[Gmsh] Transfinite line algorithm

[David Colignon]

Martin Vymazal wrote:
> Dear gmsh users,
> 
>  I'm trying to generate a second order (P2) unstructured mesh around a 
> symmetric airfoil. I managed to create a mesh which is fine so far, but 
> I would like to control precisely the number of points on the airfoil. 
> To make things a bit more complicated, I'd also like to have more points 
> close to the leading and trailing edges of the airfoil. I thought I 
> could use the transfinite algorithm with 'progression' or 'bump' 
> options, but the progression gives me a bad mesh for a progression ratio 
> which is not close to 1.0 (I attached my *.geo files, progression 1.3 at 
> the end of the naca0012 geo file already gives strange results. 

Hi Martin,

I think displaying the nodes of the 1D mesh on the lines with "shift-alt-p" or Tools->Options->Mesh->Visibility->Nodes 
could help you understanding what happens, the way "bump" and "progression" works in Gmsh and what you call "strange" 
results (I see what you mean).

http://geuz.org/gmsh/doc/texinfo/gmsh-full.html#SEC45

The optional argument `Using Progression expression' instructs the transfinite algorithm to distribute the nodes 
following a geometric progression (Progression 2 meaning for example that each line element in the series will be twice 
as long as the preceding one). The optional argument `Using Bump expression' instructs the transfinite algorithm to 
distribute the nodes with a refinement at both ends of the line.

and if you search for "bump" in the mailing-list archives, you get this:

http://www.geuz.org/pipermail/gmsh/2008/003526.html


btw , point 5000 is exactly the same as point 381. It is perhaps not a problem but you should avoid it.

Cheers,

Dave

--
David Colignon, Ph.D.
Collaborateur Logistique du F.R.S.-FNRS
CÉCI - Consortium des Équipements de Calcul Intensif
ACE - Applied & Computational Electromagnetics
Institut Montefiore B28
Université de Liège
4000 Liège - BELGIQUE
Tél: +32 (0)4 366 37 32
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It seems
> that gmsh calculates the lengths of the lines on the wall of the airfoil 
> incorrectly (not precisely enough?). I hoped I could somehow set the 
> precision with which the spline representing the wall of the airfoil is 
> meshed and thus fix the problem, but found nothing relevant in the 
> documentation.
>  On the other hand, the 'bump' option seems to have no effect whatever 
> value I use (I tried values between 0.5 and 3.5, but I must admit I 
> don't know what this number actually means).
> 
>  I'd appreciate any help/hints or at least an info whether this can be 
> done in gmsh or not.
> 
> Thank you very much,
> 
>         Martin Vymazal
> 
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