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Flow separation
All solid objects travelling through a fluid (or alternatively a stationary object exposed to a moving fluid) acquire a boundary layer of fluid around them where friction between the fluid molecules and the object's rough surface occurs. Boundary layers can be either laminar or turbulent. A calculation of the Reynolds number of the local flow conditions is necessary to determine which form the flow will take.
Flow separation occurs when the boundary layer encounters a sufficiently large adverse pressure gradient. 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 aero- or hydrodynamic surfaces which keep the local flow attached for as long as possible; examples of this include the dimples on a golf ball, turbulators on a glider, vortex generators on light aircraft and leading edge extensions on aircraft such as the F/A-18 Hornet for high angles of attack.
