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211.
By equipping structures with appropriate actuators, sensors and microprocessors, it is possible to suppress actively the undesirable vibrations of the structures. Due to small energy requirements, the vibration suppression in large space trusses orbiting the Earth by controlling the elongations and contractions of length-adjustable bars of the truss has been shown to be feasible both theoretically and experimentally. This method of vibration suppression is part of the ‘adaptive structures technology’. It can be used for suppressing the vibrations of a building subjected to earthquake or wind excitations, provided that the much higher energy and power requirements are met. In this work the use of gravitational energy of the mass of the building is proposed for active vibration control. 相似文献
212.
Issam Mouayn Ben Aissa Tadili Lahsen Aït Brahim Mohamed Ramdani Mohamed Limouri Nacer Jabour 《Pure and Applied Geophysics》2004,161(5-6):1061-1080
— The first empirical duration magnitude (MD) formula is developed and tested for the Northern Morocco Seismic Network (NMSNET). This relationship is obtained by relating the IGN (Instituto Geografico National, Madrid) body-waves mbLgIGN to the duration (), and the epicentral distance (), at 25 analogue stations of the NMSNET for 479 earthquakes with 2.5 mb 5.4, from March 1992 to February 2001. MD estimates are significantly more precise while introducing a correction term for each of these stations, cStaj. The magnitude for the ith event (MD)i is the mean value of individual MDij=–0.14+1.63log10(ij)+0.031(ij)+cStaj. The cStaj corrections reduce considerably the local site effects which influence the recorded durations and cause stations to either overestimate, or underestimate MD up to 0.5 magnitude units. Average station MD residuals (–cStaj) are found to be independent of the distance from the epicenter to at least 10 degrees. It seems evident that regional geological features in the immediate behavior of stations have a systematic effect on the corresponding obtained residuals: older well-consolidated Precambrian crystalline rocks produce high negative residuals (shorter durations), younger unconsolidated sediments produce high positive residuals (longer durations), whereas, intermediate MD site residuals appear to be the result of the effect of various factors, principally age and state of consolidation of the bedrock, combined with the local tectonic. 相似文献
213.
Jielun Sun Donald H. Lenschow Sean P. Burns Robert M. Banta Rob K. Newsom Richard Coulter Stephen Frasier Turker Ince Carmen Nappo Ben B. Balsley Michael Jensen Larry Mahrt David Miller Brian Skelly 《Boundary-Layer Meteorology》2004,110(2):255-279
Using the unprecedented observational facilities deployed duringthe 1999 Cooperative Atmosphere-Surface Exchange Study (CASES-99),we found three distinct turbulent events on the night of 18October 1999. These events resulted from a density current,solitary wave, and internal gravity wave, respectively. Our studyfocuses on the turbulence intermittency generated by the solitarywave and internal gravity wave, and intermittent turbulenceepisodes associated with pressure change and wind direction shiftsadjacent to the ground. Both the solitary and internal gravitywaves propagated horizontally and downward. During the passage ofboth the solitary and internal gravity waves, local thermal andshear instabilities were generated as cold air was pushed abovewarm air and wind gusts reached to the ground. These thermal andshear instabilities triggered turbulent mixing events. Inaddition, strong vertical acceleration associated with thesolitary wave led to large non-hydrostatic pressure perturbationsthat were positively correlated with temperature. The directionaldifference between the propagation of the internal gravity waveand the ambient flow led to lateral rolls. These episodic studiesdemonstrate that non-local disturbances are responsible for localthermal and shear instabilities, leading to intermittentturbulence in nocturnal boundary layers. The origin of thesenon-local disturbances needs to be understood to improve mesoscalenumerical model performance. 相似文献
214.
215.
Quantifying Damage, Saturation and Anisotropy in Cracked Rocks by Inverting Elastic Wave Velocities 总被引:1,自引:0,他引:1
Alexandre Schubnel Philip M. Benson Ben D. Thompson Jim F. Hazzard R. Paul Young 《Pure and Applied Geophysics》2006,163(5-6):947-973
Crack damage results in a decrease of elastic wave velocities and in the development of anisotropy. Using non-interactive
crack effective medium theory as a fundamental tool, we calculate dry and wet elastic properties of cracked rocks in terms
of a crack density tensor, average crack aspect ratio and mean crack fabric orientation from the solid grains and fluid elastic
properties. Using this same tool, we show that both the anisotropy and shear-wave splitting of elastic waves can be derived.
Two simple crack distributions are considered for which the predicted anisotropy depends strongly on the saturation, reaching
up to 60% in the dry case. Comparison with experimental data on two granites, a basalt and a marble, shows that the range
of validity of the non-interactive effective medium theory model extends to a total crack density of approximately 0.5, considering
symmetries up to orthorhombic. In the isotropic case, Kachanov's (1994) non-interactive effective medium model was used in
order to invert elastic wave velocities and infer both crack density and aspect ratio evolutions. Inversions are stable and
give coherent results in terms of crack density and aperture evolution. Crack density variations can be interpreted in terms
of crack growth and/or changes of the crack surface contact areas as cracks are being closed or opened respectively. More
importantly, the recovered evolution of aspect ratio shows an exponentially decreasing aspect ratio (and therefore aperture)
with pressure, which has broader geophysical implications, in particular on fluid flow. The recovered evolution of aspect
ratio is also consistent with current mechanical theories of crack closure. In the anisotropic cases—both transverse isotropic
and orthorhombic symmetries were considered—anisotropy and saturation patterns were well reproduced by the modelling, and
mean crack fabric orientations we recovered are consistent with in situ geophysical imaging.
