[Gmsh] Virtual topology problem

[Christophe Geuzaine]

Hi Martin,

The "little bit of work" has been done to automate the procedure so that compounds can now be used for high-order meshes. In short:

- With Mesh.CompoundClassify=1, triangles are magically reclassified on the original surfaces. This is great for keeping e.g. Physical groups persistent across compound mesh definitions, but will prevent e.g. high-order meshing, as the parametric coordinates of an element can span several surfaces.

- With Mesh.CompoundClassify=0, triangles are classified on a new (discrete) surface, and the volume mesh now automatically uses these surfaces instead of the original ones, which allows for correct high-order meshing and other post-processing algorithms. Any reference to the original surfaces (e.g. in physical groups) must however be updated.

We have added your example with the 2 scenarios to our testsuite: https://gitlab.onelab.info/gmsh/gmsh/blob/master/benchmarks/step/part_compound_curved_mesh.geo

Let us know if we should add a reference in the .geo file to the original source/author/...

Cheers,

Christophe



> On 23 Jul 2019, at 20:40, Christophe Geuzaine <cgeuzaine at uliege.be> wrote:
> 
> 
> 
>> On 23 Jul 2019, at 09:13, Martin Kraska <martin.kraska at th-brandenburg.de> wrote:
>> 
>> Hello,
>> 
>> thanks for your immediate response. Yet adding the compound constraint to surfaces 21 and 22 doesn't solve the problem. The virtual topology is still ignored.
> 
> No, with the latest dev snapshot this works perfectly:
> 
> ***
> Merge "part.step";
> 
> // Virtual topology to avoid enforcing small elements
> Compound Curve {63,64};
> Compound Curve {65,66};
> Compound Surface {23,25};
> Compound Surface {9,24,26};
> Compound Surface {21};
> Compound Surface {22};
> 
> // Set definitions
> Physical Surface("support") = {5};
> Physical Surface("load") = {17};
> Physical Volume("part") = {1};
> 
> // Mesh control and meshing
> Mesh.CharacteristicLengthMax = 7;
> ***
> 
> As explained in my previous email the flow for compounds + second order meshing still needs a little bit of work, as the default (Mesh.CompoundClassify=1) cannot be used for 2nd order meshes, since elements can be classified across surfaces (and hence across parametrizations).
> 
> Christophe
> 
> 
>> 
>> Is that because I didn't "redefine the volume in terms of  the new surfaces"? How would I do this? As I understand your concept of virtual topology, there aren't any new surfaces at all, just meshing constraints. So what should I do to redefine the volume?
>> 
>> Furthermore, I have to set a rather small element size in order to avoid negative jacobians.
>> 
>> How about imposing local mesh refinement in areas with invalid elements? That might be a way to automatically resolve such problems in a smarter way than just global refinement.
>> 
>> <41086788.png>
>> Best regards, Martin Kraska
>> 
>> ----- Am 22. Jul 2019 um 22:42 schrieb Christophe Geuzaine <cgeuzaine at uliege.be>:
>> On 22 Jul 2019, at 21:47, Martin Kraska <martin.kraska at th-brandenburg.de> wrote:
>> 
>> Hello,
>> 
>> in the attached script I try to mesh a step part using virtual topology (trying
>> to update the example
>> https://github.com/mkraska/CalculiX-Examples/tree/master/CAD/OnshapeTutorial )
>> I see that I have to use a new syntax for virtual topology but otherwise did not
>> make any changes compared to gmsh 3.0.3 which I tried last.
>> 
>> In gmsh 4.3.0 I get a boundary mesh issue.
>> In 4.4.0 the virtual topology is just ignored but the part is meshed.
>> in the git version of today (2019-07-22) I get a slightly different boundary
>> mesh issue.
>> 
>> Did I miss some settings? Or is the part just too nasty?
>> 
>> 
>> You forgot to set:
>> 
>> Compound Surface {21};
>> Compound Surface {22};
>> 
>> With these the first order mesh looks OK.
>> 
>> Note that the 2nd order mesh will not work - this is currently expected with
>> Mesh.CompoundClassify=1 (the default), as elements are reclassified across the
>> boundaries.
>> 
>> We will improve this in the future. In the meantime, you can set
>> Mesh.CompoundClassify=0; but you will need to redefine the volume in terms of
>> the new surfaces. We haven't decided yet what the best/most  natural workflow
>> should be in that case. Any feedback is welcome!
>> 
>> Christophe
>> 
>> 
>> 
>> 
>> Thanks for any advice, Martin Kraska
>> <vt.zip>_______________________________________________
>> gmsh mailing list
>> gmsh at onelab.info
>> http://onelab.info/mailman/listinfo/gmsh
>> 
>> 
>> —
>> Prof. Christophe Geuzaine
>> University of Liege, Electrical Engineering and Computer Science
>> http://www.montefiore.ulg.ac.be/~geuzaine
>> 
>> <part.step><partVT1.geo>
> 
> — 
> Prof. Christophe Geuzaine
> University of Liege, Electrical Engineering and Computer Science 
> http://www.montefiore.ulg.ac.be/~geuzaine

— 
Prof. Christophe Geuzaine
University of Liege, Electrical Engineering and Computer Science 
http://www.montefiore.ulg.ac.be/~geuzaine