Strength Of Current Research Articles

Deuterium Recovery from H-D Gas Mixture by Thermal Diffusion in the Device of Branch Columns for Improved Performance

The strength of forced convection currents, as well as the remixing effect in a continuous-flow thermal diffusion column for recovery of deuterium from H-D gas mixture, can be reduced by employing the device of branch columns, instead of the device of single column, with the same total column length. The improvement in separation increases when the flow rate or the total column length increases. It is found that the improvement in recovery can reach about 50% for the numerical example given.

Numerical modeling of tidal currents and the effects of power extraction on estuarine hydrodynamics along the Georgia coast, USA

The tidal stream power potential along the coast of the state of Georgia is evaluated based on numerical modeling and validated with the available data. The Georgia coast consists of a complex network of tidal rivers and inlets between barrier islands that funnel and locally amplify the strength of the ambient tidal currents in the region. The number of existing tidal current prediction locations is not sufficient to resolve the temporal and spatial changes in the current speeds and patterns. Therefore, the currents are modeled with the Regional Ocean Modeling System (ROMS) to determine the locations with high tidal stream power potential and the results are validated against measurements. The wetlands and the topographical features are integrated in the computational model with wetting and drying of computational cells. The locations with the largest mean tidal stream power density are identified and their characteristics are provided. The effect of power extraction on estuarine hydrodynamics is simulated by implementing an additional retarding force in the governing momentum equations in ROMS. Two different power extraction schemes are simulated at the Canoochee River. The first scheme involves extracting 20% of the original kinetic power across the entire cross-section of the river, and is found to have substantially lower impact on the original flow than the second scheme with 45% extraction. The summation of removed and residual kinetic powers is found to be larger than the original kinetic power in the cross-section, which is attributed to the recovery in the flow momentum through reorganization of stream flow energy. In both of the cases the major impact on the currents is limited to a partial reach of the river. The change in the maximum and minimum water levels is observed to be on the order of centimeters.

A dissected ring current model for assessing magnetic aromaticity: A general approach for both organic and inorganic rings

A model based on classical electrodynamics is used to measure the strength of ring currents of different molecular orbitals, i.e., σ- and π-orbitals, and characteristics of ring current loops, i.e., ring current radii and height of current loops above/below the ring planes, among a number of organic as well as inorganic molecules. For the π-current, the present model represents an improvement of previous approaches to determine ring current intensity. It is proven that the present model is more precise than previous models as they could not explain presence of the minimum in the plot of NICS(πzz) versus distance close to the ring plane. Variations in the charge of molecules and the types of constituent atoms of each species affect the ring current radii of both σ- and π-current loops as well as the height of π-current loops above/below the ring plane. It is suggested that variation in the distribution of the one-electron density in different systems is the main source of differences of the ring current characteristics.

Thread-drifting juvenile Mytilus spp. in continental shelf waters off Coos Bay, Oregon, USA

ABSTRACT: On 4 sample dates in summer 2007, vertically stratified zooplankton samples and oceano-graphic data were collected at 7 stations located from 0.5 to 28 km from shore near Coos Bay, Oregon,USA. Two of the sample dates (27 June and 14 August) were characterized by upwelling conditionsand the other two (3 and 18 July) by downwelling or relaxation. We identified larval and thread-drifting Mytilus californianus and M. trossulus/galloprovincialis . Thread-drifting M. californianus and M. trossulus/galloprovincialis ranged in size from 306 to 627 μm and from 305 to 832 μm, respectively.On all sample dates, larval and thread-drifting mytilids were rare in surface waters (0 to 10 m depth),abundant below these depths at the 3 inshore stations (0.7 to 4.5 km offshore) and rare or absent at alldepths at more seaward stations. Thread drifters were caught within 10s of meters of the bottom.There was no relationship between either the strength of the alongshore currents or wave energy andthe abundance of thread drifters. Thread drifters were significantly more abundant during down-welling/relaxation events than during upwelling events. The higher abundances during downwellingmay be due to favorable conditions for thread drifting within the benthic boundary; density instabilitygenerated by the downwelling circulation interacting with the cross-shelf density gradient createsvertical currents, which may aid lift-off of thread drifters from the bottom.KEY WORDS:

