Analysis of spatial motion attitude and droplet deposition effect of tree leaves in response to wind vibration

Air-delivered spraying can enhance the effective transport and uniform coverage of fog droplets into the interior of the tree crown. However, it is still unclear on the effect of the intensity of air-delivered flow on the response of a single leaf to the motion state and the microcosmic settlement of fog drop. In this study, a single leaf of a pear tree was placed under the stable air flow in the test section of the wind tunnel equipment. The wind vibration response test was carried out on the leaves of a single pear tree. Four feature points were then marked on the leaves, in order to represent the leaf morphology. Binocular high-speed cameras were used to track the feature points on the leaves. The three-dimensional spatial motion model of simulated leaves was constructed to decompose into several sub-stages for subsequent analysis. A systematic analysis was made on the spatial motion trajectory of feature points, in order to explore the effect of droplet deposition on the leaf surface. The research results were as follows. The leaves showed static stability, swing, and a large number of twisting and turning complex motions under the action of wind-driven air flow. The critical response speed of wind vibration was defined as the generation of a large number of twisting motions. Once the wind speed was lower than the critical airflow speed of wind vibration, the leaves were basically lifted at the angle position in a static and stable state. When the air velocity was greater than or equal to the critical, the dynamic stable state of the leaves was destroyed into a compound motion response state of large spatial swing, turnover, and torsion, indicating the periodic motion state of the leaves under the action of strong air. Specifically, the total travel distances of leaf tips in a space movement period were 180.61, 1140.77, and 766.33 mm, respectively, at the speed of 5, 7, and 9 m/s, and the torsion angles were 290.7°, 896.8°, and 716.2°, respectively. In the test of leaf droplet deposition, the spray droplets were deposited mainly on the side of the leaf facing the airflow direction, when the airflow velocity was lower than 5 m/s. There was also an increase in the airflow velocity. The coverage rate of droplet deposition on the front of the leaf gradually decreased from 43.2% to 10.3%. When the airflow velocity was greater than the critical wind response velocity of 5 m/s, the fog droplets were deposited on both sides of the leaves. However, the droplets were deposited mainly on the opposite side of the leaves with better hydrophilicity. The spatial pose and motion of leaves were also simulated, according to the actual coordinates in the instantaneous position of leaves under the space motion. The finding can further reveal the interaction and deposition between the air flow carrying fog droplets and the leaves in the air-driven sprayer.