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boundary layer 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. |
bounded karânmand (#), karândâr (#) Fr.: limité General: Having bounds or limits. Adj. from → bound. |
bounded function karyâ-ye karânmand, ~ karândâr Fr.: fonction bornée The function y = f(x) in a given range of the argument x if there exists a positive number M such that for all values of x in the range under consideration the inequality | f(x) | ≤ M will be fulfilled. → unbounded function. |
Boussinesq approximation nazdineš-e Boussinesq Fr.: approximation de Boussinesq A simplification in the equations of → hydrodynamics that treats the density as constant except in the → buoyancy term. This approximation is motivated by the fact that when pressure and temperature differences in a flow are small, then it follows from the thermodynamic → equation of state that a change in the density is also small. Named after Joseph Valentin Boussinesq (1842-1929), a French physicist who made significant contributions to the theory of hydrodynamics, vibration, light, and heat; → approximation. |
bow 1) kamân; 2) farâl Fr.: proue 1a) A bent, curved, or arched object. 1) M.E., from O.E. boga "archery bow, arch, rainbow" (cf.
O.Norse bogi, Du. boog, Ger. Bogen "bow");
PIE root *bheug- "to bend;" cf. Skt. bhujati "bends;"
O.H.G. boug, O.E. beag "a ring"). 1) Kamân "bow, arc," from Mid.Pers. kamân, related to xam "curve," cf. Breton kamm "curved, bent," Gk. kampe "a corner, a joint," L. campus "a field," Lith. kampus "corner," PIE *kamb- "to bend, crook." Farâl, from farâ "forward" (farâ raftan "to go forward, proceed," farâ rândan "to drive forward"), equivalent to → pro-, + relation suffix -âl, → -al. Compare farâl with prow "bow," Fr. la proue "prow, bow," from dialectal It. proa, prua, from L. prora "bow," from Gk. proira, related to pro "before, forward." |
bow shock farâl-toš farâl-šok Fr.: choc de proue A → shock wave created in front of an object moving through a medium with a velocity higher than that of the → sound waves in that medium. See, for example, → magnetosphere. |
bow wave farâl-mowj, mowj-e farâl Fr.: onde de proue The wave which appears in front of a speeding boat and goes out behind it in a distinctive "V". It is due to the fact that waves pile up on each other before they can move away. |
Bowen fluorescence mechanism sâzokâr-e fluoresti-ye Bowen Fr.: mécanisme de fluorescence de Bowen A mechanism, made possible by certain chance coincidences between → spectral lines of He II, O III and N III in some → planetary nebulae , that explains the presence with a high intensity of a selected group of O III and N III lines while all other lines of these elements are missing. After I. S. Bowen who first discovered this mechanism in 1935; → fluorescence; → mechanism. |
box ja'bé (#), quti (#) Fr.: boîte A container, case, or receptacle, usually rectangular, of wood, metal, cardboard, etc.
