Chapter 11: Number Control and Element Reordering

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11.1 Number Control

The commands and GUI paths described in this chapter give you control of the numbering of keypoints, lines, areas, volumes, elements, nodes, element types, real constant sets, materials, coupled DOF sets, constraint equations, and coordinate systems. Control of the numbering is useful, and sometimes essential, when combining separately modeled portions of a model into one.

Caution: The numbering of Boolean output entities is not 100% predictable. That is, sometimes the same Boolean operation, when performed on different computer systems, will assign different numbers to output entities. If you plan to generate an input stream file interactively on one machine and rerun that input stream on another system, you should avoid using entity identification numbers in your command stream. Instead, use selecting logic to identify specific entities as needed.

11.1.1 Merging Coincident Items

If two separate entities have the same location, you can merge these entities together into a single entity using one of these methods:

Command(s):

NUMMRG

GUI:

Main Menu>Preprocessor>Numbering Ctrls>Merge Items

Note-Use of the NUMMRG command or its equivalent GUI path does not cause changes to a model's geometry; only the topology is affected.

For example, suppose that you have two separate but coincident nodes. If you use the command NUMMRG,NODE (or the equivalent GUI path) to merge the nodes, the higher numbered node will be deleted and will be replaced with the lower numbered coincident node. Two coincident nodes will thus be replaced by a single node.

If two regions that have already been meshed are to be joined, you need to execute three separate merge operations. If, for example, you are using the command input method, you would first issue a merge command for nodes [NUMMRG,NODE], then one for elements [NUMMRG,ELEM], and last, one for all solid model entities [NUMMRG,KP]. (Or, you could simply use NUMMRG,ALL to merge all selected coincident items in the proper sequence. NUMMRG,ALL will also merge non-geometric items such as MAT, CE, etc.)

Many solid modeling operations create coincident keypoints, lines, and/or areas. You can use NUMMRG,KP (or the equivalent GUI path) to merge such coincident items. Keypoint locations are used as the basis for merging. Once coincident keypoints are merged, any higher order solid model entities (lines, areas, and volumes) attached to those keypoints are automatically considered for merging. The definition of coincident changes depending on the tolerances used. By default, merging of keypoints attached to lines is done when the distance between keypoints falls within the following criteria:

1. 1E-4 units of each other (see Figure 11-1), and

2. 1E-5 times the length of the longest line connected to the keypoints in consideration

Criterion 1 above describes the consideration tolerance field (TOLER) on NUMMRG. TOLER is a consideration tolerance. If a keypoint is within TOLER of another keypoint, then those two keypoints are candidates to be merged. If when "moving" the higher numbered keypoint, the distance exceeds the internal relative solid model tolerance (criterion 2 above), the keypoints will not be merged. Lines, areas, and volumes are considered for merging in a similar manner. Criterion 2 describes the default internal relative solid model tolerance, a tolerance designed to protect you from eliminating tiny lines in the model by the merge operation. Both criteria must be satisfied for keypoints to be merged.

Figure 11-1 Default merge tolerances

The internal relative tolerance (criterion 2) can be overridden by an option to specify a global solid model tolerance (GTOLER) on NUMMRG. GTOLER is a global, absolute tolerance, rather than a relative tolerance. If GTOLER is used, the size of the lines attached to keypoints is no longer considered and it is fairly easy to defeature your model by using too large a value for GTOLER. You should save your database before attempting to merge, especially when using the GTOLER option.

The following example, which corresponds to Figure 11-2, illustrates the use of NUMMRG to merge entities:

PCIRC,...	! Create a partial circle
RECTNG,...	! Create a rectangle
NUMMRG,KP	! Default merge tolerances used

For a model having coincident keypoints, the NUMMRG operation will probably be more economical (that is, faster) than AGLUE (Main Menu>Preprocessor> Operate>Glue>Areas).

Figure 11-2 Example of NUMMRG application

If you merge keypoints that are very nearly coincident, any very short line connecting those keypoints will be deleted. If the keypoints are too far apart to be merged, you can use the LCOMB command (Main Menu>Preprocessor> Operate> Add>Lines) to eliminate the very short line. LCOMB will produce a continuous (but not necessarily smooth) line. If the resulting line is kinked, it cannot be used as a drag path [ADRAG, VDRAG], nor can it be used in any Boolean operations.

11.1.2 Compressing Item Numbers

As you build your model, you might, by deleting, clearing, merging, or performing other operations, create unused slots in the numbering sequence for various items. These slots will remain empty for some items (such as elements) but will be filled in for other items (such as keypoints) as new items are created. To save data storage space (by eliminating otherwise empty numbers) or to preserve desired sequencing (by forcing newly-created items to be assigned numbers greater than those of existing items), you can eliminate these gaps by "compressing" your numbering, using one of these methods:

Command(s):

NUMCMP

GUI:

Main Menu>Preprocessor>Numbering Ctrls>Compress Numbers

The compression operation may be selectively re-executed for chosen item groups (elements, keypoints, etc.), or may be simultaneously applied to all valid items (using NUMCMP,ALL).

