Default Contact Stiffness Behavior for Bonded Contact

p7It recently came to my attention that the default contact stiffness factor for bonded contact can change based on other contact regions in a model. This applies both to Mechanical as well as Mechanical APDL. If all contacts are bonded, the default contact stiffness factor is 10.0. This means that in our bonded region, the stiffness tending to hold the two sides of contact together is 10 times the underlying stiffness of the underlying solid or shell elements.

However, if there is at least one other contact region that has a type set to anything other than bonded, then the default contact stiffness for ALL contact pairs becomes 1.0. This is the default behavior as documented in the ANSYS Mechanical APDL Help, in section 3.9 of the Contact Technology Guide in the notes for Table 3.1:

“FKN = 10 for bonded. For all other, FKN = 1.0, but if bonded and other contact behavior exists, FKN = 1 for all.”

So, why should we care about this? It’s possible that if you are relying on bonded contact to simulate a connection between one part and another, the resulting stress in those parts could be different in a run with all bonded contact vs. a run with all bonded and one or more contact pairs set to a type other than bonded. The default contact stiffness is now less than it would be if all the contact regions were set to bonded.

This can occur even if the non-bonded contact is in a region of the model that is in no way connected to the bonded region of interest. Simply the presence of any non-bonded contact region results in the contact stiffness factor for all contact pairs to have a default value of 1.0 rather than the 10.0 value you might expect.

Here is an example, consisting of a simple static structural model. In this model, we have an inner column with a disk on top. There are also two blocks supporting a ring. The inner column and disk are completely separate from the blocks and ring, sharing no load path or other interaction. Initially all contact pairs are set to bonded for the contact type. All default settings are used for contact.
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Loading consists of a uniform temperature differential as well as a bearing load on the disk at the top. Both blocks as well as the column have their bases constrained in all degrees of freedom.
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After solving, this is the calculated maximum principal stress distribution in the ring. The max value is 41,382.
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Next, to demonstrate the behavior described above, we changed the contact type for the connection between the column and the disk from bonded to rough, all else remaining the same.
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After solving, we check the stresses in the ring again. The max stress in the ring has dropped from 41,283 to 15,277 as you can see in the figure below. Again, the only change that was made was in a part of the model that was in no way connected to the ring for which we are checking stresses. The change in stress is due solely to a change in contact type setting in a different part of the model. The reason the stress has decreased is that the stiffness of the bonded connection is less by a factor of 10, so the bonded region is a softer connection than it was in the original run.

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So, what do we as analysts need to do in light of this information? A good practice would be to manually specify the contact stiffness factor for all contact pairs. This behavior only crops up when the default values for contact stiffness factor are utilized. We can define these stiffness factors easily in ANSYS Mechanical in the details view for each contact region. Further, we need to always remember that ANSYS as well as other analytical tools are just that – tools. It’s up to us to ensure that the results of interest we are getting are not sensitive to factors we can adjust, such as mesh density, contact stiffness, weak spring stiffness, stabilization factors, etc.

Using Probes to Obtain Contact Forces in ANSYS Mechanical

Recently we have had a few questions on obtaining contact results in ANSYS Mechanical. A lot of contact results can be accessed using the Contact Tool, but to obtain contact forces we use Probes. Since not everyone is familiar with how it’s done, we’ll explain the basics here.

Below is a screen shot of a Mechanical model involving two parts. One part has a load that causes it to be deflected into the other part.

p1

We are interested in obtaining the total force that is being transmitted across the contact elements as the analysis progresses. Fortunately this is easy to do using Probes in Mechanical.

The first thing we do is click on the Solution branch in the tree so we can see the Probes button in the context toolbar. We then click on the Probe drop down button and select Force Reaction, as shown here:

p2

Next, we click on the resulting Force Reaction result item under the Solution branch to continue with the configuration. We first change the Location Method from Boundary Condition to Contact Region:

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We then specify the desired contact region for the force calculation from the Contact Region dropdown:

p4

Note that the coordinate system for force calculation can either be Cartesian or Cylindrical. You can setup a coordinate system wherever you need it, selectable via the Orientation dropdown.

There is also an Extraction dropdown with various options for using the contact elements themselves, the elements underlying the contact elements, or the elements underlying the target elements (target elements themselves have no reaction forces or other results calculated). Care must be taken when using underlying elements to make sure we’re not also calculating forces from other contact regions that are part of the same elements, or from applied loads or constraints. In most cases you will want to use either Contact (Underlying Element) or Target (Underlying Element). If contact is non-symmetric, only one of these will have non zero values.

In this case, the setting Contact (Contact Element) was a choice that gave us appropriate results, based on our contact behavior method of Asymmetric:

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Here are the details including the contact force results:

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This is a close up of the force vs. ‘time’ graphs and table (this was a static structural analysis with a varying pressure load):

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***** SUMMATION OF TOTAL FORCES AND MOMENTS IN THE GLOBAL COORDINATE SYSTEM *****

FX = -0.4640219E-04
FY = -251.1265
FZ = -0.1995618E-06
MX = 62.78195
MY = -0.1096794E-04
MZ = -688.9742
SUMMATION POINT= 0.0000 0.0000 0.0000

We hope this information is useful to you in being able to quickly and easily obtain your contact forces.

Integrating ANSYS Fluent and Mechanical with Flownex

Component boundaries generated in Flownex are useful in CFD simulation (inlet velocities, pressures, temperatures, mass flow). Generation of fluid and surface temperature distribution results from Flownex can also be useful in many FEA simulations. For this reason the latest release of Flownex SE was enhance to include several levels of integration with ANSYS.  

ANF Import

By simply clicking on an Import ANF icon on the Flownex Ribbon bar users can select the file that they want to import. The user will be requested to select whether the file must be imported as 3D Geometry which conserves the coordinates system or as an isometric drawing.

The user can also select the type of component which should be imported in the Flownex library. Since the import only supports lines and line related items this will typically be a pipe component.

Following a similar procedure, a DXF importer allows users to import files from AutoCAD.

This rapid model construction gives Flownex users the ability to create and simulate networks quicker. With faster model construction, users can easily get to results and spend less time constructing models.

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ANSYS Flow Solver Coupling and Generic Interface

The Flownex library was extended to include components for co-simulation with ANSYS Fluent and ANSYS Mechanical.
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These include a flow solver coupling checks, combined convergence and exchanges data on each iteration, and a generic coupling that can be used for cases when convergence between the two software programs is not necessary.

The general procedure for both the Fluent and Mechanical co-simulation is the same:

  1. By identifying specified named selections, Flownex will replace values in a Fluent journal file or ds.dat file in the case of Mechanical.
  2. From Flownex, Fluent/Mechanical will then be run in batch mode
  3. The ANSYS results are then written into text files that are used inputs into Flownex.
  4. When applicable, specified convergence criteria will be checked and the procedure repeated if necessary.

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Learn More

To learn more about Flownex or how Flownex and ANSYS Mechanical contact PADT at 480.813.4884 or roy.haynie@padtinc.com.  You can also learn more about Flownex at www.flownex.com.

Some Tips on Configuring RSM as a User

rsm1If you’re not familiar with it, RSM is the ANSYS Remote Solve Manager.  In short, it allows you to submit solutions from various ANSYS tools so they can be solved remotely, such as on a compute cluster, remote number cruncher, or perhaps just another computer that isn’t being used very much.  Note that there is no additional licensing or installation is required (other than perhaps ANSYS HPC licensing to take advantage of multiple cores).  RSM is installed automatically when ANSYS is installed; it just needs to be configured to be activated.

In version 14.5 and 15.0, there is a nicely documented Setup Wizard that helps with the setup and configuration of RSM on compute servers.  This setup wizard as well as the rest of the RSM documentation in the ANSYS Help does a great job of explaining RSM and what must be done to setup and configure it.  This Focus entry assumes that your crack IT staff has installed RSM on your compute machine(s) and has decided where the Compute Server will be (can be on your local machine or on your ‘number cruncher’ or on a different machine).  So, our focus here is on what needs to be done as a user to send your solutions off to the remote solver using RSM.

As an example, we have RSM 15.0 configured with the Compute Server on a remote computer named cs3a. The first time running RSM, using Start > All Programs > ANSYS 15.0 > Remote Solve Manager > RSM 15.0, we get the window shown here:

rsm2

Notice that it only shows our local machine (My Computer) and nothing about the actual remote computer on which we want to solve.

Therefore, we need to add the information on our cluster node which contains the compute server.

To do this, click on Tools > Options.  This is the resulting window.  Notice the Add button at lower left is grayed out:

rsm3

What it’s waiting for us to do is type in the name of our desired remote computer, like this:

rsm4

Now that a new name has been typed in the Name field, the Add button is active.  After clicking Add, we get this:

rsm5

After clicking OK, we will now see that the new remote computer has been added in the RSM window:

rsm6

The next step is to set your login password for accessing this computer.  Right click on the new hostname in the RSM window in the tree at left, and select Set Password.

rsma

Then enter your network login and password information in the resulting window:

rsmb

If your accounts are fully setup, at this point you can run a test by right clicking on the localhost item in the tree under the remote computer name and selecting Test Server:

rsm7

If the test is successful, you will see that the test job completed with a green checkmark on the folder icon in the upper right portion of the RSM window:

rsm8

If your login is not configured properly, you will likely get an error like this one shown below.  Notice that the upper right portion now states that the job has failed and there is a red X rather than a green checkmark on the folder icon.  By clicking on the job in the upper right panel, we can see the job log in the lower right panel.  In this case, it says that the login failed due to an incorrect password.

rsm9

The fix for the password problem is to ensure that the correct login is being accessed by RSM on the remote computer.  This is done from the RSM window by right clicking on the remote computer name and selecting Accounts.

rsm10

If your account and/or password are different on the remote computer than they are on your local machine, you will need to establish an alternate account so that RSM knows to use the correct login on the remote computer.  Right click on your account in the Accounts pane, and select Add Alternate Account:

rsm11

Enter your username and password for the remote computer in the resulting window.  Next, we need to associate that login with localhost on the remote computer.  This is down by checking the localhost box in the Compute Servers pane, like this:

rsm12

Another problem we have seen is that the user doesn’t have permission for ANSYS to write to the default solve directory on the remote computer.  In that case, the test job log will have an error like this:

rsm13

This fix in this case is to establish a solve directory manually, first by creating one on the remote computer, if needed, and second by specifying that RSM use that directory rather than the default.  The second step is accomplished in the RSM window via right clicking on the localhost item for the remote computer, then selecting Properties.  On the General tab, you should be able to change the Working Directory Location to User Specified, then enter the desired directory location as shown in the image below.  If that option is greyed out, either your password for the remote machine has not been entered correctly, or you are not part of the admin group on the remote computer.  In the case of the latter, either your RSM administrator has to do it for you, or you have to be granted the admin access.

rsm14

At this point, if the test server runs have completed successfully you should be ready to try a real solution using RSM.  We’ll use Mechanical to show how it’s done.  In the Mechanical editor, click on Tools > Solve Process Settings.  Here we will need to specify the remote computer and queue we’ll be using for the solution.  Click on the Add Remote button:

rsm15

In the resulting Rename Solve Process Settings button, type in a name for your remote solve option that makes sense to you.  We called ours RemoteSolve1.  This new option will now show up on the left side of the Solve Process Settings window:

rsm16

The next step is to type in the name of the Solve Manager over on the right side.  In our case, the Solve Manager is on computer cs3a.  Any queues that are available to RSM for this Solve Manager will show in the Queue field, after a brief period of time to make the connection.  In our case, the only queue is a local queue on cs3a.

rsm17

We are now ready to solve our Mechanical model remotely, using RSM.  Instead of clicking the Solve button in Mechanical, we will click on the drop down arrow to the right of the solve button.  From the dropdown, we select the remote solve option we created, RemoteSolve1:

rsm18

Assuming the solution completes with no errors, this job will show up in the RSM window with a status of Finished when it is done.

rsm19

The final step in this case is to download the results from the remote computer back to the client machine.  In the Mechanical editor, this is done by right clicking on the Solution branch and selecting Get Results as shown below.  Also note that you can monitor a nonlinear solution via Solution Information.  You’ll just need to right click during solution to have a snapshot of the nonlinear diagnostics brought back from the remote computer.

rsm20

We hope this helps with the setup and utilization of RSM from a user perspective.  There are other options and applications for RSM that we didn’t discuss, but hopefully this is useful for those needing to get ‘over the hump’ in using RSM.

ACT Extension for a PID Thermostat Controller (PART 2)

(View part one of this series here.)

So, I’ve done a little of this Workbench customization stuff in a past life. My customizations involved lots of Jscript, some APDL, sweat and tears. I literally would bang my head against Eric Miller’s office door jamb wondering (sometimes out loud) what I had done to be picked as the Workbench customization guy. Copious amounts of alcohol on weeknights helped some, but honestly it still royally sucked. Because of these early childhood scars, I’ve cringed at the thought of this ACT thingy until now. I figured I’d been there, done that and I had zero, and I mean zero desire to relive any of it.

So, I resisted ACT even after multiple “suggestions” from upper management that I figure out something to do with it. That was wrong of me; I should have been quicker to given ACT a fair shot. ACT is a whole bunch better than the bad ol’ days of JScript. How is it better? Well, it has documentation… Also, there are multiple helpful folks back at Canonsburg and elsewhere that know a few things about it. This is opposed to the days when just one (brilliant) guy in India named Rajiv Rath had ever done anything of consequence with JScript. (Without him, my previous customization efforts would simply have put me in the mad house. Oh, and he happens to know a thing or two about ACT as well…)

Look Ma! My First ACT Extension!

In this post we are going to rig up the PID thermostat boundary condition as a new boundary condition type in Mechanical. In ACT jargon, this is known as adding a pre-processing feature. I’m going to refer you primarily to the training material and documentation for ACT that you can obtain from the ANSYS website on their customer portal. I strongly suggest you log on to the portal and download the training material for version 15.0 now.

Planning the Extension

When we create an ACT extension we need to lay things out on the file system in a certain manner. At a high level, there are three categories of file system data that we will use to build our extension. These types of data are:

  1. Code. This will be comprised of Python scripts and in our case APDL scripts.
  2. XML. XML files are primarily used for plumbing and for making the rest of the world aware of who we are and what we do.
  3. Resources. These files are typically images, icons, etc… that spice things up a little bit.

Any single extension will use all three of these categories of files, and so organizing this mess becomes priority number one when building the extension. Fortunately, there is a convention that ACT imposes on us to organize the files. The following image depicts the structure graphically.

image

We will call our extension PIDThermostat. Therefore, everywhere ExtensionName appears in the image above, we will replace that with PIDThermostat in our file structure.

