High-accuracy, high-resolution downwash flow field measurements of an unmanned helicopter for precision agriculture

Abstract

Unmanned helicopters are used widely in precision agriculture, especially in East Asia. However, the development of the downwash flow field of unmanned helicopters, which strongly influences the movement of spray droplets in the air, is poorly understood. Therefore, an indoor measurement technique was developed to investigate the downwash flow field of a 1:10 scaled unmanned agricultural helicopter (AF-25B; Copterworks). High-speed particle image velocimetry was employed to obtain the instantaneous and average velocity field of the downwash flow, with a spatial resolution less than 0.005 m and temporal resolution less than 0.001 s. Results showed that radial distance of the point at which downwash flow reached the ground was proportional to the hovering height of the helicopter at a hovering height of z/R < 2.5 (where z is the rotor height above the ground, and R is the rotor radius), whereas the maximum radial velocity near the ground was inversely proportional to the hovering height. In addition, the spatial distribution of the turbulence kinetic energy at different hovering heights was determined. Intense fluctuations were concentrated along the outer edge of the main downwash flow and secondary vortex near the ground, which might result in more uniform droplet distribution. Based on the instantaneous analysis of the downwash flow structures, velocity profile induced by the rotor tip vortex, the moving tracks of the vortices, and decay rate were obtained. A typical burst-type flow structure in the downwash flow field was also observed, which may contribute to droplet drift. These indoor test results were comparable to previous field test results. This study offers high-accuracy indoor test data that can be used as a reference for future simulation studies.