3.6.           Expanding Test Mode Shapes

The expand_modes alter implements a number of test mode expansion algorithms including static, dyamic, XORTHO, and SEREP.

·         File Assignment Deck. If the SEREP expansion method is used, or if cross-orthogonalities between test and analysis are desired, the alter requires the FEM mode shapes as output from the write_modes alter. If the OUTPUT2 or OUTPUT4 options are chosen, the corresponding file must be assigned using an INPUTT2 or INPUTT4 assignment in the file assignment deck.

·         Executive Control Deck. The expand_modes alter must be included before the CEND card.

·         Case Control Deck. Some data recovery request such as DISP(PLOT)=ALL should be included.  If modal effective mass is desired a MEFFMASS request should also be included.

·         Bulk Data Deck. The bulk data deck must include the standard information for performing a normal modes analysis including ASET cards to define the measured DOF. The test modes must be included on DMIG cards with a matrix name of PHITEST, and the test frequencies on DMI cards with a matrix name of TLAMA.  If self- and cross-orthogonalities are desired and the DMIG option was chosen for the FEM mode shapes, these must also be included in the bulk data. Otherwise the OMODES parameter must be set as follows:

OMODES < 0            Read the modal frequencies and shapes in OUTPUT2 for­mat from FORTRAN UNIT |OMODES|

OMODES > 0            Read the modal frequencies and shapes in OUTPUT4 for­mat from FORTRAN UNIT OMODES

The expand_modes alter is controlled by the following parameters:

ORTHO = ‘NO’           Do not calculate cross-orthogonality (default)

ORTHO = ‘YES’          Calculate cross-orthgonality

EXPAND = ‘GUYAN’             Use TAM shapes for expansion (default)

EXPAND = ‘DYNAMIC’        Use dynamic expansion

EXPAND = ‘SEREP’   Use SEREP expansion

EXPAND = ‘XORTHO’           Use cross orthogonality expansion

RESFOR = ‘NO’          Do not calculate residual forces (default)

RESFOR = ‘YES’         Calculate residual forces and MRPT stiffness

A sample mode expansion run is illustrated in Figure 3-23. This case uses the dynamic expansion option, calculates residual forces and orthogonalities.  The expanded mode shapes are written to the OUTPUT2 file using PARAM,POST,-2 and the residual forces are written as single point constraint force (SPCF) data.

gpsc_expand.dat

 

ASSIGN INPUTT4='gpsc_fem.op4', UNIT=13

ASSIGN MASTER ='gpsc_expand.MASTER', DELETE

ASSIGN DBALL  ='gpsc_expand.DBALL', DELETE

$

DIAG 8 $

SOL     103     $ Normal modes

$

$  Write modal data

$

INCLUDE 'expand_modes.v2001'

$

CEND

TITLE    =GENERAL PURPOSE SPACECRAFT (GPSC)

SUBTITLE =DYNAMIC EXPANSION OF TEST MODE SHAPES

$

SPC    = 10             $ Constrain booster interface points

METHOD = 70             $ Modes to 70 Hz

$

DISP(PUNCH)   = ALL     $ Recover but do not print mode shapes

MEFFMASS(ALL) = YES     $ Calculate modal effective mass

$

BEGIN BULK

$

PARAM   WTMASS  .00259

PARAM     POST      -2

PARAM     EXPAND    DYNAMIC

PARAM     RESFOR    YES

PARAM     ORTHO     YES

PARAM     OMODES    13

$

EIGRL   70      1.0     70.0

$

$  Spacecraft bulk data

$

INCLUDE 'gpsc.blk'

INCLUDE 'gpsc.prp'

$

INCLUDE 'gpsc_ei.aset'

$

INCLUDE 'gpsc_test.dmig'

$

ENDDATA

Figure 3-23. Sample input file for modal expansion.