Analysis of sand particles’ lift-off and incident velocities in wind-blown sand flux

In the research of windblown sand movement,the lift-off and incident velocities of saltating sand particles play a significant role in bridging the spatial and temporal scales from single sand particle’s motion to windblown sand flux.In this paper,we achieved wind tunnel measurements of the movement of sand particles near sand bed through improving the wind tunnel experimental scheme of paticle image velocimetry(PIV) and data processing method.And then the influence of observation height on the probability distributions of lift-off and incident velocities of sand particles was analyzed.The results demonstrate that the observation height has no obvious influence on the distribution pattern of the lift-off and incident velocities of sand particles,i.e.,the probability distribution of horizontal and vertical velocities of lift-off and incident sand particles follow a Gaussian distribution and a negative exponential distribution,respectively.However,it influences the center of the Gaussian distribution,the decay constant and the amplitude of the negative exponential distribution.     

Effects of the particle Stokes number on wind turbine airfoil erosion

Under natural conditions, wind turbines are inevitably eroded by the action of sand-wind flow. To further investigate the effects of dust drift on the erosion of the wind turbine blades in sand-wind environments, the effects of the wind velocity, particle diameter, and particle density on the erosion of wind turbine airfoils are studied, and the effects of the particle Stokes number on the airfoil erosion are discussed. The results show that, when the angle of attack(AOA) is 6.1°, there will be no erosion on the airfoil surface if the particle Stokes number is lower than 0.013 5, whereas erosion will occur if the particle Stokes number is higher than 0.015 1. Therefore, there exists a critical range for the particle Stokes number. When the particle Stokes number is higher than the maximum value in the critical range, airfoil erosion will occur. The result is further confirmed by changing the particle diameter, particle density, and inflow speed. It is shown that the erosion area on the airfoil and the maximum erosion rate are almost equal under the same particle Stokes number and AOA. The extent of airfoil erosion increases when the particle Stokes number increases, and the critical particle Stokes number increases when the AOA increases. Moreover, the geometric shape of the airfoil pressure surface greatly affects the airfoil erosion, especially at the curvature near the leading edge.     

Discrete particle simulation of mixed sand transport

An Eulerian/Lagrangian numerical simulation is performed on mixed sand transport. Volume averaged Navier–Stokes equations are solved to calculate gas motion, and particle motion is calculated using Newton's equation, involving a hard sphere model to describe particle-to-particle and particle-to-wall collisions. The influence of wall characteristics, size distribution of sand particles and boundary layer depth on vertical distribution of sand mass flux and particle mean horizontal velocity is analyzed, suggesting that all these three factors affect sand transport at different levels. In all cases, for small size groups, sand mass flux first increases with height and then decreases while for large size groups, it decreases exponentially with height and for middle size groups the behavior is in-between. The mean horizontal velocity for all size groups well fits experimental data, that is, increasing logarithmically with height in the middle height region. Wall characteristics greatly affects particle to wall collision and makes the flat bed similar to a Gobi surface and the rough bed similar to a sandy surface. Particle size distribution largely affects the sand mass flux and the highest heights they can reach especially for larger particles.     

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 filling speed.     

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.     

机器人单足系统沙土跳跃刚?散耦合动力学分析

在行星探索过程中涉及探测器在星壤上着陆、运动以及收集、存储某些样本材料等问题, 因此需要建立探测机器人在沙土上运动的动力学模型, 从而优化其系统构型. 近年来, 对跳跃型探测机器人研究得到了越多越多的关注. 本文采用离散元法对颗粒场进行建模, 以及采用多体动力学方法对机械系统进行建模, 对机器人单足系统在沙土上的跳跃问题进行耦合动力学仿真分析. 基于经典土力学Prandtl-Reissne理论, 从颗粒场受压分层的形式和动量传递出发, 对描述颗粒侵入阻力的惯性力动阻力项进行了修正, 提出了一种修正的Poncelet公式. 通过与离散元仿真结果对比, 说明所提出修正公式比原始的Poncelet公式更准确地计算了机械足受到的沙土侵入阻力, 尤其在达到一定侵入深度表现出更好的收敛性. 最后分析了机械腿足部的不同尺寸和形状对沙土中跳跃效果的影响, 给出了锥形足部和柱形足部的体积对跳跃效果影响的近似计算公式. 本研究将拓展刚?散耦合动力学理论, 并且为新型探测器在行星土壤上运动的系统设计提供技术支撑.      

