156 research outputs found
Impact of trailing wake drag on the statistical properties and dynamics of finite-sized particle in turbulence
We study by means of an Eulerian-Lagrangian model the statistical properties
of velocity and acceleration of a neutrally-buoyant finite-sized particle in a
turbulent flow statistically homogeneous and isotropic. The particle equation
of motion, beside added mass and steady Stokes drag, keeps into account the
unsteady Stokes drag force - known as Basset-Boussinesq history force - and the
non-Stokesian drag based on Schiller-Naumann parametrization, together with the
finite-size Faxen corrections. We focus on the case of flow at low
Taylor-Reynolds number, Re_lambda ~ 31, for which fully resolved numerical data
which can be taken as a reference are available (Homann & Bec 651 81-91 J.
Fluid Mech. (2010)). Remarkably, we show that while drag forces have always
minor effects on the acceleration statistics, their role is important on the
velocity behavior. We propose also that the scaling relations for the particle
velocity variance as a function of its size, which have been first detected in
fully resolved simulations, does not originate from inertial-scale properties
of the background turbulent flow but it is likely to arise from the
non-Stokesian component of the drag produced by the wake behind the particle.
Furthermore, by means of comparison with fully resolved simulations, we show
that the Faxen correction to the added mass has a dominant role in the particle
acceleration statistics even for particle with size in the inertial range.Comment: 9 pages, 9 figure
Acceleration statistics of finite-sized particles in turbulent flow: the role of Faxen forces
The dynamics of particles in turbulence when the particle-size is larger than
the dissipative scale of the carrier flow is studied. Recent experiments have
highlighted signatures of particles finiteness on their statistical properties,
namely a decrease of their acceleration variance, an increase of correlation
times -at increasing the particles size- and an independence of the probability
density function of the acceleration once normalized to their variance. These
effects are not captured by point particle models. By means of a detailed
comparison between numerical simulations and experimental data, we show that a
more accurate model is obtained once Faxen corrections are included.Comment: 10 pages, 4 figure
: THE BRONZE AGE PAINTED CAVE OF " LES FRAUX ", DORDOGNE (FRANCE)
International audienceFor six years, an interdisciplinary team of archaeologists, surveyors, environmentalists and archaeometrists have jointly carried out the study of a Bronze Age painted cave, registrered in the French Historical Monuments. The archaeological cave of Les Fraux (Saint-Martin-de-Fressengeas, Dordogne) forms a wide network of galleries, characterized by the exceptional richness of its archaeological remains such as ceramic and metal deposits, parietal representation and about domestic fireplaces. This cave is the only protohistorical site in Europe wherein are gathered testimonies of domestic, spiritual and artistic activities. Fortunately, the cave was closed at the end of the Bronze Age, following to the collapse of its entrance. The site was re-discovered in 1989 and its study started in 2007. The study in progress takes place in a new kind of tool founded by the CNRS's Institute of Ecology and Environment. The purpose of this observatory is the promotion of new methodologies and experimental studies in Global Ecology. In that framework, 3D models of the cave constitute the common work support and the best way for scientific communication for the various studies conducted on the site by nearly forty researchers. In this specific context, a partnership among archaeologists and surveyors from INSA Strasbourg allows the team to develop, in an interdisciplinary way, new methods of data acquiring based on contact-free measurements techniques in order to acquire a full 3D-documentation. This work is conducted in compliance with the integrity of the site. Different techniques based on Terrestrial Laser Scanning, Digital Photogrammetry and Spatial Imaging System have been used in order to generate a geometric and photorealistic 3D model from the combination of point clouds and photogrammetric images, for both visualization and accurate documentation purposes. Various scales of acquiring and diverse resolutions have been applied according to the subject: global volume cave, parietal representations, deposits... The aim of this paper is to issue a statement of the different 3D technologies tested in this outstanding site and the integration of magnetic measurements in the models
Thermal characterization of ancient hearths from the cave of Les Fraux (Dordogne, France) by thermoluminescence and magnetic susceptibility measurements
