[Kina]

Hi there,
I am currently investigating the flow around a flatback airfoil and I have built a script that calculates the flow and mesh data for me and puts it into a usable BlockMeshDict file. My calculated first cell layer height correlates well to the CFD Online yPlus calculator so the mistake should not be in my formulas. When I view the mesh in ParaFoam and take the ruler to measure my first cell height it also correlates well to the input data.
However, when I use the yPlus command, I get values of 3 to 3.5 whereas my input y+ is 1 or lower.
Also, the wall shear stress calculated by OF is about 70 times higher than the shear calculated by the Schlichting formula.

Does anyone know the physics behind the OF yPlus calculation?
I am using the yPlus command for t=0 so the values should match my estimation with the initial conditions.

I am using incompressible pisoFoam with kOmegaSST model.

Cheers
Alex

[fabian_roesler]

Hi

OpenFOAM is actually calculating a yStar value instead of yPlus. This can lead to different values.
Check this link:
http://www.cfd-online.com/Forums/ope...lus-ystar.html

Cheers

Fabian

[Kina]

Hi Fabian,

thanks for you reply! So even if OF is saying 'yPlus', it's just the y* value?
That's still a little confusing.
I thought they would have sorted that out by v3.0.1.

Cheers
Alex

[Fauster]

Hi !

I am a bit lost (I am a beginners with OF). I read all these threads about yplus calculation with openfoam but I still don't know how to calcul y+.

I understand that the command yPlus compute y* and not y+. Then I download the utility of Mojtaba.a on post 8. But how it works ? What do I need to do ? I try to replace in

openfoam30/src/postProcessing/functionObjects/utilities/yplus but without success

I guess I need to recompile openfoam. How can I do that ?

Thanks for your help !

[Fauster]

Still any idea about how to use this utility ?

[Fauster]

I guess I found out how to install/compile this application.

1. replace the old directory "yPlus" (locate at openFoam30/applications/utilities/postProcessing/wall) by the new one :

https://github.com/OpenFOAM/OpenFOAM...ing/wall/yPlus

2. go in the directory "make" (openFoam30/applications/utilities/postProcessing/wall/yPlus) and type

wmake

Maybe it's better to recompile all the applications by lauching Allwmake in the directory applications. I can't tell you.

[Bashar]

Thanks for the information.


Sent from my iPhone using CFD Online Forum mobile app

[fredicenci]

Hey guys, I've been trying to simulate the flow over a 3-D cylinder. I am using wall functions and it is known that Y+ should be 30 < y+ <100 not over 300. I used this calculator http://www.pointwise.com/yplus/ to estimate my first cell hight.

Well, I used the function yPlus in Openfoam 4.0 and seems like the results are different from what I calculated. I've taken a look over this issue, seems like y+ in older versions of openfoam is actually y* and I believe that this is my problem.

How can i get out of this? should I stick with y* and control the y* of my mesh instead y+? which is the ideal value of y* for wall functions?

Thanks a lot.

[fredicenci]

Originally Posted by Fauster

I guess I found out how to install/compile this application.

1. replace the old directory "yPlus" (locate at openFoam30/applications/utilities/postProcessing/wall) by the new one :

https://github.com/OpenFOAM/OpenFOAM...ing/wall/yPlus

2. go in the directory "make" (openFoam30/applications/utilities/postProcessing/wall/yPlus) and type


Maybe it's better to recompile all the applications by lauching Allwmake in the directory applications. I can't tell you.

Hey guys, I couldn't find the directory (openFoam30/applications/utilities/postProcessing/wall/yPlus). I guess I can't find it because i am using OpenFoam 4.. Do you know where is the directory of the file that I should replace?

Thanks a lot!

[wyldckat]

Greetings to all!

I'm late to the following questions, but since I did some research into this, here are the answers:

Originally Posted by fredicenci

How can i get out of this? should I stick with y* and control the y* of my mesh instead y+? which is the ideal value of y* for wall functions?

In this thread: y+ and y* issue - is mostly explained what is y+ and y* for.

From what I can figure out, the y* is the approximation made to y+ based on the wall function values. This also means that it your cell on the wall is large enough and has a y* or y+ high enough, the wall function has to approximate the result either way. But if the cell is small enough, the layer being modelled is for the laminar sub-layer, in which case the y+ calculation is more appropriate, i.e. based on the wall shear stress.

The possible reason that OpenFOAM does not provide the y+ definition, is most likely because the wallShearStress application (OpenFOAM version 2.4.x and older) or function object (as of OpenFOAM 3.0.0, if I'm not mistaken) provides a more accurate depiction of the context in which is should be used, i.e. laminar flow, at least as far as I can figure out (please correct me if I'm wrong).

Originally Posted by fredicenci

Hey guys, I couldn't find the directory (openFoam30/applications/utilities/postProcessing/wall/yPlus). I guess I can't find it because i am using OpenFoam 4.. Do you know where is the directory of the file that I should replace?

You don't need to place it inside OpenFOAM's source code folder. You can place it in the OpenFOAM-assigned user folder and build it there. If you don't know where the user folder is located, run:

echo $FOAM_RUN

and it's the parent folder of the "run" sub-folder.

Best regards,
Bruno