[Gmsh] Problem creating Line Loops with the OpenCASCADE geometry kernel

[Christophe Geuzaine]

Hi Lucas,

With the built-in kernel we automatically reorder the edges to form a loop, while the current OpenCASCADE implementation does not. Currently you thus need to sort the edges in the list (we'll implement this for OpenCASCADE - I have added it in our TODO list).

The bad news is that after fixing your file I tried to define a "Surface" on your loop, which uses the N-side surface filling algorithm from OpenCASCADE to automatically compute a BSpline surface from the boundary... and it's atrociously slow. You'll probably need to split the surface in smaller pieces for now - or look at the OpenCASCADE BRepOffsetAPI_MakeFilling class to see if changing some parameters could help.

Christophe


> On 19 Jun 2018, at 14:03, Lucas Blattner Martinho <lucas.blattner at g2elab.grenoble-inp.fr> wrote:
> 
> Dear all,
> 
> I am having trouble creating line loops while using the OpenCASCADE geometry kernel. A line loop that seems to be well defined for  Gmsh's native kernel is regarded as an open loop by OpenCASCADE...
> 
> A more detailed description of the problem is provided below and the .geo files required to repeat the issue are available attached to this message.
> 
> Does anybody understand why Gmsh exhibits sjuch a behavior ? Could anybody provide some help ? 
> 
> 
> Best regards,
> 
> Lucas Blattner
> G2Elab - Laboratoire de Génie Électrique de Grenoble
> 
> 
> 
> Problem description :
>  
> 
> I am working on the geometry of a complex coil in the shape of a horse saddle (see .geo files attached to this message).
> 
> File '"Lower.geo" contain the points and lines of the geometry. These were generated with the help of a computer program and are loaded  by the main file "Saddle.geo" with an "Include" command.
> 
> I am trying to make line loops using the lines contained  in the aforementioned file to proceed creating surfaces and volumes. The lines ids are stored in vectors such as Lower_Line2[], Lower_InputLine[] and so on, all of which are defined in "Lower.geo".
> 
> I am particularly interested in generating a thin volume with the aid of the "ThruSections"  function provided by OpenCASCADE. The first step is the creation of the line loops required by this function..
> 
> That  said, I created a line loop  in the main file "Saddle.geo". The loop seem to be OK, that is, it  is  closed and is  created without console errors or warnings. This can be verified by taking a look at the physical regions "myLoop", which correspond to the same list of lines composing the loop.
> 
> However, loading "Saddle.geo" after uncommenting the first line of the code (to enable the OpenCASCADE functions) results in the following error :
> 
> Error   : Line Loop is not closed
> Error   : 'C:\xxxxxx\saddle.geo', line 6 : Could not add line loop
> 
> This error makes it impossible to continue using ThruSections. Since the line loop seems to be correctly defined, I believe that it could be caused by a bug....
> 
> 
> 
> <Lower.geo><Saddle.geo>_______________________________________________
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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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