y+ for cells located away from the wall

aylalisa · June 20, 2014, 05:58

Dear Foamers,

according to my understanding y+ values that are computed in the cells next to the wall show the ratio between the laminar and turbulent influences in that cell.

$y^+ = y * \frac{u_\tau}{\nu}$

Therefore $\nu$ = $\nu_{lam}$ and $u_\tau$ = wall shear stress velocity (computed with the velocity gradient between a point on the wall and a certain distance, usually first cell center, away from the wall).

Which values do I have to use for $u_\tau$ and $\nu$ to compute y+ for cells that are located away from the wall

Aylalisa

jherb · June 28, 2014, 17:54

Have a look at the yPlusRAS utility: https://github.com/OpenFOAM/OpenFOAM...ities/yPlusRAS

aylalisa · June 30, 2014, 14:04

yPlusRAS computes yPlus values of the boundary patches, right?

Code:

yPlus.boundaryField()[patchi] = nutPw.yPlus();
const scalarField& Yp = yPlus.boundaryField()[patchi];

How do you compute yPlus for inner cells (for graphs like u+/y+)?

I've done this with help of a utility that calculates yPlus at given positions for inner cells (defined in a probesDict). I am not yet clear about the kinematic viscosity $\nu$ that should be used for that purpose. Due to the physical meaning of yPlus, this dimensionless number is the ratio of turbulent (nominator, $u_{tau}$ ) and laminar (denominator, $\nu$ ) influences. If the first cell center is really close to the wall $\nu$ = $\nu_{laminar}$ ~ 1 or even smaller, because this point is located in the viscous sublayer and friction dominates in this laminar flow regime. $u_{tau}$ = velocity gradient between the wall and the location of the first cell center. In that case the inertial forces and fritcion forces are of the same magnitude. This is apparently the idea of yPlus. y+ is the Re-Number of the viscous sublayer. The graph dimensionless velocity versus yPlus (

), shown in papers and books, imply y+ values of inner cells too. How are these yPlus values computed correctly (which $\nu$ and which $\tau$ are used?

jherb · June 30, 2014, 15:06

Have a look at http://www.cfd-online.com/Forums/ope...-testcase.html
and this program, which calculates yplus values for all cells in a mesh: https://github.com/chegdan/yPlusUplus
I am not sure that you can use them if you use wall functions, but then you probably have to merge these tools with the yPlusRAS utility.

aylalisa · June 30, 2014, 15:55

I've found that code some time ago.

If I understand correctly, this code maps to each inner cell the utau value of the closest boundary face. And for nu it uses the scalarVolumeField nu, which means nuEff (in case of an LES, not nu_laminar).

Is that code, according to the physical interpretation of yPlus, correct?

fly_light · August 23, 2023, 05:24

Hello everyone,

I have modified the code of chegdan for calculating yPlus for all cells. Thanks for his contribution.
This is the link to the old code: https://github.com/chegdan/yPlusUplus.
This is the link to my code: https://github.com/Ruansh233/RshOpen.../main/yPlusCFD

The old version is run on OF-2.1. And my new version can run on OF-v2106
I delete some redundant steps and omit to create too many new objects. My code is running much faster compared to the old version.

June 20, 2014, 05:58	y+ for cells located away from the wall	#1
aylalisa Senior Member Join Date: Nov 2012 Location: Bavaria Posts: 145 Rep Power: 13	Dear Foamers, according to my understanding y+ values that are computed in the cells next to the wall show the ratio between the laminar and turbulent influences in that cell. $y^+ = y * \frac{u_\tau}{\nu}$ Therefore $\nu$ = $\nu_{lam}$ and $u_\tau$ = wall shear stress velocity (computed with the velocity gradient between a point on the wall and a certain distance, usually first cell center, away from the wall). Which values do I have to use for $u_\tau$ and $\nu$ to compute y+ for cells that are located away from the wall Aylalisa

June 30, 2014, 14:04		#3
aylalisa Senior Member Join Date: Nov 2012 Location: Bavaria Posts: 145 Rep Power: 13	yPlusRAS computes yPlus values of the boundary patches, right? Code: yPlus.boundaryField()[patchi] = nutPw.yPlus(); const scalarField& Yp = yPlus.boundaryField()[patchi]; How do you compute yPlus for inner cells (for graphs like u+/y+)? I've done this with help of a utility that calculates yPlus at given positions for inner cells (defined in a probesDict). I am not yet clear about the kinematic viscosity $\nu$ that should be used for that purpose. Due to the physical meaning of yPlus, this dimensionless number is the ratio of turbulent (nominator, $u_{tau}$ ) and laminar (denominator, $\nu$ ) influences. If the first cell center is really close to the wall $\nu$ = $\nu_{laminar}$ ~ 1 or even smaller, because this point is located in the viscous sublayer and friction dominates in this laminar flow regime. $u_{tau}$ = velocity gradient between the wall and the location of the first cell center. In that case the inertial forces and fritcion forces are of the same magnitude. This is apparently the idea of yPlus. y+ is the Re-Number of the viscous sublayer. The graph dimensionless velocity versus yPlus (), shown in papers and books, imply y+ values of inner cells too. How are these yPlus values computed correctly (which $\nu$ and which $\tau$ are used?

Similar Threads
Thread	Thread Starter	Forum	Replies	Last Post
Natural convection in a closed domain STILL NEEDING help!	Yr0gErG	FLUENT	4	December 2, 2019 00:04
Very technical question about solving wall boundary layer ...	jlb001	FLUENT	6	December 27, 2014 05:56
Wall Shear on concave plate	sherifkadry	CFX	7	April 26, 2013 10:07
Error finding variable "THERMX"	sunilpatil	CFX	8	April 26, 2013 07:00
Wall functions?	Pr	Main CFD Forum	7	April 8, 2004 06:15

June 28, 2014, 17:54		#2
jherb Senior Member Joachim Herb Join Date: Sep 2010 Posts: 650 Rep Power: 21	Have a look at the yPlusRAS utility: https://github.com/OpenFOAM/OpenFOAM...ities/yPlusRAS

June 30, 2014, 15:06		#4
jherb Senior Member Joachim Herb Join Date: Sep 2010 Posts: 650 Rep Power: 21	Have a look at http://www.cfd-online.com/Forums/ope...-testcase.html and this program, which calculates yplus values for all cells in a mesh: https://github.com/chegdan/yPlusUplus I am not sure that you can use them if you use wall functions, but then you probably have to merge these tools with the yPlusRAS utility.

June 30, 2014, 15:55		#5
aylalisa Senior Member Join Date: Nov 2012 Location: Bavaria Posts: 145 Rep Power: 13	I've found that code some time ago. If I understand correctly, this code maps to each inner cell the utau value of the closest boundary face. And for nu it uses the scalarVolumeField nu, which means nuEff (in case of an LES, not nu_laminar). Is that code, according to the physical interpretation of yPlus, correct?

August 23, 2023, 05:24		#6
fly_light New Member Shenhui Ruan Join Date: Nov 2021 Location: Karlsruhe Posts: 14 Rep Power: 4	Hello everyone, I have modified the code of chegdan for calculating yPlus for all cells. Thanks for his contribution. This is the link to the old code: https://github.com/chegdan/yPlusUplus. This is the link to my code: https://github.com/Ruansh233/RshOpen.../main/yPlusCFD The old version is run on OF-2.1. And my new version can run on OF-v2106 I delete some redundant steps and omit to create too many new objects. My code is running much faster compared to the old version.