With the increasing application of plant protection unmanned aerial vehicle (UAV) in precision agriculture, the numerical simulation methods for the development of the downwash flow field of the plant protection UAV and the deposition and drift process of droplets affected by the downwash flow field have achieved rapid and diversified development, but the advantages, disadvantages, scope of application, and verification of each method still lack a systematic review. This article discusses the inviscid model, computational fluid dynamics model and lattice Boltzmann model (LBM) respectively. The advantage of the inviscid wake vortex model based on the vortex element method is that the calculation process is simple. Moreover, integrated with the most widely used aerial spray drift prediction software AGricultural DISPersal (AGDISP), it can be a promising way to do real-time UAV spray drift prediction. But due to lack of viscosity and turbulence models, the droplet deposition and drift simulation accuracy of inviscid model is relatively lower than other models. The computational fluid dynamics (CFD) model includes the finite volume method (FVM) and the finite difference method (FDM). The FVM in the computational fluid dynamics model has high robustness and can be applied to the simulation of various complex environments. Many commercial CFD software are based on FVM and achieved a fast development in aerial spray modeling recently. However, the FVM is greatly affected by the quality of the mesh, and its commonly used upwind style has limited accuracy (second-order accuracy). Under the same mesh density, it is easier to generate artificial dissipation when simulating the rotor tip vortex than the finite difference method. As a result, the simulated rotor tip vortex dissipation speed is much faster than the actual situation. Compared with the FVM, the structured grid used in the FDM is easier to construct a high-order precision numerical format. Which can reach 4-5 orders of accuracy, and with adaptive grid technology, FDM can simulate the evolution of rotor tip vortex with high temporal and spatial accuracy, and can reproduce the typical flow structure development process of the real rotor downwash flow field. However, it also has problems such as high grid structure requirements and excessive computing power requirements. LBM has advantages in computing three-dimensional flow field problems with complex boundary conditions and non-stationary moving objects. However, there are still shortcomings in its functional diversity and completeness. The accuracy of the numerical models mentioned above still needs field test and indoor experiment such as high-speed Particle Image Velocimetry (PIV)/ Phase Doppler Interferometry (PDI) method to verify and optimize. The authors finally pointed out the future direction of plant protection UAV application simulation and verification.
Unmanned Aerial Vehicle(UAV) application in agriculture and forestry has the unique advantages of high efficiency, water and pesticide saving, and strong adaptability to complex terrain. The application research of UAV in agriculture and forestry has shown a fast growing trend. In order to explore the research hotspots and the scientific impact of countries/regions and institutions on UAV application in agriculture and forestry, the relevant literatures in the Web of Science(WoS) core collection database (2011-2020) were collected. The bibliometrics analysis was performed on the journal articles of UAV application in agriculture and forestry based on VOSviewer, WoS analysis tools and Microsoft Excel. The analysis results showed that the number of published papers increased rapidly since 2017, the researches on UAV application in agriculture and forestry were carried out in 94 countries/regions, including1778 institutions. Due to the strong scientific research group in the application of UAV in agriculture and forestry of the United States, China and Australia, a large number of papers had been published, resulting in a great academic influence. Remote sensing was the most widely used technology field of UAV application in agriculture and forestry, mainly involving remote sensing technology, ecological environment science, image processing technology, geological science, etc. Engineering was an important technical field of UAV application in agriculture and forestry, mainly involving control technology, sensor technology and fluid computing modeling technology related to UAV aerial pesticide application.There were 1508 articles and reviews been published in 398 journals, about 1.90% of all journals included in WoS core collection database, indicating that more and more journals paid attention to the application research of UAV in agriculture and forestry. Remote Sensing sponsored by MDPI (Multidisciplinary Digital Publishing Institute) was the journal that published the most of papers, the most cited paper mainly focused on the research status of UAV system in photogrammetry and remote sensing, including sensing, navigation, positioning and general data processing, etc. In addition, the analysis of the research hotspots of UAV application in agriculture and forestry showed that UAV pesticide application, UAV remote sensing of diseases and pests, plant phenotype acquisition were the research hotspots. This study can provide references for innovation research and cooperation between research teams of UAV application in agriculture and forestry.
