[rogerliu]

My case is unsteady flow past a cylinder using LES. Re=3900
How can I calculate the y-plus?
does y-plus refer the distance from the first grid to the wall? Is it the smaller, the better?

Thanks!

[immortality]

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

[fredo490]

Hello, y+ refers to the size of the mesh next to the wall compared to the fluid behavior. This number helps you to know where is your first cell center compared to the boundary layer thickness.

If you didn't study any boundary layer theory, you have to know that a boundary layer is composed of different parts. The one very close to the wall is dominated by viscous effects (= the viscous layer) and a bit outer you have the log layer.

To get an accurate simulation, your first cell center must be inside the viscous layer. We usually consider that a y+ smaller than 4 is required. Ideally, you should have a y+ smaller than 1 on all your surface. But there is no need to go to 1e-3 !

Be careful, the size of your first cell must correspond to your turbulence model. For example a RAS k-omega SST requires a y+ smaller than 1 but a k-epsilon standard requires a y+ between 30 and 60. I don't know about LES simulation but I guess it should be smaller than 1.

To get the yPlus, you can only do it as a post treatment (because you need to compute the velocity first). To get it, simply type "yPlusRas" or "yPlusLes" depending of your turbulence model. You can also write "yPlusLes -h" to get some help.

For example: yPlusRas -latestTime -compressible will only compute the yplus for your last export and it will apply a compressible correction (only use it with a compressible solver).

To get a rough estimation of the cell size you need next to the wall, you can use this tool:
http://www.cfd-online.com/Tools/yplus.php

[immortality]

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

[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.

[VSass]

I have some problems calculating y+ during a compressible flow. I use Spalart-Allmaras model and the output in the results files is mut, not nut, which is prerequisite for the calculation of y+. Any ideas?

[immortality]

hi
search this site.there are some good threads with y+ for compressible flows.
do you use low-Re or high-Re?

[VSass]

I use high Re.

[immortality]

then use yPlusRAS -compressible its for high-Re models Vasilios.

[VSass]

I use yPlusRAS but it needs the "nut" values for each time step. My solution outputs "mut" files. That's the problem.

[immortality]

type: "yPlusRAS -compressible" not only yPlusRAS

[VSass]

Honestly,thanks!

[VSass]

Well, using yPlusRAS -compressible works, but it plots a value of "0" in the field. Mut has an accepted distribution , but how come the distribution of yPlus is zero?

[immortality]

where in the field? whats your case? if its something like shockTube maybe the flow has not reached there.

[caoyinyue]

Originally Posted by VSass

Well, using yPlusRAS -compressible works, but it plots a value of "0" in the field. Mut has an accepted distribution , but how come the distribution of yPlus is zero?

Hello, I used the yPlusLES for calculating the incompressible flow field. the distribution of yplus is alos zero. It is very strange. Have you found any solution? Could you give me some hint? Thanks very much

[JasonWang3]

Originally Posted by immortality

hi
search this site.there are some good threads with y+ for compressible flows.
do you use low-Re or high-Re?

Hi
I use LowRe model. When I type in ''yPlusRAS -compressible'', y+ is zero in all cells.
I noticed that you mentioned that this command was for HighRe model.
What can I do when I use LowRe model?

[Blanco]

yPlusRAS works both for HighRe and LowRe turbulence models, the only difference is that it uses nut for incompressible flow while it uses mut for compressible flow.

There should be some other problem that gives you y+=0 in your cells...how do you check y+ value?

[JasonWang3]

Originally Posted by Blanco

yPlusRAS works both for HighRe and LowRe turbulence models, the only difference is that it uses nut for incompressible flow while it uses mut for compressible flow.

There should be some other problem that gives you y+=0 in your cells...how do you check y+ value?

I noticed that there is no rho file in the folders. When I look into the file '' yPlusRAS.C ''

IOobject rhoHeader
(
"rho",
runTime.timeName(),
mesh,
IOobject::MUST_READ,
IOobject::NO_WRITE
);

if (!rhoHeader.headerOk())
{
Info<< " no rho field" << endl;
return;
}

Info<< "Reading field rho\n" << endl;
volScalarField rho(rhoHeader, mesh);

This probably means that the rho file must be required when using yPlusRAS.
And I am using sonicFoam, there is phi instead of rho.
So this is the reason why all the plus values is zero.
But still have no idea how to obtain the yPlus value, maybe rho from the state equation and then use yPlusRAS.

Do you have other suggestions? And thanks for your replying.

[Blanco]

Hi Xinguang,

I don't have experience with sonicFoam, but I think for sure you need to define mu and mut -> cinematic viscosity with varying density, laminar and turbulent, in order to perform your compressible simulation. In the results you will get the rho field and it will be used by yPlusRAS to calculate y+ values.

[JasonWang3]

Hi Andrea

As you said, yPlusRAS could give the yplus at the wall after I generate the rho fields. But the value is quite small than I expected, since I calculate yplus using the yplus calculator:
http://geolab.larc.nasa.gov/APPS/YPlus/
yPlusRAS give the value of 0.03 compare with 1 of the yplus calculator.

But in the website, there is no information about the calculation. So I think I should calculate the yplus by hand to compare with the results of OF.

Thanks for your reply.