[Gmsh] Defining Physical Surface

[Tom Peachey]

I am planning to run OpenFoam batch jobs, airfoil simulations, using gmsh
for the meshing. For this I need to automate the creation of .geo files.
It’s straightforward, following the Ingram tutorial
*https://community.dur.ac.uk/g.l.ingram/download/gmshtutorial2012.pdf
<https://community.dur.ac.uk/g.l.ingram/download/gmshtutorial2012.pdf>*
 until the final stage where one defines the “Physical Surfaces”
Physical Surface("back") = {1027};
presumably  a surface created by the previous “Extrude” command. As Ingram
says: “Finding the number associated with a particular surface is a bit of
a pain.” It seems he uses the gmsh GUI which is not available in my
automated scenario.

The Octave code by César A. Vecchio Toloy gives some more detail:
fprintf (fid, 'j5[] = Extrude {0,0,%.10g}
{Surface{5};Layers{1};Recombine;};\n', cuerda/10);
fprintf (fid, 'j6[] = Extrude {0,0,%.10g}
{Surface{6};Layers{1};Recombine;};\n', cuerda/10);
followed by
fprintf (fid, 'Physical Surface("inlet") = {j5[5],j6[5]};\n');
But again it is not clear how to choose the correct components of j5 and j6.

Any help greatly appreciated.
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