DEM simulations of die filling during pharmaceutical tabletting

The flow behaviour of powders from a stationary shoe into a moving die, which mimics the die filling process in a rotary tablet press, was analysed using a discrete element method （DEM）, in which 2D irregular shaped particles were considered. The influence of the particle shape, size and size distribution, the number of particles used in the simulation, the initial height of powder bed in the shoe, and the filling speed on the average mass flow rate and the critical filling speed （the highest speed at which the die can be completely filled） were explored. It has been found that a maximum flow rate is obtained at the critical filling speed for all systems investigated and poly-disperse systems have higher mass flow rates and higher critical filling speeds than mono-disperse systems. In addition, the powder with particles which can tessellate generally has a lower filling rate and a lower critical titling speed.     

Modelling of massive particulates for breakwater engineering using coupled FEMDEM and CFD

The seaward slope of many breakwaters consists of thousands of interlocking units of rock or concrete comprising a massive granular system of large elements each weighing tens of tonnes.The dumped quarry materials in the core are protected by progressively coarser particulates.The outer armour layer of freely placed units is intended to both dissipate wave energy and remain structurally stable as strong flows are drawn in and out of the particulate core.Design guidance on the mass and shape of these units is based on empirical equations derived from sealed physical model tests.The main failure mode for armour layers exposed to severe storms is hydraulic instability where the armour units of concrete or rock are subjected to uplift and drag forces which can in turn lead to rocking,displacement and collisions sufficient to cause breakage of units.Recently invented armour unit designs making up such granular layered system owe much of their success to the desirable emergent properties of interlock and porosity and how these combine with individual unit structural strength and inertial mass. Fundamental understanding of the forces governing such wave-structure interaction remains poor.We use discrete element and combined finite-discrete element methods to model the granular solid skeleton of randomly packed units coupled to a CFD code which resolves the wave dynamics through an interface tracking technique.The CFD code exploits several methods including a compressive advection scheme, node movement, and general mesh optimization.We provide the engineering context and report progress towards the numerical modelling of instability in these massive granular systems.     

New discrete element models for elastoplastic problems

The discrete element method （DEM） has attractive features for problems with severe damages, but lack of theoretical basis for continua behavior especially for nonlinear behavior has seriously restricted its application. The present study proposes a new approach to developing the DEM as a general and robust technique for modeling the elastoplastic behavior of solid materials. New types of connective links between elements are proposed, the interelement parameters are theoretically determined based on the principle of energy equivalence and a yield criterion and a flow rule for DEM are given for describing nonlinear behavior of materials. Moreover, a numerical scheme, which can be applied to modeling the behavior of a continuum as well as the transformation from a continuum to a discontinuum, is obtained by introducing a fracture criterion and a contact model into the DEM. The elastoplastic stress wave propagations and the tensile failure process of a steel plate are simulated, and the numerical results agree well with those obtained from the finite element method （FEM） and corresponding experiment, and thus the accuracy and efficiency of the DEM scheme are demonstrated.     

塔里木河下游数字地下水埋深模型的构建

根据数字地面模型的概念,利用航空拍摄的地面高程数据和地下水观测孔观测得到的地下水水位数据,建立了塔里木河下游地区的数字高程模型和数字地下水水位模型,并在这两个数字模型的基础上得到了塔里木河下游的数字地下水埋深模型．从数字地下水埋深模型获得的地下水埋深分布状况可知,地下水埋深大于10m的面积占统计面积的65％．     

宝钢码头前沿沙体移动的DEM模拟

利用地理信息系统（GIS）与数字高程模型（DEM）技术定量模拟了1994年~2002年宝钢码头前的沙体冲淤演变过程,结果表明：1994年~1999年,宝钢码头前河床总体表现为冲刷,冲刷量为53.286×106m3,年平均冲刷率为10.657×106m3,1999年~2002年,仍为冲刷,冲刷量为60.277×106m3,年平均冲刷率为20.092×106m3.1994年~2002年8 a共冲刷113.563×106m3;年平均冲刷率为14.195×106m3.淤积区域主要分布在上段沙体的上部及上段沙与新浏河沙体之间的区域.宝钢码头河床的演变是河床的边界条件、来水来沙及人类活动的耦合结果.上段沙体的下移南偏对宝钢码头存在潜在的不利.     

