## Constitutive Modeling of 3D Printed FDM Parts: Part 2 (Approaches)

In part 1 of this two-part post, I reviewed the challenges in the constitutive modeling of 3D printed parts using the Fused Deposition Modeling (FDM) process. In this second part, I discuss some of the approaches that may be used to enable analyses of FDM parts even in presence of these challenges. I present them below in increasing order of the detail captured by the model.

• Conservative Value: The simplest method is to represent the material with an isotropic material model using the most conservative value of the 3 directions specified in the material datasheet, such as the one from Stratasys shown below for ULTEM-9085 showing the lower of the two modulii selected. The conservative value can be selected based on the desired risk assessment (e.g. lower modulus if maximum deflection is the key concern). This simplification brings with it a few problems:
• The material property reported is only good for the specific build parameters, stacking and layer thickness used in the creation of the samples used to collect the data
• This gives no insight into build orientation or processing conditions that can be improved and as such has limited value to an anlayst seeking to use simulation to improve part design and performance
• Finally, in terms of failure prediction, the conservative value approach disregards inter-layer effects and defects described in the previous blog post and is not recommended to be used for this reason
• Orthotropic Properties: A significant improvement from an isotropic assumption is to develop a constitutive model with orthotropic properties, which has properties defined in all three directions. Solid mechanicians will recognize the equation below as the compliance matrix representation of the Hooke’s Law for an orthortropic material, with the strain matrix on the left equal to the compliance matrix by the stress matrix on the right. The large compliance matrix in the middle is composed of three elastic modulii (E), Poisson’s ratios (v) and shear modulii (G) that need to be determined experimentally.

Good agreement between numerical and experimental results can be achieved using orthotropic properties when the structures being modeled are simple rectangular structures with uniaxial loading states. In addition to require extensive testing to collect this data set (as shown in this 2007 Master’s thesis), this approach does have a few limitations. Like the isotropic assumption, it is only valid for the specific set of build parameters that were used to manufacture the test samples from which the data was initially obtained. Additionally, since the model has no explicit sense of layers and inter-layer effects, it is unlikely to perform well at stresses leading up to failure, especially for complex loading conditions.  This was shown in a 2010 paper that demonstrated these limitations  in the analysis of a bracket that itself was built in three different orientations. The authors concluded however that there was good agreement at low loads and deflections for all build directions, and that the margin of error as load increased varied across the three build orientations.

• Laminar Composite Theory: The FDM process results in structures that are very similar to laminar composites, with a stack of plies consisting of individual fibers/filaments laid down next to each other. The only difference is the absence of a matrix binder – in the FDM process, the filaments fuse with neighboring filaments to form a meso-structure. As shown in this 2014 project report, a laminar approach allows one to model different ply raster angles that are not possible with the orthotropic approach. This is exciting because it could expand insight into optimizing raster angles for optimum performance of a part, and in theory reduce the experimental datasets needed to develop models. At this time however, there is very limited data validating predicted values against experiments. ANSYS and other software that have been designed for composite modeling (see image below from ANSYS Composite PrepPost) can be used as starting points to explore this space.
• Hybrid Tool-path Composite Representation: One of the limitations of the above approach is that it does not model any of the details within the layer. As we saw in part 1 of this post, each layer is composed of tool-paths that leave behind voids and curvature errors that could be significant in simulation, particularly in failure modeling. Perhaps the most promising approach to modeling FDM parts is to explicitly link tool-path information in the build software to the analysis software. Coupling this with existing composite simulation is another potential idea that would help reduce computational expense. This is an idea I have captured below in the schematic that shows one possible way this could be done, using ANSYS Composite PrepPost as an example platform.

Discussion: At the present moment, the orthotropic approach is perhaps the most appropriate method for modeling parts since it is allows some level of build orientation optimization, as well as for meaningful design comparisons and comparison to bulk properties one may expect from alternative technologies such as injection molding. However, as the application of FDM in end-use parts increases, the demands on simulation are also likely to increase, one of which will involve representing these materials more accurately than continuum solids.

## Activating Hyperdrive in ANSYS Simulations

With PADT and the rest of the world getting ready to pile into dark rooms to watch a saga that we’ve been waiting for 10 years to see, I figured I’d take this opportunity to address a common, yet simple, question that we get:

“How do I turn on HPC to use multiple cores when running an analysis?”

For those that don’t know, ANSYS spends a significant amount of resources into making the various solvers it has utilize multiple CPU processors more efficiently than before.  By default, depending on the solver, you are able to use between 1-2 cores without needing HPC licenses.

With the utilization of HPC licenses, users can unlock hyperdrive in ANSYS.  If you are equipped with HPC licenses it’s just a matter of where to look for each of the ANSYS products to activate it.

## ANSYS Mechanical

Whether or not you are performing a structural, thermal or explicit simulation the process to activate multiple cores is identical.

1. Go to Tools > Solve Process Settings
2. The Solve Process Settings Window will pop up
3. Click on Advanced to open up the Advanced Settings window
4. You will see an option for Max number of utilized cores
5. Simply change the value to your desired core count
6. You will see below an option to allow for GPU acceleration (if your computer is equipped with the appropriate hardware)
7. Select the GPU type from the dropdown and choose how many GPUs you want to utilize
8. Click Ok and close

### Distributed Solve in ANSYS Mechanical

One other thing you’ll notice in the Advanced Settings Window is the option to turn “Distributed” On or Off using the checkbox.

In many cases Distributing a solution can be significantly faster than the opposite (Shared Memory Parallel).  It requires that MPI be configured properly (PADT can help guide you through those steps).  Please see this article by Eric Miller that references GPU usage and Distributed solve in ANSYS Mechanical

## ANSYS Fluent

Whether launching Fluent through Workbench or standalone you will first see the Fluent Launcher window.  It has several options regarding the project.

1. Under the Processing Options you will see 2 options: Serial and Parallel
2. Simply select Parallel and you will see 2 new dropdowns
3. The first dropdown lets you select the number of processes (equal to the number of cores) to use in not only during Fluent’s calculations but also during pre-processing as well

## ANSYS CFX

For CFX simulations through Workbench, the option to activate HPC exists in the Solution Manager

1. Open the CFX Solver Manager
2. You will see a dropdown for Run Mode
3. Rather than the default “Serial” option choose from one of the available “Parallel” options.
4. For example, if running on the same machine select Platform MPI Local Parallel
5. Once selected in the section below you will see the name of the computer and a column called Partitions
6. Simply type the desired number of cores under the Partitions column and then either click “Save Settings” or “Start Run”

## ANSYS Electronics Desktop/HFSS/Maxwell

Regardless of which electromagnetic solver you are using: HFSS or Maxwell you can access the ability to change the number of cores by going to the HPC and Analysis Options.

1. Go to Tools > Options > HPC and Analysis Options.
2. In the window that pops up you will see a summary of the HPC configuration
3. Click on Edit and you will see a column for Tasks and a column for Cores.
4. Tasks relate to job distribution utilizing Optimetrics and DSO licenses
5. To simply increase the number of cores you want to run the simulation on, change the cores column to your desired value
6. Click OK on all windows

There you have it.  That’s how easy it is to turn on Hyperdrive in the flagship ANSYS products to advance your simulations and get to your endpoint faster than before.

If you have any questions or would like to discuss the possibility of upgrading your ship with Hyperdrive (HPC capabilities) please feel free to call us at 1-800-293-PADT or email us at support@padtinc.com.

## PID Thermostat Boundary Condition ACT Extension for ANSYS Mechanical

PADT is pleased to announce that we have uploaded a new ACT Extension to the ANSYS ACT App Store.  This new extension implements a PID based thermostat boundary condition that can be used within a transient thermal simulation.  This boundary condition is quite general purpose in nature.  For example, it can be setup to use any combination of (P)roportional (I)ntegral or (D)erivate control.   It supports locally monitoring the instantaneous temperature of any piece of geometry within the model.  For a piece of geometry that is associated with more than one node, such as an edge or a face, it uses a novel averaging scheme implemented using constraint equations so that the control law references a single temperature value regardless of the reference geometry.

