CHAPTER 6
ANSYS Implementation
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Chapter 6. ANSYS Implementation - 1
Main Steps
Begins with building a parametric model and creating the analysis file.
Initial Design
Explore the
Design Domain
Parametric Modeling
& Loading
Solution
Parametric
Results
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Analysis
File
Optimize the
Design
Chapter 6. ANSYS Implementation - 2
Analysis File

To optimize a design, an analysis file must be available.

The analysis file contains input for the parametric model:
 Initial
parameter values
 Model
geometry and meshing
 Boundary
 Results
conditions, loads, and solution
review and retrieval
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Chapter 6. ANSYS Implementation - 3
Procedure

Four main steps (assuming that the
analysis file is available):
1
1. Identify the analysis file
2. Identify optimization variables - DVs, SVs,
and objective function
2
3
3. Run the optimization
4. Review results

4
These steps correspond to the menus in
Main Menu > Design Opt as shown on the
right.
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Chapter 6. ANSYS Implementation - 4
Example - Problem Definitions

We will use the rotating disk example to illustrate each step:
A high-speed rotating disk made of steel is to be designed for nearly
uniform stress distribution at an operating speed of 15,000 rpm.

Properties: E = 30E6 psi, r = 7.2E-4 lb-s2/in4, n = 0.3
1.6
qhub
ymid
qrim
0.5
xmid
4.0R
0.6
0.4
10.0R
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Chapter 6. ANSYS Implementation - 5
Example - Limitations

The von Mises stress should not exceed 25,000 psi

The disk’s lowest natural frequency should be 1000 Hz or
greater.

The hub and the rim are of fixed dimensions, but you are
allowed to modify the shape in between.
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Chapter 6. ANSYS Implementation - 6
Optimizing the Design - Procedure
Identify the Analysis File
 Identify the analysis file

Identify optimization variables

Run the optimization

Review results
Recall that the analysis file contains the input required
for one complete loop of the optimization run:
 Parametric
modeling and loading
 Solution
 Parametric
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results
Chapter 6. ANSYS Implementation - 7
Optimizing the Design - Procedure
Identify the Analysis File

To identify the analysis file, simply choose the file name
from the Assign Analysis File dialog:
 Design
 Or
Opt > -Analysis File- Assign…
use the OPANL command
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Chapter 6. ANSYS Implementation - 8
Optimizing the Design - Procedure
Identify Optimization Variables

Identify the analysis file
 Identify optimization variables

Run the optimization

Review results
This step involves choosing the appropriate parameters
and designating them as:
 Design
 State
variables
variables
 Objective
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function
Chapter 6. ANSYS Implementation - 9
Optimizing the Design - Procedure
Identify Optimization Variables
Design Variables (DVs)

Design characteristics that are allowed to change in order to
minimize the volume (or whatever the objective function is).

For the rotating disk example, the
DVs are

THETAHUB, 30°~ 90°

THETARIM, 45°~ 135°

XMID, 0.5 ~ 4.5 in

YMID, 0.25 ~ 1.5 in
qhub
ymid
qrim
xmid
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Chapter 6. ANSYS Implementation - 10
Optimizing the Design - Procedure
Identify Optimization Variables
To identify DVs,

Design Opt > Design Variables… > Add…

Or use the OPVAR command:
OPVAR,name,DV,min,max,toler
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Chapter 6. ANSYS Implementation - 11
Optimizing the Design - Procedure
Identify Optimization Variables

Maximum value (MAX) is required and must be > 0.0.

Minimum value (MIN), if specified, must be > 0.0. If not specified,
MIN defaults to 0.001*MAX.

Tolerance (TOLER) is the acceptable change in the DV between
loops for convergence.
 Has
same units as the DV and is not a percentage or a fraction. Default
= 0.01*current value.
For example, if THETAHUB = 54.2° in loop 6 and 55.0° in loop 7,
convergence based on DVs has not occurred since |54.2-55.0| = 0.8,
which is > 0.55.
(But convergence may have occurred based on the objective function…
more on this later.)
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Chapter 6. ANSYS Implementation - 12
Optimizing the Design - Procedure
Identify Optimization Variables

You can define up to 60 DVs, but we recommend using no
more than 10~20 DVs. The more DVs, the higher the
chance of converging to a local minimum.

