دانلود رایگان مقاله تشکیل استریمرهای برف در لایه مرزی جو آشفته

abstract

The drifting snow in the turbulent atmosphere boundary layer is an important type of aeolian multiphase flow. Current theoretical and numerical studies of drifting snow mostly consider the flow field as steady wind velocity. Whereas, little is known about the effects of turbulent wind structures on saltating snow particles. In this paper, a 3-D drifting snow model based on Large Eddy Simulation is established, in which the trajectory of every snow grain is calculated and the coupling effect between wind field and snow particles is considered. The results indicate that the saltating snow particles are reorganized by the suction effect of high-speed rotating vortexes, which results in the local convergence of particle concentration, known as snow streamers. The turbulent wind leads to the spatial nonuniform of snow particles lifted by aerodynamic entrainment, but this does not affect the formation of snow streamers. Whereas the stochastic grain-bed interactions make a great contribution to the final shapes of snow streamers. Generally, snow streamers display a characteristic length about 0.5 m and a characteristic width of approximately 0.16 m, and their characteristic sizes are not sensitive to the wind speed. Compared to the typical sand streamer, snow streamer is slightly narrower and the occurrence of other complex streamer patterns is later than that of sand streamers due to the better follow performance of snow grains with air flow.

4. Conclusions

In this study, a 3-D drifting snow model with mixed particle size in the turbulent boundary layer is established through tracking every saltation particle with explicit consideration of the coupling effect between snow particles and airflow. The snow streamers are reproduced in the turbulent boundary layer. For snow streamers, the transition from streamer families to nested streamers or other more complex streamer patterns generally needs a much larger wind speed than that of sand due to the good following performance of snow particles. The characteristic sizes of snow streamers are analyzed based on a position correlation method. The mean characteristic length and width of snow streamers are approximately 0.5 m and 0.16 m, respectively. The average coverage rate of snow streamers during drifting snow is about 9% and it is constant under different wind speeds. On average, one square meter of space contains 1.4 independent streamers and their lateral spacing is around 1.3 m. All these characteristic parameters are not sensitive to the wind speed. The rotation speed of the turbulent vortexes is found to be directly related to the formation of snow streamers. That is, highspeed rotating vortexes can trap the saltating snow particles like a ‘tornado’. Statistical results show a good correlation between local particle concentration and rotation speed of the vortex. The spatial non-uniform aerodynamic entrainment has no obvious effect on snow streamers, but the grain-bed interactions have important influence on the final shape of snow streamers.