Effects of Rotor-Induced Downwash and Crosswind on Downstream Droplet Size and Velocity in Agricultural UAV Spraying

To clarify the effects of rotor-induced downwash and crosswind on liquid sheet breakup and spray atomization characteristics of agricultural UAVs, an experimental platform integrating particle image velocimetry (PIV), a UAV spray system, and a wind tunnel was established. The droplet size and velocity characteristics of a flat-fan nozzle were investigated under different rotor speeds, crosswind conditions and spray pressures. The results showed that rotor-induced airflow significantly altered the post-breakup droplet characteristics. As the rotor speed increased from 0 to 2200 rpm, the volume median diameter (DV0.5) increased from 206.45 to 245.06 μm (18.7%), while the volume fraction of droplets smaller than 150 μm (V<150 (%vol)) decreased from 12.86% to 10.26%, indicating a shift toward coarser droplets under stronger downwash. Crosswind exhibited a limited influence on the primary breakup process but substantially modified droplet transport. Without rotor operation, increasing crosswind velocity from 0 to 6 m/s reduced the mean horizontal droplet velocity by 77.0%, promoting lateral droplet displacement. Under rotor operation at 2000 rpm, the downwash effectively enhanced spray plume stability and mitigated crosswind-induced distortion. Furthermore, increasing spray pressure from 0.10 to 0.50 MPa reduced DV0.5 from 274.29 to 222.24 μm and increased the proportion of fine droplets, demonstrating that spray pressure was the dominant factor controlling primary atomization. Overall, rotor-induced airflow primarily regulated droplet redistribution after atomization, whereas crosswind mainly affected droplet transport behavior. These findings provide theoretical guidance for optimizing UAV spray parameters and improving precision pesticide application efficiency.