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bumps on flat snaped surface

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Old   September 17, 2012, 10:18
Default bumps on flat snaped surface
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I am meshing a geometry which includes (among other parts) two simple blocks on the top of each other (see fig1).
I generate first a background mesh with blockMesh and continue the meshing in snappyHexMesh. I obtain after meshing some ugly bumps in the upper block surface (see fig2).
Does one of you has already had this problem? Do you know how to fix it?

Other info:
- the castellated mesh is visible in fig3.
- I've obtained the same result with OF2.0.x and 2.1.1
- There is not a second surface inside the upper block to which the mesh could try to snap.
- I do not need to capture the edges of the box so I do not use feature curves on this specific case (the whole geometry is anyway too complex and the use of feature edges leads to the message "Did not succesfully snap mesh. Giving up.")

Thanks for your help!
Attached Images
File Type: jpg Geometry.JPG (40.3 KB, 80 views)
File Type: jpg SurfaceMesh.jpg (99.3 KB, 98 views)
File Type: jpg Castellated.jpg (100.0 KB, 88 views)

Last edited by Eloise; September 18, 2012 at 03:19.
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Old   September 18, 2012, 07:47
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I did manage to get rid of this problem. In sHM output I had:
Moving mesh ...

Iteration 0
Moving mesh using diplacement scaling : min:1  max:1
Checking faces in error :
    non-orthogonality >  65 degrees                        : 0
    faces with face pyramid volume < 1e-13                 : 0
    faces with face-decomposition tet quality < 1e-30      : 29
    faces with concavity >  80 degrees                     : 0
    faces with skewness >   4 (internal) or  20 (boundary) : 0
    faces with interpolation weights (0..1)  <  0.02       : 0
    faces with volume ratio of neighbour cells <  0.01     : 0
    faces with face twist <  0.02                          : 0
    faces on cells with determinant < 0.001                : 0
Therefore, I changed my mesh quality settings to :
minTetQuality -1e30;
The bumps disappeared, and according to checkMesh, this has not altered my final mesh quality:
Checking geometry...
    Overall domain bounding box (-900 -500 -300) (600 500 0)
    Mesh (non-empty, non-wedge) directions (1 1 1)
    Mesh (non-empty) directions (1 1 1)
    Boundary openness (1.47399e-15 4.92283e-16 3.96052e-14) OK.
    Max cell openness = 4.32074e-16 OK.
    Max aspect ratio = 23.3478 OK.
    Minumum face area = 0.00130051. Maximum face area = 997.836.  Face area magnitudes OK.
    Min volume = 0.000863934. Max volume = 30402.1.  Total volume = 4.49955e+08.  Cell volumes OK.
    Mesh non-orthogonality Max: 62.5227 average: 9.58441
    Non-orthogonality check OK.
    Face pyramids OK.
    Max skewness = 3.16795 OK.

Mesh OK.
If you think I may have some issues in my simulation related to that modification, please let me know!!

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Old   August 14, 2015, 10:19
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Great post - I had exactly the same issue (bumpy surface mesh from snappyHexMesh) and managed to fix it by changing the minTetQuality to -1e30; Thanks!
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