Dynamic Response with External Superelements...5. Repeat for the second superelement. The form...

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Joe Brackin, Senior Software Engineer

Dynamic Response with External Superelements


Siemens PLM Software

Who am I?

Joe Brackin

Senior Software Engineer

Femap Product Development Group; Siemens PLM Software

What you will learn

Application of external superelements to system dynamic

analysis

Femap capabilities

External superelement support

Demonstrations

Benefits of this topic

More efficient dynamic analysis

How to learn more

Agenda

Dynamic Response with External Superelements



Abstract

One efficient technique for performing system level dynamic analysis is to use Craig-

Bampton style external superelements for some components. Femap now supports the

creation and use of external superelements. By adding the Craig-Bampton modal

information to the standard external superelement, we can very efficiently increase the

accuracy of the dynamic behavior of the component. We will demonstrate the creation and

use of Craig-Bampton style external superelements in a system level normal modes

analysis in Femap.



Background

• External Superelements provide an efficient method to create and transfer the

information required to perform many types of system level static and dynamic

analysis. • By default, external superelement creation performs only a Guyan or static reduction of the FEM to the

chosen physical boundary nodes.

• The stiffness, mass and loads information is reduced to just the selected boundary grids and is then passed

along to represent this component in a subsequent system analysis.

• This static reduction(Guyan) method is exact for static analysis.

• However, for dynamics, even with careful selection of boundary grids, this method is may not be sufficient to

accurately represent the dynamic behavior of the component.

• One efficient technique for supplementing the superelement with information to

increase the accuracy in dynamics is the Craig-Bampton(component modes)

reduction technique. • This technique adds fixed boundary based modal information to the reduced physical model to insure that all

dynamic behavior for a chosen frequency range is included in the external superelement.

• Femap now supports creation of both types of external superelements as

well creation of the system run which references these external

superelements.



Overview

Femap will used to demonstrate the use of external superelements to

perform a normal modes solution of a rocket system composed of 3

components.

Example steps:

1) Solve for the normal modes of the rocket system without superelements.

2) Create an external superelement representing each booster.

3) Create a system normal modes solution using the detailed center tank and

an external SE for each booster.

4) Create two new booster external SE with Craig-Bampton modes added.

5) Perform a second rocket system normal modes solution using the Craig-

Bampton booster to demonstrate the increased accuracy.



Rocket FEM



Rocket FEM Description

• Model Description:

• 39735 nodes and 41838 elements

• 238170 DOF

• Beam, Shell, Rigid Element Types

• Nonstructural Mass Regions

• Plot only elements



Steps to Create External Superelement

• Identify the boundary nodes of the

superelement • Nodes that attach to other components

• Nodes with applied loads

• Nodes with constraints in the system

analysis

• Concentrated mass locations

• Nodes that help visualize the motion of the

component

• Identify nodes and elements for

OTM creation • Output requests in the creation run

automatically create an Output

Transformation Matrix(OTM) which is added

to the output matrices. “ALL” requests

generate large files.

• Setup the analysis options



Select Boundary Nodes for the External

Superelement

1) create nodal constraint set named “left booster boundary nodes"

2) select nodes

3) Select “fixed”



Create groups to identify any node/element results

required in the system run

1) Create New Group “left booster otm"

2) Use Group/Node/ID and select nodes

3) Use Group/Element/ID and select elements



Setup the Analysis to Create the External

Superelement

1) Create analysis set “left booster extse 10

creation"

2) use "2..Normal Modes/Eigenvalue" as the

analysis type, click OK to accept analysis

defaults

3) in the Analysis Set Manager, expand

"Master Requests and Conditions

4) Highlight "External Superelement

Creation" and click on "Edit"



Set up External Superelement Creation Form

1. Check " Create External Superelement" and enter 10 for

the EXTID

2. Check the desired Output Matrices "Stiffness" and

"Mass", uncheck others

3. Select the format of the output matrices by choosing

"1..DMIGOP2" in the "Output To" drop down menu; Set

the unit ID to "36"; Note: this unit ID is written to the output files

and must be used for the input to a system run. Any unit not

reserved by NASTRAN is valid input.

4. Enter a unique File Name for the .op2 file

“left_booster_se10”



Set up External Superelement Creation Form

5) Under "Output Options" check "ASMBULK" ; This will

create a text file with .asm extension which contains the Nastran

information needed by the system run to attach the external

superelement..