Creating a UI for our Load

The beauty of ACT is that it allows us to easily create professional looking user experiences for custom loads and results. Let’s start by creating a user interface for our load that looks like the following:

clip_image004

As you can see in the above user interface, all of the controls and inputs for our little PID controller that we designed in Part 1 of this blog series are present. Before we discuss how we create these user elements, let’s start with a description of what they each mean.

The first item in the UI is named Heat Source/Sink Location. This UI element presents to the user a choice between picking a piece of geometry from the model and specifying a named selection. Internal to the PID controller, this location represents where in the model we will attach the control elements such that they supply or remove energy from this location. ACT provides us two large areas of functionality here. First, it provides a way to graphically pick a geometric item from the model. Second, it provides routines to query the underlying mesh associated with this piece of geometry so that we can reference the nodes or elements in our APDL code. Each of these pieces of functionality is substantial in its size and complexity, yet we get it for free with ACT.

The second control is named Temperature Monitor Location. It functions similarly to the heat source/sink location. It gives the user the ability to specify where on the model the control element should monitor, or sense, the temperature. So, our PID controller will add or remove energy from the heat sink/source location to try to keep the monitor location at the specified set point temperature.

The third control group is named Thermostat Control Properties. This group aggregates the various parameters that control the functionality of the thermostat. That is, the user is allowed to specify gain values, and also what type of control is implemented.

The forth control group is named Thermostat Setpoint Properties. This group allows the user to specify how the setpoint changes with time. An interesting ACT feature is implemented with this control group. Based on the selection that the user makes in the “Setpoint Type” dropdown, a different control is presented below for the thermostat setpoint temperature. When the user selects, “User Specified Setpoint” then a control that provides a tabular input is presented. In this case, the user can directly input a table of temperature vs time data that specifies how the setpoint changes with time. However, if the user specifies “Use Model Entity as Setpoint” then the user is presented a geometry picker similar to the controls above to select a location in the model that will define the setpoint temperature. When this option is selected, the PID controller will function more like a follower element. That is, it will try to cause the monitor location to “follow” another location in the model by adding or removing energy from the heat source/sink location. The offset value allows the user to specify a DC offset that they would like to apply to the setpoint value. Internally, this offset term will be incorporated into the constraint equation averaging method to add in the DC offset.

Finally, the last control group allows the user to visualize plots of computed information regarding the PID controller after the solution is finished. Normally this would be presented in the results branch of the tree; however, the results I would like to present for these elements don’t map cleanly to the results objects in ACT. (At least, I can’t map them cleanly in my mind…) More detail on the results will be presented below.

So, now that we know what the control UI does, let’s look at how to specify it in ACT

Specifying the UI in XML

As mentioned at the beginning, ACT relies on XML to specify the UI for controls. The following XML snippet describes the entire UI.

<extension version=“1” name=“ThermalTools”>

  <guid shortid=“thermaltools”>852acb16-4731-4e91-bd00-b464be02b361</guid>

  <script src=“thermaltools.py” />

 

  <interface context=“Mechanical”>

    <images>images</images>

    <callbacks>

      <oninit>initThermalToolsToolbar</oninit>

    </callbacks>

    <toolbar name=“thermtools” caption=“Thermal Tools”>

      <entry name=“PIDThermostatLoad” icon=“ThermostatGray”

             caption=“PID Thermostat Control”>

        <callbacks>

          <onclick>createPIDThermostatLoad</onclick>

        </callbacks>

      </entry>

    </toolbar>

  </interface>

 

  <simdata context=“Mechanical”>

    <load name=“pidthermostat” version=“1” caption=“PID Thermostat”

    icon=“ThermostatWhite” issupport=“false” isload=“true” color=“#0000FF”>

      <callbacks>

        <getcommands location=“solve”>writePIDThermostatLoad</getcommands>

        <getcommands location=“post”>writePIDThermostatPost</getcommands>

        <onadd>addPIDThermostat</onadd>

        <onremove>removePIDThermostat</onremove>

        <onsuppress>suppressPIDThermostat</onsuppress>

        <onunsuppress>unsuppressPIDThermostat</onunsuppress>

        <canadd>canaddPIDThermostat</canadd>

        <oncleardata>cleardataPIDThermostat</oncleardata>

      </callbacks>

      <property name=“ConnectionGeo”  caption= “Heat Source/Sink Location”

                control=“scoping”>

        <attributes selection_filter=“vertex|edge|face” />

      </property>

      <property name=“MonitorGeo”  caption= “Temperature Monitor Location”

                control=“scoping”>

        <attributes selection_filter=“vertex|edge|face|body” />

      </property>

      <propertygroup name=“ControlProperties” caption=“Thermostat Control Properties”

                     display=“caption”>

        <property name=“ControlType” caption=“Control Type”

                  control=“select” default=“Both Heat Source and Sink”>

          <attributes options=“Heat Source,Heat Sink,Both Heat Source and Sink”/>

        </property>

        <property name=“ProportionalGain” caption=“Proportional Gain”

                  control=“float” default=“0”/>

        <property name=“IntegralGain” caption=“Integral Gain”

                  control=“float” default=“0”/>

        <property name=“DerivativeGain” caption=“Derivative Gain”

                  control=“float” default=“0”/>

      </propertygroup>

      <propertygroup name=“SetpointProperties” caption=“Thermostat Setpoint Properties”

                     display=“caption”>

        <propertygroup name=“SetpointType” display=“property” caption=“Setpoint Type”

                       control=“select” default=“User Specified Setpoint”>

          <attributes options=“User Specified Setpoint,Use Model Entity as Setpoint”/>

          <propertytable name=“SetPointTemp” caption=“Thermostat Set Point Temperature”

                         display=“worksheet” visibleon=“User Specified Setpoint”

                         control=“applycancel” class=“Worksheet.PropertyGroupEditor.PGEditor”>

            <property name=“Time” caption=“Time” unit=“Time” control=“float”></property>

            <property name=“SetPoint” caption=“Set Point Temperature”

                      unit=“Temperature” control=“float”></property>

          </propertytable>

          <property name=“SetpointGeo”  caption= “Setpoint Geometry”

                    visibleon=“Use Model Entity as Setpoint” control=“scoping”>

            <attributes selection_filter=“vertex|edge|face|body” />

          </property>

        </propertygroup>

        <property name=“SetpointOffset” caption=“Offset” control=“float” default=“0”/>

      </propertygroup>

      <propertygroup name=“Results” caption=“Thermostat Results” display=“caption”>

        <property name=“ViewResults” caption=“View Results?” control=“select” default=“No”>

          <attributes options=“Yes,No”/>

          <callbacks>

            <onvalidate>onViewPIDResults</onvalidate>

          </callbacks>

        </property>

      </propertygroup>

    </load>

  </simdata>

</extension>

Describing this in detail would take far longer than I have time for now, so I’m going to direct you to the ACT documentation. The gist of it is fairly simple though. XML provides a structured, hierarchical mechanism for describing the layout of the UI. Nested structures appear as child widgets of their parents. Callbacks are used within ACT to provide the hooks into the UI events so that we can respond to various user interactions. Beyond that, read the docs!! And, hey, before I hear any whining remember that in the old days of Jscript customization there wasn’t any documentation! When I was a Workbench Customization Kid we had to walk uphill, both ways, barefoot, in 8’ of snow for 35 miles… So shut it!

Making the Magic Happen

So, the UI is snazzy and all, but the heavy lifting really happens under the hood. Ultimately, what ACT provides us, when we are creating new BCs for ANSYS, is a clever way to insert APDL commands into the ds.dat input stream. Remember that at its core all Mechanical is, is a glorified APDL generator. I’m sure the developers love me reducing their hard labor to such mundane terms, but it is what it is… So, at the end of the day, our little ACT load objects are nothing more than miniature APDL writers. You thought we were doing something cool…

So, the magic happens when we collect up all of the input data the user specifies in our snazzy UI and turn that into APDL code that implemented the PID controller. This is obviously why I started by developing the APDL code first outside of WB. The APDL code is the true magic. Collecting up the user inputs and writing them to the ds.dat file occurs inside the getcommands callback. If you look closely at the XML code, you will notice two getcommands callbacks. The first one calls a python function named: writePIDThermostatLoad. This callback is scheduled to fire when Mechanical is finished writing all of the standard loads and boundary conditions that it implements natively and is about to write the first APDL solve command. Our commands will end up in the ds.dat file right at this location. I chose this location for the following reason. Our APDL code for the PID thermostat will be generating new element types and new nodes and elements not generated by Workbench. Sometimes workbench implements certain boundary conditions using surface effect element types. So, these native loads and boundary conditions themselves may generate new elements and element types. Workbench knows about those, because it’s generating them directly; however, it has no idea what I might be up to in my PID thermostat load. So, if it were to write additional boundary conditions after my PID load, it very well might reuse some of my element type ids, and even node/element ids. The safer thing to do is to write our APDL code so that it is robust in the presence of an unknown maximum element type/real constant set/node number/etc… Then, we insert it right before the solve command, after WB has written all of its loads and boundary conditions. Thus, the likelihood of id collisions is greatly reduced or eliminated.

Note, too, that ACT provides some utility functions to generate a new element type id and increment the internal counter within Workbench; however, I have found that these functions do not account for loads and boundary conditions. Therefore, in my testing thus far, I haven’t found them safe to use.

The second getcommands callback is setup to fire when Workbench finishes writing all of the solve commands and has moved to the post processing part of the input stream. I chose to implement a graphing functionality for displaying the relevant output data from the PID elements. Thus, I needed to retrieve this data from ANSYS after the solution is complete so that I can present it to the user. I accomplished this by writing a little bit of APDL code to enter /post26 and export all of the data I wish to plot to a CSV file. By specifying this second getcommands callback, I could instruct Workbench to insert the APDL commands after the solve completed.

Viewing the Results

Once the solution has completed, clicking on the “View Results?” dropdown and choosing “Yes” will bring up the following result viewer I implemented for the PID controller. All of the graphing functionality is provided by ACT in an import module called “chart”. This result viewer is simply implemented as a dialog with a single child control that is the ACT chart widget. This widget also allows you to layout multiple charts in a grid, as we have here. As you can see, we can display all of the relevant output data for the controls cleanly and efficiently using ACT! While this can also be accomplished in ANSYS Mechanical APDL, I think we would all agree that the results are far more pleasing visually when implemented in ACT.

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Where Do We Go from Here?

Now that I’ve written an ACT module, my next steps are to clean it up and try to make it a little more production ready. Once I’m satisfied with it, I’ll publish it on this blog and on the appropriate ANSYS library. Look for more posts along the way if I uncover additional insights or gotchas with ACT programming. I will leave you with this, however. If you have put off ACT programming you really should reconsider! Being mostly new to ACT, I was able to get this little boundary condition hooked up and functioning in less than a week’s time. Given the way the user interface turned out and the flexibility thus far of the control, I’m quite pleased with that. Without the documentation and general availability of ACT, this effort would have been far more intense. So, try out ACT! You won’t be disappointed.

ACT Extension for a PID Thermostat Controller (PART 1)

I’m going to embark on a multipart blog series chronicling my efforts in writing a PID Thermostat control boundary condition for workbench. I picked this boundary condition for a few of reasons:

  1. As far as I know, it doesn’t exist in WB proper.
  2. It involves some techniques and element types in ANSYS Mechanical APDL that are not immediately intuitive to most users. Namely, we will be using the Combin37 element type to manage the control.
  3. There are a number of different options and parameters that will be used to populate the boundary condition with data, and this affords an opportunity to explore many of the GUI items exposed in ACT.

This first posting goes over how to model a PID controller in ANSYS Mechanical APDL.  In future articles I will share my efforts to refine the model and us ACT to include it in ANSYS Workbench.

PID Controller Background

Let’s begin with a little background on PID controllers. Full disclaimer, I’m not controls engineer, so take this info for what it is worth. PID stands for Proportional Integral Differential controller. The idea is fairly simple. Assume you have some output quantity you wish to control by varying some input value. That is, you have a known curve in time that represents what you would like the output to look like. For example:

image

The trick is to figure out what the input needs to look like in time so that you get the desired output. One way to do that is to use feedback. That is, you measure the current output value at some time, t, and you compare that to what the desired output should be at that time, t. If there is no difference in the measured value and the desired value, then you know whatever you have set the input to be, it is correct at least for this point in time. So, maybe it will be correct for the next moment in time. Let’s all hope…

However, chances are, there is some difference between what is measured and what is desired. For future reference we will call this the error term. The secret sauce is what to do with that information? To make things more concrete, we will ground our discussion in the thermal world and imagine we are trying to maintain something at a prescribed temperature. When the actual temperature of the device is lower than the desired temperature, we will define that as a positive error. Thus, I’m cold; I want to be warmer: that equals positive error. The converse is true. I’m hot; I want to be colder: that equals negative error.

One simple way we could try to control the input would be to say, “Let’s make the input proportional to the error term.” So, when the error term is positive, and thus I’m cold and wish to be warmer, we will add energy proportionate to the magnitude of the error term. Obviously the flip side is also true. If I’m hot and I wish to be cooler my negative error term would mean that remove energy proportionate to the magnitude of the error term. This sounds great! What more do you need? Well, what happens if I’m trying to hold a fixed temperature for a long time? If the system is not perfectly adiabatic, we still have to supply some energy to make up for whatever the system is losing to the surroundings. Obviously, this energy loss occurs even with the system is in a steady state configuration and at the prescribed temperature! But, if the system is exactly at the prescribed temperature, then the error term is zero. Anything proportionate to zero is… zero. That’s a bummer. I need something that won’t go to zero when my error term goes to zero.

What if I could keep a record of what I’ve done in the past? What if I accumulated all of the past error from forever? Obviously, this has the chance of being nonzero even if instantaneously my error term is zero. This sounds promising. Integrating a function of time with respect to time is analogous to accumulating the function values from history past. Thus, what if I integrated my error term and then made my input also proportional to that value? Wouldn’t that help the steady state issue above? Sure it would. Unfortunately, it also means I might go racing right on by my set point and it might take a while for that “mistake” to wash out of the system. Nothing is free. So, now I have kept a record of my entire past and used that to help me in the present, what if I could read the future? What if could extrapolate out in time?

Derivatives allow us to make a local extrapolation (in either direction) about a curve at a fixed point. So, if my curve is a function of time, which in our case the curves are, forward extrapolation is basically jumping ahead into the future. However, we can’t truly predict the future, we can only extrapolate on what has recently happened and make the leap of faith that it will continue to happen just as it has. So, if I take the derivative of my error term with respect to time, I can roll the dice a little a make some of my input proportional to this derivative term. That is, I can extrapolate out in time. If I do it right, I can actually make the system settle out a little faster. Remember that when the error term goes to zero and stays there, the derivative of the error term also goes to zero. So, when we are right on top of our prescribed value this term has no bearing on our input.

