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25 September 2019, Volume 36 Issue 5 Previous Issue    Next Issue

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A Cell-centered Lagrangian Scheme Based on Characteristics Theory for Condensed Explosive Detonation LI Shiyao, YU Ming 2019, 36(5): 505-516.  DOI: 10.19596/j.cnki.1001-246x.7913 Abstract ( )   HTML ( )   PDF (10553KB) ( )   We present a cell-centered Lagrangian scheme for numerical simulation of condensed-explosive detonation. It utilizes finite volume method to discrete detonation equations. Velocity and pressure of grid nodes are obtained with characteristics theory of hyperbolic partial differential equations. They are used to update position of grid nodes and calculate numerical flux of grid cells. Solution of grid nodes obtained by characteristics theory is a "genuinely multi-dimensional" theoretical solution, which is a generalization of one-dimensional Godunov scheme in two-dimensional Riemann problem. Semi-discrete system of detonation equations obtained by finite volume scheme is solved with an implicit-explicit Runge-Kutta method. Convection terms are explicitly treated, and stiff source terms of chemical reactions are implicitly treated. The numerical schemes select ZND model for detonation, JWL equations of state for unreacted explosives and detonation products, and use Ignition-Growth model to simulate evolution process in reaction zone. Typical numerical results show that the cell-centered Lagrangian scheme simulates condensed-explosive detonation problems well.

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A Local Discontinuous Petrov-Galerkin Method for Partial Differential Equations with High Order Derivatives ZHAO Guozhong, YU Xijun, GUO Hongping, DONG Ziming 2019, 36(5): 517-532.  DOI: 10.19596/j.cnki.1001-246x.7919 Abstract ( )   HTML ( )   PDF (10257KB) ( )   A local discontinuous Petrov-Galerkin method is proposed for solving three types of partial differential equations with second, third and fourth order derivatives, respectively. They are Burgers type equations, KdV type equations and bi-harmonic type equations. The method extends discontinuous Petrov-Galerkin method for conservation laws by rewriting corresponding equations into a first order system and solving the system instead of the original equation. The method has a fourth order accuracy and maintains advantages of discontinuous Petrov-Galerkin method. Numerical simulations verify that the method reaches optimal convergence order and simulates well complex wave interaction such as soliton propagation and collision.

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DSMC Analysis of Unsteady Chemical Reactions of Hydrogen Plasma in Cylinder SU Yongyuan, LI Jie, FAN Zhenglei 2019, 36(5): 533-541.  DOI: 10.19596/j.cnki.1001-246x.7891 Abstract ( )   HTML ( )   PDF (11238KB) ( )   Unsteady flow characteristics of particles in plumes are studied with direct simulation Monte-Carlo method (DSMC) in an 8×10-6 s period. Dissociation model, recombination model and charge exchange model are included based on Bird's chemical reaction models. Molecular hydrogen H2, atomic hydrogen H, atomic metal X, ion H2+ and ion H+ are included. Influence on density distribution of particles in the field due to dissociation and recombination is studied. It shows that the number density of molecular hydrogen H2 decreases and the number density of atomic H increases since dissociation rate of molecular hydrogen H2 is faster than recombination of atomic H in the flow field. A charge exchange model is added in dissociation and recombination models. It shows that the number density of ion H2+ decreases and the number density of ion H+ increases obviously in charge exchange model.

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Effect of Free-slip Boundaries on Flow and Heat Transport Characteristics in Two-dimensional Rayleigh-Bénard Convection HE Peng, BAO Yun 2019, 36(5): 542-550.  DOI: 10.19596/j.cnki.1001-246x.7923 Abstract ( )   HTML ( )   PDF (7093KB) ( )   Temperature fields in two-dimensional (2D) Rayleigh-Bénard (RB) convection with no-slip and free-slip walls were simulated with parallel direct method of direct numerical simulation (PDM-DNS) at different Rayleigh numbers (Ra) and aspect ratio numbers (Γ). Different from no-slip boundary thermal convection with random plume, free-slip boundary thermal convection eventually forms a large-scale circulation without turbulent features and temperature distributes only on four walls. Temperature distribution characteristics of time-average field near floor changes gradually for no-slip boundary but overshoot occurs for free-slip boundary. Dependences of Nusselt numbers (Nu) with Ra at Γ=1 have same scale index Nu~Ra0.3. Free-slip boundary thermal convection enhances heat transfer. Changes of Nu with Γ are obvious in free-slip boundary thermal convection. They are in two stages. At Γ=0.5, Numax≈250, it is five times of Nu in no-slip boundary thermal convection.

