The sound of a pulsating sphere in a rarefied gas: continuum breakdown at short length and time scales

The pressure field of a pulsating sphere is a canonical problem in classical acoustics, used to illustrate the acoustic efficiency of a monopole source at continuum conditions. We consider the counterpart vibroacoustic and thermoacoustic problems in a rarefied gas, to investigate the effect of continuum breakdown on monopole radiation. Focusing on small-amplitude normal-to-boundary mechanical and heat-flux excitations, the perturbation field is analysed in the entire range of gas rarefaction and input frequencies. Numerical calculations are carried out via the direct simulation Monte Carlo method, and are used to validate analytical predictions in the free-molecular and near-continuum regimes. In the latter, the regularized thirteen moments model (R13) is applied, to capture the system response at states where the Navier–Stokes–Fourier (NSF) description breaks down. Comparing with the continuum inviscid solution, the results quantitate the dampening effect of gas rarefaction on source point-wise strength and acoustic power. At near-continuum conditions, the acoustic field is composed of exponentially decaying ‘compression’, ‘thermal’ and ‘Knudsen-layer’ modes, reflecting thermoviscous and higher-order rarefaction effects. With reducing rarefaction, the contributions of the latter two modes vanish, and the former degenerates into the ideal-flow inverse-to-distance decaying wave. Stronger attenuation is obtained with increasing rarefaction, where boundary sphericity results in a ‘geometric reduction’ of the molecular layer affected by the source. Notably, while the R13 model at low frequencies appears valid up to moderate gas rarefaction rates, both the NSF and R13 descriptions break down at common low Knudsen numbers at higher frequencies. Further study should therefore be carried out to extend the applicability of moment models to unsteady flows with short time scales.

Extending a Hybrid Continuum-Molecular Simulation Method to Solve the Micro/Nanoscale Gas Mixing Problems

Despite vast efforts in developing hybrid continuum-molecular methods, there has been no specific work focused on the gas mixture flow simulations including the mixing and/or separation of species. In present study, we extend a hybrid method to analyze such phenomena suitably and study the gas mixing problems in micro/nano length scales reliably. The results of current hybrid simulations are compared against the results of full-molecular simulations to evaluate the physical accuracy of developed hybrid method. The effect of continuum breakdown criterion is investigated to find out the achieved accuracy of developed hybrid simulation method from different perspectives. The current results indicate that using a reasonable breakdown parameter can result in very good physical accuracy. The results also indicate that using the hybrid simulation can be quite effective to avoid the statistical fluctuations, which are inherent to the molecular simulation methods.