土的工程力学性质的颗粒流模拟

基于颗粒流理论,引入不同的颗粒接触连接本构模型,分别建立了砂土和粘性土的颗粒流模型．通过颗粒流数值模型试验,对砂土和粘性土的室内平面应变试验及其剪切带形成和发展进行了数值模拟,分别对比了不同围压下颗粒流试样与室内试验的应力应变关系曲线,基本再现了砂土和粘性土试样应力．应变关系．通过砂土和粘性土PFC试样剪切带模拟表明,当围压较小时试样内部颗粒位移量小而且分布范围较广,当围压增大时,试样内部颗粒位移量也增大,而且发生较大位移颗粒的分布范围趋于集中,同时随着围压的增大试样内部形成明显的剪切带．无论砂土还是粘性土的PFC试样,随着围压的增加剪切带的形状趋于集中,而且剪切带宽度在减小．在围压很小时,试样内形成大的破坏区域,在围压较大时出现明显的线破坏区．这些规律基本与室内试验结果相似。     

沉积方向和粒径组成对砂土力学特性的影响

运用砂土毛细效应所形成的假凝聚力制备具有不同沉积方向的试样,开展三轴固结排水剪切试验,研究粒径组成对初始各向异性砂土力学特性的影响,分析了粒径组成、沉积方向、强度特性和体积变化之间的相互关系。研究结论表明,试样剪切带的形成受到颗粒级配的影响,它是应力应变关系中陡降阶段的产生原因。沉积方向对粗砂和细砂的偏应力峰值强度均有影响,对残余强度的影响受制于颗粒级配。沉积方向与试样破坏面接近时,试样强度低、剪胀变形小且更容易破坏,粒径组成对砂土三轴剪切力学特性的影响更显著。     

一种初始各向异性砂土本构模型的数值应用及试验验证

基于砂土颗粒的毛细效应特性制备砂土试样,在0°～90°范围内选取若干沉积方向,在中围压条件下进行三轴固结排水剪切试验。试验表明:沉积方向对试样强度和变形特性有显著影响。通过对试验数据进行拟合得到沉积方向和偏应力的关系,结合试验结果对Mohr-Coulomb屈服准则进行修正,建立一种密砂初始各向异性本构模型;对本构模型进行数值积分,并对密砂固结排水剪切试验进行数值模拟,分析偏应力、体积变化以及主应力方向的变化。将数值分析结果与试验结果进行对比,结果表明:二者相吻合,本构模型能够合理地预测试验结果。所建立的本构模型可用于砂土初始各向异性的研究工作中。     

岩石试件端面效应的变形局部化数值模拟研究

试件端面效应直接影响试件内的应力分布、试件破坏形式和剪切带图案。采用FLAC - 3D模拟了端面效应对岩石试件剪切带图案、速度场和位移场的影响。试样端部的缚束作用较强时 ,仅在试样中部出现X形的剪切带 ;反之 ,在试样端部出现了倾斜的剪切带。压头刚性越大 ,剪切带倾角越大。块体具有整体平移的特性 ,剪切带边缘位移梯度却较大 ,反映了块体沿剪切带发生滑动的剪切破坏本质。数值模拟结果可在众多的试验结果中找到佐证。     

颗粒摩擦对颗粒材料剪切行为影响的试验研究

通过对一种类似于土的颗粒材料--玻璃珠开展一系列室内直剪试验,研究颗粒间摩擦对颗粒材料剪切行为的影响. 试验一共考虑了4种不同的摩擦情况:干燥状态、用水浸润状态、完全淹没在水中和用油浸润状态. 分析试验结果发现,与干燥状态试样相比,用油浸润能明显降低试样的剪胀性和抗剪强度,而用水浸润和淹没在水中的方法没有产生显著的影响. 此外,通过在剪胀关系式中引入可变剪胀系数来考虑颗粒摩擦对颗粒材料剪胀性的影响,并从颗粒滑动与滚动的细观机理上初步解释了颗粒滑动摩擦角对临界状态摩擦角的影响规律.     

