Development and Calibration of a Segmented Variable-Diameter Flexible Discrete Element Model of Green Foxtail Stems for Orchard Mowing Simulations

Green foxtail (Setaria viridis) is a common orchard weed, but discrete element simulations of mowing operations lack geometric, contact, and flexible-connection parameters for its stems. This study measured the geometry and physical and contact properties of post-mowing regrowth stems and developed a segmented, variable-diameter bonded sphero-cylinder model based on five internodes. Contact parameters were calibrated against the measured 20.97° angle of repose of 15-mm stem segments using Plackett–Burman, steepest-ascent, and Box–Behnken designs. Bonding V2 parameters were screened with a single-stem lateral push-over test. From IN1 to IN5, mean internode lengths were 183.9, 124.2, 82.7, 55.7, and 36.5 mm, and mean diameters were 0.8, 1.0, 1.4, 1.6, and 1.8 mm. The optimized equivalent Poisson’s ratio and stem–stem static and rolling friction coefficients were 0.338, 0.339, and 0.010, respectively. The simulated angle of repose was 21.20°, with a relative error of 1.10%. With normal and tangential stiffnesses per unit area of 5.0 × 1011 and 2.5 × 1011 N m−3 and a Bonded Disk Scale of 1.1, errors in horizontal tip displacement and tip angle were 1.31% and 2.88%. The model reproduced short-stem accumulation and macroscopic bending under the specified conditions and supports DEM studies of orchard mowing component–stem interactions.