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marine April 15, 2010 11:53

snappyHexMesh layers part

I'm trying to use snappyhexmesh for an external flow around a ship and I don't really understand how the differents functions in addlayerscontrols work. Can someone explain me how I choose the values for finalLayerThickness, expansionRatio and minThickness? I've searched for posts about that on the forum but maybe missed them, if they already exit can you give me the link?

thank you very much!!


marine April 16, 2010 05:43

I have a very high Reynolds, 10^9, and I'd like yplus about 100 wich give me a first mesh size of 1mm (using the online tool y+estimation). I guess I put "minthickness" to this value.
I don't understand why I have to give a value for "finalLayerThickness" if I specify the "minThickness", the "expansionRatio" and the "nsurfaceLayer" I want. Can someone explain me that?
thank you very much.

marine April 19, 2010 04:16

Does someone know if the "expansionratio" is the ratio between two successives layers or between the furthest and the closest to the surface?
I tried to change it but my cells always have the same size as the ones on my surface (I defined nsurfacelayer = 10).

fra76 April 20, 2010 13:12

The layers are controlled by the last element size (with "last" I mean the one far from the surface, close to the volume cells). You specify its desired size, in terms of ratio with the near volume cell, the expansionRatio, which I think is the ratio between the height of two consecutive layer cells, and all the elements are determined in this way. The minThickness is a quality parameter. If snappy can only build layers so that the relative first element height is below the minThickness value, layer extrusion from that face is disabled.

See the comment in the example snappyHexMeshDict:

    //- Minimum thickness of cell layer. If for any reason layer
      //  cannot be above minThickness do not add layer.
      //  See relativeSizes parameter.
      minThickness 0.25;

As far as I know, there's no way of specifying the height of the near-wall cell, neither absolute nor relative.

Hope this helps,

marine April 22, 2010 05:30

Thank you, I'm having a lot of dificulties with the boundary layer, sometimes I only have BL on the interior of my surface and sometimes only on the borders...
Do you know if it's possible to have a boundary layer on a surface with differents refinement levels? I'm trying with 2 differents levels (but not one on the boundaries and one on the middle), that could be the origin of my problem.



fra76 April 25, 2010 13:51

Yes, it is.
The transition in layers height above the two refinement levels is a bit too fast, but it works for sure.

braennstroem April 25, 2010 14:09

Hi Franceso,

hope your are doing well! I wonder, if you would recommend to use absolute layer sizes to circumvent the above problem...


fra76 April 27, 2010 07:09

Hi Fabian!
Good point. Depending on the geometry, this is a good solution.


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