International audienceNumerous sorts of evidence of fires were observed in the cave of Les Fraux (Dordogne, France) and in particular the effects of fire on sediment were studied for a better understanding of their use centuries ago. Our present objective is the evaluation of the firing intensity by determining the past temperature (paleotemperature) attained by the topmost sediment of the archaeological fires. The principle of paleotemperature determination is based on the thermoluminescence (TL) properties of quartz and the magnetic susceptibility of the sediment. By comparing the TL signal of anciently heated quartz to the TL signal of thermal references made in the laboratory, we were able to obtain a maximal equivalent temperature attained for each sample extracted from ancient fires. The magnetic susceptibility (previously measured on the hearth surface) could thus be estimated as a function of temperature. The main result of this study is the temperature mapping of the hearth which can be used as meaningful information about the ancient firing intensity
Caractérisation thermique de foyers de l'ùge du Bronze de la grotte des Fraux (Dordogne)
International audienceThe Les Fraux cave (Dordogne, France) was occupied during the Bronze Age. It contained numerous hearth features. The questions related to the function of the cave and more particularly of the hearth features involved a multidisciplinary study of these remains. The aim of this paper is to present the methods applied for the study of the hearths. The analysis combined thermoluminescence (TL) of the quartz grains and magnetic susceptibility principally registered by the iron oxides contained in the sediments (whether heated or not). For each sample, the TL permits to obtain the corresponding temperature (palaeotemperature) by comparison between the TL curves of the samples and those of the thermal references. The relationship established between the palaeotemperature and the magnetic susceptibility allows converting magnetic susceptibility to palaeotemperatures reached by the sediment on a larger surface. This permits to characterize the intensity of heat attained in these hearths and to contribute to the determination of the function of this site through the comparison with experimental firesLa grotte des Fraux (Dordogne, France), occupée à l'ùge du Bronze, présente de nombreuses structures de combustion. Les questions relatives à la fonction du site et plus particuliÚrement des foyers, ont entrainé la mise en place d'une étude pluridisciplinaire de ces structures. L'objet de cet article est de présenter la méthodologie mise en oeuvre pour l'étude de ces foyers. Elle s'appuie sur un couplage entre la thermoluminescence (TL) des grains de quartz et la susceptibilité magnétique principalement portée par les oxydes de fer contenus dans les sédiments (chauffés ou non). Pour chaque prélÚvement, la TL permet d'obtenir une température équivalente (paléotempérature) par comparaison entre les courbes TL correspondantes et celles de références thermiques. La relation établie entre la paléotempérature et la susceptibilité magnétique permet de traduire la carte de susceptibilité magnétique en paléotempératures atteintes par le sédiment sur une surface étendue. L'objectif est de caractériser l'intensité de chauffe de ces foyers et, par une comparaison avec des feux expérimentaux, de contribuer à préciser la fonction de ce site
Characterisation of iron (II) sulfides in wet archaeological woods: the wreck of Mandirac (IV th century, antique ports of Narbonne, France)
International audienceThe wreck of Mandirac (Narbonne, France, IV century) was excavated in 2013 and 2014 from waterlogged soil. A magnetic prospection campaign performed in 2014 revealed that the wood was abnormally magnetic. A plank extracted from the hull, containing an iron nail, was analysed using Environmental Scanning Electron Microscopy, micro-Raman spectroscopy, X-Ray diffraction and magnetic characterization methods in order to identify the mineral composition inside the wood and of the nail. Results revealed an accumulation of greigite and pyrite in the wood around the nail, where the pH was measured below 3. The nail was completely corroded into pyrite and siderite, with no metal left. Marcasite was also identified far from the nail, where the pH was measured around 5. Greigite was the only phase responsible of the magnetic signal emitted from the wood and proved to be present in varying amounts throughout the entire wreck. The iron sulfides were probably formed via anaerobic sulphidogenic bacteria influenced corrosion processes, which occurred on the nails. These results are very different from those obtained from more recent shipwrecks (XIX century). So the nature of these iron sulfides in wet archaeological woods is discussed
Assemblages bois-fer et biocorrosion : étude des sulfures de fer formés en conditions anoxiques dans des bois d'épaves
International audienceLes consĂ©quences de la corrosion des renforts de fer dans les assemblages de bois peuvent compromettre la durabilitĂ© d'une structure. Si le bois est gorgĂ© d'eau, les conditions Ă la surface du mĂ©tal deviennent anoxiques. La prĂ©sence de fer et l'activitĂ© microbiologique favorisĂ©e par la prĂ©sence de matiĂšre organique font des systĂšmes composites bois-fer des milieux propices Ă l'apparition de sulfures de fer. Or ces derniers se transforment soit en milieux anaĂ©robies sulfurĂ©s, soit lorsque l'oxygĂšne diffuse dans le bois, soit lors d'une remise Ă l'air de la structure, avec pour consĂ©quence de contribuer Ă la dĂ©gradation du bois. Afin de mieux comprendre les mĂ©canismes impliquĂ©s, nous Ă©tudions des bois d'Ă©paves archĂ©ologiques. Cet article sera illustrĂ© par des exemples d'Ă©poques rĂ©centes (18 Ăš s. 19 Ăš s.) et antiques et par une approche analytique multi-techniques originale. La mackinawite, la greigite et la pyrite ont Ă©tĂ© identifiĂ©es et il semble que la nature des sulfures de fer prĂ©sents soit liĂ©e Ă l'Ăąge du vestige conformĂ©ment aux schĂ©mas proposĂ©s dans la littĂ©rature d'Ă©volution de ces composĂ©s. La greigite est intĂ©ressante Ă considĂ©rer. Seule phase dĂ©tectĂ©e quel que soit lâĂąge, elle peut sâavĂ©rer un bon marqueur de la biocorrosion du fer dans les assemblages bois-fer
