Simcenter 3D Solutions Nastran and Test: Compare Mode Shapes and FRFs

Nastran and Test: Compare Mode Shapes and FRFs

Trying to compare simulation and test data? It can be a confusing smorgasbord of file formats, unit issues, and more. This article explains how to make comparing these data types easier.

Using Simcenter Testlab (formerly LMS Test.Lab), it is possible to view and compare Nastran simulation results with test measurement data. Nastran is a finite element solver that can be used for predicting modal performance of a simulated structure.

Possible results of a Nastran Finite Element Analysis (FEA) modal simulation include:

• Normal mode shapes
• Frequency Response Functions (FRFs)

The following modal simulation result files and solutions are viewable with Simcenter Testlab:

• Nastran Solution 103 – Natural frequencies and mode shapes in output2 file (*.op2)
• Nastran Solution 111 – Frequency Response Functions (FRFs) in punch file (*.pch)
• Nastran Bulk Data File (BDF) - Ascii file with geometry elements of model (*.bdf)
• Associated Data File (ADF) - The SDRC IDEAS file formats are still used in Simcenter3D (part of NX) to store time (*.ati), frequency (*.afu), and mode shape data (*.ash)

Data stored in any of these formats can be viewed and directly overlaid with test data in the Simcenter Testlab Desktop ‘Navigator’ worksheet.

This article contains the following sections:
1. Simcenter Testlab and Nastran
    1.1 Getting Started
    1.2 Output2 Files: Mode Shapes
    1.3 Punch Files: Frequency Response Functions (FRFs)
    1.4 Units
    1.5 Node Names
2. Associated Data File (ADF)
3. Conclusion
4. Appendix Supported Data Types
 

1. Simcenter Testlab and Nastran

In Simcenter Testlab, the Nastran Data Driver addresses some of the challenges overlaying test and measurement data:

• UNITS - Nastran simulation results are unitless. In the simulation, the analyst must enforce unit consistency. For example, if units of mm-ton-seconds were used, this must be understood to have proper overlay of results with test data. Test data has measurement units.
• NODE NAMES - Simulation and test node names are not the same. Finite element results may have node numbers like 102344:+Z that correspond to test names like wing:1:+Z. This makes overlay of data difficult if not addressed.

Units and labelling of node names are addressed using the Simcenter Testlab Nastran Data Driver and a few extra tricks.

1.1 Getting Started

Just browse to the Nastran data in the Simcenter Testlab Desktop Navigator! Nastran punch files and output2 files can now be viewed in Simcenter Testlab. It is possible to read and view the contents of the file directly without any translation. Simply navigate to the directory containing the files and drag and drop into a display.

For Version 17 and lower of SImcenter Testlab, turn on the Nastran Data Driver under “Tools -> Add-ins” in the main Simcenter Testlab menu. (Note: With Simcenter Testlab Revision 18 and higher, the add-in will not be in the list as it is already included with the Desktop).

For Simcenter Testlab 2021, the Simcenter 3D driver should be installed separately.  More details in the knowledge article "Simcenter 3D Driver for Simcenter Testlab".

1.2 Output2 Files: Mode Shapes

For example, navigate to a directory that contains a Nastran OP2 file. The file should appear with a light blue icon as shown in Figure 2.

Figure 2: Nastran files have blue icons in Simcenter Testlab Navigator

A Nastran output2 file contains modes created by Solution 103. The file is binary. To view the modes in the file (Figures 3 and 4):

1. Open a Geometry display
2. From the middle pane of the Navigator worksheet, drag and drop the Geometry into the display (Note for the OP2 to contain the geometry set PARAM OGEOM YES in the Nastran input file)
3. Double click on “ModeSet” entry in the OP2 file to view the modes contained in the file listed by frequency
4. Drag and drop the modes from the list into the display

This is the same process as displaying test based modes in Simcenter Testlab.

Figure 3: Open a geometry display and then drag and drop the geometry into the display

Figure 4: Drag and drop the mode shape into the display

It is possible to create a side-by-side display of the test and simulation mode shapes as shown in Figure 5.

Figure 5: Side-by-side animation of simulation and test mode shapes

See the Simcenter Testlab Display Layout Knowledge Base article to create a side-by-side display. The modal assurance criterion is useful for comparing test and simulation mode shapes.

1.3 Punch Files: Frequency Response Functions (FRFs)

The Nastran Solution 111 is used to calculate FRFs. The file results can be in punch format, which is ASCII.

To view the Frequency Response Functions in Simcenter Testlab (Figure 6):

1. Highlight the Nastran punch file in the navigation area on the left side of the ‘Navigator’ worksheet
2. Open a Front/Back or other display
3. Drag and drop FRF functions from the middle pane into the display

This is the same way that test based FRF functions are displayed. Dropping test FRF data of the same units in the display with the simulation FRFs allows them to be compared.

Figure 6: Drag and drop FRFs from the punch file into the display

Because the punch files are ASCII, they can be viewed in a text editor. It is not necessary to open the punch file with an editor to view the data in Simcenter Testlab. However, it can be useful to know the structure and contents of the file if questions should arise.

