Browsing by Author "Smith, M. R."

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A true-direction reconstruction of the quiet direct simulation method for inviscid gas flows
(2013-11-01)
Lin, Y. -J.
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Smith, M. R.
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Kuo, F. -A.
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Cave, H. M.
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Huang, J. -C.
In this paper, a true-direction flux reconstruction of the second-order quiet direct simulation (QDS-2N) Smith et al. (2009)[3] as an equivalent Euler equation solver, called QDS-N-2, is proposed. Because of the true-directional nature of QDS, where volume-to-volume (true-direction) fluxes are computed, as opposed to fluxes at cell interfaces as employed by traditional finite volume schemes, a volumetric reconstruction is required to reach a totally true-direction scheme. The conserved quantities are permitted to vary (according to a polynomial expression) across all simulated dimensions. Prior to the flux computation, QDS particles are introduced using properties based on weighted moments taken over the polynomial reconstruction of the conserved quantity fields. The resulting flux expressions are shown to exactly reproduce the existing second-order extension for a one-dimensional flow, while providing a means for true multi-dimensional reconstruction. The new reconstruction is demonstrated in several verification studies. These include a shock-bubble interaction problem, an Euler-four-shock interaction problem, and the advection of a vortical disturbance. These results are presented, and the increased computation time and the effect of higher-order extension are discussed in this paper. The results show that the proposed multi-dimensional reconstruction provides a significant increase in the accuracy of the solution. We show that, despite the increase in the computational expense, the computational speed of the proposed QDS-N-2 method is several times higher than that of the previously proposed QDS-2N scheme for a fixed degree of numerical accuracy, at least, for the test problem of the advection of vertical disturbances. (C) 2013 Published by Elsevier B.V.
An improved Quiet Direct Simulation method for Eulerian fluids using a second-order scheme
(2009-04-01)
Smith, M. R.
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Cave, H. M.
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Wu, J. -S.
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Jermy, M. C.
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Chen, Y. -S.
In this paper, a second-order scheme for the Quiet Direct Simulation (QDS) of Eulerian fluids is proposed. The QDS method replaces the random sampling method used in Direct Simulation Monte Carlo (DSMC) methods with a technique whereby particles are moved, have their properties distributed onto a mesh, are destroyed and then are recreated deterministically from the properties stored on the mesh using Gauss-Hermite quadrature weights and abscissas. Particles are permitted to move in physically realistic directions so flux exchange is not limited to cells sharing an adjacent interface as in conventional, direction decoupled finite volume solvers. In this paper the method is extended by calculating the fluxes of mass, momentum and energy between cells assuming a linear variation of density, temperature and velocity in each cell and using these fluxes to update the mass, velocity and internal energy carried by each particle. This Euler solver has several advantages including large dynamic range, no statistical scatter in the results, true direction fluxes to all nearby neighbors and is computationally inexpensive. The second-order method is found to reduce the numerical diffusion of QDS as demonstrated in several verification studies. These include unsteady shock tube flow, a two-dimensional blast wave and of the development of Mach 3 flow over a forward facing step in a wind tunnel, which are compared with previous results from the literature wherever is possible. Finally the implementation of QUIETWAVE, a rapid method of simulating blast events in urban environments, is introduced and the results of a test case are presented. (C) 2008 Elsevier Inc. All rights reserved.
CVD Flow Field Modeling Using the Quiet Direct Simulation (QDS) Method
(2009)
Cave, H. M.
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Lim, C. -W.
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Jermy, M. C.
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Wu, J. -S.
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Smith, M. R.
In this paper, the Quiet Direct Simulation (QDS) method is used to model the unsteady jet development in a Pulsed Pressure Chemical Vapour Deposition (PP-CVD) reactor. QDS is a novel method of gas flow simulation which is able to compute true-direction fluxes of mass, momentum and energy in a computationally efficient and accurate manner. The scheme is ideal for the simulation of novel CVD processes like PP-CVD which include highly unsteady flow structures and which has previously proved extremely difficult to simulate. Here, the axisymmetric QDS solver is outlined and the injection phase of a PP-CVD reactor is simulated.
