An Introduction to Unstructured Mesh Generation Material tret
- Slides: 39
An Introduction to Unstructured Mesh Generation Material tret de: S. J. Owen, "A Survey of Unstructured Mesh Generation Technology", Proceedings 7 th International Meshing Roundtable, 1998.
Tri/Tetrahedral Meshing Triangle and tetrahedral meshing are by far the most common forms of unstructured mesh generation. Most techniques currently in use can fit into one of three main categories: • Delaunay; • Quadtree/Octree; • Advancing Front.
Delaunay A typical approach is to first mesh the boundary of the geometry to provide an initial set of nodes. The boundary nodes are then triangulate according to the Delaunay criterion. Nodes are then inserted incrementally into the existing mesh, redefining the triangles or tetrahedra locally as each new node is inserted to maintain the Delaunay criterion. It is the method that is chosen for defining where to locate the interior nodes that distinguishes one Delaunay algorithm from another.
Delaunay • Begin with Bounding Triangles (or Tetrahedra)
Delaunay • Insert boundary nodes using Delaunay method (Lawson or Bowyer-Watson)
Delaunay • Insert boundary nodes using Delaunay method (Lawson or Bowyer-Watson)
Delaunay • Insert boundary nodes using Delaunay method (Lawson or Bowyer-Watson)
Delaunay • Insert boundary nodes using Delaunay method (Lawson or Bowyer-Watson)
Delaunay • Insert boundary nodes using Delaunay method (Lawson or Bowyer-Watson)
Delaunay • Recover boundary • Delete outside triangles • Insert internal nodes
Delaunay h Grid Based • Nodes introduced based on a regular lattice • Lattice could be rectangular, triangular, quadtree, etc… • Outside nodes ignored Node Insertion
Delaunay Grid Based • Nodes introduced based on a regular lattice • Lattice could be rectangular, triangular, quadtree, etc… • Outside nodes ignored Node Insertion
Delaunay Centroid • Nodes introduced at triangle centroids • Continues until edge length, Node Insertion
Delaunay l Centroid • Nodes introduced at triangle centroids • Continues until edge length, Node Insertion
Delaunay Circumcenter (“Guaranteed Quality”) • Nodes introduced at triangle circumcenters • Order of insertion based on minimum angle of any triangle • Continues until minimum angle > predefined minimum (Chew, Ruppert, Shewchuk) Node Insertion
Delaunay Circumcenter (“Guaranteed Quality”) • Nodes introduced at triangle circumcenters • Order of insertion based on minimum angle of any triangle • Continues until minimum angle > predefined minimum (Chew, Ruppert, Shewchuk) Node Insertion
Delaunay A B C Advancing Front • “Front” structure maintained throughout • Nodes introduced at ideal location from current front edge (Marcum, 95) Node Insertion
Delaunay Advancing Front • “Front” structure maintained throughout • Nodes introduced at ideal location from current front edge (Marcum, 95) Node Insertion
Delaunay Voronoi-Segment • Nodes introduced at midpoint of segment connecting the circumcircle centers of two adjacent triangles (Rebay, 93) Node Insertion
Delaunay Voronoi-Segment • Nodes introduced at midpoint of segment connecting the circumcircle centers of two adjacent triangles (Rebay, 93) Node Insertion
Delaunay h h h Edges • Nodes introduced at along existing edges at l=h • Check to ensure nodes on nearby edges are not too close (George, 91) Node Insertion
Delaunay Edges • Nodes introduced at along existing edges at l=h • Check to ensure nodes on nearby edges are not too close (George, 91) Node Insertion
Delaunay Boundary Intersection • Nodes and edges introduced where Delaunay edges intersect boundary Boundary Constrained
Delaunay Boundary Intersection • Nodes and edges introduced where Delaunay edges intersect boundary Boundary Constrained
Delaunay Local Swapping • Edges swapped between adjacent pairs of triangles until boundary is maintained Boundary Constrained
Delaunay Local Swapping • Edges swapped between adjacent pairs of triangles until boundary is maintained Boundary Constrained
Delaunay Local Swapping • Edges swapped between adjacent pairs of triangles until boundary is maintained Boundary Constrained
Delaunay Local Swapping • Edges swapped between adjacent pairs of triangles until boundary is maintained Boundary Constrained
Delaunay Local Swapping • Edges swapped between adjacent pairs of triangles until boundary is maintained (George, 91)(Owen, 99) Boundary Constrained
Octree/Quadtree • Define intial bounding box (root of quadtree) • Recursively break into 4 leaves per root to resolve geometry • Find intersections of leaves with geometry boundary • Mesh each leaf using corners, side nodes and intersections with geometry • Delete Outside • (Yerry and Shephard, 84), (Shepherd and Georges, 91)
Octree/Quadtree QMG, Cornell University
Octree/Quadtree QMG, Cornell University
Advancing Front C A B • Begin with boundary mesh - define as initial front • For each edge (face) on front, locate ideal node C based on front AB
Advancing Front r C A • Determine if any other nodes on current front are within search radius r of ideal location C (Choose D instead of C) D B
Advancing Front D • Book-Keeping: New front edges added and deleted from front as triangles are formed • Continue until no front edges remain on front
Advancing Front • Book-Keeping: New front edges added and deleted from front as triangles are formed • Continue until no front edges remain on front
Advancing Front • Book-Keeping: New front edges added and deleted from front as triangles are formed • Continue until no front edges remain on front
Advancing Front • Book-Keeping: New front edges added and deleted from front as triangles are formed • Continue until no front edges remain on front
Advancing Front r C A B • Where multiple choices are available, use best quality (closest shape to equilateral) • Reject any that would intersect existing front • Reject any inverted triangles (|AB X AC| > 0) • (Lohner, 88; 96)(Lo, 91)
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