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A few questions about LES filtering in OpenFOAM

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Old   May 1, 2016, 06:17
Default A few questions about LES filtering in OpenFOAM
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Hello,
In the OpenFOAM source codes,I found "laplace Filter" in the source codes, so I guessed the dynLagrangian must be used accompany with "laplace Filter".

Actually, I am going to implement the dynamic K-E or dynLagrangian SGS model to another software. But I don't know how to implement the "simple Filter".

It seems that implementing laplace Filter is more easier. Could you tell me does the SGS models must be used accompany with particular Filters? For example, dynamicKEqn + simple filter; dynamicLagrangian + laplace Filter.

Another question: If you understand the meaning of "simple Filter", Could you tell me the physical meaning of it?

Please forgive my poor English.

simple Filter:

tmp<volScalarField> filteredField = fvc::surfaceSum
(
mesh().magSf()*fvc::interpolate(unFilteredField)
)/fvc::surfaceSum(mesh().magSf())

laplace Filter:

tmp<volTensorField> filteredField =
unFilteredField() + fvc::laplacian(coeff_, unFilteredField())


Thanks,
Zhang Yan

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Old   May 4, 2016, 10:50
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Hi Zhang Yan,

I am trying to figure out LES implementation in OpenFOAM myself but I'll still try to give an answer. Please correct me if I am wrong.

simpleFilter
If you look at the code, you will see that the unFilteredField is first interpolated from the cell centers to the cell faces using 'fvc::interpolate'. Then, the filteredField is simply the area-weighted average of the interpolated values using 'fvc::surfaceSum'.

laplaceFilter
I am not so sure about this but I'll try. I'm borrowing some ideas from image processing since that's all the relevant information that I could find on Laplace filters.

Let's say you have a random signal, u which has high frequency noise that you wish to filter/smooth. We can derive the finite difference Laplace operator for the signal as:
\nabla^2 u = \frac{u_{i-1} - 2u_i + u_{i+1}}{\Delta^2}
According to the code, the Laplacian is first multiplied by 'coeff_' which is proportional to V^{2/3} and, hence, has the dimensions of \Delta^2. This result is then added to the unFilteredField. Let's try to do that by letting the proportionality parameter 'widthCoeff_' as 4:
\tilde{u_i} = u_i +\frac{ \Delta^2}{4}  \left(  \frac{u_{i-1} - 2u_i + u_{i+1}}{\Delta^2} \right)  = \frac{u_{i-1} + 2u_i + u_{i+1}}{4}
As it can be seen, the result is an average over the adjacent cells.

Conclusion
I think filtering operation can be viewed as smoothing or averaging operation. I would love to know if I am getting any of this wrong. Also, I am not too sure as to the role of 'widthCoeff_' and would like to know more as well.

As to which filter to use with which dynamic SGS model, I think it does not really matter. Any filter should be fine as long as you define the relevant '<LESFilter>Coeffs'. However, according to this, the laplace filter is not generally used. Not too sure why.

Anyway, hope this helps you.
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Old   May 4, 2016, 11:11
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Thank you very much!
Since it's too late today (in my country), I will study your admirable answer carefully tomorrow.
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Old   May 7, 2016, 03:53
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Quote:
Originally Posted by usv001 View Post
Hi Zhang Yan,

I am trying to figure out LES implementation in OpenFOAM myself but I'll still try to give an answer. Please correct me if I am wrong.

simpleFilter
If you look at the code, you will see that the unFilteredField is first interpolated from the cell centers to the cell faces using 'fvc::interpolate'. Then, the filteredField is simply the area-weighted average of the interpolated values using 'fvc::surfaceSum'.

laplaceFilter
I am not so sure about this but I'll try. I'm borrowing some ideas from image processing since that's all the relevant information that I could find on Laplace filters.

Let's say you have a random signal, u which has high frequency noise that you wish to filter/smooth. We can derive the finite difference Laplace operator for the signal as:
\nabla^2 u = \frac{u_{i-1} - 2u_i + u_{i+1}}{\Delta^2}
According to the code, the Laplacian is first multiplied by 'coeff_' which is proportional to V^{2/3} and, hence, has the dimensions of \Delta^2. This result is then added to the unFilteredField. Let's try to do that by letting the proportionality parameter 'widthCoeff_' as 4:
\tilde{u_i} = u_i +\frac{ \Delta^2}{4}  \left(  \frac{u_{i-1} - 2u_i + u_{i+1}}{\Delta^2} \right)  = \frac{u_{i-1} + 2u_i + u_{i+1}}{4}
As it can be seen, the result is an average over the adjacent cells.

