Vertical distribution and vortex structure of rotor wind field under the influence of rice canopy

Abstract Rotor unmanned aerial vehicle (UAV) for plant protection has been widely applied in China in recent years. As a unique and crucial parameter of rotor UAV, the distribution shape and law of rotor wind field determine the spatial distribution of spraying droplets and influence the outcome of low-altitude pesticide spraying. However, the exploration of the interaction between rice canopy and wind field as well as the vertical decay mechanism is still an uncharted field in the study of three-dimensional wind field. In this paper, the author acquired vertical wind speed through S pitot sensor array, wind speed sensors were installed to acquire vertical wind speed data in the canopy, 30 cm beneath the canopy and 60 cm beneath the canopy. Contour maps at three height levels with wind speed of equal difference were drawn to calculate the equivalent area (the area surrounded by the contour line and the upper boundary) and maximum wind field width, all of which are direct parameters represent the distribution pattern of different wind speeds at equal time span. Under the condition that rice plants are well-distributed with consistent density, as the height decays arithmetically, coefficient variance (CV) of equivalent area is no more than 5% with basically equal reduction in equivalent area, the vertical wind field formed by the decayed airflow as it travels through rice plants distributes hierarchically. The decay rate of equivalent area and wind speed follow linear relation. When the height reduces from Ptop to Pmedium, the decay rate monotonically changes from 47.9% to 8.4%. When the height reduces from Pmedium to Pbottom, the decay rate monotonically changes from 53.7% to 22.4%. The lower the height is, the greater the decay rate is. Having analyzed the abnormal data ranges, the author found that the vortex formed under the interaction of wind field and rice canopy is the primary cause of abnormal data. Moreover, an ideal cone model was built on the basis of the rotor area and decay rate of equivalent area at equal height. The cone model not only justifies the vortex structure of rotor wind field but also provides theoretical foundation for the permeability study of droplets in rice canopy and the study of pollination parameters.