I would like to do frequency response analysis of a nonlinear system. I am applying load with a function of time. As harmonic response in for linear system, I am doing the transient response analysis now. Is there any way to convert the transient response output to frequency domain in ANSYS workbench?

# Frequency response analysis of Nonlinear system

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I don't know how to do that in ANSYS Workbench, but I know how to export a time history result from Workbench and I have matlab with a wonderful set of free scripts called vibrationdata that give me a GUI interface to compute all sorts of frequency domain information from a time-history signal.

Does the time history result mean the data that I receive as tabular data in ANSYS?

Yes, tabular data of any result quantity from a transient structural analysis has time in the first column.

I have received the transient data fora particular frequency and now I would like to export it in the matlab as per your suggestion. Can you please give me some hints/sources from where I can get the idea to to compute all sorts of frequency domain information from a time-history signal by using matlab? thanks.

I like the vibrationdata GUI for computing various frequency content from time-history data.

Download the Vibrationdata Signal Analysis Package, which is a zip file of a large number of scripts.

Put the folder you extract the zip file into in the matlab path, then type vibrationdata at the matlab command line to start the GUI.

Attach your time-history data in a zip file if you want me to show you an FFT of that signal.

Hello,

I have attached the zip file of time-history data in my previous comment.

Here is the data from your first tab plotted.

This doesn't look like it is the output from a Transient simulation. This looks like the nonlinear convergence plot of a Static Structural analysis.

I was expecting thousands of rows of data sampled at each millisecond for several seconds.

If you want to attach your ANSYS Project Archive, I will take a look at your model.

I have added the file in .wbpj format. Let me know if it works or not. After applying gravitational load in the static structural structure module, I applied acceleration varying with time in the transient structural module. In the acceleration tab, I used the formula containing 30 Hz and 0.4mm amplitude. I used substeps in the analysis setting.So can I calculate the outputs (acceleration, stress, strain) for other frequencies in the matlab VibrationData?

You can't just send the .wbpj file, that is not enough. The folder Ansys vib practice 2_files is also required. If you use ANSYS to create a Workbench Project Archive, then those two things are combined into a compressed file called a .wbpz archive. Here is more information.

I will understand what you have done once I have your archive or the complete zip file.

I am trying to make wbpz file but facing some problems in the server. Anyway, I will send you immediately once I am done with it. Would is meant by the time history data? Does it mean the tabular data after solving?

How should I proceed in the vibrationdata initially with these time history data? Thanks a lot.

Would you please suggest me a way to proceed with the time history data (obtained from Transient Structural module) in Matlab Vibration data so that I can perform the nonlinear frequency response analysis? Thanks a lot.

I will show you when you attach some transient data to analyze.

Thanks for your reply. Your guidance is helping me a lot to proceed step by step. Currently, I am using a simple structure as my practice where I put maximum 30 substeps. If I need to create large thousands of rows of data sampled at each millisecond for several seconds in transient structural module, what should I need to do? Will increasing the substeps/step time help me in this regard? Thanks

In a transient dynamics solution, you provide an initial time step, and a maximum time step and an end time. Say your end time is 1 second, if your initial time step is 1 ms and your maximum time step is 1 ms (1000 Hz sampling frequency), then you will get a minimum of 1000 rows in your output.

ANSYS can integrate through time internally, without reporting any output, so if you were to allow a maximum time step of 10 ms (100 Hz sampling frequency), then you would get no less than 100 rows in your output.

Have you heard of the Nyquist frequency? That is half the sampling frequency. If you want to perform an FFT, you can only plot data up to 500 Hz if your sampling frequency is 1000 Hz. Likewise, you can only plot data up to 50 Hz if your sampling frequency was 100 Hz.

If you want to plot data up to 1000 Hz, then you must have a sampling frequency > 2000 Hz or a maximum time step of 0.0005 seconds.

I hope this clarifies things.

You can also try APDL command "RESP" which can convert input in time domain to frequency domain. There are some knowledge materials on that topic if you search for "RESP" in customer portal. Hope it helps.

Mr. **Peteroznewman,** I have gathered some knowledge on Nyquist frequency. If I want to initiate the FRF analysis in vibrationdata, how should I proceed through vibration data? Which tab will allow me to process those data obtained from transient structural analysis?

vibrationdata provides a Modal FRF analysis, which assumes you have the force time history of the input location and the acceleration response at the output location, in the correct format.

kcao mentioned the "RESP" function, which computes the shock response spectrum. This can also be computed by vibrationdata, but is not a FRF.

Please attach a zip file of your data and your Workbench Project Archive so I can look at it.

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