[Bernhard]

Maybe T-RANS?

[Djub]

I have heard about U-RANS (for Unsteady RANS), but never about T-RANS. What is this?

[Bernhard]

Transient RANS, I think in general they refer to the same thing. However, long ago I have seen a paper with a triple decomposition with a third, periodic term.

[niaz]

Dear Djub
there is 3 formula for kolmogorv laws and one of them is about time.
if you have mesh smaller then kolmogrov, you will approach to DNS
but you should maintain time step small.
you should look to that it is worth to simulate your case with LES
LES is very expensive model.

[Djub]

Sorry Niaz,
I can perceive very interesting features in your message, but I didn't fully catch them...
May I ask you to remind me these Kolmogorov laws? I know one aout the -5/3 slope in the intertial domain; another one, something as I mentioned (shortest eddies corresponding to Re=1, so l=nu/V). Could you tell me about what you are thinking exactly?
"you should maintain time step small". How to do this ??? I thought the Cfl number (Current Flow, maybe noted Co elsewhere) could not be greater than 1 in LES?
Remark: in fact PISO is implicit so it is not an obligation of PISO. But I use Central Differenciating for diffusion scheme which requires Cfl<~1 . Using Backward would not solve the problem, because it adds temporal diffusion so delta t has to be small. Small but larger than Cfl = 1? Thus it could be interesting! Thanks both of you, Niaz and Bernhard... I will try!
Do you think I should use LES in this kind of case? So what else are you thinking about? U-RANS or T-RANS, as Bernhard 's advices ?

[Bernhard]

Niaz is referring to the Kolmogorov scales: http://en.wikipedia.org/wiki/Kolmogorov_microscales
It your mesh is as fine as the smallest scales in the flow, there is no need for modelling the turbulence of course.

Also, CFL stands for Courant-Friedrichs-Lewy.

[Djub]

Thanks Bernhard, you are a very wise and knowing people (and I apologize to M. Courant, M. Friedrichs and M. Lewy for my mystake...).
But what about epsilon? Nobody knows its value in my problem!!!
How to "estimate [my] time step from kolmogrov law" in this case?
Excuse me: I thought a bit and find out the solution: exploiting law 1/ and 2/, I found delta T = (delta x)² / nu . Thanks Bernhard and Niaz! Nevertheless, it is mush more "expensive" than the limitation with Cfl (or Co ) !!! Here refining delta x costs the square for delta T !!!
And if I go deep to these small scales, what is the difference between LES and DNS ?

I feel uncumfortable: Bernhard seems to guide me to "simplier" solution, with RANS, while NIAZ's direction seems to be going deeper in the fluid, something closer to DNS, thus more complicated features...
Isn't LES exactely designed for this: an interscale between DNS and RANS ?

[Bernhard]

Computational resources often dictate what kind of treating turbulence you use. It also depends on what you want to know. I don't advise against Large Eddy Simulations, but you have to realize that due to different mesh requirements, LES will be more expensive. You can perform LES on a RANS mesh, then your LES results will be bogus.

The dissipation rate of turbulent kinetic energy can be estimated, to get the right order of magnitude. See e.g. Chapter 5 in the book by Pope there should be some copies in your department)

[niaz]

Dear Djub
I did not conduct you to go to DNS. I said if you create smaller mesh, your solution will go near the DNS solution. I only guide you to refine mesh near the square.
it is obvious that URANS is very cheaper than LES specifically in higher Reynolds. and in your case, I decide you to use SpalartAllmaras.
and also be aware, sometimes numerical dissipation is more than your nuSGS and you should use ILES instead of LES.
but Bernard suggestion is very good, if you do not have to use LES, it will be better to use URANS.

[Djub]

" if you do not have to use LES" ?
I do not HAVE to use LES, or URANS, or DNS or whatever. I have to compute vortex shedding the more acurately possible with low computing capabilities ( about 8 cores 2GHz5).
I thought LES was the best compromise. But if you think I need DNS or U-RANS, I will fallow your advices...

I am just surprised that a quite simple question (well, in my scientific background, which is not yet Computational Fluid Dynamics...) has no answer for the moment. This question beeing: how to model the vortex shedding of a simple geometry. The Eurocode I (2005) gives:

Note: for round cylinder, it seems simplier since St is constant 0,18, but everybody knows the the round surface is incredibly difficult to model because of Reynolds problems. Thue I think the rectangular shape is the simpliest...

[niaz]

if you don`t have to, I think it was better to start with URANS

[Djub]

Maybe you are right. Is there a Foam Tuto ? I ran LES 'cause I found Motorbike and it seems OK for me. How to run URANS with OpenFoam? PIMPLEFOAM with RANS turbulent model?

[niaz]

yes of course. you can find many cases in tut. of openfoam.

[Djub]

I managed with LES.
Some news about this thread: see
http://www.cfd-online.com/Forums/ope...tml#post407110