Wind Tunnel Experiments And Numeriaal Simulation Of Aeolian Saltaion On Micro-topogrphy

The current theoretic and numerical models of wind-blown sand mostly consider ideal circumstances such as steady wind velocity, flat sand surface, etc. The research works about the effects of the complex microtopography in real desert on sand saltation appear to be much fewer. Therefore,It is necessary to further study about the wind fields and sand movement over the surfaces with complex micro-topography. This dissertation presents some basic wind tunnel experiments and some simulation work about the aeolian saltation on different bed form, and the main achievements are as following aspects:1. Aeolian or wind-induced sand ripple is the basic geomorphologic form of desert under the action of wind. It is formed by the interaction between air-flow and sandy surface. For further understand the characteristics of the flow field on sand ripples, the PDPA (Phase Doppler Particle Analyzer) was used to measured the velocity, turbulence intensity and the turbulence kinetic energy of the flow. We also used the PIV (Particle Image Velocimetry) to measure the velocity of saltation particles and analyzed the impact and take-off velocity, the influence of sand ripples to the particles motion etc in detail.2. We aim to elucidate the sophisticated dynamic processes of aeolian sand transport on different bed morphology, so we propose a theoretical model of sand saltation process considering the coupling between wind field and sand grains on two bed forms, i.e., the flat bed and the sand ripples. The flow field was solved by the large eddy simulation (LES) precisely to obtain the wind velocity, and the discrete element method (DEM) was used to obtain the velocity and position of moving particles. The splash process of sand particle and bed can be described by the splash function. By using this method, the results of sand flux with time, the distribution of sand particles in saltation layer and the velocity of sand particles etc. were discussed in detail.3. The bed form is more complicated variable in desert including the sand ripples, sand dune, sand piles and vegetations etc. It is difficult to find a pure flat bed in the field. Therefore, the effect of a little sand pile to the saltation particles motion was investigated by wind tunnel experiment and numerical model which has been proposed and verified in section3. The results of the sand flux with height, the velocity of sand particles, and the wind velocity with height at different position were analyzed in detail.4. The coherent structure in turbulent boundary layer near the wall was investigated by the LES (large eddy simulation) method, the quadrant analysis was used to obtain the signal of the turbulent burst. Then the equilibrium equation about the threshold of sand particles was established base on the results of turbulent burst. The results show that the turbulent burst plays an important role in the particles' taking off.