[Lucas_Gasparino]

There are ways of predicting yplus before meshing, but honestly they're very rough. After simulation, it's a standard, plottable value in CFD post or any other software. In general, you want to keep yplus between 0m1 and 1 for complex models; less than 0.1 is overkill and also starts producing some bad results as well due to bad aspect ratios and volumes close to zero. LES I'd a different beast however: your mesh near the walls should be able to capture large eddies in the layer itself, which will be small nonetheless. I'd advise you to use LES WALE or WMLES, which introduce a bit o Wallace modelling to correct mesh effects. From experience, I used smagorinsky model with a bad yplus and, although it works, you're going to lose a lot of information. For the cylinder however, focus on getting the wake modelled nicely: most of your turbulence will be happening downstream. Make sure to enable data statistics to get mean flow data as well, so later you can separate the fluctuation and verify the rate of dissipation.

Sent from my XT1635-02 using CFD Online Forum mobile app

[calf.Z]

Originally Posted by Blanco

tmp<scalarField> nutLowReWallFunctionFvPatchScalarField::yPlus() const
{
    const label patchi = patch().index();
    const turbulenceModel& turbModel =
        db().lookupObject<turbulenceModel>("turbulenceModel");
    const scalarField& y = turbModel.y()[patchi];
    const tmp<volScalarField> tnu = turbModel.nu();
    const volScalarField& nu = tnu();
    const scalarField& nuw = nu.boundaryField()[patchi];
    const fvPatchVectorField& Uw = turbModel.U().boundaryField()[patchi];

    return y*sqrt(nuw*mag(Uw.snGrad()))/nuw;
}

Andrea

I want to define y+ in my code, and there are yPlus defined in nutLowReWallFunction. So I want to quote them.

But there are some errors. How can I use the definition of yPlus in nutLowReWallFunction to apply it in my code? Thank you.

[esha]

Originally Posted by immortality

I wonder how we can calculate y+ at a laminar flow without any turbulent model? is it possible?

I also want to calculate the value of Y+for laminar case. if you have calculated please let me know. thanks!

[Hughtong]

Originally Posted by fredo490

At first the y+ has nothing to do with CFD but it has to do with the boundary layer theory (this number was created in the 1930's). The definition is here : http://www.cfd-online.com/Wiki/Dimen...stance_(y_plus)

Yes this number has a meaning for a laminar flow but we don't really care for CFD cases. It is mainly used for turbulent flow because the models we use are based on some assumptions that need to be verified.

But when doing DNS in OpenFoam, the turbulenproperties will be laminar, the yPlus also should be calculated.

[fredo490]

Indeed, this is a very good comment and nice corner case

I would also say that if someone is going to run a DNS simulation, he should have at least the basic fundamentals of the theory and this question should not even exist in his mind.

[fredo490]

By the way, if someone wants to learn more about this topic, I would highly recommend Fluid Mechanics 101 videos:
https://www.youtube.com/watch?v=nSdVaF3JnI0
https://www.youtube.com/watch?v=fJDYtEGMgzs
https://www.youtube.com/watch?v=1gSHN99I7L4

[Hughtong]

Originally Posted by fredo490

Indeed, this is a very good comment and nice corner case

I would also say that if someone is going to run a DNS simulation, he should have at least the basic fundamentals of the theory and this question should not even exist in his mind.

Thank you for sharing, the videos are very helpful for me! Recently i'm using OpenFoam to do channel DNS, but I can only calculate yplus after extracting U and the first mesh length, there's not a utility for the "turbulenceProperties -laminar". So I'm quite curious why OpenFoam don't consider this condition, and can only use the yPlus utility when using turburlence model.

[shail546]

Please tell me where to type yPlusRAS to know the y+ in openFoam?
I am typing in the terminal but its showing command not found.

[shail546]

Originally Posted by immortality

hi
Type yPlusRAS or yPlusLES
If you have ras or les turbulency.

Please tell me where to type yPlusRAS to know the y+ in openFoam?
I am typing in the terminal but its showing command not found.