►Coreform Cubit™ Reference Manual
▼4 Geometry Operations
►4.5 Geometry Decomposition
On this page:
4.5.1 Splitting Geometry
4.5.1.1 Split Curve
4.5.1.2 Split Periodic Surfaces
4.5.1.3 Split Surface
4.5.1.3.3.1 Logical Rectangle
4.5.1.3.3.2 Split Orientation
4.5.1.3.3.3 Corner Specification
4.5.1.3.3.4 Direction
4.5.1.3.3.5 Segment|Fraction|Distance
4.5.1.3.3.6 Through Vertex
4.5.1.3.3.7 Parametric
4.5.1.3.3.8 Tolerance
4.5.1.3.3.9 Preview
4.5.1.3.3.10 Settings
4.5.2 Web Cutting
4.5.2.2 Web Cutting with an Arbitrary Surface
4.5.2.3 Chop Command
4.5.2.4 Web Cutting with a Planar or Cylindrical Surface
4.5.2.5 Web Cutting by Sweeping Curves or Surfaces
4.5.2.6 Web Cutting using a Tool or Sheet Body
4.5.2.6.1 Webcut with Existing Tool Body
4.5.2.6.2 Webcut with Curve Loop
4.5.2.6.3 Webcut with Offset Surfaces
4.5.2.7 Web Cutting Options
4.5.3 Section Command
4.5.4 Separating Surfaces from Bodies
4.5.5 Separating Multi-Volume Bodies
8.5
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4.5 Geometry Decomposition

Geometry decomposition is often required to generate an all-hexahedral mesh for three-dimensional solids, as fully automatic all-hex mesh generation of arbitrary solids is not yet possible in Cubit. While geometry booleans can be used for decomposition (and are the basis of the underlying implementation of advanced decomposition tools described here), Cubit has a webcut capability specially tuned for decomposition. It is also useful to split periodic surfaces to facilitate quad and hex meshing.

4.5.1 Splitting Geometry

The Split command divides curves or surfaces into multiple entities. The command results are similar to imprinting. However, vertex and/or curve creation is not necessary for the split command.

4.5.1.1 Split Curve

The Split Curve command will split a curve without the need for geometry creation (unlike imprinting).

To split a curve

  1. On the Command Panel, click on Geometry and then Curve.

  2. Click on the Modify action button.

  3. Select Split from the drop-down menu.

  4. Enter the appropriate value for Curve ID. This can also be done using the Pick Widget function.

  5. Select Fraction, Vertex ID, By Location, Distance or Pick from the Split Method/Location menu.

  6. Click Apply.

Split Curve <id> [location on curve options] [Merge] [Preview]

To split a curve, simply specify a location or a location on curve (see location specification). Using the preview keyword will draw the splitting location on the curve. The merge keyword will merge any topology that contains the newly created vertex.

4.5.1.2 Split Periodic Surfaces

Solids which contain periodic surfaces include cylinders, torii and spheres. Splitting periodic surfaces can in some cases simplify meshing, and will result in curves and surfaces being added to the volume.

To split periodic surfaces

  1. On the Command Panel, click on Geometry and then Volume.

  2. Click on the Modify action button.

  3. Select Split Periodic from the drop-down menu.

  4. Enter the appropriate value for Volume ID(s). This can also be done using the Pick Widget function.

  5. Click Apply.

Split Periodic Body <id_range|all>

This command splits all periodic surfaces in a body or bodies.

4.5.1.3 Split Surface

The Split Surface command divides one or more surfaces into multiple surfaces. The command results are similar to imprint with curve. However, curve creation is not necessary for splitting surfaces. Three primary forms of the command are available.

The first form splits a single surface using locations while the second splits by extending a surface hard-line until it hits a surface boundary. The split automatic splits either a single surface or a chain of surfaces in an automatic fashion.

4.5.1.3.1 Split Across

Two forms of Split Across are available

  1. On the Command Panel, click on Geometry and then Surface.

  2. Click on the Modify action button.

  3. Select Split from the drop-down menu.

  4. Select By Location from the drop-down menu.

  5. Enter in the appropriate value for Surface ID(s). This can also be done using the Pick Widget function.

  6. Enter in the appropriate settings on this menu.

  7. Click Apply.

Split Surface <id> Across [Pair] Location <options multiple locs> [Preview [Create]]

Split Surface <id> Across Location <multiple locs> Onto Curve <id> [Preview] Create

This command splits a surface with a spline projection through multiple locations on the surface. See Location, Direction, and Axis Specification for a detailed description of the location specifier.Figure 127 shows a simple example of splitting a single surface into two surfaces.A temporary spline was created through the three specified locations (Vertex 5 6 7), and this curve was used to split the surface.

split surface 1 across location vertex 5 6 7

Figure 127: Splitting across with multiple locations.

