Volume 43,Issue 6,2020 Table of Contents

1 A modified micromorphic model based on micromechanics for granular materials

XIU Chenxi , CHU Xihua

2020, 43(6):1-11. DOI: 10.11835/j.issn.1000-582X.2020.06.001

[Abstract](1076) [HTML](774) [PDF 659.48 K](952)

Abstract:
A modified micromorphic continuum model is proposed for granular materials based on a micromechanics approach. In this model, a continuum material point is considered as a granular volume element whose deformation behavior is influenced by the translation and the rotation of particles. And a hypothesis is proposed that the microscopic actual motion is decomposed into a microscopic average motion and a fluctuation related to the average motion. By symmetric correction of micro curvature, a symmetric Cauchy stress and a symmetric couple stress conjugated with a symmetric strain and a symmetric curvature respectively, and asymmetric relative stress measures conjugated with asymmetric relative strain measures are obtained. Based on this decomposition and the symmetry correction, first-order micromorphic constitutive relationships are derived for granular materials with inclusion of stress-strain relation, even stress-micro curvature relation and relative stress-strain relation. Furthermore, the macroscopic constitutive moduli in the micromorphic model are obtained in the expressions of the microstructural information such as the contact stiffness and the internal length.

2 Finite element analysis of the stability of unsaturated soil slope under surface water drawdown conditions

ZHANG Tao , ZHANG Hui , HUANG Wenxiong

2020, 43(6):12-20. DOI: 10.11835/j.issn.1000-582X.2020.06.002

[Abstract](982) [HTML](1018) [PDF 1.94 M](926)

Abstract:
Seepage flow is a negative factor to slope stability under surface water drawdown conditions, while the increase of the effective stress and strength induced by the water drawdown is positive to slope stability. Both of the negative and the positive factors should be considered in stability analysis of unsaturated soil slopes and both of them are influenced by soil permeability coefficient, range and speed of water drawdown. In this paper,finite element analysis was conducted and it showed that the slope safety factor initially decreased, and then increased in the process of water drawdown. The most dangerous status appeared in the early stage of water drawdown, and its appearance instant and the corresponding safety factor value were influenced by water drawdown speed, especially for the slopes with low permeability. A slope with lower permeability rendered a smaller minimum-value for the safety factor, and needed a longer time to recover its initial value. Sharp decrease of safety factor could be avoided by controlling water drawdown speed in a reasonable range according to different soil permeability coefficients.

3 Numerical analysis of aircraft dynamic behavior in ditching based on S-ALE fluid-structure interaction method

WANG Mingzhen , CAO Dongfeng , WU Bin , HU Haixiao , AN Zejun , YUAN Zhidan

2020, 43(6):21-29. DOI: 10.11835/j.issn.1000-582X.2020.06.003

[Abstract](1373) [HTML](740) [PDF 2.45 M](1389)

Abstract:
For the numerical analysis of aircraft dynamic behavior in ditching, how to define the coupling effect between fluid and structure is the key problem, which determines the success or failure of the numerical analysis to some extent. A coupling method based on structural-arbitrary Lagrangian-Euler algorithm (S-ALE) was used to characterize the interaction between the aircraft and the water during the ditching. S-ALE fluid-structure interaction method was adopted to analyze the dynamical behavior of aircraft in ditching to extract the history of aircraft pitch attitude and overload, and the results were compared with those obtained by the traditional ALE penalty-based coupling method and experiments. It is shown that the S-ALE fluid-structure interaction method can prevent the fluid leakage and effectively predict the phenomenon of aircraft secondary rise, and the pitch attitude angle and overload curve predicted by the S-ALE fluid-structure interaction method are basically consistent with the experimental data.

4 Dynamic load identification based on the cuckoo search algorithm

GAO Wenjing , ZHOU Huanlin , TAO Ran

2020, 43(6):30-39. DOI: 10.11835/j.issn.1000-582X.2020.06.004

[Abstract](958) [HTML](459) [PDF 1.08 M](822)

Abstract:
Engineering structures are often subjected to dynamic loads, which have negative influence on the structures. In order to accurately and effectively detect the load states of structures, a cuckoo search (CS) algorithm based on the Newmark-β method is developed to identify dynamic loads. Firstly, the time is discretized into several time steps, and the discretization equation of motion is obtained. The motion equation of the discrete motion system is solved and the response of the structure under dynamic load is calculated by the Newmark-β method. Secondly, the dynamic response is selected as the optimization variable. The objective function of the dynamic load identification problem is defined as the difference between the calculated response and the measured response of the dynamic load. The dynamic load is determined through minimization of the objective function with the CS algorithm. Finally, a simply supported beam bridge subjected to dynamic load is taken as an example to verify the accuracy and effectiveness of the algorithm. The effects of the nest number, the locations of measurement points, the number of measurement points and the measurement noise on the inversed results are discussed in numerical examples. The results show that the CS algorithm can be an accurate and effective inverse system method for dynamic load identification.