椭圆颗粒在剪切流中旋转特性的数值研究

研究颗粒在流体剪切作用下的运动特性是理解和预测颗粒悬浮流流动行为的关键.当流体的惯性不能忽略时,颗粒的运动往往变得非常复杂.本文采用格子Boltzmann方法对中等雷诺数下椭圆颗粒在剪切流中的旋转运动进行了模拟.首先,研究了雷诺数(0Re 170)的影响,结果表明当雷诺数低于临界值时,颗粒以周期性的方式旋转,角速度最小时对应的长轴方向随着雷诺数的增大而逐渐远离水平方向,而且这一倾角与雷诺数呈分段线性关系;当雷诺数大于临界值时,椭圆形颗粒最终保持静止状态,且静止时的转角与雷诺数呈幂函数关系,雷诺数越大,转角越小,椭圆的长轴越远离水平位置.其次,研究了椭圆颗粒的长短轴之比α(1α10)的影响,结果表明颗粒旋转的周期与α呈幂函数关系,α越大,颗粒旋转周期越小.此外,当α超过临界值时,颗粒也在水平位置附近保持静止状态,此时的转角与α也呈幂函数关系,α越大,转角越小.研究还发现,当雷诺数较大时椭圆颗粒在旋转过程中会产生过冲现象.     

Simultaneous PIV and PTV measurements of wind and sand particle velocities

Wind-blown sand is a typical example of two-phase particle-laden flows. Owing to lack of simultaneous measured data of the wind and wind-blown sand, interactions between them have not yet been fully understood. In this study, natural sand of 100–125 μm taken from Taklimakan Desert was tested at the freestream wind speed of 8.3 m/s in an atmospheric boundary layer wind tunnel. The captured flow images containing both saltating sand and small wind tracer particles, were separated by using a digital phase mask technique. The 2-D PIV (particle imaging velocimetry) and PTV (particle tracking velocimetry) techniques were employed to extract simultaneously the wind velocity field and the velocity field of dispersed sand particles, respectively. Comparison of the mean streamwise wind velocity profile and the turbulence statistics with and without sand transportation reveal a significant influence of sand movement on the wind field, especially in the dense saltating sand layer (y/δ < 0.1). The ensemble-averaged streamwise velocity profile of sand particles was also evaluated to investigate the velocity lag between the sand and the wind. This study would be helpful in improving the understanding of interactions between the wind and the wind-blown sand.     

Probability of rebound and eject of sand particles in wind-blown sand movement

When incident particles impact into a sand bed in wind-blown sand movement, rebound of the incident particles and eject of the sand particles by the incident particles affect directly the development of wind sand flux. In order to obtain rebound and eject lift-off probability of the sand particles, we apply the particle-bed stochastic collision model presented in our pervious works to derive analytic solutions of velocities of the incident and impacted particles in the post-collision bed. In order to describe randomness inherent in the real particle-bed collision, we take the incident angle, the impact position and the direction of resultant action of sand particles in sand bed on the impacted sand particle as random variables, and calculate the rebound and eject velocities, angles and coefficients (ratio of rebound and eject velocity to incident velocity). Numerical results are found in accordance with current experimental results. The rebound and eject lift-off probabilities versus the incident and creeping velocities are predicted. The project was supported by the National Natural Science Foundation of China (10532040, 10601022). The English text was polished by Yunming Chen.     

