Downwash Airflow Research Articles

Influence of the downwash airflow distribution characteristics of a plant protection UAV on spray deposit distribution

During the aerial spraying of pesticides by plant protection UAVs (unmanned aerial vehicles), droplets are deposited below the aircraft under the influence of rotor airflow. The existence of a downwash airflow can improve the deposition of spray on target crops. However, during operation, variations in flight altitude and the gradual decrease in the payload of pesticide solution in the tank leads to changes in the flow field below the UAV which affects droplets distribution. In order to study this, the downwash airflow speed and droplet distributions below a UAV under different flight altitudes (1.5–3 m) and payloads (0-10 kg) were measured and analysed. The results indicated that the changes in operational parameters normally found during operation can change distribution of downwash airflow in all directions but particularly vertical and this can affect the distribution of spray below the UAV. Through correlation analysis, it was shown that the vertical component of airflow (downwash) had a significant positive correlation with spray deposition and penetration (P < 0.05, B = 0.082, B = 0.029), and a negative correlation with droplet deposition uniformity (coefficient of variation) and drift (P < 0.05, B = −3.818, B = −0.042). Thus, it can be seen that downwash is conducive to spray deposition and penetration, which can improve the droplet distribution uniformity and reduce drift. Thus, the influence mechanism of UAV airflow on droplet distribution was explored and shown to strongly influence have the quality of spraying by plant protection UAVs.

A YOLOv3-Based Rice Vortex Detecting System Using Dual Collaborative UAVs

Highlights Used dual collaborative UAVs to track the rice vortex formed on rice canopy. A trained YOLOv3-tiny model was used to recognize and calculate the rice vortex parameters with high accuracy. Discovered a quantitative relationship between rice vortex parameters and UAV flight parameters. Aligned the time axis of data process to enhance the accuracy of the vortex parameter calculation. Abstract. A rice vortex generated by the downwash airflow of the agricultural UAV has a great connection to pesticide droplet deposition. In this study, an unmanned aircraft system (UAS) was designed to obtain rice vortex parameters including position coordinates, area, and relative distance to the UAV. The system uses the flight control system mounted Beidou RTK positioning system and Manifold onboard computer to localize the spraying UAV and trains a YOLOv3-tiny model to recognize motion images of dynamic rice vortices in real-time, achieving a mean accuracy of 98.15% with 0.9326 IOU. Moreover, to optimize the system, a data time axis alignment method based on the robot operating system (ROS) was proposed to alleviate the transmission delay and image processing time-consumption, which increased the accuracy of the vortex parameter by 3.48%. The rice vortex area acquired by the UAS was highly relevant to UAV flight parameters and the relative distance was just correlated to the UAV height. The instantaneous status of the rice vortex helps to lay a data foundation for adjusting the flight parameters of the plant protection UAV. Keywords: Precision agriculture, Rice vortex, ROS, Spray operation, UAV, YOLOv3-tiny.

WSPM-System: Providing Real Data of Rotor Speed and Pitch Angle for Numerical Simulation of Downwash Airflow from a Multirotor UAV Sprayer

The accurate setting of input parameters in the numerical simulation of downwash airflow from a UAV sprayer is important for acceptable simulation results. To provide real data of simulation parameters (rotor speed and pitch angle) for the numerical simulation of downwash airflow, a wireless simulation parameter measurement system (WSPM-System) was designed and tested in this study. The system consists of hardware and software designed based on Arduino and LabVIEW, respectively. Wireless communication was realized by nRF24L01. The lattice Boltzmann method (LBM) was applied for the numerical simulation of downwash airflow. The results showed that the valid communication distance of the WSPM-System was 100 m, with a packet loss rate of less than 1%. While hovering, the rotor speed dropped by about 30% when the load of the UAV sprayer changed from 16 kg to 4 kg, which resulted in the maximum vertical downward velocity (VVD) on the horizontal detection surface dropping by about 23%. Under forward flight, the rotor speed in the front (n1, n6) and rear (n3, n4) of the UAV sprayer, respectively, showed a negative linear correlation and positive linear correlation with flight speed (R2 &gt; 0.95). Meanwhile, the rotor speed in the middle (n2, n5) was consistent with the rotor speed while hovering under the same load; the pitch angle showed a positive linear correlation with flight speed (R2 &gt; 0.94). A correlation analysis of measured and simulated values of the VVD revealed that the numerical simulation of downwash airflow with the parameters provided by the WSPM-System was reliable (R2 = 0.91). This study confirmed that the input value of the rotor speed in the fluid software needed to be determined according to the application parameters of the UAV sprayer, thus providing a feasible method and system for obtaining real simulation parameters.

