Simulation and verification of coupling characteristics of downwash airflow field and droplet field of agricultural unmanned aerial vehicle

• The coupling model of air-droplet two-phase flow at different rotor speeds is established. • The PIV experimental verification of CFD model based on rotor flow field. • The droplet velocity interval distribution diagram is displayed. • The law of droplet velocity attenuation is proposed. The downwash airflow generated by agricultural UAV rotors significantly affects droplet dispersion, thereby influencing spray performance and increasing the risk of drift. However, the coupled dynamics between rotor-induced airflow and droplet motion remain insufficiently understood. In this study, a two-phase CFD model incorporating various rotor speeds was developed and validated through particle image velocimetry (PIV) experiments, achieving fully coupled modeling of the entire pesticide spraying process of agricultural UAV. A functional relationship was established between rotor speed and droplet velocity distribution. The effects of rotor speed on droplet motion—velocity, velocity attenuation rate (K), and spatial distribution—were quantitatively analyzed. Results showed that the maximum droplet velocity occurred near the nozzle, reaching up to 20 m/s. As rotor speed increased, both droplet velocity and concentration increased significantly, while the velocity attenuation rate (K) decreased markedly. Within 1 m below the nozzle, all K values remained below 50 %. PIV measurements agreed well with CFD simulation results, with a minimum relative error of 1 %. The integration of CFD and PIV methods provides a reliable framework for studying the interaction between UAV rotor downwash and droplet dynamics. The findings offer theoretical support for optimizing UAV spraying parameters and improving pesticide application efficiency, contributing to enhanced precision in aerial plant protection operations.