GiD Basics

GiD is a geometrical system in the sense that, having defined the geometry, all the attributes and conditions (i.e. material assignments, loading, conditions, etc.) are applied to the geometry without any reference to a mesh. Only when everything has been defined is the meshing of the geometrical domain carried out. This methodology facilitates alterations to the geometry while maintaining the definitions of the attributes and conditions. Alterations to the attributes or conditions can be made simultaneously without needing to reassign the geometry. New meshes can also be generated if necessary and all the information will automatically be assigned correctly.

GiD also provides the option of defining attributes and conditions directly to the mesh once it has been generated. However, if the mesh is regenerated, it is not possible to maintain these definitions and therefore all attributes and conditions must then be redefined.

In general, the entire workflow for a numerical simulation can be defined as:

1. Define the geometry of the domain: it can be done by generating the geometry using GiD or importing it from another CAD software.
2. Define attributes and conditions and assign it onto the geometrical model (both meshing properties and data needed for the solver).
3. Generate the mesh.
4. Carry out the simulation.
5. Visualize the results.

Depending upon the results in step (3), it may be necessary to return to the previous steps to make alterations and re-run the simulation.
Building a geometrical domain in GiD is based on four levels of geometrical entity: points, lines, surfaces and volumes. Entities of higher level are constructed over entities of lower level; two adjacent entities can therefore share the same level entity. Here are a few examples:

• Example 1: One line has two lower level entities (points), each of them at an extreme of the line. If two lines are sharing one extreme, they are really sharing the same point, which is a unique entity.

• Example 2: When creating a new line, what is really being created is a line plus two points or a line with existing points created previously.

• Example 3: When creating a volume, it is created over a set of existing surfaces, which are joined to each other by common lines. The lines are, in turn, joined to each other by common points.

All domains are considered in 3-dimensional space but if there is no variation in the third coordinate (into the screen) the geometry is assumed to be 2-dimensional for the purposes of analysis and the visualization of results. Thus, to build a geometry with GiD, the user must first define the points, join these together to form lines, create closed surfaces from the lines and define closed volumes for the surfaces. Many other facilities are provided for creating the geometrical domain; these include: copying, moving points, automatic surface creation, etc.

The geometrical domain can be created in a series of layers where each one is a separate part of the geometry. Any geometrical entity (points, lines, surfaces or volumes) can belong to a particular layer. It is then possible to view and manipulate some layers and not others. The main purpose of these layers is to offer a visualization and selection tool as they are not used in the analysis. An example of the use of layers might be a chair where the four legs, seat, backrest and side arms are the different layers.

With GiD you can import a geometry or mesh created with an external CAD program.
The geometry formats supported at present are: IGES, STEP, Parasolid, ACIS , VDA, DXF, KML (Google Earth), Shapefile and Rhinoceros file formats together with several cartographical and topographical formats.

Mesh data can be read in NASTRAN, STL, VRML, 3DStudio, CGNS, VTK and other formats

Attributes and conditions are applied to the geometrical entities (points, lines, surfaces and volumes) using data input dialog boxes. These menus are specific to the particular solver that will be employed for the simulation and, therefore, the solver needs to be defined before attributes are defined. The form of these menus can also be configured for the user's own solver module, as is explained below and later in this manual.

Once the geometry and attributes have been defined, a mesh can be generated using the mesh generation tools supplied within the system. User can generate the mesh with the default parameters, or can set specific meshing properties or tune some parameters in order to get the desired final mesh. Structured, semi-structured, unstructured and cartesian meshes can be generated, containing triangular,quadrilateral or circle elements for surface meshes and tetrahedral, hexahedral, prism or sphere elements for volume meshes. Different kinds of quadratic type can be generated for each element type. The automatic mesh generation facility uses a background mesh concept for which the user is required to supply a minimum number of parameters.

Simulations are carried out from within GiD by using the calculate menu. Indeed, specific solvers require specific data that must have been prepared previously. A number of solvers may be incorporated together with the correct preprocessing interfaces.

The final stage of graphic visualization is flexible in order to allow the user to critically evaluate the results quickly and easily. The menu items are generally determined by the results supplied by the solver module. This not only reduces the amount of information stored but also allows a certain degree of user customization.

One of the major strengths of GiD is that the user can define and configure his own graphic user interface within GiD. The first step is to create some configuration files which define new windows, where the final user will enter data, such as materials or conditions. The format that GiD uses to write a file containing the necessary data in order to run the numerical simulation program must also be defined in a similar way. This preprocessor or data input interface will thus be tailored specifically to the user's simulation program, but employing the facilities and functionality of the GiD system. The second step is to include the user's simulation program within GiD so that it may be run utilizing the calculate menu option. The third step consists of writing an interface program, or using the 'gidpost' library, which provides the results information in the format required by the GiD graphic visualizer, thereby configuring the postprocessing menus. This post-analysis interface may be included fully in the GiD system so that it runs automatically once the simulation run has terminated.

Details on this configuration can be found in later chapters.