Flow separation occurs when the boundary layer travels far enough against an adverse pressure gradient that the speed of the boundary layer relative to the object falls almost to zero. The fluid flow becomes detached from the surface of the object, and instead takes the forms of eddies and vortices. In aerodynamics, flow separation can often result in increased drag, particularly pressure drag which is caused by the pressure differential between the front and rear surfaces of the object as it travels through the fluid. For this reason much effort and research has gone into the design of aerodynamic and hydrodynamic surfaces which delay flow separation and keep the local flow attached for as long as possible. Examples of this include the fur on a tennis ball, dimples on a golf ball, turbulators on a glider, which induce an early transition to turbulent flow regime; vortex generators on light aircraft, for controlling the separation pattern; and leading edge extensions for high angles of attack on the wings of aircraft such as the F/A-18 Hornet.
Boundary layer separation occurs when the portion of the boundary layer closest to the wall or leading edge reverses in flow direction. As a result, the overall boundary layer initially thickens suddenly and is then forced off the surface by the reversed flow at its bottom.
Read more about Flow Separation: Adverse Pressure Gradient, Influencing Parameters, Internal Separation, Effects of Boundary Layer Separation
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