CFD Online Logo CFD Online URL
www.cfd-online.com
[Sponsors]
Home > Forums > Main CFD Forum

LES Filtering basic question - help

Register Blogs Members List Search Today's Posts Mark Forums Read

Like Tree15Likes

Reply
 
LinkBack Thread Tools Display Modes
Old   September 5, 2012, 04:54
Default
  #21
Senior Member
 
sbaffini's Avatar
 
Paolo Lampitella
Join Date: Mar 2009
Location: Italy
Posts: 518
Blog Entries: 14
Rep Power: 17
sbaffini will become famous soon enough
Quote:
Originally Posted by francesco_capuano View Post
I would like to go deeper into the highlighted sentence together with Prof Denaro. Besides vanishing for laminar flows, a good SGS model should also vanish (or at least diminish) automatically if the local filter width (i.e. a measure of the grid size, for implicit filtering), is comparable to the Kolmogorov scale: if we apply an LES model to a DNS grid, the subgrid stress should be ideally zero. But also, if an LES grid has regions with very fine resolution (down to the Kolmogorov scale), then the SGS stress should vanish in those regions but not in the rest of the grid.

Now, my doubt is: which part of a (let's say dynamic) SGS model is responsible for this? Is it the dynamic calculation of the constant? I guess the standard Smagorinsky is not capable of doing that, as the constant is fixed and is calculated assuming, a priori, a certain position of the filter width along the spectrum, is that right?
I'm not completely sure 100% but i'm pretty confident that no part of the classical dynamic procedure has the capability to make the constant null in the case of a fully resolved DNS.

Actually, it can be shown that assuming the constant to be equal at the two filter levels imply that the computed dynamic constant is that at the test filter level and not at the basic filter level (either explicit or implicit).

Of course, for a very well resolved DNS this is not a real issue, also because the delta^2 factor will still be at work. Nonetheless, it is a well known inconsistency of the classical dynamic procedure.

There are two known modifications of the dynamic procedure to overcome this issue. The first one is proposed by Portè-Agel, Menevau and Parlange; the other ones are by Tejada-Martinez and Jansen:

Porté-Agel F, Meneveau C, Parlange MB. 2000a. A scale-dependent dynamic model for large-eddy simulation: application to a neutral atmospheric boundary layer. J Fluid Mech 415: 261-284
“A parameter-free dynamic subgrid-scale model for large-eddy simulation”, A.E. Tejada Martinez and K.E. Jansen, Computer Methods in Applied Mechanics and Engineering,195 (2006), pp 2919-2938
“A Dynamic Smagorinsky Model with a Dynamic Filter Width Ratio”, A.E. Tejada-Martinez and K.E. Jansen, Physics of Fluids, 16 (2004) 2514-2528

The work of Tejada-Martinez also adresses the issue of determining the delta ratio when the basic filter is implicit and you can't simply fix a value based on the computational cell. However, i'm not pretty confident on this work as in some cases (i.e., the finite volume based dynamic procedure of Prof. Denaro) i found some inconsistencies in applying such procedure.
hityangsir likes this.

Last edited by sbaffini; September 5, 2012 at 05:07. Reason: corrected the incomplete references
sbaffini is offline   Reply With Quote

Old   September 5, 2012, 05:45
Default
  #22
Member
 
Francesco Capuano
Join Date: May 2010
Posts: 78
Rep Power: 7
francesco_capuano is on a distinguished road
Dear Paolo, thank you very much! That is exactly what I was looking for, and I'll go through the references you suggested. I think the underlying problem for classical SGS models is that they imply a certain filter, rather then adapting to the actual filter.

However, one more reference I suggest is a part from the book by Pope (p. 594-597), when he talks about the limiting behaviors of the Smagorinsky model (filter in the dissipative range, filter being large compared to the integral scale and laminar flow).

In any case, as you also said, this is just a theoretical issue: from a practical point of view there is no concrete inconsistency.
francesco_capuano is offline   Reply With Quote

Old   September 5, 2012, 06:44
Default
  #23
Senior Member
 
Filippo Maria Denaro
Join Date: Jul 2010
Posts: 1,584
Rep Power: 20
FMDenaro will become famous soon enough
Remember that the Germano identity is exact, it is the SGS model you introduce in it that generates the approximations (and the inconsistences).
You can think about several improvements ... different functions model at different grid levels, or some modification in the eddy viscosity assumption...
I hope you can work on it
FMDenaro is offline   Reply With Quote

Old   December 5, 2012, 06:04
Default
  #24
Senior Member
 
Albrecht vBoetticher
Join Date: Aug 2010
Location: Zürich, Swizerland
Posts: 178
Rep Power: 6
vonboett is on a distinguished road
Adressing the filtering, there is aswll the point of averaging the SGS model constant when using dynamic SGS models. Averaging along homogenious flow directions is a theoretical case when simulating natural flows, the dynamic localization approach has its own drawbacks, so I wonder what you would think about a lagrangian dynamic mixed smagorinsky / scale similarity SGS model? (not implemented in OpenFOAM yet)
vonboett is offline   Reply With Quote

