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531.
We have developed a preliminary version of a new type of code to simulate the outcomes of impacts between solid bodies, which we plan to further refine for application to both asteroid science and space debris studies. In the current code, colliding objects are modeled as two-dimensional arrays of finite elements, which can interact with each other in both an elastic and a shock-wave regime. The finite elements are hard spheres with a given value for mass and radius. When two of them come into contact the laws of inelastic scattering are applied, thus giving rise to the propagation of shock waves. Moreover each spherical element interacts elastically with its nearest neighbours. The interaction force corresponds to that of a spring having an equilibrium length equal to the lattice spacing, and results into the propagation of elastic waves in the lattice. Dissipation effects are modeled by means of a dissipative force term proportional to the relative velocity, with a given characteristic time of decay. The possible occurrence of fractures in the material is modeled by assuming that when the distance of two neighbouring elements exceeds a threshold value, the binding force between them disappears for ever. This model requires finding a plausible correspondence between the input parameters appearing in the equations of motion, and the physical properties of real solid materials. Some of the required links are quite obvious (e.g., the relationship between mass of the elements and elastic constant on one side, and material density and sound velocity on the other side), some others a priori are unclear, and additional hypotheses on them must be made (e.g., on the restitution coefficient of inelastic scattering). Despite the preliminary character of the model, we have obtained some interesting results, which appear to mimic in a realistic way the outcomes of actual impacts. For instance, we have observed the formation of craters and fractures, and (for high impact energies) the occurrence of catastrophic breakup. The masses and velocities of the fragments resemble those found in laboratory impact experiments. 相似文献
532.
We describe the use of the magneto-optic filter (MOF) to observe solar magnetic fields in the potassium line at 7699 Å. The filter has been used in the Big Bear video-magnetograph since 23 October. It gives a high sensitivity and dynamic range for longitudinal magnetic fields and enables us to measure transverse magnetic fields using the sigma component. Examples of the observations are presented.This paper was presented at the third meeting of the Solar Cycle Workshop, held in Sydney, Australia, January 9–13, 1989.NRC Senior Res. Fellow. 相似文献
533.
Experimental and Numerical Studies on the Seismic Response of R.C. Hollow Bridge Piers 总被引:3,自引:2,他引:1
Gian?Michele?CalviEmail author Alberto?Pavese Alessandro?Rasulo Davide?Bolognini 《Bulletin of Earthquake Engineering》2005,3(3):267-297
Appropriate seismic assessment of reinforced concrete bridges is an important challenge in structural engineering in consideration
of the number and relevance of bridges built without applying advanced codes of practice and of the strategic role of transportation
networks in modern economy. This paper focuses on some relevant aspects of the damage development and collapse modes of hollow
piers, as commonly designed in the past. The following aspects have been considered: absence of confinement, inadequate shear
strength, shifting of the critical section, insufficient length of lap splices. A series of experimental tests has been designed
and performed; the capacity of predicting appropriate results of formulations recommended in codes of practice or proposed
in recent research documents has been checked comparing experimental and analytical results. 相似文献
534.
Alessandro Iannace Glauco Bonardi Marco D'Errico Stefano Mazzoli Vincenzo Perrone Stefano Vitale 《Comptes Rendus Geoscience》2005,337(16):373
A new structural–stratigraphic synthesis of the Apennine units of northern Calabria is presented. The Meso-Cenozoic successions are grouped into two tectonic units, named Pollino–Ciagola Unit (PCU) and Lungro–Verbicaro Unit (LVU), comprising terrains formerly attributed to five different tectonic units. FeMg carpholite and blue amphibole record HP–LT metamorphism in the LVU, followed by progressive decompression leading to final greenschist facies re-equilibration during dominantly extensional deformation. Final tectonic emplacement of the LVU over the PCU post-dated the metamorphism of the LVU and was accompanied by intense ductile deformation along zones of strain localisation in footwall rocks. All of the units were later affected by folding and minor thrusting during subsequent Apennine tectonics. To cite this article: A. Iannace et al., C. R. Geoscience 337 (2005). 相似文献
535.
