# [Gmsh] Problems with mesh quality

Jean-Francois Remacle jean-francois.remacle at uclouvain.be
Fri Feb 13 13:48:39 CET 2009

```Le 13-févr.-09 à 11:04, Alexander Sjögren a écrit :

> Hi,
>
> I'm pretty new to GMSH and my problem may be due to a basic mistake,
> nevertheless I'm really stuck. I'm trying to make a mesh of the
> volume around a spacecraft. The spacecraft consists of an almost
> cubic body, large solar panels and two booms. The most interesting
> volumes for my simulations are around the two booms, so the mesh
> around the booms need to be in good quality. The program I'm doing
> my simulation in needs the quite large simulation box (ellipsoidal
> in the pictures).
>
> The problem is that when I make a 3D mesh of the volume around the
> spacecraft, the mesh around the two booms consists largely of
> straight lines to the booms, not of a mesh in all (random)
> directions. I've tried using different characteristic lengths for
> the booms, both making it larger and smaller than the characteristic
> length for the ellipsoid and rest of the spacecraft. I've also tried
> using the two different meshing algorithms (Tetgen+Delaunay and
> Netgen), the result looks maybe a little bit better with Netgen but
> the problem is still there. The result of the mesh-problem is that
> when I run simulations you can tell from the results that the mesh
> affects the result, too much, in a bad way.
>
> Does anyone have an idea of what I'm do

You shoud define a size field. By default, Gmsh interpolates mesh
sizes from boundaries :
-) in 1D mesh, we take mesh sizes at vertices and interpolate linearly
-) in 2D, we use edge sizes at boundaries and interpolate using
baricentric coordinates
-) in 3D, we use triangle sizes and  interpolate using baricentric
coordinates

If you have huge sizes in your outside box, this causes gmsh to
believe that the initial
3D mesh is ok.

JF

> ing wrong and what I can do to make my mesh better? I've attached
> three pictures:
>
> 1.jpg: This is the spacecraft inside the ellipsoid I'm using for the
> calculations.
> 2.jpg: A close-up of one of the booms.
> 3.jpg: Same as 2.jpg, now with the mesh that I'm not happy with.
>
>
> Best regards
> Alexander Sjögren, Uppsala, Sweden
> alex.ph.sjogren at gmail.com
> <1.jpg><2.jpg><3.jpg>_______________________________________________
> gmsh mailing list
> gmsh at geuz.org
> http://www.geuz.org/mailman/listinfo/gmsh

----------------
Prof. Jean-Francois Remacle
Universite catholique de Louvain (UCL)
Tel : +32-10-472082 -- Mobile : +32-473-909930

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