Contact Parameter Calibration for a Rigid Discrete Element Model of Rapeseed Pod at Suitable Harvest Period Using Angle of Repose

The absence of discrete element contact parameters for suitable harvest rapeseed pods hinders accurate DEM simulations. This study presents the first systematic calibration of discrete element contact parameters for “Ningza 158” rapeseed pods using physical and simulation experiments. Experimentally determined properties included external dimensions, density, pod–steel coefficients (restitution, static friction, rolling friction), and pod–pod restitution. Using the measured angle of repose as the objective, significant parameters were screened, optimized, and modeled via Plackett–Burman, steepest climbing, and central composite response surface tests. The optimal parameter combination was then validated against the angle of repose, completing the calibration. The results showed that the density, Poisson’s ratio, and shear modulus of rapeseed pods at the suitable harvest period were 346.63 kg·m−3, 0.35, and 10.5 MPa, respectively. The coefficients of restitution, static friction, and rolling friction for pod–steel and pod–pod were 0.2994, 0.4206, 0.0435, 0.1024, 0.3110, and 0.0380, respectively. Under this simulation parameter combination, the angle of repose obtained from the simulation was 71.23°, yielding a relative error of 2.12% compared with the actual value of 69.75°. Note that this validation is limited to static accumulation behavior. The calibrated parameters can be a basis for future dynamic validation studies on harvest loss mechanisms, provided additional dynamic validation is performed.