A syn-eruptive ground deformation episode measured by GPS,during the 2001 eruption on the upper southern flank of Mt Etna 总被引:4,自引:0,他引:4
Alessandro?BonforteEmail author Francesco?Guglielmino Mimmo?Palano Giuseppe?Puglisi 《Bulletin of Volcanology》2004,66(4):336-341
Ground deformation occurring on the southern flank of Mt Etna volcano during the July–August 2001 eruption was monitored by GPS measurements along an E–W profile crossing the fissure system. This profile was measured eight times during the eruption, using the 'stop and go' semi-kinematic technique. Horizontal and vertical displacements between GPS surveys are reported for each station. The most significant event is a deformation episode occurring during the first week of the eruption, between 25–27 July. Displacements were measured on benchmarks close to the eruptive fissure and the tensile 1989 fracture. Data inversions for measured displacements were performed using the Okada model. The model shows the narrowing of the 2001 dyke accompanied by a dextral dislocation along an east-dipping fault, parallel to the 1989 fracture.Editorial responsibility: T. Druitt 相似文献
536.
A. A. Deshpande F. D’Alessandro P. M. McCulloch 《Journal of Astrophysics and Astronomy》1996,17(1-2):7-16
Spectral analysis of the residual pulsearrival times of pulsars is a useful tool in understanding the nature of the underlying
processes that may be responsible for the timing noise observed from pulsars. Power spectra of pulsar timing residuals may
be described by one or a combination of powerlaws. As these spectra are expected to be very steep, it is important to ensure
a high dynamic range in the estimation of the spectrum. This is difficult in practice since one is, in general, dealing with
timing measurements made at unevenly placed epochs. In this paper, we present a technique based on, ‘CLEAN’ to obtain high
dynamic range spectra from unevenly sampled data. We compare the performance of this technique with other techniques including
some that were used earlier for estimation of power spectra of pulsar timing residuals. 相似文献
537.
Massimilliano Guzzo Alessandro Morbidelli 《Celestial Mechanics and Dynamical Astronomy》1996,66(3):255-292
We investigate the possibility of obtaining a Nekhoroshev like result for the dynamical system describing the motion of an asteroid in the main belt, From the mathematical point of view this is a new result since the problem is degenerate and we want to control also the motion of degenerate actions, We find that there are regions, such as the resonances of low order among the fast angles (mean motion resonances), where a Nekhoroshev like result cannot be proved a priori, Conversely, we are able to confine the motions in the mean motion resonances of logarithmically large order in the perturbation parameters, as well as in the non-resonant region, We discuss also the connection with the existence of invariant tori. 相似文献
538.
Fabrice Thomas Alessandro Morbidelli 《Celestial Mechanics and Dynamical Astronomy》1996,64(3):209-229
We consider the secular evolution of the orbits of bodies in the Outer Solar System under the perturbations of the jovian planets assumed on coplanar and circular orbits. Through the approach used for asteroidal belt by Yoshihide Kozai in 1962, we obtain that the Kozai resonance do not affect the behavior of bodies belonging to the Kuiper belt but concerns the long-timescale evolution of long-period comets. In particular this resonance appears as a process contributing to produce Sun-grazer comets. 相似文献
539.
Fabienne Delhaise Alessandro Morbidelli 《Celestial Mechanics and Dynamical Astronomy》1993,57(1-2):155-173
The luni-solar effects of a geosynchronous artificial satellite orbiting near the critical inclination is investigated. To tackle this four-degrees-of-freedom problem, a preliminary exploration separately analyzing each harmonic formed by a combination of the satellite longitude of the node and the Moon longitude of the node is opportune. This study demonstrates that the dynamics induced by these harmonics does not show resonance phenomena. In a second approach, the number of degrees of freedom is halved by averaging the total Hamiltonian over the two non-resonant angular variables. A semi-numerical method can now be applied as was done when considering solely the inhomogeneity of the geopotential (see Delhaise et Henrard, 1992). Approximate surfaces of section are constructed in the plane of the inclination and argument of perigee. The main effects of the Sun and Moon attractions compared to the terrestrial attraction alone are a strong increase in the amplitude of libration in inclination (from 0.6° to 3.2°) and a decrease of the corresponding libration period (from the order of 200 years to the order of 20 years).Research Assistant for the Belgian National Fund for Scientific Research 相似文献
540.
Adrian J. Boyce Paolo Fulignati Alessandro Sbrana Anthony E. Fallick 《Journal of Volcanology and Geothermal Research》2007
High-sulfidation (HS) epithermal systems have elements in common with passively degassing volcanoes associated with high T, acid fumarole fields or acid crater lakes. They are considered to form in two stages, the first of which involves advanced argillic alteration resulting from intense, strongly acidic fluid–rock interaction. The La Fossa hydrothermal system (Vulcano Island) represents a classic example of such an active HS system and can be considered as a modern analogue of this early stage of alteration, resulting in a core of intense silicic (90–95% pure SiO2) alteration surrounded by alunitic alteration zones. 相似文献