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25 September 2012, Volume 29 Issue 5 Previous Issue    Next Issue

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Positivity Preserving Time Step for Lagrange Method TANG Weijun, YUAN Guangwei, SHEN Zhijun, CHENG Junbo 2012, 29(5): 633-640.  Abstract ( )   PDF (1240KB) ( )   We propose a method of time step for predicator-corrector scheme of Lagrangian method on stagger grids.It is different from CFL time step based on tradional stable theory.The method considers nonlinear effect of original partial differential equations,and gives an adaptive time step based on physical quantities positivity preserving.Numerical results confirm validity of the method.

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Discontinuous Finite Element Method for 1D Non-equilibrium Radiation Diffusion Equations ZHANG Rongpei, YU Xijun, CUI Xia, FENG Tao 2012, 29(5): 641-646.  Abstract ( )   PDF (1177KB) ( )   We discuss numerical simulation of one-dimensional non-equilibrium radiation diffusion equations.A weighted numerical flux between adjacent grid cells is obtained by solving heat conduction equation with discontinuous coefficient.With this numerical flux of diffusive generalized Riemann problem(dGRP),a discontinuous finite element method is proposed for radiation diffusion equations. A backward Euler time diseretization is applied for semi-discrete form and a Picard iteration is used to solve nonlinear system of equations.Numerical results demonstrate that the method has a capability of capturing strong gradients and can be accommodated to discontinuous diffusion coefficient.

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Implicit-explicit Integration Factor Discontinuous Galerkin Method for 2D Radiation Diffusion Equations、 ZHANG Rongpei, YU Xijun, CUI Xia, FENG Tao 2012, 29(5): 647-653.  Abstract ( )   PDF (1269KB) ( )   A numerical method is developed for two-dimensional nonequilibrium radiation diffusion equations.Discontinuous Galerkin method is applied in spatial diseretization in which numerical flux is constructed with weighted flux averages.Implicit-explicit integration factor method for time discretization is applied to nonlinear ordinary differential equations which is obtained with discontinuous Galerkin method. Radiation diffusion equations with multiple materials are solved on unstructured grids in numerical tests.It demonstrates that the method is effective for high nonlinear and tightly coupled radiation diffusion equations.

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Iteration Process of JFNK Method and Physical Constraints AN Hengbin, MO Zeyao 2012, 29(5): 654-660.  Abstract ( )   PDF (769KB) ( )   Jacobian.free Newton-Krylov(JFNK) method is analyzed and improved by using physical constraints in iteration. Consequently.physical constraints are always satisfied in iteration process of the improved JFNK method.Non。physical phenomenon is avoided.In particular.there is no negative temperature as the method is used for 2-D 3-T energy equations.Robustness of JFNK method is improved.

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Treatment of Edge in Characteristic Non-reflecting Boundary Conditions LI Debo, FAN Jianren, YI Fuxing, CEN Kefa 2012, 29(5): 661-666.  Abstract ( )   PDF (249KB) ( )   A mathematical treatment of edge in characteristic non-reflecting boundary conditions is developed.Several numerical simulations show that this method is benefit for treating of waves.A spatially evolving compressible round jet is simulated with direct numerical simulation method.Simulation results are compared with experiments.It suggests that the method on edge boundary conditions is convincible.

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Technique for Changing Connectivity of Mesh and Closed Void Interface Simulation WANG Ruili, LIU Quan, LIN Zhong 2012, 29(5): 667-674.  Abstract ( )   PDF (1683KB) ( )   We present a closed void interface method for computational models with void interface in multi-material problems.It simulates void successfully in complicated engineering.It bases on a technique of changeable mesh topology.It avoids using a straight line to replace a curve in traditional method,and overcomes difficulty in solving contact points.Numerical simulations confirm that the technique is effective.It shows characteristic of no void connectivity.

