[Gmsh] question about the direction of normals on the surface

[Christophe Geuzaine]

Toru Takahashi wrote:
> 
> Hi.
> 
> I'm a beginner of 'gmsh' and interested in structuring surface data rather than
> volume data or FE analysis.
> 
> In my situation, it is required that all the normals of triangle elements on the surface,
> generated by the function of 2D-meshing, must be directed outward to the surface.
> Here, the surface is simply modeled with the some commands (Point, Line, Line Loop,
> Plane Surface or Extrude). And the method of constructing of normals using the gmsh's
> mesh data is determined uniquely i.e. if 72 th triangle element is defined as
> "72 2 1 1 3 45 48 50" in the mesh data under consideration, the direction of the normal
> is determined as same as that of the vector calculated by
> (position of 48th element - 45th ) x (50th - 45th),
> where 'x' means the vector product of two vectors in 3D.
> 
> How should I satisfy the above requirement ?
> 

All the triangles belonging to a given surface will have the same
orientation. You can choose the orientation by setting a + or - sign in
the definition of the Physical Surface. (Check the result by reloading
your mesh, and setting a nonzero value for the "Normals" option in
Options->Mesh->Visibility).

Christophe

-- 
Christophe Geuzaine

Tel: (626) 395-4552    http://www.geuz.org
Fax: (626) 578-0124    mailto:geuzaine at acm.caltech.edu