So, a PID controller simply takes the three concepts of how to specify an input value based on a function of the error term and mixes them together with differing proportions to arrive at the new value for the input. By “tuning” the system we can make it such that it responds quickly to change and it doesn’t wildly overshoot or oscillate.

Implementing a PID controller in ANSYS MAPDL

We will begin by implementing a PID controller in MAPDL before moving on to implementing the boundary condition in ANSYS Workbench via the ACT. We would like the boundary condition to have the following features:

  1. Ultimately we would like to “connect” this boundary condition to any number of nodes in our model. That is, we may want to have our energy input occur on a vertex, edge or face of the model in Workbench. So, we would like the boundary condition to support connecting to any number of nodes in the model.
  2. Likewise, we would like the “measured output” to be influenced by any number of nodes in our model. That is, if more than one node is included in the “measured value” set, we would like ANSYS to use the average temperature of the nodes in that set as our “measured output”. Again, this will allow us to specify a vertex, edge, face or body of the model to function as our measurement location. The measured value should be the average temperature on this entity. Averaging needs to be intelligent. We need to weight the average based on some measure that accounts for the relative influence of a node attached to large elements vs one attached to small elements.
  3. We would like to be able to independently control the proportional, integral and derivative components of the control algorithm.
  4. It would be nice to be able to specify whether this boundary condition can only add energy, only remove energy or if it can do both.
  5. We would like to allow the set point value to also be a function of time so that it too can change with time.
  6. Finally, it would be nice to be able to post process some of the heat flow quantities, temperature values, etc… associated with this boundary condition.

This is a pretty exhaustive list of requirements for the boundary condition. Fortunately, ANSYS MAPDL has built into it an element type that is perfectly suited for this type of control. That element type is the combin37.

Introducing the Combin37 Element Type

Understanding the combin37 element in ANSYS MAPDL takes a bit of a Zen state of mind… It’s, well, an element only a mother could love. Here is a picture lifted from the help:

clip_image003

OK. Clear as mud right? Actually, this thing can act as a thermostat whether you believe me from the picture or not. Like most/all ANSYS elements that can function in multiple roles, the combin37 is expressed in its structural configuration. It is up to you and me to mentally map it to a different set of physics. So, just trust me that you can forget the damping and FSLIDE and little springy looking thing in the picture. All we are going to worry about is the AFORCE thing. Mentally replace AFORCE with heat flow.

Notice those two little nodes hanging out there all by their lonesome selves labeled “control nodes”. I think they should have joysticks beside them personally, but ANSYS didn’t ask me. Those little guys are appropriately named. One of them, NODE K actually, will function as our set point node. That is, whatever temperature value we specify in time for NODE K, that same value represents the set point temperature we would like our “measured” location take on in time as well. So, that means we need to drive NODE K with our set point curve. That should be easy enough. Just apply a temperature boundary condition that is a function of time to that node and we’re good to go. Likewise, NODE L represents the “measured” temperature somewhere else in the model. So, we need to somehow hook NODE L up to our set of measurement nodes so that it magically takes on the average value of those nodes. More on that trick later.

Now, internally the combin37 subtracts the temperature at NODE K from NODE L to obtain an error term. Moreover, it allows us to specify different mathematical operations we can perform on the error term, and it allows us to take the output from those mathematical operations and drive the magical AFORCE thingy, which is heat flow. Guess what those mathematical operations are? If you guessed simply making the heat flow through the element proportional to the error, proportional to the time integral of the error and proportional to the time derivative of the error you would be right. Sounds like a PID controller doesn’t it? Now, the hard part is making sense of all the options and hooking it all up correctly. Let’s focus on the options first.

Key Option One and the Magic Control Value

Key option 1 for the combin37 controls what mathematical operation we are going to perform on the error term. In order to implement a full PID controller, we are going to need three combin37 elements in our model with each one keyed to a different mathematical operation. ANSYS calls the result of the mathematical operation, Cpar. So, we have the following:

KEYOPT(1) Value Mathematical Operation
0,1 image
2 image
3 image
4 image
5 image

Thus, for our purposes, we need to set keyopt(1) equal to 1,4 and 2 for each of the three elements respectively.

Feedback is realized by taking the control parameter Cpar and using it to modify the heat flow through the element, which is called AFORCE. The AFORCE value is specified as a real constant for the element; however, you can also rig up the element so that the value of AFORCE changes with respect to the control parameter. You do this by setting keyopt(6)=6. The manner in which ANSYS adjusts the AFORCE value, which again is heat flow, is described by the following equation:

image

Thus, the proportionality constant for the Proportional, Integral and Derivative components are specified with the C1 variable. RCONST, C3 and C4 are all set to zero. C2 is set to 1. Also note that ANSYS first takes the absolute value of the control parameter Cpar before plugging it into this equation. Furthermore, the direction of the AFORCE component is important. A positive value for AFORCE means that the element generates an element force (heatflow) in the direction specified in the diagram.  That is, it acts as a heat sink. So, assuming the model is attached to node J, the element acts as a heat sink when AFORCE is positive. Conversely, when AFORCE is negative, the element acts like a heat source. However, due to the absolute value, Cpar can never take on a negative value. Thus, when this element needs to act as an energy source to add heat to our model, the coefficient C1 must be negative. The opposite is true when the element needs to act as an energy sink.

Key Option Four and Five and when is it Alive?

If things weren’t confusing enough already, hold on as we discuss Keyopt 4 and 5. Consider the figure below, again lifted straight from the help.

clip_image016

The combination of these two key options controls when the element switches on and becomes “alive”. Let’s take the simple case first. Let’s assume that we are adding energy to the model in order to bring it up to a given temperature. In this case, Cpar will be positive because our set point is higher than our current value. If the element is functioning as a heat source we would like it to be on in this condition. Furthermore, we would like it to stay on as long as our error is positive so that we continue adding energy to bring the system up to temperature. Consider the diagram in the upper left. Imagine that we set ONVAL = 0 and OFFVAL = 0.001. Whenever Cpar is greater than ONVAL.  So this sounds like exactly what we want when the element is functioning as a heat source. Thus, keyopt(4)=0 and keyopt(5)=0.001 with OFFVAL=ONVAL=0 is what we want when the element needs to function as a heat source.

What about when it is a heat sink?  In this case we want the element to be active when the error term is negative; that is, when the current temperature is higher than the set point temperature.  Consider the diagram in the middle left.  This time let OFFVAL=0 and OFFVAL=-0.001.  In this case, whenever Cpar is negative (less than OFFVAL) then the element will be active.  Thus, keyopt(4)=0 and keyopt(5)=1 with OFFVAL=-0.001 ONVAL=0 is what we want when the element needs to function as a heat sink.  Notice, that if you set ONVAL=OFFVAL then the element will always stay on; thus, we need to provide the small window to activate the switching nature of the element.

Thus, we see that we need six different combin37 elements, three for a PID controlled heat sink and three for a PID controlled heat source, to fully specify a PID controlled thermal boundary condition. Phew… However, if we set all of the proportionality constants for either set of elements defining the heat sink or heat source to zero, we can effectively turn the boundary condition into only a heat source or only a heat sink, thus meeting requirement four listed above. While we’re marking off requirements, we can also mark off requirements three and five. That is, with this combination of elements we can independently control the P, I and D proportionality constants for the controller. Likewise, by putting a time varying temperature constraint on control node K, we can effectively cause the set point value to change in time. Let’s see if we can now address requirements one and two.

How do we Hook the Combin37 to the Rest of the Model?

We will address this question in two parts. First, how do we hook the “business” end of the combin37 to the part of the model to which we are going to add or remove energy? Second, how do we hook the “control” end of the combin37 to the nodes we want to monitor?

Hooking to the Combin37 to the Nodes that Add or Remove Energy

To hook the combin37 to the model so that we can add or remove energy we will use the convection link elements, link34. These elements effectively act like little thermal resistors with the resistance equation being specified as:

image

In order to make things nice, we need to “match” the resistances so that each node effectively sees the same resistance back to the combin37 element. We do this by varying the “area” associated with each of these convective links. To get the area associated with a node we use the arnode() intrinsic function. See the listing for details.

Hooking the Combin37 to the Nodes that Function as the Measured Value

As we mentioned in our requirements, we would like to be able to specify more than one or more nodes to function as the measured control value for our boundary condition. More precisely, if more than one node is included in the measurement node set, we would like ANSYS to average the temperatures at those nodes and use that average value as the measurement temperature. This will allow us to specify, for example, the average temperature of a body as the measurement value, not just one node on the body somewhere. However, we would also like for the scheme to work seamlessly if only one node is specified. So, how can we accomplish this? Constraint equations come to our rescue.

Remember that a constraint equation is defined as:

image

How can we use this to compute the average temperature of a set of nodes, and tie the control node of the combin37 to this average? Let’s begin by formulating an equation for the average temperature of a set of nodes. We would like this average to not be simply a uniform average, but rather be weighted by the relative contribution a given node should play in the overall average of a geometric entity. For example, assume we are interested in calculating the average temperature of a surface in our model. Obviously this surface will have associated with it many nodes connected to many different elements. Assume for the moment that we are interested in one node on this face that is connected to many large elements that span most of the area of this face. Shouldn’t this node’s temperature have a larger contribution to the “average” temperature of the face as say a node connected to a few tiny elements? If we just add up the temperature values and divide by the number of nodes, each node’s temperature has equal weight in the average. A better solution would be to area weight the nodal temperatures based on the area associated with each individual node. Something like:

image

That looks a little like our constraint equation. However, in the constraint equation I have to specify the constant term, whereas in the equation above, that is the value (Tavg) that I am interested in computing. What can I do? Well, let’s add in another node to our constraint equation that represents the “answer”. For convenience, we’ll make this the control node on our combin37 elements since we need the average temperature of the face to be driving that node anyway. Consider:

image

Now, our constant term is zero, and our Ci’s are Ai/AT and -1 for the control node. Voila! With this one constraint equation we’ve compute an area weighted average of the temperature over a set of nodes and assigned that value to our control node. CE’s rock!

An Example Model

This post is already way too long, so let’s wrap things up with a little example model. This model will illustrate a simple PI heat source attached to an edge of a plate with a hole. The other outer edges of the plate are given a convective boundary condition to simulate removing heat. The initial condition of the plate is set to 20C. The set point for the thermostat is set to 100C. No attempt is made to tune the PI controller in this example, so you can clearly see the effects of the overshoot due to the integral component being large. However, you can also see how the average temperature eventually settles down to exactly the set point value. clip_image026

The red squiggly represents where heat is being added with the PI controller. The blue squiggly represents where heat is being removed due to convection. Here is a plot of the average temperature of the body with respect to time where you can see the response of the system to the PI control.

clip_image027

Here is another run, where the set point value ramps up as well. I’ve also tweaked the control values a little to mitigate some of the overshoot. This is looking kind of promising, and it is fun to play with. Next time we will look to integrate it into the workbench environment via an actual ACT extension.

clip_image028

Part 2 is here

Model Listing

I’ve included the model listing below so that you can play with this yourself. In future posts, I will elaborate more on this technique and also look to integrate it into an ACT module.

finish

/clear

 

/prep7

*get,etmax_,etyp,0,num,max

P_et=etmax_+1

I_et=etmax_+2

D_et=etmax_+3

Link_et=etmax_+4

mass_et=etmax_+5

 

 

 

et,P_et,combin37

et,I_et,combin37

et,D_et,combin37

et,Link_et,link34

et,mass_et,mass71

Kp=1

Ki=2

Kd=0

 

keyopt,P_et,1,0    ! Control on UK-UL

keyopt,P_et,2,8    ! Control node DOF is Temp

keyopt,P_et,3,8    ! Active node DOF is Temp

keyopt,P_et,4,0    ! Wierdness for the ON/OFF range

keyopt,P_et,5,0    ! More wierdness for the ON/OFF range

keyopt,P_et,6,6    ! Use the force, Luke (aka Combin37)

keyopt,P_et,9,0    ! Use the equation, Duke (where is Daisy…)

 

 

keyopt,I_et,1,4    ! Control on integral wrt time

keyopt,I_et,2,8    ! Control node DOF is Temp

keyopt,I_et,3,8    ! Active node DOF is Temp

keyopt,I_et,4,0    ! Wierdness for the ON/OFF range

keyopt,I_et,5,0    ! More wierdness for the ON/OFF range

keyopt,I_et,6,6    ! Use the force, Luke (aka Combin37)

keyopt,I_et,9,0    ! Use the equation, Duke (where is Daisy…)

 

 

keyopt,D_et,1,2    ! Control on first derivative wrt time

keyopt,D_et,2,8    ! Control node DOF is Temp

keyopt,D_et,3,8    ! Active node DOF is Temp

keyopt,D_et,4,0    ! Wierdness for the ON/OFF range

keyopt,D_et,5,0    ! More wierdness for the ON/OFF range

keyopt,D_et,6,6    ! Use the force, Luke (aka Combin37)

keyopt,D_et,9,0    ! Use the equation, Duke (where is Daisy…)

 

keyopt,mass_et,3,1 ! Interpret real constant as DENS*C*Volume

 

 

 

!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

!!        S M A L L   T E S T   M O D E L       !!

!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

test_et=etmax_+10

et,test_et,plane55

 

mp,kxx,test_et,70

mp,dens,test_et,8050

mp,c,test_et,0.4

! Thickness of plate

r,test_et,0.1

 

! Plane55 element

keyopt,test_et,3,3

 

! Make a block

k,1

k,2,1,0

k,3,1,1

k,4,0,1

a,1,2,3,4

! Make a hole

cyl4,0.5,0.5,0.25

! Punch a hole

asba,1,2

 

type,test_et

mat,test_et

real,test_et

esize,0.025

amesh,all

 

! create an nodal component for the

! ‘attachment’ location

lsel,s,loc,x,0

nsll,s,1

cm,pid_attach_n,node

 

! create a nodal component for the

! ‘monitor’ location

allsel,all

cm,pid_monitor_n,node

 

!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

!!        B E G I N   P I D   M O D E L         !!

!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

 

! Real constant and mat prop for the mass element

mp,qrate,mass_et,0 ! Zero heat generation rate for the element

r,mass_et,1e-10  ! Extremely small thermal capacitance

 

! Material properties for convection element

! make the convection “large”

mp,hf,Link_et,1e5

 

! Real constant for the combin37 elements

! that ack as heaters

on_val_=0

off_val_=1e-3

r,P_et,0,0,0,on_val_,off_val_,0

rmore,0,1,-Kp,1

 

r,I_et,0,0,0,on_val_,off_val_,0

rmore,0,1,-Ki,1

 

r,D_et,0,0,0,on_val_,off_val_,0

rmore,0,1,-Kd,1

 

! build the PID elements

*get,nmax_,node,0,num,max

BASE_NODE=nmax_+1

P_NODE_J=nmax_+2

I_NODE_J=nmax_+3

D_NODE_J=nmax_+4

NODE_K=nmax_+5

NODE_L=nmax_+6

! Create the nodes.  They can be all coincident

! as we will refer to them solely by their number.

