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

doi:10.1016/j.compag.2021.106425

Computers and Electronics in Agriculture

Volume 190, November 2021, 106425

https://doi.org/10.1016/j.compag.2021.106425 Get rights and content

Highlights

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The downwash airflow field of the six-rotor plant protection UAV in the flight state was simulated.
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The distribution characteristics at multi-feature parameters were studied.
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The evaluating index was proposed to analyze the influence of multi-feature parameters to the operation effect.
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The prediction model based on the evaluating index was established.

Abstract

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.

Introduction

The application of agricultural aerial spraying is to provide an efficient and effective method for crop pest control, and achieves the unified prevention for large-scale crop pest and diseases (Mogili and Deepak, 2018; Chen et al., 2021). Compared with the manned aerial applications which is commonly used in US for large cropping areas, Unmanned Aerial Vehicle (UAV) has been developed rapidly in the past ten years in Asia, especially in Japan, China and South Korea, due to the characteristic of small complex field plots across Asia’s diversified crop planting zones, terrace and mountainous cropping (John, 2013; Wen et al., 2019; Tang et al., 2021). Moreover, UAV can perform in lower altitude with strong automatic control capabilities and flexible operations. However, the drift of droplets which is also caused by the downwash airflow of UAV is a major technical risk (Preftakes, 2017; Wen et al., 2019). Therefore, the development of the downwash airflow field of UAV plays an important role in plant protection research (Tang et al., 2019). The mechanism of the interaction between the downwash wind field of a UAV and the vortex generated in a rice canopy was studied and the results indicated that the movement parameters of UAV and the vortex movement of the rice canopy could improve the precise target accuracy of the spraying operation of the UAV (Li et al.; 2019). A high-speed particle image velocimetry (PIV) system was used to record the spatial droplets distribution of an eight-rotor agricultural UAV, and the results showed the movement and deposition of droplets in the downwash airflow field was influenced by the rotation speed of the rotor and the nozzle position (Tang et al., 2017). The downwash airflow of the rotor will affect the operation of agricultural UAVs, and the coverage area and the penetrable area between the downwash airflow and the crop canopy directly affect the deposition area and penetration ability of the droplets (Li et al., 2019; Zhang et al., 2019).

Scholars conducted a series of research on the distribution of the downwash airflow field of UAVs. The mature approach to numerical analysis flow filed was computational fluid dynamics (CFD) simulations (Ferziger and Peri, 2002). Based on compressible Reynolds-averaged Navier-Stokes (RANS) equations with the RNG k-ε turbulence model, the efficient three-dimensional CFD model was established to simulate the downwash airflow in hover (Yang et al., 2017; Guo et al., 2020), and the simulation results showed that the z-direction velocity was the main body of the wind velocity in the downwash airflow. The method of model migration was proposed to develop CFD models of a real-used UAV (JF01-10), and verification trials by Particle image velocimetry (PIV) were conducted, which showed the relative error between the PIV tests and the CFD models of the small UAV was less than 12% (Yang et al., 2020). Sliding mesh and discrete models were used to simulate droplet deposition distribution characteristics under the influence of the downwash flow field of a UAV under the conditions of a flight speed of 3 m/s and an altitude of 1.5 m. Under the influence of the rotor flow field, with the downward development of the downwash flow field, the overall velocity of the flow field gradually decreased, and the influence interval of the flow field gradually expanded. (Shi et al., 2019). However, the computational accuracy of CFD depended on the definition of manually generated grid and physical property parameters which were difficult to selected, CFD simulation results were not easy to be verified by wind tunnel experiment or actual field test, and it was only used in fundamental research on algorithms (Lakshminarayan et al., 2013).

Recently, the Lattice Boltzmann Method (LBM) had been applied to solve fluid dynamics problems at a mesoscopic scale and realized high fidelity calculation and simulation of complex CFD problems (Janssen and Krafczyk, 2010). The LBM was used to simulated the downwash flow fields of a quad-rotor drone, and the numerical simulation showed the flight speed and altitude of the drone would cause horseshoe-shaped eddy current in the downwash flow field, which lead to the entrainment of droplets, uneven deposition and drift of droplets eventually (Wen et al., 2019). A large-eddy simulation was performed and the lattice Boltzmann method was used to accurately capture the development of the rotor-tip vortex to simulate the instantaneous downwash flow field and droplet movement of a commonly used unmanned helicopter model (AF25B; Copterworks) at different droplet sizes, application heights, and crosswind speeds. The flow structure angle was inversely proportional to the application height and followed a second-order law with respect to the crosswind speed, and the droplet deposition rate was more likely to be affected by the quadratic term of the droplet diameter, while the coefficient of variation of deposition was affected by the quadratic terms of the droplet diameter and cross wind speed (Tang et al., 2020, 2021). The lattice Boltzmann method (LBM) based on a mesoscopic kinetic model was used to simulate the airflow field of a six-rotor plant protection drone. The airflow field of drone in hover and at varied flight speeds and various altitudes was investigated, and the results indicated that the flight speed and altitude had a significant effect on the distribution of the airflow field. The predicted values in the vertical direction using the average velocity attenuation model corresponded well with experimental measurements (Zhang et al., 2020). Although the reliability of the LBM method was verified in previous study, most of the studies focused on the distribution characteristics of downwash airflow in the hover state which differ from the flight case of UAVs, and there is no in-depth research in downwash airflow models in flight of UAVs.

