# What LES should do for you?

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November 18, 2012, 19:03
#21
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Filippo Maria Denaro
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Quote:
 Originally Posted by cfdnewbie 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?
No, I wanted to say "model", not "recover"... recovering outside the Nyquist freqeuncy is impossible as you correctly said.
Therefore, modelling all means modelling physical unresolved scale as well as taking into account for numerical errors. A report on CTR was based on a dynamic procedure taking into account also for numerical error

 November 21, 2012, 00:00 #22 Senior Member   Martin Hegedus Join Date: Feb 2011 Posts: 480 Rep Power: 12 What should LES do for me? Simple, fix the base flow issue with RANS!! And, I assume URANS has the same problem. http://www.hegedusaero.com/examples/...celerator.html This can be a HUGE problem with RANS. So, for base flows, is URANS or LES better?

November 21, 2012, 05:10
#23
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Filippo Maria Denaro
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Quote:
 Originally Posted by Martin Hegedus What should LES do for me? Simple, fix the base flow issue with RANS!! And, I assume URANS has the same problem. http://www.hegedusaero.com/examples/...celerator.html This can be a HUGE problem with RANS. So, for base flows, is URANS or LES better?

Hi Martin,
first I am curious to know what do you expect from RANS in your case... Any computed variable is statistical, that means you have a function like <f>(x) therefore, in your opinion, in what your RANS plots are wrong?

Second, have you tried to do a simple test, running your code without any turbulence model? This should give you the feeling of what the models really do...

Third, try LES ....

November 21, 2012, 12:19
#24
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Martin Hegedus
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Sorry, I may have stated my post incorrectly. With the eddy viscosity model and with how flow features set themselves up, I don't expect RANS to be different. The results were not a surprise. However, I don't believe that in reality RANS is truly statistical. Yes that's what the math says for small local areas, but the flow in small local areas sets up the flow for the entire problem and this then feeds into other small areas. In other words, the flow state trajectory for solution convergence leads to a fictitious (to a degree) answer. For example, base flows. I believe, in general, the time averaged true pressure values along many base flows are (more or less) constant along the base. The deaccelerator is an example of it. However the RANS code gives a different statistical time averaged answer. In the end, a RANS result for base flow is an answer for a flow with a lot of local (created locally and/or transported in) viscosity. So, IMO, RANS flow does not necessarily represent flow values which have been averaged over an infinite amount of time. That's a problem with RANS. And that is the nature of the beast.

So I would like LES, URANS, DES, or anything else, to get me a better physical model.

Unfortunately Reynolds number is too high, so going without a turbulence model is questionable. It definitely will not get skin friction correctly. And, that feeds into some types of base flow.

I haven't tried LES/DES for this problem. Too expensive. The intent for the deaccelerator example was just to point out the issues of RANS to people. But, given what I've heard, I'm not confident I'm aware of all the ins and outs with LES. LES/DES "should" give me better base flow results, but I am also not sure where it falls apart.

For base flows, will URANS and DES results be similar? I've never tried URANS for it.

Quote:
 Originally Posted by FMDenaro Hi Martin, first I am curious to know what do you expect from RANS in your case... Any computed variable is statistical, that means you have a function like (x) therefore, in your opinion, in what your RANS plots are wrong? Second, have you tried to do a simple test, running your code without any turbulence model? This should give you the feeling of what the models really do... Third, try LES ....

 November 21, 2012, 12:30 #25 Senior Member   Filippo Maria Denaro Join Date: Jul 2010 Posts: 3,488 Rep Power: 40 I believe that RANS solution are statistical in global sense, often they fail in providing correct (statistical) results only because the model is not perfect to take into account the fluctuations over the entire spectrum. For your problem I suggest to run an unresolved DNS just beacause it is wrong.... see the solution and then compare to a LES solution to assess the relevance of the model. LES and DES are different formulations.... in DES you get mixed solutions type

 November 21, 2012, 14:28 #26 Senior Member   Martin Hegedus Join Date: Feb 2011 Posts: 480 Rep Power: 12 Since there were a lot of discussion about RANS vs. URANS, how do people view the acceptability of using URANS on base flow?

November 21, 2012, 17:33
#27
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Filippo Maria Denaro
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Quote:
 Originally Posted by Martin Hegedus Since there were a lot of discussion about RANS vs. URANS, how do people view the acceptability of using URANS on base flow?
Just to address an issue ... if someone sees the continuous URANS equations and compare to the LES equations (for implicit filter) he would for sure wonder what about the difference ... The answer is that there are apparently no differences ... is the type of closure that distinguish URANS from LES.
But is a model really capable to do this distinction?

 November 28, 2012, 12:29 #28 New Member   RZA Join Date: Nov 2012 Posts: 25 Rep Power: 6 LES is the process in which instead of modeling everything like RANS we apply filtering process and separate large eddies form the smaller one. Based on Kalmogorov principle the smaller scales of motion are universal (isotropic) hence can be modeled similar to RANS. As the larger scales depend upon the boundary and flow conditions, hence solved like DNS. So we can say LES is compromise between DNS and RANS both in terms of accuracy and computational cost.

