Reynolds Number

Published: February 27, 2019 | Last updated: July 5, 2023

What Does Reynolds Number Mean?

Reynolds number is used to classify flow pattern in a pipe as laminar or turbulent, referring to properties such as velocity, viscosity, length and type of flow. Reynolds number is a dimensionless quantity and is the ratio of inertial forces to viscous forces. If the fluid movement is caused by inertial forces that resist the change in velocity of an object, the flow is turbulent, and if viscous forces are dominant, the flow is laminar.

Trenchlesspedia Explains Reynolds Number

When calculating critical velocity, Reynolds number plays an important role. Critical velocity is that velocity at which flow of a fluid turns from laminar to turbulent.

Reynolds number is given by the relation:

NR = ρVcritD/ μ


NR – Reynolds number

μ – coefficient of viscosity or resistance to flow in m2/ sec

D – internal pipe diameter in meter

ρ – density of fluid in kg/m2

If the Reynolds number lies between 0-2320, the flow is considered streamlined or laminar. Reynolds number between 2320-4000, indicates an unstable flow condition ranging from streamlined to turbulent. Reynolds number greater than 4000 indicates a turbulent flow meaning the flow velocity is critical.

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