On 05/11/2018 08:45 PM, 'Maxi Miller' via deal.II User Group wrote:
Now I would expect that A_x and A_y will look exactly the same, after I use
the same equation. But according to my viewer (ParaView/Visit) A_x is
different compared to A_y (and the same goes for B and C). Partially I get
wild fluctuations in the result for A_x, while the result for A_y is smooth.
Why does that happen?
That's hard to tell from your description :-)
The code snippet looks correct to me. You should assume that what you see from
Paraview is correct. That means that the problem must have happened *before*
you even get into this snippet -- i.e., that your assumption that the two
fields were computed using the same equation is not correct. Fortunately, this
is a testable assumption, and it will help you debug your program.
I don't know how exactly you solve your program, and as a consequence I can't
tell you where your problem lies. But here are a few ideas to test: If you
solve the problem with a block matrix/block vector approach, then testing
whether A_x and A_y are the same comes down to just comparing the
corresponding blocks in the vector. You can do that by looking at the first
few entries of these vectors, or by considering their norm, for example. If
they are not the same, you need to ask why. This may be because (i) you
assembled the matrix the wrong way, (ii) you assembled the right hand side
vector incorrectly, (iii) because the solver was incorrect. All of these,
again, are testable hypotheses. For example, if your equations are completely
decoupled, then for A_x and A_y to be the same, the corresponding matrix
blocks must be the same and the same holds for the right hand side blocks. If
they are not, then you need to figure out why.
Best
W.
--
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Wolfgang Bangerth email: bange...@colostate.edu
www: http://www.math.colostate.edu/~bangerth/
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