Experimental analysis of sand particles'' lift-off and incident velocities in wind-blown sand flux

The probability distributions of sand particles' lift-off and incident velocities in a wind-blown sand flux play very important roles in the simulation of the wind-blown sand movement. In this paper, the vertical and the horizontal speeds of sand particles located at 1.0 mm above a sand-bed in a wind-blown sand flux are observed with the aid of Phase Doppler Anemometry (PDA) in a wind tunnel. Based on the experimental data, the probability distributions of not only the vertical lift-off speed but also the lift-off velocity as well as its horizontal component and the incident velocity as well as its vertical and horizontal components can be obtained by the equal distance histogram method. It is found, according to the results of the χ²-test for these probability distributions, that the probability density functions (pdf's) of the sand particles' lift-off and incident velocities as well as their vertical components are described by the Gamma density function with different peak values and shapes and the downwind incident and lift-off horizontal speeds, respectively, can be described by the lognormal and the Gamma density functions. These pdf's depend on not only the sand particle diameter but also the wind speed. The project supported by the National Natural Science Foundation of China (10532040) and the Hundred Talents Project, the Knowledge Innovation Project of Chinese Academy of Sciences (KZCX2-304). The English text was polished by Keren Wang.     

Experimental analysis of sand particles＇ lift-off and incident velocities in wind-blown sand flux

The probability distributions of sand particles' lift-off and incident velocities in a wind-blown sand flux play very important roles in the simulation of the wind-blown sand movement. In this paper, the vertical and the horizontal speeds of sand particles located at 1.0 mm above a sand-bed in a wind-blown sand flux are observed with the aid of Phase Doppler Anemometry (PDA) in a wind tunnel. Based on the experimental data, the probability distributions of not only the vertical lift-off speed but also the lift-off velocity as well as its horizontal component and the incident velocity as well as its vertical and horizontal components can be obtained by the equal distance histogram method. It is found, according to the results of the χ ²-test for these probability distributions, that the probability density functions (pdf's) of the sand particles' lift-off and incident velocities as well as their vertical components are described by the Gamma density function with different peak values and shapes and the downwind incident and lift-off horizontal speeds, respectively, can be described by the lognormal and the Gamma density functions. These pdf's depend on not only the sand particle diameter but also the wind speed.     

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.     

基于碰撞模型的斜坡滚石颗粒速度预测

滑坡滚石灾害是西部山区常见的地质灾害类型,具有突发性和随机性强的特点,是山区地质灾害预测和防治工作的重点和难点. 本文基于颗粒接触理论,考虑影响斜坡滚石碰撞过程中的随机因素,建立了用于预测斜坡滚石颗粒碰撞后速度的理论模型. 根据冲量及冲量矩定理建立滚石颗粒碰撞基本方程,得到斜坡滚石颗粒碰撞后反弹速度的解析解. 结果表明:斜坡滚石碰撞后反弹速度的解析解包含了坡角、坡体上被碰颗粒速度以及角度、入射速度和角度以及撞击角度等随机因素;当考虑入射滚石颗粒与坡体上被碰颗粒的撞击角度变化时,模型预测结果与试验结果吻合较好;本文进一步预测了滚石颗粒碰撞后颗粒反弹线速度和角度以及反弹旋转角速度的概率分布情况. 结果显示,反弹颗粒速度和角度以及反弹旋转角速度的概率分布均服从高斯分布;当坡体上被碰颗粒速度和坡角发生变化时,其对反弹颗粒速度和角度以及反弹旋转角速度概率分布定性上没有影响,但是对概率分布的中心参数有显著影响.      

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.     

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.     

沙粒跃移运动的数值模拟

沙粒在风场作用下形成的风沙运动,直接导致一系列环境问题.现有的数值模拟并没有揭示风沙流的二维性质.本文提出一种合理有效的模拟方法来揭示风沙流时空的变化.气相运动采用大涡模型,而固相(沙粒)处理为高散体系(DEM),并利用已有的沙粒与床面碰撞的激溅函数,以获得整个风沙系统运动特性.结果表明:其发展过程与以往的试验结果相比较符合:风沙流达到稳定后,不同截面输沙率沿高度分布与平均输沙率相比差别很大,并大约在9cm高处,平均输沙率出现极值,单宽输沙率沿风向分布同样具有非均匀性.     