! They will be located at the origin

*do,i_,1,6

    n,nmax_+i_

*enddo

 

! Put a thermal mass on the K and L nodes

! for each control element to give them

! thermal DOFs

type,mass_et

mat,mass_et

real,mass_et

e,NODE_K

e,NODE_L

 

! Proportional element

type,P_et

mat,P_et

real,P_et

e,BASE_NODE,P_NODE_J,NODE_K,NODE_L

 

! Integral element

type,I_et

mat,I_et

real,I_et

e,BASE_NODE,I_NODE_J,NODE_K,NODE_L

 

! Derivative Element

type,D_et

mat,D_et

real,D_et

e,BASE_NODE,D_NODE_J,NODE_K,NODE_L

 

 

 

! Ground the base node

d,BASE_NODE,temp,0

 

! Get a list of the attachment nodes

cmsel,s,pid_attach_n

*get,numnod_,node,0,count

attlist_=

*dim,attlist_,,numnod_

*vget,attlist_(1),node,0,nlist

*get,rmax_,rcon,0,num,max

 

! Hook the attachment nodes to the

! end of the control element with

! convection links

*do,i_,1,numnod_

    n1_=attlist_(i_)

    a1_=arnode(n1_)

    r1_=rmax_+i_

    r,r1_,a1_

    type,link_et

    mat,link_et

    real,r1_

    e,P_NODE_J,n1_

    e,I_NODE_J,n1_

    e,D_NODE_J,n1_

*enddo

 

! Hook up the monitor nodes

cmsel,s,pid_monitor_n

*get,numnod_,node,0,count

monlist_=

monarea_=

*dim,monlist_,,numnod_

*dim,monarea_,,numnod_

*vget,monlist_(1),node,0,nlist

! We are going to need these areas

! so, hold on to them

*do,i_,1,numnod_

    monarea_(i_)=arnode(monlist_(i_))

*enddo

*vscfun,totarea_,sum,monarea_(1)

! Write the constraint equations

/pbc,ce,,0

ce,next,0,NODE_L,temp,-1

*do,i_,1,numnod_

    ce,high,0,monlist_(i_),temp,monarea_(i_)/totarea_

*enddo

 

! Create a transient setpoint temperature

*dim,setpoint_,table,2,,,time

setpoint_(1,0)=0

setpoint_(2,0)=3600

setpoint_(1,1)=100

setpoint_(2,1)=100

 

! Constrain the temperature node to be

! at the setpoint value

allsel,all

d,NODE_K,temp,%setpoint_%

 

 

! Apply an initial condition of

! 20 C to everything

allsel,all

ic,all,temp,20

 

/solu

antype,trans,new

time,1000

deltim,1,1,1

lsel,s,loc,x,0.1,1

nsll,s,1

sf,all,conv,20,20

 

allsel,all

outres,all,all

solve

/post26

! Plot the response temperature

! and the setpoint temperature

nsol,2,NODE_L,temp,,temp_r

nsol,3,NODE_K,temp,,temp_s

xvar,1

plvar,2,3

ANSYS, Inc. Unleashes New ACT Extensions for Version 15.0

act_150_1If you haven’t noticed, ANSYS, Inc. has been making quite a few ACT extensions available for ANSYS 15.0 on the ANSYS Customer Portal. If you are not familiar with ACT (ANSYS Customization Toolkit) Extensions, please see our earlier blog entry, “There’s an Extension for That,” here.

As of this writing, there are 20 ACT extensions available for download from the Customer Portal for version 15.0. There is also a set of training files available from the same link.

Among the new additions is an extension allowing the use of Mechanical APDL User Programmable Features in the Mechanical editor. Previously this could only be done in MAPDL. You will still need to install the customization files as part of the ANSYS installation, and you will still need the proper versions of the FORTRAN compiler and Visual Studio. However, this extension unleashes a significant capability within the Workbench Mechanical tool that wasn’t there previously, access to UPF’s.

The documentation states that it works with both versions 14.5 and 15.0 of ANSYS.
To get to the 15.0 ACT Extensions download area, login to the ANSYS Customer Portal and navigate through Downloads > Extension Library > 15.0. We urge you to browse the list of available extension available from the Customer Portal to see which might have benefits to your simulations.

Here is a list of all of the current extensions:

ACT Intro Training_R150_v1

ACT Introductory Training Materials

ACT Templates for DM_R150_v1

Templates for educational purpose; demonstrates most common scenarios of ACT-based development needs in DesignModeler

ACT Templates for DX_R150_v1

Templates for educational purpose; demonstrate integration of external optimizers

ACT Templates_R150_v1

Templates for educational purpose; cover most common ACT-based development needs

Acoustics Extension R150_v42

Expose 3D acoustics solver capabilities

Convection Extension_R150_v4

Expose convection with pilot node capability in steady-state and transient thermal analyses

Coupled Diffusion_R150_v3

Introduce coupled diffusion analysis (structural diffusion, thermal diffusion, and structural thermal diffusion) in both static and full transient analysis

Coupled Field Physics Extension_R150_v1

Expose piezoelectric, thermal-piezoelectric and thermal-structural solver capabilities

DDAM_R150_v2

Expose the Dynamic Design Analysis Method (DDAM) in Mechanical interface

Design Modeler Utility_R150_v1

Expose some useful functions in DM interface

Distributed Mass_R150_v2

Add distributed mass (rather than a point mass) to a surface as either “total mass” or “mass per unit area”

Enforced Motion_R150_v3

In Mode-Superposition Harmonic and Transient Analyses, allows applying base excitation (displacement or acceleration). Excitation can be either constant or frequency/time dependent

FE Info Extension_R150_v9

Expose node and element related information

FSI_Transient_Load_Mapping_R150_v4

Map temperature and pressure loads (from a CFD calculation) to a multi-step Mechanical analysis for transient one-way FSI. Includes CFD-Post macros

Follower Loads Extension_R150_v2

Create follower forces and moments to follow geometric deformation

Hydrostatic Fluid_R150_v5

Expose hydrostatic fluid elements in Mechanical

MATLAB_Optimizers_for_DX_R150_v1

Expose MATLAB optimization algorithms and user programs in the Optimization component of ANSYS DesignXplorer

MatChange_R150_v2

Change material ID to user specified value for the selected bodies

Offshore_R150_v4

Expose the MAPDL offshore features in Mechanical

Piezo Extension_R150_v8

Expose piezo-electric solver capabilities

UPF Extension_R150_v1

Allow for the use of User Programmable Features (UPF) within Workbench

Named Selections + Object Generator = Awesome

Guess who’s back…back again.  Yes, just like Slim Shady, I’m back (returned to PADT and writing Focus blogs).

So run and go tell your friends that horrible pop cultural references have returned to ANSYS blog posts.  It’s been too long.

Getting back on track, the object generator debuted in R14.5 Mechanical.  You can access this feature in the toolbar (image below taken from R15):

How the pro's generate objects

What exactly does the object generator do?  Simple answer…it makes your life better.  It uses named selections and a single instance of an object (joint, spring, bolt pretension, etc) and replicates it across all entities in the named selection.  Let’s play around with this feature on the following (dummy) assembly:

image

Above is a t-pipe with three covers, one of them has bolt ‘bodies’ modeled.  We’ll use fixed-fixed joints to connect the two ‘bolt-less’ bodies together, and then define bolt preloads on the bolt pattern.  To get started, we need to build up the named selections. 

I’m planning on defining the fixed-fixed joint between the two cylindrical surfaces:

image

This is a pretty simple assembly, and I could easily just manually select them all, right-mouse-click, and generate the named selection.  In the real world, things aren’t always so easy, so we’ll get a little fancy.  First, create a named selection of the bodies that contain faces we want to joint together:

image

I’ve created two named selections, called ‘joint_cover’ and ‘joint_pipe’ and utilized the ‘random colors’ option to display them in different colors.  Next, I insert a named selection but set the scoping method to be ‘by worksheet’:

image

I’ll then use selection logic (MAPDL hipsters will recognize the logic as the xSEL commands):

image

Now, order is important here, as the selection logic ‘flows’ from top to bottom.  First, this named selection selects the bodies contained in the existing named selection ‘joint_cover’ (note:  this object MUST exist above the worksheet-created named selection in the tree).  At this point in time, we have two bodies selected.  Next, it converts my body selection to faces belonging to those bodies.  Finally, it filters out any face that has a radius less than .05m (units are set by the ‘units’ drop-down menu, values entered in worksheet scale when units are changed).  Hit ‘generate’ and you get the following:

image

You may need to switch to the ‘graphics’ tab (circled in red in the above image).  This is great, we now have all of our faces highlighted.  Next, we need to reproduce this behavior on the pipe.  Rather than redo all of this work, just right-mouse-click on our new named selection and select ‘duplicate’. 

image image

Select the duplicated named selection, and edit the first line to use a different named selection.  Hit generate:

image

Perfect.  We can go back and add/remove bodies to the existing named selections and re-generate the named selections to have it automatically re-create these named selections. 

Next, we’ll create the original ‘joint’ we want to re-create across the two flanges. 

image

After making the joint, make note of which part is the ‘reference’ and ‘mobile’.  For the image above, the cover is the ‘reference’ while the pipe is the ‘mobile’.  Highlight this joint and select the object generator:

image

If we use the object generator on a joint, it will ask us to define the named selections that contain the reference and mobile faces.  From above, we know that the cover faces are contained in the ‘cover_faces’ named selection.  We then duplicated that and swapped the body selection, meaning the faces for the pipe are contained in ‘cover_faces 2’ (I’m lazy and didn’t rename it…sorry).  Next, we define the minimum distance between centroids.  This acts as a filter for re-creating each joint.  What happens when we hit ‘generate’ is it looks at the distance between the centroids of each face in the two named selections.  If it finds ‘matching’ faces within that distance it creates the joint. 

image

In the image above, if I use a distance equal to the red line, I will get incorrect joints defined.  I’ll get the following (a=cover, b=pipe): 1a-1b, 1a-2b, 2a-2b, 2a-1b…

What I need to do is limit the distance to the blue line, which is big enough to find the correct pairs but filter out the wrong ones.  To figure out a proper distance, you can use the ‘selection information’ window to figure out the centroid information:

image

Once you’re set, hit ‘generate’:

image

What a time to be alive!  It’s always a good idea to go through joint-by-joint to make sure everything is correct…or you can always just count the number of joints created and confirm that the number is correct (I have 15 total faces in the cover_faces named selection…so I should have 15 joints…and I do).

Next, let’s look at the bolt pretension definition.  We start with a named selection of the face where the bolt pretension will be applied:

image

Next, we create our original bolt pretension load:

image

I’ve setup my bolt pretension to solve for a 100N axial load in load step 1 and then lock the solved-for relative displacement in for load step 2.  We select the bolt pretension in the tree, then select the object generator:

image

Select the named selection that contains the bolt faces, and hit generate:

image

This is incredibly useful for bolt pretension for two reasons.  The first reason is obvious…it significantly cuts down on the amount of work you need to do for large bolt patterns.  The second reason…you can only make changes to bolt pretension objects one at a time.  By that, I mean you cannot multi-select all your bolt pretensions and change the load and step behavior (e.g. change load to 200N, open in load step 2, etc). 

image

If you select all the bolt pretensions, the changes you make in the tabular data window are only applied to the first selected object.  All other bolt pretensions are kept the same.  So if you suddenly realize the pretension was setup incorrectly, it’s best to delete all but one of the pretension object, make the necessary changes, then duplicate it.  That way you can be sure all the bolt pretensions are correct (unless you’re simulating a bolt opening up…then ignore). 

One very important thing to note is that the object generator is not parametrically linked to anything.  If I go back and change the number of holes/bolts/etc in my model, I may need to re-generate the duplicated joints/bolts/etc.  The named selections should update just fine, assuming you didn’t open the hole up bigger than the selection tolerance.  I would recommend deleting all but the original joint/bolt pretension and just re-create everything after the CAD update (this may actually speed up the CAD transfer as it’s not trying to link up incorrect entity IDs).

Hopefully this will save you some time/frustration in your next analysis.  The documentation in R15 can be accessed here:  help/wb_sim/ds_object_generator.html

ANSYS & 3D Printing: Converting your ANSYS Mechanical or MAPDL Model into an STL File

image3D printing is all the rage these days.  PADT has been involved in what should be called Additive Manufacturing since our founding twenty years ago.  So people in the ANSYS world often come to us for advice on things 3D Printer’ish.  And last week we got an email asking if we had a way to convert a deformed mesh into a STL file that can be used to print that deformed geometry.  This email caused neurons to fire that had not fired in some time. I remembered writing something but it was a long time ago.

Fortunately I have Google Desktop on my computer so I searched for ans2stl, knowing that I always called my translators ans2nnn of some kind. There it was.  Last updated in 2001, written in maybe 1995. C.  I guess I shouldn’t complain, it could have been FORTRAN. The notes say that the program has been successfully tested on Windows NT. That was a long time ago.

So I dusted it off and present it here as a way to get results from your ANSYS Mechanical or ANSYS Mechanical APDL model as a deformed STL file.

UPDATE – 7/8/2014

Since this article was written, we have done some more work with STL files. This Macro works fine on a tetrahedral mesh, but if you have hex elements, it won’t work – it assumes triangles on the face.  It also requires a macro and some ‘C’ code, which is an extra pain. So we wrote a more generic macro that works with Hex or Tet meshes, and writes the file directly. It can be a bit slow but no annoyingly slow.  We recommend you use this method instead of the ones outlined below.

Here is the macro:  writstl.zip

The Process

An STL file is basically a faceted representation of geometry. Triangles on the surface of your model. So to get an STL file of an FEA model, you simply need to generate triangles on your mesh face, write them out to a file, and convert them to an STL format.  If you want deformed geometry, simply use the UPGEOM command to move your nodes to the deformed position.

The Program

Here is the source code for the windows version of the program:

/*
---------------------------------------------------------------------------

 PADT--------------------------------------------------- Phoenix Analysis &
                                                        Design Technologies

---------------------------------------------------------------------------
                             www.padtinc.com
---------------------------------------------------------------------------

       Package: ans2stl

          File: ans2stl.c
          Args: rootname
        Author: Eric Miller, PADT
		(480) 813-4884 
		eric.miller@padtinc.com

	Simple program that takes the nodes and elements from the
	surface of an ANSYS FE model and converts it to a binary
	STL file.