APRĂS LE DĂLUGE : ĂVOLUTION GĂOMORPHOLOGIQUE DU DELTA DU DANUBE APRĂS LA RECONNEXION MER NOIRE - MĂDITERRANĂE ET SES IMPLICATIONS SUR LE PEUPLEMENT ĂNĂOLITHIQUE / CHALCOLITHIQUE
International audienceIn this work, we document the evolution of the upper Danube delta (i.e. blocked delta) using archaeological, geomorphological and geoarchaeological approaches, after the discovery, in the 90's, of the oldest human settlement (Eneolithic/Chalcolithic) close to Mila 23 district. In-site and out-site fieldwork allows us to refine the Danube delta early evolution from 5,600 to 4,000 cal. yr BC. During this period, it functions like a fresh bayhead delta, flowing into a huge lagoon isolated from the open sea by the Letea-Caraorman spit. Then, lagoon water level rose rapidly due to an eustatic rise, and possibly from an unusual highstand of sealevel resulting from wet conditions. The settlement was then partially flooded and abandoned around 4,450 cal. yr BC.Dans ce travail, l'Ă©volution de la zone interne du delta du Danube est apprĂ©hendĂ©e Ă partir de travaux gĂ©oarchĂ©ologiques et gĂ©omorphologiques rĂ©alisĂ©s dans le district de Mila 23, suite Ă la dĂ©couverte de la plus vieille occupation humaine (Ă©nĂ©olithique/ chalcolithique) du delta dans les annĂ©es 90. Les travaux sur site et hors site ont permis de prĂ©ciser les stades prĂ©coces de l'Ă©volution du delta entre 5 600 et 4 000 ans cal. av. J.-C. Celui-ci Ă©volue comme un delta lagunaire en eau douce, s'Ă©pandant dans un large lagon sĂ©parĂ© de la mer par la ride de Letea-Caraorman. La remontĂ©e rapide du niveau lagunaire, sous l'effet de la remontĂ©e eustatique et peut-ĂȘtre d'une sur cote liĂ©e Ă un Ă©vĂ©nement humide, ont conduit Ă une submersion partielle du site et probablement Ă son abandon vers 4 450 ans cal. av. J.-C
Adaptive LagrangianâEulerian computation of propagation and rupture of a liquid plug in a tube
Liquid plug propagation and rupture occurring in lung airways can have a detrimental effect on epithelial cells. In this study, a numerical simulation of a liquid plug in an infinite tube is conducted using an EulerianâLagrangian approach and the continuous interface method. A reconstruction scheme is developed to allow topological changes during plug rupture by altering the connectivity information about the interface mesh. Results prior to the rupture are in reasonable agreement with the study of Fujioka et al . in which a Lagrangian method is used. For unity nonâdimensional pressure drop and a Laplace number of 1000, rupture time is shown to be delayed as the initial precursor film thickness increases and rupture is not expected for thicknesses larger than 0.10 of tube radius. During the plug rupture process, a sudden increase of mechanical stresses on the tube wall is recorded, which can cause tissue damage. The peak values of those stresses increase as the initial precursor film thickness is reduced. After rupture, the peaks in mechanical stresses decrease in magnitude as the plug vanishes and the flow approaches a fully developed behavior. Increasing initial pressure drop is shown to linearly increase maximum variations in wall pressure and shear stress. Decreasing the pressure drop and increasing the Laplace number appear to delay rupture because it takes longer for a fluid with large inertial forces to respond to the small pressure drop. Copyright © 2010 John Wiley & Sons, Ltd.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/88023/1/2422_ftp.pd
Projected SO(5) Hamiltonian for Cuprates and Its Applications
The projected SO(5) (pSO(5)) Hamiltonian incorporates the quantum spin and
superconducting fluctuations of underdoped cuprates in terms of four bosons
moving on a coarse grained lattice. A simple mean field approximation can
explain some key feautures of the experimental phase diagram: (i) The Mott
transition between antiferromagnet and superconductor, (ii) The increase of T_c
and superfluid stiffness with hole concentration x and (iii) The increase of
antiferromagnetic resonance energy as sqrt{x-x_c} in the superconducting phase.
We apply this theory to explain the ``two gaps'' problem found in underdoped
cuprate Superconductor-Normal- Superconductor junctions. In particular we
explain the sharp subgap Andreev peaks of the differential resistance, as
signatures of the antiferromagnetic resonance (the magnon mass gap). A critical
test of this theory is proposed. The tunneling charge, as measured by shot
noise, should change by increments of Delta Q= 2e at the Andreev peaks, rather
than by Delta Q=e as in conventional superconductors.Comment: 3 EPS figure
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