Development of a parallel semi-implicit two-dimensional plasma fluid modeling code using finite-volume method
(2012-06-01)
Lin, K-M.
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Hung, C-T.
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Hwang, F-N.
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Smith, M. R.
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Yang, Y-W.
In this paper, the development of a two-dimensional plasma fluid modeling code using the cell-centered finite-volume method and its parallel implementation on distributed memory machines is reported. Simulated discharge currents agree very well with the measured data in a planar dielectric barrier discharge (DBD). Parallel performance of simulating helium DBD solved by the different degrees of overlapping of additive Schwarz method (ASM) preconditioned generalized minimal residual method (GMRES) for different modeling equations is investigated for a small and a large test problem, respectively, employing up to 128 processors. For the large test problem, almost linear speedup can be obtained by using up to 128 processors. Finally, a large-scale realistic two-dimensional DBD problem is employed to demonstrate the capability of the developed fluid modeling code for simulating the low-temperature plasma with complex chemical reactions. (C) 2012 Elsevier B.V. All rights reserved.
General Higher Order Extension to the Quiet Direct Simulation Method
(2011)
Lin, Y. -J.
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Smith, M. R.
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Cave, H. M.
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Huang, J. -C.
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Wu, J. -S.
Presented is the multi-dimensional extension of the Quiet Direct Simulation (QDS) method to general, higher order spatial accuracy. Due to the true directional nature of QDS, where volume-to-volume fluxes are computed as opposed to fluxes at cell interfaces, a volumetric reconstruction is required. In this approach, the conserved quantities are permitted to vary (according to a polynomial expression) across all simulated dimensions. Prior to flux computation, QDS particles are introduced using properties based on weighted moments taken over the polynomial reconstruction of conserved quantity fields. The resulting flux expressions are shown to exactly reproduce existing 2(nd) order extensions (for one-dimensional flows) while providing means for true multi-dimensional reconstruction and general higher order extensions. Here we will show a typical comparison among different spatial order extensions of QDS and other WENO (5(th) order accurate) based numerical scheme. The extension is demonstrated up to 5(th) order spatial accuracy and is shown to be comparable to or more accurate than the other higher order schemes.
Large-scale simulations on multiple Graphics Processing Units (GPUs) for the direct simulation Monte Carlo method
(2012-10-01)
Su, C. -C.
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Smith, M. R.
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Kuo, F. -A.
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Wu, J. -S.
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Hsieh, C. -W.
In this study, the application of the two-dimensional direct simulation Monte Carlo (DSMC) method using an MPI-CUDA parallelization paradigm on Graphics Processing Units (GPUs) clusters is presented. An all-device (i.e. GPU) computational approach is adopted where the entire computation is performed on the GPU device, leaving the CPU idle during all stages of the computation, including particle moving, indexing, particle collisions and state sampling. Communication between the GPU and host is only performed to enable multiple-GPU computation. Results show that the computational expense can be reduced by 15 and 185 times when using a single GPU and 16 GPUs respectively when compared to a single core of an Intel Xeon X5670 CPU. The demonstrated parallel efficiency is 75% when using 16 GPUs as compared to a single GPU for simulations using 30 million simulated particles. Finally, several very large-scale simulations in the near-continuum regime are employed to demonstrate the excellent capability of the current parallel DSMC method. (C) 2012 Elsevier Inc. All rights reserved.
Multi-Species Fluxes for the Parallel Quiet Direct Simulation (QDS) Method
(2011-01-01)
Cave, H. M.
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Lim, C. -W.
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Jermy, M. C.
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Krumdieck, S. P.
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Smith, M. R.
Fluxes of multiple species are implemented in the Quiet Direct Simulation (QDS) scheme for gas flows. Each molecular species streams independently. All species are brought to local equilibrium at the end of each time step. The multi species scheme is compared to DSMC simulation, on a test case of a Mach 20 flow of a xenon/helium mixture over a forward facing step. Depletion of the heavier species in the bow shock and the near-wall layer are seen. The multi-species QDS code is then used to model the flow in a pulsed-pressure chemical vapour deposition reactor set up for carbon film deposition. The injected gas is a mixture of methane and hydrogen. The temporal development of the spatial distribution of methane over the substrate is tracked.
