parametric modeling chapter two. training manual january 30, 2001 inventory #001449 2-2 2....

Parametric Modeling

Chapter Two

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Training Manual2. Parametric Modeling

• The basic requirement for optimization in ANSYS (except topological optimization) is a parametric model (in file form.)

• In this module, we will:A. Define parametric modelingB. Review some APDL basicsC. Present guidelines to build a parametric model and create an

analysis fileD. Work on one or two workshop exercises

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Parametric ModelingA. Definition & Purpose

What is a Parametric Model?

• A model built and analyzed in terms of parameters (variables) instead of numbers.

• By simply changing the values of certain parameters in the model, you can build and analyze a new model.

H1 H2

A1

A2

A3H1

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Parametric Modeling...Definition & Purpose

• ANSYS allows you to “parameterize” not only the geometry but also other characteristics, e.g:– material properties– loads– number of constraints– maximum vertical deflection– minimum temperature– maximum stress

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• The basic requirement is that quantities to be used as DVs, SVs, and OBJ must be parameters.

For example:– If volume is to be minimized (objective function), then the total

volume must be available as a parameter.– If height H1 is to be declared as a design variable, the model

geometry must be built in terms of H1.– If the design is constrained by a limit on compressive stress

(state variable), the maximum compressive stress must be available as a parameter.

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• Every optimization run begins with a parametric model.

• The “output” from the parametric model is the analysis file, which is used as “input” for the optimizer.

AnalysisFile

Explore theDesign Domain

Optimize theDesign

Initial Design

Parametric Model& Loading

Solution

ParametricResults

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• The analysis file contains ANSYS commands for the complete parametric analysis:– Modeling– Loading– Solution– Results review

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• Notes on the analysis file:– Same as the .log file with non-essential commands (plots, lists,

graphics controls, etc.) removed.– Think of the analysis file as a format in which the parametric

model must be “stored” for optimization purposes.– The optimizer runs the analysis file multiple times, using new

parameter values each time.

• Since parameters are the essence of a parametric model, we will first review some basic APDL concepts.

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Parametric Modeling B. APDL Basics

What is APDL?

• Acronym for ANSYS Parametric Design Language, a scripting language that allows you to parameterize your model and automate common tasks.

• Using APDL, you can:– input model dimensions, material properties, etc. in terms of

parameters rather than numbers.– retrieve information from the ANSYS database, such as a node

location or maximum stress.

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Parametric Modeling ...APDL Basics

• APDL also allows you to:– perform mathematical calculations among parameters.– use array parameters to create and operate on vectors and

matrices.– define abbreviations (short cuts) for frequently used commands

or macros.– create a macro to execute a sequence of tasks, with if-then-else

branching, do-loops, and user prompts.

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Parametric Modeling ...APDL Basics

• In this section, we will concentrate on just the basic APDL capabilities:– how to define parameters– how to use them– how to retrieve information from the ANSYS database and

assign them to parameters

• For details on other APDL capabilities, please refer to your APDL Programmer’s Guide.

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Parametric Modeling - APDL Basics Defining Parameters

• To define a parameter, use the format

• Name=Value– Can be typed in the input

window or in the Scalar Parameters dialog (Utility Menu > Parameters > Scalar Parameters...)

– Name is the parameter name, eight alphanumeric characters or less.

– Value may be a number, a previously defined parameter, a mathematical function, a parametric expression, or a character string.

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Parametric Modeling - APDL Basics ...Defining Parameters

• Examples:inrad=2.5

outrad=8.2

numholes=4

thick=outrad-inrad

e=2.7e6

density=0.283

bb=cos(30)

pi=acos(-1)

g=386

massdens=density/g

circumf=2*pi*rad

area=pi*r**2

dist=sqrt((y2-y1)**2+(x2-x1)**2)

slope=(y2-y1)/(x2-x1)

theta=atan(slope)

jobname=‘proj1’

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Parametric Modeling - APDL Basics ...Defining Parameters

Some naming rules:

• Parameter names must be eight characters or less (see note), beginning with a letter.

• Only letters, numbers, and the underscore character _ are allowed.

• Avoid underscore _ as starting character… reserved for ANSYS use.

• Names are not case-sensitive, i.e, “RAD” and “Rad” are the same. All parameters are internally stored in capital letters.

• Avoid common ANSYS labels such as STAT, DEFA, and ALL.

Note: There are several undocumented features – use at your own risk!1. Design variable names with up to 16 characters are recognized2. Parameter names with a maximum of 32 characters may be entered

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Parametric Modeling - APDL Basics Using Parameters

• To use a parameter, simply enter its name in the appropriate field in the dialog box or on the command.

