CFD Online Logo CFD Online URL
www.cfd-online.com
[Sponsors]
Home > Forums > Software User Forums > OpenFOAM > OpenFOAM Post-Processing

uPlus vs yPlus ?

Register Blogs Members List Search Today's Posts Mark Forums Read

Like Tree1Likes
  • 1 Post By HPE

Reply
 
LinkBack Thread Tools Search this Thread Display Modes
Old   February 13, 2021, 16:11
Exclamation uPlus vs yPlus ?
  #1
New Member
 
Kermit
Join Date: Jan 2021
Location: France
Posts: 19
Rep Power: 5
tyskies is on a distinguished road
Hello everyone

I'm currently computing the turbulent flow over a flate plate in order to prove that wall functions are useful to reproduce the theoretical laws of the wall.
My simulation settings are quickly summed up below:
- solver: simpleFoam.
- boundary layer mesh spec: around 120 cells in 0.1m with refinement near the wall.
- turbulence model: k-epsilon.
- wall functions: kqRWallFunction, epsilonWallFunction, nutkWallFunction.

Here is how I evaluate y+ and u+:
1) First, I get the value of the wallShearStress t_w (simpleFoam -postProcessing -func wallShearStress):

t_w = sqrt(t_w(x) + t_w(y) + t_w(z))

2) Then, I evaluate the friction velocity value:

u_tau = sqrt(t_w)

NB: Units of wallShearStress values in openFoam are m/s instead of kg.m^-1/s. So we don't need do divide by rho (density).

3) Third, I evaluate y+ and u+ for each y:

y+ = y * u_tau / nu, where nu is the kinematic viscosity.

u+ = u(y) / u_tau, where u(y) is the x-component of the velocity field

Is there anything wrong in my methodology ?
Someone know how yPlus is calculated by openFoam with the utility: simpleFoam -postProcessing -func yPlus ?

I hope someone will be able to help me. Thanks in advance!
tyskies is offline   Reply With Quote

Old   February 13, 2021, 16:17
Default
  #2
HPE
Senior Member
 
HPE's Avatar
 
Herpes Free Engineer
Join Date: Sep 2019
Location: The Home Under The Ground with the Lost Boys
Posts: 932
Rep Power: 12
HPE is on a distinguished road
The line of computations is OK to my knowledge. Yet didn't quite understand the main question.

Two points:
- For the zero-gradient flat plate case, consider t_w(x) only - and ignore the other two components.
- Make sure using mag(t_w(x)) to avoid the negative sign - otherwise sqrt will throw a domain error.
tyskies likes this.
HPE is offline   Reply With Quote

Old   February 13, 2021, 16:38
Default
  #3
New Member
 
Kermit
Join Date: Jan 2021
Location: France
Posts: 19
Rep Power: 5
tyskies is on a distinguished road
Thanks for your quick reply and advice !

I just dont understand why my y+ and u+ dont match with theoretical values (Spalding law). Is that because my mesh is too refined near the wall (openfoam gives me a yPlus of 0.32) for a kEpsilon model ?

Otherwise, I noticed that the pressure isnt really converged (residual=10^-3).

NB: My case geometry is based on the following one: https://www.openfoam.com/documentati...plate-zpg.html
tyskies is offline   Reply With Quote

Old   February 13, 2021, 17:10
Default
  #4
HPE
Senior Member
 
HPE's Avatar
 
Herpes Free Engineer
Join Date: Sep 2019
Location: The Home Under The Ground with the Lost Boys
Posts: 932
Rep Power: 12
HPE is on a distinguished road
`kEpsilon` is tricky for y+<30 meshes since the derivation assumption of this turbulence closure model is y+>30 (i.e. it cannot model below the inertial sublayer). Having said that you are using wall functions - which should allow you to model below y+ 30, so I presume the issue is relevant to the settings used for `kEpsilon` or `k`-`epsilon` fields.

Note that in the link you have shared, the turbulence model being used is `kOmegaSST`.

See if the ZPG flat plate tutorial helps you: https://develop.openfoam.com/Develop...ulentFlatPlate where you can find some settings for `kEpsilon`. This tutorial is more or less the replica of the ZPG case available in the NASA turbulence website.

Hope these help.
HPE is offline   Reply With Quote

Old   February 14, 2021, 05:01
Default
  #5
New Member
 
Kermit
Join Date: Jan 2021
Location: France
Posts: 19
Rep Power: 5
tyskies is on a distinguished road
Thanks, you help a lot !

I computed the case from the link you shared. It seems that with kOmegaSST it's easier to reproduce the log-law and I'm not surprised with that results because this model is way better for modelling the boundary layer. However, for the kEpsilon model it still not good... It looks like wall functions aren't working well in my case.
tyskies is offline   Reply With Quote

Reply

Tags
simplefoam, uplus, wallfunction, yplus

Thread Tools Search this Thread
Search this Thread:

Advanced Search
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are On
Refbacks are On


Similar Threads
Thread Thread Starter Forum Replies Last Post
Uplus vs Yplus with K-Epsilon Turbulent Model in Fluent sandpants FLUENT 2 December 17, 2022 12:08
[Other] Contribution a new utility: refine wall layer mesh based on yPlus field lakeat OpenFOAM Community Contributions 58 December 23, 2021 02:36
Uplus vs Yplus Non-Newtonian Fluids C.C FLUENT 1 November 19, 2012 12:09
Uplus vs Yplus C.C FLUENT 7 November 19, 2012 09:00
Compressible yPlus and uPlus for LES owayz OpenFOAM Running, Solving & CFD 0 August 13, 2012 19:03


All times are GMT -4. The time now is 08:14.