[Gmsh] Gmsh ASCII post-processing file format

[Axel Gerstenberger (TU Dresden)]

Dear Professor Geuzaine:

I am a graduate student from TU Dresden, Germany.

So far, I successfully used Gmsh for 2D and 3D mesh generation and
post-processing and I was able to create movies. So far I used only the
parsed post-processing format.

Now, I want to solve a problem including adaptive meshing by element
subdivision and encountered several problems with the post-processing format
for which I seek your help. I checked your mailing list, but couldn't find a
similar question and answer for the following problems:

1:
  I think, the ASCII post-processing file format (and, eventually, its
binary variant) is a good choice, since I am want to process transient data
and going to use a large number of elements later on. However, I was not
able to create the ASCII post-processing file format. The Gmsh manual
explains the format for a triangle with vector data.
  I can produce the scalar output for 1 triangle, however if I change to 2
or more triangles, all elements appear distorted. I would appreciate very
much, if you could take a look at the attached ".pos" file and suggest, what
I do wrong. I also don't understand, how the connectivity for the elements
is stored.

2:
  Can I use the provided possibility to save multiple time steps in one .pos
file as suggested in the manual, if the number of nodes and the connectivity
changes in time (I am using element subdivision and incompatible elements
(-> hanging nodes) as shown in the attachment)? Since I could not master the
output format yet, I could not answer this question yet. My current idea is
to store each time step in a different file and therefore I am allowed to
use independent data and output settings for each ".pos" file.

3:
  Since my mesh is incompatible, I see "jumps" or discontinuities at the
element edges. This seems to be logical, since the linear interpolation in a
3 node triangle  can't "see" the neighboring mid-point (see attachment).
However, my nodal values represent the correct value. I could display the
scalar field as a number of points without elements, but then I can't see a
smooth solution as in standard FEM meshes.
One partly satisfying solution is to show iso-values and set the number of
lines to 255. This smoothes the field a little, but does not always look
good.

Another way would be to create a new mesh using all my nodes just for the
post-processing (e.g. Delauney Triangulation), but I wonder if I can solve
the third problem within Gmsh.


I would be very happy if you find time to answer my questions, especially
the first one.

Best wishes,

Axel Gerstenberger




My .pos file look like that:

$PostFormat
1.2 0 8
$EndPostFormat
$View
Test 1
0 0 0
0 0 0
           2           0           0
0 0 0
0 0 0
0 0 0
0 0 0
0 0 0
0 0 0
  0.100000000000000
   2.70000000000000        2.75000000000000        2.75000000000000
   1.00000000000000        1.00000000000000        1.00000000000000
  0.000000000000000E+000  4.000000000000000E-002  0.000000000000000E+000
  0.000000000000000E+000
  0.000000000000000E+000
  0.000000000000000E+000
  0.200000000000000       0.150000000000000       0.150000000000000
   1.00000000000000        1.00000000000000        1.00000000000000
  4.000000000000000E-002  0.000000000000000E+000  4.000000000000000E-002
  0.000000000000000E+000
  0.000000000000000E+000
  0.000000000000000E+000
$EndView
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