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March 13, 2014, 13:20 
Questions to utility yPlusLES

#1 
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I'd like to use yPlusLES (./applications/utilities/postProcessing/wall/yPlusLES/yPlusLES.C). code snippet: Code:
volScalarField::GeometricBoundaryField d = nearWallDist(mesh).y(); volScalarField nuEff(sgsModel>nuEff()); const fvPatchList& patches = mesh.boundary(); const volScalarField nuLam(sgsModel>nu()); forAll(patches, patchi) { const fvPatch& currPatch = patches[patchi]; if (isA<wallFvPatch>(currPatch)) { yPlus.boundaryField()[patchi] = d[patchi] *sqrt ( nuEff.boundaryField()[patchi] *mag(U.boundaryField()[patchi].snGrad()) ) /nuLam.boundaryField()[patchi]; const scalarField& Yp = yPlus.boundaryField()[patchi]; Info<< "Patch " << patchi << " named " << currPatch.name() << " y+ : min: " << gMin(Yp) << " max: " << gMax(Yp) << " average: " << gAverage(Yp) << nl << endl; } } 1) Computation of yPlusLES according to the code snippet above: Calculation of uses nuEff, instead of nu. Whereas nu in the denominator is nuLam (nu). Does this proceeding have a physical or mathematical background? (nuEff could become zero, which is not allowed to happen in the denominator?) 2) Code:
*mag(U.boundaryField()[patchi].snGrad()) Is there a case where you have to replace U by a time averaged field U (UMean?) according to the formula for (RANS)? a) RANS with turbulence model (wall region not resolved) Usage of time averaged U field maybe yes, but since a wall function is used, must anyway be computed differently with regard to the wall function's results??? b) RANS with low Reynolds turbulence model (wall region resolved) Case b) seems to be the only case where it is necessary to use a time averaged field for U. Is this assumption right? c) LES (wall region resolved) Which velocity vector field U has to be used to compute to catch the real physical situation very close to the wall in a LES computation? Is it assumed that we have a laminar flow very close to the wall and therefore no statistical fluctuations in the viscous sublayer? Therefore use mag(U) as the code shows? 3) In case I want to compute not only close to the wall but for the complete flow domain, how do I have to compute ? something like > mag(U.snGrad())??? Thanks to everyone who answers! Aylalisa 

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March 16, 2014, 07:41 

#2  
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Bruno Santos
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Greetings Aylalisa,
Quote:
The details (i.e. background) for y+ can be found here:
In general, the viscosity should never be zero. Quote:
Quote:
Problem is that "nearWallDist" was designed to provide only the "y" values only near the patches/walls, therefore it's not just a matter of changing the calculation around "U". Best regards, Bruno
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March 17, 2014, 12:43 

#3 
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1) agree with Bruno
2) y+ should not be calculated with mean velocity. However for RANS UMean = U (wether you resolve or resolve the viscous sublayer is not important: this is handled by different values of nut at the wall). For LES you could, if respecting the instantaneous velocity constraint is a too tight requirement, timeaverage yPlus directly instead of using time averaged velocity. 3) As far as I remember, the right class to export wall data in the internal field is the wallDistData class (have a look at vanDriestDelta for usage). In that case you could use the tau_wall of the closest wall distance to define internal field y+. Anyhow I agree with Bruno, I don't see the point in doing that. Hope this help, Cosimo
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Cosimo Bianchini Ergon Research s.r.l. Via Panciatichi, 92 50127 Florence  ITALY Tel: +39 055 0763716 Mob: +39 320 9460153 email: cosimo.bianchini@ergonresearch.it URL: www.ergonresearch.it 

March 20, 2014, 08:10 

#4  
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Dear Bruno, Dear Cosimo,
I would like to calculate für a channel flow (LES). So far I can follow Bruno's extension of equation: (1), (2) , (3) , (4) end up with (5), (3) in (2), (2)in (1) > (5) (according to the links). To 1) In formula (5) shows up twice (nominator and denominator). In Bruno's code the used in the nominator is "nuEff", in the denominator whereas is "nuLam". Is the definition of "nuEff" ? Why did you use different values for in the formula for computation of ? Probably stupid question but why didn't you use "nuEff" in denominator as well? From a physical point of view can never be zero, but from a mathematical point of view, it could. Is that the reason for using "nuEff" in the nominator? Does it mean that "nuLam" can't get zero but "nuEff" can? To 2) Quote:
I thought in LES I have to use UMean instead of mag(U)? In the official file "yPlusLES.C" mag(U) is used. Does this mean that you have to timeaverage ? In which flow situation is it possible that the instantaneous velocity constraint is a too tight requirement as you said? To 3) Case: the duct flow (LES) is wall bounded according to bottom and top (z direction), cyclic according to left wall and right wall (y direction). The wall regions (bottom/top) are resolved, no wall function is used. I would like to compare the logarithmic law of the wall with the dimensionless wall units, computed from the results of an LES. Is the plot (yaxis) (xaxis) from y = 0 to y = channel half height the correct way to do that? florian_krause started a discussion and published his code for such a plot. I've seen that distance to the wall y can be computed with wallDist y(mesh, true). Did florian_krause implement what Cosimo described in answer 3)? Code:
if( diffDist <= matchTol){ uTau[cellI] = uTau.boundaryField()[patchi][facei]; break;} Maybe you could clarify these points a bit more Aylalisa 

