2D mesh generation using DeViSoR Grid and GiD

Transcription

1 2D mesh generation using DeViSoR Grid and GiD Dipl.-Math. Jens Friedrich Acker Dipl.-Inform. (FH) Thomas Rohkämper Mathematics III: Applied Mathematics and Numerics TU Dortmund University Oct. 28, 2010

2 Contents 1 Definitions and naming conventions Geometry definition Mesh definition 2 Using DeViSoR Grid About... General approach Demonstration 3 Using GiD as mesh generator The converter suite Step by step instructions

3 Overview 1 Definitions and naming conventions Geometry definition Mesh definition 2 Using DeViSoR Grid About... General approach Demonstration 3 Using GiD as mesh generator The converter suite Step by step instructions

4 Preliminaries The description of the computational domains used by FeatFlow (and Feast ) is split into two different parts: The geometry of the domain. A computational mesh for that geometry. Remark: It is possible to create several computational meshes using the same geometry.

5 Boundaries The geometry of the computational domain is given by describing all external and internal boundaries. Each of these boundaries is in turn described as a consecutive sequence of geometric primitives, so called segments and has to be closed. Segments can be lines or arcs/circles (or even NURBS with Feast or modified parq2d.f). The segments of each boundary additionally define a parametrization that goes from 0.0 to the number of segments in that boundary. The correct orientation of each boundary parametrization is, that going along the boundary, the interior is to the left and the exterior to the right.

6 Boundary types Lines Lines are defined by a start point at local parameter value 0.0 and an end point at local parameter value 1.0. Arcs/Circles Arcs are defined by a middle point and a radius and a start angle in radians at local parameter value 0.0 and an end angle in radians at local parameter value degree radians is at the 3 o clock position. All angles have a mathematically positive orientation!

7 Mesh definition Finite Elements In a 2D mesh, finite elements are quadrangles ( quads ) or triangles ( tris ). These are the building blocks of the mesh. Quads consist of four edges, Tris consist of three edges. An edge is made up by two nodes. Two types of nodes: Inner nodes: position given as cartesian coordinates (x, y), Boundary nodes: position given as parameter value on a given boundary. Boundary node example On a circle boundary (parameterized as [0.0, 1.0)) a boundary node with parameter value 0.25 is at the 12 o clock position.

8 Mesh definition Pitfalls, 1 of 2 Depending on the configuration you use for the calculation, you have to keep in mind the following points: No two elements may share the same space. Make sure your mesh is more dense (i.e. more and smaller elements) where you expect more action. Avoid hanging nodes, i.e. nodes which are not part of a finite element. Mixed element types may not be allowed (don t mix tris and quads).

9 Mesh definition Pitfalls, 2 of 2 Depending on the configuration you use for the calculation, you have to keep in mind the following points: The program may need at least one boundary node with parameter value 0.0. Deformed elements may cause the program to crash; an element is deformed, if the inner angles are too sharp,... the aspect ratio of the edges is too great (element is very high or very wide).

10 Overview 1 Definitions and naming conventions Geometry definition Mesh definition 2 Using DeViSoR Grid About... General approach Demonstration 3 Using GiD as mesh generator The converter suite Step by step instructions

11 DeViSoR Grid About... DeViSoR= Design and Visualization Software Resource Grid = coarse mesh editor of the chair of applied mathematics of the TU Dortmund Creation of coarse grids in PRM/TRI or FEAST-format for use with the FeatFlow - and Feast -software suite. Platform-independent (Java 1.4) Available locally through the module system # module load devisor/grid/ # module load devisor/grid/ or as download from

12 Geometry and mesh generation General approach in Grid 3 Bottom-Up, bigger objects are built from smaller objects; meaning: combine nodes to edges, and edges to elements. Parts of a 2D mesh Geometry (segmentlist, arc and circle, NURBS curve) Mesh (node, edge, finite element: tris- or quads) Parts of a 3D mesh Geometry (cuboid, sphere, triangulation) Mesh (node, edge, triangle and quadrangle faces, finite element: tetrahedron or octahedron)

13 Geometry and mesh generation General approach in Grid 4 Be aware that Grid 4 is still development and does not support all features of Grid 3. Finite elements are created as a whole (element + edges + nodes). This avoids problems with hanging nodes and edges. Moving nodes on top of each other merges them. Merging start and end nodes of edges merges the edges. Deletion of items also deletes dependent items, e.g. deleting a node also deletes the edges which use the node, which in turn deletes the elements using the delete edges. Build a very coarse version of the mesh and then use the different editing modes to create a finer mesh. Editing modes Select and transform items (move, scale, rotate), geometry editing, mesh editing.

14 Demonstration Blueprint of the domain

15 Overview 1 Definitions and naming conventions Geometry definition Mesh definition 2 Using DeViSoR Grid About... General approach Demonstration 3 Using GiD as mesh generator The converter suite Step by step instructions

16 What is GiD? GiD is a general purpose pre-postprocessor for computer analysis. All the data, geometry and mesh generation can be performed inside. Also, the visualization of all types of results can be performed. We use only its mesh generator!

17 Converter suite overview

18 Importing a geometry into GiD PRM BLST: prm reader <bench1.prm >bench1.blst BLST DXF: dxf writer <bench1.blst >bench1.dxf Import: File Import DXF Collapsing points: Geometry Edit Collapse Points y z x GiD Definitions and naming conventions Using DeViSoR Grid Using GiD as mesh generator

19 Creating NURBS surfaces Geometry Create NURBS surface By contour You can split domains into a collection of subdomains, if you later want to have different mesh types in those subdomains. y z x GiD Definitions and naming conventions Using DeViSoR Grid Using GiD as mesh generator

20 Mesh generation Assign mesh types: Mesh Structured Surfaces Assign Mesh Element type Quarilateral Assign properties: Mesh Unstructured Assign sizes on... Mesh Structured Lines Assign number of cells Mesh Generate mesh Settings: Utilities Preferences Meshing y z x GiD Definitions and naming conventions Using DeViSoR Grid Using GiD as mesh generator

21 Exporting the mesh Export: Files Export Gid mesh MSH MESH: gid mesh reader <bench1.msh >bench1.mesh MESH & BLST DOMAIN: cat bench1.mesh bench1.blst project2boundary >bench1.domain DOMAIN TRI: tri param2d writer <bench1.domain >bench1.tri Checks & Tuning: Mesh control, editing and finetuning with DeViSoR Grid.