CFD Online Discussion Forums - [snappyHexMesh] non-orthogonal faces in layers

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non-orthogonal faces in layers

Hi Foamers,

I have a hull geometry on which I try to insert layers. In order to reach a reasonable yPlus, I need to insert about 20 layers. When I do so, I nearly don't have any layer cells added on my geometry.
In order to understand why the cells are not inserted, I relaxed the mesh quality in sHM. I obtained a mesh in which the layers are inserted and many non-orthogonal faces are present. They are probably the reason why the layers are not inserted when I keep un-relaxed mesh quality requirement.

I do not understand how to get rid of those non-orthogonal faces. They are located inside the layer, between polyhedral cells which are perfectly piled up, and I don't see any reason why those faces are detected as non-orthogonal. :( In attached image, the layer's cells are in grey and the non-orthogonal faces in red.

Help needed please!
Regards,
Eloïse

Hi,

I'm still stuck with this issue of non-orthogonal faces. Here is a typical output of checkMesh of the snapped mesh (Time2) and for the layer mesh (Time 3):

Code:

Time = 2



Mesh stats

    points:           2354368

    faces:            6789693

    internal faces:   6645664

    cells:            2219640

    boundary patches: 6

    point zones:      0

    face zones:       0

    cell zones:       0



Overall number of cells of each type:

    hexahedra:     2170238

    prisms:        1958

    wedges:        0

    pyramids:      0

    tet wedges:    75

    tetrahedra:    0

    polyhedra:     47369



Checking topology...

    Boundary definition OK.

    Cell to face addressing OK.

    Point usage OK.

    Upper triangular ordering OK.

    Face vertices OK.

    Number of regions: 1 (OK).



Checking patch topology for multiply connected surfaces ...

    Patch               Faces    Points   Surface topology                  

    frontAndBack        15060    15624    ok (non-closed singly connected)  

    outlet              4650     4836     ok (non-closed singly connected)  

    inlet               4650     4836     ok (non-closed singly connected)  

    upperWall           38905    39312    ok (non-closed singly connected)  

    lowerWall           40342    41260    ok (non-closed singly connected)  

    HULL        40422    43904    ok (non-closed singly connected)  



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 (1.07829e-15 4.53941e-16 4.62619e-14) OK.

    Max cell openness = 3.80173e-16 OK.

    Max aspect ratio = 23.3478 OK.

    Minumum face area = 0.0110912. Maximum face area = 997.836.  Face area magnitudes OK.

    Min volume = 0.00975568. Max volume = 30402.1.  Total volume = 4.49867e+08.  Cell volumes OK.

    Mesh non-orthogonality Max: 60.4405 average: 4.73595

    Non-orthogonality check OK.

    Face pyramids OK.

    Max skewness = 3.25394 OK.



Mesh OK.



Time = 3



Mesh stats

    points:           2773606

    faces:            7985817

    internal faces:   7842477

    cells:            2608303

    boundary patches: 6

    point zones:      0

    face zones:       0

    cell zones:       0



Overall number of cells of each type:

    hexahedra:     2508558

    prisms:        2220

    wedges:        0

    pyramids:      0

    tet wedges:    113

    tetrahedra:    1

    polyhedra:     97411



Checking topology...

    Boundary definition OK.

    Cell to face addressing OK.

    Point usage OK.

    Upper triangular ordering OK.

    Face vertices OK.

    Number of regions: 1 (OK).



Checking patch topology for multiply connected surfaces ...

    Patch               Faces    Points   Surface topology                  

    frontAndBack        15060    15624    ok (non-closed singly connected)  

    outlet              4650     4836     ok (non-closed singly connected)  

    inlet               4650     4836     ok (non-closed singly connected)  

    upperWall           38905    39312    ok (non-closed singly connected)  

    lowerWall           40342    41260    ok (non-closed singly connected)  

    HULL        39733    43297    ok (non-closed singly connected)  



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 (1.70792e-15 7.25352e-16 4.68483e-14) OK.

    Max cell openness = 1.16192e-14 OK.

    Max aspect ratio = 506.676 OK.

    Minumum face area = 3.9423e-05. Maximum face area = 997.836.  Face area magnitudes OK.

    Min volume = 6.24616e-05. Max volume = 30402.1.  Total volume = 4.49867e+08.  Cell volumes OK.

    Mesh non-orthogonality Max: 88.4737 average: 5.90217

   *Number of severely non-orthogonal faces: 3562.

