[Gmsh] Ensuring conformal meshing when using geomtry from Freecad

Juan E. Sanchez juan.e.sanchez at gmail.com
Fri Apr 6 19:35:26 CEST 2018


Hello Jeremy,

Is my understanding correct that there would only be one material volume 
per brep file?  I would use freecad, but it seems painful to try to 
import one brep per material into gmsh, and then attempt to get gmsh to 
merge the interfaces.  I thought Coherence might not work very well, and 
performing a boolean fragments operation on multiple brep volumes might 
not work either.

Regards,

Juan

On 4/6/18 9:44 AM, jeremy theler wrote:
> apply the boolean operation in freecad (in the options you can choose 
> "compsolid" mode) and then export to brep
> alternatively use the gmsh command "Coherence"
> I suggest you search online for further information on compsolids in 
> freecad and thr coherence command in gmsh
> 
> 
> On Fri, Apr 6, 2018, 03:05 Benedikt Oswald <benedikt.oswald at lspr.swiss> 
> wrote:
> 
>     Dear juan, thank you very much for your detailed answer!
> 
>     In fact, our workflow definitely requires geometry construction in
> 
>     Freecad and then meshing in gmsh; nevertheless, I
> 
>     will try to debug the problem along your suggested
> 
>     procedure.
> 
>     thanks again and greetings, Benedikt
> 
> 
> 
>     Am 05.04.18 um 17:22 schrieb Juan Sanchez:
>      > Hello Benedikt,
>      >
>      > I am using the OpenCascade factory to do the Boolean Operations in
>      > gmsh.  This is using the pygmsh python module to help automate the
>      > process:
>      >
>      > https://github.com/nschloe/pygmsh/tree/master/pygmsh
>      >
>      > The boolean operations can then be performed using code like this:
>      > https://github.com/nschloe/pygmsh/issues/164
>      >
>      > import pygmsh
>      > geom = pygmsh.opencascade.Geometry(
>      >   characteristic_length_min=0.1,
>      >   characteristic_length_max=0.1,
>      >   )
>      >
>      > def create_vol_volume(geom, h, w, l, x, z, cr, lcar=100):
>      >   f = 0.5*w
>      >   y = [-f,-f+cr, +f-cr, +f]
>      >   z = [z, z + (h-cr), z + h]
>      >   f = 0.5 * cr
>      >   points = []
>      >   points.append(geom.add_point((x, y[0], z[0]), lcar=lcar))
>      >   points.append(geom.add_point((x, y[0], z[1]), lcar=lcar))
>      >   points.append(geom.add_point((x, y[1], z[1]), lcar=lcar))
>      >   points.append(geom.add_point((x, y[1], z[2]), lcar=lcar))
>      >   points.append(geom.add_point((x, y[2], z[2]), lcar=lcar))
>      >   points.append(geom.add_point((x, y[2], z[1]), lcar=lcar))
>      >   points.append(geom.add_point((x, y[3], z[1]), lcar=lcar))
>      >   points.append(geom.add_point((x, y[3], z[0]), lcar=lcar))
>      >
>      >   lines = []
>      >   lines.append(geom.add_line(points[0], points[1]))
>      >   lines.append(geom.add_circle_arc(points[1], points[2], points[3]))
>      >
>      >   lines.append(geom.add_line(points[3], points[4]))
>      >   lines.append(geom.add_circle_arc(points[4], points[5], points[6]))
>      >   lines.append(geom.add_line(points[6], points[7]))
>      >   lines.append(geom.add_line(points[7], points[0]))
>      >
>      >   line_loop=geom.add_line_loop(lines)
>      >   surface=geom.add_plane_surface(line_loop)
>      >   vol = geom.extrude(surface, translation_axis=[l, 0, 0])[1]
>      >   return vol
>      >
>      >
>      > h_top=40
>      > corner_radius=2.5
>      >
>      > z_vol=0
>      > h_vol=25
>      > w_vol=10
>      > l_vol=100
>      > x_vol=-0.5*l_vol
>      > vol=create_vol_volume(geom, h=h_vol, w=w_vol, l=l_vol, x=x_vol,
>      > z=z_vol, cr=corner_radius)
>      >
>      > z_vol2=0
>      > l_vol2=50
>      > t_vol2=1
>      > w_vol2=w_vol+2*t_vol2
>      > h_vol2=h_vol+t_vol2
>      > x_vol2=x_vol + 0.5*(l_vol-l_vol2)
>      > vol2=create_vol_volume(geom, h=h_vol2, w=w_vol2, l=l_vol2, x=x_vol2,
>      > z=z_vol2, cr=corner_radius)
>      > vol2=geom.boolean_difference([vol2], [vol], delete_first=True,
>      > delete_other=False)
>      > geom.boolean_fragments([vol2, vol], [], delete_first=True,
>     delete_other=False)
>      > print(geom.get_code())
>      >
>      >
>      > And then you can label the volumes as physical volumes and mesh the
>      > resulting geo file.
>      >
>      > Regards,
>      >
>      > Juan
>      >
>      > On Thu, Apr 5, 2018 at 5:53 AM, Benedikt Oswald
>      > <benedikt.oswald at lspr.swiss> wrote:
>      >> Dear gmsh developers
>      >>
>      >> I recently tried to mesh a geometry (made in Freecad) which
>     consists of
>      >> two volumes where a smaller volume was
>      >>
>      >> subtracted in Freecad from the larger one. When I mesh the two
>     volumes I
>      >> would need tetrahedra
>      >>
>      >> and that are conformal between the two volumes. Would you have a
>      >> suggestion on how
>      >>
>      >> to achieve this ? At the moment, gmsh does not really produce a
>      >> conformal mesh.
>      >>
>      >> I attach the files in question.
>      >>
>      >> Thanks a lot in advance and greetings, Benedikt
>      >>
>      >>
>      >>
>      >>
>     ----------------------------------------------------------------------------------------------------------------------
>      >> Dr. sc. techn. Benedikt Oswald - first engineer - LSPR AG -
>     phone +41 43 366 90 74
>      >> Grubenstrasse 9, CH-8045 Zürich, benedikt.oswald at lspr.swiss
>      >>
>     ----------------------------------------------------------------------------------------------------------------------
>      >>
>      >>
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>     --
>     ----------------------------------------------------------------------------------------------------------------------
>     Dr. sc. techn. Benedikt Oswald - first engineer - LSPR AG - phone
>     +41 43 366 90 74
>     Grubenstrasse 9, CH-8045 Zürich, benedikt.oswald at lspr.swiss
>     ----------------------------------------------------------------------------------------------------------------------
> 
> 
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> -- 
> --
> jeremy theler
> www.seamplex.com <http://www.seamplex.com>
> 
> 
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