CFD simulation and experimental verification of the spatial and temporal distributions of the downwash airflow of a quad-rotor agricultural UAV in hover

•The distribution characteristics of the downwash airflow were simulated based on CFD.•The downwash airflow develops downward in an approximately “cylindrical” shape.•The lower the height, the greater the coverage area of the z-direction velocity.•The relative error between the test value and the simulated value is within 10%. 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.