The set-point value for the controller can be specified in one of two ways.  First, it can be specified as a simple table that is a function of time.  In this scenario, the PID ACT Extension will attempt to inject or remove energy from some location on the model such that a potentially different location of the model tracks the tabular values.   Alternatively, the PID thermostat boundary condition can be set up to “follow” the temperature value of a portion of the model.  This location again can be a vertex, edge or face and the ACT extension uses the same averaging scheme mentioned above for situations in which more than one node is associated with the reference geometry.  Finally, an offset value can be specified so that the set point temperature tracks a given location in the model with some nonzero offset.

For thermal models that require some notion of control the PID thermostat element can be used effectively.  Please do note, however, that the extension works best with the SI units system (m-kg-s).

## Be a Pinball Wizard with Contact Regions in ANSYS Mechanical

I had a very cool music teacher back in 6th or 7th grade in the 1970’s in upstate New York.  Today we’d probably say she was eclectic.  In that class we listened to and discussed fairly recent songs in addition to general music studies.  Two songs I remember in particular are ‘Hurdy Gurdy Man’ by Donovan and ‘Pinball Wizard’ by The Who.  If you’re not familiar with Pinball Wizard, it’s from The Who’s rock opera Tommy, and is about a deaf, mute, blind young man who happens to be adept at the game of pinball.  Yes, he is a Pinball Wizard.  This sing popped into my head recently when we had some customer questions here at PADT regarding the pinball region concept as it pertains to ANSYS contact regions.

I’m not sure if the developers at ANSYS, Inc. had this song in mind when they came up with the nomenclature for the 17X (latest and greatest) series of contact elements in ANSYS, but regardless, you too can be a pinball wizard when it comes to understanding contact elements in ANSYS Mechanical and MAPDL.

Fans of this blog may remember one of my prior posts on contact regions in ANSYS that also had a musical theme (bringing to mind Peter Gabriel’s song “I Have the Touch”):

In this current entry we will go more in depth on the pinball region, also known as the pinball radius.  The pinball region is involved with the distance from contact element to target element in a given contact region.  Outside the pinball region, ANSYS doesn’t bother to check to see if the elements on opposite sides of the contact region are touching or not.  The program assumes they are far away from each other and doesn’t worry about any additional calculations for the most part.

Here is an illustration.  The gray elements on the left represent the contact body and the red elements on the right represent the target body (assuming asymmetric contact).  Target elements outside the pinball radius will not be checked for contact.  The contact and target elements actually ‘coat’ the underlying solid elements so they are shown as dashed lines slightly offset from the solid elements for the sake of visibility.  Here the pinball radius is displayed as a dashed blue circle, centered on the contact elements, with a radius of 2X the depth of the underlying solid elements.

So, outside the pinball region, we know ANSYS doesn’t check to see if the contact and target are actually in contact.  It just assumes they are far away and not in contact.  What about what happens if the contact and target are inside the pinball region?  The answer to that question depends on which contact type we have selected.

For frictionless contact (aka standard contact in MAPDL) and frictional contact, the program will then check to see if the contact and target are truly touching.  If they are touching, the program will check to see if they are sliding or possibly separating.  If they are touching and penetrating, the program will check to see if the penetration exceeds the allowable amount and will make adjustments, etc.  In other words, for frictionless and frictional contact, if the contact and target elements are close enough to be inside the pinball region, the program will make all sorts of checks and adjustments to make sure the contact behavior is adequately captured.

The other scenario is for bonded and no separation contact.  With these contact types, the program’s behavior when the contact and target elements are within the pinball region is different.  For these types, as long as the contact and target are close enough to be within the pinball region, the program considers the contact region to be closed.  So, for bonded and no separation, your contact and target elements do not need to be line on line touching in order for contact to be recognized.  The contact and target pairs just need to be inside the pinball region.  This can be good, in that it allows for some ‘slop’ in the geometry to be automatically ignored, but it also can have a downside if we have a curved surface touching a flat surface for example.  In that case, more of the curved surface may be considered in contact than would be the case if the pinball region was smaller.  This effect is shown in the image below.  Reducing the pinball radius to an appropriate smaller amount would be the fix for eliminating this ‘overconstraint’ if desired.

There is a default value for the pinball region/radius.  It can be changed if needed.  We’ll add more details in a moment.  First, why is it called the “pinball” region?  I like to think it’s because when it’s visualized in the Mechanical window, it looks like a blue pinball from an actual pinball arcade game, but I’ll admit that the ANSYS terminology may predate the Mechanical interface.  The image below shows what I mean.  The blue balls are the different pinball radii for different contact regions.

Note that you don’t see the pinball region displayed as shown in the above image unless you have manually changed the pinball size in Mechanical.  The pinball region can be changed in the Mechanical window in the details view for each contact region by changing Pinball Region from Program Controlled to Radius, like this:

In MAPDL, the pinball radius value can be changed by defining or editing the real constant labeled PINB.

By now you’re probably wondering what is the default value for the pinball radius?  The good news is that it is intelligently decided by the program for each contact region.  The default is always a scale factor on the depth of the underlying elements of each contact region.  In the first pinball region image shown near the beginning of this article, the example plot shows the pinball region/radius as two times the depth of the underlying elements.

The table below summarizes the default pinball radius values for most circumstances for 2D and 3D solid element models.  More detailed information is available in the ANSYS Help.

Default Pinball Radius ValuesLarge Deflection Off
Flexible-Flexible
Large Deflection On
Flexible-Flexible
Frictionless and Frictional1* Underlying Element Depth2*Underlying Element Depth
Bonded and No Seperation0.25*Underlying Element Depth0.5*Underlying Element Depth
Rigid-Flexible Contact: Typically the Default Values are Doubled

Summing it all up:  we have seen how the default values are calculated and also how to change them.  We have seen what they look like as blue balls in a plot of contact regions in Mechanical if the pinball radius has been explicitly defined.  We also discussed what the pinball radius does and how it’s different for frictionless/frictional contact and bonded/no separation contact.

You should be well on your way to becoming a pinball wizard at this point.

Does performing simulation in ANSYS make you think of certain songs, or are there songs you like to listen to while working away on your simulations an addition to The Who’s “Pinball Wizard” and Peter Gabriel’s “I Have the Touch”?  If so, we’d love to hear about your song preferences in the comments below.

## 7 Reasons why ANSYS AIM Will Change the Way Simulation is Done

When ANSYS, Inc. released their ANSYS AIM product they didn’t just introduce a better way to do simulation, they introduced a tool that will change the way we all do simulation.  A bold statement, but after PADT has used the tool here, and worked with customers who are using it, we feel confident that this is a software package will drive that level of change.   It enables the type of change that will drive down schedule time and cost for product development, and allow companies to use simulation more effectively to drive their product development towards better performance and robustness.

## It’s Time for a Productivity Increase

If you have been doing simulation as long as I have (29 years for me) you have heard it before. And sometimes it was true.  GUI’s on solvers was the first big change I saw. Then came robust 3D tetrahedral meshing, which we coasted on for a while until fully associative and parametric CAD connections made another giant step forward in productivity and simulation accuracy. Then more recently, robust CFD meshing of dirty geometry. And of course HPC improvements on the solver side.

That was then.  Right now everyone is happily working away in their tool of choice, simulating their physics of choice.  ANSYS Mechanical for structural, ANSYS Fluent for fluids, and maybe ANSYS HFSS for electromagnetics. Insert your tool of choice, it doesn’t really matter. They are all best-in-breed advanced tools for doing a certain type of physical simulation.  Most users are actually pretty happy. But if you talk to their managers or methods engineers, you find less happiness. Why? They want more engineers to have access to these great tools and they also want people to be working together more with less specialization.

