Aerodynamics is a branch of fluid dynamics, which is the study of fluids in motion. The fundamental laws governing the movements of gases, such as air, and liquids, such as water, are identical.
Even today, it takes the most powerful computers to solve the complex equations which govern the flow of fluids around irregularly shaped objects. It is a sobering thought to realize that we may be able to design craft which can enter space and glide back to Earth, but the detailed description of the way a river erodes its banks and changes course still relies a great deal on experiment rather than calculation.
Aerodynamics is of crucial importance in the design of jet engines, the turbines which drive electricity generators and even the family automobile. Reducing aerodynamic drag on anything that moves through the atmosphere, be it a car, an airplane, or a train, means greater efficiency and less fuel consumption. The study of aerodynamics in the modern world has received a huge boost from the need to conserve energy.
Air is by no means as insubstantial as it might at first appear. At sea level on a mild day the density of air is about 14.7 psi (1.23 kg/m2). This means that a large sedan car with a cross section of about 70.6 ft3 (2 m3) moving at 30 mph (50 km/h) must shift about 66 lbs (30 kg) of air every second. Good aerodynamic design helps the air flow over and around the car in a smooth controlled sweep, minimizing the distance each molecule of air must be moved, and thus minimizing drag forces.
Think of the vast area of the wing surface of a jumbo jet and the high speeds at which it travels through the air, and it is not difficult to see what keeps it aloft although many other effects contribute lift as well.
The equations which describe in a general fashion the motion of fluids were first developed by C. L. M. H. Navier in 1820 and subsequently perfected by G. G. Stokes in 1845. These equations, now called the Navier-Stokes equations, relate velocity, density, pressure, compressibility, viscosity and the spatial dimensions of the fluid. Because of the number of variables involved, the subject of fluid dynamics has been broken down into a number of subdivisions where certain conditions predominate and others can be ignored. This results in a whole series of solutions – each applying in a limited range of circumstances.
Historically, hydrodynamics came first and consequently includes the greater number of assumptions. Water is, however, almost incompressible, which means that the density of water does not change with the pressure applied to it. This property of water and other liquids simplifies the original Navier-Stokes equations.