aerodynamics

Branch of fluid physics that studies the forces exerted by air or other gases in motion. Examples include the airflow around bodies moving at speed through the atmosphere (such as land vehicles, bullets, rockets, and aircraft), the behaviour of gas in engines and furnaces, air conditioning of buildings, the deposition of snow, the operation of air-cushion vehicles (hovercraft), wind loads on buildings and bridges, bird and insect flight, musical wind instruments, and meteorology. For maximum efficiency, the aim is usually to design the shape of an object to produce a streamlined flow, with a minimum of turbulence in the moving air. The behaviour of aerosols or the pollution of the atmosphere by foreign particles are other aspects of aerodynamics.

Air Air has a density at sea level of 1.283 kg/m3. This falls off with increasing altitude to about one-tenth as much at 64,000 m, one-hundredth at 146,000 m, and only one-millionth at 293,000 m. Air is also viscous, so that a solid body moving through it experiences drag not only from the direct displacement of the air molecules but also from the sheer resistance as the molecules slide over each other.

Subsonic speeds At relatively low speeds aerodynamic drag forces are proportional to the local air density, the dimensions of the surface on which the air acts, and the square of the velocity difference between the air and the surface; thus, doubling the size of the surface doubles the forces, whereas doubling the speed multiplies the forces by four. According to Bernoulli's principle, the total energy in a given fluid flow remains everywhere constant; thus if the velocity is increased the pressure is correspondingly decreased. For this reason air flowing at less than the speed of sound through a pipe of varying diameter will have maximum velocity but minimum pressure at the points of minimum diameter and vice versa.

Supersonic speeds At supersonic speeds air passing through a pipe of varying cross section behaves in the reverse manner: a reduction in duct diameter slows the airflow down and increases its pressure, while the thrust chamber of a rocket engine terminates in a diverging bell mouth to accelerate the supersonic gas and reduce its pressure. A body moving with supersonic speed cannot send signals through the atmosphere ahead of it; the disturbances that it creates can move away only sideways or to the rear. The forward limit of disturbances is a very precisely defined boundary, only about 2 × 10-8 m thick, called a shock wave. Upstream of a shock wave the airflow is always supersonic. As the air passes through the shock wave it experiences an essentially instantaneous rise in pressure, density, and temperature. Energy is transferred to the air via the wave, and this results in a considerable increase in the backwards drag force experienced as a solid body accelerates through the speed of sound. The wave form of a shock wave is unlike an ordinary sound wave in that it is not sinusoidal but flat fronted so that it instantly reaches its maximum amplitude. Passage of a shock wave is heard as a sharp crack (small amplitude, as from a whip), a sudden bang (from a pistol), or one or more heavy booms (large amplitude, from thunder or an aeroplane). At supersonic speeds the density of the air has to be taken into account, and flows may undergo violent changes in compressibility and temperature. Above Mach 5 (five times the speed of sound) these effects become so extreme that the gas molecules themselves may dissociate into atoms in an ionized state. The laws of a ‘perfect gas’ no longer apply at such speeds and such flow is termed hypersonic.

Hypersonic speeds Whereas the most efficient of supersonic aircraft shapes are slender, thin-winged, and provided with sharp edges and pointed noses, the most efficient hypersonic shapes are blunt wedges with rounded noses and in some cases no wings at all. Excessive kinetic heating constrains hypersonic vehicles to fly relatively high; in fact, a safe ‘corridor’ exists for such vehicles, with a lower boundary determined by structural temperature and an upper boundary set by the available lift.