## Spring 2013 PADT Company Meeting

Twice a year, PADT employees gather from around the country in our main office in Tempe for a company meeting.  We get together in one room for about two hours to review the past 6 months, talk about what we have accomplished, what we are working on now, and where we want to be.

The spring meeting always starts with our annual company picture.  We all get a new shirt with a PADT logo proudly embroidered on the left breast, and we stand in the heat glaring into the sun, each of us wondering if it really has already been 12 months since we last did this.  The result came out pretty good:

With just over 70 employees on the payroll, we were lucky to get 60 who could make it to the picture and the meeting. That may be a record.

## CFX Expression Language–Part 3: Applying Boundary Conditions Using CEL

In two previous entries we introduced CFX Expression Language, CEL:

Part 1: Accessing CFD Simulation Information in CFX (and FLUENT)

Part 2: Augmenting Material Property Assignments in ANSYS CFX

In this third installment we will see how to use CEL to apply boundary conditions as equations rather than constant values. For example, if a non-constant velocity profile can be defined as an equation, we can use CEL to define as well as apply the profile.

Let’s look at an example in which the velocity profile is a function of y coordinate:

u(y) = 6 * Umax * y / H * (1 – y/H) (m/s)

Using the procedure we learned in part 1 of this series, in CFX Pre we have defined expressions for H and Umax. We then defined the equation for the velocity profile as Uprofile:

Next we go to the Plot tab within the Expressions editor to verify that our velocity profile matches expectations:

To use our new expression in CFX Pre, we just enter the expression name in the appropriate field when defining the inlet velocity:

Finally, this velocity plot from CFD Post shows that indeed our desired velocity profile was applied at the inlet.

Hopefully this demonstrates how easy it can be to use CFX Expressions to define non-constant boundary conditions. In the next part of the series, we will look at using expressions to ramp or step apply loads.

## PADT’s Norman Stucker Nominated for The Brian R. Vogt Community Leader of the Year

We are always humbled when a great organization takes the time to recognize PADT or one of our employees.  We are pleased to announce that our General Manager, and Salesguy, Norman Stucker was nominated for the South Metro Denver Chamber of Commerce Brian R. Vogt Community Leader of the Year.

Besides a reminder that this particular group has an even longer full name than PADT, we are pleased that the business community in the South Denver area has recognized the contribution that Norm makes to the community. We will definitely post

## 2000 Core Milestone Passed for CUBE HVPC Systems

As we put the finishing touches on the latest 512 core CUBE HVPC cluster, PADT is happy to report that there are now 2,042 cores worth of High Value Performance Computing (HVPC) power out there in the form of PADT’s CUBE computer systems.  That is 2,042 Intel or AMD cores crunching away in workstations, compute servers, and mini-clusters chugging on CFD, Explicit Dynamics, and good old fashioned structural models – producing more accurate results in less time for less cost.

When PADT started selling CUBE HVPC systems it was for a very simple reason: our customers wanted to buy more compute horsepower but they could not afford it within their existing budgets. They saw the systems we were using and asked if we could build one for them.  We did. And now we have put together enough systems to get to 2,042 cores and over 9.5TB of RAM.

### Our Latest Cluster is Ready to Ship

We just finished testing ANSYS, FLUENT, and HFSS on our latest build, a 512 core AMD based cluster. IT is a nice system:

• 512 2.5GHz AMD Opteron 6380 Processors: 16 cores per chip, 4 chips per node, 8 nodes
• 2,048 GB RAM, 256GB per node, 8 nodes
• 24 TB disk space – RAID0:  3TB per node, 8 nodes
• 16 Port 40Gbps Infiniband Switch (so they can connect to their older cluster as well)
• Linux

All for well under \$180,000.

It was so pretty that we took some time to take some nice images of it (click to see the full size):

And it sounded so awesome, that we took this video so everyone can here it spooling up on an FLUENT benchmark:

If that made you smile, you are a simulation geek!

