Study On Mechanism Of Near-Surface Aeolian Transport And Evolution Of Sand Ripples

The aeolian sand transport is responsible for desertification processes and bed topography evolution.Therefore,the prediction of the sand transport rate under turbulent wind conditions is fundamental for the management of land resources and sustainable environmental development.From a physical point of view,sand transport is the dynamic response of a granular bed under the shearing of turbulent wind.The two main types of aeolian sand transport are saltation(grains move by successive jumps over the bed)and creep(rolling and sliding over the bed surface).Aeolian sand ripple is a typical geomorphic type resulting from the interaction between topography and sediment transport,whose formation,evolution process are closely related to the characteristics of airflow movement,sand bed topography and sand bed composition.So far,saltation has been extensively investigated in the past decades.In contrast to saltation,creep still remains virtually unexplored.In addition,the existing mathematical models are mostly based on the assumption of uniform sediment size.However,the sediment is usually a mixture of particles with different characteristic grain size which essentially effects on particle dynamics and consequently has significance on formation of sandy landform.Therefore,on the basis on theoretical study,combined with wind tunnel experiments and numerical simulations,this thesis studies the aeolian sand transport and the formation and evolution of aeolian sand ripples on non-uniform sand bed,we emphatically analyzed the movement characteristic of creep and the influence of graded sand on the behavior of sand ripples and the physical mechanism of grain sorting.The detail contents and results are as follows:(1)A comprehensive model of aeolian sand transport is established,which considers different physical processes rule sand transport at various heights.We present a model for aeolian creep,making quantitative predictions for creep fluxes,which we verify experimentally.We discover that the creep transport rate scales like the Shields number to the power 5/2,clearly different from the laws know for saltation.We derive this 5/2 power scaling law from our theory and confirm it with meticulous wind tunnel experiments.We calculate the creep flux and layer thickness in steady state exactly and for the first time study the relaxation of the flux towards saturation,obtaining an analytic expression for the relaxation time.(2)A new continuum model is constructed to investigate the effects of sediment heterogeneity on the formation of aeolian sand ripples and the grain size sorting over the bed forms.The evolutions of small perturbations of both bed elevation and bed composition are studied analytically by a linear stability approach.In this model the transport of polydisperse sediment is considered as the integrated processes of saltation,reptation caused by impact-driven and creep caused by gravity-driven and drag-driven.Theoretical analysis indicates that size-selective sand transport leads to grain sorting phenomena,which appreciably affect the dynamics of the formation of aeolian sand ripples.The sediment heterogeneity is beneficial to the stability of sand bed,and that the wavelength is lengthened compared with the case of uniform sand.Longer ripples appear when the sediment heterogeneity is characterized by a poorly sorted.On the other hand,the relative volume fraction of the different grain size in the sediment heterogeneity appreciably affects the characteristics of sand ripples.Moreover,it is found that the theoretical model successfully reproduces the observed tendency of coarser grains to accumulate towards the crests of sand ripples,while finer grains accumulating towards the troughs.Lastly,a physical interpretation of the theoretical model results which qualitatively agree with field observations and wind tunnel experiments is given.Finally,the temporal and spatial evolution of sand ripples and the sand sorting process under the mixed grain size scenario are numerically simulated.