15,672 research outputs found
Tiltrotor CFD part II: aerodynamic optimisation of tiltrotor blades
This paper presents aerodynamic optimisation of tiltrotor blades with high-fidelity computational
fluid dynamics. The employed optimisation framework is based on a quasi-Newton
method, and the required high-fidelity flow gradients were computed using a discrete adjoint
solver. Single-point optimisations were first performed, to highlight the contrasting requirements
of the helicopter and aeroplane flight regimes. It is then shown how a trade-off blade
design can be obtained using a multi-point optimisation strategy. The parametrisation of the
blade shape allowed to modify the twist and chord distributions, and to introduce a swept tip.
The work shows how these main blade shape parameters influence the optimal performance
of the tiltrotor in helicopter and aeroplane modes, and how a compromise blade shape can
increase the overall tiltrotor performance. Moreover, in all the presented cases, the accuracy
of the adjoint gradients resulted in a small number of flow evaluations for finding the optimal
solution, thus indicating gradient-based optimisation as a viable tool for modern tiltrotor
design
Magnetic hour-glass dispersion and its relation to high-temperature superconductivity in iron-tuned FeTeSe
High-temperature superconductivity remains arguably the largest outstanding
enigma of condensed matter physics. The discovery of iron-based
high-temperature superconductors has renewed the importance of understanding
superconductivity in materials susceptible to magnetic order and fluctuations.
Intriguingly they show magnetic fluctuations reminiscent of the superconducting
(SC) cuprates, including a 'resonance' and an 'hour-glass' shaped dispersion,
which provide an opportunity to new insight to the coupling between spin
fluctuations and superconductivity. Here we report inelastic neutron scattering
data on FeTeSe using excess iron concentration to tune
between a SC () and a non-SC () ground states. We find
incommensurate spectra in both samples but discover that in the one that
becomes SC, a constriction towards a commensurate hourglass shape develop well
above . Conversely a spin-gap and concomitant spectral weight shift happen
below . Our results imply that the hourglass shaped dispersion is most
likely a pre-requisite for superconductivity, whereas the spin-gap and shift of
spectral weight are consequences of superconductivity. We explain this
observation by pointing out that an inwards dispersion towards the commensurate
wave-vector is needed for the opening of a spin gap to lower the magnetic
exchange energy and hence provide the necessary condensation energy for the SC
state to emerge
Angular momentum distribution of hot gas and implications for disk galaxy formation
We study the angular momentum profiles both for dark matter and for gas
within virialized halos, using a statistical sample of halos drawn from
cosmological hydrodynamics simulations. Three simulations have been analyzed,
one is the ``non-radiative'' simulation, and the other two have radiative
cooling. We find that the gas component on average has a larger spin and
contains a smaller fraction of mass with negative angular momentum than its
dark matter counterpart in the non-radiative model. As to the cooling models,
the gas component shares approximately the same spin parameter as its dark
matter counterpart, but the hot gas has a higher spin and is more aligned in
angular momentum than dark matter, while the opposite holds for the cold gas.
After the mass of negative angular momentum is excluded, the angular momentum
profile of the hot gas component approximately follows the universal function
originally proposed by Bullock et al. for dark matter, though the shape
parameter is much larger for hot gas and is comfortably in the range
required by observations of disk galaxies. Since disk formation is related to
the distribution of hot gas that will cool, our study may explain the fact that
the disk component of observed galaxies contains a smaller fraction of low
angular momentum material than dark matter in halos.Comment: 30 pages, 12 figures, 4 tables, accepted for publication in Ap
Galaxy Evolution, Deep Galaxy Counts and the Near-IR Cosmic Infrared Background
Accurate synthetic models of stellar populations are constructed and used in
evolutionary models of stellar populations in forming galaxies. Following their
formation, the late type galaxies are assumed to follow the Schmidt law for
star formation, while early type galaxies are normalized to the present-day
fundamental plane relations assumed to mimic the metallicity variations along
their luminosity sequence. We then compute predictions of these models for the
observational data at early epochs for various cosmological parameters and . We find good match to the metallicity data from the
damped systems and the evolution of the luminosity density out to
. Likewise, our models provide good fits for low values of
to the deep number counts of galaxies in all bands where data is available;
this is done without assuming existence of extra populations of galaxies at
high . Our models also match the data on the redshift distribution of galaxy
counts in and bands. We compute the predicted mean levels and angular
distribution of the cosmic infrared background produced from the early
evolution of galaxies. The predicted fluxes and fluctuations are still below
the current observational limits, but not by a large factor. Finally, we find
that the recent detection of the diffuse extragalactic light in the visible
bands requires for our models high redshift of galaxy formation, (3-4); otherwise the produced flux of the extragalactic light at optical
bands exceeds the current observational limits.Comment: Accepted to Ap
Loss of star forming gas in SDSS galaxies
Using the star formation rates from the SDSS galaxy sample, extracted using
the MOPED algorithm, and the empirical Kennicutt law relating star formation
