gas kinetics
   HOME

TheInfoList



OR:

Gas kinetics is a science in the branch of fluid dynamics, concerned with the study of motion of gases and its effects on physical systems. Based on the principles of
fluid mechanics Fluid mechanics is the branch of physics concerned with the mechanics of fluids ( liquids, gases, and plasmas) and the forces on them. It has applications in a wide range of disciplines, including mechanical, aerospace, civil, chemical and ...
and
thermodynamics Thermodynamics is a branch of physics that deals with heat, work, and temperature, and their relation to energy, entropy, and the physical properties of matter and radiation. The behavior of these quantities is governed by the four laws of th ...
, gas dynamics arises from the studies of gas flows in
transonic Transonic (or transsonic) flow is air flowing around an object at a speed that generates regions of both subsonic and supersonic airflow around that object. The exact range of speeds depends on the object's critical Mach number, but transoni ...
and supersonic flights. To distinguish itself from other sciences in fluid dynamics, the studies in gas dynamics are often defined with gases flowing around or within physical objects at speeds comparable to or exceeding the speed of sound and causing a significant change in
temperature Temperature is a physical quantity that expresses quantitatively the perceptions of hotness and coldness. Temperature is measurement, measured with a thermometer. Thermometers are calibrated in various Conversion of units of temperature, temp ...
and
pressure Pressure (symbol: ''p'' or ''P'') is the force applied perpendicular to the surface of an object per unit area over which that force is distributed. Gauge pressure (also spelled ''gage'' pressure)The preferred spelling varies by country and e ...
. Some examples of these studies include but are not limited to:
choked flow Choked flow is a compressible flow effect. The parameter that becomes "choked" or "limited" is the fluid velocity. Choked flow is a fluid dynamic condition associated with the venturi effect. When a flowing fluid at a given pressure and temperatu ...
s in
nozzles A nozzle is a device designed to control the direction or characteristics of a fluid flow (specially to increase velocity) as it exits (or enters) an enclosed chamber or pipe. A nozzle is often a pipe or tube of varying cross sectional area, ...
and
valves A valve is a device or natural object that regulates, directs or controls the flow of a fluid (gases, liquids, fluidized solids, or slurries) by opening, closing, or partially obstructing various passageways. Valves are technically fitting ...
,
shock waves In physics, a shock wave (also spelled shockwave), or shock, is a type of propagating disturbance that moves faster than the local speed of sound in the medium. Like an ordinary wave, a shock wave carries energy and can propagate through a med ...
around jets,
aerodynamic heating Aerodynamic heating is the heating of a solid body produced by its high-speed passage through air. In science and engineering, an understanding of aerodynamic heating is necessary for predicting the behaviour of meteoroids which enter the earth's ...
on atmospheric reentry vehicles and flows of gas fuel within a jet engine. At the molecular level, gas dynamics is a study of the kinetic theory of gases, often leading to the study of gas diffusion, statistical mechanics,
chemical thermodynamics Chemical thermodynamics is the study of the interrelation of heat and work with chemical reactions or with physical changes of state within the confines of the laws of thermodynamics. Chemical thermodynamics involves not only laboratory measurem ...
and
non-equilibrium thermodynamics Non-equilibrium thermodynamics is a branch of thermodynamics that deals with physical systems that are not in thermodynamic equilibrium but can be described in terms of macroscopic quantities (non-equilibrium state variables) that represent an ext ...
. Gas dynamics is synonymous with
aerodynamics Aerodynamics, from grc, ἀήρ ''aero'' (air) + grc, δυναμική (dynamics), is the study of the motion of air, particularly when affected by a solid object, such as an airplane wing. It involves topics covered in the field of fluid dy ...
when the gas field is
air The atmosphere of Earth is the layer of gases, known collectively as air, retained by Earth's gravity that surrounds the planet and forms its planetary atmosphere. The atmosphere of Earth protects life on Earth by creating pressure allowing f ...
and the subject of study is
flight Flight or flying is the process by which an object moves through a space without contacting any planetary surface, either within an atmosphere (i.e. air flight or aviation) or through the vacuum of outer space (i.e. spaceflight). This can be a ...
. It is highly relevant in the design of
aircraft An aircraft is a vehicle that is able to fly by gaining support from the air. It counters the force of gravity by using either static lift or by using the dynamic lift of an airfoil, or in a few cases the downward thrust from jet engine ...
and
spacecraft A spacecraft is a vehicle or machine designed to fly in outer space. A type of artificial satellite, spacecraft are used for a variety of purposes, including communications, Earth observation, meteorology, navigation, space colonization, p ...
and their respective propulsion systems.