The following example demonstrates one application of the NUMCMP command:

VMESH,...
VCLEAR,...    ! Node and element numbers will not be reused.
...
! Change meshing controls, element attributes, etc.
...
VMESH,...     ! Node and element numbering will contain "gaps"
NUMCMP,NODE   ! Optional step - NUMCMP can free up some computer memory
NUMCMP,ELEM   ! by eliminating gaps in numbering sequences.

Please see the description of the NUMCMP command in the ANSYS Commands Reference for more information.

11.1.3 Setting Starting Numbers

When creating new, automatically-numbered items, you might want to set the starting number of your new series of items higher than the greatest number used by existing items. By doing so, you will ensure that your newly created entities will be consecutively numbered, and will thus be prevented from occupying gaps in the existing numbering sequence. Another reason for specifying a set of starting numbers would be if you were creating portions of your model independently of one another, and you wanted to avoid numbering conflicts when they were combined together into one model. You can specify such user-defined starting numbers using one of these methods:

Command(s):

NUMSTR

GUI:

Main Menu>Preprocessor>Numbering Ctrls>Set Start Number

If using command input, you must issue this command for each separate class of items (nodes, elements, keypoints, etc.).

This example demonstrates the command input method:

! Create one portion of model:
...
...
! Create a separate, distinctly-numbered portion of your model:
NUMSTR,KP,100
NUMSTR,LINE,100
NUMSTR,AREA,100
NUMSTR,VOLU,100

For more information, see the description of the NUMSTR command in the ANSYS Commands Reference.

11.1.4 Adding Number Offsets

If you wish to combine two independently created portions of a model and want to prevent numbering conflicts, you can renumber all selected items by adding an offset value to their existing numbers. Use one of these methods:

Command(s):

NUMOFF

GUI:

Main Menu>Preprocessor>Numbering Ctrls>Add Num Offset

You must re-execute this operation for each item group (nodes, elements, keypoints, etc.) that is to be renumbered.

The CDWRITE command (Main Menu>Preprocessor>Archive Model>Write), which writes all selected model data to a text file, automatically puts a series of NUMOFF commands at the beginning of the file to push any existing data out of the way when the file is read. Gaps in numbering that may result from these NUMOFF commands can be removed with the NUMCMP command.

Solid model data (in IGES format) can be written to a text file in PREP7. You can also transfer solid model data from an externally-generated IGES file into the ANSYS database.

To write solid model data to a file, use one of these methods:

Command(s):

IGESOUT

GUI:

Utility Menu>File>Export

To transfer IGES data from a file into ANSYS PREP7 data, use one of these methods:

Command(s):

IGESIN

GUI:

Utility Menu>File>Import

New solid model entities created (in the AUX15 processor) by the IGESIN command (or GUI path) will automatically be numbered to avoid conflicts with other solid model entities that already exist in the database. If an IGES file is read into an empty database, ANSYS solid modeling entity numbering will not necessarily start at 1. This is because intermediate entities might be created (and later deleted) in the translation process. See Chapter 6 for more information on the IGES interface.

11.2 Element Reordering

At the start of a direct frontal solution, the ANSYS program will automatically reorder your elements to reduce the maximum wavefront size. This operation will not affect the element numbers used to identify elements in pre- and postprocessing of your database. It is anticipated that most users will allow the program to reorder their elements automatically. If you do not want the program to perform this automatic reordering, you can re-establish the original element ordering using one of these methods:

Command(s):

NOORDER

GUI:

Main Menu>Preprocessor>Numbering Ctrls>Element Reorder>Reset Elem Order

For more hands-on control of element reordering, use any of the methods listed below to initiate reordering within PREP7. (For any of these operations, the program knows not to overwrite your "manual" reordering with an automatic reordering.)

To initiate element reordering based upon a geometric sort, use one of these methods:

Command(s):

WSORT

GUI:

Main Menu>Preprocessor>Numbering Ctrls>Element Reorder>Reorder by XYZ

To define a starting wave list, use one of these methods:

Command(s):

WSTART

GUI:

Main Menu>Preprocessor>Numbering Ctrls>Element Reorder>Define Wave List

To add more nodes to the starting wave list, use one of these methods:

Command(s):

WMORE

GUI:

Main Menu>Preprocessor>Numbering Ctrls>Element Reorder>Extend Wave List

To erase all reordering wave lists, use one of these methods:

Command(s):

WERASE

GUI:

Main Menu>Preprocessor>Numbering Ctrls>Element Reorder>Erase Wave List

To initiate reordering, use one of these methods:

Command(s):

WAVES

GUI:

Main Menu>Preprocessor>Numbering Ctrls>Element Reorder>Reorder by List

To obtain an estimate of the wavefront statistics for your model, as currently ordered, use one of these methods:

Command(s):

WFRONT

GUI:

Main Menu>Preprocessor>Numbering Ctrls>Element Reorder>Est. Wavefront

You should avoid placing point elements (for example, MASS21 and MASS71) on element midside nodes if you intend to reorder your elements using the WAVES command; otherwise, the WAVES command may fail.

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