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25 September 2006, Volume 23 Issue 5 Previous Issue    Next Issue

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Dirac Delta Function Discrete Ordinate Method for a One-Speed Transport Equation with Anisotropic Scatterings YANG Bo, LI Mao-sheng 2006, 23(5): 505-510.  Abstract ( )   PDF (310KB) ( )   With the Dirac delta function discrete ordinate method,a one-speed time-dependent transport equation with anisotropy scatterings is studied in a slab.The scattering function is assumed a combination of linearly anisotropic and strongly forward-backward scattering.The angular flux is split into a delta function and a low order function.The source term is obtained by integrating the delta function and quadrature the low order function.Numerical results for criticality eigenvalues are obtained and tabulated with different scattering parameters including extreme regions.

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A Conservative Remapping Algorithm for Polygonal Staggered Meshes WEN Wan-zhi, LIN Zhong, WANG Rui-li, FU Shang-wu 2006, 23(5): 511-517.  Abstract ( )   PDF (499KB) ( )   A discrete conservative remapping algorithm based upon refinement and numerical integrals,named particle remapping algorithm,is presented.The mass density distribution is chosen as either a piecewise constant with first-order accuracy or a piecewise linear distribution with second-order accuracy.It results in a first-order and a second-order algorithm.The density gradient is evaluated by an area average method with a piecewise linear distribution.On a staggered mesh,in which velocity is vertex-centered,an auxiliary mesh is introduced,and the velocity is remapped.The particle remapping algorithm can be applied to a structured or an unstructured mesh.It does not require a one-to-one mapping between the old and the new meshes.Numerical results show that the first-order algorithm is robust but has an excessive diffusion.The second-order one is better in shape-preservation but violates the monotonicity sometimes.To improve the monotonicity,a conservative mass repair algorithm for structured grids is extended to unstructured grids preserving upper and lower bounds of the density.Several remapping results are presented and the errors are analyzed.

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A Level-Set Discontinuity Tracking Method for Compressible Multifluids ZHANG Xue-ying, ZHAO Ning, WANG Chun-wu 2006, 23(5): 518-524.  Abstract ( )   PDF (468KB) ( )   The Level-Set discontinuity tracking technology is extended to the simulation of general compressible multimaterial flows.A Level-Set function is used to keep the track of the interface and discontinuity.A conservative high order accurate WENO scheme is employed.The ghost fluids are constructed in the narrow band region with solution of the Riemann problem.The GFM (ghost fluid method) is implemented for the study of material interface and general discontinuity such as detonation discontinuities.The algorithm is applied to simulate multi-material fluid flows and satisfactory numerical results are obtained.

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On the Background Integral in a Meshless Method and Simulation of One Particle Dropping QIU Yi, YOU Chang-fu, QI Hai-ying, XU Xu-chang 2006, 23(5): 525-529.  Abstract ( )   PDF (284KB) ( )   In a direct numerical simulation of gas-particle flow with meshless method,if the integral is carried out with common background integral cells numerical pressure fluctuation occurs.It results in unsuccessful computations.We analyze the reason and develope a solution strategy.It is used in the simulation of one particle dropping.Reasonable results are obtained.It shows that the new strategy avoids pressure fluctuation.

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A Model for Two-Phase Cavitation Flows LI Jun, LIU Li-jun, FENG Zhen-ping 2006, 23(5): 530-536.  Abstract ( )   PDF (428KB) ( )   An algorithm is developed for the flow field in a two-phase cavitation region.The algorithm is implemented in a viscous calculation based on a Reynolds-Averaged Navier-Stokes(RANS) equation solver considering the effect of turbulence.A prior solution of wall detachment point and bubble length is not required.They are simulated by a cavitation model.The liquid/vapor interface is tracked and obtained by an iteration of flow field and interface updating.The algorithm is well validated with a comparison of computational and experimental data available for external flows of a cone/cylinder body and a hemisphere/cylinder body.The liquid/vapor interface shape and bubble length are calculated.The pressure coefficient distributions obtained along the cavitation bubble surface agree well with experimental results.The feasibility and accuracy of the algorithm are illustrated.