With the continuous increase of the spraying area, the problem of droplet drift risk in the spraying process of UAV is becoming increasingly prominent, especially the herbicide drift. In order to clarify the effect of the herbicide solution on the droplet size and the deposition and drift distribution characteristics sprayed by UAVs, the droplet sizes of 15 herbicide solutions sprayed by the centrifugal rotary atomizer nozzle installed in the plant protection UAV were measured in the laboratory, and the distribution of droplet deposition and drift in the spraying area and drift area were measured by adding a fluorescent tracer (60 g/hm2) to the tank in the field. The results showed that the herbicide solution had a significant effect on the droplet size distribution. The DV50 of all the other solutions was reduced after sprayed by the centrifugal atomizer except the Carfentrazone-ethyl water dispersible granule, and the maximum decrease ratio was 22.0%. The proportion of small droplets (V<150 μm) increased, with the maximum value of 50.8%. When the environmental crosswind speed was 3.76 m/s, the coverage and number of droplets in the spraying area were only 41.3% and 42.2% of that at 0.74 m/s, and the deposition uniformity was significantly reduced. In the drift zone, the deposition amount of droplets was under 10% of in-swath zone at the downwind of 12 m, and the deposition of all the treatments at 50 m was lower than detection limits (0.0002 μL/cm2). The drift ratio increased with the wind speed increased. When the crosswind speed reached 3.76 m/s, the drift ratio of droplets was 46.4%. Under different crosswind, 90% of the total measured spray drift were 4.8?22.4 m. By fitting the deposition in the drift zone with drift distance and crosswind speed, the downwind deposition was proportional to the crosswind speed. This study provides data support for droplet drift distance of plant protection UAV spraying in wheat fields at different wind speeds in winter and provides a basis for spray drift buffer zone, drift risk assessment, and relevant standard formulation.
To explore the effect of pesticide-adjuvants mixture on the control efficacy against Rice Chilo Suppressalis(walker). This study designed a three-factor, three-level orthogonal experiments with pesticides (10% emamectin benzoate·indoxacard SC, 5% chlorantraniliprole SC and 0.8% rotenone SC), adjuvants(organosilicon, lecithin and mineral Oil) , spray volume (21,24 and 27 L/hm2), referred to the three-factor, three-level orthogonal experimental scheme. And made the blank factor the deviation to analyze its rationality. Analysis of variance (ANOVA) statistical method was used to analyze the significance level of each factor. Duncan's new multiple range test (DMRT) method was used to analyze the order of the influence of different levels of each factor on the control efficacy against Rice Chilo Suppressalis(walker). The results showed that, under the experiment conditions of this research, the mean square value of the deviation factor was smaller than the mean square value of the pesticides, the adjuvants and the spray volume, and the deviation of the orthogonal experiment was within a reasonable range. The main order of the effect of the three factors on the control efficacy of Rice Chilo Suppressalis(walker) was: adjuvants > pesticides > spray volumn. On the 14th day after spraying, pesticides showed a significant effect on the control efficacy (P<0.05) and adjuvants showed a highly significant effect on the control efficacy (P<0.01), and spray volume showed no significant effect on the control efficacy. On the 14th day after spraying, the level 3 of the factor "pesticides" was more effective, in the order of Rotenone > Chlorantraniliprole > Emamectin Benzoate·Indoxacard. The level 1 of the factor "adjuvants" was more effective, in the order of Organosilicon > Lecithin > Mineral Oil. The level 3 of the factor "spray volume" was more effective, in the order of 27 L/hm2 > 24 L/hm2 > 21 L/hm2. Therefore, a preferred pesticide-adjuvants mixture method was 0.8% rotenone SC, organosilicon adjuvants and 27 L/hm2 of spray volume, which had a rapid and long-lasting control efficacy, and its control efficacy in the field reached 81.45% on the 14th day after spraying. Additionally, there was also a satisfactory pesticide-adjuvants mixture method that was 5% Chlorantraniliprole, organosilicon adjuvants and 24 L/hm2 of spray volume. This mixture method also performed well, achieving 79.3% control efficacy in the field on the 14th day after spraying. This study could provide a reference for the optimization of the mixture methods of solutions (pesticides, adjuvants and spray volume) for controlling Rice Chilo Suppressalis(walker).
