Showing papers by "Centre national de la recherche scientifique published in 1978"

A simple dynamical model of intermittent fully developed turbulence

Abstract: We present a phenomenological model of intermittency called the P-model and related to the Novikov-Stewart (1964) model. The key assumption is that in scales N &2-” only a fraction /3n of the total space has an appreciable excitation. The model, the idea of which owes much to Kraichnan (1972, 1974)’ is dynamical in the sense that we work entirely with inertial-range quantities such as velocity amplitudes, eddy turnover times and energy transfer. This gives more physical insight than the traditional approach based on probabilistic models of the dissipation. The P-model leads in an elementary way to the concept of the self-similarity dimension D, a special case of Mandelbrot’s (1974, 1976) ‘fractal dimension’. For threedimensional turbulence, the correction B to the Q exponent of the energy spectrum is equal to +( 3 - D) and is related to the exponent p of the dissipation correlation function by B = Qp (0.17 for the currently accepted value). This is a borderline case of the Mandelbrot inequality B < Qp. It is shown in the appendix that this inequality may be derived from the Navier-Stokes equation under the strong, but plausible, assumption that the inertial-range scaling laws for second- and fourth-order moments have the same viscous cut-off. The predictions of the P-model for the spectrum and for higher-order statistics are in agreement with recent conjectures based on analogies with critical phenomena (Nelkin 1975) but generally diasgree with the 1962 Kolmogorov lognormal model. However, the sixth-order structure function (8v6(Z)) and the dissipation correlation function (e(r) e(r + 1)) are related by

Fracture mechanics and the adherence of viscoelastic bodies

Abstract: The strain energy release rate G and its derivative delta G/ delta A are used to provide a general picture of the adherence of viscoelastic bodies. Two bodies in contact on an area A are in equilibrium if G=w, where w is the thermodynamic (or Dupre's) work of adhesion. The quasistatic force of adherence is the load corresponding to delta G/ delta A=0. When G>w, the separation of the two bodies starts, and can be seen as the propagation of a crack in mode I. Three geometries are investigated: adherence of spheres, adherence of punches, and peeling. The variation of energies with the area of contact is given, and the kinetics of crack propagation are studied. The theory is supported by experiments on the adherence of polyurethane to glass.

On the Segal-Shale-Weil representations and harmonic polynomials

Abstract: In this paper, we give the answer to the following two intimately related problems. (a) To decompose the tensor products of the harmonic representations into irreducible components to get a series of new unitary irreducible representations with highest weight vectors of the group G= Mp(n), two-sheeted covering group of the symplectic group, or G = U(p, q). (b) To describe the representations of the group GL(n, ~ ) x O(k, ~) (resp. GL(p, C) x GL(q, C) x GL(k, C)) in the space of pluriharmonic polynomials on the space M(n, k; C) of n • k complex matrices (resp. M(p, k; C) x M~q, k; ~)). The second problem arises when we construct an intertwining operator from the tensor product of the harmonic representation into a space of vector-valued holomorphic functions on the associated hermitian symmetric space G/K, or equivalently when we consider highest weight vectors in the tensor products. Some of our motivations are the following: 1) Apart from special cases the unitary dual G of a real semi-simple Lie group is not known. There exist isolated points in G which are not members of discrete or "mock-discrete" series, (for exemple for Sp(2, C) where the unitary dual has been computed by M. Duflo [-18], there are two isolated points in ~, the trivial representation and the odd component of the Segal-Shale-Weil representation) and we are interested to produce series of such representations. 2) We are extending to matrix spaces classical results for harmonic polynomials on N".

Fully developed turbulence and statistical mechanics

Abstract: This paper gives a self contained review of some recent progress of the statistical theory of fully developed turbulence. The emphasis is on both analogies and differences with Hamiltonian statistical mechanics, in particular critical phenomena. The method of spectral equations, which plays to a certain extent the role of a mean field theory, is discussed in detail. It is here viewed as a reformulation of the Kolmogorov 1941 theory leading to quantitative insight into the energetics of turbulence (power-law spectra, direct and inverse energy cascades, energy dissipation in the limit of zero viscosity, etc.). In addition, it sheds light on the proven and conjectured properties of the Navier-Stokes and Euler equations which are reviewed in terms more accessible than those of the mathematical literature. There are strong experimental indications (intermittency) that the Kolmogorov 1941 theory is only approximate. Some of the current efforts to handle higher than second order statistics by formal methods inspired from quantum field theory or critical phenomena are also discussed.