## Putting it all Together in One Place

ANSYS AIM is, among many other things, an answer to this need.  Instead of one new way of doing something or a new breakthrough feature, it is more of a product that puts everything together to deliver a step change in productivity. It is built on top of these same world class best-in-bread solvers. But from the ground up it is an environment that enables productivity, processes, ease-of-use, collaboration, and automation. All in one tool, with one interface.

## Changing the Way Simulation is Done

Before we list where we see things changing, let’s repeat that list of what AIM brings to the table, because those key deliverables in the software are what are driving the change:

• Improved Productivity
• Standardized Processes
• True Ease-of-Use
• Inherent Collaboration
• Intuitive Automation
• Single Interface

Each of these on their own would be good, but together, they allow a fundamental shift in how a simulation tool can be used. And here are the seven way we predict you will be doing things differently.

### 1) Standardized processes across an organization

The workflow in ANSYS AIM is process oriented from the beginning, which is a key step in standardizing processes.  This is amplified by tools that allow users, not just programmers, to create templates, capturing the preferred steps for a given type of simulation.  Others have tried this in the past, but the workflows were either too rigid or not able to capture complex simulations.  This experience was used to make sure the same thing does not happen in ANSYS AIM.

### 2) No more “good enough” simulation done by Design Engineers

Ease of use and training issue has kept robust simulation tools out of the hands of design engineers.  Programs for that group of users have usually been so watered down or lack so much functionality, that they simply deliver a quick answer. The math is the same, but it is not as detailed or accurate.  ANSYS AIM solves this by give the design engineer a tool they can pick up and use, but that also gives them access to the most capable solvers on the market.

### 3) Multiphysics by one user

Multiphysics simulation often involves the use of multiple simulation tools.  Say a CFD Solver and a Thermal Solver. The problem is that very few users have the time to learn two or more tools, and to learn how to hook them together. So some Multiphysics is done with several experts working together, some in tools that do multiple physics, but none well, or by a rare expert that has multi-tool expertise.  Because ANSYS AIM is a Multiphysics tool from the ground up, built on high-power physics solvers, the limitations go away and almost any engineer can now do Multiphysics simulation.

### 4) True collaboration

The issues discussed above about Multiphysics requiring multiple users in most tools, also inhibit true collaboration. Using one user’s model in one tool is difficult when another user has another tool. Collaboration is difficult when so much is different in processes as well.  The workflow-driven approach in ANSYS AIM lends itself to collaboration, and the consistent look-and-feel makes it happen.

### 5) Enables use when you need it

This is a huge one.  Many engineers do not use simulation tools because they are occasional users.  They feel that the time required to re-familiarize themselves with their tools is longer than it takes to do the simulation. The combination of features unique to ANSYS AIM deal with this in an effective manner, making accurate simulation something a user can pick up when they need it, use it to drive their design, and move on to the next task.

### 6) Stepping away from CAD embedded Simulation

The growth of CAD embedded simulation tools, programs that are built into a CAD product, has been driven by the need to tightly integrate with geometry and provide ease of use for the users who only occasionally need to do simulation. Although the geometry integration was solved years ago, the ease-of-use and process control needed is only now becoming available in a dedicated simulation tool with ANSYS AIM.

### 7) A Return to home-grown automation for simulation

If you have been doing simulation since the 80’s like I have, you probably remember a day when every company had scripts and tools they used to automate their simulation process. They were extremely powerful and delivered huge productivity gains. But as tools got more powerful and user interfaces became more mature, the ability to create your own automation tools faded.  You needed to be a programmer. ANSYS AIM brings this back with recording and scripting for every feature in the tool, with a common and easy to use language, Python.

## How does this Impact Me and or my Company?

It is kind of fun to play prognosticator and try and figure out how a revolutionary advance in our industry is going to impact that industry. But in the end it really does not matter unless the changes improve the product development process. We feel pretty strongly that it does.  Because of the changes in how simulation is done, brought about by ANSYS AIM, we feel that more companies will use simulation to drive their product development, more users within a company will have access to those tools, and the impact of simulation will be greater.

To fully grasp the impact you need to step back and ponder why you do simulation.  The fast cars and crazy parties are just gravy. The core reason is to quickly and effectively test your designs.  By using virtual testing, you can explore how your product behaves early in the design process and answer those questions that always come up.  The sooner, faster, and more accurately you answer those questions, the lower the cost of your product development and the better your final product.

Along comes a product like ANSYS AIM.  It is designed by the largest simulation software company in the world to give the users of today and tomorrow access to the power they need. It enables that “sooner, faster, and more accurately” by allowing us to change, for the better, the way we do virtual testing.

The best way to see this for yourself is to explore ANSYS AIM.  Sign up for our AIM Resource Kit here or contact us and we will be more than happy to show it to you.

## To Use Large Deflection or Not, That Is the Question

It seems like I’ve been explaining large deflection effects a lot recently. Between co-teaching an engineering class at nearby Arizona State University and also having a couple of customer issues regarding the concept, large deflection in structural analyses has been on my mind.

Before I explain any further, the thing you should note if you are an ANSYS Mechanical simulation user is this: If you don’t know if you need large deflection or not, you should turn it on. There is really no way to know for certain if it’s needed or not unless you perform a comparison study with and without it.

So, what are large deflection effects? In simple terms the inclusion of large deflection means that ANSYS accounts for changes in stiffness due to changes in shape of the parts you are simulating. The classic case to consider is the loaded fishing rod.

In its undeflected state, the fishing rod is very flexible at the tip. With a heavy fish on the end of the line, the rod deflects downward and it is then easy to observe that the stiffness of the rod has increased. In other words, when the rod is lightly loaded, a small amount of force will cause a certain downward deflection at the top. When the rod is heavily loaded however, a much larger amount of force will be needed to cause the tip to deflect downward by the same amount.

This change in the force amount required to achieve the same change in displacement implies that we do not have a linear relationship between force and displacement.
Consider Hooke’s law, also known as the spring equation:

F = Kx

Where F is the force applied, K is the stiffness of the structure, and x is the deflection. In a linear system, doubling the force results in double the displacement. In our fishing rod case, though, we have a nonlinear system. We might need to triple the force to double the displacement, depending on how much the rod is loaded relative to its size and other properties, and then to double the displacement again we might need to apply four times that force, just using numbers out of my head as examples.

So, in the case of the fishing rod, Hooke’s law in a linear form does not apply. In order to capture the nonlinear effect we need a way for the stiffness to change as the shape of the rod changes. In our finite element solution in ANSYS, it means that we want to recalculate the stiffness as the structure deflects.

This recalculation of the stiffness as the structure deflects is activated by turning on large deflection effects. Without large deflection turned on, we are constrained to using the linear equation, and no matter how much the structure deflects we are still using the original stiffness.

So, why not just have large deflection on by default and use it all the time? My understanding is that since large deflection adds computation expense to have it on, it’s off by default. It’s the same as for a lot of advanced usage, such as frictionless or frictional contact vs. the default bonded (simpler) behavior. In other words, turning on large deflection will trigger a nonlinear solution, meaning multiple passes through the solver using the Newton Raphson method instead of the single pass needed for a linear problem.

Here is an example of a simplified fishing rod. The image shows the undeflected rod (top), which is held fixed on the left side and has a downward force load applied on the right end. The bottom image shows the final deflected shape, with large deflection effects included. The deflection at the tip in this case is 34 inches.

In comparison running the same load with large deflection turned off resulted in a tip deflection of 40 inches. Thus, the calculated tip deflection is 15% less with large deflection turned on, since we are now accounting for change in stiffness with change in shape as the rod deflects.

Below we have a force (horizontal axis) vs. deflection (vertical axis) plot for a nonlinear simulation of a fishing rod with large deflection turned on. The fact that the curve is not a straight line confirms that this is a nonlinear problem, with the stiffness (slope of the curve) not constant. We can also see that as the force gets higher, the slope of the curve is more horizontal, meaning that more force is needed for each incremental amount of displacement. This matches our observations of the fishing rod behavior.

So, getting back to our original point, it’s often the case that we don’t know if we need to include large deflection effects or not. When in doubt, run cases with and without. If you don’t see a change in your key results, you can probably do without large deflection.

