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# Geometry modeling and file import

Questions about geometry, NURBS and modeling operations.

#### Which are the interfaces with CAD systems?

For geometries:

- IGES (input and output)
- VDA (input)
- DXF (input and output)
- PARASOLID [*.x_t, *.x_b] (input)
- ACIS [*.sat] (input)

For meshes:

- NASTRAN
- STL
- GiD format

Other information can be entered by the means of a batch file.

#### Does GiD support Super-surfaces?

It is not possible to do it inside GiD. We have recently added some power to the structured meshing of NURBS surfaces (structured trimmed surfaces, any number of boundary lines, etc), but we rely on the shape of the original geometry. The idea of creating super-surfaces has been considered for a long time and probably, one day it will be there. The problem is that this approach give some difficulties to both, to the development stage and to the final user. The meshing of these complex surfaces would be more difficult and time consuming than the normal one. In the other side, this is an additional complication for the end user, because he must know of another GID capability.

Basically, there are to approaches when meshing:

- The user only controls mesh density and type of elements all around the domain.
- The user also controls the general shape and orientation of the generated elements.

The first approach can be made by GiD and is enough for most applications. The second approach can be made partly, or with more human effort inside GiD.

#### Can GiD create multiple volumes at the same time?

Gid have a function to create automatic 6-sided volumes (Geometry Create Volume Automatic Struct).

In the current version, several volumes can be created simultaneously, selecting all the boundary surfaces. GiD automatically detects the holes.

To add manually a hole to a volume, invoke the menu: Geometry->Create->Volume->Hole, then select the outer volume and then select the volumes of the hole.

Option `Search` in volume creation permmits to select one surface and GiD tries to find one volume that contains this surface.

#### How can a create 2D contacts?

To create a 2D contact the process is:

- Create one side lines and put them in one layer.
- Set another layer as active
- Create another line that exactly duplicates the first one. Points must be also duplicated (Typically using copy with translation 0 and with flag 'Duplicate entities' activated).
- Set another layer as active
- Create a contact surface selection the two lines or more at the same time (if using copy, it has the option 'Create contacts').
- Several element types can be generated in the contact (degenerated elements):
  - 2-noded elements (by default).
  - No elements (useful to force the same mesh in both sides)
  - Quadrilateral elements
- Layer in this process have only been set for viewing and selecting purposes. In fact, they are not necessary.

If 2-noded elements are meshed, later it is easy in the .bas file to list them by pairs.

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