Gas dynamics - the branch of fluid mechanics that studies the motion of compressible continuous media (gas, plasma) and their interaction with solid bodies. As a part of physics, gas dynamics is connected with thermodynamics and acoustics.
Compressibility is the ability of a substance to change its initial volume under a difference of pressure or with a change of temperature. Compressibility becomes significant at high velocities of the medium, comparable with the speed at which sound propagates in that medium or exceeding it, because at such velocities large differences of pressure (see Bernoulli's equation) and large temperature gradients can arise in the medium. Modern gas dynamics also studies gas flows at high temperatures, accompanied by chemical processes (dissociation, combustion and other chemical reactions) and physical ones (ionisation, radiation). The motion of electrically conducting gases in the presence of magnetic and electric fields is the subject of magnetic gas dynamics. The motion of gases under conditions in which the gas cannot be regarded as a continuous medium, and the interaction of its constituent molecules with one another and with solid bodies has to be considered instead, belongs to the field of rarefied gas dynamics, which rests on the molecular-kinetic theory of gases. The dynamics of large masses of air at low velocities of motion forms the basis of dynamic meteorology. Historically gas dynamics arose as a further development and generalisation of aerodynamics, and for that reason one often speaks of a single science — aerogasdynamics.
The governing equations of gas dynamics follow from applying the basic laws of mechanics and thermodynamics to a moving volume of compressible gas. Unsteady motions of a viscous compressible gas, in which the parameters of the gas flow at each of its points change with time, are described by the full Navier — Stokes equations. One of the main physical features of the motion of compressible media is that shock waves can form and propagate in them; these move at a velocity exceeding the propagation speed of sound waves and constitute a narrow region of extremely large gradients of pressure, density, temperature and gas velocity.
The intensive development of gas dynamics is bound up with the rapid development of the corresponding fields of technology: jet aviation, rocket weapons, and the creation of atomic and hydrogen bombs, whose explosion brings with it the propagation of powerful blast and shock waves. In the design of various craft, engines and gas machines, the tasks of gas dynamics are to determine the forces of pressure and friction, the temperature and the heat flux at any point of the surface of a body or of a channel washed by the gas, at any instant of time. In the study of the propagation of gas jets, of blast and shock waves, of combustion and detonation, the methods of gas dynamics determine the pressure, the temperature and other parameters of the gas throughout the region of propagation. The study of the complex problems set by technology has turned modern gas dynamics into a science of the motion of arbitrary mixtures of gases, which may also contain solid and liquid particles (for example, the exhaust gases of rocket engines running on liquid or solid fuel), while the parameters that characterise the state of these gases — pressure, temperature, density, electrical conductivity and others — can vary within the widest limits.
The laws of gas dynamics are widely used in external and internal ballistics, and in the study of explosion, combustion, detonation and condensation in a moving flow.
Modern gas dynamics is marked by an inseparable combination of computational and theoretical methods, the use of computers and the setting up of complex aerodynamic and physical experiments. Theoretical ideas, resting in part on regularities found by experiment, make it possible to describe, by means of the appropriate equations, the motion of gas mixtures of complex composition, including multiphase mixtures in which physico-chemical and phase transformations occur. The methods of applied mathematics are used to develop efficient ways of solving these equations on a computer. Finally, experimental data yield the required values of the physical and chemical characteristics proper to the medium under study and to the processes considered (the coefficients of viscosity and of thermal conductivity, the rates of chemical reactions, relaxation times and others).
Many of the problems that modern technology has set gas dynamics cannot yet be solved by computational and theoretical methods; in these cases wide use is made of gas-dynamic experiments set up on the basis of similarity theory and of the laws of hydrodynamic and aerodynamic modelling. Gas-dynamic experiments are carried out in supersonic and hypersonic wind tunnels, on ballistic installations, in shock and pulse tubes and on other gas-dynamic facilities of special purpose (see Aerodynamic measurements). Applied gas dynamics, which usually employs simplified theoretical ideas about the parameters of the gas flow averaged over the cross-section, together with the basic laws of motion found by experiment, is used in the design of compressors and turbines, nozzles and diffusers, rocket engines, wind tunnels, ejectors, gas pipelines and many other technical devices.
