Fluid Dynamics
Fluid dynamics, sometimes referred to as fluid mechanics, is the study of fluids at rest or in motion. Biologists define a fluid as a water-based liquid that contains and transports the fundamental fundamental elements vital to life, but physicists, taking a broader view, define a fluid as any substance which flows under pressure. Essentially then, matter is either solid or fluid, with two basic types of fluids: gasses, which expand to fill the container or volume they are in, and liquids, which settle to the bottom of their container.
Over a wide range of situations gasses and liquids flow in similar fashions and can be described by the same equations. Some examples are flow through a pipe, flow around a body such as a rock in a stream, and flow due to temperature differences. Such cases arise in a wide variety of fields, such as aeronautical, chemical, civil, and mechanical engineering.
The scientific laws that describe fluid flow depend on whether the fluid is at rest. Fluid statics described fluids at rest (liquids), while fluid mechanics describe fluids in motion. The laws of fluid statics are well-established, some of them from the time of Archimedes (287 - 212 B.C. For example, Pascal's principle states that any pressure on a confined liquid is transmitted to every point in the liquid. Another important law is Archimedes principle, which states that a object in a fluid feels a buoyant force upward equal to the weight of the fluid it has displaced, explaining why ice, and people, float.
Fluid mechanics is a much more difficult subject, and one of the most complex branches of physics. An essential factor in fluid flow is viscosity, which is a measure of a fluid's resistance to flow. Viscosity depends on many things, especially temperature. (Molasses is thicker if taken from the refrigerator than the cupboard shelf.) Viscosity brings friction into consideration--with the walls of a pipe, a rock in a stream, or even within the liquid itself. These types of forces can cause a liquid to have exceedingly complex types of flow, as in a choppy stream or river. Such flow is called turbulent, and is very complicated to describe. One approach scientists are taking lately to understand such flow is chaos theory.
It is more typical to analyze simpler situations, such as streamline, or laminar flow--smooth flow in which no part of the fluid accelerates. An "ideal fluid" is one which has no viscosity and is not compressible. Physicists have been more successful in describing this type of flow, which is close enough to many real-world situations to be useful. Starting from Newton's laws of motions, Daniel Bernoulli (1700-1782), often called the first mathematical physicist, published his book Hydrodynamica in 1738. Bernoulli derived an equation now known as Bernoulli's principle, which states that the pressure of a moving fluid decreases as its velocity increases. (This explains the concept of the airfoil.)
In the 18th century Bernoulli's principle was generalized to include fluids with viscosity by Claude Louis Marie Henri Navier (1785-1836) and George Gabriel Stokes (1819-1903) with their Navier-Stokes equation., which is the starting point for much fluid analysis. In 1889 Osbourne Reynolds (1842-1912) developed the framework to study turbulent flows, introducing a concept now called the "Reynolds number" used in modeling fluid flow is named after him. The Reynolds number characterizes to what degree a fluid flow is laminar or turbulent. For Reynolds numbers below about 2,000 flow is laminar, whereas above 3,000 it is turbulent. Between 2,000 and 3,000 the flow is unstable and is some complex combination of laminar and turbulent flow.
Finally, no discussion of fluid dynamics would be complete without mention of the German physicist Ludwig Prandtl (1875-1953), called the father of modern fluid dynamics. In 1904 Prandtl discovered the "boundary layer" on the surface of a body moving in a fluid, which led to an understanding of skin friction drag and of the way in which streamlining reduces the drag of airplane wings and other moving bodies. Prandtl's work on wing theory, published in 1918 - 1919, became the basic material of aeronautics. Prandtl also made important contributions to the theories of supersonic flow and of turbulence, and contributed much to the development of wind tunnels and other aerodynamic equipment.
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