Variable spray technology is an important means to improve pesticide utilization rate and save pesticide. Fruit tree is a kind of three-dimensional space, and the densities of branches and leaves in the canopy of fruit trees at different locations are different at the same time. The ideal state of spray is to adjust the amount of spray according to local characteristics, so as to realize the application of the spray on the canopy of fruit trees as required and improve the utilization rate of pesticide. In order to achieve the effect of reducing the dosage and increasing the efficiency of pesticide application, a variable spray control system was developed and the methods for computing leaf area density parameter and pulse width modulation(PWM)'s duty ratio of actuators were proposed. As the dosage parameter, the leaf area density was derived based on the point cloud density detected by LiDAR sensor on the upper computer. Then PWM's duty ratio was calculated based on the leaf area density and sent to the slave computer-PLC in real time. The communication between upper and slave computer was carried out through RS485 standard. So the spray flow of each nozzle was controlled by the switching frequency of the solenoid valve with PWM's duty ratio signal. Key parameters were obtained by the test including the net size of spray unit, delay time of the system and the function relationship between the PWM's duty ratio and the spray flow of nozzle. The test results showed that there was a linear relationship between the PWM's duty ratio and the spray flow of nozzle under the pressure of 0.2, 0.3 and 0.4 MPa, and the linear goodness of fit were all above 0.98. Finally, the effectiveness of the variable spray system was verified by the spray test. The test results showed that the minimum number of droplets per unit area (cm2) on the water-sensitive paper was 35 drops at the sampling point, which was higher than the 25 drops defined by the common method for the spray amplitude of aerosol in the air supply spray. Under 39.9% of the canopy ratio between the target canopy area and the whole area, the variable spraying mode saved 71.96% of the pesticide dosage compared with the continuous spraying mode, and 29.72% compared with the target spraying mode, achieving the dose reduction effect.
The tower-type sprayer produces swirling and irregular vertical airstream. The complex swirling results in airflow asymmetry between sides of the sprayer, and the vertical air velocity profile can be unpredictable when the rotational speed of the fan changes. The spray deposition is directly linked to the airflow pattern obtained from the sprayers. In order to study airflow field of this type of air-assisted sprayer, a CFD (Computational Fluid Dynamics) model for the tower-type sprayer was developed. A boundary condition setting method of UDF (User-Defined Function) sectional 3D air velocity was proposed. And the influences of turbulence models and the size of computational domain on CFD airflow simulation were studied. Using Fluent software, three different CFD models were established. The Model 1 took the average air velocity of 11 regions as the velocity inlet. The Model 2 used UDF segmented three-dimension air velocity line as the boundary condition. In order to further study the influence of the computational domain size on simulation, the Model 3 with a smaller computational domain was established. The turbulence model based on reynolds-averaged navier-stokes (RANS) control equation was used to calculate the airflow field in all models. In order to verify the reliability of the model, a three-dimensional measurement system of airflow field was designed, which was used for accurate and fast velocity measurement. The results showed that the Standard k-ε turbulence model, Realizable k-ε turbulence model, BSL k-w turbulence model, SST k-w turbulence model were suitable, and the Standard k-ε turbulence model was the best one. The CFD boundary condition setting method of UDF sectional three-dimension air velocity could improve the accuracy of simulation, and reduce the calculation complexity. With the same settings of other parameters, the performance of the CFD model with larger scale calculation domain was slightly better than that with smaller computational domain. The size of computational domain should be set to the appropriate extent, considering the calculation capacity and practical requirements of modelling. The research results could provide an important reference for CFD modeling of spray airflow field.
In order to solve the problems of low transmission efficiency, high carbon emissions, environmental pollution, low intelligence, and poor flexibility in traditional fuel-driven and front-wheel steering high ground clearance sprayers, a new type of high ground clearance four-wheel independent drive (4WID) sprayer which was suitable for the unmanned driving was proposed in this research. The sprayer adopted the hybrid power of fuel and battery and was steered by the 4WID driving mode of the front and rear double steering axles. For this reason, the turning radius of the proposed 4WID sprayer was small, and the running track of the front and rear wheels were uniform in height, which reduced the phenomenon of seedling crushing during field plant protection operations. Considering the slippage and sinking of the driving wheel in the extremely complex operating environment of the paddy field, based on the linear time-varying kinematics model (LTV) of the sprayer, a layered path tracking control considering the slippage of the driving wheel was constructed. The upper model predictive controller (MPC) obtained the steering angle and movement speed of the sprayer according to the expected path and the current position of the vehicle to realize path tracking. The lower layer used fuzzy control and integral separation PID control to construct a driving wheel slip controller, so as to achieve effective control of path tracking, speed, and driving wheel slip, which improved the stability and path tracking accuracy of the sprayer in a complex operating environment. The co-simulation results of Adams and Matlab showed that under complex working conditions, the slip rate of the driving wheel of the sprayer was controlled within ±20%, so as to prevent excessive slip of the driving wheel from having adverse effects on the speed and steering angle, which was conducive to the improvement of the stability of the sprayer. The sprayer could be tracked quickly and accurately the desired path, the path tracking in road conditions outside attached coefficients were 0.3 and 0.7 of the lateral deviation could be controlled within ±0.018 m. In stage C roughness 3D road conditions, the sprayer could adjust the steering angle of the front wheels in time to stabilize the body posture and the lateral deviation could be controlled within ±0.054 m. Compared with the controller that didn't consider the slip of the driving wheel, the stability and path tracking accuracy of the sprayer had been significantly improved.
