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The effect of finite conductivity on the Rayleigh-Taylor instability of an incompressible, viscous rotating fluid through a porous medium has been studied in the presence of a two-dimensional horizontal magnetic field. It has been shown that the solution is characterized by a variational principle. By making use of the existence of the variational principle, proper solutions have been obtained for a semi-infinite fluid in which density has a one-dimensional (exponential) vertical stratification. The dispersion relation has been derived and solved numerically. It is found that finite resistivity and porosity have a destabilizing effect on the Rayleigh-Taylor instability while rotation has a stabilizing effect. 相似文献
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Pradeep Srivastava Indra Bir Singh Shikha Sharma Uma Kant Shukla Ashok Kumar Singhvi 《Geomorphology》2003,54(3-4):279-292
Abandoned channel belts, ponds and point bar deposits of palaeochannels in the interfluve regions of the central Ganga Plain suggest changes in the morphohydrologic conditions during the Latest Pleistocene–Holocene. Stratigraphy of these ponds comprises channel sand at the base overlain by shell-bearing clayey silt. The contact of the two facies marks the phase when channels converted into standing water bodies. Point bar deposits of some palaeochannels are overlain by oxidised aeolian sand, indicating that the channel abandonment possibly occurred due to the desiccation and aridity in the region.Optically stimulated luminescence (OSL) chronometry of the pond sediments suggests that the deposition of the basal channel sand started before 13 ka and continued up to 8 ka. The ponds formed around 8–6 ka when the channel activity ceased. Evidence from the point bar deposits also indicates that the fluvial activity in the region ended sometime during 7–5 ka. This was followed by aeolian aggradation. The present study thus suggests that the hydrologic conditions in the Gangetic plains, i.e. initiation of channels and their abandonment, formation of microgeomorphologic features such as ponds and their eventual siltation, were controlled largely by climatic changes (i.e. monsoon changes) supported by tectonic activity. For the past 2 ka, increasing human and related agricultural activity has substantially accentuated the natural siltation rate of ponds. 相似文献
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The mechanism and kinetics of superoxide-mediated reduction of a variety of organic iron(III) complexes has been investigated over the pH range 7-9. Our experimental results show that the rate of iron(II) formation is a function of pH, ligand type and ligand concentration with the measured rate varying between 0.44 ± 0.07 and 39.25 ± 1.77 pM s−1 in the systems investigated. Additionally, our results show that the presence of competing cations such as Ca2+ have a significant impact on iron(II) formation if the organic ligand is strongly complexed by Ca2+. Formation of iron(II) occurs by either (or, in some instances, both) reaction of superoxide with inorganic iron(III) after its dissociation from the complex (dissociative reduction) or by direct reaction of superoxide with the complex (non-dissociative reduction). In the presence of weak ligands, dissociative reduction (DR) dominates; however non-dissociative reduction (NDR) becomes important in the presence of either strongly binding ligands or high concentrations of weakly binding ligands. The major factors contributing to the pH dependence of the iron(II) formation rate are the complexation kinetics of inorganic iron(III) (which controls the DR contribution) and the reduction kinetics of the iron(III) complex (which controls the NDR contribution). The relative NDR contribution increases with increasing superoxide and ligand concentration and decreasing pH for all ligands examined. Since iron(II) formation occurring via NDR results in a significantly larger increase in the proportion of iron in free aquated form than does DR, this non-dissociative pathway of superoxide-mediated iron(III) reduction is particularly effective in increasing the lability of iron in aquatic systems. 相似文献
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In recent years, geotechnical engineers are using tyre waste to alter the properties of soil for providing sustainable solutions to complex engineering problems. It is noted that black cotton soil, which is expansive in nature, covers a major portion of the Indian sub-continent, such as the Deccan plateau, Malwa plateau and a portion of Gujarat and poses challenging problems to infrastructural development in the region. In the present study, authors have characterised the geotechnical properties of black cotton soil, which is partially replaced with 4.75 mm passing – 2 mm retained (coarse fraction) and 2.0 mm passing - 75 micron retained (fine fraction) shredded tyre waste. The results of the experimental studies clearly show that partial replacement of black cotton soil with shredded tyre waste alters the geotechnical properties favourable to engineering applications and greatly reduces the swelling potential of the black cotton soil. It is noted that instead of fine fraction (2.0–0.075 mm), replacement with coarse fraction (4.75–2.00 mm) of shredded tyre waste can better be used for light weight fill behind the retaining walls as well as for embankment construction as it helps in reducing swelling pressure as well as improving the strength properties. 相似文献
