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- Aerodynamics, a branch of fluid mechanics, is concerned with the
- interaction between air and solid bodies. A solid body
- travelling through the air above 20 m.p.h. will experience some
- resistance, though the exact amount depends on several factors
- related to the nature of the air and the solid body. One of the
- aims of aerodynamics is to design shapes that will experience
- minimum air resistance. Such shapes are said to be streamlined.
-
- Experiments to achieve this aim are usually carried out in a
- wind tunnel. Here, the object under study, or a model of it, is
- suspended in a controlled flow of air produced by powerful fans.
- The airflow around the object is made visible by means of
- ultraviolet light or special optical instruments. Although the
- model is stationary in moving air, this relative motion is
- comparable to that of a moving object, like an aeroplane, in
- still air. A number of complex calculations must first be made
- to equate the performance of the model with the full-sized
- object.
-
- The various patterns formed by the movement of air around the
- object indicate the amount of air resistance. If the object has
- a streamlined shape like an aeroplane or racing car, the air
- will flow relatively undisturbed around it. Airflow of this type
- is known as laminar flow. If, however, the object is square or
- cylindrical, then the air in front of and behind it will be more
- disturbed. We call this turbulent flow.
-
- A second factor is size. Air resistance increases as the square
- of the size. Thus, object X, whose speed and proportions are
- identical to those of object Y, but whose size is twice as
- great, will experience four times as much air resistance.
-
- Speed is a third factor affecting air resistance. Air resistance
- increases as the square of the velocity. So, the resistance
- experienced by an object travelling at 60 m.p.h. will be four
- times as great as that experienced by an identical object
- travelling at half the speed. However, this rule is only valid
- up to the speed of sound (approximately 750 m.p.h.), after which
- air resistance increases greatly. This is because, at moderate
- speeds, a moving object compresses the air in front of it,
- thereby sending ahead a wave to prepare a path for it. However,
- there will be no such preparation for an object which itself
- travels faster than the wave. As a result, the object is
- continually striking the air, creating a shock wave which can be
- heard as a loud explosion. For this reason, supersonic objects
- need to be sharply pointed so that they can continually deal
- with an undisturbed wall of air.
-
- Aircraft design is an important application of aerodynamic
- theory, but it is not the only one. Civil engineers, architects,
- car designers and many others all depend heavily on the valuable
- information that aerodynamics supplies.
-