fluid dynamics In physics and engineering, fluid dynamics is a subdiscipline of fluid mechanics that describes the flow of fluids— liquids and gases. It has several subdisciplines, including ''aerodynamics'' (the study of air and other gases in motion) an ...

, an adverse pressure gradient is a

pressure gradient In atmospheric science, the pressure gradient (typically of air but more generally of any fluid) is a physical quantity that describes in which direction and at what rate the pressure increases the most rapidly around a particular location. The p ...

in which the

static pressure In fluid mechanics the term static pressure has several uses: * In the design and operation of aircraft, ''static pressure'' is the air pressure in the aircraft's static pressure system. * In fluid dynamics, many authors use the term ''static pres ...

increases in the direction of the flow. Mathematically this is expressed as for a flow in the positive -direction. This is important for

boundary layer In physics and fluid mechanics, a boundary layer is the thin layer of fluid in the immediate vicinity of a bounding surface formed by the fluid flowing along the surface. The fluid's interaction with the wall induces a no-slip boundary condi ...

s. Increasing the fluid pressure is akin to increasing the

potential energy In physics, potential energy is the energy held by an object because of its position relative to other objects, stresses within itself, its electric charge, or other factors. Common types of potential energy include the gravitational potentia ...

of the fluid, leading to a reduced

kinetic energy In physics, the kinetic energy of an object is the energy that it possesses due to its motion. It is defined as the work needed to accelerate a body of a given mass from rest to its stated velocity. Having gained this energy during its accele ...

and a

deceleration In mechanics, acceleration is the rate of change of the velocity of an object with respect to time. Accelerations are vector quantities (in that they have magnitude and direction). The orientation of an object's acceleration is given by t ...

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 In fluid dynamics, flow separation or boundary layer separation is the detachment of a boundary layer from a surface into a wake. A boundary layer exists whenever there is relative movement between a fluid and a solid surface with viscous f ...

. This has very significant consequences in

aerodynamics Aerodynamics, from grc, ἀήρ ''aero'' (air) + grc, δυναμική (dynamics), is the study of the motion of air, particularly when affected by a solid object, such as an airplane wing. It involves topics covered in the field of fluid dyn ...

since flow separation significantly modifies the pressure distribution along the surface and hence the

lift Lift or LIFT may refer to: Physical devices * Elevator, or lift, a device used for raising and lowering people or goods ** Paternoster lift, a type of lift using a continuous chain of cars which do not stop ** Patient lift, or Hoyer lift, mobil ...

and drag characteristics.

Turbulent In fluid dynamics, turbulence or turbulent flow is fluid motion characterized by chaotic changes in pressure and flow velocity. It is in contrast to a laminar flow, which occurs when a fluid flows in parallel layers, with no disruption between ...

boundary layers tend to be able to sustain an adverse pressure gradient better than an equivalent

laminar Laminar means "flat". Laminar may refer to: Terms in science and engineering: * Laminar electronics or organic electronics, a branch of material sciences dealing with electrically conductive polymers and small molecules * Laminar armour or "band ...

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 In Newtonian mechanics, momentum (more specifically linear momentum or translational momentum) is the product of the mass and velocity of an object. It is a vector quantity, possessing a magnitude and a direction. If is an object's mass an ...

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 In fluid mechanics, the Reynolds number () is a dimensionless quantity that helps predict fluid flow patterns in different situations by measuring the ratio between inertial and viscous forces. At low Reynolds numbers, flows tend to be domi ...

s; the dimples on a

golf ball A golf ball is a special ball designed to be used in the game of golf. Under the rules of golf, a golf ball has a mass no more than , has a diameter not less than , and performs within specified velocity, distance, and symmetry limits. Like g ...

, the fuzz on a

tennis ball A tennis ball is a ball designed for the sport of tennis. Tennis balls are fluorescent yellow in organised competitions, but in recreational play can be virtually any color. Tennis balls are covered in a fibrous felt which modifies their aerodyna ...

, or the seams on a

baseball Baseball is a bat-and-ball sport played between two teams of nine players each, taking turns batting and fielding. The game occurs over the course of several plays, with each play generally beginning when a player on the fielding tea ...

are good examples. Aeroplane wings are often engineered with

vortex generator A vortex generator (VG) is an aerodynamic device, consisting of a small vane usually attached to a lifting surface (or airfoil, such as an aircraft wing) or a rotor blade of a wind turbine.

s on the upper surface to produce a turbulent boundary layer.

References

* Ernst R. G. Eckert & Robert M. Drake Jr. (1959) ''Heat and Mass Transfer'',

McGraw-Hill McGraw Hill is an American educational publishing company and one of the "big three" educational publishers that publishes educational content, software, and services for pre-K through postgraduate education. The company also publishes referenc ...

. *

Hermann Schlichting Hermann Schlichting (22 September 1907 – 15 June 1982) was a German fluid dynamics engineer. Life and work Hermann Schlichting studied from 1926 till 1930 mathematics, physics and applied mechanics at the University of Jena, Vienne and G� ...

(1960) ''Boundary Layer Theory'', McGraw-Hill. Fluid dynamics {{fluiddynamics-stub

See also

References