A modal TAM requires the modal_tam alter and the FEM mode shapes in DMIG, OUTPUT2 or OUTPUT4 format. No other parameters or input are required.
A sample input file to create a Modal TAM is shown in Figure 3-5. This also includes the ortho alter to calculate pseudo- and cross-orthogonality. Note that in the case of the modal TAM the FEM mode shapes are required whether or not the orthogonality calculations are made.
gpsc_mtam.dat
ASSIGN INPUTT4='gpsc_fem.op4', UNIT=13
ASSIGN MASTER ='gpsc_mtam.MASTER', DELETE
ASSIGN DBALL ='gpsc_mtam.DBALL', DELETE
$
SOL 103 $ Normal modes
INCLUDE 'ortho.v2001'
INCLUDE 'modal_tam.v2001'
CEND
TITLE =GENERAL PURPOSE SPACECRAFT (GPSC)
SUBTITLE =TAM - MODAL REDUCTION AND ORTHOGONALITY
$
SPC = 10 $ Constrain booster interface points
METHOD = 70 $ Modes to 70 Hz
$
DISP(PLOT) = ALL $ Recover but do not print mode shapes
$
BEGIN BULK
$
$ PARAMeter cards
$ ---------------
$
PARAM GRDPNT 0
PARAM USETPRT 0
PARAM WTMASS .00259
PARAM OMODES 13
$
$ Compute eigenvalues using the Lanczos method
$ --------------------------------------------
$
EIGRL 70 70.
$
$ Spacecraft bulk data
$ --------------------
$
INCLUDE 'gpsc.blk'
INCLUDE 'gpsc.prp'
$
$ Static reduction data
$ ---------------------
$
INCLUDE 'gpsc_rke1.aset'
$
ENDDATA
Figure 3-5. An alter and FEM mode shapes are required for a modal TAM.
For this example, the pseudo-orthogonality of the FEM mode shapes with respect to the modally reduced mass matrix is presented in Table 3-7. The cross-orthogonality between the TAM and FEM mode shapes is presented in Table 3-8. The modal TAM generates “exact” pseudo- and cross-orthogonality as expected[15].
Table 3-7. Pseudo-orthogonality of FEM modes w.r.t. modally reduced mass matrix.
Table 3-8. Cross-orthogonality of TAM/FEM modes w.r.t. modally reduced mass matrix.