13. Interfaces With Other Programs
5.62. Scaling Entities
Figure 5.62: Scaling Entities
5.8. Solid Model Loads
You can define loads directly on the solid model at any time before you actually initiate the solution. Thus, solid model loads may be defined before or after finite element meshing. (Details of how to define solid model loads are discussed in Loading in the Basic Analysis Guide.) The remainder of this section describes other functions related to solid model loads.
5.8.1. Transferring Solid Model Loads
Solid model loads will be transferred to the finite element model automatically when you begin the solution calculations (when you issue a SOLVE command (Main Menu> Solution> Solve> Current
LS)), or you may transfer them "manually" using the following methods:
• To transfer solid model loads and boundary conditions to the finite element model:
Command(s): SBCTRAN
GUI: Main Menu> Preprocessor> Loads> Define Loads> Operate> Transfer to FE> All Solid Lds Main Menu> Solution> Define Loads> Operate> Transfer to FE> All Solid Lds
• To transfer solid model body force loads to the finite element model:
GUI: Main Menu> Preprocessor> Loads> Define Loads> Operate> Transfer to FE> Body Loads Main Menu> Solution> Define Loads> Operate> Transfer to FE> Body Loads
• To transfer solid model DOF constraints to the finite element model:
Command(s): DTRAN
GUI: Main Menu> Preprocessor> Loads> Define Loads> Operate> Transfer to FE> Constraints Main Menu> Solution> Define Loads> Operate> Transfer to FE> Constraints
• To transfer solid model forces to the finite element model:
Command(s): FTRAN
GUI: Main Menu> Preprocessor> Loads> Define Loads> Operate> Transfer to FE> Forces Main Menu> Solution> Define Loads> Operate> Transfer to FE> Forces
• To transfer solid model surface loads to the finite element model:
Command(s): SFTRAN
GUI: Main Menu> Preprocessor> Loads> Define Loads> Operate> Transfer to FE> Surface Loads Main Menu> Solution> Define Loads> Operate> Transfer to FE> Surface Loads
5.8.2. Displaying Load Symbols
Solid model loads may be displayed at any time after you have turned on the appropriate load symbols: • To show boundary condition symbols on displays, use the /PBC command. This command includes
an option to display boundary condition values, as well as symbols. (Turning off /VSCALE scaling before issuing the /PBC command may help.)
• To show body force loads as contours on displays, use the /PBF command. • To show surface load symbols on model displays, use the /PSF command.
All of these commands can be accessed in the GUI by picking Utility Menu> PlotCtrls> Symbols.
5.8.3. Turning Off Large Symbols for Node and Keypoint Locations
Large symbols for node and keypoint locations are displayed by default (/PSYMB,DOT,1).
• To display smaller symbols for node and keypoint locations, use one of the following methods:
Command(s):/PSYMB,DOT,0
GUI: Utility Menu> PlotCtrls> Symbols
5.8.4. Selecting a Format for the Graphical Display of Numbers
You can indicate the field length and the number of digits that are displayed for the numbers appearing on a model.
• To specify a format for the display of numbers, use the following command:
Command(s): /GFORMAT
5.8.5. Listing Solid Model Loads
You can list all solid model loads or you can list separate types of solid model loads using the following methods:
• To list all solid model loads:
Command(s):SBCLIST
GUI: Utility Menu> List> Loads> Solid Model Loads
• To list the body force loads at keypoints:
Command(s):BFKLIST
GUI: Utility Menu> List> Loads> Body> On All Keypoints Utility Menu> List> Loads> Body> On Picked KPs
• To list the DOF constraints at keypoints:
Command(s):DKLIST
GUI: Utility Menu> List> Loads> DOF Constraints> On All Keypoints Utility Menu> List> Loads> DOF Constraints> On Picked KPs
• To list the DOF constraints on a line:
Command(s):DLLIST
GUI: Utility Menu> List> Loads> DOF Constraints> On All Lines Utility Menu> List> Loads> DOF Constraints> On Picked Lines
• To list the DOF constraints on an area:
Command(s):DALIST
GUI: Utility Menu> List> Loads> DOF Constraints> On All Areas Utility Menu> List> Loads> DOF Constraints> On Picked Areas
• To list the forces at keypoints:
Command(s):FKLIST
GUI: Utility Menu> List> Loads> Forces> On All Keypoints Utility Menu> List> Loads> Forces> On Picked KPs
• To list the surface loads on lines:
Command(s):SFLLIST
GUI: Utility Menu> List> Loads> Surface Loads> On All Lines Utility Menu> List> Loads> Surface Loads> On Picked Lines
• To list the surface loads on areas:
Command(s):SFALIST
GUI: Utility Menu> List> Loads> Surface Loads> On All Areas Utility Menu> List> Loads> Surface Loads> On Picked Areas
5.9. Mass and Inertia Calculations
The xSUM commands calculate and print geometry items associated with solid model entities. See
Lumped Calculation of Mass Related Information in the Mechanical APDL Theory Reference for details.
