4,957 research outputs found
Dependence of the drag over super hydrophobic and liquid infused surfaces on the textured surface and Weber number
Direct Numerical Simulations of a turbulent channel flow have been performed. The lower wall of the channel is made of staggered cubes with a second fluid locked in the cavities. Two viscosity ratios have been considered, m=ÎŒ1/ÎŒ2=0.02 and 0.4 (the subscript 1 indicates the fluid in the cavities and 2 the overlying fluid) mimicking the viscosity ratio in superâhydrophobic surfaces (SHS) and liquid infused surfaces (LIS) respectively. A first set of simulations with a slippery interface has been performed and results agree well with those in literature for perfect slip conditions and Stokes approximations. To assess how the dynamics of the interface affects the drag, a second set of DNS has been carried out at We=40 and 400 corresponding to We+â10â3 and 10â2. The deformation of the interface is fully coupled to the Navier-Stokes equation and tracked in time using a Level Set Method. Two gas fractions, GF=0.5 and 0.875, have been considered to assess how the spacing between the cubes affects the deformation of the interface and therefore the drag. For the dimensions of the substrate here considered, under the ideal assumption of flat interface, staggered cubes with GF=0.875 provide about 20% drag reduction for We=0. However, a rapid degradation of the performances is observed when the dynamics of the interface is considered, and the same geometry increases the drag of about 40% with respect to a smooth wall. On the other hand, the detrimental effect of the dynamics of the interface is much weaker for GF=0.5 because of the reduced pitch between the cubes
Heat transfer and wall temperature effects in shock wave turbulent boundary layer interactions
Direct numerical simulations are carried out to investigate the effect of the
wall temperature on the behavior of oblique shock-wave/turbulent boundary layer
interactions at freestream Mach number and shock angle of the wedge
generator . Five values of the
wall-to-recovery-temperature ratio () are considered, corresponding to
cold, adiabatic and hot wall thermal conditions. We show that the main effect
of cooling is to decrease the characteristic scales of the interaction in terms
of upstream influence and extent of the separation bubble. The opposite
behavior is observed in the case of heating, that produces a marked dilatation
of the interaction region. The distribution of the Stanton number shows that a
strong amplification of the heat transfer occurs across the interaction, and
the maximum values of thermal and dynamic loads are found in the case of cold
wall. The analysis reveals that the fluctuating heat flux exhibits a strong
intermittent behavior, characterized by scattered spots with extremely high
values compared to the mean. Furthermore, the analogy between momentum and heat
transfer, typical of compressible, wall-bounded, equilibrium turbulent flows
does not apply for most part of the interaction domain. The pre-multiplied
spectra of the wall heat flux do not show any evidence of the influence of the
low-frequency shock motion, and the primary mechanism for the generation of
peak heating is found to be linked with the turbulence amplification in the
interaction region.Comment: submitted to PRFluid
On the Optical -- X-ray correlation from outburst to quiescence in Low Mass X-ray Binaries: the representative cases of V404 Cyg and Cen X-4
Low mass X-ray binaries (LMXBs) show evidence of a global correlation of
debated origin between X-ray and optical luminosity. We study for the first
time this correlation in two transient LMXBs, the black hole V404 Cyg and the
neutron star Cen X-4, over 6 orders of magnitude in X-ray luminosity, from
outburst to quiescence. After subtracting the contribution from the companion
star, the Cen X-4 data can be described by a single power law correlation of
the form , consistent with disk reprocessing. We
find a similar correlation slope for V404 Cyg in quiescence (0.46) and a
steeper one (0.56) in the outburst hard state of 1989. However, V404 Cyg is
about times optically brighter, at a given keV X-ray
luminosity, compared to Cen X-4. This ratio is a factor of 10 smaller in
quiescence, where the normalization of the V404 Cyg correlation also changes.
