ABSTRACT
Janssen published his work on wall pressures in 1895. It took more than 50 years before the first idea was published on the way the stored bulk solid moves into and out of the silo, when Brown and Hawksley (1947) reported on experiments with cohesionless bulk solids. During discharge five areas can be distinguished (Fig. 3.1.1): single particles roll freely along layer A, which itself is rapidly moving as a block over layer B, moving very slowly compared to layer A. Area E is not moving at all. The inclination of the dividing plane between B and E is greater than the angle of repose which can be observed on the free surface. As soon as particles or groups of particles reach zone C they are accelerated. Particles in area D lose contact with neighbours and fall freely. At incipient flow the areas D, C, A and B are formed one after the other. If the hopper walls are steep enough, so that all material is in motion, layers A and B can hardly be distinguished and the transition into zone C is fluent.
