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A new methodology is proposed to couple Molecular Dynamics (MD) and Direct Simulation Monte Carlo (DSMC) methods to simulate high Knudsen number (Kn) flows. For this purpose a two-dimensional hybrid MD-DSMC code is developed. In this method gas-surface interactions are modeled using MD, and gas-gas interactions are modeled using DSMC method. Two-way coupling between MD and DSMC is implemented by employing buffer zones for both MD and DSMC regions. Bootstrap sampling and energy minimization algorithms are employed for dynamic coupling of these two methods since MD utilizes real number of molecules during simulation whereas DSMC utilizes a lesser number of simulated molecules. The hybrid methodology combines the advantages of both methods; it has the capability of modeling the gas-surface interaction accurately considering the effect of the presence of neighboring real number of gas molecules, while in the bulk it utilizes DSMC with only the simulated number of molecules thus increasing the computational efficiency significantly compared to pure MD codes. As a result comparatively large domain sizes can be simulated with realistic behavior at the walls. The utility of the hybrid method is demonstrated by simulating high Kn flows through a micro-channel, micro-nozzle and micro-scale shock tube. The effect of partial accommodation of gas molecules with the wall is seen to be captured dynamically with this approach. (C) 2015 Elsevier Inc. All rights reserved.

Wall effects in a micro-scale shock tube are investigated using the Direct Simulation Monte Carlo method as well as a hybrid Molecular Dynamics-Direct Simulation Monte Carlo algorithm. In the Direct Simulation Monte Carlo simulations, the Cercignani-Lampis-Lord model of gas-surface interactions is employed to incorporate the wall effects, and it is shown that the shock attenuation is significantly affected by the choice of the values of tangential momentum accommodation coefficient. A loosely coupled Molecular Dynamics-Direct Simulation Monte Carlo approach is then employed to demonstrate incomplete accommodation in micro-scale shock tube flows. This approach uses fixed values of the accommodation coefficients in the gas-surface interaction model, with their values determined from a separate dynamically similar Molecular Dynamics simulation. Finally, a completely coupled Molecular Dynamics-Direct Simulation Monte Carlo algorithm is used, wherein the bulk of the flow is modeled using Direct Simulation Monte Carlo, while the interaction of gas molecules with the shock tube walls is modeled using Molecular Dynamics. The two regions are separate and coupled both ways using buffer zones and a bootstrap coupling algorithm that accounts for the mismatch of the number of molecules in both regions. It is shown that the hybrid method captures the effect of local properties that cannot be captured using a single value of accommodation coefficient for the entire domain.

A Direct Simulation Monte Carlo code is coupled with a continuum unsteady conduction finite volume code to study the effects of wall conduction on shock propagation in a micro-channel. In addition, the effects of rarefaction and wall accommodation are also investigated. Wall accommodation is seen to influence the shock attenuation more significantly than wall conduction. The results indicate that both the shock front Mach number and shock strength are comparatively higher in case of adiabatic wall condition as there is no loss of energy from the shocked region. At higher rarefaction, the effect of wall accommodation as well as that of wall conduction are seen to become more noticeable. The simulation results show that it is not necessary to exactly model the wall conduction heat transfer for studying shock attenuation, and the two limiting situations of isothermal and adiabatic approximations are satisfactory as the thermal wall boundary conditions.

Direct Simulation Monte Carlo (DSMC) [1] is a well known method to study rarefied gas flows and high Knudsen number (Kn) situations. Though DSMC predicts such flows with reasonable accuracy in the bulk, it relies on various models for gassurface interactions,with the Cercignani-Lampis-Lord (CLL) model being one of the most accepted models

Two-dimensional direct simulation Monte Carlo (DSMC) was used to investigate the effect of wall accommodation on different modes of operation of a nozzle under high Knudsen number conditions. Substantial performance enhancement was seen at higher values of converging–diverging angles, because a reduction in the surface area-to-volume ratio decreased the wall effects. Though the normal energy accommodation had a negligible influence on the nozzle performance, it was seen to be a critical factor for accurate prediction of gas temperature along the wall. At higher Knudsen numbers, the flow tended to become one-dimensional even for the case of complete accommodation.

The use of micro shock tubes in novel applications such as drug delivery, microengines and chemical kinetics demand accurate predictions of flow parameters. A shock tube operating at micro scales or under rarefied conditions has a relatively high Knudsen number (Kn), and under this condition the gas-wall interactions significantly affect the flow duration time. Numerous studies of low driver pressure (0.2-5 mm of Hg) shock tubes have been carried out, wherein the decrease in the flow duration time due to wall effects was established [1, 2, 3]. The nature of the flow in a shock tube is unsteady and typically high speed, and hence the time for a molecule to be in complete equilibrium with walls is less. As a consequence the reflection of the molecules is expected to take place with incomplete accommodation [4]. Zeitoun et al.[5] performed the numerical simulations of micro shock tubes using Direct Simulation Monte Carlo (DSMC) method with diffuse reflection at the wall and observed decreases in shock wave strength and velocity. Diffuse reflection occurs as a result of complete accommodation at the wall, whereas the reflection is specular when there is no accommodation at all. For intermediate levels of accommodation, the CLL model [6] is often employed. In a previous investigation by the present authors [7], DSMC analysis of a high Knudsen number shock tube was performed with the CLL model of gas-surface interaction using arbitrary values of accommodation coefficients.

Two‐dimensional Direct Simulation Monte Carlo (DSMC) simulations are carried out for investigating the shock wave propagation and boundary layer effects in a shock tube flow, for a wide range of rarefied conditions. Nitrogen is used for all simulations. The Cercignani‐Lampis‐Lord (CLL) model of gas surface interactions is implemented to study the effects of boundary layer which develops behind the moving shock front. The effect of surfaces is analyzed with different tangential momentum accommodation coefficient (TMAC) values in the CLL model. The Larsen‐Borgnakke (LB) model of inelastic collisions is implemented for diatomic gases. The shock front deceleration and the contact surface acceleration phenomena due to viscous effects are studied. The simulation at higher Knudsen number flows shows disappearance of the shock front and the contact surface.