•    When a fluid is passed upwards at increasing velocities through a column of granular material of size d, a stage is reached when all the paticles are entrained in the flow and the bed is in a fluid state. This stage is called as fluidization.

•    The corresponding superficial velocity, V_f as fluidization velocity and is given by

V_f  = ( γs - γ ) d² e² / 18 μ

where   e = porosity of material

•    It is used in the back washing of gravity filters for water purification.

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