Editing Adverse pressure gradient (section)

{{Short description|Pressure gradient in which pressure increases in the direction of fluid flow}}

In [[fluid dynamics]], an '''adverse pressure gradient''' is a [[pressure gradient]] in which the [[static pressure]] increases in the direction of the flow. Mathematically this is expressed as {{math|''dP''/''dx'' > 0}} for a flow in the positive {{mvar|x}}-direction. 

This is important for [[boundary layer]]s. Increasing the fluid pressure is akin to increasing the [[potential energy]] of the fluid, leading to a reduced [[kinetic energy]] and a [[deceleration]] of the fluid. Since the fluid in the inner part of the boundary layer is slower, it is more greatly affected by the increasing pressure gradient. For a large enough pressure increase, this fluid may slow to zero velocity or even become reversed causing a [[flow separation]]. This has very significant consequences in [[aerodynamics]] since flow separation significantly modifies the pressure distribution along the surface and hence the [[Lift (force)|lift]] and [[Drag (physics)|drag]] characteristics.

[[Turbulent]] boundary layers tend to be able to sustain an adverse pressure gradient better than an equivalent [[laminar flow|laminar]] boundary layer. The more efficient mixing which occurs in a turbulent boundary layer transports kinetic energy from the edge of the boundary layer to the low-[[momentum]] flow at the solid surface, often preventing the separation that would occur for a laminar boundary layer under the same conditions. This physical fact has led to a variety of schemes to actually produce turbulent boundary layers when boundary layer separation is dominant at high [[Reynolds number]]s; the dimples on a [[golf ball]], the fuzz on a [[tennis ball]], or the seams on a [[Baseball (ball)|baseball]] are good examples. [[Aeroplane wing|Aeroplane wings]] are often engineered with [[vortex generator]]s on the upper surface to produce a turbulent boundary layer.