# Energy Spectrum in Three Dimension?

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 May 23, 2020, 21:27 Energy Spectrum in Three Dimension? #1 Senior Member   Farzad Faraji Join Date: Nov 2019 Posts: 205 Rep Power: 7 Energy Spectrum in Three Dimension? Dear Friends I want to do 3D spectral analysis for energy. My domain is rectangular and it is (0,0,0)*(0.5, 1.0, 0.1), and it is almost like a jet flow(rectangular bubble flow). Now my question is that for FFT, my nodes needs to be equal(Nx=Ny=Nz) OR mesh size needs to be equal(dx=dy=dz)? Thanks, Farzad

May 24, 2020, 04:38
#2
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Filippo Maria Denaro
Join Date: Jul 2010
Posts: 6,811
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Quote:
 Originally Posted by farzadmech Energy Spectrum in Three Dimension? Dear Friends I want to do 3D spectral analysis for energy. My domain is rectangular and it is (0,0,0)*(0.5, 1.0, 0.1), and it is almost like a jet flow(rectangular bubble flow). Now my question is that for FFT, my nodes needs to be equal(Nx=Ny=Nz) OR mesh size needs to be equal(dx=dy=dz)? Thanks, Farzad

You need one or more directions of periodicity and equal step size along such direction

May 24, 2020, 12:14
#3
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Join Date: Nov 2019
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Dear professor Denaro
Based on what you said for powerspectrum in 3D(rectangular), I need to have dx=dy=dz. But what about Nx(kmax=Nx/2 based on Nyquist theory). Since my Ny is larger than Nx what should I do?(Ny is my streamwise direction).

Can I just transfer rectangle to square and do the powerspectrum inside the square?

Thanks,

Quote:
 Originally Posted by FMDenaro You need one or more directions of periodicity and equal step size along such direction

 Tags energy spectrum, fft, three dimensional