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

2. The Boundary Layer Equations

3. 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.

4. Scale the x-direction Momentum Equation
RESULT:

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

7. The Boundary Layer Procedure

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