Gas dynamics
Encyclopedia
Gas dynamics is a branch of fluid dynamics
Fluid dynamics
In physics, fluid dynamics is a sub-discipline of fluid mechanics that deals with fluid flow—the natural science of fluids in motion. It has several subdisciplines itself, including aerodynamics and hydrodynamics...

 concerned with studying the motion of gases and its consequent effects. Gas dynamics combines the principles of fluid mechanics
Fluid mechanics
Fluid mechanics is the study of fluids and the forces on them. Fluid mechanics can be divided into fluid statics, the study of fluids at rest; fluid kinematics, the study of fluids in motion; and fluid dynamics, the study of the effect of forces on fluid motion...

 and thermodynamics
Thermodynamics
Thermodynamics is a physical science that studies the effects on material bodies, and on radiation in regions of space, of transfer of heat and of work done on or by the bodies or radiation...

. This study often concentrates on the behaviour of gases flowing at speeds comparable to the speed of sound
Speed of sound
The speed of sound is the distance travelled during a unit of time by a sound wave propagating through an elastic medium. In dry air at , the speed of sound is . This is , or about one kilometer in three seconds or approximately one mile in five seconds....

, making it relevant in the design of aircraft
Aircraft
An aircraft is a vehicle that is able to fly by gaining support from the air, or, in general, the atmosphere of a planet. An aircraft 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 engines.Although...

 and spacecraft
Spacecraft
A spacecraft or spaceship is a craft or machine designed for spaceflight. Spacecraft are used for a variety of purposes, including communications, earth observation, meteorology, navigation, planetary exploration and transportation of humans and cargo....

 and their propulsion systems.

Introduction

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 commonly referred to as ideal gases. Real gases are characterized by their compressibility (z) in the equation PV=zRT. 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, 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

A shock wave is a compressional force that is created by an abrupt change in fluid properties such as pressure, temperature, and density. Shockwaves can be established in two types of flows: subsonic and supersonic. The subsonic flow is adjusted by changes in the flow properties while the supersonic flow is the adjusted through the change of the presence of an object.

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 along side 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 .

See also


Important concepts
  • Mach number
    Mach number
    Mach number is the speed of an object moving through air, or any other fluid substance, divided by the speed of sound as it is in that substance for its particular physical conditions, including those of temperature and pressure...

  • Mach wave
    Mach wave
    In fluid dynamics, a Mach wave is a pressure wave traveling with the speed of sound caused by a slight change of pressure added to a compressible flow. These weak waves can combine in supersonic flow to become a shock wave if sufficient Mach waves are present at any location. Such a shock wave is...

  • Normal shock
  • Oblique shock
    Oblique shock
    An oblique shock wave, unlike a normal shock, is inclined with respect to the incident upstream flow direction. It will occur when a supersonic flow encounters a corner that effectively turns the flow into itself and compresses. The upstream streamlines are uniformly deflected after the shock wave...

  • Moving shock
    Moving shock
    In fluid dynamics, a moving shock is a shock wave that is traveling through a fluid medium with a velocity relative to the velocity of the fluid already making up the medium. As such, the normal shock relations require modification to calculate the properties before and after the moving shock...

  • Prandtl-Meyer expansion fan
    Prandtl-Meyer expansion fan
    A Prandtl–Meyer expansion fan is a centered expansion process, which turns a supersonic flow around a convex corner. The fan consists of an infinite number of Mach waves, diverging from a sharp corner. In case of a smooth corner, these waves can be extended backwards to meet at a point. Each wave...


Flows of interest
  • Compressible flow
    Compressible flow
    Compressible flow is the area of fluid mechanics that deals with fluids in which the fluid density varies significantly in response to a change in pressure. Compressibility effects are typically considered significant if the Mach number of the flow exceeds 0.3, or if the fluid undergoes very large...

  • Transonic flow
  • Supersonic flow
  • Hypersonic flow
  • Choked flow
    Choked flow
    Choked flow is a compressible flow effect. The parameter that becomes "choked" or "limited" is the velocity or the mass flow rate.Choked flow is a fluid dynamic condition associated with the Venturi effect...

  • De Laval nozzle
    De Laval nozzle
    A de Laval nozzle is a tube that is pinched in the middle, making a carefully balanced, asymmetric hourglass-shape...

