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July 28, 2010, 19:53 

#61  
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Alberto Passalacqua
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The effective viscosity is nuEff = nu + nuSGS, being nu the molecular viscosity. As a consequence, clipping the effective viscosity nuEff to zero does not prevent backscattering, or all the effort of the dynamic procedure would lose at least part of its importance. It is not possible to "directly remove the averaging", you need to implement some procedure to ensure that you do not divide by zero when computing the dynamic coefficients. There are different ways of achieving this reported in the literature. We used a very simple one, which consists in performing a local average over the neighbours. In terms of stability, it seems to work fine, and results are in agreement with what reported by other Authors. Best,
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Alberto Passalacqua GeekoCFD  A free distribution based on openSUSE 64 bit with CFD tools, including OpenFOAM. Available as in both physical and virtual formats. OpenQBMM  An opensource implementation of quadraturebased moment methods. To obtain more accurate answers, please specify the version of OpenFOAM you are using. Last edited by alberto; July 28, 2010 at 19:55. Reason: Typo 

July 28, 2010, 20:16 

#62  
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Alberto Passalacqua
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I think I saw part of those results, since they were in the paper Andrea sent me. Did you try to run one case with the incompressible version of the model to see if the same behaviour is present? Best,
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July 29, 2010, 00:31 

#63 
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July 29, 2010, 03:42 

#64  
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Filippo Maria Denaro
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yes it is possible you have seen them since Andrea was asking for getting opinions from people involved in OF. At present I am coordinator of the group LESinItaly, collecting activities from the Politecnico of Milano, Torino, Unvisersity of Pisa/Udine and Napoli. We have a project for testing several CFD/LES codes on the turbulent channel flow. With Andrea just decided a couple of days ago to repeat the same simulation with the incompressible version of the code and see what happens in the spectra. We have now to run and wait for the solution to get an energy equilibrium state and sample the fields for the statistics. Some time is therefore required.... However, the compressible code gave no peaks in the energy spectra at Re_tau=180. But the lenght of the longitudinal domain is wider, maybe permits to dissipate acoustic waves, also because the dissipation is greater.... Do you have some experience of spurious solutions with the compressible version of PISO in lowMach flows?? Thanks Filippo 

July 29, 2010, 03:45 

#65 
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Filippo Maria Denaro
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Hi, perhaps if the filter lenght is ensured to lie in the inertial region for all scales, it is common to suppose that SGS scales have a quite universal (and homogeneous) behaviour .... Often the HIT is at basis of several SGS models used for nonhomogenous flows... But this is one of the controversial issues in LES ... 

August 4, 2010, 06:06 
Clip!

#66 
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Hi,
Does anyone has a journal paper for "clipping" of effective viscosity such that (nuSGS+nu)>0 in dynamic LES mehod? Sincerely, Maani 

August 4, 2010, 07:56 

#67  
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Filippo Maria Denaro
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Quote:
F.M. Denaro, G. De Stefano, A new development of the dynamic procedure in largeeddy simulation based on a Finite Volume integral approach. Application to stratified turbulence. Theor. Comp. Fluid Dyn., online DOI10.1007/s001620100202x, 2010 

August 4, 2010, 11:47 
QUICK shceme

#68 
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Thanks Filippo
Please excuse me. But also I wanted to know if anyone has used QUICK scheme in LES despite it is too dissipative. According to Mittal and Moin (1997) the QUICK is too dissipative for reactive flows and generated noise flows and there central schemes should be used. But for ordinary flows it is nearly sufficient. I would appreciate very much it if anyone could tell me about any resent journal articles in which QUICK (or BQUICK) is used. Sincerely, Maani Last edited by mmahdinia; August 4, 2010 at 12:06. 

August 4, 2010, 12:02 

#69  
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Filippo Maria Denaro
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Quote:
There are many argumentations about using upwind schemes, I also published a paper on JCP regarding their using ... However: 1) QUICK is not suitable at all for unsteady flows, it was developed for steady flows, therefore is not only a problem of using it in LES. The unsteady version was developed by Leonard, called QUICKEST, and it is quite different having a timeintagration builtin inside. 2) More in general, upwind schemes are wellsuited for DNS since their artificial dissipation lie in the part of the spectrum where the molecular dissipation is acted on. But in LES, the action of the numerical dissipation acts on the resolved scales, prevalently in the highest part of the resolved spectrum, therefore even if you provide an "ideal" perfect SGS model it would not work properly since of the artificial smoothing of the resolved components. 3) However, some high order (more than third order) upwind schemes implemented in the FV framework, along with some suitable recovering of the smoothed components (deconvolution) can be used. Mittal and Moin showed unsatisfactory results by using FD upwind. You can find on my paper on JCP the discussion about the differences Regards Filippo 

August 4, 2010, 15:42 

#70  
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Alberto Passalacqua
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Quote:
Best,
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Alberto Passalacqua GeekoCFD  A free distribution based on openSUSE 64 bit with CFD tools, including OpenFOAM. Available as in both physical and virtual formats. OpenQBMM  An opensource implementation of quadraturebased moment methods. To obtain more accurate answers, please specify the version of OpenFOAM you are using. 

