[Gmsh] I need help

[Geordie McBain]

2011/8/13 Alfred Gimpel <agimpel at gmail.com>:
> Dear sirs,
>
> I need help about numbering local node, because in a region the triangulares
> elements are numbering in clockwise and in another region the numbering is
> in the counter clockwise direction. I need that the numbering in the two
> regions have to do a counterclockwise direction. I attached the file of
> geometry and the mesh with my problem.
> Thank you.
>
> Regards,
>  Alfred Gimpel

I think the problem here arises because Gmsh always thinks in
three-dimensions and there's no such thing as "clockwise" in space,
only in the plane can one unambiguously assign signs to loops.  I have
encountered this before on merging plane-figures from STEP files, in
which case the only recourse I found was to redefine the geometry
upstream so that the shape did have a counterclockwise boundary loop
and then resave it as STEP.

Here the remedy is easier as the geometry is defined in Gmsh.  I note
that eight Circle arcs are all clockwise, so Line Loop (10) should
reverse each of its four arcs; i.e., change "Line Loop(10) = {6, 5, 8
, 7};" to "Line Loop(10) = {-6, -5, -8 ,-7};", as in the attached
modified input file.  The annular Line Loop (12) looks fine though;
the external boundary is counterclockwise and the internal clockwise.

I haven't tested the output, as I'm assuming the difficulty doesn't
show up until you use the mesh for something, but I hope this should
help.
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