Next we are building two 64 core compute servers, for another repeat customer, with an Infiniband switch to hook up to their two existing CUBE systems. This will get them to a 256 core cluster.

We will let you know when we get to 5000 cores out there!

Are you ready to step out of the box, and step into a CUBE?  Contact us to get a quote for your next simulation workstation, compute server, or cluster.

## CFX Expression Language – Part 2: Augmenting Material Property Assignments in ANSYS CFX

In a previous entry we introduced CFX Expression Language, CEL.  You can view that post here

Before we get started, there are some key things to remember:

1. Expressions can be easily created by right-clicking in the Expressions tab after double clicking on Expressions in the CFX Pre object tree.
2. Expressions and their contents are case sensitive.

In this next part of the series, we’ll show how to use CEL to augment your material property definitions in CFX. If material properties are constants then their input is straightforward. However, if the properties are defined as equations, we can use CEL to input those equations in CFX.

For example, if viscosity is defined as a function of shear strain rate, we need to define viscosity using an equation that captures that relationship, such as

m = K * gn-1

Below are shown two ways in which that equation can be captured using CFX Expression Language, visc1 and visc2. The second equation, visc2, is more flexible in that we have defined the constant terms as expressions themselves.

It’s always a good idea to verify the input. Most expressions can be easily plotted by clicking on the Plot tab in the Details view. Here is a plot of the viscosity vs. shear strain rates between 0 and 1, as calculated by expression visc2:

Similarly, the Evaluate tab can be used to evaluate the expression for desired values of the inputs.

So, we have defined an expression for a material property, viscosity in this case. How do we get CFX to use that expression? In the material property input, we click on the expression icon to the right of the particular material property we are defining, then enter the name of the expression, as shown here for expression visc2:

Summing it up, we can use CFX Expression Language to define material property equations for non-constant material values. In the next installment we will look at how to use CEL to define changing boundary conditions, such as a ramped load.

## Lighted Speakers show Off Power of 3D Printing

We came across this very cool application that uses a Stratasys Connex 500 spreading like wildfire across the blog-sphere.  We thought we would put our own technical spin on it since we have that same system.  Evan Atherton from Autodesk did the model working with a company called LumiGeek to do the lighting.
Here is a  good video that explains the project:

As you can see, Evan built a very cool 3D model of the speakers then printed them on the Connex 500. That particular system uses an inkjet technology called polyjet to print out a photocurable resin (a plastic that hardens when you expose it to ultraviolet light) in layers. What is special about this application of the polyjet technology is that the machine can print two different materials at the same time. So you can mix those materials as you build a part to change properties. This allowed him to get a mixture of rubber-like and semi-transparent plastic parts to fit into his design.

Once printed they assembled the parts with a programmable LED strip from LumiGeek. And presto – they now have their own custom speakers that blink and shimmy in time with the music they are playing.

Endgadget has a pretty good slide show and a video that shows some of the details.

## So What?

As most of the blogers/vlogers point out this is not a way to mass produce speakers.  The material and labor costs are extensive. But it does show how a truly unique idea for a whole new type of product can be quickly and easily visualized using 3D Printing technology. Within a week a full working prototype of a complex product can be produced.

It also shows the power of custom product development. There is a significant market out there for custom applications where people need a unique application like this.  Although the cost of producing these was not small, it is much less than any traditional manufacturing process when you only need one or two copies.  This is another example of how someone can create a new service around taking peoples unique design requirements and creating a stand-out solution in relatively little time.

Plus, they just look cool. That is “what” enough for us.

If you have an idea you want to realize using 3D Printing, contact PADT.  We have the skill, the experience, and the equipment to make it happen.

## CFX Expression Language – Part 1: Accessing CFD Simulation Information in CFX (and FLUENT)

This week we are presenting an introduction to CFX Expression Language. If you’re not familiar with CFX, it is one of the two CFD tools available from ANSYS, Inc., the other being Fluent. CFX has been part of the ANSYS family of engineering tools since 2003. It is relatively easy to use and can be run stand-alone or tightly integrated with other ANSYS products within ANSYS Workbench. We have some general information on CFX available at this link.

