Hello, I was taking a look
 

Hello,

I was taking a look at the calculation of y-plus with the checkYPlus utility, but isn't this in fact a check for y-star?

Y-star = rho*Cmu^0.25*sqrt(k)*y/mu
y-plus = rho*sqrt(tau_w/rho_w)*y/mu

Thus Y-star is related to the turbulent kinetic energy and y-plus to the wallshearstress, or am I now overlooking something?

kind regards
Guido

Hey Guido

I'm running some calculations with RSTMs and while checking y+ for the first cell I recognized the same.


y+ = Utau*y/nu with Utau = SQRT(Tau_wall/rho)

Meanwhile did you figured out something to avoid this curio?

Thanks and regards,
Sven

although its old.but I have found that yPlusRAS uses y* furmula in return of y+.could anyone guide me or give an explanation on why?:)

Dear Ehsan,
Check out this post, Niklas has came up with a solution by modifying the file yPlusLES.C:

http://www.cfd-online.com/Forums/ope...tml#post280296

best

thanks Mojtaba.
but my case is compressible.do you know how to change the incompressible yPlus code?

and also it doesn't compile in OF 2.2.0:

I don't know how but maybe it is something like what Niklas did. modifying yPlusLES to be used for RAS for compressible flows.

Which file did you use to compile?

the second file he has put there.are they only for LES?

I think you got to modify them in order to use them with RAS models.
the uploaded file is the utility I used for incompressible flows using RAS models.

it gives this error.maybe it should modified for 2.2.0 version.

Well, this is strange. I compiled for OF 2.1 and it worked fine.
There is no nutWallFunctionFvPatchScalarField.H inside yPlus.C that it can't find. instead nutkWallFunctionFvPatchScalarField.H is used. Are you sure you are using the file I uploaded?
I don't know if it has changed in OF 2.2 or not. maybe its dependency issues.

yes.I use that file.
can anyone help how change it to adapt to 2.2.0 version?

in mutkWallFunctionFvPatchScalarField the equation uses for y-plus is related to y-star not y-plus

Code:
Code:
y-star is related to turbulent kinetic energy while y-plus relates to friction velocity(that relates to wall skin friction in its own)

in the code of mut
:
Code:
thanks.

could anyone have a look to resolve the error?

Hi Mojtaba,

I used your yPlus utility. I ran a case using KKLOmega turbulence model with simpleFoam solver. I ran your yPlus utility after the simulation was completed. For every time step I am getting values of yPlus as "min=0, max=0, average=0". How to fix this?

Also, the yPlusRAS utility that comes with OF-2.1.1 also has the problem of reporting y* instead of y+?

Regards,
Mayank

Hi,

My understanding of this issue is that the quantities of y-plus and y-star are approximately the same for equilibrium turbulent boundary layers. In equilibrium turbulent boundary layers, the friction velocity u_tau is approximately equal to Cmu^0.25*sqrt(k) in the log-law region, which is exactly the definition of u_star.

However, in local non-equilibrium conditions such as separation and re-attachment, a deviation exists between u_tau and u_star, as u_tau is no longer approximately equal to Cmu^0.25*sqrt(k).

I look forward to anyone correcting me if I am wrong here.

As regards OpenFOAM , nutLowReWallFunctions/mutLowReWallFunctions calculates y-plus according to the wall shear stress and the friction velocity. Other wall functions for nut/mut, such as mutkWallFunction, calculates y-plus in the way of calculating y-star.

You can borrow the code for the calculation of y-plus in nutLowReWallFunctionFvPatchScalarField.C/mutLowReWallFunctionFvPatchScalarField.C to write your own utility or function object to calculate y-plus.

Whether the utility calculates y+ or y* seems to depend on the OpenFOAM version you're using. The latest versions of both OpenFOAM and OpenFOAM+ seems to use the equation for y+. In older versions, it can be either y+, y* or depending on the turbulence model.

OpenFOAM 3.0.x:
OpenFOAM-3.0.x/src/postProcessing/functionObjects/utilities/yPlus/yPlus.C
in void Foam::yPlus::execute()
where
OpenFOAM 4.x, OpenFOAM latest (dev):
OpenFOAM-4.x/src/functionObjects/field/yPlus/yPlus.C
in void Foam::functionObjects::yPlus::calcYPlus(turbModel, mesh, yPlus)
OpenFOAM-dev/src/functionObjects/field/yPlus/yPlus.C
in void Foam::functionObjects::yPlus::calcYPlus(turbModel, yPlus)
OpenFOAM+ v1706:
src/functionObjects/field/yPlus/yPlus.C
in bool Foam::functionObjects::yPlus::execute()

That's what I want to do. But how can I borrow the the calculation of y-plus in my own turbulence model code successfully? There appears no declaration of yPlus or other mistakes. Thank you.