不同尺度分布散粒材料砂堆形成过程的二维离散元模拟

用作者开发的离散元程序,模拟不同尺寸分布的砂堆形成过程.把散体颗粒简化为圆形颗粒,模拟过程分三步:首先利用参考网格生成颗粒的松散堆积结构;为了避免颗粒下落的冲击作用对砂堆安息角的影响,先模拟颗粒在重力作用下在圆柱容器内的自由下落与堆积,直至堆积达到稳定;最后,移除容器,只保留一个底部边界,模拟颗粒体系的散落过程,直至形成一个稳定的砂堆.模拟结果表明,在其他参数保持相同的情况下,随着颗粒尺寸的减小,砂堆的安患角逐步减小并趋向于一常值.对模拟中的两组颗粒体系进行相同条件下的砂堆形成实验,结果表明,模拟与实验所得安息角大体相当.     

颗粒流动力学及其离散模型评述

颗粒流是由众多颗粒组成的具有内在相互作用的非经典介质流动. 自然界常见颗粒流都是密集流, 颗粒间接触形成力链, 诸多力链相互交接构成支撑整个颗粒流重量和外载荷的网络, 其局部构型及强度在外载荷下演化, 是颗粒流摩擦特性和接触应力的来源.本文介绍球形颗粒间无粘连作用时的Hertz法向接触理论和Mindlin-Deresiewicz切向接触理论. Campbell依据是否生成较为稳定的力链把颗粒流分为弹性流和惯性流两大类, 其中弹性-准静态流和惯性-碰撞流分别对应准静态流和快速流, 作为两种极端流动情况通常处理成连续体, 分别采用摩擦塑性模型和动理论予以描述, 但是表征接触力链的颗粒弹性参数并不出现这两个模型和理论框架中, 如何进一步考虑颗粒弹性参数将非常困难. 目前离散动力学方法逐渐成为复现其复杂颗粒流动现象、提取实验不可能获得的内部流动信息进而综合起来探索颗粒流问题的一种有效工具, 其真实性强于连续介质理论的描述. 软球模型对颗粒间接触力简化处理, 忽略了切向接触力对法向接触力及其加载历史的依赖, 带来了法向和切向刚度系数如何标度等更艰难的物理问题, 但由于计算强度小而广泛应用于工程问题中. 硬球模型不考虑颗粒接触变形, 因而不能描述颗粒流内在接触应变等物理机理, 仅适用于快速颗粒流, 这不仅仅是由于两体碰撞的限制. 因此基于颗粒接触力学的离散颗粒动力学模型是崭新的模型,适用于准静态流到快速流整个颗粒流态的模拟, 可以细致考虑接触形变及接触力的细节,建立更为合理的颗粒流本构关系, 进而有力的促进颗粒流这一非经典介质流动的研究.      

基于DEM方法的不同密度颗粒材料砂堆形成过程的数值研究

颗粒堆积是地质工程中较为普遍的现象,而其堆积稳定后的安息角可以在一定程度上反映颗粒堆积的基本特征.用DEM方法研究不同密度分布的颗粒堆积过程及堆积结果,有助于我们在细观层面研究颗粒密度对颗粒堆积结果的影响和验证DEM方法分析颗粒堆积的合理性.本研究用圆形颗粒简化模拟散体颗粒:第一步,用EDEM软件建立相关模型,在模型内生成简化的颗粒,模拟颗粒在重力作用下自由下落和堆积稳定的过程;第二步,模型中生成颗粒下落堆积稳定后,保留模型底部边界,数值模拟研究移除模型上部边界后颗粒散落堆积稳定的过程并测定堆积体的安息角;第三步,对应数值模型,做相关的实验进行对比验证.研究结果表明:在只考虑堆积颗粒密度而忽略其他参数影响的条件下,堆积颗粒材料的密度越小,其堆积稳定后的安息角越大.     