• For example, to define a rectangle using the parameters w=10 and h=5,

– you can use the menu:Preprocessor > Create > Rectangle > By 2 Corners +

– or commands:/prep7blc4,,,w,h

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Parametric Modeling - APDL Basics ...Using ParametersNote:• Whenever you use parameters, ANSYS immediately

substitutes their values.• The rectangle in the previous example is stored as a 10x5

area, not as w x h. That is, if you change the value of w or h after creating the rectangle, the area will NOT be updated.

• This is true of most parametric substitutions and is the reason for storing the parametric model in the form of a command file (the analysis file).

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Parametric Modeling - APDL Basics ...Using Parameters

• Other examples of using parameters:jobname=‘proj1’

/filnam,jobname ! Jobname

/prep7

ex=30e6 ! Exval would be safer

mp,ex,1,ex ! Young’s modulus

force=500

fk,2,fy,-force ! Force at KP 2

fk,6,fx,force/2 ! Force at KP 6

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Parametric Modeling - APDL Basics ...Retrieving Database Information

• To retrieve information from the database and assign it to a parameter, use the *GET command or Utility Menu > Parameters > Get Scalar Data...

• A vast amount of information is available, including model and results data. Refer to the *GET command description for details.

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• Examples:*get,x1,node,1,loc,x ! x1 = X coordinate of node 1 [CSYS]*

/post1

*get,sx25,node,25,s,x ! sx25 = X stress at node 25 [RSYS]*

*get,uz44,node,44,u,z ! uz44 = UZ displacement at node 44 [RSYS]*

nsort,s,eqv ! Sort nodes by von Mises stress

*get,smax,sort,,max ! smax = maximum of last sort

etable,vol,volu ! Store element volumes as vol

ssum ! Sum all element table columns

*get,totvol,ssum,,ITEM,vol ! totvol = sum of vol column

*CSYS = In the active coordinate system (CSYS)

RSYS = In the active results coordinate system (RSYS)

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• Some data can be retrieved with a get function.

Examples:x1=nx(1) ! x1 = X coordinate of node 1 [CSYS]*

nn=node(2.5,3,0) ! nn = node at or near (2.5,3,0) [CSYS]*

k1=kpnext(0) ! k1 = lowest numbered keypoint (of selected)

L1=lsnext(3) ! L1 = lowest numbered line > 3 (of selected lines)

/post1

ux25=ux(25) ! ux25 = UX at node 25 [RSYS]*

temp93=temp(93) ! temp93 = temperature at node 93

width=distnd(23,88) ! width = distance between nodes 23 & 88


RSYS = In the active results coordinate system (RSYS)

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Parametric Modeling - APDL Basics...Retrieving Database Information

• You can use a get function directly in a field, just like a parameter. For example:

k,10,kx(1),ky(3) ! KP 10 at X of KP 1, Y of KP 3 [CSYS]*

k,11,kx(1)*2,ky(3) ! [CSYS]*

f,node(2,2,0),fx,100 ! FX force at node(2,2,0) [CSYS]*

lfillt,lsnext(0),lsnext(lsnext(0)),.5

! Forms a fillet between 2 selected lines

! without referring to their numbers


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Parametric Modeling - APDL Basics...Retrieving Database Information

• APDL Summary:– Define parameters using the format Name=Value.– Value may be a number, a previously defined parameter, a

mathematical function, a parametric expression, or a character string.

– Use *GET or get functions to retrieve data from the ANSYS database.

– ANSYS stores data in their actual form (numbers or strings), not in the form of parameter names.

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Parametric ModelingC. Procedure / Guidelines

• The procedure to build and analyze a parametric model is the same as for a normal ANSYS analysis except that parameters are used wherever appropriate:

1. Define parameters

2. Build the model using parameters where appropriate

3. Apply loads and obtain the solution using parameters where appropriate

4. Review results and parameterize desired data

5. Create the analysis file.

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Parametric Modeling...Procedure / Guidelines

Step 1 - Define Parameters• Use the standard Name=Value format to define parameters, in the

Input window or in the Scalar Parameters dialog ( Utility Menu > Parameters > Scalar Parameters… ).

• No need to define all parameters in this step, but keeping all parameter definitions together makes for easier editing later (if needed).

• For parameters to be used as DVs, the values specified are used for the initial design only.

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Steps 2, 3 - Modeling, Meshing & Loading Using Parameters

• Use parameters instead of numbers wherever appropriate.

• Avoid using graphical picking or entity numbers for modeling and loading operations.

Use select logic, component names, or get functions instead.

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For example, the following method is not recommended in a parametric model (except in very simple cases):SFL,4,PRES,500 ! Pressure on line 4

F,32,FY,-7500 ! Force on node 32

Instead, use select logic or get functions:LSEL,S,LOC,… ! Select line by location

SFL,ALL,PRES,500 ! Pressure on all selected lines

LSEL,ALL

F,NODE(1.5,3,0),FY,-7500 ! Node number by get function

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H1 H2

A1, A2, A3


• Remember that you can use parametric expressions and/or if-then-else branching to modify the parameters. Example:– If diameter is defined as a parameter, you can use diameter/2 to

define a cylindrical volume using the CYLIND or CYL4 command.– If the truss bridge is modeled using half-symmetry, the cross-

sectional area of the vertical member at the symmetry plane would be A2/2.