CFX Expression Language, or CEL, is the scripting language that allows us to define inputs as variables, capture outputs as variables, and perform operations on those variables. Through the use of CEL we can be more efficient in our CFD runs and better capture results that we need. With CEL we can access and manipulate information without needing to recompile code or access separate routines besides the main CFX applications.

Also note that since CEL can be used in CFD Post, it is useful for postprocessing FLUENT solutions in addition to CFX, since CFD Post is common to both CFX and FLUENT. There are some things to be aware of regarding FLUENT In CFD Post. This link in to the ANSYS 14.5 Help system explains it:

// User’s Guide :: 0 // 7. CFD-Post File Menu // 7.15. File Types Used and Produced by CFD-Post // 7.15.10. Limitations with FLUENT Files

If you are a user of APDL, ANSYS Parametric Design Language, what I have written above about CEL should look familiar. One difference, though, is that while Mechanical APDL is dimensionless, CFX is not. Therefore, CEL definitions contain units where appropriate.

CEL is typically used in CFX-Pre and CFD-Post. A handy editor is available to assist in the definition of the expressions. Most of the activity is enabled by right clicking.

Virtually any quantity in CFX that requires a value input can make use of CEL, including boundary conditions and material properties. CEL can also be used to access and enhance results information. Expressions defined in CEL can be used in design point studies in ANSYS Workbench, either as input or output parameters.

So, what kind of things can you do in an expression? In addition to accessing simulation information and storing it as a variable, you can manipulate values using operators such as add, subtract, multiply, divide, and raise to a power. You can also use built-in functions such as sine, cosine, tangent and other trig functions, exponent, log, square root, absolute value, minimum, maximum, etc.

There are many predefined values, including some common CFD constants such as pi, the universal gas constant, and Avogadro’s number. The available options are different in CFX pre vs. CFD Post, with relevant choices for each.

In CFX Pre, expressions are accessed by double clicking on Expressions in the tree. That takes you to the expression editor, as shown here:

Notice how units are defined for each expression, but they can be mixed if desired.

Regarding CFD Post, the example below shows three expressions defined in CFD Post. The expressions within the box are user-defined. The other expressions listed are setup automatically.

The values for forceX1 and forceX2 are calculated by extracting X-direction forces on two different surfaces. The surface names were defined in ANSYS Meshing in this case, as Named Selections. The value fdiffx is calculated by subtracting forceX1 from forceX2. The resulting value, fdiffx, has been specified as an output parameter in Workbench; hence the P-> symbol next to the name.

New expressions are created by right-clicking in the Expressions tab. The new expression value is given a name, then the definition is input, typically by right clicking and selecting from the menus of available quantities, like this:

The location of application for an expression can also be selected by right clicking:

So we’ve got our variables defined using CEL. Now what? Here are some things we can do with CEL variables:

1. Use them as inputs such as material properties or boundary condition values in CFX. If we are running multiple cases, it is typically much easier to define quantities that we want to vary this way. The values can then be changes in the Expression Window, or if defined as a parameter in Workbench, in the parameters view as part of a parameter study.

2. Use them for reporting results quantities of interest, such as forces at a desired location.

3. Use them as input or output parameters in a design point study or design optimization.

Hopefully this brief introduction gives you a glimpse at the power of CEL. In a future article we will look at using CEL for more advanced functionality, such as applying ramped or time varying boundary conditions, using IF statements, and monitoring expression values during solution.

## ANSYS Acquires EVEN, the Makers of the ANSYS Composite PrepPost Tool (ACP)

Good news out there in ANSYS land.  ANSYS, Inc.  just made the relationship with EVEN as close as possible – by acquiring them.  Here at PADT it was love at first sight when we first were introduced to the ANSYS Composite PrepPost (ACP) add-on.  The solver capabilities in ANSYS Mechanical APDL have been very strong for composite modeling for some time.  But the pain and suffering required to set up a complex composite geometry kept many users from accessing those fantastic elements.  ACP solved that problem by providing a tool that takes care of the bookeeping and geometry issues involved in building an accurate model of composite layups.

