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This works concerns the interplay between surface properties and nanohydrodynamics of simple liquids in the vicinity of solid surfaces.In a first part, we have developed a new method to characterize the hydrodynamic boundary condition (HBC) relying solely on the measurement of the thermal motion of confined colloids, using a home build fluorescence correlation spectroscopy setup. We show that, for smooth surfaces, the HBC depends on the wetting properties of the wall: while the no-slip assumption is fulfilled on hydrophilic surfaces, a nanometric slippage is measured on hydrophobic surfaces.In a second part, we investigated the consequences of such modifications of the HBC on the dynamics of charged systems. For wetting surfaces, this work provides a clear interpretation for the traditional continuum description. In contrast, for non-wetting surfaces, we show that electrokinetic effects are considerably amplified by the existence of a slippage at the solid substrate., Ce travail a pour objectif d'étudier l'influence des propriétés de surface sur la nanohydrodynamique de liquides simples au voisinage de parois solides.Dans une première partie, nous avons développé une nouvelle méthode pour la détermination de la condition limite hydrodynamique (CLH) fondée sur la mesure, par spectroscopie de corrélation de fluorescence, du mouvement thermique de colloïdes confinés. Nous montrons que, sur des surfaces lisses, la CLH est influencée par les propriétés de mouillage de la paroi : tandis que l'hypothèse de non-glissement est respectée sur les parois mouillantes, nous observons un glissement nanométrique du liquide sur les parois non-mouillantes.Dans une deuxième partie, nous avons exploré, à l'aide de simulations de dynamique moléculaire, les conséquences de ces modifications sur la dynamique des systèmes chargés. Sur des parois non-mouillantes, nous mettons en évidence la possibilité d'une forte amplification des différents effets électrocinétiques.

National audience

International audience; The developmentof atmospheric pressure plasma biomedical applications led numberof teams to develop various types of rare gas plasma jets. It’sgenerally thought that the plasma plume expanding in air, wheremost of the reactive species are produced, just follow the rare gascanal which is produced at the outlet of the plasma reactor capillary.Depending on Reynolds number (i.e. capillary diameter, gasflow rate), the rare gas flow will undergo a laminar or turbulentregime. This directly affects the reactive species distribution at theplasma/target interface, thus potentially having an influence on therealized treatment. In this work, not only we demonstrate that PAPS(Pulsed Atmospheric pressure Plasma Stream) production parameters(frequency, capillary length) directly affect the rare gas flowstructure in addition to flow parameters mentioned above, but thatthe PAPS themselves control the gas column development outsidethe capillary. At low flow rate, there exists a real build up of the gascolumn within the first hundred of pulses and a rapid return to initialsituation as soon as the plasma is off. This allows a very precisemodulation of plasma application. Conversely, it can lead to misinterpretationsif not taken into account properly in the treatmentprotocol.∗ANR PAMPA, APR Plasmed Region Centre, CG Loiret.

In this paper, neon and helium plume length, generated by a plasma gun (PG) are studied. The combination of ICCD imaging, Schlieren visualization and Pitot glass probe allow to characterize the strong influence of the plasma on rare gas flow. Beyond the shifting of the transition from laminar to turbulent, a plasma induced channeling of turbulent flow is achieved. Finally, the benefit of using a capillary for plasma propagation before in-air expansion is evidenced through the generation of long plumes in ambient air at low flow rates.

International audience; The present study focuses on the inclusion behaviour near the liquid metal/slag interface. Inclusion turbulent deposition is investigated using direct numerical simulation of the liquid flow combined with Lagrangian particle tracking under conditions of one-way coupling. The interface is modelled as a non-deformable free-slip surface. Unsheared turbulence is generated by random forcing in a finite-height region parallel to the free-slip surface. In between, the turbulence diffuses toward the free surface. The inclusions are randomly introduced in the forcing region and tracked through the diffusion region up to the interface. In the particle dynamic equation, the buoyancy force, the Stokes drag, the pressure drag and the added mass are considered. Close to the interface, the hydrodynamic interactions (i.e. lubrication effects) between the inclusion and the free surface may be taken into account as well as the Van der Waals forces. Numerical simulations were performed with surface Reynolds numbers ranging from 68 to 235. The inclusion diameter varied between 10-5 m and 5.10-5 m and the particle to liquid density ratio between 0.5 and 1. For these sets of parameters, it appears that the inertia effects are very weak. The deposition of buoyant inclusions is controlled by sedimentation whereas for nonbuoyant inclusions, direct interception is the only deposition mechanism. In the latter case, the deposition velocity strongly depends on. It is shown that the deposition velocity made dimensionless by the free surface characteristic velocity scales as the inclusion diameter made dimensionless by the Kolmogorov length scale calculated at the free surface. Lastly, the effect of lubrication is examined: it can significantly reduce the direct interception contribution of the deposition velocity.

