
[Sponsors] 
July 18, 2021, 17:24 

#21 
Senior Member
Filippo Maria Denaro
Join Date: Jul 2010
Posts: 6,764
Rep Power: 71 

July 25, 2021, 04:03 

#22 
New Member
N/A
Join Date: Feb 2021
Posts: 22
Rep Power: 5 
I have read during this week.
I still have some questions regarding the implicit filter. How the SubGrid Scale Model can act as an additional filter?? The papers that I read regarding this, do not represents spectra, so it is difficult to understand how they act as filter. I always thought that the subGrid Scale Model act after pi/Delta . Can you give me a hint regarding this? 

July 25, 2021, 04:38 

#23  
Senior Member
Filippo Maria Denaro
Join Date: Jul 2010
Posts: 6,764
Rep Power: 71 
Quote:
Be careful ... the SGS model mimics the contribution of the unresolved terms (for k>pi/h) but its action is to add a contribution to the resolved ones (k<pi/h). Generally, this contribution is in terms of an energy dissipation, like a further filtering. But from a rigorous point of view the SGS is not a filtering 

July 25, 2021, 05:06 

#24  
New Member
N/A
Join Date: Feb 2021
Posts: 22
Rep Power: 5 
Quote:


July 25, 2021, 05:07 

#25  
Senior Member

Quote:
First of all, let me distinguish the Sub Grid Scale (SGS) models from the Sub Filter Scale (SFS) models. As you mentioned, the former are meant to represent contributions from scales below the grid but, if you think about it, when you use them, you are doing it on your grid, not a finer one, so they necessarily act on your solved scales. SFS models, which are meant to represent interactions explicitly involving the resolved scales, by the same reasoning also act on the solved scales. Basically, whatever you add to your equations, necessarily acts on the solved scales, but may be meant to represent things happening at some other scale, represented or not. Now, SGS models are typically in the form of an added dissipative term. More specifically, a larger viscosity. If you have an idea of how spectra change when Re is lowered, it should be pretty clear why a SGS model acts as an additional filter. Another way to look at it is by considering how a typical filter would act: Then you can rearrange this into: So filtering is like advancing your solution with an artificial viscosity Now, I cannot speak for all the SFS models, but in the FV framework, when properly formulated without commutation errors, turns out that including a scale similar model also for the diffusive term is equivalent, at the lowest order, to the inclusion of an hyperdissipative term (i.e., 4th order Laplacian), something similar to the RhieChow term if you know it (which indeed appears in the pressure equation if you include a scale similar pressure term as well). Now, this might be less obvious but, in fact, any even order derivative term would act as a filter. The higher the order, the more the filter would resemble a spectral cut off one. So a 4th order Laplacian as implied by a proper scale similar model in the FV context would still imply, at the lowest order, an additional dissipative filter (just more compact than the one implied by the SGS model). I speak of lowest order because such reasoning is based on expanding the model in a Taylor series (something that was not necessary for the SGS model to reason about it). For the convective scale similar term however, which is more common, things are more complex and this is not obviously true in general (otherwise simple dissipative models would work much better). Nonetheless, there is at least one SFS LES approach, the approximate deconvolution modeling approach (ADM) that, in one of its formulations (due to Mathew) can be rearranged into just an high order explicit filtering of the equations (higher than a fourth order Laplacian). So, again, SGS modeling is reduced to filtering (in fact there are indeed additional LES formulation that just require high order filtering). Last edited by sbaffini; July 25, 2021 at 12:06. Reason: further clarified some aspects related to the dissipative effect of SFS models 

July 25, 2021, 05:25 

#26  
Senior Member
Filippo Maria Denaro
Join Date: Jul 2010
Posts: 6,764
Rep Power: 71 
Quote:
You cannot find this specific sentence in the literature, what I wrote above is implied in the LES philosophy, since from its origin. The SGS adopts the filtered field, therefore the results acts only on the filtered field. In no way you can add components outside pi/h. 

July 25, 2021, 05:46 

#27 
Senior Member
Filippo Maria Denaro
Join Date: Jul 2010
Posts: 6,764
Rep Power: 71 

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


Similar Threads  
Thread  Thread Starter  Forum  Replies  Last Post 
Principal variable in energy equation is total enthalpy or total internal energy?  lostking18  CFX  3  June 11, 2019 01:20 
Filtered navier stokes equation..LES::  Palani Velladurai  Main CFD Forum  7  September 6, 2013 02:51 
Constant velocity of the material  Sas  CFX  15  July 13, 2010 08:56 
CFX Additional Variable Transport Equation  Scott Nordsen  CFX  3  January 30, 2010 05:36 
Additional Variable  Diffusive Transport Equation  RadheShyam Das  CFX  3  October 18, 2007 12:35 