Calibration of the discrete element parameters for the soil model of cotton field after plowing in Xinjiang of China

Abstract: A multi-layer fertilization has been considered as an efficient way to meet the needs of fertilizers at different growth stages of crops. A layered fertilization device is usually used for the process of ditching and covering soil after ploughing. In this case, the movement of soil particles is very complicated. In this study, an EDEM discrete element software was used to simulate the process of soil accumulation and sliding in the layered fertilization area, in order to calibrate soil contact parameters. A Hertz-Mindlin non-slip model was selected to simulate the contact surface of soil-soil and soil-layered fertilization device (65 Mn steel), according to the soil characteristics of cotton fields after ploughing. Three common shapes of soil particles were represented by dual surface, square four, and straight four. The calibration parameters were also selected to determine the ranges. Specifically, the static friction coefficient, rolling friction coefficient, and collision recovery coefficient between soil-soil and soil-65 Mn steel were used as test factors, while the soil angle of repose, and sliding friction angle of soil-Mn steel were used as evaluation indicators. The universal rotation center combination test was conducted to verify the model. The Design-Expert software was then utilized to perform the regression on the test data. The results showed that the coefficient of recovery from the collision of soil-soil and soil-65 Mn steel presented no significant effect on the angle of repose and sliding friction of soil. Taking the measured soil angle of repose and the sliding friction angle between the soil and 65 Mn steel as the optimization objectives, an optimal combination of discrete element contact parameters was obtained: the coefficient of restoration between soils was 0.48, the coefficient of rolling friction between soils was 0.56, the coefficient of static friction between soils was 0.24, the coefficient of restitution between the soil and 65 Mn steel was 0.5, the coefficient of rolling friction between soil and 65 Mn steel was 0.1, and the coefficient of static friction between soil and 65 Mn steel was 0.31. A soil accumulation test and the sliding test were also compared with the actual test, in order to verify the accuracy of the optimized parameters. The relative errors of the two tests were 1.7% and 2.5%, respectively, under the optimal combination of calibration parameters. Consequently, the discrete elements can be expected to simulate the ditching and soil covering process of the layered fertilization device. The relative errors of simulation and field test were 10.2%, and 7.95%, respectively, at the operating speed of 5, 6, and 7 km/h of layered fertilization device. Among them, the error of 7.04% was within the acceptable range. Consequently, the simulation and field test presented basically the same effect of ditching and covering soil, indicating the high accuracy and reliability for the calibration of soil contact parameters. The finding can provide strong theoretical and technical support for the later research on drag reduction of layered fertilization devices.