Modeling the interaction of soil and a vibrating subsoiler using the discrete element method

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Abstract

A vibrating subsoiler can effectively reduce the draft force compared to a rigid one. Several theoretical and experimental methods have been developed to study the interaction of soil and vibrating tools, enabling the exploration of the mechanism of a vibrating subsoiler to reduce the draft force. However, these methods are unable to predict the dynamic force of tools and soil behavior because of nonlinear soil properties. This paper employs the discrete element method (DEM) to simulate vibrating tillage. The aim is to clearly understand the dynamic force of vibrating tools and soil behavior. Three different types of representative vibrating subsoilers and one rigid one were selected for this study. These subsoilers varied in their tines and vibrating structures. The DEM model of soil and subsoilers was established. Results showed that the draft and vertical dynamic forces of the vibrating tines periodically changed and had distinct peak and valley phases in one period. The value of the draft and vertical forces in the valley phase of a vibrating subsoiler was much less than that of the rigid one, which led to the decrease of mean draft force. The soil disturbance widths, velocities, and horizontal and vertical forces of soil particles also periodically changed. Soil ruptures mainly occurred when a tine vibrated in the forward direction. Compared with a rigid tine, a vibrating tine had more time to disturb soil particles and thus to achieve better soil fracture. The DEM model was validated by comparing it with an actual soil bin test. The errors of the simulation model in predicting the soil disturbance and forces were less than 13.23% and 9.65%, respectively, indicating that the model is a useful tool to simulate the dynamic interaction of soil and a vibrating subsoiler.

Introduction

Tillage is essential in agricultural production. It can effectively remove weeds, prepare a good seedbed, and facilitate air and water movement in the soil (Hamzei and Seyyedi, 2016, Romaneckas et al., 2015). Subsoiling is especially suitable to break up the compacted layer in conservation fields (Mouazen and Ramon, 2002, Harisson and Licsko, 1989). It was reported that a vibrating tine can effectively reduce the draft force of tractors (Sakai et al., 1993, Butson and MacIntyre, 1981). To explore the mechanism by which a vibrating tine reduces draft force, as well as the dynamic interaction between soil and vibrating tools, is useful to optimize its structure and working parameters (Shmulevich, 2010, Ucgul et al., 2014).

The common methods used to study the interaction of soil and vibrating tools are theoretical analysis and actual experiments. Yow and Smith (1976) established a theoretical model of the displacement and velocity of a vibrating tine. It was found that the relative velocity between the tine and soil changed periodically. Based on its speed relative to the soil, the motion of a vibrating tine is divided into three phases: rewind, acceleration, and catch-up. Kofoed (1969) established the power theoretical model. It was proposed that power consumption was related to the trajectory of the tine. Vibration in the horizontal direction could reduce the power more than that in the vertical direction. Gupta and Rajput (1993) studied the effect of vibration frequency on soil fragmentation by a soil bin test. The energy efficiency was found to be highest when the vibration frequency was close to the natural crushing frequency of the soil, and to further increase the vibration frequency would lead to more power consumption without improving soil fragmentation. Niyamapa and Saloke (2000) carried out soil bin tests to explore the effect of vibration frequency and amplitude on the surface positive pressure of the tine. The results showed that the positive pressure increased with the vibration frequency. Shahgoli et al. (2009) conducted field tests to study the influence of the tine angle on the draft force and power. It was proven that a tine with a negative angle could achieve better draft reduction and energy saving than one with a positive angle.

Theoretical analysis has mostly focused on modeling the movement of the vibrating tine, and experimental methods have concentrated on the macroscopic force of tools. However, the dynamic force of vibrating tools in a period is unpredictable by theoretical analysis because of the nonlinear force properties of soil (Mak et al., 2012, Shmulevich et al., 2007), and soil behavior, such as cracking, turning, and moving, is also less focused.

