[Gmsh] suggested new meshing algorithm in Gmsh: hex gridding

[Billy Araujo]

Hi,

I don't think you should underestimate the problem. Creating a good
and conforming hex mesh is difficult. I can help you with the writing
it in C or C++ if you want.

For cartesian approach you can find a very simple hex mesher I wrote:

http://www.cfd-online.com/Forums/openfoam-meshing-opensource/61973-generating-unstructured-grids-stl-files.html


Regards,

Billy.




On Wed, Jul 29, 2009 at 4:18 PM, John Blackburn<John.Blackburn at npl.co.uk> wrote:
> Dear Sir,
>
> I have recently been trying out Gmsh and am most impressed with its
> capabilities. I am writing an FE code of my own which uses hexahedrons. I've
> noticed that, while Gmsh does generate hex's this is quite a painful process
> and involves splitting the geometry into many more parts than would be the
> case for tets. Hex meshes can only be generated for transfinite volumes or
> ones which can be made from extrusions. I would like to suggest another
> method to generate (structured) hex meshes which is to have a "logical
> mesh", which is just a cubic grid of nodes, and then "snap" the nodes onto
> nearby interface points. You then end up with a mesh with the same topology
> as the original but distored to fit the required shapes.
>
> I would like to add this functionality to Gmsh as I think it would greatly
> improve the program. However, I am mainly a Fortran programmer. I can code
> in C with some difficulty but full OO C++ is beyond me! I was wondering, if
> I send you a detailed write up of the algorithm I have in mind (with all the
> maths/geometry written out explicitly), could you code it in for me? I
> suspect this simple algorithm would be easy to implement given Gmsh's
> existing capablities. Basically two things are needed:
>
> * a function to decide whether an arbitrary point (x,y,z) is inside a given
> volume (defined, as usual by plane or ruled surfaces themselves defeined by
> curves: just the way Gmsh works now). I'm sure Gmsh already has a function
> like this?
>
> * a funciton to "snap" a point to the nearest point on the volume surface.
> Note that only points at the interface between two regions are candidates
> for snapping.
>
> I've written a little program to implement this algorithm as I see (in 2-D
> for the moment but 3-D would not be much more difficult). The program
> outputs a .MSH file which I've plotted using Gmsh and attached. Notice how
> the algorithm involves INSERTING shapes into each other: the orange square
> was defined first, then the green circle, blue circle, and finally, yellow
> polygon. This is very different from Gmsh's current approach but its a very
> powerful technique.
>
> I also intend to write a final "relaxation" part to the meshing so the
> unecessarily distorted nodes in the blue circle would return to their
> original positions.
>
> What do you think about incorporating this functionality? Basically, the
> user would do everything he normally does to prepare the GEO file. Then
> there would be an extra button on the Mesh menu, "Grid" for example.
>
> Best Regards,
>
> John Blackburn
>
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