Testing the necessity of transient spikes in the storm time ring current drivers

[1] Inner magnetospheric numerical simulations of the 17 April 2002 storm are conducted to explore the importance of transient spikes in the driving parameters at controlling the strength of the storm time ring current. The two main factors considered in this study are convection electric field strength and nightside plasma boundary condition. These quantities were smoothed and/or despiked across intervals of 20–180 min. It is found that the spikes contribute linearly to the ring current total energy content. Exceptions to this finding include too much resulting ring current after despiking or smoothing (relative to a linear response). This indicates that at best the relationship is linear and, if the timing of transient spikes in one driving parameter is not coincident with high values in the other main driving parameter, then the response is sublinear (that is, the transient spikes could be less effective than long-time duration increases in the input parameters).

A parameterization of the wavelength of tidal dunes

The idealized model of Besio et al. (On the formation of sand waves and sand banks. Journal of Fluid Mechanics 2006; 557: 1–17) is used to predict the wavelength of tidal dunes (sand waves) generated by tidal currents in estuaries and shallow seas. The predictions are then analysed and a formula is proposed to estimate the wavelength of tidal dunes as a function of the parameters of the problem. The wavelength of the dunes is found to increase when the water depth is increased and/or the strength of the tidal current is decreased. On the other hand, the size of the bottom material (if medium sand is considered) and the tidal ellipticity are found to have a relatively small influence on the length of the bottom forms. The formula proposed provides results which are consistent with field observations of different authors. Copyright © 2011 John Wiley & Sons, Ltd.

Optical properties of metamaterials based on asymmetric double-wire structures

We performed theoretical and experimental investigations of the magnetic properties of metamaterials based on asymmetric double-wire structures. Using the multipole model for the description of metamaterials, we investigated the influence of the geometrical asymmetry of the structure on the macroscopic effective parameters. The results show that the larger wire in the system dominates the dynamics of the structure and defines the orientation and the strength of the microscopic currents. As a result the magnetization of the structure can be significantly enhanced for certain asymmetric configurations of the double-wire structure.

Variation in the strength of continental boundary currents determines continent-wide connectivity in kelp

Summary 1. Determining the extent to which coastal oceanographic processes facilitate connectivity of marine organisms underpins our understanding of the ecology and evolution of marine communities. Continental boundary currents are a dominant physical influence on marine connectivity, but determining their effect has proved elusive because of difficulties in achieving replication of currents within the distribution of a single species. 2. Australia provides an unparalleled opportunity to address such questions because it has three replicate boundary currents within narrow latitudinal ranges that share continentally distributed species. We tested whether the strength of continental boundary currents influences coastal connectivity of a dominant foundation species (the kelp Ecklonia radiata). 3. Variation in the strength of different boundary currents produced entirely different patterns of connectivity in kelp with high connectivity in strong currents and low connectivity in weak currents. Spatial patterns of genetic structuring were also correlated with the nature and strength of currents. 4. Synthesis. This result has global implications; continental boundary currents are key drivers of marine connectivity and give predictive ability with which to understand variable ecologies of temperate coastlines world-wide.

Effect of tidal stream power generation on the region-wide circulation in a shallow sea

