Discrete Meta-Modeling and Parameter Calibration of Harvested Alfalfa Stalks

Addressing the problem of lacking accurate and reliable contact parameters and bonding parameters in the simulation of the mashing process during the harvesting of alfalfa, this study takes the stems of alfalfa at the harvesting stage as the research object. The geometric dimensions and related intrinsic parameters of the stems were measured. Using the Enhanced Discrete Element Method (EDEM) software, a multi-scale discrete element flexible bonding model of alfalfa stems was established based on region-specific parameters. The entire alfalfa stem was divided into three parts: the top, middle, and root sections. A multi-scale particle aggregation model of hollow stems was created using the Hertz-Mindlin with bonding model. The contact parameters between alfalfa stems at the harvesting stage and PU rubber were determined using a mathematical model based on quadratic polynomial fitting curves. The results showed that the shear modulus of the top, middle, and root sections of the alfalfa stems were 24.96 MPa, 29.60 MPa, and 10.48 MPa, respectively. The coefficients of restitution between the top, middle, and root sections of the alfalfa stems and PU rubber were 0.426, 0.375, and 0.386, respectively; the static friction coefficients were 0.613, 0.667, and 0.422, respectively; and the rolling friction coefficients were 0.213, 0.226, and 0.292, respectively. The relative error between the simulated and measured values of the angle of repose was less than 3%, effectively representing the mechanical characteristics of alfalfa stems at the harvesting stage bending and breaking under impact. This study aims to establish a discrete element flexible model of alfalfa stems at the harvesting stage and accurately calibrate the contact parameters with typical rubber materials, thereby addressing the lack of reliable bonding and contact parameters in existing simulations of the mashing process.