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Modified PEBI Grid Generation Method with Advancing Front Approach WANG Daigang, HOU Jian, XING Xuejun, ZHANG Xiansong, ZHONG Hongjiao 2012, 29(5): 675-683.  Abstract ( )   PDF (3143KB) ( )   A modified PEBI grid generation method with frontal approach is proposed,which is used to investigate reservoir modeling.Validity of the PEBI grid generation method is shown by comparing results of a self-developed reservoir simulator with that of a software CMG.A synthetic case with irregular boundary is investigated.It shows that PEBI grids reduce influence of grid orientation compared with conventional Cartesian grids.It is favorable to describe complex reservoir boundaries accurately and to implant internal constraints such as vertical well,fault in PEBI grid generation method.

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An Implicit Time·integration Algorithm for Diffusion Equations with Structured AMR and Applications XU Xiaowen, MO Zeyao, LIU Qingkai, AN Hengbin 2012, 29(5): 684-692.  Abstract ( )   PDF (3169KB) ( )   We consider solution of diffusion equations using structured adaptive mesh refinement(SAMR).In SAMR hierarchy, each level is organized as a union of uniform rectangular patches.An implicit time-integration algorithm with temporal refinement strategy is shown.In the algorithm,timestepping advances from the coarsest level to the finest level sequentially,and a multilevel synchronization process is required for fixing fluxes dismatch at coarse-fine interface.A criterion for algorithm complexity is introduced. Numerical results show validation and performance of the algorithm.Finally,the algorithm is applied to radiation hydrodynamics simulations,where nonlinear non-equilibrium radiation diffusion equations are solved.Simulation result shows that,compared with uniform refinement mesh,performance of the method is improved by 33 times.

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An Improved SPH Method with Physical Viscosity and Application in Dam.break Problem HAN Yawei, QIANG Hongfu 2012, 29(5): 693-699.  Abstract ( )   PDF (1285KB) ( )   In physical viscosity SPH method with low Reynolds number,density reinitialization is performed with reproducing kernel particle method(RKPM).It can avoid numerical dissipation resulting from artificial viscosity.Numerical stability of physical viscosity SPH method with low Reynolds number is improved.Taking dam-break problem for example,it is shown that the method can remove numerical instability efficiently.The method can be applied to problems with higher Reynolds numbers or viscous fluid.

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Improvement on Algorithm for Pressure Equation in PCCSAP-3D ZHANG Yaoli, YANG Yanning, ZHOU Zhiwei, WANG Yan 2012, 29(5): 700-706.  Abstract ( )   PDF (2615KB) ( )   An algorithm for pressure equation in a computational code PCCSAP-3 D,which is dedicated to analyze transient process of PCCS(Passive Containment Cooling System),is introduced.GMRES is introduced to accelerate calculation speed.Pre-conditioning of GMRES is applied.Convergence rates of GMRES and ML-ADI are compared.It shows that pre-conditioned GMRES can be used to solve pressure equations more efficiently.Since pressure equation coefficient matrix is structural and sparse,pre-conditioned GMRES is more efficient than ML-ADI as pressure equation coefficient matrix is larger.

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Analysis of Non-equilibrium Condensative Flow in a Shock Tube ZHAO Jiaquan, LIU Peiqi, Liu Fengxia, ZHAO Wenjing, WANG Liang, HU Dapeng 2012, 29(5): 707-712.  Abstract ( )   PDF (768KB) ( )   An explicit equation for critical quantity of heat in condensing flow in a shock tube was derived.The equation is of general validity and applies tO vapor flow with or without cagier gas.It shows that critical quantity of heat in internal heat flow for phase change is greater than external heat flow.As flow is choked thermally,a shock wave is generated and propagates upstream.A location where local static pressure begins to rises,which has long been regarded as onset of unsteady condensation in a steady supersonic nozzle,is not acceptable in shock tube flow.It is before the zone of condensation.Furthermore,a second-order AUSM scheme is applied tO two-dimensional compressible flow with condensation.Computation results reproduce integral predictions well.

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Calculation of Multi-source Looped--pipe Networks Based on Object-oriented Method WANG Hai, WANG Haiying, ZHU Tong, ZHOU Haizhu 2012, 29(5): 713-720.  Abstract ( )   PDF (862KB) ( )   An algorithm based on object-oriented(00) method is proposed.All pipe components in network topology structure are given special object,properties.Hydraulic calculation models are expressed by object methods.Boundary conditions variations are reflected by object events.Then hydraulic calculation can be solved as traversing all pipe components.Two cases are calculated and analyzed.It indicates that the method can effectively handle hydraulic imbalance caused in structure or in heat sources of a multi-source looped-pipe network with small deviations.

