3,114 research outputs found
2DV modelling of sediment transport processes over full-scale ripples in regular asymmetric oscillatory flow
Wave-induced, steep vortex ripples are ubiquitous features in shallow coastal seas and it is therefore important to fully understand and model the sediment transport processes that occur over them. To this end, two two-dimensional vertical (2DV) models have been critically tested against detailed velocity and sediment concentration measurements above mobile ripples in regular asymmetric oscillatory flow. The two models are a k–ω turbulence-closure model and a discrete-vortex, particle-tracking (DVPT) model, while the data are obtained in the Aberdeen oscillatory flow tunnel (AOFT). The models and the data demonstrate that the time-dependent velocity and suspended sediment concentration above the ripple are dominated by the generation of lee-side vortices and their subsequent ejection at flow reversal. The DVPT model predicts the positions and strengths of the vortices reasonably well, but tends to overpredict the velocity close to the ripple surface. The k–ω model, on the other hand, underpredicts the height to which the vortices are lifted, but is better able to predict the velocity close to the bed. In terms of the cycle- and ripple-averaged horizontal velocity, both models are able to reproduce the observed offshore flow close to and below the ripple crest and the DVPT model is able to produce the onshore flow higher up. In the vicinity of the vortices, the DVPT model better represents the concentration (because of its better prediction of vorticity). The k–ω model, on the other hand, better represents the concentration close to the ripple surface and higher up in the flow (because of the better representation of the near-bed flow and background turbulence). The measured and predicted cycle- and ripple-averaged suspended sediment concentrations are in reasonable agreement and demonstrate the expected region of exponential decay. The models are able to reproduce the observed offshore cycle- and ripple-averaged suspended sediment flux from the ripple troughs upwards, and as a result, produce net offshore suspended sediment transport rates that are in reasonable agreement. The net measured offshore suspended transport rate, based on the integration of fluxes, was found to be consistent with the total net offshore transport measured in the tunnel as a whole once the onshore transport resulting from ripple migration was taken into account, as would be expected. This demonstrates the importance of models being able to predict ripple-migration rates. However, at present neither of the models is able to do so
Probing the Interstellar Medium using HI absorption and emission towards the W3 HII region
HI spectra towards the W3 HII complex are presented and used to probe the
Galactic structure and interstellar medium conditions between us and this
region. The overall shape of the spectra is consistent with the predictions of
the Two-Arm Spiral Shock model wherein the gas found in the -40 km/s to -50
km/s range has been accelerated by some 20 km/s from its rotation curve
velocity. Spin temperatures of ~100 K are derived for the Local Arm gas, lower
than found in a previous, similar study towards DR 7. For the interarm region,
values on the order of 300 K are found, implying a negligible filling factor
for the Cold Neutral Medium (<< 1%). Some of the absorbing gas at velocities
near -40 km/s is confirmed to be associated with the HII regions.Comment: 23 pages, 6 figures, accepted for publication in the Astronomical
Journa
Detection of CO from SMM J16359+6612, The Multiply Imaged Submillimeter Galaxy Behind A2218
We report the detection of CO (=32) line emission from all three
multiple images (A,B and C) of the intrinsically faint ( 0.8 mJy)
submillimeter-selected galaxy SMM J16359+6612. The brightest source of the
submm continuum emission (B) also corresponds to the brightest CO emission,
which is centered at =2.5168, consistent with the pre-existing redshift
derived from \Ha. The observed CO flux in the A, B and C images is 1.2, 3.5 and
1.6 Jy \kms respectively, with a linewidth of \kms. After
correcting for the lensing amplification, the CO flux corresponds to a
molecular gas mass of \Msun, while the
extent of the CO emission indicates that the dynamical mass of the system
\Msun. Two velocity components are seen in the CO spectra;
these could arise from either a rotating compact ring or disk of gas, or
merging substructure. The star formation rate in this galaxy was previously
derived to be 100--500 \Msun \yr. If all the CO emission arises from the
