| 摘要(英) |
The purpose of the study is to investigate the convection mechanism of granular assemblies in quasi-2D vibrating beds by using discrete element method (DEM). The study investigates the effects of side walls, bottom walls and sidewall inclination on the flow behavior of particles in the vibrating beds. The vibration condition is at a frequency of f=25Hz and an acceleration amplitude of 12g (g is acceleration of gravity). The physical properties include the translational velocity vector, convection rate, granular temperature, coordination number, solid volume fraction and granular stress. The relationships amongst these physical properties are also explored. Numerical results corroborate the existence of shear bands and these shear bands drive particles to circulate in the vibrating beds. The strength of the shear bands corresponds to the convection rate, and the direction of shear stresses on both sides determine the direction of the convection cells. The shear stress of the positive convection shows a negative value in the right side and a positive value in the left side, and vice versa. As the direction of the shear bands reverses, the direction of the convection cells also reverses correspondingly.
Numerical results also show that under this vibration condition, the particle convection phenomenon takes place and exhibits two main convection cells. If there are particles glued on the vertical side walls, the particle bed shows positive convection. If there are no particles glued on the vertical side walls, even when some particles are glued on the bottom wall, the particle bed still exhibits negative (reverse) flow. For the vibrating beds with the side walls having particles glued one by one, as the inward inclination angle is less than or equal to 0o, the particle beds show positive convection. Conversely, as the outward inclination angle is greater than or equal to 15 o, the particle beds show negative (reverse) convection. As the inclination angle varies from inward inclination to outward inclination, there exists a transition regime indicating that positive convection is transformed into negative (reverse) convection and that the convection rate increases with the magnitude of the inclination angle. For the vibrating beds with four different side walls and with four different bottom walls, the convection rate decreases with the increase of particle spacing on the particle walls. However, the granular temperature near the particle walls increases with particle spacing. The normal and shear stresses of the side walls decrease with the increase of the particle spacing, and the convection rate also has the same trend. The normal and shear stresses of the inclined side walls generally increase with the magnitude of the inclination angle, and the convection rate also shows the same trend. |
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