*DEFEM Method and Its Application in Pebble Flows DOI: http://dx.doi.org/10.5772/intechopen.109347*

It can be seen from **Figure 9** that the peak stress is smaller when the bottom inclination is 45 degrees. The initial positions of particles at different bottom inclination angles are the same. When the bottom inclination is 45 degrees, the particles have contact with the wall on both sides earlier in the process of falling, which can also be verified as shown in **Figure 9**(**b**). When the bottom inclination is 15 degrees, the stress remains zero at the beginning, and then the stress begins to become non-zero around 0.185 seconds gradually. If the bottom inclination angle is smaller, the acceleration time of particles is longer due to the action of gravity, and the velocity of collision with the wall is larger. Hence, the contact force and stress are larger. In the stable state formed during the accumulation process, it can be seen that there is a great difference in the average stress of particles between the near-wall region and central region when the bottom inclination is 45 degrees. When the bottom inclination is 15 degrees, there is little difference between the two. When the bottom inclination is larger, the pressure on the side wall of the pebble bed is larger, and the average stress difference between the particles in the near-wall region and central region is larger.

[21] Differences between the near-wall region and central region.

Due to the structure of the pebble bed, particles come into contact with the side wall first, so the stress value of particles in the near-wall region changes first compared with that in the central region. However, the collision of particles in the central region is more intense, so the peak stress in the central region is greater than that in the near-wall region. Moreover, the velocity of particles changes after being affected by the wall, and the velocity-changed particles are more likely to collide with the particles in the central region. Therefore, there is a certain phase difference between the average stress change of particles in the near-wall region and the central region; that is, the average stress change of particles in the central region always lags behind that in the near-wall region. The average stress of particles in the central region is greater than that in the near-wall region.
