CFD and field-validated investigation of forward-flight downwash airflow of a quadrotor over fruit trees at varying growth stages

The downwash airflow generated by multi-rotor plant protection UAVs during forward flight critically influences droplet deposition and drift control. While most previous studies have focused on hovering conditions or a single canopy stage, comprehensive forward-flight analyses across multiple phenological stages remain scarce. To bridge this gap, this study integrates transient multi-stage CFD simulations with field measurements to quantitatively characterize stage-specific downwash dynamics. A CFD model based on the RANS equations coupled with the SST k-ω turbulence model was established to simulate three-dimensional airflow at a height of 3.5 m above fruit canopies and flight speeds ranging from 1–3 m s⁻¹. Field validation using hot-wire anemometry confirmed strong agreement between simulations and measurements (average relative error = 12.3%, maximum < 20%). Results showed that at 1.5 m s⁻¹, the downwash penetration depth increased by 40% compared with 1.0 m s⁻¹, reaching 0.8 m in the half-leaf stage, and maximum vertical velocity reached 9.2 m s⁻¹. Distinct stage-dependent flow regimes were identified: sparse and uneven airflow at sprouting, deep and symmetric penetration at half-leaf, and uniform but weakened flow at full-leaf. These insights provide a scientific basis for optimizing UAV flight parameters—including flight speed, altitude, and swath width—to achieve more uniform spray coverage, reduced drift, and improved chemical utilization in precision orchard management.