8,922 research outputs found
CRASH3: cosmological radiative transfer through metals
Here we introduce CRASH3, the latest release of the 3D radiative transfer
code CRASH. In its current implementation CRASH3 integrates into the reference
algorithm the code Cloudy to evaluate the ionisation states of metals,
self-consistently with the radiative transfer through H and He. The feedback of
the heavy elements on the calculation of the gas temperature is also taken into
account, making of CRASH3 the first 3D code for cosmological applications which
treats self-consistently the radiative transfer through an inhomogeneous
distribution of metal enriched gas with an arbitrary number of point sources
and/or a background radiation. The code has been tested in idealized
configurations, as well as in a more realistic case of multiple sources
embedded in a polluted cosmic web. Through these validation tests the new
method has been proven to be numerically stable and convergent. We have studied
the dependence of the results on a number of physical quantities such as the
source characteristics (spectral range and shape, intensity), the metal
composition, the gas number density and metallicity.Comment: accepted for publication in MNRA
UV background fluctuations traced by metal ions at
Here we investigate how LyC-opaque systems present in the intergalactic
medium at can distort the spectral shape of a uniform UV background
(UVB) through radiative transfer (RT) effects. With this aim in mind, we
perform a multi-frequency RT simulation through a cosmic volume of
~cMpc scale polluted by metals, and self-consistently derive the ions
of all the species. The UVB spatial fluctuations are traced by the ratio of
He and H column density,
, and the ratio of C and Si optical depths, . We find that: (i)
spatially fluctuates through over-dense systems () with statistically
significant deviations \% in 18\% of the volume ; (ii) same
fluctuations in are also present in \% of the enriched domain (only
8\% of the total volume) and derive from a combination of RT induced effects
and in-homogeneous metal enrichment, both effective in systems with .Comment: Accepted for pub. in MNRAS after very minor re
Evidence for Two Distinct Morphological Classes of Gamma-Ray Bursts from their Short Timescale Variability
We have analyzed the 241 bursts for which peak counts \C exist in the
publicly available Burst and Transient Source Experiment (BATSE) catalog.
Introducing peak counts in 1024 ms as a measure of burst brightness \B and
the ratio of peak counts in 64 and 1024 ms as a measure of short timescale
variability \V, we find a statistically significant correlation between the
brightness and the short timescale variability of \g-ray bursts. The bursts
which are smoother on short timescales are both faint and bright, while the
bursts which are variable on short timescales are faint only, suggesting the
existence of two distinct morphological classes of bursts.Comment: 9 pages + 2 Postscript figures available upon request; LATEX v. 2.0
JT90 thermal barrier coated vanes
The technology of plasma sprayed thermal barrier coatings applied to turbine vane platforms in modern high temperature commercial engines was advanced to the point of demonstrated feasibility for application to commercial aircraft engines. The three thermal barrier coatings refined under this program are zirconia stabilized with twenty-one percent magnesia (21% MSZ), six percent yttria (6% YSZ), and twenty percent yttria (20% YSZ). Improvement in thermal cyclic endurance by a factor of 40 times was demonstrated in rig tests. A cooling system evolved during the program which featured air impingement cooling for the vane platforms rather than film cooling. The impingement cooling system, in combination with the thermal barrier coatings, reduced platform cooling air requirements by 44% relative to the current film cooling system. Improved durability and reduced cooling air requirements were demonstrated in rig and engine endurance tests. Two engine tests were conducted, one of 1000 cycles and the other of 1500 cycles. All three coatings applied to vanes fabricated with the final cooling system configuration completed the final 1500 cycle engine endurance test. Results of this test clearly demonstrated the durability of the 6% YSZ coating which was in very good condition after the test. The 21% MSZ and 20% YSZ coatings had numerous occurrences of significant spalling in the test
Galaxy formation with radiative and chemical feedback
Here we introduce GAMESH, a novel pipeline which implements self-consistent
radiative and chemical feedback in a computational model of galaxy formation.
By combining the cosmological chemical-evolution model GAMETE with the
radiative transfer code CRASH, GAMESH can post process realistic outputs of a
N-body simulation describing the redshift evolution of the forming galaxy.
After introducing the GAMESH implementation and its features, we apply the code
to a low-resolution N-body simulation of the Milky Way formation and we
investigate the combined effects of self-consistent radiative and chemical
feedback. Many physical properties, which can be directly compared with
observations in the Galaxy and its surrounding satellites, are predicted by the
code along the merger-tree assembly. The resulting redshift evolution of the
Local Group star formation rates, reionisation and metal enrichment along with
the predicted Metallicity Distribution Function of halo stars are critically
compared with observations. We discuss the merits and limitations of the first
release of GAMESH, also opening new directions to a full implementation of
feedback processes in galaxy formation models by combining semi-analytic and
numerical methods.Comment: This version has coloured figures not present in the printed version.