Compressibility is the ability of a substance to change its initial volume under a difference of pressure or with a change of temperature. Compressibility becomes significant at high velocities of the medium, comparable with the speed at which sound propagates in that medium or exceeding it, because at such velocities large differences of pressure (see Bernoulli's equation) and large temperature gradients can arise in the medium. Modern gas dynamics also studies gas flows at high temperatures, accompanied by chemical processes (dissociation, combustion and other chemical reactions) and physical ones (ionisation, radiation). The motion of electrically conducting gases in the presence of magnetic and electric fields is the subject of magnetic gas dynamics. The motion of gases under conditions in which the gas cannot be regarded as a continuous medium, and the interaction of its constituent molecules with one another and with solid bodies has to be considered instead, belongs to the field of rarefied gas dynamics, which rests on the molecular-kinetic theory of gases. The dynamics of large masses of air at low velocities of motion forms the basis of dynamic meteorology. Historically gas dynamics arose as a further development and generalisation of aerodynamics, and for that reason one often speaks of a single science — aerogasdynamics.
The governing equations of gas dynamics follow from applying the basic laws of mechanics and thermodynamics to a moving volume of compressible gas. Unsteady motions of a viscous compressible gas, in which the parameters of the gas flow at each of its points change with time, are described by the full Navier — Stokes equations. One of the main physical features of the motion of compressible media is that shock waves can form and propagate in them; these move at a velocity exceeding the propagation speed of sound waves and constitute a narrow region of extremely large gradients of pressure, density, temperature and gas velocity.
The intensive development of gas dynamics is bound up with the rapid development of the corresponding fields of technology: jet aviation, rocket weapons, and the creation of atomic and hydrogen bombs, whose explosion brings with it the propagation of powerful blast and shock waves. In the design of various craft, engines and gas machines, the tasks of gas dynamics are to determine the forces of pressure and friction, the temperature and the heat flux at any point of the surface of a body or of a channel washed by the gas, at any instant of time. In the study of the propagation of gas jets, of blast and shock waves, of combustion and detonation, the methods of gas dynamics determine the pressure, the temperature and other parameters of the gas throughout the region of propagation. The study of the complex problems set by technology has turned modern gas dynamics into a science of the motion of arbitrary mixtures of gases, which may also contain solid and liquid particles (for example, the exhaust gases of rocket engines running on liquid or solid fuel), while the parameters that characterise the state of these gases — pressure, temperature, density, electrical conductivity and others — can vary within the widest limits.
The laws of gas dynamics are widely used in external and internal ballistics, and in the study of explosion, combustion, detonation and condensation in a moving flow.
Modern gas dynamics is marked by an inseparable combination of computational and theoretical methods, the use of computers and the setting up of complex aerodynamic and physical experiments. Theoretical ideas, resting in part on regularities found by experiment, make it possible to describe, by means of the appropriate equations, the motion of gas mixtures of complex composition, including multiphase mixtures in which physico-chemical and phase transformations occur. The methods of applied mathematics are used to develop efficient ways of solving these equations on a computer. Finally, experimental data yield the required values of the physical and chemical characteristics proper to the medium under study and to the processes considered (the coefficients of viscosity and of thermal conductivity, the rates of chemical reactions, relaxation times and others).
Many of the problems that modern technology has set gas dynamics cannot yet be solved by computational and theoretical methods; in these cases wide use is made of gas-dynamic experiments set up on the basis of similarity theory and of the laws of hydrodynamic and aerodynamic modelling. Gas-dynamic experiments are carried out in supersonic and hypersonic wind tunnels, on ballistic installations, in shock and pulse tubes and on other gas-dynamic facilities of special purpose (see Aerodynamic measurements). Applied gas dynamics, which usually employs simplified theoretical ideas about the parameters of the gas flow averaged over the cross-section, together with the basic laws of motion found by experiment, is used in the design of compressors and turbines, nozzles and diffusers, rocket engines, wind tunnels, ejectors, gas pipelines and many other technical devices.