ML-LBM: Predicting and Accelerating Steady State Flow Simulation in Porous Media with Convolutional Neural Networks

Transport in Porous Media - Fluid mechanics simulation of steady state flow in complex geometries has many applications, from the micro-scale (cell membranes, filters, rocks) to macro-scale...     

Dispersion Stability and Transport of Nanohybrids through Porous Media

Single-walled carbon nanotube-silica nanohybrid particles are a very promising material that could be used for enhanced oil recovery because of their interfacial activity. To demonstrate the basic principle, aqueous nanohybrid particle dispersions were evaluated by looking at the effect of pH, surfactant, and polymer. The results showed that pH did not have significant effect on the dispersion stability of nanohybrid particles. Although surfactant could improve the dispersion stability, it reduced the interfacial activity of the nanohybrid particles, causing them to stay in the aqueous phase. The best nanohybrid particle dispersion stability was found upon polymer addition, where the dispersions were stable for more than a week even at low polymer concentration (50?ppm). One-dimensional sand-pack studies were performed to evaluate the flow of the nanohybrid particles through porous media. The results showed that most of the nanohybrid particles (>99%) could pass through a column packed with glass beads while a measurable fraction of the particles was retained in the column packed with crushed Berea. When the columns contained a residual saturation of decane, additional nanohybrid particles were retained at the oil/water interface in both glass beads and crushed Berea sand media. The sand pack studies showed that not only can the nanohybrid particles flow through porous media but also about half of the particles injected will go the O/W interface when the porous medium contains a residual saturation of hydrocarbon, where they could be used to support a catalytic conversion of components of the oil in reservoirs.     

Micromechanical study of elastic moduli of three-dimensional granular assemblies

In micromechanics of granular materials, relationships are investigated between micro-scale characteristics of particles and contacts and macro-scale continuum characteristics. For three-dimensional isotropic assemblies the macro-scale elastic characteristics are described by the bulk and the shear modulus, which depend on the micro-scale characteristics of the coordination number (i.e. the average number of contacts per particle) and the interparticle contact stiffnesses in directions normal and tangential to the contact.     

考虑颗粒形状和破碎的胶结钙质砂力学行为离散元模拟研究

质砂作为一种建筑材料,近年来广泛应用于我国南海岛礁工程建设中。本文通过建立考虑钙质砂真实颗粒形状和颗粒破碎的胶结钙质砂离散元模型,研究了二维剪切条件下试样的宏微观力学行为,包括应力-应变行为、颗粒破碎、胶结破坏、位移场和裂纹随剪应变的演化规律,讨论了颗粒形状、颗粒粒径范围、颗粒强度和水泥胶结强度对胶结钙质砂力学行为的影响规律。结果表明,钙质砂颗粒粒径区间越宽,胶结钙质砂的强度越高。同一级配条件下,考虑真实颗粒形状的胶结钙质砂试样比圆颗粒试样的强度更高,试样总体颗粒破碎率也更高。钙质砂颗粒的强度越高,胶结钙质砂的性能越好。但是提高水泥的强度对胶结钙质砂力学性能的影响并不显著。本文的研究结果可为实际工程中钙质砂的加固提供理论依据。     

复杂加载条件下的砂土本构模型

试验表明,饱和砂土的应力应变关系具有显著的密度以及压力依存性,上述两点构成了描述砂土静力加载下变形特性无法忽视的因素. 此外,在循环加载等复杂加载作用下,砂土还会表现出明显的应力诱导各向异性以及相变转换特性. 基于在e--p空间中存在唯一的临界状态线这一基本假定,通过在e--p空间中引入当前状态点与临界状态线的距离R来作为反映密度与压力依存特性的状态参量, 将变相应力比以及峰值应力比表达为状态参量的指数函数,将上述应力比参量引入到统一硬化参量中可准确地反映初始状态下围压、密度 对于单调加载下应力应变关系的影响规律,能描述砂土剪缩、剪胀,应变软化、硬化等特性. 采用非相关联流动法则,p--q空间中采用水滴型屈服面,塑性势面为椭圆面,松砂在单调加载下的静态液化现象也可描述. 为反映循环加载下塑性体积应变的累积特性以及塑形偏应变的滞回特性,在循环加载下将状态参量R表达为应力比参量,并在硬化参数中引入描述应力诱导各向异性特性的旋转硬化部分,所提模型可有效地描述循环加载下剪切模量的衰减特性、刚度衰化性质、强度减小特性,在不排水约束作用下,则会产生往返活动性现象. 通过一系列的模型模拟与试验结果对比,验证了本构模型的有效性及适用性.      

