Browsing by Author "Lian, YY"

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Assessment of conservative weighting scheme in simulating chemical vapour deposition with trace species
(2003-09-10)
Wu, JS
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Hsiao, WJ
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Lian, YY
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Tseng, KC
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機械工程學系
Low-pressure or ultra-high vacuum chemical vapour deposition often involves important trace species in both gas-phase and surface reactions. The conservative weighting scheme (J. Thermophys. Heat Transfer 1996; 10(4):579) has been used to deal with the trace species often involved in some non-reactive physical processes, which is otherwise considered computationally impossible using the conventional DSMC method. This conservative weighting scheme (CWS) improves greatly the statistical uncertainties by decreasing the weighting factors of trace-species particles and ensures the conservation of both momentum and energy between two colliding particles with large difference of weighting factors. This CWS is further extended to treat reactive processes for gas-phase and surface reactions with trace species, which is called extended conservative weighting scheme (ECWS). A single-cell equilibrium simulation is performed for verifying both the CWS and ECWS in treating trace species. The results of using CWS show that it is most efficient and accurate for weight ratio (trace to non-trace) equal to or less than 0.01 for flows with two and three species. The results of a single-cell simulation using ECWS for gas-phase reaction and surface reactions show that only ECWS can produce acceptable results with reasonable computational time. Copyright (C) 2003 John Wiley Sons, Ltd.
Development of a general parallel three-dimensional Direct Simulation Monte Carlo Code
(2005)
Wu, JS
;
Tseng, KC
;
Lee, UM
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Lian, YY
;
機械工程學系
Development of a general parallel three-dimensional direct Simulation Monte Carlo Code, termed as PDSC, is reported in this paper. The PDSC is implemented on the memory-distributed parallel machine (e.g., PC-cluster system) using message passing interface (MPI) with several important features, including tetrahedral unstructured mesh, solution-based adaptive mesh refinement, variable time-step method, dynamic load balancing technique, extended conservative weighting scheme for trace species, very low-speed flows, iterative pressure boundary treatment method and hybrid NS-DSMC method. In addition, a graphic user interface (GUI) using VTK (Visualization Toolkit) library and Qt is developed to ease the settings of boundary conditions and domain decomposition for parallel processing and to preserve future possibility of including more features into the code.
Parallel DSMC simulation of a single under-expanded free orifice jet from transition to near-continuum regime
(2005-11-01)
Wu, JS
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Chou, SY
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Lee, UM
;
Shao, YL
;
Lian, YY
This paper describes the numerical analysis of the flow structure of a single underexpanded argon free jet issuing into a lower-pressure or vacuum environment using the parallel three-dimensional direct simulation Monte Carlo (DSMC) method employing dynamic domain decomposition. Unstructured and tetrahedral solution-based refined mesh depending on the local mean free path is used to improve the resolution of solution. Simulated Knudsen numbers of the stagnation conditions based on orifice diameter, Reynolds numbers based on the conditions at the orifice exit, and stagnation-to-background pressure ratios are in the range of 0.0005-0.1, 7-1472, and 5-infinity, respectively, where "infinity" represents vacuum condition in the background environment. Results shou, that centerline density decays in a rate proportional to the inverse of the square of the axial distance (z(-2)) from the orifice for all ranges of flow in the current stud's The more rarefied the background condition is, the longer the z(-2)-regime is. In addition, a distinct flow structure, including barrel shock, Mach disk and jet boundary, is clearly identified as the Knudsen number reaches as low as 0.001. Predicted location and size of Mach disk in the near-continuun limit (Kn = 0.001, 0.0005) are found to be in reasonable agreement with experimental results in the continuum regime.
Parallel three-dimensional direct simulation Monte Carlo method and its applications
(2003-09-01)
Wu, JS
;
Lian, YY
;
機械工程學系
;
Department of Mechanical Engineering
The development of a parallel three-dimensional direct simulation Monte Carlo (DSMC) method using unstructured cells is reported. Variable hard sphere molecular model and no time counter method are used for the molecular collision kinetics, while the cell-by-cell ray-tracing technique is implemented for particle movement. Developed serial code has been verified by comparing the results of a supersonic corner flow with those of Bird's three-dimensional structured DSMC code. In addition, a benchmark test is performed for an orifice expanding flow to verify the parallel implementation of DSMC method by comparing with available experimental data. Static physical domain decomposition is used to distribute the workload among multiple processors by considering the estimated particle weighting distribution. Two-step multi-level graph partitioning technique is used to perform the required domain decomposition. Completed code is then applied to compute a hypersonic flow over a sphere (external flow) and the flow field of a spiral drag pump (internal flow), respectively. Results of the former are in good agreement with previous numerical results using axisymmetric DSMC method and experimental data. Results of the latter also agree well with previous numerical results. (C) 2002 Elsevier Science Ltd. All rights reserved.