Experiments and Numerical Simulations for Aeolian Saltation in Natural Environments
Studies of erosion saltation are currently concentrating on wind tunnel experiments, theoretical analyses and numerical simulations under ideal as well as controllable conditions, such as under time-invariant wind speed and flat sand bed. Whereas the researches of effects of fluctuations characteristics of wind velocity and the complex microtopography in real desert on sand saltation appear to be much fewer although these topics have obtained increasing recognition of importance in recent decades. This dissertation presents some basic field experiments and theoretical modeling and computational simulation for erosion saltation, and the main achievements are made in the following aspects:1. Based on cooperation with Wind Erosion and Water conservation Research Unit of US Agriculture Department, we installed and debugged a system with a former station master of Big Spring Field Station, a soil scientist, Donald (Bill) Fryrear, which was designed to synchronously measure physical quantities, such as fluctuating near-surface wind velocity, sand transport intensity temperature and humidity with 1Hz frequency at two points on the barchan dune in Minqin area, which is located between edges of the Badain Jaran Desert and the Tengger Desert.2. The measured wind speed and saltation intensity are used to calculate threshold wind speed in local environmental conditions. The results show that the threshold wind speed varies in complex ways, as a result of the variation of local environmental conditions during the dust storm. Through analyzing observational data, it is found that the correlation coefficient between horizontal wind velocity and sand saltation intensity logarithmically increases with the increment of time step when it is less than 40 seconds and then it gradually tends to stable.3. Using a numerical model of sand saltation, we analyzed the field sand fluxes under the fluctuating wind speed, and the numerical results are fairly coincident with the field experimental results. 4. Through take into consideration of the forces on a grain resting on the slope, we predict the upstream threshold friction velocity for sand particles on sloping beds. Further, quantitative analyses of slope gradient and particle position on the initiation of particle movement are processed based on the CFD-based model.5. The wind speeds are simulated over the surfaces of complex microtopography in Chihuahuan Desert by FLUENT. The results indicate that the simulated wind speeds are in good agreement with the measured wind speeds. Then the sand fluxes have been calculated at some positions of a dune in Minqin Area during a sand storm event based on the wind field simulated by FLUENT and the variation regularities of sand transport rate along the brink of the dune are analyzed.