Study On Three-Dimensional Lift-off Velocities Of Sand Grains In Wind-Blown Sand Flux

It is obvious that the probability distribution of lift-off velocities is the important parameter for the prediction of wind sand flux. The subject of investigated in this thesis focuses on the probability distribution of lift-off velocities of sand particles in windblown sand flows. Firstly experimental measurements were conducted in a wind tunnel with the Particle Image Velocimeter (PIV) which gave the probability distribution function (PDF) of the lift-off velocities of sand particles regarding wind velocities and sand diameters. Then a3D stochastic particle/bed collision model was established to analysis the dynamical characteristic of the particle/bed collision process in saltation layer. Further an empirical formula concerning the mass flux varying with height can be obtained through the measured data. Finally, a prediction of the PDF of lift-off velocities and angular velocities of sand particles was made combing with the3D stochastic model. The context includes the following sections:1. Employing the PIV combing an improved algorithm of the Particles Tracing Velocimeter (PTV), this study realized synchronous measurements on the spatial positions, velocities and sizes of saltating sand particles in windblown sand flows on a size-mixed sand bed generated in a wind tunnel. The PDFs as well as their variations of the horizontal, vertical and lateral velocities of incident and lift-off sand particles were investigated considering the influence of wind velocities, particles sizes and heights. Empirical formulas describing the PDFs of incident and lift-off velocities in the saltating layer were put forward.2. Based on the classic collision theory, this study established a3D stochastic particle/bed collision model on the collision process between an incident particle and bed particles with respect to the incident velocity and angle, contact positions, creeping velocities as well as the counterforce acted on the incident particle from bed particles. With the model, the horizontal, vertical and lateral velocities, rolling and side-spinning angular velocities of the incident particle and bed particles were given in terms of analytical solutions. The model is proved plausible compared with experimental results which can reveal the characteristics of rebound and ejection of sand particles after collisions and the influence of above random variables to the sand particles’lift-off in quantity, including rebound velocities, the ratio of ejection velocities and incident velocities, rebound and ejection angles and the rebound probability of incident particles.3. Based on the3D stochastic particle/bed collision model and the probability theory of multiple random variables, the PDFs of lift-off quantities of sand particles were presented including the horizontal, vertical and lateral components of rebound and ejected sand particles, rolling and top-and side-spinning angular velocities. With these PDFs, the PDFs of3D lift-off velocities and angular velocities can be obtained. The predicted results have good agreement with the experimental data which proved the reliability of the method used in this study. Eventually, the influence of above random variables to the PDF of lift-off velocities of sand particles in the saltating layer.4. The variation law of windblown sand flux and lift-off velocities along with distance was investigated through wind-tunnel experiments. The changing law of the horizontal component of sand transport rate was revealed by sand traps, which demonstrates a trend of first increasing, then decreasing and finally reaching saturation. The measured saturation length was found decrease with the increasing friction wind speed which was expressed by an empirical formula. Further, an empirical relation between the horizontal sand transport rate and height was for the first time put forward to reflect the profile of windblown sand flows.