ansys-optymalizacja(1).pdf

Chapter 1. Design Optimization

Strona 1 z 36

Main TOC • Using Help • Copyright

Advanced Guide >Chapter 1. Design Optimization

Chapter 1. Design Optimization

1.1. What Is Design Optimization ?

Design optimization is a technique that seeks to determine an optimum design. By "optimum design," we mean

one that meets all specified requirements but with a minimum expense of certain factors such as weight, surface

area, volume, stress, cost, etc. In other words, the optimum design is usually one that is as effective as possible.

Virtually any aspect of your design can be optimized: dimensions (such as thickness), shape (such as fillet

radii), placement of supports, cost of fabrication, natural frequency, material property, and so on. Actually, any

ANSYS item that can be expressed in terms of parameters can be subjected to design optimization. (See Use

ANSYS Parameters in the ANSYS Modeling and Meshing Guide for a description of ANSYS parameters.)

The ANSYS program offers two optimization methods to accommodate a wide range of optimization problems.

The subproblem approximation method is an advanced zero-order method that can be efficiently applied to

most engineering problems. The first order method is based on design sensitivities and is more suitable for

problems that require high accuracy.

For both the subproblem approximation and first order methods, the program performs a series of analysis-

evaluation-modification cycles. That is, an analysis of the initial design is performed, the results are evaluated

against specified design criteria, and the design is modified as necessary. This process is repeated until all

specified criteria are met.

In addition to the two optimization techniques available, the ANSYS program offers a set of strategic tools that

can be used to enhance the efficiency of the design process. For example, a number of random design iterations

can be performed. The initial data points from the random design calculations can serve as starting points to

feed the optimization methods mentioned above.

Advanced Analysis Techniques Guide

Home

1.2. Basic Definitions

Main TOC • Using Help • Copyright

Advanced Guide > Chapter 1. Design Optimization >1.2. Basic Definitions

1.2. Basic Definitions

Before describing the procedure for design optimization, we will define some of the terminology: design

variable, state variable, objective function, feasible and infeasible designs, the analysis file, iterations, loops,

file://C:\Temp\~hh235A.htm

10.04.03

Chapter 1. Design Optimization

Strona 2 z 36

design sets, etc. We will start with a typical optimization problem statement:

Find the minimum-weight design of a beam of rectangular cross section subject to the following constraints:

Total stress σ should not exceed σ max [ σ < σ max ]

Beam deflection ∆ should not exceed ∆ max [ ∆ < ∆ max ]

Beam height h should not exceed h max [h < h max ]

Figure 1.1. Example of a Beam for Design Optimization

Design Variables (DVs) are independent quantities that are varied in order to achieve the optimum design.

Upper and lower limits are specified to serve as "constraints" on the design variables. These limits define the

range of variation for the DV. In the above beam example, width b and height h are obvious candidates for

DVs. Both b and h cannot be zero or negative, so their lower limit would be b,h > 0.0. Also, h has an upper

limit of h max . Up to 60 DVs may be defined in an ANSYS design optimization problem.

State Variables (SVs) are quantities that constrain the design. They are also known as "dependent variables,"

and are typically response quantities that are functions of the design variables. A state variable may have a

maximum and minimum limit, or it may be "single sided," having only one limit . Our beam example has two

SVs: σ (the total stress) and ∆ (the beam deflection). You may define up to 100 SVs in an ANSYS design

optimization problem.

The Objective Function is the dependent variable that you are attempting to minimize. It should be a function of

the DVs, that is, changing the values of the DVs should change the value of the objective function. In our 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.

The design variables, state variables, and objective function are collectively referred to as the optimization

variables . 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.

A design set (or design ) is simply a unique set of parameter values that represents a particular 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.

A feasible design is one that satisfies all specified constraints - constraints on the SVs as well as constraints on

the DVs. If any one of the constraints is not satisfied, the design is considered infeasible . The best design is the

one which 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.)

The analysis file is an ANSYS input file (which can be created in a variety of ways) that contains a complete

analysis sequence (preprocessing, solution, postprocessing). It must contain a parametrically defined model,

using parameters to represent all inputs and outputs which will be used as DVs, SVs, and the objective function.

From this file, an optimization loop file ( Jobname.LOOP ) is automatically created and used by the optimizer to

perform analysis loops.

A loop is one pass through the analysis cycle. (Think of this as one pass through the analysis file.) Output for

file://C:\Temp\~hh235A.htm

10.04.03

Chapter 1. Design Optimization

Strona 3 z 36

the last loop performed is saved on file Jobname.OPO . An optimization iteration (or simply iteration ) is one or

more analysis loops which 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.

The optimization database contains the current optimization environment, 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.

Some of the above concepts can be better understood through an illustration. Optimization Data Flow shows the

flow of information during an optimization analysis. The analysis file must exist as a separate entity, and note

that the optimization database is not part of the ANSYS model database.