Here is an example using an idealized compressor vane. In this case, the deflections and stresses with and without large deflection effects are nearly the same (the stress difference is about 0.2%).

Large Deflection On:

Small Deflection:

Bottom line: when in doubt, try it out, with and without large deflection. In ANSYS Mechanical, Large Deflection effects are turned on or off in the details of the Analysis Settings branch.

It’s worth noting that turning on large deflection in ANSYS actually activates four different behaviors, known as large deflection which include large rotation, large strain, stress stiffening, and spin softening. All of these involve change in stiffness due to deformation in one way or another.

If you like this kind of info, or find it useful, we cover topics like this in our training classes. For more info, check out our training pages at http://www.padtinc.com/support/software/training.html.

## Donny Don’t – Thin Sweep Meshing

It’s not a series of articles until there’s at least 3, so here’s the second article in my series of ‘what not to do’ in ANSYS…

Just in case you’re not familiar with thin sweep meshing, here’s an older article that goes over the basics.  Long story short, the thing sweep mesher allows you to use multiple source faces to generate a hex mesh.  It does this by essentially ‘destroying’ the backside topology.  Here’s a dummy board with imprints on the top and bottom surface:

If I use the automatic thin sweep mesher, I let the mesher pick which topology to use as the source mesh, and which topology to ‘destroy’.  A picture might make this easier to understand…

As you can see, the bottom (right picture) topology now lines up with the mesh, but when I look at the top (left picture) the topology does not line up with the mesh.  If I want to apply boundary conditions to the top of the board (left picture), I will get some very odd behavior:

I’ve fixed three sides of the board (why 3?  because I meant to do 4 but missed one and was too lazy to go back and re-run the analysis to explain for some of future deflection plots…sorry, that’s what you get in a free publication) and then applied a pressure to all of those faces.  When I look at the results:

Only one spot on the surface has been loaded.  If you go back to the mesh-with-lines picture, you’ll see that there is only a single element face fully contained in the outline of the red lines.  That is the face that gets loaded.  Looking at the input deck, we can see that the only surface effect element (how pressure loads are applied to the underlying solid) is on the one fully-contained element face:

If I go back and change my thin sweep to use the top surface topology, things make sense:

The top left image shows the thin sweep source definition.  Top right shows the new mesh where the top topology is kept.  Bottom left shows the same boundary conditions.  Bottom right shows the deformation contour.

The same problem occurs if you have contact between the top and bottom of a thin-meshed part.  I’ll switch the model above to a modal analysis and include parts on the top and bottom, with contact regions already imprinted.

I’ll leave the thin sweeping meshing control in place and fix three sides of the board (see previous laziness disclosure).  I hit solve and nothing happens:

Ah, the dreaded empty contact message.  I’ll set the variable to run just to see what’s going on.  Pro Tip:  If you don’t want to use that variable then you would have to write out the input deck, it will stop writing once it gets to the empty contact set.  Then go back and correlate the contact pair ID with the naming convection in the Connections branch.

The model solves and I get a bunch of 0-Hz (or near-0) modes, indicating rigid body motion:

Looking at some of those modes, I can see that the components on one side of my board are not connected:

The missing contacts are on the bottom of the board, where there are three surface mounted components (makes sense…I get 18 rigid body modes, or 6 modes per body).  The first ‘correct’ mode is in the bottom right image above, where it’s a flapping motion of a top-mounted component.

So…why don’t we get any contact defined on the bottom surface?  It’s because of the thin meshing.  The faces that were used to define the contact pair were ‘destroyed’ by the meshing:

Great…so what’s the take-away from this?  Thin sweep meshing is great, but if  you need to apply loads, constraints, define contact…basically interact with ANYTHING on both sides of the part, you may want to use a different meshing technique.  You’ve got several different options…

1. Use the tet mesher.  Hey, 2001 called and wants its model size limits back.  The HPC capabilities of ANSYS make it pretty painless to create larger models and use additional cores and GPUs (if you have a solve-capable GPU).  I used to be worried if my model size was above 200k nodes when I first started using ANSYS…now I don’t flinch until it’s over 1.5M

Look ma, no 0-Hz modes!
2. Use the multi-zone mesher.  With each release the mutli-zone mesher has gotten better, but for most practical applications you need to manually specify the source faces and possibly define a smaller mesh size in order to handle all the surface blocking features.

Look pa, no 0-Hz modes!Full disclosure…the multi-zone mesher did an adequate job but didn’t exactly capture all of the details of my contact patches.  It did well enough with a body sizing and manual source definition in order to ‘mostly’ bond each component to the board.
3. Use the hex-dominant mesher.  Wow, that was hard for me to say.  I’m a bit of a meshing snob, and the hex dominant mesher was immature when it was released way back when.  There were a few instances when it was good, but for the most part, it typically created a good surface mesh and a nightmare volume mesh.  People have been telling me to give it another shot, and for the most part…they’re right.  It’s much, much better.  However, for this model, it has a hard time because of the aspect ratio.  I get the following message when I apply a hex dominant control:

4. The warning is right…the mesh looks decent on the surface but upon further investigation I get some skewed tets/pyramids.  If I reduce the element size I can significantly reduce the amount of poorly formed elements:
5. That’s going on the refrigerator door tonight!

And…no 0-Hz modes!
• Lastly…go back to DesignModeler or SpaceClaim and slice/dice the model and use a multi-body part.
3 operations, ~2 minutes of work (I was eating at the same time)

Modify the connection group to search/sort across parts

That’s a purdy mesh!  (Note:  most of the lower-quality elements, .5 and under, are because there are 2-elements through thickness, reducing the element size or using a single element thru-thickness would fix that right up)

And…no 0-Hz modes.

Phew…this was a long one.  Sorry about that.  Get me talking about meshing and look what happens.  Again, the take-away from all of this should be that the thin sweeper is a great tool.  Just be aware of its limitations and you’ll be able to avoid some of these ‘odd’ behaviors (it’s not all that odd when you understand what happens behind the scenes).

## Taking NASTRAN Input Files Into ANSYS Mechanical via External Model in ANSYS 16.0

I found another very nice enhancement to version 16.0 of the ANSYS Workbench/ANSYS Mechanical toolset.  If you happen to have a NASTRAN input file (.dat, .nas, and .bdf) that you need to get into ANSYS Mechanical, no longer do you have to use FE Modeler in ANSYS Workbench to perform the translation.  In fact, not only can you move the NASTRAN model into ANSYS Mechanical, but you get the existing mesh as well as newly-created geometry that can be used for boundary condition application, etc.  As with most translations from one FE tool to another, you can’t expect everything will be translated.  However, this new technique can be an incredible time saver in addition to giving us capabilities to continue and augment simulations that were previously performed in NASTRAN, now in ANSYS.

Here is an example of this new procedure.  (Note that we don’t have NASTRAN here at PADT, so I couldn’t create a generic sample of a NASTRAN model in NASTRAN.  Instead, I created a model in ANSYS, then converted it into NASTRAN using ANSYS FE Modeler to get a NASTRAN input file for the purpose of this exercise.)

Once I have the NASTRAN input file that I need to convert into ANSYS Mechanical, I launch ANSYS Workbench 16.0 and insert an External Model branch.  I then click the … button to browse to the NASTRAN input file.  In this case, the file is NASTRAN.nas.

Next, I drag and drop a new analysis type block into the Project Schematic.  In this case, it was a modal analysis.  Note that you can’t drop this onto the Setup cell in the External Model block as you might expect.  You set it up as a separate block and establish the link in the next step.

Next, we drag and drop the Setup cell from the External Model block onto the Model cell of the Modal analysis block.  This establishes the link from the NASTRAN model to the new Modal analysis.

We also need to right click on the Setup cell in the External Model block and select Update to get a green checkmark in that cell:

Notice that there is no Geometry cell in the resulting Modal analysis block.  If all goes well, there will be geometry within the Mechanical model that can be used for selection purposes (in addition to the mesh that comes in from NASTRAN).

