[Gmsh] Surface mesh with holes

[Christophe Geuzaine]

Martin Aunskjær wrote:
> Hi,
> 
> I'm just starting to use gmsh. I want to generate a
> surface mesh of a rocket, but I'm having two problems.
> 
> (1)
> The way I go about this is that I have divided the
> surface of the rocket into distinct sections, ie.
> nosetip, upper body etc. Barring fins the rocket is
> rotationally symmetric, so I proceeded to model the
> generating curve of each section in the longitudinal
> direction either as a spline or a line. Along the
> circumference, I subdivide the two bounding circles of
> each section into a number of circular arcs. I then
> construct the surface of each section as a series of
> line loops consisting of two rotated versions of the
> longitudinal spline/line curves and two end circle
> arcs. These surface pacthes are set up by the 'Ruled
> Surface' primitive.
> 
> The problem with this approach is that unless I
> subdivide each section into the same number of
> circular segments, gmsh will generate a nonconforming
> 2d surface mesh, i.e. the boundary nodes of triangles
> on one rocket section will not match of with boundary
> nodes of triangles on a neighboring section.
> 
> If you want to see the problem, try and generate a
> surface mesh from the two included .geo files.
> 
> My question is, if this is not the intended way of
> constructing such a surface? Could I do it another way
> so I wouldn't have to worry about non-conforming
> surface meshes ?

Gmsh's CAD module is fairly limited : it will not automatically 
intersect/sew curves/surfaces. For example, in wr_nonconforming.geo the 
circle arcs 115 and 167 are partially overlapping. Gmsh will not 
reconcile those, and the adjacent surfaces will thus not be connected. 
Hence you get a nonconforming grid.


> 
> (2)
> 
> The lower part of the rocket I haven't done yet has
> three fins separated by 120 deg attached to the lower,
> curved body. It is not clear to me - keeping the
> problems mentioned above in mind - how I could
> generate surface patches for the lower body and still
> avoid non-conformancy.
> 
> The missing lower body is similar to the mirror of the
> upper curved section. What I need to do is to carve
> three holes of some small width in it where the fins
> should be attached. But how can I carve holes in a
> Ruled Surface?
> 
> If more information is needed, do not hestitate to ask
> for it.
> 
> Thanks in advance,
> Martin Aunskjaer
> 
> 
>       ________________________________________________________ 
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> 
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-- 
Prof. Christophe Geuzaine
University of Liege, Electrical Engineering and Computer Science
http://www.montefiore.ulg.ac.be/~geuzaine