Reynolds’s number is the ratio of inertial forces and viscous forces.

In practice, the Reynolds number used to predict if the flow will be laminar or turbulent.

This means that the Reynolds number is a unitless quantity. If the Reynolds number is less than 2300, the flow will be laminar. Any number over 4000 indicates the turbulent flow.

 Re = Inertial Force / Viscous Force

        =   VDρ/ μ

 V  = Velocity based on the actual cross-sectional area of pipe or duct.

 D  = Hydraulic Diameter of the pipe

 ρ  =  Density of the fluid.

  μ  = Dynamic Viscosity.

Laminar region Re = 2300

Laminar flow generally happens when dealing with small pipes and low velocities. Laminar flow can be regarded as a series of liquid cylinders in the pipe, where the innermost parts flow the fastest, and the cylinder touching the pipe is not moving at all.

Turbulent region Re > 4000

Turbulent flow generally happens in large flow rates and with large pipelines.

Transitional region 2300 < Re < 4000

Transitional flow is a mixture of laminar flow and turbulent flow, with turbulence at the center of the pipeline and laminar flow near the edges.