Next we open the Mechanical editor by double clicking on one of the cells in the Modal analysis blocks (other than the Engineering Data cell).  It may take several minutes to bring in the NASTRAN model depending on the size of the NASTRAN model.  The Mechanical window doesn’t really let you know that it’s working, but if it’s sitting there with nothing being displayed, it’s probably churning away at bringing in the NASTRAN mesh and creating surface geometry on it.

Here is what the Mechanical window looks like after the mesh is read in and geometry is automatically created.  This is the mesh from the NASTRAN file, but in this case both solid and surface geometry is also present.  It’s not guaranteed that everything will come across.  I’ve seen contact elements come through for certain types of contact but not for other types of contact for example.

The next image shows that geometry was created that can be used for the purposes of inserting fixed supports, just as if the geometry had come in from a CAD system.  Note that the NASTRAN input file had NO geometry, just finite element entities.  ANSYS is creating the geometry for use in Mechanical from the information in the NASTRAN input file.

Finally, after manually creating a needed contact region, I was able to solve the modal analysis, demonstrating that further simulation can be performed in ANSYS Mechanical from this model which originally came from NASTRAN.

So, the main take away here is that with version 16.0 of ANSYS, we can take a NASTRAN input file and through the use of the External Model block, go directly into ANSYS Mechanical.  Not only do we get the nodes and elements as well as other finite element entities from the NASTRAN model, but if all goes well we get geometry that facilitates further processing within ANSYS Mechanical.

We certainly hope this new capability makes it easier for you to perform additional simulations in ANSYS when the starting point is a NASTRAN model.  The other formats documented for version 16.0 are ABAQUS, Fluent input files, and ICEM CFD input files.

## Tech Tips and Videos for ANSYS Mechanical and CFD

A few weeks ago we added some great free resources to our website for existing and potential users of ANSYS Structural and CFD tools.  It includes some great videos from ANSYS, Inc. on a variety of topics as well as productivity kits. It dawned on us that many of you are faithful readers of The Focus but don’t often check out our ANSYS product web pages. So, we are including the material here for your viewing pleasure.

(7/9/2015: We just added the Electromechanical kit here.)

For structural users, we have a link to “The Structural Simulation Productivity Kit ” here. The kit includes:

• Analyzing Vibration with Acoustic–Structural Coupling Article
• Contact Enhancements in ANSYS Mechanical and MAPDL 15.0 Webinar
• ANSYS Helps KTM Develop a 21st Century Super Sports Car Case Study
• A Practical Discussion on Fatigue White Paper
• Designing Solid Composites Article

We also have a collection of videos from ANSYS, Inc that we found useful:

For CFD users, we have a link to “The CFD Simulation Productivity Kit ” here. The kit includes:

• Simulating Erosion Using ANSYS Computational Fluid Dynamics Presentation,
• Cutting Design Costs: How Industry leaders benefit from Fast and Reliable CFD  White Paper,
• Introduction to Multiphase Models in ANSYS CFD Three Part Webinar,
• Advances in Core CFD Technology: Meeting Your Evolving Product Development Needs White Paper,
• Turbulence Modeling for Engineering Flows Application Brief.

We also have a collection of videos from ANSYS, Inc that we found useful:

Interested in learning more, contact us or simply request a quote.

## Press Release: Structural Optimization from VR&D Added to PADT Portfolio

We are very pleased to announce that we have added another great partner to our product portfolio: Vanderplaats Research  Development.  VR&D is a leading provider of structural optimization tools for simulation, and a strong partner with ANSYS.  We came across their Genesis and GTAM products when we were looking for a good topological optimization tool for one of our ANSYS customers. We quickly found it to be a great compliment, especially for the growing need to support optimization for parts made with 3D Printing.

Please find the official press release below or as a PDF file.  You can also learn more about the products on our website here. We hope to schedule some webinars on this tool, and publish some blog articles, in the coming months.

Press Release:

PADT is now a reseller of the GTAM and GENESIS optimization tools from Vanderplaats R&D, offering leading structural geometry and topological optimization tools to enable simulation for components made with 3D Printing

Tempe, AZ – March 24, 2015 – Phoenix Analysis & Design Technologies, Inc. (PADT, Inc.), the Southwest’s largest provider of simulation, product development, and 3D Printing services and products, is pleased to announce that an agreement has been reached with Vanderplaats Research & Development, Inc. (VR&D) for PADT to become a distributor of VR&D’s industry leading structural optimization tools in the Southwestern United States. These powerful tools will be offered alongside ANSYS Mechanical as a way for PADT’s customers to use topological optimization and shape optimization to determine the best geometry for their products.

The GENESIS program is a Finite Element solver written by leaders in the optimization space. It offers sizing, shape, topography, topometry, freeform, and topology optimization algorithms.  No other tool delivers so many methods for users to determine the ideal configuration for their mechanical components. These methods can be used in conjunction with static, modal, random vibration, heat transfer, and buckling simulations.  More information on GENESIS can be found at http://www.vrand.com/Genesis.html

PADT recommends that ANSYS Mechanical users who require topological optimization access GENESIS through the GENESIS Topology for ANSYS Mechanical tool, or GTAM. This extension runs inside ANSYS Mechanical, allowing users the ability to use their ANSYS models and the ANSYS user interface while still accessing the power of GENESIS.  The extension allows the user to setup the topology optimization problem, optimize, post-processing, export optimized geometry all within ANSYS Mechanical user interface.

“We had a customer ask us to find a topological optimization solution for optimizing the shape of a part they were manufacturing with 3D Printing. We tried GTAM and immediately found it to be the type of technically superior tool we like to represent” commented Ward Rand, a co-owner of PADT.  “It didn’t take our engineers long to learn it and after receiving great support from VR&D, we knew this was a tool we should add to our portfolio.”

Besides reselling the tool, PADT is adopting both GENESIS and GTAM as their internal tools for shape optimization in support of their growing consulting in the area of design and simulation for Additive Manufacturing, popularly known as 3D Printing. PADT combines these with ANSYS SpaceClaim and Geomagic Studio to design and optimize components that will be created using 3D Printing.

“We are thrilled to partner with PADT because of their deep knowledge in simulation, additive manufacturing, and 3D printing and for their extraordinary ability to help their clients”, stated Juan Pablo Leiva, President and COO of VR&D, “We feel that their unique talents are crucial in supporting clients in today’s demanding and changing market.”

To learn more about the GENESIS and GTAM products, visit http://www.padtinc.com/vrand or contact our technical sales team at 480.813.4884 or sales@padtinc.com.

About Phoenix Analysis and Design Technologies
Phoenix Analysis and Design Technologies, Inc. (PADT) is an engineering service company that focuses on helping customers who develop physical products by providing Numerical Simulation, Product Development, and Rapid Prototyping products and services. PADT’s worldwide reputation for technical excellence and an experienced staff is based on its proven record of building long term win-win partnerships with vendors and customers. Since its establishment in 1994, companies have relied on PADT because “We Make Innovation Work.“  With over 75 employees, PADT services customers from its headquarters at the Arizona State University Research Park in Tempe, Arizona, its Littleton, Colorado office, Albuquerque, New Mexico office, and Murray, Utah office, as well as through staff members located around the country. More information on PADT can be found at www.PADTINC.com.

About Vanderplaats Research & Development
Since its founding in 1984, Vanderplaats Research & Development, Inc. (VR&D) has advocated for the advancement of numerical optimization in industry. The company is a premier software company, developing and marketing a number of design optimization tools, providing professional services and training, and engaging in ongoing advanced research. VR&D products include GENESIS, GTAM, VisualDOC, Design Studio, SMS, DOT, and BIGDOT. For more information on VR&D, please visit:  www.vrand.com.

## First, some good news…

In Workbench R14.5, ANSYS introduced nodal Named Selections, and in R15.0, they have added the ability to create Named Selections of elements. So now you can make groups of nodes or elements just like you can in MAPDL.  You can use these name selections for result plots to show just specific portion of the results.