In order to improve the mixing properties of injectors, we investigate the potential synergy between azimuthal and axial shear. To this end, we examine the linear modal stability of a simplified analytical model which consists of a temporally evolving swirling jet surrounded by an annular jet with a different axial velocity. We denote L = V2/V1 the ratio between the axial velocity of the non-swirling outer jet V2 and the axial velocity of the central jet V1; and q = Oc*r1/V1 the swirl number of the central jet where Oc is the rotation rate on the axis and r1 the central jet radius. The present study extends the results of Gallaire & Chomaz (2003) where a single swirling jet was considered. For all values of the swirl number up to q = 2, adding the outer nonswirling jet substantially increases the growth rate of the most amplified mode, which can be more than doubled when L > 1. This is the result of the collaborative axial and azimuthal shear instabilities localised in between the two jets. The mode selection of larger azimuthal wavenumbers with increasing q, identified by Gallaire & Chomaz, is no longer observed when the outer jet is at least as fast as the central jet L > 1, the axisymmetric mode being the most amplified.

[Pierre-André Taguieff (dir.) - ISBN : 978-2-13-055057-0 - Collection "Quadrige dicos poche"]

[Pierre-André Taguieff (dir.) - ISBN : 978-2-13-055057-0 - Collection "Quadrige dicos poche"]

[Pierre-André Taguieff (dir.) - ISBN : 978-2-13-055057-0 - Collection "Quadrige dicos poche"]

[Pierre-André Taguieff (dir.) - ISBN : 978-2-13-055057-0 - Collection "Quadrige dicos poche"]

Colloque avec actes et comité de lecture. Internationale.; International audience

In this paper, we present a study of the influence of non-equilibrium microplasma jets (“plasma bullets”) on the hydrodynamics of a helium jet operating in open air. The helium jet is produced by a flow of helium through a hollow, cylindrical microdischarge configuration. A plasma jet, triggered by the microdischarge, can propogate in the laminar zone of the helium jet which extends some distance from the exit plane of the microdischarge. We used Schlieren photography to visualize the point of transition from laminar to turbulent regime of the helium jet and the change in the transition point due to the plasma jet for different operating conditions.

Alain Michel's book, Vichy et la Shoah: Enquête sur le paradoxe français, argues the following thesis: despite Vichy's collaboration with Nazi anti-Jewish policy, the logic of rescue was at the heart of the regime's actions. Michel regrets that since the early 1970s and the appearance of Paxton's Vichy France, the "doxa" of the "absolute negativity of Vichy" has become de rigueur. True, the context of the 1980s and 1990s, with their strong social demand and saturation media coverage of cases of crimes against humanity, influenced the writing of history. True, Vichy never intended to contribute to the deportation of Jews who had ancient roots in France. But Michel's thesis does not stand up to scrutiny. In fact, the Pétain-Laval government did precisely the same as that of the other states that collaborated in the genocidal policy: they all sacrificed the foreigners and protected their nationals to whatever extent was possible. The name of the game was definitely collaboration, not rescue.

