[Gmsh] How are set up U and V vectors for elements ?

[Christophe Geuzaine]

> On 5 Oct 2020, at 07:05, bilou at netc.it wrote:
> 
> Hello. I'm trying to parse a mesh.msh generated by gmsh.
> 
> I work with 9-quad elements. I see in documentation that inside of the elements, nodes are ordered like this :
> 
>            v
>            ^
>            |    
> 3-------6-------2
> |          |         |
> |          |         |
> 7        8-------5 --> u 
> |                    |
> |                    |
> 0-------4-------1
> 
> I made a 2D mesh (all surfaces normals are oriented in the same direction)
> 
> When i look at my 2D mesh (in plan XY) the element 265 :
> 265 2 44 1047 533 49 1102 1103 544 1104
> 
> is oriented like this related to the golbal X and Y axis
> 
> ie, when looking at the screen i see:
>   44-------49-------2     
>      |           |         |     Y
>      |           |         |     ^
> 1102   1104    544    |
>      |           |         |     |
>     |           |         |     |
> 1047--1103----533    x------> X
> 
> it seems that u anv v vectors are oriented like this :
>         1-----4-----0
>         |        |       |
>         |        |       |
> u <---5-----8      7
>         |        |       |
>         |        |       |
>         2-----6-----3
>                  |
>                 V
>                 v
>                    
> 
> How are V and U vectors defined for each element ?

It depends on the definition of the surface and the meshing algorithm.

>  Is there a way to orient them along the natural X and Y axis ?

With the transfinite algorithm you could force the elements to be generated in a certain order by specifying the interpolation corners. But in general you won't know the correspondance - Gmsh produces unstructured meshes! So you would use the (u,v,w) -> (x,y,z) mapping (through e.g. Lagrange basis functions) to make the connection.

Christophe

> 
> Thanks
> 
> 
> 
> 
> _______________________________________________
> gmsh mailing list
> gmsh at onelab.info
> http://onelab.info/mailman/listinfo/gmsh

— 
Prof. Christophe Geuzaine
University of Liege, Electrical Engineering and Computer Science 
http://www.montefiore.ulg.ac.be/~geuzaine