Improved documentation of Mesh::MakePeriodic()

[najlkin]

This PR reacts on #4299 and adds a note about usage of a nodal function to Mesh::MakePeriodic().

PR	Author	Editor	Reviewers	Assignment	Approval	Merge
#4302	@najlkin	@pazner	@mlstowell + @pazner	5/16/24	5/17/24	5/22/24

PR Checklist

• Code builds.
• Code passes make style.
• Update CHANGELOG:
  • Is this a new feature users need to be aware of? New or updated example or miniapp?
  • Does it make sense to create a new section in the CHANGELOG to group with other related features?
• Update INSTALL:
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• Update .gitignore:
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• New miniapps:
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• Update this checklist, if the new pull request affects it.
• Run make unittest to make sure all unit tests pass.
• Run the tests in tests/scripts.
• (LLNL only) After merging:
  • Update internal tests to include the new features.

[Tomcat4891]

Guess it's better than nothing.

I would also add some notes to the documentation on the web page
https://mfem.org/howto/periodic-boundaries/

[najlkin]

That would be nice as well 😉 , but we need a separate PR for that in the corresponding repo...

[najlkin]

Ok, the note in the HowTo is done in mfem/web#264 😉.

[pazner]

This PR is now under review (see the table in the PR description). To help with the review process, please do not force push to the branch.

[pazner]

Looks good, thanks @najlkin!

[mlstowell]

Your addition looks good. However, I would take this opportunity to change the word "connected" to "separated". I think this would be more accurate. Technically the two sides of the mesh don't need to be connected at all.

[najlkin]

Your addition looks good. However, I would take this opportunity to change the word "connected" to "separated". I think this would be more accurate. Technically the two sides of the mesh don't need to be connected at all.

I think what is meant is that the periodic mesh has such connectivity (or "edges") that the end node is connected to the first node (along the direction of periodicity), so there cannot be two edges between the same nodes. Therefore, there must be at least three nodes and two edges going forward and one looping backward. The doc string does not really help much to understand this, but I do not know how to formulate it in a concise and clear way and replacing "connected" by "separated" makes it maybe even more unclear in my head 🤔 Maybe how about saying that "periodic boundaries must be separated by at least two interior nodes"?

[najlkin]

Is it ok like this, @mlstowell ? 🙂 (I just used "vertices" instead of "nodes" to make it less ambiguous)

[mlstowell]

Yes, @najlkin, that sounds better. Thank you for making that change.

[pazner]

Merged in next for testing...