Cyclic symmetry

[Anonymous]

Hi,

is it possible to analyze only a slice of an axisymmetric solid.

Since it is axisymmetric could I analyze only a a slice of 30° or 15° (instead of a quarter)?

thx

luca

[Andrew.Sartorelli · Accepted solution]

Hi Luca,

We do not currently have the ability to take advantage of cyclic symmetry in Nastran In-CAD. Cyclic symmetry typically requires a mesh that matches on the symmetry surfaces which is not something we can currently generate with our meshing inside of Nastran In-CAD. We have a similar limitation in Simulation Mechanical, the mesh would need to be generated in another utility or by hand.

Regards,
Andrew

[John_Holtz]

Hi everyone,

Just one small correction to Andrew's post. Simulation Mechanical can use cyclic symmetry with a different mesh on each "matching" face. It is no longer a requirement to have a matched mesh. (The Simulation Mechanical documentation will be updated to reflect this capability.)

[Anonymous]

Hi, will the cyclic symmetry be possible in the future? I think this should be a point for the Autodesk Ideas!

Best regards

Stefan

[Andrew.Sartorelli]

Hi,

One of my colleagues actually just gave me a solution to this the other day. You could acheive cyclic symmetry by defining a cylindrical coordinate system for part or assembly and then using a theta constraint. This should acheive the same affect as a pure cyclic symmetry constraint.

Regards,

Andrew

[Anonymous]

Hi Andrew,

i used a cylindrical coordinate system in my "wedge" model previously. I´m just wondering about the mentioned mesh efforts to bring the grids on the same positions in the mirrore plane.

Best regards

Stefan

[johnster100]

Hi,

could you attach a screen shot of how to set this up?

thanks,

John

[John_Holtz]

Hi,

There may be some confusion here between axisymmetric (the question in Luca's original post) and cyclic symmetry (the title of this thread). In both cases, you are generally dealing with something that has a centerline or axis of symmetry, but let me try to clarify the differences:

• Axisymmetric means that any cross-section through the model is the same: the geometry is the same, and the results are the same. If you use the 2-D cross-section and revolve it around the axis, you would create the entire model. A hydraulic cylinder under an axial load is an example of an axisymmetric model. (A hydraulic cylinder with a side load on the rod is not an axisymmetric analysis because the results are not the same through all cross-sections.) (edit) More importantly, if you use a section of a 3D model, the nodes on the symmetry plane cannot displace off of the symmetry plane. The nodes can only displace on the symmetry plane.
• Cyclic symmetry means that the model can be split into N number of sectors. If you copy and revolve the one 3-D sector N-1 times, you have the complete model. The results on the one "symmetry" plane are the same as on the other "symmetry" plane, but the results at any other location in the sector can be different. Note that the geometry is not necessarily axisymmetric. A fan blade is an example of a cyclic symmetry model. If all of the blades are the same geometry, and if all of the blades deform the same way, you only need to model one blade and define cyclic symmetry conditions on the two planes. (edit) More importantly, the nodes on the symmetry plane can displace off of the "symmetry" plane. What makes it cyclic is that the corresponding node on the opposite plane moves the same distance in the radial, tangential, and axial direction. See the attached image.

In-CAD can perform an axisymmetric analysis, but you need to use a solid to perform such an analysis. This article describes how to do it: How to represent 2D axisymmetric elements in Nastran In-CAD

In-CAD cannot perform a cyclic symmetry analysis (like the fan blade).

Any questions?

[johnster100]

thanks for your reply.

I've attached an example of my geometry. The model does have cyclic symmetry but cannot be split by a single plane as you can see.

Can this be analysed or do I need to model the full geometry?

thanks,

John

[John_Holtz]

Hi John,

Your image made me realize that I missed a critical point in my description of axisymmetric versus cyclic symmetry. (I updated my original reply to include this information, too)

In axisymmetric, or regular symmetry, the nodes cannot displace off of the symmetry plane. They can only displace in the symmetry plane.

Cyclic "symmetry" is misleading in that the displacements are not symmetric (mirror-image) about the symmetry plane. The nodes can displace off of the "symmetry" plane, but the corresponding node on the other "plane" displaces in the same direction, the same magnitude. Some other terminology is probably more accurate for this than "cyclic symmetry". Maybe "periodic" meaning that it repeats.

So in your case, are the displacements mirrored about the symmetry planes? If so, you can use symmetry constraints and perform the analysis in In-CAD. If they are periodic ("cyclic symmetry"), then you cannot perform the analysis in In-CAD.

[johnster100]

thanks for that.

I also hadn't realised you could choose the cylindrical coordinate system as shown here:

https://knowledge.autodesk.com/support/nastran-in-cad/learn-explore/caas/CloudHelp/cloudhelp/2016/EN...

thanks again,

John

[Anonymous]

@John_Holtz, @Andrew.Sartorelli

Is the cyclic symmetry possible in latest Nastran In-Cad ? I am asking becasue i need to analyse only a 5mm thick piece of the entire model.

Regards,
Bartosz

[shigeaki.k]

Hello @Anonymous,

you can define a cylindrical coordinate system and use that to apply symmetry as per the post above. If you are asking if Nastran In-CAD has axisymetric 2D element, it does not at the moment.

Regards,

Shigeaki K.

[viktor_velichko]

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If they are periodic ("cyclic symmetry"), then you cannot perform the analysis in In-CAD.

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Hello everyone! What about ver. 2020? Has anything changed? I found command CYSYMGEN, CYSYMTOL, but I suspect they are not suitable for this analysis. MSC and NX Nastran have command CYSYM and another for this analyses. Is there anything similar in Autodesk?

[John_Holtz]

Hi @viktor_velichko 

There is nothing new in versions 2020 or 2021 regarding cyclic symmetry.

The CYSYMGEN and CYSYMTOL parameters are only applicable to models where the nodes on the two "symmetry" planes are matched. This can only be done in a hand-built model, so it does not apply to CAD models that are meshed in Inventor Nastran. 

[viktor_velichko]

Thank you, John!
Hypothetically, If i used Femap with Autodesk Nastran, could i do it?

[johnster100]

Hi John,

will this feature be added any time soon?

It would be very useful for the pump / fan industries.

It would also be very useful if Autodesk were to publish a roadmap of features they intend to add and rough dates of when they should be available.

thanks,

John

[John_Holtz]

My understanding is we (meaning publicly traded companies) are prohibited by government regulations from publishing roadmaps.