2,641 research outputs found
NASA/DOD Aerospace Knowledge Diffusion Research Project. Paper 21: Technological innovation and technical communications: Their place in aerospace engineering curricula. A survey of European, Japanese, and US Aerospace Engineers and Scientists
Aerospace engineers and scientists from Western Europe, Japan, and the United States were surveyed as part of the NASA/DoD Aerospace Knowledge Diffusion Research Project. Questionnaires were used to solicit their opinions regarding the following: (1) the importance of technical communications to their profession; (2) the use and production of technical communications; and (3) their views about the appropriate content of an undergraduate course in technical communications. The ability to communicate technical information effectively was very important to the aerospace engineers and scientists who participated in the study. A considerable portion of their working week is devoted to using and producing technical information. The types of technical communications used and produced varied within and among the three groups. The type of technical communication product used and produced appears to be related to respondents' professional duties. Respondents from the three groups made similar recommendations regarding the principles, mechanics, and on-the-job communications to be included in an undergraduate technical communications course for aerospace majors
Universal Impedance Fluctuations in Wave Chaotic Systems
We experimentally investigate theoretical predictions of universal impedance
fluctuations in wave chaotic systems using a microwave analog of a quantum
chaotic infinite square well potential. Our approach emphasizes the use of the
radiation impedance to remove the non-universal effects of the particular
coupling from the outside world to the scatterer. Specific predictions that we
test include the probability distribution functions (PDFs) of the real (related
to the local density of states in disordered metals) and imaginary parts of the
normalized cavity impedance, the equality of the variances of these PDFs, and
the dependence of the universal PDFs on a single control parameter
characterizing the level of loss. We find excellent agreement between the
statistical data and theoretical predictions.Comment: 5 pages, 3 figures, submitted to Phys. Rev. Let
Structure prediction based on ab initio simulated annealing for boron nitride
Possible crystalline modifications of chemical compounds at low temperatures
correspond to local minima of the energy landscape. Determining these minima
via simulated annealing is one method for the prediction of crystal structures,
where the number of atoms per unit cell is the only information used. It is
demonstrated that this method can be applied to covalent systems, at the
example of boron nitride, using ab initio energies in all stages of the
optimization, i.e. both during the global search and the subsequent local
optimization. Ten low lying structure candidates are presented, including both
layered structures and 3d-network structures such as the wurtzite and zinc
blende types, as well as a structure corresponding to the beta-BeO type
Universal Statistics of the Scattering Coefficient of Chaotic Microwave Cavities
We consider the statistics of the scattering coefficient S of a chaotic
microwave cavity coupled to a single port. We remove the non-universal effects
of the coupling from the experimental S data using the radiation impedance
obtained directly from the experiments. We thus obtain the normalized, complex
scattering coefficient whose Probability Density Function (PDF) is predicted to
be universal in that it depends only on the loss (quality factor) of the
cavity. We compare experimental PDFs of the normalized scattering coefficients
with those obtained from Random Matrix Theory (RMT), and find excellent
agreement. The results apply to scattering measurements on any wave chaotic
system.Comment: 10 pages, 8 Figures, Fig.7 in Color, Submitted to Phys. Rev.
Phase Structure of Z(3)-Polyakov-Loop Models
We study effective lattice actions describing the Polyakov loop dynamics
originating from finite-temperature Yang-Mills theory. Starting with a
strong-coupling expansion the effective action is obtained as a series of
Z(3)-invariant operators involving higher and higher powers of the Polyakov
loop, each with its own coupling. Truncating to a subclass with two couplings
we perform a detailed analysis of the statistical mechanics involved. To this
end we employ a modified mean field approximation and Monte Carlo simulations
based on a novel cluster algorithm. We find excellent agreement of both
approaches concerning the phase structure of the theories. The phase diagram
exhibits both first and second order transitions between symmetric,
ferromagnetic and anti-ferromagnetic phases with phase boundaries merging at
three tricritical points. The critical exponents nu and gamma at the continuous
transition between symmetric and anti-ferromagnetic phases are the same as for
the 3-state Potts model.Comment: 20 pages, 22 figure
Associations between Sleep Problems and Anxiety in Youth
Most of us are able to recall sleepless nights during periods of heightened anxiety, and the associations between sleep and anxiety are sufficiently well established to be acknowledged in the DSM-IV (American Psychiatric Association, 1994). Indeed, sleeprelated difficulties are included in the diagnostic criteria for posttraumatic stress disorder, acute stress disorder and generalized anxiety disorder. In contrast to the wealth of literature highlighting associations between sleep problems and anxiety in adults, less is known about these associations in children and adolescents.
It is important to understand associations between disorders as there is evidence to suggest that co-occurring difficulties may result in greater impairment than those occurring alone. For example, a study focusing upon depression found that individuals with co-occurring disorders were more likely to demonstrate suicidal behaviour as compared to those with pure depression (Rohde et al., 1991). Furthermore, focusing upon children is important as various difficulties, including anxiety, may appear early in life and persist into adulthood (Kim- Cohen et al., 2003). This article summarises some key findings with regards to the associations between sleep problems and anxiety in youth and proposes possible clinical implications of this research
Quantifying the Uncertainty in Ground-Based GNSS-Reflectometry Sea Level Measurements
Global Navigation Satellite System reflectometry (GNSS-R) tide gauges are a promising alternative to traditional tide gauges. However, the precision of GNSS-R sea-level measurements when compared to measurements from a colocated tide gauge is highly variable, with no clear indication of what causes the variability. Here, we present a modeling technique to estimate the precision of GNSS-R sea-level measurements that relies on creating and analyzing synthetic signal-to-noise-ratio (SNR) data. The modeled value obtained from the synthetic SNR data is compared to observed root mean square error between GNSS-R measurements and a colocated tide gauge at five sites and using two retrieval methods: spectral analysis and inverse modeling. We find that the inverse method is more precise than the spectral analysis method by up to 60 for individual measurements but the two methods perform similarly for daily and monthly means. We quantify the contribution of dominant effects to the variations in precision and find that noise is the dominant source of uncertainty for spectral analysis whereas the effect of the dynamic sea surface is the dominant source of uncertainty for the inverse method. Additionally, we test the sensitivity of sea-level measurements to the choice of elevation angle interval and find that the spectral analysis method is more sensitive to the choice of elevation angle interval than the inverse method due to the effect of noise, which is greater at larger elevation angle intervals. Conversely, the effect of tropospheric delay increases for lower elevation angle intervals but is generally a minor contribution
All-Optical Production of a Degenerate Fermi Gas
We achieve degeneracy in a mixture of the two lowest hyperfine states of
Li by direct evaporation in a CO laser trap, yielding the first
all-optically produced degenerate Fermi gas. More than atoms are
confined at temperatures below K at full trap depth, where the Fermi
temperature for each state is K. This degenerate two-component mixture
is ideal for exploring mechanisms of superconductivity ranging from Cooper
pairing to Bose condensation of strongly bound pairs.Comment: 4 pgs RevTeX with 2 eps figs, to be published in Phys. Rev. Let
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