CFD Online Discussion Forums (http://www.cfd-online.com/Forums/)
-   OpenFOAM (http://www.cfd-online.com/Forums/openfoam/)
-   -   Are you relying on wall functions? (http://www.cfd-online.com/Forums/openfoam/63935-you-relying-wall-functions.html)

 sek April 23, 2009 16:21

Are you relying on wall functions?

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.

 svens November 8, 2009 19:27

Hello Sek