Using AUTOCAD to prepare geometric data for PHOENICS

Liao Ganli (CHAM)

23 February 1998

An interface program is currently been created in CHAM. It will enable AUTOCAD users to export their geometry design and turn it into input files for PHOENICS VR.

However, some AUTOCAD designs can not be easily converted into VR input files because they contain too many details, and, more importantly, they do not contain certain geometry entities which are required by PHOENICS.

This document describes what the AUTOCAD user should do in the design stage so that its output can be exported to data files that can be converted into PHOENICS VR input by the interface program.

An AUTOCAD design for inputing to PHOENICS can belong to one of the following three categories

  1. Single Solid Object
  2. 3D Mesh
  3. Entire Geometry
The AUTOCAD design for a single solid object will be exported to an STL file for converting into a VR geometry data file. The AUTOCAD design for a 3D mesh will be exported to a DXF file for converting into a VR geometry data file. The AUTOCAD design for the entire geometry will be exported to a DXF file for converting into a VR Q1 file plus some VR geometry files.

As the three categories of AUTOCAD designs are treated rather differently from the interface point of view, what the user should do in AUTOCAD is described separately in the following sections.

Single Solid Object

A single solid object should be a 3D object that is created using solid modeling in AUTOCAD. You may use all necessary 3D modeling tools to create complex solids as long as the object can eventually be exported to an STL file.

Even though surface modeling can also be used in AUTOCAD to create a 3D object, at the moment, it is not known to us at CHAM whether a 3D object created in this way can be exported to STL format or not.

In general, a single solid object in AUTOCAD needs to be exported to an STL file which contains facet description for the complete surface of the object.

3D Mesh

A 3D mesh should be created as a rectangular mesh which specifies the M and N size values and all the necessary vertex values in 3D space. The mesh should represent a contour in Z direction and its outer boundaries should show a rectangular box when projected to X-Y plane.

The 3D mesh data should be exported to a DXF file for PHOENICS interface to handle.

Entire Geometry

An AUTOCAD design showing more than one identifiable objects needs to make sure of the following before it can be imported into PHOENICS.

A 3D rectangular box in wireframe shape is required to cover the intended domain for PHOENICS simulation. As the entire geometry will be converted to a VR Q1 file, it is important to let PHOENICS know the relative positions between objects and the whole domain.

One or more BLOCKS can be defined within the domain box. Each BLOCKS entity can have its own LOCATION, SCALE and ROTATION attributes, and will be treated as a single object in PHOENICS VR.

Within each BLOCKS bounding box, 3DFACE entities are required for describing the outer surfaces of the shape. It is important to define the four corner vertexes of each 3DFACE in the correct sequence so that the normal vector points outside. If 3DFACE entities are not used for a BLOCKS definition, it will be treated as a 3D rectangular box.

A BLOCKS entity can be 2D in space. It corresponds to a 2D object in PHOENICS VR. In this case, 3DFACE entities must be defined on both visible sides.

When the whole design is saved into a DXF file, the PHOENICS interface program will use the general layout to create a VR Q1 file, while the 3DFACE definitions are turned into VR geometry data files.

If some of the BLOCKS type of objects are indeed 3DSOLIDS, they need to be exported to STL files separately. The interface will treat such BLOCKS as rectangular boxes and PHOENICS VR Editor will later on load the shapes from those STL files and fit them back into the objects.