How to plot time dependent energy spectrum

LuckyTran · January 13, 2021, 13:56

The peak at 0 Hz is likely because you are forgetting to remove the means. You should be doing the fft of the velocity fluctuations only (which is equivalent of course to simply omitting the plot at 0 Hz).

If you are using matlab, you yourself don't need to zero-pad the data, just pass the proper arguments in the callout to the fft function which is fft(X,NFFT). NFFT is the number of frequency bins in the fft. The data (X) is automatically zero-padded if it's shorter than NFFT and truncated if it's longer than NFFT. zero padding doesn't actually do anything to the spectrum plot except that it allows the frequency bins to land on a desired number, i.e. a frequency of 30 Hz (nice and round) as opposed to 30.2312312312 Hz.

Falu · January 16, 2021, 09:41

I didnt set periodic boundary conditions, so i guess the time-fft is the way to go.

Thanks LuckyTran for the remarks, i updated my script accordingly.

January 13, 2021, 13:56		#21
LuckyTran Senior Member Lucky Join Date: Apr 2011 Location: Orlando, FL USA Posts: 5,754 Rep Power: 66	The peak at 0 Hz is likely because you are forgetting to remove the means. You should be doing the fft of the velocity fluctuations only (which is equivalent of course to simply omitting the plot at 0 Hz). If you are using matlab, you yourself don't need to zero-pad the data, just pass the proper arguments in the callout to the fft function which is fft(X,NFFT). NFFT is the number of frequency bins in the fft. The data (X) is automatically zero-padded if it's shorter than NFFT and truncated if it's longer than NFFT. zero padding doesn't actually do anything to the spectrum plot except that it allows the frequency bins to land on a desired number, i.e. a frequency of 30 Hz (nice and round) as opposed to 30.2312312312 Hz. aero_head likes this.

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January 16, 2021, 09:41		#22
Falu New Member Fab Join Date: Nov 2020 Posts: 25 Rep Power: 6	I didnt set periodic boundary conditions, so i guess the time-fft is the way to go. Thanks LuckyTran for the remarks, i updated my script accordingly.