[Gmsh] Surface Patches on Extruded Surfaces (e.g.: Cylinder)

[James Snyder]

I have looked around on the list a bit yesterday and have noticed that
the question of doing intersections or surface patches has come up
quite a few times.  I realize that Gmsh is not a solid modeler
(although I might be interested in hearing of a simple, perhaps open
source, command line, solid modeler that one could provide geometry to
and get a file importable to Gmsh for meshing... is this essentially
Open CASCADE?), and I've also seen that there is the ruled surface
functionality that provides a mechanism for interpolating or pinning a
patch to a sphere, but there isn't a convenient way of doing this with
cylinders.

For me this has become of interest for doing electrostatics
simulations where one would like to have patches to define as
electrodes rather than points or ad-hoc selection of neighboring faces
to fit a geometry after mesh generation, e.g.:
http://eidors3d.sourceforge.net/tutorial/netgen/netgen_gen_models.shtml
or http://eidors3d.sourceforge.net/tutorial/EIDORS_basics/forward_solvers_3d.shtml

What I've come to ask/suggest is perhaps something could be done with
fields/attractors/thresholds for defining these patches?  Since this
functionality already provides a way to control element sizes as a
function of distance, including restrictions of whether to apply these
parameters to given surfaces or volumes, could there not be a
relatively simple way to define a patch from these or the code that is
providing this functionality (with or without actually varying mesh
density).  Maybe there's already a convenient way to select elements
resulting from the current functionality just based on size and
distance, but it would be nice to have form a clean line/arc rather
than just selecting triangles under a given distance (either in the
surface or euclidean distance would be fine).

It seems like the easiest way to get the desired effects currently is
to use splines where one calculates out the line/arc along curved
surfaces and the resulting spline can be used, but this has some
downsides in that I'm not completely sure if the splines really do
always end up in the desired curved face.  It seems like if one uses
enough or if the geometry is correct it may just fall below the
threshold of complaint one might find from embedding a line in a
surface if enough points are used?

Best.

-jsnyder



-- 
James Snyder
Biomedical Engineering
Northwestern University
http://fanplastic.org/key.txt
ph: (847) 448-0386