inner few kpc of the galaxy and the galactic CO-to-H conversion factor
holds, then the gas consumption timescale is a relatively short 40 Myr, and so
the submm emission from SMM J16359+6612 may be produced by a powerful, but
short-lived circumnuclear starburst event in an otherwise normal and
representative high-redshift galaxy.Comment: Appearing in the 2004 October 10 issue of the Astrophysical Journal
Letters, Volume 614, L5-L
A Risk Assessment Model on Pine Wood Nematode in the EU
Pine wood nematode, B. xylophilus poses a serious threat for the European forest industry. This study applies a quantitative risk assessment to analyze the risk of pine wood nematode in the EU, by estimating the reduction expected within forestry stock available for wood supply and its downstream roundwood market. Spatial analysis is used to join information on climate suitability, host distribution, pest spread and value of assets. Economic impacts are presented spatially on a NUTS-2 scale based on partial budgeting technique and for the EU as a whole based on partial equilibrium modeling. Results highlight the Southern regions of Europe as high risk areas with a total impact on available forestry stock of 19,000 M € after 20 years of an outbreak and no regulatory control measures. Welfare analysis of the roundwood market, in which its production represents 2,5% of forestry stock, demonstrates the ability of the producers to pass most of the negative impact to the consumers by charging higher prices. Reduction in social welfare estimated at 2,043 M €, where consumer surplus decreased by 2,622 M € and net producer surplus, affected and non-affected producers, increased by 579 M €.Risk assessment, pine wood nematode, economic analysis, EU, Crop Production/Industries, Risk and Uncertainty,
Uniqueness and Non-uniqueness in the Einstein Constraints
The conformal thin sandwich (CTS) equations are a set of four of the Einstein
equations, which generalize the Laplace-Poisson equation of Newton's theory. We
examine numerically solutions of the CTS equations describing perturbed
Minkowski space, and find only one solution. However, we find {\em two}
distinct solutions, one even containing a black hole, when the lapse is
determined by a fifth elliptic equation through specification of the mean
curvature. While the relationship of the two systems and their solutions is a
fundamental property of general relativity, this fairly simple example of an
elliptic system with non-unique solutions is also of broader interest.Comment: 4 pages, 4 figures; abstract and introduction rewritte
Use of antibiotics and asthma medication for acute lower respiratory tract infections in people with and without asthma:retrospective cohort study
BACKGROUND:
Antibiotics are overused in patients with acute lower respiratory tract infections (ALRTIs), but less is known about their use in patients with asthma, or the use of asthma medication for ALRTI in patients without asthma. Our aim was to describe the frequency, variation and drivers in antibiotic and asthma medication prescribing for ALRTI in adults with and without asthma in primary care.
METHODS:
A retrospective cohort analysis of patients aged ≥12 years, diagnosed with an ALRTI in primary care in 2014–15 was conducted using data from the Clinical Practice Research Datalink. Current asthma status, asthma medication and oral antibiotic use within 3 days of ALRTI infection was determined. Treatment frequency was calculated by asthma status. Mixed-effect regression models were used to explore between-practice variation and treatment determinants.
RESULTS:
There were 127,976 ALRTIs reported among 110,418 patients during the study period, of whom 17,952 (16%) had asthma. Respectively, 81 and 79% of patients with and without asthma received antibiotics, and 41 and 15% asthma medication. There were significant differences in between-practice prescribing for all treatments, with greatest differences seen for oral steroids (odds ratio (OR) 18; 95% CI 7–82 and OR = 94; 33–363, with and without asthma) and asthma medication only (OR 7; 4–18 and OR = 17; 10–33, with and without asthma).
Independent predictors of antibiotic prescribing among patients with asthma included fewer previous ALRTI presentations (≥2 vs. 0 previous ALRTI: OR = 0.25; 0.16–0.39), higher practice (OR = 1.47; 1.35–1.60 per SD) and prior antibiotic prescribing (3+ vs. 1 prescriptions OR = 1.28; 1.04–1.57) and concurrent asthma medication (OR = 1.44; 1.32–1.57). Independent predictors of asthma medication in patients without asthma included higher prior asthma medication prescribing (≥7 vs. 0 prescriptions OR = 2.31; 1.83–2.91) and concurrent antibiotic prescribing (OR = 3.59; 3.22–4.01).
