The LES of a 3D mixing layer spatially developing downstream of a flat plate has been conducted for a high Reynolds number (Reθ=2835). To overcome the problem of the pressure condition on the free boundaries, use of the (v–ω) formulation of the Navier–Stokes equations has been preferred over the primitive (v–p) formulation. To deal with the difficult problem of the divergence free constraint on both the velocity and the vorticity field, a new and efficient numerical algorithm has been devised which turns out to be very attractive. The velocity components are computed as a solution of a Cauchy–Riemann problem using a fractional step method. As one of the main advantages of the vorticity-based formulation is the treatment of the free flow boundary conditions, special care has been devoted to these boundary conditions. An optimum approximation of the outflow boundary condition has been carried out which satisfies the conservation of mass, making the long time integration easier and more accurate. The numerical results are compared to a reference experiment [Appl. Sci. Res. 53 (1994) 263; J. Delville, PhD thesis, University of Poitiers, 1995] for a rather high Reynolds number. Using inlet perturbations and the mixed scale model, the LES results agree very well with the reference experiments. The validation of the numerical procedure is reviewed on the mean and fluctuating quantities. Good prediction of the spatial evolution is demonstrated for the distribution of the vorticity thickness as well as for the Reynolds stress profiles and spatial correlations. In order to estimate the quality of the spatio-temporal development, a spectral analysis is also reported on the space and time spectra, pointing out a highly 3D arrangement with length scales and frequencies in rather good agreement with the ones generally admitted.

The active control of fully developed turbulent flows is of particular interest for many industrial applications. In such flow fields, the large-scale coherent structures contain most of the turbulent kinetic energy and are mainly responsible for vibrations, noise generation, etc… Therefore, in term of control, it seems important to describe correctly the characteristics of these structures and to predict precisely their time evolution using models as simple as possible. One of the methods proposed to mimic the dynamics of the flow is to develop low-order dynamical systems (Aubry et al., 1988) (Glauser et al., 1989) derived from the Proper Orthogonal Decomposition (POD) (Lumley, 1967; Sirovich, 1987).

The spatial development of a 3D plane mixing layer in incompressible flow has been performed using a L.E.S. approach. The resolutions of the Navier-Stokes equations, written in velocity-vorticity formulation, is based on an original fractional step algorithm (Bertagnolio et al. (1996)). The results presented are obtained through the use of two subgrid scale models (a vorticity model and a mixed scale model). To validate the L.E.S. approach, a comparison with the experiments is provided. The main statistical values, expansion factor, Reynolds stress tensor and 3D arrangement are well predicted.

High concentrations of hydroxycinnamic acids in the hemicellulosic fraction of dry season tropical grasses may influence the rate of microbial degradation of arabinoxylans by ruminant animals. The ability of 22 strains of Butyrivibrio fibrisolvens, other ruminal bacteria (Ruminococcus albus SY3, Ruminococcus flavefaciens RF1,Prevotella ruminicola AR20) and the ruminal phycomycete Neocallimastix patriciarum CX to digest the tropical grass Heteropogon contortus(spear grass) and hydrolyse esterified ferulic and p-coumaric acid was examined. Significant digestion (8-36%) of spear grass occurred with the B. fibrisolvens strains H17c, A38, LP92-1-1, 49,R. albus SY3 and N. patriciarum. Hydrolysis of ester-linked ferulic and p-coumaric acid occurred with all organisms except B. fibrisolvens strains GS113, OB156 and LP1028 and P. ruminicola AR20. The ratio of ferulic to p-coumaric acid hydrolysed by different strains of Butyrivibrio spp. varied markedly from 0.96 for AR 51 to 0.16 for A38. Butyrivibrios which were fibrolytic (H17c and A38) had higher extracellular cinnamoyl esterase activity than bacteria that did not digest spear grass fibre (LP 91-4-1 and AR 20) which had low activities or only produced cell associated enzyme. Cell associated and extracellular esterase activity were induced when Butyrivibrio spp. strains H17c, A38 and E14 and the Ruminococcus spp. were grown on birchwood xylan but induction did not occur to the same extent with N. patriciarum. This is the first reported observation of cinnamoyl esterase activity in the genus Ruminococcus. The fungus N. patriciarum had significantly higher digestibility of spear grass and solubilisation of phenolic acids than the bacteria. The study shows that high levels of extracellular cinnamoyl esterases are characteristic of a selection of fibre-degrading ruminal bacteria and fungi which probably indicates that these enzymes are common amongst xylanolytic ruminal microorganisms.