[Pierre-André Taguieff (dir.) - ISBN : 978-2-13-055057-0 - Collection "Quadrige dicos poche"]

[hors série du Bulletin de la Société archéologique et historique de Nantes et de la Loire-Atlantique - Actes du colloque organisé à Nantes les 17 et 18 novembre 2011 par l'université de Nantes et les Archives départementales, sous la direction de Betrand Joly]

[Michel Leymarie, Olivier Dard, Jeanyves Guérin (éds.) - ISBN 978-2-7574-0401-0]

[Ont contribué à cet ouvrage : François-Xavier Nérard, Virginie Sansico, Fabrice Grenard, Raphaël Spina, Marc Bergère, Fabrice Cahen, Christophe Capuano, François Rouquet, Cédric Neveu, Christiane Kohser-Spohn et Benn E. Williams. - (Collection : Syntheses Economiques)]; La délation dans la France de la Seconde Guerre mondiale est un sujet fascinant et douloureux. À peine l'historien l'évoque- t-il que les commentaires fleurissent sur le rôle néfaste des concierges, la noirceur de l'âme humaine, la félonie du deuxième sexe ou le tropisme des Français, qui seraient des champions en la matière. Il est vrai que les Français n'ont jamais autant dénoncé que durant les années noires. Mais plus que le caractère massif du phénomène, ce sont ses conséquences qui ont profondément marqué les esprits : entre 1940 et 1944, des milliers d'individus ont payé de leur vie les dénonciations portées à la connaissance des autorités vichystes ou nazies. Indéniablement, le choc de la défaite et les traumatismes de la guerre, l'occupation allemande et le régime de Vichy ont bouleversé et perverti les relations entre la société et le pouvoir. Cet ouvrage original et inédit explore les deux faces, politique et sociale, de la dénonciation sous l'Occupation, qu'elle ait visé les insultes au maréchal Pétain, l'activisme communiste, les juifs, les trafiquants du marché noir, les avorteurs ou les réfractaires au STO. Un pan capital et pourtant méconnu de l'histoire de France.

An experimental study of the compressible mixing generated by the Richtmyer-Meshkov instability (RMI) is carried out in a vertical shock tube by means of two-components Laser Doppler Velocimetry (2C-LDV) measurements and Time-resolved Schlieren visualizations. An attempt is made to quantify the RMI-induced air/sulphurhexafluoride (SF6) mixing by measuring turbulence levels inside the mixing zone at a given stage of its development and by extracting the growth rate of the mixing zone from the Schlieren images.

Many experimental and numerical studies have found that the pairing of primary Klevin-Helmholtz (KH) vortices in mixing layers generally inhibits the growth of infinintesimal three-dimensional disturbances, delaying the transition to turbulence. In this work, we investigate the existence of 3D perturbations that grow fast enough to survive the subharmonic merging instability. For this purpose, we perform a numerical study of the transient linear evolution of 3D perturbations emerging in a homogeneous time-evolving mixing layer which undergoes pairing. We look at the optimal perturbation that yields the largest gain of energy at a specific time horizon, by use of an optimization method which solves iteratively the linearized direct and adjoint Navier-Stokes equations. In particular, we consider the influence of the time horizon relative to the saturation time of both the primary KH and subharmonic pairing instabilities.

The Richtmyer–Meshkov instability (RMI) occurs when a shock wave impulsively accelerates a perturbed interface between two gases of different densities. The interaction of the shock wave with the perturbed interface produces vorticity through baroclinic effects, potentially leading to the development of a turbulent mixing zone (TMZ). The RMI is found in some engineering application, e.g. inertial confinement fusion (ICF) or supersonic combustion and also occurs in natural phenomena such as supernova explosion (M. Brouillette [2002]). A thorough understanding of the TMZ evolution requires quantifying the turbulence levels produced by the RMI. In this context, the present study constitutes a first step towards a deeper knowledge of the fundamental mechanisms driving such mixing. The linear and non-linear stages of the RMI-induced perturbations of the interface have been and are still widely studied. However the consecutive stages, corresponding first to the TMZ development, secondly to its interaction with a reshock, is nowadays an active field of research (S. Ukai et al. [2011]). The present study aims at completing an experimental database for the validation of numerical codes and will also help to decipher the physics underlying the above-mentioned phenomena. The experimental setup consists in a 5m long, 130mm square cross section vertical shock tube. In this investigation, the initial interface between light (air) and heavy (SF6) gases is generated by a thin 0.5μm nitrocellulose membrane. This membrane is trapped between two grids, the upper one imposing a three-dimensional sinusoidal initial perturbation of wavelength equal to 1.8mm. The length of the test section has been fixed to L=250mm. The test section is equipped with transparent walls to allow flow visualizations and laser measurements (Two-components Laser Doppler Velocimetry, hereafter denoted 2C-LDV). Pressure histories of the flow in the shock tube are obtained using five piezoelectric pressure transducers. The Mach number of the incident shock wave and the Atwood number just before mixing are fixed at 1.2 and 0.699 respectively. 2C-LDV measurements of the velocity in the center of the test section are performed at two different locations downstream the initial interface: 43mm and 135mm respectively. The experimental procedure and the characterization of the experimental test rig in terms of background turbulence levels are fully described in (G. Bouzgarrou et al. 2011). The study will focus on the evolution of the turbulent velocity fluctuations inside the TMZ, before and after reshock.