DVs are restricted to positive values. Since most DVs are
geometric parameters such as thickness and radius, this
restriction does not generally pose a problem.
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Optimizing the Design - Procedure
Identify Optimization Variables
State Variables (SVs)

Constraints placed on the design, such as a
maximum stress or deflection.

For the rotating disk example, SVs are:
 Maximum
 First
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hoop stress  25000 psi
natural frequency  1000 Hz
Chapter 6. ANSYS Implementation - 14
Optimizing the Design - Procedure
Identify Optimization Variables

To identify SVs,

Design Opt > State Variables… > Add…

Or use the OPVAR command:
OPVAR,name,SV,min,max,toler
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Optimizing the Design - Procedure
Identify Optimization Variables


SVs can be one-sided or two-sided.
 One-sided:
only MIN or only MAX specified
 Two-sided:
both MIN and MAX specified.
TOLER is the feasibility tolerance… how far the design can go
beyond MAX and MIN before it is considered infeasible.
 Has
same units as the SV and is not a percentage or a fraction.
Default = 0.01*(MAX-MIN).
For example, if the disk’s first natural frequency is FREQ1 = 991.3
Hz, the design is still feasible since the actual threshold is 1000(0.01*1000) = 990 Hz.
MIN
Infeasible
toler
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MAX
Feasible Region
toler
Infeasible
Chapter 6. ANSYS Implementation - 16
Optimizing the Design - Procedure
Identify Optimization Variables

State variables are not required for
optimization, but they are usually
specified since most designs need to be
constrained in some fashion.

You can define up to 100 SVs.
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Optimizing the Design - Procedure
Identify Optimization Variables
Objective Function (OBJ)

The parameter that is minimized by the optimizer.
Examples are volume, weight, and the temperature at a
location.

For the rotating disk example, the objective is to minimize
the variation in stress. Hence the OBJ is the standard
deviation of the von Mises stress, SDEV.
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Chapter 6. ANSYS Implementation - 18
Optimizing the Design - Procedure
Identify Optimization Variables

To identify the objective function,

Design Opt > Objective…

Or use the OPVAR command:
OPVAR,name,OBJ,,,toler
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Chapter 6. ANSYS Implementation - 19
Optimizing the Design - Procedure
Identify Optimization Variables

Tolerance (TOLER) is the acceptable change in the OBJ
between loops for convergence.
 Has
same units as the OBJ and is not a percentage or a fraction.
Default = 0.01*current value.
For example, if the standard deviation SDEV = 3900 in loop 7 and
3850 in loop 6, convergence based on OBJ has not occurred since
|3900-3850| = 50, which is > 39.0.
(But convergence may have occurred based on DVs… more on this
later.)
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Chapter 6. ANSYS Implementation - 20
Optimizing the Design - Procedure
Identify Optimization Variables

You can specify only one OBJ.

ANSYS always minimizes the OBJ. If you want to maximize
something, say the parameter y, specify 1/y or A-y as the
OBJ (where A >> y).

We recommend that the OBJ remain positive. Add a
constant value to the parameter if needed to ensure this.
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Chapter 6. ANSYS Implementation - 21
Optimizing the Design - Procedure
Run the optimization

Identify the analysis file

Identify optimization variables
 Run the optimization

Review results
This step involves:
A. Specifying run-time controls
B. Choosing the optimization method
C. Saving the optimization database
D. Initiating the optimization run
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Chapter 6. ANSYS Implementation - 22
Optimizing the Design - Procedure
Run the optimization
A. Run-Time Controls

These include:
 Optimization
database file name (OPDATA).
Default: jobname.opt.
 Controls
on how to read the analysis file (OPLOOP).
Default: read from the first line, ignore DV parameter
definitions.
 Printout
controls (OPPRNT). Default: suppress printout.
 Save-best-design
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option (OPKEEP). Default: OFF.
Chapter 6. ANSYS Implementation - 23
Optimizing the Design - Procedure
Run the optimization

Most controls are best left to default values except perhaps
the save-best-design option.
This option saves the best design to date as it occurs:
 model
 results
geometry, mesh, loading, etc. on jobname.bdb
on jobname.brst.
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Chapter 6. ANSYS Implementation - 24
Optimizing the Design - Procedure
Run the optimization

Design Opt > Controls...
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Chapter 6. ANSYS Implementation - 25
Optimizing the Design - Procedure
Run the optimization
Notes on the Save-Best-Design Option

The best database is saved by a SAVE operation to the file
jobname.bdb.

