[dune-pdelab] artifact in DiscreteGridfunctionGradient in parallel run

[Jö Fahlke]

Am Mo,  5. Aug 2019, 10:39:11 +0200 schrieb Michael Wenske:
> It still leaves me a bit worried for other cases though. Let's say somebody
> wants to evaluate the gradient-convergence of a new method, then
> this behavior could result in a false-negative result. I have not
> checked the situation
> when there are even more ranks. If I remember correctly, starting from 8
> ranks
> or so, the domain decomposition also divides in the y-direction, which
> might hold
> more interesting behavior at the crossing points.

The funny thing is: the the cells with the gradients that looked strange were
the only cells where the gradient was actually correct.  The bulk of the
gradient was wrong.

> The only consistent remedies I can think of would be to do an average
> of the available gradients at each vertex, which would lead to smoothing in
> high curvature regions. The second option would be to limit writing the
> gradient
> to nonconforming mode in some way, to not hide the inherent ambiguity.

I think we actually had that issue before, but could not immediately come up
with clean way to warn about it, and over time forgot about it.  And back then
we mostly used VTK output for visualization anyway, not for further data
processsing.  This is an unfortunate situation, but alas, I don't think I have
the time left to do anything about it.

Regards,
Jö.

-- 
Jorrit (Jö) Fahlke, Institute for Computational und Applied Mathematics,
University of Münster, Orleans-Ring 10, D-48149 Münster
Tel: +49 251 83 35146 Fax: +49 251 83 32729

Of all the things I've lost, I miss my mind the most.
-- Ozzy Osbourne
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