# How to calculate wavenumber while using fft?

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 June 23, 2019, 08:09 How to calculate wavenumber while using fft? #1 Senior Member   Join Date: Jan 2018 Posts: 121 Rep Power: 7 I have 192 grids of two point correlation data of a 3.14(m) length domain. I use the "fft" command in matlab to plot the energy spectrum. How can I calculate wavenumber(k) of each grid? Code: ```R= load('2pcor.txt'); PHI = fft(R); % K=?? ; %loglog(K,abs(PHI));```

June 23, 2019, 12:12
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Filippo Maria Denaro
Join Date: Jul 2010
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Quote:
 Originally Posted by KevinSpears I have 192 grids of two point correlation data of a 3.14(m) length domain. I use the "fft" command in matlab to plot the energy spectrum. How can I calculate wavenumber(k) of each grid? Code: ```R= load('2pcor.txt'); PHI = fft(R); % K=?? ; %loglog(K,abs(PHI));```

On your grid of Nx=192 nodes you have 192/2=96 resolvable wavenumbers. Each wavelength is k=n*2*pi/L. Thus, if you L is exactly pi (in meters) you have k=2*n. The maximum in your range is the Nyquist frequency pi/h=pi/(L/Nx)=Nx

Last edited by FMDenaro; June 23, 2019 at 16:16.

 Tags fft, matlab, turbulence