The best results file is saved by simply copying the results
file jobname.rst to jobname.brst (or .rth to .brth, .rmg
to .brmg, .rfl to .brfl).

If multiple analyses are done, such as thermal followed by
stress, only the last results file is copied.
 Consider
using the /ASSIGN function ( Utility Menu >
File > ANSYS File Options… ) if you want a different
results file to be copied.
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Optimizing the Design - Procedure
Run the optimization
B. Optimization Method

Two optimization methods are available in ANSYS:
 Sub-problem
 First-order
approximation method
method

The sub-problem approximation method is generally
recommended for most applications because of its
generality of approach and speed of execution.

Details of when to choose the first-order method will be
presented later.
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Chapter 6. ANSYS Implementation - 27
Optimizing the Design - Procedure
Run the optimization

Use the OPTYPE and OPSUBP commands.

Or Design Opt > Method/Tool…
 Choosing
a method brings up a second dialog box with
additional options. These options (discussed later) are best
left to default values.
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Optimizing the Design - Procedure
Run the optimization
C. Save the Optimization Database

The optimizer works with its own database, which
consists of:
 DV,
SV, and OBJ specifications.
 Analysis
file name, optimization method and controls, total
number of iterations completed to date, etc.
 Parameter

values for each design generated to date.
At the end of each iteration, ANSYS automatically saves
this information on the OPT database file specified in the
Run-Time Controls dialog (which defaults to
jobname.opt).
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Chapter 6. ANSYS Implementation - 29
Optimizing the Design - Procedure
Run the optimization

You can check the OPT
database status at any
time using:

Design Opt > -Opt
Database- Status

Or the STATUS command
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Chapter 6. ANSYS Implementation - 30
Optimizing the Design - Procedure
Run the optimization

Saving the OPT database before initiating the optimization run
allows you to resume from this point conveniently if needed.

Use the OPSAVE command.

Or Design Opt > -Opt Database- Save...

Choose a name different from the default (since the default file gets
updated each iteration). Example: jobname.opt0.
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Optimizing the Design - Procedure
Run the optimization
D. Initiate the Optimization Run

Use the OPEXE command.

Or Design Opt > Run…
 Check
the settings, then press OK to start the
optimization.
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Optimizing the Design - Procedure
Run the optimization

The optimizer will then loop through the analysis file multiple
times with new DV values each time until the design has
converged or until the maximum number of iterations is reached.
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Chapter 6. ANSYS Implementation - 33
Optimizing the Design - Procedure
Run the optimization
What is convergence?

A design has converged to a possible optimum if it is feasible
and if one of the following four conditions is true:
1.
The change in objective function between the current
design and the best feasible design is less than the
tolerance.
|OBJcurrent - OBJbest| < TOLERobj
2.
The change in objective function between the current and
previous designs is less than the tolerance.
|OBJcurrent - OBJcurrent-1| < TOLERobj
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Optimizing the Design - Procedure
Run the optimization
3.
For each DV, the change between the current design and the
best feasible design is less than its tolerance.
|DVcurrent - DVbest| < TOLERdv for all DVs
4.
For each DV, the change between the current and previous
designs is less than its tolerance.
|DVcurrent - DVcurrent-1| < TOLERdv for all DVs

Again, if one of these four conditions is true and if the current
design is feasible, we have a converged or optimum design.
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Optimizing the Design - Procedure
Run the optimization

The ANSYS Output window (or output file if batch) shows the basis for
convergence.

For the rotating disk example, design no. 9 (also called set 9)
converged based on OBJ comparison to the best design (set 3). And
set 9 is considered the “optimum” design since it has a lower OBJ
value than set 3.
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Optimizing the Design - Procedure
Run the optimization
Does convergence indicate an optimum design?

