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March 3, 2016, 08:16 
Wrong yPlus calculation in OF 3.0.1?

#1 
Senior Member
Alex
Join Date: Jan 2014
Posts: 126
Rep Power: 10 
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 

March 7, 2016, 04:01 
yStar value instead of yPlus

#2 
Senior Member
Fabian Roesler
Join Date: Mar 2009
Location: Germany
Posts: 213
Rep Power: 16 
Hi
OpenFOAM is actually calculating a yStar value instead of yPlus. This can lead to different values. Check this link: http://www.cfdonline.com/Forums/ope...lusystar.html Cheers Fabian 

March 7, 2016, 04:41 

#3 
Senior Member
Alex
Join Date: Jan 2014
Posts: 126
Rep Power: 10 
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 

May 18, 2016, 04:22 

#4 
Member
Paul Palladium
Join Date: Jan 2016
Posts: 90
Rep Power: 8 
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 ! 

September 7, 2016, 05:19 

#5 
Member
Paul Palladium
Join Date: Jan 2016
Posts: 90
Rep Power: 8 
Still any idea about how to use this utility ?


September 8, 2016, 09:31 

#6  
Member
Paul Palladium
Join Date: Jan 2016
Posts: 90
Rep Power: 8 
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 Quote:


December 17, 2016, 08:40 

#7 
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Bashar
Join Date: Jul 2015
Posts: 74
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January 27, 2017, 10:53 
Help with Y+ openfoam 4.0

#8 
Member
Fredi Cenci
Join Date: Dec 2016
Posts: 38
Rep Power: 7 
Hey guys, I've been trying to simulate the flow over a 3D 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. 

February 1, 2017, 22:04 
File

#9  
Member
Fredi Cenci
Join Date: Dec 2016
Posts: 38
Rep Power: 7 
Quote:
Thanks a lot! 

April 30, 2017, 12:00 

#10  
Retired Super Moderator
Bruno Santos
Join Date: Mar 2009
Location: Lisbon, Portugal
Posts: 10,965
Blog Entries: 45
Rep Power: 126 
Greetings to all!
I'm late to the following questions, but since I did some research into this, here are the answers: Quote:
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 sublayer, 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). Quote:
Code:
echo $FOAM_RUN Best regards, Bruno
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Tags 
komega sst model, openfoam, wall shear stress, y+ value, yplus 
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