## 1 Introduction  
According to the data from the National Bureau of Statistics of the People's Republic of China, the fruit planting area had reached 126.46 million hectares in 2020, and the fruit output had reached 286.92 million tons in China [1]. In the management of orchards, plant protection is important to ensure sustainable and steady development of fruit yield, and its workload accounts are about 25% of the total workload [2]. At present, orchard plant protection mainly depends on spraying chemical pesticides to control diseases and pests. Advanced pesticide application technology and plant protection machines are important to improve the pesticide utilization rate and work efficiency. Mechanical facilities have been widely used in developed countries to achieve professional plant protection. With the acceleration of urbanization in China and the transfer of rural labor, the costs of artificial application of drugs increase, and the work efficiency is low, which has caused the reduction in fruit planting benefits and the slow development of the fruit industry. The mechanical application of pesticides is an inexorable trend for the development of orchard plant protection in China in the future [3]. In order to reduce labor intensity and improve work efficiency, air-assisted spraying is widely used. By using the strong airflow generated by a high-speed fan, the droplets atomized by the pesticide pump and sprayer are blown to the fruit trees' canopy to eliminate pests [4]. The technology can not only ensure the spray distance but also enhance the penetration and deposition uniformity of the droplets [5]. With the development of technology, plant protection machines have also been developed rapidly. At present, the air-assisted sprayer is the most widely used in orchard plant protection, and its core components are the fan and diversion device [6]. In developed countries, air-assisted spraying technology is more advanced [7; 8], and the machines' performance is more reliable [9]. There are many famous manufacturers such as the CAFFINI company, MUNCKHOF company, and HARDI company. These sprayers made by these companies are suitable for standardized orchards. In China, various air-assisted sprayers [10] were developed according to the characteristics of orchard planting. In addition to the circular sprinkler, some scholars have improved various diversion devices, such as vertical sprinklers, multi-flexible ducts, vertical tubes, and butterfly bellows [11], which meet the needs of various orchards [12]. However, due to the continuous spray mode and the strong airflow, the problem of off-target drift still exists, which has caused pesticide waste and environmental pollution. Therefore, it is necessary to further research on improving the anti-drift ability of sprayers. Many researchers have conducted a lot of research on the drift phenomenon and its influencing factors, including the spray parameters [13; 14], airflow parameters [15; 16], and other factors [17]. By analyzing the droplets' drift process, [PERSON] [18] concluded that the main factors were the spray method and droplet size and proposed that protective shields and buffer zones should be used to reduce droplet drift. [PERSON] [19] designed a leaf-like sensor to simulate the swing of branches and leaves and changed the spray speed and the air outlet size to explore their effect on the airflow distribution, which provided a basis for further research on the airflow distribution in the canopy. By using artificial fruit trees to simulate the target canopy, [PERSON]. [20] compared the wind velocity and air volume entering the canopy when the sprayer was moving and stationary, respectively. The result showed that the air distribution on both sides of the axial fan was not uniform, which affected the droplets' deposition. [PERSON] [21] designed a multi-channel air-assisted sprayer, in which the air outlet position was changed to realize air volume adjustment and improve droplet coverage and uniformity.