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: 90 Search : city
velocity gradient
  زینه‌ی ِ تندا   
zine-ye tondâ

Fr.: gradient de vitesse   

Fluid Mechanics: The rate at which the velocity changes with the distance across the flow. When a fluid flows past a stationary wall, the fluid right close to the wall does not move. However, away from the wall the flow speed is not zero. Therefore a velocity gradient exists, which is due to adhesive, cohesive, and frictional forces. The amount of the velocity gradient is characteristic of the fluid.

velocity; → gradient.

velocity law
  قانون ِ تندا   
qânun-e tondâ

Fr.: loi de vitesse   

In the theory of → radiation-driven winds, an equation that describes the behavior of the → wind velocity of → hot stars as a function of distance from the star. This velocity β-law is given by the expression: v(r) = v(1 - R*/r)β, where v is the → terminal velocity, R* is the stellar radius, and r the distance from the center. For → O-type stars, the exponent is estimated to be β = 0.8.

velocity; → law.

velocity of light
  تندی ِ نور، تندای ِ ~   
tondi-ye nur, tondâ-ye ~

Fr.: vitesse de la lumière   

A → physical constant which represents the ultimate speed limit for anything moving through space, according to the theory of → special relativity. It is the speed of propagation of → electromagnetic waves in a vacuum, equal to 299,792.458 km/s (nearly 3 x 108 m/s). The velocity of light appears as the connecting link between mass and energy in the → mass-energy relation. Usually denoted by c, from L. celeritas "swiftness," from celer "swift," → acceleration.

velocity; → speed; → light.

velocity pressure
  فشار ِ توانیک   
fešâr-e tavânik

Fr.: pression dynamique   

dynamic pressure.

velocity; → pressure.

velocity profile
  فراپال ِ تندا   
farâpâl-e tondâ

Fr.: profil de vitesse   

A plot of the fluid velocity as a function of position.

velocity; → profile.

velocity space
  فضای ِ تندا، ~ تنداها   
fazâ-ye tondâyi, ~ tondâhâ

Fr.: espace de vitesses   

Of a dynamical system, a three-dimensional space which consists of the set of values that the velocity can take (vx, vy, vz). → phase space.

velocity; → space.

velocity-distance relation
  باز‌آنش ِ تندا-دورا   
bâzâneš-e tondâ-durâ

Fr.: relation vitesse-distance   

The linear relation wherein all galaxies are moving away from one another, with velocities that are greater with increasing distance of the galaxy. Same as → Hubble's law.

velocity; → distance; → relation.

vorticity
  گردشاریگی   
gerdšârigi

Fr.: vorticité   

In fluid mechanics, a measure of the rate of rotational spin in a fluid. Mathematically, vorticity is a vector field defined as the curl of the velocity field: ω = ∇ x v.
Meteo.: The rotation of air around a vertical axis.

From L. vortic-, from → vortex + → -ity.

Gerdšârigi, from gerdšârvortex + -igi, → -ity.

wind velocity
  تندای ِ باد   
tondâ-ye bâd

Fr.: vitesse de vent   

The speed at which the → stellar wind is forced away from the star. Wind velocities of → hot stars are directly measured from → P Cygni profiles, which indicate velocities from several hundred to several thousand km s-1. See also → escape velocity, → terminal velocity, → velocity law, → radiation-driven wind, → CAK model.

wind; → velocity.

zero-velocity surface
  رویه‌ی ِ تندای ِ صفر   
ruye-ye tondâ-ye sefr

Fr.: surface de vitesse nulle   

In the → restricted three-body problem, a surface which limits the region of space in which a small body can move. In the expression for the → Jacobi integral, the left side value is always positive or nul; hence the particle motion is confined to the region where U ≤ CJ. The surface that limits this region, defined by U = CJ, is called the zero-velocity surface.

zero; → velocity; → surface.

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