Investigations On The Formation And Evolution Of Aeolian Sand Ripples

As one of the most common patterns of granular materials driven by external forces, sand ripples have always been a research paradigm of sediment geomorphology, complex system and nonlinear sciences. Investigations on the formation and evolution of Aeolian sand ripples are on one hand beneficial to the understanding of surface erosion, transportation and sedimentation of sand/dust materials, on the other hand helpful to describe the paleo-climate of some unexploited regions and the microscopic wind structure of various landforms. The nonlinearity, self-organization behaviors, criticality and the surprising processes developing from disorderly sand moving to inerratic and orderly patterns are also fundamental issues in numerous scientific forefronts. It has universality and extreme significance in science. Besides a direct connection with desertification and wind-blown sand environmental problems, it has been widely applied in engineering and industrial fields including the discovery of the water on Martian surface, microscopic structures of crystal and many other materials etc.In this dissertation, systematical field observations and wind tunnel experiments have been conducted to investigate the formation and evolution of Aeolian sand ripples, a program code based on the discrete element method (DEM) was designed to simulate the particle-bed splash process which plays a key role in the formation and evolution of Aeolian sand ripples from a flat sand bed. Basing on these, a discrete particle tracing method (DPTM) aiming for the tempo-spatial evolution of Aeolian sand ripples was established to reflect the granular property and the of wind-blown sands system, which further discussed the key factors determining or influencing the formation of sand ripples. Main results are included in the following aspects:1. Field observations and wind tunnel experiments on the formation and evolution of Aeolian sand ripples, which describe the influence of wind velocity and sand diameter on the size of sand ripple, the developing of ripples'wavelength and amplitude along with time, saturation time, self-reparation and critical wind velocities.2. Employing the DEM, it successfully replayed particle-bed splash process in wind-blown sand movements, presented splash functions appropriate for various single-size and size-mixed beds.3. Based on the discrete particle tracing model (DPTM) which involves the wind structure and wind-blown sand flux around a sand ripple, particle-bed collisions, and saltation and creeping of sand particles, it presents the formation ande evolution process of Aeolian wind ripples from a flat surface. The simulation results agree well with laboratory measuring, which further quantitatively studied the influence of wind velocity and sand diameter to ripples'morphology, size, migration speed, saturation time etc.4. Combing the results of three dimensional splash function, the DPTM model was extended in three dimension, which realized the simulation of 3D sand ripples, presented the evolution of wavelength, amplitude, Y-junctions and lateral length, replayed the self-reparation behavior and the nested structure of sand ripples under binary wind directions, and quantitatively described the development of Y-junctions.