Browsing by Author "Chen, Y. -S."

Now showing 1 - 9 of 9

A parallel hybrid numerical algorithm for simulating gas flow and gas discharge of an atmospheric-pressure plasma jet
(2012-12-01)
Lin, K. -M.
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Hu, M. -H.
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Hung, C. -T.
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Wu, J. -S.
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Hwang, F. -N.
Development of a hybrid numerical algorithm which couples weakly with the gas flow model (GFM) and the plasma fluid model (PFM) for simulating an atmospheric-pressure plasma jet (APPJ) and its acceleration by two approaches is presented. The weak coupling between gas flow and discharge is introduced by transferring between the results obtained from the steady-state solution of the GFM and cycle-averaged solution of the PFM respectively. Approaches of reducing the overall runtime include parallel computing of the GFM and the PFM solvers, and employing a temporal multi-scale method (TMSM) for PFM. Parallel computing of both solvers is realized using the domain decomposition method with the message passing interface (MPI) on distributed-memory machines. The TMSM considers only chemical reactions by ignoring the transport terms when integrating temporally the continuity equations of heavy species at each time step, and then the transport terms are restored only at an interval of time marching steps. The total reduction of runtime is 47% by applying the TMSM to the APPJ example presented in this study. Application of the proposed hybrid algorithm is demonstrated by simulating a parallel-plate helium APPJ impinging onto a substrate, which the cycle-averaged properties of the 200th cycle are presented. The distribution patterns of species densities are strongly correlated by the background gas flow pattern, which shows that consideration of gas flow in APPJ simulations is critical. (C) 2012 Elsevier B.V. All rights reserved.
An Improved Parallelized Hybrid DSMC-NS Algorithm
(2009)
Lian, Y-Y.
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Tseng, K-C.
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Chen, Y. -S.
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Wu, M. -Z.
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Wu, J. -S.
An improved parallelized hybrid DSMC-NS algorithm presented and verified. In previous work [11, it showed the slow convergence of coupling between two numerical solvers if the normal velocities across the breakdown interfaces are very low. By detailed kinetic velocity sampling study from the pure DSMC simulation of a two-dimensional supersonic nitrogen flow (M-infinity=4) past a 25 finite wedge, we have found most of boundary layer region is in nearly thermal equilibrium, even with very high continuum breakdown Kn(max)[10]. Thus, a new continuum breakdown parameter based on pressure gradient, pressure and local mean free path, which can be used to effectively "exclude" the "false" breakdown region like the boundary layer, is proposed in this paper. Results show that the improved algorithm can greatly reduce the computational cost while maintaining essentially the same accuracy.
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.
Development of an improved spatial reconstruction technique for the HLL method and its applications
(2011-02-01)
Smith, Matthew R.
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Lin, K. -M.
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Hung, C. -T.
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Chen, Y. -S.
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Wu, J. -S.
The integral form of the conventional HLL fluxes are presented by taking integrals around the control volume centred on each cell interface. These integrals are demonstrated to reduce to the conventional HLL flux through simplification by assuming spatially constant conserved properties. The integral flux expressions are then modified by permitting the analytical inclusion of spatially linearly varying conserved quantities. The newly obtained fluxes (which are named HLLG fluxes for clarification, where G stands for gradient inclusion) demonstrate that conventional reconstructions at cell interfaces are invalid and can produce unstable results when applied to conventional HLL schemes. The HLLG method is then applied to the solution of the Euler Equations and Shallow Water Equations for various common benchmark problems and finally applied to a 1D fluid modeling for an argon RF discharge at low pressure. Results show that the correct inclusion of flow gradients is shown to demonstrate superior transient behavior when compared to the existing HLL solver and conventional spatial reconstruction without significantly increasing computational expense. (C) 2010 Elsevier Inc. All rights reserved.
Direct numerical simulation of the interaction of an ultra short-pulsed intense laser with a H(2)(+) molecule
(2008-01-01)
Lee, Y. -M.
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Wu, J. -S.
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Jiang, T. -F.
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Chen, Y. -S.
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機械工程學系
In this paper, interactions of a linearly polarized ultra short-pulsed intense laser with a single H(2)(+) molecule at various angles of incidence are studied by directly solving the time-dependent three-dimensional Schrodinger equation (TDSE), assuming Born-Oppenheimer approximation. An explicit stagger-time algorithm is employed for time integration of the TDSE, in which the real and imaginary parts of the wave function are defined at alternative times, while a cell-centered finite-volume method is utilized for spatial discretization of the TDSE on Cartesian grids. The TDSE solver is then parallelized using domain decomposition method on distributed memory machines by applying a multi-level graph-partitioning technique. The solver is applied to simulate laser-molecular interaction with test conditions including: laser intensity of 0.5* 10(14) W/cm(2), wavelength of 800 nm, three pulses in time, angle of incidence of 0-90 degrees and inter-nuclear distance of 2 a.u.. Simulation conditions include 4 million hexahedral cells, 90 a.u. long in z direction, and time-step size of 0.005 a.u.. Ionization rates, harmonic spectra and instantaneous distribution of electron densities are then obtained from the solution of the TDSE. Future possible extension of the present method is also outlined at the end of this paper.
Direct numerical simulation of the interaction of an ultra short-pulsed intense laser with a H-2(+) molecule
(2008-01-01)
Lee, Y. -M.
;
Wu, J. -S.
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Jiang, T. -F.
;
Chen, Y. -S.
