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

[John Blackburn]

Thanks, I tried your program out and it works. All you need now is to snap the elements at the edges to the original surface. However, I noticed that the STL file you provided cannot be read by Gmsh, although Gmsh does claim to read such files (actually, there is very little documentation in Gmsh about input/output files other than .geo and .msh). Therefore the STL file format may not be very standard. What package did you use to create it? How difficult would it be to adapt your code to take a Gmsh .geo file as input rather than the .stl?

John.

-----Original Message-----
From: Billy Araujo [mailto:billyaraujo at gmail.com] 
Sent: 30 July 2009 09:03
To: John Blackburn
Cc: gmsh at geuz.org
Subject: Re: [Gmsh] suggested new meshing algorithm in Gmsh: hex gridding

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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