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# 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 or knowledge of a mesh. Only when everything is defined, the meshing of the geometrical domain is carried out. This methodology facilitates alterations to the geometry while maintaining the attributes and conditions definitions. Alterations to the attributes or conditions can simultaneously be made without the need of reassigning to the geometry. New meshes can also be generated if necessary and all the information will be automatically assigned correctly.

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

In general, the complete solution process can be defined as:

1. define geometry - points, lines, surfaces, volumes.
   - use other facilities.
   - import geometry from CAD.
2. define attributes and conditions.
3. generate mesh.
4. carry out simulation.
5. view results.

Depending upon the results in step (5) it may be necessary to return to one of the steps (1), (2) or (3) to make alterations and rerun the simulations.

Building a geometrical domain in GiD is based on the following four geometrical levels of entities: 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. A few examples are given:

- 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 being really created is a line plus two points or a line with existing points created previously.
- example 3: When creating a volume, this 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 analysis and results visualization purposes. Thus, to build a geometry with GiD, the users must first define 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 the use of layers is to offer a visualization and selection tool, but 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.

GiD has the option of importing a geometry or a mesh that has been created by a external CAD program. At present, this can be done via a DXF, IGES, Parasolid, ACIS, VDA, Shapefile, STL or NASTRAN interfaces available inside GiD.

Attributes and conditions are applied to the geometrical entities (points, lines, surfaces and volumes) using the data input dialog box. These menus are specific to the particular solver that will be utilized 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 explained below and later in this manual.

Once the geometry and attributes have been defined, the mesh can be generated using the mesh generation tools supplied within the system. Structured and unstructured meshes containing triangular and quadrilateral surface meshes or tetrahedral and hexahedral volume meshes may be generated. The automatic mesh generation facility utilizes a background mesh concept for which the users are 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 pre-processing interfaces.

The final stage of graphic visualization is flexible in order to allow the users 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 the ability for the users to define and configure their own graphic user interface within GiD. This is done by creating 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 pre-processor or data input interface will thus be tailored specifically for the users simulation program, but using the facilities and functionality of the GiD system.

The user's simulation program can then be included within GiD so that it may be run utilizing the calculate menu option.

The third step consists of writing an interface program that provides the results information in the format required by the GiD graphic visualizer, thereby configuring the post-processing menus. This post analysis interface may be included fully into the GiD system so that it runs automatically once the simulation run has terminated.

Details on this configuration can be found in Chapters 16 and 17.

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