Conclusion
I think filtering operation can be viewed as smoothing or averaging operation. I would love to know if I am getting any of this wrong. Also, I am not too sure as to the role of 'widthCoeff_' and would like to know more as well.

As to which filter to use with which dynamic SGS model, I think it does not really matter. Any filter should be fine as long as you define the relevant '<LESFilter>Coeffs'. However, according to this, the laplace filter is not generally used. Not too sure why.

Anyway, hope this helps you.


Hi,
According to your reply, I got my answer about simple filter:
filteredField = sum(A_i * S_i) / sum(S_i)
A_i is face value after interpolate, S_i is face area.
Is it right?

Besides, I have another question:
If I use simple filter as the test filter for a dynamic SGS model,
as a test filter, how does simple filter show the double width compared to the first filter?

Last edited by zhangyan; May 7, 2016 at 07:51.
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Old   May 8, 2016, 10:34
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Hi there,

Yup, I have the same idea of the simple filter.

I am reading up on filtering myself (I am new to LES/OpenFOAM as well) but here is what I think is happening:

The filtering of the original LES equations is implicit in OpenFOAM, i.e. the finite volume method (grid+discretization) itself acts as a filter! This is known as implicit LES. So, when you apply any dynamic model, it performs a further filtering process, the test filter.

The factor of two does appear in the code. For example, for the 'homogeneousDynSmagorinsky' model:

Code:
const volSymmTensorField MM
    (
        sqr(delta())*(filter_(mag(D)*(D)) - 4*mag(filter_(D))*filter_(D))
    );
Notice the factor '4'. That is the result of...
MM_{ij} = \Delta^2 \left[ \widetilde{D_{ij}\sqrt{D_{ij} D_{ij}}} - \frac{\Delta_t^2}{\Delta^2} \widetilde{D_{ij}}{\sqrt{\widetilde{D_{ij}} \widetilde{D_{ij}}}} \right] where {\Delta_t}/{\Delta} = 2.

Hope this helps.
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Last edited by usv001; May 8, 2016 at 10:37. Reason: Error in formula
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Old   May 8, 2016, 11:05
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Thanks for your explanation!
Now I am implementing dynamic procedure to KIVA (the software I'm using).
In KIVA, velocity is stored on the vertices.
So one of the face(bottom) value is : 0.25(u1+u2+u3+u4).
Code:
     7/--------/|6
     /        / |
    / |      /  |
   /        /   |
 8/--------/5   |
  |   |    |    |
  |        |    |
  |  3|-  -| -  |2
  |   /    |   /
  |        |  /
  | /      | /
  |        |/
 4----------1
However, when I am calculate the filter_Sij.
You know, I must interpolate Sij to the cell faces.
For example, S11=dudx, in KIVA, dudx=0.25*(u3+u4+u7+u8-u1-u2-u5-u6)/delta_X

I know velocity is stored on the vertice, but I don't know where Sij is stored.
So I don't know how to interpolate Sij to the cell faces.
Do you have any idea?
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Old   July 13, 2017, 10:08
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Quote:
Originally Posted by usv001 View Post
Hi there,

Yup, I have the same idea of the simple filter.

I am reading up on filtering myself (I am new to LES/OpenFOAM as well) but here is what I think is happening:

The filtering of the original LES equations is implicit in OpenFOAM, i.e. the finite volume method (grid+discretization) itself acts as a filter! This is known as implicit LES. So, when you apply any dynamic model, it performs a further filtering process, the test filter.

The factor of two does appear in the code. For example, for the 'homogeneousDynSmagorinsky' model:

Code:
const volSymmTensorField MM
    (
        sqr(delta())*(filter_(mag(D)*(D)) - 4*mag(filter_(D))*filter_(D))
    );
Notice the factor '4'. That is the result of...
MM_{ij} = \Delta^2 \left[ \widetilde{D_{ij}\sqrt{D_{ij} D_{ij}}} - \frac{\Delta_t^2}{\Delta^2} \widetilde{D_{ij}}{\sqrt{\widetilde{D_{ij}} \widetilde{D_{ij}}}} \right] where {\Delta_t}/{\Delta} = 2.

Hope this helps.