[Laurent JOLY (dir.)]

We look at the effects of the radial density stratification on the stability of the q-vortex, a commonly accepted model for aircraft trailing vortices. It has been demonstrated that the 2D Lamb-Oseen vortex develops a Rayleigh-Taylor instability when its core is heavier than the surrounding uid (Joly,Fontane & Chassaing 2005, Sipp et al. 2005). The underlying mechanism relies on baroclinic vorticity generation due to any misalignment between the density gradient and the centripetal acceleration field. The instability is triggered provided that the density decreases radially somewhere in the vortex core. This mechanism is also active in the 3D trailing vortex and affects its stability characteristics due to the addition of an axial component in the acceleration field. We show that the unstable center modes of the homogeneous case (Fabre & Jacquin 2004) are promoted in a q-vortex with a heavy core. Their growth rate increases while their m-spiral structure is preserved. For an Atwood number At = 0:5, their predicted growth rate can be ten times the ones found in the homogeneous case. Furthermore, the unstable domain is extended far beyond the neutral curve in the homogeneous case, with unstable modes observed for Swirl numbers up to q = 5. It is argued here that corresponding density perturbations could eventually lead to the development of new and original strategies to decrease the lifespan of aircraft trailing vortices and greatly reduce their unwanted side-effects on contrails persistence and air traffic regulations.

[Olivier Dard, Michel Grunewald, Michel Leymarie et Jean-Michel Wittmann (éds.) - Collection : Convergences - volume 62 - ISBN 978-3-0343-0613-3 rel. - ISBN 978-3-0352-0067-6 (eBook)]

Le transport et la capture des inclusions d’oxyde à une interface métal liquide/laitier sont étudiés par simulation numérique directe. La turbulence est générée dans une zone de forçage et diffuse vers l’interface modélisée ici par une surface libre plane indéformable. Les inclusions sont introduites dans la zone de forçage et leurs trajectoires sont étudiées par suivi lagrangien : les forces de pesanteur, de masse ajoutée, d’accélération en volume et de trainée sont prises en compte dans la résolution de l’équation de la dynamique des particules. Nous rapportons ici les résultats de premières simulations réalisées dans le cas d’inclusions de diamètre 10 μm et de densité 3000 kg/m3 en suspension dans l’acier liquide.

[ISBN : 978-2-246-73691-2]; Préfecture de Police de Paris, 3e étage. Sous la férule d'André Tulard, chef du " service juif ", une centaine de fonctionnaires appliquent la réglementation antisémite. Les cadres intermédiaires, sous-chefs de bureau, organisent le recensement des Juifs. Les agents du bureau 91 examinent les cas " douteux " et reçoivent les déclarations tardives. Les employés d'ordre fabriquent les fiches du " fichier juif ". D'autres s'occupent de leur classement puis de l'exécution des rafles... Chronique ordinaire de l'antisémitisme de bureau entre 1940 et 1944. Un quotidien marqué par les intérêts du service, les stratégies de carrière, le zèle de quelques-uns et le conformisme de la plupart. Qui était ces agents de l'État et comment comprendre leur pratique administrative ? Comment distinguer leur activité de celle du commissariat général aux Questions juives, administration ultra créée par Vichy en 1941 afin d'élaborer une législation implacable et de gérer la politique de spoliation ? En répondant à ces questions, et à beaucoup d'autres, Laurent Joly signe la première étude comparée de ces deux institutions publiques qui ont joué, sous l'occupation, le rôle le plus important dans le processus de persécution. À partir de sources inédites, dossiers de carrière, dossiers d'épuration, fonds privés et entretiens individuels avec d'anciens fonctionnaires de Vichy, il met en œuvre un questionnement sociologique, adapté au passé, afin de saisir les mécanismes et les logiques de la politique antijuive telle qu'elle a été appliquée, au quotidien, dans les bureaux de l'administration française.

