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## Mesh quality check

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

- Statistics about number of points, faces, cells, etc.

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