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81.
82.
In this contribution we present our preliminary investigation on pulsar sensitivity of the Large Area Telescope, the main
instrument aboard the GLAST mission. In particular we concentrated our attention to pulsars located at low galactic latitudes.
We created a set of simulated pulsars having different fluxes in an array of galactic coordinates separated by a distance
greater than the LAT Point Spread Function in order to avoid confusion between adjacent sources. Galactic gamma-ray sky background
as used during the second LAT Data Challenge (DC2) is also included. We then run an automatic routine for testing periodicity for all the pulsars considering an opportune
timing solution. In this way we can obtain a map of the sensitivity of the periodic searches for different fluxes and for
various Galactic latitudes. Some assumptions have been made by simulating the pulsar sources, but this study is a first step
toward an estimate for pulsed emission sensitivity of the GLAST LAT. The pulsed flux sensitivity profile we generate could
also be input to a population synthesis code of Galactic pulsars in order to obtain more accurate predictions of the number
of expected pulsar detections by GLAST.
On behalf of the GLAST LAT Collaboration. 相似文献
83.
Alessandro Morbidelli Massimiliano Guzzo 《Celestial Mechanics and Dynamical Astronomy》1996,65(1-2):107-136
The present paper reviews the Nekhoroshev theorem from the point of view of physicists and astronomers. We point out that Nekhoroshev result is strictly connected with the existence of a specific structure of the phase space, the existence of which can be checked with several numerical tools. This is true also for a degenerate system such as the one describing the motion of an asteroid in the so called main belt. The main difference is that in some parts of the belt, the Nekhoroshev result cannot apply a priori. Mean motion resonances of order smaller than the logarithm of the mass of Jupiter and first order secular resonances must be excluded. In the remaining parts, conversely, the Nekhoroshev theorem can be proved, provided someparameters, such as the masses, the eccentricities and the inclinations of the planets are small enough. At the light of this result, a massive campaign of numerical integrations of real and fictitious asteroids should allow to understand which is the real dynamical structure of the asteroid belt. 相似文献
84.
Projection, or conjugate gradient like, methods are becoming increasingly popular for the efficient solution of large sparse sets of unsymmetric indefinite equations arising from the numerical integration of (initial) boundary value problems. One such problem is soil consolidation coupling a flow and a structural model, typically solved by finite elements (FE) in space and a marching scheme in time (e.g. the Crank–Nicolson scheme). The attraction of a projection method stems from a number of factors, including the ease of implementation, the requirement of limited core memory and the low computational cost if a cheap and effective matrix preconditioner is available. In the present paper, biconjugate gradient stabilized (Bi‐ CGSTAB) is used to solve FE consolidation equations in 2‐D and 3‐D settings with variable time integration steps. Three different nodal orderings are selected along with the preconditioner ILUT based on incomplete triangular factorization and variable fill‐in. The overall cost of the solver is made up of the preconditioning cost plus the cost to converge which is in turn related to the number of iterations and the elementary operations required by each iteration. The results show that nodal ordering affects the perfor mance of Bi‐CGSTAB. For normally conditioned consolidation problems Bi‐CGSTAB with the best ILUT preconditioner may converge in a number of iterations up to two order of magnitude smaller than the size of the FE model and proves an accurate, cost‐effective and robust alternative to direct methods. Copyright © 2001 John Wiley & Sons, Ltd. 相似文献
85.
