Two flows are similar if their Reynold's numbers correspond. This means that the relations of pressure forces, frictional forces, and inertia forces correspond at identical points.
With the dimensionless Reynold's number, flow relations can be generally characterized. The transfer at a plate with longitudinal flow with pressure remaining constant over the characteristic length takes place, for example, at Re=5∙105 (previous slide). Increasing pressure and higher wall roughness shorten the area of laminar flow while decreasing pressure shifts the transfer point towards larger characteristic lengths. At a vehicle's surface, the length of the laminar boundary layer amounts to approx. 20 30 cm (8-12 inches).
The turbulent boundary layer of the flow is always moving. The flow velocities differs in magnitude as well as direction. Only a velocity Umean in time exclusively runs parallel to the wall. Besides an entire velocity gradient, the additional turbulent interactions produce higher shear stresses and an quicker increasing boundary layer thickness over the characteristic length.
Thus:
