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211.
New observations of fracture nucleation are presented from three triaxial compression experiments on intact samples of Westerly
granite, using Acoustic Emission (AE) monitoring. By conducting the tests under different loading conditions, the fracture
process is demonstrated for quasi-static fracture (under AE Feedback load), a slowly developing unstable fracture (loaded
at a `slow' constant strain rate of 2.5 × 10−6 /s) and an unstable fracture that develops near instantaneously (loaded at a `fast' constant strain rate of 5 × 10−5 /s). By recording a continuous ultrasonic waveform during the critical period of fracture, the entire AE catalogue can be
captured and the exact time of fracture defined. Under constant strain loading, three stages are observed: (1) An initial
nucleation or stable growth phase at a rate of ~ 1.3 mm/s, (2) a sudden increase to a constant or slowly accelerating propagation
speed of ~ 18 mm/s, and (3) unstable, accelerating propagation. In the ~ 100 ms before rupture, the high level of AE activity
(as seen on the continuous record) prevented the location of discrete AE events. A lower bound estimate of the average propagation
velocity (using the time-to-rupture and the existing fracture length) suggests values of a few m/s. However from a low gain
acoustic record, we infer that in the final few ms, the fracture propagation speed increased to 175 m/s. These results demonstrate
similarities between fracture nucleation in intact rock and the nucleation of dynamic instabilities in stick slip experiments.
It is suggested that the ability to constrain the size of an evolving fracture provides a crucial tool in further understanding
the controls on fracture nucleation. 相似文献
212.
YAO Tandong GUO Xuejun Lonnie Thompson DUAN Keqin WANG Ninglian PU Jianchen XU Baiqing YANG Xiaoxin SUN Weizhen 《中国科学D辑(英文版)》2006,49(1)
The 213 m ice core from the Puruogangri Ice Field on the Tibetan Plateau facilitates the study of the regional temperature changes with its δ18O record of the past 100 years. Here we combine information from this core with that from the Dasuopu ice core (from the southern Tibetan Plateau), the Guliya ice core (from the northwestern Plateau) and the Dunde ice core (from the northeastern Plateau) to learn about the regional differences in temperature change across the Tibetan Plateau. The δ18O changes vary with region on the Plateau, the variations being especially large between South and North and between East and West. Moreover, these four ice cores present increasing δ18O trends, indicating warming on the Tibetan Plateau over the past 100 years. A comparative study of Northern Hemisphere (NH) temperature changes, the δ18O-reflected temperature changes on the Plateau, and available meteorological records show consistent trends in overall warming during the past 100 years. 相似文献
213.
One-dimensional turbulence (ODT) is a single-column simulation in which vertical motions are represented by an unsteady advective process, rather than their customary representation by a diffusive process. No space or time averaging of mesh-resolved motions is invoked. Molecular-transport scales can be resolved in ODT simulations of laboratory-scale flows, but this resolution of these scales is prohibitively expensive in ODT simulations of the atmospheric boundary layer (ABL), except possibly in small subregions of a non-uniform mesh.Here, two methods for ODT simulation of the ABL on uniform meshes are described and applied to the GABLS (GEWEX Atmospheric Boundary Layer Study; GEWEX is the Global Energy and Water Cycle Experiment) stable boundary-layer intercomparison case. One method involves resolution of the roughness scale using a fixed eddy viscosity to represent subgrid motions. The other method, which is implemented at lower spatial resolution, involves a variable eddy viscosity determined by the local mesh-resolved flow, as in multi-dimensional large-eddy simulation (LES). When run at typical LES resolution, it reproduces some of the key high-resolution results, but its fidelity is lower in some important respects. It is concluded that a more elaborate empirically based representation of the subgrid physics, closely analogous to closures currently employed in LES of the ABL, might improve its performance substantially, yielding a cost-effective ABL simulation tool. Prospects for further application of ODT to the ABL, including possible use of ODT as a near-surface subgrid closure framework for general circulation modeling, are assessed. 相似文献
214.
δ~(18)O record and temperature change over the past 100 years in ice cores on the Tibetan Plateau 总被引:6,自引:0,他引:6
Lonnie Thompson 《中国科学D辑(英文版)》2006,(1)
1 IntroductionMeteorological stations on the Tibetan Plateau are few in number and uneven in distribution, with a majority concentrated in the east and south. No stations exist so far within the large expanse in the middle or west of the Plateau. Moreover… 相似文献
215.
Alan E. Rubin Josep M. Trigo-Rodríguez Heinz Huber 《Geochimica et cosmochimica acta》2007,71(9):2361-2382
CM chondrites are aqueously altered rocks that contain ∼9 wt% H2O+ (i.e., indigenous water) bound in phyllosilicates; also present are clumps of serpentine-tochilinite intergrowths (previously called “poorly characterized phases” or PCP), pentlandite and Ni-bearing pyrrhotite. We studied 11 CM chondrites that span the known range from least altered to most altered. We used various petrologic properties (many previously identified) that provide information regarding the degree of aqueous alteration. There are no known unaltered or slightly altered CM chondrites (e.g., rocks containing numerous chondrules with primary igneous glass). Some CM properties result from processes associated with early and intermediate stages of the alteration sequence (i.e., hydration of matrix, alteration of chondrule glass, and production of large PCP clumps). Other petrologic properties reflect processes active throughout the alteration sequence; these include oxidation of metallic Fe-Ni, alteration of chondrule phenocrysts, changes in PCP composition (reflecting an increase in the phyllosilicate/sulfide ratio), and changes in carbonate mineralogy (reflecting the development of dolomite and complex carbonates at the expense of Ca carbonate).On the basis of these parameters, we propose a numerical alteration sequence for CM chondrites. Because there are no known CM samples that display only incipient alteration, the least altered sample was arbitrarily assigned to subtype 2.6. The most altered CM chondrites, currently classified CM1, are assigned to subtype 2.0. These highly altered rocks have essentially no mafic silicates; they contain chondrule pseudomorphs composed mainly of phyllosilicate. However, their bulk compositions are CM-like, and they are closer in texture to other C2 chondrites than to CI1 chondrites (which lack chondrule pseudomorphs). Using several diagnostic criteria, we assigned petrologic subtypes (±0.1) to every CM chondrite in our study: QUE 97990, CM2.6; Murchison, CM2.5; Kivesvaara, CM2.5; Murray, CM2.4/2.5; Y 791198, CM2.4; QUE 99355, CM2.3; Nogoya, CM2.2; Cold Bokkeveld, CM2.2; QUE 93005, CM2.1; LAP 02277, CM2.0; MET 01070, CM2.0.The proposed CM numerical alteration sequence improves upon the existing scheme of Browning et al. (1996) in that it does not require a complicated algorithm applied to electron-microprobe data to determine the average matrix phyllosilicate composition. The new sequence is more comprehensive and employs petrologic subtypes that are easier to use and remember than mineralogic alteration index values.New neutron-activation analyses of QUE 97990, QUE 93005, MET 01070, Murchison and Crescent, together with literature data, confirm the compositional uniformity of the CM group; different degrees of alteration among CM chondrites do not lead to resolvable bulk compositional differences. This suggests that the textural differences among individual CM chondrites reflect progressive alteration of similar hypothetical CM3.0 starting materials in different regions of the same parent body, with minimal aqueous transport of materials over appreciable (e.g., meters) distances. 相似文献
216.
Alan E. Rubin 《Geochimica et cosmochimica acta》2007,71(9):2383-2401
Acapulcoites are modeled as having formed by shock melting CR-like carbonaceous chondrite precursors; the degree of melting of some acapulcoites was low enough to allow the preservation of 3-6 vol % relict chondrules. Shock effects in acapulcoites include veins of metallic Fe-Ni and troilite, polycrystalline kamacite, fine-grained metal-troilite assemblages, metallic Cu, and irregularly shaped troilite grains within metallic Fe-Ni. While at elevated temperatures, acapulcoites experienced appreciable reduction. Because graphite is present in some acapulcoites and lodranites, it seems likely that carbon was the principal reducing agent. Reduction is responsible for the low contents of olivine Fa (4-14 mol %) and low-Ca pyroxene Fs (3-13 mol %) in the acapulcoites, the observation that, in more than two-thirds of the acapulcoites, the Fa value is lower than the Fs value (in contrast to the case for equilibrated ordinary chondrites), the low FeO/MnO ratios in acapulcoite olivine (16-18, compared to 32-38 in equilibrated H chondrites), the relatively high modal orthopyroxene/olivine ratios (e.g., 1.7 in Monument Draw compared to 0.74 in H chondrites), and reverse zoning in some mafic silicate grains. Lodranites formed in a similar manner to acapulcoites but suffered more extensive heating, loss of plagioclase, and loss of an Fe-Ni-S melt.Acapulcoites and lodranites experienced moderate post-shock annealing, presumably resulting from burial beneath material of low thermal diffusivity. The annealing process repaired damaged olivine crystal lattices, lending acapulcoites and lodranites the appearance of unshocked (i.e., shock-stage S1) rocks. Any high-pressure phases that may have formed during initial shock reverted to their low-pressure polymorphs during annealing. Some samples were subsequently shocked again; several acapulcoites reached shock-stage S2 levels, ALH 84190 reached S3, and the lodranite MAC 88177 reached S5. 相似文献
217.
The formal stereological transformation equation for particle sieve size distribution from measurements in lower dimensional spaces is applied to laser diffractometer measurements. The transformation function for iron ore particles is measured experimentally, and modeled. The solution is tested against the measured transformation function data as well as synthetic composite distributions of the original sample. The natural size distribution of a sample taken from a grinding circuit stream was measured by a combination of standard sieving and cyclosizer, and the result is compared to the transformed size distribution calculated from laser diffractometer measurements. The stereological transformation technique performed well in all cases. 相似文献
218.
A non-linear three-dimensional unstructured grid model of the M2 tide in the shelf edge area off the west coast of Scotland is used to examine the spatial distribution of the M2 internal tide and its higher harmonics in the region. In addition, the spatial variability of the tidally induced turbulent
kinetic energy and associated mixing in the area are considered. Initial calculations involve only tidal forcing, although
subsequent calculations are performed with up-welling and down-welling favourable winds to examine how these influence the
tidal distribution (particularly the higher harmonics) and mixing in the region. Both short- and long-duration winds are used
in these calculations. Tidal calculations show that there is significant small-scale spatial variability particularly in the
higher harmonics of the internal tide in the region. In addition, turbulence energy and mixing exhibit appreciable spatial
variability in regions of rapidly changing topography, with increased mixing occurring above seamounts. Wind effects significantly
change the distribution of the M2 internal tide and its higher harmonics, with appreciable differences found between up- and down-welling winds and long- and
short-duration winds because of differences in mixing and the presence of wind-induced flows. The implications for model validation,
particularly in terms of energy transfer to higher harmonics, and mixing are briefly discussed. 相似文献
219.
A cross-sectional non-hydrostatic model with idealized topography was used to examine the processes influencing tidal mixing
in the region of sills. Initial calculations with appropriate parameters for the sill at the entrance to Loch Etive showed
that the model could reproduce the main features of the observed mixing in the region. In particular, the hydraulic jump in
the sill region was reproduced, as was an intense mid-water jet that was observed to separate from the lee side of the sill.
Shear instabilities associated with the jet appeared to be a source of mixing within the thermocline. In addition, internal
lee waves were generated on the lee side of the sill, with the observed amplification because of trapping during the flood
stage. Their magnitude and hence the mixing increased with increasing Froude number (F
r). In the case of vertically varying buoyancy frequency, its value near the sill top determined the F
r number, with its value below influencing internal waves magnitude at depth. At high F
r values particularly with strong currents, short waves and overturning occurred. 相似文献
220.
Robert Mendelsohn Alan Basist Ariel Dinar Pradeep Kurukulasuriya Claude Williams 《Climatic change》2007,81(1):85-99
This paper measures the influence of climate normals (average long-term surface wetness and temperature) and interannual climate
variance on farms in the United States and Brazil using satellite data. The paper finds that just climate normals or just
climate variance variables can explain both net revenues and how much land is used for cropland. However, because they are
correlated with each other, it is important to include both normals and variance in the same statistical model to get accurate
measures of their individual contribution to farm outcomes. In general, higher climate variance increases the probability
that land is used for cropland in both countries and higher temperatures reduce both cropland and land values. Other annual
effects were not consistent across the two countries. 相似文献