Browsing by Author "Tseng, KC"

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Analysis of internal micro-scale gas flows with pressure boundaries using the DSMC method
(2001)
Wu, JS
;
Tseng, KC
;
機械工程學系
;
Department of Mechanical Engineering
The development and applications of a two-dimensional DSMC (Direct Simulation Monte Carlo) program for pressure boundaries using unstructured cells and its applications to typical internal micro-scale gas flows, including a micro-manifold, a micro-nozzle and a slider air bearing of computer hard disk, are described. This is aimed to further teat the treatment of pressure boundaries by particle flux conservation, especially at subsonic speed, to gas flows involving many exits, more complicated geometry and moving boundaries. Firstly, results of a T-shaped micro-manifold with inlet Knudsen number of 0.2 show that excellent mass flow conservation between the inlet and two exits is obtained at low subsonic gas flows. Secondly, a micro-nozzle with the fixed inlet Knudsen number of 0.067 is simulated. For higher specified pressure ratio (exit to inlet), the location of maximum Mach number moves further downstream as the pressure ratio decreases; while, for lower specified pressure ratio, the Mach number increases all the way through the nozzle to the exit. Eventually, supersonic speed is observed at the exit for pressure ratio equal to or less than 0.143. Thirdly, for the gas flows of a slider air bearing of computer hard drive, the results agree very well with those of Alexander et al. (Phys. Fluids, 1994) for all the simulated conditions. In summary, the particle flux conservation concept has been proved successfully at multiple (more than two) pressure boundaries with complicated geometries and moving solid boundaries.
Analysis of micro-scale gas flows with pressure boundaries using direct simulation Monte Carlo method
(2001-07-01)
Wu, JS
;
Tseng, KC
;
機械工程學系
;
Department of Mechanical Engineering
The development of a two-dimensional direct simulation Monte Carlo program for pressure boundaries using unstructured cells and its applications to typical micro-scale gas flows are described. For the molecular collision kinetics, variable hard sphere molecular model and no time counter collision sampling scheme are used, while the cell-by-cell particle tracing technique is implemented for particle movement. The program has been verified by comparison of simulated equilibrium collision frequency with theoretical value and by comparison of simulated non-equilibrium profiles of one-dimensional normal shock with previous reported work. Applications to micro-scale gas flows includes micro-manifold, micro-nozzle and slider air bearing. The aim is to further test the treatment of pressure boundaries, developed previously by the first author, by particle flux conservation for gas flows involving many exits, complicated geometries and moving boundaries. For micro-manifold gas flows, excellent mass flow conservation between the inlet and two exits is obtained at low subsonic flows. For micro-nozzle gas flows, with fixed inlet pressure, the mass flow rate increases with decreasing pressure ratio (exit to inlet), but remains essentially the same at pressure ratios much lower than that obtained by continuum inviscid analysis. For higher specified pressure ratios, the locations of maximum Mach number moves further downstream as the pressure ratio decreases; while, for lower specified pressure ratios, the Mach number increases all the way through the nozzle to the exit. Eventually, supersonic speed is observed at the exit for pressure ratios equal to or less than 0.143. Finally, for slider air bearing gas flows of the computer hard drive, the simulated gas pressures, at different rotating speeds, agree very well with previous studies. However, there exists strong translational nonequilibrium in the gas flows at the high rotating speeds. The applicability of the treatment of pressure boundaries using the equilibrium Maxwell-Boltzmann distribution function is discussed in terms of the magnitude of the local Knudsen number at the pressure boundary for micro-nozzles and slider air bearing applications. (C) 2001 Elsevier Science Ltd. All rights reserved.