Characterization of artificial spherical particles for DEM validation studies

This paper describes a study in which advanced particle-scale characterization was carried out on spherical particles that can be used in experimental tests to validate discrete element method (DEM) simulations. Two types of particle, alkaline and borosilicate glass beads, made from two different materials, were considered. The particle shape, stiffness, contact friction properties and surface roughness were measured. The influences of hardness and roughness on the mechanical response of the particles were carefully considered. Compared to the alkaline beads, the borosilicate beads were more spherical and more uniform in size, and they exhibited mechanical characteristics closer to natural quartz sand. While only two material types were studied, the work has the broader implication as a methodology for selecting particles suitable for use in DEM studies and the key parameters that should be considered in the selection process are highlighted.     

Multi-resolution analysis of wavelet transform on pressure fluctuations in an L-valve

A novel diagnostic method to characterize the flow patterns in an 80 mm-i.d. L-valve had been developed by using multi-resolution analysis (MRA) of wavelet transformation on the pressure fluctuation signals which were acquired from the standpipe and the horizontal part of L-valve. Parameters including the aeration rate, aeration positions, riser gas velocity and composition of binary particle mixture (194-μm and 937-μm sand particles) were used to investigate the relationship of performance of L-valve and its pressure fluctuations. By means of MRA, the original pressure fluctuations were divided into multi-scale signals. They were macro-scale, meso-scale and micro-scale successfully described the structures of gas–solid flow in the L-valve, such as the gas bubbles/slugs, dune-ripple flow, suspension particle flow, etc.     

Flow characterization of high-pressure dense-phase pneumatic conveying of coal powder using multi-scale signal analysis

Flow characterization of high-pressure dense-phase pneumatic conveying of coal powder is not fully understood. To further reveal the dynamic behavior of coal particles in dense-phase pneumatic conveying pipelines, a method for the scale decomposition of particle motion based on empirical mode decomposition and Hurst analysis of experimental electrostatic signals is reported. This allows the multi-scale motion characteristics of single coal particles and particle clusters to be determined. Micro-, meso-, and macro-scale subsets were reconstructed, which reflected the different behaviors of the coal particles: specifically, dynamic features of the micro-scale subset represented features of single particle collisions and frictional interactions; dual fractal characteristics of the meso-scale subset described the motion of coal particle clusters; and features of the macro-scale subset reflected persistent dynamic behavior of the entire pneumatic conveying system. Motion behavior of single particles and particle clusters could be respectively investigated by considering the relative energies of the micro- and meso-scale contributions to the electrostatic signal. This was verified both by theoretical analysis and experiment.     

A FRACTURE-ENERGY-BASED ELASTO-SOFTENING-PLASTIC CONSTITUTIVE MODEL FOR JOINTS OF GEOMATERIALS

On the basis of plasticity and fracture mechanics for quasi-brittle materials , this article presented a constitutive model for gradual softening behavior of joints of geomateri-als . Corresponding numerical tests are carried out at the local level. Characteristics of the model proposed are 1) plastic softening and dilatancy behavior are directly related to the fracture process of joint, and much less material and model parameters are required compared with those proposed by references ; 2) the process of decohesion coupled with friction-al sliding at both micro-scale and macro-scale is described.     

Relative Permeability Analysis of Tube Bundle Models

The analytical solution for calculating two-phase immiscible flow through a bundle of parallel capillary tubes of uniform diametral probability distribution is developed and employed to calculate the relative permeabilities of both phases. Also, expressions for calculating two-phase flow through bundles of serial tubes (tubes in which the diameter varies along the direction of flow) are obtained and utilized to study relative permeability characteristics using a lognormal tube diameter distribution. The effect of viscosity ratio on conventional relative permeability was investigated and it was found to have a significant effect for both the parallel and serial tube models. General agreement was observed between trends of relative permeability ratios found in this work and those from experimental results of Singhal et al. (1976) using porous media consisting of mixtures of Teflon powder and glass beads. It was concluded that neglecting the difference between the average pressure of the non-wetting phase and the average pressure of the wetting phase (the macro-scale capillary pressure) – a necessary assumption underlying the popular analysis methods of Johnson et al. (1959) and Jones and Roszelle (1978) – was responsible for the disparity in the relative permeability curves for various viscosity ratios. The methods therefore do not account for non-local viscous effects when applied to tube bundle models. It was contended that average pressure differences between two immiscible phases can arise from either capillary interfaces (micro-scale capillary pressures) or due to disparate pressure gradients that are maintained for a flow of two fluids of viscosity ratio that is different from unity.     

