Some Investigations Into Wind Snow Two-phase Flow In Wind Tunnel

Drifting snow is a major natural hazard, endangering basic installation and human life in cold regions. Drifting snow can cause snow grains to accumulate on the road to block transportation, reduces visibility, and even lead to fatal traffic accidents. For many snow-covered mountains, redistribution of snow by wind result in irregular deposit, which induce locally increased loading rate in leeward slopes and has a crucial influence on avalanche danger. Furthermore, drifting snow plays a very important role in snow water equivalent of the transport in plains regions and keeping a mass balance of sea-ice in Arctic and Antarctic ice sheet. The research of drifting snow has important significance for economic development and environment protection.In the existing experimental researches on drifting snow, artificial snow, compacted snow as well as some other materials such as sands, seeds et al. were used to substitute natural snow grains. Because the physical and mechanical properties of those materials are quite different from natural snow, it is inappropriate to reflect the motion characteristics for drifting snow in natural conditions with these substitution materials. Some characteristic physical quantities in the avalanche model were limited by measurement instrument, thus, model predictions just show qualitative relevance with experimental data. There are some quantities differences between the theoretical results and the experimental ones. Motivated by the focus on some microcosmic and macroscopic problems related to the motions of drifting snow, as well as the drifting snow in our country, this dissertation presents experimental studies on some microcosmic and macroscopic characteristic physical quantities in the mechanism of the wind-snow motions. The main topics in this dissertation are as follows:1. Researches on the snow grains in drifting snow in the case of flat bed are conducted by Particle Image Velocimetry. Through statistical analyzing, the general form of velocities distribution function for snow particles under various wind speeds as well as the relationship between the mean velocity of snow particles and wind speed have been obtained. The key characteristic quantities-the probability distribution functions of liftoff velocities and impact velocities of particles as well as the relation between liftoff particles and impact ones are revealed by the analysis of the velocities and angles of grains in the height from4mm to10mm. The function relationship between the mean velocity of snow grains and wind speed, the velocity profile in the air versus height are shown by the analysis of particle velocities at different heights. Thus, the varying patterns of erosion intensity of snow drift with height are determined.2. The distributions of snow grain size are obtained through analyzing PIV images, and the macroscopic characteristic physical quantities of drifting snow such as drifting snow structures and the snow transport rate can also be derived by above methods. The function relationship between the the snow transport rate and friction velocity is presented and its validity is discussed. These results are compared with the field experimental data. Then, the relations of the mean height of the particles saltation with wind speed are determined based on the variations of drifting snow structures.3. For the drifting snow under high turbulent conditions such as in mountains, the drifting snows under different turbulent intensity are studied and the effects of turbulent intensity on the transport of drifting snow are obtained. With the aid of snow gather, vertical profiles of drifting snow under several turbulent intensities are given, and the relations among the snow transport rate and wind speed and turbulent intensity are also discussed. The variations of the saltation height and length of snow particles as well as the mean saltation height and length are given based on the analysis of the horizontal and vertical flux of drifting snow.4. Measurements on the threshold wind velocity of snow with different deposition time (fresh snow and old snow) are carried out. Comparing the measurement results with the field experimental data, the structures of drifting snow passing through a flat bed are studied, and the influence of deposition time on drifting snow is discussed.5. Aim at the highway snow disaster in north part of China, the movements of snow particles near sub-grade and the variations of drifting snow structure over highway subgrade are analyzed, and some quantitive results are presented. These results will be of benefit to preventing the drifting snow probems in the highway.