Mean and turbulent flow characteristics of downwash air flow generated by a single rotor blade in agricultural drones

Recently, agricultural drones have been widely used for precise agricultural control. In the case of drone control, improved coverage and reduced drift potential of chemical sprays are of major concern, and this is greatly influenced by the downwash flow characteristics formed by the rotor blades. In this study, the mean and turbulent flow characteristics of the downwash flow generated by a single blade of a four-sided agricultural control drone were quantitatively evaluated. The blade radius used to generate the downwash flow was 370 mm, and the measurement position was normalized using this length. In addition, the rotational speed of the blade was 3000 rpm, and the velocity components were dimensionlessly expressed using the blade tip velocity (VBtip) corresponding to this rotational speed. The velocity of downwash flow was measured using CTA-type hot-wire anemometry with an X-type probe, and data were collected at a sampling rate of 30 kHz for turbulence characteristics analysis. The mean velocity, vorticity distribution, turbulence intensity, and Reynolds stress of the fluctuation velocity were quantitatively measured. From extensive experimental works, it was revealed that a high axial velocity is formed at a specific position (r/Lb = 0.4–0.8) along the blade length. The radial velocity shows an asymmetric distribution in the plane close to the blade, and the tangential velocity is higher in the central part of the downwash flow than in the region of a high axial and negative radial velocity. An asymmetric spiral rotational flow with low axial velocity and low-pressure distribution exists inside the downwash flow due to the shear flow, and the influence of the driving motor almost disappears at the axial position (Z/Lb ∼ 1). The averaged mean value of the axial, radial, and tangential velocity components measured at six axial planes and normalized by blade tip velocity (VBtip) is composed of the ratio of the axial velocity component of 12.8%, the radial velocity component of 0.19%, and the tangential velocity component of 1.3%. The maximum values of the turbulence intensity of the axial, radial, and tangential velocity components were estimated as 6.8%, 3.0%, and 4.3%, respectively.

Experimental Analysis of the Downwash Airflow Created by a Single Rotor Blade in Agricultural Drones

Experimental Analysis of the Downwash Airflow Created by a Single Rotor Blade in Agricultural Drones

Numerical verification on influence of multi-feature parameters to the downwash airflow field and operation effect of a six-rotor agricultural UAV in flight

The airflow field produced by UAV is an important factor to determine the movement of spray droplets, which can enhance deposition and reduce drift. Therefore, in this study, the downwash airflow field of the six-rotor plant protection UAV in the flight state was simulated based on the Lattice Boltzmann method (LBM), and the distribution characteristics at multi-feature parameters including different flight speed, flight height, crosswind speed and work load were studied. In addition, take the coverage area and the penetrable area as evaluating index, the influence of multi-feature parameters and prediction model to the operation effect were investigated. The results showed that, with the increase of flight speed and height, the angle of backward tilt of the downwash airflow field gradually increased, and the length and width of longitudinal and horizontal spread on the ground decreased. The optimal flight speed and height for UAV operations respectively were 2 m or 2.5 m, and 4 m/s based on the evaluating index. As the flight height increased from 2 m to 3 m, the eddies became larger, and they would entrain and affect the spatial movement and distribution of droplets. Furthermore, the distribution of the airflow structure was obviously changed with the increase of crosswind speed, and the coverage area and the penetrable area had a huge decline when the crosswind speed was greater than 3 m/s, which was not fit for operations. By the designed orthogonal test, the prediction models of the coverage area and the penetrable area were established, the R square was 0.952 and 0.759, respectively; the influence of multi-parameters to the operation effect was analyzed, and the prediction of the models for the dependent variables were relatively accurate. The results of the study could provide reference for exploring the downwash airflow field and droplet deposition pattern in the complex airflow field when UAV sprayings.

