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

Explicit filtering in LES

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

Like Tree27Likes

Reply
 
LinkBack Thread Tools Search this Thread Display Modes
Old   September 9, 2002, 12:15
Default Explicit filtering in LES
  #1
TJ
Guest
 
Posts: n/a
Hi,

I have been doing some LES simulations using the Finite-Volume formulation as an implicit top-hat filter. I would now like to implement explicit filtering into my code. However I have so far only succeded in confusing myself.... If anyone can answer the following questions I would be most gratefull.

In the momentum equation the velocities become filtered velocities and the pressures become filtered pressures. Do I need to determine these from the actual velocities using the filter?? can I do the opposite and determine the actual velocity (and therefore the fluctuation) by reversing the filter and applying it to the filtered variables????

Additionally I am unsure about whether to represent the subgrid dissipation as a source/sink term in the equation or as an additional viscosity - what do you recommend??

Thanks
  Reply With Quote

Old   September 9, 2002, 17:54
Default Re: Explicit filtering in LES
  #2
Kalyan
Guest
 
Posts: n/a
TJ,

One thing to note before getting to your questions. Use of a finite-volume formulation does not necessarily imply top-hat filter. There are approximate ways of estimating what the implicit-filter looks like and it's width (if needed). But in general the implicit filter depends on your discretization schemes (numerical dispersion and dissipation terms).

If you had actual velocities and knew what your filter is, then you can compute filtered velocities. However, in LES, you have the problems in reverse. You have filtered velocities. Even if you knew what the implicit filter was, you can not back out the actual velocity field except to within an approximation. This is because a lot of different actual velocity fields can result in the same filtered velocity fields (upon filtering). i.e.,

Filter(F) = <F>

For a given field "F", <F> is unique, but for a given <F>, "F" is not.

Often when you reverse a filter operation, you might end up with unbounded operators or operators with broad support which make the reverse filtering cumbersome. There are however models based on approximate inverse operators to filters that are used in LES modeling. Here is a good website that discuss such models.

http://www.ifd.mavt.ethz.ch/cfd/resles.htm

Explicit filtering : When people talk about explicit filtering what that often means is you take the LES generated field and filter it using an explicit filter. The LES generated field being implicitly filtered already, explicit filtering involves filtering of a field that is already implicitly filtered. (Hope I haven't confused anyone here).

EF : explicit filter IF : implicit filter

actual variable q produces an LES variable IF(q).

Upon explicit filtering what you end up with is EF[IF(q)].

If the characteristic width of the explicit filter is much larger than that of the implicit filter, you can safely assume that the resulting field is an actual velocity field filtered using an explicit user-specified filter. i.e.,

EF[IF(q)] ~ EF(q)

Thus the implicit filter, which is often the unknown, becomes inconsequential. Since the implicit filter also depends on the mesh spacing, any dependence on the mesh is also removed. Sometimes, the numerics create heuristic high wavenumber behavior that needs to kept in check (eg. de-aliasing errors), explicit filtering removes all those heuristic scales. That's why explicit filtering is highly recommended for unstructured meshes.
aerosjc, hz283 and wenxu like this.
  Reply With Quote

Old   September 10, 2002, 08:50
Default Re: Explicit filtering in LES
  #3
TJ
Guest
 
Posts: n/a
Kaylan,

Thanks for your reply - It answers most of my questions. can I just confim a couple of points...

If I explicitly filter an implicitly filtered field I need to make the explicit filter width much greater than that of the implicit filter. My implicit filter width is equal to the cube root of the cell volume - how much bigger does the explicit filter have to be ? It seems I could sensibly do 2 or 3 times the implicit filter length - is this enough - if I did this would the resolution of the simulation be effected ? If I used a germano SGS model would my test filter be 4 or 6 * grid scale ??

If I feed the explicitly filtered velocities into the momentum eqns and Explicitly filtered pressures into my SIMPLEC algorythm do I get actual or filtered pressures & velocities out at the end of the time step ??

Thanks ever so much

TJ

  Reply With Quote

Old   September 10, 2002, 10:20
Default Re: Explicit filtering in LES
  #4
Kalyan
Guest
 
Posts: n/a
Typically, it has been assumed that filter width is about twice the cube root of cell volume. However, it is true for near-isotropic grids. For highly anisotropic grid, characteristic filter width is hard to define since the implicit filter may itself be highly anisotropic. This however seems to have minimal impact of dynamic LES since the test filter also is typically anisotropic (i.e., the anisotropy of the implicit filter and the test filter may be nearly analagous).