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Feasibility of Simulation on Flow in Porous Media with Gas Kinetic Scheme ZENG Wei, CHEN Songze, GUO Zhaoli 2019, 36(5): 551-558.  DOI: 10.19596/j.cnki.1001-246x.7922 Abstract ( )   HTML ( )   PDF (9037KB) ( )   We extend gas kinetic scheme (GKS) to low-speed porous media flow, and test feasibility and effectiveness of the method. It shows that GKS is second-order accurate in space and is capable of predicting permeability of porous media. Compared with single-relaxation-time lattice Boltzmann method, GKS implements precisely no slip boundary condition, thus reflects correctly characteristics of viscosity-independent permeability. For complex flow in Berea sandstone slice structure, simulation results are in good agreement with experimental data, and permeability is calculated accurately. A criteria of Ma number in GKS is proposed for Darcy flow. It shows that GKS could be a promising scheme for porous media flows.

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Characteristics of Transient Pressure for Multiple Fractured Horizontal Wells in Fractal Fractured Shale Gas Reservoirs DING Mingcai, WU Minglu, LI Xuan, YAO Jun 2019, 36(5): 559-568.  DOI: 10.19596/j.cnki.1001-246x.7929 Abstract ( )   HTML ( )   PDF (11349KB) ( )   Based on theory of fractal and assumption of continuity, a well test model for multiple fractured horizontal wells in fractal fractured shale gas reservoirs was established in which adsorption, desorption and cross flow between matrix and fractures are considered. Solution of the model was obtained with Laplace transform, point source solution and pressure drop superposition principles. Double logarithmic curves of dimensionless pressure changing over time were obtained. Characteristics of pressure of multiple fractured horizontal well in fractal fractured shale gas reservoirs and influence of parameters, such as fractal index and fractal dimension, on pressure and production curves were analyzed. It shows that the curves can be divided into 7 flow stages. Slope of unstable pressure well-test curves in the middle and late stages is greater with bigger fractal index or smaller fractal dimension.

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Finite Difference Time Domain Method for Composite Electromagnetic Scattering from Soil Surface and Partly Buried Multiple Targets REN Xincheng, ZHU Xiaomin, GUO Lixin 2019, 36(5): 569-576.  DOI: 10.19596/j.cnki.1001-246x.7920 Abstract ( )   HTML ( )   PDF (4433KB) ( )   Dobson semi-empirical model and dielectric complex permittivity are used to represent real and imaginary parts of soil dielectric constant. Soil surface is simulated with exponential distribution model and Monte Carlo method. Composite electromagnetic scattering from soil surface and partially buried multiple columns with rectangular cross-section is studied with finite difference time domain method. It shows that composite scattering coefficient oscillates with scattering angle. Root mean square of soil surface, soil moisture content, dielectric constant of target and incidence angle have great influence on composite scattering coefficient. Correlation length of soil surface, width, height, distance of target section, and dip angle have weak influence on composite scattering coefficient. Buried depth of target hardly has effect on composite scattering coefficient. Compared with other numerical methods, finite difference time domain method obtains higher accuracy, and reduces calculation time and amount of memory occupying as well. It can be used to calculate complex scattering from rough ground and sea surface with nearby arbitrary multiple targets.

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Parameter Inversion of Rough Surface Optimization Based on Multiple Algorithms for SVM WANG Lixiang, WANG Anqi, HUANG Zhixiang 2019, 36(5): 577-585.  DOI: 10.19596/j.cnki.1001-246x.7900 Abstract ( )   HTML ( )   PDF (3519KB) ( )   Support vector machine (SVM) is one of the most widely used algorithms in parameter inversion of rough surface. However, the penalty parameters (C) and kernel function parameters (G) in SVM affects accuracy of results. If the parameters are not used properly, the model will lead to "over learning"or "less learning", which reduces greatly prediction accuracy. Several optimization algorithms for C and G of SVM are shown, such as K-fold cross validation (K-CV), genetic algorithm (GA) and particle swarm optimization (PSO). An improved PSO algorithm based on K-CV and GA (GA-CV-PSO) is proposed. The training set and test set are constructed with rough surface backscattering coefficient obtained by the moment method (MoM). Inversion precision and calculation time of different optimization algorithms are compared. It shows that GA-CV-PSO algorithm overcomes shortcomings of single optimization algorithms, with more accurate inversion precision and stronger generalization ability.

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Cluster Dynamics Modeling with Spatial Correlations in Cascades TANG Panfei, ZHENG Qirong, LI Jingwen, WEI Liuming, ZHANG Chuanguo, LI Yonggang, ZENG Zhi 2019, 36(5): 586-594.  DOI: 10.19596/j.cnki.1001-246x.7904 Abstract ( )   HTML ( )   PDF (3801KB) ( )   Cluster dynamics (CD) is a fast method for simulating long-term defect evolution in materials under irradiation. However, CD models based on mean-field rate theory do not account for spatial correlations between defects/clusters in cascades. Object kinetic Monte Carlo (OKMC) models take intrinsically account of defect spatial correlations, but they are limited by time scale and irradiation dose. A CD model with spatial correlation effect, termed as CD-SC, is developed based on a simple constant-time annealing method to determine CD source term by coupling Monte Carlo (IM3D) and OKMC (MMonCa) models. It shows that, results by CD-SC match those by full OKMC well. It is helpful for accuracy improvement of sequential multi-scale models of long-term radiation damage under typical irradiation conditions.

