Experiments and Numerical Simulations for Aeolian Saltation in Natural Environments
【Author in Chinese】 史锋；
【Author's Information】 兰州大学， 固体力学， 2010， 博士
【Abstract in Chinese】 现阶段风沙运动研究的理论分析与数值模拟主要是在平坦地面、给定风向风速等可控条件下进行的,能够考虑野外真实环境下风速脉动和复杂地形对其影响的研究较少。本学位论文开展了有关野外真实环境下风沙运动机理微宏观方面的一些基础性实验与理论建模及数值模拟,主要研究内容如下：1、通过与美国农业部土壤风蚀与水土保持研究所合作,配合该研究所大斯普林野外观测站前站长Fryrear研究员对野外测量仪器进行了安装调试,利用调试的一套可以多点同步实时测量风速、风向、温湿度和输沙强度的测量系统,在巴丹吉林沙漠和腾格里沙漠交汇的甘肃民勤地区对近地层风速、风向、输沙强度、温度和湿度等物理量进行了频率为1Hz的测量；2、利用野外实验观测得到的风速和输沙强度数据,计算了野外真实环境下的临界起动风速,结果表明由于局部地表特征的变化导致临界起动风速随时问波动很大；通过分析实验观测数据还发现时间步长不同时,水平瞬时风速和跃移强度的相关系数随时间步长的增长成对数增加,当时间步长大于40s时,相关系数趋于稳定；3、利用建立的风—沙相互耦合的风沙跃移运动模型,研究了脉动风速作用下的输沙率,与实测数据对比说明该模型的合理性；4、从坡面颗粒受力出发,推导了坡面沙粒起动要求的来流临界摩阻风速计算公式,并利用FLUENT软件模拟了流过沙丘的风场,进一步分析了坡度和坡面位置对临界起动风速的影响；5、利用FLUENT软件模拟了沙尘暴发生期间18种来流条件下奇华胡安沙漠复杂地形的流场,并与实测数据进行了对照；而后对沙尘暴发生期间不同来流条件下民勤典型新月型沙丘的流场进行了数值模拟,分析了沙丘丘基线上单宽输沙率的变化规律。
【Abstract】 Studies of erosion saltation are currently concentrating on wind tunnel experiments, theoretical analyses and numerical simulations under ideal as well as controllable conditions, such as under time-invariant wind speed and flat sand bed. Whereas the researches of effects of fluctuations characteristics of wind velocity and the complex microtopography in real desert on sand saltation appear to be much fewer although these topics have obtained increasing recognition of importance in recent decades. This dissertation presents some basic field experiments and theoretical modeling and computational simulation for erosion saltation, and the main achievements are made in the following aspects:1. Based on cooperation with Wind Erosion and Water conservation Research Unit of US Agriculture Department, we installed and debugged a system with a former station master of Big Spring Field Station, a soil scientist, Donald (Bill) Fryrear, which was designed to synchronously measure physical quantities, such as fluctuating near-surface wind velocity, sand transport intensity temperature and humidity with 1Hz frequency at two points on the barchan dune in Minqin area, which is located between edges of the Badain Jaran Desert and the Tengger Desert.2. The measured wind speed and saltation intensity are used to calculate threshold wind speed in local environmental conditions. The results show that the threshold wind speed varies in complex ways, as a result of the variation of local environmental conditions during the dust storm. Through analyzing observational data, it is found that the correlation coefficient between horizontal wind velocity and sand saltation intensity logarithmically increases with the increment of time step when it is less than 40 seconds and then it gradually tends to stable.3. Using a numerical model of sand saltation, we analyzed the field sand fluxes under the fluctuating wind speed, and the numerical results are fairly coincident with the field experimental results. 4. Through take into consideration of the forces on a grain resting on the slope, we predict the upstream threshold friction velocity for sand particles on sloping beds. Further, quantitative analyses of slope gradient and particle position on the initiation of particle movement are processed based on the CFD-based model.5. The wind speeds are simulated over the surfaces of complex microtopography in Chihuahuan Desert by FLUENT. The results indicate that the simulated wind speeds are in good agreement with the measured wind speeds. Then the sand fluxes have been calculated at some positions of a dune in Minqin Area during a sand storm event based on the wind field simulated by FLUENT and the variation regularities of sand transport rate along the brink of the dune are analyzed.