Here is the official press release.

With this acquisition ANSYS, Inc. has secured the future development of this tool and given all of us in the ANSYS world even better access to the consulting team at EVEN.  You can learn more about the ACP tool on our ACP page.  We also have an older blog posting on ACP when it came out.  We also did a seminar on the last release, here is the recording to that. [probably time to write an updated posting on newer capabilities…].

This is great news, and we can not wait to see further improvements in the composite modeling capabilities for the ANSYS Product family.

## Yes! Concurrent Design Point Solves Using New ANSYS HPC Parametric Pack Licensing

### Design Optimization – Design Point Studies.

These are terms that for many years now have been tossed about as powerful simulation tools. Indeed they are powerful tools, but for anything but relatively small models, the computing resources and time involved to get solutions have been prohibitive in many cases.

We are now in the 2010’s and computing power is far greater than it was just a few years ago. To help us better take advantage of those horsepower increases, ANSYS, Inc. has released a new license product with version 14.5, called the ANSYS HPC Parametric Pack.

How does a six minute turnaround time for 4 design points look when compared to a two hour time for a single design point? If you find that intriguing, please keep reading.

Simply put, the Parametric Pack license allows us to solve simultaneous design points on multi-core systems. For the most part, design point runs have been serial up to now. With Parametric Packs, you can solve several design points at the same time, each running in parallel.

What ANSYS, Inc. has done with the Parametric Pack concept is to allow you to multiply your existing licenses for use in simultaneous solutions of design points. Each Parametric Pack license provides a multiplier on existing licenses. If you currently have one Mechanical or ANSYS CFD license, with a Parametric Pack license it now becomes equivalent to 4 licenses for the purposes of solving concurrent design points. The more parametric pack licenses, the greater the multiplier, as shown in the following table. Note that the maximum allowed number of Parametric Pack licenses for a given study is 5.

 # Parametric Pack Licenses # Simultaneous Design Point Solves 1 4 2 8 3 16 4 32 5 64

The Parametric Pack license multipliers apply in two scenarios. With scenario one, a design point study has been setup in ANSYS Workbench in which there is a set of input parameters and a set of output parameters. A table of various values of the input parameters has been defined for which we want to track the outputs. An example of this is shown below. The other scenario in which Parametric Pack licenses can be used is with design optimization using an ANSYS DesignXplorer license. We will focus on scenario one in this article, while a future article will address scenario two.

The example we will use is a Fluent study. It could just as well be an ANSYS structural or thermal solution, CFX solution, coupled field solution, etc.

In this case, we just have one varying input parameter (inlet velocity) and one varying output parameter (mass flow at the outlet) for the sake of simplicity.

Design point updates with the Parametric Pack license work through the ANSYS Remote Solve Manager, RSM. The runs can be made either on the local machine or on a remote number cruncher, but either way they need to be submitted with RSM. RSM comes with ANSYS automatically, but needs to be configured the first time you use it.

For the example shown here, I set it up to run on one of our Linux PADT Cube systems. The submission to RSM was made from my local Windows box while the solving was done on the remote Cube on PADT’s cluster.

ANSYS has to be told to use an available Parametric Pack license. It also has to be told which licenses to be used on conjunction with the Parametric Pack license. This information is defined from within Workbench, by right-clicking on the Parameter Set box and displaying Properties. Once License Checkout is set to Reserved, we click on the Reserve Licenses link to select the desired licenses to be used:

In the window below you can see I have reserved 1 ANSYS CFD license which allows for 1 Fluent solve. I have also reserved one ANSYS HPC Pack which allows for up to 8 parallel tasks per solve. By also reserving one ANSYS HPC Parametric Pack license, the other two are amplified. As the last column shows, the reported number of concurrent licenses is 4 for the ANSYS CFD license and 4 for the ANSYS HPC Pack license (meaning 4*8 or 32 total cores for 4 simultaneous solves).