	USAGE:
		Create and ANSYS surface mesh one of two ways:
			1: amesh the surface with triangles
			2: esurf an existing mesh with triangles
         	Write the triangle surface mesh out with nwrite/ewrite
		Run ans2stl with the rootname of the *.node and *.elem files
		   as the only argument
		This should create a binary STL file

	ASSUMPTIONS:
		The ANSYS elements are 4 noded shells (MESH200 is suggested)
		in triangular format (nodes 3 and 4 the same)

		This code has been succesfully compiled and tested
		on WindowsNT

		NOTE: There is a known issue on UNIX with byte order
				Please contact me if you need a UNIX version

	COMPILE:
		gcc -o ans2stl_win ans2stl_win.c

       10/31/01:       Cleaned up for release to XANSYS and such
       1/13/2014:	Yikes, its been 12+ years. A little update 
       			and publish on The Focus blog
			Checked it to see if it works with Windows 7. 
			It still compiles with GCC just fine.

---------------------------------------------------------------------------
PADT, Inc. provides this software to the general public as a curtesy.
Neither the company or its employees are responsible for the use or
accuracy of this software.  In short, it is free, and you get what
you pay for.
---------------------------------------------------------------------------
*/
/*======================================================

   SAMPLE ANSYS INPUT DECK THAT SHOWS USAGE

finish
/clear
/file,a2stest
/PREP7  
!----------
! Build silly geometry
BLC4,-0.6,0.35,1,-0.75,0.55 
SPH4,-0.8,-0.4,0.45 
CON4,-0.15,-0.55,0.05,0.35,0.55 
VADD,all
!------------------------
! Mesh surface with non-solved (MESH200) triangles
et,1,200,4
MSHAPE,1,2D   ! Use triangles for Areas
MSHKEY,0      ! Free mesh
SMRTSIZE,,,,,5
AMESH,all
!----------------------
! Write out nodes and elements
nwrite,a2stest,node
ewrite,a2stest,elem
!--------------------
! Execute the ans2stl program
/sys,ans2stl_win.exe a2stest

======================================================= */

#include 
#include 
#include 

typedef struct vertStruct *vert;
typedef struct facetStruct *facets;
typedef struct facetListStruct *facetList;

        int     ie[8][999999];
        float   coord[3][999999];
        int	np[999999];

struct vertStruct {
  float	x,y,z;
  float	nx,ny,nz;
  int  ivrt;
  facetList	firstFacet;
};

struct facetListStruct {
  facets	facet;
  facetList	next;
};

struct facetStruct {
  float	xn,yn,zn;
  vert	v1,v2,v3;
};

facets	theFacets;
vert	theVerts;

char	stlInpFile[80];
float	xmin,xmax,ymin,ymax,zmin,zmax;
float   ftrAngle;
int	nf,nv;  

void swapit();
void readBin();
void getnorm();
long readnodes();
long readelems();

/*--------------------------------*/
main(argc,argv)
     int argc;
     char *argv[];
{
  char nfname[255];
  char efname[255];
  char sfname[255];
  char s4[4];
  FILE	*sfile;
  int	nnode,nelem,i,i1,i2,i3;
  float	xn,yn,zn;

  if(argc <= 1){
        puts("Usage:  ans2stl file_root");
        exit(1);
  }
  sprintf(nfname,"%s.node",argv[1]);
  sprintf(efname,"%s.elem",argv[1]);
  sprintf(sfname,"%s.stl",argv[1]);

  nnode = readnodes(nfname);
  nelem = readelems(efname);
  nf = nelem;

  sfile = fopen(sfname,"wb");
  fwrite("PADT STL File, Solid Binary",80,1,sfile);
  swapit(&nelem,s4);    fwrite(s4,4,1,sfile);

  for(i=0;i<nelem;i++){ 
      i1 = np[ie[0][i]];
      i2 = np[ie[1][i]];
      i3 = np[ie[2][i]];
      getnorm(&xn,&yn,&zn,i1,i2,i3);

      swapit(&xn,s4);	fwrite(s4,4,1,sfile);
      swapit(&yn,s4);	fwrite(s4,4,1,sfile);
      swapit(&zn,s4);	fwrite(s4,4,1,sfile);

      swapit(&coord[0][i1],s4);	fwrite(s4,4,1,sfile);
      swapit(&coord[1][i1],s4);	fwrite(s4,4,1,sfile);
      swapit(&coord[2][i1],s4);	fwrite(s4,4,1,sfile);

      swapit(&coord[0][i2],s4);	fwrite(s4,4,1,sfile);
      swapit(&coord[1][i2],s4);	fwrite(s4,4,1,sfile);
      swapit(&coord[2][i2],s4);	fwrite(s4,4,1,sfile);

      swapit(&coord[0][i3],s4);	fwrite(s4,4,1,sfile);
      swapit(&coord[1][i3],s4);	fwrite(s4,4,1,sfile);
      swapit(&coord[2][i3],s4);	fwrite(s4,4,1,sfile);
      fwrite(s4,2,1,sfile);
  }
  fclose(sfile);
    puts(" ");
  printf("  STL Data Written to %s.stl \n",argv[1]);
    puts("  Done!!!!!!!!!");
  exit(0);
}

void  getnorm(xn,yn,zn,i1,i2,i3)
	float	*xn,*yn,*zn;
	int	i1,i2,i3;
{
	float	v1[3],v2[3];
	int	i;

        for(i=0;i<3;i++){
	  v1[i] = coord[i][i3] - coord[i][i2];
	  v2[i] = coord[i][i1] - coord[i][i2];
	}

	*xn = (v1[1]*v2[2]) - (v1[2]*v2[1]);
	*yn = (v1[2]*v2[0]) - (v1[0]*v2[2]);
	*zn = (v1[0]*v2[1]) - (v1[1]*v2[0]);
}
long readelems(fname)
        char    *fname;
{
        long num,i;
        FILE *nfile;
        char    string[256],s1[7];

        num = 0;
        nfile = fopen(fname,"r");
		if(!nfile){
			puts(" error on element file open, bye!");
			exit(1);
		}
        while(fgets(string,86,nfile)){
          for(i=0;i<8;i++){
            strncpy(s1,&string[6*i],6);
            s1[6] = '\0';
            sscanf(s1,"%d",&ie[i][num]);
          }
          num++;
        }

        printf("Number of element read: %d\n",num);
        return(num);
}

long readnodes(fname)
        char	*fname;
{
        FILE    *nfile;
        long     num,typeflag,nval,ifoo;
        char    string[256];

        num = 0;
        nfile = fopen(fname,"r");
		if(!nfile){
			puts(" error on node file open, bye!");
			exit(1);

		}
        while(fgets(string,100,nfile)){
          sscanf(string,"%d ",&nval);
          switch(nval){
            case(-888):
                typeflag = 1;
            break;
            case(-999):
                typeflag = 0;
            break;
            default:
                np[nval] = num;
                if(typeflag){
                        sscanf(string,"%d %g %g %g",
                           &ifoo,&coord[0][num],&coord[1][num],&coord[2][num]);
                }else{
                        sscanf(string,"%d %g %g %g",
                           &ifoo,&coord[0][num],&coord[1][num],&coord[2][num]);
                        fgets(string,81,nfile);
                }
num++;
            break;
        }

        }
        printf("Number of nodes read %d\n",num);
        return(num);

}

/* A Little ditty to swap the byte order, STL files are for DOS */
void swapit(s1,s2)
     char s1[4],s2[4];
{
  s2[0] = s1[0];
  s2[1] = s1[1];
  s2[2] = s1[2];
  s2[3] = s1[3];
}

ans2stl_win_2014_01_28.zip

Creating the Nodes and Elements

I’ve created a little example macro that can be used to make an STL of deformed geometry.  If you do not want the deformed geometry, simply remove or comment out the UPGEOM command.  This macro is good for MAPDL or ANSYS Mechanical, just comment out the last line  to use it with MAPDL:

et,999,200,4

type,999

esurf,all

finish ! exit whatever preprocessor your in

! move the RST file to a temp file for the UPCOORD. Comment out if you want

! the original geometry

/copy,file,rst,,stl_temp,rst

/prep7 ! Go in to PREP7

et,999,200,4 ! Create a dummy triangle element type, non-solved (200)

type,999 ! Make it the active type

esurf,all ! Surface mesh your model

!

! Update the geometry to the deformed shape

! The first argument is the scale factor, adjust to the appropriate level

! Comment this line out if you don’t want deformed geometry

upgeom,1000,,,stl_temp,rst

!

esel,type,999 ! Select those new elements

nelem ! Select the nodes associated with them

nwrite,stl_temp,node ! write the node file

ewrite,stl_temp,elem ! Write the element file

! Run the program to convert

! This assumes your executable in in c:\temp. If not, change to the proper

! location

/sys,c:\temp\ans2stl_win.exe stl_temp

! If this is a ANSYS Mechanical code snippet, then copy the resulting STL file up to

! the root directory for the project

! For MAPDL, Comment this line out.

/copy,stl_temp,stl,,stl_temp,stl,..\..

An Example

To prove this out using modern computing technology (remember, last time I used this was in 2001) I brought up my trusty valve body model and slammed 5000 lbs on one end, holding it on the top flange.  I then inserted the Commands object into the post processing branch:

image

When the model is solved, that command object will get executed after ANSYS is done doing all of its post processing, creating an STL of the deformed geometry. Here is what it looks like in the output file. You can see what it looks like when APDL executes the various commands:

/COPY FILE FROM FILE= file.rst

TO FILE= stl_temp.rst

FILE file.rst COPIED TO stl_temp.rst

1

***** ANSYS – ENGINEERING ANALYSIS SYSTEM RELEASE 15.0 *****

ANSYS Multiphysics

65420042 VERSION=WINDOWS x64 08:39:44 JAN 14, 2014 CP= 22.074

valve_stl–Static Structural (A5)

Note – This ANSYS version was linked by Licensee

***** ANSYS ANALYSIS DEFINITION (PREP7) *****

ELEMENT TYPE 999 IS MESH200 3-NODE TRIA MESHING FACET

KEYOPT( 1- 6)= 4 0 0 0 0 0

KEYOPT( 7-12)= 0 0 0 0 0 0

KEYOPT(13-18)= 0 0 0 0 0 0

CURRENT NODAL DOF SET IS UX UY UZ

THREE-DIMENSIONAL MODEL

ELEMENT TYPE SET TO 999

GENERATE ELEMENTS ON SURFACE DEFINED BY SELECTED NODES

TYPE= 999 REAL= 1 MATERIAL= 1 ESYS= 0

NUMBER OF ELEMENTS GENERATED= 13648

USING FILE stl_temp.rst

THE SCALE FACTOR HAS BEEN SET TO 1000.0

USING FILE stl_temp.rst

ESEL FOR LABEL= TYPE FROM 999 TO 999 BY 1

13648 ELEMENTS (OF 43707 DEFINED) SELECTED BY ESEL COMMAND.

SELECT ALL NODES HAVING ANY ELEMENT IN ELEMENT SET.

6814 NODES (OF 53895 DEFINED) SELECTED FROM

13648 SELECTED ELEMENTS BY NELE COMMAND.

WRITE ALL SELECTED NODES TO THE NODES FILE.

START WRITING AT THE BEGINNING OF FILE stl_temp.node

6814 NODES WERE WRITTEN TO FILE= stl_temp.node

WRITE ALL SELECTED ELEMENTS TO THE ELEMENT FILE.

START WRITTING AT THE BEGINNING OF FILE stl_temp.elem

Using Format = 14(I6)

13648 ELEMENTS WERE WRITTEN TO FILE= stl_temp.elem

SYSTEM=

c:\temp\ans2stl_win.exe stl_temp

Number of nodes read 6814

Number of element read: 13648

STL Data Written to stl_temp.stl

Done!!!!!!!!!

/COPY FILE FROM FILE= stl_temp.stl

TO FILE= ..\..\stl_temp.stl

FILE stl_temp.stl COPIED TO ..\..\stl_temp.stl

image

The resulting STL file looks great:

image

I use MeshLab to view my STL files because… well it is free.  Do note that the mesh looks coarser.  This is because the ANSYS mesh uses TETS with midside nodes.  When those faces get converted to triangles those midside nodes are removed, so you do get a coarser looking model.

And after getting bumped from the queue a couple of times by “paying” jobs, our RP group printed up a nice FDM version for me on one of our Stratasys uPrint Plus machines:

image

It’s kind of hard to see, so I went out to the parking lot and recorded a short video of the part, twisting it around a bit:

Here is the ANSYS Mechanical project archive if you want to play with it yourself.

Other Things to Consider

Using FE Modeler

You can use FE Modeler in a couple of different ways with STL files. First off, you can read an STL file made using the method above. If you don’t have an STL preview tool, it is an easy way to check your distorted mesh.  Just chose STL as the input file format:

image

You get this:

image

If you look back up at the open dialog you will notice that it reads a bunch of mesh formats. So one thing you could do instead of using my little program, is use FE Modeler to make your STL.  Instead of executing the program with a /SYS command, simply use a CDWRITE,DB command and then read the resulting *.CDB file into FE Modeler.  To write out the STL, just set the “Target System” to STL and then click “Write Solver File”

image

You may know, or may have noticed in the image above, that FE Modeler can read other FEA meshes.  So if you are using some other FEA package, which you should not, then you can make an STL file in FE Modeler as well.

Color Contours

The next obvious question is how do I get my color contours on the plot. Right now we don’t have that type of printer here at PADT, but I believe that the dominant 3D Color printer out, the former Z-Corp and now 3D Systems machines, will read ANSYS results files. Stratasys JUST announced a new color 3D Printer that makes usable parts. Right now they don’t have a way to do contours, but as soon as they do we will publish something.

Another option is to use a /SHOW,vrml option and then convert that to STL with the color information.

Scaling

Scaling is something you should think about. Not only the scaling on your deformed geometry, but the scaling on your model for printing.  Units can be tricky with STL files so make sure you check your model size before you print.

Smoother STL Surfaces

Your FEA mesh may be kind of coarse and the resulting STL file is even coarser because of the whole midside node thing.  Most of the smoothing tools out there will also get rid of sharp edges, so you don’t want those. Your best best is to refine your mesh or using a tool like Geomagic.

Making a CAD Model from my Deformed Mesh

Perhaps you stumbled on this posting not wanting to print your model. Maybe you want a CAD model of your deformed geometry.  You would use the same process, and then use Geomagic Studio.  It actually works very well and give you a usable CAD model when you are done.

The 10 Coolest New Features in R15 of ANSYS Mechanical

It’s that time of year again, time for a new release of ANSYS, Inc’s products.  R15 is being released in stages to the user community this week so we thought we would take some time to point out ten features in R15 of ANSYS Mechanical that we find useful, important, or just plain cool.  There are a ton of new features and we will try and cover most of them in the coming months, but these are the ones we felt every user should know about.

