[Gmsh] Errors when increasing NNodesByEdge on a small edge

[Christophe Geuzaine]

> On 10 Apr 2019, at 21:57, Nathan RICKS <Nathan.James.Ricks at vub.be> wrote:
> 
> Hi all,
> 
> I have quite a particular problem, but I'm hoping someone can help. I'm meshing an airfoil using transfinite surfaces in the boundary layer.
> 
> I've just had to add a blunt trailing edge (mesh was working well with sharp trailing edge) to the airfoil, and now I have this one error stopping me, and I just can't figure it out...
> 
> When I increase the NNodesByEdge on Field[11] from 6 to 15, the mesh ends up bad quality, with cell corners not matching other cells at the boundary of surfaces, and it causes SU2 to diverge instantly. It runs fine with 6, but crashes at 15. I've attached two images showing that it is a mesh issue causing this (nothing else is being changed in the setup).
> 

I cannot seem to reproduce the issue on my Mac. Attached is the mesh generated on your .geo file:

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> Other things such as reducing lcMin on Field[12] can also make it crash for example, however the above has visible errors in the mesh.
> 

Idem... What OS are you running on?

PS: instead of generating the BL by hand, why not using the boundary layer field directly? Cf. e.g. 

https://gitlab.onelab.info/gmsh/gmsh/blob/master/benchmarks/2d/naca12BL.geo
https://gitlab.onelab.info/gmsh/gmsh/blob/master/benchmarks/2d/naca12_2d_trailing.geo

> Does anyone have an idea on why I can't use the Distance/Threshold field as normal on this edge?
> 
> If you can provide any guidance that would be greatly appreciated, it's so close...! I've already tested with the latest release with the same results.
> 
> Thanks a lot,
> Kind Regards,
> 
> Nathan Ricks
> 
> 
> 
> 
> 
> <NNodes_15.png><NNodes_6.png><airfoil_level1.geo>_______________________________________________
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? 
Prof. Christophe Geuzaine
University of Liege, Electrical Engineering and Computer Science 
http://www.montefiore.ulg.ac.be/~geuzaine