- **OpenFOAM Post-Processing**
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- - **yPlusRAS Question**
(*https://www.cfd-online.com/Forums/openfoam-post-processing/98096-yplusras-question.html*)

yPlusRAS QuestionHello all,
I am running a 2-D airfoil simulation at M=0.3, Re = 6e6, and I want to set up my simulation with y+ ~ 30 to use the SST turbulence model effectively. When I run yPlusRAS, I get the following values for yPlus: min: 2.89107 max: 48.1331 average: 18.4332. I plotted yPlus vs x/c to find the locations where y+ deviates most from the desired value, and now I want to plot y vs x/c to find out how I can alter the mesh to achieve a more consistent y+. However, when I run yPlusRAS the data I get in the 'y' file is not of much use: Code:
`wing` y+ = u* y / nu u* = sqrt(tau_w / rho) tau_w = mu * (du/dy)@y=0 I calculate a different tau_w than OF when I use OF's wallGradU for (du/dy)@y=0; I calculate a different y+ than OF when I use OF's tau_w & y. I think it is clear that y is not being calculated properly, at least not output to file properly, and it does not make sense to me that tau_w is not reproducible. I believe that y+ is being reported properly, but I am frustrated that I cannot reproduce it. I think y is wrong because this website: http://geolab.larc.nasa.gov/APPS/YPlus/ Suggests I need a y of 0.0001356 m to get y+ = 30, and that is the number I used to generate the mesh. Has anyone tried to calculate y+ manually like this and succeeded? I would appreciate any advice that could be offered. Thanks, Dan |

Hi Dan,
Have you seen this thread? yPlusRANS gives you ystar ( and yes this is confusing), you should be able to calculate yPlus using the tool from Niklas in post 8. Cheers. Eysteinn |

Thanks Eysteinn,
The file "y" that it produces is identical. The file "yPlus" that it produces gives me lower values of y+, that are zero in some cases. Unfortunately, without an accurate list of y values it is not possible to check the y+ calculation manually. Any ideas? yPlusRAS: min: 2.70987 max: 40.6757 average: 17.2329 yPlus: min: 0 max: 36.9822 average: 9.29129 A second issue is the calculation of tau_w: I cannot reproduce this from dU/dx, which concerns me for two reasons: 1) I use tau_w in determining y+ manually, and 2) I use tau_w in determining Cd. Any advice? Thanks, Dan |

Hi Dan,
Sorry for my late answer, I think the y values are given in the cell centers which should explain your value on the wing. I get a list in my y file with values for the cells. So you can use the spreadsheet view in paraview and select a cell to get the exact cell value and then compare to the yPlus value This was useful for me: http://www.paraview.org/Wiki/Data_Selection /Eysteinn |

Overview yPlus methods1 Attachment(s)
Hi,
I spend some hours to find out what the yPlusRAS utility is doing. I calculated yplus with four different methods: 1) yPlusRAS utility 2) ystar from the k at the wall: y*= Cmu^0.25*k^0.5*rho*y_wall /mu. 3) yplus using the wallShearStress utility: y+=tau^0.5*rho*y_wall /mu 4) yplus using the wallgradU utility: y+=(mu*du/dy)^0.5*rho*y_wall /mu Be carefull: - The tau openfoam is calculating is nu*du/dy. Multiplying with rho will give the wallShear stress in Pa as in most formulas in books and on the web.- Use the cell center (height of the cell devided by two) for y_wall. I get exactly the same values for method 1) and 2). Method 3) and 4) are giving different results, see plot. Looking in the code, the wallGradU utility is using: wallGradU.boundaryField()[patchi] = -U.boundaryField()[patchi].snGrad(); which is the same as in the yplus utility going around on this forum. The wallShearStress utility is using: wallShearStress.boundaryField()[patchi] = (-mesh.Sf().boundaryField()[patchi]/mesh.magSf().boundaryField()[patchi]) & Reff.boundaryField()[patchi]; But what is correct? I don't know yet. However, I think I will use the yPlusRAS utility since fluent is doing the same. |

Hello Arnout,
For 3) and 4) to be the same, you must consider mut (compressible) or nut (incompressible). See this thread: http://www.cfd-online.com/Forums/ope...tml#post439666 As for 1 & 2 and 3 & 4 being different from each other, that is the subject of many threads - yPlusRAS calculates y* and not y+. Regards, Dan |

Quote:
So using the corrected yPlus Utility from here: http://www.cfd-online.com/Forums/ope...earstress.html calculates the right y+ values for a completed simulation am I right ? My question is how do we now determine the Y distance which will be varying through out the airfoil and implement it within the mesh ? for capturing the flow better closer to the airfoil. Kind Regards, Hasan K.J |

Hello Hasan,
It has been a while since I looked at it but I believe you are correct, that utility will calculate y+ in place of y*, though the Fluent guide states that the two should be similar under some conditions - I do not recall what conditions, though - suggest you check the guide. As for determining the wall distance of the first layer of cells, you could calculate the minimum required distance to achieve the maximum desired y+ and apply that single value all over the airfoil, or calculate it by cell on the airfoil (probably unnecessary and difficult to mesh). Hope this helps. Good luck! Dan |

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