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NGSolve / ngsolve

Netgen/NGSolve is a high performance multiphysics finite element software. It is widely used to analyze models from solid mechanics, fluid dynamics and electromagnetics. Due to its flexible Python interface new physical equations and solution algorithms can be implemented easily.
https://ngsolve.org/
GNU Lesser General Public License v2.1
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Integrals over named internal faces behaves unexpectedly for DG #63

Closed blakseth closed 8 months ago

blakseth commented 1 year ago

For the purpose of solving a heat transfer problem with thermal contact resistance, I wish to include an integral over a named internal interface in my bilinear form. For standard discontinuous Galerkin, this produces unexpected results (cf. gists [1] and [2]).

In [1], I attempt to compute the lifting of an arbitrary CoefficientFunction that lives on an internal face. The resulting lifting function is non-zero only on neighboring elements on one side of the face (cf. [3]). I expected it to be non-zero on both sides. Moreover, the

In [2], I have copied the code from tutorial 2.8 [4] and applied it to a domain with a named internal interface. The definition of the domain is the only modification I have made to the code (apart from renaming some variables to distinguish DG and hybrid DG, and removing the wildcard import). I have not made any specifications with regards to the named interface, so the facets it contains should be treated as all other facets. For hybrid DG, this is what I observe, and I get the same output as when using the geom2d.unit_square as my domain. However, for DG, the results are not the same as for the geom2d.unit_square.

PS: I saw that you recommended HDG over DG on the forum recently. Since HDG appears to be working, I will try to make progress on my problem using that. In any case, I thought it would be nice if someone could look into the identified DG issue.

[1] https://gist.github.com/blakseth/4f6313b9d9e200801d29f4bf3bd3600e [2] https://gist.github.com/blakseth/460bac09d60b064c6fa1938f5f63c0f5 [3] https://github.com/blakseth/images/blob/main/MWE%20Lifting%20output.PNG [4] https://docu.ngsolve.org/latest/i-tutorials/unit-2.8-DG/DG.html

Best, Sindre

mhochsteger commented 8 months ago

Such specific questions are better suited for the forum: https://forum.ngsolve.org

blakseth commented 8 months ago

It was not a question though. Just an observation about something that appeared to be a bug. Do you want bug reports on the forum instead of here?