An Etymological Dictionary of Astronomy and Astrophysics
English-French-Persian

A dimensionless quantity that governs the conditions for hydrodynamic stability and the occurrence of turbulence in fluids. It is defined by the ratio, R, of the inertial force (ρ u²) and the viscous force (μ u / L), i.e. R = L u ρ/μ, where L is a typical dimension of the system, u is a measure of the velocities that prevail, ρ the density, and μ the kinematic viscosity. At low Reynolds numbers the flow is steady, since the viscous forces are predominant in controlling the flow. At a critical value of R, corresponding to a critical velocity, the flow becomes turbulent.

Named after Osborne Reynolds (1842-1912), a British physicist who pioneered the study of turbulent flows; → number.

A → manifold on which there is a defined → Riemannian metric (Douglas N. Clark, 2000, Dictionary of Analysis, Calculus, and Differential Equations).

→ Riemannian; → metric.

Fluid mechanics: A second order differential equation that governs the vertical structure of a perturbation in a stratified parallel flow.

Named after G. I. Taylor (Effect of variation in density on the stability of superposed streams of fluid, 1931, Proc. R. Soc. Lond. A, 132, 499), → Taylor number, and S. Goldstein (On the stability of superposed streams of fluids of different densities, 1931, Proc. R. Soc. Lond. A, 132, 524); → equation.

The level that must be reached for a physical effect to begin or be noticeable.

M.E. threschold, O.E. threscold, threscwald "doorsill, point of entering."

Âstâné "threshold; a place of rest or sleeping," variant âstân; Mid.Pers. âstânak; ultimately from Proto-Iranian *ā-stānaka-, from *stā- "to stand;" cf. O.Pers./Av. sta- "to stand, stand still; set;" Av. hištaiti; Mid.Pers. êstâtan "to stand;" Mod.Pers. istâdan "to stand;" cf. Skt. sthâ- "to stand;" Gk. histemi "put, place, weigh," stasis "a standing still;" L. stare "to stand;" Lith. statau "place;" Goth. standan; PIE base *sta- "to stand."

The minimum energy necessary for the occurrence of some chemical/physical effect.

→ threshold; → energy.

The minimum energy, for an incident particle or photon, below which a particular reaction does not occur.

→ threshold; → reaction.

The minimum intensity of a signal that can be detected and recognized.

→ threshold; → signal.