
[Sponsors] 
July 1, 2019, 11:15 
LES filter frequency limit

#1 
Member
Ashish bhigah
Join Date: Jan 2019
Posts: 31
Rep Power: 5 
Hi,
I have a theorical doubt about the les filter. The filter, in my software, is the cubic root of the the cell volume. So if I use, for istance, the grid spacing delta=0,125 m, the resolved frequency should go from 0 to 2*pi/delta, which in this case is almost 50 Hz. But now I have a doubt, since 2*pi/delta has the unit m^1 , it should be wavenumber and not frequency. So I am getting confused. Can you help me to understand which is the frequency limit of the resolved area. Thank you in advance 

July 1, 2019, 13:49 

#2  
Senior Member
Filippo Maria Denaro
Join Date: Jul 2010
Posts: 6,151
Rep Power: 66 
Quote:
You have an implicit filter, I suppose...You are doing confusion between the filter width (which is a lenght) and the cutoff wavenumber. That is the filter is implicitly due to the grid and to the numerical scheme. In such s case, the cutoff frequency is simply due to the grid size. In a simple 1D example, a grid of step h introduces the cutoff spatial frequency (Nyquist) kc= pi/h. It is dimensional since the frequency is expressed as k = n*2*pi/L this way you see that n is the wavenumber and k is the frequency. Be aware that if L=2*pi then k=n*(1). This means that n is always a number but, to be trasformed in a frequency k, that requires to be multiplied by 1 that has the dimension 1/L. Of course, this is never explicitly written and you can read in papers n or k. 

July 15, 2019, 09:01 

#3 
Member
Ashish bhigah
Join Date: Jan 2019
Posts: 31
Rep Power: 5 
Hi, I have another question related to this. Is the cutoff related to the fact the we can considered the filter a top hat and doing the FT of a top hat function the first zero is at kc=pi/delta ??


July 15, 2019, 09:40 

#4  
Senior Member
Filippo Maria Denaro
Join Date: Jul 2010
Posts: 6,151
Rep Power: 66 
Quote:
No, the transfer function of the tophat filter has an infinite number of zeros and is a function going to zero only for k>+Inf. The the cutoff is due the superimposition of the grid size. 

Tags 
filter, frequency, les 
Thread Tools  Search this Thread 
Display Modes  


Similar Threads  
Thread  Thread Starter  Forum  Replies  Last Post 
OpenFOAM LES capability questions  siw  OpenFOAM Running, Solving & CFD  3  January 6, 2015 08:50 
Problem of InterFoam with LES SpalartAllmarasIDDES  keepfit  OpenFOAM  3  August 29, 2013 11:21 
LES filter for dieselfoam (or in general)  Rickard.Solsjo  OpenFOAM  0  August 25, 2010 08:12 
calculation of test filter quantites for desired parameters(equations) in les  mrn  FLUENT  0  July 15, 2010 05:04 
Basic LES question  toyCFD  Main CFD Forum  3  August 23, 2006 15:09 