Modeling liquefaction of water saturated granular material under undrained cyclic shearing

The tendency of particles in a water-saturated granular mass to re-arrange into a denser state during cyclic shearing under pressure results in an increase in pore water pressure. The increase in the pore water pressure causes a reduction in the inner particle contact forces, and in turn easier re-arrangement of the particles. Eventually, the material loses its shear strength, partially or almost completely. In this paper, a general three-dimensional continuum mechanics model is presented for the deformation of granular materials. A physically based model is also presented for characterization of liquefaction of the water saturated granular material under undrained cyclic shearing. The model incorporates the fabric of the granular mass, which develops as the frictional granular mass is deformed in shear. It includes the coupling between shearing and excess pore water pressure. The model parameters are estimated, based on the results of cyclic shearing experiments on large hollow cylindrical samples of silica sand. Basically, the calculation results utilizing this model can embody liquefaction phenomena of the water saturated granular material under undrained cyclic shearing.     

ROTATIONAL RESISTANCE AND SHEAR-INDUCED ANISOTROPY IN GRANULAR MEDIA

This paper presents a micromechanical study on the behavior of granular materials under confined shear using a three-dimensional Discrete Element Method （DEM）. We consider rotational resistance among spherical particles in the DEM code as an approximate way to account for the effect of particle shape. Under undrained shear, it is found rotational resistance may help to increase the shear strength of a granular system and to enhance its resistance to liquefaction. The evolution of internal structure and anisotropy in granular systems with different initial conditions depict a clear bimodal character which distinguishes two contact subnetworks. In the presence of rotational resistance, a good correlation is found between an analytical stress-force-fabric relation and the DEM results, in which the normal force anisotropy plays a dominant role. The unique properties of critical state and liquefaction state in relation to granular anisotropy are also explored and discussed.     

A micro-mechanically based continuum damage model for fatigue life prediction of filled rubbers

This paper presents a continuum damage model based on two mechanisms: decohesion between fillers and matrix at a micro-scale followed by a crack nucleation at a macro-scale. That scenario was developed considering SEM observations and an original experimental procedure based on simple shear and tension specimens. Damage accumulation is related to fatigue life using the continuum damage mechanics (CDM). The material behavior is investigated using the statistical framework introduced by Martinez et al. (2011). A Finite Element implementation is proposed and some numerical examples are provided.     

Heat transfer and pressure loss of a very shallow fluidized-bed heat exchanger Part 2. Experiment with multirow tube banks

Experiments were carried out on heat transfer and pressure-loss characteristics of a low-pressure-loss fluidized-bed heat exchanger with an extremely small ststic bed height of glass beads. This heat exchanger was composed of a tube bankd in a staggered arrangement and a multislit distributor designed especially to reduce the pressure loss. The effect of the cross-sectional shape of the distributor, the particle size of the glass beads, the static bed height, and the number of rows on heat transfer performance were examined. The heat transfer and pressure-loss characteristics are compared with experimental data in terms of fan power consumption and the compactness of the heat exchanger.     

一种改进的颗粒移动床μ(I)拟流体模型及应用

颗粒移动床在工业领域应用广泛, 发展实用可靠的颗粒移动床模型具有理论和应用价值. 本文基于颗粒流μ(I)模型, 补充局部颗粒体积分数与颗粒局部压力和局部颗粒流密度的关系式, 将移动床内密集颗粒处理成可压缩拟流体, 建立了颗粒流单相可压缩流μ(I)模型, 并建立了颗粒流?壁面摩擦条件, 在计算中对颗粒流拟黏度和拟压力项进行正则化处理. 采用上述模型与方法对3种典型散料在移动床缩口料仓内的流动进行模拟, 与实验对比, 得到了玻璃珠、刚玉球和粗沙的μ(I)模型参数, 分析了3种不同散料在料仓内的颗粒速度、体积分数等分布特性, 模拟结果较好地揭示了料仓内不同物料的整体流和漏斗流特性; 进而以玻璃珠为例, 对移动床颗粒单管绕流流动进行了模拟, 所得结果合理揭示了管流附近的流动特性. 计算结果表明, 对于本文的计算工况, 颗粒体积分数变化最大范围为0.510 ~ 0.461, 绝大部分区域流动惯性数小于0.1, 改进的单相μ(I)模型能合理预测出密集颗粒流移动床内的流动特性, 方法可行且较多相流算法能明显减小计算量.      