Drone Payload and Flying Speed Effects on Rotor Blades' RPM and Traveling Pattern for Agricultural Chemical Spraying

The sprayed chemicals by drones have been widely reported to be off-targeted and not uniformly distributed. This study aims to evaluate the drone blade’s revolutions per minute (RPM) and its travelling pattern at different payloads and flight speeds. The obtained results were used to relate to the potential effects on the quantity and quality of spraying. In a test flight on an area of 1000 m2, a hexacopter, Advansia A1 was tested in 6 different flying paths of 56 m length. The drone was set to fly at 5 payloads (10, 8, 6, 4, and 2 kg) and 4 flying speeds (i.e. 1, 3, 5, and 7 m.s-1) combinations. The drone travelling pattern and individual rotor blade rpm at each payload-flying speed combinations were analysed. From the result, the RPM of each rotor blade were found to decrease by 14 to 20% as the payload was decreased from 10kg to 0kg. Thus, in actual spraying activities, the changes in RPM could produce a downwash airflow pattern that continually varies from starting point up to the finishing point that would effect on pesticide's distribution along the flying path. On drone travelling pattern, at higher flying speed, a much lesser time and distance was required for the drone to be stabilized to the targeted speed. This relates to the longer time needed by the drone to accelerate and decelerate. The average real speed of the drone was notably reduced to 0.96, 2.72, 3.83 and 4.05 m.s-1, in which, it was, far less than the initial specified speed set at 1, 3, 5, and 7 m.s-1, respectively. The drone flying pattern during spraying needs to be considered for application rate determination to avoid for the crops to be under or over pesticide applications. The obtained finding is remarkably critical and useful in ensuring the efficiency of agricultural chemical spraying activities using drone.

The Airflow Field Characteristics of UAV Flight in a Greenhouse

The downwash airflow field of UAVs is insufficient under the dual influence of greenhouse structure and crop occlusion, and the distribution characteristics of the flight flow field of UAVs in greenhouses are unclear. In order to promote the application of UAVs in greenhouses, the flow field characteristics of UAVs in a greenhouse were studied herein. In a greenhouse containing tomato plants, a porous media model was used to simulate the obstacle effect of crops on the airflow. The multi-reference system model method was selected to solve the flow field of the UAV. Studies have shown that the airflow field generated by UAV flight in a greenhouse is mainly affected by the greenhouse structure. With the increase in UAV flight height, the ground effect of the downwash flow field weakened, and the flow field spread downward and around. The area affected by the flow field of the crops became larger, while the development of the crop convection field was less affected. The simulation was verified by experiments, and linear regression analysis was carried out between the experimental value and the simulation value. The experimental results were found to be in good agreement with the simulation results.

Distribution regularity of downwash airflow under rotors of agricultural UAV for plant protection

In the plant protection spray operation of UAVs, the process of droplet from formation to sedimentation target is affected by airflow, easy to form uneven deposition. Accurately description of rotor downwash flow field, clarification of velocity vector distribution at different heights of the UAV rotor flow field, simulation of the flow field with high precision, which are the prerequisites for accurately analyzing the droplet deposition distribution in rotor downwash flow field. Based on CFD method, the detail of rotor flow field was numerically calculated. Taking LTH-100 single-rotor agricultural UAV as the research object, the three-dimensional solid model of UAV was established, the Reynolds average N-S equation was used as the control equation and the RNG κ-e as the turbulence model to simulate the flow field of UAV in hover and lateral wind conditions, the wind velocity distribution at different altitudes of rotor downwash flow field was studied. The simulation results of the hover state showed that: In the flow field, the peak velocity appears in a circular distribution below the distal axis of the rotor. With the decrease of height, the peak velocity distribution area showed a tendency to expand gradually after small shrinkage; When the distance from the rotor was not more than 1.5 m, the downwash flow field presented an axisymmetric distribution based on the rotor axis, and the variation rate of velocity in the peak velocity was basically the same, turbulence in downwash flow field made the flow field more complex when the distance from rotor was larger than 2.0 m. On this basis, the optimal flight altitude of UAV is 1.5 m. Wind velocity test of the flow field was carried out on a rotor test bench, wind velocities at four altitudes of 0.5 m, 1.0 m, 1.5 m and 2.0 m were measured to verify the coincidence between the simulated and measured values. The test results showed that: the relative error between the measured and simulated values at four measurement heights were between 0.382-0.524, and the overall average relative errors was 0.430, which verified the confidence level of simulated values for measured values. When the lateral wind velocity was 3 m/s, 4 m/s and 5 m/s, the simulation results showed that: The distribution trend of airflow velocity at the same altitude in lateral-wind flow field with different wind speeds was similar; When the lateral wind speed was 5 m/s, the coupling field formed by the lateral wind and rotor airflow cannot reach the height of 2 m below the rotor. The results of this study can provide more accurate environmental conditions for theoretical analysis of droplet deposition regularity in the flow field, and also provide methodological guidance for the related research on rotor flow field of multi-rotor UAV. Keywords: aviation plant protection, downwash flow field, distribution regularity, numerical simulation, single-rotor UAV DOI: 10.25165/j.ijabe.20211403.4036 Citation: Liu X, Zhang W, Fu H B, Fu X M, Qi L Q. Distribution regularity of downwash airflow under rotors of agricultural UAV for plant protection. Int J Agric & Biol Eng, 2021; 14(3): 46–57.

