Reynolds Number

This is a non-dimensional number (consistent units) which describes whether fluid flow is laminar, turbulent, or transitional between the two. It is significant in the design of a model of any system in which the effect of viscosity is important in controlling the velocities or the flow pattern of a fluid.

where
Re = Reynolds Number
ρ = density [kgm-3]
U = characteristic velocity [ms-1]
L = characteristic length scale [m]
η = dynamic viscosity [Nsm-2]
Note:
Re < 2000 - laminar flow.
2000 < Re < 3000 - unstable flow.
Re > 3000 - turbulent flow.

Critical Reynolds Number

The Reynolds number at which some significant change occurs, e.g., the Reynolds number at which a transition from laminar to turbulent flow begins, or at which the drag of a cylinder or sphere drops sharply.

See also: Froude Number, Poise, Prandtl Number, Scale Effect, Strouhal Number.

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Subjects: Mechanical Engineering Physics