[ISBN : 978-2-357-66045-8]; Si les principaux acteurs du collaborationnisme, les Doriot, Déat, Brasillach ou Darnand - incarnant ce courant de la politique française qui a prôné le ralliement total à l'Allemagne nazie sous l'Occupation -, sont bien connus, on connaît moins ses seconds couteaux, ses hommes et ses femmes de l'ombre. Cette série de portraits, fondés sur l'exploitation d'archives inédites produites par la police et l'institution judiciaire (dossiers des RG et des services secrets de Vichy, archives de l'épuration), plonge le lecteur dans le monde de la collaboration ultra, dévoilant ses ressorts intimes et ses logiques cachées. Les documents - affiches, cartes d'adhésion, tracts et papillons dont raffolaient les contemporains - et les photographies qui accompagnent les textes révèlent, comme par confrontation, l'image flamboyante que nos personnages voulaient donner d'eux-mêmes lorsqu'ils discouraient en meeting, portaient beau l'uniforme du parti et même lorsqu'ils devaient rendre des comptes à la justice de la Libération.

[dans le chapitre 3 : Le régime de Vichy, p.70-97]

[dans le chapitre 3 : Le régime de Vichy, p.70-97]

[dans le chapitre 7 : Les résistances, p. 158-187]

With the aim to get a better insight into wall-turbulence interaction, an original configuration has been designed where a random forcing replaces the turbulence production by the mean flow. The synthesized turbulence therefore self-diffuses from a central layer, towards a solid wall or a free-slip surface. This situation allows one to perform a refined analysis of the blocking effect at the surface, and the calibration of the intercomponent energy transfer occurring in this region. The database includes six test cases at three different Taylor microscale Reynolds numbers ranging from 190 to 500 and the two aforementioned boundary conditions. The highest Reynolds number corresponds to a computational domain involving 10242 x 896 grid-points. These results are well suited to the assessment of Reynolds-stress transport models, and in particular the closure scheme for the slow part of the pressure-strain correlation. In the vicinity of the wall, existing models for the dissipation tensor εij might also be tested against the database.

A turbulent deposition model initially developed for aerosols is adapted to the specific case of inclusion deposition at liquid metal/slag interface. In this model, the eddies near the interface, responsible for inclusion deposition, are modeled analytically by a sequence of stagnation point flows. The equation governing inclusion motion includes the steady drag force, the buoyancy force and the pressure gradient force. Lubrication effects are taken into account close to the interface. Lagrangian tracking is performed in order to identify the particle limiting trajectory and calculate the particle deposition velocity. The numerical simulations show that the lubrication effects are negligible in the case of aerosols but are significant in the case of hydrosols (like inclusions in liquid metal). Indeed, lubrication effects reduce the deposition velocity by one order of magnitude.

We perform direct numerical simulations of the relaxation of variable-density two-dimensional turbulence. An initial collection of vortices is evolved in time together with an initially bimodal density field corresponding to an Atwood number A = 2/3. The initial vortex collection counts a thousand Gaussian vortices and spans a radius decade. The vortex Reynolds number of the smaller ones Γ/ν = 300 and we consider a unit Schmidt number. The misaligned density-gradient and fluid acceleration trigger baroclinic sources and sinks of vorticity on density fronts. The adaptive spatial resolution increase up to 6144² at peak enstrophy production time. We analyze the bias on both the evolution of the vortex population (number and radii) and the mixing progress, against a passive scalar reference simulation started from the same initial fields. Vortex carrying high-density fluid are mass-depleted or break-up into filamentary debris rapidly removed by mass diffusion. Meanwhile, intense vortices merge and trap almost unmixed low-density fluid. The resulting density field is negatively skewed and the mass exchange between vortices and the interlacing medium is shown to be the result of vortex interactions of different types among which asymmetric binary interactions are playing a central role in the mass-segregation by vorticity.

[BATTINI Michele, MATARD-BONUCCI Marie-Anne (a cura di)]

[dans le chapitre 8 : Répressions et persécutions, p. 188-215]

[dans le chapitre 8 : Répressions et persécutions, p. 188-215]