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211.
A dynamical model of lava flows cooling by radiation 总被引:1,自引:0,他引:1
Michele Dragoni 《Bulletin of Volcanology》1989,51(2):88-95
The behaviour of a lava flow is reproduced by a two-dimensional model of a Bingham liquid flowing down a uniform slope. Such a liquid is described by two rheological parameters, yield stress and viscosity, both of which are strongly temperature-dependent. Assuming a flow rate and an initial temperature of the liquid at the eruption vent, the temperature decrease due to heat radiation and the consequent change in the rheological parameters are computed along the flow. Both full thermal mixing and thermal unmixing are considered. The equations of motion are solved analytically in the approximation of a slow downslope change of the flow parameters. Flow height and velocity are obtained as functions of the distance from the eruption vent; the time required for a liquid element to reach a certain distance from the vent is also computed. The gross features of observed lava flows are reproduced by the model which allows us to estimate the sensitivity of flow dynamics to changes in the initial conditions, ground slope and rheological parameters. A pronounced increase in the rate of height increase and velocity decrease is found when the flow enters the Bingham regime. The results confirm the observation according to which lava flows show an initial rapid advance, followed by a marked deceleration, while the final length of a flow is such that the Graetz number is in the order of a few hundreds. 相似文献
212.
Michele Caputo 《Tectonophysics》1983,91(1-2)
It is found that the results of the experiments made with constant strain-rate give also the creep functions. This is determined with three different assumptions on the creep modelling function, and it is seen that, to find the long-term reaction of the material, experiments with duration longer than that of these already carried out are needed. It is also found that the classic stress-strain relations representing the anelasticity of materials imply stress hardening and possibly fatigue; a method to estimate the number of cycles which could give fatigue is suggested.As an application the creep of polycrystalline halite is obtained finding also its temperature dependence and the relation between the activation energy and the temperature. 相似文献
213.
Calcium-olivine, γ-Ca2SiO4, larnite, β-Ca2SiO4, merwinite, Ca3Mg(SiO4)2, and monticellite, CaMgSiO4, are considered. According to a rigid oxyanion scheme, eulerian orientation angles of the SiO4 tetrahedra and translation coordinates of Ca and Si atoms are specified as structural variables τk. All derivatives of the static energy (Born model) contain atomic charges and repulsive parameters as unknowns; the minimum energy conditions ?E L/?τk=0 yield 34 equations which are solved by a least-squares method. The set of energy parameters fitting structural properties of all four phases together is: z Ca=1.50, z o=?1.10 e, r Ca=1.05, ρ=0.25 Å; the Mg charge was fixed at 1.38 e, from a previous study on forsterite. An average shift of 0.04 Å is observed between experimental and least-energy calculated atomic positions. Results are compared with those of Mg2SiO4, where the fit was based both on thermoelastic and on structural properties. If no charge values were fixed “a priori”, just ratios between charges could be determined by fitting them to structural data only. 相似文献
214.
The authors present a statistical procedure to estimate the probability distributions of storm characteristics. The approach uses recent advances in stochastic hydrological modeling. The temporal dynamics of rainfall are modeled via a reward alternating renewal process that describes wet and dry phases of storms. In particular, the wet phase is modeled as a rectangular pulse process with dependent random duration and intensity; the global dependence structure is described using multidimensional copulas. The marginal distributions are described by Generalized Pareto laws. The authors derive both the storm volume statistics and the rainfall volume distribution within a fixed temporal window preceding a storm. Based on these results, they calculate the antecedent moisture conditions. The paper includes a thorough discussion of the validity of the assumptions and approximations introduced, and an application to actual rainfall data. The models presented here have important implications for improved design procedures of water resources and hydrologic systems. 相似文献
215.
In this paper, we argue that a feminist geographical analysis that examines women as active agents in their daily lives, pays attention to the multiplicity of women's experiences in relational space, and values knowledge for transformative purposes, provides insights as to how GIScience might develop in the near future. We draw upon our research with two different community organizations in North Philadelphia to show how a feminist geographical analysis can shape the conceptualization of a community GIS. We argue that collaborative work with community organizations based on “a view from below” necessitates alternative institutional arrangements while providing rich data to better understand the intersection of daily life and information and communication technologies (ICT) as experienced, in the particular case of our work, by poor women. Our research illustrates that understanding ICT frameworks from the perspective of women and in the context of their daily lives has important implications for GIScience. 相似文献
216.
