Numerical Simulation Of Stochastic Collision Between Grain And Bed In Aeolian Sand Saltation

This thesis focuses on the key problem in the wind blown-sand transport, the sand particle-bed collision. Firstly, a 2D mixed sand bed is prepared by stochastic method and the direct numerical simulation program is created based on Discrete Element Method (DEM). And then we insight the grain-bed collision on flat mixed size bed using this program and deal with the results by a statistic approach. Secondly, the effect of particle sizes on grain-bed impact is discussed, meanwhile the relationship between incident and lift-off quantities for different particle size are presented. Finally, we extend our program to different slope to estimate the effect of slope angle on grain-bed impact. The relationships of incident and lift-off quantities proposed by this thesis, including impact speed, impact grain size and slope angle, will be significant for windblown sand transport and sand ripple simulation. The main works are concluded as follows:1. Considering the Probability Density Function (PDF) of natural sand particle size has unimodal distribution, this thesis presents one approach to propagation an approximate natural sand bed used to target bed. Moreover, a program code is created based on Discrete Element Method (DEM) or Particle Dynamic Method (PDM) in two dimensional to realize the mixed size grain-bed impact process.2. Due to grain-bed interactions are stochastic processes, we propose a statistical method to deal with results, and also find the appropriate coefficients of steady state program and statistical method when the number consisted of sand bed, number of sample and impact points vary.3. Using the grain-bed impact program and statistical method, to each different incident particle size, different incident speed and angle a lot of samples will be used to simulate impact. We find that there are distinct deviations between mixed grain size and uniform grain size simulation results, and the results of this thesis well agree with the experimental data of the multiple size sand grains, which suggests us that the simulations of windblown saltation and sand ripple should be based on our results. It is also justified the necessary to study mixed grain-bed impact.4. Based on the simulation on flat bed, we insight the effect of particle size on the impact processes. For the first time, we argue the relationship between particle size and random physical quantities of lift-off particles. These simulation results well consist with measurements. 5. The efforts have been done on the flat bed are prepared for us to discuss the grain-bed impact on more natural sand beds which always are undulate in field desert. In order to find the effect of slope on different location of one sand ripple, we simulate the impact processes on a series of slopes. The results demonstrate that the random physical quantities of lift-off particles on different slop have markedly deviations. Some random physical quantities change regularly varying with the angle of slope. For the first time this thesis presents the relationship between incident and lift-off physical quantities of particles effected by incident speed, incident particle size and slope angle.