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:
The following modal simulation result files and solutions are viewable with Simcenter Testlab:
Data stored in any of these formats can be viewed and directly overlaid with test data in the Simcenter Testlab Desktop ‘Navigator’ worksheet.
Simcenter Testlab and Nastran
In Simcenter Testlab, the Nastran Data Driver addresses some of the challenges overlaying test and measurement data:
Units and labelling of node names are addressed using the Simcenter Testlab Nastran Data Driver and a few extra tricks.
Turn on the Nastran Data Driver under “Tools -> Add-ins” in the main Simcenter Testlab menu as shown in Figure 1.
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.
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.
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):
This is the same process as displaying test based modes in Simcenter Testlab.
It is possible to create a side-by-side display of the test and simulation mode shapes as shown in Figure 5.
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):
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.
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):
The units of the punch file must be known for viewing in Simcenter Testlab:
LMSPCHDefaultUnit.xml describes the punch file quantities and their exact unit as shown in Figure 8.
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 Simcenter Testlab versions 16 and higher. To view an AFU file containing FRFs in Simcenter Testlab (Figure 9):
This is the same way that test based FRF functions are displayed.
AFU files already contain units. They can be directly overlaid with test based FRFs of the same unit in Simcenter Testlab.
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? Contact us!
Appendix Supported Data Types
Related Structural Analysis Links: