiphysics enables it to effectively couple these physical phenomena and analyze their interactions to create more efficient MEMS devices (Figure 4). The technology also helped SilMach develop an electromagnetic actuator that has a power output of 100 W/g, compared with 1 W/g for standard devices.

Engineers can now perform multiphysics analysis as a routine part of development, especially in the early stages of the cycle. Companies and individuals that previously could not afford to invest in this technology can now readily put multiphysics analysis to work. In this way, multiphysics has become a strategic tool for systemengineering methodologies that account for all relevant physical phenomena that influence design and, thus, enable companies to develop innovative, winning products in less time and at lower cost.

Further reading

Biography

Paul Lethbridge is product manager for multiphysics at ANSYS, Inc., in Canonsburg, Pennsylvania

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