Establishment of a Discrete Element Model for Ramie Stalk Phloem and Xylem

Simulation tests require the first step of building an accurate particle model. Ramie stalk phloem and xylem are nonspherical, and in DEM simulations, the multispherical method is usually used to construct irregular particle models [41–44]. In nonspherical particle discrete element simulations, the multichemical packing model has the advantages of fast calculation rate and simple contact judgment; however, as the number of filling particle elements increases, the simulation time will greatly increase, so the number of filling particle elements needs to be reasonable [45,46].

Since no significant deformation occurs during stacking, this study did not consider introducing bonding. When building the model, the ramie stalk phloem and xylem are first modeled and meshed using the Mesh submodule in the Workbench module of ANSYS 16.0 software. Then, the saved .msh file is imported into Fluent. The coordinate file containing the mesh coordinate information is obtained by reading and compiling the source file CalcRadius.c through the userdefine module, followed by executing CalcRadiusVolume. The coordinate data of the model are then imported into the .xml file saved by EDEM software, and the particle coordinate import is completed by entering EDEM. Based on the size measurement results of the ramie xylem, 732 circular spheres with a radius of 1.15 mm are used to form a cylindrical ramie stalk xylem with an outer diameter and inner diameter of 12.8 mm and 8 mm, respectively, and a length of 140 mm, as shown in Figure 7a.

The related study [40] showed that when calibrating the parameters of the discrete element model if the modeling is performed exactly according to the actual size of the material, it will greatly prolong the simulation time, increase the computational volume, and thus reduce the simulation efficiency. The correct calibration of the discrete element model with a moderately enlarged particle radius can truly reflect the contact parameters of the target material. The ramie phloem was relatively thin, and the filling particles according to the actual size will result in a low simulation efficiency due to the large number of calculations. In the simulation, the thickness of the ramie phloem was doubled, and 16 circular spheres with a radius of 0.7 mm were used to form a rectangular ramie stalk phloem with a length and width of 5.6 × 5.6 mm and a height of 1.4 mm, as shown in Figure 7b. Since the thickness of the ramie phloem is magnified in the simulation, it needs to be redetermined. The mass of a single sphere particle remained unchanged, and the

radius was doubled, so the volume increased eight times, and the density was one-eighth of the physical test value of 1618.95 kg/m3, which is 202.37 kg/m3.

**Figure 7.** Ramie stalk discrete element model: (**a**) xylem discrete element model; (**b**) phloem discrete element model.

2.4.3. Calibration of Contact Parameters for Phloem–Xylem Discrete Element Model Calibration of the Phloem–Xylem Restitution Coefficient

In the physical measurement tests of phloem–xylem restitution coefficients, the coefficient was measured by the rebound height of the phloem and xylem attached to the steel plate. A discrete element simulation test was set up with the same experimental conditions. The collision simulation test was set with an initial collision height of 205 mm, a time step length of 20% of the Rayleigh time, and a save interval of 0.001 s. The rebound height was obtained by reading the data through the analyst module.
