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Old   February 25, 2014, 06:09
Default Sails Meshing with snappyHexMesh
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PaoloFariselli
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Hello everyone!

I'm simulating the air flow around a sailing boat...
I am trying to mesh with snappyHexmesh (I have an .stl file of the hull, the main sail and the jib ) and then I will solve with the SimpleFoam Solver.

Unfortunately I have found the meshing process harder than I expected ( previously I have made simulations around an airfoil and a flat plate).

The sails are surfaces (without thickness) and they are twisted , so even if they have no thickness I have to handle a 3D model.

I can achieve a quite good mesh, but I have to use really a lot of elements (4.5M cells) and I can't obtain BoundaryLayers on the sail's surface. This brings the solver to overestimate lift and drag coefficients (expecially CD)

Does anybody has any idea to help me introducing the BL?
Thanks !!!
paolo

this is my SnappyHexMeshDict:
Attached Files
File Type: txt SnappyHexMeshDict.txt (12.3 KB, 7 views)

Last edited by Paolo.F; February 25, 2014 at 06:29. Reason: post was too long
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Old   February 25, 2014, 15:00
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Julian Langowski
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Hi Paolo,
what is the problem with layer generation?
What does the log say? The file looks OK...

Best regards
Julian
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Old   February 25, 2014, 19:55
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PaoloFariselli
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Hello Julian,
unfortunately the log doesn't say anything wrong ( just a few faces with skewness)...the problem is that I get a very poor percentage of added layers...in fact snappy tries to add layers, and I can see them, but not all around the sail. They appear only in a small flat area of the sail (very far from the curved part or around the leading/trailing edge).
Paolo
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Old   February 26, 2014, 02:39
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Julian Langowski
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Dear Paolo,
do you have any pictures of the layers?

What might help:
- set higher refinement level => layers can follow surface easier
- reduce relaxation iterations => layers don`t get destructed as much

How much layer coverage do you have in the first layer iteration?

Best regards
Julian
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Old   February 26, 2014, 04:49
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PaoloFariselli
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Hi Julien,

Thanks for your hints! I would like to raise the refinement lev. but I already have many cells so it will be hard to handle more...

Now I will try to reduce the relaxation iterations to see if anything changes...

this is what I get from the snappy log:

Detected 1528121 illegal faces (concave, zero area or negative cell pyramid volu
me)
Extruding 0 out of 788781 faces (0%). Removed extrusion at 0 faces.
Added 0 out of 1577562 cells (0%).

You have asked me :" How much layer coverage do you have in the first layer iteration?"
I don't know if I get you the right answer....

These are a couple of screenshots of my "best" mesh till now. You are watching the sails from above; wind is coming from the top ad the left side (22 angle of attack; x-axis is 0)
I hope I've been clear...
Thanks again!!!
Paolo
Attached Images
File Type: jpg mesh1.jpg (98.6 KB, 25 views)
File Type: jpg mesh2.jpg (99.7 KB, 27 views)
File Type: jpg mesh3.jpg (100.2 KB, 24 views)
File Type: jpg mesh4.jpg (98.3 KB, 19 views)
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Old   February 26, 2014, 10:49
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Julian Langowski
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Dear Paolo,
you have NLayerIter 65, which means 65 iterations of layer generation and adoption. Do you get the full layer coverage after the first iteration? Its the log part directly after the end of snapping

You also could play with:
-featureAngle

Thinking about it: Did you check, if you have 8 layers, with the outmost being 0.3 dx, that the innermost layer isn`t below your minThickness of 0.00001? Just an idea..

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Old   February 26, 2014, 17:21
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PaoloFariselli
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Dear Julian,
I've tried to follow your hints, but unfortunately nothing seems to work....I checked the minThickess parameter but everything seems to be right...

I checked the log file, and I found that i get a coverage near to 0%

Anyway thanks again for your help! I you get any other idea please tell me!
Regards,
paolo
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