November 28, 2012, 12:36
#29
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Filippo Maria Denaro
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Quote:
 Originally Posted by Engr.RZA LES is the process in which instead of modeling everything like RANS we apply filtering process and separate large eddies form the smaller one. Based on Kalmogorov principle the smaller scales of motion are universal (isotropic) hence can be modeled similar to RANS. As the larger scales depend upon the boundary and flow conditions, hence solved like DNS. So we can say LES is compromise between DNS and RANS both in terms of accuracy and computational cost.

ok, but this is just the theory...

- the filter process is never applied in practical LES (apart the explicit filtering approach). Filtering is only an implicit process due to discretization of equations and domain.
- the universailty of the SGS model based on isotropy of the unresolved scales is almost never obtained in practical LES since you can not produce a filter width so small in all the regions of a wall-bounded flow. As a consequence, often you have unresolved scales that are far from being isotropic.

Thus, what do you expect when analyse your LES solution?

 December 5, 2012, 05:35 what to expect from LES #30 Senior Member   Albrecht vBoetticher Join Date: Aug 2010 Location: Zürich, Swizerland Posts: 214 Rep Power: 10 ...modelling mountain torrent flows, I can define what I expect from LES and how far it works: I need to get the influence of the complex geometry on the turbulence structure and that of the presence of the free surface. I accept huge errors, because there is nothing better available. Based on the thesis of I. Wendling (2007) "Dynamische Large-Eddy Simulationen turbulenter Strömungen in komplexen Geometrien" I started to consider the dynamic mixed SGS model as that one that accounts at least for some influence of the complex geometry, and with Keylock et. al (2005) "The theoretical foundations and potential for large-eddy simulations (LES) in fluvial geomorphic and sedimentological research" in Earth-Science Reviews 71 I started to see that the backscatter is important to catch the influence of the free surface, and that the dynamic mixed scale similarity model is ok. So in the current version of OpenFOAM, the dynLagrangian SGS model is promising, but since the authors of the corresponding paper ('A lagrangian dynamic subgrid-scale model of turbulence' by Charles Meneveau 1996) themselves point out that the scale similarity should be included, I am looking forward for the day when there will be a lagrangian dynamic mixed SGS model in OpenFOAM. One more thing I would like to see is a discussion about how to determine if the grid resolution is fine enough for LES. I use two-point correlations to check if the bigger structures are resolved well, since Lars Davidson shows in "Quality and Reliability of Large-Eddy Simulations II" Vol. 16, pp 269-286, Springer (2010) that the energy spectra and ratios of SGS viscosity to physical viscosity are not reliable quantities to estimate the grid resolution.

December 5, 2012, 09:11
#31
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Filippo Maria Denaro
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Quote:
 Originally Posted by vonboett ...modelling mountain torrent flows, I can define what I expect from LES and how far it works: I need to get the influence of the complex geometry on the turbulence structure and that of the presence of the free surface. I accept huge errors, because there is nothing better available. Based on the thesis of I. Wendling (2007) "Dynamische Large-Eddy Simulationen turbulenter Strömungen in komplexen Geometrien" I started to consider the dynamic mixed SGS model as that one that accounts at least for some influence of the complex geometry, and with Keylock et. al (2005) "The theoretical foundations and potential for large-eddy simulations (LES) in fluvial geomorphic and sedimentological research" in Earth-Science Reviews 71 I started to see that the backscatter is important to catch the influence of the free surface, and that the dynamic mixed scale similarity model is ok. So in the current version of OpenFOAM, the dynLagrangian SGS model is promising, but since the authors of the corresponding paper ('A lagrangian dynamic subgrid-scale model of turbulence' by Charles Meneveau 1996) themselves point out that the scale similarity should be included, I am looking forward for the day when there will be a lagrangian dynamic mixed SGS model in OpenFOAM. One more thing I would like to see is a discussion about how to determine if the grid resolution is fine enough for LES. I use two-point correlations to check if the bigger structures are resolved well, since Lars Davidson shows in "Quality and Reliability of Large-Eddy Simulations II" Vol. 16, pp 269-286, Springer (2010) that the energy spectra and ratios of SGS viscosity to physical viscosity are not reliable quantities to estimate the grid resolution.

from my experience, I agree that dynamic mixed model is superior, in order to use on non regular grids (e.g. unstructured) the LES procedure must be properly written.

As far the grid resolution is concerned, this issue is somehow misleading for LES based on implicit filtering. The computational grid is also a measure of the filter grid, therefore the question is "what do you want that a filtered field resolve for you?"
I know the work of Davidson as I was present at the QLES conference and some conclusions he draw were criticized

 December 5, 2012, 10:30 #32 Senior Member     Paolo Lampitella Join Date: Mar 2009 Location: Italy Posts: 761 Blog Entries: 17 Rep Power: 21 I would like to add that, still in my opinion, LES is also the most natural and (in a certain sense) trivial way to approach a fluid flow computation when the computational resources are not enough for a DNS. The main problem here is that we still don't know what is the most appropriate large scale discrete system of equations to be solved in order to remain consistent with the original Navier-Stokes equations... that is, a universal framework taking also the full numerical implementation into account seems far to come at the moment. Of course, in this sense, the task is also far from trivial

