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1.
2.
Hot and cold deserts have been thoroughly searched for meteorites in the past decades, which has led to a large inventory of classified meteorites. H‐ and L‐chondrites are the most abundant meteorites in all collections, and many authors used the H/L ratio as a characteristic parameter in comparing meteorite populations. H/L ratios (after pairing) vary from 0.90 in observed falls up to 1.74 in El Médano (Atacama Desert). In this study, we investigate the H/L ratio of 965 unpaired H‐ and L‐chondrites collected in Oman and compare this population with observed falls and other hot desert collections. We find a mass dependence of the H/L ratio among hot desert finds and identify mechanisms such as fragmentation during weathering and fall that have an impact on the H/L ratio. We employ the Kolmogorov–Smirnov and Mann–Whitney U statistical tests to compare the mass distributions of H‐ and L‐chondrites and to test the relationship between the similarity of mass distributions and the H/L ratio. We conclude that the variations of the H/L ratios observed in various populations are a sampling artifact resulting from secondary effects and observational bias, expressed in differences of the H and L mass distributions which are not observed in falls, and not due to variations in H/L of the meteorite flux. The H/L ratio of 0.90 observed among recent falls is considered as most representative for the overall meteorite flux, at least since the Late Pleistocene. 相似文献
3.
A finite differences formulation for the linear and nonlinear dynamics of 2D catenary risers 总被引:1,自引:0,他引:1
A finite differences (FD) solution method is proposed for the numerical treatment of the dynamic equilibrium problem of 2D catenary risers. The method is based on the so-called Box approximation, which in the scope of the present contribution is applied to the complete nonlinear model as well as to the reduced linearized formulation. The application of the Box method transforms the original governing systems into convenient sets of algebraic equations, which in turn are solved efficiently by the relaxation method. Extensive numerical calculations are presented that describe the dynamic behaviour of the structure and evaluate the amplification in loading due to the dynamic components. The effect of the geometric nonlinearities is assessed through comparative calculations that concern both mathematical formulations examined in the present, i.e. the complete nonlinear, and the reduced linearized model. Special attention is paid to the heave excitations as they amplify significantly the magnitudes of the loading components. 相似文献
4.
Ioannis Politopoulos 《地震工程与结构动力学》2010,39(3):325-342
In this paper, the origin of rocking‐type excitations and their effects on the response of base isolated structures are studied. In particular, the role of kinematic interaction in the determination of the rocking excitation is highlighted. The cases of surface foundations subjected to horizontally propagating waves, as well as of embedded foundations under vertically incident shear waves are examined. The validity of the kinematic interaction based on the rigid base mat assumption is discussed. It is shown that, in the case of classical horizontal isolation, rocking input may amplify significantly the response of the lower non‐isolated modes. The examination of full three‐dimensional isolation and active and semi‐active control methods demonstrates the efficacy of these methods to improve the performance of seismically isolated structures subjected to rocking excitations. Copyright © 2009 John Wiley & Sons, Ltd. 相似文献
5.
The in situ observations of the Earth magnetosphere performed over the past decades of space research have provided a rather good understanding of many partial localized processes of the magnetospheric substorm. The continuing lack of global observations inhibits the construction of a coherent picture of the substorm as a whole, which is actually determined by the coupling of the partial processes. In this context the importance of global observations for the advancement of magnetospheric substorm studies is critical. This paper presents briefly a promising technique of global observations, namely the imaging of charge exchange neutral atoms, or neutral atom imaging (NAI) of the magnetosphere. Model and theoretical estimates of charge-exchange neutral atom fluxes, as well as appropriate spacecraft orbit and instrumentation requirements are presented and discussed for specific regions of interest and vantage points. The potential merits of NAI for substorm research are presented along with possible combinations with other types of observational methods. Substorm issues that would benefit from NAI should include among others the assessment of the ionospheric contribution to the hot magnetospheric plasma, the relative importance of various ionospheric ion source regions, the resolution of spatial and temporal characteristics of substorm ion injections. NAI observations can be precious complements to local observations and lead to the understanding of how local processes, many of which are resolved quite well today, combine to form the global process of the magnetospheric substorm. 相似文献
6.
