Étude expérimentale d'un plasma d'air créé au cours de l'interaction d'un faisceau laser CO2 et d'une cible solide

A plane target in air is irradiated by a CO2 laser beam which is focused sufficiently to produce a laser flux from 107 to 1.5 × 108 W cm-2 at the surface. Emission and spectral absorption results describe the state of the plasma created on the target. The continued deposition of the laser energy to this plasma finally results in a movement upstream from the surface. Measurements related to this motion are presented.Une cible solide plane placée dans l'air est illuminée par le faisceau d'un laser CO2 focalisé, produisant un flux de 107 à 1,5 × 108 W cm-2 sur la surface. Des mesures d'émission et d'absorption décrivent l'état du plasma créé près de la cible. La déposition d'énergie laser dans ce plasma conduit à un mouvement en amont de la surface. Des mesures relatives à ce mouvement sont présentées

Étude expérimentale du développement et du chauffage d'une colonne de plasma par le rayonnement d'un laser CO2 de puissance

A plasma column is created and heated by the energy of a CO2 laser and is radially confined by the magnetic field of a solenoid. The measurement of the plasma front development velocity for varying W/p (ratio of the laser flux to the initial gas density) show three regimes. To aid in the understanding of these regimes some measured values of the temperature and electron density are presented.L'étude porte sur la création et le chauffage au moyen d'un laser CO 2 de puissance,. d'une colonne de plasma d'hydrogène confiné radialement par un champ magnétique. La mesure de la vitesse longitudinale du front de plasma suivant la valeur du paramètre W/p (flux laser rapporté à la densité initiale du gaz), met en évidence trois régimes de développement. Des mesures de la température et de la densité électronique axiale sont effectuées pour caractériser les différents régimes

Doorway flow from a reduced scale isothermal air/helium approach

International audienceThis paper deals with an experimental study of buoyancy-induced gas flow at a doorway-like opening as part of a fire smoke propagation application. The experimental approach is based on a reduced scale air/helium protocol, which allows a wide range of density conditions to be studied. The flow at the opening has been studied for wide ranges of density (0.17-1.22) kg/m(3), flow rate (20-600) l/min and doorway width (0.4-14) cm usually associated with smoke propagation in life-size applications. The data set consists of measurements of flow thicknesses, spilling angles and laser tomography visualizations. The results show that flow behaviour is in harmony with the basic Bernoulli theory commonly applied to this type of flow. The value of the flow coefficient obtained with several density conditions corresponds to the behaviour found by Emmons for a given flow density. The magnitude of the spilling angle has been investigated and its dependency on the Froude number has been demonstrated. The opening width has a significant effect on flow behaviour and the associated spill plume. The reduction of the opening width enhances the mechanism of air entrainment and modifies the position of the transition height at which plume cross sections change from being rectangular to circular in shape. Published by Elsevier Masson SAS

Study Of The Cavitation Induced By A High Power Laser Beam

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RECENT PROGRESS AT IMFM ON EXCIMER LASERS AND THEIR APPLICATIONS

Recent progress made at IMFM on excimer lasers and their applications is presented. It mainly concerns high power and pulse rate frequency discharge excimer lasers, high energy E-Beam pumped excimer lasers, new excitation schemes, photolysis excitation of uv-visible lasers by sliding discharges, new vuv sources using ionic excimers and processing or elaboration of materials with excimer lasers

Twin inclined jets in crossflow: experimental investigation of different flow regimes and jet elevations

International audienceThe present experimental study is dedicated to unfolding the mixing process generated by double-inclined, variably elevated jets in crossflow. Twin tandem jets in crossflow are very common in the industry and are closely dependent on several parameters. Detailing the induced interactions in such a model would ultimately enhance our understanding and help optimize related applications. The jets handled in the present work are placed three diameters apart, arranged in line with the oncoming crossflow and sent at variable levels (h = 0, 1, 2, 5 cm) from the ground of a wind tunnel and are discharged from elliptic discharging cross sections. Elliptic jets are of particular interest in applications like industrial and boat chimneys, and are practically easily obtained by razing circular cylinders at the desired inclination and height. Particle image velocimetry and coupled charge device CCD camera were used. The dynamic and turbulent behavior of the resulting flowfield was characterized in terms of streamlines, velocity components and vortical structures. The obtained data helped highlight the impact of jets' elevation over their mixing mechanism among the surrounding free stream: establishment of the resulting flowfield dynamics and settlement of the induced vortical structures. The mixing process evolution was also considered under different flow regimes. For the matter an injection ratio (defined as jets to the mainstream's velocity ratio) raging between 0.67, 1 and 1.29 was considered. These cases correspond to jets dynamically dominated by, equivalent to or more dominant than the mainstream. The impact of jets' elevation combined to that of the injection ratio affects both the developed vortical structures and the established dynamic fields, which in turn highly affects the induced mixing process