In R15.0, you can right-click on a Name Selection in the tree and hit, “Create Nodal Name Selection”. This creates a Name Selection of all the nodes associated with the particular piece of geometry in the original Named Selection, whether that is a body, surface, edge, or vertex. Highlighting the nodal named selection in the tree will then take you to the Worksheet where you can add rows for limiting the selection of nodes to a location value or some other criteria.

This is also where you can add a row to “Convert” the “Mesh Node” entity type to “Mesh Element”. The Mesh Element entity type has a criterion choice for how the elements are selected from the nodes.

“Any Node” will select all the elements that have any of their nodes in the list of nodes that make up the current named selection.  “All Nodes” will select only those elements that have all of their nodes in the current set. Many of you may already know this, and it is a great new feature, but there is a catch, and that brings us to the telling of the “Bad News”.

## The Bad News…

After noticing the generation time of the name selection drastically increase when using the “All Nodes” criteria, I ran a small test case. With just a cube meshed to two different refinement levels, I tracked the generation time for the element name selection using the two different criterion. Here is what I found.

I am not even going to speculate what is different with the “All Nodes” node-checking algorithm, but an increase in element count by a factor of eight caused more than a 13300% increase in generation time. But look at the generation time for the “Any Node” criteria. It stayed right on par for the different mesh sizes.

## So, back to the Good News, and the Really Good News…

The Good News is that you can avoid the long generation times, in R15.0, by not using the “All Nodes” criteria. The Really Good news is that when I ran the same test in R16.0, I got 6.0 Sec for the “Any Node” criteria, and 6.3 Seconds for the “All Nodes” criteria. So ANSYS has already fixed the problem in R16.0, which just gives you another reason to upgrade. If you are going to continue using R15.0, then just stay away from the “All Nodes” criteria for the element named Selections. It is much better to use the location based filtering to cut down your nodal selection so that you can use the “Any Node” criteria.

## 10 Useful New Features in ANSYS Mechanical 16.0

PADT is excited about the plethora of new features in release 16.0 of ANSYS products.  After sorting through the list of new features in Mechanical, here are 10 enhancements that we found to be particularly useful for general applications.

## 1: Mesh Display Style

This new option in the details view for the mesh branch makes it easy to visualize mesh quality items such as aspect ratio, skewness, element quality, etc.  The default style is body color, but it can be changed in the details to element quality, for example, as shown here:

Figure 1. A. – Mesh Display Style Set to Element Quality

Figure 1. B. – Element Quality Plot After Additional Mesh Settings

Figure 1. C. – Accessing Display Style in the Mesh Details

## 2: Image to Clipboard

How many times have you either done a print screen > paste into editing tool > crop or done an image to file to get the plots you need into tools such as Word and PowerPoint?  The new Image to Clipboard menu pick streamlines this process.  Now, just get the image the way you want it in the geometry view, right click, and select Image to Clipboard.  Or just use Ctrl + C.  When you paste, you’ll be pasting the contents of that view window directly.  Here’s what it looks like:

Figure 2 – Right Click, Image to Clip Board

## 3: Beam Contact Formulation

This was a beta feature at 15.0, but if you didn’t get a chance to try it out, it’s now fully supported at 16.0.  The idea here is that instead of the ‘traditional’ bonded contact methods (using the augmented Lagrange or pure penalty formulation) or the Multi-Point Constraint (MPC) bonded option, we now have a new choice of beam contact.  This option utilizes internally-created massless linear beam elements to connect the two sides of a contact interface together.  This can be more efficient than the traditional formulations and can avoid the over constraints that can happen if multiple contact regions utilizing the MPC option end up generating constraint equations that tend to conflict with each other.

Figure 3 – Beam Formulation for Bonded Contact

## 4: Nonlinear Adaptive Region

If you have ever been frustrated by the error message in the Solution Information window that says, “Element xyz … has become highly distorted…”, version 16.0 adds a new tool to our toolbox with the Nonlinear Adaptive Region capability.  This capability is in its infancy stage at 16.0, but in the right circumstances it allows the solution to recover from highly distorted elements by pausing, remeshing, and then continuing.  We plan on publishing more details on this capability soon, but for now please know that it exists and more can learned in the 16.0 Mechanical Help.  There are a lot of restrictions on when it can work, but a big one is that it only works for elements that become overly deformed due to large and nonuniform deformation, meaning not due to unstable materials, numerical instabilities, or structures that are unstable due to buckling effects.

As shown in figure 4. A., a Nonlinear Adaptive Region can be inserted under the Solution branch.  It is scoped to bodies.  Options and controls are set in the details view.

Figure 4. A. – Nonlinear Adaptive Region

If the solver encounters a ‘qualifying event’ that triggers a remesh, the solver output will inform us like this:

**** REGENERATE MESH AT SUBSTEP     5 OF LOAD STEP      1 BECAUSE OF

AmsMesher(ANSYS Mechanical Solver Mesher),Graph based ANSYS Meshing EXtension,v0.96.03b
(c)ANSYS,Inc. v160-20141009
Platform           :  Windows 7 6.1.7601
Arguments          :  F:\Program Files\ANSYS Inc\v160\ANSYS\bin\winx64\AnsMechSolverMesh.exe
:  -m
:  G:\Testing\16.0\_ProjectScratch\Scr692\file_inpRzn_0001.cdb
:  –slayers=2
:  –silent=0
:  –aconcave=15.0000
:  –aconvex=15.0000
:  –gszratio=1.0000
Seed elements      :  _RZNDISTEL block

– 17:6:17 2015-2-11

===================================================================
== Mesh quality metrics comparison
===================================================================
Element Average    :  ——–Source——–+——–Target——–
..Skewness(Volume) :    4.0450e-001             4.1063e-001
..Aspect Ratio     :    2.3411e+000             2.4331e+000
Domain Volume      :    8.6109e-003             8.6345e-003

Worst Element      :  ——–Source——–+——–Target——–
..Skewness(Volume) :    0.8564  (e552     )      0.7487  (e2217    )
..Aspect Ratio     :    4.9731  (e434     )      6.8070  (e2236    )

===================================================================
== Remeshing result statistics
===================================================================
Domain(s)          :   1
Region(s)          :   1
Patche(s)          :   7
nNode[New]         :   39
nElem[New/Eff/Src] :   79 / 92 / 2076

Peak memory        :   10 MB

– 17:6:17 2015-2-11
– AmsMesher run completed in 0.225 seconds

========================= End Run =================================
===================================================================

**** NEW MESH HAS BEEN CREATED SUCCESSFULLY. CONTINUE TO SOLVE.

Results item tabular listings will show that a remesh has occurred, as shown in figure 4. B.

Figure 4. B. – Results Table Indicating a Remesh Occurred in the Nonlinear Adaptive Region

Figure 4. C. – Before and After Remesh Due to Nonlinear Adaptive Region

## 5: Thermal Fluid Flow via Thermal ‘Pipes’

This has also been a beta option in prior releases, but nicely, at 16.0 it becomes a production feature.  The idea here is that we can use the ANSYS Mechanical APDL FLUID116 elements in Mechanical, without needing a command object.  These fluid elements have temperature as their degree of freedom in this case, and enable the effects of one dimensional fluid flow.  This means we have a reduced order model for capturing heat transfer due to a fluid moving through some kind of cavity without having to explicitly model that cavity.  The pipe ‘path’ is specified using a line body.

The line body gets defined with a cross section in CAD, and is tagged as a named selection in Mechanical.  This thermal pipe can then interact on appropriate surfaces in your model via a convection load.  Once the convection load is applied on appropriate surfaces in your model, the Fluid Flow option can then be set to Yes, and the line body is specified as the appropriate named selection.  Appropriate BC’s need to be applied to the line body, such as temperature constraints and mass flow rate, as shown in figure 5.