Three dimensional simulation of lunar dust levitation under the effect of simulated sphere body

Four typical models of irregular lunar dust are built based on the particle shape. Parameters of the simulated sample are determined by dimensional analysis and simulated triaxial test. The simulation was performed by a simulated sphere body acting as the lunar explorer experiencing free fall landing on moon surface from a certain height. Tracking the movement of lunar dust, the motion characteristics and levitation distribution were statistically analyzed. The influences of the landing speed of the simulated sphere body, friction coefficient and stiffness of the particles on the levitation of the dust were also discussed. Results show that the landing speed and the friction coefficient have greater influence on the number and height of levitating particles than other factors. The number and height of levitating particles increase with landing speed. While the friction coefficient increases, the number of levitating particles increases but the levitation height decreases. The stiffness of particle also has some influence on levitation height. The larger the stiffness of particles is, the smaller the levitation height. But it has little effect on the number of levitating particles.     

Experimental study of planar shock wave interactions with dense packed sand wall

A dense packed sand wall is impacted by a planar shock wave in a horizontal shock tube to study the shock-sand wall interaction. The incident shock Mach number ranges from 2.18 to 2.38. A novel device for actively rupturing diaphragm is designed for the driver section of the shock tube. An apparatus for loading particles is machined by the electrical discharge cutting technique to create a dense packed particle wall. High-speed schlieren imaging system and synchronized pressure measurement system are used together to capture the wave structures and particle cloud velocity. The dynamic evolution model from dense packed particles to dense gas–solid cloud at the initial driving stage is established. The blockage and permeation effects of the sand wall work together and influence each other. The high pressure gas behind the incident shock wave blocked by the sand wall pushes the upstream front of the wall forward like a piston. Meanwhile, the high speed gas permeating through the sand wall drags the sands of the most downstream layer forward. The incident shock strength, initial sand wall thickness and particle diameter are varied respectively to investigate the shock attenuation and the wall acceleration. Increasing the sands diameter or mixing in small diameter sands can significantly attenuate the incident shock. The smaller particles or the particles in thinner wall can be dispersed into a larger range in the process of transform from dense packed particles to dense gas–solid cloud. Moreover, the stronger incident shock can disperse the particles into a larger region.     

The influence of relative fluid depth on initial bedform dynamics in closed,horizontal pipe flow

Measurements of time-dependent bedforms produced by the deposition of solid plastic particles in two-phase liquid-solid flows were performed using a novel ultrasonic echo method and via video image analysis in a 100-liter, closed-pipe slurry flow loop. Results are presented for the settled bed thicknesses over a range of nominal flow rates and initial bed depths and are combined into several phase diagrams based on various combinations of parameters, with the bedforms categorized into five types. The novel observation is made that the type of bedform that arises depends on both the flow rate and the initial relative bed or fluid depth, with both ripples and dunes being observed in the same system and in a single experiment. In addition, the critical Shields number at incipient particle motion is measured to be θ_sc = 0.094 ± 0.043, hysteretic behavior is observed, and the evolution and scaling of each time-dependent type of bedform is analyzed in detail and compared against several expressions for initial and equilibrium dimensions from the literature. A number of universal scalings for bedforms in any type of conduit are proposed with a view ultimately to unifying the observations of bedforms in pipes with those in channels and natural flows.     

NUMERICAL STUDY ON THE SEDIMENTED MOTION CHARACTERISTICS OF THREE ALIGNED CIRCULAR PARTICLES IN THE INCLINED CHANNELS

主要应用浸没边界的格子玻尔兹曼方法（immersed boundary-lattice Boltzmann method, IB–LBM） 对处于不同倾斜角度通道内的三个刚体圆形颗粒在重力作用下下落的动力学特性进行了计算研究. 首先分析通道倾斜角度的影响, 结果显示当通道倾斜角处于59°90°的范围时会发生后一个颗粒超越前一个颗粒的现象. 其次, 研究了Re对颗粒沉降特性的影响, 结果表明Re 越大, 颗粒间发生聚集的时间越早. 研究还发现当3 个颗粒的直径大小不均匀时, 颗粒由大到小纵向依次排列, 或者出现中间小球直径较相邻两个小球直径大的排列情况, 均能促使颗粒加快聚集. 本文的研究结果可为环境工程及地质学中的颗粒沉降问题提供有价值的参考.     