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The effect of solar and interplanetary disturbances on geomagnetospheric conditions leading to 121 moderate geomagnetic storms (MGS) have been investigated using the neutron monitor, solar geophysical and interplanetary data during the period 1978–99. Further, the duration of recovery phase has been observed to be greater than the duration of main phase in most of the cases of MGS. It has further been noted that Ap-index increases on sudden storm commencement (SSC) day than its previous day value and acquires maximum value on the day of maximum solar activity. Generally, the decrease in cosmic ray (CR) intensity and Dst begins few hours earlier than the occurrence of MGS at Earth. Furthermore, negative Bz pointing southward plays a key causal role in the occurrence of MGS and the magnitude and the duration of Bz and Bav also play a significant role in the development of MGS. The solar features Hα, X-ray solar flares and active prominences and disappearing filaments (APDFs) which have occurred within lower helio-latitudinal/helio-longitudinal zones produce larger number of MGS. Solar flares seem to be the major cause for producing MGS. 相似文献
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Shikha Sharma Michon L. Mulder Andrea Sack Karl Schroeder Richard Hammack 《Ground water》2014,52(3):424-433
Water and gas samples were collected from (1) nine shallow groundwater aquifers overlying Marcellus Shale in north‐central West Virginia before active shale gas drilling, (2) wells producing gas from Upper Devonian sands and Middle Devonian Marcellus Shale in southwestern Pennsylvania, (3) coal‐mine water discharges in southwestern Pennsylvania, and (4) streams in southwestern Pennsylvania and north‐central West Virginia. Our preliminary results demonstrate that the oxygen and hydrogen isotope composition of water, carbon isotope composition of dissolved inorganic carbon, and carbon and hydrogen isotope compositions of methane in Upper Devonian sands and Marcellus Shale are very different compared with shallow groundwater aquifers, coal‐mine waters, and stream waters of the region. Therefore, spatiotemporal stable isotope monitoring of the different sources of water before, during, and after hydraulic fracturing can be used to identify migrations of fluids and gas from deep formations that are coincident with shale gas drilling. 相似文献
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Jonathan S. Friedman Craig A. Tepley Shikha Raizada Qihou H. Zhou Jonas Hedin Rubn Delgado 《Journal of Atmospheric and Solar》2003,65(16-18):1411-1424
We have developed a lidar to study the temperature structure of the nighttime mesopause region over the Arecibo Observatory (18.35°N, 66.75°W) by measuring the lineshape of the fluorescence spectrum of atomic potassium that is deposited in the mesosphere and lower thermosphere (MLT) by meteors. To demonstrate how the potassium lidar can enhance MLT studies at Arecibo, we show recent results for: (1) comparisons with airglow temperature measurements; (2) simultaneous operations with stratospheric and mesospheric temperature profiling by Rayleigh lidar; (3) simultaneous observations of K, Ca+, and E-region electron density profiles; and (4) occurrences of sporadic K layers, and relationships to sporadic E layers. 相似文献
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Shikha Sharma Michael M Joachimski Indra B Singh Rakesh Tewari 《Quaternary Research》2004,62(1):19-28
Oxygen isotope analysis was performed on enamel phosphate of mammalian teeth from archaeological sites Kalli Pachchhim and Dadupur in the central Ganga plain and Charda in the northern Ganga plain. The bulk oxygen isotopic compositions of enamel phosphate from third molars (M3) of Bos indicus individuals belonging to different cultural periods were used to understand the climatic changes during the past 3600 cal yr B.P. Oxygen isotope ratios indicate humid conditions around 3600 cal yr B.P., followed by a trend toward drier conditions until around 2800 cal yr B.P. Then from 2500 to 1500 cal yr B.P. there is a trend toward higher humidity, followed by the onset of a dry period around 1300 cal yr B.P. The study of intratooth δ18O variations in teeth from different periods demonstrates that the monsoon seasonality was prominent. Spatial changes in the amount of annual rainfall are also reflected in the δ18O values. Teeth derived from areas with intense rainfall have lighter isotope ratios compared to teeth from regions receiving less rain, but they show similar seasonal patterns. The long-term paleoclimatic variations reflected by fluctuations in bulk δ18Op values from M3 teeth match well with the regional paleoenvironmental records and show a good correlation to the cultural changes that took place during this time span in Ganga plain. 相似文献