Influential factors on the repose angle of desert sands

The repose angle is one of the most significant macroscopic parameters in describing the behavior of granular materials. Under a static condition, the repose angle is the steepest angle at which sediment particles can rest without motion. In this paper, we use existing data and aeolian physics to analyze the main factors that influence the repose angle of sand dunes, and we investigate different repose angles involving various states and types of materials. We have determined that different factors have differential influence on the magnitude of the repose angle. Our results show that for powdery （〈400-μm diameter） desert sands, the main influential factor on the magnitude of repose angle is the molecular force among particles. Particle size does not influence the repose angle of desert sands directly, but has an indirect impact by affecting the grit sphericity and surface roughness, of which the grit sphericity acts as a major factor. Even at the same average particle size, the repose angle differs with different grain compositions. Furthermore, with increasing unevenness in grain composition, the repose angle increases correspondingly. Sand texture also has a direct influence on the repose angle of desert sands. In two sand samples having the same grain composition but different textures, the repose angles may be different. Water content has a stronger influence on the repose angle than any other factor. However, the relationship between the repose angle and water content is not a simple direct proportion. In fact, with increasing water content, the repose angle first increases and then decreases. These research results will be useful for understanding the mechanisms of dune transport, variations of dune morphology, and the stability and fluidity of dune sands.

Determination of key soil characteristic parameters using angle of repose and direct shear stress test

Discrete element modelling(DEM)is a numerical method for examining the dynamic behavior of granular media.In order to build an accurate simulation model and provide more comprehensive soil characteristic parameters for the design and optimization of various soil contact machinery,in this paper,the discrete element simulation method(EDEM)combined with experimental approach is used to investigate the soil contact characteristic parameters in East Asia.In this study,Hertze-Mindlin(no slip)was used as a particle contact model by taking particle contact parameters and soil JKR(Johnson-Kendall-Roberts)surface energy as determinants,and repose angle,internal friction angle,and cohesive force as evaluation indexes.The method of Plackett-Burman,Stepest ascent,and Box-Behnken were used to gradually reduce the range of parameters needed for simulation until the most accurate value was determined.The results show that the restitution coefficient,static friction coefficient,and rolling friction coefficient between soil particles have significant effects on the DEM model,and their value of them are 0.596,0.725,and 0.16,respectively.Based on these parameters used for the repose angle test and direct shear stress test,the value of repose angle is 31.97°,the internal friction angle is 27.61°,and the cohesive force is 33.06 kPa.The relative errors with the actual measured values are 9.54%,1.87%,and 2.31%,respectively.In order to further test whether the simulation parameters of soil obtained by repose angle test and direct shear stress test are consistent with the real soil,comparison test between field test and discrete element simulation was used.The results show that the error in height of ridge between the simulated soil and the actual soil is 4.06%,which is within the acceptable range.It also indicates that the calibrated and optimized soil simulation model can accurately represent the real soil.The research provides theoretical basis and technical support for the study of soil contact parts by using the discrete element method,combined with repose angle test and direct shear stress test.

基于离散元法的面粉颗粒接触参数标定试验

为设计和优化面粉输送设备,应用离散元法对面粉进行准确地工程建模和分析,需要对其接触参数进行必要的标定。该研究依据颗粒缩放理论,用“Hertz-Mindlin with Johnson-Kendall-Roberts”接触模型表征面粉颗粒间黏性的影响,提出了一种基于静/动态休止角的接触参数标定方法。运用正交试验方法,对接触参数的敏感性和方差分析,表明面粉颗粒间的滚动摩擦系数、面粉颗粒与不锈钢表面间的静摩擦系数、表面能对静态休止角的影响极显著(P<0.01),并且多组接触参数都可以模拟出与试验相同的静态休止角。进一步研究表明,面粉颗粒与不锈钢表面间的静摩擦系数的合理取值范围为0.2~0.4。通过2种填充率、4种转速下基于动态休止角的参数标定,将其中与试验最为吻合的一组参数作为标定结果,其值如下:面粉颗粒之间恢复系数为0.6、面粉颗粒之间静摩擦系数为0.2、面粉颗粒之间滚动摩擦系数为0.1、面粉颗粒与不锈钢容器表面之间恢复系数为0.6、面粉颗粒与不锈钢容器表面之间静摩擦系数为0.6、面粉颗粒与不锈钢容器表面之间滚动摩擦系数为0.5、表面能为0.12 J/m^(2)。使用该组参数对矩形容器中物料自由坍塌试验进行仿真,其结果与试验结果相符,验证了该标定方法的有效性。该研究提出的标定方法简单、易执行,对粉料输送设备的设计及优化具有一定的工程应用价值。

