Effect of wet paddy and stalk parameter for DEM simulation based on Hertz–Mindlin with JKR model

The cleaning of wet rice threshed outputs presents a critical constraint on the operational efficiency of harvesters during humid conditions. This study aimed to quantify the specific impact of wet-state conditions on the physical properties of rice threshed outputs. Utilizing the Hertz–Mindlin with Johnson–Kendal–Roberts contact model, Plackett–Burman screening experiments were designed in Design-Expert to identify contact parameters significantly influencing the dynamic angle of repose. For the six most significant parameters, a six-factor, three-level Box–Behnken experimental design was implemented. Analysis of variance was performed on the experimental data, establishing a multivariate nonlinear regression model relating the dynamic angle of repose to the key contact parameters. Systematic measurements were conducted to determine the compositional content and key physical parameters of the constituent materials: the elastic modulus of paddy was 216.3 MPa; the elastic moduli of stalks (categorized by diameters of 4, 5, and 6 mm) were 5.59, 6.28, and 8.93 MPa, respectively; the paddy friction coefficient was 0.518; the stalk sliding friction coefficient was 0.566 and rolling friction coefficient was 0.115; and the surface moisture contents were 7.76% for paddy and 11.8% for stalks, respectively. The optimized contact parameter group of wet paddy and stalks was compared with the measured values. The relative error of the predicted angle of repose for the paddy was only 0.23%, and the error for the rice stalks was 1.3%. The precise contact parameters established in this study provide crucial foundational support for the development of subsequent CFD-DEM coupled numerical models.