Review of agricultural spraying technologies for plant protection using unmanned aerial vehicle (UAV)

With changing climate and farmland ecological conditions, pest outbreaks in agricultural landscapes are becoming more frequent, increasing the need for improved crop production tools and methods. UAV-based agricultural spraying is anticipated to be an important new technology for providing efficient and effective applications of crop protection products. This paper reviews and summarizes the status of the current research and progress on UAV application technologies for plant protection, and it discusses the characteristics of atomization by unmanned aircraft application systems with a focus on spray applications of agrichemicals. Additionally, the factors influencing the spraying performance including downwash airflow field and operating parameters are analyzed, and a number of key technologies for reducing drift and enhancing the application efficiency such as remote sensing, variable-rate technologies, and spray drift models are considered. Based on the reviewed literature, future developments and the impacts of these UAV technologies are projected. This review may inspire the innovation of the combined use of big data analytics and UAV technology, precision agricultural spraying technology, drift reduction technology, swarm UAV cooperative technology, and other supporting technologies for UAV-based aerial spraying for scientific research in the world. Keywords: UAV, plant protection, spraying technology, drift reduction, pesticide efficacy, spraying model, big data analytics DOI: 10.25165/j.ijabe.20211401.5714 Citation: Chen H B, Lan Y B, Fritz B K, Hoffmann W C, Liu S B. Review of agricultural spraying technologies for plant protection using unmanned aerial vehicle (UAV). Int J Agric & Biol Eng, 2021; 14(1): 38–49.

Review of agricultural spraying technologies for plant protection using unmanned aerial vehicle (UAV)

With changing climate and farmland ecological conditions, pest outbreaks in agricultural landscapes are becoming more frequent, increasing the need for improved crop production tools and methods. UAV-based agricultural spraying is anticipated to be an important new technology for providing efficient and effective applications of crop protection products. This paper reviews and summarizes the status of the current research and progress on UAV application technologies for plant protection, and it discusses the characteristics of atomization by unmanned aircraft application systems with a focus on spray applications of agrichemicals. Additionally, the factors influencing the spraying performance including downwash airflow field and operating parameters are analyzed, and a number of key technologies for reducing drift and enhancing the application efficiency such as remote sensing, variable-rate technologies, and spray drift models are considered. Based on the reviewed literature, future developments and the impacts of these UAV technologies are projected. This review may inspire the innovation of the combined use of big data analytics and UAV technology, precision agricultural spraying technology, drift reduction technology, swarm UAV cooperative technology, and other supporting technologies for UAV-based aerial spraying for scientific research in the world. Keywords: UAV, plant protection, spraying technology, drift reduction, pesticide efficacy, spraying model, big data analytics DOI: 10.25165/j.ijabe.20211401.5714 Citation: Chen H B, Lan Y B, Fritz B K, Hoffmann W C, Liu S B. Review of agricultural spraying technologies for plant protection using unmanned aerial vehicle (UAV). Int J Agric & Biol Eng, 2021; 14(1): 38–49.

