| 摘要: | 本研究採用離散元素法(Discrete Element Method, DEM)探討類二維顆粒振動床的迴流機制，在振動頻率為f=25Hz與振動加速度為12g(g為重力加速度)的條件下，探討不同側壁、不同底壁與不同邊壁傾角對顆粒體在振動床內流動行為的影響。本研究探討振動床邊界條件對顆粒體迴流現象的影響，探討的物理量包含顆粒體的平移速度向量、迴流率、粒子溫度、配位數、粒子體積佔有率及內部應力，並探討這些物理量相互之間的關聯性。本研究證實剪力帶的存在，而且剪力帶趨動顆粒體產生迴流現象，剪力帶的強度亦反映迴流強度，兩側剪應力的方向決定迴流的方向，正向迴流在邊壁上的剪應力為右負左正，逆向迴流在邊壁上呈現右正左負，當剪力帶的方向改變時，迴流的方向隨之改變。研究結果顯示在該振動條件下，顆粒振動床均呈現迴流現象，而且均具有兩個主要迴流包。當垂直側壁有顆粒時，顆粒體呈現正向迴流，當垂直側壁沒有顆粒時，即使底壁有顆粒，顆粒體仍呈現逆向迴流。當兩側邊壁為連續顆粒排列時，內傾角度小於或等於0度，顆粒體呈現正向迴流，相反地，外傾角度大於或等於15度時，顆粒體呈現逆向迴流。當傾角由內傾轉為外傾，中間存在著過渡區，由正向迴流轉變至逆向迴流，而且迴流強度隨著向內(外)傾斜的角度增加而增強。四種不同側壁的振動床與四種不同底壁的振動床，迴流強度皆隨著顆粒間距的增加而下降，但靠近顆粒壁面區域的粒子溫度皆隨著顆粒間距的增加而增加。兩側邊壁的正向應力與剪應力皆隨著顆粒間距的增加而下降，迴流強度亦有相同的趨勢。兩傾斜邊壁的正向應力與剪應力皆隨著向內(外)傾斜的角度增加而增強，迴流強度亦有相同的趨勢。;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. |
