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Yoshikawa H.N.,CNRS Laboratory of Waves and Complex Media | Crumeyrolle O.,CNRS Laboratory of Waves and Complex Media | Mutabazi I.,CNRS Laboratory of Waves and Complex Media
Physics of Fluids | Year: 2013

The thermal convection driven by the dielectrophoretic force is investigated in annular geometry under microgravity conditions. A radial temperature gradient and a radial alternating electric field are imposed on a dielectric fluid that fills the gap of two concentric infinite-length cylinders. The resulting dielectric force is regarded as thermal buoyancy with a radial effective gravity. This electric gravity varies in space and may change its sign depending on the temperature gradient and the cylinder radius ratio. The linear stability problem is solved by a spectral-collocation method. The critical mode is stationary and non-axisymmetric. The critical Rayleigh number and wavenumbers depend sensitively on the electric gravity and the radius ratio. The mechanism behind the instability is examined from an energetic viewpoint. The instability in wide gap annuli is an exact analogue to the gravity-driven thermal instability. © 2013 American Institute of Physics.


The effective wavenumber of the coherent wave propagating in a fluid containing parallel porous cylinders randomly distributed in space is derived at the Rayleigh limit for (i) explicit formulas: Independent Scattering Approximation (ISA), Waterman and Truell (WT) and Linton and Martin (LM) and (ii) for implicit formulas: Coherent Potential Approximation (CPA) and Generalized Self Consistent Method (GSCM) applied to WT and to LM. The effective mass density and bulk modulus are also derived. The validity of all the effective quantities is checked by recovering, when the porosity of the scatterers tends to zero, the case of an inhomogeneous medium of elastic cylinders. © 2016 Elsevier B.V.


Lique F.,CNRS Laboratory of Waves and Complex Media
Monthly Notices of the Royal Astronomical Society | Year: 2015

We report nearly exact quantum time-independent calculations of rate coefficients for the collisional (de-)excitation of H2 by H, from low to high temperatures. Our calculations are based on a highly accurate global potential energy surface. The reactive hydrogen exchange channels are taken into account rigorously. New collisional data are obtained for the rovibrational relaxation of highly excited H2 (with internal excitation up to ≃22 000 K) for temperatures ranging from 100 to 5000 K. We also provide a comparison with the available experimental rate coefficients at room temperature. The good agreement between theory and experiment is an illustration of the accuracy of the present calculations. The new results significantly differ from previous data presently used in astrophysical models, especially at low temperatures, the difference being essentially due to the inclusion of the reactive channels. The impact of these new data in astrophysics is discussed. In particular, the coolingmechanism will have to be reviewed for several astrophysical media. © 2015 The Author.


Roueff E.,Laboratoire Univers et Theories | Lique F.,CNRS Laboratory of Waves and Complex Media
Chemical Reviews | Year: 2013

Collisional excitation is the basic process, and its efficiency relies on the composition of the medium providing the density of the main perturbers and the temperature, which establishes the degree of excitation and, consequently, drives the intensities of the radiated emission. Nevertheless, other excitation mechanisms can be at work such as radiative or chemical pumping. The computation of collisional inelastic rate coefficients usually takes place within the Born-Oppenheimer approximation for the separation of electronic and nuclear motions. Scattering cross sections are thus obtained by solving the motion of the nuclei on an electronic potential energy surface (PES) that is independent of the masses and spins of the nuclei. Recent studies have demonstrated that computational techniques employing advanced treatments for both the electronic and nuclear motion problems can rival experimental measurements, in terms of the achieved accuracy.


Duchemin B.J.C.,CNRS Laboratory of Waves and Complex Media
Green Chemistry | Year: 2015

In this study, mercerisation of native cellulose I was achieved in an aqueous sodium hydroxide solution at a concentration of only 1 wt% NaOH by processing at temperatures below 0 °C. This represents a tenfold reduction in the use of NaOH to accomplish this very common transformation. The cellulose sample was a form of hydrolysed cotton with a high crystallinity. The samples were mixed with aqueous sodium hydroxide at various concentrations and stored at -17 °C. The samples were then defrosted, neutralised and dried before being analysed by Fourier-transform infrared spectroscopy, wide-angle X-ray diffraction and field-emission scanning electron microscopy. In the route described here, transformation from cellulose I to cellulose II was possible without greatly affecting the crystallinity or the microstructure of the samples. © The Royal Society of Chemistry 2015.


Kalugina Y.,CNRS Laboratory of Waves and Complex Media | Kalugina Y.,Tomsk State University | Lique F.,CNRS Laboratory of Waves and Complex Media | Klos J.,University of Maryland University College
Monthly Notices of the Royal Astronomical Society | Year: 2012

Modelling of molecular emission spectra from interstellar clouds requires the calculation of rate coefficients for excitation by collisions with the most abundant species. Among the interstellar molecules, the CN radical is of particular interest since it is a good probe of dense region and can be used as a tracer of magnetic fields. We calculate fine- and hyperfine-structure-resolved excitation rate coefficients of CN(X 2Σ +) by H 2(j= 0), the most abundant collisional partner in the interstellar medium. The calculations are based on a new potential energy surface obtained from highly correlated ab initio calculations. State-to-state rate coefficients between the first fine and hyperfine levels of CN were calculated for low temperatures ranging from 5 to 100K. The new results are compared to the CN-He rate coefficients. Significant differences are found between the two sets of rate coefficients. This comparison shows that the CN-H 2 rate coefficients have to be used for observation interpretations and we expect that their use will significantly help the astronomers in the interpretation of the CN emission lines observed with current and future telescopes. © 2012 The Authors Monthly Notices of the Royal Astronomical Society © 2012 RAS.


