A dynamic TAM requires the use of the dynamic_tam alter and the definition of the PARAM,DFREQ to define a central frequency in Hz. As in the case of all other TAMs the instrumented DOF are selected on ASET or ASET1 cards.
No special differences are needed for superelement or non-superelement models beyond those listed in Section 1.5.
A sample input file to create a dynamic TAM is shown in Figure 3-3. This also includes the ortho alter to calculate pseudo- and cross-orthogonality. The center frequency (PARAM,DFREQ) is chosen as 20 Hz for this example. This is approximately the frequency of the fourth FEM mode and will improve the accuracy of the TAM in the range of this frequency.
gpsc_dtam.dat
ASSIGN INPUTT4='gpsc_fem.op4', UNIT=13
ASSIGN MASTER ='gpsc_dtam.MASTER', DELETE
ASSIGN DBALL ='gpsc_dtam.DBALL', DELETE
$
SOL 103 $ Normal modes
INCLUDE 'ortho.v2001'
INCLUDE 'dynamic_tam.v2001'
CEND
TITLE =GENERAL PURPOSE SPACECRAFT (GPSC)
SUBTITLE =TAM - DYNAMIC 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
PARAM DFREQ 20.0
$
$ 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 3-3. An alter
and one parameter are required for a Dynamic TAM.
For this example, the pseudo-orthogonality of the
FEM mode shapes with respect to the dynamically reduced mass matrix is
presented in Table 3-3. The
cross-orthogonality between the TAM and FEM mode shapes is presented in
Table 3-4. The dynamic TAM improves both the pseudo- and cross-orthogonality
when compared to the static reduction, though it is not exact.
Table 3-3. Pseudo-orthogonality of FEM modes w.r.t. dynamically reduced mass matrix.
Table 3-4. Cross-orthogonality of TAM/FEM modes w.r.t. dynamically reduced mass matrix.