Effects of application height and crosswind on the crop spraying performance of unmanned helicopters

Unmanned helicopters are increasingly used for crop protection in Asia. However, the moving mechanism of droplets from helicopters, which is strongly related to the complexity of the downwash flow, is still unclear. In this study, the lattice Boltzmann method was used to simulate the flow field and movement of the droplets released from the helicopter. The application height was set to 1.5, 2.5, and 3.5 m, while the crosswind speed was varied from -3 to 3 m/s. The vortex structures and downwash flow velocity field were found to be asymmetric, and the maximum downwash flow area was observed at the left side of the helicopter. The flow structure angle was inversely proportional to the application height and followed a second-order law with respect to the crosswind speed. The yaw angle was inversely proportional to the crosswind speed and was found to be more significantly affected by the crosswind speed at higher application heights. To study the effects of the droplet size on their movement in the downwash flow, droplets with diameters of 50, 100, 200, and 400 mu m were studied. The back view of the small droplets with diameter of 50 and 100 mu m has a double-peak pattern. Larger droplets, which were hardly moved up by the vortex, reached much nearer distances and floated at lower altitude. When the application height was 2.5 and 3.5 m, the droplets deposited nearly symmetrically at the cross wind with opposite direction. At an application height of 1.5 m, the droplets, especially the large ones (200 and 400 mu m), deposited asymmetrically at the cross wind with opposite direction. The cross wind blew from the left side due to a stream vortex near the ground on the right side of the helicopter, which carried the big droplets and move them back into the air. When the cross wind blew from the right side, large droplets (200 and 400 mu m) deposited faster and concentrated near the swath, while small droplets (50 and 100 mu m) were moved by the crosswind blowing from the right side with strong non-uniform spatial distribution and floated on the far left side. This finding could help to greatly reduce the spray drift. The droplet deposition rate is more likely to be affected by the quadratic term of the droplet diameter, while the coefficient of variation of deposition is affected by the quadratic terms of the droplet diameter and cross wind speed. The results of this study will be useful to analyze and optimize the downwash flow development and droplet movement/deposition of water from plant-protection unmanned aerial vehicles.