# [Gmsh] Normals

Christophe Geuzaine geuzaine at acm.caltech.edu
Sun Feb 22 17:31:06 CET 2004

```Nacho Andres de la Fuente wrote:

> Dear all,
>
> I am using gmsh in combination with getdp and I am a newbie so I would
> (i.e. passing from conductivity_1 to conductivity_2) in getdp and
> therefore I need the normals defined outwards for all the surfaces to
> impose the appropriate boundary conditions.
>
> I have done two things in gmsh:
> 1) define the inner surface and the outer surface different (i.e. with
> different geometrical elements) corresponding to the same geometrical
> location => gmsh gets crazy when meshing since it finds out that the

Yes, you cannot do that...

> 2) create an inner volume giving it the right normals and the outer
> volume being the first one with the gap of the second => the problem
> with the normals appear: the outwards direction for the inner volume is
> right but for the outer volume gets the inwards direction (opposite to
> what I would need for the boundary condition for getdp)
>
> I am certainly doing something wrong and sure some of you have the right
>

Simply define two Physical Surfaces for the boundary:

// interior boundary:
Physical Surface(1) = {10,11,-12};

// exterior boundary, with opposite orientation:
Physical Surface(2) = {-10,-11,12};

Christophe

--
Christophe A. Geuzaine
Applied and Computational Mathematics, Caltech
geuzaine at acm.caltech.edu - http://geuz.org

```