The pair keyword will pair locations to create multiple surface splitting curves (each defined with two locations). An even number of input locations is required.Figure 128 shows an example:

split surface 1 across pair vertex 5 7 6 8

Figure 128: Splitting across with pair option.

The preview keyword will show a graphics preview of the splitting curve.If the create keywordis also specified, a free curve (or curves) will be created - these are the internal curves that are used to imprint the surfaces.

The onto curve format of the command takes one or more locations on one side of the surface and projects them onto a single curve on the other side of the surface.Figure 129 shows an example:

split surface 1 across vertex 5 6 onto curve 4

Figure 129: Splitting across with onto curve.

4.5.1.3.2 Split Extend

The Split Extend function can be called with the following command:

Split Surface <id_list> Extend [Vertex <id_list> | AUTO] [Preview [Create]]

With the following settings:

Set Split Surface Extend Normal {on|OFF}

Set Split Surface Extend Gap Threshold <val>

Set Split Surface Extend Tolerance<val>

This command splits a surface by extending a surface hard-line until it hits a surface boundary. Figure 130 shows a simple example of extending a curve. The hard-line curve was extended from the specified vertex until it hit the surface boundary.

split surface 1 extend vertex 2

Figure 130: Splitting by extending hard-line.

The auto keyword will search for all hard-lines and extend them according to the Split Surface Extend settings. Figure 131 shows an example:

split surface 1 extend auto

Figure 131: Splitting by extending with auto option.

The preview keyword will show a graphics preview of the splitting curve. If the create keyword is also specified, a free curve (or curves) will be created - these are the internal curves that are used to imprint the surfaces.

The normal setting can be turned on or off. When it is on, Cubit will attempt to extend the hard-line so that it is normal to the curve it will intersect. An example of this is in Figure 132:

set split surface normal on
split surface 1 extend vertex 2

Figure 132: Splitting by extending a hard line with normal setting on.

Cubit uses the gap threshold to decide whether or not to extend a hard-line when the user specifies auto. If the distance between a vertex on a hard-line and the curve it will hit is greater than the gap threshold, then Cubit will not extend that hard-line. The default value is INFINITY, and can be set to any value. To reset the value back to INFINITY, set the gap threshold to -1.0.

This setting only applies when using the keyword auto.

An example of using the gap threshold is shown in Figure 133:

set split surface gap threshold 2.0
split surface 1 extend auto.

Figure 133: Extending Hard-lines with Gap Threshold = 2.0.
(Notice vertex 1 was not extended because it exceeded the gap threshold).

The tolerance setting can be used to avoid creating short curves on the surface boundary. If Cubit tries to extend a hard-line that comes within tolerance of a vertex, it will instead snap the extension to the existing vertex. An example of this is shown in Figure 134:

set split surface tolerance 1.0
split surface 1 extend vertex 2

Figure 134: Extending Hard-lines with Tolerance
(Notice the extension snapped to vertex 3)

4.5.1.3.3 Split (Automatically)

This form of the command splits a single surface or a chain of surfaces in an automatic fashion.It is most convenient for splitting a fillet or set of fillets down the middle - oftentimes necessary to prepare for mesh sweeping. These surfaces cannot have multiple curve loops.

To Split a surface

  1. On the Command Panel, click on Geometry and then Surface.

  2. Click on the Modify action button.

  3. Select Split from the drop-down menu.

  4. Select Along Fillet from the drop-down menu.

  5. Enter in the appropriate value for Surface ID(s). This can also be done using the Pick Widget function.

  6. Click Apply.

Split Surface <id_list>[Corner Vertex <id_list>] [Direction Curve <id>] [Segment|Fraction|Distance <val> [From Curve <id>]] [Through Vertex <id_list>] [Parametric <on|OFF>] [Tolerance <val>] [Preview [Create]]

The volume shown in Figure 135 was quickly prepared for sweeping by splitting the fillets and specifying sweep sources as shown (with the sweep target underneath the volume). The surface splits are shown in blue.

