RANS modelling and URANS methods in unsteady flow simulations
 

Hi
I have read in some texts and papers that RANS turbulent simulation is not appropriate for unsteady cases and for example in https://www.google.com/url?sa=t&rct=...47534661,d.bGE
this subject in page 24 has mentioned and has proposed to use a URANS method.
so I have two clear question:
1)how much may be the error in using RANS models in unstedy flow simulations?(so we should give up it totally?)
2)is there a(or some) URANS method in OpenFOAM?
thanks for consideration.

Hi Ehsan,

AFAIK, OpenFOAM only has these: http://www.openfoam.com/features/turbulence.php
From what I can see, the closest you've got to something like "URANS" is "DES".

As for using RANS in unsteady cases: it depends on the nature of the simulation. As I've said in a previous email or thread, possibly you'll need to switch to LES or DES, in order to properly simulate your specific cases. At the very least, for comparing if the solutions are approximate or not.
But the problem with LES is that it's far more heavy computationally, although DES might ease on the computational power needed...

On the topics of DES, all I know are on these two threads:

• http://www.cfd-online.com/Forums/ope...tml#post424797
• http://www.cfd-online.com/Forums/ope...tml#post424652

Best regards,
Bruno

edit: continue reading down to and including post #13 for an updated answer.

Hi Bruno
are there any test case to see whats the settings for LES or DES?

On the first link I pointed out on the paragraph "On the topics of DES", has a case that uses DES.
I have not tried to compare with LES, because that case never entered into the LES region, because there was no obstacle inside it or anything that unleashed complex turbulence :(


In reference to your thesis: since it's a master thesis, you can always indicate, in the section "future work", that there should be a comparison with LES, in order to ascertain the validity of the simulations.
Keep in mind that 5-10 years ago, LES could only be properly performed using supercomputers! Nowadays, a home made cluster or a top grade workstation can handle some of these kinds of LES simulations, but it could still take a week or so to do a single simulation, depending on the mesh resolution!

then in DES it sometimes turns LES(when flow be very turbulent,yes?)
Oh! my case is complicated and slow enough.maybe I test a DES not LES with these circumstances you prescribed!
thanks so much.

From what I could figure out: yes, it switches from RAS to LES in the regions it thinks it should use LES. And from OpenFOAM's website, RAS (RANS) is always used near the walls.

well,i try a DES method.
But i don't know exactly which method of LES is better for a compressible unsteady case that i have.
Any suggestion and explanation is thanked.

URANS question
 

Hi,

I also have a URANS question. It seems like when I use URANS (with all 2nd order upwind solvers), the oscillations decay away. This is even though the flow configuration (flow around a blunt body) should have unsteadiness. Do you have any advice on why this would be happening? My inflow conditions are all steady. Do I have to use forced inlet conditions to sustain the oscillations?

Thanks!

Hi
how you use URANS?which model in OF is of type of URANS?
what do you mean by forced inlet condition?

I'm using RKE in Fluent. By "forced", I mean "unsteady."

Thanks.

Greetings to all!

I'm a bit late to the conversation, but here goes anyway...

@HaveSomeQuestions:
Looks to me that you didn't read the OpenFOAM User Guide... namely the section 4.4: http://www.openfoam.org/docs/user/fvSchemes.php
From there:
Therefore, it's only natural that with a first order divergence scheme, that you get such nice results ;)

Then I think that's not exactly URANS... technically that should be the "Realizable k-Epsilon" turbulence model... but I could be wrong.

Best regards,
Bruno

there is not any URANS in OF,is?

Hi Ehsan,

Since you asked once again... I did some more searching and researching... and apparently I can now stand corrected, I suppose - here's a quote from an old post (from this thread: http://www.cfd-online.com/Forums/ope...algorithm.html):
Therefore, looks like HaveSomeQuestions has a very good reason to say that RKE is URANS.

Also, from my research, here's what I found that might come in handy:

• http://www.cfd-online.com/Forums/cfx...onal-cost.html
• http://www.cfd-online.com/Forums/mai...rans-rans.html
• http://www.cfd-online.com/Forums/mai...ans-urans.html
• http://scicomp.stackexchange.com/que...r-turbulent-tr
• http://en.wikipedia.org/wiki/Computa...fluid_dynamics
• http://www.os-cfd.ru/cfd_docs/chalmers/urans.pdf
• http://www.cfd-online.com/Forums/mai...osity-les.html
• http://www.cfd-online.com/Forums/mai...-2d-vs-3d.html
• http://www.cfd-online.com/Forums/flu...nce-2d-3d.html
• http://www.cfd-online.com/Forums/ope...tml#post443838
• http://www.cfd-online.com/Forums/ope...tml#post426028
  Quote:

  Originally Posted by vonboett (Post 426028) The LeMoS extensions for OpenFOAM provide an URANS-LES Detatched Eddy approach where the regions of URANS and LES are decomposed dynamically in every timestep dependent on flow properties and local mesh resolution. They have very good validation results using ship hydrodynamic test cases. So if you want to use LES with an unstructured grid, this might be helpful.

  • About LeMoS: http://www.lemos.uni-rostock.de/cfd-software/

Last but not least: I found at least around 3 or 4 threads here at CFD-Online of people who apparently didn't search the forum for more information... and ironically, they got no answer in return.

Best regards,
Bruno

thanks Bruno for searching despite of your tiredness,
only please correct the first link"on the pimpleFoam".
and is Jose your brother? :)
I'll edit this post after reading links.

Thanks! I've fixed it!

:confused: No. "Santos" is a popular family name here in Portugal.
Better yet, there should be more than 10 "Bruno Santos" in just Portugal, not even counting Brazil and other countries. Which is why people here in Portugal have 4 or more names, not just first and last names.

sorry,I was thinking that user is in your family since a long time :)

Don't you call "URANS" every RANS simulation that is unsteady? E.g. by unsteady inlet or globally unstable flow, such as bluff-body flows? So basically you can use every RANS model for URANS by switching to any unsteady solver (Piso, pimple, ...).

Hi Philip
do you mean if I use any of turbulent models in unsteady solvers,the RANS model will become URANS automatically?
is there any other comment?
thanks.

Dear Ehsan,
When you specify ddtSchemes (i.e. not steady state), the simulation becomes unsteady, i.e. you are using URANS.

I do not know how far URANS is physically meaningful.

Any comments ?
--
KANNAN

Ehsan, yes, that is right.
I found from Jochen Fröhlich, Progress in Aerospace Sciences 44 (2008) 349-377, Hybrid LES/RANS methods for the simulation of turbulent flows:
"...It has become common to name RANS modeling as URANS whenever the computed solution is time-dependent."

Kannan, I don't see why this wouldn't be physically meaningful. RANS model derivation does not say anything about the meaning of the <...> operator. This can be temporal mean, phase average,... Just some basic features must be supported by <...> such as linearity and so on. Now, the turbulence model doesn't know about this.
Only the turbulent fluctuations of certain time-scales are damped out by the model. If the inlet is unsteady, with a time-scale slower than the turbulence, you will get a time-dependent solution. On the other hand, for bluff body flows, you will get such things as vortex streets, which are physically meaningfull as well.