GEOMETRY FLIP part/ALL
or: ... CONSISTENT senam [snam]
or: ... FOCAL senam [snam] [pnam]

          

This command is used to reverse the direction of the local Z-axis of a surface or a group of surfaces. This is particularly important for pressure loading or situations where surfaces surfaces need to be oriented consistently. Where necessary, this enables the local Z-axis of the elements generated on the surface(s) to be `flipped' in order to be consistent with the local Z-axis of elements generated on other surfaces.

The effect of the command is to change the order of the lines that define a surface. In the case of a three- or four-sided surface, line 1 and line 3 are swapped in the surface definition. This gives the local axis system a rotation of 180 degrees about the local X-axis. In the case of a region, the sense of the outer loop (defined by the first set in the region definition) is reversed. Thus, under a flip, clockwise becomes anti-clockwise and vice versa.

part = name of an existing surface or set of surfaces to be flipped.

= /PICK-S and a cursor hit to select an existing surface to be flipped.

ALL = All surfaces in the current model will be flipped.

senam = name of a set of surfaces (in the case of CONSISTENT or

= FOCAL mode).

snam = name of an optional reference surface (in the case of CONSISTENT or

= FOCAL mode).

pnam = name of an optional user-specified focal point (in the case of FOCAL mode).

A group of surfaces may be flipped in normal, consistent or focal mode.

Normal mode

When GEOMETRY FLIP is used without the CONSISTENT or FOCAL keywords, the command is assumed to be working in normal mode. In normal mode, the local Z-direction for each surface is flipped, regardless of its topological or geometrical status in relation to other surfaces.

Consistent mode

Use of the keyword CONSISTENT indicates that the surfaces will be flipped in consistent mode.

In consistent mode, the surfaces within a set are selectively flipped such that the normals of topologically connected surfaces within that set are aligned in a consistent manner. The next two figures demonstrate a consistent flip. The first depicts a set of surfaces whose normals are aligned inconsistently. The second figure demonstrates the result of GEOMETRY FLIP CONSISTENT on the same set of surfaces.

 

Figure 2.70: A set of inconsistently aligned surfaces, prior to a flip

 

Figure 2.71: A set of consistently aligned surfaces, after a flip

Topological information only is used to assess whether two surfaces have their normals aligned consistently. The angles between surfaces are immaterial, only the order of the lines that define the outer, bounding loop of each surface is taken into account. For two adjacent surfaces to have consistent normals, their common (or shared) line must be used such that its use in the definition of one surface is opposite (in sense) to its use in the definition of the other surface. For a well defined shell of surfaces (in which an outside and an inside exists), the effect of GEOMETRY FLIP CONSISTENT will be to orient the surfaces such that their normals either (i) all point outwards, or (ii) all point inwards with respect to the inside of the shell.

A reference surface may be specified optionally in order to select a reference normal that defines a sense of alignment. For a well defined shell of surfaces, a reference normal may be used to specify whether the surfaces should be aligned in an inward or an outward sense. In general, the specification of a reference surface will ensure that the normals of all surfaces that are topologically connected to the reference surface are aligned consistently with the normal of the reference surface. The reference surface itself is never flipped. If no reference surface is specified, then the first surface in the set is taken as a default reference surface.

Notes:

1.

Use of reference surface
If a reference surface is specified as part of a CONSISTENT flip, then only those surfaces that are topologically connected to that surface will be flipped. If the reference surface is not contained in the specified set, then a warning message will notify the user of this, and no surfaces will be flipped.

2.

Scope of sets that can be consistently flipped
There exist sets of surfaces for which it is not possible to perform a consistent flip and subsequently obtain predictable results. This applies in particular to the case in which a line is referred to by three or more surfaces from the specified set. In this instance, the concept of topological consistent normals becomes ambiguous, and the user is notified that a flip will not take place.

3.

Orientability
Sets of surfaces that constitute definitions of objects that are not properly orientable (such as Klein bottles and Möbius strips) cannot be flipped consistently. If a consistent flip is shown to be impossible, then the user will be notified of this.

4.

Holes
For the pruposes of GEOMETRY FLIP CONSISTENT, surfaces that are attached to the lines that define the hole(s) in a region are not considered to be topologically attached to the lines that define the outer boundary of the region.

Focal mode

Use of the keyword FOCAL indicates that the surfaces will be flipped in focal mode.

In focal mode, the surfaces within a set are selectively flipped such that the normal of each surface is aligned with a direction defined by the surface's 'focal vector', (defined later). Compared to consistent mode, the sense of alignment (or otherwise) is determined entirely by geometric information. In general, the result of a focal flip on a set of surfaces is to redefine surfaces, where necessary, such that all the normals of surfaces in the set are either (i) pointing away from a focal point, or (ii) pointing towards a focal point.

The focal technique involves the specification (directly or indirectly) of a focal point for the set in question. The focal point may be defined explicitly by the user as a point name, or specified indirectly (the default) as the centroid of the set. A focal vector is then associated with each surface in the set. The focal vector is taken from the set's focal point to the centroid of the surface in question. For each surface, a sense of alignment can then be determined by assessing the direction of the surface's normal against its focal vector.

Under a focal flip, a surface is flipped if its sense of alignment is opposite to the sense of alignment of a reference surface. The user may explicitly specify the reference surface, otherwise the reference surface is taken as the first surface in the set.

Notes:

1.

Indeterminate surfaces in focal mode
Under certain circumstances, the alignment and location of the reference surface with respect to the focal point may cause indeterminacy. This is the case where the focal vector is perpendicular to the surface normal, and where the centroid of a surface coincides with the focal point. In these cases, no flip is undertaken and a warning message is issued. Non-reference surfaces that are indeterminate will not be flipped.

Return Level: GEOMETRY FLIP

Examples:

1.

GEOMETRY FLIP S23

Flips the definition (and thus local axis system) of surface S23.

2.

GEOMETRY FLIP ALL

Flip the definition of all the surfaces in the current model.

3.

GEOMETRY FLIP SE4

Flip the definition of all the surfaces in the set SE4.

4.

GEOMETRY FLIP CONSISTENT SE13

Flip any surface in SE13 that is topologically connected to the first surface in SE13, such that its normal is topologically consistent with that of the first surface.

5.

GEOMETRY FLIP CONSISTENT SE4 S13

Flip any surface in SE4 that is topologically connected to the surface S13, such that its normal is topologically consistent with that of S13.

6.

GEOMETRY FLIP FOCAL SE13

Flip any surface in SE13 whose normal is not aligned with a focal vector taken from the centroid of the set to the centroid of the surface in question.

7.

GEOMETRY FLIP FOCAL SE13 P2

Flip any surface in SE13 whose sense of alignment is opposite to that of the first surface in the set. Use the point P2 as the focal point.

8.

GEOMETRY FLIP FOCAL SE13 S7

Flip any surface in SE13 whose sense of alignment is opposite to that of surface S7. The focal point is the centroid of SE13.

9.

GEOMETRY FLIP FOCAL SE13 S7 P4

Flip any surface in SE13 whose sense of alignment is opposite to that of surface S7. The focal point is located at point P4.

See also the following commands

'LABEL GEOMETRY NORMALS'
'LABEL GEOMETRY AXES'
'LABEL MESH AXES'
'UTILITY TABULATE GEOMETRY SURFACE'