Application of local mesh refinement in the DSMC method
(2001)
Wu, JS
;
Tseng, KC
;
Kuo, CH
;
機械工程學系
;
Department of Mechanical Engineering
The implementation of an adaptive mesh embedding (h-refinement) schemes using unstructured grid in two-dimensional Direct Simulation Monte Carlo (DSMC) method is reported. In this technique, local isotropic refinement is used to introduce new meshes where local cell Knudsen number is less than some preset value. This simple scheme, however, has several severe consequences affecting the performance of the DSMC method. Thus, we have applied a technique to remove the hanging mode, by introducing anisotropic refinement in the interfacial cells. This is completed by simply connect the hanging node(s) with the other non-hanging node(s) in the non-refined, interfacial cells. In contrast, this remedy increases negligible amount of work; however, it removes all the difficulties presented in the first scheme with hanging nodes. We have tested the proposed scheme for Argon gas using different types of mesh, such as triangular and quadrilateral or mixed, to high-speed driven cavity flow. The results show an improved flow resolution as compared with that of unadaptive mesh. Finally, we have triangular adaptive mesh to compute two near-continuum gas flows, including a supersonic flow over a cylinder and a supersonic flow over a 35 degrees compression ramp. The results show fairly good agreement with previous studies. In summary, the computational penalties by the proposed adaptive schemes are found to be small as compared with the DSMC computation itself. Nevertheless, we have concluded that the proposed scheme is superior to the original unadaptive scheme considering the accuracy of the solution.
Assessment of conservative weighting scheme in simulating chemical vapour deposition with trace species
(2003-09-10)
Wu, JS
;
Hsiao, WJ
;
Lian, YY
;
Tseng, KC
;
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.
Concurrent DSMC method using dynamic domain decomposition
(2003)
Wu, JS
;
Tseng, KC
;
機械工程學系
;
Department of Mechanical Engineering
In the current study, a parallel two-dimensional direct simulation Monte Carlo method is reported, which incorporates a multi-level graph-partitioning technique to dynamically decompose the computational domain. The current DSMC method is implemented on an unstructured mesh using particle ray-tracing technique, which takes the advantages of the cell connectivity information. Standard Message Passage Interface (MPI) is used to communicate data between processors. In addition, different strategies applying the Stop at Rise (SAR) [7] scheme is utilized to determine when to adapt the workload distribution among processors. Corresponding analysis of parallel performance is reported using the results of a high-speed driven cavity flow on IBM-SP2 parallel machines (memory-distributed, CPU 160 MHz, RAM 256 MM each) up to 64 processors. Small, medium and large problems, based on the number of particles and cells, are simulated. Results, applying SAR scheme every two time steps, show that parallel efficiency is 57%, 90% and 107% for small, medium and large problems, respectively, at 64 processors. In general, benefits,of applying SAR scheme at larger periods decrease gradually with increasing problem size. Detailed time analysis shows that degree of imbalance levels off very rapidly at a relatively low value (30%-40%) with increasing number of processors applying dynamic load balancing, while it, at a value of 5similar to6 times larger, increases with increasing number of processors without dynamic load balancing. At the end, the completed code is applied to compute a near-continuum gas flow to demonstrate its superior computational capability.
Development of a general parallel three-dimensional Direct Simulation Monte Carlo Code
(2005)
Wu, JS
;
Tseng, KC
;
Lee, UM
;
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 method using dynamic domain decomposition
(2005-05-07)
Wu, JS
;
Tseng, KC
;
機械工程學系
;
Department of Mechanical Engineering
A general parallel direct simulation Monte Carlo method using unstructured mesh is introduced, which incorporates a multi-level graph-partitioning technique to dynamically decompose the computational domain. The current DSMC method is implemented on an unstructured mesh using particle ray-tracing technique, which takes the advantages of the cell connectivity information. In addition, various strategies applying the stop at rise (SAR) (IEEE Trans Comput 1988; 39:1073-1087) scheme is studied to determine how frequent the domain should be re-decomposed. A high-speed, bottom-driven cavity flow, including small, medium and large problems, based on the number of particles and cells, are simulated. Corresponding analysis of parallel performance is reported on IBM-SP2 parallel machine up to 64 processors. Analysis shows that degree of imbalance among processors with dynamic load balancing is about 1/6 - 1/2 of that without dynamic load balancing. Detailed time analysis shows that degree of imbalance levels off very rapidly at a relatively low value with increasing number of processors when applying dynamic load balancing, which makes the large problem size fairly scalable for processors more than 64. In general, optimal frequency of activating SAR scheme decreases with problem size. At the end, the method is applied to compute two two-dimensional hypersonic flows, a three-dimensional hypersonic flow and a three-dimensional near-continuum twin-jet gas flow to demonstrate its superior computational capability and compare with experimental data and previous simulation data wherever available. Copyright (c) 2005 John Wiley A Sons, Ltd.