History

Progress in gas dynamics coincides with the developments of transonic and supersonic flights. As aircraft began to travel faster, the density of air began to change, considerably increasing the air resistance as the air speed approached the speed of sound. The phenomenon was later identified in
wind tunnel Wind tunnels are large tubes with air blowing through them which are used to replicate the interaction between air and an object flying through the air or moving along the ground. Researchers use wind tunnels to learn more about how an aircraft ...
experiments as an
effect Effect may refer to: * A result or change of something ** List of effects ** Cause and effect, an idiom describing causality Pharmacy and pharmacology * Drug effect, a change resulting from the administration of a drug ** Therapeutic effect, a ...
caused by the formation of shock waves around the aircraft. Major advances were made to describe the behavior during and after
World War II World War II or the Second World War, often abbreviated as WWII or WW2, was a world war that lasted from 1939 to 1945. It involved the vast majority of the world's countries—including all of the great powers—forming two opposing ...
, and the new understandings on
compressible In thermodynamics and fluid mechanics, the compressibility (also known as the coefficient of compressibility or, if the temperature is held constant, the isothermal compressibility) is a measure of the instantaneous relative volume change of a f ...
and high speed flows became theories of gas dynamics. As the construct that gases are small particles in
Brownian motion Brownian motion, or pedesis (from grc, πήδησις "leaping"), is the random motion of particles suspended in a medium (a liquid or a gas). This pattern of motion typically consists of random fluctuations in a particle's position insi ...
became widely accepted and numerous quantitative studies verifying that the
macroscopic The macroscopic scale is the length scale on which objects or phenomena are large enough to be visible with the naked eye, without magnifying optical instruments. It is the opposite of microscopic. Overview When applied to physical phenomena a ...
properties of gases, such as temperature, pressure and
density Density (volumetric mass density or specific mass) is the substance's mass per unit of volume. The symbol most often used for density is ''ρ'' (the lower case Greek letter rho), although the Latin letter ''D'' can also be used. Mathematical ...
, are the results of collisions of moving particles, the study of kinetic theory of gases became increasingly an integrated part of gas dynamics. Modern books and classes on gas dynamics often began with an introduction to kinetic theory. The advent of the
molecular modeling Molecular modelling encompasses all methods, theoretical and computational, used to model or mimic the behaviour of molecules. The methods are used in the fields of computational chemistry, drug design, computational biology and materials scien ...
in computer simulation further made kinetic theory a highly relevant subject in today's research on gas dynamics.


Introductory terminology

*
Compressibility In thermodynamics and fluid mechanics, the compressibility (also known as the coefficient of compressibility or, if the temperature is held constant, the isothermal compressibility) is a measure of the instantaneous relative volume change of a f ...
* Mach number *
Diffusion Diffusion is the net movement of anything (for example, atoms, ions, molecules, energy) generally from a region of higher concentration to a region of lower concentration. Diffusion is driven by a gradient in Gibbs free energy or chemica ...
Gas dynamics is the overview of the average value in the distance between two molecules of gas that has collided with out ignoring the structure in which the molecules are contained. The field requires a great amount of knowledge and practical use in the ideas of the kinetic theory of gases, and it links the kinetic theory of gases with the solid state physics through the study of how gas reacts with surfaces.


Definition of a fluid

Fluids are substances that do not permanently change under an enormous amount of stress. A solid tends to deform in order to remain at equilibrium under a great deal of stress. Fluids are defined as both liquids and gases because the molecules inside the fluid are much weaker than those molecules contained in a solid. When referring to the density of a fluid in terms of a liquid, there is a small percentage of change to the liquid’s density as pressure is increased. If the fluid is referred to as a gas, the density will change greatly depending on the amount of pressure applied due to the equation of state for gases (p=ρRT). In the study of the flow of liquids, the term used while referring to the little change in density is called incompressible flow. In the study of the flow of gases, the rapid increase due to an increase of pressure is called compressible flow.


