Estimating the maximum impact force of dry granular flow based on pileup characteristics

The maximum normal impact resultant force(NIRF)is usually regarded as the sum of the static earth pressure of the dead zone and the dynamic impact pressure of the flowing layer.The influence of the interaction between the flowing layer and dead zone on the impact force is ignored.In this study,we classified two impact models with respect to the pileup characteristics of the dead zone.Then,we employed the discrete element method to investigate the influences of the pileup characteristics on the impact force of dry granular flow on a tilted rigid wall.If the final pileup height is equal to the critical value,the maximum NIRF can be estimated using a hydrostatic model,because the main contribution to the maximum NIRF is the static earth pressure of the dead zone.If the final pileup height is less than the critical value,however,the particles in the dead zone are squeezed along the slope surface by the impact ofthe flowing layer on the dead zone,and because of shear effects,the flowing layer causes an entrainment in the dead zone.This results in a decrease in the volume of the dead zone at the moment of maximum NIRF with increases in the slope angle.As such,the maximum NIRF mainly comprises the instant impact force of the flowing layer,so hydro-dynamic models are effective for estimating the maximum NIRF.Impact models will benefit from further study of the components and distribution of the impact force of dry granular flow.

Evaluation of the Impact Force of Dry Granular Flow onto Rock Shed

In the design of rock sheds for the mitigation of risk due to rapid and long landslides, a crucial role is played by the evaluation of the impact force exerted by the flowing mass on the rock sheds. This paper is focused on the influencing factors of the impact force of dry granular flow onto rock shed and in particular on the evaluation of the maximum impact force. The coupled DEM-FEM model calibrated with small-scale physical experiment is used to simulate the movement of dry granular flow coupled with impact forces on the rock-shed. Based on the numerical results, three key stages were identified of impact process, namely startup streams slippery, impact and pile-up. The maximum impact force increases linearly with bulk density, and the maximum impact force exhibits a power law dependence on the impact height and slop angle respectively. The sensitivities of bulk density, impact height, and slope angle on the maximum impact force are: 1.0, 0.496, and 2.32 respectively in the benchmark model. The parameters with high sensitivity should be given priority in the design of the rock shed. The results obtained from this study are useful for facilitating design of shed against dry granular flow.

Solid Boundary as Energy Source and Sink in a Dry Granular Dense Flow: A Comparison between Two Turbulent Closure Models

Solid boundary as energy source and sink of the turbulent kinetic energy of the grains, and its influence on the mean and turbulent features of a dry granular dense flow, are investigated by using the proposed zero- and first-order turbulent closure models. The first and second laws of thermodynamics are used to derive the equilibrium closure relations, with the dynamic responses postulated by a quasi-static theory for weak turbulent intensity. Two closure models are applied to analyses of a gravity-driven flow down an inclined moving plane. While the calculated mean porosity and velocity correspond to the experimental outcomes, the influence of the turbulent eddy evolution can be taken into account in the first-order model. Increasing velocity slip on the inclined plane tends to enhance the turbulent dissipation nearby, and the turbulent kinetic energy near the free surface. The turbulent dissipation demonstrates a similarity with that of Newtonian fluids in turbulent boundary layer flows. While two-fold roles of the solid boundary are apparent in the first-order model, its role as an energy sink is more obvious in the zero-order model.

草原干涸湖床地表粒度特征及其对沙尘释放的影响

[目的]为探究干涸湖盆典型地表风蚀过程及盐碱尘暴形成的机理。[方法]以锡林郭勒盟阿巴嘎旗查干淖尔干涸盐湖为研究对象,从湖心到湖岸根据盐湖地表的动态演化过程选择4种典型地表(结皮地表、破碎地表、活化地表和沙化地表),分析从湖心到湖岸近地表土壤粒度分布特征、风速廓线及输沙规律等。[结果](1)从湖心向湖岸延伸,地表0-2 cm沉积物颗粒粒配整体呈逐渐变粗的趋势,且以黏粒和粉粒为主;(2)干涸盐湖从湖心到湖岸风速整体呈降低趋势,不同地表的风速廓线基本符合Karman的速度对数分布规律,且拟合效果较好(R 2>0.90);(3)破碎地表的输沙量最大,约为结皮地表的50倍,且由于有限沙尘供应,导致结皮地表的输沙率随高度变化呈现线性规律(R^(2)=0.80),其他3种地表的输沙率随高度变化呈现指数函数形式(R^(2)>0.97);(4)在近地表0~50 cm高度内,风沙流中携带的沙粒平均粒径为2~8μm,属于粉粒,分选性较差,偏度以负偏为主,峰态以中等和尖窄为主。[结论]整体来看,破碎地表上的风蚀作用最强,结皮地表上的风蚀作用最弱。可通过提高沙粒的临界启动风速阈值和下垫面的覆盖度等方式减弱该地区的风蚀作用。

块矿干燥特性的实验研究

提高高炉炉料结构中块矿配比对降低铁水生产成本具有重要作用,而天然块矿一般湿度较大,天然块矿在入高炉前有必要进行干燥。以块矿为研究对象,通过实验研究了干燥介质温度、流速和块矿粒度对块矿的干燥特性的影响。结果表明,升高干燥温度、提高空气流速、减小块矿粒度可以有效缩短块矿干燥时间,提高干燥效率,适宜的干燥工艺条件为:干燥气体温度400℃,干燥气体流速12 m^(3)/h;在此基础上对块矿干燥进行了动力学分析,发现与Henderson and Pabis模型、Lewis模型相比,Page模型与实际干燥水分比MR-时间曲线吻合程度最高。同时得到了干燥介质温度T、流速V与干燥水分比MR的干燥动力学方程。发现干燥过程依次出现升速干燥、恒速干燥、第一降速干燥和第二降速干燥4个阶段,且升速干燥和恒速干燥过程时间相对较短,分别为干燥总时间的5%和10%。

碎石垫层对碎屑流冲击棚洞的缓冲效应研究

棚洞常用于保护山区道路免受山体滑坡或崩塌而形成的碎屑流的冲击破坏,为了进一步明确缓冲垫层对棚洞的防护作用,通过室内模型实验研究碎屑流对棚洞的冲击机制以及利用碎石作为缓冲垫层的缓冲效应。实验中记录的高速影像显示碎屑流冲击过程中碎石缓冲垫层有明显的变形,测力元件测得冲击力沿法向和切向在棚洞上的分布。实验结果揭示在没有碎石垫层的情况下,棚洞上的最大法向力和切向力分量作用在受冲击侧的拱脚位置,增加碎石垫层后,最大冲击力的值变小,并且最大法向力和切向力分量从受冲击侧的拱脚偏移30°~45°,这表明缓冲层不仅能减小冲击力的大小,还能改变其分布模式。以冲击力作为输入对棚洞运用力法分析其结构内力,发现碎石垫层可以有效减少结构最大弯矩和最大剪力并增加最大轴力,增加了结构的安全性。同时根据冲击过程中能量的变化,发现颗粒粒径较细的碎石垫层耗散冲击能量的效果更好。实验结果充分表明,碎石垫层在碎屑流冲击棚洞时发挥了显著的缓冲和耗能作用。