Node & Element Selection in ANSYS Mechanical: Some Good News and Some Bad News (fixed)… And Some More Good News

ansys-mechanical-selection-f1First, some good news… 

In Workbench R14.5, ANSYS introduced nodal Named Selections, and in R15.0, they have added the ability to create Named Selections of elements. So now you can make groups of nodes or elements just like you can in MAPDL.  You can use these name selections for result plots to show just specific portion of the results. ansys-mechanical-selection-f2

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

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

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

The Bad News…

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

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

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

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

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Linearized Stress – Using Nodal Locations for Path Results in Workbench Mechanical 14.5

Postprocessing results along a path has been part of the Workbench Mechanical capability for several rev’s now. We need to define a path as construction geometry on which to map the results unless we happen to have an edge in the model exactly where we want the path to be or can use an X axis intersection with our model. You have the option to ‘snap’ the path results to nodal locations, but what if you want to use nodal locations to define the path in the first place? We’ll see how to do this below.

For more information on “picking your nodes”, see the Focus blog entry written by Jeff Strain last year: http://www.padtinc.com/blog/the-focus/node-interaction-in-mechanical-part-1-picking-your-nodes

The top level process for postprocessing result along a path is:

  • Define a Path as construction geometry
  • Insert a Linearized Stress result
  • Calculate the desired results along the path using the Linearized Stress item

The key here is to define the path using existing nodes. Why do that? Sometimes it’s easier to figure out where the path should start and stop using nodal locations rather than figure out the coordinates some other way. So, let’s see how we might do that.

  • First, turn on the mesh via the “Show Mesh” button so that it’s visible for the path creation

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  • From the Model branch in Mechanical, insert Construction Geometry
  • From the new Construction Geometry branch, insert a Path

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  • Note that the Path must be totally contained by the finite element model, unlike in MAPDL.
  • If you know the starting and ending points of the path, enter them in the Start and End fields in the Details view for the Path.
  • Otherwise, click on the “Hit Point Coordinate” button:

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  • Pick the node location for the start point, click apply

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  • Pick the node location for the end point, click apply

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  • In the Solution branch, insert Linearized Stress (Normal Stress in this case); set the details:
  • Scoping method=Path
  • Select the Path just created
  • Set the Orientation and Coordinate System values as needed
  • Define Time value for results if needed

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Results are displayed graphically along the path…

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…as well is in an X-Y plot and a table

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Besides normal stresses, membrane and bending, etc. results can be accessed using these techniques. So, the next time you need to list or plot results along a path, remember that it can be done in Mechanical, and you can use nodal locations to define the starting and ending points of the path.

Visualizing Nodal Connectors in Mechanical

As I noted in my series on nodal interactions in Mechanical, ANSYS has been exposing more capabilities to interact with the underlying finite element model over the past couple of versions. Additionally, Mechanical’s visual verification capabilities have improved as well, as it is now possible to view nodal connectors created by remote forces and displacements, weak springs, and MPC contact.

To demonstrate this, I’ve modeled a ball valve as shown below.

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The model is set up with the following options and boundary conditions. (Don’t try to make real-life sense of these; I’m just demonstrating capabilities here.)

  • Weak springs are turned On under Analysis Settings.
  • The bonded contact between the handle and shaft is set to MPC behavior (the bonded contact between between the valve body and ball is kept as Program Controlled).
  • A 50 lb remote load is applied just off the end of the handle and scoped to the end face of the handle (B in the figure above).
  • A 5 degree Z-rotation is applied as a remote displacement and scoped to the front face of the valve body (C in the figure above).

Now, you won’t be able to view the “spider webs,” “bicycle spokes,” etc. generated by the nodal connections yet. The weak springs, MPCs, and beams are not created until the matrices are assembled. So, at this point you will want to solve the model.

When the solution is complete, highlight the Solution Information folder in the Model tree. You will see two tabs at the bottom of the graphics window: Graphics and Worksheet. Click on the Graphics tab.

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You will now see all the nodal connections displayed for your finite element edification, and they are glorious. Note: Constraint equations (CEs) include multi-points constraints (MPCs).

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Click the Show Mesh button for the full finite element display.

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The “clumping” of the MPCs on the front face of the valve body might look a little odd, and it is—you’re not imagining it—but it deflects the way I expect it to, so I’m good with it.

Now right about now, you’re yelling at me through your monitor and I can hear what you’re saying. “Hey, Strain, I don’t have the luxury of working with these little Mickey Mouse sample models that you create for sales demos or training courses or Focus articles! The models I make are real-life models that take hours or days to solve. Do you really expect me to wait for hours or days before I can verify that my connectors are correct?” Fret not, dear ANSYS user; there is a simple workaround to this. When the Solution Status says “Solving the mathematical model,” simply click [Stop Solution] and continue to display the connectors as described above. Maybe give it a minute or two first, though, just to make sure the matrices have been assembled and the connectors generated.

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The default is that you see everything, displayed as lines, but if you take a look at the Solution Information details, you’ll see that you have some additional display options under FE Connection Visibility.

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By default, we see All FE Connectors, but we can switch the Display option to CE Based, Beam Based, or Weak Springs. (We can also change it to None, but that would defeat the purpose of this article.) Here is the same model with Display set to Weak Springs.

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By default, the connections for all nodes are displayed, but you can isolate the display to a nodal named selection under the Draw Connections Attached To option. For example, here is the connector display for the front valve body face nodes, named “front face nodes.” (Note: I’ve turned all FE connectors back on.)

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Finally, if you want a bit more visual clarity, you can change the Display Type to Points instead of Lines.

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This is another example of direct finite element interaction being enabled in Mechanical. With this capability, the user will no longer need to export the model to Mechanical APDL for visual node connector verification. Expect even further finite element interaction capability in future versions; ANSYS is on a roll in this area.