Parallel implementation of DSMC using unstructured mesh
(2003-09-01)
Wu, JS
;
Tseng, KC
;
Yang, TJ
;
機械工程學系
;
Department of Mechanical Engineering
The parallel implementation of the Direct Simulation Monte Carlo (DSMC) method on memory-distributed machines using unstructured mesh is reported. Physical domain decomposition is used to distribute the workload among multiple processors. A high-speed driven cavity flow is used as the benchmark problem for the validation of the parallel implementation. Three static partitioning techniques including simple coordinate partitioning, two-step partitioning (JOSTLE) and multi-level partitioning (METIS) are used for static domain decomposition, respectively. A cell renumbering technique is used to improve the memory management efficiency. Results of parallel efficiency show that two-step partitioning using JOSTLE performs the best, with 63% up to 25 processors, due to better load balancing among the processors. The powerful computational capability of the parallel implementation is demonstrated by computing a 2-D, near-continuum, hypersonic flow over a cylinder as well as a 3-D hypersonic flow over a sphere, respectively, both using 25 processors. Results compare reasonably well with previous simulated and experimental studies.
Parallel particle simulation of the near-continuum hypersonic flows over compression ramps
(2003-01-01)
Wu, JS
;
Tseng, KC
;
機械工程學系
;
Department of Mechanical Engineering
This paper describes the analysis of the near-continuum hypersonic flow over a compression ramp using the two-dimensional parallel direct simulation Monte Carlo (DSMC) method. Unstructured and triangular solution-based adaptive mesh depending on the local mean free path is used to improve the resolution of solution for the flow field with highly varying properties. In addition, a freestream parameter is defined to help reduced the cell numbers in the freestream area, resulting in appreciable decrease of the computational time (20-30%) without sacrificing the accuracy of the solution. The two-step multilevel graph partition technique is used for physical domain decomposition, employing estimated particle number distribution in each cell as the graph vertex weight. 32 IBM-SP2 processors are used throughout the study unless otherwise specified. The Effect of the outflow vacuum boundary condition, compression ramp angle, freestream condition, and length of the ramp to the flow field are investigated. Computational results are compared with previous numerical results whenever available.
Parallel three-dimensional DSMC method using mesh refinement and variable time-step scheme
(2004-10-01)
Wu, JS
;
Tseng, KC
;
Wu, FY
;
機械工程學系
;
Department of Mechanical Engineering
Application of variable time-step and unstructured adaptive mesh refinement in parallel three-dimensional Direct Simulation Monte Carlo (DSMC) method is presented. A variable time-step method using the particle fluxes conservation (mass, momentum and energy) across the cell interface is implemented to reduce the number of simulated particles and the number of iterations of transient period towards steady state, without sacrificing the solution accuracy. In addition, a three-dimensional h-refined unstructured adaptive mesh with simple but effective mesh-quality control, obtained from a preliminary parallel DSMC simulation, is used to increase the accuracy of the DSMC solution. Completed code is then applied to compute several external and internal flows, and compared with previous results wherever available. (C) 2004 Elsevier B.V. All rights reserved.
The direct simulation Monte Carlo method using unstructured adaptive mesh and its application
(2002-02-10)
Wu, JS
;
Tseng, KC
;
Kuo, CH
;
機械工程學系
;
Department of Mechanical Engineering
The implementation of an adaptive mesh-embedding (h-refinement) scheme using unstructured grid in two-dimensional direct simulation Monte Carlo (DSMC) method is reported. In this technique, local isotropic refinement is used to introduce new mesh where the local cell Knudsen number is less than Some Preset Value. This simple scheme, however, has several severe consequences affecting the performance of the DSMC method. Thus, we have applied a technique to remove the hanging node, by introducing the an-isotropic refinement in the interfacial cells between refined and non-refined cells. Not only does this remedy increase a negligible amount of work, but it also removes all the difficulties presented in the originals scheme. We have tested the proposed scheme for argon gas in a high-speed driven cavity flow, The results show an improved flow resolution as compared with that of un-adaptive mesh. Finally, we have used triangular adaptive mesh to compute a near-continuum gas flow, a hypersonic flow over a cylinder, The results show fairly good agreement with previous studies. In summary, the proposed simple mesh adaptation is very useful in computing rarefied gas flows, which involve both complicated geometry and highly non-uniform density variations throughout the flow field. Copyright (C) 2002 John Wiley Sons, Ltd.