Reynolds Number
This is a nondimensional 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.

 Note:
 R_{e} < 2000  laminar flow.
 2000 < R_{e} < 3000  unstable flow.
 R_{e} > 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.
Subjects: Mechanical Engineering Physics