Mechanical APDL Structural Analysis Guide

• Mode Selection Based on DDAM Procedure (p. 75)

Zero-frequency modes are ignored when mode selection is based on a criterion (Modal Effective Mass,
Mode Coefficients, and DDAM methods). Only the User Defined Array procedure can be used to select
zero frequency modes.

3.9.6.1. Mode Selection Based on a User Defined Array

The mode selection is performed during a modal analysis or a modal analysis restart. It is applicable to
real and complex eigensolvers.

The user defined arra y is a *DIM arra y with a dimension equal to the number of extracted modes. The
array values are 1 for selected modes and 0 otherwise.

Damped eigensolver solutions (MODOPT,DAMP) are pairs of complex conjugate modes. When filling
in the table array, it is important to select the pairs (and not individual modes) to ensure that the MODE
file remains consistent. (For example, modes #3 and #4 are a pair, so indices 3 and 4 in the arra y should
have the same value, 0 or 1.)

Note

For the selection of complex modes from the QR damped eigensolver (MODOPT,QRDAMP),

you must input MXPAND,-1 in the modal solution.This will force the generation of a complex

MODE file that will enable the complex mode selection.

A sample input listing of the procedure follows:

! Build the Model

/filnam,...

/title,...

/prep7

...

finish

! Obtain the Modal Solution

/solu

antype, MODAL

modopt, UNSYM, 10! Unsymmetric eigensolver, 10 modes are requested

solve

finish

save

/post1

...! Post-process the modes to identify the modes of interest

finish

/clear, nostart! Start a new session

resume,, db

*dim, tab, ARRAY, 10! Create the array

tab(1) = 1

tab(2) = 0

tab(3) = 0

tab(4) = 0

tab(5) = 1

tab(6) = 1

tab(7) = 1

tab(8) = 0

tab(9) = 1

tab(10) = 1

! Perform the modal selection and expansion

/solu

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Modal Analysis