7,309 research outputs found
Coupling Between Thermal Oscillations in the Surface of a Micro-Cylinder and Vortex Shedding
his article studies the coupling between prescribed thermal oscillations in the surface of a micro-cylinder and vortex shedding. We deal with the unsteady, laminar, compressible flow regime where the aerodynamics forces have a periodic behavior. It is shown that appropriate spatial and time-dependent temperature oscillations on the surface of the micro-cylinder create a resonance that controls the amplitude and frequency of both lift and drag coefficients. In practice, what we study is a mechanism to modulate the amplitude and frequency of mechanical loads of aerodynamics origin in a micro-structure by using surface temperature fluctuations as the control parameter
Spatial-distribution of recombination centers in gaaste - effects of the doping level
The distribution in liquid-encapsulated-Czochralski (LEC) GaAs:Te wafers of point and complex defects has been investigated together with their influence on the minority-carrier diffusion length L. Three wafers with different Te-doping concentration (2.2 X 10(17), 4.5 X 10(17), and 1.5 X 10(18) cm-3) have been studied by means of the electron-beam-induced-current (EBIC) mode of scanning electron microscopy and of the surface photovoltage (SPV) method. The morphology and electrical activity of the defects observed across each wafer have been correlated to the formation and distribution of deep electronic levels, which are significantly affected by the tellurium concentration. The diffusion length has been found to be mainly controlled by deep levels associated with dislocations. EBIC localized measurements of L and of the net ionized free-carrier concentration provide evidence for the influence of Te concentration on impurity segregation at complex defects
Metallicity inhomogeneities in local star-forming galaxies as sign of recent metal-poor gas accretion
We measure the oxygen metallicity of the ionized gas along the major axis of
seven dwarf star-forming galaxies. Two of them, SDSSJ1647+21 and SDSSJ2238+14,
show 0.5 dex metallicity decrements in inner regions with enhanced
star-formation activity. This behavior is similar to the metallicity drop
observed in a number of local tadpole galaxies by Sanchez Almeida et al. (2013)
and interpreted as showing early stages of assembling in disk galaxies, with
the star formation sustained by external metal-poor gas accretion. The
agreement with tadpoles has several implications: (1) it proves that galaxies
other than the local tadpoles present the same unusual metallicity pattern. (2)
Our metallicity inhomogeneities were inferred using the direct method, thus
discarding systematic errors usually attributed to other methods. (3) Taken
together with the tadpole data, our findings suggest a threshold around one
tenth the solar value for the metallicity drops to show up. Although galaxies
with clear metallicity drops are rare, the physical mechanism responsible for
them may sustain a significant part of the star-formation activity in the local
Universe. We argue that the star-formation dependence of the mass-metallicity
relationship, as well as other general properties followed by most local disk
galaxies, are naturally interpreted as side effects of pristine gas infall.
Alternatives to the metal poor gas accretion are examined too.Comment: Accepted for publication in ApJ. 10 pages. 5 Fig
Galactic Kinematics Towards the South Galactic Pole. First Results from the Yale-San Juan Southern Proper-Motion Program
The predictions from a Galactic Structure and Kinematic model code are
compared to the color counts and absolute proper-motions derived from the
Southern Proper-Motion survey covering more than 700 toward the South
Galactic Pole in the range . The theoretical assumptions
and associated computational procedures, the geometry for the kinematic model,
and the adopted parameters are presented in detail and compared to other
Galactic Kinematic models of its kind. The data to which the model is compared
consists of more than 30,000 randomly selected stars, and it is best fit by
models with a solar peculiar motion of +5 km s in the V-component
(pointing in the direction of Galactic rotation), a large LSR speed of 270 km
s, and a (disk) velocity ellipsoid that always points towards the
Galactic center. The absolute proper-motions in the U-component indicate a
solar peculiar motion of km s, with no need for a local
expansion or contraction term. The fainter absolute motions show an indication
that the thick-disk must exhibit a rather steep velocity gradient of about -36
km s kpc with respect to the LSR. We are not able to set
constraints on the overall rotation for the halo, nor on the thick-disk or halo
velocity dispersions. Some substructure in the U & V proper-motions could be
present in the brighter bins , and it might be indicative
of (disk) moving groups.Comment: 24 double-column pages, 12 tables, AAS Latex macros v4.0, 19 B&W
figures, 1 color figure. Accepted for publication on The Astronomical Journa
Security Policies to Mitigate Attacks VLAN Hopping in the Data Link Layer of LA Networks
A proposal of security policies based on the ISO 27002 standard is presented, which allows to mitigate VLAN HOPPING attacks at the data link layer level in LAN networks, as it is evident that network administrators pay more attention to policies to ensure the layers of the OSI model, so that internal users with certain privileges can take advantage of these vulnerabilities to access valuable information of the organization. For this purpose, a base network infrastructure of the companies in the city of Riobamba-Ecuador was determined as a case study. In this scenario, a standard four-phase Pentesting was performed to test VLAN HOPPING attacks (Switch Spoofing and Double Tagging) before and after applying the proposed policies, resulting in a 100% mitigation of the technological vulnerabilities found and 90% of organizational, operational, and physical vulnerabilities.
