# Reynold’s Number | Definition, Formula – Hydrodynamics

Reynold’s Number Definition:
Reynold’s number is a pure number. It is equal to the ratio of the inertial force per unit area to the viscous force per unit area for a flowing fluid.

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## Reynold’s Number | Definition, Formula – Hydrodynamics

Reynold’s Number Formula:
Reynold number,

K = $$\frac{\text { Inertial force }}{\text { Force of viscosity }}=\frac{v_{c} \rho r}{\eta}$$

where, vc = critical velocity.

For pure water flowing in a cylindrical pipe, K is about 1000.

• When 0 < K < 2000, the flow of liquid is streamlined.
• When 2000 < K < 3000, the flow of liquid is variable between streamlined and turbulent.
• When K > 3000, the flow of liquid is turbulent.

It has no unit and dimension.

Hydrodynamics:
In physics, hydrodynamics of fluid dynamics explains the mechanism of fluid such as flow of liquids and gases. It has a wide range of applications such as evaluating forces and momentum on aircraft, prediction of weather, etc.