Nonlocal microplane model with strain-softening yield limits

The paper deals with the problem of nonlocal generalization of constitutive models such as microplane model M4 for concrete, in which the yield limits, called stress–strain boundaries, are softening functions of the total strain. Such constitutive models call for a different nonlocal generalization than those for continuum damage mechanics, in which the total strain is reversible, or for plasticity, in which there is no memory of the initial state. In the proposed nonlocal formulation, the softening yield limit is a function of the spatially averaged nonlocal strains rather than the local strains, while the elastic strains are local. It is demonstrated analytically as well numerically that, with the proposed nonlocal model, the tensile stress across the strain localization band at very large strain does soften to zero and the cracking band retains a finite width even at very large tensile strain across the band only if one adopts an “over-nonlocal” generalization of the type proposed by Vermeer and Brinkgreve [In: Chambon, R., Desrues, J., Vardoulakis, I. (Eds.), Localisation and Bifurcation Theory for Soils and Rocks, Balkema, Rotterdam, 1994, p. 89] (and also used by Planas et al. [Basic issue of nonlocal models: uniaxial modeling, Tecnical Report 96-jp03, Departamento de Ciencia de Materiales, Universidad Politecnica de Madrid, Madrid, Spain, 1996], and by Strömberg and Ristinmaa [Comput. Meth. Appl. Mech. Eng. 136 (1996) 127]). Numerical finite element studies document the avoidance of spurious mesh sensitivity and mesh orientation bias, and demonstrate objectivity and size effect.     

振动对充填裂隙渗透率影响的实验研究

为了研究振动和充填泥沙的共同作用对裂隙岩石渗透率的影响,通过对裂隙试件在充填和振动后条件下的渗流实验,得到了频率和振幅与充填裂隙岩石渗透率的关系曲线。结果表明,充填泥沙后的相对渗透率低于充填前,最小降幅为10.19%,最大降幅为59.74%。在相同的振幅下,试件的相对渗透率随频率的增加都基本呈下降趋势;振动后的渗透率低于振动前的渗透率;充填试件在较大频率振动下,渗透率减小的更为显著。在相同的频率下,充填试件振动后的渗透率基本上都减小了,最小降幅为8.90%,最大降幅为51.23%;而未充填试件振动后的渗透率与振幅存在一定关系;相对渗透率与振幅的变化之间存在拐点,相对渗透率并没有随着振幅的增加而一直减小。     

Density functional theory investigation of carbon monoxide adsorption on the kaolinite(001) surface

Carbon monoxide(CO) is a gaseous pollutant with adverse effects on human health and the environment. Kaolinite is a natural mineral resource that can be used for different applications, including that it can also be used for retention of pollutant gases. The adsorption behavior of carbon monoxide molecules on the(001) surface of kaolinite was studied systematically by using density-functional theory and supercell models for a range coverage from 0.11 to 1.0 monolayers(ML). The CO adsorbed on the three-fold hollow, two-fold bridge, and one-fold top sites of the kaolinite(001) was tilted with respect to the surface. The strongest adsorbed site of carbon monoxide on the kaolinite(001) surface is the hollow site followed by the bridge and top site. The adsorption energy of CO decreased when increasing the coverage, thus indicating the lower stability of surface adsorption due to the repulsion of neighboring CO molecules. In addition to the adsorption structures and energetics, the lattice relaxation, the electronic density of states, and the different charge distribution have been investigated for different surface coverages.     

基于第一性原理的石墨吸附一氧化碳倾向性研究

利用基于量子力学的第一性原理计算方法,从微观角度研究煤层对一氧化碳气体的吸附机理.根据煤层的基本成分及结构特点,用结构类似的石墨模拟复杂的煤层,建立石墨吸附一氧化碳分子的模型,讨论石墨表面的三种高对称吸附位置,分别是顶位（top位）、桥位（bridge位）、洞位（hollow位）对一氧化碳分子的吸附.结果表明：当一氧化碳分子垂直于石墨表面,且碳原子端靠近碳层时,在石墨（001）表面的桥位（bridge位）有较强的吸附能.     

Two and three dimensional modeling of fluidized bed with multiple jets in a DEM–CFD framework

Fluidized beds with multiple jets have widespread industrial applications. The objective of this paper is to investigate the jet interactions and hydrodynamics of a fluidized bed with multiple jets. Discrete element modeling coupled with in-house CFD code GenIDLEST has been used to simulate a bed with nine jets. The results are compared with published experiments. Mono dispersed particles of size 550 μm are used with 1.4 times the minimum fluidization velocity of the particles. Both two and three dimensional computations have been performed. To the best of our knowledge, the results presented in this paper are the first full 3D simulations of a fluidized bed performed with multiple jets. Discrepancies between the experiment and simulations are discussed in the context of the dimensionality of the simulations. The 2D solid fraction profile compares well with the experiment close to the distributor plate. At higher heights, the 2D simulation over-predicts the solid fraction profiles near the walls. The 3D simulation on the other hand is better able to capture the solid fraction profile higher up in the bed compared to that near the distributor plate. Similarly, the normalized particle velocities and the particle fluxes compare well with the experiment closer to the distributor plate for the 2D simulation and the freeboard for the 3D simulation, respectively. A lower expanded bed height is predicted in the 2D simulation compared to the 3D simulation and the experiment. The results obtained from DEM computations show that a 2D simulation can be used to capture essential jetting trends near the distributor plate regions, whereas a full scale 3D simulation is needed to capture the bubbles near the freeboard regions. These serve as validations for the experiment and help us understand the complex jet interaction and solid circulation patterns in a multiple jet fluidized bed system.     