I agree with this statement that normal LES models use implicit filtering in OpenFOAM.
Also dynamic models use explicit filter (with different choices for shape and size) as the test filter. But if you check the code for one of the dynamic models like "Dynamic Smagorinsky" they use the same delta that is used in normal "Smagorinsky" which is implicit. Isn't it contradictory?

Because as you showed in the equation above, the factor 4 appears because the test filter width (explicit) is twice the grid filter (implicit), but what we see as the delta in both
models is grid size (i.e. cuberoot ---> deltaCoeff_*pow(mesh().V(), 1.0/3.0) )
and this delta coefficient is always 1 in every "LESProperties". Do you have any comment on that?
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Old   July 13, 2017, 12:17
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Hello Mahdi,

My understanding is that if you don't use any SGS model, then you are letting the inherent numerical dissipation to play its place. So, under this definition the Smagorinsky model is not an implicit LES but an explicit one - you specify an SGS model. So, I see no issues in the SGS models (Smagorinsky and its dynamic counterpart) using the same delta. The only thing that matters is that the test filter width \Delta_t must be larger than the normal filter width \Delta. However, I am no expert in this area. So, correct me if I wrong about any of this.

Cheers,
USV
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Old   July 13, 2017, 15:33
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Quote:
Originally Posted by usv001 View Post
Hello Mahdi,

My understanding is that if you don't use any SGS model, then you are letting the inherent numerical dissipation to play its place. So, under this definition the Smagorinsky model is not an implicit LES but an explicit one - you specify an SGS model. So, I see no issues in the SGS models (Smagorinsky and its dynamic counterpart) using the same delta. The only thing that matters is that the test filter width \Delta_t must be larger than the normal filter width \Delta. However, I am no expert in this area. So, correct me if I wrong about any of this.

Cheers,
USV
This is true that the test filter must be larger, but as far as I understand from different posts in forum, in OpenFOAM the normal filter (implicit, grid) is operating by the discretization of equation. I mean in a normal/static LES, the grid acts as filter by means of discretization. So it uses \Delta x for as the filter width, or not?
That's why we do not see any "filter" in the formulation of, for instance, Smagorinsky.
But when you check the code of "dynSmagorinsky" or "dynOneEddy", the term "filter" is clearly seen in the code. This is of course the "test" filter, and its type is usually chosen "simple". But it uses the same width. So, the crucial condition of "\Delta_t > \Delta" doesn't get satisfied. Isn't it a problem?
Or you mean the \Delta in the turbulence model and filter width are two different entities?
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Old   July 16, 2017, 09:21
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Hi Mahdi,

I have the same question and am thinking in the following manner.
In the simple filter, the fvc::interpolate function is used to calculate the filtered field,
so the neighbour cells' values are considered in the test filtering operation and \Delta_t > \Delta is satisfied.

Best regards,
Fumiya
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Old   July 16, 2017, 09:40
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Quote:
Originally Posted by fumiya View Post
Hi Mahdi,

I have the same question and am thinking in the following manner.
In the simple filter, the fvc::interpolate function is used to calculate the filtered field,
so the neighbour cells' values are considered in the test filtering operation and \Delta_t > \Delta is satisfied.

