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25 July 2020, Volume 37 Issue 4 Previous Issue    Next Issue

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A Positivity-preserving Finite Volume Scheme Based on Second-order Scheme ZHAO Fei, SHENG Zhiqiang, YUAN Guangwei 2020, 37(4): 379-392.  Abstract ( )   HTML ( )   PDF (3565KB) ( )   Based on a second-order accurate linear scheme, a normal flux is reconstructed to obtain a nonlinear finite volume scheme with a two-point flux discrete stencil on tetrahedral meshes. It is suitable for discontinuous and anisotropic diffusion coefficient problems, and can be generalized to general polyhedral meshes. It is unnecessary to assume that auxiliary unknowns are non-negative, and avoids artificial processing of "setting negative to be zero" in calculating auxiliary unknowns. Moreover, it is proved that the linearized scheme at each nonlinear iteration step satisfies strong positivity-preserving, i.e., as the source term and boundary condition are non-negative, non-zero solution of the scheme is strictly greater than zero. Numerical tests verify that the scheme has second-order accuracy and is strong positivity-preserving.

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An Inverse Distance Weighting Spatial Interpolation Algorithm with Second Order Accuracy CHAI Guoliang, SU Junwei, WANG Le 2020, 37(4): 393-402.  Abstract ( )   HTML ( )   PDF (3344KB) ( )   For low precision of traditional inverse distance weighting (IDW) interpolation, a high-precision interpolation algorithm is developed. Iterative defect correction (IDeC) is applied to correct IDW result, with which a second order accuracy is achieved theoretically within finite iterations. Numerical validations based on structured and unstructured grid are performed. It shows that, with this algorithm the second order accuracy is kept. Furthermore, the algorithm is adopted to surface reconstruction of a 2D circle and a 3D sphere, in which smoothness of reconstructed surface is improved and second order accuracy is maintained. In a two-layer mesh interpolation experiment, absolute error of velocity and pressure is reduced by more than 45%, and pressure contours are closer to the initial field.

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3D Lagrangian Methods for Ideal Magnetohydrodynamics on Unstructured Meshes XU Xiao, GAO Zhiming, DAI Zihuan 2020, 37(4): 403-412.  Abstract ( )   HTML ( )   PDF (8849KB) ( )   Interaction between magnetic field and fluids is crucial in Z pinch process. Since Z pinch involves with multi materials and severe deformation, we develop 3D compatible Lagrangian staggered and cell-centered schemes for ideal MHD on unstructured meshes. Both of the schemes are of first order in space and time discretization. Accuracy and robustness of the schemes are validated with typical numerical tests.

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Periodicity Analysis and Benchmark Solution of Lid-driven Cavity at Re=15 000 ZHAN Jiemin, BIAN Shiqi, LUO Yingying, HU Wenqing, GONG Yejun 2020, 37(4): 413-421.  Abstract ( )   HTML ( )   PDF (9295KB) ( )   We study periodic cavity flow at Re=15 000, with a high quality non-uniform mesh obtained by boundary-fitted coordinated method and extra quadtree encryption method based on structural grid. Fourier power spectra analysis and phase-space trajectory were applied to understand periodic evolution and kinetic energy distribution of the flow field. Fundamental frequency and corresponding period were determined. It was found that during periodic evolution process of the flow field, the primary vortex maintained a low frequency of kinetic energy, while high-order vortices near boundaries and corners showed high frequencies of kinetic energy. Benchmark solutions of two characteristic states in period evolution were obtained.

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Numerical Simulation of Fluid-Thermal Coupled in Droplet Impact onto Liquid Film: Front Tracking Method GAO Hui, GAO Ruifeng, YAO Mengjun, ZHANG Daoxu, PENG Chengyu, ZHANG Ying 2020, 37(4): 422-430.  Abstract ( )   HTML ( )   PDF (6672KB) ( )   Flow and heat transfer characteristics of a droplet impact onto a relatively high temperature thin film during distillation was studied by means of front tracking method. The proposed model was validated by numerical results with analytical solutions. Evolution of gas-liquid interface and heat flux distribution were investigated. Meanwhile, effect of Weber number and dimensionless liquid film thickness on heat transfer was analyzed. It indicates that according to heat flux distribution after impact liquid film can be classified into three zones: The impact zone, the transition zone and the static zone. Forced convection is the main heat transfer mechanism inside the impact zone mainly due to impact. Increasing Weber number or decreasing dimensionless film thickness could enhance heat transfer. With the increase of Weber number, disturbance caused by impact on liquid film strengthens, which makes synergy of momentum and energy more obvious, thus increasing average heat flux between liquid and solid wall, and coronal spray in the impact zone becomes more obvious. The smaller the dimensionless liquid film thickness is, the greater the average heat flux density is, and the longer the heat transfer duration of high heat flux density is.