Distribution regularity of downwash airflow under rotors of agricultural UAV for plant protection

In the plant protection spray operation of UAVs, the process of droplet from formation to sedimentation target is affected by airflow, easy to form uneven deposition. Accurately description of rotor downwash flow field, clarification of velocity vector distribution at different heights of the UAV rotor flow field, simulation of the flow field with high precision, which are the prerequisites for accurately analyzing the droplet deposition distribution in rotor downwash flow field. Based on CFD method, the detail of rotor flow field was numerically calculated. Taking LTH-100 single-rotor agricultural UAV as the research object, the three-dimensional solid model of UAV was established, the Reynolds average N-S equation was used as the control equation and the RNG κ-ε as the turbulence model to simulate the flow field of UAV in hover and lateral wind conditions, the wind velocity distribution at different altitudes of rotor downwash flow field was studied. The simulation results of the hover state showed that: In the flow field, the peak velocity appears in a circular distribution below the distal axis of the rotor. With the decrease of height, the peak velocity distribution area showed a tendency to expand gradually after small shrinkage; When the distance from the rotor was not more than 1.5 m, the downwash flow field presented an axisymmetric distribution based on the rotor axis, and the variation rate of velocity in the peak velocity was basically the same, turbulence in downwash flow field made the flow field more complex when the distance from rotor was larger than 2.0 m. On this basis, the optimal flight altitude of UAV is 1.5 m. Wind velocity test of the flow field was carried out on a rotor test bench, wind velocities at four altitudes of 0.5 m, 1.0 m, 1.5 m and 2.0 m were measured to verify the coincidence between the simulated and measured values. The test results showed that: the relative error between the measured and simulated values at four measurement heights were between 0.382-0.524, and the overall average relative errors was 0.430, which verified the confidence level of simulated values for measured values. When the lateral wind velocity was 3 m/s, 4 m/s and 5 m/s, the simulation results showed that: The distribution trend of airflow velocity at the same altitude in lateral-wind flow field with different wind speeds was similar; When the lateral wind speed was 5 m/s, the coupling field formed by the lateral wind and rotor airflow cannot reach the height of 2 m below the rotor. The results of this study can provide more accurate environmental conditions for theoretical analysis of droplet deposition regularity in the flow field, and also provide methodological guidance for the related research on rotor flow field of multi-rotor UAV. Keywords: aviation plant protection, downwash flow field, distribution regularity, numerical simulation, single-rotor UAV DOI: 10.25165/j.ijabe.20211403.4036 Citation: Liu X, Zhang W, Fu H B, Fu X M, Qi L Q. Distribution regularity of downwash airflow under rotors of agricultural UAV for plant protection. Int J Agric & Biol Eng, 2021; 14(3): 46–57.

Influence of the inner tilt angle on downwash airflow field of multi-rotor UAV based on wireless wind speed acquisition system

Influence of the inner tilt angle on downwash airflow field of multi-rotor UAV based on wireless wind speed acquisition system

Design and Development of an Indoor Testing Facility for Downwash and Spray Distribution Evaluations of Agricultural UAV

The usage of UAV as a pesticide application technology is becoming a common practice in South East Asia with a very significant proportion of agricultural areas have now being treated using such method. Up to date, about more than 169 of various designs and configurations of UAV have been produced to cater the demand, however it is in doubt whether all these available models were optimally integrated with the spray systems to give the required spraying qualities. Despite of each UAV configuration has their own unique aerodynamic effects, it is crucial to determine their downwash airflow patterns, the spray distributions and the effective application parameters that could give high spraying quality. This research outlines the development of an indoor testing facility and its standardised testing procedure for evaluating the spraying performances of any model UAVs that are used in agricultural applications. The testing facility was developed to imitate the UAV in the actual field spraying operation. With 23 m travel length, the developed testing facility was designed to carry the test UAV up to 100 kg, adjust the test UAV at three levels of spraying heights (1.5, 2.5, and 3.5 m), and moves the test UAV up to a maximum travelling speed of 10 m/s. The 6x6 m sampling platform structure for the pressure sensors and water sensitive papers was built underneath the rail support structure to measure and collect data for spraying distributions and downwash 3D profile spectrum. With the availability of this indoor UAV Testing Facility, it is expected that any efforts to increase the spraying efficiency using UAV could be tested repeatedly with same standard protocols, so that proper recommendations could be made on the flying requirements of the UAV in order to achieve an efficient agricultural chemical spraying operation.