Note
For very narrow (sliver) areas or very thin volumes, such that the ratio of the minimum to the maximum dimension is less than 0.01, the ASUM and VSUM commands can provide erroneous area or volume information. To ensure that such calculations are accurate, make certain that you subdivide such areas and volumes so that the ratio of the minimum to the maximum is at least 0.05.
• To calculate and print the center of mass location, moments of inertia, and so on, associated with selected keypoints:
Command(s):KSUM
GUI: Main Menu> Preprocessor> Modeling> Operate> Calc Geom Items> Of Keypoints
• To calculate and print the length, center of mass location, moments of inertia, and so on, for selected lines:
Command(s):LSUM
GUI: Main Menu> Preprocessor> Modeling> Operate> Calc Geom Items> Of Lines
• To calculate and print the area, center of mass location, moments of inertia, and so on, for selected areas:
Command(s):ASUM
GUI: Main Menu> Preprocessor> Modeling> Operate> Calc Geom Items> Of Areas
• To calculate and print the volume, center of mass location, moments of inertia, and so on, for selected volumes:
Command(s):VSUM
GUI: Main Menu> Preprocessor> Modeling> Operate> Calc Geom Items> Of Volumes
• To calculate and print all of the previously mentioned keypoint, line, area, and volume geometry items at one time:
Command(s):GSUM
GUI: Main Menu> Preprocessor> Modeling> Operate> Calc Geom Items> Of Geometry
5.10. Considerations and Cautions for Solid Modeling
5.10.1. Representation of Solid Model Entities
During the solid modeling process, it is helpful to understand the underlying mathematical operations used by ANSYS. This knowledge is particularly useful when you encounter degeneracies and discontinu- ities. For example, an error stating that a degeneracy has been detected in a solid model Boolean oper- ation may occur. Knowledge of the mathematical terminology involved can aid in overcoming such an error.
Internally, solid model entities are mathematically represented by trimmed parametric surfaces. Trimmed parametric surfaces consist of two components: parametric geometry and topology. Parametric geometry defines the underlying surface of a model. The term "parametric" refers to the parametric space that mathematically represents the geometric space. Refer to Figure 5.63: Cone Surface Maps to a Parametric Square (p. 87) for an illustration of the relationship between the geometric model and the parametric model. Nonuniform Rational B-splines, or NURBS, are used in the definition of the parametric geometry. The term "topology" refers to the trimming surfaces that bound a model's geometry.
5.10.2. When a Boolean Operation Fails
Boolean operators provide a powerful set of tools that enable you to create complicated geometries with a minimum amount of input. However, you might encounter situations in which a Boolean operation will have difficulties. If this happens, you can still sometimes work around the problem to achieve the desired solid model. Let's examine some of the causes of and cures for Boolean failures.
5.10.2.1. Degeneracies
Boolean operations can fail due to degeneracies. Knowing what constitutes a degeneracy, how a degen- eracy is formed, and why degeneracies sometimes cause Booleans to fail can help you overcome such
errors. Degeneracies can arise due to issues in geometry or topology. ANSYS will classify degeneracies (which cause a Boolean failure) as either parametric (geometry) or topological.
Parametric degeneracies result from the representation of geometric space with an underlying parametric space. When this "shadow world" of the parametric representation is not in dimensional harmony with the real world of the geometric model, a degeneracy is created. For example, at the apex of a cone, a single point on the geometric model is represented by an edge in the parametric representation (see
Figure 5.63: Cone Surface Maps to a Parametric Square (p. 87)). Such a point is termed a degenerate
edge, or more simply, a degeneracy.Figure 5.65: Examples of Degeneracies (p. 89) illustrates several common degeneracies formed during modeling.
A degeneracy of this kind is not harmful in and of itself. A model that contains a degeneracy can often be used in Boolean operations, can be meshed successfully, and can yield excellent analysis results. It is only when a degeneracy happens to cause a problem in a Boolean operation that you even need to be aware of its existence.
Figure 5.63: Cone Surface Maps to a Parametric Square
Poi nti srep re sente db yapa ra me tri cdege ne ra cy
Ge ome tri cmodel Pa rame t ricm odel
5.10.3. Graphically Identifying Degeneracies
Degeneracies in areas or volumes can be graphically identified using the methods described below. If you are using the command input method, including the DEGEN label will place a red star at keypoints associated with degeneracies (see Figure 5.64: Plotting of Geometric Degeneracies (p. 88)).
