SnappyHexMesh is checking the mesh quality parameters during mesh generation and trying to fix any violation. Nevertheless, it may happen that not all violation are fixed. Therefore, it is very important to analyze the quality of the final mesh.

• checkMesh is a utility which analyzes and evaluates mesh statistics and quality parameters.
• It can be run anytime by a command:
  # checkMesh > log.checkMesh &
• The log contains following information:
  • Statistics about number of points, faces, cells, etc.

    Mesh stats
        points:           144661
        faces:            356007
        internal faces:   332729
        cells:            106949
        faces per cell:   6.439855
        boundary patches: 6
        point zones:      0
        face zones:       0
        cell zones:       0
    

  • Number of cells of each type and number of cells with respect to their face numbers…

    Overall number of cells of each type:
        hexahedra:     77446
        prisms:        2136
        wedges:        0
        pyramids:      0
        tet wedges:    192
        tetrahedra:    0
        polyhedra:     27175
        Breakdown of polyhedra by number of faces:
            faces   number of cells
                4   756
                5   617
                6   6387
                7   6550
                8   2076
                9   8312
               10   168
               11   85
               12   1687
               13   6
               14   38
               15   476
               18   17
    

  • Topology checks and statistics:

    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).
    

  • Patch topology checks and statistics:

    Checking patch topology for multiply connected surfaces...
                       Patch    Faces   Points                  Surface topology
                defaultPatch     1694     1842  ok (non-closed singly connected)
                       inlet       70       90  ok (non-closed singly connected)
                      outlet       88      115  ok (non-closed singly connected)
                           C     1718     1855      ok (closed singly connected)
                           F    15342    17499      ok (closed singly connected)
                           D     4366     4719      ok (closed singly connected)
    

  • Geometry checks and quality parameters:

    Checking geometry...
        Overall domain bounding box (-0.3 -0.2005916 -0.2501288) (2 1.200554 0.2501306)
        Mesh has 3 geometric (non-empty/wedge) directions (1 1 1)
        Mesh has 3 solution (non-empty) directions (1 1 1)
        Boundary openness (2.337983e-17 -1.285235e-17 -1.306927e-16) OK.
        Max cell openness = 5.136153e-16 OK.
        Max aspect ratio = 34.99396 OK.
        Minimum face area = 4.305349e-08. Maximum face area = 0.01093973.  Face area magnitudes OK.
        Min volume = 3.810619e-11. Max volume = 0.001116064.  Total volume = 1.442543.  Cell volumes OK.
        Mesh non-orthogonality Max: 64.37104 average: 12.90798
        Non-orthogonality check OK.
        Face pyramids OK.
        Max skewness = 3.127895 OK.
        Coupled point location match (average 0) OK.
    
    Mesh OK.
    

  • If any of the evaluated parameter is out of the safe range, it is highlighted by  sign. An example of a log with several problems is printed below:

    Checking geometry...
        Overall domain bounding box (0.02025607 -0.1065428 -0.14) (0.2423177 0.1190007 0.22)
        Mesh has 3 geometric (non-empty/wedge) directions (1 1 1)
        Mesh has 3 solution (non-empty) directions (1 1 1)
        Boundary openness (5.491257e-16 -7.362733e-17 1.666132e-16) OK.
     ***High aspect ratio cells found, Max aspect ratio: 1036.362, number of cells 58
      < 70 degrees) faces: 5192.
        Non-orthogonality check OK.
      <
    
    

  • high aspect ratio cells usually appears in very fine boundary layers. It is not fatal for the solver stability, but can significantly decrease convergence speed.
  • non-orthogonality (nO) is a very important parameter, we can define three ranges of its values:
    • nO  70 – safe values
    • 70  nO < 90 – require special treatment of , e.g., nonOrthoCorrectors in fvSolution or numerical schemes in fvSchemes
    • nO  9O – bad mesh, which can not be used for simulation
  • skewness – higher values may impair quality (accuracy) of the results, anyway any reasonably high value (20) of skewness parameter can be used for simulation