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

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.

Radar/infrared integrated stealth optimization design of helicopter engine intake and exhaust system

Along with the diversification and sophisticated development of detection methods in modern warfare, helicopters are increasingly subject to unilateral or simultaneous threats from radar and infrared detectors. In order to improve the survivability and operational effectiveness of the helicopter, a comprehensive stealth approach based on Pareto solution is presented. Considering the geometric constraints and aerodynamic characteristics of the engine intake and exhaust system, the model of the system is established by the full factorial design, the internal, central and external flow fields are constructed, then the high-precision computational fluid dynamics method is used to simulate the total flow field under the rotor downwash airflow in hovering state. The radar cross section of the system is evaluated by the physical optics and physical theory of diffraction. Based on the Monte Carlo and ray tracking method, the infrared signature of the system is calculated and analyzed in detail. Under the comprehensive evaluation and selection of comprehensive stealth approach, the optimization model of the system is continuously established and updated. The ultimate design has achieved good results in both radar cross section reduction and infrared radiation suppression and the proposed method is effective and efficient for radar/infrared integrated stealth of helicopter engine intake and exhaust systems.

Distribution characteristics on droplet deposition of wind field vortex formed by multi-rotor UAV.

When the unmanned aerial vehicle (UAV) is used for aerial spraying, the downwash airflow generated by the UAV rotor will interact with the crop canopy and form a conical vortex shape in the crop plant. The size of the vortex will directly affect the outcome of the spraying operation. Six one-way spraying were performed by the UAV in a rice field with different but random flying altitude and velocities within the optimal operational range to form different vortex patterns. The spraying reagent was clear water, which was collected by water sensitive paper (WSP), and then the WSP was analyzed to study the droplets deposition effects in different vortex states. The results showed that the formation of the vortex significantly influenced the droplet deposition. To be specific, the droplet deposition amount in the obvious-vortex (OV) state was about 1.5 times of that in the small-scale (SV) vortex state, and 7 times of that in the non-vortex (NV) state. In the OV state, the droplets mainly deposited directly below and on both sides of the route. The deposition amount, coverage rate and droplet size increased from top to bottom of the crops with the deposition amount, coverage rate, and volume median diameter (VMD) ranging 0.204–0.470 μL/cm2, 3.31%-7.41%, and 306–367μm, respectively. In the SV state, droplets mainly deposited in the vortex area directly below the route. The deposition amount in the downwind direction was bigger than that in the upwind direction. The maximum of deposition amount, coverage rate and droplet size were found in the middle layer of the crops, the range are 0.177–0.334μL/cm2, 2.71%-5.30%, 295–370μm, respectively. In the NV state, the droplet mainly performed drifting motion, and the average droplet deposition amount in the downwind non-effective region was 29.4 times of that in the upwind non-effective region and 8.7 times of the effective vortex region directly below the route. The maximum of deposition amount, coverage rate and droplet size appeared in the upper layer of the crop, the range are 0.006–0.132μL/cm2, 0.17%-1.82%, 120–309μm, respectively, and almost no droplet deposited in the middle and lower part of the crop. The coefficient of variation (CV) of the droplet deposition amount was less than 40% in the state of obvious-vortex and small-scale vortex, and the worst penetration appeared in the non-vortex amounting to 65.97%. This work offers a basis for improving the spraying performance of UAV.

Design of UAV Downwash Airflow Field Detection System Based on Strain Effect Principle.

