udf: dimensionless wall coordinate
 

I need to determine the dimensionless wall coordinate u+=u/u_tau from within an udf. The missing link is the friction velocity u_tau=sqrt(tau_w/rho). Obviously enough, the wall friction tau_w is only available for cells adjacent to a wall. While Fluent does know a macro which delivers the distance from a cell to the nearest wall, it lacks the possibility to specify which cell that would correspond to (at least I haven't found that possibility).

So, in short, my question is: Does anybody have an idea how to get a pointer to the nearest cell adjacent to a wall from any given cell inside the fluid thread?

regards, Chris