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Nastran versus LMS Test.Lab: Mode Shapes and FRFs

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Siemens Legend

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Nastran versus LMS Test.Lab: 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 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:

 

The following modal simulation result files and solutions are viewable with LMS Test.Lab:

  • 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 LMS Test.Lab Desktop ‘Navigator’ worksheet.

 

LMS Test.Lab and Nastran

 

In LMS Test.Lab, 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 LMS Test.Lab Nastran Data Driver and a few extra tricks.

 

Getting Started

 

Turn on the Nastran Data Driver under “Tools -> Add-ins” in the main LMS Test.Lab menu as shown in Figure 1.

addin.pngFigure 1: Turn on “Nastran Data Driver” under “Tools -> Add-ins”

Nastran punch files and output2 files can now be viewed in LMS Test.Lab.  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.

 

Output2 Files: Mode Shapes

 

With the Nastran driver activated, navigate to a directory that contains a Nastran OP2 file.  The file should appear with a light blue icon as shown in Figure 2.

nastran_navigator.pngFigure 2: Nastran files have blue icons in LMS Test.Lab 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
  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 LMS Test.Lab.

 

nastran_geometry.pngFigure 3: Open a geometry display and then drag and drop the geometry into the display

nastran_mode.pngFigure 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.

 

side_by_side_modes.pngFigure 5: Side-by-side animation of simulation and test mode shapes

See the LMS Test.Lab 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.

 

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 LMS Test.Lab (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.

 

nastran_FRF.pngFigure 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 LMS Test.Lab.  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 LMS Test.Lab (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

 

punch.pngFigure 7: ASCII contents of Nastran punch file

 The units of the punch file must be known for viewing in LMS Test.Lab:

  • 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 TL, LMSPCHDefaultUnit.xml is installed on the “central/configuration” directory where LMS Test.Lab 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.

 

punch_units.pngFigure 8: LMSPCHDefault.xml file defines units used

For example, if the simulation user was using “mm-ton-seconds” as the units, the LMSPCHDefaultUnit.xml file 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).

 

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 LMS Test.Lab versions 16 and higher. To view an AFU file containing FRFs in LMS Test.Lab (Figure 9):

  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.

 

AFU.pngFigure 9: Viewing ADF files in LMS Test.Lab

AFU files already contain units.  They can be directly overlaid with test based FRFs of the same unit in LMS Test.Lab.

 

Conclusion

 

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

 

Questions?  Contact us!

 

Appendix Supported Data Types

  • LMS Test.Lab Project file (*.lms) – A LMS binary project file can contain 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

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