Wrong yPlus calculation in OF 3.0.1?
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 |
yStar value instead of yPlus
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 |
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 |
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 ! |
Still any idea about how to use this utility ?
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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:
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Help with Y+ openfoam 4.0
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. |
File
Quote:
Thanks a lot! |
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 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). Quote:
Code:
echo $FOAM_RUN Best regards, Bruno |
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