Terminal falling velocity and particle drag coefficient

Terminal falling velocity and particle drag coefficient 

At its terminal falling velocity the particle no longer accelerates and 

Stokes (1851) first showed that the drag force F on a sphere was given by 

This is Stokes law which is valid in the particle Reynolds number range 10 Re 0.20 where the Reynolds number is defined by 

A particle drag coefficient Cd can now be defined as the drag force divided by the product of the dynamic pressure acting on the particle (i.e. the velocity head expressed as an absolute pressure) and the cross-sectional area of the particle. This definition is analogous to that of a friction factor in conventional fluid flow. Hence 

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