;
機械工程學系
In this paper, interactions of a linearly polarized ultra short-pulsed intense laser with a single H-2(+) molecule at various angles of incidence are studied by directly solving the time-dependent three-dimensional Schrodinger equation (TDSE), assuming Born-Oppenheimer approximation. An explicit stagger-time algorithm is employed for time integration of the TDSE, in which the real and imaginary parts of the wave function are defined at alternative times, while a cell-centered finite-volume method is utilized for spatial discretization of the TDSE on Cartesian grids. The TDSE solver is then parallelized using domain decomposition method on distributed memory machines by applying a multi-level graph-partitioning technique. The solver is applied to simulate laser-molecular interaction with test conditions including: laser intensity of 0.5* 10(14) W/cm(2), wavelength of 800 nm, three pulses in time, angle of incidence of 0-90 degrees and inter-nuclear distance of 2 a.u.. Simulation conditions include 4 million hexahedral cells, 90 a.u. long in z direction, and time-step size of 0.005 a.u.. Ionization rates, harmonic spectra and instantaneous distribution of electron densities are then obtained from the solution of the TDSE. Future possible extension of the present method is also outlined at the end of this paper.
Investigation of Dual-Vortical-Flow Hybrid Rocket Engine without Flame Holding Mechanism
(2018-01-01)
Lai, A.
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Chou, T. -H.
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Wei, S. -S.
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Lin, J. -W.
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Wu, J. -S.
A 250 kgf thrust hybrid rocket engine was designed, tested, and verified in this work. Due to the injection and flow pattern of this engine, this engine was named dual-vortical-flow engine. This propulsion system uses N2O as oxidizer and HDPE as fuel. This engine was numerically investigated using a CFD tool that can handle reacting flow with finite-rate chemistry and coupled with the real-fluid model. The engine was further verified via a hot-fire test for 12 s. The ground I-sp of the engine was 232 s and 221 s for numerical and hot-fire tests, respectively. An oscillation frequency with an order of 100 Hz was observed in both numerical and hot-fire tests with less than 5% of pressure oscillation. Swirling pattern on the fuel surface was also observed in both numerical and hot-fire test, which proves that this swirling dual-vortical-flow engine works exactly as designed. The averaged regression rate of the fuel surface was found to be 0.6 similar to 0.8 mm/s at the surface of disk walls and 1.5 similar to 1.7 mm/s at the surface of central core of the fuel grain.
Parallel solver for the three-dimensional Cartesian-grid-based time-dependent Schrodinger equation and its applications in laser-H(2)(+) interaction studies
(2008-01-01)
Lee, Y. -M.
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Wu, J. -S.
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Jiang, T. -F.
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Chen, Y. -S.
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機械工程學系
A parallelized three-dimensional Cartesian-grid-based time-dependent Schrodinger equation (TDSE) solver for molecules with a single electron, assuming the motion of the nucleus is frozen, is presented in this paper. An explicit stagger-time algorithm is employed for time integration of the TDSE, in which the real and imaginary parts of the wave function are defined at alternate times, while a cell-centered finite-volume method is utilized for spatial discretization of the TDSE on Cartesian grids. The TDSE solver is then parallelized using the domain decomposition method on distributed memory machines by applying a multilevel graph-partitioning technique. The solver is validated using a H(2)(+) molecule system, both by observing the total electron probability and total energy conservation without laser interaction, and by comparing the ionization rates with previous two-dimensional axisymmetric simulation results with an aligned incident laser pulse. The parallel efficiency of this TDSE solver is presented and discussed; the parallel efficiency can be as high as 75% using 128 processors. Finally, examples of the temporal evolution of the probability distribution of laser incidence onto a H(2)(+) molecule at inter-nuclear distance of 9 a.u. (chi=0 degrees and 90 degrees) and the spectral intensities of harmonic generation at internuclear distance of 2 a.u. (chi=0 degrees, 30 degrees, 60 degrees, and 90 degrees) are presented to demonstrate the powerful capability of the current TDSE solver. Future possible extensions of the present method are also outlined at the end of this paper.
Simulations of the FORMOSAT-5 Cold Gas Propulsion System by Using the Hybrid Continuum-Particle Method
(2012)
Tseng, K. -C.
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Lien, Y. -Y.
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Chen, Y. -S.
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Kuo, T. -C.
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Gu, B. -R.
Reaction control subsystem (RCS) is an onboard satellite propulsion system used to provide required thrusting for orbit raising, orbit maintenance, and attitude control etc.. High pressure flow with high temperature could be generated in a chamber by chemical reactions or other power resources then expelled through a convergent-divergent nozzle to obtain thrust. In order to optimize the thrusting performance, numerical simulation is an efficient method to study the physics and parameters in design phase. In the current study, a hybrid method coupled continuum and particle methods is proposed to simulate flows involving continuum and rarefied regions. The Navier-Stokes (NS) solver named UNIC is developed by Chen and his coworkers. It employs the cell-centered finite-volume method with a hybrid 2D/3D unstructured-grid topology. The proposed particle code named Parallel DSMC Code (PDSC) is a parallelized solver based on the well-known Direct Simulation of Monte Carlo (DSMC) method, which was proposed by Bird in 1976. The physical domain is decomposed into several regimes and each sub-domain executes the serial DSMC code at different processor for speeding up the computing. A practical cold gas thruster with different chamber pressures is simulated by using this hybrid code to study the potential malfunction of the pressure regulator. Then the curve-fitting thrusting equation can be referred to satellite control operations.