# [Gmsh] Mesh with cubic elements

RAFAEL ZAMORANO razamorano at uc.cl
Mon Sep 11 22:34:23 CEST 2017

```Dear Gmsh Team,

My name is Rafael Zamorano, and I'm writing from Santiago (Chile) with a
doubt about how to mesh with cubic elements a particular 3D domain (with
interior holes) in Gmsh. As a context, I'm preparing a CFD simulation for a
MSc in Pontificia Universidad Católica de Chile (Santiago, Chile), working
along Dr. Wolfram Jahn from the Mechanical Engineering Department. The work
consists in validating the fireFoam solver (from the OpenFOAM utility) for
large-scale LES turbulent diffusion flames. The experimental measurements
come from Edinburgh, Scotland.

The mesh is for a 2x5x18 meter compartment with 12 squared-outlet burners,
which code (".geo" file) is attached in this mail. The domain for the
simulation looks like this:

The interior cubes are the burners, which inject fuel through the superior
face. Note that the additional 3D rectangular prism (in the right hand side
of the image) is an outlet domain placed to capture the air entrainment and
combustion gas outlet fluxes. The meshed 3D domain (with default
Tetrahedral elements) looks like this:

The mesh has local refinement on the burner inlets, with 9cm elements. The
general resolution per element is 30cm. The interior of the 12 cubic
burners are UNMESHED. For this, I defined a Volume consisting of the outer
surfaces as exterior walls, and the burner surfaces for inner surfaces
(leaving the 12 burners as un-meshed 3D holes). The burners had to be a
little bit lifted from the ground, because the mesher couldn't understand
the holes sharing surfaces with the outer surfaces (Volume function
restriction).  Now, we* want to implement a cubic element mesh with the
same local refinement*, but we can't get it to work right. We tried using
the Extrude function, but the mesher keeps meshing the interior of the
cubic burners (these need to remain unmeshed), basically the whole
extrusion. Also, when using the extruding function, the Transfinite
function for square elements wont work on the XZ plane, because the number
of sides is incorrect. For reference, this XZ plane is the one shown here
(not including the burner faces):

The mesh with tetrahedral elements works fine with the fireFoam solver, but
we need to test the same one but with cubic elements. I attached the
sceenshots shown, the ".geo" file, and the ".msh" file. It would be of
great help if you have a solution for this issue. The Gmsh utility is
excellent and intuitive (i saw all the Youtube tutorials and revised the
manual), but this issue is troubling us.

With nothing more to add, i wish you a great year.

Sincerely,
RZ

--
Rafael Zamorano Z.
*MSc(c)*
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```