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11.
In the upper Chesapeake Bay (Maryland, U.S.A.) field surveys were conducted at 18 multiple longshore sand bar sites. The multiple bar systems were found in water depths less than approximately 2 m (mean sea level), and exhibited mild bottom slopes of 0·0052 or less. The number of bars composing each system ranged from four to 17 and the spacing between the crests typically increased in the offshore direction, ranging from 12 to 70 m. Bar height also typically increased with distance offshore and ranged from 0·03 to 0·61 m. A grain size analysis of crest and trough sediment did not reveal any significant differences and the sediment was categorized as ‘fine sand’. A review of the literature data indicated that the Chesapeake Bay multiple bars possessed similar characteristics to those found in Gelding Bay (Baltic Sea); similarities in fetch, wave height and tidal range between the two bays may account for this finding. The surf-scaling parameter indicated that the multiple bar systems were extremely dissipative with regard to wave energy, and wave height appeared to be an important factor in controlling bar spacing and bar height. A multiple wave break point hypothesis was discussed as a possible mechanism for the formation of Chesapeake Bay multiple longshore bars, and limited observational evidence appeared to support such a mechanism. 相似文献
12.
The Bloomington meteorite, a 67.8 gram veined, brecciated chondrite, fell during the summer of 1938 in Bloomington, Illinois. Its olivine, orthopyroxene and metal compositions (fo69, en74 and Fe52 Ni48 respectively) and its texture identify it as a brecciated LL6 chondrite of shock facies d. Shock melt glasses occur in Bloomington as sparse melt pockets and veins in clasts and as isolated masses in the black, clast-rich matrix. The vein glasses chemically resemble bulk LL-group chondrites and thus appear to reflect total melting of the host meteorite. The melt pocket and matrix glasses, like those described previously in L-group chondrites, have more varied compositions and are typically enriched in normative plagioclase. All glasses that we analyzed in Bloomington have FeO/MgO and Na/Al ratios similar to those of LL-group chondrites, indicating that melting of this meteorite involved neither a significant change in the oxidation state of iron nor loss of sodium to a vapor phase. Bloomington is a monomict breccia whose components formed in place as a result of a single episode of shock and attendant melting. 相似文献
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14.
The finite‐element formulation and integration algorithms developed in Part I are used to analyse a number of practical problems involving unsaturated and saturated soils. The formulation and algorithms perform well for all the cases analysed, with the robustness of the latter being largely insensitive to user‐defined parameters such as the number of coarse time steps and error control tolerances. The efficiency of the algorithms, as measured by the CPU time consumed, does not depend on the number of coarse time steps, but may be influenced by the error control tolerances. Based on the analyses presented here, typical values for the error control tolerances are suggested. It is also shown that the constitutive modelling framework presented in Part I can, by adjusting one constitutive equation and one or two material parameters, be used to simulate soils that expand or collapse upon wetting. Treating the suction as a strain variable instead of a stress variable proves to be an efficient and robust way of solving suction‐dependent plastic yielding. Moreover, the concept of the constitutive stress is a particularly convenient way of handling the transition between saturation and unsaturation. Copyright © 2003 John Wiley & Sons, Ltd. 相似文献
15.
R. M. Hueckstaedt S. H. Batha M. M. Balkey N. D. Delamater J. R. Fincke R. L. Holmes N. E. Lanier G. R. Magelssen J. M. Scott J. M. Taccetti C. J. Horsfield K. W. Parker S. D. Rothman 《Astrophysics and Space Science》2005,298(1-2):255-259
Observations of the interstellar medium reveal a dynamic realm permeated by shocks. These shocks are generated on a large
range of scales by galactic rotation, supernovae, stellar winds, and other processes. Whenever a shock encounters a density
interface, Richtmyer-Meshkov instabilities may develop. Perturbations along the interface grow, leading to structure formation
and material mixing. An understanding of the evolution of Richtmyer-Meshkov instabilities is essential for understanding galactic
structure, molecular cloud morphology, and the early stages of star formation. An ongoing experimental campaign studies Richtmyer-Meshkov
mixing in a convergent, compressible, miscible plasma at the Omega laser facility. Cylindrical targets, consisting of a low
density foam core and an aluminum shell covered by an epoxy ablator, are directly driven by fifty laser beams. The aluminum
shell is machined to produce different perturbation spectra. Surface types include unperturbed (smooth), single-mode sinusoids,
multi-mode (rough), and multi-mode with particular modes accentuated (specified-rough). Experimental results are compared
to theory and numerical simulations. 相似文献
16.
We present new 1.45-1.75 μm spectra of Europa's dark non-ice material with a spectral resolution (λ/δλ) of 1200, obtained by using adaptive optics on the Keck telescope to spatially separate the spectrum of the non-ice material from that of the surrounding ice-rich regions. Despite the great increase in spectral resolution over the previous best spectra of the non-ice material, taken with Galileo's near-infrared mapping spectrometer (NIMS) with λ/δλ=66, no new fine-scale spectral structure is revealed. The smoothness of the spectra is inconsistent with available laboratory spectra of crystalline hydrated salts at Europa temperatures, but is more consistent with various combinations of non-crystalline hydrated salts and/or hydrated sulfuric acid, as have been matched to the lower-resolution NIMS spectra. 相似文献
17.
Jean-Pierre Macquart Kinwah Wu Diana C. Hannikainen Robert J. Sault David L. Jauncey 《Astrophysics and Space Science》2003,288(1-2):105-119
We compare the observational properties of the circular polarization in Galactic jet sources with that observed in AGN, and outline the constraints they place on the mechanism responsible for the circular polarization. We also discuss the implications of the time scale of polarization variations on the mechanism responsible for the circular polarization. 相似文献
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19.
A new compilation of N‐isotope and abundance data for metasedimentary rocks, and hyrdothermal micas that proxy for bulk crust, show systematic patterns. (1) δ15N values of kerogen in Precambrian cherts are more negative relative to siliciclastic counterparts, probably due to a mantle hydrothermal component. (2) There is a secular trend from average δ15N 15.3 ± 1.8‰ in Archean shales, through intermediate values in the Proterozoic, to Phanerozoic counterparts where δ15N averages +3.5‰. (3) Hydrothermal micas in metamorphic hydrothermal systems of Palaeozoic and Mesozoic age that proxy for crust have δ15N within the range of contemporaneous sedimentary rocks. (4) Hydrothermal micas track the secular trend of δ15N for kerogen from 2.7 Ga to the Phanerozoic. (5) Within Precambrian datasets δ15N does not increase with decreasing N content; accordingly, high δ15N values cannot stem either from metamorphism or form Rayleigh fractionation. (6) Previous studies show isotopic shifts during metamorphism are only +1 to +3‰ up to amphibolite facies. Values of 10–24‰ are attributed to a high δ15N Archean atmosphere, a residual signature of CI carbonaceous chondrites where δ15N is +30‰ to + 42‰. 相似文献
20.
This paper compares lead-acid batteries, sodium-sulfur batteries, solid polymer fuel cells and closed-cycle diesel engines for autonomous underwater vehicle (AUV) applications. The service is described in terms of a parametric mission and life cycle. A generic AUV is used as a basis for comparison. Power systems are evaluated by two criteria: (1) submerged endurance capability and (2) life cycle cost. This study determines categories of service for which each power system is preferred. The solid polymer fuel cell can provide greater submerged endurance than other power systems examined. For extremely long duration AUV missions, the fuel cell is the required system, indicating a possible market niche for today's fuel cell technology. Considering cost projections for each power system, the results also show that the SPFC can become cost-competitive with conventional technologies, particularly for services characterized by high levels of utilization 相似文献