CFD Modeling Research On Orchard Spatial Airflow Field And Interaction Between Airflow And Tree For Orchard Spraying

Most of orchards have the characteristics of large canopy and dense branches and leaves.In order to improve the penetration of liquid,air-assisted spraying technology is widely used in the world.If the airflow of the sprayer is too large,it will cause a large number of pesticide drift problems.If the airflow is too small,it will not be able to run through the canopy of fruit trees,and cannot achieve the purpose of pest control.The research and development of airflow control technology and equipment is based on the study of airflow field distribution and interaction between airflow and fruit tree canopy.CFD uses computer to quickly calculate the approximate solution of fluid control equation,which can be used for efficient visualization modeling of airflow field.According to the requirements of airflow field measurement and modeling verification of orchard spraying,a three-dimensional airflow field measurement system was designed.Based on CFD,the spatial airflow field model of orchard sprayer was established and verified.Based on the porous medium model,a CFD model of interaction between airflow field and fruit tree canopy was established,and the validity of the model was verified by the measurement of airflow field in the canopy.The interactive CFD model of spatial airflow field and wind tree can provide model support for airflow control of orchard spraying.The main research contents are as follows.(1)In view of the characteristics of the airflow generated by tower-type air-assisted sprayer,the complex wind and environment conditions,a test bench for the spatial airflow field was designed.The test bench consists of three subsystems:airflow measurement system,meteorological station and reference platform.The airflow measurement system is the core part.The air velocity measurements were carried out by using hot-film anemometer,moving along the guide rail in a straight line driven by the stepping motor,measuring air velocity with a horizontal interval of 2cm to evaluate the complex airflow uniformity.The width of the measuring plane is 5m,and the height is 3m.The resolution of the anemometer is 0.001m/s with an air velocity range of 0.3-30m/s.The meteorological station is composed by ultrasonic anemometer and upper computer,to measure wind speed and direction,during measuring process.The range of the anemometer of air velocity is 0-60m/s,with a wind direction range of 0?-359?.The benchmark is used to ensure the accurate positioning of sampling points,and the positioning accuracy is less than 1cm.The system can measure the airflow field quickly and accurately.(2)A CFD model for simulating the airflow produced by tower-type air-assisted sprayer was developed.Field testing of spatial airflow field was carried out.CFD models were developed with a commercial CFD program Fluent 16.0.Cells in the computational domain were created using ICEM CFD software.The influence of different air velocity inlet on CFD simulation are compared.Based on the RANS governing equations,the SIMPLE algorithm and the second-order upwind discrete scheme,the spatial airflow field calculated by different turbulence models are compared.Through the analysis of the determination coefficient between the simulating velocity and the measuring velocity,the results show that the modeling method,using UDF to put profiles of measured jet velocity components,including horizontal velocity,vertical velocity and lateral velocity,with Standard k-?turbulence model,is the most accurate and simple airflow field modeling method among the 12 models.CFD simulations made it possible to estimate fluid velocities at 37%of the measuring points,which shows that the CFD modeling method can reasonably predict the airflow distribution discharged from air-assisted sprayer.(3)CFD modeling and experiment of interaction between airflow field and fruit tree canopy was carried out.The canopy was established based on porous medium model using Fluent 16.0.UDFs were developed and used to set proper pressure loss coefficient C_ir into cells,define momentum source term,turbulence source term and turbulence quantities for tree canopy.The modeling method is simple and general CFD modeling method.The model was able to predict accurately the air velocity inside the canopy with relative errors of 24%,obtained with the model and those obtained with experiment test,a total of 105 data,which below the error of usual validation criterion of CFD model.This study can conclude that the simulation achieve the expected air distributions inside tree canopy.