[Gmsh] Adding a boundary surface on a STL model

[Christophe Geuzaine]

> On 17 May 2020, at 15:00, Dimitrios Stoupis <dstoupis at auth.gr> wrote:
> 
> Hi,
> 
> I want to run brain simulations with models from the PHM repository
> (not relevant to the problem but suits the narrative) which are
> shipped in STL format. The meshes of the STL have been fixed using
> MeshFix and the attached .geo file has been used to generate the 3D
> mesh.
> 
> [file: brain_mesh.geo]
> 
> The script successfully meshes the model and the generated mesh is
> correct. The problem is how can I add electrodes on the head
> (cylindrical volumes) so that later I use those as the boundary
> surfaces to add the boundary conditions in the FEM solver.
> Illustration
> 
> I have tried creating a cylinder and applying boolean operation with
> the STL, but obviously this did not work. How can I create such a
> thing on the STL surface? There is no CAD available just the STL
> files. For copyright reasons I am not able to share the STL, but I
> will try to illustrate below the problem using a sphere in paint.

You'll need to preprocess the STL with an external tool first: currently there is nothing in Gmsh to modify STLs - only remesh them.

Christophe


> 
> [image: electrode.png]
> 
> I want the electrode (red volume) to be tangent with the surface of
> the skin (black sphere). The black sphere is only in STL format.
> Triangle position problem
> 
> Finally the next problem that will arise is how can I get the
> individual triangle positions on the STL surface through the file in
> order to be able and add the electrode perpendicular to the surface
> and to the right spot, without doing trial and error work to find the
> location.
> 
> Thank you!
> <brain_mesh.geo><electrode.png>_______________________________________________
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— 
Prof. Christophe Geuzaine
University of Liege, Electrical Engineering and Computer Science 
http://www.montefiore.ulg.ac.be/~geuzaine