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 flow separation ： ウィキペディア英語版
flow separation

All solid objects traveling through a fluid (or alternatively a stationary object exposed to a moving fluid) acquire a boundary layer of fluid around them where viscous forces occur in the layer of fluid close to the solid surface. Boundary layers can be either laminar or turbulent. A reasonable assessment of whether the boundary layer will be laminar or turbulent can be made by calculating the Reynolds number of the local flow conditions.
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.〔Anderson, John D. (2004), ''Introduction to Flight'', Section 4.20 (5th edition)〕〔Clancy, L.J., ''Aerodynamics'', Section 4.14〕 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 is the detachment of a boundary layer from the surface into a broader wake.〔White (2010), "Fluid Mechanics", Section 7.1 (7th edition)〕 Boundary layer separation occurs when the portion of the boundary layer closest to the wall or leading edge reverses in flow direction. The separation point is defined as the point between the forward and backward flow, where the shear stress is zero. The overall boundary layer initially thickens suddenly at the separation point and is then forced off the surface by the reversed flow at its bottom.〔Wilcox, David C. Basic Fluid Mechanics. 3rd ed. Mill Valley: DCW Industries, Inc., 2007. 664-668.〕
:$u = - +$
where $s,y$ are streamwise and normal coordinates.
An adverse pressure gradient is when $dp/ds > 0$, which then can be seen to cause the velocity $u$ to decrease along $s$ and possibly go to zero if the adverse pressure gradient is strong enough.〔Balmer, David. "Separation of Boundary Layers." School of Engineering and Electronics. 2 December 2007. University of Edinburgh. 12 March 2008 .〕