[Gmsh] Boundary Layer

[Christophe Geuzaine]

Patrick Bourdin wrote:
> Hi Dave,
> 
> I just tried the BL extrusion command. Neat!
> 1st question: Is there any means to control the expansion ratio of the 
> cells within the extruded mesh?

You can specify the expansion ratio by giving a list of "heights" for 
each layer of cells. For example, the following command will extrude 2 
layers with one cell per layer, the first layer with height 0.1, the 
second with height 0.3-0.1=0.2:

Extrude { Surface{1}; Layers{ {1,1}, {0.1,0.3}}; }

If you need a complex grading, the easiest is probably to define lists 
for the layers and heights, e.g.:

cells[0] = 1;
heights[0] = 0.1;
For i In {1:10}
  cells[i] = 1;
  heights[i] = heights[i-1] * 2;
EndFor
Extrude { Surface{1}; Layers{ cells[], heights[] }; }

> 2nd question: How to avoid the collapsing of the upper and lower layers 
> at the *sharp* trailing edge of an airfoil (see pic attached)? Ideally I 
> would like to extend those layers in the wake. I tried to append a wake 
> surface (in fact a wedge with a very thin apex angle) to the trailing 
> edge and grow a BL mesh on it, but didn't work, I still have the same 
> problem at the trailing edge. Any idea?
> 

We need to implement one of the classical methods proposed in the
literature to deal with this (as well as with reentrant corners): normal
smoothing, multiple normals per point on sharp edges, ...



> Cheers,
> 
> Patrick
> 
> 
> 
> 
> David Colignon wrote:
> 
>>
>>
>> Patrick Bourdin wrote:
>>
>>> spot on Dave!
>>>
>>> my mistake indeed, there was a typo in tmp2.geo: some colons were 
>>> used in lieu of commas in the transfinite line expression-lists, 
>>> definitively not the intended effect, I should stop gmsh-scripting 
>>> late at night ;-)
>>>
>>> thx again,
>>>
>>> Patrick
>>
>>
>>
>> De nada.
>>
>> Your geometry and your scripts are very nice !
>>
>> Do you know that we have a new _experimental_ boundary layer command :
>>
>> http://www.geuz.org/gmsh/doc/texinfo/gmsh-full.html#SEC36
>>
>>
>> Extrude { Surface { expression-list }; layers }
>>    Extrudes a boundary layer along the normals of the specified surfaces.
>>
>>
>> Here is a small example (t1_boundary_layer.geo).
>>
>> Dave
>>
> 
> ------------------------------------------------------------------------
> 
> 
> ------------------------------------------------------------------------
> 
> _______________________________________________
> 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