
[Sponsors] 
January 29, 2013, 16:40 

#21  
Senior Member
Join Date: Nov 2012
Posts: 168
Rep Power: 5 
Quote:
Thank you for your reply. About the Smagorinksy model (for compressible flows) in openfoam, I still have three questions: 1, The compressible form of Smagorinksy model use the equation B.D + epsilon =0, this is discussed in the paper (but the models in this paper are for incompressible models): http://pof.aip.org/resource/1/phfle6...sAuthorized=no But they did not the normal form, Eq. (4), and instead they use B.D + epsilon =0, which can be derived from Eq. (5) when the local equilibrium assumption is introduced. In fact, there is another paper by the same author and this paper discussed the compressible LES models: http://pof.aip.org/resource/1/phfle6...sAuthorized=no In compressible Openfoam, they do not use the Smagorinsky model from this paper. Does any one know what is the reason? 2, Return to the first paper, it was mentioned that B.D + epsilon =0 can make model B1 reduce to A1 model. Actually in Openfoam, for TKE k, a algebraic equation is solved. Thus, how B1 can be reduced to A1? 3, I also found that it is difficult to relate these model constants ck and ce in compressible Openfoam to the standard Smagorinky model: nusgs=(Cs*delta)**2*S. Your any comments and suggestions are welcome. 

January 31, 2013, 05:32 

#22  
Member
Gregor Olenik
Join Date: Jun 2009
Location: http://greole.github.io/
Posts: 80
Rep Power: 9 
Quote:
 c_B*rho*K^2*Delta^(1) = 2/3 rho*K*trace(D)2*c_k*Delta*D^2 in the incompressible case: 2/3 rho*K*trace(D) = 0 (continuity) thus k=c_B/c_K Delta^(2) D^2 which gives with nu_t = c_k Delta sqrt(k) = sqrt(c_k^3/c_e)Delta^2 D Quote:
I dont know if i am getting Q1 ?! 

June 19, 2013, 14:52 

#23 
New Member
Rajesh Kumar
Join Date: Apr 2009
Posts: 24
Rep Power: 9 
Hi Mr. Gregor
Please throw some light on why k is defined in two ways in the smagorinsky model. // Member Functions // Return SGS kinetic energy // calculated from the given velocity gradient tmp<volScalarField> k(const tmp<volTensorField>& gradU) const { volSymmTensorField D(symm(gradU)); volScalarField a(ce_/delta()); volScalarField b((2.0/3.0)*tr(D)); volScalarField c(2*ck_*delta()*(dev(D) && D)); return sqr((b + sqrt(sqr(b) + 4*a*c))/(2*a)); } // Return SGS kinetic energy virtual tmp<volScalarField> k() const { return k(fvc::grad(U())); } 

June 26, 2013, 05:32 

#24 
Member
Gregor Olenik
Join Date: Jun 2009
Location: http://greole.github.io/
Posts: 80
Rep Power: 9 
well if you have a close look you'll see that the second function
Code:
virtual tmp<volScalarField> k() const Code:
tmp<volScalarField> k(const tmp<volTensorField>& gradU) const 

June 28, 2013, 19:21 

#25 
New Member
Rajesh Kumar
Join Date: Apr 2009
Posts: 24
Rep Power: 9 
Thanx Gregor for the explanation. I observed a peculiar thing in the Smagorinsky Model.
For compressible Smagorinsky Ck = 0.02 For Incompressible Smagorinsky Ck = 0.094 I think this is wrong. 

November 16, 2013, 13:12 

#26 
Senior Member
Join Date: Jan 2013
Posts: 310
Rep Power: 6 
Hi All,
In Openfoam, the model constants for compressible Smagorisnky model are: Ck=0.01 Ce=1.048 I checked the following paper: Fureby, C On subgrid scale modelling in large eddy simulation of compressible fluid flow. Physics of Fluid, 8(5) 1996. In this paper, there are four models for the subgrid scale modelling, but I found that no models exactly correspond to the one used in the Openfoam code (compressible). Actually, in openfoam, the compressible Smagorisnky model is not a standard. I did not found how the two model constant (ck and Ce) come from and if there are validated by the experimental data. Does anybody know how these two constants come from? Thank you. 

September 9, 2014, 07:22 

#27 
New Member
Hans Barósz
Join Date: May 2014
Posts: 22
Rep Power: 4 
Hi openfoammaofnepo,
have you found an answer to your question about the coefficients? I find it very difficult to resolve the OpenFoam coefficients c_k and c_e from the compressible smagorinsky model, and I wasnt able to do so yet. I would appreciate any help. From what gregor already posted (I think there is a mistake in it), I was able to derive the quadratic equation for k, which is solved in the model. But I cant figure out the relationship between the OpenFOAM smagorinsky and the one published by Fureby (1996). Fureby says: muSgs = rho*c_d*delta^2*D with the coefficient c_d = 0.038 OpenFoam says: muSgs = ck*rho*sqrt(k)*delta with ck = 0,02. There are of course coefficients in sqrt(k), but my main problem is that I cant calculate the resulting coefficient! In http://croccolab.umd.edu/publication...FD00_vol13.pdf on page 5 it is said that the final coefficient should be the square of the standard smagorinsky coefficient Ck = 0.16. I am very confused now. Please dont hesitate to give me your hints. 

Thread Tools  
Display Modes  


Similar Threads  
Thread  Thread Starter  Forum  Replies  Last Post 
LESSmagorinsky model  CFDUSERIN  CFX  5  August 28, 2008 00:43 
Smagorinsky model  sylvain91  OpenFOAM Running, Solving & CFD  0  June 22, 2006 06:29 
Smagorinsky Subgrid Model  Craig Johansen  CFX  1  October 13, 2004 03:10 
Smagorinsky closure model for LES  Jimmy  FLUENT  0  December 18, 2002 05:33 
Dynamic Smagorinsky model  Tim  Main CFD Forum  7  May 29, 2002 07:37 