The objective of this study was to simulate the downwash airflow field of a six-rotor UAV in the flight state based on the LBM, and study the distribution characteristics with multi-feature parameters including flight speed, flight height, crosswind speed, and work load. Coverage area and penetrable area were proposed as evaluating index (Li et al., 2019; Zhang et al., 2020), and based on them, the influence of multi-feature parameters to the operation effect were discussed in detail and a more complete model was established. The simulation results help to provide reference for exploring the rotor downwash airflow field and droplet deposition pattern in the complex airflow field under the fusion of multiple parameters in the operation of UAVs.

Section snippets

Lattice Boltzmann method

Lattice Gas Cellular Automata (LGCA) is a microscopic molecular dynamics model that discrete the velocity, space and time. The central idea is to assume that the particles move in an N-dimensional lattice with discrete, specified time (t = 0, 1, 2, 3…) and velocity (c_i, i = 0, 1, …, b). When different particles reach the same lattice position, they collide according to specific rules to keep the mass and linear momentum unchanged (Malaspinas and Sagaut, 2014, Mohamad, 2017).

The velocity

Distribution characteristics of the flight speeds and heights to the downwash airflow field

Fig. 6 shows the front-view of velocity in the downwash airflow field of the helicopter with a flight speed of 1 m/s, 2 m/s, 3 m/s, and 4 m/s, respectively and a flight height of 1.5 m, 2.0 m, 2.5 m, and 3.0 m, respectively. Posterior slope defined as β is the angle of backward tilt of rotor downwash airflow field in the flight state of UAV. It can be seen that at the same flight height, the posterior slope gradually increases with the increasing flight speed; the posterior slope goes up by

Conclusion

In this paper, a three-dimensional model of a six-rotor UAV in the flight state based on the LBM was simulated, and on this basis, the distribution characteristics of the downwash airflow field of a six-rotor UAV in flight at different flight speed, flight height, crosswind speed and work load were studied. The y-direction velocity of the downwash airflow was as a basis for proposing the evaluating index of the coverage area S₁ and the penetrable area S₂ to analyze the influence of

CRediT authorship contribution statement

Ling Wang: Conceptualization, Writing – original draft. Mao Xu: Supervision. Qihang Hou: Simulation, Data curation. Zhiwei Wang: Simulation, Data curation. Yubin Lan: Resources. Shumao Wang: Revision-final draft.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

Funding: This work was supported by the National Natural Science Foundation of China (Grant No. 31801783).

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Citation Excerpt :
The results showed that the phenomenon of downwash airflow spiral downward airflow could blow open the fruit tree canopy and improve the penetration ability of droplets. Based on the lattice Boltzmann (LBM) method, Zhang et al. (2020) and Wang et al. (2021b) configured the computational domain as a virtual wind tunnel and set different wind velocities at the velocity entrance boundary to simulate the different flight velocities of UAV. The numerical simulation results showed that when the flight velocity exceeded 4 m/s, the downwash flow was horseshoe in the transverse section directly behind the UAV.
To improve the effectiveness of unmanned aerial vehicle (UAV) spray, it is necessary to clarify the flow characteristics and penetration inside the canopy of the downwash airflow of UAV. This study used a three-dimensional computational fluid dynamic (CFD) model for the flow characteristics and velocity distribution of the downwash airflow generated by a quadrotor plant protection UAV at different flight velocities (1 ∼ 5 m/s) was established. The measurement tests for airflow velocity distribution inside the canopies were conducted. The results indicated that the spiral vortices appeared below the rotors and then evolved into inclined horseshoe vortices. The horseshoe vortices at the flight velocity of 5 m/s exceeded the flight height of the UAV and tended to cause droplet drift. When the flight velocity was larger than 4 m/s, it was difficult for the horseshoe vortex to enter the bottom of the canopy, which was not conducive to improving the droplet penetration performance. The diffusion and attenuation of the horseshoe vortex during the flow process led to an increase and then a decrease in the velocity distribution area in the Z-direction of airflow. In order to improve the droplet penetration performance, the optimal operating distance range between UAV and canopy was given when the flight velocity was less than 3 m/s based on the variation law of the velocity distribution in the Z-direction. The validation results showed that the simulated and measured values maintained the same trend.

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

Highlights

Abstract

Introduction

Section snippets

Lattice Boltzmann method

Distribution characteristics of the flight speeds and heights to the downwash airflow field

Conclusion

CRediT authorship contribution statement

Declaration of Competing Interest

Acknowledgements

Comput. Electron. Agric

Comput. Math. Appl.

Comput. Electron. Agric.

J. Comput. Phys.

Proc. Comput. Sci.

Computers and Electronics in Agriculture

Comput. Electron. Agric.

Lattice-Boltzmann method for complex flows

Annu. Rev. Fluid Mech.

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