Not necessarily. It simply indicates that the current design is
feasible and meets one of the four convergence criteria.

It is up to you, the engineer, to determine whether the design is
indeed optimum.
 One
way to do this is by continuing (restarting) the
optimization with different tolerance values or with a different
set of designs. More on restarts later.
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Optimizing the Design - Procedure
Run the optimization

The output window also lists a design sensitivity summary table,
which can be used as a guide to determine which DV has the
most (or least) effect on design parameters.

For the rotating disk example, notice that a unit change in YMID
has a large effect on the maximum stress. The design is also
sensitive to changes in XMID.
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Chapter 6. ANSYS Implementation - 38
Optimizing the Design - Procedure
Review the Results

Identify the analysis file

Identify optimization variables

Run the optimization
 Review results

You can review optimization results by:

first restoring the OPT database (if needed)

listing design sets

creating graphs

restoring the geometry and results for the best design
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Optimizing the Design - Procedure
Review the Results
Restoring the OPT Database

This step is needed only if you exited ANSYS after the
optimization run.

Use the OPRESU command.

Or Design Opt > -Opt Database- Resume…
 File
name defaults to the OPT database file name
specified in the Run-Time Controls dialog (which in turn
defaults to jobname.OPT).
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Optimizing the Design - Procedure
Review the Results
Listing Design Sets

Allows you to review parameter values for a desired design or
a range of designs.

You can choose to list optimization parameters only or all
parameters.

Use the OPLIST command.

Or Design Opt > -Design Sets- List…
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Chapter 6. ANSYS Implementation - 41
Optimizing the Design - Procedure
Review the Results

Note that the listing
indicates whether a design
is feasible or infeasible and
also the reason for
infeasibility.

For the rotating disk
example:

The initial design is
infeasible because SMAX is
out of range.

The two feasible designs
are also the best designs.
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Optimizing the Design - Procedure
Review the Results
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Chapter 6. ANSYS Implementation - 43
Optimizing the Design - Procedure
Review the Results
Creating Graphs

Use the PLVAROPT command.

Or Design Opt > Graphs/Tables…

You can graph optimization
variables versus set number
(default) or versus other
optimization variables.
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Optimizing the Design - Procedure
Review the Results

This is a graph of objective function SDEV vs. set number.

PLVAROPT,SDEV
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Optimizing the Design - Procedure
Review the Results

State variable FREQ1 vs. set number.

PLVAROPT,FREQ1
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Optimizing the Design - Procedure
Review the Results

SMAX (maximum von Mises stress) vs. set number.

PLVAROPT,SMAX
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Optimizing the Design - Procedure
Review the Results
Working with Graphs

Many graph controls are available to alter the way the graph
shows on the screen:
 Axis
controls: change the axis label, number of divisions, range of
values, etc.
 Curve
 Grid

controls: thickness of curve, filling of area under curve, etc.
controls: grid on/off; X-grid only, Y-grid only, or both; etc.
Utility Menu : PlotCtrls > Style > Graphs
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Optimizing the Design - Procedure
Review the Results

As an example, we will change the previous graph as
follows:
 Set
Y-axis label to SMAX
 Change
number of X-axis divisions to 8 (9 sets, therefore 8
divisions)
 Increase
curve thickness
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Optimizing the Design - Procedure
Review the Results
Utility Menu : PlotCtrls > Style >
Graphs > Modify Axes ...
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Utility Menu : PlotCtrls > Style >
Graphs > Modify Curve ...
Chapter 6. ANSYS Implementation - 50
Optimizing the Design - Procedure
Review the Results
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Chapter 6. ANSYS Implementation - 51
Optimizing the Design - Procedure
Review the Results
Restoring the Best Design Geometry and Results

To do this, simply exit the optimizer and resume the file
jobname.bdb (best db).
 Main

Menu > Finish
 Utility
Menu > File > Resume from…
 Utility
Menu > Plot > Elements
Or issue these commands:
 FINISH
 RESUME,
jobname,bdb
 EPLOT
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Optimizing the Design - Procedure
Review the Results

