[Gmsh] how to make a periodic cubic mesh

[Aleksejs Fomins]

Dear Gmsh,

I would like to create a cubic domain with a few complicated 3D shapes inside, and produce a corresponding tetrahedral mesh. The domain boundaries of the cube should be periodically conformal, e.g. each triangle on the surface with normal -x should be equivalent to a triangle on the surface with normal +x under translation along the x-axis.

Using google I have found a solution proposed earlier on this list:

http://onelab.info/pipermail/gmsh/2016/010305.html

The .geo code suggested in the above link produces a conformal tetrahedral mesh of a cube using Transfinite surfaces and edges. It seems to work, but has a few problems:
1) It completely ignores the value of the characteristic lengthscale variable "lc", always splitting each edge in exactly 4 parts
2) The characteristic lengthscale changes depending on when index number of the Transfinite Line. Currently it is 5. If one would set it to 14, each edge would be split into exactly 25 segments. Why is that?

GMSH version 2.13.2, Ubuntu 16.04

Best regards,
Aleksejs Fomins
PhD student at Nanophotonics and Metrology Lab, EPF Lausanne, Switzerland