Stoke’s law - Reynold's Number

Stoke’s law

Suppose that a spherical body of radius r moves at a speed v through a fluid of viscocity η. The viscous force F acting on the body is:


F = 6 π ηrv


Reynold’s number

The quantity

N = ρvD/ η

Where
ρ = density of the liquid
v = velocity of the liquid
D = diameter of the tube through which the liquid is flowing
η = coefficient of viscocity of the liquid

is called Reynold’s number.

If the Reynold’s number is less than 2000, the flow is steady.
If the Reynold’s number is more than 3000, the flow is turbulent
If the Reynold’s number is more than 2000 but less than 3000, the flow is unstable.