September 3, 2010, 12:21 
Coeffs/averaging planes

#71 
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Matt James
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With all this talk of openFOAM's unfaithful implementation of dynSmag, it seems a good place to ask what the code is using the supplied coefficient for
i.e. in CASE/constant/LESProperties : dynSmagorinskyCoeffs { filter simple; ce 1.048; } is ce used to define the second filtering size (~ in Germano 1991) or what? I've looked through dynSmagorinsky.C for the answer but can't seem to find "ce" even used anywhere. Also, I suspect it's not a trivial endeavor to change the code to average over horizontal planes rather than the entire domain? 

September 3, 2010, 19:55 

#72  
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Alberto Passalacqua
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Hello,
Quote:
Quote:
Best,
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Alberto Passalacqua GeekoCFD  A free distribution based on openSUSE 64 bit with CFD tools, including OpenFOAM. Available as in both physical and virtual formats. OpenQBMM  An opensource implementation of quadraturebased moment methods. To obtain more accurate answers, please specify the version of OpenFOAM you are using. 

September 7, 2010, 15:01 

#73  
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Matt James
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RIght, Cs is determined by the algorithm. I was unsure of what you meant by dissipation rate, but it appears as though we're talking about the same thing in different ways. I wondered if Ce is just the ratio of the coarse filter to the gridsized filter. (Ce=delta_tilde/delta_bar). After some more reading it appears that this is indeed what Ce adjusts.
Quote:
Thanks very much for your speedy and helpful response, as usual. 

December 22, 2010, 14:36 

#74 
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Filippo Maria Denaro
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Hi,
just coming back to this topic in order to acquire further insight on the dynamic procedure implemented in the compressible version of OF. In this case, the computed eddy viscosity is local, not averaged on the whole volume, that's right? It is allowed to have some negative value (corresponding to an energy backscatter) or the value has a clipping? Many thanks Filippo 

December 22, 2010, 16:18 

#75 
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Alberto Passalacqua
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Hi Filippo (and happy holiday!),
there is no dynamic Smagorinsky model for compressible flow implemented in OpenFOAM currently, just the standard Smagorinsky for compressible flows. There is the dynOneEqEddy model, however, which averages the coefficients on the whole domain. Notice that nuSgs is local also in the dynSmagorinsky implementation in OpenFOAM, since it depends on the local value of the magnitude of the filtered deformation rate tensor. What is not local, but averaged, is the coefficient Cs, which should be local instead. Best,
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Alberto Passalacqua GeekoCFD  A free distribution based on openSUSE 64 bit with CFD tools, including OpenFOAM. Available as in both physical and virtual formats. OpenQBMM  An opensource implementation of quadraturebased moment methods. To obtain more accurate answers, please specify the version of OpenFOAM you are using. 

December 22, 2010, 17:03 

#76  
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Filippo Maria Denaro
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Quote:
thanks for your reply, I wish you a Merry Christmas Filippo 

January 10, 2011, 08:11 

#77 
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Thomas Jung
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talking about dynamic smagorinsky reminded me of this this thread:
Improved implementation of dynamic Smagorinsky 

February 15, 2011, 20:21 

#78 
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hello!
im working on the tutorial pitzDaily (incompressible/pisoFoam) in LES mode with the dynSmagorinsky model and i have a question: if i'm right, i only need the p, U and k files for this case. according to dynSmagorinsky.C . but with deleting the nusgs file in folder 0 i get the error message which tells me, that nusgs file is missing. why?? best regards grandgo 

February 16, 2011, 05:32 

#79 
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Dr. Fabian Schlegel
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This is easy to answer. Please have a look to dynSmagorinsky.C:
Code:
void dynSmagorinsky::updateSubGridScaleFields(const volSymmTensorField& D) { nuSgs_ = cD(D)*sqr(delta())*sqrt(magSqr(D)); nuSgs_.correctBoundaryConditions(); } 

February 21, 2011, 08:39 

#80  
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