# [Gmsh] 3D Periodic Boundary Conditions with test.

Dorin Niculescu niculescu_dorin85 at yahoo.com
Tue Apr 16 10:59:43 CEST 2013

```Hello Gustavo,

I've attached a ".geo" that i'mcurrently using for PBCs. If you want a structured mesh on the cube faces you have to uncomment the Transfinite Surface lines. You are right, the points aren't exactly the same, you can see a difference after 10-6, which is not so bad... If you really need a perfect match you can copy the points by your self in your program, it's true, it becomes a little complicated but not impossible.

Hope that helps,

Dorin

________________________________
From: Gustavo Charles Peixoto de Oliveira <tavolesliv at gmail.com>
To: niculescu_dorin85 at yahoo.com
Cc: gmsh at geuz.org; felipe.alves.portela at gmail.com
Sent: Monday, 15 April 2013, 19:02
Subject: 3D Periodic Boundary Conditions with test.

Dear all,

http://www.geuz.org/pipermail/gmsh/2013/008054.html

and I'd like to give some contribution. I'm a GMSH newbie and my need seems similar to yours.

I've been trying to generate 3D PBC meshes for simple geometries like square, triangular and rectangular channels with the Periodic Surface command by using the version 2.7.0. However, before running my code, I had to write a function to verify if the periodicity really was being satisfied like Dorin did. It's surprisingly that the test fails for 1 point for some refined meshes. However, it succeeds for coarser meshes of the same kind. I didn't know why...

For instance, my script test outputs something like that for an x-extrusion:

Warning! Left and right y-components are not equal.
Entry: 72
yU = 0.5; yV = 0.625
Warning! Left and right z-components are not equal.
Entry: 72
zU = 0.25; zV = 0.625

As you can see, the y-coordinate is "slightly" different, whereas z-component is a bit more. In the end, both differs, but for an unique point! Although the error is small, it is problematic for my purposes.

I didn't test the output with Transfinite option.

Dorin, I will test your suggestion to use Extrude + Transfinite, but have you already found some solution concerning extrusion for further directions?

Would be this case a bug of the Periodic Surface function?

Kind regards,

--

Gustavo Oliveira, Ph.D. Student
State University of Rio de Janeiro
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