We show that once the bolometric X-ray emission is considered and the known
main differences between V404 Cyg and Cen X-4 are taken into account (a larger
compact object mass, accretion disk size, and the presence of a strong jet
contribution in the hard state for the black hole system) the two systems lie
on the same correlation. In V404 Cyg, the jet dominates spectrally at
optical-infrared frequencies during the hard state, but makes a negligible
contribution in quiescence, which may account for the change in its correlation
slope and normalization. These results provide a benchmark to compare with data
from the 2015 outburst of V404 Cyg and, potentially, other transient LMXBs as
well.Comment: Accepted on ApJ, 12 pages, 4 figures, 4 table
Role of Dirac cones in magnetotransport properties of REFeAsO (RE=rare earth) oxypnictides
In this work we study the effect of the rare earth element in iron
oxypnictides of composition REFeAsO (RE=rare earth). On one hand we carry out
Density Functional Theory calculations of the band structure, which evidence
the multiband character of these compounds and the presence of Dirac cones
along the Y-{\Gamma} and Z-R directions of the reciprocal space. On the other
hand, we explore transport behavior by means of resistivity, Hall resistance
and magnetoresistance measurements, which confirm the dominant role of Dirac
cones. By combining our theoretical and experimental approaches, we extract
information on effective masses, scattering rates and Fermi velocities for
different rare earth elements.Comment: 13 pages, 5 figures accepted for publication on European Journal of
Physics
Theoretical correlation between possible evidences of neutrino chiral oscillations and polarization measurements
Reporting about the formalism with the Dirac equation we describe the
dynamics of chiral oscillations for a fermionic particle non-minimally coupling
with an external magnetic field. For massive particles, the chirality and
helicity quantum numbers represent different physical quantities of
representative importance in the study of chiral interactions, in particular,
in the context of neutrino physics. After solving the interacting Hamiltonian
(Dirac) equation for the corresponding {\em fermionic} Dirac-{\em type}
particle (neutrino) and quantifying chiral oscillations in the Dirac wave
packet framework, we avail the possibility of determining realistic neutrino
chirality conversion rates by means of (helicity) polarization measurements. We
notice that it can become feasible for some particular magnetic field
configurations with large values of {\boldmath} orthogonal to the direction
of the propagating particle.Comment: 12 pages, 2 figure
Events leading up to the June 2015 outburst of V404 Cyg
On 2015 June 15 the burst alert telescope (BAT) on board {\em Swift} detected
an X-ray outburst from the black hole transient V404 Cyg. We monitored V404 Cyg
for the last 10 years with the 2-m Faulkes Telescope North in three optical
bands (V, R, and i). We found that, one week prior to this outburst, the
optical flux was 0.1--0.3 mag brighter than the quiescent orbital modulation,
implying an optical precursor to the X-ray outburst. There is also a hint of a
gradual optical decay (years) followed by a rise lasting two months prior to
the outburst. We fortuitously obtained an optical spectrum of V404 Cyg 13 hours
before the BAT trigger. This too was brighter () than
quiescence, and showed spectral lines typical of an accretion disk, with
characteristic absorption features of the donor being much weaker. No He II
emission was detected, which would have been expected had the X-ray flux been
substantially brightening. This, combined with the presence of intense
H emission, about 7 times the quiescent level, suggests that the disk
entered the hot, outburst state before the X-ray outburst began. We propose
that the outburst is produced by a viscous-thermal instability triggered close
to the inner edge of a truncated disk. An X-ray delay of a week is consistent
with the time needed to refill the inner region and hence move the inner edge
of the disk inwards, allowing matter to reach the central BH, finally turning
on the X-ray emission.Comment: Accepted by ApJ Letter, 7 pages, 5 figure
Equilibrium and stability of neutrino lumps as TOV solutions
We report about stability conditions for static, spherically symmetric
objects that share the essential features of mass varying neutrinos in
cosmological scenarios. Compact structures of particles with variable mass are
held together preponderantly by an attractive force mediated by a background
scalar field. Their corresponding conditions for equilibrium and stability are
given in terms of the ratio between the total mass-energy and the spherical
lump radius, . We show that the mass varying mechanism leading to lump
formation can modify the cosmological predictions for the cosmological neutrino
mass limits. Our study comprises Tolman-Oppenheimer-Volkoff solutions of
relativistic objects with non-uniform energy densities. The results leave open
some questions concerning stable regular solutions that, to an external
observer, very closely reproduce the preliminary conditions to form
Schwarzschild black holes.Comment: 20 pages, 5 figure
Effects of macroscopic polarization in III-V nitride multi-quantum-wells
Huge built-in electric fields have been predicted to exist in wurtzite III-V
nitrides thin films and multilayers. Such fields originate from heterointerface
discontinuities of the macroscopic bulk polarization of the nitrides. Here we
discuss the background theory, the role of spontaneous polarization in this
context, and the practical implications of built-in polarization fields in
nitride nanostructures. To support our arguments, we present detailed
self-consistent tight-binding simulations of typical nitride QW structures in
which polarization effects are dominant.Comment: 11 pages, 9 figures, uses revtex/epsf. submitted to PR
Deep Learning-Based Wave Digital Modeling of Rate-Dependent Hysteretic Nonlinearities for Virtual Analog Applications
Electromagnetic components greatly contribute to the peculiar timbre of analog audio gear. Indeed, distortion effects due to the nonlinear behavior of magnetic materials are known to play an important role in enriching the harmonic content of an audio signal. However, despite the abundant research that has been devoted to the characterization of nonlinearities in the context of virtual analog modeling over the years, the discrete-time simulation of circuits exhibiting rate-dependent hysteretic phenomena remains an open challenge. In this article, we present a novel data-driven approach for the wave digital modeling of rate-dependent hysteresis using recurrent neural networks (RNNs). Thanks to the modularity of wave digital filters, we are able to locally characterize the wave scattering relations of a hysteretic reluctance by encapsulating an RNN-based model into a single one-port wave digital block. Hence, we successfully apply the proposed methodology to the emulation of the output stage of a vacuum-tube guitar amplifier featuring a nonlinear transformer
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