剪胀性饱和砂土弹塑性模型

在修正剑桥模型基础上,本文建立的剪胀性饱和砂土弹塑性模型使用相变状态参数描述剪胀性饱和砂土剪胀特性,克服了修正剑桥模型不能直接模拟剪胀砂土力学行为这一局限性。该模型有两方面的改进:一方面,模型将剪胀应力比Md引入剪胀方程;另一方面,模型在塑性功基础上提出用与应力路径无关的硬化参数来替代修正剑桥模型中的塑性体积应变增量。通过试验验证及与修正剑桥模型计算结果对比,结果表明,该模型较适合模拟剪胀性饱和砂土的力学性能,同时也能较好地体现较密实砂土的硬化及软化现象。模型共8个参数,用常规三轴试验就可获取。     

胶结砂砾石层面直剪试验及破坏行为细观研究

本文采用宏观试验和细观模拟相结合的方法研究胶结砂砾石层面在剪切过程中的破坏行为．首先进行了不同法向应力作用下的胶结砂砾石层面直剪试验,获得了不同的剪切面破坏特征：随着法向应力的不断增加,剪切破坏面凹凸起伏程度、骨料脱落现象越发明显．其次为了深入探究层面破坏现象,按照室内试验采用的骨料级配粒径建立细观颗粒数值模型,结合物理试验对模型进行参数标定,并进行数值模型的层面直剪模拟和细观分析．结果表明,数值模型可以再现宏观直剪试验层面破坏特征;法向应力越大,层面区域颗粒发生错动和翻转的数量越多;层面破坏方式为颗粒间的张拉和剪切混合破坏,裂隙均集中在层面位置,随着法向应力的增加,裂隙的集中区域逐渐由“面”向“带”转变．     

基于球单元组合颗粒模型的筒仓卸粮速度场演化机理研究

由于现有的单一球单元模型不能客观反映颗粒之间以及颗粒与筒仓仓壁之间接触的实际情况,本文提出了一种组合颗粒模型。以室内卸粮物理试验为基础,基于该组合颗粒模型进行离散元模拟,研究了卸粮速度场的时空变化,并对比分析了仓壁动态压力变化。研究结果表明:(1)改进的组合颗粒模型相比传统的ball单元模型,卸粮时间大幅增加,且动态侧压力波动幅度增大;(2)组合颗粒模型相比原模型中心颗粒垂直速度更小,而仓壁处颗粒由于剪切带颗粒堆积,两模型的垂直速度并无明显差别;(3)颗粒的水平速度波动与仓壁的动态压力波动呈同种变化趋势,且组合模型颗粒水平速度变化幅度较原模型更大;(4)组合模型由于颗粒间空隙较小、摩擦较为充分,在中心卸粮过程中会周而复始地出现压力拱现象,因此其中心颗粒的转动角速度(较仓壁处)的增长幅度更大。本文建立的组合颗粒模型能更准确地模拟筒仓卸粮动态物理特征变化,可广泛应用于粮食散体细观力学模拟等领域。     

砂土统一模型的显式计算及加载速度影响研究

以砂土体渐进破坏问题为背景，采用显式方法进行高、低压下砂土统一模型的二次开发以克服隐式算法在软化计算中的强非线性不收敛问题，实现具有剪胀软化、剪缩硬化特性材料的模拟计算．在此基础上开展三个压力等级、不同位移加载速度条件下的平面应变压缩试验，分析位移加载速度对显式计算结果的影响效应．通过分析得到结论：(1)采用动力学显式算法对常压至高压范围的砂土统一模型进行计算是可行的，受荷响应随加载速度的降低逐渐趋于稳定并最终得到静力学计算结果，能够反映砂土在低压下的剪胀软化以及高压下的剪缩硬化特性；(2)砂土统一模型的显式计算结果受加载速度的影响呈双折线趋势，并存在加载速度界限值，试样在加载速度界限值前后呈现不同的受荷变形破坏形态；(3)采用显式计算方法开展砂土统一模型的准静态模拟过程中，须结合计算结果偏差面积比随加载速度的关系曲线，来确定满足一定偏差面积比并且小于界限值的准静态位移加载速度．     

剪胀性砂土边界面模型的研究

剪胀性对于砂土,尤其是中密以及密实砂土,是一个非常显著的特性。相变线是剪胀性砂土的特征曲线,能够反映砂土的围压以及初时孔隙比对变形特性的影响。本文在边界面塑性理论的框架内,把相变状态参量引入到剪胀方程以及塑性硬化模量中,建立了一个能够描述砂土剪胀性以及循环特性的本构模型。本模型采用一套参量可以模拟不同初时孔隙比、不同围压、排水(或不排水)条件下单调(或循环)加载的应力-应变特性。验证表明本模型数值计算与试验结果相吻合。     