 December 7, 2012, 07:23 #33 Senior Member   Albrecht vBoetticher Join Date: Aug 2010 Location: Zürich, Swizerland Posts: 214 Rep Power: 10 ...concerning the dynamic mixed SGS models and the lagrangian dynamic mixed model, I am happy they have been realized for OF 2.1. Here is the link, I post it here because I did not find it by searching cfd-online: http://www.lemos.uni-rostock.de/en/cfd-software/

January 17, 2014, 12:48
#34
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MMS
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Quote:
 Originally Posted by FMDenaro 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
Hi All! this thread is very useful, there are lots of information about LES. But the fact is that most of them are advance level information and hard to get into it for the beginner. Moreover, most of the people in CFD are using different black box (fluent, cfx, openfoam etc)and they actually don't know how LES works and what are the criteria to meet the LES requirements and other relevant things. For the new comer in this challenging field you all could do a great help!

If some of you (it seems most of you are expert in LES, I will definitely mention the name of FMDenaro and Sbafinni, they are always helping lots of people and students like us regarding LES all the time in CFD online -respect!) post some lectures on YouTube including.......how LES equations are derived and filterd, .....how filtering operation accomplished, .......why FFT and other mathematical stuffs are required for the LES solution, .....how discretization method and modelling parameters impacts the solution etc. in sequential manner with pictorial presentation and required diagrams, it will be a great help for all new comers to get on the wheels and a great contribution to the knowledge in the field of LES as well.

I can definitely say that most of the new comers often go to the YOUTUBE (best open source e-learning resource) to have some visual idea about LES and actually how it works? And most of them are disappointed after wasting their plenty of time for nothing. Addition, there is no information about LES in Youtube.

Its my personal suggestion, and would like to place in front of you all for kind consideration.

Regards

 January 17, 2014, 13:13 #35 Senior Member   Filippo Maria Denaro Join Date: Jul 2010 Posts: 3,488 Rep Power: 40 Hi, what are you asking for is something that is only possible in the academic site...attending CFD and turbulence courses is necessary and have a practice with other students is very helpful. I don't think that forums, youtube or other can go further in details. Of course, don't forget that many books about LES are now available

January 17, 2014, 13:24
#36
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Alex
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Quote:
 Originally Posted by cfdmms YOUTUBE (best open source e-learning resource)

Many of the tutorials there are crap and for a beginner in CFD it is impossible to tell which ones.

Sorry for picking up this specific aspect, I know it has nothing to do with the subject of the thread.
I didnt mean to distract from the topic, I just could not leave this statement uncommented.
Because apparently, many of the misconceptions that pop up in the subforums on commercial cfd software after long discussions about what appears to be a simulation results could be traced back to the errors and omissions made in the tutorials on said platform.
I am not saying that the "bad practice" was necessarily acquired there, but it might be.
And as Paolo has just shown, there is no lack of good material to learn from, it just has to be digested in a less convenient form.

Last edited by flotus1; January 21, 2014 at 04:19.

 January 21, 2014, 15:17 #38 Member   Join Date: May 2012 Posts: 68 Rep Power: 7 1) In Pope's book, he has explained the difference between LES, DNS & RANS based on the spectrum. I highly doubt any "better" explanation is possible atm 2) In one of Dr Moin's interviews, he has pointed out the effective use of LES in acoustics which was one of the main reasons for which the funding was possible to arrange to continue the research on LES. 3)I think it has been explicitly mentioned by other members but still let me point out:- DNS > LES > RANS Conclusion:- LES can be used to:- 1)show-off (which is the most common case now-a-days) 2)acoustics 3)turbulence and related work __________________ Best wishes, Somdeb Bandopadhyay

January 21, 2014, 15:38
#39
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cfdnewbie
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Quote:
 Originally Posted by som87 Conclusion:- LES can be used to:- 1)show-off (which is the most common case now-a-days)
Hello,
I don't agree with your statement at all. In my experience, there are usually two scenarios:
a) Industrial applications of LES, where people either check the "Do LES" Box in their solver and don't know much about what happens behind the scenes, and it just like "better URANS" to them or they realize that they cannot do LES due to the parallel licences of their commercial codes or the CPU time required- because while LES is of course a lot cheaper than RANS, it is usually the temporal averaging that requires a lot of CPU time. So they consider LES unnecessary and stay away from it
b) researchers in the field, who usually are a humble crowd and are aware of the tons of open issues in LES and the complexity involved.

Using LES to show off is a very stupid idea, because it is so easy to point out the open issues / unresolved problems and shortcomings

 January 21, 2014, 15:54 #40 Senior Member   Filippo Maria Denaro Join Date: Jul 2010 Posts: 3,488 Rep Power: 40 From my experience, people working in the industrial field consider LES as a more accurate turbulence modelling than RANS, but then are frustrated while discovering the computational cost for problems with complex geometries and quite large lenght scales. RANS is still largely used. Conversely, during the last years I observed a reduced interest in LES researches at CTR, see for example http://ctr.stanford.edu/ResBriefs/2012/index.html. What do you think about?

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