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Lattice Boltzmann Numerical Simulation of Heat-Flow Coupling in a Square Cavity Filled with Porous Media with Partially Colding Wall Layout WAN Qikun, LUO Song, SHANG Wenqiang, ZHANG Ying, LIU Haotian, ZHU Baojie 2020, 37(4): 431-438.  Abstract ( )   HTML ( )   PDF (11894KB) ( )   Lattice Boltzmann method is used to simulate natural convection in a cavity fill with porous medium under REV scale. Effects of Rayleigh number, Darcy number and porosity on heat transfer and flow in a square enclosure were studied. It is found that Darcy number has a great influence on fluid flow pattern in the cavity. Increasing Rayleigh number,Darcy number, and porosity enhance the heat dissipation effect. Effect of porosity on average Nusselt number is related to Darcy number. As Darcy number is small, porosity has little effect on average Nusselt number. As Darcy number is great, porosity has a great influence on average Nusselt number. As cold source is placed above the wall, average Nusselt number changes dramatically with the increase of Rayleigh number. Under high Rayleigh number condition, cooling effect is improved by arranging the cold source above the boundory.Among the six schemes, Case 6 has the best heat dissipation effect.

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Pore-scale Oil-Water Two-phase Flow Simulation Based on Phase Field Method FENG Qihong, ZHAO Yunchang, WANG Sen, ZHANG Yigen, SUN Yeheng, SHI Shubin 2020, 37(4): 439-447.  Abstract ( )   HTML ( )   PDF (9212KB) ( )   Based on a typical particle size distribution of conventional sandstone, we establish a three-dimensional porous media of unconsolidated sandstone reservoir with process method. By using phase field method(PFM), a two-phase flow model is established on the basis of two-dimensional porous media model, which is solved with finite element method. Influences of displacement velocity, fluid properties and wettability on remaining oil distribution and recovery are discussed. It shows that displacement speed increase and inviscosity ratio decrease lead to a higher capillary number, thereby facilitating enhancing oil recovery. As for wettability, capillary force is the driving force for displacement under-water wet condition and it is a resistance under oil-wet condition,which results in a higher oil recovery in water-wet core.Meanwhile, this study reveals mechanism of two-phase flow and remaining oil distribution in pore scale. It can be seen that due to complex pore structure, fluid exhibits various characteristics as flowing through pores,which have an important impact on pressure distribution and fluid velocity distribution of oil-water two-phase flow.

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A Vertical Node-wise Non-structural Superstructure Model for Heat Exchanger Network Optimization LI Wanzong, CUI Guomin, SUN Tao, XIAO Yuan 2020, 37(4): 448-458.  Abstract ( )   HTML ( )   PDF (1150KB) ( )   As node-wise non-structural superstructure model is applied to heat exchanger network optimization, heat exchanger crossover may appear. It is found that crossover structure causes an increase of total heat transfer area for heat exchange unit with same heat loads. At the same time, it reduces obviously computational efficiency of the algorithm. A vertical node-wise non-structural superstructure model is established. Compared with node-wise non-structural superstructure, the vertical node-wise non-structural superstructure model reduces computational complexity in the optimization process and eliminates adverse effects of crossover structure, which improves efficiency and accuracy of optimization. Finally, optimization performance of RWCE algorithm based on vertical non-structural superstructure model is analyzed in two examples. The results are superior to those in literature.

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Axial Acoustic Radiation Force of Gaussian Beam Incident on an Off-axis Spherical Particle Near Boundary ZANG Yuchen 2020, 37(4): 459-466.  Abstract ( )   HTML ( )   PDF (1028KB) ( )   Based on scattering theory of sound waves, beam factor of a Gaussian beam is obtained with series expanding theory. Analytical expression of acoustic radiation force on a spherical particle near an impedance boundary is deduced. Numerical simulations of rigid and fluid particles are carried out and comparison with those in free space is made. Effects of reflecting coefficient, distance between particle and boundary and waist radius on acoustic radiation force are discussed. It shows that the acoustic radiation force increases along with reflecting coefficient of the boundary, while the peaks are not changed. At some appropriate frequencies, negative acoustic radiation force can be obtained. The acoustic radiation force changes periodically with the distance between particle and boundary. Moreover, the waist radius mainly affects acoustic radiation force at middle and high frequencies. As the particle off-axis distance and angle deviation increases, attenuation of acoustic radiation force becomes obvious. This work provides a theoretical basis for particle manipulation using Gaussian beam.

