Experimen And Numerical Simulation Of WindBlown Sand Cloud

This thesis focused on some basic physical quantities in windblown sand movement, such as, the probability density function distribution of the initial liftoff velocity, mass flux, etc., and experimental and numerical method is used. Firstly, According to some qualitative conclusions in windblown sand movement, the probability density function distribution of the initial liftoff velocity is studied in theoretical method; with the steady-state and unsteady wind-sand coupling model, the windblown sand cloud is discussed in detail; with building grain/bed collision experiments of multi-grain size, the impact of sand size on the collision process is studied, and a model of heterogeneous saltation is suggested to study the evolution of mixed-sized windblown sand movement. The main works are concluded as follows:1. Based on some qualitative conclusions in windblown sand movement and the constitution theory, the probability density function distribution of the initial liftoff velocity is studied in theoretical method, we find that the probability density function distribution of the initial liftoff velocity of bed sands in the steady-state and the probability density function distribution of the initial liftoff velocity of the ejected sands are in agreement with Rayleigh distribution.2. With the steady-state and unsteady wind-sand coupling model, the windblown sand cloud is discussed in detail. It is found that the windblown sand cloud is influenced by the probability density function distribution of the initial liftoff velocity; the wind speed in the reptating layer is strongly reduced and varies little at the different friction velocities, and as the Friction velocity increases, the wind speed at the same height in the reptating layer decreases appreciably; the trajectories of the reptating grains vary slightly at the different friction velocity. Like the reptating grains whose liftoff velocity, on average, and the probability density function distribution of the initial liftoff velocity are decided by the characteristics of grains, the height of the peak value of the mass flux also scales on the property of the grains and will be independent of the wind strength.3. A grain/bed collision experiment of multi-grain size is builded to study the impact of sand size on the collision process. It is found that the coefficient of restitution, the rebound angle, the probability density distribution of the rebound angle and the number of ejected sands are affected by the size of the impact sands and bed sands. When the size of the impact sands and bed sands are homology. the coefficient of restitution and the rebound angle will be independent of sands size and impact velocity.4. A model of heterogeneous saltation is builded to study the evolution of mixed-sized windblown sand flux. It is found that the time needed for the mixed-sized windblown sand flux to reach the steady state is much longer than that for the single-sized windblown sand flux, and the streamwise sand transport and the bed shear stress in the steady state are bigger than the ones in the single-sized windblown sand flux. The grain size distribution in air is related to the grain size distribution of bed. average grain size offset to the small grain, but the trend remained the same.