# [Gmsh] Fwd: Re: Boundary layers in gmsh?

Christophe Geuzaine cgeuzaine at ulg.ac.be
Mon Mar 1 17:42:22 CET 2010

```On 27/02/10 10:33, David Colignon wrote:
>
>
> Dave,
>
>
> Thanks, that's just the function I need! (I always feel stupid when I
> have to be pointed to the userguide, but sadly it happens more often
> than not)
>
> I attempted the Boundary command you mentioned, with the "Volume
> Method" I previously proposed.
> I attempted it on Christophe's sphere example, but unfortunatly I get
> 50 errors similar to this:
> Error : ERROR: Edge 1 - 9 multiple times in surface mesh
> Error : ERROR: Edge 4 - 6 multiple times in surface...
>
> I tried the Coherence; command, but it did not influence the issue
> (long shot). My .geo file is attached if you are curious.
>
> Do you know if there is a way to do boolean addition of volumes in
> gmsh?(not critical, but it would be another possible avenue)
> I know that boolean subtraction is: Volume(4) = {1, 2, 3}; where 1 is
> the positive volume, 2 and 3 are negative volumes and 4 is the final
> volume.

The easiest is to just use the values returned by the extrusion: cf.
attached example.

Hope this helps,

Christophe

>
>
> Thanks again, Eric Nutsch
>
>
>
>
> On Thu, Feb 25, 2010 at 11:46 PM, David Colignon
> <David.Colignon at ulg.ac.be> wrote:
>> Hi Eric,
>>
>> there is the "Boundary{}" command :
>>
>> http://www.geuz.org/gmsh/doc/texinfo/gmsh.html#Transformations
>>
>> Boundary { transform-list }
>> (Not a transformation per-se.) Returns the boundary of the elementary
>> entities in transform-list.
>>
>> with
>>
>> transform-list:
>> Point | Line | Surface | Volume { expression-list };
>>
>> Regards,
>>
>> Dave
>>
>> --
>> David Colignon, Ph.D.
>> Collaborateur Logistique du F.R.S.-FNRS
>> CÉCI - Consortium des Équipements de Calcul Intensif
>> ACE - Applied & Computational Electromagnetics
>> Sart-Tilman B28
>> Université de Liège
>> 4000 Liège - BELGIQUE
>> Tél: +32 (0)4 366 37 32
>> Fax: +32 (0)4 366 29 10
>> WWW: http://hpc.montefiore.ulg.ac.be/
>> Agenda:
>>
>>
>> On 26/02/10 08:39, Eric Nutsch wrote:
>>>
>>> Christophe,
>>>
>>>
>>> It appears that the surface loop method to the boundary layer will
>>> work in most all cases, but I may have found a more efficient way.
>>>
>>> The BL extrude command returns a volume number as well. If it were
>>> possible to take the farfield and preform a boolean operation with the
>>> BL volumes, it would make the process very simple even for complex
>>> BLs.
>>>
>>> I attempted this for in your sphere example:
>>> Volume(1001) = {1002, 1,2,3,4,5,6,7,8};
>>> ,but gmsh replied with "Unknown surface loop 1"
>>>
>>> Is there any way to convert a volume (created via extrude for example)
>>> to be converted into a surface loop?
>>> Perhaps I am missing an important leap in the coding structure, but if
>>> it was possible it would be an extremely handy function.
>>>
>>> What are your thoughts?
>>>
>>>
>>> Thanks,
>>> Eric Nutsch
>>>
>>>
>>>
>>>
>>> On Wed, Feb 24, 2010 at 4:03 AM, Christophe Geuzaine
>>> <cgeuzaine at ulg.ac.be> wrote:
>>>>
>>>> On 24/02/10 11:26, julien hoessler wrote:
>>>>>
>>>>> Hi Eric,
>>>>> If it is a flat surface or if you know where the interface would be
>>>>> located you can just move your domain line/surface to the interface
>>>>> and
>>>>> extrude it towards the outside, but otherwise I can't seem to find
>>>>> a way
>>>>> to link the interface with the rest of the mesh
>>>>
>>>> Hi Guys - boundary layers are still a bit experimental, but the basics
>>>> are
>>>> there to connect them with other geometrical entities. Attached are two
>>>> examples:
>>>>
>>>> * sphere_boundary_layer.geo shows how to connect volumes to boundary
>>>> layer
>>>> faces ;
>>>>
>>>> * t1_boundary_layer_connection.geo shows how you can also connect
>>>> things
>>>> to
>>>> the lines or vertices created during the boundary layer generation.
>>>>
>>>> Retrieving the id numbers of the extruded entities can be done in a
>>>> script
>>>> the usual way (list[] = Extrude...), but as usual this only gives
>>>> access
>>>> to
>>>> the ids of the "top" surface, the new volume and the "lateral"
>>>> surfaces.
>>>>
>>>> To retrieve the ids of the new lines and the new points, you have at
>>>> the
>>>> moment to resort to the GUI (in "Tools->Options->General->General"
>>>> select
>>>> "Enable mouse hover over meshes" to see the numbers displayed at the
>>>> bottom
>>>> of the graphical window).
>>>>
>>>> Hope this helps,
>>>>
>>>> Christophe
>>>>
>>>>> regards
>>>>> Julien
>>>>> On 24 Feb 2010, at 02:03, Eric Nutsch wrote:
>>>>>
>>>>>> Hi,
>>>>>>
>>>>>>
>>>>>> Has anyone successfully gotten an extruded boundary layer to
>>>>>> interface
>>>>>> with a mesh?
>>>>>>
>>>>>>
>>>>>> Thanks,
>>>>>> Eric Nutsch
>>>>>>
>>>>>> _______________________________________________
>>>>>> gmsh mailing list
>>>>>> gmsh at geuz.org
>>>>>> http://www.geuz.org/mailman/listinfo/gmsh
>>>>>
>>>>> Julien Hoessler
>>>>> PhD Student
>>>>> Department of Aeronautics
>>>>> Imperial College London
>>>>>
>>>>> Phone: +44 (0) 20759 45042
>>>>> Email: julien.hoessler06 at imperial.ac.uk
>>>>>
>>>>>
>>>>> _______________________________________________
>>>>> gmsh mailing list
>>>>> gmsh at geuz.org
>>>>> http://www.geuz.org/mailman/listinfo/gmsh
>>>>>
>>>>>
>>>>
>>>>
>>>> --
>>>> Prof. Christophe Geuzaine
>>>> University of Liege, Electrical Engineering and Computer Science
>>>> http://www.montefiore.ulg.ac.be/~geuzaine
>>>>
>>>
>>> _______________________________________________
>>> gmsh mailing list
>>> gmsh at geuz.org
>>> http://www.geuz.org/mailman/listinfo/gmsh
>>
>>
>

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