Pressures from flowing granular solids in silos

Abstract

Large displacement flows in granular solids occur whenever silos are discharged. Measurements of pressures during flow, combined with visual observations of flow patterns and control tests on the ensiled solids, have revealed several phenomena contributing to pressure variations during flow. Not only are the pressure variations time dependent within a single silo, but significant systematic differences are found between one silo and another, even when the two are superficially identical and contain similar ensiled materials. One serious outcome is that silo pressures are quite unsymmetrical even in symmetrical silos, and this is a most dangerous phenomenon for the safety of the silo structure.The loss of symmetry can largely be traced to inhomogeneity and anisotropy in the ensiled material, developed by the initial packing during filling. Thus, unless the mechanism of packing is understood, the mechanics of these solids cannot be modelled. The sensitivity of granular materials to stress history and the role of geometrically imperfect boundaries present other complications in interpreting observations and understanding the mechanics of silo pressures and flow regimes.No current theory of silo loads covers these phenomena, which themselves are only illustrative examples of current shortcomings. More comprehensive constitutive models are needed for application to silos and large-scale granular solids flows.

The role of particle shape on the stress distribution in a sandpile

“…Indeed, previous investigations of the behaviour of granular media in other configurations have shown important effects of particle shape. These include investigations of the particle packing fraction by Sugden (1980), Donev et al (2004) and Lumay & Vandewalle (2004), wall pressures in the discharge of a silo by Nielsen (1998) and Zhong et al (2001) and jamming in the flow of grains through an orifice by Zuriguel et al (2005). The general consensus from all of these studies is that orientational effects of the particles are important.…”

Influence of the feeding mechanism on deposits of square particles

“…In deposits formed under the action of gravity, this particular stress transmission leads to characteristic phenomena such as the pressure dip below the apex of a pile [5,6], or the Janssen effect [7], i.e., pressure saturation in silos. Moreover, it is well known [8][9][10] that these phenomena strongly depend on the packing formation history. In particular, the pouring process has been shown to have a remarkable influence on the global mechanical properties of the packing [8,11].…”

Granular packings of elongated faceted particles deposited under gravity

“…In the last decade many different studies have proved that the introduction of modifications in the particle shape may give rise to completely different physical scenarios [10]- [14]. These include the effect of particle shape on packing fraction [11,12], wall pressures in a silo discharge [13], coordination number [14], jamming [15], and stress propagation in granular piles [16]. Despite these efforts, the aspect ratio has only very recently been systematically modified to analyze the special features that particle shape asymmetry may introduce in the packing [11], [17]- [19].…”