294 research outputs found
Acoustic Emission Monitoring of the Syracuse Athena Temple: Scale Invariance in the Timing of Ruptures
We perform a comparative statistical analysis between the acoustic-emission time series from the ancient Greek Athena temple in Syracuse and the sequence of nearby earthquakes. We find an apparent association between acoustic-emission bursts and the earthquake occurrence. The waiting-time distributions for acoustic-emission and earthquake time series are described by a unique scaling law indicating self-similarity over a wide range of magnitude scales. This evidence suggests a correlation between the aging process of the temple and the local seismic activit
SIP-based mobility management in next generation networks
The ITU-T definition of next generation networks includes the ability to make use of multiple broadband transport technologies and to support generalized mobility. Next generation networks must integrate several IP-based access technologies in a seamless way. In this article, we first describe the requirements of a mobility management scheme for multimedia real-time communication services; then, we report a survey of the mobility management schemes proposed in the recent literature to perform vertical handovers between heterogeneous networks. Based on this analysis, we propose an application-layer solution for mobility management that is based on the SIP protocol and satisfies the most important requirements for a proper implementation of vertical handovers. We also implemented our proposed solution, testing it in the field, and proving its overall feasibility and its interoperability with different terminals and SIP servers
Fast ray-tracing algorithm for circumstellar structures (FRACS). II. Disc parameters of the B[e] supergiant CPD-57° 2874 from VLTI/MIDI data
B[e] supergiants are luminous, massive post-main sequence stars exhibiting
non-spherical winds, forbidden lines, and hot dust in a disc-like structure.
The physical properties of their rich and complex circumstellar environment
(CSE) are not well understood, partly because these CSE cannot be easily
resolved at the large distances found for B[e] supergiants (typically \ga
1~kpc). From mid-IR spectro-interferometric observations obtained with
VLTI/MIDI we seek to resolve and study the CSE of the Galactic B[e] supergiant
CPD-57\degr\,2874. For a physical interpretation of the observables
(visibilities and spectrum) we use our ray-tracing radiative transfer code
(FRACS), which is optimised for thermal spectro-interferometric observations.
Thanks to the short computing time required by FRACS (~s per monochromatic
model), best-fit parameters and uncertainties for several physical quantities
of CPD-57\degr\,2874 were obtained, such as inner dust radius, relative flux
contribution of the central source and of the dusty CSE, dust temperature
profile, and disc inclination. The analysis of VLTI/MIDI data with FRACS
allowed one of the first direct determinations of physical parameters of the
dusty CSE of a B[e] supergiant based on interferometric data and using a full
model-fitting approach. In a larger context, the study of B[e] supergiants is
important for a deeper understanding of the complex structure and evolution of
hot, massive stars
Fast ray-tracing algorithm for circumstellar structures (FRACS) I. Algorithm description and parameter-space study for mid-IR interferometry of B[e] stars
The physical interpretation of spectro-interferometric data is strongly
model-dependent. On one hand, models involving elaborate radiative transfer
solvers are too time consuming in general to perform an automatic fitting
procedure and derive astrophysical quantities and their related errors. On the
other hand, using simple geometrical models does not give sufficient insights
into the physics of the object. We propose to stand in between these two
extreme approaches by using a physical but still simple parameterised model for
the object under consideration. Based on this philosophy, we developed a
numerical tool optimised for mid-infrared (mid-IR) interferometry, the fast
ray-tracing algorithm for circumstellar structures (FRACS) which can be used as
a stand-alone model, or as an aid for a more advanced physical description or
even for elaborating observation strategies. FRACS is based on the ray-tracing
technique without scattering, but supplemented with the use of quadtree meshes
and the full symmetries of the axisymmetrical problem to significantly decrease
the necessary computing time to obtain e.g. monochromatic images and
visibilities. We applied FRACS in a theoretical study of the dusty
circumstellar environments (CSEs) of B[e] supergiants (sgB[e]) in order to
determine which information (physical parameters) can be retrieved from present
mid-IR interferometry (flux and visibility). From a set of selected dusty CSE
models typical of sgB[e] stars we show that together with the geometrical
parameters (position angle, inclination, inner radius), the temperature
structure (inner dust temperature and gradient) can be well constrained by the
mid-IR data alone. Our results also indicate that the determination of the
parameters characterising the CSE density structure is more challenging but, in
some cases, upper limits as well as correlations on the parameters
characterising the mass loss can be obtained. Good constraints for the sgB[e]
central continuum emission (central star and inner gas emissions) can be
obtained whenever its contribution to the total mid-IR flux is only as high as
a few percents. Ray-tracing parameterised models such as FRACS are thus well
