Gas Properties Definitions (2024)

Aerodynamics involves the interactionsbetween an object and the surrounding air. To better understand theseinteractions, 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 ofmatter present (oxygen, lead, silver, neon ...). Individual atoms cancombine with other atoms to form molecules. In particular, oxygen andnitrogen, which are the major components of air, occur in nature asdiatomic (2 atom) molecules. Under normal conditions, matterexists as either a solid, a liquid, or a gas. Air is a gas. Inany gas, we have a very large number of molecules that are onlyweakly attracted to each other and are free to move about in space.When studying gases, we can investigate the motions and interactionsof individual molecules, or we can investigate the large scale actionof the gas as a whole. Scientists refer to the large scale motion ofthe gas as the macro scale and the individual molecularmotions as the micro scale. Some phenomenon are easier tounderstand and explain based on the macro scale, while otherphenomenon are more easily explained on the micro scale. Macro scaleinvestigations are based on things that we can easily observeand measure. But micro scale investigations are based on rathersimple theories because we cannot actuallyobserve an individual gas molecule in motion. Macro scale and microscale 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 averagedfrom all the individual components (oxygen, nitrogen, watervapor...). On the macro scale, we are dealing with large scaleeffects that we can measure, such as the gasvelocity, the pressure exerted on thesurroundings, or the temperature of thegas. A gas does not have a fixed shape or size but will expand tofill any container. Because the molecules are free to move about in agas, the mass of the gas is normally characterized by the density.On the macro scale, the properties of the gas can change withaltitude and depend on the thermodynamicstate of the gas. The state of the gas can be changed bythermodynamic processes.

Individual Molecular Motion of a Gas--Micro Scale
On the micro scale, air is modeled by the kinetic theory ofgases. The model assumes that the molecules are very small relativeto the distance between molecules. The molecules are in constant,random motion and frequently collide with each other and with thewalls of any container. The molecules have the standard physicalproperties of mass, momentum, and energy. And these properties arerelated to the macro properties of density, pressure, andtemperature. The interactions of the molecules introduce some otherproperties that we normally do not encounter when dealing withsolids. In a solid, the location of the molecules relative to eachother remains almost constant. But in a fluid, the molecules can movearound and interact with each other and with their surroundings indifferent ways. As mentioned above, there is always a randomcomponent of molecular motion. But the entire fluid can be made tomove as well in an ordered motion. As the molecules move, theproperties of the fluid move as well. If the properties aretransported by the random motion, the process is calleddiffusion. (An example of diffusion is the spread of an odorin a perfectly still room). If the properties are transported by theordered motion, the process is called convection. (An exampleof 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 flowis rotational. (No net angular momentum in the fluid isirrotational.)

Viscosity
As an object moves through the air, the viscosity (stickiness)of the air becomes very important. Air molecules stick to anysurface, creating a layer of air near the surface (called aboundary layer) that, in effect, changes the shape of theobject. To make things more confusing, the boundary layer may liftoff or "separate" from the body and create an effective shape muchdifferent from the physical shape of an object. And to make it evenmore confusing, the flow conditions in and near the boundary layerare often unsteady (changing in time). The boundary layer isvery important in determining both the dragand lift of an object.

Compressibility
As an object moves through the air, the compressibility of theair also becomes important. Air molecules move around an object as itpasses through. If the object passes at a low speed (typically lessthan 200 mph), the density of the fluidremains constant. But for high speeds, someof the energy of the object goes into compressing the fluid, movingthe molecules closer together and changing the air density, whichalters the amount of the resulting force on the object. This effectis more important as speed increases. Near and beyond the speed ofsound (about 700 mph), shock waves are produced that affect both thelift and drag of an object.