# [Gmsh] Bisecting Subdomains of Imported Geometry/Mesh

Christophe Geuzaine cgeuzaine at uliege.be
Fri Apr 24 09:24:09 CEST 2020

```Dear Randal,

If you can import the geometry as e.g. a STEP file in Gmsh, you can indeed split it with cutting planes. See t20 : http://gmsh.info/dev/doc/texinfo/gmsh.html#t20

If you cut the tetrahedral mesh in a brute-force way you will end up with polyhedra, and generally very poorly shaped elements.

Christophe

> On 24 Apr 2020, at 06:49, Andress <randalandress1507 at comcast.net> wrote:
>
> Background
>
> I am doing Finite Element Analysis to study the flow of electric current in persons and animals due to stray electric current in bodies of water. I am using FEAtool(a MATLAB plugin) which employs gmsh to generate grids from geometries.
>
> Part of my analysis involves the calculation of the total current through (perpendicular to) the cross section of a sub-domain which is bordered by the intersection of a cut plane and the subdomain surface. This current is the integral of current density over the area of the cross section.
>
> FEAtool does this integration but only over a surface that is a boundary of the subdomain.
>
> Here is an example (See attached illustrations.):
>
> Geometry: a 1m cube (subdomain 2) containing a component (subdomain 1) made of 4 joined cylindrical sections.
>
> Boundaries: 5V/0V on the opposite faces of the cube that are perpendicular to the axes of the cylinders and zero current on the other cube faces.
>
> Solution: current density is plotted and a YZ plane is cut through it in the center (at x = 0.5).
>
> I want to calculate the current flowing through each of the small connecting cylinders by integrating the current density over the two circular, planar intersections of the cut plane and the two small cylinders.
>
> In order to do this wholly within FEAtool, I must create (geometry) the small cylinders so that they are in two pieces and simply share a face/boundary.  That way there is a pre-defined boundary over which FEAtool can do the integration.
>
> Sometimes this is not possible/convenient because the model geometry has been imported and/or is quite complex (such as that of a human body) or because the decision to make such a measurement/cut-plane is made only after the model and mesh is completed and the solution is being examined.
>
> Question
>
> If I export the model geometry and/or the mesh from FEAtool to gmsh can the model/mesh be split by a designated plane within gmsh so that, when exported and imported back into FEAtool, the sub-domains intersected by the plane would have true faces/boundaries so they could be designated in an integration?
>
> If possible, but not required, I would like to be able to designate the sub-domains to be split by the cut plane in gmsh so as to not split a sub-domain over which integration is not needed.
>
> From the FEAtool perspective, see: http://forum.featool.com/Integration-of-Current-Density-over-Slice-Intersection-td180.html
>
> Kind regards,
> Randal
>
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> <cylinders-in-box-geometry.jpg><cylinders-cut-plane-to-integrate.jpg><cylinders-cut-plane-to-integrate-YZplane at 50cm.jpg>_______________________________________________
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—
Prof. Christophe Geuzaine
University of Liege, Electrical Engineering and Computer Science
http://www.montefiore.ulg.ac.be/~geuzaine

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