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Abstract Fokker-Planck kinetic models have been devised as an approximation of the Boltzmann collision operator. Cubic Fokker-Planck-DSMC hybrid method is employed to simulate the diatomic gas flow through a thin slit and a thin orifice. Pressure driven nitrogen expansion gas flows with two different pressure ratios are investigated at Knudsen number 0.001. The DSMC method is physically accurate for all flow regime; however it is computationally expensive in high density or near continuum regions. The Fokker-Planck-DSMC hybrid scheme employs DSMC in rarefied regions and Fokker-Planck method in near continuum flow regions for an efficient and accurate solution. Numerical procedures of the cubic Fokker-Planck method are implemented within the framework of an existing DSMC-solver, SPARTA. The Fokker-Planck-DSMC hybrid solution reproduces pure DSMC solution with improved computational efficiency up to a factor of five for vacuum flow through a thin orifice. In addition, breakdown of translational equilibrium is investigated. Domain criterion of FP-DSMC is safely smaller than Bird's breakdown criterion. Highlights • Rarefied diatomic gas flow through a thin slit and a thin orifice. • Access of accuracy and efficiency of cubic FP-DSMC for diatomic gas flow. • Investigation of breakdown of translational equilibrium on cubic FP-DSMC hybrid. • Comparison of diatomic and monoatomic flow through a thin slit and a thin orifice. [ABSTRACT FROM AUTHOR]

Abstract: A method to take into account the influence of the inlet/outlet ends on rarefied gas flows through moderately long capillaries proposed previously [Vacuum 97, 26 (2013)] is applied to gas flows through circular tubes. The method is based on the concept of effective tube length, representing a sum of its real length and an increment. To calculate the length increment, a flow field near the inlet/outlet of tube is calculated on the basis of the linearized kinetic equation. It is shown that the value of the length increment is independent of the tube length, but depends only on the rarefaction parameter so that it can be used to obtain accurate predictions of the mass flow rate and the axial pressure distribution without great computational effort. Comparisons with results obtained by considering the complete flow domain have shown the efficiency of the end correction concept. [Copyright &y& Elsevier]

Abstract: A rarefied gas flow through channels (i.e. flow through parallel plates) of finite length has been modeled based on the direct simulation Monte Carlo method. The reduced flow rate and the flow field have been calculated as function of the gas rarefaction, the length-to-height ratio and the pressure ratio upstream and downstream of the channel. The whole range of the gas rarefaction including the free-molecular, transitional and hydrodynamic regimes and a wide range of the length-to-height ratio representing both short and long channels have been considered. Several values of the pressure ratio between 0 and 0.5 have been used in the calculations. It is shown that the rarefaction parameter has the most significant effect on the flow field characteristics and patterns, followed by the pressure ratio, while the length-to-height ratio has a rather modest impact. The Mach belt phenomenon is discussed in detail. [Copyright &y& Elsevier]

Abstract: The flow of a monatomic gas through a slit and an orifice due to an arbitrarily large pressure difference is examined on the basis of the nonlinear Bhatnagar–Gross–Krook (BGK) model equation, subject to Maxwell diffuse boundary conditions. The governing kinetic equation is discretized by a second-order control volume scheme in the physical space and the discrete velocity method in the molecular velocity space. The nonlinear fully deterministic algorithm is optimized to reduce the computational effort by introducing memory usage optimization, grid refinement and parallelization in the molecular velocity space. Results for the flow rates and the macroscopic distributions of the flow field are presented in a wide range of the Knudsen number for several pressure ratios. The effect of the various geometric and physical parameters on the flow field is examined. Comparison with previously reported corresponding Direct Simulation Monte Carlo (DSMC) results indicates a very good agreement, which clearly demonstrates the accuracy of the kinetic algorithm and furthermore the reliability of the BGK model for simulating pressure driven flows. [Copyright &y& Elsevier]