Fund Project:Project supported by the National Natural Science Foundation of China(Grant No. 11802094)and the Fundamental Research Fund for the Central Universities, China(Grant No. 2018MS043).
Received Date:17 December 2018
Accepted Date:08 April 2019
Available Online:06 June 2019
Published Online:05 July 2019
Abstract:Granular medium is ubiquitous in nature, and is an important issue in many infrastructural construction projects. In particular, the gravity discharge of fine particles from a silo constitutes an important problem of research, because of its many industrial applications. However, the physical mechanism of this system remains unclear. In this work, we study the discharge of silo from the bottom or lateral orifice, by performing pseudo-three-dimensional (3D) continuum simulations based on the local constitutive theory. The simulation is two-dimensional (2D), in order to study the 3D silo, we add the lateral frictional force in the averaged momentum equation. For a rectangular silo with an orifice of height $D$ and the silo thickness $W$, we study the influence of the orifice size ($W$ and $D$) on the granular pressure and velocity. The force analysis and simulation results reveal that for the relation between the granular pressure and the orifice size, there exist two regimes: when $D/W$ is small enough, the pressure near the orifice varies only with $D$; when $D/W$ is large enough, the pressure varies only with $W$. These scaling laws are the same for both bottom and lateral orifice. Somewhat surprisingly, the simulation results also show that when the orifice is at the bottom, the scaling law of the vertical velocity is different from that of the pressure; when it is on the lateral side, the scaling law of the horizontal velocity is consistent with that of the pressure. This observation contradicts a hypothesis that the flow rate of discharge is controlled by the granular pressure near the orifice, and validates the recent experimental results reported in the literature. Furthermore, the relationship between the vertical velocity and the orifice size reveals that when the orifice is at the bottom, the critical value of $D/W$ for the transition of regime is much larger than the lateral orifice case, the flow rate will depend only on $W$ when $D/W\gg50$. This condition is hardly satisfied in practice, so the new scaling law has not yet been observed for the bottom orifice case in the literature. Furthermore, this work demonstrates that the stagnant zone has an important effect on the discharge of silo, especially for the lateral orifice case. Since a non-local constitutive law can well describe the quasi-static flow, it will be interesting to modify the local constitutive model into a non-local constitutive model, and to compare the results from the two models. Keywords:local constitutive law/ continuum simulation/ granular medium/ discharge of silo
选取筒仓卸载过程中流量稳定情况下的某一时刻$ t/\sqrt{L/g} = 4 $(其中$ L $为容器宽度), 该时刻颗粒物压强场、速度场以及无量纲数$ I $的分布在图2中画出. 从容器顶部至底部看压强场云图, 压强变化呈现如下几个特点: 1) 在颗粒物球床较高的位置($ z \approx 3L $), 颗粒物压强随着$ z $降低增大; 2) 很快压强不再升高, 而是在$ 1<z/L<2 $区域几乎保持不变; 3) 对于$ z<L $区域, 压强迅速下降直至出口处压强为$ 0 $; 4) 靠近容器壁处压强比中心处更大, 可见容器壁承担了部分颗粒物的重量, 体现了Janssen 效应. 从速度场云图可以看出, 在$ z>L $的区域内, 竖直方向速度大小几乎相等, 而位于容器底部, 在尺寸约为$ D $的区域内, 速度随着$ z $减小显著增大. 另外, 颗粒物停滞区域(即速度大小为$ 0 $)的边界线与竖直方向所成夹角几乎保持不变. 从无量纲常数$ I $的分布图可以看出, 其数值在整个容器区域内都比较小, 沿着容器壁以及停滞区域的颗粒物剪切力较大, 此时$ I $的值也较大(浅蓝色区域), 但$ I $值最大的区域是颗粒物加速度较大并且压强较小的出口处. $ I $的最大值为$ 0.2 $, 表示临近出口处颗粒物仍旧为密集颗粒流. 图 2$ D = 0.3125L $以及$ W = 0.25L $情况下$ t/\sqrt{L/g} = 4 $时刻连续数值模拟结果, 从左至右: 容器内压强与其最大值之比; 竖直方向速度与其最大值之比, 其中黑色实线表示颗粒物流线; 无量纲常数$ I = d\sqrt{2}D_2/(\sqrt{p/\rho}) $ Figure2. Continuum simulation results with $ D = 0.3125L $ and $ W = 0.25L $ at time $ t/\sqrt{L/g} = 4 $, from the left to the right: pressure $ p^{\rm p} $ normalized by it's maximum value within the silo; the vertical velocity $ v^{\rm p} $ normalized by it's maximum value within the silo, the black lines represent the streamlines; dimensionless number $ I = d\sqrt{2}D_2/(\sqrt{p/\rho}) $.
可见在靠近出口处, 动能项开始逐渐增大并且可以部分补偿压强降低, 但由于摩擦力的存在, 二者之间的关系并不满足伯努利方程. 接下来重点分析距离出口较近处的颗粒物压强$ p^p $和速度$ v^p $与容器厚度$ W $及出口高度$ D $的关系. 33.1.1.压强结果分析 -->
其中$ \bar{F}_{\rm g} $代表颗粒物之间的摩擦力(图3中实心箭头), $ \bar{F}_{\rm w} $代表颗粒物与前后容器壁之间的摩擦力(图3中空心箭头), $ P $为重量. 假设$ p^{\rm p} $在这个区域内几乎无变化. 图 3 出口在底部情况下容器内不同区域受力图 Figure3. Force diagram of different zones within the silo for the case with orifice at the bottom.
图6显示了出口在侧面的情况下流量恒定期间某一时刻压强、速度以及无量纲常数$ I $的分布图. 与出口在底部的情况非常相似, 压强在出口处降低而速度显著增大, $ I $的值在出口处最大, 且最大值为$ 0.1 $左右. 接下来分析不同区域内的压强$ p^{\rm p} $和速度$ v^{\rm p} $值与容器厚度及出口尺寸的关系. 图 6$ D = 0.40125L $以及$ W = 0.25L $情况下$ t/\sqrt{L/g} = 4 $时刻连续数值模拟结果, 从左至右: 容器内压强与其最大值之比; 总速度与其最大值之比, 其中黑色实线表示颗粒物流线; 无量纲常数$ I = d\sqrt{2}D_2/(\sqrt{p/\rho}) $ Figure6. Continuum simulation results with $ D = 0.40125L $ and $ W = 0.25L $ at time $ t/\sqrt{L/g} = 4 $, from the left to the right: pressure $ p^{\rm p} $ normalized by it's maximum value within the silo; the total velocity $ U^p $ normalized by it's maximum value within the silo, the black lines represent the streamlines; dimensionless number $ I = d\sqrt{2}D_2/(\sqrt{p/\rho}) $.
33.2.1.压强结果分析 -->
3.2.1.压强结果分析
如图7所示, 研究沿着流线方向距离出口约为$ D $的区域, 可假设颗粒物几乎沿着竖直方向运动, 且流动区域沿$ x $方向的尺寸约为$ D $, 同出口在底部情况做相同的受力分析, 可以得到沿中心流线方向距离出口$ D $处的压强与$ D $和$ W $的关系为 图 7 出口在侧面情况下容器内不同区域受力图, 图中阴影部分代表颗粒物停滞区域 Figure7. Force diagram of different zones within the silo for the case with lateral orifice, the dashed area represents the stagnant zone.