Experimental Investigation On Aerodynamic Wall Shear Fluctuation Of Wind-blown Sand In A Wind Tunnel

Concerning problems of fluid mechanics in wind-blown sand and soil erosion,this Ph D thesis presents innovative experiments and theoretical modeling on the phenomenon of aerodynamic wall shear fluctuation in wind-blown sand flow.The probability of aerodynamic entrainment of surface particles during the development of a wind-blown sand flow is proposed.The prediction of the initial velocity of the aerodynamic entrained particle in the saturated wind-blown sand flow is established.The effect of airborne particles on turbulent coherent structures in the boundary layer is analyzed.The wall shear stress and wind erosion potential near a building is predicted by simulation based on the results of above experiments.Firstly,the time-averaged and fluctuation of aerodynamic wall shear stress were measured in a developing wind-blown sand flow.Based on the wind velocity profile and sand mass flux along the flow direction,the saturation fetch of the wind-blown sand was determined to be within 4 m and 5 m.In the saturated wind-blown sand flow,the probability that the instantaneous aerodynamic wall shear stress exceeding the fluid threshold was 21 %,indicating the existence of potential aerodynamic entrainment.With the development of wind-blown sand flow,the probability of aerodynamic entrained saltation decreased from 82.9 % to 15.1 %,the probability of rocking increased from 1.3 % to 5.2 %,and the probability of creeping increased from 2.1 % to6.3 %.Secondly,the phenomenon that the instantaneous aerodynamic wall shear stress exceeding the fluid threshold named impulse event was studied.The frequency of aerodynamic entrainment and the initial velocity of the entrained particles in the saturated wind-blown sand flow were analyzed.If a surface sand particle is elevated by one diameter in an impulse event,it is defined as an effective aerodynamic entrainment.Results show that there is an exponential relationship between the total number of impulse events and the number of effective aerodynamic entrainment.The probability density of the initial velocity of aerodynamic entrained sand particles shows lognormal distribution.The average value of the initial velocity of aerodynamic entrained sand particles is insensitive to the wind speed,but is affected by the averaged sand particle diameter of the sand bed.At the same time,the characteristic parameters including turbulent intensity,probability density distribution and energy spectrum of the aerodynamic wall shear stress in the sand-free and sand-laden flows were compared.The effects of the airborne particles on the coherent structures in the boundary layer were analyzed.In a windblown sand flow,the turbulent intensity of aerodynamic wall shear stress increased rapidly with Reynolds number,and the growth rate was much higher than in the airflow,indicating that the turbulent intensity of aerodynamic wall shear stress is more sensitive to particle mass flux than Reynolds number.The blocking effect of airborne sand particles resulted in 2.5 %-6.7 % lower wind velocity in the near wall region.The energy spectra show that the fluctuation with larger scales were enhanced in the wind-blown sand flow and the intermittent occurring sand streamers were responsible to this enhancement of large-scale fluctuation.By analyzing the structure function of the friction velocity,it was found that the friction velocity is a 1.07 order power-law parameter of the wind velocity in the airflow,and the order increased to 1.52 in the wind-blown sand flow.By comparing the scaling exponent in sand-free and sand-laden flows,it was found that the sand-laden flow is more intermittent.Finally,considering the process of airborne sand particles splashing surface particles and the aerodynamic entrainment of surface sand particles,the wind-blown sand flow around a railway bridge in the reversed flow region of an isolated sand dune was simulated by particle tracking method.The wind-sand flow around bridge was predicted,as well as the corresponding sand hazard prevention project was proposed.The aerodynamic wall shear stress on the sand dune was studied.In the middle of two adjacent bridge piers,the aerodynamic wall shear stress increases at the bottom of the leeward slope,indicating possible wind erosion in this area.