Diagnostics have been developed and fielded at the Nova laser facility that, for the first time, image nearly all the light scattered within 20° of the laser axis, including the light collected by the laser focusing lens as well as that just outside the lens. Absolute calibration of optical components exposed to the target debris have been achieved by a combination of routine in situ calibration and maintenance. Measurements from plasmas relevant to ignition experiments indicate that scattering is peaked in the direction of backscatter with significant energy scattered both into the lens and just outside the lens. The scattering outside the lens is found to be dominant when the f number is large (f/8).

The experimental images of the sidescattered light from a plasma, created by the multiterawatt laser pulse propagating in a hydrogen gas jet, exhibit clear dependence on both gas jet pressure and laser power. Two- and three-dimensional simulations of wave propagation, in presence of the relativistic electron mass increase and the ponderomotive expel of electrons, have been performed to reproduce the Thomson radiation from the plasma electrons. They show electron cavitation induced by the beam focusing, self-focusing, self-guiding, smoothing of the beam nonuniformities and, at larger power, beam filamentation. A bremsstrahlung model with account of the ionization, heating, expansion, and recombination dynamics of the gas, provides the plasma emission background. Both Thomson emission and bremsstrahlung are required to recover the experimental emission patterns. Among the interpretations, a scenario of laser self-guiding over five Rayleigh lengths can be found for 10 TW laser power and 5×1018 cm−3 electron density, which surprisingly disappears at larger powers and densities.

Calibrated thermoluminescence dosimeters have been used to measure the angular and spectral dependence of hard x-ray emission produced from intense subpicosecond laser irradiation of solid targets. A dosimeter detector set with nine filter channels (13.5–400 keV) has been tested successfully. Total bremsstrahlung conversion up to 0.23% and conversion of 8×10−5 from laser energy to Ta Kα line emission (photon energy ≊57 keV) was determined. The scaling of the hard x-ray yield with laser intensities ranging from 3×1016 to 3×1018 W/cm2 was investigated.

Scattering of laser light by stimulated Brillouin scattering (SBS) and stimulated Raman scattering (SRS) is a concern for indirect drive inertial confinement fusion (ICF). The hohlraum designs for the National Ignition Facility (NIF) raise particular concerns due to the large scale and homogeneity of the plasmas within them. Experiments at Nova have studied laser–plasma interactions within large scale length plasmas that mimic many of the characteristics of the NIF hohlraum plasmas. Filamentation and scattering of laser light by SBS and SRS have been investigated as a function of beam smoothing and plasma conditions. Narrowly collimated SRS backscatter has been observed from low density, low-Z, plasmas, which are representative of the plasma filling most of the NIF hohlraum. SBS backscatter is found to occur in the high-Z plasma of gold ablated from the wall. Both SBS and SRS are observed to be at acceptable levels in experiments using smoothing by spectral dispersion (SSD).

Temperature profiles in the bulk and at the front facet of a 20 emitter GaAs/AlGaAs double‐quantum‐well laser array are studied by spatially resolved luminescence and micro‐Raman spectroscopy. For optical output powers of about 1 W, the facet temperature of the individual emitters differs by up to 90 K. In contrast, the temperature distribution inside the resonator is highly uniform with temperature differences of less than 2 K. The facet temperature distribution correlates with the near‐field intensity pattern of the laser array. Reabsorption of laser emission close to the facet and subsequent surface recombination of the photogenerated carriers represent the main heating mechanism.

We report the first clear experimental demonstration of relativistic self-channeling of a multiterawatt laser pulse interacting with an underdense plasma. We show via Thomson scattering observations that for laser power above the critical power for self-focusing the beam remains trapped and guided over the plasma length. These observations are in fairly good agreement with a numerical model describing the laser propagation and taking into account the plasma response to the ponderomotive force.

