Dynamic Response with External Superelements

Transcription

1 Dynamic Response with External Superelements Joe Brackin, Senior Software Engineer

2 Agenda Dynamic Response with External Superelements Who am I? Joe Brackin Senior Software Engineer Femap Product Development Group; 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 Page 2

3 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. Page 3

4 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. Page 4

5 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. Page 5

6 Rocket FEM Page 6

7 Rocket FEM Description Model Description: nodes and elements DOF Beam, Shell, Rigid Element Types Nonstructural Mass Regions Plot only elements Page 7

8 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 Page 8

9 Select Boundary Nodes for the External Superelement 1) create nodal constraint set named left booster boundary nodes" 2) select nodes 3) Select fixed Page 9

10 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 Page 10

11 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" Page 11

12 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 Page 12

13 Set up External Superelement Creation Form Continued: 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 Page 13

14 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. Page 14

15 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. Page 15

16 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" Page 16

17 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. Page 17

18 Submit System Analysis 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. External superelement reference Page 18

19 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 Page 19

20 Create a new booster external SE with Craig- 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. Page 20

21 Create a new booster external SE with Craig- Bampton modes added 1. Expand "Master Requests and Conditions 2. Highlight "External Superelement Creation" and click on "Edit" Page 21

22 Create a new booster external SE with Craig- Bampton modes added 1. Fill out form as shown previously. 2. Under "DOF Sets", go to the line labeled "QSET"; 3. select " 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. Page 22

23 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 Page 23

24 Results Comparison Full Model: Mode Hz 1min 12sec Default EXTSE Model: Mode 4.58 Hz 18 sec CB EXTSE Model: Mode 3.60 Hz 48sec Page 24

25 Results Comparison Full vs Default EXTSE Full Model: Mode Hz Default External SE Model: Mode 4.58 Hz Page 25

26 Results Comparison Full vs Craig-Bampton EXTSE Full Model: Mode Hz CB External SE Model: Mode 3.60 Hz Page 26

27 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 Page 27