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Edge and Face Meshing

Edge and Face Meshing. Meshing - General. To reduce overall mesh size, confine small cells to areas where they are needed (e.g., where high gradients are expected). Controlling cell size distribution Edges, Faces and Volumes can be directly meshed

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Edge and Face Meshing

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  1. Edge and Face Meshing

  2. Meshing - General • To reduce overall mesh size, confine small cells to areas where they are needed (e.g., where high gradients are expected). • Controlling cell size distribution • Edges, Faces and Volumes can be directly meshed • A uniform mesh is generated unless pre-meshing or sizing functions are used. • Pre-meshing • Edge meshes can be graded (varying interval size on edge) • Graded edge mesh can be used to control distribution of cell size in face mesh. • Controlling distribution of cell size in face mesh also controls distribution of cell size in volume mesh. • Sizing Functions • New feature to control cell size in faces and volumes directly.

  3. Edge Meshing • Edge mesh distribution is controlled through the spacing and grading parameters. • Using the Edge meshing form • Picking • Temporary graphics • Links, Directions • Grading/Spacing • Special characteristics • Apply and Defaults • Invert and Reverse • Options

  4. Picking Edges for Meshing • Picking • Temporarily meshed edges • When you pick an edge, the edge is temporarily meshed using white nodes • Displayed edge mesh is based on current grading and spacing parameters • If you modify the scheme or spacing, the temporary mesh will be immediately updated • When you Apply, the mesh nodes will turn blue • Sense • Sense is used to show direction of grading • Every picked edge will show its sense direction using an arrow • The sense can be reversed by a shift-middle click on the last edge picked (this is in addition to the “next” functionality) or by clicking on the Reverse button

  5. Soft Links • Picking and soft links • Pick with links • By enabling this option, Soft-linked edges can be selected in a single pick • Linked edges share the same information and can be picked in a single pick • Modifying soft links • You can anytime: • Form links • Break links • Maintain links • By default, GAMBIT will form links between unmeshed edges that are picked together • By default, GAMBIT will maintain links between meshed edges that are picked together

  6. Single-sided grading Symmetric grading Asymmetric grading Grading • Controls mesh density distribution along an edge. • Grading can produce single-sided or double-sided mesh • Doubled-sided mesh can be symmetric or asymmetric. • Symmetric schemes produce symmetric mesh about edge center. • Asymmetric schemes can produce asymmetric mesh about edge center. • Single-sided grading: • Uses a multiplicative constant, R, to describe the ratio of the length of two adjacent mesh elements, i.e., • R = l(i+1) / li • R can be specified explicitly (Successive Ratio) or determined indirectly • Gambit also uses edge length and spacing information to determine R.

  7. Double Sided Grading • Symmetric grading schemes implicitly generate double sided grading that is symmetric. • Asymmetric schemes are accessible when Double-Sided Option is used with: • Successive Ratio, First Length, Last Length, First-Last Ratio, and Last-First Ratio • The mesh is symmetric if R1 and R2 are equal. • The mesh is asymmetric if R1 and R2 are not equal. • Edge center is determined automatically.

  8. Spacing • Spacing • In all meshing forms, the following spacing functions can be specified: • Interval count - recommended for edge meshing only • A value of 5 creates 5 intervals on the edge (6 nodes, including ends) • % of edge length - recommended for edge meshing only • An edge length of 10 and a value of 20 creates 5 intervals on the edge • Interval size - the default setting • Identifies the interval size relative to overall dimensions of geometry • Identifies “average” interval size if used with grading • An edge-length of 10 and a value of 2 creates 5 intervals on the edge • Average size of elements/grid is 2

  9. First Edge Settings • Use first edge settings enabled • First edge selected in pick list updates form • Useful to copy settings from one meshededge to other edges. • Use first edge settings disabled • Any time you pick two or more meshed edges where there is a difference in: • the Type • the Spacing the local Apply button for that option will be turned off • This allows you to maintain pre-existing grading and/or spacing settings for each edge. • Enforce a change in grading and/or spacing by enabling Apply button.

  10. Mesh Options • Apply without meshing • This option is useful in cases where you want to impose a scheme without fixing the number of intervals • The higher level meshing scheme will decide (and match) the intervals • Example • Specify fixed interval and no grading • Specify double sided grading andApply without Meshing on bottom edges • Face meshing will automatically match mesh • Remove Old Mesh • Deletes old mesh • Ignore Sizing Function • Sizing function has precedence on meshingunless this option is enabled.

