**3.1 Geometric parameter settings**

The particle accumulation process in a two-dimensional pebble bed is simulated. As shown in **Figure 6**, the bottom inclination angle of the pebble bed is changed in a certain range. The bottom diameter of the pebble bed is 0.2 m, and the bottom inclination angle is 15 degrees or 45 degrees, as shown in **Figure 6**. In the initial state, the particles with a regular arrangement move downward vertically at a certain velocity. The particles collide with the wall of the pebble bed or other particles and finally form a stable accumulation structure at the bottom of the pebble bed. Two regions in the pebble bed are defined: the central and the near-wall regions. The particle motion characteristics in two different regions are discussed. In the case of this paper, the near-wall region is within 0.2 m from the left and right walls, and the central region is within 0.2 m from the central axis of the pebble bed (the selection of values with careful consideration of particle diameter and pebble bed size). The velocity, stress, and deformation during particle accumulation are discussed and analyzed under the different pebble bed geometry structures. The specific parameters in the numerical simulation are shown in **Table 1**.