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Algorithm and Test of MCDB for BNCT LI Gang, DENG Li, CHEN Chaobin, YE Tao, MO Zeyao 2012, 29(5): 721-726.  Abstract ( )   PDF (1665KB) ( )   A BNCT treating planning system MCDB is developed.Three-dimensional material matrix and tally matrix are designed, which are used to describe voxel models and tally.A fast track technique is used in simulation.Computation time is greatly decreased compared with MCNP code.Same dosimetry results with MCNP are achieved by MCDB,which is faster by 3.1-3.4 times with respect to MCNP.Computation time and accuracy of most voxels reach clinical BNCT requirement in a scale of 10 million particles.

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Critical Effect in Lattice Homogenization via Monte Carlo Method LI Mancang, WANG Kan, YAO Dong 2012, 29(5): 727-732.  Abstract ( )   PDF (1211KB) ( )   In practice of homogenization with Monte Carlo method,critical effect is studied and BN theory is applied taking critical effect into account.Fundamental mode with buckling B is used as a measure of finite size.A critical spectrum in solution of fine-group Bl equations is used to correct weighted spectrum for homogenization.A PWR prototype core is examined to verify that the method generates few group constants effectively.In addition,a zero power physical experiment verification is performed.Numerical validations show that BN theory is adequate for critical correction in multi-group constants generation via Monte Carlo method.

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Implementation of Monte Carlo Collision Algorithm in Code CHIPIC YANG Chao, WANG Huihui, CHEN Ying, XIA Mengzhong, WANG Xiaomin, LIU Dagang 2012, 29(5): 733-738.  Abstract ( )   PDF (1152KB) ( )   Collisions between electrons in an ion source are analyzed theoretically.Processing models for null collisions between electron and other particles and Coulomb collisions among electrons are analyzed.With different storage and call mechanism,two Monte Carlo collision(MCC) processing models are developed.Then,MCC numerical models are added to a particle simulation software CHIPIC.With ion source JAERI 10A,the models ale verified.

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Particle Simulation on Non-resonance and Stochastic Heating of Ions with Low-frequency Alfvén Waves LI Kehua, GONG Xueyu, LU Xingqiang, LI Jingchun 2012, 29(5): 739-744.  Abstract ( )   PDF (1222KB) ( )   Heating of magnetized plasmas by low-frequency Alfvén waves propagating along background magnetic field is studied with test particle simulation.It shows that ions in perpendicular and parallel directions of the background magnetic field are heated significantly.In a stage of non-resonance heating,perpendicular heating is more significant than parallel heating,and anisotropic temperature comes into being.In a stage of stochastic heating,perpendicular and parallel temperatures of ions finally reach saturation and convergence.In heating process,the maximum kinetic temperature of ions is dependent on the ratio of magnetic field energy density and plasma density,which has no relevance with Alfvön-wave frequency and amplitude.Ions are significantly accelerated in parallel direction.and attained a bulk speed that is roughly equal to phase velocity of Alfvén waves.

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SAR Imaging of One-dimensional Rough Surface with a Buried Object JI Weijie, TONG Chuangming 2012, 29(5): 745-752.  Abstract ( )   PDF (2180KB) ( )   A fast radar imaging method for a buried object is presented.At first,backscatter data is computed with propagation. inside-layer expansion combining forward and backward method with spectral acceleration(PILE+FB.SA).Then,a conventional synthetic aperture radar(SAR)imaging procedure called back projection method is used to generate 2-D images.The choice of window functions is studied.A random rough surface with Gauss spectrum is used to simulate the ground.A tapered incident wave is chosen to reduce truncation errors.This method is proper for rough surface with any parameters with buried complex object.It is useful in realistic object detecting.