5 Identification of elasticity boundary conditions based on cuckoo search algorithm

WU Xiuzhuang , ZHOU Huanlin , CHEN Haolong

2020, 43(6):40-49. DOI: 10.11835/j.issn.1000-582X.2020.06.005

[Abstract](1142) [HTML](753) [PDF 1.02 M](954)

Abstract:
For Cauchy boundary condition inverse problems in 2-D elasticity, all the boundary conditions on accessible part of the boundary are known,and the boundary conditions on the rest inaccessible part of the boundary need to be solved. In this paper, based on the boundary element method, and with a polynomial function to approximate the unknown traction boundary conditions, the inverse problem was transformed into a problem with the identification of unknown coefficients of the polynomial. The objective function was defined as the least square error between the calculated values and the given values of the tractions on the measurable part of the boundary. The unknown tractions on the immeasurable boundary were recognized by minimizing the objective function through the cuckoo search (CS) algorithm. Then, the unknown boundary displacements were obtained by solving the direct problem with the inversed tractions and the other known conditions. The calculation results with and without using polynomial approximation were compared, and the influences of nest number, polynomial order and measurement noise on the numerical inversion were also discussed. Numerical examples verify that the CS algorithm combined with polynomial approximation can accurately and effectively solve the Cauchy problem in elasticity.

6 Topology optimization design of microstrip antenna with metamaterial cover for high-gain at 24 GHz

DONG Yanzhang , ZHOU Jinghao

2020, 43(6):50-57. DOI: 10.11835/j.issn.1000-582X.2020.290

[Abstract](981) [HTML](655) [PDF 1.74 M](1072)

Abstract:
In this paper, a topology optimization design method is proposed for high gain metamaterial cladding microstrip antenna based on genetic algorithm. Adopting a holistic approach to metamaterial elements and the maximum gain value of the microstrip antenna is selected as the objective function. The binary 0-1 coding of the lattice copper patch behind the discretization of the metamaterial copper-clad area is selected as the optimization variable. The topology optimization model of metamaterial microstrip antenna at 24 GHz is established. The redundant design method of patch square grid is used to eliminate the point connection phenomenon and a suitable genetic algorithm strategy is adopted to solve the optimal design problem of 10×10 square lattice size, thus obtaining a new metamaterial microstrip antenna without point connection. The results show that metamaterial cladding microstrip antenna has better matching performance compared with the conventional microstrip antenna, and its gain performance and directivity are significantly improved. Thereinto, the maximum gain performance is raised from 7.51 dB to 11.54 dB, and the improvement rate is 53.66%. Finally, the topological optimization design of metamaterial microstrip antennas with different square lattice sizes such as 12×12 and 14×14 is studied. The results show that the creative configuration design of the metamaterial microstrip antenna is convergent, and the microstructure configuration for 10×10 lattice size is the most cost-effective.

7 DEM simulation of silo discharge of wet granular materials based on liquid bridge model

ZHOU Jianping , DU Zepeng

2020, 43(6):58-64. DOI: 10.11835/j.issn.1000-582X.2020.06.007

[Abstract](972) [HTML](862) [PDF 2.40 M](1025)

Abstract:
This paper presented two numerical methods(contact constitutive model and user-defined constitutive model)to simulate the liquid bridge force of wet granular materials with small amount of liquid during silo discharge. The discrete element methods was employed to analyze the impact of liquid bridge force and moisture content on the granular fluidity. By the analysis of the differences of the macro-features between dry particles and wet particles, according to the observations of the peak pressure at the bottom of the silo and the jamming probability of particles, we came to the conclusion that the peak pressure at the bottom of the silo appeared during the discharge process and the peak pressure of dry particles was higher than that of wet particles. In the process of silo discharge, the jamming probability of particles with uniform particle size distribution was greater than that of single particle size, and the jamming probability of wet particles was larger than that of dry particles. And the liquid bridge force had a hindrance to the particle flow. The results demonstrate the feasibility and effectiveness of the proposed methods for modeling the flow behaviors of the wet granular materials.

