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|ID||Project||Category||View Status||Date Submitted||Last Update|
|0000099||OpenFOAM||[All Projects] Bug||public||2010-12-05 21:24||2010-12-06 08:53|
|Fixed in Version|
|Summary||0000099: Dynamic Smagorinsky model with local coefficient values|
|Description||The current implementation of the dynamic Smagorinsky model in OpenFOAM uses a domain-averaged coefficient to compute the SGS stress tensor. This is not appropriate, since it removes the major advantage of the Smagorinsky model, which should tend to the correct limit near walls, and should be able to describe backscattering (negative SGS viscosities). I replaced the domain average with a local face average to compute the coefficients: cI=<K*m>_face/<m*m>_face, and cD=<L.M>_face/<M.M>_face, where <...>_face is fvc::average(...). Additionally, to stabilize the numerical procedure, the SGS viscosity is limited so that it cannot become negative and smaller than -nu (laminar viscosity).|
|Steps To Reproduce||N/A|
|Additional Information||- I called the model dynLocalAverageSmagorinsky to distinguish it from the currently implemented dynSmagorinsky, but it is actually the same model, with the differences explained above.|
- The header contains my copyright statement, since the code is in my GitHub repository ( git://github.com/AlbertoPa/dynLocalAverageSmagorinsky.git ). If you include the code in OpenFOAM, consider the copyright transferred to OpenCFD, and replace the header.
|Tags||No tags attached.|
dynLocalAverageSmagorinsky.tar.gz (15,648 bytes)
||The dynamic Smagorinsky model dynSmagorinsky does what it is designed to do which is model the sub-grid turbulence in homogeneous turbulence. If you want to use a dynamic model with local averaging you can use locDynOneEqEddy but the averaging region is small and rather arbitrary and causes problem for complex flows. An alternative would be to use a form of Lagrangian averaging for the coefficients....|
I disagree. The dynamic Smagorinsky model, as implemented in OpenFOAM, does not reflect the original implementation. The current implementation uses a constant value of the coefficients in the whole computational domain, which is not what is done in the dynamic model, where Cs should properly tend to small values when approaching, for example, a wall, or where the flow laminarizes.
The current implementation in OpenFOAM is simply a traditional Smagorinsky model with Cs determined from the flow, but it loses one of the fundamental advantages of the dynamic procedure, which is being able to describe what happens near walls, or where the flow laminarizes, without any need of damping functions.
The original model of Germano relied on averaging along the homogeneous direction, since one existed in his channel flow, but Cs could change normally to walls. However the averaging was a justified workaround to avoid numerical difficulties.
Averaging on cell faces does not really introduce any major error, given the grid resolution required to perform LES, and it mitigates the chances of having a very small number in the division to compute Cs.
The implementation I attached has been widely tested on channel flows, and in much more complicated cases in chemical reactors for mixing applications, without observing numerical difficulties and with results in excellent agreement with PIV experiments.
||Different averaging is appropriate for different purposes and the averaging in dynSmagorinsky is the most appropriate for the purpose for which it was written. Perhaps you would prefer that dynSmagorinsky were renamed but given that it is a name we created and not the name Germano or others used I am not sure what the problem is with it.|
||In the literature, "dynamic Smagorinsky" identifies a model where the coefficient is computed locally and based on the flow conditions. Your implementation follows the dynamic procedure suggested by Germano, but then it throws away its advantages by averaging on the whole computational domain. I understand why this was done, however it confused quite some users, who believed to deal with the dynamic Smagorinsky model they find described in the literature and implemented in other codes.|
||There are many many forms of dynamic SGS models which use various form of averaging. The one under the name dynSmagorinsky uses the recommended and appropriate averaging for simulating homogeneous turbulence which is what we implemented it for. If you are not simulating homogeneous don't use it.|
Yes, I am aware there are various approaches to define the dynamic coefficient. However, I do not see the assumption of homogeneous turbulence clarified anywhere (you have to check the implementation to see it), and it is surely less general and more arbitrary than a local average, if implemented in a general code as OpenFOAM is.
To conclude, I based my implementation on what is usually done for LES on unstructured grids (S.E. Kim, Large Eddy Simulation Using an Unstructured Mesh Based Finite-Volume Solver, American Institute of Aeronautics and Astronautics, 34th Fluid Dynamics Conference and Exhibit, AIAA-2004-2548, June 2004), but allowing back-scattering. Their approach has been implemented in commercial codes, which additionally limit the coefficient to zero, and they also fix an upper limit for it. Both these assumptions are however arbitrary, and I did not include them. Clipping the effective viscosity might be rough, but it has a physical foundation, and successfully stabilizes the code.
||I agree that the description in dynSmagorinsky is not sufficient and we will elaborate in the next version.|
|2010-12-05 21:24||albertop||New Issue|
|2010-12-05 21:24||albertop||File Added: dynLocalAverageSmagorinsky.tar.gz|
|2010-12-05 21:42||henry||Note Added: 0000156|
|2010-12-05 21:42||henry||Status||new => closed|
|2010-12-05 21:42||henry||Assigned To||=> henry|
|2010-12-05 21:42||henry||Resolution||open => fixed|
|2010-12-05 22:36||albertop||Note Added: 0000158|
|2010-12-05 22:36||albertop||Status||closed => feedback|
|2010-12-05 22:36||albertop||Resolution||fixed => reopened|
|2010-12-05 22:40||henry||Note Added: 0000159|
|2010-12-05 22:41||henry||Status||feedback => closed|
|2010-12-05 22:41||henry||Resolution||reopened => fixed|
|2010-12-05 23:05||albertop||Note Added: 0000160|
|2010-12-05 23:05||albertop||Status||closed => feedback|
|2010-12-05 23:05||albertop||Resolution||fixed => reopened|
|2010-12-05 23:08||henry||Note Added: 0000161|
|2010-12-05 23:08||henry||Status||feedback => closed|
|2010-12-05 23:08||henry||Resolution||reopened => fixed|
|2010-12-06 03:30||albertop||Note Added: 0000162|
|2010-12-06 03:30||albertop||Status||closed => feedback|
|2010-12-06 03:30||albertop||Resolution||fixed => reopened|
|2010-12-06 08:38||henry||Note Added: 0000164|
|2010-12-06 08:52||henry||Status||feedback => acknowledged|
|2010-12-06 08:53||henry||Status||acknowledged => resolved|
|2010-12-06 08:53||henry||Resolution||reopened => suspended|