Figure 135: Splitting fillets to facilitate sweeping.

Each surface is always split with a single curve along the length of the surface (or multiple single curves if the Segment option is used). The splitting curve will either be a spline, arc or straight line.

4.5.1.3.3.1 Logical Rectangle

The Split Surface command analyzes the selected surface or surface chain to find a logical rectangle, containing four logical sides and four logical corners. Each side can be composed of zero, one or multiple curves. If a single surface is selected (with no options), the logical corners will be those closest to 90 and oriented such that the surface will be split parallel to the longest aspect ratio of the surface. If a chain of surfaces is selected, the logical corners will include the two corners closest to 90 on the starting surface of the chain and the two corners closest to 90 on the ending surface of the chain.

The split will always occur along the chain.

In Figure 127 , the logical corners selected by the algorithm are Vertices 1-2-5-6. Between these corner vertices the logical sides are defined. These sides are described in Table 1. The default split occurs from the center of Side 1 to the center of Side 3 (parallel to the longest aspect ratio of the surface), and is shown in blue.

Figure 136: Split surface logical properties.

Logical Side

   

Corner Vertices

   

Curve Groups

1

   

1-2

   

1

2

   

2-5

   

2,3,4

3

   

5-6

   

5

4

   

6-1

   

6

Table 4: Listing of logical sides for Figure 127.

Figure 127 shows a surface along with two possibilities for its logical rectangle and the resultant splits.

Figure 137: Different possible logical rectangles for same surface.

Table 2 shows various surfaces and the resultant split based on the automatically detected or selected logical rectangle. Note that surfaces are always traversed in a counterclockwise direction.

    Surface(s) (resultant split in blue)    

   

Ordered corners (to form the logical rectangle)

   

1-2-3-4

(using aspect ratio)

   

4-1-2-3

(user selected)

   

1-2-5-6

   

2-5-6-1

   

1-2-3-4 (split is always along the chain)

   

1-2-3-4 (notice triangular surfaces along the chain)

   

1-1-2-3 (note side 1 of the logical rectangle is collapsed. Side 3 is from vertex 2 to 3.)

   

1-2-2-3 (note side 2 of the logical rectangle is collapsed)

   

1-2-3-4

   

1-2-4-4

   

1-1-2-2

   

1-1-2-2 (selected automatically)

Table 4: Sample surfaces and logical rectangles.

4.5.1.3.3.2 Split Orientation

If a chain of surfaces are split, the surfaces will always be split along the chain. The command will not allow disconnected surfaces.

For a single surface, the split direction logic is a bit more complicated. If no options are specified, the surface aspect ratio determines the split direction - the surface will be split parallel to the longest aspect ratio side through the midpoint of each curve. This behavior can be overridden by the order the Corner vertices are selected (the split always starts on the side between the first two corners selected), the Direction option, the From Curve option, or the Through Vertex list.

Table 3 shows examples of the various split orientation methods. These options are explained in more detail in the sections below.

Surface Example

   

Split Orientation Method

   

Multiple surfaces are always split along the chain

   

Parallel to longest surface aspect ratio (default)

   

corner vertex 4 1 2 3 (split always starts on side 1 of the logical rectangle.)

   

direction curve 1

   

from curve 1 fraction .75 or from curve 1 distance 7.5

   

through vertex 5 6

Table 5: Split orientation methods.

4.5.1.3.3.3 Corner Specification

The corner option allows you to specify corners that form logical rectangle the algorithm uses to orient the split on the surface. When analyzing a surface to be split, the software automatically selects the corners that are closest to 90. The Preview option displays the automatically selected corners in red. Sometimes incorrect corners are chosen, so you must specify the desired corners yourself. The split always starts on the side between the first two corners selected and finishes on the side between the last two corners selected. Figure 127 shows a situation where the user had to select corners to get the desired split.

Figure 138: Selecting the desired corners.

The split can be directed to the tip of a triangular shaped surface by selecting that corner vertex twice (at the start or end of the corner list) when specifying corners, creating a zero-length side on the logical rectangle. A shortcut exists wherein if you specify only three corner vertices, the zero-length side will be directed to the first corner selected. If you specify only two corner vertices, a zero-length side will be directed to both the first and second corner you select. Table 6 shows these examples. Note the software will automatically detect triangle corners based on angle criteria - the corner selection methods for zero-length sides explained in this section need only be applied if the angles are outside of the thresholds specified in the Set Split Surface Auto Detect Triangle settings.

