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Boundary layer addition at corners with sHM

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Old   July 21, 2011, 07:54
Default Boundary layer addition at corners with sHM
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Ben Vernieres
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Hi everyone,

I'm trying to mesh a wing using snappyHexMesh and have been struggling for quite some time now with the layer addition setup.

This wing has a 1mm thick TE and a square tip so quite a few sharp edges. I'm exporting the geometry in stl from UG NX7 and tried from Rhino 3 too. It didn't change the problem I have.

What is happening is that basically sHM creates a reasonably nice BL around the section of the wing, at the TE too (see pic attached), but, when reaching the tip of the wing, it stops making the layer addition (see second pic attached), to a point that it doesn't even add layers on the tip face.

Now if I change the front wall patch type from "empty" to "symmetryPlane", it does work a bit better (not a lot though) at the tip but then doesn't add layers up to the symmetry plane... (Note that I'm only using "empty" in the meshing process, I later change it back to "symmetryPlane" to run simpleFoam)

I've been playing around with the settings of layer addition in a systematic way but can't find a workaround... Refining or coarsening the mesh doesn't seem to change anything neither (tried from 200k cells up to 4M cells)

I've been looking around in this forum and some others but still can't find a solution for my problem. Lots of threads that are asking similar questions that are not answered too...

Has anyone got a remedy for this by any chance? sHM is a great tool, would be so nice to have this layer addition part to work nicely!

I'm also attaching the sHMDict file.

Looking forward to a solution!

Cheers,

Ben

TE.jpg Tip.jpg

snappyHexMeshDict.txt
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Old   July 21, 2011, 10:30
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lore
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Why you have the mesh inside the wing as well?
have you used the new featureEdge funcionality for the mesh around the trailing edge ad tips?
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Old   July 21, 2011, 10:44
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Ben Vernieres
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Hi Lore,

Thanks for the reply.

Mesh isn't inside, it's only the background, the view is looking towards the tip of the wing. Sorry for the confusing picture.

I'm not trying to make a 2D mesh but a 3D one, hence the issue at the tip.

I've been using the featureEdge tool yes. I didn't see any change to the meshing when having it on and it messed up the surface snapping when its refinement was set to a higher level than the surface itself.

Regards,

Ben
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Old   July 22, 2011, 10:22
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Balakrshnan Ramakrishnan
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Hi Ben,
I just made some changes to your sHMDict and have reattached.. Try to use this one and let me know how this works.

I am not sure that it will work.

Cheers,

Bala
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File Type: txt snappyHexMeshDict.txt (10.9 KB, 39 views)
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Old   July 22, 2011, 11:59
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Ben Vernieres
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Hey Bala!

Thanks for the post.

I've tried those settings but having the minArea set to 0.02 prevents any layer to be added.
I've tried with this value set back to -1 and it did create layers although still didn't carry on round the tip corners everywhere.
When running checkMesh, I also get about 1600 skewed faces but I guess that's because the maxInternalSkewness is set to 6 instead of 4. (I've run with maxInternalSkewness set back to 4 and still get those highly skewed cells)

What are the reasons that made you change the settings I had? I struggle to understand how some of them work!

I was thinking that maybe the problem I have is coming from the stl file because it doesn't create a nice corner at the tip / trailing edge junction.

My TE is 1mm thick, I've also tried with a 0mm thick one with no success. When going extreme: meshing a 12%thick rectangular section, I get a "perfect" BL mesh all around the "wing".

I will post later the screenshots of what I get at the moment.

Again, thanks a lot for your time.

Ben
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Old   July 25, 2011, 10:47
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Ben Vernieres
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Hi All,

I don't really feel I'm progressing on meshing that basic foil...

I've changed the tip geometry in order to have it thicker and get the TE splits in 2 in the vertical direction otherwise the snapping would mess up the tip / TE corner and I suspect it doesn't help to add layers to this messy area.

I'm still struggling to get some layers on the tip although it does add a few in some areas but the corners are still not meshed correctly and, with symmetryPlane boundary at the root, I'm still not getting the layers addition to go all the way to the symmetry plane.

Now, if I switch this boundary to empty, it does the job nicely at the symmetry plane but fails to add layers at the tip.

See screen dumps attached for better clarity.

Has anyone got an idea of what I should do to fix this mess? Its getting pretty frustrating!

Thanks,

Ben
Attached Images
File Type: png Root-emptyBC.png (5.1 KB, 67 views)
File Type: jpg root-symPlaneBC.jpg (49.5 KB, 60 views)
File Type: jpg Tip-emptyBC.jpg (65.7 KB, 73 views)
File Type: jpg Tip-symPlaneBC.jpg (60.9 KB, 56 views)
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Old   July 27, 2011, 08:32
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Ben Vernieres
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Hi All,

This is a bit of a monologue but anyway....

I'm wondering if the problem is coming from the snapping part of the mesh creation. Here is a shot of how the wing tip / TE zone looks like after snapping.

It can clearly be seen some sort of triangular faces at the tip corner. these are not triangles in reality and have 2 edges almost aligned. That can't be good for sure! I didn't get those when meshing a rectangular section wing so I assume that might be the cause of the pain...

I've tried playing with minArea in order to try to get rid of those faces since they seem quite smaller than the other ones but with no success.

Anyone's got some tip / tricks to solve that issue?

Thanks,

Ben
Attached Images
File Type: jpg TipTE.jpg (71.5 KB, 96 views)
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