March 21, 2014, 05:44 

#5 
Member

1) This is to deal with wallfunction approach. In your case at the wall mu_eff = mu so you could use it in both terms without errors but for wall function approach mu_eff != mu so it is more robust to follow the correct definition of tau_w (i.e. using mu_eff) so it can be applied to both cases.
If your nu is going to zero then your flow is inviscid thus I do not see the point in calculating y+. 2) As I said previously I think using UMean is wrong (at least in principle) since its gradients could largely differ from instantaneous velocity one. On the other side verifying on a single instantaneous solution might not be representative because y+ constraints should be verified on each wall point for each time step. In order to avoid this tedious operation you could try timeaveraging y+ and verify which is your margin to 1. 3) This is a code extract showing how to use wallDistData class. It was written to implement Chien LowReynolds kepsilon turbulence model for OpenFOAM1.3. Might be a but outofdate but I guess the relevant part is unchanged. Good luck Code:
forAll(mesh_.boundary(), patchi) { const fvPatch& patch = mesh_.boundary()[patchi]; //Calculate wall properties : velocity gradient, shear stress, friction velocity if (typeid(patch) == typeid(wallFvPatch)) { fvPatchScalarField& wallData = yStar.boundaryField()[patchi]; wallGradU.boundaryField()[patchi] =  U_.boundaryField()[patchi].snGrad(); Tau_wall.boundaryField()[patchi] = ( mesh_.Sf().boundaryField()[patchi] / mesh_.magSf().boundaryField()[patchi]) & Dev.boundaryField()[patchi]; U_tau.boundaryField()[patchi] = sqrt(mag(Tau_wall.boundaryField()[patchi])); wallData = mu().boundaryField()[patchi]/ rho_.boundaryField()[patchi] / U_tau.boundaryField()[patchi]; } } // Assign yStar value to y_.data() wallDistData<wallPointData<scalar> > y_(mesh_, yStar, true); yPlus = y_/y_.data();
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Cosimo Bianchini Ergon Research s.r.l. Via Panciatichi, 92 50127 Florence  ITALY Tel: +39 055 0763716 Mob: +39 320 9460153 email: cosimo.bianchini@ergonresearch.it URL: www.ergonresearch.it 

May 4, 2014, 08:24 

#6  
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shawn qin
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Quote:
But can you also explain why it uses nuLam in the denominator instead of nu, cosimo? Thank you in advance. 

May 6, 2014, 07:01 

#7  
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Dear All,
I still stick on the answers of Bruno and Cosimo and my remaining questions are: I can follow the explanation about whereas I don't understand why is used in the code, respectively why it is necessary to introduce ? According to Cosimo's explanation I understand why the best approach for LES cases finally is to directly timeaverage y+. Which information gives me this verification? Quote:
Is the computation of internalFields (for and ) in principle possible by using functionObjects (type swakExpression)? kind regards, Aylalisa 

May 9, 2014, 04:25 

#8 
Member

Hi Elisabeth and Qin,
1) nuLam is introduced due to y+ definition which is based on a comparison between effective shear on the wall and laminar viscosity. 2) if your timeaveraged y+ is well below 1 you can substantially sure that your grid satisfy LowReynolds y+ requirements in most time steps. 3) Well if it is for post processing you can make y dimensionless simply computing tau_w on the reference location. The other option is to use wallDistData as previously suggested. 4) Don't think it is easily feasible.
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Cosimo Bianchini Ergon Research s.r.l. Via Panciatichi, 92 50127 Florence  ITALY Tel: +39 055 0763716 Mob: +39 320 9460153 email: cosimo.bianchini@ergonresearch.it URL: www.ergonresearch.it 

May 9, 2014, 05:37 

#9 
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Hello Cosimo,
thanks a lot, your answers are absolutely helpful for me! I want to show u+/y+ from a point on the wall, in the viscous layer, in the transition layer and finally in the area where wall turbulence comes up and the last point should be positioned in an area where free shear flow exists. For that graph I need more y+ (and u+) values, therefore I stick on the idea to access/compute internal volume scalar fields for y+ and u+ (instead of a single values). Is that idea wrong? Aylalisa 

May 9, 2014, 09:33 

#10 
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shawn qin
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Thank you, cosimo. So nuLam represents molecular dynamic viscosity in the Laminar flow, right? If so, in a LES computation without use of wall function, the yPlusLES utility gives the y+ defined by the equation in the 1st post: y+=y*u_tau/nu. Am I right?


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