Unless the material surrounding the flow channel holds together to form a pipe or rathole, the local surface depression of the flow channel allows adjacent material to collapse and form a drained cone of repose. The flowing core then passes the material running from the surface layers. The surface cone of repose extends, until it eventually meets the walls of the container. At this stage the level of material against the wall starts to decrease, the material moving away from the wall rather than sliding down the wall surface. The characteristic shape of this combined patterns of drained repose and core flow led Jenike to term this mode of behaviour funnel flow (see Fig. 1). A drawback of this form of flow channel is that freshly charged product is discharged before material that has been previously filled into the container. This occurs on first fill, and also if the container is refilled before all the previous contents have been discharged. When a core flow type storage container is normally refilled before completely discharging it is not possible to predict the maximum residence time of the product, as some regions will never discharge until the hopper is completely emptied.

A material loaded into a container in a fluid state is prone to flush through the outlet unless held to settle to a stable condition. The nature of materials which favour loose fluidity when dilate, i.e. fine powders of low density, are also those which leads to a poor flow condition when settled. A further hazard with this class of product occurs when it settles to a marginal flow condition and temporary voids form under unstable arches. The collapse of the arch allows the material to entrain air, become highly fluid, and pour through an outlet if not positively restrained. Such a falling arch can progress through the entire bulk, or even worse, develop a route to release more recently-filled material in a fluid condition from the top of the hopper. Core flow installations handling this kind of material can offer problems of both intransigent flow and uncontrolled flow.

Nevertheless it is a fact that the majority of bulk storage containers used by industry are of the core flow type. The main reason is that for given available headroom this form of container holds a greater volume than a mass flow type, and hence the cost per unit volume is lower for the container and also the equipment that fills the unit. With bulk materials that offer no flow problem this is the preferred choice, as it often is for abrasive products that cause aggressive wear on container walls if they slide under high contact pressures. Industries greatest problem appears to be that of recognising and evaluating potential flow problems at the design stage. A close second is not employing expanded flow often enough to help counter these problems.