11.2. Using Component Mode Synthesis

As in substructuring, a component mode synthesis (CMS) analysis involves three distinct steps, called passes:

1.Generation pass (SEOPT, CMSOPT)

2.Use pass (SETRAN, SESYMM, CPINTF)

3.Expansion pass (EXPASS, SEEXP, EXPSOL, NUMEXP)

The CMS generation pass condenses a group of finite elements into a single CMS superelement, which includes a set of master degrees of freedom (DOFs) and truncated sets of normal mode generalized coordinates. The master DOFs serve to define the interface between the superelements or other elements.

The following CMS usage topics are available:

11.2.1.The CMS Generation Pass: Creating the Superelement

11.2.2.The CMS Use and Expansion Passes

11.2.3.Superelement Expansion in Transformed Locations

11.2.4.Plotting or Printing Mode Shapes

CMS Wizard

A user-friendly wizard is available to help you better understand the CMS process as well as to guide you through the generation, use, and expansion passes for the fixed-interface (CMSOPT,FIX) and freeinterface (CMSOPT,FREE) methods for modal analyses. The wizard also provides file organization and management support as files are generated by a CMS analysis. Use the Solution (/SOLU) processor to access the CMS Wizard.

11.2.1. The CMS Generation Pass: Creating the Superelement

The process for generating a CMS superelement consists of two primary tasks:

1.Building the model

This step is identical to building the model for a substructuring analysis. Define density (DENS)--or mass in some form--because CMS must generate both stiffness and mass matrices.

2.Creating the superelement matrices

The "solution" from a CMS generation pass consists of the superelement matrices (generalized stiffness and mass matrix). This flowchart illustrates the process necessary for creating the superelement

matrix file:

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Figure 11.2: Process Flow for Creating a CMS Superelement Matrix

Specifying the CMS method When specifying the CMS method, also specify the number of modes and, optionally, the frequency range used to generate the superelement. The program supports the fixed-interface (CMSOPT,FIX), free-interface (CMSOPT,FREE), and residual-flexible free-interface (CMSOPT,RFFB) CMS methods. If using the free-interface method, also specify the rigid body modes (CMSOPT,,,,FBDDEF). If using the residual-flexible free-interface method, specify pseudo-constraints (D,,,SUPPORT).

Naming the superelement matrix file The program assigns the .sub extension to the superelement matrix file name that you specify (SEOPT,Sename); therefore, the complete file name is Sename.SUB. The default file name is the Jobname (/FILNAME).

Specifying the lumped mass matrix formulation Specify the lumped mass matrix formulation (LUMPM) if necessary. For most applications, ANSYS, Inc. recommends the default formulation (depending upon the element type); however, for dynamic analyses involving "skinny" structures such as slender beams or very thin shells, the lumped mass approximation typically yields better results.

Defining master DOFs In a substructure, master degrees of freedom (DOFs) serve as the interface between the superelements or other elements. Define master DOFs (M) at all nodes that connect

to non-superelements (Lab1 = ALL), as shown in Example of a Substructuring Application. You must define master DOFs even if you intend to have no elements in the model other than a superelement.

If this superelement is to be transformed (SETRAN) later in the use pass or used in a large deflection analysis (NLGEOM,ON), all nodes that have master DOFs must have all six DOFs (UX, UY, UZ, ROTX, ROTY, ROTZ) defined and active.

For large deflections, master DOFs are typically defined at the joints of the flexible body and are at the nodes connected to a joint element (MPC184), another rigid or flexible body node, or ground. At least two master DOFs must be defined for each substructure, as the average rotation of the su-

perelement is computed from the average rotation of its master DOF. If only one node is a joint node, then another must be chosen at the free end. See the Multibody Analysis Guide for more details.

Specifying pseudo-constraints Required for the residual-flexible free-interface method (CMSOPT,RFFB). For each superelement where rigid-body modes exist, specify pseudo-constraints. Apply only the minimum number of displacement constraints (D,,,SUPPORT) necessary to prevent rigid body motion: three constraints (or fewer, depending on the element type) for 2-D models and six (or fewer) for 3-D models.

Saving a copy of the database Saving a copy of the database (SAVE) is necessary because you must work with the same data in the expansion pass.

Obtaining the CMS generation pass solution Output from the solution (SOLVE) consists of the superelement matrix file (Sename.SUB), where Sename is the file name you assigned (via the SEOPT command).

When the Coriolis effect in the stationary reference frame is activated (CORIOLIS,ON,,,ON), the stiffness, mass, and damping matrices are treated as non-symmetric by the sparse solver in the generation pass. This doubles the amount of memory used when compared to symmetric systems. For a model with a large number of degrees of freedom, the memory options for the sparse solver (BCSOPTION) should be set using the guidelines outlined in Direct (Sparse) Solver Memory Usage in the Performance Guide.

After obtaining the CMS superelement matrices, proceed to the use pass and then the expansion pass, as you would in a substructuring analysis.

For a detailed example of how to use CMS, see Example Component Mode Synthesis Analysis (p. 290).

11.2.2. The CMS Use and Expansion Passes

The CMS use pass and expansion pass are identical to those in a substructuring analysis. The CMS use pass supports the following analysis types:

•Modal (ANTYPE,MODAL)

•Static (ANTYPE,STATIC)

•Transient (ANTYPE,TRANS) -- full or mode superposition method

•Harmonic (ANTYPE,HARMIC) -- full or mode superposition method

•Spectrum (ANTYPE,SPECT)

The use pass also supports substructuring analysis prestress.

As in substructuring, the generation and expansion passes occur for each part (CMS superelement) of the entire structure, and the use pass occurs only once because it uses all superelements together to build the full model. The use pass extracts the eigenvalues of the full model (but not the eigenvectors, because the expansion pass recovers them).

In a modal analysis using the free-interface CMS method, the use pass may not always extract all of the modes requested via the MODOPT command. In such cases, increase or decrease the number of modes to extract and run the use pass eigensolution again.