Geostationary secular dynamics revisited: application to high area-to-mass ratio objects     

Fabien Gachet  Alessandra Celletti  Giuseppe Pucacco  Christos Efthymiopoulos 《Celestial Mechanics and Dynamical Astronomy》2017,128(2-3):149-181

The long-term dynamics of the geostationary Earth orbits (GEO) is revisited through the application of canonical perturbation theory. We consider a Hamiltonian model accounting for all major perturbations: geopotential at order and degree two, lunisolar perturbations with a realistic model for the Sun and Moon orbits, and solar radiation pressure. The long-term dynamics of the GEO region has been studied both numerically and analytically, in view of the relevance of such studies to the issue of space debris or to the disposal of GEO satellites. Past studies focused on the orbital evolution of objects around a nominal solution, hereafter called the forced equilibrium solution, which shows a particularly strong dependence on the area-to-mass ratio. Here, we (i) give theoretical estimates for the long-term behavior of such orbits, and (ii) we examine the nature of the forced equilibrium itself. In the lowest approximation, the forced equilibrium implies motion with a constant non-zero average ‘forced eccentricity’, as well as a constant non-zero average inclination, otherwise known in satellite dynamics as the inclination of the invariant ‘Laplace plane’. Using a higher order normal form, we demonstrate that this equilibrium actually represents not a point in phase space, but a trajectory taking place on a lower-dimensional torus. We give analytical expressions for this special trajectory, and we compare our results to those found by numerical orbit propagation. We finally discuss the use of proper elements, i.e., approximate integrals of motion for the GEO orbits.  相似文献   

A cervelleite-like mineral and other Ag-Cu-Te-S minerals [Ag2CuTeS and (Ag,Cu)2TeS] in gold-bearing veins in metamorphic rocks of the Cycladic Blueschist Unit, Kallianou, Evia Island, Greece     

Panagiotis Christos Voudouris  Paul G. Spry  Gregory Aarne Sakellaris  Constantinos Mavrogonatos 《Mineralogy and Petrology》2011,101(3-4):169-183

A cervelleite-like mineral, two unnamed silver sulfotellurides in the system Ag-Cu-Te-S [Ag₂CuTeS, (Ag,Cu)₂TeS], Te-rich polybasite and cadmian tetrahedrite occur in gold-bearing quartz veins in metapelites and faults within brecciated marbles of the Cycladic Blueschist Unit in the Kallianou area (southern Evia Island, Greece). The quartz veins and faults are discordant to syn-metamorphic structures and formed during ductile to brittle deformation in the final stages of exhumation of the Styra Nappe extrusion wedge (~21?Ma). Te-rich polybasite (up to 7.4 wt. % Te), cadmian tetrahedrite (up to 12.4 wt. % Cd), together with electrum (23?C54 wt. % Ag) and the sulfotellurides, are the main silver carriers in the mineralization. The two unnamed sulfotellurides, Ag₂CuTeS and (Ag,Cu)₂TeS are believed to be new quaternary minerals in the system Ag-Cu-Te-S. These minerals and the cervelleite-like phase could have exsolved from galena during cooling (below 200°C). Initial temperatures for the formation of the sulfotellurides, in the form of hessite-intermediate solid solution, at Kallianou may be up to 300°C under logfS₂ values between?~ ?11.5 to ?8.3, and logfTe₂ from?~ ?14.8 to ?7.8. The values of logfTe₂ and logfS₂ during re-equilibration (at ~200°C) were constrained to ?19.5 to ?15.2 and to ?15.8 to ?11.5 respectively.  相似文献   

Macrobenthic community changes due to dystrophic events and freshwater inflow: Changes in space and time in a Mediterranean lagoon (Gialova lagoon, SW Greece)     

Georgios ChatzigeorgiouSophia Reizopoulou  Maria MaidanouMaria Naletaki  Eleni OrnerakiEugenia Apostolaki  Christos Arvanitidis 《Estuarine, Coastal and Shelf Science》2011,94(1):111-121