6) Under "DOF Sets" select the constraint set “left

booster boundary nodes" which identifies the boundary

nodes for the external superelement.

7) Use the Entity ID Range Checks to verify the model

has the desired node and element numbering

ranges(optional)

8) Click OK and return to the Analysis Set Manager

Continued:



Create the Output Requests for the External

Superelement

1) Highlight "Output Requests" and select “Edit”;

2) Check the desired output types and then use

the dropdown to pick the group “left booster otm"; Note: Output requests automatically create an Output

Transformation Matrix(OTM) which is added to the output

file. Using "0..Full model" can result in a very large OTM

3) Under Results Destination select "1..Print Only"

to avoid creation of the default op2 results file.



Submit the analysis

Save model file

Click analyze to run the analysis.

Check for existence of the new .op2 file

containing the mass, stiffness, and OTM

matrices. You should also see the

corresponding .asm text file which contains

Nastran bulk data needed to attach the

external superelement to the system run.



Create a normal modes system solution for the

rocket with 2 attached booster external SE

1. Create a new analysis set

2. use type "2..Normal Modes/Eigenvalue" as

the analysis type, click OK to accept defaults

3. in the Analysis Set Manager, expand

"Options”

4. highlight "External Superelement Reference"

and select "edit"



Create the external superelement reference for the 2

attached booster external SE

1. click on the "create new external superelement

reference " icon.

2. the file select dialog will open, select the .op2

file from the external creation run.

3. Set the Unit ID to "36" and File Type to “OP2”;

this must match the unit and type from the

creation run.

4. now the file select dialog opens again; select

the .asm file created by the same creation run.

5. Repeat for the second superelement. The form

summarizes the attached extse’s and files.

6. Check Param,SECOMB to have all individual

SE results combined by Nastran into one

datablock for postprocessing.



Submit System Analysis

External superelement reference

Select Analyze

Note: If SECOMB has been checked,

results for all superelements will imported

by Femap. The op2 will contain the

residual(SE 0) results and any EXTSE

OTM results requested in the creation run.



Create a new booster external SE with Craig-

Bampton modes added

1. Open Analysis Set Manager

2. create standard normal modes analysis set

3. expand "Options" and then "Modal/Buckling“

4. Highlight "Modal/Buckling" and click "edit“




Bampton modes added

1. The "NASTRAN Modal Analysis" form will open

2. Use the frequency range to limit the modes

calculated.

3. Set the "number desired" to the number of

Craig-Bampton Modes that you want to add to

the external superelement.




Bampton modes added

1. Expand "Master Requests and Conditions“

2. Highlight "External Superelement Creation"

and click on "Edit"




Bampton modes added

1. Fill out form as shown previously.

2. Under "DOF Sets", go to the line labeled

"QSET";

3. select "...”

4. The "Create SPOINTS" form will open. Type

in “10” for the number of SPOINTS.

• Note: The number of SPOINTS should be set to the

number of Craig-Bampton modes that you want to add

to the external SE. This should match the "number

desired" on the "Modal/Buckling" form. If the number

of SPOINTS is less than the number of modes

calculated, the extra modes will not be saved.

5. Run analysis to create new external superelement.

6. Femap creates the required Nastran SPOINT and

QSET input.



Perform a second rocket normal modes solution

using the Craig-Bampton boosters to demonstrate

the increased accuracy

The system analysis setup

procedure is the same for a

Craig-Bampton style EXTSE.

Use External SE reference to attach

the Craig-Bampton external

superelements



Results Comparison

Full Model:

Mode 9 @ 3.899 Hz

1min 12sec

Default EXTSE Model:

Mode 10 @ 4.58 Hz

18 sec

CB EXTSE Model:

Mode 9 @ 3.60 Hz

48sec



Results Comparison Full vs Default EXTSE

Full Model:

Mode 9 @ 3.899 Hz

Default External SE Model:

Mode 10 @ 4.58 Hz



Results Comparison Full vs Craig-Bampton EXTSE

Full Model:

Mode 9 @ 3.899 Hz

CB External SE Model:

Mode 9 @ 3.60 Hz



Conclusions

• External Superelements can reduce the resources required for

dynamic analysis

• Use of Craig-Bampton Style External Superelements can

increase the accuracy of the reduced component models

• Creation and use of external superelements is supported by

Femap

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