玉米果穗离散元模型构建与脱粒仿真验证

针对玉米脱粒离散元仿真中果穗模型难以表征籽粒分离和芯轴破碎的问题，该研究构建了玉米果穗聚合体离散元模型并进行脱粒仿真验证。基于玉米芯轴3层结构采用分层建模与网格划分方法建立玉米芯轴离散元模型，结合Plackett-Burman试验、最陡爬坡试验、Box-Behnken试验和仿真弯曲试验标定粘结参数；以马齿型玉米籽粒为原型，采用五球粘结的籽粒-芯轴连接方式建立玉米果穗聚合体离散元模型，仿真标定籽粒与芯轴的连接力；最后模拟梯形杆齿、圆头钉齿和纹杆块3种脱粒分离机构的玉米脱粒进程。结果表明：玉米芯轴弯曲破坏力和弯曲刚度仿真结果与实测平均值的相对误差分别为-0.12%和-0.14%，籽粒果柄轴向压缩力和径向压缩力仿真结果与实测平均值的偏差分别为-1.8和2.46 N，3种脱粒分离机构脱粒段仿真区域内籽粒平均法向接触力依次为12.50、12.32和8.03 N，3种脱粒元件对籽粒平均法向接触力的递减趋势与台架试验的籽粒破碎率变化一致，根据籽粒与脱粒元件接触合力的累积频率曲线确定籽粒破碎率的临界接触合力为550 N，仿真未脱净率依次为0.15%、0.37%、0.35%，较台架试验结果分别偏小0.07、偏高0.04和偏小0.25个百分点，沿滚筒轴向籽粒质量分布百分比曲线均表现为正偏态单峰分布，脱粒仿真试验的曲线峰值分别比台架试验高1.03、1.86和0.85个百分点，两者脱粒质量相近。该玉米果穗聚合体离散元模型参数标定准确，能够准确反映籽粒和芯轴的力学特性差异，可还原玉米脱粒分离过程，为后续脱粒分离机构的优化提供参考依据。

Construction of the discrete element model for maize ears and verification of threshing simulation

The corn ear model makes it difficult to represent the kernel separation and cob breakage in the corn threshing during discrete element simulation. In this study, a new discrete element model was constructed for the corn ear aggregate using threshing simulation. Hertz-Mindlin (no-slip) and Bonding contact models were combined in the EDEM software. According to the layered structure of the corn cob, a three-layered discrete element model with adhesive bonding was established using a layered modelling and meshing method. The three-point bending mechanical test was carried out, where the bending breaking force and stiffness were taken as evaluation indicators. The simulated bending tests were conducted with the Plackett-Burman, steepest ascent, and Box-Behnken. The bonding parameters between particles were calibrated in each layer of the corn cob discrete element model. A corn ear aggregate discrete element model was established using a horse-tooth-shaped corn kernel prototype and a five-ball bonding kernel-cob connection. The forces of the axial and radial connectivity were simulated and then calibrated for the kernels and cobs. Finally, the corn threshing process was simulated in three threshing mechanisms: trapezoidal tooth, round-headed nail tooth, and rasp bar. The results showed that the bending breaking force and stiffness of the corn cob were simulated as 168.76N and 13.08 N/mm, respectively, under the optimal bonding parameter combination, with relative errors of -0.12%, and -0.14%, respectively. The relative errors were -12.84% and 13.25%, respectively, for the axial and radial compression of the kernel pedicel strength. The average normal contact forces of the grains on three threshing elements were 12.50, 12.32, and 8.03 N, respectively, in the simulation area of the threshing. The decreasing trend was consistent with the grain breakage rate in the bench tests. Furthermore, the critical contact force was determined to quantify the rate of broken kernels, according to the cumulative frequency curve of the contact force between the kernels and the threshing elements. The simulation rates of unthreshed kernels were 0.15% and 0.35%, respectively, for the trapezoidal tooth and rasp bar, which were lower by 0.07 and 0.25 percentage points than that in the bench test, respectively. The simulation unthreshed rate was 0.37% for the round-headed nail tooth, which was 0.04 percentage points higher than that in the bench test. There was a positively skewed unimodal distribution in the proportion of threshed material along the axis of the drum. Among them, the peak values in the proportion of the simulated discharge grain mass were 1.03, 1.86, and 0.85 percentage points higher than those in the bench tests, respectively. There were similar threshing qualities between threshing simulation and bench tests. The reason was that there were different threshing mechanisms in the various threshing elements. Specifically, the trapezoidal tooth and round-headed nail tooth relied mainly on the impact and strike to the thresh kernels, indicating the lower frequency of the contacts with kernels. The greater force of contact with the kernel was obtained to cause the higher broken kernel rate during effective threshing. By contrast, the lower broken kernels rate was achieved in the rasp bar to thresh kernels. The larger contact area was obtained to rub the ears at a relatively smaller average contact force, leading to the more frequent contact with the kernels. Although there were still differences between the simulations and the actual, the parameters were accurately evaluated in the discrete element model of corn ear aggregate. The corn threshing and separation were simulated to clarify the mechanical characteristics between the kernel and cob. The corn ear model can be expected to evaluate the threshing performance of the threshing and separation using various indicators, such as the kernel force, contact frequency, the rate of unthreshed kernels, and discharged material distribution. This finding can provide a strong reference for the subsequent optimization of threshing and separation.