Large Eddy Simulation Of Atmospheric Surface Layer Gas-solid Particle Flow

This thesis study the interaction between particles and turbulence in turbulent boundary layer based on large eddy simulation of wall turbulence and point force model of particle.First,an improved near-wall correction scheme for the vertical gradient of the resolved streamwise velocity in the strain-rate tensor was poposed to calculate the eddy viscosity coefficient in the subgrid-scale?SGS?model.The LES code is realized for the simulation of Very-high-Reynolds-number?Re??3×10⁷?ABL flow under the neutral stratification condition with a second-order finite-difference scheme,scale-dependent dynamic SGS stress model,equilibrium wall stress model and the proposed correction scheme.It is found that the logarithmic profile of mean streamwise velocity and some key features of large-scale coherent structures can be reasonably predicted by adopting the proposed correction method and a low-order numerical scheme.Using wall resolved large eddy simulation and particle point force model to simulate the two-phase flow of turbulent particles with high Reynolds number horizontal channel on the surface of the erosive particles,to analyze the modulation effect of the particles on turbulence and explain the mechanism of turbulent modulation.The results show that particle concentrate in the middle of the high and low flow speed fluid.The structure of average three-dimensional particle concentration near the wall is large and small far from the wall.like hill-like.The average particle velocity is lower than the average turbulence velocity except very near the wall(y^?10??27?3);The average flow velocity in the particle flow is reduced compared to the single phase flow over the entire calculated height.The flow velocity pulsation and Reynolds stress caused by near-wall particles are attenuated compared with single-phase flow and enhanced away from wall surface.The turning point of attenuation and enhancement at about y^?10??40,decreases as the Reynolds number increases.The vertical and horizontal turbulence velocity pulsation is enhanced by particles over the all calculated height.The streamwise and spanwise scales of turbulent structures increased in the inner region of near the wall and decreased in the outer region.Quadrant analysis of Reynolds stress shows that the amplitude and occurrence frequency of stress in quadrant Q2 and Q4 weaken near the wall and strengthens far form the wall.The amplitude of stress in quadrant Q1 and Q3 is almost unchanged but the frequency increases near the wall,while the amplitude increases far form the wall.The condition analysis shows that the particle saltation layer is an active rough element to the outer fluid,which is the main reason for the inner fluid velocity fluctuation and the Reynolds stress decrease,and the outer fluid velocity fluctuation and the Reynolds stress increase.In addition,the analysis of conditional average flow field during particle rise and fall shows that the falling process of particles caused by gravity is the main reason why the turbulence structure is affected.For the aeolian sand transport,sand streamers phenomenon of near-surface sand flow transport with an area of 600m×12m×75m successfully reproduced by combining particle Lagrangian trajectory simulation with turbulent large eddy simulation?WMLES?.The characteristic scale of streamers extracted by the two-point correlation of sand concentration,and found that streamers increased with the increase of wind speed and particle size.At a typical 0.2mm particle size,sand streamers are30¹00m long and 1.2².4m wide for Rcc=0.05,while 2.0⁴.5 m long and 0.3⁰.4 m wide corresponding to the correlation threshold of Rcc=0.4,consistent with the field observation.This paper shows that,sand streamers on a flat and erodible surface is related to the presence of very-large/large scale motions in the wind field.Sand streamer of high concentration occurs in the wake area when the very large/large scale motions sweeps the surface with conditional average,which can regarded as the footprint of outer turbulent structure in the near-wall.Moreover,the characteristic scales of sand streamers affected by the splashing process between sand and surface in the sand flow.On a non-erodible surface,sand streamers trailing from upstream sandy surface become narrow though it can propagate downstream for a long distance.