(94b) Flow Rate Interference Effects in a Silo with Two Openings

Here we study the mass flow-rate of grains from a flat-bottomed rectangular silo with two symmetrically-spaced openings, each of width W. In recent DEM and experimental studies [1,2] it was found that as the spacing between these two openings was increased the flow rate decreased monotonically from a maximum (when the spacing was zero, i.e. a single large opening of size 2W) to a minimum when the openings are far apart. However, in an analogous system, the evacuation of pedestrians from a room with two exits, as the separation between exits doors was increased the flow rate dropped, reached a minimum value, and then rose again to an equilibrium rate.
Using a load-cell measuring device we have experimentally measured the mass flow rate of grains from our silo, modifying the spacing between the openings. Our results were performed with mustard seeds (d = 2.15mm), amaranthus (d = 1.08mm), and screened sand (d = 0.46 mm). We found that we also recovered non-monotonic behaviour. Initially, with a single opening of size 2W the flow rate was at a maximum. Similarly to the pedestrian case [3], as the spacing, L, between the openings was increased, the flow rate rapidly dropped, reached a minimum, then rose to a rate close to twice that of a single opening of size W. This is the first experimental evidence of such behaviour in a granular silo system.

Our experiments were then simulated using DEM (MFix). For values of low inter-granular friction monotonic behaviour was observed (as in [1,2]), but for larger values, the flow-rate behaved as in our physical experiment and the pedestrian dynamics simulation [3]. These observations illustrate the importance of granular friction in the drainage dynamics of these systems.

[1] Zhang, X., Zhang, S., Yang, G., Lin, P., Tian, Y., Wan, J. F., & Yang, L. (2016). Investigation of flow rate in a quasi-2D hopper with two symmetric outlets. Physics Letters A, 380(13), 1301-1305.

[2] Xu, C., Wang, F. L., Wang, L. P., Qi, X. S., Shi, Q. F., Li, L. S., & Zheng, N. (2018). Inter-orifice distance dependence of flow rate in a quasi-two-dimensional hopper with dual outlets. Powder Technology.

[3] Sticco, I. M., Frank, G. A., Cerrotta, S., & Dorso, C. O. (2017). Room evacuation through two contiguous exits. Physica A: Statistical Mechanics and its Applications, 474, 172-185.