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Fast Fourier Transform and CFD

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Old   January 29, 2017, 04:14
Default Fast Fourier Transform and CFD
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Sarah M
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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
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Old   January 29, 2017, 04:21
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Filippo Maria Denaro
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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?
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Old   January 31, 2017, 03:55
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Sarah M
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You are correct.

Last edited by Fun_Doge; February 1, 2017 at 05:25. Reason: wrong file
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Old   January 31, 2017, 04:00
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Filippo Maria Denaro
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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....
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Old   February 1, 2017, 05:25
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Thank you, I will try it!
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