By doing explicit filtering, you lose some small scale information. This is OK since smallest scales in almost all numerical methods are affected the most by the numerical discretization errors and are thus not usually fully representative of the fluid physics at that scale. You are right that the test filter needs to be fairly wide when using explicit filtering. You have to balance your accuracy requirements with your computational expense.

Regarding your last question, I can only give my opinion. If you feed explicitly filtered fields (v,p) into a numuerical solver with an LES model (corresponding to the explicit filter), you should end up with variables filtered at the explicit filter level. But in reality, you may end up with a explicitly filtered variable + some small scale field. I am reluctant to give this field a name but I guess the term "numerical noise" is probably fits it well. Upon filtering this field, the "numerical noise" is removed and you get the explicitly filtered variable. SIMPLE and it variants (SIMPLER, SIMPLEC) use implicit time-stepping which may be problematic in terms of convergence if you explicitly filter your velocity field after each implicit iteration. Typically most LES is done using small time steps (CFL < 1) using predictor-corrector type schsmes and there is no need to iterate on the velocity.
aerosjc likes this.
  Reply With Quote

Old   December 28, 2010, 12:49
Thumbs up
  #5
Member
 
SM
Join Date: Dec 2010
Posts: 97
Rep Power: 15
canopus is on a distinguished road
Quote:
Originally Posted by Kalyan
;21283
TJ,

Explicit filtering : When people talk about explicit filtering what that often means is you take the LES generated field and filter it using an explicit filter. The LES generated field being implicitly filtered already, explicit filtering involves filtering of a field that is already implicitly filtered. (Hope I haven't confused anyone here).

EF : explicit filter IF : implicit filter

actual variable q produces an LES variable IF(q).

Upon explicit filtering what you end up with is EF[IF(q)].

If the characteristic width of the explicit filter is much larger than that of the implicit filter, you can safely assume that the resulting field is an actual velocity field filtered using an explicit user-specified filter. i.e.,

EF[IF(q)] ~ EF(q)

Thus the implicit filter, which is often the unknown, becomes inconsequential. Since the implicit filter also depends on the mesh spacing, any dependence on the mesh is also removed. Sometimes, the numerics create heuristic high wavenumber behavior that needs to kept in check (eg. de-aliasing errors), explicit filtering removes all those heuristic scales. That's why explicit filtering is highly recommended for unstructured meshes.
This is very helpful Kalyan Sir! If you by any chance visit this please let me know I have few things to ask.
canopus is offline   Reply With Quote

Old   May 5, 2013, 02:03
Default Re: Explicit filtering in LES
  #6
New Member
 
Mahfuz Sarwar
Join Date: Nov 2011
Posts: 13
Rep Power: 14
mahfuzsarwar is on a distinguished road
Thanks Kalyan sir for the post. The information regarding explicit filtering technique is very helpful.

But I have a question to Kalyan sir that whether this mentioned concept

EF[IF(q)] ~ EF(q)

is only applicable to Dynamic Smagorinsky Model (DSM) or it can be implemented on the Constant Smagorinsky Model (CSM) as well??


Quote:
Originally Posted by Kalyan
;21283
TJ,

One thing to note before getting to your questions. Use of a finite-volume formulation does not necessarily imply top-hat filter. There are approximate ways of estimating what the implicit-filter looks like and it's width (if needed). But in general the implicit filter depends on your discretization schemes (numerical dispersion and dissipation terms).

If you had actual velocities and knew what your filter is, then you can compute filtered velocities. However, in LES, you have the problems in reverse. You have filtered velocities. Even if you knew what the implicit filter was, you can not back out the actual velocity field except to within an approximation. This is because a lot of different actual velocity fields can result in the same filtered velocity fields (upon filtering). i.e.,

Filter(F) = <F>

For a given field "F", <F> is unique, but for a given <F>, "F" is not.

Often when you reverse a filter operation, you might end up with unbounded operators or operators with broad support which make the reverse filtering cumbersome. There are however models based on approximate inverse operators to filters that are used in LES modeling. Here is a good website that discuss such models.

http://www.ifd.mavt.ethz.ch/cfd/resles.htm

Explicit filtering : When people talk about explicit filtering what that often means is you take the LES generated field and filter it using an explicit filter. The LES generated field being implicitly filtered already, explicit filtering involves filtering of a field that is already implicitly filtered. (Hope I haven't confused anyone here).