Figure 1.2. Optimization Data Flow

Chapter 1. Design Optimization

Home

1.3. About GUI Paths and Command

Syntax

Main TOC • Using Help • Copyright

Advanced Guide > Chapter 1. Design Optimization >1.3. About GUI Paths and Command

Syntax

file://C:\Temp\~hh235A.htm

10.04.03

Chapter 1. Design Optimization

Strona 4 z 36

1.3. About GUI Paths and Command Syntax

Throughout this document, you will see references to ANSYS commands and their equivalent GUI paths. Such

references use only the command name, because you do not always need to specify all of a command's

arguments, and specific combinations of command arguments perform different functions. For complete syntax

descriptions of ANSYS commands, consult the ANSYS Commands Reference .

The GUI paths shown are as complete as possible. In many cases, choosing the GUI path as shown will perform

the function you want. In other cases, choosing the GUI path given in this document takes you to a menu or

dialog box; from there, you must choose additional options that are appropriate for the specific task being

performed.

For all types of analyses described in this guide, specify the material you will be simulating using an intuitive

material model interface. This interface uses a hierarchical tree structure of material categories, which is

intended to assist you in choosing the appropriate model for your analysis. See Material Model Interface in the

ANSYS Basic Analysis Guide for details on the material model interface.

1.2. Basic Definitions

Home

1.4. How to Do Design Optimization

Main TOC • Using Help • Copyright

Advanced Guide > Chapter 1. Design Optimization >1.4. How to Do Design Optimization

1.4. How to Do Design Optimization

You can approach an ANSYS optimization in two ways: as a batch run or interactively through the Graphical

User Interface (GUI). The approach you take will depend on your ANSYS expertise and your preference for

interacting with the ANSYS program.

If you are very familiar with ANSYS commands, you may choose to perform the entire optimization by

creating an ANSYS command input file and submitting it as a batch job. This may be a more efficient method

for complex analyses (for example, nonlinear) that require extensive run-time.

On the other hand, the interactive features of optimization offer greater flexibility and immediate feedback for

review of loop results. When performing optimization through the GUI, it is important to first establish the

analysis file for your model. Then all operations within the optimizer can be performed interactively, allowing

the freedom to probe your design space before the actual optimization is done. The insights you gain from your

initial investigations can help to narrow your design space and achieve greater efficiency during the

optimization process. (The interactive features can also be used to process batch optimization results.)

file://C:\Temp\~hh235A.htm

10.04.03

Chapter 1. Design Optimization

Strona 5 z 36

The usual procedure for design optimization consists of the following main steps. These steps may vary

slightly, depending on whether you are performing optimization interactively (through the GUI), in batch mode,

or across multiple machines.

1. Create an analysis file to be used during looping. This file should represent a complete analysis sequence

and must do the following:

Build the model parametrically (PREP7).

Obtain the solution(s) (SOLUTION).

Retrieve and assign to parameters the response quantities that will be used as state variables and

objective functions (POST1/POST26).

2. Establish parameters in the ANSYS database which correspond to those used in the analysis file; this step

is typical, but not required (Begin or OPT).

3. Enter OPT and specify the analysis file (OPT).

4. Declare optimization variables (OPT).

5. Choose optimization tool or method (OPT).

6. Specify optimization looping controls (OPT).

7. Initiate optimization analysis (OPT).

8. Review the resulting design sets data (OPT) and postprocess results (POST1/POST26).

Details of the optimization procedure are presented below. Differences in the procedure for a "batch" versus

"interactive" approach are indicated, where appropriate.

1.4.1. Create the Analysis File

The analysis file is a key component which is crucial to ANSYS optimization. The program uses the analysis

file to form the loop file, which is used to perform analysis loops. Any type of ANSYS analysis (structural,

thermal, magnetic, etc.; linear or nonlinear) may be incorporated in the analysis file.

Note

An explicit dynamics analysis using ANSYS/LS-DYNA cannot be optimized.

In this file, the model must be defined in terms of parameters (which are usually the DVs), and results data

must be retrieved in terms of parameters (for SVs and the objective function). Only numerical scalar parameters

are used by the design optimizer. See Use ANSYS Parameters in the ANSYS Modeling and Meshing Guide for a

discussion of parameters. See the ANSYS APDL Programmer's Guide for a discussion of the ANSYS

Parametric Design Language (APDL).

It is your responsibility to create the analysis file and make sure that it is correct and complete. It must represent

a clean analysis that will run from start to finish. Most nonessential commands (such as those that perform

graphic displays, listings, status requests, etc.) should be stripped off or commented out of this file. Maintain

only those display commands which you would like to see during an interactive session [ EPLOT , etc.], or

direct desired displays to a graphics file [ /SHOW ]. Keep in mind that the analysis file will be used over and

over again during optimization looping. Any commands that do not have direct bearing on the analysis will

produce wasted action and therefore decrease looping efficiency.

There are two ways to create an analysis file:

file://C:\Temp\~hh235A.htm

10.04.03

Plik z chomika:

Inne pliki z tego folderu:

Inne foldery tego chomika: