[Gmsh] List of Questions

[Christophe Geuzaine]

Joe Koski wrote:

> I've been trying to create meshes with some success, but some questions have
> regarding the proper use of GMSH have arisen. I'm using GMSH 1.52.1 on a
> Macintosh with OS X 10.3.4.
> 
> Here are the questions, in no particular order:
> 
> 1) My .msh files follow the Version 1 format, not Version 2. For example,
> the node section starts with $NOD, not $Nodes. Is this correct? When will
> Version 2 be used?

Yes, the default is still version 1. You can save your mesh in the new
format with File->Save as->Gmsh v2.0. To make version 2.0 the default
format, just add "Mesh.MshFileVersion = 2;" in your option file
($HOME/.gmsh-options).

> 
> 2) For Diriclet type boundary conditions, is it possible to associate a
> unique value or a flag with a geometric line or surface (other than line or
> surface number) that can be used later to trigger a translator to recognize
> that particular boundary? For example, if the temperature at a boundary is
> to be 293 K, is it possible to enter the value 293 somewhere during the
> geometry construction? Should I use physical entities for this or is there
> another mechanism?

Physical entities are exactly what you need. Look for example at
tutorial/t1.geo to see how they work.

> 
> 3) If I zoom and pan to a particular view on the screen, is it possible to
> interactively write a .jpg of that view? I know how to do a screen capture,
> but resolution can be lost that way.

Sure: File->Save as (or Ctrl+Shift+s) and enter "file.jpg" in the input
field. By default, Gmsh tries to recognize the format by looking at the
extension you provide. If you don't like that, just select the
appropriate format from the drop-down list in the dialog.

> 
> 4) So far I have been unsuccessful in recombining a simple 1 x 1 square mesh
> of triangles to obtain some quadrilaterals. First, when I try to create a
> Plane Surface, I am told to "Select Exterior Boundary" and I successfully
> pick a line loop. Then I am asked to "Select Hole Boundary," but there is no
> hole. What to do? 

Press 'q'.

Note that the geometrical selection interface will be greatly improved
in version 1.53 (which should be released really soon now).

> I've tried selecting the exterior boundary again, without
> success. When I do fiddle somehow and successfully get a mesh, I do
> "Define," "Recombine," and "Select Surface" then 'e' for end, and nothing
> happens to the mesh. Any suggestions? My .geo file keeps changing, or I
> would include it.
> 
> 5) I have Gmsh.app in my /Applications folder. Is there a way of running
> gmsh from the terminal command line on a Mac? Otherwise, how do you access
> the command line options?

If you want to use Gmsh in batch mode, you can simply call, e.g.,

/Applications/Gmsh.app/Contents/MacOS/Gmsh file.geo -3

Launching the GUI version will not work well, as I cannot include the
resource forks in the binary in a tar.gz archive (I use the much nicer
OS X XML scheme for that). If you really need to launch the GUI version
from the command line, you will have to recompile Gmsh from the sources.

Best,

Christophe

-- 
Christophe Geuzaine
Applied and Computational Mathematics, Caltech
geuzaine at acm.caltech.edu - http://geuz.org