Old   December 5, 2012, 09:02
Default
  #25
Senior Member
 
Filippo Maria Denaro
Join Date: Jul 2010
Posts: 1,584
Rep Power: 20
FMDenaro will become famous soon enough
Quote:
Originally Posted by vonboett View Post
Adressing the filtering, there is aswll the point of averaging the SGS model constant when using dynamic SGS models. Averaging along homogenious flow directions is a theoretical case when simulating natural flows, the dynamic localization approach has its own drawbacks, so I wonder what you would think about a lagrangian dynamic mixed smagorinsky / scale similarity SGS model? (not implemented in OpenFOAM yet)
recently, this problem was superseeded by the integral-based dynamic procedure, no averaging of the model function is required as the extraction from filter is automatic
FMDenaro is offline   Reply With Quote

Old   December 10, 2012, 06:44
Default
  #26
Senior Member
 
Albrecht vBoetticher
Join Date: Aug 2010
Location: Zürich, Swizerland
Posts: 178
Rep Power: 6
vonboett is on a distinguished road
I see. So I wonder what you 'd think about the dynamic mixed version of Meneveau's lagrangian SGS model in comparison to the dynamic mixed model, both provided for OpenFOAM by Prof. Kornev: http://www.lemos.uni-rostock.de/en/cfd-software/

Found it due to a link by Hannes Kröger, thanks Hannes!
vonboett is offline   Reply With Quote

Old   December 10, 2012, 08:28
Default
  #27
Senior Member
 
Filippo Maria Denaro
Join Date: Jul 2010
Posts: 1,584
Rep Power: 20
FMDenaro will become famous soon enough
Quote:
Originally Posted by vonboett View Post
I see. So I wonder what you 'd think about the dynamic mixed version of Meneveau's lagrangian SGS model in comparison to the dynamic mixed model, both provided for OpenFOAM by Prof. Kornev: http://www.lemos.uni-rostock.de/en/cfd-software/

Found it due to a link by Hannes Kröger, thanks Hannes!
In general, taking into account the literature, I see an improvemnt in the Lagrangian version, some comparative testing were presented since 1999 (Sarghini et. al, PoF). Unfortunately, I have no knowledge of the perfomances when tested using OF.
FMDenaro is offline   Reply With Quote

Old   April 17, 2013, 11:44
Default Gaussing Filters and LES codes
  #28
New Member
 
sankarv
Join Date: Feb 2010
Posts: 26
Rep Power: 7
sankarv is on a distinguished road
Quote:
Originally Posted by FMDenaro View Post
Just to add a comment, at best of my knowledge explicit filtering is only an acadamic issue, no commercial LES code (FLuent, TransAT, etc) or open-source (OpenFOAM, Code_Saturne) use that, the filter is always implicitly defined by the discretization.
That makes very often confusing to distinguish in the manuals the LES from URAN equations, they are written formally in the same way..
Dear Fillipo

I have a general question about the use of filters other than top-hat filter in CFD codes.

Let us consider implicitly filtered LES approach, Smagorinsky model without dynamic coefficient estimation and finite volume CFD code.

If I want to use Gaussian filter, where will the Gaussian filter kernel appear in the cfd code ? The filtered LES equation only needs filter length scale for the subgrid model. If I want to use Gaussian filter how will the CFD code see the filter kernel shape ?

Will the filter length scale be different from cube root of cell volume if I want to use Gaussian filter ?

If I use dynamic smagorinsky or mixed models for subgrid stresses, there will be a step to explicit filter where the filter shape information is used by the code. If I use non-dynamic models, then I do not see where the filter shape will be used in the CFD code.

Can you please clarify ?

Thanks
Vaidya
sankarv is offline   Reply With Quote

Old   April 17, 2013, 12:09
Default
  #29
Senior Member
 
Filippo Maria Denaro
Join Date: Jul 2010
Posts: 1,584
Rep Power: 20
FMDenaro will become famous soon enough
Quote:
Originally Posted by sankarv View Post
Dear Fillipo

I have a general question about the use of filters other than top-hat filter in CFD codes.

Let us consider implicitly filtered LES approach, Smagorinsky model without dynamic coefficient estimation and finite volume CFD code.

