[Gmsh] surface normals in gmsh 4.0

[Christophe Geuzaine]

> On 26 Aug 2018, at 23:01, Daniel Winklehner <winklehn at mit.edu> wrote:
> 
> Hi All,
>  
> The following would produce correctly oriented surface normals in gmsh 3.0 (surfaces 1 to 277 form a “half-shell” of my symmetric part):
>  
> new_surfs[] = Symmetry {1, 0, 0, 0} {Duplicata{Surface {1:277};}};
> Physical Surface(2) = {1:277, -new_surfs[]};
>  
> The minus before ‘new_surfs’ was necessary to flip the normals in the mirrored surfaces. However, in gmsh 4.0, it does not produce the desired result anymore and the normal of the mirrored half now all point inwards. Removing the minus has no effect.

Indeed: with the new MSH4 format Physical groups are just groups, that don't change the numbering or orientation of elements anymore. 

Best solution: use "ReverseMesh Surface{-new_surfs()};" (or mesh::setReverse in the API) to explicitly request the mesh orientation to be reversed w.r.t the natural geometrical orientation of the geometrical entity.

(You can also still use the old MSH2 file format, which will behave as before for backward compatibility.)

Christophe


>  
> I’m using the 64 bit windows binary.
>  
> Any suggestions would be appreciated!
>  
> Best,
>     Daniel
>  
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— 
Prof. Christophe Geuzaine
University of Liege, Electrical Engineering and Computer Science 
http://www.montefiore.ulg.ac.be/~geuzaine

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