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[cfMesh] tichkness Transition top of boundary layer

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Old   September 17, 2018, 12:18
Default tichkness Transition top of boundary layer
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FredPieri
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Hello ,


Using cfMesh , I wondering how can I smooth the transition in thickness between the top boundary layer cell and the first cell of the mesh ?

I mean it looks like my boundary layer is always one mesh size thick , and it is divide by the number of layer I ask. So teh transistion is size is big.

Please find attach a picture , with 5 and 15 layers. Overall thickness looks the same.

Reading the user Guide I tried to play with the parameters: featureSizeFactor and relThicknessTol . But without sucess.

I found this thread, but it doesn't give me the answer.
boundary layers



What tricks do you use?


Thanks in advance.


Special thanks to franjo_j ans the CfMesh developers.



Fred


ps: Here my boundarylayer MeshDict:


Code:
boundaryLayers
{
   

//       untangleLayers 0;
 
    patchBoundaryLayers
    {
        w_wall
        {
            nLayers  1;
            thicknessRation 1.1;
             allowDiscontinuity  1;
        }

    }
     optimiseLayer             1;       

    optimisationParameters
    {
        nSmoothNormals             5;       
        relThicknessTol         0.15;        
        featureSizeFactor         0.4;       
        reCalculateNormals         1;        
        maxNumIterations         5;   
    }

}
Attached Images
File Type: jpg boundary_cell_size.jpg (69.8 KB, 120 views)

Last edited by Fredvol; September 17, 2018 at 12:19. Reason: cleaning
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Old   September 17, 2018, 13:19
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I guess the source of your problem is a wrong keyword.


Try "thicknessRatio" instead of "thicknessRation"


Then you can play with values...
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Old   September 18, 2018, 05:09
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Hi mzzmrt


Thanks for your reply and the typo mistake.


unfortunatly , I continue to have a problem.
I think it is geometric problem.

If I use this naming ( see overview.jpg):

The position on each layer can be found using a geometric progression:
https://en.wikipedia.org/wiki/Geometric_progression


I use the mesh ( without BL) size as référence unit.

If I want 10 layers with an expension ratio to 1.2.
the last boudarylayer is only 0.198 thick.
Which make a big gap with the next cell.


If I accept the last boundary cell size to be at least 0.6 thick.
I only have 2 choices:
- nb layer = 101 expension ration = 2.5 !
- expension ratio = 1.2 but only 2 layers.


Please see the other attachement.


am I missing something?
Is there a way to spread more the boundary layer toward the mesh?


Thanks.
Fred
Attached Images
File Type: jpg overview.jpg (48.2 KB, 74 views)
File Type: jpg nb2.jpg (32.3 KB, 60 views)
File Type: jpg exp2-5.jpg (38.1 KB, 54 views)
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Old   September 18, 2018, 06:22
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Well, I am not an expert for the cfMesh and have limited experience with it.

But you can also use the "maxFirstLayerThickness" keyword for declaring it according to your yPlus needs and also try to increase nLayers...

A sample:


w_wall
{
nLayers 20;
maxFirstLayerThickness 0.002; // use a sutiable size for the geometry here
thicknessRatio 1.2;
allowDiscontinuity 1;
}
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Old   September 18, 2018, 08:29
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Hi mzzmrt


Thanks again ! Very kind !

Even playing with "maxFirstLayerThickness" I could not get the BL as smmoth as I would like.
I will try more and let this thread knows if I found something.


Cheers
fred
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Old   September 24, 2018, 15:54
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here an other thread I found having this questions:


2015:
version 1.1 - Total boundary layer thickness
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Old   October 18, 2018, 03:57
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It seems CFMesh is coded so that the total boundary layer is always around 1 cell height (I hope I am wrong though).
But if the gradient in that area is not high, this should be acceptable I guess?

maxFirstLayerThickness is to set the maximum first layer thickness and it seems it will not give what you want..
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Old   October 19, 2018, 09:03
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Hello Lava12005


Yes I contacted CfMesh on this topic , here their answer:


Quote:
Thank you for your enquiry. Indeed, smooth transition between boundary layer and inner cells is a feature available only in our commercial meshing software (CF-MESH+).

In general, there is significant difference between capabilities of our commercial (CF-MESH+) and open-source (cfMesh) software packages. Especially when it comes to boundary layers and new functionalities that are being developed (e.g. multi-material meshing, etc.) and to be implemented only in the commercial version. If you wished to try CF-MESH+, please submit your request via: https://cfmesh.com/free-trial/
I hope they will improve the next open-source version of CfMesh.
As it looks very promising, but with this way of doing the boundary layer, I had to swith back to snappyHexMesh.


Regards
fred
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Old   October 21, 2018, 09:22
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Ow, thanks for the information..
Yes CFMesh is a nice tool to have, hope they improve the open-source version..
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