Wall-Modeled Large-Eddy Simulation Of Near-Surface Wind-Blown Sand

Wind-blown sand is a turbulent multiphase flow with high Reynolds number,in which there exists strong interaction between turbulent flow field and particles.Turbulent flow and particle movement near the surface are more complex,which is the key to the complexity of two-phase wall turbulence.In order to obtain the influence of turbulence characteristics on near-surface wind-blown sand,the following work is carried out by wall-modeled large-eddy simulation.It includes testing different flow fields and particle near-wall models in two-phase flow,appling a more reliable near-wall model to the numerical simulation of wind-blown sand and analyzing the effects of turbulent fluctuation and atmospheric boundary layer thickness on the near-wall flow field and particle movement.As compared with the previous numerical simulation,the innovations of this thesis were:1)the near-wall model of flow field adopts the integral wall model,which includes the non-equilibrium item of the momentum equation;2)the influence of impact parameters on velocity restitution coefficient after collision is considered in the three-dimensional particle-bed collision model.Through the numerical simulation of near-surface wind-blown sand,the innovative results obtained in this thesis mainly include:1)Integral wall model can better predict flow field and particle statistics,such as wall shear stress fluctuation,particle volume fraction and transport flux.The splash function with impact parameters on restitution coefficient can reflect the change law of total restitution coefficient with impact velocity.Compared with empirical or semi-empirical splash function based on experiment,the impact and lift-off velocities obtained by three-dimensional particle-bed collision model are closer to the actual observation.2)In the experimental study,the low-energy particles with relatively low trajectory height were ignored,and the critical height was 5 times particle diameter obtained from our results,in which the impact particles with a trajectory height less than 5times particle diameter accounted for 35%?40%of the total impact particles.The grain-scale distributions with trajectory height greater than 5 times particle size are consistent with previous studies.However,some new phenomena appear in the simulation results when all particle motions are considered.In addition,considering only the velocity distribution with the trajectory height greater than 5 times diameter overestimates the aeolian sand flux.When the friction wind speed varies from 0.3 to0.8m·s^-1,the relative error can reach 55.6%to 6.6%by wall-modeled large-eddy simulation.3)The boundary layer thickness influences the particle motion near the surface by affecting the turbulent characteristic scale.The average particle velocity and maximum saltation height are proportional to the boundary layer thickness and friction wind speed.The lower boundary layer thickness(H=0.3m)limits the development of aeolian sand flow,while the sand flux of H=12.0m is smaller than the real field observation,indicating that the sediment transport will further increase under the real field boundary layer.The larger the boundary layer thickness is,the larger the characteristic scale of sand streamers obtained by correlation of vertical integral concentration is.That is,the aggregation effect of particles is more obvious.Otherwise,the particles tend to be evenly distributed.