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).
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| 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.
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| 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.
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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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