Numerical study on inverse grading segregation mechanism of single coarse particles with different particle sizes under two-dimensional cyclic shear

Granular mixtures with size differences can segregate when subjected to shaking or shear. This study investigates the mechanism underlying the inverse grading segregation of single coarse particles with varying sizes under cyclic shear. A self-developed two-dimensional testing device combined with three-dimensional printing technology and the image identification capabilities of the segment anything model enabled the construction of a cyclic shear numerical model based on rigid blocks. The analysis concentrated on the movement of coarse particles and the evolution of the macroscopic structure of the particle system, and the local topological structures surrounding single coarse particles. The findings reveal the following: (1) Larger coarse particle sizes and lower shape factors under cyclic shear result in shorter times to free surface and higher vertical velocities. (2) Throughout the cycles, the vertical net force acting on each coarse particle fluctuates around zero, while its vertical position displays a zigzag upward trend. (3) Within a single typical cycle, larger coarse particles increase the local void ratio, aiding their lift. Vertical displacement and net force exhibit a double peak pattern inversely related to coordination number, while horizontal displacement fluctuates periodically around zero. (4) Weighted local degree centrality negatively correlates with vertical displacement of single coarse particles, reflecting the dual influence of particle size and importance on segregation velocity. Fine particles occupying the two lower corners of single coarse particles create the lifting effect, driving their zigzag upward motion. Additionally, larger coarse particles enhance their importance, accelerating the segregation process.