• To display degeneracies in areas:
Command(s): APLOT, , , ,DEGE
GUI: Main Menu> Preprocessor> Modeling> Operate> Booleans> Show Degeneracy> Plot Degen Areas
• To display selected volumes:
Command(s): VPLOT, , , ,DEGE
GUI: Main Menu> Preprocessor> Modeling> Operate> Booleans> Show Degeneracy> Plot Degen Volus
5.10.4. Listing the Keypoints Associated with Degeneracies
You may also choose to list the keypoints associated with degeneracies: • To list keypoints of an area that lie on a parametric degeneracy:
Command(s): ADGL
GUI: Main Menu> Preprocessor> Modeling> Operate> Booleans> Show Degeneracy> List Degen Areas
• To list keypoints of a volume that lie on a parametric degeneracy:
Command(s): VDGL
GUI: Main Menu> Preprocessor> Modeling> Operate> Booleans> Show Degeneracy> List Degen Volus
Figure 5.64: Plotting of Geometric Degeneracies
Another kind of degeneracy can be detected if a Boolean operation attempts to create a degenerate
boundary during any phase of its operation. A degenerate boundary is an incomplete or zero-area loop,
or an incomplete or zero-volume shell. This type of degeneracy is commonly referred to as a topological
degeneracy. A Boolean operation will produce an error message if a degeneracy of this nature is detected.
Topological degeneracies cannot be plotted since they do not exist prior to a Boolean operation. Examples of topological degeneracies are illustrated in Figure 5.66: Topologically Degenerate Loop (p. 90) and
Figure 5.67: Examples of Topologically Degenerate Loops and Shells (p. 90). An example of a Boolean operation resulting in failure is shown in Figure 5.66: Topologically Degenerate Loop (p. 90). In this ex- ample, the triangular prism cannot be subtracted [VSBV] from the block, because a degenerate loop would be formed on the top surface of the block. Other Boolean operations such as VADD,VOVLAP, etc. would also fail for these volumes due to this degeneracy.
Figure 5.65: Examples of Degeneracies
A topologically degenerate loop will be created during VSBV,VADD,VOVLAP, or other Boolean oper- ations involving these volumes
Figure 5.66: Topologically Degenerate Loop Edgeoftr ian gular pr isml ies
ontopsur fa ceofbl ock(p ot en t ial degen era t eloop)
Figure 5.67: Examples of Topologically Degenerate Loops and Shells
L 1 L2 L3 L4 L5 L 6 A7 I y 5 h , w . I 5,6 , 7 h , h yw z - . I 5 6 h ,h yw . I 7 h v ,w h . 1 2 3 4 5 L 1 L2 L3 L4 L7 L 6 L5
5.10.4.1. Discontinuities
Generally, discontinuities are sharp "kinks" in solid model entities. They can result from the combination of lines [LCOMB] with inconsistent end tangents or from an IGES import. See Figure 5.68: Lines and Areas Containing Discontinuities (p. 91) for an illustration of entities containing discontinuities.
Figure 5.68: Lines and Areas Containing Discontinuities
L1 1
3
di sconti nui ti e s
Many solid model operations will support entities containing discontinuities. However, Boolean operations do not directly support discontinuities. Boolean operations will split or divide entities at points or along lines of discontinuities prior to the Boolean operation. For example,Figure 5.69: Boolean Operation In- volving Entities With Discontinuities (p. 92) shows an area containing a discontinuity involved in a Boolean subtract operation. Prior to the subtract operation area 1 along with lines 1 and 3 are split at discontinuities.
Figure 5.69: Boolean Operation Involving Entities With Discontinuities
Subtr acti nglinefr om ar e ac ont ai ning di sc onti nuiti e s L2 A 1 L6 6 7 A 1 L1 L3 L5 3 5
O utputare asandl i ne s fr omB oole anoperation ar espli talongdi sc ontinui tie s
L2 A L7 1 L8 L9 8 L14 9 L13 L5 L10 3 L11 5 L12 10 11 ASB L
Note
Discontinuities can be associated with both direction and magnitude of tangent vectors. Boolean operations will detect both types of discontinuities.
5.10.4.2. Other Causes of Boolean Failures
Boolean operations can fail due to other circumstances besides degeneracies. For instance, intersections at tangent points can sometimes be difficult for the Boolean operations to handle, particularly with non-primitive constructions. Also, entities that share a boundary (such as two volumes with bounding areas on the same surface) can potentially cause problems in a Boolean operation. Problems might also be encountered if your geometry contains small regions with very high curvatures, or regions with very sharp angles or transitions.
5.10.5. Some Suggested Corrective Actions
If a Boolean operation fails, you can try using one or more of the following procedures to work around the problem.
You need not always follow these guidelines as you first build your model. You can often create a model any way you like, and never encounter a Boolean failure. These guidelines are provided as suggested ways to try to recover from a Boolean failure.
Adjust the input geometries, using the following guidelines:
• Whenever possible, try to use geometric primitives as you create your solid model. The results of Boolean operations on non-primitives can sometimes be less accurate and efficient.
• Try to avoid creating geometries that contain degeneracies, if the degeneracy will lie on a potential intersection curve. A few specific examples of such geometries would include:
– An untruncated cone primitive. (See Figure 5.63: Cone Surface Maps to a Parametric Square (p. 87).)