The "average" value that you specify is not what is applied to the boundary. If you specify an average value, then the instantaneous mapped data is scaled such that the average of the mapped data matches the specified average.

If you want to use mapping for a developing boundary layer, then you need something different. I suggest you try and contact Gavin Tabor at Exeter University. They published some work on this type of stuff.

SGS stress
 

Hi, Eugene.
I am simulating the plane channel flow using LES and I look into your PhD thesis as a reference. It helps alot, thanks.
BTW, I also want to see SGS stress on channel as you presented (in your case SGS shrear stress, Sigma_xy). So I am wondering if there is any option to calculate SGS stress. I think I need to calculate SGS stress and U simultaneously because SGS stress = nu_SGS * dU_i/dx_j (U_i is resolved velocity vector, not mean velocity). Correct me If I am wrong.
Thanks in advance.
Yusik

Unfortunately, to calculate the mean sgs stress, will require some coding. You need to add something like this to your solver:

Before the time loop in pisoFoam, e.g. in createFields.H add:

"tmp<volSymmTensorField> R(turbulence->R());"

for the oodles solver replace with sgsModel->() . Also add the following inside the time loop:

R = turbulence->R();

Now, use the fieldAverage function object to get RMean.

Hope this helps.

Thanks, Eugene.

To generate R field, I added this code in 'createField.H' :
Info<< "Reading field U\n" << endl;
volSymmTensorField R
(
IOobject
(
"R",
runTime.timeName(),
mesh,
IOobject::MUST_READ,
IOobject::AUTO_WRITE
),
mesh
);

rather than
tmp<volSymmTensorField> R(turbulence->R());

because if I use only one line above, I had this error message ' turbulence was not declared in this scope'.

The others are fine. Thanks again.
Yusik

:)
He was not wrong, I guess you have misunderstood him, "turbulence" in his post is more a general type than a specific variable. Bye

I had a basic question on LES. In OF, we use the same equations for momentum while we selected every type of turbulence models including LES, K-e and so on. So, where is the effect of volume filtering in LES in momentum equation?

Thanks

Generally speaking, filter operation is done "implicitly", which is a common practice in many finite volume code.

And you are right, the idea of "implicit" filtering or averaging, makes the gap between RANS and LES no longer big from the programming point of view, and then hybrid methods arise!

Go hybrid!

Hi Daniel

O.K, But does it mean all LES models in Openfoam are implicit? When we select some models which permit us selecting filter (like Laplace), I think the LES in explicit in these cases, am I wrong?

Ehsan

And I had another key question:

If our Momentum equation is the same for RANS and LES, and all the effects of small scales eddies are given by: B=-2v_sgs*D, the final momentum equation in LES reads the same as any other turbulence modelling: v_effective=v+v_sgs (in RANS it is v+v_turb),
assume we use OneEqEddy model and solve an equation for k to get v_sgs. This OneEqEddy is typically close to the one in RANS. So, what is the different LES and RANS finally? Why LES gives much better results?
Only because this v_sgs is limited to sub grid sclae unlike RANS where v_t is for whole domain?

Ehsan

Are LES and RANS big different?

In short: Yes and No.

1. The stress scaling with delta or scaling with wall distance makes a huge difference, the former we call LES, the latter we call RANS. 2. Of cause, there are more differences in numerics so it is not easy to convert a RANS-based code to LES, but if you just consider the turbulence modeling part, it is not that difficult, as you already noticed, their formula are quite similar 3. Both ways, both formulations are correct, so once you know how to use them, their answers should be bother correct and accurate, so saying LES is always better is not correct. 4. Since LES only try to model a small portion of energy in the spectrum, and these small eddies tends to be isotropic away from the wall, so the model accuracy is not so sensitive as a rans model, so even with a smagrinsky model, you can have a pretty good results, no needs always go dynamics, however, as you may experience, building a powerful RANS model is such a great challenge because it is not easy to find an universal one, but to be sure, we still need a new and powerful generation of RANS model.

Thanks a lot, could you please give a clue on my former question, explicit LES in OpenFOAM as well.

Regards

That I dont know very well, hope someone else could shed some lights on it and share some comparison results!

My understanding is that through some mathematical operations, the "effect" of of filter is actually only working on the SGS stress term, that is the viscosity formulation in terms of EVM theory. At least for some filters.

To my knowledge an explicit LES model means that the sgs viscosity is modeled explicitly in some way (e. g. with an algebraic expression which uses flow variables, such as the Smagorinsky formulation). On the other side an implicit LES does not have a separately modeled sgs viscosity, but it simply introduces the necessary viscosity level into the flow domain in form of numerical viscosity generated by the numerical schemes. So, generally speaking OpenFOAM treats LES modeling in an explicit way.

Regards

V.

If you see in the implementations of LES models in OF, some of theme does not require any filter (like OneEqEddy) but some other categories require it. I think that implicit models does not need filter while explicit ones need.

Regards

In the OneEqEddy model there is still an explicitly calculated sgs viscosity, which depends on the filtering spatial cutoff value "delta", so to me this cannot be defined as an implicit LES model. However, if someone else wants to enter the discussion it would be interesting to further clarify this point.

Regards

V.

1- Please see this nice paper which comes from Furbey,

Numerical Simulation of an Oscillating Cylinder Using Large Eddy Simulation and Implicit Large Eddy Simulation

DOI: 10.1115/1.4005766

Journal of Fluids Engineering, 2012, MARCH 2012, Vol. 134

See Eq. 2 and 5 about implicit and explicit LES. This work is done with OpenFOAM.

2- And as I see the 2008 paper (attached some pages), oneEqEddy is explicit, as you told. So where are the implicit models that Bensow used?

Regads

Honestly I don't know how the implicit LES modeling has been implemented in these papers (I don't have access to the full copies), but remember that OpenFOAM is an open code, so the fact that apparently there isn't a "standardized" implicit LES modeling approach in the official releases does not mean that such modeling cannot be done (probably you should ask one of the authors of the papers for more information about it). Anyway, I think that an interesting passage extracted from the second of your attachments is the following:

"The discretization scheme used is a central differencing scheme with different TVD limiters applied to the convective term to ensure stability and introduce the necessary dissipation for the implicit modeling."

Regards

V.

I was saying "implicit filtering", not Implicit LES (ILES). ILES is a different matter.

I Agree!

solvers for LES in steady flow
 

Hi Eugene,

You told about the solver pisoFoam to solve a flow for LES, but I´ve read it´s for transient flow... do you know which solver can be used to solve a steady one?

Thanks in advance

Paula

Hi, Alberto!

Could you please give me any suggestion on how should I set up k at wall boundary in my LES simulation? The y+ is smaller than 2 in my case.

Best regards,
Peter