• Laminar Boundary Layer: Transverse transport of momentum and vorticity is driven by VISCOUS SHEAR STRESS
  
  
• Turbulent Boundary Layer: Transverse transport is driven by turbulence itself, (TURBULENT FLUCTUATIONS)

Example:

Persons in two side by side vehicles (moving with different velocities) exchanging water melons. The vehicles achieve same velocity after some time.

The estimate for the tickness of a turbulent bounadry layer is,

or,

The reasonable but rough estimates are

The maximum value of turbulent fluctuations,

Where is the velocity scale given by wall shear stress .

For a given flow,

and

=constant

Skin friction data indicate that δ grows slightly slower than x itself as x increases

,     for moderately high Reynolds numbers

One can derive the above result from the experimental result of Blasius

Following Nikuradse's experiments, one obtains

,   for high Reynolds numbers