4,609 research outputs found
Direct numerical simulation of supersonic pipe flow at moderate Reynolds number
We study compressible turbulent flow in a circular pipe, at computationally
high Reynolds number. Classical related issues are addressed and discussed in
light of the DNS data, including validity of compressibility transformations,
velocity/temperature relations, passive scalar statistics, and size of
turbulent eddies.Regarding velocity statistics, we find that Huang's
transformation yields excellent universality of the scaled Reynolds stresses
distributions, whereas the transformation proposed by Trettel and Larsson
(2016) yields better representation of the effects of strong variation of
density and viscosity occurring in the buffer layer on the mean velocity
distribution. A clear logarithmic layer is recovered in terms of transformed
velocity and wall distance coordinates at the higher Reynolds number under
scrutiny (\Rey_{\tau} \approx 1000), whereas the core part of the flow is
found to be characterized by a universal parabolic velocity profile. Based on
formal similarity between the streamwise velocity and the passive scalar
transport equations, we further propose an extension of the above
compressibility transformations to also achieve universality of passive scalar
statistics. Analysis of the velocity/temperature relationship provides evidence
for quadratic dependence which is very well approximated by the thermal analogy
proposed by Zhang et Al.(2014). The azimuthal velocity and scalar spectra show
an organization very similar to canonical incompressible flow, with a
bump-shaped distribution across the flow scales, whose peak increases with the
wall distance. We find that the size growth effect is well accounted for
through an effective length scale accounting for the local friction velocity
and for the local mean shear
Target-adaptive CNN-based pansharpening
We recently proposed a convolutional neural network (CNN) for remote sensing
image pansharpening obtaining a significant performance gain over the state of
the art. In this paper, we explore a number of architectural and training
variations to this baseline, achieving further performance gains with a
lightweight network which trains very fast. Leveraging on this latter property,
we propose a target-adaptive usage modality which ensures a very good
performance also in the presence of a mismatch w.r.t. the training set, and
even across different sensors. The proposed method, published online as an
off-the-shelf software tool, allows users to perform fast and high-quality
CNN-based pansharpening of their own target images on general-purpose hardware
Iron line afterglows: how to produce them
We discuss how a powerful iron line emission can be produced if ~1-5 iron
rich solar masses are concentrated in the close vicinity of the burst.
Recombination, thermal and fluorescent reflection are discussed. We find that
recombination suffers the high Compton temperature of the plasma while the
other two scenarios are not mutually exclusive and could account for the
claimed iron line detected in two afterglows.Comment: 2 pages, A&AS in press, proceedings of the Workshop "Gamma Ray Bursts
in the Afterglow Era" held in Rome, November 199
Iron line in the afterglow: a key to unveil Gamma-Ray Burst progenitors
The discovery of a powerful and transient iron line feature in the X-ray
afterglow spectra of gamma-ray bursts would be a major breakthrough for
understanding the nature of their progenitors. Piro et al. (1999) and Yoshida
et al. (1999) report such a detection in the afterglow of GRB 970508 and GRB
970828, respectively. We discuss how such a strong line could be produced in
the various scenarios proposed for the event progenitor. We show that the
observed line intensity requires a large iron mass, concentrated in the
vicinity of the burst. The previous explosion of a supernova, predicted in the
Supranova scenario, is the most straightforward way to account for such a large
amount of matter. We discuss three different physical processes that could
account for the line: recombination, reflection and thermal emission. Among
these, reflection and thermal emission may explain the observed line features:
reflection should be important if the remnant is optically thick, while thermal
lines can be produced only in a thin plasma. The recombination process requires
extremely high densities to efficiently reprocess the burst photons, whereas
this process could work during the X-ray afterglow. Future key observations for
discriminating the actual radiating process are discussed.Comment: 5 pages, 1 figure, MNRAS letters in pres
High-Reynolds-number effects on turbulent scalings in compressible channel flow
The effect of the Reynolds number in a supersonic isothermal channel flow is studied using a direct numerical simulation (DNS). The bulk Mach number based on the wall temperature is 1.5, and the bulk Reynolds number is increased up to Reτ ≈ 1000. The use of van Driest velocity transformation in the presence of heated walls has been questioned due to the poor accuracy at low Reynolds number. For this reason alternative transformations of the velocity profile and turbulence statistics have been proposed, as, for instance, semi-local scalings. We show that the van Driest transformation recovers its accuracy as the Reynolds number is increased. The Reynolds stresses collapse on the incompressible ones, when properly scaled with density, and very good agreement with the incompressible stresses is found in the outer layer
Iron line afterglows: general constraints
The discovery of a powerful and transient iron line feature in the X-ray
afterglow spectra of gamma-ray bursts would be a major breakthrough for
understanding the nature of their progenitors, strongly suggesting the presence
of a large, iron rich, mass in the vicinity of the burst event.
Model-independent limits to the size and the mass of the the iron line emitting
region are derived and discussed. We also discuss how these results can be used
to constrain the amount of beaming or anisotropy of the burst emission.Comment: 2 pages, A&AS in press, proceedings of the Workshop "Gamma Ray Bursts
in the Afterglow Era" held in Rome, November 199
Reaction spreading on percolating clusters
Reaction-diffusion processes in two-dimensional percolating structures are
investigated. Two different problems are addressed: reaction spreading on a
percolating cluster and front propagation through a percolating channel. For
reaction spreading, numerical data and analytical estimates show a power-law
behavior of the reaction product as M(t) \sim t^dl, where dl is the
connectivity dimension. In a percolating channel, a statistically stationary
traveling wave develops. The speed and the width of the traveling wave are
numerically computed. While the front speed is a low-fluctuating quantity and
its behavior can be understood using a simple theoretical argument, the front
width is a high-fluctuating quantity showing a power-law behavior as a function
of the size of the channelComment: 7 pages, 8 figure
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