More HPC Parametric Packs would amplify the licenses further. It’s important to note that not all ANSYS licenses can be amplified by the Parametric Pack license. In general, the licenses that can’t are products that rely on a third party for some of the technology, such as DesignModeler which uses the Parasolid kernel. That doesn’t mean that DesignModeler can’t be part of a study that utilizes the Parametric Pack licenses, though. It just means that that the DesignModeler tasks will be automatically completed before the jobs are submitted for simultaneous solving.

Getting back to the example, we asked ANSYS Workbench to solve 4 design points. Without Parametric Pack licensing, that would have been done sequentially. On my local Workstation, solving on a single core each design point takes about 2 hours to solve. Using 8 cores on our Cube machine, each design point takes about 6 minutes to solve. What happens when I activate the simultaneous solution with the Parametric Pack license? All 4 design points solve in 6 minutes. This particular Cube has 64 cores, so solving a single design point on 8 cores or four design points concurrently using 32 total cores both take six minutes. That is a very significant speedup. I say it’s a game changing speedup.

Here is a graph of CPU utilization during the concurrent design point solution. 32 processors utilized and the elapsed time was about 6 minutes.

The resulting design point info including the as-solved output parameters:

The bottom line:

What do you need to be able to take advantage of this capability?

1. A regular license enabling the solver you need, such as ANSYS Mechanical, Multiphysics, ANSYS CFD, ANSYS Fluent, ANSYS CFX, etc.

2. ANSYS HPC or ANSYS HPC Pack licenses which allow you to solve on more than two processors/cores for each design point.

3. At least one ANSYS HPC Parametric Pack license which allows the simultaneous design point studies and the amplification of the existing licenses. Talk to your local ANSYS rep or ANSYS Channel Partner for more info.

In a future article we will look at the use of the HPC Parametric Pack license in conjunction with a design optimization study.

## Arizona Science and Engineering Fair: April 1-3 2013

It’s that time of year again. We just got the brochures for the 2013 Arizona Science and Engineering Fair (AzSEF) back from our printer.

This is one of PADT’s favorite STEM events in the state, it is amazing what the kids show off at this event.

This is a statewide science and engineering fair for grades 5 through 12.

Public viewing is on Wednesday from 9:00 am to 3:00 pm and it is held in the South Hall of the Phoenix Convention Center.  We can’t wait to see what the students have done this year.

The general public’s interest in 3D Printing has taken us all a bit by surprise. We know it is a popular topic but we were shocked when a local TV station (ABC15 – KNXV) called us up and wanted to do some filming of 3D printers in action, answer some questions they had about the technology so they could make sure they got it right in the story,  and talk to someone about 3D Printing.  Tey ended up getting a copy of the reporters head as well. Here is the result:

The List: The Futurist: 3D Printing a beak, a break and something in vain

It is not the first time that PADT has been on TV, but the first time we have made it on beyond background shots or public access.  They edited out all of the brilliant and insightful comments, which is expected.  What was nice is they really did not get anything wrong in the story and they spelled our name right!

## A Look at the “First to File” Change to Patents in the US

On March 16th of this year (last Saturday) there was a major change to how you claim rights to an idea in the US, and it came and went without much fanfare.  That is when the US changed from a “first-to-invent” system for deciding who gets to rights to an idea to a “first-to-file” system.  It snuck up on me as well. I remember when the “America Invents Act” was passed and signed in 2011, and I remember thinking “I should remember this”  but it got here and I forgot.  So I did a little research to make sure I understood what was going on and I have shared what I found here.

