Reynold's Number:

Reynold's number is a pure number which gives an idea whether the flow of fluid is laminar or turbulent. The Reynold's number is given as (see above) :

k = vc ρ d/η

Eq. can also be written as:

k =   ρ v²_c /( η v_c  / d) = Inertial Force/Force of Viscosity

Thus, the Reynold's number k is the ratio of the force of inertia and the force of viscosity. The flow of the fluid is said to be laminar when the value of k is less than 2000. For values of k above 3000, the flow is turbulent. For k between 2000 and 3000 the flow is unstable and may switch over from laminar to turbulent and vice-versa.