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161.
Abstract The breakdown and separation or reattachment of boundary layers adjoining a mainstream are studied in the three related situations (i)-(iii) of the title. For (i) the classical steady boundary layer generally admits a logarithmic singularity in the displacement when breakdown occurs on a downstream-moving surface whereas the corresponding singularity for an upstream-moving surface can be logarithmic or of minus-one-sixth form. Conversely, the breakdown can be delayed to the onset of zero mainstream flow, in which case the displacement singularity is again logarithmic. In certain flows these singularities prove to be removable locally, yielding a breakaway separation or reattachment and including the first known successes of a classical strategy in describing large-scale separation. Other flows, by contrast, require an interactive strategy. Again, even on a fixed surface a breakdown different from Goldstein's can be produced if there is a moving section of surface further upstream. The application to (ii), semi-similar unsteady boundary layers, e.g. near an impulsively started wedge-like trailing edge, then follows readily and predicts analogous forms of singularity. The corresponding singularity in displacement predicted for fully unsteady classical boundary layers, (iii), occurs within a finite time and, like (i) (usually) and (ii), a three-tiered breakdown is involved at first. Subsequently interaction comes into play. Comparisons with numerical and/or earlier work are noted. In all three situations (i)-(iii), although the dynamics involved near breakdown, separation or reattachment are predominantly inviscid, the presence of small viscosity is of significance in enforcing smoothness of the local velocity profiles. 相似文献
162.
Agricultural and forested land use impacts on floodplain shallow groundwater temperature regime 下载免费PDF全文
Given the importance of groundwater temperature to the biogeochemical health of aquatic ecosystems, a floodplain study was implemented to improve understanding of rural land use impacts on shallow groundwater (SGW) temperature. Study sites included a historic agricultural field (Ag) and bottomland hardwood forest (BHF), each with nine piezometers in an 80 × 80 m grid. Piezometers were equipped with pressure transducers to monitor SGW temperature and level at 30 min intervals during the 2011, 2012, 2013, and 2014 water years. The study is one of the first to utilize long‐term, continuous, automated, in situ monitoring to investigate rural land use impacts on shallow groundwater temperatures. Average SGW temperature during the study period was 11.1 and 11.2 °C at the Ag and BHF sites, respectively. However, temperature range at the Ag site was 72% greater than at the BHF site. Results indicate a greater responsiveness to seasonal climate fluctuations in Ag site SGW temperature related to absence of forest canopy. Patterns of intra‐site groundwater temperature differences at both study sites illustrate the influence of stream–aquifer thermal conduction and occasional baseflow reversals. Considering similar surface soil temperature amplitudes and low average groundwater flow values at both sites, results suggest that contrasting rates of plant water use, groundwater recharge, and subsurface hydraulic conductivity are likely mechanistic causes for the observed SGW temperature differences. Results highlight the long‐term impact of forest removal on subsurface hydrology and groundwater temperature regime. Copyright © 2015 John Wiley & Sons, Ltd. 相似文献
163.
In this study, we evaluate four different parameterizations of the turbulent Prandtl (Schmidt) number Prt = νt/Γt where νt is the eddy viscosity and Γt is the scalar eddy diffusivity, for stably stratified flows. All four formulations of Prt are strictly functions of the gradient Richardson number Ri, which provides a measure of the strength of the stratification. A zero-equation (i.e. no extra transport equations are required) turbulence model for νt in a one-dimensional, turbulent channel flow is considered to evaluate the behavior of the different formulations of Prt. Both uni-directional and oscillatory flows are considered to simulate conditions representative of practical flow problems such as atmospheric boundary layer flows and tidally driven estuarine flows, to quantify the behavior of each of the four formulations of Prt. We perform model-to-model comparisons to highlight which of the models of Prt allow for a higher rate of turbulent mixing and which models significantly inhibit turbulent mixing in the presence of buoyancy forces resulting from linear (continuous) stratification as well as two-layer stratification. The basis underlying the formulation of each model in conjunction with the simulation results are used to emphasize the considerable variability in the different formulations and the importance of choosing an appropriate parameterization of Prt given a model for νt in stably stratified flows. 相似文献
164.
Olenin S Elliott M Bysveen I Culverhouse PF Daunys D Dubelaar GB Gollasch S Goulletquer P Jelmert A Kantor Y Mézeth KB Minchin D Occhipinti-Ambrogi A Olenina I Vandekerkhove J 《Marine pollution bulletin》2011,62(12):2598-2604
Adverse effects of invasive alien species (IAS), or biological pollution, is an increasing problem in marine coastal waters, which remains high on the environmental management agenda. All maritime countries need to assess the size of this problem and consider effective mechanisms to prevent introductions, and if necessary and where possible to monitor, contain, control or eradicate the introduced impacting organisms. Despite this, and in contrast to more enclosed water bodies, the openness of marine systems indicates that once species are in an area then eradication is usually impossible. Most institutions in countries are aware of the problem and have sufficient governance in place for management. However, there is still a general lack of commitment and concerted action plans are needed to address this problem. This paper provides recommendations resulting from an international workshop based upon a large amount of experience relating to the assessment and control of biopollution. 相似文献
165.
