Structured quadrilateral/hexahedral meshes will generallyforce cells to be placed in regions where they are not needed.Unstructured quadrilateral/hexahedralmeshes offermany of theadvantages of triangular/tetrahedral meshes for moderatelycomplexgeometries.A characteristic of quadrilateral/hexahedral elements thatmight make them more economical in some situations is thattheyratiothanpermitmuchlargeraaspecttriangular/tetrahedralcells36/253
36/253 Structured quadrilateral/hexahedral meshes will generally force cells to be placed in regions where they are not needed. Unstructured quadrilateral/hexahedral meshes offer many of the advantages of triangular/tetrahedral meshes for moderatelycomplex geometries. A characteristic of quadrilateral/hexahedral elements that might make them more economical in some situations is that they permit a much larger aspect ratio than triangular/tetrahedral cells
A large aspect ratio in a triangular/tetrahedral cell willinvariably affect the skewness of the cell, which is undesirableas it mayimpede accuracy and convergence.Therefore, if it is a relatively simple geometry in which theflow conforms well to the shape of the geometry, such as a longthin duct, use a mesh of high-aspect-ratio quadrilateral/hexahedral cells.The mesh is likely to have far fewer cells thanifweusetriangular/tetrahedral cells.37/253
37/253 A large aspect ratio in a triangular/tetrahedral cell will invariably affect the skewness of the cell, which is undesirable as it may impede accuracy and convergence. Therefore, if it is a relatively simple geometry in which the flow conforms well to the shape of the geometry, such as a long thin duct, use a mesh of high-aspect-ratio quadrilateral/ hexahedral cells. The mesh is likely to have far fewer cells than if we use triangular/tetrahedral cells
The following practices are generallyrecommended:For simple geometries,use quadrilateral/hexahedral(四边形/六面体)meshes.Forunstructuredmoderatelycomplexgeometries,usequadrilateral/hexahedralmeshes.For relatively complex geometries, use triangular/tetrahedral(=角形和四面体）meshes withwedge elements inthe boundarylayers.For extremely complex geometries, use pure triangular/tetrahedralmeshes.38/253
38/253 The following practices are generally recommended: ② For moderately complex geometries, use unstructured quadrilateral/hexahedral meshes. ① For simple geometries, use quadrilateral/hexahedral(四边形/六 面体) meshes. ③ For relatively complex geometries, use triangular/tetrahedral(三 角形和四面体) meshes with wedge elements in the boundary layers. ④ For extremely complex geometries, use pure triangular/ tetrahedral meshes
Tri/Tet vs. Quad/Hex Meshes1) For simplegeometries,quad/hexmeshesprovidehigher-qualitycancellssolutionswithfewerthanacomparabletri/tet mesh.2) For complex geometries, quad/hexmeshes show no numerical advantage,and we can save meshing effort by usinga tri/tet mesh.39/253
39/253 Tri/Tet vs. Quad/Hex Meshes 1) For simple geometries, quad/hex meshes can provide higher-quality solutions with fewer cells than a comparable tri/tet mesh. 2) For complex geometries, quad/hex meshes show no numerical advantage, and we can save meshing effort by using a tri/tet mesh
HybridMeshExampleValve port gridtet mesh) Specific regions can be meshedwithdifferent cell types.hex mesh2)Both efficiency and accuracy areenhanced relative to a hex or tetmesh alone.wedge mesh3)Tools for hybrid mesh generationareavailableinICEMHybrid mesh for an engine valve port40/253
40/253 Hybrid Mesh Example Valve port grid Hybrid mesh for an engine valve port tet mesh hex mesh wedge mesh 1) Specific regions can be meshed with different cell types. 2) Both efficiency and accuracy are enhanced relative to a hex or tet mesh alone. 3) Tools for hybrid mesh generation are available in ICEM