Martin Vymazal wrote:
Hello Dave,
thank you very much for your reply. I had an idea how 'progression'
works, the problem is that this is not what I see in my mesh when I
visualize the nodes on the wall of the airfoil.
can you send us an example (.geo plus picture or an indication on where to
zoom) ?
As I wrote in my
previous email, making progression slightly bigger than 1.0 (say, 1.3 or
bigger), gmsh gives me a completely incorrect mesh.
the spacing between the 1D meshing nodes obeying to a geometric law, it is very sensible to the progression parameter,
and 1.3 is a rather high value
Sometimes it even
tries to mesh the inside of the airfoil.
I also found the question/reply about 'bump' in gmsh mailing list
archives to which you pointed me, but it does not help me to understand
whether I'm doing something wrong or if gmsh simply cannot mesh the
airfoil the way I want. My only question is simply: can I control the
number and distribution of the mesh points on the airfoil in the geo
file I attached yesterday? If so, how?
With the transfinite command, you can precisely control the number (and spacing) of the _1D_ meshing nodes on the
boundary (splines) of your airfoil.
But the problem is that you mix the 1D transfinite mesh with (various) Fields for the 2D meshing, and if there is a huge
discrepancy between the spacing of the 1D nodes and the background 2D field of characteristic lengths, the 2D meshing
algorithm sometimes do not succeed to match the conflicting constraints, and you sometimes get these strange results.
So Gmsh is perhaps able to mesh the airfoil the way you want, but it is a little bit "tricky" and need a lot of trials
and errors...
Good luck !
Dave
Once again, thanks for any help.
Best,
Martin Vymazal
Cituji David Colignon <[email protected]>:
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
Fax: +32 (0)4 366 29 10
WWW: http://hpc.montefiore.ulg.ac.be/
Agenda:
http://www.google.com/calendar/embed?src=david.colignon%40gmail.com
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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