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A Fractional Model of Metal Fiber Sintering Process ZHENG Zhoushun, LIU Zhen, GENG Tingting, WU Xiaoxin, TANG Huiping, WANG Jianzhong 2019, 36(5): 595-602.  DOI: 10.19596/j.cnki.1001-246x.7910 Abstract ( )   HTML ( )   PDF (9965KB) ( )   Based on geometric model of metal fiber sintering nodes, with Caputo fractional differential equations, a time fractional surface diffusion model is established. Numerical solution by finite difference method is made. Numerical simulation of metal fiber sintering process is realized. Numerical simulation of sintering process and variation of neck length as fractional order varies from 0 to 1 are obtained. As the order is fixed at 0.9, sintering process at initial included angles of 0°, 30°, 60° and 90° are simulated. It shows that as the order is equal to 1 the result is consistent with integer order diffusion model. Neck radius with integer order and fractional order grows rapidly in initial stage of sintering. With progress of sintering, fractional simulation of sintering neck length appears local fluctuation, and finally grows at an increase rate greater than the integer order. As the order is fixed, the smaller the initial angle, the greater the rate of growth. The fractional order surface diffusion model describes well the complex change of sintering node during fiber sintering process than the integer order surface diffusion model.

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Plasmon Excitations in Two-dimensional Binary Silicon Carbide Nanostructures YIN Haifeng, ZENG Chunhua, CHEN Wenjing 2019, 36(5): 603-609.  DOI: 10.19596/j.cnki.1001-246x.7905 Abstract ( )   HTML ( )   PDF (8623KB) ( )   Plasmon excitations in two-dimensional binary silicon carbide (SiC) nanostructures are investigated with time-dependent density functional theory. SiC nanostructures have two plasmon resonance bands. Compared with silicene nanostructures and graphene nanostructures, due to change of bond length, two plasmon resonance bands of SiC nanostructures are blue-shifted and red-shifted, respectively. Plasmon resonance excitations of one kind SiC nanostructures depend on edge configuration and nanostructure morphology. However, for another kind of SiC nanostructure, dependence on edge configuration and nanostructure morphology decreases. Plasmon resonance properties of this kind SiC nanostructure are basically the same for impulse excitation polarized in different directions.

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Fixed Investment Cost Relaxation Strategy for Heat Exchanger Network Synthesis DENG Weidong, CUI Guomin, ZHU Yushuang 2019, 36(5): 610-620.  DOI: 10.19596/j.cnki.1001-246x.7902 Abstract ( )   HTML ( )   PDF (8407KB) ( )   In view of structural evolution difficulty caused by integer variables in heat exchanger network with fixed investment cost, a fixed investment cost relaxation strategy is proposed. It simplifies mathematical model of the problem by relaxing fixed investment cost. As heat exchanger is very small, fixed investment cost is almost zero. With increase of heat exchanger, fixed investment cost is gradually increased with a certain slope, and finally equals to the actual value. Controlling the change slope by the relaxation strength coefficient, on the basis of ensuring reliability of optimization results, heat exchangers with structural evolutionary obstacles are guided to generate or eliminate. The strategy is used to two cases in the literature, and effect of this strategy on generation or elimination of heat exchanger under different relaxation strength is investigated. Finally, a random walk algorithm with compulsive evolution based on fixed investment cost relaxation strategy is proposed. The algorithm is applied to a heat transfer network case. The result is superior to existing literature.

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Parameter Estimation of Chaotic System Based on Hybrid Swarm Intelligence Optimization Algorithm SHI Jianping, LI Peisheng, LIU Guoping 2019, 36(5): 621-630.  DOI: 10.19596/j.cnki.1001-246x.7909 Abstract ( )   HTML ( )   PDF (5809KB) ( )   Estimation of unknown parameters of chaotic systems is a primary problem in chaos control and synchronization, which could be transformed into an optimization problem with multi-dimensional parameter space by constructing a proper fitness function. A hybrid swarm optimization algorithm combining improved bare bones particle swarm optimization algorithm and improved differential evolution is proposed. Particle position update mechanism, mutation operation, crossover operation, and crossover probability factor design are improved, taking into account both diversity of population and convergence rate of algorithm. On this basis, fusion optimization strategy of bare bones particles swarm optimization algorithm and differential evolution algorithm is discussed. Co-evolution of two algorithms is realized. To test algorithm optimization performance, simulation experiments were carried out with six benchmark functions. It shows that the proposed algorithm has powerful global optimizing ability, more stability, fast convergence speed and higher optimizing precision and so on. Lorenz chaotic system was taken as an example to estimate three unknown system parameters.