[Gmsh] Holes in curved surfaces / merging surfaces

[Sean Victor Hum]

Hello,

First off I would like to commend the gmsh team on an amazing piece of
software. I have found many uses for gmsh ranging from electromagnetic
scattering problems to mechanical problems. Thanks for making it
publically available.

I have a question about the geometry module. All the problems I use
gmsh for involve surfaces (no 3D elements). I have not found a way to
easily make a hole in a curved surface. I know it is quite easy for
plane surfaces since holes can be defined in the expression list, but I
am interested in making a hole in a curved surface, specifically a hole
in a surface that was made by extruding a curve. If cutting a hole in a
surface extruded from a spline is a problem, I can reformulate the
problem using a series of lines for the extrusion instead of a spline,
thus using a piecewise linear model of the curve (but it would still
involve many, many points, making defining the hole 'manually' using
multiple extrusions a daunting task.

On a similar topic, I am also learning if there is a way to merge 2D
meshes from two intersecting surfaces. For example, in the problem
above, if I had a hole in a surface and wanted to attach a curved
surface to cover the hole, how would I go about defining the
geometry/mesh so that the mesh flows smoothly over the composite
surface? If I were to use an example from the gmsh website, I would use
the famous fighter jet screen shot: the problem I am trying to solve is
very similar to trying to attach the canopy of the fighter jet to the
fuselage starting with two separate surfaces defining those two objects.

Any help on how to approach these tasks would be appreciated.

Thanks, and best regards,

Sean

[Christophe Geuzaine]

Post by Sean Victor Hum
Hello,
First off I would like to commend the gmsh team on an amazing piece of
software. I have found many uses for gmsh ranging from electromagnetic
scattering problems to mechanical problems. Thanks for making it
publically available.
I have a question about the geometry module. All the problems I use
gmsh for involve surfaces (no 3D elements). I have not found a way to
easily make a hole in a curved surface. I know it is quite easy for
plane surfaces since holes can be defined in the expression list, but I
am interested in making a hole in a curved surface, specifically a hole
in a surface that was made by extruding a curve. If cutting a hole in a
surface extruded from a spline is a problem, I can reformulate the
problem using a series of lines for the extrusion instead of a spline,
thus using a piecewise linear model of the curve (but it would still
involve many, many points, making defining the hole 'manually' using
multiple extrusions a daunting task.

Unfortunately, Gmsh does not support this yet. Jean-Francois hacked some
partial support for trimmed surfaces a while ago, but I don't know the
current status of his efforts. He will have to comment on this himself.

Post by Sean Victor Hum
On a similar topic, I am also learning if there is a way to merge 2D
meshes from two intersecting surfaces. For example, in the problem
above, if I had a hole in a surface and wanted to attach a curved
surface to cover the hole, how would I go about defining the
geometry/mesh so that the mesh flows smoothly over the composite
surface? If I were to use an example from the gmsh website, I would use
the famous fighter jet screen shot: the problem I am trying to solve is
very similar to trying to attach the canopy of the fighter jet to the
fuselage starting with two separate surfaces defining those two objects.

That would indeed be very useful.

I recently added support for "Discrete Surfaces" in Gmsh (it's available
in the latest nightly cvs snapshots), which could serve as the basis for
such developments--that is, if you want to compute the intersections at
the discrete level (between two meshes). The drawback is that you will
probably have to modify some elements in the intersected mesh close to
the intersection in order to avoid small/badly shaped elements.

The other solution would be to actually compute the surface
intersections at the CAD level. This is perfectly feasible, but a robust
implementation is not trivial. You can find some (untested and
non-functional) routines aiming at this in Geo/CAD.cpp. But it might be
better to start from scratch...

Best,

Christophe

Post by Sean Victor Hum
Any help on how to approach these tasks would be appreciated.
Thanks, and best regards,
Sean
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