Management of the marine environment: integrating ecosystem services and societal benefits with the DPSIR framework in a systems approach 总被引:4,自引:0,他引:4
Ever increasing and diverse use of the marine environment is leading to human-induced changes in marine life, habitats and landscapes, making necessary the development of marine policy that considers all members of the user community and addresses current, multiple, interacting uses. Taking a systems approach incorporating an understanding of The Ecosystem Approach, we integrate the DPSIR framework with ecosystem services and societal benefits, and the focus this gives allows us to create a specific framework for supporting decision making in the marine environment. Based on a linking of these three concepts, we present a set of basic postulates for the management of the marine environment and emphasise that these postulates should hold for marine management to be achieved. We illustrate these concepts using two case studies: the management of marine aggregates extraction in UK waters and the management of marine biodiversity at Flamborough Head, UK. 相似文献
166.
Philip A.E. Pogge von Strandmann Tim Elliott Chris Coath Alistair B. Jeffcoate 《Geochimica et cosmochimica acta》2011,75(18):5247-5268
We present whole rock Li and Mg isotope analyses of 33 ultramafic xenoliths from the terrestrial mantle, which we compare with analyses of 30 (mostly chondritic) meteorites. The accuracy of our new Mg isotope ratio measurement protocol is substantiated by a combination of standard addition experiments, the absence of mass independent effects in terrestrial samples and our obtaining identical values for rock standards using two different separation chemistries and three different mass-spectrometric introduction systems. Carbonaceous, ordinary and enstatite chondrites have irresolvable mean stable Mg isotopic compositions (δ25Mg = −0.14 ± 0.06; δ26Mg = −0.27 ± 0.12‰, 2SD), but our enstatite chondrite samples have lighter δ7Li (by up to ∼3‰) than our mean carbonaceous and ordinary chondrites (3.0 ± 1.5‰, 2SD), possibly as a result of spallation in the early solar system. Measurements of equilibrated, fertile peridotites give mean values of δ7Li = 3.5 ± 0.5‰, δ25Mg = −0.10 ± 0.03‰ and δ26Mg = −0.21 ± 0.07‰. We believe these values provide a useful estimate of the primitive mantle and they are within error of our average of bulk carbonaceous and ordinary chondrites. A fuller range of fresh, terrestrial, ultramafic samples, covering a variety of geological histories, show a broad positive correlation between bulk δ7Li and δ26Mg, which vary from −3.7‰ to +14.5‰, and −0.36‰ to + 0.06‰, respectively. Values of δ7Li and δ26Mg lower than our estimate of primitive mantle are strongly linked to kinetic isotope fractionation, occurring during transport of the mantle xenoliths. We suggest Mg and Li diffusion into the xenoliths is coupled to H loss from nominally anhydrous minerals following degassing. Diffusion models suggest that the co-variation of Mg and Li isotopes requires comparable diffusivities of Li and Mg in olivine. The isotopically lightest samples require ∼5-10 years of diffusive ingress, which we interpret as a time since volatile loss in the host magma. Xenoliths erupted in pyroclastic flows appear to have retained their mantle isotope ratios, likely as a result of little prior degassing in these explosive events. High δ7Li, coupled with high [Li], in rapidly cooled arc peridotites may indicate that these samples represent fragments of mantle wedge that has been metasomatised by heavy, slab-derived fluids. If such material is typically stirred back into the convecting mantle, it may account for the heavy δ7Li seen in some oceanic basalts. 相似文献
167.
168.
Hydrography and copepod abundances (Acartia tonsa, Eurytemora affinis, and nauplii) were regularly monitored for 2 years in sub-estuaries of the lower Chesapeake Bay. Copepod vital status was
determined using neutral red. Abundances of A. tonsa copepodites and nauplii peaked in late summer and were related to water temperature. E. affinis was present in early fall and winter–spring. Copepod carcasses were a persistent feature in the plankton from 2007 to 2009,
with similar annual patterns of occurrence during both years. The relative abundance of carcasses varied among species and
developmental stages, with means of 30% dead for stages NI–NIII copepod nauplii, 12–15% for stages NIV–NVI nauplii and A. tonsa copepodites, and 4–8% for E. affinis copepodites. Percent dead was also higher for adult male than female A. tonsa. No strong relationships were found between measured hydrographic variables and percent dead, but the higher percent dead
in young nauplii and adult male A. tonsa may indicate greater susceptibility of these stages to death from environmental stressors. 相似文献
169.
García-Alonso J Greenway GM Munshi A Gómez JC Mazik K Knight AW Hardege JD Elliott M 《Marine environmental research》2011,71(4):295-303
Due to the ecological importance of estuaries, it is necessary to understand the biological effects that potentially toxic contaminants induce in bioindicator species. A key aspect is whether effects at lower levels of biological organisation transfer through the system to higher levels. In understanding such processes, characterising multivariate relationships between contaminants, sediment toxicities and detoxification processes are important. Worms (Hediste diversicolor) and sediments were collected along the Humber Estuary, England, and inorganic and organic contaminants were quantified. Sediment toxicities and glutathione-S-transferases (GSTs) activity in the ragworm were analysed. Concentrations of metals were highest near urban and industrial areas, whereas organic contaminants appeared at upstream locations. GST activity correlated with heavy metals. The genotoxicity, oestrogenicity, dioxin and dioxin-like activity were higher at upstream locations. Oestrogenicity correlated with alkylphenols and some organochlorines, whilst genotoxicity correlated with organochlorines and heavy metals. Despite this, higher level biological responses could not be predicted, indicating that homeostasis is operating. 相似文献
170.