Reynolds number

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The Reynolds number (Re) is the ratio of inertial resistance to viscous resistance for a flowing fluid.

It is named after the British physicist and engineer Osborne Reynolds who is generally regarded as the first to realize its importance in 1883.

Reynolds number, in fluid mechanics, a criterion of whether fluid (liquid or gas) flow is absolutely steady (streamlined, or laminar) or on the average steady with small unsteady fluctuations (turbulent).

Whenever the Reynolds number is less than about 2,000, flow in a pipe is generally laminar, whereas, at values greater than 2,000, flow is usually turbulent.

Actually, the transition between laminar and turbulent flow occurs not at a specific value of the Reynolds number but in a range usually beginning between 1,000 to 2,000 and extending upward to between 3,000 and 5,000.