EF : explicit filter IF : implicit filter

actual variable q produces an LES variable IF(q).

Upon explicit filtering what you end up with is EF[IF(q)].

If the characteristic width of the explicit filter is much larger than that of the implicit filter, you can safely assume that the resulting field is an actual velocity field filtered using an explicit user-specified filter. i.e.,

EF[IF(q)] ~ EF(q)

Thus the implicit filter, which is often the unknown, becomes inconsequential. Since the implicit filter also depends on the mesh spacing, any dependence on the mesh is also removed. Sometimes, the numerics create heuristic high wavenumber behavior that needs to kept in check (eg. de-aliasing errors), explicit filtering removes all those heuristic scales. That's why explicit filtering is highly recommended for unstructured meshes.
mahfuzsarwar is offline   Reply With Quote

Old   May 5, 2013, 05:09
Default
  #7
Senior Member
 
Filippo Maria Denaro
Join Date: Jul 2010
Posts: 6,760
Rep Power: 71
FMDenaro has a spectacular aura aboutFMDenaro has a spectacular aura aboutFMDenaro has a spectacular aura about
this post is very old and many new studies appeared in literature, I suggest a search to find the state-of-the-art in the field of implicit/explicit filtering
FMDenaro is offline   Reply With Quote

Old   June 17, 2013, 01:25
Default Explicit filtering in LES
  #8
New Member
 
Mahfuz Sarwar
Join Date: Nov 2011
Posts: 13
Rep Power: 14
mahfuzsarwar is on a distinguished road
Quote:
Originally Posted by FMDenaro View Post
this post is very old and many new studies appeared in literature, I suggest a search to find the state-of-the-art in the field of implicit/explicit filtering
Thank you sir for your suggestion.

However, as I am quite new in the field of LES, I have some questions regarding implicit/ explicit filtering techniques in LES. Say for an example: if any LES code (which is basically physics based model) is operating by deafault in implicit technique and some one would like to implement the explicit filtering in it (as from CTR briefs it assumes that explicit filter will be able to give better solution over implicitly filtered LES), what would be the best possible way to do it?
mahfuzsarwar is offline   Reply With Quote

Old   June 17, 2013, 04:31
Default
  #9
Senior Member
 
Filippo Maria Denaro
Join Date: Jul 2010
Posts: 6,760
Rep Power: 71
FMDenaro has a spectacular aura aboutFMDenaro has a spectacular aura aboutFMDenaro has a spectacular aura about
Quote:
Originally Posted by mahfuzsarwar View Post
Thank you sir for your suggestion.

However, as I am quite new in the field of LES, I have some questions regarding implicit/ explicit filtering techniques in LES. Say for an example: if any LES code (which is basically physics based model) is operating by deafault in implicit technique and some one would like to implement the explicit filtering in it (as from CTR briefs it assumes that explicit filter will be able to give better solution over implicitly filtered LES), what would be the best possible way to do it?
You need to implement first a filtering subroutine, the type of filtering can be somehow arbitrary. Then, it is a consistent way to apply (at each time step) the explicit filtering to the resolved convective term.
Conversely, the explicit filter applied to the updated resolved velocity is not consistent to the filtered equations.
FMDenaro is offline   Reply With Quote

Old   March 29, 2016, 15:27
Default
  #10
Senior Member
 
Julio Mendez
Join Date: Apr 2009
Location: Fairburn, GA. USA
Posts: 290
Rep Power: 17
juliom is on a distinguished road
Send a message via Skype™ to juliom
Quote:
Originally Posted by FMDenaro View Post
You need to implement first a filtering subroutine, the type of filtering can be somehow arbitrary. Then, it is a consistent way to apply (at each time step) the explicit filtering to the resolved convective term.
Conversely, the explicit filter applied to the updated resolved velocity is not consistent to the filtered equations.
Dear Professor;
I have basically two questions regarding this post. If we have to implement an specific filter (explicit filter) let's say that the kernel is the spectral cut-off which indeed reduces the degree of freedom of the computation. According to your previous statement, implementing an explicit is basically to filter the velocity field (u_i) from every time steps using the filter kernel decided? However, you mentioned the convective term; which is made by u_i*u_j. I lean towards the idea of applying the filter procedure on the u_i field at every time steps and then use that field for the next time step.