An image processing method for feature extraction and dynamic tracking of particle clusters in CFBs

A new image processing method based on the high-speed camera is proposed to identify, locate, and track clusters. The instantaneous characteristic parameters of particle clusters in the riser of the circulating fluidized bed (CFB) can be acquired, such as solids holdup, vertical velocity, lateral displacement, aspect ratio and near-circularity. Experiments were carried out with glass bead particles, river sand particles and FCC particles. The time series of images of gas–solid flow in a CFB riser with a 100 mm × 25 mm cross-section and 3.2 m in length were obtained using high-speed cameras. The k-means++ clustering algorithm is utilized to identify the clusters, centroid is applied to locate the clusters, and the cross-correlation algorithm is employed to track the specific clusters and number them to get the instantaneous characteristic parameters. The results illustrate that the shapes of clusters in the center area are closest to circle, moving upwards at a uniform speed, while the clusters in the side-wall area are mostly elongated or long chain-like, moving slowly downwards. In the transition area, the clusters are more complex, moving upwards at a constant speed, and having large lateral displacement. The results show that the image processing method used in this study is successful in acquiring the dynamic and structural parameters of the clusters simultaneously.     

地表温度对颗粒跃移轨迹的影响

为了研究由地表温度变化引起的向上的垂向气流对沙粒跃移运动的影响,本文给出了考虑近地表温度变化和水平来流风场作用下的沙粒的跃移运动。在定量给出不同时刻的近地表温度和垂向风速的基础上,计算了由于太阳辐射所引起的近地表层垂向气流对沙粒跃移运动的影响,发现:垂向风速在午后可达到1.5m/s并使得沙粒跃移轨迹的最大高度和长度分别增加55.56%和73.68%;同时,与不考虑温度效应的情况不同的是,沙粒跃移轨道最大高度将随粒径变化。     

沙丘背风侧不同铁路结构形式对风沙环境的适应性分析

当铁路穿越大风沙漠地区时,风沙灾害会对铁路工程及其正常运营产生严重威胁,而设计一种合理的铁路结构形式能够减小风沙沉积对铁路工程的危害. 在本文中,以敦煌至格尔木铁路沙山沟段落为研究对象,采用多相流的方法对越过沙丘的风沙运动过程进行数值模拟,分别讨论了风沙运动对位于沙丘背风坡的铁路路基工程和桥梁工程的影响. 主要的模拟结果显示:路基工程明显降低了风速并且将沙丘后的回流区分成了两部分,而桥梁工程的导流效应则压缩了沙丘背风坡的回流区;轨道间的道碴增大了铁路表面的粗糙度,在轨道间有少量沙粒沉积,而路基工程两侧则有大量积沙;铁路表面的积沙量与摩阻风速呈现出非线性关系,随着摩阻风速的增大,路基工程沙粒沉积的增加速度大于风蚀能力的增加速度,而桥梁工程则正好相反. 在防止风沙危害铁路方面,设置桥梁工程明显优于路基工程. 本研究为风沙运动对铁路工程的影响提供了理论支持,也为今后的铁路工程设计提供了新的思路与研究工具.      

CFD-DEM simulation of spouting of corn-shaped particles

Three dimensionally coupled computational fluid dynamics (CFD) and discrete element method (DEM) were used to investigate the flow of corn-shaped particles in a cylindrical spouted bed with a conical base. The particle motion was modeled by the DEM, and the gas motion by the k-? two-equation turbulent model. A two-way coupling numerical iterative scheme was used to incorporate the effects of gas–particle interactions in terms of momentum exchange. The corn-shaped particles were constructed by a multi-sphere method. Drag force, contact force, Saffman lift force, Magnus lift force, and gravitational force acting on each individual particle were considered in establishing the mathematical modeling. Calculations were carried out in a cylindrical spouted bed with an inside diameter of 200 mm, a height of 700 mm, and a conical base of 60°. Comparison of simulations with experiments showed the availability of the multi-sphere method in simulating spouting action with corn-shaped particles, but it depended strongly on the number and the arrangement of the spherical elements. Gas–solid flow patterns, pressure drop, particle velocity and particle concentration at various spouting gas velocity were discussed. The results showed that particle velocity reaches a maximum at the axis and then decreases gradually along the radial direction in the whole bed. Particle concentration increases along the radial direction in the spout region but decreases in the fountain region, while it is nearly constant in the annulus region. Increasing spouting gas velocity leads to larger pressure drop, remarkably increased speed of particle moving upward or downward, but decreased particle concentration.     