Sand particle dislodgement in windblown sand

The incipient motion of sand particle from sand bed plays a very important role in the prediction of windblown sand.In this paper,we proposed a new method for predicting the incipient motion of sand particle based on wind speed fluctuation as follows,when the wind speed is larger than the critical wind speed,if the total impulse on sand particle is larger than the critical impulse,incipient motion of sand particle would take place,otherwise if not.Furthermore,from the analysis of entrainment in the rolling and lifting modes,we come to the following conclusion.When the average wind speed is smaller than the critical wind speed,if the average wind speed is used to judge the incipient motion of sand particle,one will underestimate the number of sand particles jumping from the bed,if the instantaneous wind speed is used to judge incipient motion of sand particle,one will overestimate the number of sand particles jumping from the bed;When the average wind speed is larger than the critical wind speed,either the average or the instantaneous wind speeds is used to judge the incipientmotion of sand particles,one will overestimate the number of sand particles jumping from the bed.     

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.      

菱形颗粒冲击材料表面冲蚀磨损特性分析

基于弹射试验装置,借助高速摄像机捕捉不同入射条件下单个菱形颗粒冲击金属表面的动态过程,同时结合试验过程建立菱形颗粒冲击金属表面的FEM-SPH耦合数值模型,通过对比试验现象与仿真结果优化数值模型参数,最后借助数值模型进一步分析菱形颗粒在临界冲击、自身初始旋转以及重复冲击等工况下的运动行为及预测的凹坑轮廓形态. 结果表明:优化后的模型能够很好地捕捉颗粒冲击过程中金属表面凹坑的产生及演化规律,并能详细记录颗粒的入射行为及反弹规律,测得颗粒反弹速度和反弹角度误差均在14%以内. 临界冲击工况下颗粒动能损失最大,且冲击角越高,残余动能越少;颗粒初始旋转能够改变其反弹后的运动行为及金属表面材料的失效方式;颗粒重复冲蚀对材料表面的作用机制与后续颗粒的入射条件有密切关系,模型成功捕捉到重复冲蚀导致的材料破坏加深和破坏减缓两种特殊现象.      

Numerical study of particle motion near a charged collector

The behavior of particles impacting the surface of a charged droplet involves adhesion, rebound, and submersion. In the present study, a numerical model for simulating particle impacts on charged droplets is presented that takes into account the various impact modes. With the droplet considered as a solid boundary, the criterion for rebounding is that the particle’s impact angle is <85°. The simulated trajectories of the particles are verified by comparing with experimental data for low-velocity particles to assess the reliability of the model. For impact angles >85°, particles undergo three distinct modes depending on normal impact velocities. The critical velocity of adhesion/rebound and rebound/submersion is used to identify the mode that the particles are undergoing. The criteria are also verified by comparing with analytical data. The results show that the impact angle of particles increases with increasing Coulomb number and decreases dramatically with increasing Stokes number, both of which lead to a high probability for particle rebound.     

Review on charging model of sand particles due to collisions

In this paper, the models describing the charge transfer between two sand particles due to collisions are reviewed. By comparing the experimental results and the calculated results by the models carried on an individual particle due to a single collision, it indicates the Mosaic model is more reasonable to describe the collision charging mechanism. The Mosaic model cannot only describe the dependence of the collision charges on the relative collision speed and the particle size, but also reveal the relationship between the collision charges with the environmental temperature, the relative humidity and the material parameters, e.g., the absorption energy. Based on the Mosaic model, the model to describe the charges transfer due to multiple collisions is also developed, which can be used to calculate the charges carried by sand particles due to multiple collisions in the wind blown sand flux.     

The velocity of sand particles relative to water and the variation of fluctuation velocity due to the concentration of suspended sand particles

In this paper, we discuss three fundamental problems of the theory of suspended sand particles based on the theory of fluid dynamics. They are: (1) the relative velocity of sand particles to the surrounding fluid in the turbulent flow, (2) the variation of the velocity fluctuation of the fluid when the suspended sand particles exist, (3) the profile of the vertical distribution of the concentration of the suspended sand particles in two-dimensional uniform steady channel flow and the modification of usual diffusion theory. The results of our theoretical analysis are somewhat different from the corresponding expressions in the textbooks of hydraulics and river mechanics.     