转动驱动圆角立方体颗粒有序堆积的离散元模拟

宏观颗粒体系的有序堆积研究可以为热系统中微观粒子的自组装提供研究模型,也有助于工业生产中提高颗粒材料的填充率.实验发现圆筒中的圆角立方体颗粒受容器往复旋转剪切作用会实现有序堆积.为探究旋转圆筒中立方体颗粒有序堆积过程的内部结构演化过程和动力学机理,采用超二次曲面方程构造了圆角立方体颗粒,基于离散元方法对旋转剪切作用下圆...     

离散型湍流多相流动的研究进展和需求

离散型多相流动,指气体-颗粒(气-固)、液体-颗粒(液-固)、液体-气泡、气体-液雾以及气泡-液体-颗粒等两相或三相流动.这种类型的多相流动广泛存在于能源, 航天和航空, 化工和冶金,交通运输, 水利, 核能等领域.本文阐述了离散型多相流动的国内外基础研究,包括颗粒/液滴/气泡在流场中受流体动力作用力的研究, 颗粒-颗粒,液滴-液滴,气泡-气泡之间以及颗粒/液滴和壁面之间碰撞和聚集规律的研究,颗粒-气体和气泡-液体湍流相互作用的研究, 和数值模拟的研究,包括多相流动的雷诺平均模拟、大涡模拟和直接数值模拟的研究进展.最后, 归纳了目前尚待研究的需求.      

粗粒土离散元非球趋真颗粒模型

颗粒形状对粗粒土的物理力学特性有着显著的影响。离散元法广泛应用于粗粒土宏观物理力学特性的细观机理研究。为了考虑颗粒形状的影响,亟待发展计算高效的离散元非球趋真颗粒模型。本文基于X射线CT扫描技术并结合数字图像处理技术,对光滑和棱角性两类典型粗粒土(鹅卵石与碎石)进行三维重构,并提出了两类趋真颗粒模型,分别采用扩展超椭球模型和球多面体模型进行趋真逼近;开展了两类颗粒试样的3D打印和单轴压缩试验,分析了配位数和局部孔隙率分布等细观特性;基于离散元开源程序SudoDEM开展了两类试样的离散元模拟,并将模拟细观分析结果与物理试验进行了对比。结果表明,提出的两类趋真颗粒模型能够较好地对粗粒土颗粒进行离散元建模。     

尼龙粉末在SLS预热温度下的离散元模型参数确定及其流动特性分析

尼龙粉末是增材制造中常用的粉体材料,温度对其流动性有重要影响. 探索尼龙粉末增材制造预热温度下的流动性是研究选择性激光烧结(selective laser sintering, SLS)工艺中粉体铺展成形的基础. 选取SLS技术中的尼龙粉末为原材料,采用离散元数值方法,研究尼龙粉末的流动行为,是增材制造工艺数值模拟和铺粉工艺优化的研究热点. 以Hertz-Mindlin模型为基础,基于Hamaker理论模型和库伦定律,在尼龙粉末的接触动力学模型中引入范德华力和静电力,建立预热温度下尼龙粉末流动的离散元模型(discrete element method, DEM),通过对比相应实验结果,标定了该模型的参数. 对加热旋转圆筒中尼龙粉末流动过程进行了DEM数值模拟,校核了所建模型的正确性,并研究了粉体粒径分布对尼龙粉末流动特性的影响规律. 研究表明,尼龙粉末黏附力是静电力与范德华力的共同作用结果;随着粉体粒径的增大,尼龙粉末崩塌角增大,流动性增强;相对于高斯粒径分布,粒径均匀分布的尼龙粉末颗粒流动性更强. 研究结果可指导SLS中铺粉工艺的优化.      

