The importance of flow patterns during discharge

The discussion above concerning pressures during emptying has omitted a key aspect that became very clear during the 1960s and 1970s. The manner in which a solid flows within the silo has a major effect on the pressures exerted on the silo wall. 

These images show an idealised version of the pattern of flow. The real boundaries of flow channels often vary a little from time to time because they depend quite sensitively on small changes in the packing of particles (Arnold 1991). Further, the idealised pattern is shown with the silo completely full, but the pattern cannot develop until some solid has come out at the bottom (unless it is being continuously replenished). However, because the critical design condition is almost always when the silo is fuU, this is the idealised reference shape. 

Finally, it must be clearly repeated that it is not yet possible to predict the geometries of pipe flow and mixed flow solids flow patterns, so this rather critical distinction is not yet quantifiable. The distinction is therefore not used in the design mles of EN 1991-4 (2007). 

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