Direct Simulation Monte Carlo Results Research Articles

Near-Field Measurement and Modeling Results for Flight-Type Arcjet: Hydrogen Atom

Hydrogen atoms were probed in the near-e eld plume of a 1.8-kW e ight-type arcjet thruster, using two-photon laser-induced e uorescence. The arcjet was operated on an N 2/H2 gas mix simulating hydrazine decomposition products. Thevelocity distributionsof Hatoms were used to obtain theirtranslational temperatures. Comparisons of the laser-induced e uorescence signal intensity, radial linewidth, and derived density distribution with direct simulation Monte Carlo results were in good agreement except near the nozzle exit plane. A relatively weak signal was observed near the exit plane, while the maximum signal was unexpectedly obtained 0.8 cm downstream. Fluorescence quenching, linewidth variation, and self-absorption effects are proposed to account for most of the discrepancy in signal intensity. The results improve the knowledge of near-e eld e ow parameters for the arcjet and ultimately enhance the accuracy and understanding of predicted plume impingement torques and other arcjet plume phenomenology on spacecraft.

EFFECTS OF RAREFACTION AND COMPRESSIBILITY OF GASEOUS FLOW IN MICROCHANNEL USING DSMC

The direct simulation Monte Carlo (DSMC) is performed for two-dimensional gaseous flow through a microchannel in both slip and transition regimes to understand the effects of compressibility and rarefaction. Results are presented in the form of axial pressure distribution, velocity profile, local friction coefficient, and local Mach number (Ma) and are compared with the available analytical and experimental results. The effect of compressibility is examined for the inlet to outlet pressure ratios ranging from 1.38 to 4.5. Low-pressure drop simulations with Knudsen numbers (Kn) ranging from 0.03 to 0.11 are performed to identify the effect of rarefaction. It was found that compressibility makes the axial pressure variation nonlinear and enhances the local friction coefficient. On the other hand, rarefaction does not affect pressure distribution but causes the flow to slip at the wall and reduces the local friction coefficient. In addition, it was found that the locally fully developed (LFD) assumption is valid for low Ma flows by comparison with DSMC results.

REPORT: A MODEL FOR FLOWS IN CHANNELS, PIPES, AND DUCTS AT MICRO AND NANO SCALES

Rarefied gas flows in channels, pipes, and ducts with smooth surfaces are studied in a wide range of Knudsen number (Kn) at low Mach number (M) with the objective of developing simple, physics-based models. Such flows are encountered in microelectromechanical systems (MEMS), in nanotechnology applications, and in low-pressure environments. A new general boundary condition that accounts for the reduced momentum and heat exchange with wall surfaces is proposed and its validity is investigated. It is shown that it is applicable in the entire Knudsen range and is second-order accurate in Kn in the slip flow regime. Based on this boundary condition, a universal scaling for the velocity profile is obtained, which is used to develop a unified model predicting mass flow rate and pressure distribution with reasonable accuracy for channel, pipe, and duct flows in the regime (0 Kn). A rarefaction coefficient is introduced into this two-parameter model to account for the increasingly reduced intermolecular collisions in the transition and free-molecular regimes. The new model is validated with comparisons against direct-simulation Monte Carlo results, linearized Boltzmann solutions, and experimental data.

Molecular Dynamics Simulation of Piston-Driven Shock Wave in Hard Sphere Gas

Molecular dynamics simulation is used to study the piston driven shock wave at Mach 1.5, 3, and 10. A shock tube, whose shape is a circular cylinder, is filled with hard sphere molecules having a Maxwellian thermal velocity distribution and zero mean velocity. The piston moves and a shock wave is generated. All collisions are specular, including those between the molecules and the computational boundaries, so that the shock development is entirely causal, with no imposed statistics. The structure of the generated shock is examined in detail, and the wave speed; profiles of density, velocity, and temperature; and shock thickness are determined. The results are compared with published results of other methods, especially the direct simulation Monte-Carlo method. Property profiles are similar to those generated by direct simulation Monte-Carlo method. The shock wave thicknesses are smaller than the direct simulation Monte-Carlo results, but larger than those of the other methods. Simulation of a shock wave, which is one-dimensional, is a severe test of the molecular dynamics method, which is always three-dimensional. A major challenge of the thesis is to examine the capability of the molecular dynamics methods by choosing a difficult task.

Parallel Monte Carlo simulation of three-dimensional flow over a flat plate

This article describes a parallel implementation of the direct simulation Monte Carlo (DSMC) method. Runtime library support is used for scheduling and execution of communication between nodes, and domain decomposition is performed dynamically to maintain a favorable load balance. Performance tests are conducted using the code to evaluate various remapping and remapping-interval policies, and it is shown that a onedimensional chain-partitioning method works best for the problems considered. The parallel code is then used to simulate the Mach 20 nitrogen flow over a finite thickness flat plate. It will be shown that the parallel algorithm produces results that are very similar to previous DSMC results, despite the increased resolution available. However, it yields significantly faster execution times than the scalar code, as well as very good load-balance and scalability characteristics.

Reply to "Comment on 'Simulation of a two-dimensional Rayleigh-Bénard system using the direct simulation Monte Carlo method' "

In response to the preceding Comment by Garcia, Baras, and Mansour [Phys. Rev. E 51, 3784 (1995)], we evaluate the Rayleigh number by taking the temperature jump at the wall into consideration. It is shown that a good agreement between the direct simulation Monte Carlo results and the linear stability theory is obtained by using the diffuse boundary condition, while there is a slight discrepancy in the case of the semislip boundary condition.

DSMC approach to nonstationary Mach reflection of strong incoming shock waves using a smoothing technique

The direct simulation Monte Carlo (DSMC) method is applied to simulation of nonstationary Mach reflection of strong shock waves. Normally the DSMC method is very time consuming in solving unsteady flow field problems especially for high Mach numbers, because of the necessity of iterative calculations to eliminate the inherent statistical fluctuation caused by a finite sample size. A central weighted smoothing technique is introduced to process the DSMC results, so that the iteration time can be significantly reduced. In spite of some relaxations of the shock wave structure, the smoothing technique is verified to be useful to estima te the flow fields qualitatively and even quantitatively by using a relatively small sample size. The comparison between the present approach and the kineticmodel approach (Xu et al. 1991a, 1991b) on the application to unsteady rarefied flow fields was also carried out.

Numerical simulation of rarefied flow through a slit. Part I: Direct simulation Monte Carlo results

The pressure-driven flow of a rarefied monatomic gas through a two-dimensional slit is simulated using the direct simulation Monte Carlo technique. Of particular interest is the change in flow field structure as pressure ratio and Knudsen number are varied. Comparisons are made to quantify the limits of validity of free-molecular theory and approximate, nearly free-molecular iterative methods. Also addressed is the sensitivity of the numerical solutions to grid structure and boundary conditions. The free-molecular theory is found to predict quantitative flow field properties (e.g., centerline velocities or downstream flux profiles) reasonably well for large finite Knudsen number with the error dependent on the pressure ratio. The nearly free-molecular corrections are shown to have limited range of applicability. A previously derived parameter is found to correlate total mass flux well as a function of pressure ratio and Knudsen number over a large portion of the transitional regime.