颗粒缩放理论标定未筛选烟煤离散元仿真参数

【目的】为了给未筛选烟煤的仿真研究提供参数依据,分析未筛选烟煤的离散元仿真参数,保证仿真与实际颗粒的几何、材料及运动学相似,实现未筛选烟煤的可靠仿真研究。【方法】采用实验和仿真相结合的方法,测得未筛选烟煤的粒径分布、密度、静摩擦因数、堆积密度、休止角等基本参数;基于颗粒缩放理论,建立不同粒径范围内典型颗粒的放大模型,通过Plackett-Burman、最陡爬坡、 Box-Behnken试验对未筛选烟煤的泊松比、切变模量、滚动摩擦因数、恢复系数、 Johnson-Kendall-Roberts(JKR)表面能等仿真参数进行标定。【结果】以实验和仿真休止角相对误差最小为优化目标,得到最优参数组合下的未筛选烟煤的仿真休止角为37.59°,与休止角实验值37.81°的误差为0.58%;仿真堆积密度为717 kg/m^(3),与堆积密度实验值722 kg/m^(3)的误差为0.69%。【结论】煤-煤恢复系数与休止角呈负相关,煤-煤和煤-钢滚动摩擦因数与休止角呈正相关,溅射现象会阻碍颗粒堆积。

基于休止角试验的颗粒钾肥离散元模型参数标定

为了获取颗粒钾肥的离散元模型参数,对一种颗粒钾肥测量三轴尺寸、质量,通过查阅文献确定离散元仿真参数的取值范围,并通过无底圆筒堆积物理试验,获得颗粒钾肥的休止角。采用EDEM软件建立颗粒钾肥仿真模型,通过试验设计,以期获得不同参数组合下的休止角仿真试验结果。通过Design Expert软件内部的Plackett-Burman功能,对仿真所需要的8个关键参数进行显著性测评,发现钾肥-钾肥静摩擦因数以及钾肥-钾肥滚动摩擦因数对休止角的角度有显著影响;通过最陡爬坡试验进一步确定这两个参数的取值范围,其余6个参数由于显著性较小全部取中值。在此基础上,根据Box-Behnken试验结果建立了休止角与显著性参数的二阶回归模型,并对模型进行了优化,最终确定参数的取值为:钾肥泊松比为0.25,钾肥剪切模量为7.3×10^(7),钾肥-钾肥恢复系数为0.35,钾肥-钢板恢复系数为0.25,钾肥-钾肥静摩擦因数为0.4,钾肥-钢板静摩擦因数为0.3、钾肥-钾肥滚动摩擦因数为0.2、钾肥-钢板滚动摩擦因数为0.245。采用上述参数组合,进行休止角仿真试验,对仿真休止角与物理试验休止角结果作对比验证,通过T检验发现二者没有显著差异(t<t_(0.05/2))。由此表明,采用响应面优化标定颗粒钾肥离散元模型关键参数可行,标定的参数可为颗粒钾肥离散元仿真提供参考。

新拌混凝土离散元参数的测量与标定

随着计算机技术的发展,可以采用仿真的方法来研究新拌混凝土的流变性能.离散元方法适合于新拌混凝土的大变形流动.颗粒的物性参数和接触参数的设定是模拟结果真实可靠的关键.在本研究中,将混凝土分为机制石和砂浆两相.首先测量了物性参数,包含密度、恢复系数、静摩擦系数和滚动摩擦系数.使用Hertz-Mindlin(no slip)接触模型表示粗骨料-边界和粗骨料-粗骨料之间的相互作用,并通过休止角的实验进行了验证;采用Hertz-Mindlin with JKR接触模型来描述粗骨料-砂浆、砂浆-砂浆、砂浆-边界之间的相互作用,用坍落度实验对JKR参数进行了标定,并采用响应曲面法确定了最佳的参数组合值.最后通过L型箱试验对新拌混凝土的离散元仿真方法进行了验证.