This posting will focus on features in ANSYS Mechanical that are unique to ANSYS Mechanical.  Later this week or next week we will do the same for ANSYS Mechanical APDL, and we will cover solver changes that impact ANSYS Mechanical there as well.

image1: Mesh Based Geometry

This is by far the most far-reaching enhancement in R15.  A fundamental limitation of ANSYS Mechanical from the beginning was the requirement that you had to have a valid BREP geometry that can be correctly meshed in ANSYS Mechanical. For most problems this is fine, you have a CAD model, you mesh it, and you move on.  But there are often situations where you have a legacy model or a mesh from another source that you want to use. And in such cases you were just stuck. Most things in ANSYS Mechanical work on geometry and if you just have a mesh, and no geometry, there area not a lot of options.

No longer.  Yipidee yapidee dooooooo daaaa!!!!  That gets a leap for joy gif:

It works through an External Model system in the Workbench project page.  There are a slew of options to copy, translate, change units, etc.. for the model. However, the most important option is the tolerance angle. The way the mesh geometry import works is that it takes the external faces of your mesh and treats them as facets. Any facets that have an angle less than the tolerance angle are considered to be on the same face. Any angle greater than the tolerance treats the edge between facets as an edge on a face.  This is key to understanding how using an mesh in Mechanical works.

I’ll resist the temptation to get into the details and save that for a future posting where we can dig deep. 

To prove it out I searched my hard drive for old *.DB files and found one from 2001. A generic turbine blade ad disk model I made for some cyclic-symmetry testing ANSYS was doing.  Here is what is looks like in ANSYS Mechanical APDL:

image

And here it what it looks like in ANSYS Mechanical:

image

That is a sight for sore eyes.  It’s not perfect, the trailing edge muffs things up a big because the mesh is a bit coarse there. But a little work playing with the tolerance angle and/or named selections in MAPDL and that can be cleaned up. More on that in that promised post on this capability.  Here is a wireframe where you can see the internal cooling passages, and the funky elements.

image

Changing the tolerance from the default 45deg to 55deg cleans up most of the problems:

image

image2: Assembly Mechanical Models

A related capability to the mesh geometry mentioned above is the ability to create an assembly of multiple external models (mesh based) and other Mechanical Models.

You can take as many External Model, Mechanical Model, or various Analysis systems as you want, and feed them all into a new mechanical model or any analysis system you want to use them in. ANSYS, Inc. provided this really cool example of a model of a pad lock:

image

As much information as is possible is transferred over, depending on what makes sense.  Also note that you can apply transformations and units conversions to each model, so if you build your parts in different coordinate systems, you can move them around and get them set up when you build the assembled model.

image3: Parallel Meshing by Part

The first step in making meshing fully parallel in ANSYS Mechanical has been released in R15: parallel part meshing.  Basically, if you have more than one part, the program will mesh each part on its own CPU.  So if you have 8 cores and 6 parts, it will mesh on 6 cores at one time.  The default setting is 0, which tells the program to run on as many cores as it can.  Most users will want to keep it there, but if you do a lot of models with lots of large parts, you may want to set it at N-1 where N is the number of cores on your machine.  Leave one open to watch YouTube on while you are meshing.

The settings are in Tools->Options->Meshing->Meshing->Meshing->Number of CPUs for Meshing Methods

image4: Mechanical Shortcut Keys

You may love this one, or it may generate a “Meh.”  Shortcut Keys are almost a religious thing.  If you are on the “true believer” side then you now can use function and control keys to do the following actions:

Tree Outline Actions

F1: opens the Mechanical User’s Guide.
F2: rename a selected tree object.
Ctrl + S: save the project.

Graphics Actions

F6: toggles between the Shaded Exterior and Edges, Shaded Exterior, and Wireframe views
F7: executes Zoom to Fit option
F8: hide selected faces.
F9: hide selected bodies.
Ctrl + A: selects all entities based on the active selection filter (bodies, faces, edges, vertices, nodes).

Selection Filters

Ctrl + B: activate Body selection.
Ctrl + E: activate Edge selection.
Ctrl + F: activate Face selection.
Ctrl + P: activate Vertex selection.

Even if you are not a Shortcut Key acolyte, the selection filters and the F7 options look pretty useful.

image5: Element Selection and Grouping

In the last release the developers at ANSYS, Inc. gave users full access to nodes in ANSYS Mechanical. At R15 they have opened up access to elements.  Using the term “opened up access” is a bit misleading, they did not just change a parameter from FALSE to TRUE and boom, you have elements.  It was a major change to how data is stored and accessed in the program. 

Selecting works as you would expect, just like nodes. You choose “Select Mesh” from the Select Type icon:

image

Then you choose the Select “Body/Element” from the type choices (the green cube).  Here is where you can use those shortcuts: Ctrl + B selects it for you. Now you can pick elements or use box select to get what you want.

Names selections for elements work just like nodes.  Very useful indeed. And they do get converted to components in Mechanical APDL, avoiding that annoying snippet where you had to convert a nodal component into an element component.

image6: Mapping Enhancements

One feature set in ANSYS Mechanical that makes users of other ANSYS products jealous are the mapping tools. And at this release they got even better, adding more functionality, feedback, and making some beta features released features. If you are not familiar with mapping tools, they allow you to take a load specified on points in space, and interpolate that load on to your model. Again, this is a topic worthy of its own posting, but here are the highlights:

  • Support for Velocity
  • Support for Initial Stress or Strain
  • Support for Body Force Density (forces from an electromagnetic solution)
  • Pressure can now be applied to nodes as well as to elements
  • Acoustic loads from a Harmonic Response Analysis can be mapped as velocities
  • UV Mapping is now available for surfaces that don’t really line up.

That last one, UV Mapping, may be one of the more powerful. It is not that uncommon for you to get pressures on a surface that does not really sit on your model surface.

image7: Better Control of Hydrostatic Pressure

This is pretty specialized, but if you work on parts that see hydrostatic loading you always had to play around with APDL snippets to get the control you needed.  At R15 they have added those controls into the program for us.

image

The first addition is that you can turn the pressure on and off for a given load step.  This is not necessarily done in an intuitive way, but it works.  Select the step in the graph or in the table when you have the load selected in the tree. Then Right Mouse Button to get the context menu and activate or deactivate the load. It doesn’t show anything in the table, but it does show on the graph that the load is turned off.  Note, you can’t change the acceleration, you can only turn it on or off.

The second addition is simply that the values for fluid density and magnitude are parameters.

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image8: Multiple GPU Support, and Intel Phi Support

GPU usage for ANSYS Mechanical solves is growing.  We are seeing good enhancements in performance at every release, and R15 is no exception.  But that is solver stuff and I said we would not talk about solver stuff…  What is important in this area for ANSYS Mechanical users is that you can now specify more than one GPU for a solve, and at R15 the new Intel Phi coprocessor, which is really not a GPU, is supported. You access the control, and all settings for HPC, under Tools-> Solve Process Settings-> Advanced.  Note that there are different settings for solving interactive and in the background.

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image9: Follower Load for Rigid Body Dynamics

Because Rigid Body Dynamics are, well rigid body dynamics, they are generally inherently large deflection.  When you apply a load to an object you usually want that load to move with the objects motion, to follow it.  In the past, you had to create a dummy rigid part and apply a pressure to get this.  Now at R15 you can set “Follower Load” to yes in the details view for a Remote Force and it will go along for the ride.  If you do RBD, this is critical.

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image10: Bearing Enhancements

The last item on our top ten list for this release are improvements to modeling bearings in ANSYS Mechanical. The ANSYS Mechanical APDL solver supports a wide range of bearing capabilities, and with this release most of them are now exposed in ANSYS Mechanical. 

The big change is that all of the solvers that support bearings are now supported in ANSYS Mechanical. In the past, it was only modal analysis. Now you can simplify your model and get the proper stiffness and damping of your bearing for transient, static, and any other type of run you want to do.

As you would expect with the support of the bearing joint on the pre-processing side, they have added a probe that allows you to get key information out of your bearing after the solve. Since a bearing joint is basically two spring-dampers, you can get spring type info for each spring: elastic force, damping force, elongation, and velocity (for transient runs).

Thoughts

If you look at this list you should notice one common thread, that most of these changes are not general, they are for specific analysis types.  As time has gone by ANSYS Mechanical has grown and matured, and there are less and less basic or general features that need to be added. So now we are in to the fun stuff, digging down into the nitty gritty and exposing more and more of the most powerful solver available (ANSYS Mechanical APDL), through what is the most powerful user interface for structural mechanics, ANSYS Mechanical.

This May Be the Fastest ANSYS Mechanical Workstation we Have Built So Far

The Build Up

Its 6:30am and a dark shadow looms in Eric’s doorway. I wait until Eric finishes his Monday morning company updates. “Eric check this out, the CUBE HVPC w16i-k20x we built for our latest customer ANSYS Mechanical scaled to 16 cores on our test run.” The left eyebrow of Eric’s slightly rises up. I know I have him now I have his full and complete attention.

Why is this huge news?

This is why; Eric knows and probably many of you reading this also know that solving differential equations, distributed, parallel along with using graphic processing unit makes our hearts skip a beat. The finite element method used for solving these equations is CPU intensive and I/O intensive. This is headline news type stuff to us geek types. We love scratching our way along the compute processing power grids to utilize every bit of performance out of our hardware!

Oh and yes a lower time to solve is better! No GPU’s were harmed in this tests. Only one NVIDIA TESLA k20X GPU was used during the test.

Take a Deep Breath and Start from the Beginning:

I have been gathering and hording years’ worth of ANSYS mechanical benchmark data. Why? Not sure really after all I am wanna-be ANSYS Analysts. However, it wasn’t until a couple weeks ago that I woke up to the why again. MY CUBE HVPC team sold a dual socket INTEL Ivy bridge based workstation to a customer out of Washington state. Once we got the order, our Supermicro reseller‘s phone has been bouncing of the desk. After some back and forth, this is how the parts arrive directly from Supermicro, California. Yes, designed in the U.S.A.  And they show up in one big box:

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Normal is as Normal Does

As per normal is as normal does, I ran the series of ANSYS benchmarks. You know the type of benchmarks that perform coupled-physics simulations and solving really huge matrix numbers. So I ran ANSYS v14sp-5, ANSYS FLUENT benchmarks and some benchmarks for this customer, the types of runs they want to use the new machine for. So I was talking these benchmark results over with Eric. He thought that now is a perfect time to release the flood of benchmark data. Well some/a smidge of the benchmark data. I do admit the data does get overwhelming so I have tried to trim down the charts and graphs to the bare minimum. So what makes this workstation recipe for the fastest ANSYS Mechanical workstation so special? What is truly exciting enough to tip me over in my overstuffed black leather chair?

The Fastest Ever? Yup we have been Changed Forever

Not only is it the fastest ANSYS Mechanical workstation running on CUBE HVPC hardware.  It uses two INTEL CPU’s at 22 nanometers. Additionally, this is the first time that we have had an INTEL dual socket based workstation continue to gain faster times on and up to its maximum core count when solving in ANSYS Mechanical APDL.

Previously the fastest time was on the CUBE HVPC w16i-GPU workstation listed below. And it peaked at 14 cores. 

Unfortunately we only had time before we shipped the system off to gather two runs: 14 and 16 cores on the new machine. But you can see how fast that was in this table.  It was close to the previous system at 14 cores, but blew past it at 16 whereas the older system actually got clogged up and slowed down:

  Run Time (Sec)
Cores Used Config B Config C Config D
14 129.1 95.1 91.7
16 130.5 99 83.5

And here are the results as a bar graph for all the runs with this benchmark:

CUBE-Benchmark-ANSYS-2013_11_01

  We can’t wait to build one of these with more than one motherboard, maybe a 32 core system with infinband connecting the two. That should allow some very fast run times on some very, very large problems.

ANSYS V14sp-5 ANSYS R14 Benchmark Details

  • Elements : SOLID187, CONTA174, TARGE170
  • Nodes : 715,008
  • Materials : linear elastic
  • Nonlinearities : standard contact
  • Loading : rotational velocity
  • Other : coupling, symentric, matrix, sparse solver
  • Total DOF : 2.123 million
  • ANSYS 14.5.7

Here are the details and the data of the March 8, 2013 workstation:

Configuration C = CUBE HVPC w16i-GPU

  • CPU: 2x INTEL XEON e5-2690 (2.9GHz 8 core)
  • GPU: NVIDIA TESLA K20 Companion Processor
  • GRAPHICS: NVIDIA QUADRO K5000
  • RAM: 128GB DDR3 1600Mhz ECC
  • HD RAID Controller: SMC LSI 2208 6Gbps
  • HDD: (os and apps): 160GB SATA III SSD
  • HDD: (working directory):6x 600GB SAS2 15k RPM 6Gbps
  • OS: Windows 7 Professional 64-bit, Linux 64-bit
  • Other: ANSYS R14.0.8 / ANSYS R14.5

Here are the details from the new, November 1, 2013 workstation:

Configuration D = CUBE HVPC w16i-k20x

  • CPU: 2x INTEL XEON e5-2687W V2 (3.4GHz)
  • GPU: NVIDIA TESLA K20X Companion Processor
  • GRAPHICS: NVIDIA QUADRO K4000
  • RAM: 128GB DDR3 1600Mhz ECC
  • HDD: (os and apps): 4 x 240GB Enterprise Class Samsung SSD 6Gbps
  • HD RAID CONTROLLER: SMC LSI 2208 6Gbps
  • OS: Windows 7 Professional 64-bit, Linux 64-bit
  • Other: ANSYS 14.5.7

You can view the output from the run on the newer box (Configuration D) here:

Here is a picture of the Configuration D machine with the info on its guts:

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What is Inside that Chip:

The one (or two) CPU that rules them all: http://ark.intel.com/products/76161/

Intel® Xeon® Processor E5-2687W v2

  • Status: Launched
  • Launch Date: Q3’13
  • Processor Number: E5-2687WV2
  • # of Cores: 8
  • # of Thread: 16
  • Clock Speed: 3.4 GHz
  • Max Turbo Frequency: 4 GHz
  • Cache:  25 MB
  • Intel® QPI Speed:  8 GT/s
  • # of QPI Link:  2
  • Instruction Se:  64-bit
  • Instruction Set Extension:  Intel® AVX
  • Embedded Options Available:  No
  • Lithography:  22 nm
  • Scalability:  2S Only
  • Max TDP:  150 W
  • VID Voltage Range:  0.65–1.30V
  • Recommended Customer Price:  BOX : $2112.00, TRAY: $2108.00

The GPU’s that just keep getting better and better:

Features

TESLA C2075

TESLA K20X

TESLA K20

Number and Type of GPU

FERMI

Kepler GK110

Kepler GK110

Peak double precision floating point performance

515 Gflops

1.31 Tflops

1.17 Tflops

Peak single precision floating point performance

1.03 Tflops

3.95 Tflops

3.52 Tflops

Memory Bandwidth (ECC off)

144 GB/sec

250 GB/sec

208 GB/sec

Memory Size (GDDR5)

6GB

6GB

5GB

CUDA Cores

448

2688

2496

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Ready to Try one Out?

