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November 16, 2011, 06:46 |
3D Energy Spectrum
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#1 |
New Member
Join Date: Nov 2011
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Hi all 1st post so be gentle!
I have a 3D (turbulent) velocity field. For the sake of example lets say this is contained in 3 n^3 arrays, ux, uy, uz. I want the 3d energy spectrum, my method at the moment is to calculate fft of these arrays (everything below is in matlab syntax) i.e. >fux=fftn(ux)/(n^3); >fuy=fftn(uy)/(n^3); >fuz=fftn(uz)/(n^3); I then get real and imaginary parts: >energyr=real(fux).^2+real(fuy).^2+real(fuz).^2; >energyi=imag(fux).^2+imag(fuy).^2+imag(fuz).^2; Define the maximum wavenumber >midpt=n/2+1; Now I compute energy spectra in shells, spect(1:1.5*n)=0.; for i=1:n for j=1:n for k=1:n ii=i; jj=j; kk=k; if ii>midpt ; ii=n-ii+1; ; end ; if jj>midpt ; jj=n-jj+1; ; end ; if kk>midpt ; kk=n-kk+1; ; end ; r=round(sqrt(ii^2+jj^2+kk^2)); spect(r)=spect(r)+energyr(i,j,k)+energyi(i,j,k); end end end My question is, whilst this works well, I do not get the energy at the box scale (k=1). Where am I going wrong? Many thanks in advance |
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December 2, 2011, 08:20 |
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#2 |
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Anyone got any ideas?
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December 3, 2011, 18:58 |
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#3 |
Member
Mike
Join Date: Apr 2011
Location: Canada
Posts: 83
Rep Power: 15 |
Hey,
You know you should first calculate the Autocorrelation or autocovariance, and then take the fft of that one not the vlocity itslf. |
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September 23, 2015, 11:16 |
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#4 | |
New Member
V
Join Date: Sep 2015
Posts: 2
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Hi
Did you figure it out? I may need the same code too Quote:
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