酒用高粱种子离散元仿真参数标定

为获得酒用高粱种子离散元仿真参数,以仁怀市大坝镇地区的茅梁糯2号高粱种子为研究对象,完成了物性参数的测量种子-尼龙、种子-ABS及种子颗粒间接触参数的标定。首先,采用物理试验测出高粱种子的三轴尺寸、密度、泊松比、弹性模量、剪切模量及休止角等参数,且获取高粱种子-尼龙与高粱种子-ABS的静摩擦因数、滚动摩擦因数和碰撞恢复系数;其次,采用Box-Behnken法设计三因素三水平试验,得到高粱种子颗粒间接触参数与休止角的二阶回归模型;最后,以实测休止角29.14°为优化目标值,求得高粱种子颗粒间的碰撞恢复系数、静摩擦因数、滚动摩擦因数分别为0.422、0.504、0.074,代入EDEM中得到仿真试验结果为28.39°,与实测值的相对误差为2.57%,进一步验证了仿真试验的可靠性和有效性。酒用高粱种子物性参数测定和接触参数标定可为高粱播种机仿真分析及优化设计提供一定参考和依据。

有机无机混合肥料离散元参数标定方法研究

[目的]在有机无机混合肥料仿真试验中,传统参数标定方法待标定参数多,标定过程复杂。本文根据有机肥对无机肥颗粒黏附特性,提出一种新的离散元仿真接触参数标定方法。[方法]利用自由落体碰撞试验、静摩擦试验、滚动摩擦试验对复合小球和钢的接触参数进行校准;设计Plackett-Burmen Design试验、爬坡试验和Box-Behnken响应面试验,得到混合肥料接触模型最优参数,通过堆积角对比试验、有机肥和无机肥混合比例预测试验、撒肥盘颗粒分布对比试验验证最优参数。[结果]复合小球与钢的最优碰撞恢复系数、静摩擦系数和滚动摩擦系数分别为0.323、0.776、0.255;有机肥肥料之间最优碰撞恢复系数、静摩擦系数和滚动摩擦系数分别为0.40、0.70、0.14;有机肥与钢板之间的最优碰撞恢复系数、静摩擦系数和滚动摩擦系数分别为0.44、0.70、0.15;Hertz-Mindlin with JKR模型参数为0.017。最优参数进行对比验证堆积角的相对误差为1.05%,最优参数能预测不同混合比例肥料的堆积角,撒肥盘抛撒试验列分布最大相对误差为35.95%,平均误差15.86%。[结论]试验结果验证了标定方法正确性,同时为后续有机肥-无机肥-机械互作系统研究提供依据。

基于MatLab的颗粒饲料休止角试验研究

颗粒饲料休止角对饲料储料箱设计、自动化饲喂站、投料装置设计等工程实践有重要指导意义。为实现休止角的快速准确测量,探究颗粒饲料休止角与粒径、表面能之间的关系,利用物理试验与离散元仿真相结合的方法对颗粒饲料堆进行基于MatLab图像处理的休止角测定。由于传统休止角的测量存在系统误差和测量精度低的问题,本文提出一种基于MatLab图像处理方法的休止角测定,利用傅里叶级数对堆积图像进行轮廓拟合,利用拟合曲线的单边斜率求出休止角大小,操作便捷,可提高测量精度。基于这种方法,对颗粒饲料堆进行分析,探究了颗粒粒径对休止角的影响并进行仿真试验,利用EDEM以物理试验得到的休止角为目标值进行仿真试验,以休止角作为参照,标定接触模型JKR表面能。试验结果表明:6.5mm颗粒饲料实际休止角为34.992°,仿真休止角为34.9535°;5mm颗粒饲料实际休止角为36.9797°,仿真休止角为35.2603°;3.2mm颗粒饲料实际休止角为37.9186°,仿真休止角为36.092°。休止角仿真结果与实际试验结果较为接近,两者的绝对误差分别为:6.5mm对应0.11%,5mm对应4.65%,3.2mm对应4.82%,表明颗粒饲料的休止角随着粒径的增大而减小,根据颗粒饲料参数建立的离散元模型,3种粒径的颗粒饲料其对应表面能分别为:3.2mm对应0.15J/m^(2),5mm对应0.22J/m^(2),6.5mm对应0.22J/m^(2)。

