Molecular Flow

molecular flow

[mə′lek·yə·lər ′flō]
(fluid mechanics)
Gas-flow phenomenon at low pressures or in small channels when the mean free path is of the same order of magnitude as the channel diameter; a gas molecule thus migrates along the channel independent of other gas molecules present.

Molecular Flow


the flow of a rarefied gas (of molecules, atoms, ions, or electrons) during which the properties of the flow depend significantly on the random motion of the molecules, in contrast to flows where the gas is considered to be a continuous medium. Molecular flow takes place during the flight of bodies in the upper layers of the atmosphere, and also in vacuum systems. During molecular flow, the molecules or other particles of a gas participate in the translational motion of the entire gas as a whole on the one hand and move randomly and independently on the other. In any particular volume the molecules of the gas may have entirely different velocities. Therefore, the kinetic theory of gases is the basis for the theoretical consideration of molecular flows. The macroscopic properties of a nonviscous, compressible isoentropic flow can be described satisfactorily by the very simple model of molecules in the form of elastic, smooth spheres that obey Maxwell’s law of velocity distribution. To describe a viscous, nonisoentropic molecular flow, a more complex model of the molecules and a distribution function that differs somewhat from Maxwell’s distribution function must be used.

Molecular flows are studied in the aerodynamics of rarefied gases.


Patterson, G. N. Molekuliarnoe techenie gazov. Moscow, 1960. (Translated from English.)
Chapman, S., and T. Cowling. Matematicheskaia teoriia neodnorodnyk gazov. Moscow, 1960. (Translated from English.)
Aerodinamika razrezhennykh gazov [collection]. Edited by S. V. Val-lander. Leningrad, 1963.
Kogan, M. N. Dinamika razrezhennogo gaza. Moscow, 1967.


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