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Validation of LES models

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Old   May 3, 2015, 09:04
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  #21
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Ashvin Chaudhari
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Hi Xianbei,

Sorry for my late reply. I just noticed it now.

Sure, we have made some comparison between the two solvers: rk4+projection and pisoFoam. I would encourage you to look the following two papers in order to see/learn the differences between RK-4 method and the built-in second order time integration.

1) http://www.sciencedirect.com/science...45793014000334
2) http://www.sciencedirect.com/science...65997814001513

- Ashvin
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Old   December 6, 2016, 08:12
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Mukesh Adlak
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Quote:
Originally Posted by ashvinc9 View Post
Dear all,

I have been running the Open channel flow simulation using wall function approach. I have done done several LES for open channel flow using different wall-functions, Sgs models, numerics, etc, and I would like to share my results for further suggestions and results corrections.

These all simulations are for relatively higher Re_tau, e.g. 7500 to 60000, and in most of the cases, the grid is really coarse. Earlier (in OF 2.0 and 2.1), I was using "nuSgsUSpaldingWallFunction" wall-function but now in the newer version of OF, it also possible to employ almost all the wall-function which are made for RANS. In my case, I have used
nuSgsUSpaldingWallFunction and nutUSpaldingWallFunction, and nutURoughWallFunction for smooth, and rough wall conditions, respectively.

The LES results for open channel flow using wall-functions are reasonably good as well as the velocity solutions agree reasonably well with the Log-law for both smooth and rough wall surfaces. Actually, in all the cases, LES velocity profile underestimate the Log-law in a near-wall region and produce a kink only for few cell centred values after the first one from the wall (please see the results in the attached file: Static_SGSModel.pdf ). I have been trying a lot in order to rid out of that kink near wall boundary but didn't find any reason for such a kink so far. I also discussed with other researcher those who use LES-wall-function approach, and they say that it is common to have a such a behaviour for few cell centred values where the velocity solution go away from the Log-law. Also, that velocity defect occurs only for few cells (or nodes) so it shouldn't be too serious problem in my opinion. Please correct me if I am wrong? Please share your ideas if you have had experienced such a behaviour of velocity using wall function in LES.

SGS model validation:
Since, the coarse grid has been used in almost all LES for open channel flow, the SGS models play important roles in the simulations, and thus, the results are very sensitive to the SGS model used.
From the open channel flow testing, it seems that one equation eddy viscosity (oneEqEddy) model gives good results for the coarse LES. I also tried standard Smagorinsky model but no improvement was observed in the near-wall agreement as well as poor agreement with Log-law compared to that of using oneEqEddy model. When using any Dynamic or Lagrangian SGS models on the same case (same grid, Re_tau, numerical schemes, averaging time, solver), the LES results are really bad, the velocity profile seems to be non-stable as well as the velocity profile is not even comparable to the Log-law.I do not know why any Dynamic or Lagrangian SGS models are not working as good as static SGS model, although they are suppose to produce better results because of the dynamic procedure of finding the model constant. I have tried, e.g. dynOneEqEddy, homogeneousDynOneEqEddy, homogeneousDynSmagorinsky and dynLagrangian SGS models on the same case. But the results are not acceptable. . The results using dynamic SGS models are attached in the file: Dynamic_SGSModel.pdf. Please find the attachments.

Does any one have idea behind this problem? Did I miss something to take into account when employing the dynamic SGS model? I appreciate your comments and suggestions.

Regards,
Ashvin


Hi ashvin,

Can u help me to run dynamicLagrangian SGS model ? I run my case but facing following error :

--> FOAM FATAL ERROR:
incompatible dimensions for operation
[flm[0 4 -5 0 0 0 0] ] + [flm[1 1 -5 0 0 0 0] ]

From function checkMethod(const fvMatrix<Type>&, const fvMatrix<Type>&)
in file /mnt/home/SW/CFDSupportFOAM3.0/OpenFOAM-3.0.x/src/finiteVolume/lnInclude/fvMatrix.C at line 1295.

FOAM aborting

#0 Foam::error:rintStack(Foam::Ostream&) at ??:?
#1 Foam::error::abort() at ??:?
#2 void Foam::checkMethod<double>(Foam::fvMatrix<double> const&, Foam::fvMatrix<double> const&, char const*) at ??:?
#3 Foam::tmp<Foam::fvMatrix<double> > Foam:perator+<double>(Foam::tmp<Foam::fvMatrix<d ouble> > const&, Foam::tmp<Foam::fvMatrix<double> > const&) at ??:?
#4 Foam::LESModels::dynamicLagrangian<Foam::EddyDiffu sivity<Foam::ThermalDiffusivity<Foam::Compressible TurbulenceModel<Foam::fluidThermo> > > >::correct() at ??:?
#5 ? at ??:?
#6 __libc_start_main in "/lib/x86_64-linux-gnu/libc.so.6"
#7 ? at ??:?
Aborted (core dumped)

If u can help, it will be great for my work.
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Old   September 28, 2017, 11:53
Default Validation of pressure-incremental projection scheme + dynLagrangian model
  #23
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Santiago Lopez Castano
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Hi guys,

I just saw your discussion and I wanted to put my 2 cents on the issue. For the last month I have developed a projection method similar (see Guermond&Shen) to that of Chaudhari for the incompressible N-S equations, and as validation I run the classic channel flows with different Re. The difference of my case with oodlesChannel (or channel365) is that I drive my flow using a certain pressure gradient, not by setting some flow.

Additionally, with the risk of sparking a salty debate, its my opinion that whenever you run openFOAM without any turbulence model you should not call your results 'DNS' but more 'NO-MODEL LES'. This given the 2nd-order FV nature of the library itself, and the boundary conditions that you can impose. In fact, calling 'cyclic' boundary conditions 'periodic' carries a promise that the developers where well aware of and thus decided to call them as the former.

In any case, you'll find attached the velocity profiles and RMS of channel flows run using Re_tau=171 and Re_tau=410. In both the dynamic lagrangian model of Meneveau was used. In general results seem decent.
Attached Files
File Type: pdf vel180.pdf (9.9 KB, 14 views)
File Type: pdf RMS180.pdf (17.4 KB, 9 views)
File Type: pdf vel410.pdf (10.4 KB, 8 views)
File Type: pdf RMS410.pdf (11.3 KB, 11 views)

Last edited by Santiago; September 28, 2017 at 12:22. Reason: ChaudharI
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Old   September 29, 2017, 04:16
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  #24
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Albrecht vBoetticher
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Tahnk you, that really helps!
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