In top down construction, you use geometric primitives (fully-defined lines, areas, and volumes) to assemble your model. As you create a primitive, the program automatically creates all the "lower" entities associated with it. A geometric primitive is a commonly used solid modeling shape (such as a sphere or regular prism) that can be created with a single Mechanical APDL command.
Because primitives are higher-order entities that can be constructed without first defining any keypoints, model generation that uses primitives is sometimes referred to as "top down" modeling. (When you create a primitive, the program automatically creates all the necessary lower-order entities, including keypoints.) Geometric primitives are created within the working plane.
You can freely combine bottom up and top down modeling techniques, as appropriate, in any model. Remember that geometric primitives are built within the working plane while bottom up techniques are defined against the active coordinate system. If you are mixing techniques, you may wish to consider using the CSYS,WP or CSYS, 4 command to force the coordinate system to follow the working plane.
Caution: Solid modeling operations in a toroidal coordinate system are not recommended. Areas or volumes generated may not be what you expect.
Any area primitives you create will lie flat on the working plane and will be oriented according to the working plane coordinate system. Area primitives must have surface areas greater than zero (that is, you cannot create a degenerate area as a means of defining a line).
The interface between two touching primitives will create a seam of discontinuity in the finite element model, unless you take steps to "weld" that seam shut, using commands such as NUMMRG, AADD, or AGLUE.
You can define area primitives using the methods described in the following table.
| Create a | Command |
|---|---|
| rectangular area anywhere on the working plane | RECTNG |
| rectangular area by corner points | BLC4 |
| rectangular area by center and corner points | BLC5 |
| circular area centered about the working plane origin | PCIRC |
| circular area anywhere on the working plane | CYL4 |
| circular area by end points | CYL5 |
| regular polygonal area centered about the working plane origin | RPOLY |
| regular polygonal area anywhere on the working plane | RPR4 |
| arbitrary polygonal area based on working plane coordinate pairs | POLY [1] |
When you define an arc segment of a circular geometric primitive
(PCIRC and CYL4 discussed above, or
CONE, CYLIND, SPHERE,
and TORUS discussed in the next section on volume primitives) the
arc sector begins at the algebraically smaller
angle, extends in a positive angular direction, and ends at the larger angle. (The input order of
THETA1, THETA2 on these
commands does not define the starting and ending
angles of the arc sector.) The following figure illustrates how these commands
work:
Volume primitives are positioned relative to the working plane as outlined in their command descriptions.
The interface between two touching primitives will create a seam of discontinuity in the finite element model, unless you take steps to "weld" that seam shut, using commands such as NUMMRG, VGLUE, or VADD.
You can define volume primitives using the methods described in the following table.
| Create a | Command |
|---|---|
| block volume based on working plane coordinates | BLOCK |
| block volume by corner points | BLC4 |
| block volume by center and corner points | BLC5 |
| cylindrical volume centered about the working plane origin | CYLIND |
| cylindrical volume anywhere on the working plane | CYL4 |
| cylindrical volume by end points | CYL5 |
| regular prism volume centered about the working plane origin | RPRISM |
| prism volume anywhere on the working plane | RPR4 |
| arbitrary prism based on working plane coordinate pairs | PRISM [1] |
| spherical volume centered about the working plane origin | SPHERE |
| spherical volume anywhere on the working plane | SPH4 |
| spherical volume by diameter end points | SPH5 |
| conical volume centered about the working plane origin | CONE |
| conical volume anywhere on the working plane | CON4 |
| toroidal volume [2] | TORUS |
You must use the PTXY command to define coordinate pairs before issuing the PRISM command.
See Creating a Torus or Toroidal Sector for more information on toroidal volumes.
You can use the
TORUS,RAD1,RAD2,RAD3,THETA1,THETA2
command to create either a torus or a
toroidal sector. To create a torus, you do not need to specify values for
THETA1 or THETA2. You
must specify three values to define the radii of the torus
(RAD1, RAD2, and
RAD3). You can specify the radii in any order.
The smallest of the values is the inner minor radius, the intermediate value is
the outer minor radius, and the largest value is the major radius. (There is one
exception regarding the order of the radii values - if you want to create a
solid torus, specify zero or blank for the inner minor radius, in which case the
zero or blank must occupy either the
RAD1 or RAD2
position.) At least two of the values that you specify must be positive values.
They will be used to define the outer minor radius and the major radius.
To create the torus shown in Figure 5.16: Torus Primitive, the command
TORUS,5,1,2 was issued. Due to the sizes of the specified
radii values relative to one another, 5, 1, and 2 were used to define the major
radius, inner minor radius, and outer minor radius of the torus, respectively.
Since no values for THETA1 and
THETA2 were specified, the default values of 0
and 360 were used as the starting and ending angles of the torus. (See Figure 5.17: Toroidal Sector for a view of a toroidal sector showing all
radii.)
See the description of the TORUS command for additional details.
To create the toroidal sector shown in Figure 5.17: Toroidal Sector, the command TORUS,5,1,2,0,180 was issued, where 5, 1, and 2 are the major radius, inner minor radius, and outer minor radius of the torus, and 0 and 180 are the starting and ending angles of the torus.
