aerodynamics

aerodynamics, study of gases in motion. As the principal application of aerodynamics is the design of aircraft, air is the gas with which the science is most concerned. Although aerodynamics is primarily concerned with flight, its principles are also used in designing automobile and train bodies for minimum drag and in computing wind stresses on bridges, buildings, smokestacks, trees, and other structures. It is also used in charting flows of pollutants in the atmosphere and in determining frictional effects in gas ducts. The wind tunnel is one of the aerodynamicist's basic experimental tools; however in recent years, it has been supplanted by the simulation of aerodynamic forces during the computer-aided design of aircraft and automobiles.

The Basic Forces of Thrust, Drag, and Lift

There are three basic forces to be considered in aerodynamics: thrust, which moves an airplane forward; drag, which holds it back; and lift, which keeps it airborne. Lift is generally explained by three theories: Bernoulli's principle, the Coanda effect, and Newton's third law of motion. Bernoulli's principle states that the pressure of a moving gas decreases as its velocity increases. When air flows over a wing having a curved upper surface and a flat lower surface, the flow is faster across the curved surface than across the plane one; thus a greater pressure is exerted in the upward direction. This principle, however, does not fully explain flight; for example, it does not explain how an airplane can fly upside down. Scientists have begun suggesting that the Coanda effect is at least partially responsible for how planes fly. Regardless of the shape of a plane's wing, the Coanda effect, in which moving air is attracted to and flows along the surface of the wing, and the tilt of the wing, called the angle of attack, cause the air to flow downward as it leaves the wing. The greater the angle of attack, the greater the downward flow. In obedience to Newton's third law of motion, which requires an equal and opposite reaction, the airplane is deflected upward. At the same time, a force that retards the forward motion of the aircraft is developed by diverting air in this way and is known as drag due to lift. Another kind of drag is caused by the slowing of air very near to the aircraft's surface; this can be reduced by making the surface area of the craft as small as possible. At low speeds (below Mach .7) the ratio between lift and drag decreases with gains in speed; accordingly, aerodynamic development for many years stressed increases in thrust over real reductions in drag.

Creation of Shock Waves

Above speeds of Mach .7 the air flowing over the wing accelerates above the speed of sound, causing a shock wave (also known as a sonic boom) as the airplane compresses air molecules faster than they can move away from the airplane. The danger of this shock wave is its effect on control surfaces and fragile wing members, and for many years it was thought to represent a near-solid barrier to faster flight. The problems associated with this shock wave were ultimately conquered through the use of swept-back wings and the moving of critical control surfaces out of the wave's direct path. Chuck Yeager, in 1947, was the first to fly at sustained supersonic speed. Other troublesome phenomena associated with supersonic flight are the shock waves that build up at engine air intakes, and the much larger wave that trails after the craft.

Effect of Hypersonic Speeds

Recently, intense research has gone into the development of planes that can fly at hypersonic speeds, approximately five times or more than the speed of sound. At these speeds the properties of air change radically, especially the rapid increase in temperature (to as much as 2,000°F;/1,080°C;) associated with the air flowing at such speeds along a plane's surface. There appears to exist an aerodynamic thermal barrier similar to the sound barrier confronted fifty years ago.

Bibliography

See A. M. Kuethe and C. Y. Chow, Foundations of Aerodynamics (5th ed. 1997); D. Anderson and S. Eberhardt, Understanding Flight (2001); G. Craig, Introduction to Aerodynamics (2003).

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aerodynamics

Branch of physics concerned with the forces acting on bodies passing through air and other gaseous fluids. It explains the principles of flight of aircraft, rockets, and missiles. It is also involved in the design of automobiles, trains, and ships, and even stationary structures such as bridges and tall buildings, which must withstand high winds. Aerodynamics emerged as a discipline around the time of Wilbur and Orville Wright's first powered flight in 1903. Developments in the field have led to major advances in turbulence theory and supersonic flight.

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aerodynamics

the study of the dynamics of gases, esp of the forces acting on a body passing through air

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aerodynamics [‚e·rō·dī′nam·iks]

(fluid mechanics)

The science that deals with the motion of air and other gaseous fluids and with the forces acting on bodies when they move through such fluids or when such fluids move against or around the bodies.

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Aerodynamics

The applied science that deals with the dynamics of airflow and the resulting interactions between this airflow and solid boundaries. The solid boundaries may be a body immersed in the airflow, or a duct of some shape through which the air is flowing. Although, strictly speaking, aerodynamics is concerned with the flow of air, in modern times the term has been liberally interpreted as dealing with the flow of gases in general.

Depending on its practical objectives, aerodynamics can be subdivided into external and internal aerodynamics. External aerodynamics is concerned with the forces and moments on, and heat transfer to, bodies moving through a fluid (usually air). Examples are the generation of lift, drag, and moments on airfoils, wings, fuselages, engine nacelles, and whole airplane configurations; wind forces on buildings; the lift and drag on automobiles; and the aerodynamic heating of high-speed aerospace vehicles such as the space shuttle. Internal aerodynamics involves the study of flows moving internally through ducts. Examples are the flow properties inside wind tunnels, jet engines, rocket engines, and pipes. In short, aerodynamics is concerned with the detailed physical properties of a flow field and also with the net effect of these properties in generating an aerodynamic force on a body immersed in the flow, as well as heat transfer to the body. See Aerodynamic force, Aerothermodynamics

Aerodynamics can also be subdivided into various categories depending on the dominant physical aspects of a given flow. In low-density flow the characteristic size of the flow field, or a body immersed in the flow, is of the order of a molecular mean free path (the average distance that a molecule moves between collisions with neighboring molecules); while in continuum flow the characteristic size is much greater than the molecular mean free path. More than 99% of all practical aerodynamic flow problems fall within the continuum category. See Rarefied gas flow