福建烟区不同含水率红壤离散元参数标定

旨在建立不同含水率下红壤的离散元参数模型,探讨适用于不同含水率红壤的标定方法。首先,利用圆筒提升法对不同含水率的红壤进行物理试验,测定其堆积角,建立含水率与堆积角的回归方程;其次,采用EDEM离散元软件,模拟不同含水率红壤的堆积角;再次,以堆积角为响应值,运用Plackett-Burman试验从10个因素中筛选出3个显著因素,并通过最陡爬坡试验确定其最优区间;最后,使用Box-Behnken试验建立堆积角与显著因素的二次回归模型,从而得到含水率与离散元参数的关系模型。为了验证模型的有效性,采用朗氏法试验进行对比分析。结果表明:建立了10%~35%范围内红壤含水率—堆积角模型,模型相关系数为0.992。显著因素及其最优区间:红壤剪切模量为4.6~8.2 MPa、红壤—红壤静摩擦系数为0.52~0.84、动摩擦系数为0.13~0.29。红壤堆积角—离散元参数的关系模型p值为0.002,仿真试验与物理试验的堆积角相对误差不超过2.13%。含水率—离散元参数的关系模型经试验验证,仿真试验与物理试验结果的相对误差不超过4.25%,说明模型与实际情况相符,模型标定的离散元参数可靠有效,可为福建烟区红壤地质条件下的烟草机械研发和仿真优化提供理论支持。

田间机收元胡离散元模型参数标定与试验

为提高机收元胡所用离散元仿真模型参数准确性,通过物理试验与仿真试验结合的方法对元胡离散元参数进行标定。首先,采用物理试验确定元胡基本物性参数范围;其次,依据物理试验测定的参数范围进行休止角的显著性试验,得到对元胡休止角影响由大到小的顺序为元胡间滚动摩擦系数、元胡与钢板间滚动摩擦系数、元胡间碰撞恢复系数;再通过最陡爬坡试验缩小显著性参数取值范围,并通过响应面试验建立休止角与显著因素的最优二阶回归方程;以物理试验休止角为目标获取最佳参数组合:元胡间滚动摩擦系数0.08、元胡与钢板间滚动摩擦系数0.32、元胡间碰撞恢复系数0.49。最后,在对比标定模型参数下仿真堆积体与实际堆积体,测得二者的休止角和堆体外形轮廓无明显差异,表明该最佳参数组合可作为元胡机收过程中离散元仿真模型参数。

土壤颗粒滚动摩擦系数测量及工程验证

随着国家对工程机械节能增效的要求越来越高,减少工程机械作业阻力是节能增效的核心技术之一。土壤作为工程机械作业的典型工况,很有必要对其物理参数进行深入研究,而土壤的滚动摩擦系数对其宏观性能有着显著的影响。现有滚动摩擦系数的测量方法并不适用土壤颗粒,论文设置了一套土壤滚动摩擦系数测量装置,可以很好的避免土壤颗粒在测试时发生破碎,并可获得足量的实测数据,通过SPC方法处理测试数据呈正态分布并满足稳定性要求。经过测量计算得到土壤颗粒-土壤颗粒之间的滚动摩擦系数为:0.045,土壤颗粒-钢之间的滚动摩擦系数为:0.033。把测得的系数设置到离散元模型(JKR模型)中,得到虚拟土壤安息角,并与实测的土壤安息角进行对比,偏差在1.7%以内,表明测量装置设计合理,测量结果准确,满足工程要求,为后续工程机械降低作业阻力研究打下坚实的基础。