A laser-fluorescence technique for turbulent two-phase flow measurements

A non-intrusive measurement technique has been developed for accurate determination of gas and particle velocities in a turbulent two-phase flow field. The principle of the technique is based on the discrimination between the scattered light from particles and the fluorescence emission from particles coated with a fluorescent dye. A high-powered, argon-ion based, single-channel, on-axis backscatter laser-Doppler velocimetry system was used. The fluorescent dye was Rhodamine 6G. A study of the gas-solid two-phase flow behaviour in the freeboard of a cold gas-fluidized bed was undertaken. The solid phase contained two particle groups: bed material (sand) and fuel particles (wood). Measurements of the axial velocity and turbulence intensity distributions of the gas phase and both particle groups within the solid phase were made along the column centre and across the freeboard. Excellent discrimination of velocities from the two phases and from the two particle groups within the solid phase was achieved.     

Simulation and analysis of the impact of micron-scale particles onto a rough surface

Normal impact of micron-scale copper particles onto a rough copper surface is investigated in the 25–150 m/s impact velocity range, by the finite element method. Surface roughness is generated numerically and incorporated into the finite element model. Particle size is varied in a range comparable to the magnitude of the standard deviation of the surface roughness. Isotropic hardening with strain rate effects and thermal softening due to plastic heat dissipation are included in the model. Analysis is carried out in plane strain mode and impact of single and multiple particles are modeled. The effects of surface roughness on the mechanics of impact, energy exchange, rebound characteristics of the particle and the residual stresses in the substrate are investigated. Impacts on peaks and valleys cause response similar to oblique impact, affecting the rebound behavior of the particle by changing the rebound direction and increasing the rebound energy of the particle. Impacts also cause surface smoothening by crushing the surface peaks; however, collapse of adjacent peaks can provoke stress concentrations and initiate crack formation. Influence of surface roughness on the aftermath of particle impacts decreases with increasing particle size and impact velocity. For impact velocities higher than 50 m/s, no significant difference is observed between impact on smooth and rough surfaces in terms of residual stress generation in the substrate. In general, it is concluded that the effect of surface roughness should be taken into account for low velocity impacts where only the surface peaks deform, or for small particles with size comparable to the standard deviation of the surface roughness.     

基于水平集-离散元方法的球谐函数颗粒材料缓冲性能分析

在自然环境与工业领域中,颗粒材料是一种常见的缓冲材料,其中大量形态各异的非球形颗粒表现出复杂的力学特性并应用于不同工程领域。本文采用球谐函数构造不同球面度和表面凹凸特性的非规则颗粒,通过水平集方法计算球谐函数颗粒间的接触点和碰撞力,并对冲击过程中球形和凹形颗粒的缓冲性能进行离散元分析。数值结果表明,颗粒床厚度、冲击速度和颗粒形状显著影响球谐函数颗粒材料的缓冲性能。颗粒床底部的冲击力峰值随着颗粒床厚度和表面凹凸性的增加而降低,同时冲击力峰值随着冲击速度和颗粒球面度的增加而增加。与球形颗粒相比,球谐函数颗粒具有凹凸表面和多接触点特性,这有利于冲击荷载向四周扩展并提高凹形颗粒的缓冲效果。     

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.     

喷砂冲蚀实验中颗粒轨迹的数值预测

针对冲击磨损实验研究中磨粒群体的运动轨迹难以准确表征的问题,在负压喷射砂粒群冲击Q235钢板的实验中宏观测量了砂粒撞击的速度与位置分布,并使用数值方法模拟了实验砂粒与空气在喷嘴内外的双向耦合过程,以实现负压喷射砂粒群的轨迹预测。计算中提出了非球形粒子在相对马赫数接近1时的曳力模型,以反映空气可压缩引起砂粒表面流动分离的现象,并合理选择Magnus升力模型及壁面反射模型,最终数值预测的砂粒碰撞速度以及撞击位置与实验情况吻合良好。     

The theory of similarity and the profile of vertical distribution of concentration of sand particles

In this article, we treat the problem of two-dimensional uniform steady channel flow of turbid water with theory of similarity. Under the condition of similarity of turbulent fluctuation velocity and fluctuation of concentration of sand particles, we obtain the profile of the vertical distribution of concentration of sand particles. This profile of vertical distribution of concentration of sand particles is slightly different from that obtained by diffusion theory, and departs from that obtained by gravitational theory.