Modeling And Simulation Of Dust Emission Process In Arid And Semi-arid Regions

Dust has an important impact on the earth system model,and it interacts in a complex way with many major systems and processes,such as physical climate system,biogeochemical cycle,geomorphological evolution,water cycle and ecosystem.As a typical natural hazard,dust storm associated with dust emission in arid and semi-arid regions is also a hot issue worldwide.The saltation transport and spatio-temporal changes of soil properties are the most important factors affecting the dust emission process.The saltation of sand sized particles near-surface is the main power of dust emission,and the influence of particle shape and rotation has currently not been effectively considered,which has been the bottleneck to improve the calculation accuracy.For the process of wind erosion and dust emission,existing studies seldom consider the spatio-temporal variation characteristics of soil moisture content,and the existing soil structure theory is difficult to explain the phenomenon of multi-stage emission in the process of dust emission.Starting from the micro-fluid-solid coupling process of ellipsoidal rotation sand saltation movement,and based on the research methods of Aeolian sand physics and soil hydrodynamics,this doctoral dissertation modifies the existing Aeolian transport model.At the same time,we proposed an evaporation model containing a description of water movement in dry soil layer and wet soil,as well as a new soil structure system including free dust layer.Finally,we established a dynamic dust emission model to describe the complex physical mechanism combining blowing wind driven,soil erosion,water movement.The following work was mainly carried out:Firstly,sand grains are regarded as ellipsoidal rigid bodies,with consideration of their shape factors and rotational motion,the fluid-solid coupling law in the saltation movement of a single sand grain and a double-sand grain system is studied based on the dynamic mesh method.It was found that the rotational motion caused a complex interaction between the ellipsoidal sand grain and the flow field,and the saltation height and distance of a single sand grain were significantly increased,but the fluid-solid coupling in the double-sand grain system consumed more energy,and the saltation height and distance were less than that of a single sand grain with the same initial motion condition.In the rotating two-particle system,the stable airflow deceleration zone between the sand grains causes the downwind sand grains to fall faster,and the transcendence phenomenon of the front sand grains passing the back sand grains occurs,thus reducing the probability of air collision between the sand grains.Secondly,the aerodynamic force of ellipsoidal rotating sand is modified,and a fully developed model of wind-sand transport process on flat bed is established,and the influence of sand shape factor and rotary motion on macroscopic transport is simulated quantitatively.The results show that the shape parameters of sand particles have little influence on the saltation transport rate when the wind speed is small,and the gain effect of the shape parameters of sand particles on the sediment transport rate becomes prominent with the increase of wind speed,which shows that the sediment transport rate increases linearly with the wind speed,and the shape parameters are correlated with the slope of this linear relationship.The sand grains with the ratio of long axis to short axis of 2 have the most similar wind-sand transport process to the experimental results,and their morphological characteristics are more consistent with the existing research results on sand morphologyAt the same time,based on the basic theory of soil hydrodynamics,a soil evaporation model which can completely describe the movement of water in the dry soil layer and the wet soil layer was established,and the distribution characteristics and variation rules of soil water in the evaporation process were analyzed.Results show that in the stable stage before the dried soil layer form,evaporation main consume its own reserves of soil moisture,and in the stage of evaporation with the moisture content before the dried soil layer form,evaporation mainly rely on deep soil moisture transport upwards.However,considering the specific environment in arid and semi-arid areas,the former is difficult to form,and the latter is also relatively short duration.After the formation of the dry soil layer,the growth rate is low at the beginning,but the growth trend is obvious.In the middle stage,the growth rate enters a relatively stable stage,while in the later stage,the growth rate decreases slightly.After the formation of the dry soil layer,the evaporation rate dropped to a low level and remained relatively stable.Finally,the wind erosion model based on the wind-sand transport process,the soil evaporation model including the dry soil layer and the wet soil layer,and the soil structure theory including the free dust layer were coupled effectively,and the dynamic dust emission model was established,and the typical situation was simulated numerically.The process of dust emission is divided into three main stages.The first stage is the stage in which free dust is the main supply body and fluid entrainment emission is the main mechanism.In this stage,the high dust emission rate decreases rapidly with time,reflecting the supply limitation of free dust.After that,the dry soil layer was taken as the main supply body,and saltation bombardment emission and aggregate crushing emission were the main mechanisms.The dust emission rate was significantly reduced,and the thickness of the dry soil layer was the main limiting factor.At last,the wet soil layer was taken as the main supply body,and saltation bombardment emission and aggregate crushing emission were the main mechanisms.The dust emission rate was at a very low level,and the soil moisture content became the main limiting factor.