    Non-orthogonality check OK.

  <<Writing 3562 non-orthogonal faces to set nonOrthoFaces

    Face pyramids OK.

    Max skewness = 2.99817 OK.



Mesh OK.

As you can see, I have a good quality mesh ("Mesh OK") at time 2 and 3, with however a warning on the non-orthogonality at time 3. Those non-orthogonal faces are located on flat faces, rounded faces (large radius) and along an edge which is covered by the layer (edge angle ~30deg). They lead to a crash during the computation, so I really need to avoid them.
I add layers while relaxing the mesh quality in the following way:

Code:

    relaxed

    {

    maxBoundarySkewness 1000; 

    maxInternalSkewness 40;

    minTetQuality -1e30; 

    minDeterminant -1;

    minTwist -1e30;

    maxNonOrtho 180;

    }

Of course, the relaxed maxNonOrtho allows those non-orthogonal faces to appear. If I don't relax this parameter, a too large part of the patches is not covered by layers. Does anyone has an idea about a way to improve the mesh quality in the layers so that the layers cover the whole patch without all those non-orthogonal errors?

Regards,
Eloïse

Hi,

there is a good thread here, but I do not find it now.
You can switch off all mesh quality parameters to create the layers on the whole patch.

What sHM are you using? I have no relaxation{} in my sHMD :)

Hello Tobi,

If you find the link to that thread, I'd really be interested! I've been browsing quite a lot around but haven't read about that topic.

I am using OF2.0.1. The relaxation appears under the meshQualityControls in the following way:

Code:

meshQualityControls

{

    maxNonOrtho 65;

    maxBoundarySkewness 20;

    maxInternalSkewness 4;

    maxConcave 80;

    minVol 1e-13;

    minTetQuality 1e-30;

    minArea -1;

    minTwist 0.02;

    minDeterminant 0.001;

    minFaceWeight 0.02;

    minVolRatio 0.01;

    minTriangleTwist -1;



    // Advanced

    nSmoothScale 4;

    errorReduction 0.75;



    relaxed

    {

    maxBoundarySkewness 1000; 

    maxInternalSkewness 40;

    minTetQuality -1e30; 

    minDeterminant -1;

    minTwist -1e30;

    }

}

If I relax the maxNonOrtho, I indeed cover the whole surface with layers, but I get non-orthogonal faces in the layers which lead to a crash when solving.
If I don't relax, I get holes in my layers.

Regards,
Eloïse

Quote:

Originally Posted by Eloise (Post 388431)

Code:

meshQualityControls

{

    maxNonOrtho 65;

    maxBoundarySkewness 20;

    maxInternalSkewness 4;

    maxConcave 80;

    minVol 1e-13;

    minTetQuality 1e-30;

    minArea -1;

    minTwist 0.02;

    minDeterminant 0.001;

    minFaceWeight 0.02;

    minVolRatio 0.01;

    minTriangleTwist -1;



    // Advanced

    nSmoothScale 4;

    errorReduction 0.75;



    relaxed

    {

    maxBoundarySkewness 1000; 

    maxInternalSkewness 40;

    minTetQuality -1e30; 

    minDeterminant -1;

    minTwist -1e30;

    }

}

Hi,

thats nice to know. I am using 2.1.x and in my sHMDict I have no entry of the relaxation factors ... :confused:
If I get it I ll send it to you

Quote:

Originally Posted by Tobi (Post 388436)

Hi,

thats nice to know. I am using 2.1.x and in my sHMDict I have no entry of the relaxation factors ... :confused:
If I get it I ll send it to you

Hi Eloïse,

do you get a output if you are setting layers that you use relaxed things?
I insert the relax entry but its not working :(
My output

Code:

Shrinking and layer addition phase

----------------------------------



Using mesh parameters 

{

    maxNonOrtho     65;

    maxBoundarySkewness 20;

    maxInternalSkewness 4;

    maxConcave      60;

    minVol          1e-13;

    minTetQuality   1e-30;

    minArea         1e-30;

    minTwist        0.03;

    minDeterminant  0.001;

    minFaceWeight   0.03;

    minVolRatio     0.01;

    minTriangleTwist -1;

    nSmoothScale    4;

    errorReduction  0.75;

}

Hmmm :/

SOLVED

I insert the lines into the wrong section :D

Hi Tobi,

Great to read that it is still an option in OF 2.1 :)

Have a nice week,
Eloïse