This paper considers the hypothesis that changes in community structure through the control of the larvae maintenance and of the biological traits of the species mostly contribute to the spatio-temporal community pattern. This is supported by the results of the study, the changes to the macrobenthic community pattern deriving from two sampling periods in Gialova lagoon (SW Greece), carried out on a seasonal basis for two yearly periods: 1994-95 and 1998-99. Cognetti’s hypothesis that populations of the tolerant species occupying coastal marine habitats may belong to different species is another alternative hypothesis still to be tested. The importance of alternative management plans aiming at the amelioration of the hydrodynamic conditions of the lagoons and supported by continuous scientific monitoring is highlighted. Two canals bringing fresh water were opened at the beginning of the second sampling period, an intervention among others, suggested by the management plan proposed after the end of the first sampling period. The results show considerable variations in the values of the key environmental variables, for instance long periods with negative Redox potential values, decreased salinity and increased concentrations of the particulate organic matter and of the nutrients. The variables were correlated with the spatio-temporal community pattern, characterized during the second sampling period by: (i) greater dissimilarities among stations/seasons; (ii) disruption of the periodic trend observed on the seasonal scale; (iii) larger relative dissimilarities among the patterns stemming from the macrobenthos and the most abundant groups (polychaetes, molluscs, crustaceans); (iv) significant decrease in abundance or even the disappearance of several marine origin species, along with increased abundance in a few brackish-water species. All of the above changes in the community pattern are considered as early warning signals leading towards degradation, which has not yet been registered in the phylogenetic/taxonomic structure of the macrobenthic community.  相似文献   

An integrated approach of flash flood analysis in ungauged Mediterranean watersheds using post-flood surveys and unmanned aerial vehicles     

Aristeidis Kastridis  Christos Kirkenidis  Marios Sapountzis 《水文研究》2020,34(25):4920-4939

This study analyzes the flash flood event of two ungauged ephemeral streams in Olympiada region (Chalkidiki, North Greece), which occurred at the 21–22 of November 2019. Aim of the study is to reconstruct the specific flash flood event, investigate the causes of flood generation mechanisms, evaluate the performance of SCS-CN hydrological and HEC-RAS hydraulic models, investigate the relation between extreme flash floods and human intervention, using the combination of ground and aerial observations obtained from the field survey and unmanned aerial vehicles (UAVs), respectively. The results of the specific discharge ranged between 9 and 11 m³ s⁻¹ km², values that are typical for flash flood events in Mediterranean region. The comparison between the observed and simulated values of flood extent showed sufficiently good performance of the hydraulic model (CSI = 82%). However, the statistical analysis of the observed and simulated flood depths displayed a flood depth overestimation by the applied model, despite that the values of the used statistic indexes are acceptable (RMSE = 0.35 m, SD = 0.53, NSE = 0.56, PBIAS = 11.26%). The model overestimation of flood depth was attributed to the DEM low resolution and quality. Ground and aerial observations depicted the alluvial fan activation, the alternation of flow paths and the huge sediment transport. Human intervention in main streams, urban sprawl, wet AMC and sediment transport were among the main factors that contributed to the flash flood generation. This integrated approach revealed the necessity of the constant evaluation and validation of hydrological and hydraulic models in small ungauged Mediterranean watersheds and ephemeral streams. The use of UAVs in combination with ground observations and hydraulic simulation could significantly contribute to the enhanced understanding of flash flood mechanisms, in the direction of flood risk mitigation, improvement of the planning efficiency of flood prevent measures, flood hazard estimation, evolution of flood warning systems and floodplain geomorphology analysis.  相似文献   

The Structure and the Partial Opacity of the Spiral Shocks on the Novalike Cataclysmic Variables     

Christos Papadimitriou  Emilios Harlaftis  Danny Steeghs 《Astrophysics and Space Science》2006,304(1-4):315-316

V347 Pup is the first novalike system with clear spiral arms in its accretion disc, as evidenced from its HeI Doppler maps. Combining the Doppler maps of the various lines of the system V347 Pup a more complex structure is revealed on the first arm. On the Doppler maps of HeI λ5875 emission line the first arm splits to two smaller and thinner structures. The Doppler map of HeI λ6678 emission line shows that this line dominates the region of thin structure with higher velocities that we met on the Doppler map of HeI λ5875 emission line. On the contrary the Hα emission line dominates the region of thin structure with lower velocities. We therefore observe, that the Hα emission line dominates on the exterior of the first arm, and the emission line of HeI on the interior. Most of NaI is probably emanated from the white dwarf or from the interior of the disc, and then being absorbed by the spiral shocks.  相似文献   

Trojan resonant dynamics,stability, and chaotic diffusion,for parameters relevant to exoplanetary systems     

Rocío Isabel Páez  Christos Efthymiopoulos 《Celestial Mechanics and Dynamical Astronomy》2015,121(2):139-170

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Structural identification of Egnatia Odos bridges based on ambient and earthquake induced vibrations     