Ioannis N. Psycharis 《地震工程与结构动力学》1983,11(4):501-521
The dynamic behaviour of a simplified model of a multi-storey building, supported by an elastic foundation and allowed to uplift, is examined. The building is modelled by an n-degree-of-freedom oscillator, while the foundation is represented by a viscously damped two-spring model which permits uplift. This model has been shown in previous studies to be an accurate approximation to the more realistic but more complex Winkler foundation. Approximate values for the characteristic frequencies of the interacting system are presented and a simple, first-mode solution is developed. The response of the system is non-linear and the apparent fundamental period increases with the amount of lift-off. In contrast to the first mode of the superstructure which participates strongly in the interaction, the second and higher modes of the building are not affected significantly by either the interaction with the soil or the uplift. The study shows that lift-off results in larger rocking motion of the structure, but it is not clear from the analysis and the example whether the interfloor displacements are consistently increased or decreased, since this appears to depend on the properties of the system and the excitation. 相似文献
7.
Sokrat Amataj Todor Anovski Ralf Benischke Romeo Eftimi Laurence L. Gourcy Liliana Kola Ioannis Leontiadis Eftim Micevski Alqiviadis Stamos Jovan Zoto 《Environmental Geology》2007,51(5):749-753
Prespa Lake and Ohrid Lake constitute a hydraulic system shared between Albania, FYR of Macedonia and Greece. Karst rocks
separate both lakes. The elevation of Prespa Lake is about 150 m higher than that of Ohrid Lake. Considering these facts,
Cvijić formulated in 1906 the hypothesis that Prespa Lake recharges the St. Naum and Tushemisht springs at Ohrid lakeside.
Environmental isotopes demonstrated that Prespa Lake recharges about 37–42 and 52–54% of water emerging in St. Naum, and Tushemisht
springs, respectively. An artificial tracer experiment carried out in 2002 physically demonstrated the underground connection
between both lakes. This experiment confirmed the supposed underground connection and brought important information about
the groundwater velocity, transit time, and karst water conduits development. 相似文献
8.
The current design of seismically isolated bridges usually combines the use of bearings and stoppers, as a second line of
defence. The stoppers allow the development of the in-service movements of the bridge deck, without transmitting significant
loads to the piers and their foundations, while during earthquake they transmit the entire seismic action. Despite the fact
that stoppers, which restrain the transverse seismic movements of the deck, are used frequently in seismically isolated bridges,
the use of longitudinal stoppers is relatively rare, mainly due to the large in-service constraint movements of bridges. The
present paper proposes a new type of external longitudinal stoppers, which are installed in stiff sub-structures-boundaries,
aiming at limiting the bridge seismic movements. The parametric investigation, which was conducted in order to identify the
seismic efficiency of the external stoppers, showed that the interaction of the bridge with the stiff boundaries can lead
to significant reductions in the seismic movements of the bridge. Serviceability is appropriately arranged in the paper by
expansion joints and approach slabs. 相似文献
9.
Ioannis A. Daglis Georgios Balasis Natalia Ganushkina Fiori-Anastasia Metallinou Minna Palmroth Risto Pirjola Ioanna Tsagouri 《Acta Geophysica》2009,57(1):141-157
Comprehensive understanding of the dynamics of the coupled solar wind-magnetosphere-ionosphere system is of utmost interest,
both from the perspective of solar system astrophysics and geophysics research and from the perspective of space applications.
The physical processes involved in the dynamical evolution of this complex coupled system are pertinent not only for the Sun-Earth
connection, but also for major phenomena in other astrophysical systems. Furthermore, the conditions in geospace collectively
termed space weather affect the ever increasing technological assets of mankind in space and therefore need to be understood,
quantified and efficiently forecasted. The present collaborative paper communicates recent advances in geospace dynamic coupling
research through modeling, simulations and data analysis and discusses future directions. 相似文献
10.
This paper presents a macroelement formulation for the prediction of the planar dynamic response of inelastic deformable rocking bodies. The formulation is based on a previous macroelement developed by the authors able to describe the cyclic response of inelastic rocking bodies, which takes into account the deformability both along the height of the member, as well as near the rocking end. Modifications of this formulation to account for other motion modes of rocking members during their dynamic response, namely, sliding and upthrow, as well as modifications to account for damping in a uniform manner during the whole motion, including impacts, are introduced. The dynamic response predicted by the macroelement for free-standing rigid and deformable rocking bodies is presented and compared with existing theoretical solutions, and the effect of deformability, damping, inelasticity, and friction on the response is discussed. 相似文献