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Lagrangian method raveš-e Lâgrânži Fr.: méthode lagrangienne Fluid mechanics: An approach in which a single fluid particle (→ Lagrangian particle) is followed during its motion. The physical properties of the particle, such as velocity, acceleration, and density are described at each point and at each instant. Compare with → Eulerian method. → Lagrangian; → method. |
Lagrangian multiplier bastâgar-e Lagrange Fr.: multiplicateur de Lagrange Math.: A constant that appears in the process for obtaining extrema of functions of several variables. Suppose that the function f(x,y) has to be maximized by choice of x and y subject to the constraint that g(x,y)≤ k. The solution can be found by constructing the → Lagrangian function L(x,y,λ) = f(x,y) + λ[k - g(x,y)], where λ is the Lagrangian multiplier. → Lagrangian point; → multiplier. |
Lagrangian particle zarre-ye Lâgrânži Fr.: particule lagrangienne Fluid mechanics: In the → Lagrangian method, a particle that moves as though it is an element of fluid. The particle concept is an approach to solving complicated fluid dynamics problems by tracking a large number of particles representing the fluid. The particle may be thought of as the location of the center of mass of the fluid element with one or more property values. → Lagrangian; → particle. |
Lagrangian point noqtehâ-ye Lagrange (#) Fr.: points de Lagrange On of the five locations in space where the → centrifugal force and the → gravitational force of two bodies (m orbiting M) neutralize each other. A third, less massive body, located at any one of these points, will be held in equilibrium with respect to the other two. Three of the points, L1, L2, and L3, lie on a line joining the centers of M and m. L1 lies between M and m, near to m, L2 lies beyond m, and L3 on the other side of M beyond the orbit. The other two points, L4 and L5, which are the most stable, lie on either side of this line, in the orbit of m around M, each of them making an equilateral triangle with M and m. L4 lies in the m's orbit approximately 60° ahead of it, while L5 lies in the m's orbit approximately 60° behind m. See also → Trojan asteroid; → Roche lobe; → equipotential surface; → horseshoe orbit. → Lagrangian; → point. |
lake daryâcé (#) Fr.: lac A body of fresh or salt water entirely surrounded by land. From O.Fr. lack, from L. lacus "pond, lake," related to lacuna "hole, pit," from PIE *lak- (cf. Gk. lakkos "pit, tank, pond," O.C.S. loky "pool, cistern," O.Ir. loch "lake, pond"). Daryâcé, from daryâ "sea" Mid.Pers. daryâp variant zrah; O.Pers. drayah-; Av. zrayah- "sea;" cf. Skt. jráyas- "expanse, space, flat surface" + -cé diminutive suffix, from Mid.Pers. -cak, variants -êžak (as in kanicak "little girl," sangcak "small stone," xôkcak "small pig"), also Mod.Pers. -ak. |
Lamb shift kib-e Lamb Fr.: décalage de Lamb A tiny change in the energy levels of the hydrogen atom between the states 2S_{1/2} and 2P_{1/2}, which creates a shift in the corresponding spectral lines. The 2P_{1/2} state is slightly lower than the 2S_{1/2} state, contrarily to the Schrodinger's solution. Named after Willis Eugene Lamb, Jr. (1913-), an American physicist who discovered this effect in 1951, and won the Nobel Prize in physics in 1955 "for his discoveries concerning the fine structure of the hydrogen spectrum;" → shift. |
lambda lâmbdâ Fr.: lambda The eleventh letter of the Greek alphabet.
In lower case, λ, it denotes → wavelength.
It is also used in the → Bayer designation system
to identify a specific star in a → constellation.
See also → lambda point. From Phoenician lamedh. |
Lambda Bootis star setâre-ye lâmbda Gâvrân Fr.: étoile lambda du Bouvier The prototype of a small class of stars (A-F types) which have weak metallic lines (indicating that they are depleted in metals heavier than Si, but with solar abundances of C, N, O, and S). Moreover, they have moderately large rotational velocities and small space velocities. Lambda Boo stars may be pre-main-sequence objects, or they may be main sequence stars that formed from gas whose metal atoms had been absorbed by interstellar dust. Named after the prototype, the star → Lambda (λ) of constellation → Bootes; → star. |
lambda cold dark matter model model-e lâmbdâ-mâde-ye-sard-e-târik Fr.: modèle ΛCDM The → standard model of → Big Bang that incorporates both → dark matter and → dark energy. See also → cold dark matter (CDM). → lambda, → cosmological constant; → cold; → dark; → matter; → model. |
lambda point noqte-ye lâmbdâ Fr.: point lambda The temperature (roughly 2.17 K) at which → liquid helium (→ helium I) becomes → superfluid (→ helium II). The name was given by the Dutch physicist Willem Hendrik Keesom (1876-1956), who discovered the behavior of helium near this transition point and successfully solidified helium in 1926 (under an external pressure of 25 atmospheres). The name was originally suggested by Paul Ehrenfest (1880-1933), who was inspired by the shape of the → specific heat curve, which resembles the Gk. letter → lambda; → point. |
lambert lambert Fr.: lambert A centimeter-gram-second (cgs) unit of luminance (or brightness) equal to 1/π candle per square centimeter. Physically, the lambert is the luminance of a perfectly diffusing white surface receiving an illuminance of 1 lumen per square centimeter. Johann Heinrich Lambert (1728-1777), German scientist and mathematician; → law. |
Lambert's cosine law qânun-e cosinus-e Lambert Fr.: loi en cosinus de Lambert The intensity of the light emanating in any given direction from a perfectly diffusing surface is proportional to the cosine of the angle between the direction and the normal to the surface. Also called → Lambert's law. |
Lambert's law qânun-e Lambert Fr.: loi de Lambert Same as → Lambert's cosine law. |
Lambertian disk gerde-ye Lamberti, disk-e ~ Fr.: disque lambertien A → planetary or → satellite disk with → Lambertian surface. Such a disk has the same → surface brightness at all angles. |
Lambertian surface ruye-ye Lamberti Fr.: surface lambertienne A surface whose → luminous intensity obeys → Lambert's cosine law. Such a source has a → reflectance that is uniform across its surface and uniformly emits in all directions from all its points. It appears equally bright from all viewing directions. Lambertian surface is a very useful concept for the approximation of radiant power transfer. |
lamina varaqé (#) Fr.: lame, lamina A thin plate, layer, or flake. From L. lamina "thin plate or layer, leaf." Varaqé "sheet, plate," from varaq "a leaf of tree or of paper," from Ar. waraq, from Pers. barg "leaf" (Tabari, Gilaki valg, balg; Kurd. belg, balk, Semnâni valg); Mid.Pers. warg "leaf;" Av. varəka- "leaf;" cf. Skt. valká- "bark, bast, rind;" Russ. volokno "fibre, fine combed flax." |
laminar varaqe-yi Fr.: laminaire Composed of, or arranged in, laminae, sheets. |
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. |
laminar flow tacân-e varaqe-yi Fr.: écoulement laminaire A flow in which the particles of fluid are moving orderly, and in which adjacent layers or laminas glide smoothly over another with little mixing between them. A laminar flow may rapidly transform into a → turbulent flow for large → Reynolds numbers. |
laminar sublayer 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. |
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