APG flows will cause the BL separation because flow direction reverses, creating a recirculating area. Classical example is an airfoil at high angle of attack.
The boundary layer concept is valid at high Reynolds numbers. In such a case the pressure in the boundary layer corresponds to the pressure imposed by the frictionless outer flow. The longitudinal pressure gradient dp/dx is related to the velocity in the main flow through the equation of Bernoulli. For an incompressible stationary main flow dp/dx=-rho U(dU/dx), where U is the magnitude of the flow velocity at the edge of the boundary layer. Separation is for a stationary flow due to a competition between the adverse pressure gradient that tends to reverse the flow and momentum transfer from the main flow across the boundary layer than tends to drag the fluid close to the wall in the flow direction.The ratio of the two time scales for these two processes depends whether the boundary layer is laminar or turbulent. For a laminar boundary layer of momentum thickness theta the time scale is (theta)^2/nu where nu is the kinematic viscosity of the fluid. The time scale for the deformation of the main flow is -1/(dU/dx). When [(theta)^2/nu](dU/dx)
In fact you could state that any sharp edge in a wall will induce flow separation.
Adverse pressure gradient and viscosity are both two necessary factors for separation. The velocity along normal direction reduced due to the adverse pressure(external factor), the near wall fluid cells can not retain the forward direction due to viscosity(inner factor), resulting the velocity changing direction and forming reverse flow(separation).
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