Abstract The plant protection effect of an agricultural unmanned aerial vehicle (UAV) with multiple rotors is closely related to its speed and direction and the spatial and temporal distributions of the effective coverage of the downwash airflow. By combining compressible Reynolds-averaged Navier-Stokes (RANS) equations, the shear stress transport (SST) k - ω turbulence model, sliding grid technology and the Semi-Implicit Method for Pressure Linked Equations (SIMPLE) algorithm, this paper establishes a computational fluid dynamics (CFD) model of the downwash airflow of the quad-rotor agricultural UAV in hover and discusses the distribution of the downwash airflow in the spatial and temporal dimensions. The simulation results show that most areas of the downwash airflow tend to be stable within 4.5 s, and due to the downward flow characteristics after the stabilization, the z-direction (perpendicular to the ground) velocity of the downwash airflow directly under the rotor is the highest with a maximum value of −8.96 m/s, which accounts for the main part of the downwash flow. The rotor downwash airflow flows downward in spiral form and the phenomenon of “contraction and expansion” is obvious during the downwash flow. Closer to the ground, the coverage area of the z-direction velocity is larger, the pressure distribution is more uniform, and the airflow exhibits an “upwash” phenomenon due to the “ground effect”. The test and simulation values at test points from 1 m to 2.5 m and 0.75 m exhibit the same variation trend with maximum relative errors of 10% and 15%, and the numerical simulation is accurate.

中文翻译：

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悬停四旋翼农用无人机下洗气流时空分布的CFD模拟与实验验证

摘要 农用多旋翼无人机的植保效果与其速度、方向以及下洗气流有效覆盖的时空分布密切相关。本文通过结合可压缩雷诺平均纳维-斯托克斯 (RANS) 方程、剪应力输运 (SST) k - ω 湍流模型、滑动网格技术和压力关联方程半隐式方法 (SIMPLE) 算法，建立了计算悬停四旋翼农用无人机下洗气流的流体动力学 (CFD) 模型，并讨论了下洗气流在空间和时间维度上的分布。仿真结果表明，下洗气流的大部分区域在 4.5 s 内趋于稳定，并且由于稳定后的向下流动特性，转子正下方的下洗气流的z向（垂直于地面）速度最高，最大值为-8.96 m/s，占主要部分的下洗流。旋翼下洗气流呈螺旋状向下流动，下洗气流中“收缩和膨胀”现象明显。离地面越近，z向速度覆盖面积越大，压力分布越均匀，气流因“地面效应”而呈现“上冲”现象。1 m~2.5 m和0.75 m测试点的测试模拟值变化趋势相同，最大相对误差分别为10%和15%，数值模拟准确。转子正下方的下洗气流的z向（垂直于地面）速度最高，最大值为-8.96 m/s，占下洗气流的主要部分。旋翼下洗气流呈螺旋状向下流动，下洗气流中“收缩和膨胀”现象明显。离地面越近，z向速度覆盖面积越大，压力分布越均匀，气流因“地面效应”而呈现“上冲”现象。1 m~2.5 m和0.75 m测试点的测试模拟值变化趋势相同，最大相对误差分别为10%和15%，数值模拟准确。转子正下方的下洗气流的z向（垂直于地面）速度最高，最大值为-8.96 m/s，占下洗气流的主要部分。旋翼下洗气流呈螺旋状向下流动，下洗气流中“收缩和膨胀”现象明显。离地面越近，z向速度覆盖面积越大，压力分布越均匀，气流因“地面效应”而呈现“上冲”现象。1 m~2.5 m和0.75 m测试点的测试模拟值变化趋势相同，最大相对误差分别为10%和15%，数值模拟准确。