Lanza M.,CNRS Laboratory of Waves and Complex Media | Lique F.,CNRS Laboratory of Waves and Complex Media
Monthly Notices of the Royal Astronomical Society | Year: 2012

Modelling of molecular emission from interstellar clouds requires the calculation of rate coefficients for excitation by collisions with the most abundant species. This paper deals with the determination of rate coefficients for the rotational and hyperfine excitation of the HCl molecule in its ground vibrational state due to collisions with He. Calculations of pure rotational (de-)excitation cross-sections of HCl by He were performed using the essentially exact close-coupling method. The calculations are based on a new potential energy surface obtained from highly correlated ab initio calculations. Cross-sections for transitions among the 11 first rotational levels of HCl were calculated for total energies up to 3000cm -1. The hyperfine cross-sections are then obtained using a recoupling technique. The rotational and hyperfine cross-sections are used to determine collisional rate coefficients for temperatures ranging from 5 to 300K. A clear propensity rule in favour of odd Δj rotational transitions is observed. The usual Δj=ΔF propensity rule is observed for the hyperfine transitions. The new rate coefficients are compared with the previous results obtained for the HCl molecule. Significant differences are found, mainly due to the use of a new potential energy surface. The new rate coefficients will significantly help in interpreting HCl emission lines observed with current and future telescopes. © 2012 The Authors Monthly Notices of the Royal Astronomical Society © 2012 RAS.


Faure A.,CNRS Grenoble Institute for Particle Astrophysics and Cosmology Laboratory | Lique F.,CNRS Laboratory of Waves and Complex Media
Monthly Notices of the Royal Astronomical Society | Year: 2012

Nuclei with non-zero spin induce hyperfine splittings in the rotational spectrum of many commonly observed interstellar molecules. Radiative transfer modelling of such species requires in general a good knowledge of hyperfine selective collisional rate coefficients. We investigate in this work the impact of collisional rate coefficients on the molecular hyperfine excitation. The approximate sudden and statistical (proportional) methods are first compared to the almost exact recoupling approach. Rate coefficients are presented for a large number of CN and HCN transitions, with para-H 2(j = 0) as a collider. The sudden approximation and the recoupling approach, which both predict the propensity rule Δj = ΔF, are found to agree within a factor of 3 or better. Radiative transfer calculations are then performed using the large velocity gradient approximation. At low and moderate total optical depths (τ ≲ 10), where the relative hyperfine populations are close to the statistical weights, both the sudden and the statistical approximations are shown to provide accurate alternatives to the recoupling approach. At higher total opacities, however, the hyperfine propensity rule appears to matter and the sudden method is found to be significantly superior to the statistical approach. © 2012 The Authors Monthly Notices of the Royal Astronomical Society © 2012 RAS.


Tizniti M.,Rennes Institute of Physics | Le Picard S.D.,Rennes Institute of Physics | Lique F.,CNRS Laboratory of Waves and Complex Media | Berteloite C.,Rennes Institute of Physics | And 3 more authors.
Nature Chemistry | Year: 2014

The prototypical F + H2 → HF + H reaction possesses a substantial energetic barrier (∼800 K) and might therefore be expected to slow to a negligible rate at low temperatures. It is, however, the only source of interstellar HF, which has been detected in a wide range of cold (10-100 K) environments. In fact, the reaction does take place efficiently at low temperatures due to quantum-mechanical tunnelling. Rate constant measurements at such temperatures have essentially been limited to fast barrierless reactions, such as those between two radicals. Using uniform supersonic hydrogen flows we can now report direct experimental measurements of the rate of this reaction down to a temperature of 11 K, in remarkable agreement with state-of-the-art quantum reactive scattering calculations. The results will allow a stronger link to be made between observations of interstellar HF and the abundance of the most common interstellar molecule, H 2, and hence a more accurate estimation of the total mass of astronomical objects. © 2014 Macmillan Publishers Limited. All rights reserved.


Travnikov V.V.,CNRS Laboratory of Waves and Complex Media
International Journal of Heat and Mass Transfer | Year: 2016

This paper describes a numerical investigation of the influence of axial and dipolar magnetic fields on the stability of the convective flow of an electrically conductive Boussinesq fluid in a spherical gap for the radius ratios η = inner radius/outer radius = 0.4, 0.5, and 0.6. The inner shell is warmer than the outer shell. We show that whereas the axial magnetic field stabilizes the flow for low Hartmann numbers, a destabilizing effect is detected if the magnetic field increases. On the other hand, numerical analysis shows that the dipolar magnetic field configuration stabilizes the flow. The critical wave number, mc, is much higher than that for the axial magnetic field. The critical Grashof numbers are presented as a function of the Hartmann number for both configurations of the B-field. The stability analysis is accompanied by calculations of the 3D flows. © 2016 Elsevier Ltd. All rights reserved.

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