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– If thk is defined as a parameter in a shell model and only three shell thicknesses are available (e.g, 1/8”, 3/16”, 1/4”), you can use an if-then-else construct (along with the appropriate OPLOOP setting for optimization looping - discussed later):

et,1,63 ! Shell element type*if,thk,lt,2.5/16,then thk=1/8 ! Use 1/8 if thk < 2.5/16*elseif,thk,gt,3.5/16,then thk=1/4 ! Use 1/4 if thk > 3.5/16*else thk=3/16 ! Otherwise use 3/16*endifr,1,thk ! Define shell thickness

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Step 4 - Parameterize the Results• This means to retrieve results data and store them in

parameters.• Usually done in POST1, the general postprocessor.• Results data, such as maximum deflection, maximum von

Mises stress, and total volume, are typically used as state variables and objective functions.

• Use the *GET command ( Utility Menu > Parameters > Get Scalar Data… ) or get functions to retrieve results data.

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Step 5 - Create the Analysis file

• The analysis file contains the command input used to build and analyze the parametric model. The optimizer uses it to loop through multiple designs.

AnalysisFile

Explore theDesign Domain

Optimize theDesign

Initial DesignParametric Model& Loading

Solution

ParametricResults

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• The sequence of commands on the analysis file follows the sequence you use to build and analyze the parametric model:– Parameter definitions (initial values)– Commands to define the geometry– Meshing commands– Commands for loads and boundary conditions– Solution commands– Commands to retrieve results data

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• Two ways to create the analysis file:– “Dump out” the command log stored in the ANSYS database.– Edit a copy of the log file (jobname.log).

We will concentrate on the first method in this seminar.

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• To dump out the command log stored in the database, choose Utility Menu > File > Write DB Log File (LGWRITE command).– The default file name is

jobname.lgw.– You can choose to write

essential commands only or to write non-essential commands (plots, lists, etc.) as comments.

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• You might want to edit the analysis file using a text editor and:– Add descriptive comments using the ! character. Any text

following ! is ignored by ANSYS as a comment.– Add some plot commands such as EPLOT and contour plots

(PLNSOL,…).This way, when you run the optimization process interactively, you can see the model changing from one iteration to the next.

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• Be sure to test your analysis file and make sure it doesn’t cause errors or unrealistic models.

• To test the analysis file, simply re-input it into a new ANSYS session.– Use the /INPUT command– Or Utility Menu > File > Read Input from…

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Parametric ModelingSummary

• A parametric model, stored in the form of an analysis file, is required for ANSYS design optimization.

• APDL parameters are used to build and analyze the parametric model.

• The analysis file typically contains commands for:– Initial parameter values– Model geometry and meshing– Boundary conditions, loads, and solution– Results review and retrieval

…or it could be a single line defining the objective function in terms of the design variables with APDL: Obj = 0.5*dv1**3 - 21*dv2**2 + 210*dv3 - 100

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Parametric ModelingD. Example• Shown here is the analysis file

for a rectangular plate model built with thickness defined as a discrete design variable thk. Other parameters defined are vol for volume and smax for maximum stress.

thk=.25/prep7 et,1,63*if,thk,lt,2.5/16,then

thk=1/8*elseif,thk,gt,3.5/16

thk=1/4*else

thk=3/16*endifvol=thk*400

r,1,thk ! Plate thicknessmp,ex,1,1e7rect,,20,,20amesh,1finish/solunsel,s,extd,all,all ! Fix exterior nodesnsel,allsf,all,pres,5 ! Pressure loadingsolvefinish/post1set,lasttopnsort,s,eqv*get,smax,sort,,maxfinish

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Parametric Modeling...Example• Optimization input for the rectangular plate is shown here. Notice

the use of the OPLOOP,PREP,PROCESS command to make sure that the thk definitions are processed. Details of the optimization procedure are presented in the next chapter.

/opt ! Optimization commandsopvar,thk,dv,1/8,1/4 ! thk is the DVopvar,smax,sv,,21000 ! smax is the SVopvar,vol,obj ! vol is the objective functionoptype,subpoploop,prep,process ! Read analysis file from /PREP7, process DV settingsopexe ! Execute optimizationoplist,allfinish

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Parametric ModelingE. Workshop

This workshop consists of two problems:

1. Parametric modeling of an axisymmetric rotating disk– A step-by-step description of the procedure.– You may choose to run this problem yourself, or your instructor

may show it as a demonstration.

2. Parametric modeling of a hexagonal steel plate– Step-by-step description of most of the procedure.

• See your Design Optimization Workshop Supplement for details.

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