Abstract. This paper quantifies the backward effect on the ocean currents caused by a tidal stream farm located in the open shallow sea. Recent studies in channels with 1-D models have indicated that the power potential is not given purely by the flux of kinetic energy, as has been commonly assumed. In this study, a 3-D ocean circulation model is used to estimate (i) practically extractable energy resource at different levels of rated generation capacity of the farm, (ii) changes in the strength of currents due to energy extraction, and (iii) alterations in the pattern of residual currents and the pathways of passive tracers. As well as tidal streams, the model also takes into account the wind-driven and density-driven ocean currents. Numerical modelling has been carried out for a hypothetical tidal farm located in the Celtic Sea north of Cornwall, an area known for its high level of tidal energy. Modelling results clearly indicate that the extracted power does not grow linearly with the increase in the rated capacity of the farm. For the case study covered in this paper, a 100-fold increase in the rated generation capacity of the farm results in only 7-fold increase in extracted power. In the case of a high power farm, kinetic energy of currents is altered significantly as far as 10–20 km away from the farm. At high levels of extracted energy the currents tend to avoid flowing through the farm, an effect which is not captured with 1-D models. Residual currents are altered as far as a hundred kilometres away. The magnitude of changes in the dispersion of tracers is highly sensitive to the location. Some of the passive drifters analysed in this study experience significant variations in the end-to-start distance due to energy extraction ranging from 13% to 238% while others are practically unaffected. This study shows that both energy extraction estimates and effects on region wide circulation depend on a complex combination of factors, and the specific figures given in the paper should be generally considered as first estimates.

Nonlinear magnetization dynamics excited by the spin-transfer torque effect

The macrospin model based on Landau-Lifshitz-Gilbert equation is used to study the current-induced magnetization dynamics in magnetic spin valves. We find that the DC spin-polarized current could either switch the magnetization of free layer or excite the steady-state precessional motion via the so-called spin-transfer torque effect. The AC current could drive the chaotic oscillations. The route to chaotic oscillation depending on the strength of current is demonstrated through a series of period doubling bifurcations.

On the role of subthreshold currents in the Huber–Braun cold receptor model

We study the role of the strength of subthreshold currents in a four-dimensional Hodgkin-Huxley-type model of mammalian cold receptors. Since a total diminution of subthreshold activity corresponds to a decomposition of the model into a slow, subthreshold, and a fast, spiking subsystem, we first elucidate their respective dynamics separately and draw conclusions about their role for the generation of different spiking patterns. These results motivate a numerical bifurcation analysis of the effect of varying the strength of subthreshold currents, which is done by varying a suitable control parameter. We work out the key mechanisms which can be attributed to subthreshold activity and furthermore elucidate the dynamical backbone of different activity patterns generated by this model.

Principles governing recruitment of motoneurons during swimming in zebrafish

Locomotor movements are coordinated by a network of neurons that produces sequential muscle activation. Different motoneurons need to be recruited in an orderly manner to generate movement with appropriate speed and force. However, the mechanisms governing recruitment order have not been fully clarified. Using an in vitro juvenile/adult zebrafish brainstem-spinal cord preparation, we found that motoneurons were organized into four pools with specific topographic locations and were incrementally recruited to produce swimming at different frequencies. The threshold of recruitment was not dictated by the input resistance of motoneurons, but was instead set by a combination of specific biophysical properties and the strength of the synaptic currents. Our results provide insights into the cellular and synaptic computations governing recruitment of motoneurons during locomotion.

Coral extension rates in the NW Indian Ocean I: reconstruction of 20th century SST variability and monsoon current strength

To date, only a few coral proxy studies have investigated coral growth as an indicator of climate variability. This study presents the first extension-rate record (Porites lutea) from the Maldives (NW Indian Ocean), inferred from skeletal δ18O chronology for the lagoon of Rasdhoo Atoll (4°N/73°W) in the central area of the Maldives, influenced by the Indian monsoon. The record spans 90 years over the period 1917–2007. The mean annual extension over this period was 9.9 mm/year, and an increase of annual extension rates until 1990 by 3 mm/year can be explained by a rise of 0.7°C in sea surface temperature (SST) in this region. After 1990, the extension rates do no continue increasing, possibly due to ecological stress caused by progressive ocean warming and acidification. The correlation between annual extension rates and SSTs is thus significant and strong in the lower part of the record until 1955 (r = +0.69, p < 0.0001), but weaker thereafter (r = +0.44, p < 0.001). The extension rates yield a distinct interannual variability of 3–4 years, caused by interannual SST fluctuations driven by the El Nino-Southern Oscillation. A variability of 8–9 years is likely driven by SST variations endemic to the Indian Ocean. Spectral peaks between 18–19 years and 6–7 years cannot be explained by SST fluctuations, but by variations in the strength of the SW monsoon currents. It is suggested that during phases of stronger monsoon activity, the coral sacrificed coral extension in favor of a denser, more robust skeleton. The geomorphology of the atoll may strengthen the potential of this new coral archive to track climate variability.

