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-   -   What LES should do for you? (http://www.cfd-online.com/Forums/main/109108-what-les-should-do-you.html)

FMDenaro November 9, 2012 06:51

What LES should do for you?
 
I often read people writing about using "LES turbulence model", therefore I open this post since I am curious about your ideas of what LES is and what LES should do for you.
You are also probably aware of the paper http://iopscience.iop.org/1367-2630/6/1/035

sbaffini November 9, 2012 08:23

Working on it since 2008... still have to find out :rolleyes:

Jokes apart, i don't trust Unsteady RANS (or RANS, for that matter)

sail November 9, 2012 09:30

I'd say provide better results than ranse (not necessarily the right ones, just better would be enough) in all the areas where ranse fail.

FMDenaro November 9, 2012 12:27

Quote:

Originally Posted by sail (Post 391261)
I'd say provide better results than ranse (not necessarily the right ones, just better would be enough) in all the areas where ranse fail.

what "better" means for you? does LES should be just "different" from RANS?

francesco_capuano November 10, 2012 08:55

Quote:

Originally Posted by FMDenaro (Post 391214)
I often read people writing about using "LES turbulence model", therefore I open this post since I am curious about your ideas of what LES is and what LES should do for you.
You are also probably aware of the paper http://iopscience.iop.org/1367-2630/6/1/035

Good question! :D

I expect from LES just what its definition foresees, i.e., an under-resolved, poorly modeled direct numerical simulation of Navier-Stokes equations. LES should provide, with a reasonable computational cost, the correct underlying physical processes (within modeling limits), should automatically distinguish between laminar and turbulent zones, catch unsteady phenomena, give accurate macroscopic results… in a few words, provide something which is as close as possible to reality.

Considering the endless modeling, numerical, filtering, mathematical and even philosophical issues inspired by LES during the last decades (see ten questions by Pope, for instance), I think this is the minimum we must expect from it!

Far November 10, 2012 11:39

Quote:

Originally Posted by sbaffini (Post 391241)
Working on it since 2008... still have to find out :rolleyes:

Jokes apart, i don't trust Unsteady RANS (or RANS, for that matter)

what an Irony: In RANS we first average the equations and filter all fluctuations and solve for the mean flow quantities only. Then we add the effect of fluctuations through turbulence model. Then in URANS we add the time factor through single time term which is not able to resolve all time scales.

Can any body tell me, what is the URANS actually do?

JBeilke November 10, 2012 15:58

Quote:

Originally Posted by Far (Post 391400)
... Then in URANS we add the time factor through single time term which is not able to resolve all time scales.

Why do you want to resolve all time scales? For uRANS you do the filtering before the calculation and when doing an LES you have to apply "the filtering" after the calculation to be able to interpret the results.

So whats the cheaper way of getting an engineering problem solved?

Far November 10, 2012 16:05

I know it is viable engineering approach. I actually cant understand the URANS philosophy. This approach may be good for the laminar transient flow, but not for the highly turbulent transient flows.

FMDenaro November 10, 2012 18:42

Just think of these two philosophies:

LES -> filter G(x-x'; Delta_x)
URANS -> filter G(t-t'; Delta_t)

Actually, an example would be useful, the cycle of a piston in engine. RANS will set a (hypotethically) statistical steady-state for all the cycles (1 cycle = 360 degree). URANS will give you the unsteady (statistical) behaviour during the 360 degree. Each time is representative of the average of N events at that time.

Far November 10, 2012 18:44

Quote:

Originally Posted by FMDenaro (Post 391433)
Just think of these two philosophies:

LES -> filter G(x-x'; Delta_x)
URANS -> filter G(t-t'; Delta_t)

Actually, an example would be useful, the cycle of a piston in engine. RANS will set a (hypotethically) statistical steady-state for all the cycles (1 cycle = 360 degree). URANS will give you the unsteady (statistical) behaviour during the 360 degree. Each time is representative of the average of N events at that time.

While in LES we get the all times and all N events at each time?

FMDenaro November 10, 2012 18:51

Quote:

Originally Posted by Far (Post 391434)
While in LES we get the all times and all N events at each time?

no.... in LES you do not have the statistical meaning of the solution at time t as the average of N sample... Indeed, in LES you have to simulate the N sample and explicitly do the average. LES require that you solve in time for all the scales, as happens in DNS

FMDenaro November 13, 2012 13:58

Does LES is useful only if RANS/URANS fails or you really need as much details as LES can provide?

cfdnewbie November 15, 2012 06:56

LES for me is one thing above all: an intellectual challenge, and a good meeting place for fluid physicists, mathematicians, numerical analysis, high performance computing people....

plus it is damn fascinating and also frustrating :)


seriously: For me, it is a great research tool to do research with and to do research on!

FMDenaro November 17, 2012 06:35

good !

just to stimulate the discussion, I do a simple example... consider the "academic" test-case of the lid-drive cavity at Re=1000. It is a laminar case.
Then you run your code as it is on a uniform grid of 10 cells for side. You have a Re cell number =100 which means you are running the code in unresolved condition.
For you, is that an LES? In other words, any time you use a CFD code in unresolved conditions, you are filtering something and implicitly doing an LES?

cfdnewbie November 17, 2012 13:05

well, my answer to this question is twofold:

1) yes, everytime you are using any type of discretization, you are doing an "unresolved DNS".... one might call that an implicit LES.... As soon as you are introducing a filter through approximation(by projection onto your approximation basis, e.g. phi=1 (the mean) in FV, dirac in FD, polynomials in FE) , you are no longer solving for u (the DNS), but for u(bar) (the filtered u). Plus you are adding approximation errors through your discretization of continuous term (like difference equations for derivatives), so, you what you are in reality dealing with is something like u(hat)_h.