EXPERIMENTAL AND COMPUTATIONAL INVESTIGATION OF PULSED LASER ENERGY TRANSMISSION THROUGH THE ATMOSPHERE

An experimental study has been conducted and compared with a computed numerical model in order to determine the limitations induced by the air breakdown on the transmission of a high energy laser pulse through the standard atmosphere. The 10.6 µm CO2 laser displays up to 300 J in a 2.5 µsec pulse with a high power peak (typically 5x108 W ; 50 ns F W H M) followed by a high energy tail. The laser beam is focused by means of a 5 X telescope at 67.5 m (f/D = 135) outside the laboratory. Envelope of the beam and energy density distribution in the focal plane are presented. Aerosol concentration and size distribution measurements are obtained by means of a particle optical counter (Knollenberg axially scattering spectrometer probe : 0.5 µm 45 µm). Energy and fluence transmitted through the air breakdown - induced plasma for given atmospherical conditions are measured as a function of time and incident fluence and then compared with results of a computed numerical hydrodynamic bi-dimensional model

Dynamic and mass transfer characteristics of the flow issued from a bent chimney around buildings

International audienceThe present work consists in an experimental investigation of the flow issuing from a bent chimney over a downstream obstacle. Our purpose is to explore the resulting flow field and its different characterizing features. These features were captured by means of the Particle Image Velocimetry technique. A numerical simulation of the problem has also been carried out and validated after comparison of the corresponding results to the experimental data. A good level of agreement was achieved between the experiments and the calculations. Then, we tried to upgrade our model by adopting large (real) scale dimensions. Our purpose consisted mainly in the observation and evaluation of the behavior of the incoming flow in presence of a double tandem obstacle. In a second step, we proposed to increase the number of the placed obstacles to four. The results given by the three-dimensional model are likely to highlight the dynamic features of the established field as well as the resulting mass transfer. Finally, we tried to evaluate the effect of further parameters on the characterizing features of the resulting flow filed such as the velocity ratio, the obstacles' gap, the arrangement of the obstacles and the obstacles' geometry

MESURE DE LA PRESSION INDUITE SUR UNE CIBLE METALLIQUE PAR UN LASER CO2 IMPULSIONNEL

Des mesures de pressions induites au cours de l'interaction d'un laser CO2 - impulsionnel de puissance avec une cible d'aluminium, ont été effectuées à l'aide de capteurs rapides. Il a été possible d'étudier l'évolution de la pression au niveau de la cible quand la pression de l'air ambiant varie de 760 Torr à 0,5 Torr. L'influence de la forme de l'impulsion laser sur le couplage mécanique I/E (impulsion mécanique I, rapportée à l'énergie laser E) a été mise en évidence. Des pressions élevées 1,7 kbar et des couplages mécaniques importants (20 dyne sec/J) ont été obtenus.Measurements of induced pressures during the interaction of a CO2 pulsed laser with an aluminium target have been carried with the help of fast gauges. It has been possible to investigate the evolution of the pressure on the target surface while the ambiant air pressure varies from 0.5 Torr to 760 Torr. The influence of the shape of the laser pulse upon the mechanical coupling I/E (ratio of the mechanical impulse I to the laser energy E) has been shown. High pressures - 1.7 kbar and significant mechanical coupling (20 dyne sec/J) have been obtained

Temperature impact on the turbulence generated by the interaction of twin inline inclined jets in crossflow

International audienceConsideration is given to the interaction of twin tandem jets with an oncoming uniform crossflow. A variable temperature is assumed for the emitted jets while the crossflow is maintained constant, equivalent to the ambient temperature. Both jet nozzles are elliptic, as initially inclined with an angle of 60A degrees, placed three diameters apart in line with the crossflow and discharge a nonreactive fume. The handled configuration is numerically simulated in the present work, by means of the finite volume method together with a non uniform grid system. The model is first validated with reference to available experimental data, in the simple isothermal case of air jets in air crossflow. It is then upgraded by considering a nonreactive fume discharged at a variable temperature. The upgraded model turbulence is described by means of the Reynolds Stress Model second order turbulent closure model. The present work is to our knowledge pioneering in the introduction of this particular model is such a configuration and its introduction proved to be highly valuable since is described satisfyingly the turbulent behavior of the resulting flowfield. This behavior is, precisely, specified in terms of shear stress components whose evolutions, explored along the different directions of the domain, showed a more pronounced vertical mixing, and gave rise to more significant vortices in most characterizing zones: near the injection plane as well as within the discharging nozzles