In case of free flow in the half space over a surface, the flow velocity at a distance asymptotically approaches the value of Bernoulli's Equation. This zone is called boundary layer. Its thickness is small in the field of accelerated flow but increases constantly in the case of constant or moreover in the case of decelerated flow. Due to the asymptotic approach towards the Bernoulli-state, as limit value a 99% approach can be defined e.g. for settling the boundary layer thickness. Outside the boundary layer, the velocity of flow once again amounts to v∞. In Fig. 1-31 the boundary layer flow over a plate in the longitudinal direction is presented.
Initially, the flow is stationary towards the parallel layers of the plate in the form of laminar flow. The boundary layer thickness → increases according to the following relation:
Eq. 1-26

The laminar boundary layer flow remains stable up to a certain length Xu. The boundary layer then becomes turbulent and its mean velocity profile significantly increases by stronger impetus exchange in the sidewall area. The position of the point of transition is defined by the Reynold's number formed with the characteristic length. The Reynold's number is defined as:

Eq. 1-27

where: V∞ = flow velocity
= kinematic viscosity