Page 198 - Using ANSYS for Finite Element Analysis Dynamic, Probabilistic, Design and Heat Transfer Analysis

Design optimization • 185
for the best design set will also be available if OPKEEP,ON was
issued before looping. The “best results” will be in file Jobname.BRST
(.BRTH, etc.), and the “best database” will be in Jobname.BDB.

5.2.5 gUiDeLineS FoR PeRFoRMing oPTiMizATion
ANAlySiS

5.2.5.1 Choosing Design Variables

DVs are usually geometric parameters such as length, thickness, diameter,
or model coordinates. They are restricted to positive values. Some points
to remember about DVs are:

• Use as few DVs as possible. Obviously, more DVs demand more itera-
tions and, therefore, more computer time. One way to reduce the num-
ber of design variables is to eliminate some DVs by expressing them
in terms of others, commonly referred to as design variable linking.
• Specify a reasonable range of values for the design variables (MIN
and MAX). Too wide a range may result in poor representation
of design space, whereas too narrow a range may exclude “good”
designs. Remember that only positive values are allowed, and that
an upper limit must be specified.
• Choose DVs such that they permit practical optimum designs.

5.2.5.2 Choosing State Variables

SVs are usually response quantities that constrain the design. Examples of
SVs are stresses, temperatures, heat flow rates, frequencies, deflections,
absorbed energy, elapsed time, and so on. A state variable need not be an
ANSYS-calculated quantity; virtually any parameter can be defined as a state
variable. Some points to keep in mind while choosing state variables are:

• When defining SVs (OPVAR command), a blank input in the MIN
field is interpreted as “no lower limit.” Similarly, a blank in the
MAX field is interpreted as “no upper limit.” A zero input in either
of these fields is interpreted as a zero limit.
• Choose enough SVs to sufficiently constrain the design. In a stress anal-
ysis, for example, choosing the maximum stress as the only SV may not
be a good idea because the location of the maximum stress may change