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.
data access layer (DAL)
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.
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.
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)
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.
lâye-ye Heaviside (#)
Fr.: couche de Heaviside
English physicist Oliver Heaviside (1850-1925).
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.
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.
Fr.: couche de Knudsen
Named after Danish physicist Martin Knudsen (1871-1949); → layer.
laminar boundary layer
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.
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.
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" + -é nuance suffix of nouns.
North Polar Layered Deposits (NPLD)
Lerdhâ-ye Laye-laye-ye Qotb-e Hudar
Fr.: couches de dépôt du pôle nord
A large area of the north polar region of Mars which is covered with alternating layers of water ice and dust. → South Polar Layered Deposits.
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.
quasi-separatrix layer (QSL)
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.
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.
South Polar Layered Deposits (SPLD)
Lerdhâ-ye Laye-laye-ye Qotb-e Daštar
Fr.: couches de dépôt du pôle sud
A large area of the south polar region of → Mars which is covered with layers of → water ice and → dust. The SPLD, like the NPLD, has a maximum relief relative to the surrounding terrain of ~ 3.5 km and ~ 1,000 km across. Above the SPLD lies a very thin temporary (1-10 m) cap of → carbon dioxide ice/frost that snows out in the winter and sublimates over the spring and summer seasons. It is believed that the rhythmic nature of the deposits is related to oscillations in Mars' → orbital parameters (J. J. Plaut et al., 2007, Science 316, 92).
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).
turbulent boundary layer
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.
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.