Numerical and Experimental Analysis of Downwash-Spray Interactions in Agricultural Sprayer UAVs

Abstract

The use of rotary-wing Unmanned Aerial Vehicles (UAVs) in agriculture for tasks such as pesticide spraying and field mapping is increasing. Their ability to target specific areas, minimize chemical waste, and adapt to diverse field conditions positions them as invaluable tools in precision farming. However, the airflow generated by the rotors can influence droplet dispersion, which in turn affects the efficacy of pesticide applications. This study examines how UAV design affects sprayer efficiency, with a particular focus on the interactions between rotor‑induced airflow and spray fluid behavior. Computational fluid dynamics (CFD) simulations are employed to assess the performance of various sprayer system configurations that change the number, spacing, and positioning of the equipped 110-degree fan nozzles. These simulations are then used to inform the design of a small experimental UAV and its integrated sprayer system. An initial literature review informs the simulation setup and the platform of the experimental UAV. An actuator disk approach is employed to model rotor-induced airflow in ANSYS Fluent, while spray behavior is simulated using the Discrete Phase Model. Particle tracking is used to find deposition area, density, and uniformity data for each configuration. These values for one configuration are compared to experimental results of deposition area and density obtained through the use of water sensitive paper for validation of the drawn conclusions. Results indicate that a two-nozzle setup in an under rotor configuration positioned beneath the adjacent front rotors of the UAV provides the most practical balance between fluid density, uniformity, and design practicality. The actuator disk method simplifies rotor modeling and allows the simulations to reasonably represent the final UAV despite the changes it undergoes. Future work will involve making the simulation setup more accurate to the UAV geometry while looking into the precise locations for nozzles in relation to rotors and observing how conclusions hold up when implemented at a commercial level.