Parallel Direct Simulation Monte Carlo Computation Using CUDA on GPUs
(2011)
Su, C. -C.
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Hsieh, C. -W.
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Smith, M. R.
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Jermy, M. C.
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Wu, J. -S.
In this study computations of the two-dimensional Direct Simulation Monte Carlo (DSMC) method using Graphics Processing Units (GPUs) are presented. An all-device (GPU) computational approach is adopted-where the entire computation is performed on the GPU device, leaving the CPU idle-which includes particle moving, indexing, collisions between particles and state sampling. The subsequent application to GPU computation requires various changes to the original DSMC method to ensure efficient performance on the GPU device. Communications between the host (CPU) and device (GPU) occur only during problem initialization and simulation conclusion when results are only copied from the device to the host. Several multi-dimensional benchmark tests are employed to demonstrate the correctness of the DSMC implementation. We demonstrate here the application of DSMC using a single-GPU, with speedups of 3 similar to 10 times as compared to a high-end Intel CPU (Intel Xeon X5472) depending upon the size and the level of rarefaction encountered in the simulation.
Quiet Direct Simulation (QDS) of Viscous Flow Using the Chapman-Enskog Distribution
(2011-01-01)
Smith, M. R.
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Kuo, F. -A.
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Cave, H. M.
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Jermy, M. C.
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Wu, J. -S.
Presented here is the QDS method modified to employ an arbitrary governing velocity probability distribution. An algorithm is presented for the computation of QDS particle "blueprints". The method, which employs a known continuous velocity probability distribution function, uses a set of fixed QDS particle "weights", which can be arbitrarily selected. Provided the weights, particle "blueprint" velocities are computed by taking multiple moments around the governing velocity probability distribution function to provide the discrete representation employed by QDS. In particular, we focus on the results obtained when the governing distribution function is the Chapman-Enskog distribution function. Results are shown for several benchmark tests including a one dimensional standing shock wave and a two dimensional lid driven cavity problem. Finally, the performance of QDS when applied to General Purpose computing on Graphics Processing Units (GPGPU) is demonstrated.
The direction decoupled Quiet Direct Simulation method for rapid simulation of axisymmetric inviscid unsteady flow in pulsed pressure chemical vapour deposition
(2013-11-05)
Lim, C. W.
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Smith, M. R.
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Cave, H. M.
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Jermy, M. C.
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Wu, J. -S.
Pulsed Pressure-Chemical Vapour Deposition (PP-CVD) is a thin film deposition process which employs a highly unsteady flow with wide dynamic range of pressure. The large, time-varying density gradient during a PP-CVD process cycle produces a flow field in which the Knudsen number varies from the near-continuum to the rarefied regimes, making Direct Simulation Monte Carlo (DSMC) prohibitively expensive. The present directional decoupled Quiet Direct Simulation (DD-QDS) method is a novel kinetic-based flux scheme that computes fluxes of mass, momentum and energy at the interface of computational cells in a highly computationally efficient manner. The Maxwell-Boltzmann equilibrium distribution is enforced locally at each computational cell at each time step. In this paper, an axisymmetric second order directional decoupled QDS scheme is used to simulate highly unsteady flows encountered in PP-CVD reactor. Two simulations were conducted to investigate the PP-CVD reactor flow field at 1 Pa and 1 kPa reactor base pressures. The assumption of the local Maxwell-Boltzmann equilibrium distribution used in the QDS scheme is verified by examining the gradient length local Knudsen number based on the density, and by estimating the average number of molecular collisions within each computational cell in one computational time step. The validity of the local equilibrium assumption is found satisfactory at 1 kPa reactor based pressure but not at 1 Pa. The limitation of the QDS scheme in modelling PP-CVD flow was also investigated. The time required to establish the quasi-steady under-expanded jet is found to be similar to 4 ms, and the jet dissipates within 0.5 ms of the end of injection. This important information is required to set up PP-CVD operating conditions which give uniform film deposition. (C) 2013 Elsevier Ltd. All rights reserved.