3.2 Simulation conditions
Large and small glass bead particles, described by the median sizes of
the actual particles, 180 and 40 µm respectively, are used in the
current study. Both particles are assumed to be spherical and possess
the same material properties. Initially, a number of large spheres and
small spheres are randomly generated in the simulation domain, periodic
in two dimensions, with a specified mixture composition. The size of the
domain is \(30d_{S}\times 30d_{S}\times 15d_{S}\), in which \(d_{S}\)is the diameter of the small particle. Based on our previous
work2, it is believed the simulated shear stress
results are independent of the domain size, although the domain size is
smaller than the shear cell size. Cases with mass fraction of large
particles ranging from 0 to 1 are conducted. After all particles deposit
on the bottom wall of the DEM shear cell and achieve a negligible
velocity, a normal pressure (2000 Pa) is specified on the top wall and a
velocity (shear rate of 0.2 s-1) is applied to the
bottom wall to start the shearing process as shown in Figure 5. The
shear rate is higher than the experimental one to reduce the
computational expense but it also assures a quasi-static state in which
the shear stress is independent of shear rate2,20.
Shearing bars (blades) on both top and bottom walls are also created to
mimic the configuration of the realistic shear cell and prevent the
sliding of particles. Simulations run sufficient time steps until the
shear process reaches a steady state and the stresses are nearly
constant with small fluctuations. The particle properties in the
simulations are given in Table 1.