I recommend you start with the Wired.com article that started me on my search, it is a good overview. After googleing around for a while I found that most of the on-line information was the same, they were just rephrasing the USPTO press releases.  This is my summary:

• This is a big shift. Instead of documenting when you came up with an idea in a note book on some other type of system, you now just have to file your idea first. This is the more common practice around the world.
• They passed this law to avoid all of the cost of litigating when you are trying to prove who came up with an idea first.  Now it does not matter, it is whoever gets their paperwork submitted first.
• The definition of prior art is also changed, meaning that if you disclose your invention to the public anywhere in the world, you have stuck your claim in the ground and you have 12 months to file a patent before anyone else can. You basically get a one-year head start if you just share your ideas with the public and can document it.
• But if you use this exception you give up some rights in other countries.
• If someone is using an idea but did not patent it, and someone else develops the same idea independently, then the first person can still use the idea even if a patent is issued to the second person.  This is prior use and it comes in to play often with trade secrets. You use your trade secret to make a product, then someone else independently develops the same idea and gets a patent, they can not sue you. You have to right to keep using it if you can prove commercial use.
• A confidential sale of a product containing patented technology no longer starts the 1 year period to file a patent.
• The cost of getting a patent has changed, mostly going up.
• How you appeal things has changed. Honestly, I do not understand the current process well enough to get a feel for the new process.

This should get your started with a basic foundation so you can go to the US Patent and Trademark Office site and really dig in to the actual details:

www.uspto.gov/aia_implementation/index.jsp

Unfortunately the law itself is not a new law, but it edits the current laws so I found it very hard to follow.  The Wikipedia page is a bit easier to follow and seems to cover most things, even some of the more obscure aspects of the law.  The National Law Review also provides a pretty good summary

What Does this Mean to Me, My Company, My Startup?

It means a lot.  The days of jotting an idea in a book and getting it notarized are now a thing of the past. Whoever is in charge of your intellectual property policies and processes should have already made changes in your organization (I missed that boat by a bit). Here are some thoughts:

• You still need to keep good track of ideas and when you had them. You still need to prove that you really invented the thing and you did not steal it.
• You should also link those ideas to when they were first used commercially. Tracking prior use of trade secrets is now very important when avoiding a patent infringement case.
• Look at filing patents or at least provisional patents much sooner.  This means:
• You need to budget a lot more for IP protection
• Or, you need to decide early on that an idea is not worth protecting, you can no longer wait to act because you have the invention event documented.
• How US patent law relates to foreign patent law has changed. And when foreign countries recognize a US patent has changed. If this impacts you, then get an understanding of it.
• As always with anything legal, you need to get hold of your lawyer and discuss this with them.  As everyone goes down this path things will change, especially as everyone starts litigating. But in the long run, this should reduce litigation and make things simpler.  Just be aware of the change and adapt to it.

## Innovation Arizona Website: InnovationAZ.com

It is here.  And it is really well done.

InnovationAZ.com

Arizona has been a place where the pioneering spirit, innovation, and good old fashioned brain power could combine to make entrepreneurial successes.  But until now, the fact that Arizona is so spread out and that Arizonans tend to be so independent meant that there has been no single place to go to learn about what is going on, what is available, and who the players in this area are.

With today’s launch of InnovationAZ.com that has changed.  After spending a some time delving through the pages I can tell you that the Arizona Commerce Authority has done a great job pulling this dynamic and changing community together in one place on the web. Sections include:

• Programs for entrepreneurs
• Resources for anyone involved in innovation
• A review of the Tech Community in Arizona including who is here, what makes the state such a successful place for technology companies, and what industries thrive here.
• A comprehensive guide to funding of all types that is available in the state

Check it out, I already learned a thing or three I did not know before I scanned the sight. While you are at it check out the sister sites:  azaerospace.com and azsolarstate.com.

## On the Road for Three Different Shows, Three Different States

It’s been a busy couple of weeks for PADT, jetting around the four-corner region for technical conferences.  Three in fact.

The first was the 2013 Society for Mining, Metallurgy & Exploration Annual Meeting & Exhibit in Denver, Colorado.   We were focusing on simulation for this event, with Flownex and ANSYS. We were not however, focusing our camera phone, this is the best picture we got:

Next was an event near and dear to our heart, the 2nd Annual Aerospace & Defense Requirements Conference put on by the Arizona Technology Council and the Arizona Commerce Authority. Since so many of our employees grew up in the Arizona Aerospace community, it was great to see old co-workers and a large number of our customers at the event, held at the Scottsdale Hilton. At this event we were able to highlight how all of PADT’s services and products can benefit our Aerospace and Defense customers.

