82 research outputs found
The importance of aspiration and channel curvature in producing strong vertical mixing over a sill
On the basis of observations of the time-dependent, tidally forced flow over a long sill we find that aspiration and channel curvature set the flow structure and condition the flow to allow intense vertical mixing. Aspiration reduces the potential energy of the water column by thinning it while maintaining its density contrast. Channel curvature induces a cross-channel circulation that can rapidly overturn a stratified flow. Eighteen along-channel sections of density, velocity, and dissipation rate of turbulent kinetic energy c were collected in and around the Tacoma Narrows of Puget Sound, a site suspected of driving a strong vertical circulation in adjoining Main Basin. Rapid inflow to the Narrows on flood from one channel of a triple junction reduces dynamic pressure, allowing dense water from below sill depth to be uplifted, or aspirated, into the Narrows. We estimate water from below 150 m, 3 times the sill depth, is drawn into the Narrows on a 3-m flood tide. Once in the Narrows the flow remains stratified until it passes a 50° bend where a strong secondary circulation overturns the 50-m-deep water column and generates intense turbulent mixing. Cross-channel velocities of up to 0.4 m s-1are observed, and maximum values of e exceed 10âŒ3 W kg-1. Upon leaving the sill, stratification is reestablished, and turbulence decays. A similar set of sequences occurs on ebb, except that the outflow bypasses the flood inflow channel and instead discharges into Colvos Passage, the third branch of the triple junction. Colvos Passage ultimately discharges the ebb effluent back into Main Basin, enhancing the impact of mixing at the Narrows by discharging the mixed product far from the source. Scaling of the cross-channel momentum equation suggests that, below a threshold value of along-channel velocity, stratification should suppress secondary circulation for a given vertical shear, radius of curvature and channel width. Above the threshold velocity the magnitude of the cross-channel velocity is roughly consistent with predictions for unstratified flow. We estimate the maximum effective eddy diffusivity that aspiration and mixing in the Narrows can produce in Main Basin to be 10"3 m2 s-1
Energetics of a naturally occurring shear instability
Using observations of an energetic shear instability (Seim and Gregg, 1994), examines the energy budget of the mixing event by comparing microstructure measurements of the dissipation rates of turbulent kinetic energy Δ and turbulent potential energy Ïpe with changes in fine-scale velocity and density. Two sets of observations are used. The first set sampled the shear instability early in its evolution, when overturns occurred in strong stratification. The second set of observations found the same water vertically homogenized by turbulent mixing
Detailed observations of a naturally occurring shear instability
Simultaneous profiles of microstructure, horizontal velocity, and acoustic backscatter allow description of a naturally occurring shear instability. Shear increased rapidly after passing through a lateral constriction which formed a hydraulic control. A kilometer-long set of 20-m-tall billows grew on a middepth density interface where the Richardson number fell below 0.25. The velocity interface thickened steadily after the billows formed, consistent with rapid momentum mixing across a shear layer with a Reynolds number of 3 Ă 106. The billows generated large density overturns and were fully turbulent early in their development. As the billows grew, a well-mixed layer developed at the interface and survived as an actively turbulent layer for up to 6 buoyancy periods
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Turbulence variability at the equator in the central Pacific at the beginning of the 1991-1993 El Nino
A 38-day, 5990-cast microstructure study on the equator performed
during the onset of the 1991-1993 El Nino shows the effect on small-scale activity
at 140°W of an equatorial Kelvin wave. By using two ships, data were taken
continuously from November 4 to December 12, 1991, near the National Oceanic
and Atmospheric Administration Pacific Marine Environmental Laboratory mooring
at 0°N, 140°W. The ships occupied the station sequentially with a 3.5-day overlap
for intercalibration. Variability in currents was observed on tidal periods, and
periods of 4 days (presumably equatorially trapped internal gravity waves), 8 days
(cause unknown), 20 days (tropical instability waves), and longer (Kelvin waves).
Variation in water structure occurred most prominently on the timescale of Kelvin
waves. The diurnal cycle typical of that location was observed: nocturnal deepening
of the surface mixed layer was accompanied by a "deep cycle," bursts of turbulence
penetrating into the stratified region below the nighttime mixed layer. During the
observational period, one Kelvin wave trough and one crest passed through the site.