2) Should every underresolved DNS be called an LES? No, I don't think so, because for me there's more to a LES than just picking a low resolution for a high frequency problem. But that's open to discussion, implicit filter implicit SGS LES-people would disagree maybe :)

Very interesting discussion! What are your thoughts about this?

Cheers, all the best!

FMDenaro November 17, 2012 13:21

I agree :)

I think that a cfd user must be aware of the fact the running any numerical simulation will allways produce an intrinsic filtering. The "label" of the simulation can be DNS or LES depending on the ratio between the Kolmogorov and the Nyquist frequency scale. I think that if this ratio is no longer than 10 we can still talk of real DNS. When this value exceeds 10 I use to call "unresolved DNS" or, better, "no-model LES".
In my experience, despite we know that is wrong, no-model LES often produces better results that LES modelled with an eddy viscosity SGS model.

So, further question arise, what an SGS model should do for you?

cfdnewbie November 17, 2012 13:43

Quote:

Originally Posted by FMDenaro (Post 392703)
I agree :)

I think that a cfd user must be aware of the fact the running any numerical simulation will allways produce an intrinsic filtering. The "label" of the simulation can be DNS or LES depending on the ratio between the Kolmogorov and the Nyquist frequency scale. I think that if this ratio is no longer than 10 we can still talk of real DNS. When this value exceeds 10 I use to call "unresolved DNS" or, better, "no-model LES".

Ok, I guess we agree on the essence of the issue, although I tend to be a little bit more strict with what I really would call a DNS... in fact, (I have never tested it), I guess one would have to resolve at least half of the Kolmogorov scale to be fully fully fully DNS, since the non-linear terms will produce (for incompressible eqn) a frequency that's two times Kolmogorov... so to capture that (without any need for dealising), one you have to resolved the non-linear terms computed from the smallest scale...

but I have never tested that, and is doesn't really matter, I guess, plus that's then often so close to the numerical double precision barrier of 10E-16.... so it is just a theoretical point...

Quote:

In my experience, despite we know that is wrong, no-model LES often produces better results that LES modelled with an eddy viscosity SGS model.
I think that's due to two issues:
a) interaction of the numerical errors and SGS model... I have the feeling that this often makes the overall results worse than without a model.
b) here's another idea, although it just a feeling, not a fact:
eddy viscosity models (at least the Smagorinsks) rely on two things: 1) the equilibrium of dissipation and production, i.e. isotropic turbulence and 2) the alignment of resolved strain and stress. Both assumptions are invalid in many cases, BUT (here comes my feeling) if you use your discretization error as the model (or the Riemann solver /flux function in FV), then your discretization has the chance of introducing the anisotropy naturally... I'm not sure if I can express it in a good way, but I have the feeling that eddy viscosity models are more "isotropic" than your discretization dissipation.

Quote:

So, further question arise, what an SGS model should do for you?
the textbook answer would be to model the effect of the unresolved on the resolved scales, I guess :)
For me, it should
a) turn off when laminar
b) allow backscatter
c) remove the energy, of course, at the correct rate
d) (maybe) even be clever enough to remove any aliasing errors I'm introducing through inexact nonlinear terms...
e) not assume isotropy of strain and stress
f) not destroy the time step
g) not interfere with parallelization

wow..... can think of more things, but have to get dinner now!

until then!

francesco_capuano November 17, 2012 16:22

Very interesting discussion! :)

In my opinion the major drawback of many "classical" SGS models is that they have been derived within a formal mathematical framework which starts from the analytically filtered Navier-Stokes equations. As a consequence, such models imply a filter to the simulation rather than adapting to the real effective filter imposed by discretization and numerical scheme. One should rather start from the discretized form of the governing equations, but unfortunately the discretization operator is often unknown... however one interesting contribution in this vein was the one by Carati & al:

http://journals.cambridge.org/action...line&aid=82969

Another interesting point is the one made by cfdnewbie about the alignment between resolved strain and SGS stress models, and the inherent anisotropy introduced by numerical schemes. In fact, in the book on ILES by Grinstein et al. the authors show via Modified-Equation Analysis how some flux reconstruction schemes are equivalent to scale-similarity SGS models, which are able to capture anisotropic effects. I also agree on what a SGS model should do, and would also add: catch correct near-wall dependence.

FMDenaro November 17, 2012 18:56

I agree that the eddy viscosity assumption suffers of many limitations. However, non-isotropic eddy viscosity SGS provides somehow disappointing results, as well ...

In my experience the two-parameter dynamic mixed model is the more accurate, it provides good zero-order statistics as in the no-model LES but also quite good high order statistics.
Deconvolution-based modelling are also advisable for smooth filters.

Now, my next question is: what do you intend for an LES?
- a numerical approximation of a mathematical system of PDE obtained by filtering the NS in continuous form
- a way to model all is unresolved by the discretized NS equations

In the first case, in order to accurate represent the filtered solution, the ideal numerical method should minimize the local truncation error, that is exactly in opposite sense of the ILES approach.

cfdnewbie November 18, 2012 15:46

Quote:

Originally Posted by FMDenaro (Post 392726)
- a way to model all is unresolved by the discretized NS equations

In what way would that be possible? by selecting your number of DOF, you restrict the dimensionality (and thus exactness) or your solution space... so there is no real way to recover all that is lost....you might get first and second order relations correct, but beyond that, I find that hard to imagine....

or am I missing something?


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