Experimental and DEM analyses on wheel-soil interaction

In this paper, the wheel-soil interaction for a future lunar exploration mission is investigated by physical model tests and numerical simulations. Firstly, a series of physical model tests was conducted using the TJ-1 lunar soil simulant with various driving conditions, wheel configurations and ground void ratios. Then the corresponding numerical simulations were performed in a terrestrial environment using the Distinct Element Method (DEM) with a new contact model for lunar soil, where the rolling resistance and van der Waals force were implemented. In addition, DEM simulations in an extraterrestrial (lunar) environment were performed. The results indicate that tractive efficiency does not depend on wheel rotational velocity, but decreases with increasing extra vertical load on the wheel and ground void ratio. Rover performance improves when wheels are equipped with lugs. The DEM simulations in terrestrial environment can qualitatively reproduce the soil deformation pattern as observed in the physical model tests. The variations of traction efficiency against the driving condition, wheel configuration and ground void ratio attained in the DEM simulations match the experimental observations qualitatively. Moreover, the wheel track is found to be less evident and the tractive efficiency is higher in the extraterrestrial environment compared to the performance on Earth.     

No more conventional reference electrode: Transition time for determining chloride ion concentration

Ion selective electrodes (ISE) are used extensively for the potentiometric determination of ion concentrations in electrolytes. However, the inherent drift in these measurements and the requirement of a stable reference electrode restrict the feasibility of this method for long-term in-situ applications. This work presents a chronopotentiometric approach to minimize drift and avoid the use of a conventional reference electrode for measuring chloride ion concentration. An anodic current pulse is applied to a Ag/AgCl working electrode which initiates a faradaic reaction that depletes the chloride ions near the electrode surface. The rate of change in potential at the Ag/AgCl electrode, due to chloride ion depletion, reaches an inflection point once the chloride ions deplete completely near the electrode surface. The moment of the inflection point, also known as the transition time, is a function of the chloride ion concentration and is described by the Sand equation. It is shown that the square root of the transition time is linearly proportional to the chloride ion concentration. Drift in the response over two weeks is negligible: 59 μM/day when measuring 1 mM of Cl⁻ ions using a 10 A m⁻² current pulse. The transition time at a specific ion concentration can be tuned by the applied current pulse, e.g., in a solution containing 5 mM chloride ions, the transition times with current pulses of 10 and 20 A m⁻² are 1.56 and 0.25 s, respectively. The moment of inflection determines the response, and thus is independent of the absolute potential of reference electrode. Therefore, any metal wire can act as a pseudo-reference electrode, enabling this approach for long-term and integrated-sensor applications such as measurement inside concrete structures.     

三维颗粒生成算法

颗粒生成是进行散粒体细观数值模拟的前提.为快速生成指定级配的颗粒并减少自由堆积过程,针对三维球形及多面体颗粒生成,改进了颗粒生成的前进面算法.其基本思想是:随机选取3种粒径,生成3个相互接触的球,3个球的球心构成初始的2个前进面;随机选取前进面和一种粒径,在前进面的外侧生成斯的球,使新球与构成前进面的3个球相切;更新前...     

基于克里金法的数字高程模型插值研究

数字高程模型(DEM)是对实际地面模拟的一种有效方式,它的数据来源于实地测量,由于客观条件的限制,实地测量数据是一种抽样数据,样本密度影响了数字高程模型的精密度。针对这一问题,探讨了运用克里金法对数据样本进行插值处理的算法,将样本进行加密,从而提高数字高程模型的适应度,满足不同的需求。     

基于VirtuoZo全数字摄影测量系统的无人机遥感数据后处理

针对无人机遥感数据特点,介绍了利用VirtuoZo全数字摄影测量系统和PATB平差软件进行空三加密及制作数字高程模型DEM以及数字正射影像DOM的无人机影像数据后处理方法。