CONCLUSION:
Findings from the study indicate that antibiotics are over-used for ALRTI, irrespective of asthma status, and asthma medication is over-used in patients without asthma, with between-practice variation suggesting considerable clinical uncertainty. Further research is urgently needed to clarify the role of these medications for ALRTI
HI Narrow Line Absorption in Dark Clouds
We have used the Arecibo telescope to carry out an survey of 31 dark clouds
in the Taurus/Perseus region for narrow absorption features in HI (
21cm) and OH (1667 and 1665 MHz) emission. We detected HI narrow
self--absorption (HINSA) in 77% of the clouds that we observed. HINSA and OH
emission, observed simultaneously are remarkably well correlated. Spectrally,
they have the same nonthermal line width and the same line centroid velocity.
Spatially, they both peak at the optically--selected central position of each
cloud, and both fall off toward the cloud edges. Sources with clear HINSA
feature have also been observed in transitions of CO, \13co, \c18o, and CI.
HINSA exhibits better correlation with molecular tracers than with CI.
The line width of the absorption feature, together with analyses of the
relevant radiative transfer provide upper limits to the kinetic temperature of
the gas producing the HINSA. Some sources must have a temperature close to or
lower than 10 K. The correlation of column densities and line widths of HINSA
with those characteristics of molecular tracers suggest that a significant
fraction of the atomic hydrogen is located in the cold, well--shielded portions
of molecular clouds, and is mixed with the molecular gas.
The average number density ratio [HI]/[\h2] is . The
inferred HI density appears consistent with but is slightly higher than the
value expected in steady state equilibrium between formation of HI via cosmic
ray destruction of H and destruction via formation of H on grain
surfaces. The distribution and abundance of atomic hydrogen in molecular clouds
is a critical test of dark cloud chemistry and structure, including the issues
of grain surface reaction rates, PDRs, circulation, and turbulent diffusion.Comment: 40 pages, 10 figures, accepted by Ap
Learning Hybrid Process Models From Events: Process Discovery Without Faking Confidence
Process discovery techniques return process models that are either formal
(precisely describing the possible behaviors) or informal (merely a "picture"
not allowing for any form of formal reasoning). Formal models are able to
classify traces (i.e., sequences of events) as fitting or non-fitting. Most
process mining approaches described in the literature produce such models. This
is in stark contrast with the over 25 available commercial process mining tools
that only discover informal process models that remain deliberately vague on
the precise set of possible traces. There are two main reasons why vendors
resort to such models: scalability and simplicity. In this paper, we propose to
combine the best of both worlds: discovering hybrid process models that have
formal and informal elements. As a proof of concept we present a discovery
technique based on hybrid Petri nets. These models allow for formal reasoning,
but also reveal information that cannot be captured in mainstream formal
models. A novel discovery algorithm returning hybrid Petri nets has been
implemented in ProM and has been applied to several real-life event logs. The
results clearly demonstrate the advantages of remaining "vague" when there is
not enough "evidence" in the data or standard modeling constructs do not "fit".
Moreover, the approach is scalable enough to be incorporated in
industrial-strength process mining tools.Comment: 25 pages, 12 figure
Physical Conditions in Orion's Veil
Orion's veil consists of several layers of largely neutral gas lying between
us and the main ionizing stars of the Orion nebula. It is visible in 21cm H I
absorption and in optical and UV absorption lines of H I and other species.
Toward the Trapezium, the veil has two remarkable properties, high magnetic
field (~100 microGauss) and a surprising lack of molecular hydrogen given its
total hydrogen column density. Here we compute photoionization models of the
veil to establish its gas density and its distance from the Trapezium. We use a
greatly improved model of the hydrogen molecule that determines level
populations in ~1e5 rotational/vibrational levels and provides improved
estimates of molecular hydrogen destruction via the Lyman-Werner bands. Our
best fit photoionization models place the veil 1-3 pc in front of the star at a
density of 1e3-1e4 cubic centimeters. Magnetic energy dominates the energy of
non-thermal motions in at least one of the 21cm H I velocity components.
Therefore, the veil is the first interstellar environment where magnetic
dominance appears to exist. We find that the low ratio of molecular to atomic
hydrogen (< 1e-4) is a consequence of high UV flux incident upon the veil due
to its proximity to the Trapezium stars and the absence of small grains in the
region.Comment: 45 pages, 20 figures, accepted for publication in Ap
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