There are several key components in the punch file that should be present for successful viewing in Simcenter Testlab (Figure 7):

• $LABEL – This is the reference, or input, location for the FRF
• $POINT ID – This is the point identification, or output location, of the FRF
• $ACCELERATIONS - The type of the curves are identified by the header which indicates PRESSURE, ACCELERATIONS OR DISPLACEMENTS. The exact unit is defined by LMSPCHDefaultUnit.xml which is explained further on in this document.
• X, Y, and Z direction – The three columns of values are the X, Y, and Z direction data respectively

Figure 7: ASCII contents of Nastran punch file

1.4 Units

The units of the punch file must be known for viewing in Simcenter Testlab:

• The default X-axis is frequency in Hz.
• Curves can be acoustic sensitivity (e.g., MPa/N) or acceleration sensitivity (e.g., mm/s^2/N) or displacement sensitivity (e.g., mm/N). The type of the curves are identified by the header which indicates $PRESSURE, $ACCELERATIONS, or $DISPLACEMENTS respectively.
• Upon installation of Testlab, LMSPCHDefaultUnit.xml is installed on the “C:\Program Files (x86)\LMS\LMS Test.Lab {Rev}\central\Configuration” or "C:\Program Files (x86)\Simcenter\Testlab {Rev}\central\Configuration" directory where Simcenter Testlab is installed. This file follows the central/group/local mechanism.

LMSPCHDefaultUnit.xml describes the punch file quantities and their exact unit as shown in Figure 8.

Figure 8: LMSPCHDefault.xml file defines units used

For example, if the simulation user was using “mm-ton-seconds” as the units, the default file LMSPCHDefaultUnit.xml shown in Figure 8 would work properly.

Usually the FRF comparison between simulation and test is restricted to translational directions (X, Y, and Z). It is difficult to measure rotational acceleration, so comparisons are not normally made for rotational degrees of freedom (RX, RY, and RZ).

More about Simcenter Testlab unit handling in the knowledge article: Simcenter Testlab Units.

1.5 Node Names

Sometimes it is desirable to have the node names of the test FRFs and simulation FRFs match for easy comparison of the data. The file LMSNastranPointMap.xml (Figure 9) can be modified to create a map or key between the test and simulation FRFs.

Figure 9: The LMSNastranPointMap.xml file can be used to map test Ids (AliasPointID) to simulation Ids (FinalPointId).

A file that can be edited is located in either C:\Program Files (x86)\LMS\LMS Test.Lab {Rev}\central\Configuration or C:\Program Files (x86)\Simcenter\Testlab {Rev}\central\Configuration. It can be copied to the user local folder as not to affect other user accounts on the same PC: C:\LMS\UserConfiguration\ms1mwm\Testlab {Rev}\Configuration.

The advantage of using the XML file is that individual Punch files will not require editing. With a modified LMSNastranPointMap.xml file, when browsing through the files, the Identifications (Ids) are changed as shown in Figure 10.

Figure 10: After modifying LMSNastranPointMap.xml, the Nastran Ids are changed.

The FRFs in the Nastran Punch file can be copied to the Simcenter Testlab project. By doing so, features like curve scrolling can be used to quickly overlay data.

ADF file output of Simcenter3D is also supported.

2. Associated Data File (ADF)

In Simcenter3D, it is possible to generate FRFs in an ADF file format from Solution 103 normal modes. ADF is an acronym for Associated Data File, which is a format used by SDRC IDEAS. The FRF file is stored in a specific frequency ADF file with a *.afu extension.

All ADF file formats are automatically recognized in Simcenter Testlab versions 16 and higher. To view an AFU file containing FRFs in Simcenter Testlab (Figure 11):

1. Highlight the AFU file in the navigation area on the left side of the screen
2. Open a Front/Back or other display
3. Drag and drop FRF functions from the middle pane into the display

This is the same way that test based FRF functions are displayed.

Figure 11: Viewing ADF files in Simcenter TestlabAFU files already contain units. They can be directly overlaid with test based FRFs of the same unit in Simcenter Testlab.

3. Conclusion

Enjoy comparing simulation and test data in Simcenter Testlab! All data types, when in a display, can have active pictures created in Powerpoint and Word documents.

Questions? Email peter.schaldenbrand@siemens.com or download the Simcenter NASTRAN Brochure.

Interested in more Structural Dynamics?  Check out the On-Demand Structural Dynamics webinars.

4. Appendix Supported Data Types

• Simcenter Testlab Project file (*.lms) – A binary format project file that contains FRFs, Spectrums, time histories, and mode shapes.
• Nastran OP2 (*.op2) – A Nastran OP2 file can contain the results of an analysis. They contain geometry, mode shapes, and 2D functions like FRFs. The can be results from Solution 103 or 111. It is a binary file format.
• Nastran Punch File (*.pch) – Frequency response results file, ASCII
• Nastran Bulk Data File (*.bdf) – Geometry only
• ADF (*.afu, *.ash, *.ati) – Associated Data file format from SDRC IDEAS and Simcenter3D

Related Structural Analysis Links:

• Index of Testing Knowledge Articles
• Simcenter 3D Driver for Simcenter Testlab
• History of Modal Analysis
• Accessing Simcenter Amesim Models in Simcenter Testlab Neo
• Accessing Simcenter3D Motion Results in Simcenter Testlab
• Using Simcenter Testlab Project Files in Simcenter 3D Correlation
• Simcenter Testlab Neo and Functional Mockup Interface
• What is Frequency Response Function (FRF)?
• Simcenter Testlab Units
• Simcenter Testlab Impact Testing
• Natural Frequency and Resonance
• How to calculate damping from a FRF?
• The FRF and it Many Forms!
• What modal impact hammer tip should I use?
• Modal Tips: Roving hammer versus roving accelerometer
• Modal Driving Point Survey: What's at Stake?
• Getting Started with Modal Curvefitting
• Modal Assurance Criterion
• Using Pseudo-Random for high quality FRF measurements
• What is an Operational Deflection Shape (ODS)?
• Simcenter Testlab Modal Analysis: Modification Prediction
• Modal Impact Testing: User Defined Impact Sequence
• Import CAD into Simcenter Testlab
• Animate CAD Geometry
• Alias Table: Mapping Test Data to Geometry
• Geometry in Simcenter Testlab
• Maximum Likelihood estimation of a Modal Model (MLMM)
• Ground Vibration Testing and Flutter