颗粒层横向推移过程离散元仿真与参数分析

针对颗粒层横向推移过程,展开了物理试验及离散元仿真分析。采用物理试验方法对颗粒的物性参数进行了测定,并实测了颗粒层推移过程的推移力大小,进行了颗粒层横向推移过程离散元仿真模型的调试,通过仿真-试验结果的对比分析验证了仿真模型的合理性。利用所建立的仿真模型分析出:推移力斜率与静摩擦系数呈近似正比关系,颗粒堆积坡角度也随静摩擦系数的增大而增大;发现了摩擦系数增大造成推移力增大的力链机理,推板倾角增大会降低推移力增大速率,使推移力转变点位置靠近终点墙及增大竖直方向附加力。     

鱼洗内颗粒流动的数值模拟

实验表明,鱼洗内的流动存在高度非线性的相互作用,并最终导致表面连续性的破坏. 在数值模拟中,这类问题是非常难处理的. 该文从离散的观点出发,直接考虑容器内部粒子的运动. 即采用分子动力学方法,模拟鱼洗内的流动. 该方法所考虑的实质是颗粒流动,不要求介质是连续的,从统计物理的观点看,当所模拟的粒子数非常大时,在统计上和连续介质相当. 该文首先发展了基于消息传递(MPI)的并行计算程序,可以用来做大规模的颗粒流动模拟. 对鱼洗内的颗粒流动模拟结果表明,在外界激励下,鱼洗内的颗粒流动表现出的现象和液体流动类似,边界的能量以波的形式向内部传播,并很快耗散掉,在边界激励点附近,颗粒因为受到边界和内部粒子的挤压作用而跳起,从直观现象上看, 和鱼洗的流动实验一致. 在能量的概率密度分布函数中,可以观察到两个明显的指数区.      

尼龙粉末在SLS预热温度下的离散元模型参数确定及其流动特性分析

尼龙粉末是增材制造中常用的粉体材料,温度对其流动性有重要影响.探索尼龙粉末增材制造预热温度下的流动性是研究选择性激光烧结(selective laser sintering, SLS)工艺中粉体铺展成形的基础.选取SLS技术中的尼龙粉末为原材料,采用离散元数值方法,研究尼龙粉末的流动行为,是增材制造工艺数值模拟和铺粉工艺优化的研究热点.以Hertz-Mindlin模型为基础,基于Hamaker理论模型和库伦定律,在尼龙粉末的接触动力学模型中引入范德华力和静电力,建立预热温度下尼龙粉末流动的离散元模型(discrete element method,DEM),通过对比相应实验结果,标定了该模型的参数.对加热旋转圆筒中尼龙粉末流动过程进行了DEM数值模拟,校核了所建模型的正确性,并研究了粉体粒径分布对尼龙粉末流动特性的影响规律.研究表明,尼龙粉末黏附力是静电力与范德华力的共同作用结果;随着粉体粒径的增大,尼龙粉末崩塌角增大,流动性增强;相对于高斯粒径分布,粒径均匀分布的尼龙粉末颗粒流动性更强.研究结果可指导SLS中铺粉工艺的优化.     

Discrete and continuum modelling of excavator bucket filling

Two-dimensional discrete and continuum modelling of excavator bucket filling is presented. The discrete element method (DEM) is used for the discrete modelling and the material-point method (MPM) for continuum modelling. MPM is a so-called particle method or meshless finite element method. Standard finite element methods have difficulty in modelling the entire bucket filling process due to large displacements and distortions of the mesh. The use of a meshless method overcomes this problem. DEM and MPM simulations (plane strain) of bucket filling are compared to two-dimensional experimental results. Cohesionless corn grains were used as material and the simulated force acting on the bucket and flow patterns were compared with experimental results. The corn macro (continuum) and micro (DEM) properties were obtained from shear and oedometer tests. As part of the MPM simulations, both the classic (nonpolar) and the Cosserat (polar) continuums were used. Results show that the nonpolar continuum is the most accurate in predicting the bucket force while the polar and DEM methods predict lower forces. The DEM model does not accurately predict the material flow during filling, while the polar and nonpolar methods are more accurate. Different flow zones develop during filling and it is shown that DEM, the polar and the nonpolar methods can accurately predict the position and orientation of these different flow zones.     

