【作者】 张欢；

【导师】 郑晓静；

【作者基本信息】 兰州大学 ， 力学·工程力学， 2016， 博士

【摘要】 沙尘带电现象普遍存在于风沙运动之中,其对沙尘颗粒从地表的起动和沙尘颗粒的输运等有显著的影响。所以,对风沙电现象的研究有助于人们更加深刻地认识风沙运动的物理规律和准确地预报沙尘天气。但是,到目前为止人们对于沙尘颗粒的带电性质以及风沙电场的定量规律等都不是十分的清楚。为了解决风沙电领域中存在的一些关键问题,本论文进行了一系列的工作,包含了跃移沙粒荷质比和三维电场的野外观测、跃移沙粒荷质比的数值模拟以及沙尘暴内外部电场的理论模型。首先,与已有的野外观测相比,本论文实验主要的创新点在于:1)采用了精度更高的荷质比测量系统和风沙电场仪VREFM;2)野外观测的测量高度达到了30 m,并且进行了多个物理量的实时同步观测,包含了三维风速、环境温度、悬移粉尘浓度、三维电场强度、跃移沙粒荷质比、跃移沙粒数浓度和悬移粉尘的粒径分布。其次,与已有的风沙跃移数值模型相比,本论文数值模型主要的改进在于:1)考虑了沙尘颗粒之间碰撞的动力学过程和带电过程,并且包含了冲击速度对颗粒之间电荷转移过程的影响;2)将地表处理为一个由混合粒径沙尘颗粒组成的沙床,而不是一个无限大的平面。最后,本论文建立了一个简化的预测沙尘暴内外部电场的理论模型,其中悬移粉尘的荷质比是基于实验测量的电场强度和粉尘浓度通过反演得到。通过野外观测和更加完善的数值模拟,本论文主要结果如下:1)揭示了单颗沙粒荷质比服从正态概率分布,以及其概率密度函数与高度、风速和沙样的粒径分布之间的关系;2)揭示了沙尘暴电场的高度廓线,即随着高度是非单调的并且会发生方向的翻转,而且基于电场和粉尘的观测数据本论文反演得到悬移粉尘的电荷结构为负-正-负;3)建立了沙尘暴内外部电场的理论模型,并且通过理论和实验证实了基于电场监测的沙尘暴预警方法是可行的。最后,本论文提出了近地表电场观测列阵的沙尘暴预警方案,其必须建立在当地沙尘暴观测数据的基础之上。更多还原

【Abstract】 Dust electrification as a common phenomenon in the dusty phenomena like wind-blown sand,dust devils and dust storms,has a significant effect on the lifting and transport of dust particles.Therefore,the investigation of electrical effects involved in the dusty phenomena is beneficial for the better understanding of its physical process and prediction.However,so far the properties of dust charging and electric field remain poorly understood.To solve some key questions related to the electrical effects,we conducted a series of electrical effects field observations,a comprehensive numerical simulation of wind-blown sand,and a theoretical model of electric field in the dust storms.Firstly,as compared with the previous observations the main innovations of our field observations were: 1)higher precision instruments,i.e.,charge-to-mass ratios measuring system and vibrating-reed electric field mill(VREFM)were adopted;2)wind velocity,ambient temperature,vertical E-field,mass concentration of PM10(dust particles with diameter of <10 mm),number density of saltating particles,and the particle size distribution of airborne dust particles were simultaneously measured in the height of up to 30 m above the ground.Secondly,there were some advantages of our numerical models: 1)the charge transfer process between dust particles during collisions was included,and the effect of the impact speed on the charging of dust particles was explicitly accounted for;2)the ground was regarded as a sand bed contained with sand/dust particles of different size obeying a lognormal distribution.Finally,this thesis proposed a theoretical model of electric field in the dust storms,in which the charge-to-mass ratios of suspended dust particles were determined by calibrating the theoretical model with the measured electric fields.The most important finding of this thesis were: 1)the charge-to-mass ratio of saltating particles could be described by a normal distribution,and the effects of height above the ground,wind speed and particle size distribution of sand sample on the probability distribution function were also investigated;2)the vertical electric field increased first,then decreased,and finally decreased with the height above the ground,reversing its direction at several heights.This suggested that the charge polarity of dust particles changed from negative to positive and back to negative again as the height increased;3)a simple theoretical model of electric fields in the dust storms was proposed,and field measurements and theoretical calculations demonstrated that substantial enhancement in electric field could be observed several hours before the arrival of the dust storms,thus indicating that electric field could be used for the dust storms early warning.Additionally,an early warning scheme based on monitoring the near-ground electric field was proposed.更多还原