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July 3, 2014, 10:51 |
FSAS coefficient in k-omega SST SAS model
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#1 |
Senior Member
Vesselin Krastev
Join Date: Jan 2010
Location: University of Tor Vergata, Rome
Posts: 368
Rep Power: 20 |
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 RANS-LES Modelling: Results of the European-Union Funded Project, 2004 - 2007 (Notes on Numerical Fluid Mechanics and Multidisciplinary Design). Chapter 2, section 8 Formulation of the Scale-Adaptive Simulation (SAS) Model during the DESIDER Project. Published in Springer-Verlag 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 SST-SAS Turbulence Model in the DESIDER Project" Y. Egorov and F. Menter, S.-H. Peng and W. Haase (Eds.): Adv. in Hybrid RANS-LES Modelling, NNFM 97, pp. 261–270, 2008. "The Scale-Adaptive 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 omega-equation 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=A-B. 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. |
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July 3, 2014, 11:14 |
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#2 |
Senior Member
Chris Sideroff
Join Date: Mar 2009
Location: Ottawa, ON, CAN
Posts: 434
Rep Power: 22 |
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.
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July 3, 2014, 11:38 |
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#3 | |
Senior Member
Vesselin Krastev
Join Date: Jan 2010
Location: University of Tor Vergata, Rome
Posts: 368
Rep Power: 20 |
Quote:
Regards V. |
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July 3, 2014, 13:08 |
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#4 | |
Senior Member
Chris Sideroff
Join Date: Mar 2009
Location: Ottawa, ON, CAN
Posts: 434
Rep Power: 22 |
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. |
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July 3, 2014, 13:25 |
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#5 | |
Senior Member
Vesselin Krastev
Join Date: Jan 2010
Location: University of Tor Vergata, Rome
Posts: 368
Rep Power: 20 |
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. |
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July 3, 2014, 14:20 |
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#6 | |
Senior Member
Chris Sideroff
Join Date: Mar 2009
Location: Ottawa, ON, CAN
Posts: 434
Rep Power: 22 |
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. |
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July 4, 2014, 04:12 |
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#7 | |
Senior Member
Vesselin Krastev
Join Date: Jan 2010
Location: University of Tor Vergata, Rome
Posts: 368
Rep Power: 20 |
Quote:
Best V. |
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January 24, 2018, 12:30 |
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#8 |
New Member
Join Date: Jul 2017
Posts: 9
Rep Power: 9 |
Hi all,
I´m using OpenFOAM 4.x and trying to do a simulation with the kOmegaSSTSAS model. I would like to compare those results with Fluent SAS results. I have been reading almost everything i could find in this forum about the SAS model, but i´m still not able to solve my problem. The problem is: It seems, that the simulation remains in RAS mode. I would be interested, if anyone was able to get good result with the kOmegaSSTSAS..... At the moment, i´m not very happy with what i get. Best regards Flo |
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January 25, 2018, 03:49 |
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#9 |
Member
Andreas P.
Join Date: May 2017
Posts: 41
Rep Power: 9 |
Hi Flo and all others,
I am also investigating on using the SAS turbulence model with OpenFOAM. Nevertheless, i am using Foam-Extend-4.0, where the SAS model is implemented as a LES type and not RAS as it is in OpenFOAM-4.x. In my opinion it would be more consistent to have it as a RAS model, as it is based on the kOmegaSST model, right? Does anyone know, why it is implemented as a LES type in foam-extend-4.0? I managed to get some resolved turbulence with the infamous cylinder flow at Re=30000 as you can see in the image attached (Q-Criterion with isosurfaces of Q=5000). Also I get a strange error message when omega is computed. I searched for this error in the forum and also googled it but all i found was the specific line in the OpenFOAM code of the omegaWallFunction. It seems that in LES there is no G-field accessable when using wall functions? Im not very familiar with LES: From function void omegaWallFunctionFvPatchScalarField::updateCoeffs( ) in file derivedFvPatchFields/wallFunctions/omegaWallFunctions/omegaWallFunction/omegaWallFunctionFvPatchScalarField.C at line 179 Cannot access RASModel::G field for patch CYLINDER. Evaluating as zeroGradient Also in the single timesteps the nonlinear residuals are not converging to my desired tolerance (1e-5 for all variables) either, I am working on that though. I use the pimpleFoam solver and my yPlus values are between 0.26 and 16, averaging around 6.36 as the mesh was originally inteded for Re=3000 as a lowReynolds mesh. As divScheme for U I use the blended scheme with a coefficient of 0.95. I think one has to use a rather high quality discretisation of the divergence Schemes as too diffusive schemes may dampen the turbulent fluctuations too much. What is the case you are looking at, and what is your setup? Especially discretisation schemes would be interesting. Best Andreas Last edited by AndreasPe; January 26, 2018 at 03:08. |
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October 25, 2018, 09:09 |
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#10 | |
Member
Amir
Join Date: Jan 2017
Posts: 32
Rep Power: 9 |
Quote:
I found that this problem comes out when you insert the zero value for the initial condition in omega Field! However it is only happening in foam-extend 4.0 as long as I find! Maybe it is result of division by zero somewhere! Regards, Amir |
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