In a two-layer model, the boundary layer is considered to comprise two layers: inner (close to the surface) and outer. The eddy viscosity is calculated separately for each layer and combined using:
where  is the smallest distance from the surface where  is equal to .
The inner-region eddy viscosity is given by:
where
with the von Karman constant  usually being taken as 0.4, and with
The eddy viscosity in the outer region is given by:
where ,  is the displacement thickness, given by
and F is the Klebanoff intermittency function given by