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Vibrata Documentation: Example Problems
66. The results are summarized in Table 4-1. The SRSS method produces the lowest
responses since it sums the modal responses as if they are all out of phase, while the
absolute value method produces the highest responses since it sums the modal
responses as if they are all in phase. The NRL results in this example are between
these two because it sums modal responses as in phase if it considers them closely
spaced and out of phase if not closely spaced. This makes the NRC results somewhat
surprising. Since it uses the same closely-spaced-mode parameter as the one we
selected for the NRL case, we might have expected its results to match the NRL
results, yet they are indistinguishable from the SRSS results. There is a significant
difference between the NRL and NRC methods that accounts for this. While both
treat closely spaced modes as in phase, NRL also treats the maximum-response
mode as in phase with all of the closely spaced modes. Looking at the effective
masses in Figure 4-81, this will clearly be mode 1. It is not “close” to any other
mode, so it will not be part of the in-phase summation for NRC, but it will be for
NRL.
Table 4-1. Maximum response level for each summation method.
Transient Analysis
4.5.
The next two examples illustrate Vibrata’s modal transient analysis capabilities. In the first
example we will simulate a thruster firing on the ISat model, and in the second example we
will simulate a hammer impacting the frame model.
4.5.1.
ISat Model with RCS Thruster Firing.
In this example, we will use the ISat in its deployed configuration (Figure 4-6). Note that
this model includes residual modes to give us improved stress results for loads applied at
the RCS thrusters.
67. In Femap, import the ISat deployed model (ISat_Dploy_Sm.dat), the normal modes
results that go with it (isat_dploy_sm.op2), and the Neutral file that defines many
useful groups for it (ISat_Dploy_Sm_groups.neu). Save the model as
ISat_Dploy_Sm.modfem.
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