Discrete element parameter calibration and the modelling of dragline bucket filling

The Discrete Element Method (DEM) is useful for modelling granular flow. The accuracy of DEM modelling is dependent upon the model parameter values used. Determining these values remains one of the main challenges. In this study a method for determining the parameters of cohesionless granular material is presented. The particle size and density were directly measured and modelled. The particle shapes were modelled using two to four spheres clumped together. The remaining unknown parameter values were determined using confined compression tests and angle of repose tests. This was done by conducting laboratory experiments followed by equivalent numerical experiments and iteratively changing the parameters until the laboratory results were replicated. The modelling results of the confined compression tests were mainly influenced by the particle stiffness. The modelling results of the angle of repose tests were dependent on both the particle stiffness and the particle friction coefficient. From these observations, the confined compression test could be used to determine the particle stiffness and with the stiffness known, the angle of repose test could be used to determine the particle friction coefficient. Usually DEM codes do not solve the equations of motion for so-called walls (non-granular structural elements). However, in this study a dynamic model of a dragline bucket is developed and implemented in a commercial DEM code which allows the dynamics of the walls to be modelled. The DEM modelling of large systems of particles is still a challenge and procedures to simplify and speed up the modelling of dragline bucket filling are presented. Using the calibrated parameters, numerical results of bucket filling are compared to experimental results. The model accurately predicted the orientation of the bucket. The model also accurately predicted the drag force over the first third of the drag, but predicted drag forces too high for the subsequent part of the drag.     

The numerical modelling of excavator bucket filling using DEM

The filling of an excavator bucket is a complex granular flow problem. In order to optimize the filling process, it is important to understand the different mechanisms involved. The discrete element method (DEM) is a promising approach to model soil-implement interactions and it was used in this study to model the filling process of an excavator bucket. Model validation was based on the accuracy with which the model predicted the bucket drag force and the development of the different flow regions. Compared to experimental measurements, DEM predicted lower bucket drag forces, but the general trend was accurately modelled. At the end of the filling process the error in predicted drag force was 20%. Qualitatively, there was a good agreement between the observed and the modelled flow regions in terms of position and transition from one stage to the other. During all stages of filling, DEM was able to predict the volume of material inside the bucket accurately to within ±6%.     

Determining the angle of repose of sand under low-gravity conditions using discrete element method

This study is a comparative investigation of data, collected through experimental and numerical means, related to the flow of sand particles through a hopper under low-gravity conditions. During a parabolic airplane flight simulating low-gravity conditions, we determined effects of gravity on the angle of repose of sand pile particles by flowing dry sand from a hopper. The gravity effects on the angle of repose of the sand were negligible. Two-dimensional discrete element method (DEM) was used to simulate the angle of repose. Results were compared to observations made during the low-gravity experiments. Effects of varying parameters such as the friction coefficient and coefficient of rolling friction were determined by running various DEM simulations. Moreover, the effect of the elemental radius on the angle of repose was investigated using DEM. The angle of repose is influenced by certain changes in the friction coefficient and rolling friction values, but the elemental radius has only a negligible effect on the angle of repose within the range of variation. Results show that the DEM model used for this study might be applicable to determine terramechanical interactions under lunar surface gravity conditions, provided that parameters are adjusted and an extended period of simulation is achieved.     

基于离散单元法和人工神经网络的近壁颗粒动力学特征研究

颗粒与壁面的相互作用往往对颗粒流动具有显著影响. 为研究颗粒与壁面作用机理, 对滚筒内颗粒流动过程进行离散单元法(DEM)数值模拟. 基于模拟结果统计分析靠近壁面处颗粒的运动特征, 结果表明, 小摩擦系数时颗粒平动和旋转速度均近似满足正态分布, 但由于壁面影响, 摩擦系数增大时颗粒沿滚筒轴向的旋转速度偏离正态分布, 颗粒动力学理论推导壁面边界条件时应考虑速度正态分布的修正及速度脉动的各向异性. 采用人工神经网络(ANN)构建了颗粒无因次旋转温度、滑移速度和平动温度之间的函数模型, 进而可以在常规双流模型壁面边界条件中考虑颗粒旋转的影响. 基于DEM模拟及结果分析可以为壁面边界条件的理论构造和半经验修正提供基础数据和封闭模型.      