The ability of the ruminal anaerobic phycomycete Neocallimastix patriciarum to digest model lignin compounds and lignified structures in plant material was studied in batch culture. The fungus did not degrade or transform model lignin compounds that were representative of the predominant intermonomer linkages in lignin, nor did it solubilize acid detergent lignin that had been isolated from spear grass. In a stem fraction of sorghum, 33.6% of lignin was apparently solubilized by the fungus. Solubilization of ester- and either-linked phenolics accounted for 9.2% of the lignin released. The amounts of free phenolic acids detected in culture fluid were equivalent to the apparent loss of ester-linked phenolics from the sorghum substrate. However, the fungus was unable to cleave the ether bond in hydroxycinnamic acid bridges that cross-link lignin and polysaccharide. It is suggested that the majority of the solubilized lignin fraction was a lignin carbohydrate complex containing ether-linked hydroxycinnamic acids. The lignin carbohydrate complex was probably solubilized through dissolution of xylan in the lignin-xylan matrix rather than by lignin depolymerization.

Les émissions des plasmas laser dans les domaines X et XUV sont suffisamment intenses pour constituer des sources d'analyse d'autres plasmas.La configuration expérimentale où les faisceaux laser disponibles sont répartis en deux catégories, l'une étant utilisée pour former le plasma à étudier et l'autre servant à la création du rayonnement d'analyse, est donc couramment rencontrée.Dans le domaine du rayonnement X (de 1 à 10 KeV) les diagnostics ainsi réalisés sont la radiographie et la spectroscopic d'absorption.Les applications de la radiographie sont relatives aux mesures de vitesse des zones denses de cibles planes ou sphériques et aux mesures de densité de la matière pour les microballons en phase finale d'implosion.Les paramètres accessibles au moyen de la spectroscopie d'absorption sont la profondeur optique et par confrontation avec des modèles théoriques la composition ionique du plasma, sa densité et sa température électroniques.Dans le domaine XUV (de la cinquantaine à la centaine d'eV), une sélection spectrale du rayonnement est obtenue par association de miroirs multicouches avec des filtres métalliques minces.L'obtention d'informations sur le gradient de densité électronique dans la zone sondée passe par des phénomènes de réfraction du rayonnement sonde.Des exemples correspondants aux deux domaines sont présentés.

In this paper we present X-UV gain measurements performed at nearly the same wavelength (≈ 100 Å) using three different pumping schemes. The amplifying plasma was produced by the PHEBUS laser. A same diagnostic apparatus was kept in al cases providing a direct comparison of the experimental results.

To study the x-ray laser beam deflection and divergence due to the refraction within the plasma, a specific diagnostic called CADIX has been developed at the Centre d'Etudes de Limeil-Valenton (CEL-V). When the beam refraction analysis is required a multilayer mirror is inserted on the x-ray laser output beam. It selects the lasing wavelengths and reflects them to an off-axis streak camera which records x rays as a function of time and angle. A detailed description of the instrument is presented including multilayer mirror calibration at the LURE (Laboratoire pour L'Utilisation du Rayonnement Synchrotron, Orsay, France). An experimental measurement on neon-like silver amplification is presented and analyzed with the response of the mirror. An important refraction effect is observed.

An important issue in x-ray laser experiments is the non-uniformities of the linear focal spot that can affect the growth of amplification. To understand have built a cross-slits x-ray camera which provides different magnifications in two perpendicular directions. Analysis of the image makes apparent details that were not visible when using a pinhole camera.Data are also compared to x-ray space-resolved spectra recorded in the 5-10 angstrom wavelength range with a focussing crystal spectrograph. Informations of the actual state of the plasma are drawn.

The starting flows past a two-dimensional oscillating and translating airfoil are investigated by visualization experiments and numerical calculations. The airfoil, elliptic in cross-section, is set in motion impulsively and subjected simultaneously to a steady translation and a harmonic oscillation in pitch. The incidence of the airfoil is variable between 0° and 45° and the Reynolds number based on the chord length is between 1500 and 10000. The main object of the present study is to reveal some marked characteristics of the unsteady vortices produced from the oscillating airfoil set at large incidences in excess of the static stall angle. Another purpose is to examine, in some detail, the respective and combined effects of the major experimental parameters on the vortex wake development. It is shown that, in general, the dominant parameter of the flow is the reduced frequency not only when the airfoil oscillates at incidences close to the static stall angle but also at larger incidences. It is also demonstrated that, as the pitching frequency is increased, the patterns of the vortex wake are dependent on the product of the reduced frequency and the amplitude rather than on the frequency itself. It is noted that the combined effect of a high reduced frequency and a large amplitude can give rise to cyclic superposition of leading-edge vortices from which a gradually expanding standing vortex is developed on the upper surface.