  The energy gradient method is used to analyze the turbulent generation in the
transition boundary layer flow. It is found that the maximum of the energy
gradient function occurs at the wall for the Blasius boundary layer flow. At
this location under a sufficiently high Reynolds number, even a low level of
free-stream disturbance can cause the turbulent transition and sustain the flow
to be in a state of turbulence. This is an excellent explanation of the physics
of self-sustenance of wall turbulence. The mechanism of receptivity for
boundary layer flow can also be understood from the energy gradient criterion.
That is, the free-stream disturbance can propagate towards the wall by the
"energy gradient" process to cause turbulent transition, and the transition
point in boundary layer can be moved forward towards the leading edge when the
level of external disturbance increases.
