Starting from the hypothesis of coalescence through inelastic collisions of small fastmoving interstellar gas clouds, an attempt has been made to study the evolutionary mass distribution of a system of fragments simulating a protocluster. The assumption of a mass spectrum with a continuous injection of newly formed entities into the primeval system, and the condition of gravitational reduction of the impact cross-section, have been considered. Comparisons of numerical experiments with the mass spectrum in some well-known young galactic clusters, confirm the mass distribution power-law already obtained by other authors. The empirical Schmidt's law concerning the rate of star formation is also confirmed. The hypothesis of the universal validity of the luminosity function, should not be a priori rejected. 相似文献
Numerical calculations have been made of the gravitational axisymmetric collapse of isothermal gas clouds endowed with angular momentum. The evolutionary study is based on the so-called Fluid-in-Cell method coupled to an efficient algebraic algorithm which allows the Poisson equation to be integrated by means of block tri-diagonal matrices. The results, at ages slight larger than the initial free-fall time, indicate that flattened disk-shaped structures are formed in the central region of the clouds-in good agreement with the previous analytical results predicted by the authors. 相似文献
Earthquake Engineering and Engineering Vibration - Self-centering earthquake-resistant structures have received increased attention due to their ability to reduce post-earthquake residual... 相似文献
The gravitational collapse mechanism of a non-magnetized isothermal rotating interstellar gas cloud is discussed analytically.
The study predicts the formation of final flattened disk-like configurations in the central region of the infalling uniform
gas cloud. The existence of critical times indicates the possibility that, beyond these times, discrepancies can arise in
the numerical calculations. It seems therefore very likely that the ring-like structures, as suggested by earlier numerical
calculations of hydrodynamic models, would be caused by pressure effects which arise through the introduction of higher order
terms in the linearized theory. 相似文献
The gravitational collapse of a spherically symmetric interstellar gas cloud has been investigated following the non-linear discontinuity waves propagation theory. It has been pointed out that macroscopic phenomena, such as the process of fragmentation, can arise (shock wave formation)-even in the case of spherical symmetry- at times smaller than the free-fall timetff, provided the initial data of the Cauchy problem be discontinuous within a sphere of radius
(caustic cases). It has also been proved that strong discontinuities outside the mentioned sphere may generate critical timestcr<tff (depending on the typical non-linear structure of the differential system). The cooling-heating function plays an important role in contrasting the formation of shock waves. 相似文献
The paper investigates the dynamic characterisation, the numerical model tuning and the seismic risk assessment of two monumental masonry towers located in Italy: the Capua Cathedral bell tower and the Aversa Cathedral bell tower. Full-scale ambient vibration tests under environmental loads are performed. The modal identification is carried out using techniques of modal extraction in the frequency domain. The refined 3D finite element model (FEM) is calibrated using the in situ investigation survey. The FEM tuning is carried out by varying the mechanical parameters and accounting for the restraint offered by the neighbouring buildings and the role of soil–structure interaction. The assessment of the seismic performance of the bell towers is carried out through a nonlinear static procedure based on the multi-modal pushover analysis and the capacity spectrum method. Through the discussion of the case studies, the paper shows that the modal identification is a reliable technique that can be used in situ for assessing the dynamic behaviour of monumental buildings. By utilising the tuned FEM of the towers, the theoretical fundamental frequencies are determined, which coincide with the previously determined experimental frequencies. The results from seismic performance assessment through a pushover analysis confirm that the masonry towers in this study are particularly vulnerable to strong damage even when subjected to seismic events of moderate intensity.
The paper investigates the dynamic characterisation, the numerical model tuning and the seismic risk assessment of two monumental masonry towers located in Italy: the Capua Cathedral bell tower and the Aversa Cathedral bell tower. Full-scale ambient vibration tests under environmental loads are performed. The modal identification is carried out using techniques of modal extraction in the frequency domain. The refined 3D finite element model (FEM) is calibrated using the in situ investigation survey. The FEM tuning is carried out by varying the mechanical parameters and accounting for the restraint offered by the neighbouring buildings and the role of soil–structure interaction. The assessment of the seismic performance of the bell towers is carried out through a nonlinear static procedure based on the multi-modal pushover analysis and the capacity spectrum method. Through the discussion of the case studies, the paper shows that the modal identification is a reliable technique that can be used in situ for assessing the dynamic behaviour of monumental buildings. By utilising the tuned FEM of the towers, the theoretical fundamental frequencies are determined, which coincide with the previously determined experimental frequencies. The results from seismic performance assessment through a pushover analysis confirm that the masonry towers in this study are particularly vulnerable to strong damage even when subjected to seismic events of moderate intensity. 相似文献
In this letter, a methodology to overcome the layover problem and obtain the 3-D reconstruction of urban areas will be discussed. Interferometric synthetic aperture radar (SAR) (InSAR) systems allow the estimation of height profiles of the Earth surface, but in the case of urban scenarios, estimation becomes a hard task due to the presence of SAR geometrical distortions, with layover above all. First, the layover signal in InSAR images is investigated; then, a procedure to specifically manage layover areas is presented. The proposed method consists of introducing an auxiliary data exploitation, optical data or SAR shadowing, in the maximum a posteriori statistical estimation technique to improve the digital elevation model reconstruction, particularly on phase discontinuities. We test the method on simulated data, showing its effectiveness. 相似文献