Thermodynamics of organic mixtures. A generalized quasichemical theory in terms of group surface interactions

Abstract: A general quasi chemical theory in terms of group surface interactions is presented. The theory is applied to several basic classes of organic mixtures comprising n-alkanes, aromatic hydrocarbons, cyclohexane, tetrachloromethane, ethers and chloroalkanes. Interchange Gibbs energies and enthalpies are tabulated for ten contact pairs. Comparison with experimental Gibbs energies and enthalpies shows quite satisfactory agreement for all of the systems investigated. Possible sources of discrepancy and ways of refining the model are discussed.

Black-hole eddy currents

Abstract: We study dissipative test electromagnetic fields in a black-hole background. Quantities such as surface velocity, tangential electric field, normal magnetic induction, total surface current, and conduction surface current are introduced and are shown to satisfy Ohm's law with a surface resistivity of $4\ensuremath{\pi}\ensuremath{\simeq}377$ ohms. Associated with these currents there exists a "Joule heating". These currents can exist when the black hole is inserted in an external electric circuit, but they can exist even in the absence of external currents. In particular, we study the eddy currents induced by the rotation of a black hole in an oblique uniform magnetic field, and we show how the computation of the ohmic losses allows a very simple derivation of the torque exerted on the hole.

Magnetic structure of V 2 O 3 in the insulating phase

Abstract: A phase diagram for all the possible collinear spin arrangements for ${\mathrm{V}}_{2}$${\mathrm{O}}_{3}$ is derived within the atomic limit. Due to the fact that the ${a}_{1g}$ electrons of the V atoms form a diamagnetic bond for the vertical pairs of V atoms, the magnetic structure of ${\mathrm{V}}_{2}$${\mathrm{O}}_{3}$ can be considered to be essentially determined by the remaining one electron per V atom in a twofold degenerate ${e}_{g}$ level. Depending on only two parameters: $\frac{{t}_{\mathrm{ij}}^{11}}{{t}_{\mathrm{ij}}^{22}}$, the ratio of the hopping integrals within the two orbital states 1 and 2 and between a certain pair ($i, j$) of V atoms in the basal plane, and $\frac{J}{U}$, the ratio of the exchange constant to Hubbard's $U$, the regions of stability for a particular magnetic and orbital order are determined. The experimentally observed magnetic order falls into a region of values of these parameters which are expected for ${\mathrm{V}}_{2}$${\mathrm{O}}_{3}$.

A new general integral theory for dielectric coated gratings

Abstract: We present a new rigorous integral formalism for the theoretical study of dielectric coated gratings and grating couplers. It applies in the resonance domain, where the wavelength of the incident field and the groove spacing are of the same order of magnitude. The computed program issued from this theory extends the domain of application of the previous differential or integral theories. It can be used to investigate, with a very good accuracy, the properties of bare or dielectric coated gratings, for any groove shape and any polarization, in the entire visible, ultraviolet, and infrared regions. Various classical criteria are used to control the validity of the numerical results and comparisons are made with the numerical results obtained using the previous integral and differential formalisms. Two examples of applications are given. First, we show that the new possibilities of our program lead to a better agreement between theoretical results and experimental data. Second, a theoretical study of a certain type of grating coupler is given.

Experimental determination of the isotopic fractionation between gaseous CO2 and carbon dissolved in tholeiitic magma

Abstract: Isotope fractionation of carbon between CO2 and carbon dissolved in a tholeiitic magma measured in the range 1120–1280 ° C, 7.0–8.4 Kb varies from 4.6 to 4‰ in favor of CO2. These results make possible to explain all deep seated 13C values from a restricted range of primary mantle 13C concentrations. They also suggest that carbon could be dissolved in basaltic magmas in a reduced form.

Spins, magnetic moments, and isotope shifts of Na 2 1 − 3 1 by high resolution laser spectroscopy of the atomic D 1 line

Abstract: A polyisotopic sodium beam produced by reactions of 20 GeV protons in an uranium target, was illuminated with a tunable cw dye laser The atomic beam is analyzed by a six-pole magnet, ionized, and detected after selection of one isotope by use of a mass spectrometer From the optical ${D}_{1}$ resonance lines the hyperfine structure, magnetic moments, and isotope shifts of $^{21\ensuremath{-}31}\mathrm{Na}$ have been determined The nuclear spins of $^{26\ensuremath{-}30}\mathrm{Na}$ have been measured by magnetic resonance The results are discussed in terms of nuclear deformation The analysis of isotope shifts shows the presence of an appreciable volume shift contributionNUCLEAR STRUCTURE $^{26\ensuremath{-}30}\mathrm{Na}$; measured $J$ $^{21\ensuremath{-}31}\mathrm{Na}$; measured isotope shifts; deduced $\ensuremath{\mu}$ Atomic beam laser spectroscopy and magnetic resonance