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Effect of Lower Hybrid Wave Current Drive with High Component N‖ on EAST LIU Zuguang, LI Xinxia, YANG Ming 2020, 37(4): 467-472.  Abstract ( )   HTML ( )   PDF (3286KB) ( )   In EAST tokamak high confinement mode discharge, a plasma current belt was found experimentally at the edge plasmas region. For EAST plasma with non-circular cross-section equilibrium configuration, effect of high parallel refractive index N‖ component of lower hybrid wave on current drive was numerically studied. Results of the ray-tracing platform shows that kA plasma current emerges in a region of normalized radius from 0.7 to 1.0 for 6≤N‖≤8. For electron temperature profile with pedestal and higher plasma edge temperature, the region of drive current is r/a>0.9.A theoretical analysis of lower hybrid Landau damping in EAST tokamak was also performed. Effects of electron density profile and formation of lower hybrid wave spectrum on current drive were discussed.

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Magnetic Properties of Fe Nanorings with Small Defects DENG Chuchu, ZHANG Xinyuan, CHEN Shuiyuan, WU Yuhan, YE Qingying, HUANG Zhigao 2020, 37(4): 473-478.  Abstract ( )   HTML ( )   PDF (8907KB) ( )   Magnetic dynamic properties of Fe nanorings with small defects were simulated with Monte Carlo method. It is found that hysteresis loops of small defect Fe nanoring have "bistable" characteristics, which is consistent with experimental results. Energy research shows that vortex states appear in the region of local energy minimum. Remanence of defect system changes with defect location: The remanence increases first and then decreases with the increase of Y, and remains relatively stable in the middle region. Spin configuration of the system explains the above phenomenon well.

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Potential Biotoxicity of Fullerene C60 on Cell Membrane: Molecular Dynamics Simulation LIU Yongzhi, XIE Liqiang, LIANG Shengde, ZHU Kaili, XI Zhonghong, YUAN Fangqiang 2020, 37(4): 479-487.  Abstract ( )   HTML ( )   PDF (9572KB) ( )   Coarse-grained simulations were adopted to investigate interaction between fullerene C60 cluster and lipid membranes. C60 aggregates in water and their interaction with lipid membrane were investigated. It was found that the aggregate can penetrate the membrane directly instead of endocytosis and pinocytosis. The penetration caused transmembrane movement of lipid and water molecules. Interaction between C60 molecules and small lipid vesicle was investigated and dependence on C60 concentration was interpreted explicitly. Conflicting experimental observations due to different preparation methods and processes are also provided. Potential mechanism of the biotoxicity of fullerene was put forward. Our results are helpful in medical applications.

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Electronic Structure and Electrochromic Property of Sulvanite Compounds: A First-principles Study LI Lin, SUN Yuxuan, SUN Weifeng 2020, 37(4): 488-496.  Abstract ( )   HTML ( )   PDF (8754KB) ( )   With first-principles pseudo-potential plane-wave method, crystal structures, electronic structures and optical properties of sulvanite compounds:Cu3VS4, Cu3NbS4 and Cu3TaS4 are calculated to investigate their electrical conductivity and electrochromic performance. They are potentially applied in solar cells and electrochromic devices as transparent semiconductors. Electronic structures indicate that the secompounds are indirect band-gap semiconductors, with conduction band minimum and valence band maximum located at X and R points, respectively, in Brillouin zone. Electronic states of conduction band minimum and valence band maximum are dominated by d-orbitals of Cu and Ⅴ-group elements, respectively. From band structure, atomic charge population analysis, electron localization function and optical adsorption/reflectivity spectrum it is suggested that these sulvanite compounds are polar covalent semiconductors with high charge transport mobility and extraordinary electrochromic characteristics. They could be applied to electrochromic devices.

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Application of Conservative Hyperchaos in Digital Image Encryption XUE Wei, ZHANG Yongchao 2020, 37(4): 497-504.  Abstract ( )   HTML ( )   PDF (17537KB) ( )   We propose a digital image encryption algorithm based on conservative hyperchaotic signals. Based upon a 5-dimensional conservative hyperchaotic system, the algorithm generates a 5-channel time series to perform scrambling in both pixel-level and bit-level, and subsequently an XOR operation on scrambled RGB components of the original image. On this basis, an encrypted image is obtained with time series of one of the channels to perform a forward XOR operation and S-BOX processing, a reverse XOR operation, S-BOX processing, and scrambling. Finally, the encryption algorithm is evaluated with several security indicators such as histogram, information entropy, and key space. Performance of the encryption algorithm in digital image encryption is compared with that of a 5-dimensional dissipative hyperchaotic system. It shows that the algorithm based on conservative chaos has higher safety and reliability.