459 research outputs found
Experimental study of the turbulent structure of a boundary layer developing over a rough surface
National audienceNous avons analysé les caractéristiques turbulentes de la couche limite neutre se développant sur une surface rugueuse. Des expériences ont été réalisées dans un canal hydraulique pour mesurer les champs bidimensionnels de vitesse via la technique de Particle Image Velocimetry (PIV). Ces données expérimentales décrivent cette couche limite en termes de quantités moyennes et turbulentes avec un haut niveau de précision. Les termes des budgets d'énergie ont ainsi pu être estimés. Il apparait que le développement de la couche limite rugueuse ne modifie pas significativement la répartition entre les termes constitutifs des différents bilans. Les échelles de longueurs intégrales ont été estimées, de manière directe, à partir des corrélations spatiales. Ces échelles de longueurs verticales permettent alors de paramétrer les longueurs de mélange et de dissipation, utilisées dans des modèles 1D de prédiction. / We analysed the turbulent characteristics of the neutral boundary-layer developing over rough surfaces. A set of hydraulic flume experiments were carried out in order to measure two-dimensional velocity fields via a particle image velocimetry (PIV) technique. The resulting experimental data describe this boundary layer in terms of the mean and turbulent quantities with a high level of accuracy. These results enabled the terms of the energy budgets to be estimated and show that the development of the rough neutral boundary layer does not significantly modify the repartition between the constitutive terms of the different budget. Spatial correlation analysis permitted the longitudinal and vertical integral length also to be estimated directly. Theses vertical length scales are then used to parametrize the mixing and dissipative lengths, used in 1D prediction models
Morphological controls on surface runoff: an interpretation of steady-state energy patterns, maximum power states and dissipation regimes within a thermodynamic framework
A stellar flare during the transit of the extrasolar planet OGLE-TR-10b
We report a stellar flare occurring during a transit of the exoplanet OGLE-TR-10b, an event not previously reported in the literature. This reduces the observed transit depth, particularly in the u'-band, but flaring could also be significant in other bands and could lead to incorrect planetary parameters. We suggest that OGLE-TR-10a is an active planet-hosting star and has an unusually high X-ray luminosity
A homogeneous spectroscopic analysis of host stars of transiting planets
The analysis of transiting extra-solar planets provides an enormous amount of
information about the formation and evolution of planetary systems. A precise
knowledge of the host stars is necessary to derive the planetary properties
accurately. The properties of the host stars, especially their chemical
composition, are also of interest in their own right. Information about planet
formation is inferred by, among others, correlations between different
parameters such as the orbital period and the metallicity of the host stars.
The stellar properties studied should be derived as homogeneously as possible.
The present work provides new, uniformly derived parameters for 13 host stars
of transiting planets. Effective temperature, surface gravity, microturbulence
parameter, and iron abundance were derived from spectra of both high
signal-to-noise ratio and high resolution by assuming iron excitation and
ionization equilibria. For some stars, the new parameters differ from previous
determinations, which is indicative of changes in the planetary radii. A
systematic offset in the abundance scale with respect to previous assessments
is found for the TrES and HAT objects. Our abundance measurements are
remarkably robust in terms of the uncertainties in surface gravities. The iron
abundances measured in the present work are supplemented by all previous
determinations using the same analysis technique. The distribution of iron
abundance then agrees well with the known metal-rich distribution of planet
host stars. To facilitate future studies, the spectroscopic results of the
current work are supplemented by the findings for other host stars of
transiting planets, for a total dataset of 50 objects.Comment: accepted for publication in A&A, 7 pages, 6 figure
Staphylococcus epidermidis glucose uptake in biofilm versus planktonic cells
The aim of this work was to compare the glucose
uptake of biofilms formed by four different Staphylococcus
epidermidis strains as well as to compare between
sessile and planktonic cells of the same strain. Biofilm cells
showed a lower level of glucose uptake compared to
planktonic cells. Moreover, glucose uptake by cells in the
sessile form was strongly influenced by biofilm composition.
