14,559 research outputs found
Seawater capacitance – a promising proxy for mapping and characterizing drifting hydrocarbon plumes in the deep ocean
Hydrocarbons released into the deep ocean are an inevitable consequence of natural seep, seafloor drilling, and leaking wellhead-to-collection-point pipelines. The Macondo 252 (Deepwater Horizon) well blowout of 2010 was even larger than the Ixtoc event in the Gulf of Campeche in 1979. History suggests it will not be the last accidental release, as deepwater drilling expands to meet an ever-growing demand. For those who must respond to this kind of disaster, the first line of action should be to know what is going on. This includes knowing where an oil plume is at any given time, where and how fast it is moving, and how it is evolving or degrading. We have experimented in the laboratory with induced polarization as a method to track hydrocarbons in the seawater column and find that finely dispersed oil in seawater gives rise to a large distributed capacitance. From previous sea trials, we infer this could potentially be used to both map and characterize oil plumes, down to a ratio of less than 0.001 oil-to-seawater, drifting and evolving in the deep ocean. A side benefit demonstrated in some earlier sea trials is that this same approach in modified form can also map certain heavy placer minerals, as well as communication cables, pipelines, and wrecks buried beneath the seafloor
A controlled experiment for the empirical evaluation of safety analysis techniques for safety-critical software
Context: Today's safety critical systems are increasingly reliant on
software. Software becomes responsible for most of the critical functions of
systems. Many different safety analysis techniques have been developed to
identify hazards of systems. FTA and FMEA are most commonly used by safety
analysts. Recently, STPA has been proposed with the goal to better cope with
complex systems including software. Objective: This research aimed at comparing
quantitatively these three safety analysis techniques with regard to their
effectiveness, applicability, understandability, ease of use and efficiency in
identifying software safety requirements at the system level. Method: We
conducted a controlled experiment with 21 master and bachelor students applying
these three techniques to three safety-critical systems: train door control,
anti-lock braking and traffic collision and avoidance. Results: The results
showed that there is no statistically significant difference between these
techniques in terms of applicability, understandability and ease of use, but a
significant difference in terms of effectiveness and efficiency is obtained.
Conclusion: We conclude that STPA seems to be an effective method to identify
software safety requirements at the system level. In particular, STPA addresses
more different software safety requirements than the traditional techniques FTA
and FMEA, but STPA needs more time to carry out by safety analysts with little
or no prior experience.Comment: 10 pages, 1 figure in Proceedings of the 19th International
Conference on Evaluation and Assessment in Software Engineering (EASE '15).
ACM, 201
Myosin VIIA is required for aminoglycoside accumulation in cochlear hair cells.
Myosin VIIA is expressed by sensory hair cells and has a primary structure predicting a role in membrane trafficking and turnover, processes that may underlie the susceptibility of hair cells to aminoglycoside antibiotics. [3H]Gentamicin accumulation and the effects of aminoglycosides were therefore examined in cochlear cultures of mice with different missense mutations in the myosin VIIA gene, Myo7a, to see whether myosin VIIA plays a role in aminoglycoside ototoxicity. Hair cells from homozygous mutant Myo7a(sh1) mice, with a mutation in a non-conserved region of the myosin VIIA head, respond rapidly to aminoglycoside treatment and accumulate high levels of gentamicin. Hair cells from homozygous mutant Myo7a(6J) mice, with a mutation at a highly conserved residue close to the ATP binding site of the myosin VIIA head, do not accumulate [3H]gentamicin and are protected from aminoglycoside ototoxicity. Hair cells from heterozygotes of both alleles accumulate [3H]gentamicin and respond to aminoglycosides. Although aminoglycoside uptake is thought to be via apical surface-associated endocytosis, coated pit numbers on the apical membrane of heterozygous and homozygous Myo7a(6J) hair cells are similar. Pulse-chase experiments with cationic ferritin confirm that the apical endocytotic pathway is functional in homozygous Myo7a(6J) hair cells. Transduction currents can be recorded from both heterozygous and homozygous Myo7a(6J) hair cells, suggesting it is unlikely that the drug enters via diffusion through the mechanotransducer channel. The results show that myosin VIIA is required for aminoglycoside accumulation in hair cells. Myosin VIIA may transport a putative aminoglycoside receptor to the hair cell surface, indirectly translocate it to sites of membrane retrieval, or retain it in the endocytotic pathway
Acoustic interactions between an altitude test facility and jet engine plumes: Theory and experiments
The overall objective of the described effort was to develop an understanding of the physical mechanisms involved in the flow/acoustic interactions experienced in full-scale altitude engine test facilities. This is done by conducting subscale experiments and through development of a theoretical model. Model cold jet experiments with an axisymmetric convergent nozzle are performed in a test setup that stimulates a supersonic jet exhausting into a cylindrical diffuser. The measured data consist of detailed flow visualization data and acoustic spectra for a free and a ducted plume. It is shown that duct resonance is most likely responsible by theoretical calculations. Theoretical calculations also indicate that the higher discrete tones observed in the measurements are related to the screech phenomena. Limited experiments on the sensitivity of a free 2-D, C-D nozzle to externally imposed sound are also presented. It is shown that a 2-D, C-D nozzle with a cutback is less excitable than a 2-D C-D nozzle with no cutback. At a pressure ratio of 1.5 unsteady separation from the diverging walls of the nozzle is noticed. This separation switches from one wall to the opposite wall thus providing an unsteady deflection of the plume. It is shown that this phenomenon is related to the venting provided by the cutback section
Non-Markovian Dynamics and Entanglement of Two-level Atoms in a Common Field
We derive the stochastic equations and consider the non-Markovian dynamics of
a system of multiple two-level atoms in a common quantum field. We make only
the dipole approximation for the atoms and assume weak atom-field interactions.