If you are as impressed as we are, then it is time for you to try out this next iteration of the Intel chip, configured for simulation by PADT, on your problems.  There is no reason for you to be using a CAD box or a bloated web server as your HPC workstation for running ANSYS Mechanical and solving in ANSYS Mechanical APDL.  Give us a call, our team will take the time to understand the types of problems you run, the IT environment you run in, and custom configure the right system for you:

http://www.padtinc.com/products/hardware/cube-hvpc,
email: garrett.smith@padtinc.com,
or call 480.813.4884

Making Charts and Tables in ANSYS Mechanical

imageOne of the nicer features in ANSYS Mechanical is the fact that when you enter in any type of tabular data, or look at any type of tabular results, you can view it as a table or as a graph.  But what if you want to make your own graph, maybe even viewing values from two different solutions?  ANSYS Mechanical has a little used feature called “New Chart and Table” that will allow you to make a table or a graph (chart) of quantities in your model tree that make sense when displayed as a graph or table: Time, loads applied over time, and results over time.

image

I have found myself exporting data to excel and making graphs all the time. And this is OK if you just do it once. But if you make a change to the model, you need to export again and redo your graph.  The Chart and Table function makes this an automatic step, right there in your model tree.

For this posting, we will just use a simple plasticity bending example. We hold the bottom of a round bar with a grove cut in the bottom part and push on the top with forces.

In its simplest form the “Chart and Table” duplicates what you see in the graph and Tabular Data windows when you click on a load or a result. Here is what you get when you click on a displacement:

image

And if you select the probe in the tree and click on the “New Chart and Table” icon you get:

image

No woop.  But even if I want to just plot one value, I can now customize the look of the graph a bit.  Take a look at the Details for the Chart:

image

With the Chart Controls you can define what is shown on the X axis; if you want lines, points or both with Plot Style, log or linear scale, and if you want horizontal, vertical, neither, or both gridlines.

image

This is what it looks like if I turn on both gridlines and use a log scale for the Y Axis.

Next, we can add axis labels with “Axis Labels:”

image

The “Report” Section tells the program what to do when a report is generated. By Default you get a table and a graph.  But you can do either, both, or you can suppress it in the report.  You can give the plot and/or table in the report a caption by filling in the Caption field.  It comes out nice:

image

Note that it actually includes a legend in the report. If you want the legend when you are looking at a graph interacively, just Right Mouse Button on the graph and choose “Show Legend” to turn it on:

image

image

Note that the legend shows the name of the branch in the tree. That is not very informative. So I change it to something useful and now the legend is useful:

image

 

So even with a basic graph, we can do a lot. But the real power is when you want to look at more. Let’s say I want to plot the force and the stress over time. I create a new chart with the icon then select the force and the stress results as my “Outline Selection”

image

I get a lot of stuff on my graph. That is because the program starts by plotting all the components for the load, and all max and min stress over time for the result. I simply change the ones I don’t want from “Display” to “Omit.”  Then I get:

image

Much more useful.  Note that it does not create two separate Y axis. Instead it normalizes the values between the min and max for each. This is not ideal, and hopefully in the future they will support multiple axis, but it still works for most cases when you want to compare things. Note that I renamed the branches in my tree so they show up in the legend correctly.  Next I will add some labels and turn on gridlines.

image

We have been neglecting the table. It also gets created:

image

As with any table in ANSYS Mechanical, it can be exported to Excel. So if you find yourself grabbing data from multiple input or result tables and pasted them into Excel, make a Chart and Table item to grab all the data you want in one place, then export it once.  To be honest, the quality of the graphs that are made are good enough for engineering, but maybe not good enough for a presentation. By making a Chart & Table of what you need, then exporting to Excel or some other graphing tool, you can still save a lot of time.

Next, let us look at plotting values from multiple simulations.  If you look at the tree, you will notice that the charts are a child of the model, not the simulations.  This signals that we can show data form the same model, but different simulations:

image

In our example I’ve simply made one with a tip force in the Y direction, and one with a tip force in the X direction. And I can show that by making a chart:

image

And I get a table:

image

HINT: If you want to make a single table or chart that shows all your input loads over time, in a single simulation or across multiple simulations, this is the way to do it.  If I add a third simulation where I vary the load in all three directions, I can capture all three cases in one table:

image

These examples show loads. Here is what it looks like if we review the deflection on the tip probe over time for two simulations:

image

Or mash it all up, and show stress and deflection for both cases:

image

In every case so far we have used time (Load Step for static) as our X axis. But you can put any value you want on the X axis.  Here is Force applied vs Tip Deflection:

image

Make sure you turn off Time and loads you don’t want to see.  This is a great way to plot hysteresis effects.

You may notice the plots in this posting are nice and big and have a good aspect ratio. And your screen looks like this:

image

Every window in ANSYS Mechanical can be dragged out of the frame and positioned/sized however you want. So I pull off the Graph window by itself and resize it to the aspect ratio I want. Now when I want to save the image all I have to do is select that window and hit Alt-Print Screen. The image is now stored in the clipboard and I can past it where I want.

image

To get the normal window configuration back, click View>Windows>Reset Layout.

As always, play with it to figure more out. I’ve included my simple test case in case you want to play with it first:

Making APDL Parameters Available in the ANSYS Parameter Manager or DesignXplorer: Prep, Solve, and Post

This is one of those questions that comes up every once in a while that is not so obvious at first glance, but that is simple once you understand how ANSYS Mechanical interacts with ANSYS Mechanical APDL.  After a couple of email exchanges around a tech support question, we thought it would be good to share with everyone.

Before we get started, if you need a refresher on Command Objects in ANSYS Mechanical, the way in which you send APDL commands to the ANSYS Mechanical APDL solver, here is a seminar from a couple of years ago that covers the whole deal:

The basic problem is this: you have an APDL script you execute as a command object that does some sort of model interrogation or stores the result of some calculation, and you want to use that parameter in the parameter manager or in DesignXplorer.  If you look at the details view for a command object you will notice that it only supports input parameters: ARG1-ARG9.

image

If you look at the example (silly) macro you will see that it:

  1. Grabs component (named selection) END1
  2. Figures out how many nodes are attached to END1 (NMND)
  3. Takes ARG1 as the total load applied load
  4. Calculates the per node load by dividing the total load by the number of loads.
  5. Applies that per node load
  6. Reselects all the nodes

If I want to know how many nodes I put the load on and what the per node load is I’m kind of stuck here.  Any command object you add to the tree above the Solution branch only allows input parameters.

But a command snippet applied in the Solution branch is different, it allows you to pull parameters back and share them through the parameter manager.

When you first insert a command object you only get input parameters (ARG1-ARG9) as usual, and an empty section called “Results”

image

The way you get result parameters, or what I think should be called “Output Parameters” is you create a parameter in the command object’s APDL script that starts with “my_”  When you click outside the text input window the program parses you script and if it finds any “my_” parameters in the text, it sticks them in the Results section:

image

Note, the default is “my_” but you can change it n the “Output Search Prefix” line in the Definition block.

Initially they will show up pinkish because the model has not been run and they are not defined. Click on the box to make them parameters that get passed outside of the program and then run:

image

If you pop back out to the project view you will see that we now have a Parameter Set bar with both input and output parameters:

image

And if you open the parameter manager up you can see the input and output parameters:

image

This works because all ANSYS mechanical is doing is making one big batch input file for ANSYS MAPDL.  That file contains any command objects you insert into the tree and extracts any parameters that you tagged in a post processing command object for return to ANSYS Mechanical.

20 APDL Commands Every ANSYS Mechanical User Should Know

One of the most powerful things about ANSYS Mechanical is the fact that it creates an input file that is sent to ANSYS Mechanical APDL (MAPDL) to solve. This is awesome because you as a user have complete and full access to the huge breadth and depth available in the MAPDL program.  MAPDL is a good old-fashioned command driven program that takes in text commands one line at a time and executes them. So to access all those features, you just need to enter in the commands you want.

For many older users this is not a problem because we grew up using the text commands. But new users did not get the chance to be exposed to the power of APDL (ANSYS Parametric Design Language) so getting access to those advanced capabilities can be tough. 

In fact, I was in a room next to one of our support engineers while they were showing a customer how to change the elements that the solver would solve (Mechanical defaults to the most common formulation, but you can change them to whatever still makes sense) and the user had to admit he had never really used or even seen APDL commands before. 

So, as a way to get ANSYS Mechanical users out there started down the road of loving APDL commands, we got together and came up with a list of 20 APDL commands that every user should know.  Well, actually, it is more than 20 because we grouped some of them together.  We are not going to give too much detail on their usage, the APDL help is fantastic and it explains everything.  In fact, if you use a copy of PeDAL you can get the help right there live as you type (yes, that was a plug for you to go out and plop down $49 and buy PeDAL).

Also note that we are not getting in to how to script with APDL. It is a truly parametric command language in that you can replace most values in commands with parameters. It also has control logic, functions and other capabilities that you find in most scripting languages.  We will focus on actual commands you use to do things in the program here. If you want to learn more about how to program with APDL, you can purchase a copy of our “Introduction to the ANSYS Parametric Design Language” book. (another plug)

Some APDL Basics

APDL was developed back in the day of punch cards.  It was much easier to use than the other programs out there because the commands you entered didn’t have to be formatted in columns.  Instead arguments for commands are separated by commas.  Therefore, instead of defining a Node in your model as:

345   12.456    17.4567   0.0034 

(note that the location of that decimal point is critical). You create a line as:

N,345,12.456,17.4567, 0.0034

Trust me, that was a big deal. But what you need to know now is that all APDL commands start with a keyword and are followed by arguments. The arguments are explained in the Command Reference in the help.  So the entry for creating a node looks like this:

image

The documentation is very consistent and you will quickly get the hang of how to get what you need out of it.  The layout is explained in the help:  // Command Reference // 3. Command Dictionary

Another key thing to know about commands in MAPDL is that most entities you create (not loads and boundary conditions) have an ID number. You refer to entities by their ID number.  This is a key concept that gets lost if you grew up using GUI’s.  So if you want to make a coordinate system and use it, you define an ID for it and then refer to that ID. Same thing goes for element definitions (Element Types), material properties, etc…  Remember this, it hangs up a lot of newer users.

To use MAPDL commands you simply enter each command on a line in a command object that you place in your model tree. We did a seminar on this very subject about two years ago that you can watch here.

The idea of entity selection is fundamental to APDL.  Above we point out that all entities have an ID.  You can interact with each entity by specifying its ID.  But when you have a lot of them, like nodes and elements, it would be a pain.  So APDL deals with this by letting you select entities of a given type and making them “selected” or “unselected”  Then when you execute commands, instead of specifying an ID, you can specify “ALL” and all of the selected entities are used for that command.  Sometimes we refer to entities as being selected, and sometimes we refer to them as “active.”  The basic concept is that any entity in ANSYS Mechanical APDL can be one of two states: active/selected or inactive/unselected.  inactive/unselected entities are not used by whatever command you might be executing.

If you want to see all of the APDL command that ANSYS Mechanical writes out, simply select the setup branch of your model tree and choose Tools->Write Input File.  You can view it in a text editor, or even better, in PeDAL.

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One last important note before we go through our list of commands: the old GUI for MAPDL can be used to modify or create models as well as ANSYS Mechanical. Every action you take in the old GUI is converted into a command and stored in the jobname.log file.  Many users will carry out the actions they want in an interactive session, then save the commands they need from the log file.

Wait, one more thing:  Right now you need these commands. But at every release more and more of the solver is exposed in ANSYS Mechanical FUI and we end up using less and less APDL scripts.  So before you write a script, make sure that ANSYS Mechanical can’t already do what you want.

The Commands

1. !

An exclamation point is a comment in APDL. Any characters to the right of one are ignored by the program. Use them often and add great comments to help you and others remember what the heck you were trying to do.

2. /PREP7 – /SOLU – /POST1 – FINISH

The MAPDL program consists of a collection of 10 processors (there were more, but they have been undocumented.) Commands only work in some processors, and most only in one.  If you enter in a preprocessor command when you are in the postprocessor, you will get an error.

When you create a command object in your ANSYS Mechanical model, it will be executed in either the Pre processor, the Solution processor, or in the Post processor.  Depending on where in the model tree you insert the command object.   If you need to go into another processor you can, you simply issue the proper command to change processors.  JUST REMEMBER TO GO BACK TO THE PROCESSOR YOU STARTED IN when you are done with your commands.

/PREP7 – goes to the pre processor. Use this to change elements, create things, or modify your mesh in any way.

/SOLU – goes to the solution processor.  Most of the time you will start there so you most often will use this command if you went into /PREP7 and need to get back. Modify loads, boundary conditions, and solver settings in this processor.

/POST1 – goes to the post processor. This is where you can play with your results, make your own plots, and do some very sophisticated post-processing.

FINISH – goes to the begin level. You will need to go there if you are going to play with file names.

3. TYPE – MAT – REAL – SECNUM

You only really need to know these commands if you will be making your own elements… but one of those things everyone should know because the assignment of element attributes is fundamental to the way APDL works…. so read on even if you don’t need to make your own elements.

Every element in your model is assigned properties that define the element.  When you define an element, instead of specifying all of its properties for each element, you create definitions and give them numbers, then assign the number to each element.  The simplest example are material properties. You define a set of material properties, give it a number, then assign that number to all the elements in your model that you want to solve with those properties.

But you do not specify the ID’s when you create the elements, that would be a pain. Instead, you make the ID for each property type “active” and every element you create will be assigned the active ID’s. 

The commands are self explanatory: Type sets the Element Type, MAT sets the material ID, REAL set the real constant number, and SECNUM sets the active section number. 

So, if  you do the following:

type,4
real,2
mat,34
secnum,112
e,1,2,3,4,11,12,13,14

you get:

     ELEM MAT TYP REL ESY SEC        NODES
      1  34   4   2   0 112      1     2     3     4    11    12    13    14
      2   3   4   4   0 200    101   102   103   104   111   112   113   114

4. ET

The MAPDL solver supports hundreds of elements.   ANSYS Mechanical picks the best element for whatever simulation you want to do from a general sense.  But that may not be the best for your model. In such cases, you can redefine the element definition that ANSYS Mechanical used.