Surface

   

Corner Specification

   

1-2-4-4- or 4-4-1-2

or

4-1-2 (shortcut method)

   

1-1-2-2 or 2-2-1-1

or

1-2 or 2-1 (shortcut method)

Table 6: Selecting corners to split to triangle tips.

4.5.1.3.3.4 Direction

The direction option allows you to conveniently override the default split direction on a single surface. Simply specify a curve from the logical rectangle that is parallel to the desired split direction. If Corners are also specified, the Direction option will override the split orientation that would result from the specified corner order. The Direction option is not valid on a chain of surfaces. Figure 127 shows an example.

Figure 139: Direction specification overrides corner order.

4.5.1.3.3.5 Segment|Fraction|Distance

The segment option allows you to split a surface into 2 or more segments that are equally spaced across the surface. The fraction option allows you to override the default 0.5 fractional split location. The distance option allows you to specify the split location as an absolute distance rather than a fraction. By specifying a from curve, you can indicate which side of the logical rectangle to base the segment, fraction or distance from (versus a random result). Table 5 gives examples of these options.

Surface

   

Command Options

   

Segment 6 From Curve 1

   

Fraction .3 From Curve 1

   

Distance 3 From Curve 1

Table 7: Segment, fraction, distance examples.

4.5.1.3.3.6 Through Vertex

The through vertex option forces the split through vertices on surface boundaries perpendicularto the splitdirection.Use this option if the desired fraction is not constant from one end of the surface to another or if a split would otherwise pass very close to an existing curve end resulting in a short curve.Through vertices can be used in conjunction with the Fraction option- the split will linearly adjust to pass exactly through the specified vertices.It is not valid with the Segment option.The maximum number of Through Vertices that can be specified is equal to the number of surfaces being split plus one.The selected vertices can be free, but must lie on the perpendicular curves.Table 6 gives several examples.

Surface(s)

   

Command Options

   

Fraction .3 From Curve 1 Through Vertex 9

   

Through Vertex 5 6 7 8

Table 8: Through vertex examples.

4.5.1.3.3.7 Parametric

By default, split locations are calculated in 3D space and projected to the surface. As an alternative, split locations can be calculated directly in the surface parametric space. In rare instances, this can result in a smoother or more desirable split. The command option parametric {on|off} can be used to split the given surfaces in parametric space. Alternatively, the default can be overridden with the Set Split Surface Parametric {on|OFF} command.

4.5.1.3.3.8 Tolerance

A single absolute tolerance value is used to determine the accuracy of the split curves. A smaller tolerance will force more points to be interpolated. The tolerance is also used when detecting an analytical curve (e.g., an arc or straight line) versus a spline. A looser tolerance will result in more analytical curves. The default tolerance is 1.0. The command option tolerance <val> can be used to split the given surfaces using the given tolerance. Alternatively, the default tolerance can be overridden with the Set Split Surface Tolerance <val> command.

It is recommended to use the largest tolerance possible to increase the number of analytical curves and reduce the number of points on splines, resulting in better performance and smaller file sizes. The Preview option displays the interpolated curve points. Table 7 shows the effect of the tolerance for a simple example.

Surface

   

Tolerance

   

2.0

   

1.0

   

0.5

   

0.01

Table 9: Effect of tolerance on split curve.

4.5.1.3.3.9 Preview

The preview keyword will show a graphics preview (in blue) of the splitting curve (or curves) and the corner vertices (in red) selected for the logical rectangle. The curve preview includes the interpolated point locations that define spline curves. Note that if no points are shown on the interior of the curve, it means that the curve is an analytical curve (line or arc). If the create keyword is also specified, a free curve (or curves) will be created – these are the internal curves that are used to imprint the surfaces. Table 10 shows some examples.

Surface

   

Curve Type

   

Spline

   

Arc (no preview points shown on interior of curve).

Table 10: Graphics preview.

4.5.1.3.3.10 Settings

This section describes the settings that are available for the automatic split surface command. To see the current values, you can enter the command set split surface, optionally followed by the setting of interest (without specifying a value).

