Getting global DoF indices of a face

[Alexander Kiselyov]

Dear deal.II users,

I'm currently developing a way to uniquely identify a boundary face on
a mesh. My idea has been to gather global DoF indices on a face and
hash them together. However, `get_dof_indices` method of a face
accessor doesn't return any useful information, and
`FESystem::max_dofs_per_face()`, surprisingly, returns 0. See the
attachment for a minimal example. The problem persists when running on
a single process. The version of deal.II used is 9.4.0.

What am I doing wrong here? Is there a better way to globally identify
a face?

Best regards,
Alexander Kiselyov

[Peter Munch]

Hi Alexander,

You are using FE_DGQ, which per definition only has DoFs in the cell. Switch to quadratic FE_Q and then your idea with the dummy DoFHandler to globally uniquely identify faces should work! Not sure why you create a system!?

Peter

[Alexander Kiselyov]

Dear Peter,

Thank you for the reply!

The thing is that the input data is a FE_DGQ DoF vector, and I would rather not project it into FE_Q elements. The original plan was not to create a separate dummy DoFHandler, but your idea seems like a solution for my issue.

Still, I'm a bit confused about why FE_DGQ doesn't contain face DoFs. Is the reason that in this case DoF indices would be different when looking on either sides of a face?

Regards,

Alexander

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[Daniel Arndt]

Alexander,

Yes, we are identifying DoFs on faces and thus FE_DGQ doesn't have any DoFs on faces.

Best,

Daniel

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[Alexander Kiselyov]

Daniel,

Thanks a lot for your reply, it answers my question fully!

Regards,

Alexander