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Multiseale Modeling of Three-dimensional Contacts DUAN Fangli, GUO Qichao 2012, 29(5): 753-758.  Abstract ( )   PDF (2093KB) ( )   A bridging domain method,in which molecular models and continuum models are coupled,is performed to study threedimensional contact of a rigid spherical tip and a smooth fiat substrate.It is compared with full-atom molecular simulations.We focus on relaxation behavior under a given load,continuity of displacement and stress at bridging domain,normal force and contact radius.It shows that the bridging domain method quickly reaches equilibrium and has weaker oscillations under a given external load.It has small relative errors as initial temperature of system is at 0 K.The bridging domain method can transmit continuously displacement and stress from molecular domain to continuum domain,and has goad coupling effect.Normal force-displacement and contact radius-displacement are nearly coincide with molecular simulations.Its computational accuracy is high.

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Molecular Dynamics Simulation of Semicrystalline Polymers DUAN Fangli, YAN Shidang 2012, 29(5): 759-765.  Abstract ( )   PDF (1732KB) ( )   Molecular dynamics simulation in coarse-grained model of polyvinyl alcohol is made to get semicrystalline polymer from a molten state with slow cooling and partial crystallization.Evolution of static structure factor shows that intensity increasing in small-angle scattering appears before the Bragg peak in early crystallization,which is consistent with experimental observations with small/wide angle X-ray scattering.Simulated semicrystalline polymer shows a structure resembling Ying-shaped micro-beam model.Different cooling stages have different formation mechanisms with ordered structures.From crystallization to glass temperature in solidification process,stretching of molecular chain and arranging in parallel between straight molecular chains exist.Adjustment of straight chains in relative positions is only allowed by material activate.

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Simulation of Particle Aggregations with Local Area Source CHEN Le, WENG Jiaqiang 2012, 29(5): 766-774.  Abstract ( )   PDF (1765KB) ( )   A modified DLA model is used to simulate particle aggregations formed by random particle released from a local source.It shows that growth structure of aggregations are relative to position of the particle source.Growth velocity toward particle source is faster.It is also shown that particle distribution in growth plane is related to relative distance and relative size of the particle source.A law of particle distribution depends on particle source is shown.It may help in understanding azimuth of particle source from the growth stiucture of aggregations,and to estimate relative distance and relative size of the particles source.

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Influence of Relative Phase on Transient Evolution of LWI Gain in an Open V-type System LIANG Ying, Wang Jun, FAN Xijun 2012, 29(5): 775-780.  Abstract ( )   PDF (324KB) ( )   With numerical solution of density matrix motion equations of an open V-type three-level atomic system with spontaneously generated coherence,we analyze transient evolution of gain of lasing without inversion(LWI).It is shown that relative phase(Ф) between probe and driving fields has a significant effect on transient evolution of gain regardless of incoherent pump;Greater transient and steady gains can be obtained by selecting φ;As incoherent pump is absent,longer time is needed to get the greatest transient gain and steady gain.Oscillation amplitude in the evolution process is greater,and gain is also greater;Changing atomic exit rate r0(ratio of atomic injection rates S) leads to detail of gain transient evolution.In certain range of r0(S),transient and steady gains increase with S(r0).

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Radiations of C4 with Two Structures in Laser Field ZHU Yun, LU Chao, WANG Zhiping 2012, 29(5): 781-785.  Abstract ( )   PDF (692KB) ( )   With time-dependent density functional theory and molecular dynamics excitations of linear and rhombic C4 in intense laser field are explored.It is found that electrons of 1incar C4 obtain mote enelrgy while ions of rhombic C4 get more from the 1aser filed. Ionization is enhanced as C4 owns 8 linear structure. As laser polarization is along the x axis there is only Dx excitation of a linear C4 while dipole oscillations of a rhombic C4 in x and y directions are excited.

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First Principles Study of Electronic Structure of BaTi2As2O XUAN Shuke 2012, 29(5): 786-790.  Abstract ( )   PDF (2039KB) ( )   With plane wave vectors and pseudopotential method within the fbme of density functional theory.we study band structure,Fermi surfaces and density of state of BaTi2As2O.It is found that BaTi2As2O is a nonmagnetic metal and density of state at Fermi level is dominated by Ti 3d electrons.Ti 3d orbitals are hybridized with As 4p orbitals strongly rather than O 2p orbitals.A magnetic ground state of BaTi2As2O is not found,which is consistent with a study on Na2Ti2Sb2O by Pickett.