Tidal amplification of seabed light

Because solar irradiance decreases approximately exponentially with depth in the sea, the increase in irradiance at the seabed from mid to low tide is greater than the decrease from mid to high tide. Summed over a day, this can lead to a net amplification of seabed irradiance in tidal waters compared to nontidal waters with the same mean depth and transparency. In this paper, this effect is quantified by numerical and analytical integration of the Lambert‐Beer equation to derive the ratio of daily total seabed irradiance with and without a tide. Greatest amplification occurs in turbid water with large tidal range and low tide occurring at noon. The theoretical prediction is tested against observations of seabed irradiance in the coastal waters of North Wales where tidal amplification of seabed light by up to a factor of 7 is both observed and predicted. Increasing the strength of tidal currents tends to increase the turbidity of the water and hence reduce the light reaching the seabed, but this effect is made less by increasing tidal amplification, especially when low water is in the middle of the day. The ecological implications of tidal amplification are discussed. The productivity of benthic algae will be greater than that predicted by simple models which calculate seabed irradiance using the mean depth of water alone. Benthic algae are also able to live at greater depths in tidal waters than in nontidal waters with the same transparency.

Comment: World without Art

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Delivery, deposition and redistribution of fine sediments within macrotidal Fitzroy Estuary/Keppel Bay: Southern Great Barrier Reef, Australia

The Fitzroy catchment in central Queensland, Australia is the largest catchment discharging to the Great Barrier Reef (GBR) Lagoon. Sediments and nutrients (both particulate and dissolved) originating in the catchment, are discharged from the Fitzroy River, into the Fitzroy Estuary (FE) and ultimately into Keppel Bay (KB). The flow in the river is characterised by summertime high-flow events typically of several weeks duration followed by virtually no flow for the remainder of the year. In this manuscript, we describe the hydrodynamics and fine-sediment dynamics in the coupled FE–KB system. Particular emphasis is placed on describing the temporal and spatial variability of these dynamics and on how the interactions between the temporal scales have important implications for system function. The greater bulk of the fresh water, sediments and nutrients are discharged into the FE in a relatively short time, but these materials are then redistributed throughout the FE–KB system and beyond by vigorous macrotidal currents during the rest of the year. Pronounced temporal variability in concentrations of suspended sediments also occurs at semi-diurnal and fortnightly timescales arising from the variation in the strength of tidal currents. The fluxes of fine sediments from the system to the lagoon of the Great Barrier Reef demonstrate pronounced variability at interannual, seasonal, fortnightly, and semi-diurnal timescales. Suspended sediments in the river discharge have very small sinking rates, but flocculation that occurs when they encounter salty water greatly increases their sinking rate and is an important agent for trapping these sediments near the estuary mouth. The large temporal variability of the hydrodynamics and fine-sediment dynamics of the FE–KB system also has important consequences for the biogeochemical function of the system and poses major challenges for the interpretation of moored, shipboard, and remote sensing measurements.

Annual variation of the southern boundary current in the Banda Sea

ADCP measurements in the southern Banda Sea, obtained with the bulk carrier “MS First Jupiter” from 1997 until 2000, have been analysed. The observations reveal the presence of an eastward flowing southern boundary current, bringing water from the Indonesian throughflow towards the connections with the Indian Ocean in Ombai Strait and the Timor Sea. The mean transport in the upper 300 m is estimated to be about 5 Sv, over 50% of the outflow towards the Indian Ocean in this layer through the eastern passages near Timor. The velocity in the boundary current shows a clear annual variation, more or less in phase with the annually varying inflow through Makassar Strait and the outflow near Timor. The phase of the annual variation cannot be explained by the monsoonal variation of the local winds. Therefore this annual variation of the throughflow is probably generated by large-scale forcing. A considerable reduction of the strength of the boundary current was observed in 1998, following the 1997–1998 El Niño with a delay of about half a year. On shorter time scales, Kelvin waves, entering the Banda Sea from the Indian Ocean, cause flow reversals of the boundary current.

