Reynolds number [for Osborne Reynolds], dimensionless quantity associated with the smoothness of flow of a fluid. It is an important quantity used in aerodynamics and hydraulics. At low velocities fluid flow is smooth, or laminar, and the fluid can be pictured as a series of parallel layers, or lamina, moving at different velocities. The fluid friction between these layers gives rise to viscosity. As the fluid flows more rapidly, it reaches a velocity, known as the critical velocity, at which the motion changes from laminar to turbulent (see turbulence), with the formation of eddy currents and vortices that disturb the flow. The Reynolds number for the flow of a fluid of density ρ and viscosity η through a pipe of inside diameter d is given by R = ρ dv /η, where v is the velocity. The Reynolds number for laminar flow in cylindrical pipes is about 1,000.