What is the significance of Reynold's number?

The Reynolds number is a dimensionless number that is used to assess the flow condition of fluids whether bounded or unbounded based on empirical results.

The Reynolds number is a ratio of momentum or inertial forces to that of viscous forces and the higher this number signifies the degree to which the inertial forces such as (forces due to rotational flow or irregular) takes over the viscous forces that arise due to boundary conditions, fluid characteristics etc..

A high Reynolds number usually indicates turbulent flow conditions whereby an assessment of the flow should be conducted via experimentally available data as there is no direct methods of evaluating the characteristics of turbulent flow.  

The Reynolds number provides a measure of the conditions or range of conditions at which a boundary layer becomes turbulent depending on the density, viscosity and velocity of a fluid in motion with respect to a surface and for a characteristic length defined for the particular geometry such as a wing surface or the ID of a tube. The turbulent transition may also depend on surface roughness.

Reynold number is the ratio of inertia force to viscous force, and it is signify the flow characteristic whether flow is bounded or unbounded. it is also shows flow pattern such as turbulence flow or laminar flow depend on property of the fluid , characteristic or equivalent diameter.

ex- Re (Reynold number) for cylindrical body,

Re<; signify laminar flow,

<Re<; signify transition flow,

> Re ; signify Turbulent flow.