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July 3, 2014, 09:51 
FSAS coefficient in komega SST SAS model

#1 
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Vesselin Krastev
Join Date: Jan 2010
Location: University of Tor Vergata, Rome
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Hi all,
I'm doing some work with the SAS model and one of the first checks I've done of the OF (2.2.x) implementation follows the original sources. In the .H source file, it is stated that the implementation follows: DESider A European Effort on Hybrid RANSLES Modelling: Results of the EuropeanUnion Funded Project, 2004  2007 (Notes on Numerical Fluid Mechanics and Multidisciplinary Design). Chapter 2, section 8 Formulation of the ScaleAdaptive Simulation (SAS) Model during the DESIDER Project. Published in SpringerVerlag Berlin Heidelberg 2009. F. R. Menter and Y. Egorov. Well, I don't have the above mentioned reference but I have the following two: "Development and Application of SSTSAS Turbulence Model in the DESIDER Project" Y. Egorov and F. Menter, S.H. Peng and W. Haase (Eds.): Adv. in Hybrid RANSLES Modelling, NNFM 97, pp. 261–270, 2008. "The ScaleAdaptive Simulation Method for Unsteady Turbulent Flow Predictions. Part 1: Theory and Model Description" Flow Turbulence Combust (2010) 85:113–138 Both of them define the QSAS term (additional term in the omegaequation which distinguishes the RANS and SAS SST formulations) in the same way, the only differences being in the formal rearrangement of some constants. The OpenFOAM implementation (either 2.2.x or 2.3.x) is different, as: 1) the whole QSAS term is multiplied by FSAS=1.25; 2) apart from multiplying constants, the QSAS term is formulated as QSAS=max(C,0), where C=AB. In the references above, the B term contains a CSAS=2 constant, which is not there in the OF version. Aside from the fact that I really don't like when people writes "I have implemented this like that" when, in fact, this is not true, can someone comment on why these discrepancies are there? I haven't find any tread about this in the forum, so I guess it will be useful to start a new one. Thank you in advance V. 

July 3, 2014, 10:14 

#2 
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Chris Sideroff
Join Date: Mar 2009
Location: Ottawa, ON, CAN
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It's not the only one. The forums are littered with such discussions. I encourage you to review all of the turbulence models to the published definitions.


July 3, 2014, 10:38 

#3  
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Vesselin Krastev
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Location: University of Tor Vergata, Rome
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Quote:
Regards V. 

July 3, 2014, 12:08 

#4  
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Chris Sideroff
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Quote:
But ultimately it's up to end user to properly verify and validate any model they use and never assume it's correct by default. Caveat emptor. That's my $0.02. 

July 3, 2014, 12:25 

#5  
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Vesselin Krastev
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Location: University of Tor Vergata, Rome
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Quote:
Anyway, thank you for the interesting discussion (and if someone knows the practical reasons for the differences in the OF SAS implementation, it is still welcome here) V. 

July 3, 2014, 13:20 

#6  
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Chris Sideroff
Join Date: Mar 2009
Location: Ottawa, ON, CAN
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Quote:
What I will say is that I've given up to trying to figure "why" they were modified and simply implement my own version following a published/accepted implementation. A good example is the OpenFOAM SA RAS model. For some reason, they choose the version with the fv3 term. However, the original authors even recommend against using it. See here: http://turbmodels.larc.nasa.gov/spalart.html and search for fv3. Rather than understand their reason for choosing that model, I simply implemented the "standard" SA with curvature correction. 

July 4, 2014, 03:12 

#7  
Senior Member
Vesselin Krastev
Join Date: Jan 2010
Location: University of Tor Vergata, Rome
Posts: 368
Rep Power: 12 
Quote:
Best V. 

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