# [Gmsh] RE The direction of normal vectors on the surface elements

Theler German Guillermo gtheler at cites-gss.com
Tue Jan 27 13:35:42 CET 2015

```Hi

This is nice, but it does not solve the issue of getting the outward normal in the sense needed to set neumann or robin boundary conditions.

BTW, I think that there is some valuable information about the meshed geometry that should be optionally included in the resulting mesh file. For example, I do not see the point of having to compute the outward normals (or the face areas or the element's neighbors) from the solver side. Moreover, with the information Gmsh has in its administrative structures it should be far more efficient to compute the geometric properties at the time of the mesh generation, let alone if the same mesh has to be used several times in different runs.

I tried to follow step-by-step the mesh() method but I am not so fond to C++ so I could not completely understand what is going on behind, but maybe someone here may be able to tell.

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On Mon, 2015-01-26 at 15:55 +0100, jean pierre aubry wrote:

hello

the normal orientation of the elements is the same that their parent
surface which cab adjusted at will try this example to understand

lx = 100.0;
ly = 100.0;
Point(1) = {0, 0, 0, cl1};
Point(2) = {lx, 0, 0, cl1};
Point(3) = {lx, ly, 0, cl1};
Point(4) = {0, ly, 0, cl1};
Line(1) = {1, 2};
Line(2) = {2, 3};
Line(3) = {3, 4};
Line(4) = {4, 1};
Line Loop(5) = {1, 2, 3, 4};
//comment or uncomment next two lines
Plane Surface(6) = {5};
//Plane Surface(6) = {-5};

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