Set Split Surface Tolerance <val>

This sets the default tolerance for the accuracy of the split curves. See the Tolerance section for more information.

Set Split Surface Parametric {on|OFF}

This sets the default for whether surfaces are split in 3D (default) or in parametric space.See the Parametric section for more information.

Set Split Surface Auto Detect Triangle {ON|off}

Set Split Surface Point Angle Threshold <val>

Set Split Surface Side Angle Threshold <val>

The split surface command automatically detects triangular shaped surfaces as explained in the section on Corners. This behavior can be turned off with the setting above. Two thresholds are used when detecting triangles - the point angle threshold and the side angle threshold, specified in degrees. Corners with an angle below the Point Angle threshold are considered for the tip of a triangle (or the collapsed side of the logical rectangle). Corners within the Side Angle threshold of 180 are considered for removal from the logical rectangle. In order for a triangle to actually be detected, corners for both the point and side criteria must be met. The default Point Angle threshold is 45, and the default Side Angle threshold is 27. Figure 127 provides an illustration.

Figure 140: Triangle detection settings.

4.5.1.3.4 Split Skew

The Split Skew function can be called with the following command:

Split Surface <id_list> Skew [Preview] [Create]

This command will split a surface or list of surfaces in a logical way to reduce the amount of skew in a quadrilateral mesh. This function uses the control skew algorithm to determine where to make these logical splits. Users should note that Split Skew can only be utilized effectively on surfaces that lend themselves to a structured meshing scheme. These surfaces cannot have multiple curve loops. Figure 127 shows a simple example of a surface being split.

split surface 1 skew

Figure 141: Split skew applied to an l-shaped surface.

The preview keyword will show a graphics preview of the splitting curves. If the create keyword is also specified, free curves will be created.

4.5.2 Web Cutting

The term "web cutting" refers to the act of cutting an existing body or bodies, referred to as the "blank", into two or more pieces through the use of some form of cutting tool, or "tool". The two primary types of cutting tools available in Cubit are surfaces (either pre-existing surfaces in the model or infinite or semi-infinite surfaces defined for web cutting), or pre-existing bodies.

The various forms of the web cut command can be classified by the type of tool used for cutting. These forms are described below, starting with the simplest type of tool and progressing to more complex types.

4.5.2.1 General Notes

The primary purpose of web cutting is to make an existing model meshable with the hex meshing algorithms available in Cubit. While web cutting can also be used to build the initial geometric model, the implementation and command interface to web cutting have been designed to serve its primary purpose. Several important things to remember about web cutting are as follows:

The Decomposition Tutorials and the Power Tools Tutorial contain some examples that demonstrate the use of web cutting operations.

4.5.2.2 Web Cutting with an Arbitrary Surface

An arbitrary "sheet" surface can also be used to web cut a body. This sheet need not be planar, and can be bounded or infinite. The following commands are used:

To webcut using a sheet

  1. On the Command Panel, click on Geometry and then Volume.

  2. Click on the Webcut action button.

  3. Select Surface Extended from the drop-down menu.

  4. Enter in the appropriate values for Body ID(s) and With Surface ID(s). This can also be done using the Pick Widget function.

  5. Click Apply.

Webcut {blank} with sheet {body|surface} <id> [webcut_options]

Webcut {blank} with sheet extended [from] surface <id> [webcut_options]

In its first form, the command uses a sheet body, either one that is pre-existing or one formed from a specified surface. Note that in this latter case the (bounded) surface should completely cut the body into two pieces. Sheet bodies can be formed from a single surface, but can also be the combination of many surfaces; this form of web cut can be used with quite complicated cutting surfaces.

Extended sheet surfaces can also be used; in this case, the specified surface will be extended in all directions possible. Note that some spline surfaces are limited in extent, and so these surfaces may or may not completely cut the blank.

4.5.2.3 Chop Command

The chop command works similarly to a web cut command, but is faster. Given two bodies, the command will find the intersection of the two bodies, and divide the main body into a body that lies outside the intersection, and a body that lies inside the intersection. The tool body will be deleted, unless the keep option is specified. The syntax of the command is:

Chop [Volume|BODY] <id> with [Volume|BODY] <id> [keep] [nonreg]

The nonreg option results in the bodies being non-regularized.

4.5.2.4 Web Cutting with a Planar or Cylindrical Surface