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: 18 Search : layer
boundary layer
  لایه‌ی ِ کرانی   
lâye-ye karâni

Fr.: couche limite   

A layer of fluid that is formed wherever a fluid flows past a solid surface and the effects of → viscosity are important. The boundary level forms because as the fluid moves past the object, the molecules which are in direct contact with the surface stick to the surface. The molecules just above the surface are slowed down in their collisions with the molecules sticking to the surface. These molecules in turn slow down the flow just above them, but less effectively. This creates a thin layer of fluid near the surface in which the velocity changes from zero at the surface to the free stream value away from the surface. The boundary layer may be either → laminar or → turbulent in character, depending on the value of the → Reynolds number. The concept of boundary level was first put forward by Ludwig Prandlt (1875-1953) in 1904.

boundary; → layer.

data access layer (DAL)
  لایه‌ی ِ دسترسی به داده‌ها   
lâye-ye dastrasi bé dâde-hâ

Fr.: couche accès aux données   

In the → software architecture, the code that deals with reading from or writing to the data store, hiding its nature and complexity.

data; → access; → layer.

depletion layer
  لایه‌ی ِ تیسایش   
lâye-ye tisâyeš

Fr.: couche de déplétion   

The region of a semiconductor in which the density of mobile carriers is too low to neutralize the fixed charge density of donors and acceptors.

depletion; → layer.

Ekman layer
  لایه‌ی ِ اکمن   
lâye-ye Ekman

Fr.: couche d'Ekman   

A kind of viscous → boundary layer in a rotating fluid system. Such a layer forms over a flat bottom that exerts a frictional → stress against the flow, bringing the velocity gradually to zero within the layer above the bottom. An Ekman layer occurs also on the fluid surface whenever there is a horizontal frictional stress, for example along ocean surface, when waters are subject to wind stress.

Named for Vagn Walfrid Ekman (1874-1954), Swedish oceanographer, who studied the phenomenon originally in his doctoral thesis (1902) and later developed it (1905, 1906); → layer.

half-value layer (HVL)
  لایه‌ی ِ نیم-تنکش   
lâye-ye nim-tonokeš

Fr.: couche de demi-atténuation   

The thickness of material required to reduce the intensity of an → X-ray beam to one half of its initial value. The HVL is an indirect measure of the photon energies of a beam.

half; → value; → layer; → attenuation.

Heaviside layer
  لایه‌ی ِ هه‌وی‌ساید   
lâye-ye Heaviside (#)

Fr.: couche de Heaviside   

Kennelly-Heaviside layer.

English physicist Oliver Heaviside (1850-1925).

inversion layer
  لایه‌ی ِ واگردانی   
lâye-ye vâgardâni

Fr.: couche d'inversion   

Meteo.: The atmospheric layer in which the temperature gradient is inverted, that is increases; → inversion. The inversion layer tends to prevent the air below it from rising, thus trapping any pollutants that are present.
Electricity: A converting of direct current into alternating current.

inversion; → layer.

Kennelly-Heaviside layer
  لایه‌ی ِ کنلی-هوی‌ساید   
lâye-ye Kennelly-Heaviside (#)

Fr.: couche de Kennelly-Heaviside   

One of several layers in the Earth's ionosphere occurring at 90-150 km above the ground. It reflects medium-frequency radio waves whereby radio waves can be propagated beyond the horizon.

Named after the American electrical engineer Arthur Edwin Kennelly (1861-1939) and the English physicist Oliver Heaviside (1850-1925), who independently predicted the existence of the reflecting layer in 1902; → layer.

Knudsen layer
  لایه‌ی ِ کنودسن   
lâye-ye Knudsen

Fr.: couche de Knudsen   

The thin layer of → vapor immediately adjacent to an irradiated surface. The thickness of the Knudsen layer is generally recognized to be in the order of a few → mean free paths from the surface.

