14.1.1 Geometry

To define the geometry, we start with GEOMETRY POINT commands for the coordinates of the necessary points of the model (by default Z = 0 ).

Points    (disc.fgc)

 
GEOMETRY POINT COORD  0.0  0.0
GEOMETRY POINT COORD -6.0 -5.1
GEOMETRY POINT COORD  6.0 -5.1
GEOMETRY POINT COORD -6.0  5.1
GEOMETRY POINT COORD  6.0  5.1
CONSTRUCT SPACE WORK-BOX 6.5 6.0 0.0 -6.5 -5.5 0.0
EYE FRAME
LABEL GEOMETRY POINTS
 

As we don't specify point names explicitly, iDIANA will name the points sequentially, in this case P1 to P5 for points O, G, H, E, and F respectively. To keep any display of the model free from the monitor in the viewport we give the CONSTRUCT SPACE WORK-BOX command to define a work box which fits widely around the model. Then the EYE FRAME and LABEL GEOMETRY POINTS commands display the points with name labels, fitted in the work box [Fig.14.3a].

Figure 14.3: Geometry definition

(a) points

(b) lines and surfaces

We will now apply the specified points to define the lines and the surface of the model geometry with some GEOMETRY commands.

Lines and surface    (disc.fgc)

 
GEOMETRY LINE CIRCLE P1 5
CONSTRUCT SET OUTER APPEND LINES ALL
GEOMETRY SURFACE REGION OUTER
GEOMETRY LINE STRAIGHT P2 P3
GEOMETRY LINE STRAIGHT P5 P4
VIEW GEOMETRY ALL VIOLET
LABEL GEOMETRY LINES ALL VIOLET
LABEL GEOMETRY SURFACES ALL WHITE
 

The definition of the circle of the disc is simple: the CIRCLE option with center point and radius. iDIANA will create four lines, i.e., four 90° arcs, along the circle and automatically call them L1 to L4 . With the CONSTRUCT SET command we collect these lines in a set called OUTER . We specify this set in the GEOMETRY SURFACE REGION command to define the outline of a `region' type surface named S1 . iDIANA offers some very useful algorithms for automatic mesh generation on such a surface [§14.1.2].

Next we define the two straight lines for the rigid plates at the top and the bottom of the disc, automatically named L5 and L6 . Here the sequence of the specified points is important, it defines the direction of the local y -axis which must point in the direction of the disc. We will discuss this later on [§14.1.2]. Note that there is a small distance between the lines and the disc: initially there is no contact.

Finally the VIEW and LABEL commands display the geometry with name labels for lines and surface [Fig.14.3b]. Note that iDIANA approximates the circle display with four straight lines per quarter. This is because the default division for meshing is four, which we have not yet overruled. In reality iDIANA `knows' that it is a real circle, the approximation is only applied for a display.