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Old   September 9, 2002, 11:15
Default Explicit filtering in LES
  #1
TJ
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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
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Old   September 9, 2002, 16:54
Default Re: Explicit filtering in LES
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Kalyan
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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.
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Old   September 10, 2002, 07:50
Default Re: Explicit filtering in LES
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TJ
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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

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Old   September 10, 2002, 09:20
Default Re: Explicit filtering in LES
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Kalyan
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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.
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Old   December 28, 2010, 12:49
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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.
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Old   May 5, 2013, 01:03
Default Re: Explicit filtering in LES
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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.
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Old   May 5, 2013, 04:09
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Filippo Maria Denaro
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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
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Old   June 17, 2013, 00:25
Default Explicit filtering in LES
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Mahfuz Sarwar
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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?
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Old   June 17, 2013, 03:31
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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.
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