Evaggelos Ntotsios  Christos Karakostas  Vasilios Lekidis  Panagiotis Panetsos  Ioannis Nikolaou  Costas Papadimitriou  Thomas Salonikos 《Bulletin of Earthquake Engineering》2009,7(2):485-501

The dynamic characteristics of two representative R/C bridges on Egnatia Odos motorway in Greece are estimated based on low amplitude ambient and earthquake-induced vibrations. The present work outlines the instrumentation details, algorithms for computing modal characteristics (modal frequencies, damping ratios and modeshapes), modal-based finite element model (FEM) updating methods for estimating structural parameters, and numerical results for the modal and structural dynamic characteristics of the two bridges based on ambient and earthquake induced vibrations. Transverse, bending and longitudinal modes are reliably identified and stiffness-related properties of the piers, deck and elastomeric bearings of the FEMs of the two bridges are estimated. Results provide qualitative and quantitative information on the dynamic behavior of the bridge systems and their components under low-amplitude vibrations. Modeling assumptions are discussed based on the differences in the characteristics identified from ambient and earthquake vibration measurements. The sources of the differences observed between the identified modal and structural characteristics of the bridges and those predicted by FEMs used for design are investigated and properly justified.  相似文献   

Identifying wildland fire ignition factors through sensitivity analysis of a neural network   总被引：6，自引：2，他引：4  

Christos Vasilakos  Kostas Kalabokidis  John Hatzopoulos  Ioannis Matsinos 《Natural Hazards》2009,50(1):125-143

Artificial neural networks (ANNs) show a significant ability to discover patterns in data that are too obscure to go through standard statistical methods. Data of natural phenomena usually exhibit significantly unpredictable non-linearity, but the robust behavior of a neural network makes it perfectly adaptable to environmental models such as a wildland fire danger rating system. These systems have been adopted by many developed countries that have invested in wildland fire prevention, and thus civil protection agencies are able to identify areas with high probabilities of fire ignition and resort to necessary actions. Since one of the drawbacks of ANNs is the interpretation of the final model in terms of the importance of variables, this article presents the results of sensitivity analysis performed in a back-propagation neural network (BPN) to distinguish the influence of each variable in a fire ignition risk scheme developed for Lesvos Island in Greece. Four different methods were utilized to evaluate the three fire danger indices developed within the above scheme; three of the methods are based on network’s weights after the training procedure (i.e., the percentage of influence—PI, the weight product—WP, and the partial derivatives—PD methods), and one is based on the logistic regression (LR) model between BPN inputs and observed outputs. Results showed that the occurrence of rainfall, the 10-h fuel moisture content, and the month of the year parameter are the most significant variables of the Fire Weather, Fire Hazard, and Fire Risk Indices, respectively. Relative humidity, elevation, and day of the week have a small contribution to fire ignitions in the study area. The PD method showed the best performance in ranking variables’ importance, while performance of the rest of the methods was influenced by the number of input parameters and the magnitude of their importance. The results can be used by local forest managers and other decision makers dealing with wildland fires to take the appropriate preventive measures by emphasizing on the important factors of fire occurrence.  相似文献   

Discussion of the Paper “Shear Modulus and Damping Ratio of Organic Soils” by P. Kallioglou,Th. Tika,G. Koninis,St. Papadopoulos,K. Pitilakis: Geotechnical and Geological Engineering,DOI 10.1007/s10706-008-9224-1     

Christos Vrettos 《Geotechnical and Geological Engineering》2009,27(4):485-487

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On the connection between the Nekhoroshev theorem and Arnold diffusion     

Christos Efthymiopoulos 《Celestial Mechanics and Dynamical Astronomy》2008,102(1-3):49-68

The analytical techniques of the Nekhoroshev theorem are used to provide estimates on the coefficient of Arnold diffusion along a particular resonance in the Hamiltonian model of Froeschlé et al. (Science 289:2108–2110, 2000). A resonant normal form is constructed by a computer program and the size of its remainder ||R _opt || at the optimal order of normalization is calculated as a function of the small parameter ${\epsilon}$ . We find that the diffusion coefficient scales as ${D \propto ||R_{opt}||^3}$ , while the size of the optimal remainder scales as ${||R_{opt}|| \propto {\rm exp}(1/\epsilon^{0.21})}$ in the range ${10^{-4} \leq \epsilon \leq 10^{-2}}$ . A comparison is made with the numerical results of Lega et al. (Physica D 182:179–187, 2003) in the same model.  相似文献