[Nek5000-users] Filtering in the turbChannel, it that possible to use another filter, such as top-hat filter or Gaussian filter?

[nek5000-users at lists.mcs.anl.gov]

Hello everyone, 

I have been working on the turbulent channel case for some time. But I am quite confused with the filtering technique in the turbChannel.usr. 
I understand well the filtering technique for improving stability (p101 and p103), thanks to the discussion on the forum. However, is the same technique used in the build_1d_filt ? basically, partly removing the high wavenumber spectral in the expansion ? 

I have a question about transfer function also: 

diag(nx-0) = 0.05 
diag(nx-1) = 0.50 
diag(nx-2) = 0.95 

It seems to me that the diag(:) would determine how much energy will be removed for the high wave number spectral? but what are the variables fh, fht in the subroutine 
build_1d_filt (fh,fht,diag,nx,nid) ? 

it seems quite difficult to apply the more traditional filters like top-hat filter or Gaussian filter in the Nek5000. 

Lastly, removing high wave number parts of Legendre expansion seems makes no sense physically. After all, the energy spectral of turbulence is represented in Fourier series. 

Does anyone has any idea on these puzzles? Any discussion is greatly appreciated. 

Best regards 

Bien cordialement 

Zhenrong JING 

Doctorant (Doctor Candidate) 

LHEEA 

Ecole Centrale de Nantes 

1 Rue de la Noë, 44321 Nantes, France 
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