Named after Danish physicist Martin Knudsen (1871-1949); → layer.

laminar boundary layer
  لایه‌ی ِ کرانی ِ ورقه‌ای   
lâye-ye karâni-ye varaqe-yi

Fr.: Couche limite laminaire   

In a fluid flow, layer next to a fixed boundary. The fluid velocity is zero at the boundary but the molecular viscous stress is large because the velocity gradient normal to the wall is large. → turbulent boundary layer.

laminar; → boundary; → layer.

laminar sublayer
  زیرلایه‌ی ِ ورقه‌ای   
zirlâye-ye varaqe-yi

Fr.: sous-couche laminaire   

A layer in which the fluid undergoes smooth, nonturbulent flow. It is found between any surface and a turbulent layer above.

laminar; sublayer, from → sub- + → layer.

layer
  لایه   
lâyé (#)

Fr.: couche   

A thickness of some material laid on or spread over a surface.

From M.E. leyer, legger + -er. The first element from layen, leggen "to lay," from O.E. lecgan; cf. Du. leggen; Ger. legen; O.N. legja; Goth. lagjan

Lâyé "layer," from lâ, lây "fold" + nuance suffix of nouns.

ozone layer
  لایه‌یِ اُزون   
lâye-ye ozon (#)

Fr.: couche d'ozone   

An atmospheric layer that contains a high proportion of oxygen that exists as ozone. It acts as a filtering mechanism against incoming ultraviolet radiation. It is located between the troposphere and the stratosphere, around 15 to 20 kilometers above the Earth's surface.

ozone; → layer.

quasi-separatrix layer (QSL)
  لایه‌ی ِ چونان‌جداگر   
lâye-ye cunân-jodâgar

Fr.: couche quasi-séparatrice   

A region of the solar atmosphere where the gradient of the field line → linkage from one boundary to another is large so that the field lines can slip-run rapidly through the → plasma. The QSL results from → magnetic reconnection without → null point.

quasi-; → separatrix; → layer.

reversing layer
  لایه‌ی ِ واگردان   
lâye-ye vâgardân (#)

Fr.: couche d'inversion   

A layer of relatively cool gas forming the lower part of the Sun's chromosphere, just above the photosphere, that gives rise to absorption lines in the Sun's spectrum.

Reversing verbal adj. of → reverse; → layer.

supra-Eddington layer
  لایه‌ی ِ ابر-ادینگتونی   
lâye-ye abar-Eddingtoni

Fr.: couche super-eddingtonienne   

In some stellar models, particularly for evolved → massive stars, such as → red supergiants, → Luminous Blue Variables, and → Wolf-Rayet stars, an outermost layer of the stellar envelope where the luminosity might exceed the → Eddington limit. This is due to the → opacity peak produced by the variation in the ionization level of hydrogen in the outer → convective envelope, beneath the surface, of very luminous stars. The opacity peak generates supra-Eddington layers and density inversion. The high opacity decreases the Eddington luminosity in these layers, possibly to fainter levels than the actual stellar luminosity. As a result, the → radiative acceleration exceeds the → gravitational acceleration leading to → mass loss enhancement (see, e.g., A. Maeder, Physics, Formation and Evolution of Rotating Stars, Springer, 2009).

supra-; → Eddington limit; → layer.

turbulent boundary layer
  لایه‌یِ کرانیِ آشوبناک   
lâye-ye karâni-ye âš:ubnâk

Fr.: couche limite turbulente   

The layer in which the Reynolds stresses are much larger than the viscous stresses. When the → Reynolds number is sufficiently high, there is a turbulent layer adjacent to the → laminar boundary layer.

turbulent; → boundary; → layer.

violet layer
  لایه‌ی ِ بنفش   
lâye-ye banafš

Fr.: couche violette   

A layer of particles in the upper Martian atmosphere that scatter and absorbs electromagnetic radiation at shorter wavelengths, making the atmosphere opaque to blue, violet, and ultraviolet light.

violet; → layer.