Experimental Investigations On Threshold Value For Aerodynamic Entrainment Of Sanddust In Wind Tunnel

Aerodynamic entrainment is a basic form of movement for particles,it is an important part of soil erosion research.When the wind velocity gradually increases to a certain value,some particles on the surface start to break away from the quiescent state and move with wind together.This threshold value of wind velocity is called threshold velocity for aerodynamic entrainment.It can be seen that aerodynamic entrainment is the decisive factor which causes soil erosion,land desertification and etc.,it can directly affect human life.Threshold friction velocity is a key parameter to describe the aerodynamic entrainment,it plays an important role in judging the occurrence of dust events,forecasting the dust emission flux and evaluating the transport intensity.Therefore,the threshold friction velocity is the key variable in the aeolian research.In this paper,we do the investigations about the threshold velocity for aerodynamic entrainment of sand-dust in the wind tunnel.We simulate the process of aerodynamic entrainment and measure the major physical quantities during the process.Through monitoring the changes of surface stress of turbulent boundary layer,and corresponding the changes with the average emission rate.Combining the analysis of instantaneous Shield number,we present the predictive model of aerodynamic entrainment which used in this paper.And meanwhile,we use the method of traditional threshold,the improved threshold method and the improved backstepping method to do systematically test for the threshold friction velocity of sand-dust.Comparing the results about the improved method with the method of traditional threshold,we the improved methods are more accurate.And the predictive model of this paper is more in accordance with the physical meaning of aerodynamic entrainment.On the contrary,the method of traditional threshold rely on empirical judgment.They lack the supports of physical mechanisms.We get a series of friction velocity by the improved methods.Comparing the results about the improved methods with previous results,we find the trend which shows the V-shaped,so we get the critical particle size which is 40?m.The threshold friction velocity in this paper are greater than previous.Therefore,we believe that the traditional criteria cannot accurately predict the threshold friction velocity for aerodynamic entrainment of sand-dust,and the predecessors underestimate the threshold friction velocity of particles.For the dust,we do statistics of emission frequency which is bigger0.001g m~3 and do the frequency analysis.Finally,we use 0.87 as the threshold frequency to determine the threshold velocity for aerodynamic entrainment of dust.