8 Dynamic analysis of a functionally graded material beam undergoing large overall motions

GAO Xiang , DU Chaofan , ZHANG Dingguo , ZHOU Xiaoting , HAN Junwen

2020, 43(6):65-76,89. DOI: 10.11835/j.issn.1000-582X.2020.294

[Abstract](938) [HTML](676) [PDF 6.45 M](1199)

Abstract:
The deformation field of the flexible beam was described by using the assumed mode method and the finite element method and the dynamic characteristics of a hub-functionally graded material beam undergoing large overall motions were studied. Assuming that the physical parameters of functionally graded materials followed certain kind of power law gradient distribution and varied along the thickness direction, considering both the longitudinal deformation and transversal deformation of the beam, and taking the nonlinear coupling term known as the longitudinal shortening caused by transversal deformation into account, we derived the rigid-flexible coupling dynamics equations of the system described by two different discrete methods with a uniform form via employing Lagrange’s equations of the second kind. The validity of the finite element method established in this paper was verified by comparison with the numerical simulation results of the assumed mode method. The limitation of the assumed mode method based on small deformation assumption was illustrated by the example of large deformation. On this basis, the influence of functional gradient distribution rules on the dynamic characteristics of flexible beams undergoing large overall motions was discussed. The results show that the assumed mode method cannot deal with large deformation problem. When other physical parameters of functionally graded materials beam remain unchanged, the maximum displacement of the beam increases with the increase of functionally graded materials index while the natural frequency of transverse bending of beam increases with the increase of rotational speed, and when rotational speed is constant, the natural frequency will decrease with the increase of functional gradient index.

9 Vibration of elastic restrained simply supported carbon nanotubes

LI Ming , FANG Kang

2020, 43(6):77-81. DOI: 10.11835/j.issn.1000-582X.2020.295

[Abstract](791) [HTML](558) [PDF 666.52 K](793)

Abstract:
This paper investigated the free vibration of simply supported single-walled carbon nanotubes(SWCNTs) with both ends restrained elastically. Based on nonlocal Euler-Bernoulli beam theory, the governing partial differential equations of motion and associated boundary conditions were derived by Hamilton’s principle. The differential transformation method (DTM) was employed to solve the equation of motion and the influences of the nonlocal parameter, the viscoelastic CNT parameter and restraining elastic coefficient on the dynamic behaviors of the SWCNT were analyzed. It can be concluded that the nonlocal small-scale parameter and the viscoelastic CNT parameter make the SWCNT natural frequency decrease. More importantly, the results show that it will be a convenient and effective way to increase the natural frequency SWCNT system through additional elastic restraining with proper coefficient on two ends while their values are low.

10 Optimization design of composite bulkhead structure based on response surface method

ZENG Yang , ZHOU Jun , SHEN Zhiyuan , YU Zixian , CHEN Hao , YU Rong

2020, 43(6):82-89. DOI: 10.11835/j.issn.1000-582X.2020.292

[Abstract](1093) [HTML](585) [PDF 5.88 M](1038)

Abstract:
An optimal design method combining response surface model with genetic algorithm was proposed in the present research to ensure reliable structural parameters of bulkhead. The Box-Behnken design method was selected to determine the location of sampling points for generating the response surface, and the model of maximum stress, total mass property and maximum displacement was established in terms of the input parameters. Then the Pareto optimal solutions were obtained by using multi-objective genetic algorithm to optimize the model. The numerical simulation results show that the optimized total mass of the composite bulkhead is reduced by 23% compared with that of the steel bulkhead, while the maximum displacement is basically consistent with that of the steel bulkhead structure. Therefore, the present research provides useful insight into the structural optimization of composite bulkhead structure in engineering application.

11 Finite element analysis on the flood discharge atomization of Shuibuya Hydropower Station based on water-air two-phase flow

LIU Gang , TONG Fuguo , TIAN Bin

2020, 43(6):90-102. DOI: 10.11835/j.issn.1000-582X.2020.06.011

[Abstract](1017) [HTML](768) [PDF 9.29 M](854)

Abstract:
Flood discharge atomization is actually a process of mixed motion of water and air two-phase fluids. Based on the mass and momentum conservation equations, the mathematical model of flood discharge atomization was established and solved by the finite element method. A non-linear solution strategy was adopted, which solved the problems of numerical convergence and stability in the solution process. The correctness of the model was verified by comparing the result with that of the square cavity driven flow case. According to the spillways of the Shuibuya Hydropower Station and the topography of the valley, a three-dimensional finite element calculation grid was established, and the flood discharge process was simulated. The consistency of the calculated data with the actual flood discharge data verifies that the numerical method has good prediction ability for the wind and rain fields in the process of flood discharge atomization.

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