Results are available on the file jobname.brst.
 Enter
POST1 and specify jobname.brst as the results file
(using the FILE command
or Main Menu : General Postproc > Data & File Opts… ).
 Then

use standard POST1 functions to review results.
Note: .bdb and .brst are written only if you activate the savebest-design option in the Optimization Controls dialog.
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Chapter 6. ANSYS Implementation - 53
Optimizing the Design - Procedure
Review the Results
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Chapter 6. ANSYS Implementation - 54
Optimizing the Design - Procedure
Review the Results
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Chapter 6. ANSYS Implementation - 55
Optimizing the Design
Procedure

Identify the analysis file

Identify optimization variables

Run the optimization

Review results
Next we will briefly discuss how to work with the optimization
database and continue (restart) the optimization if needed.
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Chapter 6. ANSYS Implementation - 56
Optimizing the Design
C. Restarts

A restart is a continuation of a previous optimization run.

Its purpose is usually one or both of the following:
 to
further hone in on the optimum by changing tolerances and/or
DV limits
 to
force a different optimization “path” by starting from a different
set of designs
OBJ
OBJ
Current optimum
Current optimum
Possible new optimum
DV
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Possible new optimum
DV
Chapter 6. ANSYS Implementation - 57
Optimizing the Design
Restarts

A typical restart involves four steps:
1.
First save the current OPT database to a named file
2.
Select a subset of design sets if needed
3.
Modify optimization variables (tolerances and limits)
4.
Run the optimization
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Optimizing the Design
Restarts
1. Save the Current OPT Database
 This
allows you to restore it later if needed.
 Design
 Be
Opt > -Opt Database- Save… (or OPSAVE)
sure to specify a non-default file name
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Optimizing the Design
Restarts
2. Select a Subset of Design Sets
 Design
Opt > -Design Sets- Select/Delete… (or OPSEL and OPDELE)
 Allows
you to keep a desired set of designs (such as all feasibles)
and discard the rest.
 Caution:
Selecting a subset
actually removes the unselected
design sets from the database.
(That’s why the first step is to
save the OPT database!)
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Optimizing the Design
Restarts
 For
the rotating disk example, selecting all feasible sets discards
seven design sets and keeps two.
 Use
the List function to list all currently selected design sets.

Design Opt > -Design Sets- List… > All Sets

Notice that the original set numbers are retained.
That is, deleted set
numbers are not reused.
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Optimizing the Design
Restarts
3. Modify Optimization Variables

You can modify tolerances, change limits, and even delete a DV or
SV for the restart.

Modifying Tolerances
 For
DVs and OBJ, allows you to change the convergence basis.
For example, tightening the OBJ tolerance may “force”
convergence based on DV tolerances.
 For
SVs, allows you to increase or decrease the feasibility
threshold.
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Optimizing the Design
Restarts
 For
the rotating disk, we will tighten the OBJ tolerance (SDEV) to
0.5.
 Design
Opt > Objective… > Modify
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Optimizing the Design
Restarts
 Changing

Limits
Allows you to constrain or expand the design
domain.

Design Opt > Design Variables… > Edit…

Design Opt > State Variables… > Edit…

For the rotating disk, we will change DV limits as
follows:
– THETARIM: 45°-90°
– XMID: 2.0-3.0 in
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Optimizing the Design
Restarts
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Optimizing the Design
Restarts
4. Run the optimization
 First
save the new settings to a named file:
 Design
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Opt > -Opt Database- Save…
Chapter 6. ANSYS Implementation - 66
Optimizing the Design
Restarts
 Then
initiate the optimization run. The new run will most likely
converge to a different design. Again, it is up to you, the engineer,
to verify the validity of the optimum design.
 Design
Opt > Run…
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Optimizing the Design
Restarts
 The
rotating disk
example took 12
iterations to converge
based on DV tolerances.
The best design is set
21.
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Optimizing the Design
Restarts

Compared to the previous best design (set 9, which is now
infeasible because of the new DV limits), the new best design
(set 21) has a lower OBJ.
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Optimizing the Design
Restarts
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