摘要
文中应用数值算法对尘卷风的发展过程进行了模拟计算与分析。通过模拟计算得到了尘卷风的内部详细结构和运动过程。研究表明尘卷风是由于地面局部增热不均匀而形成的一种特殊的旋转对流运动。在尘卷风形成的过程中 ,外围空气通过贴近地面的薄层被地面加热后流向中心部位 ,外围空气的旋转能量在中心部位得到加强形成尘卷风 ,其旋转能量是热泡原来具有的旋转能量的局部集中和一部分势能转化而形成的 ,其旋转方向是由热对流泡的初始旋转方向所决定。尘卷风是一种类兰金涡 ,旋转速度和压力的分布具有兰金组合涡的特点 ;在成熟阶段 ,尘卷风的详细结构可以粗略地分为 4个区域———地面附近的气流汇聚区域、柱状的涡核区域、旋风与地面作用形成的转角风区域以及涡核外部的外围气流区域。转角区域可以细分为两个子区域———外围的方位角风区域(在该区域 ,上升气流运动方向与轴线之间有一定的夹角 ,称为方位角 )和中心的下沉停滞气流区域。尘卷风中心的低压和急速的上升气流可使大量沙尘扬起 ,不同直径的沙尘颗粒在尘卷风的作用下运动轨迹不同 ,因此卷起不同大小沙尘的尘卷风的外形也是不同的。
Dust devil is a small whirlwind, usually in a short duration, which swirls dust, debris, and sand to a certain heights. In this paper Eulerian/Lagrangian methods and PISO algorithm are employed to simulate dust devil to get its fine structure and its development. During the course of developing of dust devil, the air near the bottom of hot-bubble is heated by the hot ground and flows to the central field, and then the swirling energy is strengthened at the center. The swirling energy of dust devil comes from the swirling energy of hot-bubble and a part of potential energy. The direction of swirl of dust devil (clockwise or anticlockwise) accords with the direction of swirl of hot-bubble. For dust devils, the tangential velocity profile behaves like Rankine vortex, so it could be regarded as a kind of Rankine-like vortex. The study indicated that dust devils have four stages as the swirl ratio is increased from zero: weak vortex stage, developing stage, developed stage and vanishing stage. The fine structure of dust devil in the developed stage could be roughly divided into four zones: the inflow surface layer near the surface, the corner flow region, the core flow region, and the outer flow region. A lot of sand and dust can be lifted by the upward airflow with high velocity, and the tracks of sand particles are different with different diameters.
基金
教育部跨世纪人才培养计划项目