The other questions is, how can I really end up with a grid independent solution if the solved velocity field (prior the filtering with the specific kernel) has an implicit filter (second order schemes Top-hat ). Hence the filter operation will be on the solved velocity field which has an implicit filter on it. From different papers, it is clearly shown that the cut-off kernel is very effective imposing the cut-off wavelength. However, the usually se spectral methods. I was wondering if the same effect is achieved even if the derivatives are computed using finite differences.

I know that this is very cumbersome, for someone who is just beginning in this field .!! thank you very much for your time.
juliom is offline   Reply With Quote

Old   March 29, 2016, 15:38
Default
  #11
Senior Member
 
Filippo Maria Denaro
Join Date: Jul 2010
Posts: 6,760
Rep Power: 71
FMDenaro has a spectacular aura aboutFMDenaro has a spectacular aura aboutFMDenaro has a spectacular aura about
Many authors suggested the correct way to adopt explicit filtering as the application of a filtering (it can be a spectral cut-off but that is not mandatory) of width Delta > h (this latter being the computational size) on the non-linear term.
It can be shown that filtering the velocity field at each time step is not consistent with the filtered equation.

This way, a LES solution will be grid-independent when Delta is maintained fixed and h goes toward zero. That does not depends on the type of the filter that can be either projective or smooth.
FMDenaro is offline   Reply With Quote

Old   March 29, 2016, 15:42
Default
  #12
Senior Member
 
Filippo Maria Denaro
Join Date: Jul 2010
Posts: 6,760
Rep Power: 71
FMDenaro has a spectacular aura aboutFMDenaro has a spectacular aura aboutFMDenaro has a spectacular aura about
https://www.researchgate.net/profile...51647c27eb.pdf

https://web.stanford.edu/group/ctr/R...08/13_bose.pdf
yuan_neu and Alexander.Ruan like this.
FMDenaro is offline   Reply With Quote

Old   March 29, 2016, 16:20
Default
  #13
Senior Member
 
Julio Mendez
Join Date: Apr 2009
Location: Fairburn, GA. USA
Posts: 290
Rep Power: 17
juliom is on a distinguished road
Send a message via Skype™ to juliom
Quote:
Originally Posted by FMDenaro View Post
It can be shown that filtering the velocity field at each time step is not consistent with the filtered equation.
Professor; does it mean that the filter operation needs to be done on the product u_i*u_j rather than in the velocity field u_i? In other words; once I compute the velocity field (u_i); it is just computing u_i*u_j and apply the filter on the solution of the previous tensor?
juliom is offline   Reply With Quote

Old   March 29, 2016, 16:41
Default
  #14
Senior Member
 
Filippo Maria Denaro
Join Date: Jul 2010
Posts: 6,760
Rep Power: 71
FMDenaro has a spectacular aura aboutFMDenaro has a spectacular aura aboutFMDenaro has a spectacular aura about
Quote:
Originally Posted by juliom View Post
Professor; does it mean that the filter operation needs to be done on the product u_i*u_j rather than in the velocity field u_i? In other words; once I compute the velocity field (u_i); it is just computing u_i*u_j and apply the filter on the solution of the previous tensor?

correct
FMDenaro is offline   Reply With Quote

Old   March 29, 2016, 16:43
Default
  #15
Senior Member
 
Julio Mendez
Join Date: Apr 2009
Location: Fairburn, GA. USA
Posts: 290
Rep Power: 17
juliom is on a distinguished road
Send a message via Skype™ to juliom
thank you professor; Now I see why it is computational expensive..!!!
juliom is offline   Reply With Quote

Old   March 30, 2016, 16:19
Default
  #16
Senior Member
 
Julio Mendez
Join Date: Apr 2009
Location: Fairburn, GA. USA
Posts: 290
Rep Power: 17
juliom is on a distinguished road
Send a message via Skype™ to juliom
Professor; In Pope's book section 13.5 (LES in wavenumber space) he considered sharp spectral filter, which is natural choice for LES in wavenumber. So, now I wondering if this type of filters are only possible for spectral or pseudo spectral methods. Nonetheless, I think that if a sharp cut-off is applied as an explicit filter in physical space, then it is possible to retain the advantages of the sharp cut-off no matter how I introduce the filtering.. Am I right?

