[wc34071209]

Hi everybody,

It seems that snappyHexMesh and checkMesh calculate face skewness in a different way.


snappyHexMesh output

Checking final mesh ...
Checking faces in error :
    non-orthogonality > 80  degrees                        : 0
    faces with face pyramid volume < 1e-30                 : 0
    faces with concavity > 80  degrees                     : 0
    faces with skewness > 4   (internal) or 20  (boundary) : 0
    faces with interpolation weights (0..1)  < 0.005       : 0
    faces with volume ratio of neighbour cells < 0.001     : 0
    faces with face twist < 0.001                          : 152
    faces on cells with determinant < 0.0001                : 54
Finished meshing with 206 illegal faces (concave, zero area or negative cell pyramid volume)
Finished meshing in = 98.52 s.
End

checkMesh output

Checking geometry...
    Overall domain bounding box (-0.600453913 -0.000453508017 -6.17837081e-08) (0.60016445 2.4400002 2.46000011)
    Mesh (non-empty, non-wedge) directions (1 1 1)
    Mesh (non-empty) directions (1 1 1)
    Boundary openness (-1.48156865e-16 8.02340479e-17 -2.54456316e-16) OK.
    Max cell openness = 3.16296583e-16 OK.
    Max aspect ratio = 26.777507 OK.
    Minimum face area = 3.60752727e-08. Maximum face area = 0.00704660078.  Face area magnitudes OK.
    Min volume = 1.40396368e-10. Max volume = 0.000584855605.  Total volume = 7.1436757.  Cell volumes OK.
    Mesh non-orthogonality Max: 79.940617 average: 12.8038228
   *Number of severely non-orthogonal (> 70 degrees) faces: 329.
    Non-orthogonality check OK.
  <<Writing 329 non-orthogonal faces to set nonOrthoFaces
    Face pyramids OK.
 ***Max skewness = 4.39520581, 1 highly skew faces detected which may impair the quality of the results
  <<Writing 1 skew faces to set skewFaces
    Coupled point location match (average 0) OK.

Failed 1 mesh checks.

End

[akidess]

I'm guessing the skew face that snappyHexMesh reports lies on the boundary.

[wc34071209]

Originally Posted by akidess

I'm guessing the skew face that snappyHexMesh reports lies on the boundary.

Yes, you are right. But do you know why this happens?

[cibanez]

Hi,

It happens because when checkMesh checks for face skewness it only uses one threshold value to issue the warning. That is skewThreshold_ = 4, which is hardcoded.

You can take a look at https://develop.openfoam.com/Develop...eck.C#L565-630 to see that checkMesh doesn't distinguish between internal or boundary faces to write out the warning. At the beginning of this file you will also find the hardcoded variables used as threshold values. For instance it can be seen that nonOrthThreshold_ = 70 degrees for non-orthogonality.

It could then be a good practice to lower maxBoundarySkewness in your meshQualityDict to 4 when creating the mesh with SHM to be sure to avoid checkMesh warnings.