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Large Eddy Simulation of Turbulent Channel Flows with an Oscillating Wall in Spanwise Direction XU Chun-xiao, WU Chao, CUI Gui-xiang 2006, 23(5): 537-544.  Abstract ( )   PDF (393KB) ( )   Large eddy simulations with three subgrid eddy viscosity models,i.e., the classic Smagorinsky model,the dynamical Smagorinsky model and a model derived from Kolmogorov equation by Cui(2004),are performed to study turbulent channel flows with a wall oscillation in the spanwise direction.The capability of the subgrid eddy viscosity models in predicting statistical three-dimensional unsteady flows is evaluated.It is found that the new model and the dynamical model are both capable of predicting three-dimensional unsteady turbulent flows;and the dynamical model can achieve a higher accuracy;the classic Smagorinsky model yields the worst result.

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A Meshless Method for Nonlinear Transient Field with Phase Change GAO Zhi-hua, ZHANG Ming-yi, LIU Zhi-qiang, ZHANG Shu-juan 2006, 23(5): 545-550.  Abstract ( )   PDF (301KB) ( )   We present an element free Galerkin method(EFGM) for nonlinear transient field involving phase change.It needs no element connectivity.Compared with other methods such as the finite element method(FEM),it is easy in tracking the growth of phase boundaries.Essential boundary conditions are enforced using a penalty function method.The MATLAB codes are developed to obtain numerical solutions.Two classical examples show that,compared with the FEM,EFGM has more advantages such as high accuracy,good convergence and simple post-process,etc.

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A Model for Image Deblurring and Denoising Based on the Rudin-Osher-Fatemi Model SHI Yu-ying, CHANG Qian-shun 2006, 23(5): 551-558.  Abstract ( )   PDF (485KB) ( )   A time dependent model for deblurring and denoising problems is proposed.Comparisons with the Rudin-Osher-Fatemi model are made in numerical experiments with antireflective boundary conditions and medium boundary conditions.The results demonstrate that the error with medium boundary conditions is smaller than that with antireflective boundary conditions.

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Simulation of Size Segregation of Granulars with Two Sizes in a Horizontal Vibrating Container ZHOU Di-wen, CHEN Shi-yi, CAI Qing-dong 2006, 23(5): 559-563.  Abstract ( )   PDF (331KB) ( )   The size segregation of grains is studied with two-dimensional molecular dynamic simulations for dissipative particles under horizontal vibration.A series of segregation that induced by horizontal vibrations at different amplitudes and frequencies are reported and the relationship between these two aspects is analyzed.A standard is given to scale the speed of the segregation.It is found that an optimal amplitude exists at a given frequency.An empiristic formula is obtained for this amplitude.It shows that another segregation mechanism exists at a large amplitude and frequency,in which the big particles are thrown directly to the top by the side wall.

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A Fast Local Optimization Algorithm for LJ and H2O Clusters HU Wei-jun, MENG Xian-mei, MEI Liang-mo, LI Hua 2006, 23(5): 564-570.  Abstract ( )   PDF (505KB) ( )   The time step in a molecular dynamic method is defined as a function of the largest acceleration,the largest velocity and the largest allowed space step of a particle.Using this algorithm,the lowest-energy geometries of smaller LJ clusters and H2O clusters are obtained,and the efficiency of this algorithm is demonstrated.A lowest-energy structure of(H2O)13 is found.

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Numerical Analysis of Microstructural Magnetization in Magnetic Fluid Materials LI Guo-bin, SONG Shun-cheng, ZHAO Bao-rong, YANG Run-tian, CAO Xue-jun 2006, 23(5): 571-575.  Abstract ( )   PDF (447KB) ( )   A boundary element model for particle dynamics simulation of magnetic fluids in magnetic fields is employed to calculate the field distributions and interactions in a particulate system.Compared with the finite element method,the boundary element method calculates sharper and more variable boundary area of particles.In the area other than boundary areas,the two methods show good agreement.With increasing node numbers,the magnetic force obtained by the boundary element method closes to the virtual force obtained by the finite element method.

