Aerodynamics involves the interactions between an object and the surrounding air. To better understand these interactions, we need to know some things about air.
Characteristics of Air
All matter is made from atoms with the configuration of the atom (number of protons, number of neutrons ..) determining the kind of matter present (oxygen, lead, silver, neon ...). Individual atoms can combine with other atoms to form molecules. In particular, oxygen and nitrogen, which are the major components of air, occur in nature as diatomic (2 atom) molecules. Under normal conditions, matter exists as either a solid, a liquid, or a gas. Air is a gas. In any gas, we have a very large number of molecules that are only weakly attracted to each other and are free to move about in space. When studying gases, we can investigate the motions and interactions of individual molecules, or we can investigate the large scale action of the gas as a whole. Scientists refer to the large scale motion of the gas as the macro scale and the individual molecular motions as the micro scale. Some phenomenon are easier to understand and explain based on the macro scale, while other phenomenon are more easily explained on the micro scale. Macro scale investigations are based on things that we can easily observe and measure. But micro scale investigations are based on rather simple theories because we cannot actually observe an individual gas molecule in motion. Macro scale and micro scale investigations are just two views of the same thing.
Large Scale Motion of a Gas--Macro Scale
Air is treated as a uniform gas with properties that are averaged from all the individual components (oxygen, nitrogen, water vapor...). On the macro scale, we are dealing with large scale effects that we can measure, such as the gas velocity, the pressure exerted on the surroundings, or the temperature of the gas. A gas does not have a fixed shape or size but will expand to fill any container. Because the molecules are free to move about in a gas, the mass of the gas is normally characterized by the density. On the macro scale, the properties of the gas can change with altitude and depend on the thermodynamic state of the gas. The state of the gas can be changed by thermodynamic processes.
Individual Molecular Motion of a Gas--Micro Scale
On the micro scale, air is modeled by the kinetic theory of gases. The model assumes that the molecules are very small relative to the distance between molecules. The molecules are in constant, random motion and frequently collide with each other and with the walls of any container. The molecules have the standard physical properties of mass, momentum, and energy. And these properties are related to the macro properties of density, pressure, and temperature. The interactions of the molecules introduce some other properties that we normally do not encounter when dealing with solids. In a solid, the location of the molecules relative to each other remains almost constant. But in a fluid, the molecules can move around and interact with each other and with their surroundings in different ways. As mentioned above, there is always a random component of molecular motion. But the entire fluid can be made to move as well in an ordered motion. As the molecules move, the properties of the fluid move as well. If the properties are transported by the random motion, the process is called diffusion. (An example of diffusion is the spread of an odor in a perfectly still room). If the properties are transported by the ordered motion, the process is called convection. (An example of convection is a blast of cold weather brought down from Canada). If the flow of a gas produces a net angular momentum, we say the flow is rotational. (No net angular momentum in the fluid is irrotational.)
As an object moves through the air, the viscosity (stickiness) of the air becomes very important. Air molecules stick to any surface, creating a layer of air near the surface (called a boundary layer ) that, in effect, changes the shape of the object. To make things more confusing, the boundary layer may lift off or "separate" from the body and create an effective shape much different from the physical shape of an object. And to make it even more confusing, the flow conditions in and near the boundary layer are often unsteady (changing in time). The boundary layer is very important in determining both the drag and lift of an object.
As an object moves through the air, the compressibility of the air also becomes important. Air molecules move around an object as it passes through. If the object passes at a low speed (typically less than 200 mph), the density of the fluid remains constant. But for high speeds, some of the energy of the object goes into compressing the fluid, moving the molecules closer together and changing the air density, which alters the amount of the resulting force on the object. This effect is more important as speed increases. Near and beyond the speed of sound (about 700 mph), shock waves are produced that affect both the lift and drag of an object.
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