Transition of the laminar boundary layer on a flat plate into a fully turbulent boundary layer (not to scale). Trip Wires:

The Boundary Layer Equations

Scale the y-direction Momentum Equation RESULT: Pressure may change along a boundary layer (x-direction), but the change in pressure across a boundary layer (y-direction) is negligible.

Scale the x-direction Momentum Equation RESULT:

Outer flow speed parallel to the wall is U(x) and is obtained from the outer flow pressure, P(x).

The Boundary Layer Procedure

Displacement thickness For a laminar flat plate boundary layer, the displacement thickness is roughly one-third of the 99 percent boundary layer thickness.