[Gmsh] PLOT3D format
mjgundry at faa-engineers.com
Fri Feb 18 21:57:34 CET 2005
OK, I have made a few simple additions to make the exporting of surfaces
to Plot3D format work. I have checked the output in Plot3D (the
visualization app) and FAST, and it appears correct. More specifically,
the format is formatted ASCII multigrid Plot3d.
I currently have elementary surfaces and volumes working, but eventually
I'd like merged physical surfaces and volumes too.
For surfaces, I've assumed that if pS->Nu and pS->Nv are defined, then I
have a structured surface. I also check that the number of quads
corresponds to Nu and Nv (throws out incompletely recombined surfaces).
I do a similar check for volumes by extracting the Nu and Nv for surface
1 and Nv for surface 2 to define the dimensions of the volume grid. I
then compare against the number of hexahedra. Is that a reasonable approach?
The orientation of quads, surfaces, hexes and volumes don't seem to
agree with the documentation (or I've assumed too much). As a first
example, say I define a square trasfinite surface, starting from the
lower left corner, and selecting vertices counter clockwise.
With this definition, the documentation indicates that the "u" direction
is from left to right, and the "v" direction is up. However, when this
surface is meshed, the quads are numbered as follows:
| 2 | 4 |
| 1 | 3 |
It's a simple enough matter to change my surface definitions (or reorder
the quads) to get what I want, but I was wondering - Do I just have it
backwards in my head, or if I found a bug?
Another related point - If you try to define the surfaces for a cube
following the numbering in the documentation, the side, top, and bottom
surfaces will give an error because the definition is out of order. The
order of nodes given for those surfaces (or quads on a hexahedron) would
result in a twisted face, or butterfly. Obviously, Gmsh creates the
quads with nodes ordered correctly - generally with the last two nodes
switched as compared to the docs.
I'll submit a patch for my Plot3D file generator if you'd like. The
physical stuff is going to take a fair amount of work reorienting
surfaces and volumes, merging them, and checking the merged surfaces. As
I am an engineer, not a programmer, that will take some time for me to
figure out. However, what I have now may be of some use to others.
Christophe Geuzaine wrote:
> Hi Matt - With the transfinite, elliptic or extrusion algorithms, the
> elements are indeed generated in the "expected" order. But keep in mind
> that the elements are stored in a tree structure according to their
> quality, not their number.
> So if you want a structured output, we will have to reorder the tree
> according to the element numbers, or use some other (geometry-related)
> sorting criterion.
Matt Gundry, PE
At Work mjgundry at faa-engineers.com
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