CFD Online Discussion Forums - CEL y+ calculation

Hi CFX experts,

I want to implement a new wall function in CFX-Pre using CEL language rather than Fortran, I found easier.

First I want to implement the standard wall function. So, I have the formula y+= $\frac{u_{\tau}*\Delta y}{\nu}$ .
where:
friction velocity: $u_{\tau}$ = $\sqrt{\frac{\tau_{w}}{\rho}}$
kinematic viscosity: $\nu$
$\Delta y$ = $\Delta n/4$ and " $\Delta n$ is the distance between the first and second grid points off the wall." - Ansys help

I have the following problems:

1. How do I compute the Wall Shear Stress, $\tau_{w}$ ? Until now I managed to computed it as $\tau_{w}$ =ave(Wall Shear)@REGION:WALLS but I know that is wrong because the values for WSS are different over the geometry.

2. How do i compute $\Delta n$ ?

3. In Ansys help they talk about some empirical formulas for the viscous sublayer, where do I find these empirical formulas ? All I know is that in the viscous sublayer u+=y+. "In the wall-function approach, the viscosity affected sublayer region is bridged by employing empirical formulas to provide near-wall boundary conditions for the mean flow and turbulence transport equations." - Ansys Help

I have done a k- $\epsilon$ simulation with the built-in standard wall function from CFX to validate the wall function implementation.
I attached the CEL code too.

The geometry that I use is the Diffuser 1 from the article "Geometric sensitivity of three-dimensional separated by Erica M. Cherry" http://www.sciencedirect.com/science...42727X08000131

Any help, tips, documentation etc. will be greatly appreciated. Thanks in advance for help.