To describe the flow behaviour of bulk material in a structured manner, definitions of local flow regimes are set out that represent specific zones of flow, as characterised by boundary conditions that are common in that region of the flow channel. The flow boundary may be a confining wall, a region of static product, exposure to an unconfined surface, or some combination of these. Velocity gradients, if any, within the flowing bulk are of interest, but these are usually secondary to the form and nature of the flow channel boundary. Bulk flow is essentially a three-dimensional phenomenon and a massive distinction exists between flow channels that change dimensions in two planes to those that only alter in cross section in a single plane

The definitions used here embrace existing terms and particularly accepts the concept of a mass flow hopper as a generic case of a bulk storage container in which the whole contents move during the discharge process, with slip taking place on all the wall contact surfaces. The term container is used throughout to represent all manner of bulk storage vessels, variously described as hoppers, silos, bunkers, vessels, bins, and similar devices for storing loose bulk materials, of all scales and geometry, and subjected to varied cycles and means of filling and discharge.

There are four basic ways in which bulk materials move during the emptying of a storage container:

Repose Flow, where surface layers of the material slide down an inclined static bed of product according to the repose condition of the bulk material (marked 1 on the top regions of Figs. 1a, 1b, and 4). For more information see Section 3.1.

Core Flow, or internal flow, where the flow channel in bounded by a static region of the stored material (marked 2 in the lower regions of Figs. 1a, 1b, 2, and 5b and the middle regions of Figs. 4 and 5a). For more information see Section 3.2.

Mass Flow, or converging mass flow, where the cross section of the flow channel extends to the confining wall of the container, but the material is subjected to internal deformation in a converging flow channel (marked 3 in Fig. 3a, and the lower regions of Figs. 3b, 4 and 5a). For more information see Section 3.3.

Bed Flow, or coherent flow, where material moves in a parallel flow channel (marked 4 on the top regions of Figs. 2, 3b, 5a and 5b). For more information see Section 3.4.

In practical bulk storage containers these zones of local flow behaviour take place in various combinations according to the geometry of the container, the properties of the bulk material and their interface characteristics, to give various global patterns of behaviour. The more common ways in which these global patterns occur during the discharge of bulk storage containers are shown in Figs. 1 - 5. Discharging devices that offer an expanded outlet, as Fig. 5b, act as an enlarged size of opening with restrained flow so far as the flow pattern is concerned, provided that extraction takes place over the whole outlet area.