Studies On The Evolution Of Wind-blown Sand Movement In Nonstationary Wind Field

Saltation sand transport shows a great spatital and temporal variability because of the turbulence of airflow in the near surface layer. Studies on the wind-blown sand saltation in nonstationary wind field will not only enhance the understanding of the physics of sand saltaion but also obtain the quantitative changes of macroscopic quantities of sand transport in realistic wind conditions, which is extreme significance for wind erosion and dust storm prediction. This dissertation focuses on the study of field observation and numerical simulation of unsteady sand saltation in nonstationary wind field. Firstly, the characteristics of gusty wind speed and intermittent saltation as well as their correlation are investigated by using the measured data of the 1Hz frequency. Secondly, theoretical model is established to numerically simulate the evolution of wind-blown sand movement, the variations of saltation transport and the influence of wind parameters are presented. Finally, the formula predicting sand transport rate is proposed with the parameters of average and fluctuating wind speed. The main conclusions are presented as following,1) The in situ measurements of average sand transport rates on different surfaces in dust storm, synchronous wind speed and saltation intensity on drifting sand surface were performed during the field experiment in the desert-oasis ecotone region in Minqin, Gansu province. The measured data of wind speed below the height of 2m and saltation intensity at the height of 5cm are statistically analyzed. The saltation response time, variations of saltation intensity with wind speed and saltation occurring when average wind speed is smaller than the threshold wind speed are discussed respectively. The surface conditions and measured transport rates are used to test the suitability of wind erosion prediction model WEPS for the surface in the desert-oasis ecotone region in Minqin. The possible reasons are analyzed.2) A one-dimensional model of unsteady sand saltation considering wind-sand coupling based on the existing steady model is established to numerically simulate the evolution of sand saltation. The variations of micro process of wind-blown sand flux and the effects of wind factors on macro physical quantities in periodic wind variations are discussed. The results show that wind variations will lead to variations of particle trajectories, liftoff number flux and probability distribution of liftoff velocity, and then influence lag time and transport rate. In addition, it is found that the response of sand transport depends on the amplitude, frequency and waveform of the wind variations. Based on the simulation results, sand saltation could be detected even when the average wind velocity equals to or somewhat smaller than the threshold velocity in sequences of periodic wind variations, and sand transport rate is larger than zero. Further analysis indicates that the impact threshold velocity in unsteady wind field is lower than that in steady wind field.3) Saltation intensities simulated using the time series of wind speed generated from the power density spectrums of measured streamwise wind speed are compared with the measured values. Furthermore, the evolutions of sand transport rate under natural wind conditions are simulated. The effect of statistic interval on transport rate and variations of transport rate with average and standard deviation of wind speed are discussed respectively. A new forecast formula of sand transport rate is proposed for the first time in terms of the simulated results, which simultaneously considers both average wind velocity and turbulence. The advance of this method is that it can takes average wind filed as well as its gusty characteristic into account, however, the traditional method predicts sand transport rate only using friction velocity.