【作者】 段绍臻；

【导师】 郑晓静；

【作者基本信息】 兰州大学 ， 工程力学， 2013， 博士

【摘要】 发生存沙床表面上的风沙运动包括：沙粒的启动、粒-床碰撞的击溅过程等,这些均涉及到沙粒与风场等复杂环境条件下的相互作用,其中沙粒的临界启动风速、沙粒起跳速度分布、冲击速度分布关系到风沙流以及更大尺度风沙运动的发生、发展,并且也是风沙运动数值模拟以及沙尘天气数值预报中的关键输入参数。本论文针对沙粒的临界启动风速、风沙流中跃移沙粒的运动特性以及粒-床碰撞后的击溅过程展开研究,主要工作如下：1.针对以往风洞PIV实验的不足,通过改变实验中光路排布,提出一种测量近地表沙粒的新实验方法,并考虑沙床的起伏,实时更新床面信息,有效降低了风洞实验的测量高度,定量分析了测量高度对实验测量结果的影响。2.基于上述改进的风洞PIV实验,利用PTV算法分析了沙粒起跃速度、冲击速度、起跳角度和冲击角度随粒径和摩阻风速的变化,并给出了其概率分布函数；首次给出了风沙流中近地表沙粒之间距离的概率分布形式,最终给出了沙粒之间距离与摩阻风速的函数关系。3.利用离散单元法(DEM)深入研究了风沙流中沙粒同时冲击床面的粒-床碰撞过程,结果显示空中跃移沙粒的间距是影响跃移沙粒的击溅过程的一个重要因素,而且存风沙流中存在一个临界间距,当跃移沙粒间距小于这一临界间距时需要考虑沙粒同时冲击,反之则不需要考虑。并给出了临界冲击距离与冲击速度之间的关系以及同时冲击床面的击溅函数表征。4.建立了临界启动风速的随机模型,推导出含有随机变量(偏心距、偏心角、支持角以及接触角)的临界启动风速的解析表达式。临界启动风速的概率分布取决于偏心距、偏心角、支持角以及接触角概率密度,利用多维随机变量分析方法给出了临界启动风速的概率密度函数以及均值和方差与粒径的关系,其与实验结果吻合较好,充分说明了沙粒不规则形状以及起伏地表对沙粒包裹的随机性是不可忽视的。5.将临界启动风速预测引入到风沙流模拟的启动过程中,将跃移沙粒间距影响引入到风沙流模拟的击溅过程,实现了对风沙流演化过程的预测。给山了两种击溅函数下的单宽输沙率随摩阻风速的变化曲线,指出当摩阻风速小于0.4m／s时,两种击溅函数得到的单宽输沙率相对误差小于5%,间距离对输沙率的影响可以忽略不计；当摩阻风速大于0.4m/s时,两种击溅函数得到的单宽输沙率相对误差随摩阻风速的增大逐渐增大,最大值为33%。更多还原

【Abstract】 The movements of particles in wind-blown sand, such as the initiation and the saltation of sand grains, are related to the interactions between sand grains and complex wind environment. The threshold friction velocity of sand particle motion, the distributions of impact velocity and lift-off velocity are crucial for the occurrence and development of the wind-blown sand, as well as the larger-scale sand movements. And, they also are the key input for sand movement numerical simulation and dust weather numerical forecast. So, this thesis focuses on the threshold friction velocity, the movement characteristics of saltation of sand in the wind-blown sand and the splash process of grain-bed collision. The main works are concluded as follows:1. In comparison to previous wind tunnel PIV experiments, we presented a new experimental design by changing the way of the laser sheet casting; and updated the information of sand-bed in time during data process responding to the variation of the sand-bed surface. The improved method effectively reduces the measuring height of wind tunnel experiments, and the effect of height on the measurement information was quantitatively analyzed.2. Based on the above the improved experimental method, we analyzed the impact velocity, lift-off velocity, impact angle and lift-off angle of saltating sand particles, and obtained the probability distribution functions of these quantities. We firstly obtained the probability distribution form of the distance between sand particles in wind-blown sand, and the relationship between the distance and friction wind velocity.3. Numerical simulation of the grain-bed collision process of particles impacting the sand-bed at the same time was conducted by the discrete element method (DEM). The results illustrate that the distance between sand particles is a key parameter, there exits a critical distance. When the distance is smaller than the critical distance, it is need to consider the sand particles impact sand-bed at same time. On the contrary there is no need to consider. We obtained the function of the critical distance and impact velocity and the splash function of multi-particles impacting the sand-bed simultaneously.4. We established the stochastic model of threshold friction velocity, and derived an analytical expression of the threshold friction velocity with random variables (the eccentricity, the eccentricity angle, support angle and contact angle). Based on the probability theory of multi-dimension random variable, the probability distribution of the threshold friction velocity, its mean value and standard deviation are calculated. The mean and standard deviation are fitted as functions of sand particle’s diameter. So we can’t ignore the effect of the microstructure of soil surface and the irregularity of particle shape.5. We introduced the threshold wind velocity prediction into stratup process and introduced the effect of distance into the splash process, and then we can predict the evolution of windblown sand flux. We obtained the sand transport rate varying with friction wind velocity under two kinds of splash function. It shows that when the friction wind velocity is smaller than0.4m/s, the relative error of sand transport rate between two kinds of splash functions is smaller than5%; when the friction wind velocity is larger than0.4m/s, the relative error increases and can be up to about33%.更多还原