Simulated configurational temperature of particles and a model of constitutive relations of rapid-intermediate-dense granular flow based on generalized granular temperature

In gas–solid flat-base spout bed with a jet, the flow of particles must go through an intermediate regime where both kinetic/collisional and frictional contributions play a role. In this paper, the statistical framework is proposed to define the generalized granular temperature which sums up the configurational temperature and translational granular temperature. The configurational temperature, translational and rotational granular temperatures of particles are simulated by means of CFD-DEM (discrete element method) in a 3D flat-base spout bed with a jet. The configurational temperatures of particles are calculated from instantaneous overlaps of particles. The translational and rotational granular temperatures of particles are calculated from instantaneous translational and angular velocities of particles. Roughly, the simulated translational and rotational granular temperatures increase, reach maximum, and then decrease with the increase of solids volume fractions. However, the configurational temperature increases with the increase of solids volume fractions. At high solid volume fraction, the predicted configurational temperatures are larger than the translational and rotational granular temperatures, indicating that the rate of energy dissipation do contributes by contact deformation of elastic particles. The generalized granular temperature is proposed to show the relation between the variance of the fluctuation velocity of deformation and the variance of the translational fluctuation velocity of particles. The constitutive relations of particle pressure, viscosity, granular conductivity of fluctuating energy and energy dissipation in rapid-intermediate-dense granular flows are correlated to the generalized granular temperature. The variations of particle pressure, shear viscosity, energy dissipation and granular conductivity are analyzed on the basis of generalized granular temperature in a flat-base spout bed with a jet. The axial velocities of particles predicted by a gas–solid two-fluid model of rapid-intermediate-dense granular flows agree with experimental results in a spout bed.     

Image processing for the study of bedload transport of two-size spherical particles in a supercritical flow

Bedload sediment transport of two-size coarse spherical particle mixtures in a turbulent supercritical flow was analyzed with image and particle tracking velocimetry algorithms in a two-dimensional flume. The image processing procedure is entirely presented. Experimental results, including the size, the position, the trajectory, the state of movement (rest, rolling, and saltation), and the neighborhood configuration of each bead, were compared with a previous one-size experiment. Analysis of the solid discharge along the vertical displayed only one peak of rolling in the two-size bed, whereas three peaks of rolling appeared in the one-size case due to a larger collective motion. The same contrast is evidenced in spatio-temporal diagrams where the two-size mixtures are characterized by the predominance of saltation and a smaller number of transitions between rest and rolling. The segregation of fine particles in a bed formed by larger particles was analyzed taking into account the neighborhood configurations.     

风沙颗粒运动的数字高速摄影图像的分割算法

为了深入理解多因素驱动下风沙颗粒起动的动态演化规律, 需要准确地获得沙质床面附近沙粒群起跳的方式、速度和运动轨迹. 以连续强激光源为照明的数字高速摄影技术是研究这类问题的有效手段, 但由于风沙运动的高速摄影图像具有运动沙粒和静止床面的对比度小、相邻两帧图像相似性小等特点, 原始图像叠加算法难以有效实现目标与背景的分割. 该文提出了基于相邻的风沙运动图像灰度差值变化原理的图像分割算法. 实例显示,只要选择合适的相邻图像灰度差值阈值和自适应二值化处理方法就能实现图像中运动沙粒与床面分割. 当起跳沙粒浓度较低情况下, 基于MATLAB平台的最小距离匹配的粒子追踪算法(PTV算法)能较为准确地恢复床面附近沙粒的运动轨迹.