[Gmsh] Hybrid mesh and combine triangles in quadrilaterals

[Fabian]

Hi Christophe,

thanks a lot!

Actually, it seems that I wasted a lot of time to create a hex mesh for 
a room airflow setup with a jet just in the middle of one wall.
I attached my approach. It looks like, that I could have done it a lot 
easier and with much less transfinite surfaces/volumnes using 'Extrude'. 
Though, do you think that it could be a problem to expand the inlet 
section just like in my attached approach using extrude? The middle 
section should expand, whereas the other part of the room would become 
smaller in the x-direction...

Regards!
Fabian


Christophe Geuzaine wrote:
> Fabian wrote:
>   
>> Hi Christophe,
>>
>> Christophe Geuzaine wrote:
>>     
>>> Fabian wrote:
>>>  
>>>       
>>>> Hi,
>>>>
>>>> I just saw the geometry and wonder, if there is a chance to extrude 
>>>> the     
>>>>         
>>> sure: cf. attached file.
>>>   
>>>       
>> Oh, thanks! Am I able to declare any progression or bump to the extrude 
>> volume as well? Would be great.
>>
>>     
>
> You can specify the number of elements and the relative height of each
> extruded layer. For example:
>
> Extrude {0, 0, 1} {
>    Surface{9, 1};
>    Layers{{1,1,1,1,1,1}, {0.05,0.15,0.5,0.85,0.95,1}};
>    Recombine;
> }
>
> You could thus generate a progression with something like this:
>
> n = 6;
> r = 2;
> a = (r - 1) / (r^n - 1);
> one[0] = 1;
> layer[0] = a;
> For i In {1:n-1}
>    one[i] = 1;
>    layer[i] = layer[i-1] + a * r^i;
> EndFor
> Extrude {0, 0, 1} { Surface{9, 1}; Layers{one[], layer[]}; Recombine; }
>
>
>
>
>
>   
>> Fabian
>>
>>
>>     
>>>  
>>>       
>>>> surfaces and create a transfinite volume?
>>>> It's probably not working due to more than six surfaces building the 
>>>> volumen!? Would be nice, if this would work, though...
>>>>
>>>> Fabian
>>>>
>>>> Christophe Geuzaine wrote:
>>>>    
>>>>         
>>>>> Durufle Marc wrote:
>>>>>        
>>>>>           
>>>>>> Hi,
>>>>>> I have two questions :
>>>>>>
>>>>>> 1- How do you create hybrid meshes in 2-D (quad-dominant with a few 
>>>>>> triangles) ?
>>>>>> Until now, I am using the Recombine option, but often I get 
>>>>>> non-convex quadrilaterals.
>>>>>> In the picture attached to this message, you can see that two 
>>>>>> elements (in black) are non-convex
>>>>>> quads and that's a problem ... So, often I change the characterisc 
>>>>>> length in the hope that I will have only
>>>>>> fine quadrilaterals. Is it possible to require that all 
>>>>>> quadrilaterals are convex ?
>>>>>>
>>>>>>             
>>>>>>             
>>>>> Yes. In your file you use
>>>>>
>>>>> Recombine Surface {1,s0[0]} = 100;
>>>>>
>>>>> which means that you allow quads with angles between 90-100 and 
>>>>> 90+100 degrees (which does not make sense).
>>>>>
>>>>> Use
>>>>>
>>>>> Recombine Surface {1,s0[0]} = 45;
>>>>>
>>>>> instead, which will allow angles between 90-45=45 and 90+45=135 
>>>>> degrees. (45 is the default if you don't specify anything, i.e., 
>>>>> with just "Recombine Surface {1,s0[0]}".)
>>>>>
>>>>> We are working on an automatic full quad algorithm, but it's not 
>>>>> ready yet. In the meantime for simple geometries you could also use 
>>>>> Transfinite meshes to generate a full quad mesh (see attached file).
>>>>>
>>>>>
>>>>>        
>>>>>           
>>>>>> 2- In 3-D, when I try "Recombine Volume", it doesn't work. So, how 
>>>>>> is it possible with gmsh
>>>>>> to create 3-D hexahedral-dominant meshes (with some tets, pyramids 
>>>>>> and wedges) ?
>>>>>>             
>>>>>>             
>>>>> Non. Your only choices for generating hex/prism/pyramid meshes are 
>>>>> extruded (see "Extrude Surface") or transfinite (see "Transfinite 
>>>>> Volume") grids.
>>>>>
>>>>>
>>>>>
>>>>>        
>>>>>           
>>>>>> Thank you for your help.
>>>>>>
>>>>>> PS : I am using gmsh 2.24
>>>>>>
>>>>>>
>>>>>>
>>>>>> ------------------------------------------------------------------------ 
>>>>>>
>>>>>>
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>>>>>> gmsh at geuz.org
>>>>>> http://www.geuz.org/mailman/listinfo/gmsh
>>>>>>             
>>>>>>             
>>>>>   
>>>>> ------------------------------------------------------------------------ 
>>>>>
>>>>>
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>>>>>           
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>>>>
>>>>     
>>>>         
>>>   
>>>       
>>
>>     
>
>
>   

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