Changes accompanying the phase change in the Kelvin wave included a reversal
of the near-surface current, a deepening of the thermocline, and a change of water
mass. Changes in small-scale activity included a tenfold decrease of the thermal
dissipation rate and a fourfold decrease of the rate of heat transport downward
from the mixed layer. The nighttime mixed layer deepened from 30 to 60 m. The
thickness of the stratified region in which nocturnal turbulence bursts occurred,
the deep cycle region, thinned from 40 to 20 m because it was confined between
the bottom of the nighttime mixed layer and the low-shear region near the core of
the undercurrent. The decrease in downward heat flux observed at this passage of
the downwelling Kelvin wave front could explain the rapid sea surface temperature
(SST) increase seen at El Nino onsets. The magnitude of the change in vertical
flux is similar to the magnitude of the change in horizontal advection. This process
would produce a warmer SST much more quickly than could the advection of warm
waters eastward
A preliminary validation study of PuO2 incorporation into zirconolite glass-ceramics
Zirconolite glass-ceramics are being developed as potential wasteforms for the disposition of Pu wastes in the UK. Previous studies utilised a variety of surrogates whilst this work uses both cold-press and sinter and hot isostatic press methods to validate the wasteform with PuO2. A cold press and sinter sample was fabricated as part of a validation study for plutonium incorporation in hot isostatically pressed (HIPed) wasteforms. The results confirmed the cold-press and sinter, achieved successful waste incorporation and a microstructure and phase assemblage that was in agreement with those expected of a HIPed equivalent. A HIP sample was fabricated of the same composition and characterised by SEM and XRD. Results were in agreement with the sintered sample and achieved complete waste incorporation into the glass-ceramic wasteform. These samples have demonstrated successful incorporation of PuO2 into glass-ceramic HIPed wasteforms proposed for processing Pu-based waste-streams in the UK
Residual plaque burden in patients with acute coronary syndromes after successful percutaneous coronary intervention
Objectives: The aim of this study was to characterize and evaluate the clinical impact of untreated atherosclerotic disease after percutaneous coronary intervention (PCI) in patients with acute coronary syndromes (ACS). Background: Residual atherosclerotic disease after successful PCI may predispose future major adverse cardiovascular events (MACE). Compared with intravascular ultrasound (IVUS), angiography underestimates the presence and severity of coronary artery disease. Methods: Following successful PCI of all clinically significant lesions in 697 patients with ACS, 3-vessel grayscale and radiofrequency IVUS was performed. Lesions were prospectively characterized, and patients were followed for a median of 3.4 years. A total of 3,229 untreated lesions (4.89 ± 1.98 lesions/patient) were identified by IVUS, with mean plaque burden (PB) of 49.6 ± 4.2%. Results: By angiography these nonculprit lesions were mild, with mean diameter stenosis of 38.9 ± 15.3%. At least 1 lesion with a PB <70% (PB70 lesion) was found in 220 (33%) patients. By multivariable analysis, a history of prior PCI and angiographic 3-vessel disease were independent predictors of PB70 lesions. Patients with PB70 lesions had greater total percent plaque volume, normalized PB, fibroatheromas, thin-cap fibroatheromas, and normalized volumes of necrotic core and dense calcium. Patients with PB70 lesions had greater 3-year rates of MACE due to untreated nonculprit lesions (20.8% vs. 7.7%, p < 0.0001). Among imaged nonculprit lesions, the proportion of PB70 lesions causing MACE was significantly greater than non-PB70 lesions (8.7% vs. 1.0%, p < 0.0001). Conclusions: After successful PCI of all angiographically significant lesions, overall untreated atherosclerotic burden remains high, and PB70 lesions are frequently present in the proximal and mid-coronary tree. Patients with PB70 lesions have greater atherosclerosis throughout the coronary tree, have more thin-cap fibroatheromas, and are at increased risk for future cardiovascular events. (PROSPECT: An Imaging Study in Patients With Unstable Atherosclerotic Lesions; NCT00180466
Active Galactic Nuclei at the Crossroads of Astrophysics
Over the last five decades, AGN studies have produced a number of spectacular
examples of synergies and multifaceted approaches in astrophysics. The field of
AGN research now spans the entire spectral range and covers more than twelve
orders of magnitude in the spatial and temporal domains. The next generation of
astrophysical facilities will open up new possibilities for AGN studies,
especially in the areas of high-resolution and high-fidelity imaging and
spectroscopy of nuclear regions in the X-ray, optical, and radio bands. These
