| Themes > Science > Physics > Fluid Dynamics > Flying the flag for fluid dynamics > Aerodynamics > Introduction to Aerodynamics |
When objects move through air, forces are generated by the relative motion between the air and the surfaces of the object. Aerodynamics is the study of these forces, generated by the motion of air. The behavior of air in motion can be described in general terms using physical theories at various levels, going from the dynamics of huge masses of air such as hurricanes, down to the tiniest scales of atomic motion. However it is unnecessary to use these general, all-inclusive theoretical descriptions to solve most problems. To design vehicles and predict their performance, we use several methods, each of which is restricted to a small range of parameters. Thus, for example, we divide the field of aerodynamics into categories based on the speed range of interest. The behavior of air flows changes depending on the ratio of the flow speed to the speed of sound. This ratio is called the Mach number. The speed of sound is the speed at which information propagates through a gas. So if the vehicle moves faster than the speed of sound, the air ahead of it cannot "move away": there is no way for it to "know" of the approaching vehicle. This leads to the formation of "shock" waves in the air ahead of the vehicle. Very close to a body surface, or at the interface betwen two streams of air moving at different speeds, we encounter friction. This leads to many strange and beautiful effects, producing the sinuous structures which make us want to keep looking at flowing streams for hours. Unfortunately, these things are quite difficult to calculate, so we argue that the primary effects of friction are confined to a region very close to the surface, called the "boundary layer". The boundary layer is a "shear layer". Likewise, the region between two streams of air, flowing at different speeds, is called a "free shear layer" because no solid surface boundary is involved. Away from surfaces, the flow can usually be considered to the "inviscid": its almost as if viscosity does not exist there. |
|
|