Reevaluation of the role of dimethyl sulphide in the sulphur budget

Abstract: THE possible existence of an oceanic source of atmospheric SO2 has recently been suggested for the following reasons. First, to account for the observed1 SO2 background concentration of 0.1 × 10−6g m−3 of air, even very far from any continental source and in areas as remote as the Antarctic ocean—despite a residence time for SO2 in the marine atmosphere of ∼0.7 d (refs 2, 3). Second, because another major source such as the ocean is necessary to complete the global atmospheric sulphur budget4–7. Because of the basic pH of seawater, SO2 cannot be emanated directly from the oceans. According to Lovelock et al.8, the dimethyl sulphide (DMS) produced in the sea by many living systems, could be evolved in a first step into the atmosphere and subsequently oxidised to SO2. These authors observed a mean DMS concentration of the order of 12 × 10−9 gl−1 in Atlantic ocean water. We have developed an improved method for the measurement of DMS in seawater and have used this technique on several recent oceanographic research cruises. Our results suggest that the oceans could contribute over 30% of the amount of sulphur required to balance the sulphur budget.

A different approach to RNA sequencing.

Abstract: DURING the past few years techniques to simplify the systematic sequence determination of nucleic acids have become available; these involve the use of polyacrylamide gels from which the sequence may be directly and rapidly read off. The most recent of these, applicable to RNA fragments, rely on the base-specific nature of cleavage by T1 and U2 RNases1,2 and pancreatic RNase and RNase I from Physarum polycephalum2 by which a spectrum of 5′ terminally-labelled fragments may be produced and comigrated with a ‘ladder’ achieved by incubating the fragment with alkali or hot formamide. We describe here a method for the sequence determination of RNA fragments bearing a blocked 5′ terminus, in which the above enzymatic hydrolysis steps and their accompanying problems may be avoided.

Neural induction of the 16S acetylcholinesterase in muscle cell cultures.

Abstract: SEDIMENTATION analysis of rat muscle acetylcholinesterase (AChE) has revealed the existence of three molecular forms, 4S, 10S and 16S (refs 1, 2). The 16S form was exclusively found in skeletal muscle segments containing neuromuscular junctions; it disappeared after denervation1,2 and reappeared following rein-nervation either at the original endplate, or at ectopic sites. The 16S AChE may therefore be referred to as an ‘endplate specific’ form, but it clearly does not represent all activity at the endplate. It took several days for the disappearance of the 16S form after denervation, suggesting a postsynaptic localisation2. In posterior leg muscles from rat embryos, the 16S form was absent before the 14th day of gestation; it appeared at day 15 and was relatively more abundant on day 16 (25% of the total AChE activity) than in the adult (5%). (The total AChE activity increased by a factor of 103 from day 16 of gestation to the adult muscle2.) We present here some data concerning the synthesis of the 16S form by embryonic rat cells in culture, which demonstrate that this form is produced by muscle cell cultures on induction by neuronal elements.

Flavonoid compounds are potent inhibitors of cyclic AMP phosphodiesterase.

Abstract: The inhibitory activity of 19 flavonoid molecules on cyclic AMP breakdown by a commercial beef heart phosphodiesterase preparation is reported. 7 compounds are active in the micromolar range, 2 of which have a potency equivalent to that of papaverine. Some structure activity relationships are drawn.

Charge-density waves in NbSe3 at 145K: crystal structures, X-ray and electron diffraction studies

Abstract: The crystal structure of NbSe3 has been refined from single crystal X-ray diffraction data. It has a monoclinic symmetry with lattice parameters: a=10.009 AA, b=3.4805 AA, c=15.629 AA, beta =109.47 AA, space group P21/m and six formulae per unit cell. The crystal structure of NbSe3 as determined at 100K shows that the 145K transition in the electrical resistivity is not accompanied by a structural distortion. Except for the decrease due to thermal contraction all interatomic distances are found to be the same. This is compatible with the proposed model of the charge-density-waves formation which has been put forward in order to explain the physical properties of the 145K transition. Electron diffraction pictures taken above and below the transition give direct evidence of the charge-density-waves formation at the transition. At 120K in agreement with Tsutsumi et al. (1977) the pictures contain superstructure spots at the (h,k+or-0.243, l) positions. Contrary to what has been reported by the same authors, the authors have found that at 3K above the transition the electron diffraction pictures do not contain the diffuse scattering streaks corresponding to the planes at k'=k+or-0.243, which would indicate fluctuations of the CDW above the transition.