Effect of calibration experiments on the micro-parameters of wheat required in discrete element simulations

Abstract. Angles of repose measured in various calibration experiments were widely used to determine the micro-parameters of agricultural bulk materials required in discrete element method. However, the effect of these calibration experiments on the calibrated micro-parameters is still uncertain. In this study, difference among the calibrated micro-parameters of wheat was studied qualitatively and quantitatively by two representative tests, namely the cylinder-lifting test (static angle of repose) and the rotating drum test (dynamic angle of repose). The Plackett-Burman design was firstly simulated to screen the micro-parameters which significantly affected the angles of repose. Based on the simulation results of Box-Behnken design, regression models for the static and dynamic angles of repose with the screened micro-parameters were developed, respectively. The qualitatively comparison was conducted by finding a set of micro-parameters which could simultaneously made the simulated static and dynamic angles of repose matching well with the experimental ones. However, no such set of micro-parameters was found, showing that the two calibration tests will lead to different sets of micro-parameters. For quantitatively comparison, two sets of micro-parameters were respectively calibrated and validated by the two tests, from which large difference was found. Specifically, the wheat-wheat coefficient of static friction, the wheat-acrylic coefficient of static friction and the wheat-wheat coefficient of rolling friction were calibrated as 0.40, 0.42 and 0.092 for the static test, 35%~100% higher than the dynamic test (0.20, 0.31 and 0.048). Both the qualitative and quantitative studies indicate that the two calibration experiments have an effect on the calibrated micro-parameters of wheat.