(Dictionary.com). M.E., O.E., probably from L.L. buxis, from L. buxis, from Gk. pyxis "boxwood box," from pyxos "box tree," of uncertain origin. Ja'bé, from Ar. ja'bah; quti, from Turk. |
box-peanut bulge kuž-e quti/bâdâm-zamini Fr.: bulbe box/peanut A → galaxy bulge that shows a boxy or peanut-like morphology. These bulges are usually featureless and show no signs of → dust obscuration, young → stellar populations, or → star-forming regions. They are also kinematically cold and usually referred to as → pseudo-bulges. A number of studies have shown that these structures are just the inner parts of → bars that grow vertically thick due to vertical → resonances. They have basically the same dynamics and stellar content as bars, just their geometry is somewhat different. Box/peanut bulges are not seen if the galaxy is not inclined enough. In a → face-on galaxy, if it has a box/peanut, it will be seen as part of the bar. The → Milky Way shows a box/peanut bulge. Another remarkable case is that of → M31, known to have a bar, with its box/peanut inner part (Combes & Sanders 1981, A&A 96, 164; Combes et al. 1990, A&A 233, 82; Kormendy & Kennicutt, 2004, ARA&A 42, 603). |
Boyle-Mariotte law qânun-e Boyle-Mariotte (#) Fr.: loi de Boyle-Mariotte In a → perfect gas where mass and temperature are kept constant, the volume of the gas will vary inversely with the absolute pressure. The law can be expressed as PV = constant, where P = absolute pressure and V = volume. After Robert Boyle (1627-1691), an Irish philosopher, chemist, and physicist, and Edme Mariotte (1620-1684), a French physicist and pioneer of neurophysiology, who discovered the law independently, the first one in 1662 and the second one in 1676; → law. |
Bq star setâre-ye Bq Fr.: étoile Bq An obsolete designation used in early objective-prism studies to denote → B-type stars with "abnormal spectra" characterized by → forbidden emission lines. → B[e] star. → star. |
bra brâ Fr.: bra In Dirac's notation for describing a quantum state, a vector which together with → ket constitutes the dual vector → bracket. A bra is shown by <|, the mirror image of the symbol for a ket vector. The scalar product of a bra vector < B| and a ket vector |A> is written < B|A >, i.e. as a juxtaposition of the symbols for the bra and the ket vectors, that for the bra vector being on the left, and the two vertical lines being contracted to one for brevity. From bra- the first syllable in → bracket. |
bracket brâket Fr.: bracket In Dirac's notation, an expression which is a → scalar product of the dual vectors → bra and → ket which describe a quantum state. The bra vector appears on the left of the ket vector. From M.Fr. braguette "codpiece armor." |
Brackett series seri-ye Brackett Fr.: série de Brackette A series of lines in the infrared spectrum of atomic hydrogen due to electron jumps between the fourth and higher energy levels (Br α has wavelength 4.052 μm, Br γ 2.166 μm). Named after the American physicist Frederick Brackett (1896-1980); → series. |
Bragg angle zâviye-ye Bragg Fr.: angle de Bragg The grazing angle between an incident beam of X-rays and a given set of crystal planes for which the secondary X-rays from the planes combine to give a single beam. → Bragg's law; → angle. |
Bragg's law qânun-e Bragg Fr.: loi de Bragg A parallel beam of monochromatic X-rays of wavelength λ, incident on a given set of parallel crystal planes at a grazing angle θ will give rise to a reflected beam whenever: n λ = 2d . sinθ, where n is an integer representing the difference in path length, and d is the perpendicular distance between a pair of adjacent planes. Named after William Lawrence Bragg (1890-1971), British physicist, who, in collaboration with his father, William Henry Bragg (1862-1942), joint Nobel Prize in Physics 1915, pioneered X-ray analysis and spectrometry; → law. |
brake 1) legâm, tormoz 2) legâmidan, tormoz kardan Fr.: 1) frein; 2) freiner 1) A device for slowing or stopping a vehicle or other moving mechanism by
the absorption or transfer of the energy of momentum, usually by means of friction. From O.Du. braeke "flax brake," from breken "to break." Legâm originally "a horse bit," on the model of Fr. frein "horse bit; motor brake;" and Ger. Bremse "horse bit; brake;" tormoz, loan from Russ. тормоз. |
braking legâmeš Fr.: freinage The act or fact of stopping by means of or as if by means of a brake. See: → magnetic braking; → radiative braking; → tidal braking; → braking index. Verbal noun of → brake. |
braking index dišan-e legâmeš Fr.: indice de freinage A parameter indicating the rate at which a → pulsar slows down. Neutron stars are powered by → rotational energy and lose energy by accelerating particle → winds and by emitting → electromagnetic radiation. The → rotation frequency, Ω, thus decreases with time and this slowdown is usually described by the relation Ω^{.} = - kΩn, where k is a positive constant which depends on the → moment of inertia and the → magnetic dipole moment of the → neutron star and n is the braking index. Conventionally, the braking index is derived by differentiation of the above equation, yielding n = ΩΩ^{..} / Ω^{.2}. In a highly simplified model in which the spin-down torque arises from dipole radiation at the rotation frequency, one expects n = 3 (Johnston, S., Galloway, D., 1999, arXiv:astro-ph/9905058). |
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