2. Meshing#

2.1. Generalities#

The*Aster* mesh file can be written (for really basic meshes) or edited manually with any text editor. It is a file read in free format, structured in blocks of information or sub-files by imposed keywords.

Various utilities have been developed to facilitate the import of meshes in*Code_Aster*. A distinction is made between:

*conversion utilities that allow the conversion of a mesh file produced by another software package (IDEAS, GIBI, GMSH…) into a mesh file in*Aster, * format

  • the command to read a mesh file in MED format, produced by Salome.

2.2. The Aster mesh file#

The structure and syntax of the **MaillageAster* file are detailed in Paper [U3.01.00].

The Aster mesh file is read from the first line to the first occurrence of a line beginning with the word FIN .**This keyword is required.**The mesh file is structured into independent subfiles starting with a **keyword and ending with the imposed keyword FINSF.

This file must have at least two sub-files:

  • the coordinates of all the nodes in the mesh in a 2D (COOR_2D) or 3D (COOR_3D) Cartesian coordinate system.

  • the description of all the meshes (TRIA3, HEXA20, etc…), on which physical properties, finite elements, boundary conditions or loads will then be assigned.

It may possibly contain groups of nodes (GROUP_NO) or meshes (GROUP_MA) to facilitate assignment operations, but also the analysis of results.

**It is essential to explicitly create the meshes located on the application borders* loads and boundary conditions at this stage. In the mesh file, we will then find:

  • the edge meshes of the required 2D elements,

  • the face meshes of the massive 3D elements required;

  • the associated edge and/or face mesh groups.

This constraint becomes bearable when using an interface, which does the work based on the indications provided during the meshing (see documents PRE_IDEAS [U7.01.01] or PRE_GIBI [U7.01.11]).

2.3. Conversion utilities#

These interfaces make it possible to convert files, with or without format, used by various software packages or calculation codes, to the conventional format of the Aster mesh file.

The interfaces currently available are those that allow the use of mesh IDEAS, mesh GIBI from CASTEM 2000 and mesh GMSH.

2.3.1. The IDEAS universal file#

The convertible file is the universal file defined by the I- DEAS documentation (see Paper [U3.03.01]). The recognition of the version IDEAS used is automatic.

A universal file IDEAS consists of several independent blocks called « data sets » . Each « data set » is surrounded by the character string -1 and numbered. The « data sets » recognized by the interface are described in the paper [U3.03.01].

2.3.2. The GIBI mesh file#

The interface is created using the command PRE_GIBI [U7.01.11]).

The convertible file is file ASCII rendered by the SAUVERFORMAT command from CASTEM 2000. The precise description of the interface is given in [U3.04.01].

2.3.3. The GMSH mesh file#

The interface is created using the command PRE_GMSH [U7.01.31]).

The convertible file is file ASCII rendered by the SAVE command from GMSH.

2.4. The mesh file in MED format#

The interface is created using the command LIRE_MAILLAGE (FORMAT =” MED “) [U4.21.01].

MED (Modeling and Data Exchanges) is a neutral data format developed by EDF R&D and CEA for data exchanges between calculation codes. MED files are binary and portable files. Reading a file MED by LIRE_MAILLAGE allows you to retrieve a mesh produced by any other code capable of creating a file MED on any other machine. This data format is in particular used for the exchange of mesh files and results between Code_Aster and Salomé or the mesh refinement tool HOMARD.

2.5. The use of incompatible meshes#

Although the finite element method recommends the use of regular meshes, without discontinuity, to obtain a correct convergence towards the solution of the continuous problem, it may be necessary for some models to use incompatible meshes: on either side of a border, the meshes do not correspond to each other’s steps. The connection of these two meshes is then managed at the level of the command file by the keyword LIAISON_MAIL of the AFFE_CHAR_MECA command. In particular, this makes it possible to connect a finely meshed zone with another zone where it is possible to be satisfied with a coarse mesh.

2.6. Adaptive mesh#

From an initial mesh, it is possible to adapt the mesh, to minimize the error committed, using the macro command MACR_ADAP_MAIL, which uses the HOMARD software. The HOMARD adaptive mesh software works on meshes made up of segments, triangles, quadrangles, tetrahedra, hexahedra, and pentahedra.

This mesh adaptation takes place after a first calculation with the Code_Aster. An error indicator will have been calculated. Depending on its value mesh by mesh, the HOMARD software will modify the mesh. It is also possible to interpolate temperature or displacement fields at the nodes of the old mesh to the new one [U7.03.01].