Investigations Into Particle-bed Collision And Sediment Transport Characteristics Of Aeolian Sand-flow In The Gobi Desert

Gobi areas are widespread in the north and northwest of China.The arid and windy climate characteristics of Gobi area make the aeolian sand-flow(such as sandstorm)occur frequently,which has caused great harm to the production,infrastructure,transportation,even health and life of the local people.Compared to desert,the surface of Gobi is mainly composed of gravel and crust,and the ambient wind speed in the area is stronger.As a result,the aeolian sand-flow in the Gobi has different characteristics from the desert.However,most of the researches on the basic theory of aeolian sandflow mainly focuses on the desert,and the understanding of the characteristics and mechanism of aeolian sand flow in Gobi is still very limited.During wind-blown sand,the saltation of sand particles advancing in the form of ballistic jumping is the main form of aeolian sand flow,which colliding with the bed surface and jumping forward is an important driving mechanism for the occurrence and development of aeolian sand flow.Moreover,some researches showed that there is a covering layer composed of metal oxides on the surface of sand particles on the earth and terrestrial planets.The covering layer will lead to the change of mechanical properties of particles and then affect the results of particle-bed collision,and the covering layer will fall off in the continuous collision of sand saltation,which makes the influence on aeolian sand movement more complicated.However,in the existing numerical models of sand saltation,sand particles are considered to be composed of a single substance,and the influence of overburden and its shedding process on aeolian sand flow is rarely studied.Due to the hard surface state and limited sand source environment of Gobi,the impact of particle bed collision process on the local aeolian sand flow structure is more significant.Therefore,it is necessary to carry out in-depth research on the impact of sand covering layer on sand-bed collision process in Gobi area and aeolian sand flow structure.In this paper,a new method of using chemical acid and alkali solution to dissolve sand covering layer is proposed for the first time,and the impact of covering layer on collision process is studied by collision experiments.Then,the quantitative formula between the coefficient of restitution and the covering layer mass was provided.Based on the collision experiment and the improved particle tracking image processing algorithm,the particle-bed impact splash function suitable for Gobi surface was obtained for the first time.Furthermore,the empirical formula of sand coefficient of restitution and the particle-bed collision splash function were embedded into the aeolian sand flow model.And the key physical processes in aeolian sand flow were simulated.The main works and detail contents are as follows:1.A hydrochloric acid solution was used to dissolve covering layer of sand surface in the Badain Jaran Desert,the Mu us Desert,the Taklimakan Desert and the shoreside of the Yellow River.High-speed photography and Particle Emitters were used to conduct particle-bed collision experiments on sand before and after dissolution in different areas.The micro-motion characteristics of sand particles were studied and the empirical formula for the variation of the sand collision coefficient of restitution in different areas with the mass of sand surface covering layer and the size of sand particles was given.It was found that the sand collision coefficient of restitution increased by 2%-9% after sand dissolved.Finally,this result was incorporated into an aeolian sand transport model to evaluate the effect of the covering layer on sand flux.It was found that the structural parameters such as wind flow field and sediment transport rate would change obviously with the change of covering layer masses.2.Based on the collision experiment platform and the improved particle tracking algorithm,the particle-bed collision experiment between natural sand and Gobi surface was carried out.Due to the Gobi surface is mainly composed of crust and gravel which has a complex boundary,this paper gave the collision results of sand particle between the crust and gravel,respectively.It was found that the sand incident speed and incident angle obeyed the normal distribution,rebound speed,rebound angle and the recovery coefficient obeyed the lognormal distribution.Then,the change law of sand collision coefficient of restitution and the rebound angle of the two bed surface collisions with the particle incident speed and incident angle,the probability density distribution of the recovery coefficient and the sand particle rebound angle of the Gobi bed surface collision were provided,which obeyed the lognormal distribution.Finally,the splash function of Gobi surface aeolian sand-flow was given.3.Combined with the above empirical collision formula and splash function,the sediment transport structure of Gobi aeolian sand-flow was simulated and analyzed.Through simulation,the distribution characteristics of the wind speed profile,sediment transportation rate,single-width sediment transportation rate,sand horizontal transition speed,transition length and height in the Gobi aeolian sand-flow were given.It was found that the transport characteristics of aeolian sand flow under the surface of Gobi area and desert were compared,and the sand particles with or without covering layer had different transport structures.A new physical explanation was given for the reason that the "Elephant Nose" of Gobi sand flow might be caused by surface obstruction.