A study of influence of gravity on bulk behaviour of particulate solid

This paper examines the influence of gravity on the bulk responses of a granular solid. The loading scenarios in this study include confined compression, rod penetration into a granular medium and discharging through an orifice. Similar loading and flow conditions are likely to be encountered in the stress and deformation regimes that regoliths are subjected to in extraterrestrial exploration activities including in situ resource utilisation processes. Both spherical and non-spherical particles were studied using the discrete element method （DEM）. Whilst DEM is increasingly used to model granular solids, careful validations of the simulation outcomes are rather rare. Thus in addition to exploring the effect of gravity, this paper also compares DEM simulations with experiments under terrestrial condition to verify whether DEM can produce satisfactory predictions. The terrestrial experiments were conducted with great care and simulated closely using DEM. The key mechanical and geometrical properties for the particles were measured in laboratory tests for use in the DEM simulations. A series of DEM computations were then performed under reduced gravity to simulate these experiments under extraterrestrial environment. It was found that gravity has no noticeable effect on the force transmission in the confined compression case; the loading gradient in the rod penetration is linearly proportional to the gravity; the mass flow rate in silo discharge is proportional to square root of the gravity and the angle of repose increases with reducing gravity. These findings are in agreement with expectation and existing scientific evidence.     

A study of influence of gravity on bulk behavior of particulate solid

This paper examines the influence of gravity on the bulk responses of a granular solid. The loading scenarios in this study include confined compression, rod penetration into a granular medium and discharging through an orifice. Similar loading and flow conditions are likely to be encountered in the stress and deformation regimes that regoliths are subjected to in extraterrestrial exploration activities including in situ resource utilisation processes. Both spherical and non-spherical particles were studied using the discrete element method (DEM). Whilst DEM is increasingly used to model granular solids, careful validations of the simulation outcomes are rather rare. Thus in addition to exploring the effect of gravity, this paper also compares DEM simulations with experiments under terrestrial condition to verify whether DEM can produce satisfactory predictions.The terrestrial experiments were conducted with great care and simulated closely using DEM. The key mechanical and geometrical properties for the particles were measured in laboratory tests for use in the DEM simulations. A series of DEM computations were then performed under reduced gravity to simulate these experiments under extraterrestrial environment. It was found that gravity has no noticeable effect on the force transmission in the confined compression case; the loading gradient in the rod penetration is linearly proportional to the gravity; the mass flow rate in silo discharge is proportional to square root of the gravity and the angle of repose increases with reducing gravity. These findings are in agreement with expectation and existing scientific evidence.     

铁路道砟破碎特性的离散元分析

为降低有砟铁路轨道维护成本并提高乘客舒适度,对铁路道砟材料破碎机理深入研究具有重要作用。本文设计并开展了测量铁路道砟材料特征强度的试验,对道砟受压破碎过程的力和位移等数据加以分析;同时,通过三维激光扫描系统获取道砟三角网格多面体边界,依此采用颗粒平行粘结方式构造具有不规则真实外观的铁路道砟,并采用离散单元方法(DEM)数值模拟道砟径向加载破碎过程,分析应力应变曲线和粘结断裂数等数据以分析颗粒破碎机制。通过对多个道砟颗粒加载试验及数值模拟计算发现,随着道砟尺寸的增加,其有效压缩强度逐渐减小。道砟颗粒的有效压缩强度符合威布尔概率分布函数,并确定了其残存概率的平均强度和威布尔参量,试验和数值模拟结果相吻合。