The discrete element method (DEM) is a numerical method that is suited to analyze the microscopic behavior of granular media and has been widely used to model the interaction of soil and tillage tools, such as sweeps, coulters, and rigid subsoilers. Its reliability has been confirmed by comparison to actual measurements (Horner et al., 2001, Hofstetter, 2002, Franco et al., 2005, Asaf et al., 2007). Shmulevich et al. (2007) used the DEM to study the interaction of soil and a wide cutting blade. The draft force and soil rupture boundaries obtained from the DEM were highly consistent with actual tests, which indicates that the DEM is a promising method for modeling soil-tillage tool interaction. Ucgul et al. (2014) used the DEM to predict draft and vertical force of a sweep. The reliability was verified by comparison with field tests by Fielke (1988). The correlation coefficient R2 ranged from 0.95 to 0.99. Li et al. (2016) used the DEM to optimize the structure of a bear claw. Aiming to acquire less draft force and more soil disturbance, a tilt angle of 30° was determined to be the best angle. Zheng et al. (2016) employed the contact model described by Ucgul et al. (2015) to simulate the sandy loam soil that is mainly found in the Huanghuaihai region of China. A soil bin model including plow, hardpan, and subsoil layers was established. The static and dynamic friction coefficient and cohesion parameters of soil particles for each layer were calibrated by a typical measuring test of the angle of response. Verification test results showed that the calibrated static and dynamic friction coefficient and cohesion parameters could better predict the soil disturbance. The error of using these simulation parameters to predict the real width of the soil furrow was 8.21%. Asaf et al. (2008) demonstrated the potential of the DEM for modeling a moldboard plow by comparing the soil arrangement after tillage to experimental results, and their results showed a good correlation between the model and the experimental results. Zeng et al. (2017) developed a DEM model to simulate a deep tillage tool. The model was validated by a real laboratory test, and results showed that the calibrated model was capable of predicting soil cutting resistance and soil disturbance characteristics with relative errors ranging from 2.63% to 10.2%. Li et al. (2014) established a 3D model of soil particles and a subsoiler by considering the liquid bridge force between soil particles. The simulated operating resistance agreed well with the field experimental results and had relative errors of 2.96%, 14.95%, and 7.15% at depths of 180, 220, and 260 mm, respectively. Tanaka et al. (2007) used the DEM to simulate the tillage process of a vibrating subsoiler. The simulated soil cross sections of the vibrating subsoiler were compared with a field test, and the disturbance areas were found to be very consistent, which indicates that DEM could be used to study microscopic soil movement under vibrating tools.

The purpose of this study is to employ the DEM in modeling the dynamic interaction of soil and vibrating tools to explore the mechanism of a vibrating subsoiler to reduce draft force. Three different representative vibrating subsoilers and one rigid subsoiler were selected for DEM simulation. The dynamic force of the four subsoilers and soil behavior were investigated.

Section snippets

Machine configuration

Three vibrating subsoilers were selected for DEM simulation: a subsoiler with two abreast vibrating negative tines (AV2N), a subsoiler with one negative and one positive tine abreast vibrating (AVNP), and a subsoiler with two staggered vibrating negative tines (SV2N). Besides, one rigid subsoiler was taken as a comparison. The four subsoilers had tillage depths of 0–600 mm and a row spacing of 600 mm. The three vibrating subsoilers differed in the vibrating mechanism and tine configuration, as

Draft force

Draft force is an important indicator for vibrating tillage tools. A smaller draft force means better performance on resistance reduction (Gunn and Tramontini, 1995, Hendrick and Buchele, 1963, Larson, 1967). The draft force of the four subsoilers versus frequency is shown in Fig. 8. The draft force of the three vibrating subsoilers showed a decreasing trend as the frequency increased, which is consistent with existing research (Sulatisky and Ukrainetz, 1972, Shahgoli et al., 2010). The draft

Conclusions

The DEM model of soil and a vibrating subsoiler was established in this paper. By simulating the tillage process, the dynamic forces of vibrating subsoilers and soil behavior in one period were analyzed. Verification tests were performed in a soil bin to determine the accuracy of the simulation model. The conclusions of this paper are as follows.

  • (1)

    The dynamic force of the vibrating tine periodically changed, and there were distinct peak and valley phases in one period. The value of the 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

This work was supported by the Special Fund for Agro-scientific Research in the Public Interest (Grant No. 201503117 and No. 201503116-16), China; and the Soil-Machine-Plant key laboratory of the Ministry of Agriculture of China, China.

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