Simulating shear behavior of a sandy soil under different soil conditions

Understanding of soil shear behavior is very important in the field of agricultural machinery and soil dynamics. In this study, a discrete element model was developed using a simulation tool, Particle Flow Code in Three Dimensions (PFC^3D). The model simulates direct shear tests of soil and predicts soil shear behavior, in terms of shear forces and displacements. To determine and calibrate model parameters (stiffness of particles, strength and stiffness of bond between particles), laboratory direct shear tests were conducted to examine effects of soil moisture content and bulk density on shear behaviors of a sandy soil. Three soil moisture levels (0.02%, 13.0%, and 21.5%) and four bulk density levels (0.99, 1.28, 1.36, and 1.50 Mg/m³) were used in the tests. The test results showed that in general drier and denser soil conditions produced higher shear forces. Based on the test results, the bond strengths of the model particles were determined from soil cohesion and internal friction angle. The model particle stiffness was calibrated based on the yield forces from the tests. The calibrated particle stiffness varied from 1.0 × 10³ to 8.2 × 10³ N/m, depending on soil moisture and density levels. The bond stiffness calibrated was 1.0 × 10⁷ Pa/m for all soil conditions.     

Mechanical behaviour of the upper layers of a sandy loam soil under shear loading

The mechanical behaviour of the upper layers of a sandy loam soil was studied under standard triaxial compression and direct shear box tests. Variations of soil material properties were investigated at four different initial dry bulk densities of 1410, 1520, 1610 and 1670 kg/m³. Soil deformation and volume change under the triaxial compression loading were also studied at these bulk densities. Results from the two tests showed increases in the soil mechanical properties with the initial dry bulk density. The internal friction angle values measured with the triaxial compression apparatus exceeded those measured with the direct shear box. In contrast, the soil cohesion values measured with the direct shear box exceeded those measured with the triaxial compression apparatus. Under the triaxial compression test, the loose soil samples underwent contraction and volume reduction, whereas the dense samples swelled and failure cracks appeared clearly at various planes. The soil contraction for the former case characterizes the occurrence of soil compaction, whereas the cracks propagation and volume increase in the latter case characterizes the breaking up and loosening of soil during tillage operations. For the loose and moderately compacted states, the engineering Poisson's ratio increased with the axial strain until loading was completed. It also increased at the compacted and very compacted states until reaching given loading stages, after which its value started to decrease. This shifting in the engineering Poisson's ratio during loading may provide another identification of the moment of soil failure occurrence, in addition to that of the maximum shear stress.     

Numerical simulation and testing verification of the interaction between track and sandy ground based on discrete element method

Tractive effort of tracked vehicles plays an important role in military and agricultural fields. In order to solve the problem of low precision in numerical simulation of the interaction between track and sandy ground, a systematic and accurate discrete element modeling method for sandy road was proposed. The sandy ground was modeled according to the mechanical parameters measured by soil mechanics tests. The interaction coefficients of sandy soil were measured by the repose angle test and triaxial compression test combined with the corresponding simulation. On this basis, a discrete element interaction model of track-sandy ground was established, which can be used to test the tractive effort of track. Numerical simulation calculation of track model at different speeds was carried out, and the simulation results were compared with the results of indoor soil bin test for verification. The verification results show that the interaction between track and sandy ground based on DEM simulation is consistent with the actual soil bin test. The discrete element modeling method in this paper can be used to model the track and sandy ground accurately, and the simulation model can be used to test the tractive effort of tracked vehicle.     