基于堆积试验的沙蓬种子离散元仿真参数标定

为了获得沙蓬种子仿真播种过程所需的参数,通过物理堆积试验与仿真堆积试验相结合的方法对沙蓬种子的离散元仿真参数进行标定。首先,对沙蓬种子相关物性参数进行了测定,建立其离散元模型,并利用物理堆积试验获得沙蓬种子的堆积角为45.995°。其次,采用Plackett-Burman试验进行仿真堆积试验,筛选出对堆积角影响显著的参数为:沙蓬种子之间恢复系数、沙蓬种子之间滚动摩擦系数、沙蓬种子与ABS板间静摩擦系数。然后,根据最陡爬坡试验确定显著参数的取值范围;设计二次正交旋转组合试验,建立堆积角相对误差与显著参数的二阶回归方程,得到离散元仿真最佳参数组合:沙蓬种子之间恢复系数为0.529、沙蓬种子之间滚动摩擦系数为0.057、沙蓬种子与ABS板间静摩擦系数为0.629。最后,将最佳参数组合的值进行仿真堆积试验验证,得到仿真试验堆积角为46.740°,与物理堆积试验相对误差为1.62%,表明最佳参数组合选取合理,可为沙蓬种子播种机械离散元仿真参数设置及优化设计提供参考依据。

三七种植土壤离散元仿真模型参数标定

[目的]获取三七Panax notoginseng种植土壤与触土部件相互作用的离散元仿真模型参数。[方法]基于Hertz-Mindlin with JKR接触模型建立三七种植土壤离散元模型并进行参数标定。首先,以土壤颗粒间及土壤-65Mn钢板间的JKR表面能、恢复系数、静摩擦系数、动摩擦系数为试验因素,以土壤堆积角、土壤在65Mn板上的滚动距离为评价指标。其次,采用基于Box-Behnken的响应面优化方法建立土壤堆积角、滚动距离回归模型。[结果]对回归模型进行寻优,得到仿真标定的土壤颗粒间JKR表面能、恢复系数、静摩擦系数和动摩擦系数的最优值分别为14.88 J/m2、0.53、0.46和0.150,标定的土壤-65Mn板间JKR表面能、恢复系数、静摩擦系数和动摩擦系数的最优值分别为7.02 J/m2、0.59、0.57和0.058。通过三七挖掘铲仿真试验与土槽试验对比分析得到,挖掘铲受X、Y轴方向平均阻力仿真值与实测值相对误差分别为9.91%、8.78%。[结论]标定的离散元土壤模型参数准确度高,研究可为三七收获机触土部件及装备优化提供理论参考。

STUDY ON THE ANGLE OF REPOSE OF NONUNIFORM SEDIMENT

A conception of exposure degree is used in this article to account for the hiding and exposure mechanism of nonuniform sediment transport. The force arms of particles on the bed slope are assumed to be a function of the exposure degree, which is stochastically related to the size and gradation of bed materials. Based on this conception, the formula for the angle of repose of nonuniform sediment is developed. The angle of repose of nonuniform sediment is experimentally investigated in rotation drum at the rolling motion of sediment particles. The nonuniform sediment is gotten from the mixture of two kinds of uniform sediment with different weight ratios and two experimental plans that one for changing the weight ratio of two kinds of uniform sediment from 1：9 to 9：1 and the other changing the uniform sediment size and weight ratio are used. The effect of the rotation speed and the filling degree on the angle of repose is investigated, too. Finally, the formula has been tested against a wide range of laboratory data, the prediction by the newly proposed formulas is very good.

DEM parameter calibration of cohesive bulk materials using a simple angle of repose test

The numerical effort of discrete element method (DEM) simulations results in a general idealisation of DEM models that makes the calibration crucial to obtaining realistic simulation results. The angle of repose test has become a standard test for the calibration of DEM parameters of cohesionless bulk materials and is extensively discussed in the literature. One of the most used test methods is the pull-up test of a hollow cylinder filled with bulk material. This paper presents how this basic pull-up test can also be used for the calibration of DEM parameters of cohesive materials by changing the analysis criteria. In contrast to analysing the static angle of repose, the focus lies on the macroscopic flow behaviour during the lifting of the cylinder. Reproducible phases of flow are identified in experiments: the build-up of a stable bulk material column, the convex bending of the column, and the beginning of collapse of the column. Furthermore, the phases are independent of the size and lifting velocity. The convex bending of the bulk-material column combined with the moment of first collapse are introduced as new measurable calibration criteria. These experimental results of wet sand are used for the calibration of related DEM parameters using a simplified JKR cohesion model. The DEM calibration including an analysis algorithm that fits the experimental flow behaviour and the final selection of the relevant parameters (i.e., sliding friction coefficient, rolling friction coefficient, and cohesion energy density) is presented in detail.

