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101.
Mark L. Porter Marcel G. Schaap Dorthe Wildenschild 《Advances in water resources》2009,32(11):1632-1640
Hysteresis in the relationship between capillary pressure (Pc), wetting phase saturation (Sw) and nonwetting–wetting interfacial area per volume (anw) is investigated using multiphase lattice-Boltzmann simulations of drainage and imbibition in a glass bead porous system. In order to validate the simulations, the Pc–Sw and anw–Sw main hysteresis loops were compared to experimental data reported by Culligan et al. [Culligan KA, Wildenschild D, Christensen BS, Gray WG, Rivers ML, Tompson AB. Interfacial area measurements for unsaturated flow through porous media. Water Resour Res 2004;40:W12413]. In general, the comparison shows that the simulations are reliable and capture the important physical processes in the experimental system. Pc–Sw curves, anw–Sw curves and phase distributions (within the pores) show good agreement during drainage, but less satisfactory agreement during imbibition. Drainage and imbibition scanning curves were simulated in order to construct Pc–Sw–anw surfaces. The root mean squared error (RMSE) and mean absolute error (MAE) between drainage and imbibition surfaces was 0.10 mm−1 and 0.03 mm−1, respectively. This small difference indicates that hysteresis is virtually nonexistent in the Pc–Sw–anw relationship for the multiphase system studied here. Additionally, a surface was fit to the main loop (excluding scanning curves) of the drainage and imbibition Pc–Sw–anw data and compared to the surface fit to all of the data. The differences between these two surfaces were small (RMSE = 0.05 mm−1 and MAE = 0.01 mm−1) indicating that the Pc–Sw–anw surface is adequately represented without the need for the scanning curve data, which greatly reduces the amount of data required to construct the non-hysteretic Pc–Sw–anw surface for this data. 相似文献
102.
Anne-Désirée Schmitt Stephen J.G. Galer Wafa Abouchami 《Earth and Planetary Science Letters》2009,277(1-2):262-272
We report a survey of natural mass-dependent cadmium isotope fractionation measured by thermal ionization mass spectrometry using a double-spike technique (DS-TIMS). Over sixty samples of natural terrestrial Cd from diverse environments, including MORB, OIB, continental loess, hydrogenic and hydrothermal ferromanganese deposits, and sphalerites (both oceanic and from major continental ore deposits) were analysed. Our results are expressed in terms of ε112/110Cd, which are deviations in 112Cd/110Cd from our in-house JMC Cd standard in parts per 104. The total ε112/110Cd variation is relatively small, with a range of only 5 ε-units, and is one-to-two orders of magnitude smaller than that previously found in meteorites.The MORB, OIB and loess ε112/110Cd values are similar and provide a good estimate for the bulk silicate Earth (BSE) value which is ? 0.95 ± 0.12 relative to our Cd standard (ε112/110Cd = + 0.16 relative to Münster JMC Cd). Taken together, these data suggest little Cd isotope fractionation takes place during crust–mantle segregation. Cd isotopic compositions of continental sphalerite (ZnS) deposits worldwide and high-temperature oceanic hydrothermal sulphides show remarkably similar ε112/110Cd values, consistent with our estimate for the BSE. In contrast, mid-temperature oceanic sulphides from a single extinct hydrothermal chimney display over 4 ε-units variation — along with the most negative values. These variations are most probably caused by precipitation/redissolution of sulphide phases en route within the hydrothermal system.The ε112/110Cd variability found in worldwide marine Fe–Mn deposits reflects the seawater Cd isotope signal upon precipitation from ambient seawater. A decrease in ε112/110Cd is observed in passing from shallow-water Fe–Mn deposits to those from deeper waters (> 2000 m depth). This shift is explained by biological fractionation related to the uptake of dissolved seawater Cd by phytoplankton in the upper water column. The relatively uniform ε112/110Cd values close to zero at great depths are consistent with regeneration and remineralization of Cd at depth. Our data suggest that Cd isotopes – much like the Cd/Ca ratio in foraminifera – could potentially serve as a proxy for past changes in biological productivity. The temporal Cd isotope record in a Fe–Mn crust archive at 2000 m depth from the NE Atlantic suggests no gross long-term changes in Cd cycling took place over the past 8 Ma. 相似文献
103.
