Wall functions

From CFD-Wiki

Based on law of the wall

A wall-function simulation normally requires that y plus of the first cell outside the walls is in the log-layer, which starts at about y plus 20 and, depending on the Re number, extends up to say y plus 200. In the log layer, there is equilibrium between production and dissipation of the turbulent kinetic energy, therefore decreasing turbulent instability in near-wall simulations.

Another empiric profile that covers both the near wall and logarithmic region is the Reichardt profile.