  11. Face Meshing • Face Meshing form • Upon picking a face • GAMBIT automatically chooses Quad elements • GAMBIT chooses the Type based on the Solver/face vertex types • Available element/scheme type combinations • Quad • Map • Submap • Tri-Primitive • Pave • Tri • Pave • Gambit also has quad-to-tri conversion utility. • Quad/Tri • Map • Pave • Wedge

  12. Face Meshing - Quad Examples • Quad: Map • Quad: Submap • Quad: Tri-Primitive • Quad: Pave

  13. Face Meshing - Quad/Tri and Tri Examples • Quad/Tri: Map • Quad/Tri: Pave • Quad/Tri: Wedge • Tri: Pave

  14. Deleting Old Mesh • Existing mesh must be removed before remeshing. • Mesh can be deleted using delete mesh form. • Lower topology mesh can also be deleted (default) • Existing mesh can also be removed in all Create mesh forms without the need for Delete mesh • Remove mesh • Leaves all lower topology mesh • Remove mesh + lower mesh • Removes all lower mesh that is not shared with another entity • Undo after meshing operation also works!

  15. + + + + Hard Linking • Mesh Links (Hard Links) • Mesh linked entities have identical mesh • created for periodic boundary conditions • Applicable to Edge, Face, and Volume entities • Best to use soft links for edge meshing • To link volume meshes, all faces must be mesh hard linked first. • Setting up Hard Links for Faces • Select faces and reference vertices • Edge ‘sense’ will appear • Reverse orientation on by default for sense • Periodic option can be used for split edges • Mesh one face before or after hard link is defined • mesh on second face generated automatically • Multiple pairs of hard links can be created.

  16.  max 0 1 bestworst  min Assessing Mesh Quality • Default measure of quality is based on EquiAngle Skew. • Definition of EquiAngle Skew: • where: • max = largest angle in face or cell • min = smallest angle in face or cell • e = angle for equiangular face or cell • e.g., 60 for triangle, 90 for square • Range of skewness:

  17. Examining the Mesh Examine Mesh Form • Display Type • Plane/Sphere • View mesh elements that fall in plane or sphere. • Range • View mesh elements within quality range. • Histogram shows quality distribution. • Select 2D/3D and Element Type • Select Quality Type • Display Mode • Change cell display attributes.

  18. Striving for Quality • A poor quality grid will cause inaccurate solutions and/or slow convergence. • Minimize EquiAngle Skew: • Hex and Quad Cells • Skewness should not exceed 0.85. • Tri’s • Skewness should not exceed 0.85. • Tet’s • Skewness should not exceed 0.9 • Minimize local variations in cell size: • e.g., adjacent cells should not have ‘size ratio’ greater than 20%. • If Examine Mesh shows such violations: • Delete mesh • Perform necessary decomposition and/or pre-mesh edges and faces. • Remesh

  19. Mesh Smoothing • Gambit generally incorporates default smoothing scheme after initial mesh on face or in volume is created. • Default scheme can be changed in Edit - Defaults • Different scheme can be selected and applied after meshing. • Face mesh smoothing • Length-weighted Laplacian • Centroid Area • Winslow • Volume mesh smoothing • Length-weighted Laplacian • Equipotential • Boundary mesh by default is fixed. • Option allows frees boundary mesh.

  20. Assigning Boundary and Continuum Types • The Boundary Type Form • Enter entities to be grouped into single zone in entity list box. • First choose entity type as face or edge. • Select boundary type for zone (entity group). • Available types depend on Solver • Name zone if desired. • Apply defines zone and boundary type. • Can also modify and delete zone/boundary. • By default, • External faces/edges are walls • Internal faces/edges are interior • The Continuum Type Form • Similar operation. • All continuum zones are by default, fluid.

  21. Linear/Quadratic Elements (FIDAP/POLYFLOW USERS ONLY) • General tools • Higher-order elements • For FEM codes (FIDAP and POLYFLOW), the element order can be changed at all three meshing levels • Only linear and quadratic elements are directly available • A change to quadratic element type at one level will automatically change the element type in other levels • The following table presents the most commonly used and recommended quadratic element types for FEM - solvers POLYFLOW FIDAP edge 3-node 3-node face 8-node quad 9-node quad volume 21-node brick 27-node brick

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