Our results point out that: (1) It is possible to predict damage, anisotropy and saturation in terms of a crack density tensor
and mean crack aspect ratio and orientation; (2) using well constrained wave velocity data, it is possible to extrapolate
the contemporaneous evolution of crack density, anisotropy and saturation using wave velocity inversion as a tool; 3) using
such an inversion tool opens the door in linking elastic properties, variations to permeability. 相似文献
216.
Background
Anthropogenic CO2 uptake by the ocean decreases the pH of seawater, leading to an 'acidification' which may have potential detrimental consequences on marine organisms [1]. Ocean warming or circulation alterations induced by climate change has the potential to slowdown the rate of acidification of ocean waters by decreasing the amount of CO2 uptake by the ocean [2]. However, a recent study showed that climate change affected the decrease in pH insignificantly [3]. Here, we examine the sensitivity of future oceanic acidification to climate change feedbacks within a coupled atmosphere-ocean model and find that ocean warming dominates the climate change feedbacks. 相似文献217.
Volume Contents
Contents of Volume 54 相似文献218.
A dynamic scheme for water movements over a flood in a small watershed was proposed in a previous paper [Ben Othman, D., Luck, J.M., Tournoud, M.G., 1997. Geochemistry and water dynamics: application to short-time scale flood phenomena in a small Mediterranean catchment: I. Alkalis, alkali-earths and Sr isotopes. Chem. Geol. [140] 9–28] based on major, alkali–alkali earth trace elements and Sr isotopes. In the present paper, metal contents and Pb isotope data are reported for the same samples, and compared to natural and anthropogenic local sources analysed for this purpose. Water treatment plants have stable
but show relatively large
variations, presumably related to yet unidentified (industrial?) Pb sources. Some Cu–Zn-rich chemicals used extensively on vineyards have unradiogenic
around 17.7. Local rains sampled over two years show roughly similar low values. Pb from rocks is variable and more radiogenic (
≈18.8–22.8). Except for the first hours, trace element concentrations in the dissolved load are similar to or slightly higher than those observed over the year, and similar to other moderately anthropogenic areas [Shiller, A.M., Boyle, E.A., 1987. Variability of dissolved trace metals in the Mississippi river. Geochim. Cosmochim. Acta [51] 3273–3277] for [Pb] (0.05–0.45 nM), [Zn] (10–75 nM) and [Cd] (0.03–0.18 nM). Dissolved [U] and [Co] show simple variations correlated to carbonate, related to local natural sources. Most trace elements in the particulate load are strongly correlated. Lead isotopes in the dissolved and particulate loads show ranges over the flood, again similar to those observed over the year (
≈17.9–18.3), implying the same sources. The very good alignments observed in
and
vs.
diagrams, especially for the particulate, are interpreted as mixing phenomena. Generally Pb isotopic signature of the dissolved load is less radiogenic than the particulate, indicating differences in sources or proportions and absence of isotopic equilibration with respect to the time of transfer. Pb isotopes shift regularly with time away from the road runoff endmember towards more radiogenic values. The strong negative correlation between Pb isotopes in the particulate load and Sr isotopes in the dissolved load, observed for the first time in a small watershed, probably reflects the local coupling between mechanical and chemical erosion, respectively. 相似文献
219.
David Ward Ben T. Ngairorue Johannes Kathena Rana Samuels Yanay Ofran 《Journal of Arid Environments》1998,40(4):357-371
In Otjimbingwe, a region of arid south-central Namibia, human population densities are high and these communal pastoralists own large numbers of livestock. Such situations are commonly perceived to lead to the ‘tragedy of the commons'. This region lends itself to a comparison of the effects of communal and commercial farming (with private land ownership) because the communal area is completely surrounded by commercial farms. In spite of far higher stocking densities on the communal areas and the absence of an overall grazing strategy, we found no evidence of the ‘tragedy of the commons' on Otjimbingwe. Indeed, the communal areas did not differ in a number of soil and vegetation parameters from the commercial farms. These results point both to the resilience of arid environments to high stocking levels and the over-riding influence of abiotic variables on environmental quality. 相似文献
220.
Ben J. Kefford 《International Journal of Salt Lake Research》1998,7(4):285-299
The effect of saline lake water disposal from the Lough Calvert Drainage Scheme on water quality of the Barwon River, in south
west Victoria, Australia, was investigated. The Scheme is used to regulate the levels of several saline lakes outside the
Barwon's catchment. This study found that the disposal of saline lake water was associated with increased total phosphorus,
PO4, TKN, suspended solids, electrical conductivity and stream discharge and lower NOx in the Barwon River. Thus, when disposing
of saline water, factors other than salinity should be considered in order to prevent an impact on the aquatic environment.
At present this is generally not done. The results are discussed with reference to the effects of saline water disposal on
aquatic biota and how the effect of saline lake water disposal on water quality may differ from the disposal of saline groundwater. 相似文献