An Etymological Dictionary of Astronomy and Astrophysics
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فرهنگ ریشه شناختی اخترشناسی-اخترفیزیک

M. Heydari-Malayeri    -    Paris Observatory

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Number of Results: 27 Search : old
Reynolds number
  عدد ِ رینولدز   
adad-e Reynolds (#)

Fr.: nombre de Reynolds   

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.

Riemannian manifold
  بسلای ِ ریمانی   
baslâ-ye Riemanni

Fr.: variété riemannienne   

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

Riemannian; → metric.

Taylor-Goldstein equation
  هموگش ِ تیلر-گلدشتاین   
hamugeš-e Taylor-Goldstein

Fr.: équation de Taylor-Goldstein   

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.

threshold
  آستانه   
âstâné (#)

Fr.: seuil   

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."

threshold energy
  کاروژ ِ آستانه   
kâruž-e âstâné

Fr.: seuil d'énergie   

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

threshold; → energy.

threshold of reaction
  آستانه‌ی ِ واژیرش   
âstâne-ye vâžireš

Fr.: seuil de réaction   

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

threshold; → reaction.

threshold signal
  آستانه‌ی ِ نشال   
âstâne-ye nešâl

Fr.: seuil de signal   

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

threshold; → signal.

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