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A Theoretical Study on the Local Structure,Optical Spectra and EPR Spectra of NiSO4·6H2O Crystals CHEN Tai-hong 2006, 23(5): 576-582.  Abstract ( )   PDF (361KB) ( )   The local structure,optical spectra,electron paramagnetic resonance(EPR) spectra and susceptibility of NiSO4·6H2O crystal at room temperature are calculated with a semi-SCF d-orbit wave function model for free Ni2+ ions,a point-charge model and Ballhausen's SCF wave functions for O2- using a complete diagonalization procedure(CDP).Theoretical results are in good agreement with experimental data.Some controversial issues,such as perturbation method,LCAO-MO method,parameter fitting and Clementi's single-zeta wave function are explained.

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Monte Carlo Simulation of Magnetic Monolayer Films DU Haifeng, DU An, HU Yong 2006, 23(5): 583-588.  Abstract ( )   PDF (330KB) ( )   Monte Carlo simulation is used to study the magnetic properties of magnetic monolayer films with an Ising-like model in which the exchange interaction and dipolar interaction are considered.The system shows an abnormal magnetization with an increasing of the dipolar interaction at low reduced temperatures.The magnetization is divided into two stages.The magnetization becomes difficult with increasing temperature in low external fields.The reverse process occurs in high external fields.This magnetization behavior results in an abnormal phenomenon that the magnetic entropy changes above zero at low temperatures in external fields.An accurate method is introduced to deal with the long-range interaction.

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Neutronic Performance in a Peaceful Nuclear Explosion Reactor BAI Yun, PENG Xian-jue 2006, 23(5): 589-593.  Abstract ( )   PDF (306KB) ( )   The neutronic behavior in a peaceful nuclear explosion reactor is analyzed.238U and 232Th are used in the reactor to produce fission materials,239Pu and 233U. MCNP is used to calculate thickness of the neutron slowing down material C2H4.The thickness of 232Th is predicted in which the neutron capturing ability is above 50%.Considering the effect of reflected saving,the thickness of 232Th is reduced.

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Potential Energies of Electronic States X3Σg- and A3Σu- of a B2 Molecule XIE An-dong, ZHU Zheng-he 2006, 23(5): 594-598.  Abstract ( )   PDF (293KB) ( )   The energies,equilibrium geometries and harmonic frequencies of the ground state X3Σg- and the first excited state A3Σu- of a B2 molecule are calculated using the GSUM(Group Sum of Operators) method of SAC/SAC-CI with the basic sets(D95(d),) 6-311g** and cc-PVTZ.It is found that the basis set D95(d) is suitable for the energy calculation of a B2 molecule.The potential curves are scanned using the SAC/D95(d) method for the ground state and the SAC-CI/D95(d) methods for the excited states.A least square is fitted to a Murrell-Sorbie function.The spectroscopy constants(Be,αe,ωe,and ωeχe) are calculated which are in good agreement with experiment.It is believed that the Murrell-Sorbie function and the SAC/SAC-CI method are suitable not only for the ground state,but also for low-lying excited states.

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On Algorithms for N-body Problems YANG Yuan-ling, NIE Qing-xiang, WU Xiao-mei, XU Shun-fu 2006, 23(5): 599-603.  Abstract ( )   PDF (276KB) ( )   The Runge-Kutta-Fehlberg algorithm(RKF),the symplectic algorithm and the Hermite algorithm for N-body problems are studied with energies errors and semimajor axis and eccentricity.It shows that the precision of RKF is the highest,but its error increases with computation time.The symplectic algorithm has no artificial dissipation,and keeps stability of the energy error.The structure of the Hermite algorithm is simple and its computation time is short,but its error is greater than that of the other two.

