Normal solutions of the Boltzmann equation for highly nonequilibrium Fourier flow and Couette flow

TL;DR

This paper investigates the behavior of monatomic gases under highly nonequilibrium conditions using analytical and numerical methods, providing solutions for the Boltzmann equation in Fourier and Couette flows and validating the results with DSMC simulations.

Contribution

It derives normal solutions of the Boltzmann equation for specific flow conditions across a range of Knudsen numbers, combining analytical and numerical approaches.

Findings

Analytical solutions match DSMC results at small Knudsen numbers.

Transport properties for Maxwell molecules are independent of heat-flux Knudsen number.

Transport properties decrease slightly with increasing Knudsen number for hard-sphere molecules.

Abstract

The state of a single-species monatomic gas from near-equilibrium to highly nonequilibrium conditions is investigated using analytical and numerical methods. Normal solutions of the Boltzmann equation for Fourier flow (uniform heat flux) and Couette flow (uniform shear stress) are found in terms of the heat-flux and shear-stress Knudsen numbers. Analytical solutions are found for inverse-power-law molecules from hard-sphere through Maxwell at small Knudsen numbers using Chapman-Enskog (CE) theory and for Maxwell molecules at finite Knudsen numbers using a moment-hierarchy (MH) method. Corresponding numerical solutions are obtained using the Direct Simulation Monte Carlo (DSMC) method of Bird. The thermal conductivity, the viscosity, and the Sonine-polynomial coefficients of the velocity distribution function from DSMC agree with CE results at small Knudsen numbers and with MH results at finite Knudsen numbers. Subtle differences between inverse-power-law, variable-soft-sphere, and variable-hard-sphere representations of Maxwell molecules are observed. The MH and DSMC results both indicate that the effective thermal conductivity and the effective viscosity for Maxwell molecules are independent of the heat-flux Knudsen number, and additional DSMC simulations indicate that these transport properties for hard-sphere molecules decrease slightly as the heat-flux Knudsen number is increased. Similarly, the MH and DSMC results indicate that the effective thermal conductivity and the effective viscosity for Maxwell molecules decrease as the shear-stress Knudsen number is increased, and additional DSMC simulations indicate the same behavior for hard-sphere molecules. These results provide strong evidence that the DSMC method can be used to determine the state of a gas under highly nonequilibrium conditions