[Gmsh] Orientation of surface loops for defining a volume

[Sacconi, Andrea]

Hi everyone,

I would like to ask you a very important question about the orientation of surfaces. It is driving me insane since I get contradictory outputs and I cannot understand where I am doing wrong.

The problem is the following: I need to generate a 3D mesh for a parallelepiped which has a spherical hole inside. Therefore, I have defined the 6 outer faces and the inner sphere. In order to correctly define the volume, I need to create two surface loops. I now have to pass the orientation: which is the correct rule? Inner normal or outer normal? In other words, clockwise orientation on the outer faces and anticlockwise on the inner ones or viceversa? I see that I experimentally get two different meshes if I switch from one to the other.
Second question: for some of my simulations, I need to remesh my domain, since the hole drifts and the quality of the tetrahedra deteriorates. I create a surface loop with all the triangular faces the hole is composed of, but sometimes I get the weird message: "GMSH is not able to orient surface XXX. Normal computed with 5, 10, ....., 10000 points. Failed". How can I fix this? Does it depend on the resolution of my mesh?

I would really appreciate an answer, since I have a deadline for my PhD thesis, and so far I haven't managed to get these simulations work.
Thank you a lot in advance !!!!!! 

Andrea
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Andrea Sacconi
PhD student, Applied Mathematics
AMMP Section, Department of Mathematics, Imperial College London,
London SW7 2AZ, UK
a.sacconi11 at imperial.ac.uk