Alessandro Ielpi Mathieu G.A. Lapôtre Alvise Finotello Massimiliano Ghinassi 《地球表面变化过程与地形》2021,46(2):357-370
River bends occasionally meander to the point of cutoff, whereby a river shortcuts itself and isolates a portion of its course. This fundamental process fingerprints a river's long-term planform geometry, its stratigraphic record, and biogeochemical fluxes in the floodplain. Although meander cutoffs are common in fast-migrating channels, timelapse imagery of the Earth surface typically does not offer a long enough baseline for statistically robust analyses of these processes. We seek to bridge this gap by quantifying cutoff kinematics along the Humboldt River (Nevada) – a stream that, from 1994 to 2019, hosted an exceptionally high number of cutoffs (specifically, 174 of the chute type and 53 of the neck type). A coincidence between major floods and cutoff incidence is first suggestive of hydrographic modulation. Moreover, not just higher sinuosity but also upstream planform skewness is associated with higher cutoff incidence and channel widening for a sub-population of chute cutoffs. We propose a conceptual model to explain our results in terms of channel-flow structure and then examine the distances between adjacent cutoffs to understand the mechanisms governing their clustering. We find that both local and nonlocal perturbations together trigger the clustering of new cutoffs, over distances capped by the backwater length and over yearly to decadal timescales. Our research suggests that planform geometry and backwater controls might sway the occurrence of cutoff clusters – both local and nonlocal – thereby offering new testable hypotheses to explore the evolution of meandering-river landscapes that have significant implications for river engineering and stratigraphic modelling. © 2020 John Wiley & Sons, Ltd. 相似文献
86.
Andrea D'Aniello Sigrún Tómasdóttir Bergur Sigfússon Massimiliano Fabbricino 《Ground water》2021,59(5):677-693
Particular attention is paid to the risk of carbon dioxide (CO2) leakage in geologic carbon sequestration (GCS) operations, as it might lead to the failure of sequestration efforts and to the contamination of underground sources of drinking water. As carbon dioxide would eventually reach shallower formations under its gaseous state, understanding its multiphase flow behavior is essential. To this aim, a hypothetical gaseous leak of carbon dioxide resulting from a well integrity failure of the GCS system in operation at Hellisheiði (CarbFix2) is here modeled. Simulations show that migration of gaseous carbon dioxide is largely affected by formation stratigraphy, intrinsic permeability, and retention properties, whereas the initial groundwater hydraulic gradient (0.0284) has practically no effect. In two different scenarios, about 18.3 and 30.6% of the CO2 that would have been injected by the GCS system for 3 days could be potentially released again into the atmosphere due to a sustained leakage of the same duration. As the gaseous leak occurs, the aquifer experiences high pressure buildups, and the presence of a less conductive layer further magnifies these. Strikingly, the dimensional analysis showed that buoyant and viscous forces can be comparable over time within the predicted gaseous plumes, even far from the leakage source. Local pressure gradients, buoyant, viscous, and capillary forces all play an important role during leakage. Therefore, neglecting one or more of these contributions might lead to a partial prediction of gaseous CO2 flow behavior in the subsurface, giving space to incorrect interpretations and wrong operational choices. 相似文献
87.
Ray A. F. Cas Heather M. N. Wright Christopher B. Folkes Chiara Lesti Massimiliano Porreca Guido Giordano Jose G. Viramonte 《Bulletin of Volcanology》2011,73(10):1583-1609
The 2.08-Ma Cerro Galán Ignimbrite (CGI) represents a >630-km3 dense rock equivalent (VEI 8) eruption from the long-lived Cerro Galán magma system (∼6 Ma). It is a crystal-rich (35–60%),
pumice (<10% generally) and lithic-poor (<5% generally) rhyodacitic ignimbrite, lacking a preceding plinian fallout deposit.
