Large Eddy Simulation Of Saltating Sand Flow With Wind-sand-electricity Multi-field Coupling

Aeolian saltating sand flow taking place in the atmospheric near surface layer is a typical,high-Reynolds-number turbulent multiphase flow,which is usually accompanied by particle charging and strong electric fields.Existing numerical simulation studies are mostly based on Reynolds-averaged flow field,uniform particle size and electric field simulation with the point-charge Coulomb formula.Such kind of numerical models cannot reproduce the actually turbulent fluctuation,the fluctuation and structure of electric field,as well as the observed stratification phenomenon of vertical average electric field.In this thesis,the wind-sand-electricity multi-field coupling simulation of saltating sand flow are performed using large eddy simulation of wall turbulence,Lagrangian tracking of point particle and a so called P3 M method for solving electric field.Two simulation cases are "uniform particle size" with constant charge-to-mass ratio and "mixed particle size" with distributed charge-to-mass ratio,respectively.The results reveal:In the case of "uniform particle size" with constant charge-to-mass ratio(-60?c/kg),the vertical wind-sand electric field decays monotonically along the height.It can reach as high as 50 k V/m near the surface bed.The root-mean-square of vertical electric field fluctuation also decays monotonically along the height and it can reach 1/2 of the average value near the wall.Although the average values of streamwise and spanwise electric field are zero,their fluctuation can reach 1/10?1/20 of the vertical electric field fluctuation near the wall.On one hand,the interaction between saltating sand particle and strong electric field reduces the average sediment transport rate and the structure scale of strongly correlated particle concentration.On the other hand,large-scale particle clusters result in coherent electric field structures.For the first time we report the shape and characteristic scale of the electric field structure.It is found that the streamwise and spanwise scales of the near-wall vertical electric field structure are close to the scales of the concentration structure.However,the scales of electric field structure beyond saltation layer increases with height.Moreover,we found the-5/3 spectral region in the energy spectrum of streamwise and spanwise electric field.In the case of "mixed particle size" with distributed charge-to-mass ratio,it is found that the average particle size firstly increases and then decreases with wall-normal height,which leads to a tri-layered structure of the average charge-to-mass ratio and the average charge density along the height.The simulation results show that there is also a similar layering phenomenon in the vertical electric field.We propose a theoretical model that can link the relationship between charge layering and electric field layering,which can explain the simulated results.This model can also be extended to explain the "non-unipolar" phenomenon up to six layers of vertical average electric field observed experimentally in the actual sandstorm.The simulations in this paper do not consider suspended particles,particle triboelectric charging,and collision charge transfer processes.At the same time,the relevant laws of the wind-sand electric field under different wind speeds are not given,which need to be further studied.