砂土地基锚板基础抗拔承载力PFC数值分析

在模型试验的基础上,采用PFC离散单元法对条形锚板基础在中密砂土地基中的抗拔性能进行了数值分析。数值模拟采用簇颗粒单元来模拟砂粒的不规则形状,颗粒级配根据模型试验福建标准砂的级配按照相似级配法生成,细观参数根据数值双轴试验确定。水平锚板数值模拟结果与模型试验结果基本一致。与模型试验结果相比,颗粒流数值模拟能得到颗粒间接触力链的分布及其演化规律,能从细观角度来探明宏观抗拔承载力特性的演化机理。在此基础上对倾斜锚板上拔过程进行了模拟,分析了锚板前后砂粒的运动趋势以及接触力链的演化规律,并与已有承载力结果进行了对比分析。     

Effects of supply air temperature and inlet location on particle dispersion in displacement ventilation rooms

The effects of supply temperature and vertical location of inlet air on particle dispersion in a displacement ventilated (DV) room were numerically modeled with validation by experimental data from the literature. The results indicate that the temperature and vertical location of inlet supply air did not greatly affect the air distribution in the upper parts of a DV room, but could significantly influence the airflow pattern in the lower parts of the room, thus affecting the indoor air quality with contaminant sources located at the lower level, such as particles from working activities in an office. The numerical results also show that the inlet location would slightly influence the relative ventilation efficiency for the same air supply volume, but particle concentration in the breathing zone would be slightly lower with a low horizontal wall slot than a rectangular diffuser. Comparison of the results for two different supply temperatures in a DV room shows that, although lower supply temperature means less incoming air volume, since the indoor flow is mainly driven by buoyancy, lower supply temperature air could more efficiently remove passive sources (such as particles released from work activities in an office). However, in the breathing zone it gives higher concentration as compared to higher supply air temperature. To obtain good indoor air quality, low supply air temperature should be avoided because concentration in the breathing zone has a stronger and more direct impact on human health.     

Influence of particle shape on the microstructure evolution and the mechanical properties of granular materials

In order to study the influence of particle shape on the microstructure evolution and the mechanical properties of granular materials, a two-dimensional DEM analysis of samples with three particle shapes, including circular particles, triangular particles, and elongated particles, is proposed here to simulate the direct shear tests of coarse-grained soils. For the numerical test results, analyses are conducted in terms of particle rotations, fabric evolution, and average path length evolution. A modified Rowe's stress–dilatancy equation is also proposed and successfully fitted onto simulation data.     

Numerical and experimental direct shear tests for coarse-grained soils

The presence of particles larger than the permissible dimensions of conventional laboratory specimens causes difficulty in the determination of shear strength of coarse-grained soils. In this research, the influence of particle size on shear strength of coarse-grained soils was investigated by resorting to experimental tests in different scale and numerical simulations based on discrete element method (DEM). Experimental tests on such soil specimens were based on using the techniques designated as "parallel" and "scalping" to prepare gradation of samples in view of the limitation of laboratory specimen size. As a second approach, the direct shear test was numerically simulated on assemblies of elliptical particles. The behaviors of samples under experimental and numerical tests are presented and compared, indicating that the modification of sample gradation has a significant influence on the mechanical properties of coarse-grained soils. It is noted that the shear strengths of samples produced by the scalping method are higher than samples by the parallel method. The scalping method for preparing specimens for direct shear test is therefore recommended. The micromechanical behavior of assemblies under direct shear test is also discussed and the effects of stress level on sample behavior are investigated.     

Numerical study of concentrated fluid–particle suspension flow in a wavy channel

The particle migration effects and fluid–particle interactions occurring in the flow of highly concentrated fluid–particle suspension in a spatially modulated channel have been investigated numerically using a finite volume method. The mathematical model is based on the momentum and continuity equations for the suspension flow and a constitutive equation accounting for the effects of shear‐induced particle migration in concentrated suspensions. The model couples a Newtonian stress/shear rate relationship with a shear‐induced migration model of the suspended particles in which the local effective viscosity is dependent on the local volume fraction of solids. The numerical procedure employs finite volume method and the formulation is based on diffuse‐flux model. Semi‐implicit method for pressure linked equations has been used to solve the resulting governing equations along with appropriate boundary conditions. The numerical results are validated with the analytical expressions for concentrated suspension flow in a plane channel. The results demonstrate strong particle migration towards the centre of the channel and an increasing blunting of velocity profiles with increase in initial particle concentration. In the case of a stenosed channel, the particle concentration is lowest at the site of maximum constriction, whereas a strong accumulation of particles is observed in the recirculation zone downstream of the stenosis. The numerical procedure applied to investigate the effects of concentrated suspension flow in a wavy passage shows that the solid particles migrate from regions of high shear rate to low shear rate with low velocities and this phenomenon is strongly influenced by Reynolds numbers and initial particle concentration. Copyright © 2008 John Wiley & Sons, Ltd.