The CGI is preserved up to 80 km from the structural margins of the caldera, but almost certainly was deposited up to 100 km
from the caldera in some places. Only one emplacement unit is preserved in proximal to medial settings and in most distal
settings, suggesting constant flow conditions, but where the pyroclastic flow moved into a palaeotopography of substantial
valleys and ridges, it interacted with valley walls, resulting in flow instabilities that generated multiple depositional
units, often separated by pyroclastic surge deposits. The CGI preserves a widespread sub-horizontal fabric, defined by aligned
elongate pumice and lithic clasts, and minerals (e.g. biotite). A sub-horizontal anisotropy of magnetic susceptibility fabric
is defined by minute magnetic minerals in all localities where it has been analysed. The CGI is poor in both vent-derived
(‘accessory’) lithics and locally derived lithics from the ground surface (‘accidental’) lithics. Locally derived lithics
are small (<20 cm) and were not transported far from source points. All data suggest that the pyroclastic flow system producing
the CGI was characterised throughout by high sedimentation rates, resulting from high particle concentration and suppressed
turbulence at the depositional boundary layer, despite being a low aspect ratio ignimbrite. Based on these features, we question
whether high velocity and momentum are necessary to account for extensive flow mobility. It is proposed that the CGI was deposited
by a pyroclastic flow system that developed a substantial, high particle concentration granular under-flow, which flowed with
suppressed turbulence. High particle concentration and fine-ash content hindered gas loss and maintained flow mobility. In
order to explain the contemporaneous maintenance of high particle concentration, high sedimentation rate at the depositional
boundary layer and a high level of mobility, it is also proposed that the flow(s) was continuously supplied at a high mass
feeding rate. It is also proposed that internal gas pressure within the flow, directed downwards onto the substrate over which
the flow was passing, reduced the friction between the flow and the substrate and also enhanced its mobility. The pervasive
sub-horizontal fabric of aligned pumice, lithic and even biotite crystals indicates a consistent horizontal shear force existed
during transport and deposition in the basal granular flow, consistent with the existence of a laminar, shearing, granular
flow regime during the final stages of transport and deposition. 相似文献
88.
Massimiliano Favalli Fabrizio Innocenti Maria Teresa Pareschi Giorgio Pasquarè Stefano Branca 《Geodinamica Acta》2013,26(5):279-290
AbstractA Digital Elevation Model (DEM) of Mt. Etna is presented; it has altimetric and planimetric resolution of 1 m and 5 m, respectively, and covers an area of about 120 km . This 3-D view of Mt. Etna allowed both recognition and location of the main morphostructural and volcano-tectonic features of the volcano. A slope map has been generated from the DEM; on the basis of slope distributions and surface textures, five acclivity domains have been recognized. The largest domain, south of the summit craters, reflects the occurrence of old plateau lavas, distinct from central volcanoes which built the present Etnean volcanic system. Interaction between the central volcanoes, with their summit calderas and failed slopes, produced the other recognised domains. Furthermore, newly identified relevant morphostructural lines are discussed. © Elsevier, Paris 相似文献
89.
GIGLI Giovanni INTRIERI Emanuele LOMBARDI Luca NOCENTINI Massimiliano FRODELLA William BALDUCCI Marco VENANTI Luca Domenico CASAGLI Nicola 《山地科学学报》2014,(6):1521-1530
The Torgiovannetto quarry(Assisi municipality,central Italy) is an example of a site where the natural equilibrium was altered by human activity,causing current slope instability phenomena which threaten two roadways important for the local transportation.The quarry front,having a height of about 140 m,is affected by a 182,000 m3 rockslide developed in intensely fractured limestone and is too large to be stabilized.In 2003 some tension cracks were detected in the vegetated area above the quarry upper sector.From then on,several monitoring campaigns were carried out by means of different instrumentations(topographic total station,extensometers,inclinometers,ground-based interferometric radar,laser scanner and infrared thermal camera),allowing researchers to accurately define the landslide area and volume.The latter’s major displacements are localized in the eastern sector.The deformational field appears to be related to the seasonal rainfall.The landslide hazard associated with the worst case scenario was evaluated in terms of magnitude,intensity and triggering mechanism.For the definition of the possible runout process the DAN 3D code was employed.The simulation results were used in order to design and construct a retaining embankment.Furthermore,in order to preserve both the safety of the personnelinvolved in its realization and of the roadways users,an early warning system was implemented.The early warning system is based on daily-averaged displacement velocity thresholds.The alarm level is reached if the prediction based on the methods of Saito(1969) and Fukuzono(1985) forecasts an imminent rupture. 相似文献