Generation and characteristics of snowdrifts and powder snow avalanches

研究了在固气两相流条件下粉末雪崩的产生及其特征。雪堆与山区粉雪崩的发生密切相关。雪堆本身是由强风产生的，强风会悬浮大量的雪粒。当斜坡上可能发生积雪时，雪颗粒与空气混合物的重力过剩会加速形成粉状雪崩。采用k-ε湍流模型对粉末雪崩的形成机理进行了数值模拟。在模拟中，将固气两相流替换为固液两相流，重力过剩用水中盐度浓度表示。利用k-ε湍流模型的数值模拟结果，精确地讨论了加速倾斜热源的流动特性。浓度等高线和速度矢量图帮助我们更好地了解了热气流的内部和外部结构。根据数值模拟结果，估计了气流在流动方向上的运动速度、高度和最大浓度的变化。这些热力学性质与室内实验结果吻合较好。利用k-ε湍流模型对积雪特征进行了数值模拟。模拟的问题是雪粒子浓度边界值的估计；即雪夹带系数。夹带雪系数的数值来源于数值模拟和雪堆现场观测和风洞试验的实际值。空气中雪的夹带系数小于水中沙的夹带系数。其中一个原因是空气中的雪与水中的沙密度差较大。因此，雪颗粒很难被空气的湍流运动悬浮。相反，沙粒很容易被水的湍流所悬浮。沙粒在水中的比重为零阶，而雪粒在空气中的比重则变得相当大。另一个原因是在光学原理下，雪粒子的直径是通过雪粒子计数器（SPG）来测量的。假设粒子的形状是球形，这种方法可能会高估直径。实际上，雪颗粒的形状不是一个球体，而是有各种各样的晶体。少

The generation and characteristics of powder snow avalanches have been studied as the treatment of solid-gas two phase flow. The snowdrifts are closely related to the occurrences of powder snow avalanches in mountainous area. Snowdrifts themselves are generated by strong wind which suspends large amount of snow particles. When snowdrifts may occur on the slope, the gravity excess of mixture of snow particles and air can accelerate to become a powder snow avalanches.These mechanism of the generation of powder snow avalanches was simulated numerically using the k-ε turbulence model. In the simulation, the solid-gas two phase flow was replaced by the solid-liquid two phase flow and the gravity excess was expressed by the salinity concentration in water.The flow characteristics of the accelerated inclined thermals were discussed precisely by the results of the numerical simulation using the k-ε turbulence model. The drawings of concentration contours and velocity vectors have helped us to … More get the idea of the inner and outside structure of thermals. The variation of the travel speed, the height and the maximum concentration of a thermal in the flow direction were estimated from the numerical results of the simulation of thermals. Those mass characteristics of thermals are well agreed with the experimental results in the laboratory.The characteristics of the snowdrifts also were discussed by the numerical simulation using the k-ε turbulence model. The problem in the simulation was estimation of the boundary value of concentration of snow particles ; i.e. the snow entrainment coefficient. The values of the snow entrainment coefficient were derived from the numerical simulation and the actual values from the field observations and wind tunnel experiments of snowdrifts. The values of snow entrainment coefficient in air were smaller then those of the sand entrainment coefficient in water. The one reason was the large density difference of snow case in air comparing with the sand case in water. Thus, the snow particles are difficult to be suspended by the turbulent motion of air. On the contrary the sand particles can be easily suspended by the turbulent motion of water. The specific gravity of sand particles in water is the zero order, however, the specific gravity of snow particles in air becomes quite large values. The other reason was that diameters of snow particles were measured by the snow particle counter(SPG) in the optic principle. This method may over-estimate of diameter assuming that the shape of the particle is a sphere. Actually the shape of snow particles is not a sphere but having wide variety of crystals. Less