8,248 research outputs found
Hamiltonian formalism and the Garrett-Munk spectrum of internal waves in the ocean
Wave turbulence formalism for long internal waves in a stratified fluid is
developed, based on a natural Hamiltonian description. A kinetic equation
appropriate for the description of spectral energy transfer is derived, and its
self-similar stationary solution corresponding to a direct cascade of energy
toward the short scales is found. This solution is very close to the high
wavenumber limit of the Garrett-Munk spectrum of long internal waves in the
ocean. In fact, a small modification of the Garrett-Munk formalism includes a
spectrum consistent with the one predicted by wave turbulence.Comment: 4 pages latex fil
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
New Modeling of the Lensing Galaxy and Cluster of Q0957+561: Implications for the Global Value of the Hubble Constant
The gravitational lens 0957+561 is modeled utilizing recent observations of
the galaxy and the cluster as well as previous VLBI radio data which have been
re-analyzed recently. The galaxy is modeled by a power-law elliptical mass
density with a small core while the cluster is modeled by a non-singular
power-law sphere as indicated by recent observations. Using all of the current
available data, the best-fit model has a reduced chi-squared of approximately 6
where the chi-squared value is dominated by a small portion of the
observational constraints used; this value of the reduced chi-squared is
similar to that of the recent FGSE best-fit model by Barkana et al. However,
the derived value of the Hubble constant is significantly different from the
value derived from the FGSE model. We find that the value of the Hubble
constant is given by H_0 = 69 +18/-12 (1-K) and 74 +18/-17 (1-K) km/s/Mpc with
and without a constraint on the cluster's mass, respectively, where K is the
convergence of the cluster at the position of the galaxy and the range for each
value is defined by Delta chi-squared = reduced chi-squared. Presently, the
best achievable fit for this system is not as good as for PG 1115+080, which
also has recently been used to constrain the Hubble constant, and the
degeneracy is large. Possibilities for improving the fit and reducing the
degeneracy are discussed.Comment: 22 pages in aaspp style including 6 tables and 5 figures, ApJ in
press (Nov. 1st issue
Spin gap in the Quasi-One-Dimensional S=1/2 Antiferromagnet: Cu2(1,4-diazacycloheptane)2Cl4
Cu_{2}(1,4-diazacycloheptane)_{2}Cl_{4} contains double chains of spin 1/2
Cu^{2+} ions. We report ac susceptibility, specific heat, and inelastic neutron
scattering measurements on this material. The magnetic susceptibility,
, shows a rounded maximum at T = 8 K indicative of a low dimensional
antiferromagnet with no zero field magnetic phase transition. We compare the
data to exact diagonalization results for various one dimensional
spin Hamiltonians and find excellent agreement for a spin ladder with
intra-rung coupling meV and two mutually frustrating
inter-rung interactions: meV and meV. The
specific heat in zero field is exponentially activated with an activation
energy meV. A spin gap is also found through inelastic
neutron scattering on powder samples which identify a band of magnetic
excitations for meV. Using sum-rules we derive an
expression for the dynamic spin correlation function associated with
non-interacting propagating triplets in a spin ladder. The van-Hove
singularities of such a model are not observed in our scattering data
indicating that magnetic excitations in Cu_{2}(1,4-diazacycloheptane)_{2}Cl_{4}
are more complicated. For magnetic fields above T specific
heat data versus temperature show anomalies indicating a phase transition to an
ordered state below T = 1 K.Comment: 9 pages, 8 postscript figures, LaTeX, Submitted to PRB 8/4/97, e-mail
Comments to [email protected]
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Analysis of Negative Revisions to Natural Gas Reserves in Texas
Early problems in overestimating effective porosity in some deep Delaware Basin carbonate reservoirs in the Permian Basin, particularly in District 8, resulted in noticeable negative revisions once these issues were resolved. However, the net negative volume of revisions for the Permian Basin (Districts 8, SA, and parts of 7B and 7C) was nearly an order of magnitude less than that for the Gulf Coast Basin. The largest negative revisions of total natural gas reserves were concentrated in the Gulf Coast within Texas Railroad Commission Districts 2, 3, and 4. District 4, with the largest volume of negative revisions, accounted for 56 percent of all negative revisions in Texas for the period 1966-1979. The total for the three districts equaled that of the whole state for the same period. Negative revisions of non-associated gas reserves in Districts 2, 3, and 4 accounted for more than two-thirds of the total gas negative revisions for the entire state from 1966 through 1979.
Large negative revisions were mainly due to a combination of interrelated factors. Among these, the original overestimation of natural gas reserves in the Texas Gulf Coast, fueled by optimism from market-related incentives, was significant. These estimates underwent insufficient critical review and reassessment, as supplies greatly exceeded demand. Continued high Reserves to Production (R/P) ratios into the 1960s further delayed reassessment. Technical variables such as water saturation, reservoir heterogeneity, and recovery factors, as well as non-technical variables including economic climate and regulatory controls, were analyzed.
Concern should be raised regarding the quality of reserve estimates declared during times of excess supply, as these reserves would not have undergone the test of extended maximum demand. However, there have been more frequent reviews of actual recoverable reserves over the last five years. Continued careful review of technical factors and awareness of the impacts of economic and regulatory environment changes suggest that extensive negative revisions over the next 10 to 20 years can be avoided.Bureau of Economic Geolog
Elastic energy of proteins and the stages of protein folding
We propose a universal elastic energy for proteins, which depends only on the
radius of gyration and the residue number . It is constructed using
physical arguments based on the hydrophobic effect and hydrogen bonding.
Adjustable parameters are fitted to data from the computer simulation of the
folding of a set of proteins using the CSAW (conditioned self-avoiding walk)
model. The elastic energy gives rise to scaling relations of the form
in different regions. It shows three folding stages
characterized by the progression with exponents , which we
identify as the unfolded stage, pre-globule, and molten globule, respectively.
The pre-globule goes over to the molten globule via a break in behavior akin to
a first-order phase transition, which is initiated by a sudden acceleration of
hydrogen bonding
A deep, high resolution survey of the low frequency radio sky
We report on the first wide-field, very long baseline interferometry (VLBI)
survey at 90 cm. The survey area consists of two overlapping 28 deg^2 fields
centred on the quasar J0226+3421 and the gravitational lens B0218+357. A total
of 618 sources were targeted in these fields, based on identifications from
Westerbork Northern Sky Survey (WENSS) data. Of these sources, 272 had flux
densities that, if unresolved, would fall above the sensitivity limit of the
VLBI observations. A total of 27 sources were detected as far as 2 arcdegrees
from the phase centre. The results of the survey suggest that at least 10% of
moderately faint (S~100 mJy) sources found at 90 cm contain compact components
smaller than ~0.1 to 0.3 arcsec and stronger than 10% of their total flux
densities. A ~90 mJy source was detected in the VLBI data that was not seen in
the WENSS and NRAO VLA Sky Survey (NVSS) data and may be a transient or highly
variable source that has been serendipitously detected. This survey is the
first systematic (and non-biased), deep, high-resolution survey of the
low-frequency radio sky. It is also the widest field of view VLBI survey with a
single pointing to date, exceeding the total survey area of previous higher
frequency surveys by two orders of magnitude. These initial results suggest
that new low frequency telescopes, such as LOFAR, should detect many compact
radio sources and that plans to extend these arrays to baselines of several
thousand kilometres are warranted.Comment: Accepted by The Astrophysical Journal. 39 pages, 4 figure
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