Experiment And Theoretical Predictions Of Sand Movement In Wind-blown Sand Flux

The movements of particles in wind-blown sand, such as the initiation and the saltation of sand grains, are related to the interactions between sand grains and complex wind environment. The threshold friction velocity of sand particle motion, the distributions of impact velocity and lift-off velocity are crucial for the occurrence and development of the wind-blown sand, as well as the larger-scale sand movements. And, they also are the key input for sand movement numerical simulation and dust weather numerical forecast. So, this thesis focuses on the threshold friction velocity, the movement characteristics of saltation of sand in the wind-blown sand and the splash process of grain-bed collision. The main works are concluded as follows:1. In comparison to previous wind tunnel PIV experiments, we presented a new experimental design by changing the way of the laser sheet casting; and updated the information of sand-bed in time during data process responding to the variation of the sand-bed surface. The improved method effectively reduces the measuring height of wind tunnel experiments, and the effect of height on the measurement information was quantitatively analyzed.2. Based on the above the improved experimental method, we analyzed the impact velocity, lift-off velocity, impact angle and lift-off angle of saltating sand particles, and obtained the probability distribution functions of these quantities. We firstly obtained the probability distribution form of the distance between sand particles in wind-blown sand, and the relationship between the distance and friction wind velocity.3. Numerical simulation of the grain-bed collision process of particles impacting the sand-bed at the same time was conducted by the discrete element method (DEM). The results illustrate that the distance between sand particles is a key parameter, there exits a critical distance. When the distance is smaller than the critical distance, it is need to consider the sand particles impact sand-bed at same time. On the contrary there is no need to consider. We obtained the function of the critical distance and impact velocity and the splash function of multi-particles impacting the sand-bed simultaneously.4. We established the stochastic model of threshold friction velocity, and derived an analytical expression of the threshold friction velocity with random variables (the eccentricity, the eccentricity angle, support angle and contact angle). Based on the probability theory of multi-dimension random variable, the probability distribution of the threshold friction velocity, its mean value and standard deviation are calculated. The mean and standard deviation are fitted as functions of sand particle’s diameter. So we can’t ignore the effect of the microstructure of soil surface and the irregularity of particle shape.5. We introduced the threshold wind velocity prediction into stratup process and introduced the effect of distance into the splash process, and then we can predict the evolution of windblown sand flux. We obtained the sand transport rate varying with friction wind velocity under two kinds of splash function. It shows that when the friction wind velocity is smaller than0.4m/s, the relative error of sand transport rate between two kinds of splash functions is smaller than5%; when the friction wind velocity is larger than0.4m/s, the relative error increases and can be up to about33%.