[Gmsh] Hexahedral mesh inside a sphere

[Felix Salazar]

Hello,

There are several things. Do you want unstructured hexas or structured ones?

If you want unstructured, you just need to change the meshing algorithms
until a certain combination provides you hexas. That's what GMSH is telling
you: The default 3D algorithm (Frontal) is only for tetrahedra, and these
tetras need triangles on the surfaces, not quads. For example, using 3D
Algorithm: MMG3D works in my machine. However, the quality of the mesh is
not good, at least to my taste.

If you pay attention to what GMSH is doing with this MMG3D case, you might
get a clue. The best option is to build an O-grid. Split your spheres in
several 6-faced blocks, 7 blocks in total: 1 cube in the middle of the
sphere, of about 0.5 radius per side, and 6 blocks around this central cube
that meets on the spherical surface. Something like this
http://catlikecoding.com/unity/tutorials/cube-sphere/02-sphere-ortho-frontal.png

If you do the last option, remember that you need to specify transfinite
lines, surface and volumes. Read the gmsh documentation about transfinite
and structured meshes for more details.

Finally, you are using a ruled surface between 3 circles to define your
spherical surface. Be careful with this, as it might create distortions
(although I haven't checked in this case). For perfectly spherical or
cylindrical surfaces, try to use a rotation instead.

Good luck

-------------------------
*Félix Salazar*
felix.a.salazar at gmail.com
-------------------------

On Tue, May 10, 2016 at 12:03 PM, Deepak Garg <deepak.garg at ingv.it> wrote:

> Hi all
>
> I am trying to generate a hexahedral 3d mesh inside a sphere of radius 1.
> I am getting the error message
>
> "Cannot tetrahedralize volume with quadrangles on boundary".
>
> How to solve this problem? I am attaching my geo file. Any kind of help or
> suggestions will be a great help.
>
> Thank you
> Deepak
>
>
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