
[Sponsors] 
November 18, 2012, 19:03 

#21  
Senior Member
Filippo Maria Denaro
Join Date: Jul 2010
Posts: 2,941
Rep Power: 34 
Quote:
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: 479
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  
Senior Member
Filippo Maria Denaro
Join Date: Jul 2010
Posts: 2,941
Rep Power: 34 
Quote:
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  
Senior Member
Martin Hegedus
Join Date: Feb 2011
Posts: 479
Rep Power: 12 
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:


November 21, 2012, 12:30 

#25 
Senior Member
Filippo Maria Denaro
Join Date: Jul 2010
Posts: 2,941
Rep Power: 34 
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: 479
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  
Senior Member
Filippo Maria Denaro
Join Date: Jul 2010
Posts: 2,941
Rep Power: 34 
Quote:
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  
Senior Member
Filippo Maria Denaro
Join Date: Jul 2010
Posts: 2,941
Rep Power: 34 
Quote:
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 wallbounded 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: 212
Rep Power: 9 
...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 LargeEddy 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 largeeddy simulations (LES) in fluvial geomorphic and sedimentological research" in EarthScience 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 subgridscale 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 twopoint correlations to check if the bigger structures are resolved well, since Lars Davidson shows in "Quality and Reliability of LargeEddy Simulations II" Vol. 16, pp 269286, 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  
Senior Member
Filippo Maria Denaro
Join Date: Jul 2010
Posts: 2,941
Rep Power: 34 
Quote:
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

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 NavierStokes 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: 212
Rep Power: 9 
...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 cfdonline:
http://www.lemos.unirostock.de/en/cfdsoftware/ 

January 17, 2014, 12:48 

#34  
New Member
MMS
Join Date: Nov 2013
Posts: 29
Rep Power: 5 
Quote:
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 elearning 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: 2,941
Rep Power: 34 
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 
Senior Member
Alex
Join Date: Jun 2012
Location: Germany
Posts: 1,392
Rep Power: 22 
Youtube is definitely not the place to learn something about CFD. 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. Now back to topic. Last edited by flotus1; January 21, 2014 at 04:19. 

January 21, 2014, 03:51 

#37 
Senior Member

If we can forget youtube for a while (sorry, but FMDenaro, flotus1 and Martin Hegedus are completely right and, definitely, that's not how we learned the few things we know), as you can imagine from my commitment, i would like to share everything i know to the best i can.
However, the required effort, especially in LES, is really far from trivial. Mostly because you cannot have a single target for your explanations. While, probably, some time, i will try to put some effort on the work you suggest, this is not certainly the way a forum is intended to work and not the way it can be best used. When i first used CFDONLINE, back in 2005, i was unable to use few lines in a UDF for Fluent. They were taken straightly from the manual and they didn't work. Stacked. Someone just answered with the correct way to do it, something i could have never achieved by myself in the time i had... even because the Fluent ufficial support could not help neither!!! The second time i really needed the forum, i had a problem with the principal value component in vortex panel methods and the way these behave for closed surfaces with lift. In this case i got the answer after few weeks. What i'm trying to say is that if you have a problem with something, you can come here and ask for an answer. Soon or later someone is gonna answer. If you need a general explanation of a method, that could still work (but notice that, ususally, in this case, you get very short answers, possibly even incorrect). If you need a full knowledge, that is not gonna work. More specifically, in LES you would need to first give details of numerical methods, how they work and how they are implemented. Then you should give details of CFD and RANS/URANS. How they work, how they are effectively implemented, how the Whole thing is actually set up to get some answers. After that you can introduce LES. But even in that case, trust me, you're not gonna get it in any case. The first time i read the book of Sagaut was in 2007 and i made i full translation, from english to italian (my native language), of almost all the first 7 chapters, including all the derivations in the models. 2 Years later i was working on my master thesis and i read it again few additional times. While i would have not changed a word of what i wrote 2 years before, i definitely had a different view on the topic, and my 80 pages thesis chapter on LES was clearly different. Today, i'm writing my Ph.D. thesis (which is on LES of course). While i'm possibly using some material from my old master thesis, my view is still completely diffferent and in 80 pages i wrote completely different things, none of which i had any chance to get 4 years before. To give you an example, consider that i just got, few weeks ago, that, possibly, the basis of LES was already put down by Reynolds in 1895, including the Favre filtering for compressible flows and some very advanced concepts which would only emerge again in 1974!!! That is certainly something you don't read in books (not all at least). So, to better clarify what i'm saying: if you need an answer, you can get it here. If you want to understand something, you can still hope. If you want a full course on a topic and hope to understand it... well, forget it. That's simply not possible, no matter how effort someone could put on it. Besides this, there is already some material on internet which is not bad at all. http://www.eng.utah.edu/~rstoll/LES/Lectures.html Still, thank you very much for the consideration Regards Edit Dear MMS, i hope you get this whole thing right. P.S. Just for curiosity, back in 2005, what i needed in my UDF was just the following line: for (n=0; n <= cell_type_nnodes[(int)C_TYPE(c,t)]; n++) { ...stuff... } which does a loop over the nodes of a finite volume cell. Now, consider the version reported in the manual: c_node_loop(c,t,n) { ...stuff... } Today i completely understand where the answer comes from (but i still don't get why the manual version should have been wrong). But, 9 years ago i was practically a child, i would have never managed it by myself. This is the best place for this kind of answers and, despite all, will always be. Last edited by sbaffini; January 21, 2014 at 15:31. 

January 21, 2014, 15:17 

#38 
Member
Join Date: May 2012
Posts: 68
Rep Power: 6 
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)showoff (which is the most common case nowadays) 2)acoustics 3)turbulence and related work
__________________
Best wishes, Somdeb Bandopadhyay 

January 21, 2014, 15:38 

#39  
Senior Member
cfdnewbie
Join Date: Mar 2010
Posts: 557
Rep Power: 12 
Quote:
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: 2,941
Rep Power: 34 
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? 

Thread Tools  
Display Modes  


Similar Threads  
Thread  Thread Starter  Forum  Replies  Last Post 
LES Filtering: how are the small and large scales equations solved?  atmcfd  Main CFD Forum  38  March 14, 2016 15:52 
Beginning to work in LES  Suggestions needed  atmcfd  Main CFD Forum  5  July 20, 2012 22:16 
Turbulence dampening due to magnetic field in LES and RAS  eelcovv  OpenFOAM  0  June 8, 2010 11:35 
LES and combustion model  Margherita Cadorin  CFX  0  October 29, 2008 06:24 
Some Questions about LES.  Bin Li  Main CFD Forum  2  February 20, 2004 10:58 