Scaling Law Of Aerodynamic Entrainment Of Particles In Turbulent Boundary Layer

Wind-driven particle transport in atmosphere boundary layer is one of the most common phenomena in nature,such as snow drifting,wind blowing sand/dust and etc.The resulting disasters like frozen weather and dust storms and other secondary disasters are the most serious environmental problems that society concerned nowadays.Aerodynamic entrainment is a process that particles are lifted into the air only by fluid force,which is one of the key processes in wind-driven particle transport.However,the early scholars believed that aerodynamic entrainment is just the induced factor and the mass proportion of particle initiated by emission is so small that can be ignored.Studies on aerodynamic entrainment rate is rare so far,while recent field observations and wind-tunnel experiments results have shown that under certain conditions,for example,low wind speed,supply-limited desert surfaces,and convective turbulence,aerodynamic entrainment plays an important role in aeolian granular system transport.Therefore,in this paper,we reviewed the main results in aerodynamic entrainment researches and summarized unsolved problems up till now.We then performed windtunnel experiments on aerodynamic entrainment rate of several particle materials with different particle sizes.From the experiments results,we found the scaling laws between aerodynamic entrainment rate and surface shear stress.And by putting a cloth in wind tunnel to change the turbulent characteristic of wind field,we also studied the turbulent effects on aerodynamic entrainment rate,as well as the definition of fluid threshold and the role of aerodynamic entrainment in the total aeolian transport.The detail conclusions are shown as below:(1)We proposed a method to change the turbulent characteristic of wind field through putting a cloth horizontally into the wind-tunnel.By spectrum analysis,we found that this method can make the power spectrum of wind field in wind tunnel has similar laws with that of unstable atmospheric boundary layer in the field.Based on that,we studied on the effect of this kind of wind conditions on aerodynamic entrainment.(2)We found different scaling laws of aerodynamic entrainment rate varies with mean surface shear stress under low wind speed and high wind speed: when wind speed is low,aerodynamic entrainment rate follows an exponential law with mean surface shear stress,while when wind speed is high,aerodynamic entrainment rate follows an exponential law with mean surface shear stress.By analyzing the distribution of instantaneous surface shear stress under different wind speed,we found that the change of proportion of instantaneous excess surface shear stress leads to the different scaling laws.Based on it,we established an aerodynamic entrainment rate model with consideration of distribution of instantaneous surface shear stress and surface shear stress threshold,which is applicable for the forecast of aerodynamic entrainment rate under different wind conditions.(3)The mean emission velocity of particle under strong wind almostly keep constant when the wind velocity changed,and only depend on the particle size.It is similar with the mean surface horizontal velocity of particles in steady saltation motion in a fully developed aeolian transportation,which keep constant with friction velocity changing.