adapted to prepare and/or interpret long wavelengths (from mid-IR to radio)
observations at present (e.g. VLTI/MIDI) and near-future (e.g. VLTI/MATISSE,
ALMA) interferometers
Scaling in temporal occurrence of quasi-rigid-body vibration pulses due to macrofractures
We subjected the time series of quasi-rigid-body vibration pulses (elastic emissions) from laboratory fracture carried out by a piezoelectric accelerometer on concrete and rock specimens under uniaxial compression to statistical analysis. In both cases, we find that the waiting-time distribution can be described by a scaling law extending over several orders of magnitude. This law is indistinguishable from a universal scaling law recently proposed for the waiting-time distributions of acoustic emissions in heterogeneous materials and earthquakes, suggesting its general validity for fracture processes independent of modes and magnitude scales
Possible Assessment of Calf Venous Pump Efficiency by Computational Fluid Dynamics Approach
Three-dimensional simulations of peripheral, deep venous flow during muscular exercise in limbs of healthy subjects and in those with venous dysfunction were carried out by a computational fluid-dynamics (CFD) approach using the STAR CCM + platform. The aim was to assess the effects of valvular incompetence on the venous calf pump efficiency. The model idealizes the lower limb circulation by a single artery, a capillary bed represented by a porous region and a single vein. The focus is on a segment of the circuit which mimics a typical deep vein at the level of the calf muscle, such as the right posterior tibial vein. Valves are idealized as ball valves, and periodic muscle contractions are given by imposing time-dependent boundary conditions to the calf segment wall. Flow measurements were performed in two cross-sections downstream and upstream of the calf pump. Model results demonstrate a reduced venous return for incompetent valves during calf exercise. Two different degrees of valvular incompetence are considered, by restricting the motion of one or both valves. Model results showed that only the proximal valve is critical, with a 30% reduction of venous return during calf exercise in case of valvular incompetence: the net flow volume ejected by the calf in central direction was 0.14 mL per working cycle, against 0.2 mL for simulated healthy limbs. This finding appeared to be consistent with a 25% reduction of the calf ejection fraction, experimentally observed in chronic venous disease limbs compared with healthy limbs
Radiative transfer in very optically thick circumstellar disks
In this paper we present two efficient implementations of the diffusion
approximation to be employed in Monte Carlo computations of radiative transfer
in dusty media of massive circumstellar disks. The aim is to improve the
accuracy of the computed temperature structure and to decrease the computation
time. The accuracy, efficiency and applicability of the methods in various
corners of parameter space are investigated. The effects of using these methods
on the vertical structure of the circumstellar disk as obtained from
hydrostatic equilibrium computations are also addressed. Two methods are
presented. First, an energy diffusion approximation is used to improve the
accuracy of the temperature structure in highly obscured regions of the disk,
where photon counts are low. Second, a modified random walk approximation is
employed to decrease the computation time. This modified random walk ensures
that the photons that end up in the high-density regions can quickly escape to
the lower density regions, while the energy deposited by these photons in the
disk is still computed accurately. A new radiative transfer code, MCMax, is
presented in which both these diffusion approximations are implemented. These
can be used simultaneously to increase both computational speed and decrease
statistical noise. We conclude that the diffusion approximations allow for fast
and accurate computations of the temperature structure, vertical disk structure
and observables of very optically thick circumstellar disks.Comment: Accepted for publication in A&
Drivers of Small-Scale Fishers’ Acceptability across Mediterranean Marine Protected Areas at Different Stages of Establishment
The success of marine protected areas (MPAs) in achieving conservation and sustainable development goals hinges on, among other things, their social acceptability by local communities. Small-scale fishing communities represent a key stakeholder category within and around MPAs. Although many authors have examined the social acceptability of MPAs, relatively few studies have addressed this issue by considering how MPA acceptability is built and can be preserved. This study assessed the latent structure of MPA social acceptability and identified the individual and institutional variables driving stakeholders’ acceptability. Using questionnaire surveys, 124 small-scale fishers’ perceptions of MPAs and their social acceptability were explored in six Mediterranean MPAs (three were implemented, and three were designated). The results show that MPA acceptability is positively related to fishers’ age. The findings also highlight that the formal establishment of MPAs is not a sufficient condition for increasing MPA acceptability among fishers. Considerations about the possibility that MPA acceptability can be increased by building support and compliance emerged. MPA managers should implement successful long-term stakeholder engagement initiatives to increase commitment around conservation measures and to improve overall MPA effectiveness
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