Numerical Simulation Of Spatio-temporal Evolution Of Unstable Wind-blown Sand

Sandstorm and desertification are major disasters in arid and semi-arid regions.The near-surface sand transport under atmospheric turbulence is the main driving mechanism for its occurrence and development,which would cause ecological disasters such as wind erosion,dune migration and land desertification.The wind speed,wind direction and the stability of stratification in the natural atmospheric turbulent boundary layer are always in dynamically changes.Sand transport occurs frequently under the conditions of time-varying wind field and unstable stratification.However,some studies mainly focus on steady-state model of wind-blown sand under steady wind and near-neutral atmospheric stratification,and researchers pay less attention to the influence of unstable conditions such as time-varying wind,unstable stratification and surface thermal heterogeneity on sand transport law,resulting in a large deviation from the actual sand transport process.At present,only a few scholars have studied the sand transport law intime-varying wind,but still have not focused on the interaction between time-varying wind and particles.Other scholars have studied the transport law under non-neutral stratification,but simply used the Reynolds average equations to simulate the wind field.Therefore,there are some shortcomings in existing studies,such as the model is simple and the experiment conditions are difficult to control.And they do not systematically and quantitatively explore the law of wind-sand blown under time-varying wind and unstable atmospheric stratification.Based on the large-eddy simulation of ARPS(Advance Regional Prediction System)model,coupling the Lagrangian sand transport model and the turbulent thermal effect,comprehensively considering the particle-bed collision process and the coupling effect between particles and turbulence,a thermal-dynamic model for wind-blown sand simulation is established.And then the numerical simulation of wind sand movement under unstable conditions are carried out as follows.A wind-blown sand model coupling particle movement and time-varying wind field is established.By setting two time-varying inflow of continuously sinusoidal wind and sudden wind at the inlet,the large eddy simulation is used to calculate the near-surface wind-blown sand under unsteady turbulence.And the transport law under the coupling effect in unstable time-varying wind with different amplitudes and frequencies is analyzed.The simulation results show that: When the frequency of sinusoidal wind is greater than 1.0 Hz,the sand transport law is similar to that of steady wind.However,when the frequency is less than 1.0 Hz,the sediment transport rate exhibits a sine-like,and its mean value is greater than that of steady wind.Moreover,it increases with the increase of the frequency and amplitude of sinusoidal wind.For unstable wind with mutation,the response time of sediment transport rate with different particles size to increasing wind speed is different,which is shown as the larger particle size is,the longer response time is.However,when the wind speed suddenly decreases,the response time is independent of the particle size,which is about 3.0s.Secondly,the effects of unstable stratification formed by bottom heating on surface shear stress and particle release rate are investigated by wind tunnel experiments.The results show that the unstable stratification would increase the release rate of particles.And then,the sand transport process under unstable stratification is simulated based on the three-dimensional wind-blown sand two-phase flow model of particle-wind-temperature coupling.With the fully developed turbulent boundary layer and Lagrange particle tracing method,the particle trajectories are depicted and the interaction between the flow and particles is reproduced under unstable stratification.The simulation show that the strong temperature gradient near the surface strongly affects the vertical movement of air,and then affects the law of sand transport.Especially,when the wind speed is around particle entrainment threshold,the error of the influence of unstable stratification on sediment transport rate is up to 4 times.However,when the wind speed is large and the wind-blown sand reaches saturation,the influence of unstable stratification on the mean value of the sediment transport rate is small,but significantly increases the fluctuation value.In order to better characterize the influence of wind speed fluctuation on sediment transport rate,the effective surface shear stress is proposed as a parameter for the first time.The analysis shows that compared with the average surface shear stress,the effective surface shear stress contains the information of particle entrainment threshold and distribution of surface shear stress,which could unify the relationship between sediment transport rate and the effective surface shear stress into a law under neutral and unstable stratification.Based on the analysis of the law,when the ratio of convective velocity scale to initial friction velocity is less than a constant related to particle size,the influence of unstable stratification on wind-blown sand can be neglected.Finally,a sand transport model affected by the heterogeneous thermal process is established by combining the sand transport model with a heterogeneous underlying surface characterized by different heat fluxes under unstable stratification.The results show that under windless conditions with only surface thermal effect,the spatial thermal inhomogeneity would prouduce a typical inhomogeneous scale circulation.When the wind speed is large,the surface thermal inhomogeneity has a great influence on the high-order statistics of turbulence,and would also change the spatial distribution of sand transport rate.It is also shown that the average sediment transport rate in the heating zone is slightly higher than that in the cooling zone,and the sediment transport rate decreases first and then increases with the increase of the extent of heterogeneity.