Submitted to MNRAS, minor revision
Proton imaging of stochastic magnetic fields
Recent laser-plasma experiments report the existence of dynamically
significant magnetic fields, whose statistical characterisation is essential
for understanding the physical processes these experiments are attempting to
investigate. In this paper, we show how a proton imaging diagnostic can be used
to determine a range of relevant magnetic field statistics, including the
magnetic-energy spectrum. To achieve this goal, we explore the properties of an
analytic relation between a stochastic magnetic field and the image-flux
distribution created upon imaging that field. We conclude that features of the
beam's final image-flux distribution often display a universal character
determined by a single, field-scale dependent parameter - the contrast
parameter - which quantifies the relative size of the correlation length of the
stochastic field, proton displacements due to magnetic deflections, and the
image magnification. For stochastic magnetic fields, we establish the existence
of four contrast regimes - linear, nonlinear injective, caustic and diffusive -
under which proton-flux images relate to their parent fields in a qualitatively
distinct manner. As a consequence, it is demonstrated that in the linear or
nonlinear injective regimes, the path-integrated magnetic field experienced by
the beam can be extracted uniquely, as can the magnetic-energy spectrum under a
further statistical assumption of isotropy. This is no longer the case in the
caustic or diffusive regimes. We also discuss complications to the
contrast-regime characterisation arising for inhomogeneous, multi-scale
stochastic fields, as well as limitations currently placed by experimental
capabilities on extracting magnetic field statistics. The results presented in
this paper provide a comprehensive description of proton images of stochastic
magnetic fields, with applications for improved analysis of given proton-flux
images.Comment: Main paper pp. 1-29; appendices pp. 30-84. 24 figures, 2 table
STOCHASTIC DYNAMICS OF LARGE-SCALE INFLATION IN DE~SITTER SPACE
In this paper we derive exact quantum Langevin equations for stochastic
dynamics of large-scale inflation in de~Sitter space. These quantum Langevin
equations are the equivalent of the Wigner equation and are described by a
system of stochastic differential equations. We present a formula for the
calculation of the expectation value of a quantum operator whose Weyl symbol is
a function of the large-scale inflation scalar field and its time derivative.
The unique solution is obtained for the Cauchy problem for the Wigner equation
for large-scale inflation. The stationary solution for the Wigner equation is
found for an arbitrary potential. It is shown that the large-scale inflation
scalar field in de Sitter space behaves as a quantum one-dimensional
dissipative system, which supports the earlier results. But the analogy with a
one-dimensional model of the quantum linearly damped anharmonic oscillator is
not complete: the difference arises from the new time dependent commutation
relation for the large-scale field and its time derivative. It is found that,
for the large-scale inflation scalar field the large time asymptotics is equal
to the `classical limit'. For the large time limit the quantum Langevin
equations are just the classical stochastic Langevin equations (only the
stationary state is defined by the quantum field theory).Comment: 21 pages RevTex preprint styl
Heat transfer in rotating serpentine passages with trips normal to the flow
Experiments were conducted to determine the effects of buoyancy and Coriolis forces on heat transfer in turbine blade internal coolant passages. The experiments were conducted with a large scale, multipass, heat transfer model with both radially inward and outward flow. Trip strips on the leading and trailing surfaces of the radial coolant passages were used to produce the rough walls. An analysis of the governing flow equations showed that four parameters influence the heat transfer in rotating passages: coolant-to-wall temperature ratio, Rossby number, Reynolds number, and radius-to-passage hydraulic diameter ratio. The first three of these four parameters were varied over ranges which are typical of advanced gas turbine engine operating conditions. Results were correlated and compared to previous results from stationary and rotating similar models with trip strips. The heat transfer coefficients on surfaces, where the heat increased with rotation and buoyancy, varied by as much as a factor of four. Maximum values of the heat transfer coefficients with high rotation were only slightly above the highest levels obtained with the smooth wall model. The heat transfer coefficients on surfaces, where the heat transfer decreased with rotation, varied by as much as a factor of three due to rotation and buoyancy. It was concluded that both Coriolis and buoyancy effects must be considered in turbine blade cooling designs with trip strips and that the effects of rotation were markedly different depending upon the flow direction
Tsunami: alla scoperta dei maremoti
Tsunami è una parola giapponese che significa “onda (nami) nel porto (tsu)”, in quanto anticamente si osservava che alcune onde diventavano devastanti avvicinandosi alla costa. Il termine italiano usato per definire uno tsunami è maremoto, cioè una serie di onde che, superando l'abituale linea costiera, provocano danni all'interno dei porti, ma anche lungo tutta la costa e a volte nell’entroterra
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