Continuum flow can be subdivided into viscous flow, which is dominated by the dissipative effects of viscosity (friction), thermal conduction, and mass diffusion; and inviscid flow, which is, by definition, a flow in which these dissipative effects are negligible. Both viscous and inviscid flows can be subdivided into incompressible flow, in which the density is constant, and compressible flow, in which the density is a variable. In low-speed gas flow, the density variation is small and can be ignored. In contrast, in a high-speed flow the density variation is keyed to temperature and pressure variations, which can be large, so the flow must be treated as compressible. See Compressible flow, Fluid flow, Incompressible flow, Viscosity

In turn, compressible flow is subdivided into four speed regimes: subsonic flow, transonic flow, supersonic flow, and hypersonic flow. These regimes are distinguished by the value of the Mach number, which is the ratio of the local flow velocity to the local speed of sound.

A flow is subsonic if the Mach number is less than 1 at every point. Subsonic flows are characterized by smooth streamlines with no discontinuity in slope. The flow over light, general-aviation airplanes is subsonic.

A transonic flow is a mixed region of locally subsonic and supersonic flow. The flow far upstream of the airfoil can be subsonic, but as the flow moves around the airfoil surface it speeds up, and there can be pockets of locally supersonic flow over both the top and bottom surfaces of the airfoil.

In a supersonic flow, the local Mach number is greater than 1 everywhere in the flow. Supersonic flows are frequently characterized by the presence of shock waves. Across shock waves, the flow properties and the directions of streamlines change discontinuously, in contrast to the smooth, continuous variations in subsonic flow. See Supersonic flow

Hypersonic flow is a regime of very high supersonic speeds. A conventional rule is that any flow with a Mach number equal to or greater than 5 is hypersonic. Examples include the space shuttle during ascent and reentry into the atmosphere, and the flight of the X-15 experimental vehicle. The kinetic energy of many hypersonic flows is so high that, in regions where the flow velocity decreases, kinetic energy is traded for internal energy of the gas, creating high temperatures. Aerodynamic heating is a particularly severe problem for bodies immersed in a hypersonic flow.

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Aerodynamics

The applied science that deals with the dynamics of airflow and the resulting interactions between this airflow and solid boundaries. The solid boundaries may be a body immersed in the airflow, or a duct of some shape through which the air is flowing. Although, strictly speaking, aerodynamics is concerned with the flow of air, in modern times the term has been liberally interpreted as dealing with the flow of gases in general.

Depending on its practical objectives, aerodynamics can be subdivided into external and internal aerodynamics. External aerodynamics is concerned with the forces and moments on, and heat transfer to, bodies moving through a fluid (usually air). Examples are the generation of lift, drag, and moments on airfoils, wings, fuselages, engine nacelles, and whole airplane configurations; wind forces on buildings; the lift and drag on automobiles; and the aerodynamic heating of high-speed aerospace vehicles such as the space shuttle. Internal aerodynamics involves the study of flows moving internally through ducts. Examples are the flow properties inside wind tunnels, jet engines, rocket engines, and pipes. In short, aerodynamics is concerned with the detailed physical properties of a flow field and also with the net effect of these properties in generating an aerodynamic force on a body immersed in the flow, as well as heat transfer to the body. See Aerodynamic force, Aerothermodynamics

Aerodynamics can also be subdivided into various categories depending on the dominant physical aspects of a given flow. In low-density flow the characteristic size of the flow field, or a body immersed in the flow, is of the order of a molecular mean free path (the average distance that a molecule moves between collisions with neighboring molecules); while in continuum flow the characteristic size is much greater than the molecular mean free path. More than 99% of all practical aerodynamic flow problems fall within the continuum category.

Continuum flow can be subdivided into viscous flow, which is dominated by the dissipative effects of viscosity (friction), thermal conduction, and mass diffusion; and inviscid flow, which is, by definition, a flow in which these dissipative effects are negligible. Both viscous and inviscid flows can be subdivided into incompressible flow, in which the density is constant, and compressible flow, in which the density is a variable. In low-speed gas flow, the density variation is small and can be ignored. In contrast, in a high-speed flow the density variation is keyed to temperature and pressure variations, which can be large, so the flow must be treated as compressible.

In turn, compressible flow is subdivided into four speed regimes: subsonic flow, transonic flow, supersonic flow, and hypersonic flow. These regimes are distinguished by the value of the Mach number, which is the ratio of the local flow velocity to the local speed of sound.

A flow is subsonic if the Mach number is less than 1 at every point. Subsonic flows are characterized by smooth streamlines with no discontinuity in slope. The flow over light, general-aviation airplanes is subsonic. See Subsonic flight

A transonic flow is a mixed region of locally subsonic and supersonic flow. The flow far upstream of the airfoil can be subsonic, but as the flow moves around the airfoil surface it speeds up, and there can be pockets of locally supersonic flow over both the top and bottom surfaces of the airfoil. See Transonic flight

In a supersonic flow, the local Mach number is greater than 1 everywhere in the flow. Supersonic flows are frequently characterized by the presence of shock waves. Across shock waves, the flow properties and the directions of streamlines change discontinuously, in contrast to the smooth, continuous variations in subsonic flow. See Supersonic flight

Hypersonic flow is a regime of very high supersonic speeds. A conventional rule is that any flow with a Mach number equal to or greater than 5 is hypersonic. Examples include the space shuttle during ascent and reentry into the atmosphere, and the flight of the X‐15 experimental vehicle. The kinetic energy of many hypersonic flows is so high that, in regions where the flow velocity decreases, kinetic energy is traded for internal energy of the gas, creating high temperatures. Aerodynamic heating is a particularly severe problem for bodies immersed in a hypersonic flow. See Hypersonic flight