  • Fanno flow
    Fanno flow
    Fanno flow refers to adiabatic flow through a constant area duct where the effect of friction is considered. Compressibility effects often come into consideration, although the Fanno flow model certainly also applies to incompressible flow. For this model, the duct area remains constant, the flow...

  • Rayleigh flow
    Rayleigh flow
    Rayleigh flow refers to diabatic flow through a constant area duct where the effect of heat addition or rejection is considered. Compressibility effects often come into consideration, although the Rayleigh flow model certainly also applies to incompressible flow. For this model, the duct area...


Experimental techniques
  • Shock tube
    Shock tube
    For the pyrotechnic initiator, see Shock tube detonatorThe shock tube is an instrument used to replicate and direct blast waves at a sensor or a model in order to simulate actual explosions and their effects, usually on a smaller scale...

  • Transonic, supersonic,
    hypersonic
    Hypersonic wind tunnel
    A hypersonic wind tunnel is designed to generate a hypersonic flow field in the working section. The speed of these tunnels vary from Mach 5 to 15. As with supersonic wind tunnels, these types of tunnels must run intermittently with very high pressure ratios when initializing.Since the temperature...

     wind tunnel
    Wind tunnel
    A wind tunnel is a research tool used in aerodynamic research to study the effects of air moving past solid objects.-Theory of operation:Wind tunnels were first proposed as a means of studying vehicles in free flight...

    s.

Visualisation methods
  • Shadowgraph
    Shadowgraph
    Shadowgraph is an optical method that reveals non-uniformities in transparent media like air, water, or glass. It is related to, but simpler than, the schlieren and schlieren photography methods that perform a similar function...

  • Schlieren photography
    Schlieren photography
    Schlieren photography is a visual process that is used to photograph the flow of fluids of varying density. Invented by the German physicist August Toepler in 1864 to study supersonic motion, it is widely used in aeronautical engineering to photograph the flow of air around objects...

  • Mach-Zehnder interferometer
    Mach-Zehnder interferometer
    The Mach–Zehnder interferometer is a device used to determine the relative phase shift between two collimated beams from a coherent light source. The interferometer has been used, amongst other things, to measure small phase shifts in one of the two beams caused by a small sample or the change in...


Computational techniques
  • Riemann solver
    Riemann solver
    A Riemann solver is a numerical method used to solve a Riemann problem. They are heavily used in computational fluid dynamics and computational magnetohydrodynamics.-Exact solvers:...

  • Smoothed particle hydrodynamics
    Smoothed particle hydrodynamics
    Smoothed-particle hydrodynamics is a computational method used for simulating fluid flows. It has been used in many fields of research, including astrophysics, ballistics, volcanology, and oceanography...

Aerodynamics
Aerodynamics
Aerodynamics is a branch of dynamics concerned with studying the motion of air, particularly when it interacts with a moving object. Aerodynamics is a subfield of fluid dynamics and gas dynamics, with much theory shared between them. Aerodynamics is often used synonymously with gas dynamics, with...

  • Wave drag
    Wave drag
    In aeronautics, wave drag is a component of the drag on aircraft, blade tips and projectiles moving at transonic and supersonic speeds, due to the presence of shock waves. Wave drag is independent of viscous effects.- Overview :...

  • Sonic boom
    Sonic boom
    A sonic boom is the sound associated with the shock waves created by an object traveling through the air faster than the speed of sound. Sonic booms generate enormous amounts of sound energy, sounding much like an explosion...

  • Whitcomb area rule
  • Supercritical airfoil
    Supercritical airfoil
    A supercritical airfoil is an airfoil designed, primarily, to delay the onset of wave drag in the transonic speed range. Supercritical airfoils are characterized by their flattened upper surface, highly cambered aft section, and greater leading edge radius compared with traditional airfoil shapes...

  • Supersonic transport
    Supersonic transport
    A supersonic transport is a civilian supersonic aircraft designed to transport passengers at speeds greater than the speed of sound. The only SSTs to see regular service to date have been Concorde and the Tupolev Tu-144. The last passenger flight of the Tu-144 was in June 1978 with its last ever...


  • External links

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