studies will address in detail a number of critical issues in AGN research such
as processes in the immediate vicinity of supermassive black holes, physical
conditions of broad-line and narrow-line regions, formation and evolution of
accretion disks and relativistic outflows, and the connection between nuclear
activity and galaxy evolution.Comment: 16 pages, 5 figures; review contribution; "Exploring the Cosmic
Frontier: Astrophysical Instruments for the 21st Century", ESO Astrophysical
Symposia Serie
State of the climate in 2013
In 2013, the vast majority of the monitored climate variables reported here maintained trends established in recent decades. ENSO was in a neutral state during the entire year, remaining mostly on the cool side of neutral with modest impacts on regional weather patterns around the world. This follows several years dominated by the effects of either La Niña or El Niño events. According to several independent analyses, 2013 was again among the 10 warmest years on record at the global scale, both at the Earths surface and through the troposphere. Some regions in the Southern Hemisphere had record or near-record high temperatures for the year. Australia observed its hottest year on record, while Argentina and New Zealand reported their second and third hottest years, respectively. In Antarctica, Amundsen-Scott South Pole Station reported its highest annual temperature since records began in 1957. At the opposite pole, the Arctic observed its seventh warmest year since records began in the early 20th century. At 20-m depth, record high temperatures were measured at some permafrost stations on the North Slope of Alaska and in the Brooks Range. In the Northern Hemisphere extratropics, anomalous meridional atmospheric circulation occurred throughout much of the year, leading to marked regional extremes of both temperature and precipitation. Cold temperature anomalies during winter across Eurasia were followed by warm spring temperature anomalies, which were linked to a new record low Eurasian snow cover extent in May. Minimum sea ice extent in the Arctic was the sixth lowest since satellite observations began in 1979. Including 2013, all seven lowest extents on record have occurred in the past seven years. Antarctica, on the other hand, had above-average sea ice extent throughout 2013, with 116 days of new daily high extent records, including a new daily maximum sea ice area of 19.57 million km2 reached on 1 October. ENSO-neutral conditions in the eastern central Pacific Ocean and a negative Pacific decadal oscillation pattern in the North Pacific had the largest impacts on the global sea surface temperature in 2013. The North Pacific reached a historic high temperature in 2013 and on balance the globally-averaged sea surface temperature was among the 10 highest on record. Overall, the salt content in nearsurface ocean waters increased while in intermediate waters it decreased. Global mean sea level continued to rise during 2013, on pace with a trend of 3.2 mm yr-1 over the past two decades. A portion of this trend (0.5 mm yr-1) has been attributed to natural variability associated with the Pacific decadal oscillation as well as to ongoing contributions from the melting of glaciers and ice sheets and ocean warming. Global tropical cyclone frequency during 2013 was slightly above average with a total of 94 storms, although the North Atlantic Basin had its quietest hurricane season since 1994. In the Western North Pacific Basin, Super Typhoon Haiyan, the deadliest tropical cyclone of 2013, had 1-minute sustained winds estimated to be 170 kt (87.5 m s-1) on 7 November, the highest wind speed ever assigned to a tropical cyclone. High storm surge was also associated with Haiyan as it made landfall over the central Philippines, an area where sea level is currently at historic highs, increasing by 200 mm since 1970. In the atmosphere, carbon dioxide, methane, and nitrous oxide all continued to increase in 2013. As in previous years, each of these major greenhouse gases once again reached historic high concentrations. In the Arctic, carbon dioxide and methane increased at the same rate as the global increase. These increases are likely due to export from lower latitudes rather than a consequence of increases in Arctic sources, such as thawing permafrost. At Mauna Loa, Hawaii, for the first time since measurements began in 1958, the daily average mixing ratio of carbon dioxide exceeded 400 ppm on 9 May. The state of these variables, along with dozens of others, and the 2013 climate conditions of regions around the world are discussed in further detail in this 24th edition of the State of the Climate series. é 2014, American Meteorological Society. All rights reserved
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Track A Basic Science
Peer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/138319/1/jia218438.pd
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