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 TJ September 9, 2002 11:15

Explicit filtering in LES

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

 Kalyan September 9, 2002 16:54

Re: Explicit filtering in LES

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.

 TJ September 10, 2002 07:50

Re: Explicit filtering in LES

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

 Kalyan September 10, 2002 09:20

Re: Explicit filtering in LES

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.

 canopus December 28, 2010 12:49

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.

 mahfuzsarwar May 5, 2013 01:03

Re: Explicit filtering in LES

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) = For a given field "F", is unique, but for a given , "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.

 FMDenaro May 5, 2013 04:09

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

 mahfuzsarwar June 17, 2013 00:25

Explicit filtering in LES

Quote:
 Originally Posted by FMDenaro (Post 425275) 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?

 FMDenaro June 17, 2013 03:31

Quote:
 Originally Posted by mahfuzsarwar (Post 434364) 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.

 juliom March 29, 2016 14:27

Quote:
 Originally Posted by FMDenaro (Post 434383) 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.

 FMDenaro March 29, 2016 14:38

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 March 29, 2016 14:42

 juliom March 29, 2016 15:20

Quote:
 Originally Posted by FMDenaro (Post 592239) 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?

 FMDenaro March 29, 2016 15:41

Quote:
 Originally Posted by juliom (Post 592251) 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 ;)

 juliom March 29, 2016 15:43

thank you professor; Now I see why it is computational expensive..!!!

 juliom March 30, 2016 15:19

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

 FMDenaro March 30, 2016 15:31

Quote:
 Originally Posted by juliom (Post 592471) 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...

 juliom March 30, 2016 15:42

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

 FMDenaro March 30, 2016 16:04

Quote:
 Originally Posted by juliom (Post 592478) 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.

 juliom March 30, 2016 17:36

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

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