Figure 5 – Thermal “Pipe” Line Body at Top, Showing Applied Boundary Conditions

## 6: Solver Pivot Checking Control

This new option under Analysis Settings > Solver Controls allows you to potentially continue an analysis that has stopped due to pivoting issues, meaning a model that’s not fully constrained or one that is having trouble due to contact pairs not being fully in contact.

The options are Program Controlled, Warning, Error, and Off.  The Warning setting is the one to use if you want the solver to continue after any pivoting issues have occurred.  The Error setting means that the solver will stop if pivoting issues occur.  The Off setting results in no pivot checking to occur, while Program Controlled, which is the default, means that the solver will decide.

Figure 6 – Solver Pivot Checking Controls Under Analysis Settings

## 7: Contact Result Trackers

This new feature allows you to more closely track contact status data while the solution is running, or after it has completed.  This capability uses the .cnd file that is created during the solution in the solver directory.  It is useful because it gives you more information on the behavior of your contact regions during solution so you can have more confidence that things are progressing well or potentially stop the solution and take corrective action if they are not.  The tracker objects get inserted under the Solution Information branch, as shown in figure 7. A.

Figure 7. A. – Contact Trackers Inserted Under Solution Information

A large variety of quantities can be selected to track, such as Number Contacting, Number Sticking, Gap, Penetration, etc.

Figure 7. B. – Contact Results Tracker Settings in the Details View

Contact results tracker quantities can be viewed in real time during the solution, as shown in figure 7. C.

Figure 7. C. – Contact Results Tracker Showing Gap Decreasing as the Solution Progresses

## 8: Tree Filtering

For large assemblies or other complex models, there are useful enhancements in how the tree can be filtered, including the ability to create Groups.  Groups can consist of tree entities that are geometry, coordinate systems, connection features, boundary conditions, or even results.  Grouping is accomplished as easily as selecting the desired items in the tree, then right clicking to specify Group, as shown in Figure 8. A.

Figure 8. A. – Grouping Displacements

A new folder in the tree is then created which can be named something useful.  Figure 8. B. shows the displacement boundary condition group (folder) after it was given a name.

Figure 8. B. – Group of Displacement BC’s, Given a Meaningful Name

It’s easy to right click and Ungroup if needed, and there is also a Group Similar Objects option which allows you to select just one item in the tree and easily group all similar items by right clicking.

## 9: Results Set Listing Enhancements

In addition to the information on remeshing that we mentioned back in useful new feature number 4, there is a new capability to right click in the tabular listing of results and then right click to create total deformation or equivalent stress results.  This capability can make it faster to create a deformation or stress plot for a particular time point or result set of interest.

The procedure to do this is:

• Left click on the Solution branch in the tree.
• Left click on the desired Results set in Tabular Data
• Right click on that results set and select Create Total Deformation Results or Create Equivalent Stress Results, as shown in figure 9.

The result of these steps will be a new result item in the tree, waiting for you to evaluate so you can see the new results plot.

Figure 9 – Right Click in Solution Tabular Data to Create Deformation or Equivalent Stress Result Items

## 10: Explode View

We’ve saved a fun one for last, the new Explode View capability.  This allows you to incrementally ‘explode’ the view of your assemblies, making it potentially easier to visualize the parts and interaction between parts that make up the assembly.  To use this feature, make sure the Explode View Options toolbar is turned on in your View settings.  There are several options for the ‘explosion center’, such as the assembly center or the global or a user defined coordinate system.

Figure 10. A. – The Explode View Options Toolbar

As you can see in figure 10. A., there is a slider that allows you to control the ‘level’ of view explosion.  Keep in mind this is just a visual tool and does nothing to the coordinates of the parts in your assemblies.

Figures 10. B. and 10. C. show various slider settings for the exploded view of an assembly.

Figure 10. B. – Explode View Level 3

Figure 10. C. – Explode View Level 4

This concludes our tour of 10 useful new features in ANSYS Mechanical 16.0.  We hope you find this information helps you get your ANSYS Mechanical simulations completed more efficiently.  There are lots and lots of other new features that we didn’t mention here.  The Release Notes in the Help covers a lot of them.  We’ll be writing more about some of the things we mentioned here as well as some of the other new features soon.

## Donny Don’t – Remote Objects

Nothing like a good ‘ol fashion Simpson’s reference.  I’m trying to start a new series of articles that address common mistakes and things to avoid, and what better reference than when Bart ‘joined’ the Junior Campers and found out he might get a knife out of the deal.

For this first article, let’s talk about remote objects (force, displacement, points, joints).  First, remote objects are awesome.  Want to add a rotational DOF to your solid-object model?  Remote Displacement.  Want to apply a load and don’t want to worry about force/moment balance?  Remote Force.  Want to apply a load but also constrain a surface?  Remote Point.  Take two points and define a open/locked degrees of freedom and you have a kinematic joint.

The thing to watch out for is how you define these remote points.  ANSYS Mechanical does an amazing job at making a pretty tedious process easy (create pilot node, create constraint-type contact, specify DOFs to include, specify formulation).  In Mechanical, all you need to do is highlight some geometry, right mouse click, and insert the appropriate object (remote point, remote force, etc).  No need to keep track of real constant sets, element tshape’s…easy.  Almost too easy if you ask me.

Once you start creating multiple remote objects, you may see the following:

If you dig into the solver output file you may see this:

The complaint is that we have multiple overlapping constraint sets.  Let’s take a step back and see the model I’ve setup:

I have a cylinder, attached to a body-to-ground spring on one face, a translational joint applied on the OD, and a remote force and moment applied on the opposite end.  If I follow the instructions shown from the ANSYS Workbench message about graphically displaying FE Connections (select the ‘Solution Information’ item, click the graphics tab):

We can see that any type of constraint equation is shown in red.  The issue here is that the nodes on the OD edge on the top and bottom of my cylinder belong to multiple constraint equation sets.  On the bottom my my cylinder those nodes are being constrained to the spring end AND the cylindrical joint.  On the top the nodes on the edge are being constrained to the joint AND remote force.  When you hit solve, ANSYS needs to figure out how to resolve the conflicting constraint sets (a node cannot be a slave term for two different constraint sets).  I don’t know exactly how the solver manages this, but I like to imagine it’s like two people fighting over who gets to keep a dog…and they place the dog in-between them and call for it, and whoever the dog goes to gets to keep it.

Now for this example, the solver is capable of handling the over-constraint because overall…the model is properly constrained.  The spring can loose some of the edge nodes and still properly connect to the cylinder.  Same goes for the other remote objects (translation joint and remote force/moment).  If we had more objects defined and more overlaps, that’s a different story.  You can introduce a pretty lengthy lag, or outright solver failure, if there are a lot of overconstraint terms in the model.

So now the question becomes, how do I fix this.  The easiest way is to not fix this and ignore the warning.  If our part behaves properly, we get the reaction forces we’d expect, then odds are the overconstraint terms that are automatically corrected are fine.  If we actually wanted to remove that warning, we would need to make sure we scope remote objects that do not touch other remote objects.  We can do this by going into DesignModeler or SpaceClaim and imprinting the surfaces.

In DM, I just extruded the edges with the operation set to imprint face.  In SpaceClaim you would just need to use the ‘copy edge’ option on the pull command:

Now this will modify the topology and will ensure we have a separation of nodes for all of our remote objects:

When we solve…no warning message about MPC conflicts:

And when we look at the FE connectivity, there are no nodes shared by multiple remote objects:

The last thing I’d like to point out is the application of a force and moment on a remote point:

Whenever you have two remote objects operating on the same surface (e.g. a moment and force, force and displacement, etc), you should really be using a remote point.  If I were to create two remote objects:

I now come right back to my original problem of conflicting constraints.  These two objects share the exact same nodal set but are creating two independent remote points.  If you want to do this, right-mouse-click on one of your remote objects and select ‘promote to remote point’:

Then modify the other remote objects to use that remote point.  No more conflict.

Very last point…in R16 it will now tell you when you have ‘duplicate’ remote objects  (like the remote force + displacement shown above).

Hope this helps!

