Chapter 5 Robust Performance Tailoring with Tuning - SSL - MIT

does not change with the design parameters, so it is generated a priori and then
loaded into the workspace at each iteration. The model components are integrated
with DOCS to form the fully-integrated system model and evaluate the performance.
It is important to keep in mind that the process outlined in Figure 6-10 is exe-
cuted once for each iteration, or random guess, in the SA algorithm. Therefore, the
NASTRAN model must be built and a finite element analysis run thousands of times.
This operation is computationally expensive and becomes more so as the model fi-
delity and number of degrees of freedom are increased. In addition, as the design
space grows, a greater number of SA iterations are required. It is clear that a brute
force method such as the one outlined here may not be appropriate for a model of
this fidelity and further investigation into the optimization implementation is war-
ranted. There are known methods for reducing the computational effort required for
these types of optimizations [28, 39] that are appropriate for this application, such as
reduced-order modeling [37] and response surface approximation [22, 11].
6.4 Results
The TPF SCI structure is designed through a SA stochastic search using the PT, RPT
and RPTT cost functions (Equations 2.1, 3.5, and 5.5, respectively). In the RPT and
RPTT optimizations the uncertainty space is searched with the vertex method. Since
there are three uncertainty parameters, there are eight vertices at which performance
must be evaluated during each iteration. The RPTT cost is considered with α =0
to obtain the most tunable design. In the PT and RPT optimizations both the
tailoring, �x and tuning, �y parameters are used as tailoring parameters to provide for
a fair comparison with the RPTT design. In this way all three optimizations have
the same design variables available to them and the only difference lies in the manner
in which the tuning parameters are incorporated.
The tailoring parameters for the nominal and optimized designs are listed in Ta-
ble 6.13 along with the mass of the structures. The three designs are distinct from
each other and very different from the nominal design. In each case the truss bays
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