  The flow of the laminar boundary layer on a flat plate is studied with
simulation of Navier-Stokes equations. The mechanisms of flow instability at
external edge of the boundary layer and near the wall are analyzed using the
energy gradient theory. The simulation results show that there is an overshoot
on the velocity profile at the external edge of the boundary layer. At this
overshoot, the energy gradient function is very large which results in
instability according to the energy gradient theory. It is found that the
transverse gradient of the total mechanical energy is responsible for the
instability at the external edge of the boundary layer, which induces the
entrainment of external flow into the boundary layer. Within the boundary
layer, there is a maximum of the energy gradient function near the wall, which
leads to intensive flow instability near the wall and contributes to the
generation of turbulence.
