Fast Fourier Transform and CFD

Fun_Doge · January 29, 2017, 04:14

Hi,

Since people here have an understanding of CFD and are good at explaining stuff to us newbies, I have to ask something that may be silly.

I was doing a FFT of the Nusselt number over time (for a RANS simulation) and got a graph where there is one peak, and the signal decreases periodically. The thing I don't understand is why you want to do a FFT of the Nu number in the first place. Is the first peak related to the first VK vortex? I am self-taught and just gave this a try by following a tutorial to learn plotting in CFD. The theory was not explained.

FMDenaro · January 29, 2017, 04:21

RANS is a steady formulation, maybe you are talking about URANS?
The Fourier analysis of a signal is a very classical tool to analyze turbulence, it produces the energy spectra and you can detect the wavelenght of characteristic flow structures.
In your case you have a periodic time-window?
Could you post your results?

Fun_Doge · January 31, 2017, 03:55

You are correct.

FMDenaro · January 31, 2017, 04:00

try a log-log scale, the peak should correspond to the frequency of max spectral density...however, I do not remember analysis of spectra of the Nu number....

Fun_Doge · February 1, 2017, 05:25

Thank you, I will try it!

January 29, 2017, 04:14	Fast Fourier Transform and CFD	#1
Fun_Doge New Member Sarah M Join Date: Oct 2016 Posts: 8 Rep Power: 9	Hi, Since people here have an understanding of CFD and are good at explaining stuff to us newbies, I have to ask something that may be silly. I was doing a FFT of the Nusselt number over time (for a RANS simulation) and got a graph where there is one peak, and the signal decreases periodically. The thing I don't understand is why you want to do a FFT of the Nu number in the first place. Is the first peak related to the first VK vortex? I am self-taught and just gave this a try by following a tutorial to learn plotting in CFD. The theory was not explained. Last edited by Fun_Doge; January 29, 2017 at 04:16. Reason: forgot a word

January 31, 2017, 03:55	plot	#3
Fun_Doge New Member Sarah M Join Date: Oct 2016 Posts: 8 Rep Power: 9	You are correct. Last edited by Fun_Doge; February 1, 2017 at 05:25. Reason: wrong file

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January 29, 2017, 04:21		#2
FMDenaro Senior Member Filippo Maria Denaro Join Date: Jul 2010 Posts: 6,764 Rep Power: 71	RANS is a steady formulation, maybe you are talking about URANS? The Fourier analysis of a signal is a very classical tool to analyze turbulence, it produces the energy spectra and you can detect the wavelenght of characteristic flow structures. In your case you have a periodic time-window? Could you post your results?

January 31, 2017, 04:00		#4
FMDenaro Senior Member Filippo Maria Denaro Join Date: Jul 2010 Posts: 6,764 Rep Power: 71	try a log-log scale, the peak should correspond to the frequency of max spectral density...however, I do not remember analysis of spectra of the Nu number....

February 1, 2017, 05:25		#5
Fun_Doge New Member Sarah M Join Date: Oct 2016 Posts: 8 Rep Power: 9	Thank you, I will try it!