And just this week, our team in Utah was at the  Wasatch Front Materials Expo (WFME) for the local Society for the Advancement of Material and Process Engineering (SAMPE) conference at the University of Utah in Salt Lake City.  This was a great opportunity to show of the use of FDM and PolyJet 3D Printing technology from Stratasys for prototyping and tooling.

Look for a PADT booth at these upcoming Events:

06/03/2013 – 06/07/2013:  ASME Turbo Expo 2013

06/12/2013 – 06/13/2013: 2013 ANSYS Convergence Conference – Houston TX

07/15/2013 – 07/16/2013:  ACTEAZ 2013 ACTE Summer Conference

We hope to see you there!

## Direct Coupled-Field Elements in Mechanical APDL

We received one of those tech support calls last week that you hate getting.  It was something like “I need to transfer my ANSYS model to this other FEA package, how do I do that?”  We of course asked “Why do you need to go to this other package?” The answer was “Because they have elements that solve for stress and thermal degrees of freedom in the same element.”  Well, so does ANSYS Mechanical APDL, and it has for years.  But as a Workbench user they had only been exposed to Multiphysics that uses Load transfer as the mechanism to solve different domains in the same run

Therefore, a The Focus posting is born.

In this posting we will go over the basics of direct coupled-field elements and simulation to make everyone aware of what is available.

### Direct Coupled-Field vs. Load Transfer

When most people talk about Multiphysics they are talking about Fluid-Structural Interaction (FSI) or some other interaction between two different models where the program solves each physics by itself and transfers the resulting values from one physics as a load on the next physics.  This is called load transfer Multiphysics and it is very useful and powerful.  But it requires a solve for each physics for each step in your solving process, and often more because you have to iterate back and forth between physics till things converge before you can move to the next substep.

There is a whole other way to do Multiphysics if you have the same mesh for each physics: you can modify your finite element equations to cover all the different physics in one set of equations, therefore in one matrix, and therefore in one pass through the solver for each solve.  This capability has been in the ANSYS Mechanical APDL solver for a very long time and has been expanded over time to cover some surprising combinations of physics.

So when should you use one over the other? That depends. Here are some thoughts:

• Your meshes need to be or are different
• Fluid flow with something other then heat-transfer
• Direct Approach:
• The interaction between two physics is strongly coupled
• The interaction is non-linear
• Acoustics is involved
• Piezoelectric is involved
• Porous fluid flow is involved
• Diffusion is involved

In general, if you can use Direct Coupling and you know MAPDL well, it is the preferred way to go, it is just a lot easier to do. But if you are not familiar with MAPDL for running and post processing, you may be better off with the Load Transfer approach.

### The Coupled-Field Elements

You access the coupled-field capabilities in the solver through the use of the coupled-field elements.  Although there are some legacy elements that can be used as well, we will focus on the three standard coupled-field elements. They all have the same capability, and just vary in topology:

• PLANE223: 2D 8 Node Quad
• SOLID226: 3D 20 Node Hex
• SOLID227: 3D 10 Node Tet

All of these support the following physics, DOF’s, forces and reaction loads:

 Field DOF Label Force Label Reaction Solution Structural UX, UY, UZ FX, FY, FZ Force Thermal TEMP HEAT Heat Flow Electric Conduction VOLT AMPS Electric Current Electrostatic/Piezo VOLT CHRG Electric Charge Diffusion CONC RATE Diffusion Flow Rate

You use a combination of KEYOPTS and material properties to enable the various types of coupling.  Take a look at the element documentation to see how it all works.

In addition to these, there are some specialty elements worth discussion. The first are FLUID29/FLUID30. These are the Acoustic field elements. These solve for displacement and pressure. They also can share the displacement DOF’s with structural elements where they touch.