rate to gas density, we calculate the time evolution of the gas fraction as a
function of the present stellar mass. We show how the gas-to-stars ratio varies
with stellar mass, finding good agreement with previous results for smaller
samples at the present epoch. For the first time we show clear evidence for
progressive gas loss with cosmic epoch, especially in low-mass systems. We find
that galaxies with small stellar masses have lost almost all of their cold
baryons over time, whereas the most massive galaxies have lost little. Our
results also show that the most massive galaxies have evolved faster and turned
most of their gas into stars at an early time, thus strongly supporting a
downsizing scenario for galaxy evolution.Comment: 29 pages, 9 figures, ApJ, accepte
On the detectability of gravitational waves background produced by gamma ray bursts
In this paper we discuss a new strategy for the detection of gravitational
radiation likely emitted by cosmological gamma ray burst. Robust and
conservative estimates lead to the conclusion that the uncorrelated
superimposition of bursts of gravitational waves can be detected by
interferometric detectors like VIRGO or LIGO. The expected signal is predicted
to carry two very distinctive signatures: the cosmological dipole anisotropy
and a characteristic time scale in the auto correlation spectrum, which might
be exploited, perhaps with ad hoc modifications and/or upgrading of the planned
experiments, to confirm the non-instrumental origin of the signal.Comment: 9 pages, 2 figures, LATEX2e, Accepted for pubblications as a Letter
to the Editor in Journal of Physics G: Nuclear and Particle Physic
Second-Generation Objects in the Universe: Radiative Cooling and Collapse of Halos with Virial Temperatures Above 10^4 Kelvin
The first generation of protogalaxies likely formed out of primordial gas via
H2-cooling in cosmological minihalos with virial temperatures of a few 1000K.
However, their abundance is likely to have been severely limited by feedback
processes which suppressed H2 formation. The formation of the protogalaxies
responsible for reionization and metal-enrichment of the intergalactic medium,
then had to await the collapse of larger halos. Here we investigate the
radiative cooling and collapse of gas in halos with virial temperatures Tvir >
10^4K. In these halos, efficient atomic line radiation allows rapid cooling of
the gas to 8000 K; subsequently the gas can contract nearly isothermally at
this temperature. Without an additional coolant, the gas would likely settle
into a locally gravitationally stable disk; only disks with unusually low spin
would be unstable. However, we find that the initial atomic line cooling leaves
a large, out-of-equilibrium residual free electron fraction. This allows the
molecular fraction to build up to a universal value of about x(H2) = 10^-3,
almost independently of initial density and temperature. We show that this is a
non--equilibrium freezeout value that can be understood in terms of timescale
arguments. Furthermore, unlike in less massive halos, H2 formation is largely
impervious to feedback from external UV fields, due to the high initial
densities achieved by atomic cooling. The H2 molecules cool the gas further to
about 100K, and allow the gas to fragment on scales of a few 100 Msun. We
investigate the importance of various feedback effects such as
H2-photodissociation from internal UV fields and radiation pressure due to
Ly-alpha photon trapping, which are likely to regulate the efficiency of star
formation.Comment: Revised version accepted by ApJ; some reorganization for clarit
Ab initio evaluation of the charge-ordering in
We report {\it ab initio} calculations of the charge ordering in
using large configurations interaction methods on
embedded fragments. Our major result is that the electrons of the
bridging oxygen of the rungs present a very strong magnetic character and
should thus be explicitly considered in any relevant effective model. The most
striking consequence of this result is that the spin and charge ordering differ
substantially, as differ the experimental results depending on whether they are
sensitive to the spin or charge density.Comment: 4 page
Identification of an Extended Accretion Disk Corona in the Hercules X-1 Low State: Moderate Optical Depth, Precise Density Determination, and Verification of CNO Abundances
We identify an accretion disk atmosphere and corona from the high resolution
X-ray spectrum of Hercules X-1, and we determine its detailed physical
properties. More than two dozen recombination emission lines (from Fe XXVI at
1.78 A to N VI at 29.08 A) and Fe K-alpha, K-beta fluorescence lines were
detected in a 50 ks observation with the Chandra High-Energy Transmission
Grating Spectrometer (HETGS). They allow us to measure the density,
temperature, spatial distribution, elemental composition, and kinematics of the
plasma. We exclude HZ Her as the source of the recombination emission. We
compare accretion disk model atmospheres with the observed spectrum in order to
constrain the stratification of density and ionization, disk atmosphere area,
elemental composition, and energetics. The atmospheric spectrum observed during
the low state is photoionized by the main-on X-ray continuum, indicating that
the disk is observed edge-on during the low state. We infer the mean number of
scatterings N of Ly-alpha and Ly-beta line photons from H-like ions. We derive
N < 69 for O VIII Ly_alpha_1, which rules out the presence of a mechanism
modeled by Sako (2003) to enhance N VII emission via a line overlap with O
VIII. The line optical depth diagnostics are consistent with a flattened
atmosphere. Our spectral analysis, the disk atmosphere model, and the presence
of intense N VII and N VI lines (plus N V in the UV), confirm the
over-abundance of nitrogen relative to other metals, which was shown to be
indicative of CNO cycle processing in a massive progenitor.Comment: 38 pages, 14 figures, accepted for publication in Ap
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