If I want to use Gaussian filter, where will the Gaussian filter kernel appear in the cfd code ? The filtered LES equation only needs filter length scale for the subgrid model. If I want to use Gaussian filter how will the CFD code see the filter kernel shape ?
In no way an implicit filtering-based CFD code contains some filter you can choose to use... in your example, the filter is defined by the type of FV discretization. If you want to use a Gaussian filter then you must introduce it in an explicit filtering formulation. But in any case you must be aware that the FV formulation always imply that a smooth filter is acted on, thus adding a Gaussian filter in an explicit formulation can result is a strong smoothing (the two filter kernel are not idempotent)


[/QUOTE] Will the filter length scale be different from cube root of cell volume if I want to use Gaussian filter ?

If I use dynamic smagorinsky or mixed models for subgrid stresses, there will be a step to explicit filter where the filter shape information is used by the code. If I use non-dynamic models, then I do not see where the filter shape will be used in the CFD code.

Can you please clarify ?

Thanks
Vaidya[/QUOTE]

actually, both in scale-similar and mixed (static) model, the shape of the second filter must be prescribed as it is really applied on the variable to formulate the SGS model.
To complete the answers, the lenght of the filter should be deduced by the transfer function that is really in action in the code. The cube root of the cell measure is a rude approximation.

Further details can be found here: http://www.sciencedirect.com/science...21999111000933
FMDenaro is offline   Reply With Quote

Old   April 17, 2013, 13:39
Default
  #30
New Member
 
sankarv
Join Date: Feb 2010
Posts: 26
Rep Power: 7
sankarv is on a distinguished road
Thanks a lot for the quick reply. Your paper is certainly interesting. I will look into it carefully.
sankarv is offline   Reply With Quote

Old   September 10, 2013, 23:46
Default
  #31
New Member
 
Mahfuz Sarwar
Join Date: Nov 2011
Posts: 13
Rep Power: 5
mahfuzsarwar is on a distinguished road
In most of the cases we are talking about the filter size is larger than the grid size in the case of explicit filtering or almost equals to grid size if implicitly filtered.


What will happen if the filter width is less than the grid cell? Whether it will create any unphysical condition? What will be the effect on the SGS modelling?
mahfuzsarwar is offline   Reply With Quote

Old   September 11, 2013, 03:58
Default
  #32
Senior Member
 
Filippo Maria Denaro
Join Date: Jul 2010
Posts: 1,584
Rep Power: 20
FMDenaro will become famous soon enough
Quote:
Originally Posted by mahfuzsarwar View Post
In most of the cases we are talking about the filter size is larger than the grid size in the case of explicit filtering or almost equals to grid size if implicitly filtered.


What will happen if the filter width is less than the grid cell? Whether it will create any unphysical condition? What will be the effect on the SGS modelling?
When the computational size h is chosen, you fixed also the Nyquist frequency Kc=pi/h. In no way you can have a filter width lesser than the computational size, simply because there is nothing to filter ...
FMDenaro is offline   Reply With Quote

Old   September 19, 2013, 22:56
Default
  #33
New Member
 
Mahfuz Sarwar
Join Date: Nov 2011
Posts: 13
Rep Power: 5
mahfuzsarwar is on a distinguished road
Quote:
Originally Posted by FMDenaro View Post
When the computational size h is chosen, you fixed also the Nyquist frequency Kc=pi/h. In no way you can have a filter width lesser than the computational size, simply because there is nothing to filter ...
Thanks Filippo, for the reply.
It would be a great help if you just clear me up in this regards, what is the difference between sharp cutoff filter and smooth filter in LES?
mahfuzsarwar is offline   Reply With Quote

Old   September 20, 2013, 11:02
Default
  #34
Senior Member
 
Filippo Maria Denaro
Join Date: Jul 2010
Posts: 1,584
Rep Power: 20
FMDenaro will become famous soon enough
Quote:
Originally Posted by mahfuzsarwar View Post
Thanks Filippo, for the reply.
It would be a great help if you just clear me up in this regards, what is the difference between sharp cutoff filter and smooth filter in LES?
In the Fourier space, the sharp cut-off retains components (unmodified) at all frequencies up to Kc=pi/h.
The smooth filter is characterized by a smoothing behaviour of the resolved frequencies. For example the top-hat filter acts as sin (k*h) / (k*h). The first zero corresponds to the same cut-off frequency Kc=pi/h, but before it the components are now smoothed
FMDenaro is offline   Reply With Quote

Reply

Tags
filter, les, navier stokes equation

Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are On
Pingbacks are On
Refbacks are On


Similar Threads
Thread Thread Starter Forum Replies Last Post
basic question for pressure BC vivien OpenFOAM 0 November 23, 2009 06:01
Basic Poiseuille flow simulation question Ashish Main CFD Forum 0 October 2, 2007 13:05
New to CFD with some basic question Newbie Main CFD Forum 3 October 23, 2006 12:22
About filtering process in LES Yang Main CFD Forum 2 January 29, 2002 14:46
Dynamic filtering for LES Takuya TSUJI Main CFD Forum 1 July 14, 1999 02:17


All times are GMT -4. The time now is 16:08.