[Gmsh] PostView format and background mesh

[J S]

On 07/30/2013 01:10 AM, Geordie McBain wrote:
> 2013/7/30 J S <j.s4403 at gmail.com>:
>>>> The missing pieces are:
>>>> 4. write out a characteristic length in a format understood by gmsh.
>>> Write out the characteristic length in the .msh format.  Then let
>>> "gmsh -0" convert that to .pos, as described.
What is the format?  It is not documented as far as a I can tell?


>> How do you specify the characteristic length in .msh format?  I don't see it
>> in the reference manual?
> The format is described at
> http://geuz.org/gmsh/doc/texinfo/gmsh.html#MSH-ASCII-file-format.  The
> characteristic length can be output as either NodeData or ElementData,
> depending on how you calculate it (which of course will be very much
> dependent on your application).
>     As a simple example, you might decide that the magnitude of the
> gradient of the solution of your partial differential equation on the
> first mesh might be a useful indicator.  This will be P0 if your
> solution is P1, so you'd output it as ElementData, or rather its
> reciprocal, since you want something that's smaller where the gradient
> is steeper to get smaller elements there.
>    A more sophisticated tool is Pascal Frey's mshmet
> <http://www.ann.jussieu.fr/~frey/software.html>.  There's an interface
> to this in FreeFem++.  It computes an indicator field on nodes, so
> that should be written as NodeData.
>
>> bgmesh.pos has 3 numbers after the 9 numbers corresponding to triangle
>> coordinates.  Which one is the characteristic length?
> I don't know.  I haven't investigated the .pos format at all.  I just
> let "gmsh -0" do its job and it does appear to.
>
> If I had to guess, I'd say that they might be the values at the
> vertices, but really I don't know.
>
>>   How do I specify this in 1d and 3d?
> The "gmsh -0" conversion certainly works in three dimensions.  I
> haven't tried one dimension, but I would think it would work there
> too.