Best regards,
Fumiya
You are right about the role of interpolation function, however
it is still unclear to me how to determine the width of filter/stencil.
In other words, is the width now twice the grid size? Because by interpolation
I still see the delta_t = grid size. Or I am mistaken?
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Old   August 17, 2017, 09:09
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Hello to everybody, I will float again this thread, since now I have some doubts. I will set my points:
  • LES in OpenFOAM is implicit, so the size of the filter (ie, delta) is provided by the grid. So why do we provide already a delta in the LESProperties?
  • When we use a dynamic model, we have to specify the filter (the origin of this thread). But, what about the width of the filter? Is it the same delta that I said above?
Thank you in advance,
Agustin
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Old   August 17, 2017, 09:38
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Quote:
Originally Posted by agustinvo View Post
  • LES in OpenFOAM is implicit, so the size of the filter (ie, delta) is provided by the grid. So why do we provide already a delta in the LESProperties?
  • When we use a dynamic model, we have to specify the filter (the origin of this thread). But, what about the width of the filter? Is it the same delta that I said above?
Hi Agustín,
  • As the source code tell us, LESdeltas are various type of delta calculation. Such as cubeRootVolDelta: delta is the cubt root of cell volume, a kind of mesh size; maxDeltaxyz: deltaCoeff*delta is the max among x y z ,also a kind of mesh size.
  • In general , width of the filter is double mesh size. Such as laplaceFilter:
    Code:
    coeff_.ref() = pow(mesh.V(), 2.0/3.0)/widthCoeff_;
    and simpleFilter:
    Code:
    filteredField = fvc::surfaceSum(mesh().magSf()*fvc::interpolate(unFilteredField))
    /fvc::surfaceSum(mesh().magSf());
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Old   August 17, 2017, 09:43
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Quote:
Originally Posted by agustinvo View Post
Hello to everybody, I will float again this thread, since now I have some doubts. I will set my points:
  • LES in OpenFOAM is implicit, so the size of the filter (ie, delta) is provided by the grid. So why do we provide already a delta in the LESProperties?
  • When we use a dynamic model, we have to specify the filter (the origin of this thread). But, what about the width of the filter? Is it the same delta that I said above?
Thank you in advance,
Agustin
The LES in OpenFOAM is implicit. So, the grid and discretization scheme act as filter. But you need a representative length scale for grid size in the sub-grid models like smagorinsky. So you have to estimate this length scale. There are different options for delta estimation such as "cubeRoot", "smooth" and etc. (Note that the implicit filter is not "exactly" equal to the grid size but almost similar. More details can be found in the theory of LES and the definition of "effective filter").


Therefore, what you give as a user input parameter in the turbulence dictionary as "delta" is the representative of grid filter size or delta_g
But the delta that you use in the dynamic models is the width of "test" filter not grid filter. It is usually considered twice the grid filter i.e. delta_t = 2*delta_g.
So, again you would need only delta_g, because you can simply multiply it by factor 2 where you need "test" filter. I mean the factor 2 can be automatically considered in the formulation of dynamic models (For instance, see the factor 4 in the formula of MM in dynamic Smagorinsky ).
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Old   August 17, 2017, 12:33
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Quote:
Originally Posted by zhangyan View Post
Hi Agustín,
  • As the source code tell us, LESdeltas are various type of delta calculation. Such as cubeRootVolDelta: delta is the cubt root of cell volume, a kind of mesh size; maxDeltaxyz: deltaCoeff*delta is the max among x y z ,also a kind of mesh size.
  • In general , width of the filter is double mesh size. Such as laplaceFilter:
    Code:
    coeff_.ref() = pow(mesh.V(), 2.0/3.0)/widthCoeff_;
    and simpleFilter:
    Code:
    filteredField = fvc::surfaceSum(mesh().magSf()*fvc::interpolate(unFilteredField))
    /fvc::surfaceSum(mesh().magSf());
Quote:
Originally Posted by Mahdi2010 View Post
The LES in OpenFOAM is implicit. So, the grid and discretization scheme act as filter. But you need a representative length scale for grid size in the sub-grid models like smagorinsky. So you have to estimate this length scale. There are different options for delta estimation such as "cubeRoot", "smooth" and etc. (Note that the implicit filter is not "exactly" equal to the grid size but almost similar. More details can be found in the theory of LES and the definition of "effective filter").


Therefore, what you give as a user input parameter in the turbulence dictionary as "delta" is the representative of grid filter size or delta_g
But the delta that you use in the dynamic models is the width of "test" filter not grid filter. It is usually considered twice the grid filter i.e. delta_t = 2*delta_g.
So, again you would need only delta_g, because you can simply multiply it by factor 2 where you need "test" filter. I mean the factor 2 can be automatically considered in the formulation of dynamic models (For instance, see the factor 4 in the formula of MM in dynamic Smagorinsky ).
Thank to both of you. I will try to expose what I got:
  • OpenFOAM uses an implicit filter, and it uses our delta definition to get the filter width
  • In a dynamic model, we filter explicitly with the "filter_" command, using the filter in our modelCoeffs.
My doubts come from this article, where they have this expression

\nu_{sgs}=C_v \Delta^{3/2}|S|^{1/2}q_c^{1/4}

Where q_c=1/2\overline{u_i}'\overline{u_i}'=1/2(\overline{u_i}-\tilde{\overline{u_i}})^2

They talk about using a coarser width, but I don see how to follow the analogy with the dynamic Smagorinsky. Cv is constant. If there was a way to specify a width directly to the filter...
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Old   August 18, 2017, 06:52
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Another point is: where the delta is used in the code of the filters?
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