Note: The new element must have the same topology. You can’t change a 4 noded shell into an 8 noded hex.  But if the node ordering is the same (the topology) then you can make that change using the ET command. 

5. EMODIF

If you define a real constant, element type, or material ID in APDL and you want to change a bunch of elements to those new ID’s, you use EMODIF.  This is the fastest way to change an elements definition.

6. MP – MPDATA – MPTEMP –TB – TBDATA – TBTEMP

Probably the most commonly needed APDL command for ANSYS Mechanical users are the  basic material property commands. Linear properties are defined with MP command for a polynomial vs. temperature or MPDATA and MPTEMP for a piece-wise linear temperature response.  Nonlinear material properties are defined with the TB, TBDATA, and TBTEMP commands.

It is always a good idea to stick your material definitions in a text file so you 1) have a record of what you used, and 2) can reuse the material model on other simulation jobs.

7. R – RMODIF

If you define an elements formulation with options on the ET command, and the material properties on the material commands, where do you specify other stuff like shell thickness, contact parameters, or hourglass stiffness?  You put them in real constants.  If you are new to the MAPDL solver the idea of Real constants is a bit hard to get used to. 

The official explanation is:

Data required for the calculation of the element matrices and load vectors, but which cannot be determined by other means, are input as real constants. Typical real constants include hourglass stiffness, contact parameters, stranded coil parameters, and plane thicknesses.

It really is a place to put stuff that has no other place.  R creates a real constant, and RMODIF can be used to change them.

8. NSEL – ESEL

As mentioned, selection logic is a huge part of how MAPDL works.  You never want to work on each object you want to view, change, load, etc… Instead you want to place entities of a given type into an “active” group and then operate on all “active” entities. (you can group them and give them names as well, see CM-CMSEL-CMDELE below to learn about components)

When accessing MAPDL from ANSYS Mechanical you are most often working with either nodes or elements.  NSEL and ESEL are used to manage what nodes and elements are active. These commands have a lot of options, so review the help.

9. NSLE – ESLN

You often select nodes and then need the elements attached to those nodes. Or you select elements and you need the nodes on those elements.  NSLE and ESLN do that.  NSLE selects all of the nodes on the currently active elements and ESLN does the opposite.

10. ALLSEL

A very common mistake for people writing little scripts in APDL for ANSYS Mechanical is they use selection logic to select things that they want to operate on, and then they don’t remember to reselect all the nodes and elements.  If you issue an NSEL and get say the nodes on the top of your part that you want to apply a load to. If you just stop there the solver will generate errors because those will be the only active nodes in the model. 

ALLSEL fixes this. It simply makes everything active. It is a good idea to just stick it at the end of your scripts if you do any selecting.

11. CM – CMSEL

If you use ANSYS Mechanical you should be very familiar with the concept of Named Selections. These are groups of entities (nodes, elements, surfaces, edges, vertices) that you have put into a group so you can scope based on them rather than selecting each time. In ANSYS MAPDL these are called components and commands that work with them start with CM.

Any named selection you create for geometry in ANSYS Mechanical gets turned into a nodal component – all of the nodes that touch the geometry in the Named Selection get thrown into the component. You can also create your own node or element Named Selections and those also get created as components in MAPDL. 

You can use CM to create your own components in your APDL scripts.  You give it a name and operate away.  You can also select components with the CMSEL command.

12. *GET

This is the single most awesomely useful command in APDL.  It is a way to interrogate your model to find out all sorts of useful information: number of nodes, largest Z value for node position, if a node is selected, loads on a node, result information, etc… 

Check out the help on the command. If you ever find yourself writing a script and going “if I only knew blah de blah blah about my model…” then you probably need to use *get.

13. CSYS – LOCAL – RSYS

Coordinate systems are very important in ANSYS Mechanical and ANSYS MAPDL.  In most cases you should create any coordinate systems you need in ANSYS Mechanical. They will be available to you in ANSYS MAPDL, but by default ANSYS Mechanical assigns a default ID. To use a coordinate system in MAPDL you should specify the coordinate system number in the details for a given coordinate system by changing “Coordinate System” from “Program Defined” to “Manual” and then specifying a number for “Coordinate System ID”

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If you need to make a coordinate system in your APDL script, use the LOCAL command. 

When you want to use a coordinate system, use CSYS to make a given coordinate system active.

Note: Coordinate system 0 is the global Cartesian system. If you change the active coordinate system make sure you set it back to the global system with CSYS,0

RSYS is like CSYS but for results. If you want to plot or list result information in a coordinate system other than the global Cartesian, use RSYS to make the coordinate system you want active.

14: NROTATE

One thing to be very aware of is that each node in a model has a rotation associated with it. By default, the UX, UY, and UZ degrees of freedom are oriented with the global Cartesian coordinate system. In ANSYS Mechanical, when you specify a load or a boundary condition as normal or tangent to a surface, the program actually rotates all of those nodes so a degree of freedom is normal to that surface.

If you need to do that yourself because you want to apply a load or boundary condition in a certain direction besides the global Cartesian, use NROTATE.  You basically select the nodes you want to rotate, specify the active coordinate system with CSYS, then issue NROTATE,ALL to rotate them.

Be careful though. You don’t want to screw with any rotations that ANSYS Mechanical specified.

15. D

The most common boundary condition is displacement, even for temperature.  To specify those in an ANSYS MAPDL script, use the D command.  Most people use nodal selection or components to apply displacements to multiple nodes.

In its simplest form you apply a single value for displacement to one node in one degree of freedom.  But you can specify multiple nodes, multiple degrees of freedom, and more powerfully, the value for deflection can be a table. Learn about tables here.

16. F

The F command is the same as the D command, except it defines forces instead of displacement.  Know, it, use it.

17. SF – SFE

If you need to apply a pressure load, you use either SF to apply to nodes ore SFE to apply to elements. It works a lot like the D and F commands.

18. /OUTPUT

When the ANSYS MAPDL solver is solving away it writes bits and pieces of information to a file called jobename.out, where jobname is the name of your solver job.  Sometimes you may want to write out specific information, say list the stresses for all the currently selected nodes, to a file. use /OUTPUT,filename to redirect output to a file. When you are done specify /OUTPUT with no options and it will go back to the standard output.

19. /SHOW

ANSYS MAPDL has some very sophisticated plotting capabilities.  There are a ton of command and options used to setup and create a plot, but the most important is /SHOW,png.  This tells ANSYS MAPDL that all plots from now on will be written in PNG format to a file. Read all about how to use this command, and how to control your plots, here.

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20. ETABLE

The ANSYS MAPDL solver solves for a lot of values. The more complex the element you are using, the more the number of values you can store.  But how do you get access to the more obscure ones? ETABLE.  Issue 38 of The Focus from 2005 goes in to some of the things you can do with ETABLE.

Where to go From Here

This is certainly not the definitive list.  Ask 20 ANSYS MAPDL users what APDL commands all ANSYS Mechanical users should know, and you might get five or six in common. But based on the support calls we get and the scripts we write, this 20 are the most common that we use.

Command help is your friend here.  Use it a lot.

The other thing you should do is open up ANSYS MAPDL interactively and play with these commands. See what happens when you execute them.

Submodeling in ANSYS Mechanical: Easy, Efficient, and Accurate

Back “in the day” when we rode horses into work as Finite Element Analysis Engineers, we had somewhat limited compute capacity.  70,000 elements was a hard and fast limit.  But we still needed accurate results with local refinement in areas of concern.  The way we accomplished that was with a  process called submodeling where you make a refined local model just of the area you care about, and a coarse mesh that modeled the whole part but still fit on the computer.  The displacement field from the coarse model was then applied as a boundary condition on the refined model.

We called the refined model a zoom model or a submodel.  It worked very well for many years. Then computers got bigger and we just started meshing the heck out of those areas of interest in the full part model.  And in many cases that is still the best solution for an accurate localized stress: localized refinement.

Submodeling is one of those “tricks” in stress analysis that used to be used all the time. But until recently it was a bit of a pain to do in ANSYS Mechanical so it fell out of use.  Now, the process of doing submodeling is easy, efficient, and accurate.  The purpose of this posting is to introduce the concept to newer users who have not used it before, and show experienced (old) users how much easier it is to do in ANSYS Mechanical vs. Mechanical APDL.

What is Submodeling?

The best description of submodeling is the illustration that has been in the ANSYS help system, in one form or another, for over 25 years:

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The basic idea is that you have a coarse model of your whole part or assembly.  You ignore small features in the mesh that don’t have an impact on the overall response of the system – the local stiffness does not have influence on the strain beyond that local region. You then make a very refined model, the submodel, of the region of interest. You use the displacement field (and temperature if you have a temperature gradient) from the coarse model and apply it to the submodel as a boundary condition to get the accurate highly-refined response in the area of interest.

The process is based on St. Venant’s principle: “… the difference between the effects of two different but statically equivalent loads becomes very small at sufficiently large distances from load.”

An aside:
What a cool name this guy had:
Adhémar Jean Claude Barré de Saint-Venant.  To top it off he was not just a mathematician, but he was given the title of Count as well… a count mathematician. And, I have to say, I have serious beard envy.  He had some very nice facial hair, I can’t even grow thick stubble.

Anyhow, what he showed was that if you are looking at the stresses in a part far away from where loads are applied, how those loads are applied does not matter. So we can replace the forces/pressures/etc… from our course model as an equivalent static deflection load and the stress field will be the same.

The way this is done in a Finite Element model is you determine what faces in your submodel are “inside” your course model. These are called the cut boundary faces and the nodes on those faces are the cut boundary nodes. and you apply the displacement field from the coarse model onto the nodes

The most common use is to add mesh refinement in an area without having to solve the whole model. Another common usage is to actually mesh small features like fillets, holes, and groves that were left out of or under-meshed in the full model.  It can also be used to capture material non-linearities if that behavior is highly localized.

But probably the most beneficial use today is to study the parametric variation of small features like the size of a fillet or a hole.  If changing the size of such features does not change the overall response of the system, then you only need to do a parametric study on the submodel – as the guy with the great beard proved, if the static load does not change with your geometric variations, you don’t have to look at the whole structure.

And don’t forget the new crack growth capabilities. You will probably want to do that on a submodel around your crack and not on your whole geometry.

Here is a more modern version of the original example geometry:

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The red highlight shows the cut boundaries. this is where you need to apply the displacement field.

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This is the nasty coarse mesh. Now if you were modeling a single part, you would just mesh the fillets and be done with it.  But assume this is in a large assembly.

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The Submodel. Nice elements in the key area.

You can even set up the radius as a parameter and do a study, where only the Submodel is modified and updated.

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The Process

The process is fairly simple:

  1. Make and solve your full model
  2. Make a geometry model of the area you want a submodel in
  3. Attach the submodel to the engineering data and solution of the full model
  4. Set up and solve the submodel

Before we get started, here is a ANSYS 14.5 archived project for both models we will discuss in this posting:  PADT-Focus-Submodeling-2013_08_14.wbpz

For the sample geometry we showed above, the system looks like this:

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When you go into ANSYS Mechanical for the sample model, you have a new model branch:

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When you first get in there, the branch is empty, you have to insert Body Temperature and/or Displacement:

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The Details for the Displacement object are as follows:

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There are a lot of options here. It is basically using the external load mapper to map the displacements. Consult the help and just play around with the options to understand them better. In most cases, all you need to do is specify the faces that you want the displacement field applied to for the Scope section.

A cool feature is that once you have specified the faces, you can “Import Load” and then view them by clicking on the object. Graphics Control –>Data = All shows vectors. Total/X/Y/Z shows the applied displacement field as a contour:

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Now you just need to make sure your Submodel is set up correctly, you have the mesh you want, and any other loads that are applied directly to the Submodel are the same as the loads in the full model (see next section).  Run and you get your refined results.

Here is that same process with a more realistic model of a beam with a tube welded on it.  The welds are not modeled in the full model and the fillets in the beam are very coarse.

So here is the geometry. Imagine that these two parts are actually part of a very large assembly so we really can’t refine them the way we want.

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This is what the systems look like. Note that the geometry comes from one source. I made the submodel in the same solid model in DesignModeler and just suppress the parts I don’t want in each Mechanical model.

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The loading is simple. I fix one end and put a force on the top of the tube.

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And here is my coarse mesh. I could probably mesh the tube with a lot more elements, especially along the axis.

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The results. Not too useful from a stress standpoint. Deflections are good, but the fillet is missing and beam is too coarse.

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So here is the submodel.  All the fillets are in there and it is just the area around the connection.

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I used advanced meshing to get a really nice refined mesh. It only solves in about 20 seconds so I can really refine it.

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Here are the cut boundaries. The bottom of the beam ribs are also selected.

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And here is the result. A really accurate look at the stresses in the fillet.  I could even put a probe in there and do some nice fatigue or crack growth.

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The other thing that showed up were some stress problems on the bottom of the beam.  Those could be an issue under a high load. The fillet stress on top my yield out but these stresses under the beam could be a fatigue problem.

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Tips and Hints

In most cases, doing a sub model is pretty simple. But there is a lot more to it than what we covered here.  Because I need to get back to some very pressing HR tasks, I’ll just list them here so you know that you are aware of them:

  1. Label your systems in the project page with some sort of “full” and “sub” terminology Things get really confusing fast if you don’t.
  2. You can do submodeling with a transient or multiple substep model. In your Imported Displacement/Body Temperature, specify what load step to grab the loads from.
  3. Don’t forget temperature. One of the most common problems is when a user applies temperature and therefore gets thermal stress.  They then forget to apply that to their submodel and everything is wrong.
  4. Make sure you don’t change material properties. Remember, these models are statically identical, you are just looking at a chunk with greater refinement.
  5. Remember that loads need to be away from the area you are zooming in on.  Don’t cut where a load is applied, or even near where one is applied. The exception is temperature. (Sometimes you can get away with pressure loads too, but you have to be very careful to get the same load over the area)
  6. Your can’t have geometry in the submodel sticking too far out of the coarse mesh. The displacement is interpolated onto the fine mesh and if a node on the fine mesh is outside the coarse mesh, the program extrapolates and that can sometimes induce errors. If you see spotty or high stresses on your cut boundaries, that is why.  There are tools in the Submodeling details to help diagnose and fix that.
  7. If you are going to do a parametric study on geometry changes in the submodel, use a separate geometry file to create that model (I just duplicate the original and suppress the full geometry in DM).  Why? Because if you change a parameter in your geometry model, both models will need to resolve since they both use the same geometry file, even if the geometry change occurs on a part that is suppressed in the full model.
  8. You can do submodels of submodels as many levels down as you want.
  9. You can have multiple submodels in one system
  10. Read the help, it is fairly detailed

That is about all for now. As always: crawl, walk, run.  Start with a very simple sub model with obvious cut boundaries and get experienced.