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Chapter 11: Component Mode Synthesis

Component mode synthesis (CMS) is a form of substructure coupling analysis frequently used in structural dynamics.

CMS allows you to derive the behavior of the entire assembly from its constituent components. First, the dynamic behavior of each of the components is formulated. Then, by enforcing equilibrium and compatibility along component interfaces, the program forms the dynamic characteristics of the full system model.

CMS is available in the ANSYS Mechanical and ANSYS Structural products.

The following CMS topics are available:

11.1.Understanding Component Mode Synthesis

11.2.Using Component Mode Synthesis

11.3.Example Component Mode Synthesis Analysis

11.1. Understanding Component Mode Synthesis

Although breaking up a single large problem into several reduced-order problems via substructuring saves time and processing resources, component mode synthesis (CMS) offers the following additional advantages:

More accurate than a Guyan reduction for modal, harmonic and transient analyses. CMS includes truncated sets of normal mode generalized coordinates defined for components of the structural model.

The ability to include experimental results, as the substructure model need not be purely mathematical.

A typical use of CMS involves a modal analysis of a large, complicated structure (such as an aircraft or nuclear reactor) where various teams each design an individual component of the structure. With CMS, design changes to a single component affect only that component; therefore, additional computations are necessary only for the modified substructure.

Finally, CMS supports these substructuring features:

Top-down substructuring

Nested superelements

Prestressed substructures (static analysis prestress approach only).

11.1.1. CMS Methods Supported

The following component mode synthesis methods are available:

• Fixed-interface (CMSOPT,FIX)

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Free-interface (CMSOPT,FREE)

Residual-flexible free-interface (CMSOPT,RFFB)

For most analyses, the fixed-interface CMS method is preferable. The free-interface method and the residual-flexible free-interface method are useful when your analysis requires more accurate eigenvalues computed at the midto high-end of the spectrum. The following table describes the primary characteristics of each interface method:

CMS Methods Supported

 

Fixed (CMSOPT,FIX)

Free (CMSOPT,FREE)

Residual-Flexible Free (CM-

SOPT,RFFB)

 

 

Interface nodes are constrained during the CMS superelement generation pass.

No requirement to specify rigid body modes.

Generally recommended when accuracy on only the lower modes of the assembled structure (use pass) is necessary.

Interface nodes remain free during the CMS superelement generation pass.

You must specify the number of rigid body modes (CMSOPT).

Generally recommended when accuracy on both lower and higher modes of the assembled structure (use pass) is required.

Interface nodes remain free during the CMS superelement generation pass.

If rigid body motion exists, you must specify pseudo-constraints (D).

Generally recommended when accuracy on both lower and higher modes of the assembled structure (use pass) is required.

For more information, see the discussion of component mode synthesis theory and methods in the

Mechanical APDL Theory Reference.

11.1.2. Solvers Used in Component Mode Synthesis

Following are the solvers and files used in a typical component mode synthesis analysis:

Figure 11.1: Applicable CMS Solvers and Files

 

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