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Solving for the behavior of a single symmetric sector (part of a circular component or assembly)

Using the single-sector solution to construct the response behavior of the full circular component or assembly (as a postprocessing step).

For example, by analyzing a single 10° sector of a 36-blade turbine wheel assembly, you can obtain the complete 360° model solution via simple postprocessing calculations. Using twice the usual number of degrees of freedom (DOFs) in this case, the single sector represents a 1/18th part of the model.

7.1.2. Commands Used in a Cyclic Symmetry Analysis

The most important command in an automated cyclic symmetry analysis is CYCLIC, which initiates a cyclic analysis and configures the database accordingly. The command automatically detects cyclic symmetry model information such as edge components, the number of sectors, the sector angles, and the corresponding cyclic coordinate system.

The ANTYPE command specifies the analysis type (for example, static, modal or buckling), and the SOLVE command obtains the cyclic solution.

Other cyclic-specific commands include:

CYCOPT for specifying solution options (/PREP7 and /SOLU)

/CYCEXPAND for graphically expanding displacements, stresses and strains of a cyclically symmetric model (/PREP7 and /POST1)

CYCPHASE for determining minimum and maximum possible modal result values from frequency couplets during postprocessing (/POST1).

Depending upon the type of cyclic symmetry analysis that you want to perform and your specific needs, it may be necessary to issue other commands. For example:

In a prestressed modal cyclic symmetry analysis, you must issue the PSTRES,ON command during the static portion of the analysis to apply the prestress effects. (The PSTRES command is not needed if the linear perturbation analysis procedure is used.)

During modal postprocessing, you may want to issue the ANCYC command to apply a traveling wave animation to your cyclic model.

The sections of this document describing various cyclic symmetry analyses mention such commands as necessary. For more information, see Solving a Cyclic Symmetry Analysis (p. 171).

7.2. Cyclic Modeling

This section describes how to set up a cyclic sector model, discusses important considerations for edge component pairs, and shows how to verify the cyclically symmetric model.

The following cyclic modeling topics are available:

7.2.1.The Basic Sector

7.2.2.Edge Component Pairs

7.2.3.Modeling Limitations

7.2.4.Model Verification (Preprocessing)

 

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Cyclic Modeling

7.2.1. The Basic Sector

A cyclic symmetry analysis requires that you model a single sector, called the basic sector. A proper basic sector represents one part of a pattern that, if repeated N times in cylindrical coordinate space, yields the complete model, as shown:

Figure 7.2: A Basic Sector in a Cyclically Symmetric Structure

A basic sector model that is cyclically symmetric can be defined in any global or user-defined cylindrical coordinate system. (For information about creating a model, see the Modeling and Meshing Guide.)

The angle α (in degrees) spanned by the basic sector should be such that Nα = 360, where N is an integer. The basic sector can consist of meshed or unmeshed geometry. The program allows user-defined

coupling and constraint equations on nodes that are not on the low or high edges of the cyclic sector. (For more information about the cyclic sector's low and high edges, see Edge Component Pairs (p. 166).)

If meshed, the basic sector may have matching (as shown in Figure 7.3: Basic Sector Definition (p. 165)) or unmatched lower and higher angle edges. Matching means that corresponding nodes exist on each edge, offset geometrically by the sector angle α. The edges may be of any shape and need not be "flat" in cylindrical coordinate space. For more information, see Identical vs. Dissimilar Edge Node Pat-

terns (p. 169).

Figure 7.3: Basic Sector Definition

Low Component Nodes

High Component Nodes

Z

Y

 

α

 

Sector

angle

 

 

X

CSYS = 1

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165