排序方式： 共有46条查询结果，搜索用时 15 毫秒
41.
Jean-Marc Huré 《Celestial Mechanics and Dynamical Astronomy》2012,114(4):365-385
We have established the exact expression for the gravitational potential of a homogeneous polar cell??an elementary pattern used in hydrodynamical simulations of gravitating discs. This formula, which is a closed-form, works for any opening angle and radial extension of the cell. It is valid at any point in space, i.e. in the plane of the distribution (inside and outside) as well as off-plane, thereby generalizing the results reported by Durand (Electrostatique. Vol. I. Les Distributions, 1953) for the circular disc. The three components of the gravitational acceleration are given. The mathematical demonstration proceeds from the incomplete version of Durand??s formula for the potential (based on complete elliptic integrals). We determine first the potential due to the circular sector (i.e. a pie-slice sheet), and then deduce that of the polar cell (from convenient radial scaling and subtraction). As a by-product, we generate an integral theorem stating that ??the angular average of the potential of any circular sector along its tangent circle is ${\frac{2}{\pi}}$ times the value at the corner??. A few examples are presented. For numerical resolutions and cell shapes commonly used in disc simulations, we quantify the importance of curvature effects by performing a direct comparison between the potential of the polar cell and that of the Cartesian (i.e. rectangular) cell having the same mass. Edge values are found to deviate roughly like ${2 \times 10^{-3}\times N/256}$ in relative (N is the number of grid points in the radial direction), while the agreement is typically four orders of magnitude better for values at the cell??s center. We also produce a reliable approximation for the potential, valid in the cell??s plane, inside and close to the cell. Its remarkable accuracy, about ${5\times 10^{-4}\times N/256}$ in relative, is sufficient to estimate the cell??s self-acceleration. 相似文献
42.
Glacier Extent and Volume Change （1966-9000） on the Su-lo Mountain in Northeastern Tibetan Plateau,China 总被引：2，自引：2，他引：0
The topographic maps of 1：50, 000 scales, aerial photographs taken in 1966, one Landsat image taken in 1999, and SRTM data from 2000 were used to quantify the losses in area and volume of the glaciers on the Su-lo Mountain, in the northeastern Tibetan Plateau, China in the past 30 years. The total glacier area decreased from 492.9 km^2 in 1966 to 458.2 km^2 in 1999. The volume loss of the studied glaciers reached 1.4 km^3 from 1966 to 2000. This agrees with documented changes in other mountain glaciers of the whole Tibetan Plateau. 相似文献
43.
The structural, spectroscopic and phenanthrene binding characteristics were compared for humic acids (HA) extracted from two different sources: eight soils and six lake sediments. The elemental analysis revealed that HA from sediments had higher H/C, N/C, and (N + O)/C ratios compared to HA from soils, reflecting a lower degree of humification and more autochthonous organic input upon the formation of the HA for lake sediments versus soil environments. HA from soils exhibited a higher content of aromatic carbon structures than the sediment HA based on 13C NMR results. The source of HA was easily distinguished by comparing the synchronous fluorescence spectra of each HA group. The presence of a protein like fluorescence was prominent for the HA from sediment while it was minor for the HA from soil. Irrespective of the HA source, however, humification index (HIX) exhibited a common positive correlation with the aromatic content, and a negative correlation with O-alkyl carbon structures of the HA. The correlations were consistent with the general structural trends of humification processes, suggesting that HIX may serve as a source independent predictor to describe the structural information and humification degree of terrigenous HA. Aromatic carbon structures did contribute to enhancing the phenanthrene binding for both sources of HA. However, the primary structures associated with non-ideal phenanthrene binding (i.e., non-linear sorption isotherm) appear to differ by the HA source because the opposed correlations were obtained between aromaticity and the isotherm linearity for the two HA groups. Our results suggest that the HA structural function associated with specific non-linear sorption for hydrophobic organic contaminants (HOCs) may be more strongly governed by the HA source than by the apparent physico-chemical properties. 相似文献
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Jean-Marc Huré Audrey Trova Franck Hersant 《Celestial Mechanics and Dynamical Astronomy》2014,118(4):299-314
The local character of self-gravity along with the number of spatial dimensions are critical issues when computing the potential and forces inside massive systems like stars and disks. This appears from the discretisation scale where each cell of the numerical grid is a self-interacting body in itself. There is apparently no closed-form expression yet giving the potential of a three-dimensional homogeneous cylindrical or spherical cell, in contrast with the Cartesian case. By using Green’s theorem, we show that the potential integral for such polar-type 3D sectors—initially, a volume integral with singular kernel—can be converted into a regular line-integral running over the lateral contour, thereby generalising a formula already known under axial symmetry. It therefore is a step towards the obtention of another potential/density pair. The new kernel is a finite function of the cell’s shape (with the simplest form in cylindrical geometry), and mixes incomplete elliptic integrals, inverse trigonometric and hyperbolic functions. The contour integral is easy to compute; it is valid in the whole physical space, exterior and interior to the sector itself and works in fact for a wide variety of shapes of astrophysical interest (e.g. sectors of tori or flared discs). This result is suited to easily providing reference solutions, and to reconstructing potential and forces in inhomogeneous systems by superposition. The contour integrals for the 3 components of the acceleration vector are explicitely given. 相似文献
46.
Park Heon-Joon Ko Kil-Wan Song Young-Hun Song Myung-Jun Jin Seokwoo Ha Jeong-Gon Kim Dong-Soo 《Acta Geotechnica》2020,15(9):2637-2648
Acta Geotechnica - A disconnected piled raft (DPR) foundation has been introduced as an effective pile design to reduce the vertical loading experienced by the pile. The characterization of DPRs... 相似文献