Study On Charges Carried By Individual Sand Particles In Field Wind-blown Sand Flux

Due to sand movement blown by wind,different aeolian landforms and sandstorms were developed,which result in desert expansion and desertification as well as environment pollution and negative impact on human beings' physical and mental health.It had been known for a long time that the moving sands may carry a set number of charges to form aeolian electric field.When a charged sand particle is acted upon by an electrostatic force,the threshold friction wind speed is reduced;further the sand trajectory in the wind will be changed,and thus the sand transport rate will be also affected.Moreover,charged sand particles also enhance the attenuation of electromagnetic waves when prorogating through the dust system.Therefore,the study of the amount of charges carried by individual sand particles can be helpful to quantitatively figure out the effect of the charges carried by individual sand on the threshold friction wind speed and sand transport rate;evaluate the attenuation degree of electromagnetic wave signal caused by charged sand particles;estimate the effect of the charges carried by individual sand particle on geomorphic evolution,and global climate change;to understand the electrification mechanism of sand particles.Therefore,the experimental studies on the amount of charges carried by individual sand particles,especially the individual sands in filed wind-blown sand flux are conducted out.The main results are as follows.First of all,considering the aeolian electric field,an experimental study is conducted to investigate the charges carried by individual sand particles directly lofted up into air by electrostatics force in this thesis.The sand samples were placed on the surface of simulated dune under electric field.With the increase of electric field,the sand particles are lofted up from the surface,and the trajectories of the sand particles are recorded by high-speed camera.By analyzing the movement trajectory of sand particles in electric field,the amount of charges carried by individual sands are reversely calculated.There is a critical electric field,beyond which the sand particles can be directly lofted by electrostatics force.Meanwhile,the experimental results show that the higher the relative humidity of the environment is,the lower the critical electric field is.The empirical formula of the charges carried by individual sand particles lofted away from sand bed is proposed.Then,the Mosaic random collision electrification model is improved by considering the influence of aeolian electric field on the charges carried by sand particles.The charges carried by sand particles calculated by this model is positively correlated with the particles size and aeolian electric field,which is consistent with the existing law of charges carried by sand particles.Furthermore,an in-situ measurement method to investigate the charges carried by individual particles in complex environment with high particle number density and multi-influencing factors is proposed,including taking sand step and measuring step.Firstly,the charged sand particles in field wind-blown sand flux are collected in a container filled with silicone oil to keep the net charge of the sand particle cannot be changed.Then,the electric charges carried by individual sand particles is reversely calculated according to the trajectories of particles under electric field.An instrument for the in-situ measurement of charges carried by individual particles is also built up,which can be used to measure the charges carried by individual sand particles in complex environmental conditions such as in field wind-blown sand flux.Finally,an experimental study is conducted to investigate the charges carried by individual sands in field wind-blown sand flux,at the Tengger Desert in China.The amount of charges carried by individual sand particles in field wind-blown sand flux are obtained for the first time.The results show that the majority of sand particles in field carry net charges with the charge-to-mass ratio ranging from-180.9?C/kg to350.0?C/kg.The average charge-to-mass ratio of positively charged and negatively charged sand particles is 15.5?C/kg and-16.1?C/kg,respectively.The proportion of positively charged sand particles is more than 80%.The field environment also has an effect on the amount of charges carried by sand particles.With the increase of the relative humidity,the charge-to-mass ratio of both positively and negatively charged sand particles are increased.As the wind speed increases,the proportion of positively charged sand particles also increases,up to 90%.Based on the statistical analysis of charges carried by individual sands,a fitted probability density distribution of the charge-to-mass ratio of individual sand particles in field wind-blown sand flux is given.