The study of the flow of air and the behaviour of objects moving relative to air; a subject which is applicable to other gases, and is part of the larger subject of fluid mechanics. Aerodynamic principles explain flight. The shape and orientation of an aircraft wing (curved upper surface, wing tilted down) mean that the air above the wing travels further than the air beneath. Air above the wing thus travels faster and so has lower pressure (Bernoulli's principle). The pressure difference provides lift to support the aircraft. The available lift increases with wing area, and decreases with altitude. The movement of an aircraft through the air produces a force which impedes motion, called drag, dependent on the aircraft size and shape. Air movement across the surface is impeded by friction, which produces additional drag called frictional drag; this causes heating, which can sometimes be extreme, as in the case of space re-entry vehicles. Friction losses increase with wing area and velocity, and decrease with altitude. At velocities greater than the speed of sound (Mach 1, approximately 331·5 m/1088 ft per second) air can no longer be treated as incompressible and new rules apply, giving rise to supersonic aerodynamics. Passing from subsonic to supersonic speeds, aircraft cross the sound barrier, marked by a dramatic increase in drag. Supersonic drag is reduced using thin, swept-back wings typical of military fighter aircraft. Aircraft flying at supersonic speeds produce sonic booms - shock waves in the air around the aircraft, produced because the aircraft's velocity is too great to allow the air pressure to adjust smoothly around it. Vehicles moving through air experience drag forces which increase fuel consumption. Also, buildings and bridges sway due to wind. These effects must be allowed for in design, and can be minimized by attending to the shape of the object as seen by the oncoming air stream. Wind tunnels allow scale models to be exposed to simulated wind conditions, and aerodynamic properties of design can be determined. Of special importance are effects of abnormal air flows such as turbulence and vortices.
Aerodynamics (shaping of objects that affect the flow of air, liquid or gas) is a branch of fluid dynamics concerned with the study of forces and gas flows. The solution of an aerodynamic problem normally involves calculating for various properties of the flow, such as velocity, pressure, density, and temperature, as a function of space and time. Understanding the flow pattern makes it possible to calculate or approximate the forces and moments acting on bodies in the flow.
Aerodynamic problems can be classified in a number of ways. External aerodynamics is the study of flow around solid objects of various shapes. Evaluating the lift and drag on an airplane, the shock waves that form in front of the nose of a rocket or the flow of air over a hard drive head are examples of external aerodynamics. Internal aerodynamics is the study of flow through passages in solid objects.
The ratio of the problem's characteristic flow speed to the speed of sound comprises a second classification of aerodynamic problems. A problem is called subsonic if all the speeds in the problem are less than the speed of sound, transonic if speeds both below and above the speed of sound are present (normally when the characteristic speed is approximately the speed of sound), supersonic when the characteristic flow speed is greater than the speed of sound, and hypersonic when the flow speed is much greater than the speed of sound. minimum Mach numbers for hypersonic flow range from 3 to 12.
The influence of viscosity in the flow dictates a third classification. The approximations to these problems are called inviscid flows. Flows for which viscosity cannot be neglected are called viscous flows.
Aerodynamics in other fields
Aerodynamics is important in a number of applications other than aerospace engineering. The aerodynamics of internal passages is important in heating/ventilation, gas piping, and in automotive engines where detailed flow patterns strongly affect the performance of the engine. In these cases, statistical mechanics is a more valid method of solving the problem than aerodynamics.
Conservation laws
Aerodynamic problems are solved using the conservation laws, or equations derived from the conservation laws.
All aerodynamic problems are therefore solved by the same set of equations.
Boundary layer
The concept of boundary layer is important in most aerodynamic problems.
Subsonic aerodynamics
In a subsonic aerodynamic problem, all of the flow speeds are less than the speed of sound. This class of problems encompasses nearly all internal aerodynamic problems, as well as external aerodynamics for most aircraft, model aircraft, and automobiles.
In solving a subsonic problem, one decision to be made by the aerodynamicist is whether or not to incorporate the effects of compressibility. The problem is then an incompressible problem. When the density is allowed to vary, the problem is called a compressible problem. In air, compressibility effects can be ignored when the Mach number in the flow does not exceed 0.3. Above 0.3, the problem should be solved using compressible aerodynamics.
Transonic aerodynamics
Transonic aerodynamic problems are defined as problems in which both supersonic and subsonic flow exist.
Transonic flows are characterized by shock waves and expansion waves. A shock wave or expansion wave is a region of very large changes in the flow properties.
Transonic problems are arguably the most difficult to solve. Flows behave very differently at subsonic and supersonic speeds, therefore a problem involving both types is more complex than one in which the flow is either purely subsonic or purely supersonic.
Supersonic aerodynamics
Supersonic aerodynamic problems are those involving flow speeds greater than the speed of sound.
Supersonic flow behaves very differently from subsonic flow. Therefore, since sound is in fact an infinitesmal pressure difference propagating through a fluid, the speed of sound in that fluid can be considered the fastest speed that "information" can travel in the flow. In Gas travelling at subsonic speed, this pressure disturbance can propagate upstream, changing the flow pattern ahead of the object and giving the impression that the fluid "knows" the object is there and is avoiding it. However, in a supersonic flow, the pressure disturbance cannot propagate upstream, akin to the case of a man walking 10 km/h backwards in a train moving 50 km/h forwards. The presence of shock waves, along with the compressibility effects of high-velocity (see Reynolds number) fluids, is the central difference between supersonic and subsonic aerodynamics problems.
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