Research on the Relationship between Grain Composition and Repose Angle of Coal Gangue in Dongkuang Mine, Heshan City, Guangxi, China

Heshan, a city in Southwest China, has been built over an old coal mining area. Due to low level of reclamation, coal gangue had been stacked in large heaps and caused geological environ- mental problems. To facilitate designing management project of coal gangue, we made an investigation of the largest coal gangue heap in Dongkuang Mine of Heshan. Firstly, the grain composition of coal gangue was estimated through the sieving method, then the representative grading curve was drawn through the statistics method of coarse grain content, and lastly the relationship between the repose an- gle and grain grading was studied by piling up coal gangue with different mix proportion. The result indicates that there is a wider range of grain size in the upper part of coal gangue heap, and the repre- sentative grain grading is well graded and easy to be compacted as filling materials. Besides, the wavy relationship curve of repose angle and coarse grain content shows that the content of 70% is the infiec- tion point of ascent stage and 85% the inflection point of descent stage of the repose angle. What＇s more, the repose angle corresponding to the representative grading of coal gangue is approximately 38.4° and this has guiding significance for management project of coal gangue.

Experimental study on the angle of repose of pulverized coal

An experimental study on the angle of repose （AoR） of pulverized coal with different particle sizes and different moisture contents （MC） was conducted. Three different measurement methods, free-base piling, fixed-base piling and sliding, were used. The data were analyzed by one-way and two-way analysis of variance. The results showed that the AoRs of pulverized coal with particle sizes smaller than 150 μm were in the range of 30-50°. The characterization of the flowability of pulverized coal was some cohesiveness or true cohesiveness. The increase of MC will increase AoR and thus decrease the flowability of the powder. However, the particle size effect is bifurcated. Below a critical size, the decrease of particle size decreases the flowability; while above the critical size, the decrease of particle size increases the flowability. It was found that the value of the critical size strongly depends on the powder density. Moreover, the AoR dependence on particle size could be linked with the Geldart＇s particle classification. The critical size at the turning point is on the boundary between Group A and Group B in Geldart＇s classification diagram. Based on the experimental results, there is no significant cross interaction between particle size and MC. The AoRs measured by free-base method and fixed-base method are close, but both remarkably smaller than that measured by the sliding method.

DEM investigation of angle of repose for super-ellipsoidal particles

We explore the effect of particle shape on the angle of repose (AoR) in granular packing using a three-dimensional discrete element method.Non-spherical particles were modeled using super-ellipsoids characterized by aspect ratio and blockiness.The relationship between AoR and particle shape was examined,followed by a series of analyses on the origin of AoR from a microscopic perspective.Results show that,with blockiness deviating from unity,AoR has an approximate "M" shape that exhibits a strong to weak trend with aspect ratio,matching results of previous reports.Another finding suggests a parabolic relationship between normalized AoR and normalized coordination number,AoR increases with anisotropy related more with the anisotropy of particle orientation than with the anisotropy of contact normal.Particle shape is more likely to affect AoR by affecting the tangential force than the normal contact force.An analysis based on traction indicates that dips in stress associated with the tangential force (compared with the normal force) and non-spherical particles (compared with spheres) are more significant.For various particle shapes,AoR is positively correlated with friction mobilization but is negatively correlated with friction mobilization for various coefficient of friction.

Effect of granular size on the angle of repose in the chute

Avalanche in the chute has been studied to examine how the sizes of granules affect the angle of repose θr when the width of the chute is fixed. The results show that the angle of repose in the chute has exponential decay with λ,which is the reciprocal of granular diameter d. The fluctuation of the angle of repose is also studied during the chute being tilted,and the results indicate that the fluctuation of the angle of repose increases with the increase of granular diameter d.