Unfortunately the electromagnetic coupled field elements have been put on legacy status, as ANSYS Maxwell is where the development effort is going in this area. But you can still use them for coupled-field simulation that involves the MAG degree of freedom.  The elements are: PLANE13, SOLID5, SOLID98. ANSYS MAPDL still has actively supported electromagnetic elements, but they are electromagnetic only and do no support displacement or thermal degrees of freedom.

Flow in a fully saturated porous media can be modeled with the Coupled Pore-Pressure elements. These elements: CPT212/213/215/216/217, solve for pressure and deflection and are used for things like modeling nuclear waste issues, soil subsidence, oil well stability, and bone deformation and healing.

We should also mention that ANSYS supports circuit simulation using the CIRCU124 element.  This element can be coupled to other elements that have VOLT, CURR, or EMF capability.

### Running Direct Coupled-Field Multiphysics in ANSYS Mechanical APDL

When I wrote this section heading it seemed like a good idea. But this is supposed to be a short blog entry and not a full one day training class. So I will wimp out and share where you can find more information in the help:

There is a whole manual dedicated to coupled-field analysis: Mechanical APDL // Coupled Field Analysis Guide. Within that guide is the Direct Coupled-Field Analysis section, Chapter 2.  In it you will not only find discussions about how to do what you need to do, but also a whole bunch of simple examples that are very helpful.

In general, you run like any other simulation.  There is really nothing special or unique and you do not have to deal with managing the load transfer like you do with load transfer coupled field simulations.

### Running Direct Coupled-Field Multiphysics in ANSYS Mechanical

This is a question that comes up a lot. Unfortunately only one type of direct coupling is supported, Thermal-Electric.  What we recommend people do is they build their models in ANSYS mechanical for one of the physics, then use code snippets to change the elements to the proper direct coupled-field type and to also do any post processing. It will run when you solve, but it will come back with an error, and you need to post processes via APDL code or you need to post process in MAPDL interactively.

NEW INFO:  Edward points out in the comment below that you can get this to work.  I’ll repeat it here:

“We’ve had some success post-processing U-TEMP-VOLT analyses in Mechanical. Mechanical seems to accept a model as solved, so long as it sees a result file of the correct type in the Solver Files directory. The coupled field analysis in this case output a .rst file, so we used a Static Structural object as the base model.
We could access the structural results directly and used User-defined results to access most of the thermal and electric results.
I seem to recall that we also had success using a Thermal analysis as a base and then changing the result file extension from .rst to .rth, but I can’t find my test model to confirm this.”

I can verify that both of these approaches work. I added a /sys, copy file.rst to file.rth to a code segment for the thermal base.  But it was simpler to just use the structural as the base.  If you do this you can do your post processing for the most part in ANSYS Mechanical. [E. Miller 3/28/2013)

### Thoughts

So this was, as promised, a very high level overview. The fact of the matter is that there are a significant number of users, especially in the MEMS industry, that use these direct coupled-field elements all the time.  They are powerful and robust with as many uses as you can dream up, truly expanding the reach of what you can model and the accuracy of those models.

Over the years we have found some good tricks for using these elements effectively:

1. Pick one of the physics and get a static run of that physics by itself running first. Debugging your model this way is usually faster and clears out any issues before you deal with the direct coupling issues. If you have more than two physics, add them in one at a time.
2. Pay attention to units. When you start mixing voltage and distance or what not, it is easy to get confused. If you are doing MEMS devices, you need to make sure you are using the MEMS units and that you are consistent.  Unlike ANSYS Mechanical, ANSYS Mechanical APLD is unitless and requires the user to make sure the are consistent across physics.
3. Try not to use the legacy elements if you don’t have to. They may not be around in the future.
4. If you are doing EMAG, you may want to look at using load coupling with Maxwell or MAPDL instead of using the legacy direct coupled elements.  Maxwell and the newer elements in MAPDL have more capabilities and are more efficient.
5. Make sure you really understand how your physics interact. Go through the thought experiment of predicting the interaction on as simple of a problem as you can, while keeping it relevant. Think about what loads interact with what structures and what that interaction implies.