Effect of WC and CeO2 on microstructure and properties of W–Cu electrical contact material

Abstract W–Cu alloys with an individual addition of WC and CeO2 particles were prepared by means of powder metallurgy and infiltration, and the effect of WC and CeO2 individual addition on microstructure and properties of W–Cu alloys was investigated. The results show that W–Cu alloys with a separate addition of 1.5 wt% WC and 0.5 wt% CeO2 have desired properties, but an excessive addition decreases hardness and electric conductivity. The vacuum electrical breakdown tests show that the individual addition of WC and CeO2 results in the enhancement of the breakdown strength and decrease of the chopping current. The microstructual analysis of W–Cu alloys with WC or CeO2 addition after vacuum electrical breakdown 50 times shows that the vacuum electrical arcs spread on the cathode surface. Meanwhile, W–Cu alloys with WC or CeO2 addition has a slight splash of molten copper, a less amount of cathode craters, and fine dispersed cathode craters on the entire surface of W–Cu alloys.

Terrestrial palynology from marine cores as an indicator of environmental change for the Waipaoa Sedimentary System and north-eastern New Zealand

Diverse terrestrial palynomorph assemblages of Pleistocene to Recent age have been recovered from mainly marine sediment cores along the eastern New Zealand margin. These assemblages record a range of environmental changes including climate, deforestation, reforestation, rates of erosion, sediment provenance and age, volcanic activity, Polynesian and European settlement, strength and direction of ocean currents, astronomical cyclicity, and other major events many of which can be correlated with shorter onshore terrestrial sequences. As such, pollen yields valuable information on the dynamic New Zealand landscape, which is of particular relevance to a source-to-sink approach. This paper provides an integrated summary of published and unpublished late Pleistocene palynological studies off eastern North Island, New Zealand that gives context for an analysis of cores within and marginal to the Waipaoa Sedimentary System. Pollen assemblages faithfully reflect the last glacial vegetation dominated by grass- and shrub-land species together with elevated Tertiary pollen from enhanced erosion of the regolith. The glacial termination and Holocene were marked by establishment of podocarp forest, a mid-Holocene disappearance of sediment-trapping mangroves, and introduction of distal pollen with establishment of coastal current systems under rising sea level. These natural signals are overprinted by abrupt changes in pollen species caused by volcanic eruptions. Finally, endemic forest and shrub pollen were markedly reduced with human colonisation, associated land use changes and introduction of exotic species.

Literature Review of Unconsolidated Sediment in San Francisco Bay and Nearby Pacific Ocean Coast

A review of the geologic literature regarding sedimentation in the San Francisco Bay estuarine system shows that the main part of the bay occupies a structural tectonic depression that developed in Pleistocene time. Eastern parts, including San Pablo Bay and Suisun Bay, have had sedimentation throughout late Mesozoic and Tertiary. Carquinez Strait and the Golden Gate may represent antecedent stream erosion. Sedimentation has included estuarine, alluvial, and eolian deposition. The ages of estuarine deposition includes the modern high sea level stand and earlier Pleistocene interglacial periods. Sediment sources can be generally divided into the Coast Ranges, particularly the Franciscan Complex, and “Sierran.” Much of the estuarine system is floored by very fine sediment, with local areas of sand floor. Near the Golden Gate, sediment size decreases in both directions away from the deep channel. Bedforms include sand waves (submarine dunes), flat beds, and rock and boulders. These are interpreted in terms of dominant transport directions. Near the Golden Gate is an ebb-tidal delta on the outside (including San Francisco bar) and a flood-tidal delta on the inside (parts of Central Bay). The large tidal prism causes strong tidal currents, which in the upper part of the estuary are normally much stronger than river currents, except during large floods. Cultural influences have altered conditions, including hydraulic mining debris, blasting of rocks, dredging of navigation channels, filling of the bay, and commercial sand mining. Many of these have served to decrease the tidal prism, correspondingly decreasing the strength of tidal currents.