叶片风振响应空间运动姿态及雾滴沉积效果分析

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

    • 摘要: 风送喷雾技术可有效提升雾滴向冠层内部的输送能力,增强雾滴在树冠各处的覆盖均匀性,但气流对单一叶片的动态响应及雾滴沉积的细观作用效果还不清楚。该研究通过风洞内的稳定气流对单一梨树叶片进行风振响应试验,追踪叶片表面代表叶片形态的4个特征点,构建模拟叶片的三维空间运动模型,分析研究特征点的空间运动轨迹及叶面雾滴沉积效果。研究结果表明:低于临界风振响应速度,叶片基本稳定于一定抬升角位置,大于等于临界风振响应速度,叶片的稳定状态才会破坏,出现大幅度的摆动与翻转的复合运动响应,且运动响应具有周期性特征。对应5、7和9 m/s风速下,在一个周期内叶尖的空间运动总行程分别为180.61、1140.77、766.33 mm,叶片总扭转角分别为290.7°、896.8°、716.2°。雾滴沉积试验中,风速低于5m/s,雾滴主要沉积于面向气流方向的叶面,随着气流速度增大,正面雾滴沉积覆盖率由43.2%逐渐减小为10.3%;大于等于风速5m/s时,叶片正反两表面均有雾滴沉积,但雾滴主要沉积亲水性较好的叶片反面上。提出了应用叶片的空间运动瞬时位置的实际坐标构建模拟叶片位姿,可清晰体现叶片的空间位姿及运动过程。研究结果可进一步揭示风送喷雾过程中载雾气流与动态叶片的沉积作用机理。

       

      Abstract: 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.

       

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