Real gases

Real gases are characterized by their compressibility (z) in the equation ''PV'' = ''zn''0''RT''. When the pressure ''P'' is set as a function of the volume ''V'' where the series is determined by set temperatures ''T'', ''P'', and ''V'' began to take hyperbolic relationships that are exhibited by ideal gases as the temperatures start to get very high. A critical point is reached when the slope of the graph is equal to zero and makes the state of the fluid change between a liquid and a vapor. The properties of ideal gases contain viscosity,
thermal conductivity The thermal conductivity of a material is a measure of its ability to conduct heat. It is commonly denoted by k, \lambda, or \kappa. Heat transfer occurs at a lower rate in materials of low thermal conductivity than in materials of high thermal ...
, and diffusion.


Viscosity

The viscosity of gases is the result in the transfer of each molecule of gas as they pass each other from layer to layer. As gases tend to pass one another, the velocity, in the form of momentum, of the faster moving molecule speeds up the slower moving molecule. As the slower moving molecule passes the faster moving molecule, the momentum of the slower moving particle slows down the faster moving particle. The molecules continue to enact until frictional drag causes both molecules to equalize their velocities.


Thermal conductivity

The thermal conductivity of a gas can be found through analysis of a gas’ viscosity except the molecules are stationary while only the temperatures of the gases are changing. Thermal conductivity is stated as the amount of heat transported across a specific area in a specific time. The thermal conductivity always flows opposite of the direction of the temperature gradient.


Diffusion

Diffusion of gases is configured with a uniform concentration of gases and while the gases are stationary. Diffusion is the change of concentration between the two gases due to a weaker concentration gradient between the two gases. Diffusion is the transportation of mass over a period of time.


Shock waves

The shock wave may be described as a compression front in a supersonic flow field, and the flow process across the front results in an abrupt change in fluid properties. The thickness of the shock wave is comparable to the mean free path of the gas molecules in the flow field. In other words, shock is a thin region where large gradients in temperature, pressure and velocity occur, and where the transport phenomena of momentum and energy are important. The normal shock wave is a compression front normal to the flow direction. However, in a wide variety of physical situations, a compression wave inclined at an angle to the flow occurs. Such a wave is called an oblique shock. Indeed, all naturally occurring shocks in external flows are oblique.


Stationary normal shock waves

A stationary normal shock wave is classified as going in the normal direction of the flow direction. For example, when a piston moves at a constant rate inside a tube, sound waves that travel down the tube are produced. As the piston continues to move, the wave begins to come together and compresses the gas inside the tube. The various calculations that come alongside of normal shock waves can vary due to the size of the tubes in which they are contained. Abnormalities such as converging-diverging nozzles and tubes with changing areas can affect such calculations as volume, pressure, and Mach number.


Moving normal shock waves

Unlike stationary normal shockwaves, moving normal shockwaves are more commonly available in physical situations. For example, a blunt object entering into the atmosphere faces a shock that comes through the medium of a non-moving gas. The fundamental problem that comes through moving normal shockwaves is the moment of a normal shockwave through motionless gas. The viewpoint of the moving shockwaves characterizes it as a moving or non-moving shock wave. The example of an object entering into the atmosphere depicts an object traveling in the opposite direction of the shockwave resulting in a moving shockwave, but if the object was launching into space, riding on top of the shockwave, it would appear to be a stationary shockwave. The relations and comparisons along with speed and shock ratios of moving and stationary shockwaves can be calculated through extensive formulas.


Friction and compressible flow

Frictional forces play a role in determining the flow properties of compressible flow in ducts. In calculations, friction is either taken as inclusive or exclusive. If friction is inclusive, then the analysis of compressible flow becomes more complex as if friction is not inclusive. If the friction is exclusive to the analysis, then certain restrictions will be put into place. When friction is included on compressible flow, the friction limits the areas in which the results from analysis in be applied. As mentioned before, the shape of the duct, such as varying sizes or nozzles, effect the different calculations in between friction and compressible flow.John, James Edward Albert., and Theo G. Keith. Gas Dynamics. 3rd ed. Harlow: Prentice Hall, 2006. 283–336. Print.


See also


References

;Specific ;General * * * * *


External links


Georgia Tech web page on gas dynamics topics
{{DEFAULTSORT:Gas Dynamics Fluid dynamics