Therefore, this work helps to confirm the phenotypic
variability of S. epidermidis strains and the different
behaviour patterns between sessile and free cells.Fundação para a Ciência e a Tecnologia (FCT) - POCTI/ESP/42688/2001;
SFRH/BD/19265/2004
Surface Incompressibility from Semiclassical Relativistic Mean Field Calculations
By using the scaling method and the Thomas-Fermi and Extended Thomas-Fermi
approaches to Relativistic Mean Field Theory the surface contribution to the
leptodermous expansion of the finite nuclei incompressibility has been
self-consistently computed. The validity of the simplest expansion, which
contains volume, volume-symmetry, surface and Coulomb terms, is examined by
comparing it with self-consistent results of the finite nuclei
incompressibility for some currently used non-linear sigma-omega parameter
sets. A numerical estimate of higher-order contributions to the leptodermous
expansion, namely the curvature and surface-symmetry terms, is made.Comment: 18 pages, REVTeX, 3 eps figures, changed conten
Activation of Hypoxia Inducible Factor 1 Is a General Phenomenon in Infections with Human Pathogens
BACKGROUND: Hypoxia inducible factor (HIF)-1 is the key transcriptional factor involved in the adaptation process of cells and organisms to hypoxia. Recent findings suggest that HIF-1 plays also a crucial role in inflammatory and infectious diseases. METHODOLOGY/PRINCIPAL FINDINGS: Using patient skin biopsies, cell culture and murine infection models, HIF-1 activation was determined by immunohistochemistry, immunoblotting and reporter gene assays and was linked to cellular oxygen consumption. The course of a S. aureus peritonitis was determined upon pharmacological HIF-1 inhibition. Activation of HIF-1 was detectable (i) in all ex vivo in biopsies of patients suffering from skin infections, (ii) in vitro using cell culture infection models and (iii) in vivo using murine intravenous and peritoneal S. aureus infection models. HIF-1 activation by human pathogens was induced by oxygen-dependent mechanisms. Small colony variants (SCVs) of S. aureus known to cause chronic infections did not result in cellular hypoxia nor in HIF-1 activation. Pharmaceutical inhibition of HIF-1 activation resulted in increased survival rates of mice suffering from a S. aureus peritonitis. CONCLUSIONS/SIGNIFICANCE: Activation of HIF-1 is a general phenomenon in infections with human pathogenic bacteria, viruses, fungi and protozoa. HIF-1-regulated pathways might be an attractive target to modulate the course of life-threatening infections
The corona and companion of CoRoT-2A. Insights from X-rays and optical spectroscopy
CoRoT-2 is one of the most unusual planetary systems known to date. Its host
star is exceptionally active, showing a pronounced, regular pattern of optical
variability caused by magnetic activity. The transiting hot Jupiter, CoRoT-2b,
shows one of the largest known radius anomalies. We analyze the properties and
activity of CoRoT-2A in the optical and X-ray regime by means of a high-quality
UVES spectrum and a 15 ks Chandra exposure both obtained during planetary
transits. The UVES data are analyzed using various complementary methods of
high-resolution stellar spectroscopy. We characterize the photosphere of the
host star by deriving accurate stellar parameters such as effective
temperature, surface gravity, and abundances. Signatures of stellar activity,
Li abundance, and interstellar absorption are investigated to provide
constraints on the age and distance of CoRoT-2. Furthermore, our UVES data
confirm the presence of a late-type stellar companion to CoRoT-2A that is
gravitationally bound to the system. The Chandra data provide a clear detection
of coronal X-ray emission from CoRoT-2A, for which we obtain an X-ray
luminosity of 1.9e29 erg/s. The potential stellar companion remains undetected
in X-rays. Our results indicate that the distance to the CoRoT-2 system is
approximately 270 pc, and the most likely age lies between 100 and 300 Ma. Our
X-ray observations show that the planet is immersed in an intense field of
high-energy radiation. Surprisingly, CoRoT-2A's likely coeval stellar
companion, which we find to be of late-K spectral type, remains X-ray dark.
Yet, as a potential third body in the system, the companion could account for
CoRoT-2b's slightly eccentric orbit.Comment: accepted for publication by A&
Shallow flow over a bed with a lateral change of roughness
River beds frequently exhibit a lateral variation of roughness. For example, in the case of an overflowing river, the main channel has a smoother topography compared to the adjacent floodplains where vegetation and land occupation yield an important hydraulic roughness. The lateral difference in roughness can induce a high lateral velocity gradient within the river crosssection that gives birth to a mixing layer. This mixing layer leads to fluid and momentum transfers between the two adjacent beds. To understand such mixing processes in rivers is important for predicting stage-discharge relationships and the velocity distribution within the cross-section. In order to address these issues in the context of a shallow water flow with a water depth h of the same order as the roughness elements of the bed, experiments were undertaken in a 26 m long and 1.1 m wide glass-walled open-channel flume. One half-side of the bed was covered with an array of cubes of height k arranged in a square configuration, the other side with smooth glass. Three different levels of cube submergence h/k were examined (h/k = 0.8, 1.5 and 2). The experiments and measurements were designed to yield the flow in the complete volume of the interstices across the cube array. To achieve this, 2C-3D linear-scanning PIV measurements with zero-parallax optics were developed and set up. The measurements revealed the complexity of the flow structure around the interface between the rough and smooth beds. The results show that the ability of the mixing layer to exchange momentum is highly dependent on the level of the cube submergence h/k
Effect of Low Temperature on Growth and Ultra-Structure of Staphylococcus spp
The effect of temperature fluctuation is an important factor in bacterial growth especially for pathogens such as the staphylococci that have to remain viable during potentially harsh and prolonged transfer conditions between hosts. The aim of this study was to investigate the response of S. aureus, S. epidermidis, and S. lugdunensis when exposed to low temperature (4°C) for prolonged periods, and how this factor affected their subsequent growth, colony morphology, cellular ultra-structure, and amino acid composition in the non-cytoplasmic hydrolysate fraction. Clinical isolates were grown under optimal conditions and then subjected to 4°C conditions for a period of 8 wks. Cold-stressed and reference control samples were assessed under transmission electron microscopy (TEM) to identify potential ultra-structural changes. To determine changes in amino acid composition, cells were fractured to remove the lipid and cytoplasmic components and the remaining structural components were hydrolysed. Amino acid profiles for the hydrolysis fraction were then analysed for changes by using principal component analysis (PCA). Exposure of the three staphylococci to prolonged low temperature stress resulted in the formation of increasing proportions of small colony variant (SCV) phenotypes. TEM revealed that SCV cells had significantly thicker and more diffuse cell-walls than their corresponding WT samples for both S. aureus and S. epidermidis, but the changes were not significant for S. lugdunensis. Substantial species-specific alterations in the amino acid composition of the structural hydrolysate fraction were also observed in the cold-treated cells. The data indicated that the staphylococci responded over prolonged periods of cold-stress treatment by transforming into SCV populations. The observed ultra-structural and amino acid changes were proposed to represent response mechanisms for staphylococcal survival amidst hostile conditions, thus maintaining the viability of the species until favourable conditions arise again
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