Recently, multi-rotor unmanned aerial vehicle (UAV) becomes more and more irreplaceable in the field of plant protection. Multi-rotor unmanned plant protection machine has been widely used in pesticide spraying, pollination operation, and so on, however, the unclearness of wind field distribution leads to non-uniform deposition problem. In this paper, the influence of the down wash flow on the motion distribution of droplet group was studied for the multi-rotor UAV in hovering state. Based on the N-S equation and realizable k-ε turbulence model, the three-dimensional numerical model was established for six-rotor unmanned plant protection machine under 3 kg load condition. In the numerical calculation, 12 observation points were set at 2 heights directly below each rotor, and the wind velocity value at each point was observed. When the computing time reached 2.28 s, the wind velocity tended to be stable, and the 12 stable wind velocity pulsation values were obtained. The wind velocity of the down wash flow was tested one by one at the 12 observation points for the multi-rotor UAV in hovering state. Comparative analysis showed that the relative error of the velocity at the z direction between the experimental measurement and numerical simulation for characteristic points was less than 9% when the down wash flow was stable, and the reliability of the wind field numerical calculation was verified. Under the rotation of airfoil and the extrusion of the external atmospheric pressure, the "shrinkage-expansion-shrinkage" phenomenon appeared in the longitudinal main section;under the influence of wing interference caused by the opposite velocity of adjacent rotor, the turbulent effect of down wash flow was obvious, and the "airflow inlet" and "airflow outlet" region appeared between the wings area at the cross section. Droplet discrete phase was introduced to N-S equation, the momentum and energy equations were also corrected for continuous phase to establish two-phase flow model, then the two-phase flow model with droplet discrete phase was established for the nozzle, and the feasibility and validity of the two-phase flow model to calculate the trajectory of the droplets were also verified combined with the spray test. The advantages of easy boundary setting in numerical computation were used to discuss the influence law of droplet size on droplet motion for the atomization nozzle. Numerical results showed that the droplet distribution was basically circular, and the area of droplets with the highest concentration was the medium-sized droplet sedimentation area, which had the maximum differential ratio. Moreover, the numerical simulation results also showed that the smaller the droplet size, the smaller the droplet spray amplitude; the larger the droplet size of the droplet, the larger the final sub-velocity value at the vertical direction. Then the three-dimensional two-phase flow model was established for six-rotor unmanned plant protection machine, and the analysis showed that droplets were mainly distributed in the 3 "airflow inlet" areas and 3 "airflow outlet" areas where wing interference was obvious. The droplets in the inner side of down wash area were interlaced, and the horizontal travel of large particles was greater and they were distributed in the outside periphery of down wash area. When the diameter was less than 200 m, the droplet movement range could not cover all the "airflow inlet" area, and the droplets were mostly distributed in the center down wash area. When the diameter was greater than 250 m, the droplet motion region gradually covered all the "airflow inlet" and "airflow outlet". The result can lay a foundation for the research of drifting and deposition of droplets under the disturbance of vertical wind flow, down wash flow and instantaneous transverse wind during the flight spraying.%多旋翼植保无人飞机在农药喷洒和授粉作业等相关领域已开展广泛应用,但存在风场分布不明晰导致的分布不均问题.针对六旋翼植保无人飞机,结合雷诺平均N-S方程及Realizable k-ε湍流模型,建立了下洗气流三维数学模型,风场测试及非定常计算表明特征点z向速度的测量和模拟值相对误差在9%以内,验证了风场数值计算的可靠性;在机翼旋转诱导及外界气压的挤压下,下洗气流纵向主截面呈现出"收缩-扩张-再收缩"现象;"旋翼间干扰"使得下洗风场湍流效应明显,横截面的旋翼间区域出现了气流"引入"及"导出"区.引入雾滴离散相,并对连续相进行动量、能量方程修正,建立喷头含雾滴离散相的两相流模型,结合喷头喷幅试验来验证了该两相流模型计算雾滴运动轨迹的有效性;结果表明,粒径越小,雾滴水平方向分速度衰减越快,喷幅越小;雾滴粒径越大,竖直方向的最终分速度越大.建立了六旋翼植保无人飞机悬停条件下含雾滴离散相的三维两相流模型,计算分析表明,雾滴主要分布在"旋翼间干扰"明显的3个"引入区"、3个"导出区",下洗区内侧雾滴群交织,外侧大雾滴周向水平行程更大进而分布在外围;当雾滴粒径小于200 μm时,雾滴运动范围无法覆盖全部的"引入区",雾滴多分布在下洗区中心;当雾滴粒径大于250 μm时,雾滴运动区域逐渐覆盖所有"引入区"、"导出区".该研究可为飞行施药过程中迎风气流、下洗气流、瞬时横风耦合风场扰动下雾滴的漂移、沉积研究提供参考.
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