Etienne Balan Michele Lazzeri Simon Delattre Merlin Méheut Keith Refson Bjoern Winkler 《Physics and Chemistry of Minerals》2007,34(9):621-625
The theoretical anharmonicity of the inner-OH stretching modes of kaolinite and lizardite is determined using a frozen-phonon
approach. The anharmonicity parameter, defined as the difference between half the first overtone frequency and the frequency
of the fundamental transition, is −95 and −92 cm−1 for kaolinite and lizardite, respectively. These values are consistent with the experimental measurements. The very good
agreement usually observed between harmonic calculations of vibrational frequencies at the density functional theory (DFT)
level, using the generalized gradient approximation (GGA), and experimental positions of vibrational bands results from the
fortuitous, but almost exact, cancelation of two types of errors. One is related to the GGA approximation and the other to
the neglect of anharmonicity. 相似文献
217.
218.
Alessandro Ellero Alessandro Malasoma Michele Marroni Luca Pandolfi Franco Urbani 《Island Arc》2007,16(1):105-123
Abstract The southern margin of the Caribbean Plate is well exposed in the Cordillera de la Costa of northern Venezuela, where amalgamated terranes consisting of continental and oceanic units occur. In the Cordillera de la Costa, metamorphosed oceanic units crop out along the coast near Caracas. Among them, the Tacagua unit is characterized by metaserpentinites and metabasites showing mid-oceanic ridge basalt geochemical affinity. These lithologies, representative of a disrupted ophiolite sequence, are associated with metasediments consisting of calcschists alternating with pelitic and psammitic schists, whose protoliths were probably represented by deep-sea hemipelagic and turbiditic deposits. In the Tacagua unit, a polyphase deformation history has been reconstructed, consisting of four folding phases from D1 to D4 . Geological setting suggests an involvement of the Tacagua unit in the processes connected with a subduction zone. The following deformations (from D2 to D4 ) observed in the field might be related to the exhumation history of the Tacagua unit. The late deformation history consists of an alternation of deformation phases characterized by displacement parallel ( D2 and D4 phases) and normal ( D3 phase) to plate boundary between the Caribbean and South America Plates. All lines of geological evidence suggest that the whole evolution of the Tacagua unit was acquired in a setting dominated by oblique convergence, in which alternation of strike-slip and pure compressional or pure extensional tectonics occurred through time. 相似文献
219.
The potential of correlating high-resolution Synthetic Aperture Radar images to estimate local water-level variations is analysed based on observing the motions of floating structures. By computing range and azimuth offsets on a COSMO-SkyMed multi-temporal data-set in an area of the French Atlantic Coast, displacement signatures were detected in several harbours of the region. They can be associated with specific floating structures, whose vertical motion is mostly driven by tides. Although the image resolution is not the best achievable for the COSMO-SkyMed sensor, comparison with ground-based tide information revealed that the obtained space-borne measurements are consistent with the tide gauges to a decimetric accuracy. Even if the requirements for traditional users of tidal data are not met by the current revisit time of the COSMO-SkyMed constellation, the method proposed here provides complementary measures, which are already well aligned to requirements of users concerned with long-term sea-level variation and river or lake water stocks. 相似文献
220.
Renato Gerdol Lisa Brancaleoni Lorenzo Lastrucci Giovanni Nobili Mauro Pellizzari Michele Ravaglioli Daniele Viciani 《Estuaries and Coasts》2018,41(7):2079-2091
Wetlands are important centers of biodiversity. Coastal wetlands are subject to anthropogenic threats that can lead to biodiversity loss and consequent negative effects on nature conservation. We investigated relationships between wetland vegetation and habitat conditions in a coastal Nature Reserve in Northern Italy that has undergone seawater intrusion and eutrophication for several decades. The wetland vegetation in the Nature Reserve consisted of nine communities of hygrophytic and helophytic vegetation and five communities of waterplant vegetation. The hygrophytic and helophytic communities were arranged according to a salinity gradient, from salt-free habitats to strongly saline habitats. The saline habitats had high nutrient levels, due to the influx of nitrate-rich saltwater from an adjacent lagoon. The waterplant communities were all typical of freshwater habitats. Water-table depth and concentration of dissolved nutrients in the water were the main factors structuring waterplant vegetation. The main driver of future changes in the wetland vegetation of the Nature Reserve is the ongoing increase in salinity levels which may enhance expansion of halophilic species and communities, thus outcompeting locally rare freshwater species. If nutrient, especially nitrate, load further increases in the next future, this may exert negative effects on wetland species and communities preferring nutrient-poor habitats. 相似文献