Fast Fourier Transform and CFD

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 January 29, 2017, 04:14 Fast Fourier Transform and CFD #1 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 29, 2017, 04:21 #2 Senior Member   Filippo Maria Denaro Join Date: Jul 2010 Posts: 6,777 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, 03:55 plot #3 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

 January 31, 2017, 04:00 #4 Senior Member   Filippo Maria Denaro Join Date: Jul 2010 Posts: 6,777 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 New Member   Sarah M Join Date: Oct 2016 Posts: 8 Rep Power: 9 Thank you, I will try it!

 Tags cfd - post, fft, plot