Consider a vessel open at the top and filled with relatively coarse particulate solids.
Suppose fluid is injected vertically through a centrally located small opening at the base
of the vessel. If the fluid injection rate is high enough, the resulting high-velocity jet
causes a stream of particles to rise rapidly in a hollowed central core within the bed of
solids. These particles, after being carried somewhat beyond the peripheral bed level,
rain back onto the annular region between the hollowed core and the column wall, where
they slowly travel downward and, to some extent, inward as a loosely packed bed. As the
fluid travels upward, it flares out into the annular region. The overall bed thereby becomes
a composite of a dilute phase central core with upward-moving solids entrained by a
cocurrent flow of fluid, and a dense phase annular region with countercurrent percolation
of fluid. A systematic cyclic pattern of solids movement is thus established, giving rise to
a unique hydrodynamic system that is more suitable for certain applications than more
conventional fluid-solid configurations.