why does boundary layer separate from surface of plate along the flow in a fluid?

Question

in what type of conditions and on what type of surfaces does separation occur. what is the mechanism behind the separation

Answer 1

Fluid dynamicists have studied this, as well as the physics of turbulence, for many years. Feynman (not a fluid dynamicist) called turbulence "the most important unsolved problem of classical physics." One explanation of boundary layer separation is given in the ref.