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841.
Pradip Roul Alexander Schinner Klaus Kassner 《Geotechnical and Geological Engineering》2011,29(4):597-610
This work is a contribution to the understanding of the mechanical properties of non-cohesive granular materials in the presence
of friction and a continuation of our previous work (Roul et al. 2010) on numerical investigation of the macroscopic mechanical properties of sand piles. Besides previous numerical results obtained
for sand piles that were poured from a localized source (“point source”), we here consider sand piles that were built by adopting
a “line source” or “raining procedure”. Simulations were carried out in two-dimensional systems with soft convex polygonal
particles, using the discrete element method (DEM). First, we focus on computing the macroscopic continuum quantities of the
resulting symmetric sand piles. We then show how the construction history of the sand piles affects their mechanical properties
including strain, fabric, volume fraction, and stress distributions; we also show how the latter are affected by the shape
of the particles. Finally, stress tensors are studied for asymmetric sand piles, where the particles are dropped from either
a point source or a line source. We find that the behaviour of stress distribution at the bottom of an asymmetric sand pile
is qualitatively the same as that obtained from an analytical solution by Didwania and co-workers (Proc R Soc Lond A 456:2569–2588,
2000). 相似文献
842.
Damien Sirjacobs Aida Alvera-Azcárate Alexander Barth Geneviève Lacroix YoungJe Park Bouchra Nechad Kevin Ruddick Jean-Marie Beckers 《Journal of Sea Research》2011,65(1):114-130
Optical remote sensing data is now being used systematically for marine ecosystem applications, such as the forcing of biological models and the operational detection of harmful algae blooms. However, applications are hampered by the incompleteness of imagery and by some quality problems. The Data Interpolating Empirical Orthogonal Functions methodology (DINEOF) allows calculation of missing data in geophysical datasets without requiring a priori knowledge about statistics of the full dataset and has previously been applied to SST reconstructions. This study demonstrates the reconstruction of complete space–time information for 4 years of surface chlorophyll a (CHL), total suspended matter (TSM) and sea surface temperature (SST) over the Southern North Sea (SNS) and English Channel (EC). Optimal reconstructions were obtained when synthesising the original signal into 8 modes for MERIS CHL and into 18 modes for MERIS TSM. Despite the very high proportion of missing data (70%), the variability of original signals explained by the EOF synthesis reached 93.5% for CHL and 97.2% for TSM. For the MODIS TSM dataset, 97.5% of the original variability of the signal was synthesised into 14 modes. The MODIS SST dataset could be synthesised into 13 modes explaining 98% of the input signal variability. Validation of the method is achieved for 3 dates below 2 artificial clouds, by comparing reconstructed data with excluded input information. Complete weekly and monthly averaged climatologies, suitable for use with ecosystem models, were derived from regular daily reconstructions. Error maps associated with every reconstruction were produced according to Beckers et al. (2006). Embedded in this error calculation scheme, a methodology was implemented to produce maps of outliers, allowing identification of unusual or suspicious data points compared to the global dynamics of the dataset. Various algorithm artefacts were associated with high values in the outlier maps (undetected cloud edges, haze areas, contrails, and cloud shadows). With the production of outlier maps, the data reconstruction technique becomes also a very efficient tool for quality control of optical remote sensing data and for change detection within large databases. 相似文献
843.
844.
Felipe Alexander Vargas Bazán Edison Castro Prates de Lima Marcos Queija de Siqueira Elizabeth Frauches Netto Siqueira Carlos Alberto Duarte de Lemos 《Applied Ocean Research》2011,33(4):344-365
In riser structural systems, the region where the riser is connected to the platform is critical due to the development of high stress levels. To reduce the stress concentration in this region, bend stiffeners and stress joints are used in order to provide a gradual stiffness transition between the riser and the platform. The present paper presents an optimization design approach for bend stiffeners and stress joints, using a slender beam procedure for the connection joint analysis and an evolution strategies multi-objective optimization algorithm. This approach produces similar results when compared to a finite element analysis of the complete riser, but with significant reduction of computational costs. Numerical examples are presented, showing the efficiency and robustness of the suggested methodology. 相似文献
845.
Timothy Garn David A. Green Sally E. G. Hales Julia M. Riley Paul Alexander 《Monthly notices of the Royal Astronomical Society》2007,376(3):1251-1260
Observations of the Spitzer extragalactic First Look Survey field taken at 610 MHz with the Giant Metrewave Radio Telescope are presented. Seven individual pointings were observed, covering a total area of ∼4 deg2 with a resolution of 5.8 × 4.7 arcsec2 , PA 60°. The rms noise at the centre of the pointings is between 27 and 30 μJy before correction for the Giant Metrewave Radio Telescope (GMRT) primary beam. The techniques used for data reduction and production of a mosaicked image of the region are described, and the final mosaic, along with a catalogue of 3944 sources detected above ∼5σ, are presented. The survey complements existing radio and infrared data available for this region. 相似文献
846.
The interaction of optically emitting clouds with warm X-ray gas and hot, tenuous radio plasma in radio jet cocoons is modelled by 2D compressible hydrodynamic simulations. The initial setup is the Kelvin–Helmholtz instability at a contact surface of density contrast 104 . The denser medium contains clouds of higher density. Optically thin radiation is realized via a cooling source term. The cool phase effectively extracts energy from the other gas which is both, radiated away and used for acceleration of the cold phase. This increases the system's cooling rate substantially and leads to a massively amplified cold mass dropout. We show that it is feasible, given small seed clouds of the order of 100 M⊙ , that all of the optically emitting gas in a radio jet cocoon may be produced by this mechanism on the propagation time-scale of the jet. The mass is generally distributed as T −1/2 with temperature, with a prominent peak at 14 000 K. This peak is likely to be related to the counteracting effects of shock heating and a strong rise in the cooling function. The volume filling factor of cold gas in this peak is of the order of 10−5 –10−3 and generally increases during the simulation time.
