995 research outputs found
Short-wavelength secondary instabilities in homogeneous and stably stratified shear flows
We present a numerical investigation of three-dimensional, short-wavelength
linear instabilities in Kelvin-Helmholtz (KH) vortices in homogeneous and
stratified environments. The base flow, generated using two-dimensional
numerical simulations, is characterized by the Reynolds number and the
Richardson number defined based on the initial one-dimensional velocity and
buoyancy profiles. The local stability equations are then solved on closed
streamlines in the vortical base flow, which is assumed quasi-steady. For the
unstratified case, the elliptic instability at the vortex core dominates at
early times, before being taken over by the hyperbolic instability at the
vortex edge. For the stratified case, the early time instabilities comprise a
dominant elliptic instability at the core and a hyperbolic instability strongly
influenced by stratification at the vortex edge. At intermediate times, the
local approach shows a new branch of instability (convective branch) that
emerges at the vortex core and subsequently moves towards the vortex edge. A
few more convective instability branches appear at the vortex core and move
away, before coalescing to form the most unstable region inside the vortex
periphery at large times. The dominant instability characteristics from the
local approach are shown to be in good qualitative agreement with results from
global instability studies for both homogeneous and stratified cases.
Compartmentalized analyses are then used to elucidate the role of shear and
stratification on the identified instabilities. The role of buoyancy is shown
to be critical after the primary KH instability saturates, with the dominant
convective instability shown to occur in regions with the strongest statically
unstable layering. We conclude by highlighting the potentially insightful role
that the local approach may offer in understanding the secondary instabilities
in other flows.Comment: Submitted to J. Fluid Mech., 20 pages, 10 figure
ORIGAMIX, a CdTe-based spectro-imager development for nuclear applications
The Astrophysics Division of CEA Saclay has a long history in the development
of CdTe based pixelated detection planes for X and gamma-ray astronomy, with
time-resolved imaging and spectrometric capabilities. The last generation,
named Caliste HD, is an all-in-one modular instrument that fulfills
requirements for space applications. Its full-custom front-end electronics is
designed to work over a large energy range from 2 keV to 1 MeV with excellent
spectroscopic performances, in particular between 10 and 100 keV (0.56 keV FWHM
and 0.67 keV FWHM at 13.9 and 59.5 keV). In the frame of the ORIGAMIX project,
a consortium based on research laboratories and industrials has been settled in
order to develop a new generation of gamma camera. The aim is to develop a
system based on the Caliste architecture for post-accidental interventions or
homeland security, but integrating new properties (advanced spectrometry,
hybrid working mode) and suitable for industry. A first prototype was designed
and tested to acquire feedback for further developments. In this study, we
particularly focused on spectrometric performances with high energies and high
fluxes. Therefore, our device was exposed to energies up to 700 keV (133Ba,
137Cs) and we measured the evolution of energy resolution (0.96 keV at 80 keV,
2.18 keV at 356 keV, 3.33 keV at 662 keV). Detection efficiency decreases after
150 keV, as Compton effect becomes dominant. However, CALISTE is also designed
to handle multiple events, enabling Compton scattering reconstruction, which
can drastically improve detection efficiencies and dynamic range for higher
energies up to 1408 keV (22Na, 60Co, 152Eu) within a 1-mm thick detector. In
particular, such spectrometric performances obtained with 152Eu and 60Co were
never measured before with this kind of detector.Comment: Nuclear Instruments and Methods in Physics Research Section A:
Accelerators, Spectrometers, Detectors and Associated Equipment. Available
online 9 January 2015, ISSN 0168-9002
(http://www.sciencedirect.com/science/article/pii/S0168900215000133).
Keywords: CdTe; X-ray; Gamma-ray; Spectrometry; Charge-sharing; Astrophysics
Instrumentation; Nuclear Instrumentation; Gamma-ray camera
The Josephson critical current in a long mesoscopic S-N-S junction
We carry out an extensive experimental and theoretical study of the Josephson
effect in S-N-S junctions made of a diffusive normal metal (N) embedded between
two superconducting electrodes (S). Our experiments are performed on Nb-Cu-Nb
junctions with highly-transparent interfaces. We give the predictions of the
quasiclassical theory in various regimes on a precise and quantitative level.
