Research on unmanned aerial vehicle (UAV) downwash has largely focused on spray applications, with millimeter-scale granular fertilizer spreading receiving considerably less attention. This study developed a coupled computational fluid dynamics and discrete element method (CFD-DEM) model to simulate wind field-particle interactions during fertilizer spreading, and its correctness was verified under specific parameters. Using this model, UAV downwash characteristics and their relationship with resultant deposition patterns were investigated, involving the three key parameters of the UAV rotor layout, flight speed, and fertilizer particle size. The results indicated that the axisymmetric rotor structure generated symmetric helical vortex tubes, which helped prevent skewness in the deposition pattern. The downwash wind field of four-rotor X-shaped under low-speed flight conditions manifested as four helical downward air columns. As flight speed increased, these columns tilted backward and subsequently detached from the ground, forming wake vortices. The symmetrical downwash airflow caused central deposition accumulation, reducing spreading range while inducing irregular patterns. Both reduced flight speeds and smaller particle sizes amplified these distortions: deposition patterns evolved from elliptical forms to irregular polygon at lower speeds/small size particles. Compared to no-wind conditions, the downwash reduced maximum effective width. Quantitative analysis via the relative deviation (RD) of lateral distribution curves evaluated the impact of wind field. The primary negative impact of downwash on spreading performance was a reduction in effective swath width. Based on the simulation results, it was reasonable to select the parameters of four-rotor X-shaped layout (RD = 19.45 %), flight speeds ≥ 5 m/s (RD ≤ 19.85 %) and fertilizer particle size ≥ 2 mm (RD ≤ 11.60 %) for operation. This study provides a valuable theoretical framework for predicting UAV fertilizer deposition patterns, while its broader applicability requires further validation across a wider parameter space, thereby contributing to the advancement of precision aerial application in agriculture.
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