Experimental Analysis of the Downwash Airflow Created by a Single Rotor Blade in Agricultural Drones

Purpose: The downwash flow produced by agricultural drones has a great influence on the coverage ratio and the drift potential of pesticide spray droplets. Therefore, the configuration of the boom and the position of the nozzle of the pesticide injection system should be adjusted on the basis of the characteristics of the downwash flow. However, although many studies have been conducted, few studies have quantitatively investigated the characteristics of downwash flow. In this study, the average characteristics and turbulence of the downwash flow formed by a single blade were quantitatively evaluated. Methods: The single rotor blade used in the test had a radius of 370 mm, and this length was used to normalize the measurement positions. Downwash flow was measured at 400 mm × 400 mm at 10-mm intervals in the X-Y plane and for 6 measurement planes at 100-mm intervals along the Z-direction. The rotational speed of the blade was set to 2000, 2500, and 3000 rpm, and the corresponding blade tip velocities were 71.17 m/s, 88.97 m/s, and 106.76 m/s, respectively; these values were used to normalize the mean and turbulent velocity components. The velocity of downwash flow was measured using a constant temperature anemometer (CTA)-type hot-wire anemometry with an X-type probe, and data were collected at a sampling rate of 30 kHz for analysis of the turbulence characteristics. Results: From this experimental study, the axial, radial, and tangential velocity components of the downwash flow, based on the maximum value, were 20%, 4.8%, and 5.9% of the blade tip velocity (V_Btip), respectively. In addition, the turbulence intensity of the axial, radial, and tangential velocity components was 8.5%, 3.0%, and 4.5%, respectively, based on the maximum value, and the axial component was approximately twice as high as the other two velocity components. High turbulent intensity appeared near the shear flow region at high axial velocities. Conclusion: The downwash flow generated by a single rotor blade showed a steep velocity gradient with the blade length and axial distance, and the turbulent intensity was high in the shear layer region of the mean flow field.