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174 • using ansys for finite eLement anaLysis
Objective Function: The dependent variable that you are attempting
to minimize. It should be a function of the DVs (i.e., changing the values
of the DVs should change the value of the objective function). In the beam
example, the total weight of the beam could be the objective function (to
be minimized). You may define only one objective function in an ANSYS
design optimization problem.
Optimization Variables: Collectively, the design variables, state
variables, and the objective function. In an ANSYS optimization, these
variables are represented by user-named variables called parameters. You
must identify which parameters in the model are DVs, which are SVs, and
which is the objective function.
Design Set or Design: A unique set of parameter values representing
a given model configuration. Typically, a design set is characterized by the
optimization variable values; however, all model parameters (including
those not identified as optimization variables) are included in the set.
Feasible Design: A design that satisfies all specified constraints
(those on the SVs as well as on the DVs). If any one of the constraints is
not satisfied, the design is considered infeasible. The best design is the one
that satisfies all constraints and produces the minimum objective function
value. (If all design sets are infeasible, the best design set is the one closest
to being feasible, irrespective of its objective function value.)
Analysis File: An ANSYS input file containing a complete analy-
sis sequence (preprocessing, solution, and postprocessing). The file must
contain a parametrically defined model, using parameters to represent all
inputs and outputs to be used as DVs, SVs, and the objective function.
Loop File: An optimization file (named Jobname.LOOP), created
automatically via the analysis file. The design optimizer uses the loop file
to perform analysis loops.
Loop: A single pass through the analysis file. Output for the last loop
performed is saved in file Jobname.OPO. An optimization iteration (or
simply iteration) is one or more analysis loops that result in a new design
set. Typically, an iteration equates to one loop; however, for the first order
method, one iteration represents more than one loop.
Optimization Database: This contains the current optimization envi-
ronment, which includes optimization variable definitions, parameters, all
optimization, specifications, and accumulated design sets. This database
can be saved (to Jobname.OPT) or resumed at any time in the optimizer.
The following Figure 5.2 illustrates the flow of information during an
optimization analysis.
The analysis file must exist as a separate entity. The optimization
database is not part of the ANSYS model database.