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Local Split Step Fourier Propagator for Wave Equation Depth Migration CHEN Sheng-chang, MA Zai-tian, WU Ru-shan 2006, 23(5): 604-608.  Abstract ( )   PDF (418KB) ( )   A frame expansion method of operator is used to the phase shift operator of split-step Fourier propagator.A local split-step Fourier propagator for wavefield propagation is presented and used in a wave equation prestack depth migration.The local split-step Fourier propagator consists of a phase shift operator in phase-space-frequency domain and a windowed time shift operator in space-frequency domain.Different from the global phase shift operator in wavenumber-frequency domain,the phase shift operator in phase-space-frequency domain shows good space localization.A numerical test of wave equation prestack depth migration on the international standard SEG-EAGE 2D salt dome model shows that the local split-step Fourier propagator is stable and is applicable to the media with velocity strong lateral variation.

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Spatial Angle Quadrature in FMM YE Hong-xia, JIN Ya-qiu 2006, 23(5): 609-613.  Abstract ( )   PDF (274KB) ( )   We analyze the integrand spectrum in the plane wave expansion of spherical functions,and present a standard of(2L,4L) point quadrature for spatial angles satisfying the sampling theorem.Numerical results with the method of moments(MOM) and fast multipole method(FMM) in different quadrature schemes are compared to show the validation of our quadrature scheme.

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Computation of Flow Electrification in Oil Pipes WANG Ju-fen, PU Jia-ning, MENG Hao-long 2006, 23(5): 614-620.  Abstract ( )   PDF (316KB) ( )   Computation of flow electrification in oil pipes is studied.The charge transportation equation in turbulent flows is a convection-dominated convection diffusion reaction equation.An operator-splitting scheme is introduced to solve this equation.In the scheme,the equation is split into a pure convection equation,a pure diffusion equation and a pure reaction equation.They are solved by a characteristics method and a finite difference method,respectively.Numerical examples show that this scheme describes the charge distribution in a pipe.A reliable method for streaming current is obtained.

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A New Chaotic Encryption Algorithm Based on Ergodicity WANG Xing-yuan, DUAN Chao-feng 2006, 23(5): 621-625.  Abstract ( )   PDF (322KB) ( )   The encryption and weaknesses of the E.Álvarez cryptosystem are analyzed.A new chaotic cryptosystem based on ergodicity is proposed.The control parameters and initial condition of a chaotic system are selected as the secret key.A bit chain from the chaotic orbit is generated,and the position at which a plaintext block appears in the chain is found.Then record the number of iterations of the chaotic map as the cipher block.Several weaknesses of the E.Álvarez cryptosystem are voided in the new scheme.The security is strengthened.Finally the new cryptosystem is studied experimentally using a logistic map.It is shown that the new cryptosystem satisfies the confusion and diffuse characteristics.Effectiveness of the proposed scheme is demonstrated.

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Synchronization of Nonlinear Coupled Hyper-Chaotic Rössler Systems and Networks YU Hong-jie, PENG Jian-hua 2006, 23(5): 626-630.  Abstract ( )   PDF (277KB) ( )   The synchronization of two symmetrically nonlinear-coupled hyperchaotic Rössler systems is discussed.A special nonlinear-coupled term is constructed by a suitable separation of the linear and nonlinear terms of a hyperchaotic system.The phenomenon of stable chaotic synchronization is found in a certain region around α=0.5.The stability of the synchronous state is examined by the stability criterion of linear systems and conditional Lyapunov exponent.The synchronization of the network linked by hyperchaotic Rössler systems using special nonlinear coupling function is studied.It is shown that the stability region of coupling units is expected from an estimating stability region of two coupling units only.And the coupling strength is in inverse proportion to the number of coupling units.Numerical results demonstrate that the proposed method is effective for hyper-chaotic systems and networks.