Oscillatory Rarefied Gas Flows in Long Capillaries

Abstract

Oscillatory, rarefied, linear and nonlinear fully developed flows of single gases and binary gas mixtures, driven by external harmonic mechanisms with arbitrary frequency, have been recently considered by the authors in a series of works. Here, these works are reviewed by focusing on the most notable findings. More specifically, the effects of the oscillation frequency on the velocity overshooting and gas separation phenomena in gas mixture flows and of the oscillation amplitude on the flow pattern in nonlinear single gas flows are presented. Modeling is based in the former case on the McCormack kinetic model and in the latter one on the DSMC method. In general, as the flow becomes more rarefied higher frequencies are needed to trigger the overshooting phenomenon, which becomes more pronounced as the molecular mass of the gas species is increased. Notably, gas separation may be present in the whole range of gas rarefaction, provided that the flow is subject to adequate high oscillation frequency. Finally, the presence of strong external harmonic forces does not significantly affect the oscillatory macroscopic quantities, including the mass flow rate (no distortion of the amplitude-frequency curve), except of the oscillatory axial heat flux, which exhibits a non-sinusoidal pattern.