Keywords: VLAN HOPPING, Security Policies, Vulnerability Mitigation, Security Mechanism
The open cluster NGC 6520 and the nearby dark molecular cloud Barnard 86
Wide field BVI photometry and CO(10) observations are presen ted
in the region of the open cluster NGC 6520 and the dark molecular cloud
Barnard~86. From the analysis of the optical data we find that the cluster is
rather compact, with a radius of 1.00.5 arcmin, smaller than previous
estimates. The cluster age is 15050 Myr and the reddening
E=0.420.10. The distance from the Sun is estimated to be
1900100 pc, and it is larger than previous estimates. We finally derive
basic properties of the dark nebula Barnard 86 on the assumption that it lies
at the same distance of the cluster.Comment: 21 pages, 8 eps figures (a few degraded in resolution), accepted for
publication in the Astronomical Journa
Suppression of hole-hole scattering in GaAs/AlGaAs heterostructures under uniaxial compression
Resistance, magnetoresistance and their temperature dependencies have been
investigated in the 2D hole gas at a [001] p-GaAs/AlGaAs
heterointerface under [110] uniaxial compression. Analysis performed in the
frame of hole-hole scattering between carriers in the two spin splitted
subbands of the ground heavy hole state indicates, that h-h scattering is
strongly suppressed by uniaxial compression. The decay time of the
relative momentum reveals 4.5 times increase at a uniaxial compression of 1.3
kbar.Comment: 5 pages, 3 figures. submitted to Phys.Rev.
Electric-Field Tuning of Spin-Dependent Exciton-Exciton Interactions in Coupled Quantum Wells
We have shown experimentally that an electric field decreases the energy
separation between the two components of a dense spin-polarized exciton gas in
a coupled double quantum well, from a maximum splitting of meV to
zero, at a field of 35 kV/cm. This decrease, due to the field-induced
deformation of the exciton wavefunction, is explained by an existing
calculation of the change in the spin-dependent exciton-exciton interaction
with the electron-hole separation. However, a new theory that considers the
modification of screening with that separation is needed to account for the
observed dependence on excitation power of the individual energies of the two
exciton components.Comment: 5 pages, 4 eps figures, RevTeX, Physical Review Letters (in press
Competing magnetic states, disorder, and the magnetic character of Fe3Ga4
The physical properties of metamagnetic FeGa single crystals are
investigated to explore the sensitivity of the magnetic states to temperature,
magnetic field, and sample history. The data reveal a moderate anisotropy in
the magnetization and the metamagnetic critical field along with features in
the specific heat at the magnetic transitions K and K. Both
and are found to be sensitive to the annealing conditions of the
crystals suggesting that disorder affects the competition between the
ferromagnetic (FM) and antiferromagnetic (AFM) states. Resistivity measurements
reveal metallic transport with a sharp anomaly associated with the transition
at . The Hall effect is dominated by the anomalous contribution which
rivals that of magnetic semiconductors in magnitude ( cm at 2 T
and 350 K) and undergoes a change of sign upon cooling into the low temperature
FM state. The temperature and field dependence of the Hall effect indicate that
the magnetism is likely to be highly itinerant in character and that a
significant change in the electronic structure accompanies the magnetic
transitions. We observe a contribution from the topological Hall effect in the
AFM phase suggesting a non-coplanar contribution to the magnetism. Electronic
structure calculations predict an AFM ground state with a wavevector parallel
to the crystallographic -axis preferred over the experimentally measured FM
state by 50 meV per unit cell. However, supercell calculations with a
small density of Fe-antisite defects introduced tend to stabilize the FM over
the AFM state indicating that antisite defects may be the cause of the
sensitivity to sample synthesis conditions.Comment: 13 pages, 14 figures, and 4 supplementary table
NPARSEC : NTT Parallaxes of Southern Extremely Cool objects. Goals, targets, procedures and first results
The discovery and subsequent detailed study of T dwarfs have provided many surprises and pushed the physics and modelling of cool atmospheres in unpredicted directions. Distance is a critical parameter for studies of these objects to determine intrinsic luminosities, test binarity and measure their motion in the Galaxy. We describe a new observational programme to determine distances across the full range of T-dwarf subtypes using the New Technology Telescope (NTT)/SOFI telescope/instrument combination. We present preliminary results for ten objects, five of which represent new distances.Peer reviewe
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