Accurate measurement of the downwash flow field of plant protection unmanned aerial vehicles (UAVs) is essential for analyzing the spatial distribution of droplets. To realize on-line rapid detection of the downwash flow field of a multi-rotor UAV, a flexible polypropylene detection device based on the principle of full bridge strain effect was proposed. Its performance principle was based on the physical deformation caused by wind pressure. The Fluid Flow and Static Structural modules of ANSYS 16.0 finite element software were used to simulate one-way fluid-solid coupling interaction. The surface of the resistive strain gauge embedded in the flexible detecting structure responded well to wind speed variation, hence it was suitable for downwash airflow wind field detection. By solving the strain force on the surface of the flexible detection structure, the length and layout of the grating wire of the strain gauge on the surface of the flexible detection structure were optimized. Meanwhile at 4 m·s−1 wind speed, the output voltage at varied bridge flexible acquisition systems in the acquisition card was measured. Results indicated coefficient of variation of 3.67%, 1.63% and 1.5%, respectively, which proved the good data acquisition consistency of the system. Through calibration test, the regression equation for the relationship between output voltage and wind speed for three unique sensor signal measuring circuits was established. The determination coefficients R2 for single bridge, half bridge and full bridge circuits were 0.9885, 0.9866 and 0.9959, respectively. In conclusion, by applying the multi-rotor plant protection UAV test platform, the results indicated the maximum relative error of the wind speed at each sampling point of the system at 1.0 m altitude was below 5.61%. Simulated and measured value had an RMSE maximum error of 0.1246 m·s−1. Moreover, downwash airflow detection not only has high accuracy but also has high sensitivity. Thus, there is convenience and practicability in the plant protection offered by this approach. The rapid measurement of UAV wind field and the established two-dimensional wind field model can provide a basis for precise application of agricultural aviation.

UAV-Borne Dual-Band Sensor Method for Monitoring Physiological Crop Status.

Unmanned aerial vehicles (UAVs) equipped with dual-band crop-growth sensors can achieve high-throughput acquisition of crop-growth information. However, the downwash airflow field of the UAV disturbs the crop canopy during sensor measurements. To resolve this issue, we used computational fluid dynamics (CFD), numerical simulation, and three-dimensional airflow field testers to study the UAV-borne multispectral-sensor method for monitoring crop growth. The results show that when the flying height of the UAV is 1 m from the crop canopy, the generated airflow field on the surface of the crop canopy is elliptical, with a long semiaxis length of about 0.45 m and a short semiaxis of about 0.4 m. The flow-field distribution results, combined with the sensor’s field of view, indicated that the support length of the UAV-borne multispectral sensor should be 0.6 m. Wheat test results showed that the ratio vegetation index (RVI) output of the UAV-borne spectral sensor had a linear fit coefficient of determination (R2) of 0.81, and a root mean square error (RMSE) of 0.38 compared with the ASD Fieldspec2 spectrometer. Our method improves the accuracy and stability of measurement results of the UAV-borne dual-band crop-growth sensor. Rice test results showed that the RVI value measured by the UAV-borne multispectral sensor had good linearity with leaf nitrogen accumulation (LNA), leaf area index (LAI), and leaf dry weight (LDW); R2 was 0.62, 0.76, and 0.60, and RMSE was 2.28, 1.03, and 10.73, respectively. Our monitoring method could be well-applied to UAV-borne dual-band crop growth sensors.

Research status and trends of downwash airflow of spray UAVs in agriculture

Downwash airflow of spray UAVs in agriculture has been gradually focused on and a few studies have already been involved in it. Thus, this paper firstly reviewed the current research of downwash airflow of UAVs for agricultural spray. It is found that Chinese scholars have published the majority of the studies in this area. Furthermore, two aspects of this kind of research, one, the work related to the caused results of UAV downwash acting on spray, the other, that regarding downwash itself for investigation, were analysed. It is illustrated that the present research usually uses water-sensitive paper, electronic devices, like wind-speed arrays and PIV, and CFD simulation. However, the existed electronic devices have some limitations, such as lacking of the ability of showing and demonstration of downwash in big enough areas in detail during recording. Moreover, the studies directly related to downwash have been just launched around the world and the examination for multi-rotor UAVs is even seriously not enough. Therefore, in terms of research trends, it is significant to develop specialised sensors for downwash measurement. The general and reliable CFD models of a class of UAVs are also needed. In addition, the combination of UAV downwash with real applications and actual production is important for the clarification of the details in different situations. Keywords: spray UAV, downwash, representation results, investigation methods DOI:  10.33440/j.ijpaa.20190201.0023  Citation : Yang S H, Zheng Y J, Liu X X.  Research status and trends of downwash airflow of spray UAVs in agriculture.  Int J Precis Agric Aviat, 2019; 2(1): 1–8. Â