Remove constraints at some DoF

[yy.wayne]

Hello my friends,

I'm working on problems with singularity fields. Is it possible to remove a constraint line/entry at specific DoF? Say at the edge of a corner in 3d, the two faces intersect at the corner are already interpolate with zero boundary condition. However I want to un-constraint the DoF exactly at the corner rather on faces (for FE_Q elements). Is there a function to remove constraint on already constrained DoFs?

[yy.wayne]

Another question relates to singular field that, do we support mixed finite elements (with cg elements and dg elements at different location)? 

I only see mixed order formulation but not  mixed element type in tutorials.

[Peter Munch]

I don't see a remove function in AffineConstraints. What you could do is to create a new AffineConstrains instance based on the old one by ignoring the unwanted entry. See also: https://github.com/dealii/dealii/blob/a654d1fe45cc11c9dbbbcaaef6f53320f459baa2/include/deal.II/lac/affine_constraints.templates.h#L526-L541

> I only see mixed order formulation but not  mixed element type in tutorials.

Maybe you could take a look at the `tests/` folders. Maybe there is an example!?

Hope this helps,
Peter

[yy.wayne]

Thansk Peter.  Appreciate your feedback.

Best,

Wayne

[Wolfgang Bangerth]

On 3/17/23 21:09, 'yy.wayne' via deal.II User Group wrote:
> Another question relates to singular field that, do we support mixed finite
> elements (with cg elements and dg elements at different location)?
> I only see mixed order formulation but not  mixed element type in tutorials.

The canonical reference for using different elements in different parts of the
domain is step-46.

Best
W.

--
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Wolfgang Bangerth email: bang...@colostate.edu
www: http://www.math.colostate.edu/~bangerth/

[yy.wayne]

That's a helpful tutorial. Thank you.

Best,

Wayne