CFD Online Logo CFD Online URL
Home > Forums > Software User Forums > OpenFOAM

Are you relying on wall functions?

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

LinkBack Thread Tools Search this Thread Display Modes
Old   April 23, 2009, 16:21
Default Are you relying on wall functions?
Sung-Eun Kim
Join Date: Mar 2009
Posts: 76
Rep Power: 17
sek is on a distinguished road
Hello FOAMers!

This is something that has bene lingering in my mind for some time. I'd like hear what OF developers/gurus have to say about what I'm going to say from now.

Let's say you want to use one of those high-Re models in OpenFOAM. And suppose your first cell off the wall is in fully turbulent region (log-layer), say, at y+ = 100.
With the wall velocity fixed to zero (no slip - yes, it's physically correct!) , Green-Gauss theorem will give you incorrect velocity gradient at wall-adjacent cells. In essence, GG will give something close to Up/yp (Up and yp are velocity and distance from the wall at cell center), whereas the real gradient should be du/dy = u_tau/(kappa*y) based on the law-of-the-wall (LOW), which is significantly smaller than Up/yp. OpenFOAM avoids the potential problems by adjusting nut_w (turbulent viscosity at the wall) so that the multiplication of nut_w*Up/yp recovers the correct wall-shear stress (therefore correct viscous flux at the wall). In addition ,OF computes turbulent kinetic energy (TKE) production at wall-adjancent cells without explicitly using velocity gradient. So, everything looks fine so far.

However, the velocity gradients at wall-adjacent cells get used for discretization of convection and diffusion through others internal faces (e.g., west, east, and north faces, if the south face is on the wall). Then, wouldn't it introduce a serious numerical error? FLUENT, for instance, uses a "slip velocity" concept in which the wall-velocity keeps being updated using the LOW.

I'll appreciate if you read it through.
sek is offline   Reply With Quote

Old   November 8, 2009, 18:27
Sven Schweikert
Join Date: Jun 2009
Posts: 38
Rep Power: 17
svens is on a distinguished road
Hello Sek

Thanks for your post!

At the moment I'm trying to validate some changes at the REqn of OpenFOAMs Launder-Gibson-RSTM. The idea was to write the pressure-shear-corelation in a way that just by changing some coefficiants you can get different RSTMs. One of them represents the Launder-Gibson-RSTM. Running OpneFOAMs formulation and the equivalent with absolutly the same initial and boundary conditions leads to different results for the Reynolds-Stress-Tensor. Having a deeper look at it shows that the field values for R are absolutly the same but the near wall region have some huge variations.

These brought my to the search for OpenFOAMs near wall treatement. And yes like you wrote - they correct the gradient near the walls with the turbulent viscocity. I tried to figure out how this can be done but didn't could find a satisfing soultion.

Your post sounds like you are pretty familiar with this way of gradiant correction. So please - could you give me some further explanation? I'm really stucked at this point and need some help from extern!

Thank you so much!
svens is offline   Reply With Quote


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
help with wall functions Nick Georgiadis Main CFD Forum 10 January 17, 2017 10:03
Errors running allwmake in OpenFOAM141dev with WM_COMPILE_OPTION%3ddebug unoder OpenFOAM Installation 11 January 30, 2008 20:30
Wall functions tutlhino OpenFOAM Pre-Processing 0 July 2, 2007 05:04
Multicomponent fluid Andrea CFX 2 October 11, 2004 05:12
Wall functions John Grimm Main CFD Forum 13 February 1, 2002 18:34

All times are GMT -4. The time now is 02:59.