From these assumptions we use a combination of non-secular open- and
closed-system perturbation theory, and we abstain from any additional
approximation schemes. These more accurate solutions are necessary to explore
several regimes: in particular, near-resonance dynamics and low-temperature
behavior. In detuned atomic systems, small variations in the system energy
levels engender timescales which, in general, cannot be safely ignored, as
would be the case in the rotating-wave approximation (RWA). More problematic
are the second-order solutions, which, as has been recently pointed out, cannot
be accurately calculated using any second-order perturbative master equation,
whether RWA, Born-Markov, Redfield, etc.. This latter problem, which applies to
all perturbative open-system master equations, has a profound effect upon
calculation of entanglement at low temperatures. We find that even at zero
temperature all initial states will undergo finite-time disentanglement
(sometimes termed "sudden death"), in contrast to previous work. We also use
our solution, without invoking RWA, to characterize the necessary conditions
for Dickie subradiance at finite temperature. We find that the subradiant
states fall into two categories at finite temperature: one that is temperature
independent and one that acquires temperature dependence. With the RWA there is
no temperature dependence in any case.Comment: 17 pages, 13 figures, v2 updated references, v3 clarified results and
corrected renormalization, v4 further clarified results and new Fig. 8-1
Linear Sigma EFT for Nearly Conformal Gauge Theories
We construct a generalized linear sigma model as an effective field theory
(EFT) to describe nearly conformal gauge theories at low energies. The work is
motivated by recent lattice studies of gauge theories near the conformal
window, which have shown that the lightest flavor-singlet scalar state in the
spectrum () can be much lighter than the vector state () and
nearly degenerate with the PNGBs () over a large range of quark masses.
The EFT incorporates this feature. We highlight the crucial role played by the
terms in the potential that explicitly break chiral symmetry. The explicit
breaking can be large enough so that a limited set of additional terms in the
potential can no longer be neglected, with the EFT still weakly coupled in this
new range. The additional terms contribute importantly to the scalar and pion
masses. In particular, they relax the inequality , allowing for consistency with current lattice data.Comment: 9 pages, 1 figure, published versio
C/C ratio in planetary nebulae from the IUE archives
We investigated the abundance ratio of C/C in planetary nebulae
by examining emission lines arising from \ion{C}{3} 2s2p ^3P_{2,1,0} \to 2s^2
^1S_0. Spectra were retrieved from the International Ultraviolet Explorer
archives, and multiple spectra of the same object were coadded to achieve
improved signal-to-noise. The C hyperfine structure line at 1909.6 \AA
was detected in NGC 2440. The C/C ratio was found to be
1.2. In all other objects, we provide an upper limit for the flux
of the 1910 \AA line. For 23 of these sources, a lower limit for the
C/C ratio was established. The impact on our current
understanding of stellar evolution is discussed.
The resulting high signal-to-noise \ion{C}{3} spectrum helps constrain the
atomic physics of the line formation process. Some objects have the measured
1907/1909 flux ratio outside the low-electron density theoretical limit for
C. A mixture of C with C helps to close the gap somewhat.
Nevertheless, some observed 1907/1909 flux ratios still appear too high to
conform to the presently predicted limits. It is shown that this limit, as well
as the 1910/1909 flux ratio, are predominantly influenced by using the standard
partitioning among the collision strengths for the multiplet --
according to the statistical weights. A detailed calculation for the fine
structure collision strengths between these individual levels would be
valuable.Comment: ApJ accepted: 19 pages, 3 Figures, 2 Table
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