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Kinetic Theory of a Weakly Coupled Fluid

Kinetic Theory of a Weakly Coupled Fluid Virtually all measurable properties of a classical fluid may be determined from the expectation value of the phase-space density operator f ( r → p → t ) = Σ α δ ( r → - r → α ( t ) ) δ ( p → - p → α ( t ) ) , and the phase-space density correlation function 〈 f ( r → p → t ) f ( r → ′ p → ′ t ′ ) 〉 - 〈 f ( r → p → t ) 〉 〈 f ( r → ′ p → ′ t ′ ) 〉 , a matrix with indices ( r → p → t ). Systematic procedures for approximating this matrix, unhindered by secular effects, are always based on approximations to its inverse. For a weakly coupled fluid, the inverse can be expanded in powers of λ , the ratio of potential to kinetic energy. The leading term in this expansion gives rise to a Vlasov equation for the phase-space correlation function. The next term is the first that includes collisions, and results in relaxation towards equilibrium. This paper is concerned with the detailed study of the resulting fundamental nontrivial approximation. It is not Markovian and is perfectly reversible. Although the approximation is complicated, it is tractable analytically in various limits, and numerically for all wavelengths and frequencies. In this paper, only the behavior in certain limits is evaluated. Particular attention is directed toward its contractions - the density correlation function, which is measured by inelastic neutron and light scattering, and the momentum correlation function. Calculation of the former at long wavelengths corroborates the Landau-Placzek expression for light scattering, and therefore demonstrates that the kinetic equation predicts hydrodynamic behavior at long times. Since the correlation function is correct to order λ 2 , it has, in contrast to a solution to the Boltzmann equation, the correct long-wavelength velocity of sound, c 2 = ( dp dmn ) s ≠ 5 3 k B T m . It also predicts different transport coefficients than those deduced from a Boltzman equation. These include a nonvanishing bulk viscosity. The transport coefficients reduce to those derived from the Boltzmann equation at low densities. Some aspects of the short-time behavior are also discussed. http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Physical Review A American Physical Society (APS)

Kinetic Theory of a Weakly Coupled Fluid

Forster, Dieter; Martin, P. C
Physical Review A , Volume 2 (4) – Oct 1, 1970
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References (18)

Publisher
American Physical Society (APS)
Copyright
Copyright © 1970 The American Physical Society
ISSN
1094-1622
DOI
10.1103/PhysRevA.2.1575
Publisher site
See Article on Publisher Site

Abstract

Virtually all measurable properties of a classical fluid may be determined from the expectation value of the phase-space density operator f ( r → p → t ) = Σ α δ ( r → - r → α ( t ) ) δ ( p → - p → α ( t ) ) , and the phase-space density correlation function 〈 f ( r → p → t ) f ( r → ′ p → ′ t ′ ) 〉 - 〈 f ( r → p → t ) 〉 〈 f ( r → ′ p → ′ t ′ ) 〉 , a matrix with indices ( r → p → t ). Systematic procedures for approximating this matrix, unhindered by secular effects, are always based on approximations to its inverse. For a weakly coupled fluid, the inverse can be expanded in powers of λ , the ratio of potential to kinetic energy. The leading term in this expansion gives rise to a Vlasov equation for the phase-space correlation function. The next term is the first that includes collisions, and results in relaxation towards equilibrium. This paper is concerned with the detailed study of the resulting fundamental nontrivial approximation. It is not Markovian and is perfectly reversible. Although the approximation is complicated, it is tractable analytically in various limits, and numerically for all wavelengths and frequencies. In this paper, only the behavior in certain limits is evaluated. Particular attention is directed toward its contractions - the density correlation function, which is measured by inelastic neutron and light scattering, and the momentum correlation function. Calculation of the former at long wavelengths corroborates the Landau-Placzek expression for light scattering, and therefore demonstrates that the kinetic equation predicts hydrodynamic behavior at long times. Since the correlation function is correct to order λ 2 , it has, in contrast to a solution to the Boltzmann equation, the correct long-wavelength velocity of sound, c 2 = ( dp dmn ) s ≠ 5 3 k B T m . It also predicts different transport coefficients than those deduced from a Boltzman equation. These include a nonvanishing bulk viscosity. The transport coefficients reduce to those derived from the Boltzmann equation at low densities. Some aspects of the short-time behavior are also discussed.

Journal

Physical Review A – American Physical Society (APS)

Published: Oct 1, 1970

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