Resonantly driven Alfvén waves are studied in non-uniform stationary magnetic flux tubes. Analytic dissipative MHD solutions are obtained for the Lagrangian displacement and the Eulerian perturbation of the total pressure. These analytic solutions are valid in the dissipative layer and in the two overlap regions to the left and the right of the dissipative layer. From these analytic solutions we obtain the fundamental conservation law and the jump conditions for resonantly driven Alfvén waves in magnetic flux tubes with an equilibriun flow. The fundamental conservation law and the jump conditions depend on the equilibrium flow in a more complicated way than just a Doppler shift. The effects of an equlibrium flow are not to be predicted easily in general terms with the exception that the polarization of the driven Alfvén waves is still in the magnetic surfaces and perpendicular to the magnetic field lines as it is in a static flux tube. 相似文献
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The present paper discusses the implementation of the SGHR method (Sakurai, Goossens, and Hollweg, 1991; Goossens, Ruderman, and Hollweg, 1995) in a numerical scheme for determining resonantly driven Alfvén waves in nonuniform magnetic flux tubes. This method is based on jump conditions over the dissipative layer which are obtained from an asymptotic analysis of analytical solutions to simplified versions of the linear non-ideal MHD equations in this dissipative layer. The emphasis is on the computational simplicity and the accuracy of the method. The method derives its computational simplicity from the fact that it circumvents the numerical integration of the non-ideal MHD equations. The implementation only requires the numerical integration of the ideal MHD equations away from the resonant position. There is no need for a special integration scheme and a PC suffices as a hardware tool.The accuracy of the method is verified by means of test computations. These test computations deal with the loss of power of acoustic waves in sunspots by resonant absorption of Alfvén waves. Results for the absorption coefficients obtained with the SGHR method are compared with known results in the literature obtained by integration of the full set of the linearized non-ideal MHD equations. The agreement is excellent and identifies the SGHR method as a powerful and extremely easy to use tool for studying resonant Alfvén waves. 相似文献
107.
We have measured apparent fluctuations in stellar radial velocities with the ELODIE fiber-fed crossed-dispersion spectrograph and the 193-cm telescope of Observatoire de Haute-Provence. Within one given night, the fluctuations consist of two terms which may be sorted out. The first comes from imperfect scrambling of the stellar beam; the second arises from photon noise and agrees closely with our published calculations. So far, scrambler noise dominates for bright stars, but a perfect scrambler could be built by combining adatative optics and a single-mode fiber. The photon-noise results confirm that extrasolar planetary searching by the radial-velocity technique may be implemented with relatively small telescopes for a large number of stars. Consequences for the detection of astrophysical noise are discussed. 相似文献
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109.
Urban Community Garden Agrodiversity and Cultural Identity in Philadelphia,Pennsylvania, U.S.
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Hamil Pearsall Sheila Gachuz Marcel Rodriguez Sosa Birgit Schmook Hans van der Wal Maria Amalia Gracia 《Geographical review》2017,107(3):476-495
Considerable research has examined the social, cultural, economic, and community benefits of urban gardening. Few studies, however, have empirically assessed factors that influence urban community garden agrodiversity or its relationship to these dimensions of gardening. We conducted an interdisciplinary study of agrodiversity and cultural identity, based certain markers of identity, including how people see themselves with respect to race, ethnicity, or place of origin, in community gardens in Philadelphia, Pennsylvania. We conducted fifty‐six semistructured interviews with gardeners with different cultural identities in eight community gardens on their motivations for urban community gardening during 2014. We conducted plant inventories of the corresponding garden plots and found 104 cultivated edible and ornamental species and 28 varieties representing 34 families. We find that although gardens with culturally diverse gardeners did not have higher species richness, the cultural identity of the gardeners influenced species selection and reason for gardening. Further, the structure, design and species composition of garden plots reflected the identities of garden members. These finding have implications for the recent institutionalization of urban agriculture into city land policies in Philadelphia and other cities in North America. 相似文献
110.
In South Africa, fishery managers have struggled with a number of challenges in terms of compliance amidst a process of legal and policy reform since the democratic elections of 1994. The past 10 years, and more specifically the past five, have seen a significant change in compliance effort, with a particular focus on increasing law enforcement activities and capacity. This paper highlights two case studies that have been instrumental in restructuring fisheries compliance in South Africa. Although law enforcement has been significantly strengthened, greater political will and government commitment is required to implement alternative strategies that will have a long-term impact on levels of compliance. 相似文献