摘要
尘卷风是地球上常见的小型风沙灾害输移系统,但在火星上却大的多。而且尘卷风内部的电场对火星探测器产生严重的电磁干扰。通过建立尘卷风及其电场形成的模型,对尘卷风结构特征及电场进行数值计算。研究表明:尘卷风的形成机理可以用热对流泡理论来解释。沙粒在尘卷风中出现分层现象,粒径小的沙尘往往在粒径大的沙尘上面。尘卷风中带正负电荷的沙粒大约各占23.4%,荷质比大约为60μC·kg-1时,尘卷风数值模拟结果与野外观测值吻合。在尘卷风发展过程中,尘卷风电场大约需要60 s达到稳定。而且电场关于尘卷风中心基本对称,并且在尘卷风中心电场强度较大,在离尘卷风较远的地方,电场趋于零。在距离尘卷风中心一定距离处电场随高度增大先增大后减小,大约10 m以下电场随高度增大而增大,在10 m以上电场随高度增大而减小。
Dust devils are very common meteorological phenomena on the Earth as well as on Mars. They are an abbreviated wind-sand conveyance system. The moving particles in dust devils may become electrically charged, to the point of arcing to spacesuit or vehicle, and creating electromagnetic interference. In this paper, taking the effect of thermal flux from the surface to the atmospheric boundary layer into consideration, a dynamic model is suggested to simulate a dust devil and to obtain its fine structure with large eddy simulation （LES）. And this mod- el not only reveals the formation mechanism of dust devil, but also can realize its process of occurrence, develop- ment. Then, in the dust devil the large amount of sands collide and chafe each other, it causes a part of sands charged positive or negative, and the charged sands will produce electric field in dust devil. So based on Cou- lomb＇ s law, the electric field model is established to simulate the electric field and its distribution in dust devils. The numerical results show that the formation mechanisms of dust devil can be explained with theory of thermal convection, and the hemispheric isothermal distribution is formed at the beginning of the simulation and iso-therms deliver upward step by step. Then isotherms declutch at a height, and the high temperature delivers further upward. This indicates that high temperature of the local surface heats the lower atmosphere and the heated atmo- sphere will transfer upward heat. This process of heat diffusion causes unstable atmosphere state and generates in- tense turbulence and convective plumes, air converging into these plumes tends to conserve any initial angular momentum and a spinning vortex develops. In dust devil, when the wind speed exceeds the threshold velocity and sand grains begin to be lifted from bed with a random lift-off velocity. With more and more sand grains with different diameters being picked up into rotational wind field, the number of grains in air becomes to attain a dy- namic constant. As sand grains with different diameters move in a dust devil, stratification will occur： smaller sand grains are usually above bigger sand grains. In dust devils, sand grains will have charge via frictional or tri- boelectric processes, and sand grains with different diameters may have different charges. Usually small sand grains have negative charge and big sand grains have positive charge. When the percent of charged sands is about 23.4% and charge-to-mass is about, the stable electric field in dust devil is in a good agreement with the observa- tion values. And at the beginning of the evolution of a dust devil the electric field strengthens with time, but after 60 s the electric field changes little, which indicates that electric field of dust devil has trended to stabilization. The electric fteld is symmetric about dust devil center, and the electric field has a maximum value at a certain height, then it weakens rapidly outward to be zero. In the meantime, the electric field increases to maximum and then decreases with the increment of height. In the interior of a dust devil the electric field increases to maximum and then decreases as height increases and the electric field attains maximum at about 6 m height. Outside the dust devil the variational trend of electric field is similar to that of inside the dust devil, but there does not exist a certain height where the electric field attains maximum. This research will improve the dust devil - electric field prediction model, and in future it plays an important role in studying the electromagnetic interference and damage of large dust devil on Mars. At the same time, the research also has the positive significance to study the internal structure and electromagnetic field of large tornado on the earth.
出处
《干旱区地理》
CSCD
北大核心
2015年第2期267-274,共8页
Arid Land Geography
基金
河南省重点学科资助项目(509919)
关键词
尘卷风
热对流
分层现象
电场
荷质比
dust devil
thermal convection
stratification
electric field
charge-to-mass
作者简介
岳高伟（1977-），男，河南临颍人，副教授，博士，从事风沙物理学、复杂系统多场耦合数值模拟等研究．Email：mxlygw@163.com
