Numerical simulation of the downwash flow field and droplet movement from an unmanned helicopter for crop spraying

Abstract Unmanned helicopter-based spray systems are developing rapidly in Asia. However, droplet movement during aerial spraying remains poorly understood, owing to the complexity of downwash flow-structure development. Therefore, a computational fluid-dynamics method was used to investigate the downwash flow field of a full-scale unmanned agricultural helicopter (AF-25B; Copterworks) and the resultant movements of the spray droplets. A large-eddy simulation was performed and the lattice Boltzmann method was used to accurately capture the development of the rotor-tip vortex. To assess model performance, a hovering case with the height of 2.7 m was simulated and compared with previous field- and indoor-test results. The simulated instantaneous flow structures are qualitatively comparable with the indoor test results, supporting the reliability of the simulation method. Then, forward flight was simulated at heights of 1.5, 2.5, and 3.5 m above ground. In the forward-flight case, the flow speed on the right side was approximately 15% lower than that on the left side. The iso-surface of the vorticity was used to represent the asymmetric flow structures produced by the rotor. This flow structure was asymmetric, and the yaw angle, θ, was approximately 3–12° inclined to the right of the helicopter. When the application height was 1.5, 2.5, and 3.5 m, the angle of the edge of the flow structure, α, was about 105, 80, and 44°, respectively, and the entire flow structure width was 13, 9, and 6 m, respectively, and inversely proportional to the application heights. The temporal developments of the flow structures and droplet movements after the helicopter flew by were also simulated. The droplet expansion speed was found to have decreased when the application height increased. The droplet distribution was observed to become more asymmetric, with fewer droplets on the left side of the helicopter, when the application height was decreased. The coefficient of variation of deposition could, therefore, get reduced by the increase in the application height and cause the droplet deposit to be more uniform. However, the deposition quality may also get rapidly decreased as a consequence. A potential method to predict aerial-spray drift and deposition under complex conditions is presented in this study.