Research On Movement Regularity Of Downwash Wind Field And Spray Field Of Multi-rotor Plant Protection UAV

It is difficult for ground spraying equipment to pass through complex terrain,and plant protection operations in hilly and mountainous areas are mainly carried out by manual application,which is costly and inefficient.With the development of agricultural aviation technology,the Unmanned Aerial Vehicle(UAV)is widely used because of its high operational efficiency,flexibility and lack of terrain control,etc.The downwash wind field generated during its operation is the main factor affecting the movement,deposition and drift of pesticide droplets.It is important to study the downwash air flow characteristics and droplet movement patterns of UAVs to improve the efficiency of pesticide spraying and reduce surface source pollution.This paper is based on computational fluid dynamics(CFD)and uses a four-axis,eight-rotor UAV as the research object to investigate the motion of its wind and spray fields under hovering conditions.The specific objectives were as follows:(1)Numerical simulation and experimental verification of the spatiotemporal distribution characteristics of the downwash airflow generated by a quadcopter agricultural spraying drone during hovering are presented.A three-dimensional computational fluid dynamics model is established based on the Reynolds-averaged Navier-Stokes(RANS)equations,Shear Stress Transport(SST)k-ω turbulence model,and sliding mesh technique to investigate the spatiotemporal distribution and separation characteristics of the downwash airflow and analyze its effects on the movement of spray droplets.The downwash airflow undergoes a transition from unsteady to steady state over time and generates vortex flow features,such as the wake vortex,ground diffusion flow,and vortex ring,during its evolution.The vortex characteristics reveal the existence of a negative value channel in the center region of the rotor’s downwash airflow.As the distance from the rotor increases,the area of the wake vortex flow becomes larger,causing adjacent wake vortices to gradually couple together.However,the "spiral effect" of the downwash airflow is compensated by the mutual coupling effect of the up and down rotor airflow.When the spray nozzle is installed in the area directly below the rotor,the droplet deposition distribution is better than when installed on the central axis,indicating that droplets located directly below the rotor can avoid the coupled area of the wake vortex,effectively reducing droplet drift.The experimental results show that the measured downwash airflow velocities have consistent trends with the simulation results,with a minimum relative error of 4% and a maximum relative error of29%.(2)Study on the Influence of Fruit Tree Canopy on the Aerodynamic Characteristics of UAV Downwash and Spray Field.A three-dimensional CFD model was established to investigate the spatial and temporal distribution of downwash airflow generated by a quadcopter octocopter agricultural UAV hovering above fruit trees,combining RANs equations,SST k-ω turbulence model,porous media model,and slip mesh technique.The downwash airflow from the rotors reached the surface of the canopy in a symmetrical pattern,enveloping the target fruit tree canopy.The airflow velocity at the center of the canopy was weakened due to the negative value channel,forming four evenly distributed high-speed regions around the canopy after reaching the target.When the airflow in the z direction first encountered the canopy,a portion of it was blocked and bounced back,diffusing along the x and y directions,causing the originally independent high-speed airflow vortices to merge earlier compared to the case without fruit trees.The peak velocity reached 6.12 m/s,and there were still small negative velocity values in the central region.As the distance from the rotors increased,the frequency of velocity changes in the x and y directions slowed down,and the diffusion range of the airflow increased,leading to an enlargement of the wake vortex airflow area.Experiments showed that the measured values of wind field velocity had an error of mostly within 25% compared to the simulated values,and the relative error between the measured and simulated values of spray deposition in the spray field was no more than30%.The airflow velocity distribution inside the experimental fruit tree canopy was in good agreement with the simulated fruit tree canopy airflow velocity distribution.