Optimizing Constriction Flow with Submerged Granular Hoppers

In granular hoppers, discrete particles flow out small hole due to gravity. Usually the role of air is considered negligible on both discharge rate and clogging probability. However, this is not so for hoppers submerged in a viscous fluid like water. Our experiments [1] and simulations [2] show a counterintuitive terminal surge, where the discharge rate increases as the hopper empties. This is accompanied and caused by a fluid-is-faster effect whereby the interstitial fluid flows downwards faster than the grains. For applications, this raises the possibility to (i) tune the discharge rate by pumping fluid through the medium at controlled rate, (ii) prevent clogging, and (iii) maximize throughput.
[1] J. Koivisto and D.J. Durian, Nat. Comm. 8, 15551 (2017).
[2] J. Koivisto, M. Korhonen, M. Alava, C.P. Ortiz, D.J. Durian, and A. Puisto, Soft Matter C7SM00806F (2017).