The simulations tend towards an isotropic scale-free Kolmogorov-type energy spectrum over the simulation time-scale. We find the same Mach-number density relation as Kritsuk & Norman and show that this relation may explain the velocity widths of emission lines associated with high-redshift radio galaxies, if the environmental temperature is lower, or the jet-ambient density ratio is less extreme than in their low-redshift counterparts. 相似文献
The simulations tend towards an isotropic scale-free Kolmogorov-type energy spectrum over the simulation time-scale. We find the same Mach-number density relation as Kritsuk & Norman and show that this relation may explain the velocity widths of emission lines associated with high-redshift radio galaxies, if the environmental temperature is lower, or the jet-ambient density ratio is less extreme than in their low-redshift counterparts. 相似文献
847.
848.
Abstract— We report in situ magnesium isotope measurements of 7 porphyritic magnesium‐rich (type I) chondrules, 1 aluminum‐rich chondrule, and 16 refractory inclusions (14 Ca‐Al‐rich inclusions [CAIs] and 2 amoeboid olivine aggregates [AOAs]) from the ungrouped carbonaceous chondrite Acfer 094 using a Cameca IMS 6f ion microprobe. Both AOAs and 9 CAIs show radiogenic 26Mg excesses corresponding to initial 26Al/27Al ratios between ~5 × 10?5 ~7 × 10?5 suggesting that formation of the Acfer 094 CAIs may have lasted for ~300,000 years. Four CAIs show no evidence for radiogenic 26Mg; three of these inclusions (a corundum‐rich, a grossite‐rich, and a pyroxene‐hibonite spherule CAI) are very refractory objects and show deficits in 26Mg, suggesting that they probably never contained 26Al. The fourth object without evidence for radiogenic 26Mg is an anorthite‐rich, igneous (type C) CAI that could have experienced late‐stage melting that reset its Al‐Mg systematics. Significant excesses in 26Mg were observed in two chondrules. The inferred 26Al/27Al ratios in these two chondrules are (10.3 ± 7.4) × 10?6 (6.0 ± 3.8) × 10?6 (errors are 2σ), suggesting formation 1.6+1.2‐0.6 and 2.2+0.4‐0.3 Myr after CAIs with the canonical 26Al/27Al ratio of 5 × 10?5. These age differences are consistent with the inferred age differences between CAIs and chondrules in primitive ordinary (LL3.0–LL3.1) and carbonaceous (CO3.0) chondrites. 相似文献
849.
Gordon T. Richards Scott M. Croom Scott F. Anderson Joss Bland-Hawthorn Brian J. Boyle Roberto De Propris Michael J. Drinkwater Xiaohui Fan James E. Gunn eljko Ivezi Sebastian Jester Jon Loveday Avery Meiksin Lance Miller Adam Myers Robert C. Nichol Phil J. Outram Kevin A. Pimbblet Isaac G. Roseboom Nic Ross Donald P. Schneider Tom Shanks Robert G. Sharp Chris Stoughton Michael A. Strauss Alexander S. Szalay Daniel E. Vanden Berk Donald G. York 《Monthly notices of the Royal Astronomical Society》2005,360(3):839-852
850.
We present a kinetic model of a disk of solid particles, orbiting a primary and experiencing inelastic collisions. In distinction to other collisional models that use a 2D (mass-semimajor axis) binning and perform a separate analysis of the velocity (eccentricity, inclination) evolution, we choose mass and orbital elements as independent variables of a phase space. The distribution function in this space contains full information on the combined mass, spatial, and velocity distributions of particles. A general kinetic equation for the distribution function is derived, valid for any set of orbital elements and for any collisional outcome, specified by a single kernel function. The first implementation of the model utilizes a 3D phase space (mass-semimajor axis-eccentricity) and involves averages over the inclination and all angular elements. We assume collisions to be destructive, simulate them with available material- and size-dependent scaling laws, and include collisional damping. A closed set of kinetic equations for a mass-semimajor axis-eccentricity distribution is written and transformation rules to usual mass and spatial distributions of the disk material are obtained. The kinetic “core” of our approach is generic. It is possible to add inclination as an additional phase space variable, to include cratering collisions and agglomeration, dynamical friction and viscous stirring, gravity of large perturbers, drag forces, and other effects into the model. As a specific application, we address the collisional evolution of the classical population in the Edgeworth-Kuiper belt (EKB). We run the model for different initial disk's masses and radial profiles and different impact strengths of objects. Our results for the size distribution, collisional timescales, and mass loss are in agreement with previous studies. In particular, collisional evolution is found to be most substantial in the inner part of the EKB, where the separation size between the survivors over EKB's age and fragments of earlier collisions lies between a few and several tens of km. The size distribution in the EKB is not a single Dohnanyi-type power law, reflecting the size dependence of the critical specific energy in both strength and gravity regimes. The net mass loss rate of an evolved disk is nearly constant and is dominated by disruption of larger objects. Finally, assuming an initially uniform distribution of orbital eccentricities, we show that an evolved disk contains more objects in orbits with intermediate eccentricities than in nearly circular or more eccentric orbits. This property holds for objects of any size and is explained in terms of collisional probabilities. The effect should modulate the eccentricity distribution shaped by dynamical mechanisms, such as resonances and truncation of perihelia by Neptune. 相似文献