Experimental And Numerical Study Of Aerial Spraying Droplet Drift

Aerial spraying can help to improve agricultural application efficiency.However,spray drift to non-target area is inevitable during the aerial spraying.Drift is a waste of pesticide and contaminates the environment.Drift can be reduced by using precision aerial spraying techniques,which includes GPS navigation,aerial spraying nozzles,drift prediction model,and variable rate spraying technology.Drift prediction model is one of the key technologies in precision aerial spraying.The model is based on the kinetics of droplets.It involves atomization of spray materials,motion of droplets in the atmospheric boundary layer,droplet evaporation,penetration and deposition in the canopy.In the present research,wind tunnel experiment,flight experiment,and numerical analysis were conducted to study the kinetics of droplets.And a model was developed to predict the drift of spray droplets.The contents are:(1)Wind tunnel measurements were made to determine initial droplet size distributions of pressure nozzles(LU-120-03 flat fan nozzle and IDK-120-03 air induction nozzle)and rotary cage atomizer(Micronair AU5000).Volume median diameter and relative span were used to assess the characteristics of the spray.Firstly,the spray at various distances were evaluated to determine the degree of atomization and an optimal measuring distance.Secondly,the influence of pressure and wind speed on the atomization of pressure nozzles were investigated.Comparison was made to determine the effect of the cavity in the air induction nozzle.Lastly,the influence of blade angle,wind speed,and flow rate on the atomization of rotary cage atomizer were investigated.Results were compared with AGDISP atomizer model.(2)Flight experiment of aerial spraying was carried.3-axis ultrasonic anemometers were used to measure the velocity field of the aerial application.Average velocity,turbulent kinetic energy,and turbulence dissipation rate of the atmospheric boundary layer were calculated.Vertical and horizontal velocity profile induced by the wake vortices were also measured.Ground deposition of spray droplets were collected with water sensitive papers.Image processing techniques were used to determine the deposit VMD and deposition volume.(3)A model of the wake vortices of Thrush 510 G was established.RANS equations were solved using CFD techniques.The case of Duponcheel's vortices pair was calculated to validate the proposed wake vortices model,as well as the numerical analysis techniques.Atmospheric boundary layer flow was modelled using the data from the flight experiment.A model of the velocity field was established combing the atmospheric boundary layer flow model and the wake vortices model.Then the cases with and without crosswind were simulated to investigate the characteristics of the velocity field affecting aerial spraying.Trajectories of vortices are symmetry without the influence of crosswind.Downwash flow was induced in between the vortices,and upwash flow beyond the vortices.Both vortices were blown downwind by crosswind,with the port vortex decaying slower.Computational results were compared with velocity profiles obtained in the flight experiment.(4)A model of the rotary cage atomizer was added to the velocity field model to make the spray drift prediction model.Euler–Lagrangian techniques were used to predict the motion of droplets in the velocity field.Droplets between the vortices are driven down to the ground by the downwash velocity,while droplets near the wing-tips are enrolled into the vortices and move aside.Droplet deposition range were larger than the length of the spray boom,which indicates drift deposition by the vortices even without crosswind.For the case with crosswind,ground deposition agreed well with flight experiment results and AGDISP predictions,which validates the model and the numerical analysis procedure.(5)The spray drift prediction model was used to investigate the influence of aircraft wake vortices,atmospheric crosswind,temperature,relative humidity,and droplet sizes on the drift and deposition of spray droplets.The sources of spray drift were identified.The investigation could be used to optimize aerial spraying application and reduce spray drift.Droplet lifetime were calculated to determine the critical driftable droplet size,which could be used to choose nozzles before aerial spraying.Ground deposition ranges could be used to determine the offset of flight path.And the optimal atmospheric conditions could be determined according to the drift and deposition statistics.