We describe the crossover between the short and the long junction regimes and
provide the temperature dependence of the critical current using dimensionless
units and where
is the Thouless energy. Experimental and theoretical results are in excellent
quantitative agreement.Comment: 5 pages, 4 figures, slighly modified version, publishe
Coherent low-energy charge transport in a diffusive S-N-S junction
We have studied the current voltage characteristics of diffusive mesoscopic
Nb-Cu-Nb Josephson junctions with highly-transparent Nb-Cu interfaces. We
consider the low-voltage and high-temperature regime eV<\epsilon_{c}<k_{B}T
where epsilon_{c} is the Thouless energy. The observed excess current as well
as the observed sub-harmonic Shapiro steps under microwave irradiation suggest
the occurrence of low-energy coherent Multiple Andreev Reflection (MAR).Comment: 4 pages, 4 figures, final versio
A GENETIC STUDY OF SUSCEPTIBILITY TO EXPERIMENTAL TUBERCULOSIS IN MICE INFECTED WITH MAMMALIAN TUBERCLE BACILLI
A study has been made of the genetic aspects of the difference between two inbred strains of mice (C57B1/6 and Swiss) in response to experimental infection with mammalian tubercle bacilli. Males and females, 4 to 6 weeks of age were inoculated intravenously with 0.2 ml of a 1/50 culture dilution of Mycobacterium tuberculosis var. bovis (Vallée strain) grown in tween albumin medium. Mean survival time for C57B1 animals was 28.1 ± 0.6 days and for Swiss, 55.3 ± 0.6 days postinfection. The characteristic survival time of the two strains was reversed in mice receiving a smaller infective dose. The age of mice at the time of inoculation also affected the results of infection: both C57B1 and Swiss, inoculated at 12 months of age, died at the same rate, but when inoculated at older ages, C57B1 survived slightly longer. Bacteriologic studies demonstrated that there was no significant difference between the two mouse strains with regard to the numbers of viable units of tubercle bacilli recovered from various organs during the 2 week period following infection with a 10–3 culture dilution of Vallée. Moreover, the standard infective inoculum (1/50 culture dilution) did not activate corynebacterial pseudotuberculosis in C57B1 mice, a strain known to be latently infected with Corynebacterium kutscheri, rapid multiplication of tubercle bacilli occurred, but no corynebacteria were recovered. When C57B1 and Swiss strains were crossed, survival tests after infection with the standard inoculum demonstrated that mice of the F1 generation were more resistant than either parent. Whether the overdominance was due to a new combination of parental genes for resistance or to heterosis was not determined. The increased litter size of the F1 mice, an evidence of increased vigor, supports the view that heterosis was involved. In backcrosses to the resistant strain (Swiss), survival time gradually became stabilized at approximately the parental level. In the 1st backcross to the susceptible strain (C57B1), survival times fell into two classes indicating segregation of genes, with perhaps dominance of genes from the Swiss. After repeated backcrosses to C57B1, mice of the 4th backcross generation had a survival time essentially the same as that of the original parental strain. On the basis of having obtained progeny characterized by the original parental susceptibilities after genetic tendencies had been intermingled by crossbreeding, it was concluded that hereditary factors influenced the response of mice to experimental infection with M. tuberculosis. The number of genes was not determined
CORYNEBACTERIAL PSEUDOTUBERCULOSIS IN MICE : II. ACTIVATION OF NATURAL AND EXPERIMENTAL LATENT INFECTIONS
Latent corynebactenai infection occurs naturally in many strains of mice. It can be evoked into the active disease, pseudotuberculosis, by a single injection of 10 mg of cortisone. The cortisone effect was tested in 21 colonies, representing 11 genetically different strains of mice. Animals of the C57B1/6, DBA/2, and RIII strains were shown to be latently infected with Corynebacterium kutscheri by the fact that they developed fatal pseudotuberculosis following cortisone treatment. Virulent C. kutscheri could not be isolated from homogenates of organs obtained from latently infected animals before cortisone administration; however, these homogenates yielded small translucent colonies of avirulent organisms. Recovery of these atypical colonies was facilitated by preincubating the organ homogenates before plating. The organisms constituting such colonies differed morphologically and immunologically from C. kutscheri, but had similar biochemical properties with the exception that they lacked urease and catalase activity. Mice treated with cortisone yielded both the avirulent bacteria and virulent C. kutscheri. The latter was the predominant organism present in the organs at the height of infection. Injection of avirulent organisms into Swiss Lynch mice, which are normally free of latent corynebacteria, occasionally established a latent infection which could be converted into corynebacterial pseudotuberculosis by cortisone. Cultures of fully virulent C. kutscheri were then obtained from the lesions. Latency was produced experimentally with a streptomycin-resistant strain of virulent C. kutscheri (CKsr) derived from the stock culture. When sublethal doses of CKsr were injected into NCS mice (Institut Pasteur colony), they induced a latent infection characterized by the presence of avirulent organisms possessing the streptomycin resistance marker. These were isolated in the form of small translucent colonies from the livers of the infected animals. Administration of cortisone to these animals subsequently evoked active infection from which virulent CKsr could be obtained. Injection of the avirulent streptomycin-resistant organisms into normal NCS mice established a latent infection which could be uniformly converted into corynebacterial pseudotuberculosis by cortisone. The virulent C. kutscheri obtained from the lesions bore the genetic marker of streptomycin resistance, thus being identical with CKsr. Except for streptomycin resistance, the avirulent organisms isolated from the experimentally induced latent infections were identical with those found in the naturally occurring latent infections. These results suggest that C. kutscheri can persist in vitro in an avirulent form which is resistant to the defense mechanisms of the host, and can thus establish a latent infection. Treatment of the animal with cortisone results in the conversion of the avirulent form into virulent C. kutscheri, and of the latent infection into active corynebacterial pseudotuberculosis. The findings are discussed with regard to their relevance to infection immunity, and to the conversion of latent infection into overt disease
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