延性金属层裂自由面速度曲线特征多尺度模拟研究

以延性金属钽为研究对象,对钽在平板撞击下的层裂行为进行了多尺度下的数值模拟研究,从微观视角对自由面速度曲线上的典型特征进行了新的解读。在宏观尺度,对比分析了光滑粒子流体动力学法（smootfied particle hydrodynamics, SPH）与Lagrange网格法以及几种本构模型的模拟结果及其适用性。通过与实验数据的对比表明,Steinberg-Cochran-Guinan本构模型在层裂模拟中与实验数据吻合较好,通过改变加载条件获得了不同应变率下的自由面速度曲线,分析了不同应变率下的自由面速度曲线中的典型特征。在微观尺度,采用分子动力学方法获得层裂区域内损伤演化情况,揭示了宏观尺度自由面速度曲线典型特征所蕴含的物理内涵。分析表明,层裂表现为材料内部微孔洞形核、长大和聚集的损伤演化过程,自由面速度曲线上的典型特征与层裂区域的损伤演化过程存在密切关联。Pullback信号是层裂区域内微孔洞形核的宏观表征;自由面速度曲线的下降幅值在一定程度上反映了微孔洞的形核条件,由此计算得到的层裂强度实际上是微孔洞的形核强度。此外,Pullback信号后的速度回跳速率反映了微损伤演化的速率。     

水电站坝的砂层地基地震液化可靠度研究

对四川地区江河上数座水电站坝基砂层的26组动力三轴试验资料进行了统计分析,基于动剪应力比法的液化判别方法推导了的地震液化的极限状态方程,使用蒙特卡洛随机抽样的方法计算了砂层液化的失效概率,并对某水电站的厂房地基砂层的液化可靠度进行了计算分析。研究表明,统计按粉砂样总体和中细砂样总体划分较为合理;砂层的动剪应力比可采用正态分布;电站砂层地基地震液化的最危险工况为,闸坝盖重加稳定的向上渗流及遭遇Ⅶ度地震荷载,为高液化风险,其液化概率随埋深加大而增大,最危险部位为砂层底板,对坝基砂层应进行抗液化处理。     

Modelling turbulence around and inside porous media based on the second moment closure

To predict turbulence in porous media, a new approach is discussed. By double (both volume and Reynolds) averaging Navier–Stokes equations, there appear three unknown covariant terms in the momentum equation. They are namely the dispersive covariance, the macro-scale and the micro-scale Reynolds stresses, in the present study. For the macro-scale Reynolds stress, the TCL (two-component-limit) second moment closure is applied whereas the eddy viscosity models are applied to the other covariant terms: the Smagorinsky model and the one-equation eddy viscosity model, respectively for the dispersive covariance and the micro-scale Reynolds stress. The presently proposed model is evaluated in square rib array flows and porous wall channel flows with reasonable accuracy though further development is required.     

基于临界状态的砂土本构模型研究

砂土孔隙比及所受压力是其力学特性的重要影响因素. 本文基于砂土临界状态线特性分析,采用以e-(p/p_a)ξ平面内的线性关系描述其等向压缩线. 通过对比分析两种不同压缩线函数 与临界状态线函数之间的关系提出更适合描述砂土在等向压缩下的参考压缩线,并给出了基于参考压缩线的等向硬化规律. 建议了适用于 描述砂土剪切特性的屈服面函数,并给出利用等向压缩和等p路径确定屈服面形状参数μ的方法. 将不同应力比对应的压缩线作为砂土状态参量参考线,以获取潜在强度M_f与特征状态应力比M_c,进而描述砂土压缩与剪切特性;基于等向压缩与等p路径建立了当前应力比与状态参量参考线之间的相关关系,从而实现了砂土状 态参量参考线由参考压缩线向临界状态线平稳过渡. 建立的砂土本构模型共11个参数,均能够通过常规土工试验或经验获取. 基于模型预测与Toyoura砂的等向压缩、三轴不排水剪切试验及排水剪切试验的对比结果,本文建立的砂土本构模型很好地描述了Toyoura 砂在不同孔隙比和不同压力下的压缩与剪切特性.