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Recently,multi-rotor unmanned aerial vehicle(UAV)becomes more and more significantly irreplaceable in the field of plant protection against diseases,pests and weeds of crops.The easy takeoff and landing performance,hover function and high spraying efficiency of UAV are urgently required to spray pesticide for crop timely and effectively,especially in dispersed plots and hilly mountains.In such situations,the current research about UAVspray application focuses on studying the influence of the UAV spraying parameters on the droplet deposition effect,such as operation height,operation velocity and wind velocity.The deposition and distribution of pesticide droplets on crops which depends on installation position of nozzle and airflow distribution characteristics of UAV are directly related to the control effect of pesticide and crop growth in different growth periods.As a preliminary step,this paper focuses on the research of the dynamic development law and distribution characteristics of the downwash air flow for the SLK-5 six-rotor agricultural UAV.Based on compressible Reynolds-averaged Navier-Stokes(RANS)equations with an RNG ? ?? turbulence model and dynamic mesh technology,the efficient three-dimensional computational fluid dynamics(CFD)method was established to analyze the flow field distribution characteristics of UAV in hover.Then the unsteady interaction flow field of the wing was investigated in detail.The downwash wind speed of the marked points for the SLK-5 UAVin hover was also tested by weather tracker.It was found that the maximum velocity value of the downwash flow was close to 10m/s; the Z direction velocity was the main body of the wind velocity in the downwash airflow,and the comparison of the wind velocity experiment test and simulation showed that the relative error was less than 12%between the experimental and simulated values of the z direction velocity at the marked points.Through the numerical simulation it was found that there was a clear "wing interference" phenomenon between the wings; the influence of wing interference on the velocity distribution was so significant in the "xoz longitudinal section" that the "two velocity peaks" in the section were completely different when the vertical height fromthe wing was more than 0.6m,while the velocity components of x,y,and z directions were substantially symmetrical at different height in the "yoz longitudinal section"; with the distance from the wings getting farther and farther,downwash air flow gradually weakens,and the velocity distribution tended to be uniform on the "xoy radial cross section".The phenomenon of "discontinuous phenomenon" appears during the development of the downwash air flow,and the wind field was basically stable after the calculation of 2.3722s.In view of the wing interference and the wind field distribution,the proposed spraying height for the SLK-5 agricultural UAVwas about 0.6m.