Magnetohydrodynamic Kelvin-Helmholtz instability for finite-thickness fluid layers

We have derived the analytical formulas for the Kelvin-Helmholtz instability (KHI) of two superposed finite-thickness fluid layers with the magnetic field effect into consideration. The linear growth rate of KHI will be reduced when the thickness of the fluid with large density is decreased or the thickness of fluid with small density is increased. When the thickness and the magnetic field act together on the KHI, the effect of thickness is more obvious when the magnetic field intensity is weak. The magnetic field transition layer destabilizes (enforces) the KHI, especially in the case of small thickness of the magnetic field transition layer. When considering the effect of magnetic field, the linear growth rate of KHI always decreases after reaching the maximum with the increase of total thickness. The stronger the magnetic field intensity is, the more obvious the growth rate decreases with the total thickness. Thus, it should be included in applications where the effect of fluid thickness on the KHI cannot be ignored, such as in double-cone ignition scheme for inertial confinement fusion.     

类岩石材料力学特性的试验及数值模拟研究

在进行多组不同配比类岩石材料单轴压缩试验和巴西试验的基础上,详细分析了石膏水泥比和石英砂含量对类岩石材料的单轴抗压强度、抗拉强度及弹性模量等力学参数的影响规律,力图找到适合模拟现场砂质泥岩的类岩石材料及配合比。利用颗粒流程序(PFC)模拟,进一步研究了高径比和围压对类岩石材料力学特性的影响。结果表明:随着石膏水泥比的增大,抗压强度和弹性模量均逐渐减小,而抗拉强度逐渐增大;随着石英粉含量的增大,抗压强度和弹性模量均先增大后减小,而抗拉强度则为先减小后增大。结合单轴压缩过程的声发射特征,揭示了裂纹扩展与声发射有密切的关系。PFC2D模拟获得的力学参数与室内试验相近,破裂模式也与实际情况相似。通过尺寸效应的研究可知试样的高径比在2.0~2.5较合理。随着围压的增大,试样的峰值强度、残余强度、峰值应变及弹性模量等力学参数均增大,且围压会改变试样的破裂模式。     

THREE-DIMENSIONAL STATIC ANALYSES FOR FGM PLATES WITH MEDIUM COMPONENTS AND DIFFERENT NET STRUCTURES

A new microelement method for the analyses of functionally graded structures was proposed. The key of this method is the maneuverable combination of two kinds of elements. Firstly, the macro elements are divided from the functionally graded material structures by the normal finite elements. In order to reflect the functionally graded distributions of materials and the microconstitutions in each macro-element, the microelement method sets up the dense microelements in every macro-element, and translates nodes to the same as the normal finite elements by the degrees of freedom of all microelemental the compatibility conditions. This microelement method can fully reflect the micro constitutions and different components of materials, and its computational elements are the same as the normal finite elements, so it is an effective numerical method for the analyses of the functionally graded material structures. The three-dimensional analyses of functionally graded plates with medium components and different micro net structures are given by using the microelement method in this paper. The differences of the stress contour in the plane of functionally graded plates with different net microstructures are especially given in this paper.     

Numerical simulation of coupled heat-fluid transport in freezing soils using finite volume method

A solution procedure using finite volume method has been established for the coupled heat-fluid transport model of freezing soils, and details about determination of the time step interval and discretization at special nodes have been introduced. Comparison between the simulation and the freezing experiments of silica flour and Zhangye loam has been conducted, and the calculated results are in general agreement with the experimental data. The research indicates that the moisture migration in the frozen zone is insignificant, and water mainly migrates from the frozen zone to the vicinity of the moving freezing front; the moving velocity of the freezing front has a great effect on the extent of moisture accumulation to the freezing front, and high extent of accumulation occurs when the freezing front advances slowly. Finally, an apparent heat capacity model has been suggested for the temperature calculation of the soil freezing process in low water content conditions; however, when the moisture migration is significant, water redistribution during the freezing process should be considered.     

Elastic bending analysis of bilayered beams by an alternative two-variable method

An alternative two-variable method is used to reanalyze thermoelastic bending problems of bilayered beams subjected to external moments and internal stresses. The differences among zero-stress axis, zero-strain axis (i.e. neutral axis), bending axis, centroidal axis, and the parameter conditions for null/single/dual zero-stress axes are investigated analytically and numerically. Comparisons of thermoelastic stress predictions by the present model with Stoney's model and Hsueh's model are discussed in a representative case of GaAs top coat/Si substrate wafers. Results showed that the neutral axis does not coincide with the zero-stress axis in the general case, and the numbers and the locations of zero-strain or zero-stress axes depend on not only elastic modulus, thickness and/or thermal expansion coefficient ratios between the film and the substrate but also mechanical/thermal loading ratio.