Thanks before hand

Respectfully
JM
juliom is offline   Reply With Quote

Old   March 30, 2016, 16:31
Default
  #17
Senior Member
 
Filippo Maria Denaro
Join Date: Jul 2010
Posts: 6,760
Rep Power: 71
FMDenaro has a spectacular aura aboutFMDenaro has a spectacular aura aboutFMDenaro has a spectacular aura about
Quote:
Originally Posted by juliom View Post
Professor; In Pope's book section 13.5 (LES in wavenumber space) he considered sharp spectral filter, which is natural choice for LES in wavenumber. So, now I wondering if this type of filters are only possible for spectral or pseudo spectral methods. Nonetheless, I think that if a sharp cut-off is applied as an explicit filter in physical space, then it is possible to retain the advantages of the sharp cut-off no matter how I introduce the filtering.. Am I right?

Thanks before hand

Respectfully
JM

using spectral methods the cut-off is a natural way to get an explicit filtering, conversely, using FV or FD methods you have to perform a Fourier transform, apply the filter and come back to the physical space, a task quite cumbersome.

The second part of your question is not very clear to me...
FMDenaro is offline   Reply With Quote

Old   March 30, 2016, 16:42
Default
  #18
Senior Member
 
Julio Mendez
Join Date: Apr 2009
Location: Fairburn, GA. USA
Posts: 290
Rep Power: 17
juliom is on a distinguished road
Send a message via Skype™ to juliom
Excuse me professor; I will make my statement clearer. Although it is a little disappointing that sharp cut-off are only possible in spectral methods. It means that any one using FD or FV can only apply the top hat (based on the numerical scheme and order)
What I wanted to say, is that since the kernel function of the convolution integral can take any form, i.e: the form of the top hat, Gaussian , spectral etc. I thought that it was a matter of changing the filter kernel of the convolution integral and use any integration method to apply the filter on the specific property.
Hence, if I apply an explicit filter (once my velocity field is computed) using the kernel I wish, then the final solution will keep the advantages of the sharp cut-off....

Thanks
Julio

Last edited by juliom; March 30, 2016 at 16:44. Reason: IT was missing information
juliom is offline   Reply With Quote

Old   March 30, 2016, 17:04
Default
  #19
Senior Member
 
Filippo Maria Denaro
Join Date: Jul 2010
Posts: 6,760
Rep Power: 71
FMDenaro has a spectacular aura aboutFMDenaro has a spectacular aura aboutFMDenaro has a spectacular aura about
Quote:
Originally Posted by juliom View Post
Excuse me professor; I will make my statement clearer. Although it is a little disappointing that sharp cut-off are only possible in spectral methods. It means that any one using FD or FV can only apply the top hat (based on the numerical scheme and order)
What I wanted to say, is that since the kernel function of the convolution integral can take any form, i.e: the form of the top hat, Gaussian , spectral etc. I thought that it was a matter of changing the filter kernel of the convolution integral and use any integration method to apply the filter on the specific property.
Hence, if I apply an explicit filter (once my velocity field is computed) using the kernel I wish, then the final solution will keep the advantages of the sharp cut-off....

Thanks
Julio

theoretically yes ... but practically is very cost-expensive working with FV/FD methods and perform at each time step FFT/IFFT only to apply the spectral cut-off. What is more, the case would be feasible ony for particolar geometry with periodical conditions. Conversely, the top-hat filter would be useful for general flow conditions.
FMDenaro is offline   Reply With Quote

Old   March 30, 2016, 18:36
Default
  #20
Senior Member
 
Julio Mendez
Join Date: Apr 2009
Location: Fairburn, GA. USA
Posts: 290
Rep Power: 17
juliom is on a distinguished road
Send a message via Skype™ to juliom
Thank you professor; It seems that the only options for FD and FV are Gaussian and Top-hat. My main concern is due to important facts that I have come across in the literature; that only cut-off reduces the degree of freedom of the problem. And, smooth filters (gaussian, top hat) are not effective to attenuate wavenumbers beyond the filter width.
The more I read, the least I understand :S I think this is not a good sign!!!

Respectfully and thank you very much
Julio Mendez
juliom is offline   Reply With Quote

Reply

Thread Tools Search this Thread
Search this Thread:

Advanced Search
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 Off
Pingbacks are On
Refbacks are On


Similar Threads
Thread Thread Starter Forum Replies Last Post
LES explicit filtering maka OpenFOAM Running, Solving & CFD 4 December 27, 2012 10:08
LES explicit filtering Paul Main CFD Forum 0 March 17, 2006 09:03
LES (explicit) filtering John Main CFD Forum 2 July 29, 2004 19:03
Channel LES and explicit filtering Luca Liberti Main CFD Forum 0 January 17, 1999 11:07


All times are GMT -4. The time now is 22:18.