## Thermal Submodeling in ANSYS Workbench Mechanical 15.0

If you've been following The Focus for a long time, you may recall my prior article about submodeling using ANSYS Mechanical APDL, which was a 'sub' model of a submarine.  The article, from 2006, begins on page 2 at this link:

Also, Eric Miller here at PADT wrote a Focus blog entry on the new-at-14.5 submodeling capability in ANSYS Workbench Mechanical.

Since both of those articles were about structural submodeling, I decided it was time we published a blog entry on how to perform submodeling in ANSYS Mechanical for thermal simulations.

Submodeling is a technique whereby we can obtain more accurate results in a small, detailed portion of a large model without having to build an incredibly refined and detailed finite element model of our complete system.  In short, we map boundary conditions onto a 'chunk' of interest that is a subset of our full model so that we can solve that 'chunk' in more detail.  Typically we mesh the 'chunk' with a much finer mesh than was used in the original model, and sometimes we add more detail such as geometric features that didn't exist in the original model like fillets.

The ANSYS Workbench Project Schematic for a thermal solution involving submodeling looks like this:

Figure 1 – Thermal Submodeling Project Schematic

Note that in the project schematic, the links are automatically established when we setup the submodel after completing the analysis on the coarse model as we shall see below.

First, here is the geometry of the coarse model.  It's a simple set of cooling fins.  In this idealized model, no fillets have been modeled between the fins and the block.

Figure 2 – Coarse Model Geometry, Idealized without Fillets

The boundary conditions consisted of a heat flux due to a  thermal source on the base face and convection to ambient air on the cooling fin surfaces.  The heat flux was setup to vary over the course of 3 load steps as follows:

Load Step        Heat Flux (BTU/s*in^2)

1                      0.2

2                      0.5

3                      0.005

Thus, the maximum heat going into the system occurs in load step 2, corresponding to 'time' 2.0 in this steady state analysis.

Figure 3 – Coarse Model Boundary Conditions – Heat Flux and Convection

The coarse model is meshed with relatively large elements in this case.  The mesh refinement for a production model should be sufficient to adequately capture the fields of interest in the locations of interest.  After solving, the temperature results show a max temperature at the base where the heat flux is applied, transitioning to the minimum temperature on the cooling fins where convection is removing heat.

Figure 4 – Coarse Model Mesh and Temperature Results for Load Step 2

Our task now is to calculate the temperature in one of these fins with more accuracy.  We will use a finer mesh and also add fillets between the fin and base.  For this example, I isolated one fin in ANSYS DesignModeler, did some slicing, and added a fillet on either side of the base of the fin of interest.

Figure 5 – Fine Model (Submodel) Isolated Fin Geometry and Mesh, Including Fillets at Base

ANSYS requires that the submodel lie in the exact geometric position as it would in the coarse model, so it's a good idea to overlay our fine model geometry onto the coarse model to verify the positioning.

Figure 6 – Submodel and Coarse Model Overlaid

Figure 7 – Submodel and Coarse Model Overlaid, Showing Addition of Fillet

The next step is to insert the submodel geometry as a stand-alone geometry block in the Project Schematic which already contains the coarse model, as shown in figure 8.  A new Steady-State Thermal analysis is then dragged and dropped onto the geometry block containing the submodel geometry.

Figure 8 – Submodel Geometry Added to Project Schematic, New Steady-State Thermal System Dragged and Dropped onto Submodel Geometry

Next, we drag and drop the Engineering Data cell from the coarse model to the Engineering Data cell in the submodel block.  This will establish a link so that the material properties will be shared.

Figure 9 – Drag and Drop Engineering Data from Coarse Model to Submodel

The final needed link is established by dragging and dropping the Solution cell from the coarse model onto the Setup cell in the submodel.  This step causes ANSYS to recognize that we are performing submodeling, and in fact this will cause a Submodeling branch to appear in the outline tree in the Mechanical window for the submodel.

Figure 10 – Solution Cell Dragged and Dropped from Coarse Model to Submodel Setup Cell

After opening the Mechanical editor for the submodel block, we can see that the Submodeling branch has automatically been added to the tree.

Figure 11 – Submodeling Branch Automatically Added to Outline Tree

After meshing the submodel I specified that all three load steps should have their temperature data mapped to the submodel from the coarse model.  This was done in the Details view for the Imported Temperature branch, by setting Source Time to All.

Figure 12 – Set Imported Temperature Source Time to All to Ensure All Loads Steps Are Mapped

Next I selected the four faces that make up the cut boundaries in the submodel and applied those to the geometry selection for Imported Temperature.

Figure 13 – Cut Boundary Faces Selected for Imported Temperature

As mentioned above, the Imported Temperature details were set to read in all load steps by setting Source Time to All.  The Imported Temperature branch can now be right-clicked and the resulting imported temperatures viewed.  I also inserted a Validation branch which we will look at after solving.

Figure 14 – Setting Source Time to All, Viewing Imported Temperature on Submodel

Any other loads that need to be applied to the submodel are added as well.  For this model, it's convection on the large faces of the fin that are exposed to ambient air.

Figure 15 – Submodel Convection Load on Fin Exposed Faces

Since there are three load steps in the coarse model and we told ANSYS to map results from all time points, I set the number of steps to three in Analysis Settings, then solved the submodel.  Results are available for all three load steps.

Figure 16 – Submodel Temperature Results for Step 2 (Highest Heat Flux Value in Coarse Model)

Regarding the Validation item under the Imported Temperature branch, this is probably best added after the solution is done.  In my case I had to clear it and recalculate it.  Validation can display either an absolute or relative (percent difference) plot on the nodes at which loads were imported.  Figure 17 shows the relative difference plot, which maxes out at about 6%.  The validation information as well as mapping techniques are described in the ANSYS Help.

Figure 17 – Submodel Imported Temperature Validation Plot – Percent Difference on Mapped Nodes

Looking at the coarse model and submodel results side by side, we see good agreement in the calculated temperatures.  The temperature in the fillets shows a nice, smooth gradient.

Figure 18 – Coarse and Submodel Temperature Results Showing Good Agreement

Hopefully this explanation will be helpful to you if you have a need to perform submodeling in a thermal simulation in ANSYS.  There is a Thermal Submodeling Workflow section in the ANSYS 15.0 Help in the Mechanical User's Guide that you may find helpful as well.

## ANSYS Workbench Installations and RedHat 6.6 – Error and Workaround

We were recently alerted by a customer that there is apparently a conflict with ANSYS installations if Red Hat Enterprise Linux 6.6 (RHEL 6.6) is installed. We have confirmed this here at PADT. This effects several versions of ANSYS, including 15.0.7, 14.5, and 14.0. The primary problem seems to be with meshing in the Mechanical or Meshing window.

The windows errors encountered can be: “A software execution error occurred inside the mesher. The process suffered an unhandled exception or ran out of usable memory.” or “an inter-process communication error occurred while communicating with the MESHER module.”

The error message popup can look like this:

or

Note that the Platform Support page on the ANSYS website does not list RHEL 6.6 as supported. RHEL is only supported up through 6.5 for ANSYS 15.0. This is the link to that page on the ANSYS website:

That all being said, there is a workaround that should allow you to continue using ANSYS Workbench with RHEL 6.6 if you encounter the error. It involves renaming a directory in the installation path:

In this directory:

/ansys_inc/v150/commonfiles/MainWin/linx64/mw/lib-amd64-linux/

Rename the folder ‘X11’ to ‘Old-X11’

After that change, you should be able to successfully complete meshes, etc,. in ANSYS Workbench. Keep in mind that RHEL 6.6 is not officially supported by ANSYS, Inc. and their recommendation is always to stick with supported levels of operating systems. These are always listed in the ANSYS Help for the particular version you are running as well as at the link shown above.

Since the renamed directory is contained within the ANSYS installation files, it is believed that this will not affect anything else other than ANSYS. Use at your own risk, however. Should you encounter one of more of the errors listed above, we hope this article has provided useful information to keep your ANSYS installations up and running.