11,284 research outputs found
Analysis of low-temperature direct-condensing vapor-chamber fin and conducting fin radiators
Analysis of flat, direct-condensing finned-tube space radiator with vapor chamber, and central fin tube geometries for low temperature Rankine space power electric generating syste
Quantum Gauge Equivalence in QED
We discuss gauge transformations in QED coupled to a charged spinor field,
and examine whether we can gauge-transform the entire formulation of the theory
from one gauge to another, so that not only the gauge and spinor fields, but
also the forms of the operator-valued Hamiltonians are transformed. The
discussion includes the covariant gauge, in which the gauge condition and
Gauss's law are not primary constraints on operator-valued quantities; it also
includes the Coulomb gauge, and the spatial axial gauge, in which the
constraints are imposed on operator-valued fields by applying the
Dirac-Bergmann procedure. We show how to transform the covariant, Coulomb and
spatial axial gauges to what we call
``common form,'' in which all particle excitation modes have identical
properties. We also show that, once that common form has been reached, QED in
different gauges has a common time-evolution operator that defines
time-translation for states that represent systems of electrons and photons.
By combining gauge transformations with changes of representation from
standard to common form, the entire apparatus of a gauge theory can be
transformed from one gauge to another.Comment: Contribution for a special issue of Foundations of Physics honoring
Fritz Rohrlich; edited by Larry P. Horwitz, Tel-Aviv University, and Alwyn
van der Merwe, University of Denver (Plenum Publishing, New York); 40 pages,
REVTEX, Preprint UCONN-93-3, 1 figure available upon request from author
Precision Measurements of Stretching and Compression in Fluid Mixing
The mixing of an impurity into a flowing fluid is an important process in
many areas of science, including geophysical processes, chemical reactors, and
microfluidic devices. In some cases, for example periodic flows, the concepts
of nonlinear dynamics provide a deep theoretical basis for understanding
mixing. Unfortunately, the building blocks of this theory, i.e. the fixed
points and invariant manifolds of the associated Poincare map, have remained
inaccessible to direct experimental study, thus limiting the insight that could
be obtained. Using precision measurements of tracer particle trajectories in a
two-dimensional fluid flow producing chaotic mixing, we directly measure the
time-dependent stretching and compression fields. These quantities, previously
available only numerically, attain local maxima along lines coinciding with the
stable and unstable manifolds, thus revealing the dynamical structures that
control mixing. Contours or level sets of a passive impurity field are found to
be aligned parallel to the lines of large compression (unstable manifolds) at
each instant. This connection appears to persist as the onset of turbulence is
approached.Comment: 5 pages, 5 figure
A Systematic Review and Meta-Analysis Estimating the Expected Dropout Rates in Randomized Controlled Trials on Yoga Interventions
© 2016 Holger Cramer et al. A reasonable estimation of expected dropout rates is vital for adequate sample size calculations in randomized controlled trials (RCTs). Underestimating expected dropouts rates increases the risk of false negative results while overestimating rates results in overly large sample sizes, raising both ethical and economic issues. To estimate expected dropout rates in RCTs on yoga interventions, MEDLINE/PubMed, Scopus, IndMED, and the Cochrane Library were searched through February 2014; a total of 168 RCTs were meta-analyzed. Overall dropout rate was 11.42% (95% confidence interval [CI] = 10.11%, 12.73%) in the yoga groups; rates were comparable in usual care and psychological control groups and were slightly higher in exercise control groups (rate = 14.53%; 95% CI = 11.56%, 17.50%; odds ratio = 0.82; 95% CI = 0.68, 0.98; p = 0.03). For RCTs with durations above 12 weeks, dropout rates in yoga groups increased to 15.23% (95% CI = 11.79%, 18.68%). The upper border of 95% CIs for dropout rates commonly was below 20% regardless of study origin, health condition, gender, age groups, and intervention characteristics; however, it exceeded 40% for studies on HIV patients or heterogeneous age groups. In conclusion, dropout rates can be expected to be less than 15 to 20% for most RCTs on yoga interventions. Yet dropout rates beyond 40% are possible depending on the participants' sociodemographic and health condition
Persistent Transport Barrier on the West Florida Shelf
Analysis of drifter trajectories in the Gulf of Mexico has revealed the
existence of a region on the southern portion of the West Florida Shelf (WFS)
that is not visited by drifters that are released outside of the region. This
so-called ``forbidden zone'' (FZ) suggests the existence of a persistent
cross-shelf transport barrier on the southern portion of the WFS. In this
letter a year-long record of surface currents produced by a Hybrid-Coordinate
Ocean Model simulation of the WFS is used to identify Lagrangian coherent
structures (LCSs), which reveal the presence of a robust and persistent
cross-shelf transport barrier in approximately the same location as the
boundary of the FZ. The location of the cross-shelf transport barrier undergoes
a seasonal oscillation, being closer to the coast in the summer than in the
winter. A month-long record of surface currents inferred from high-frequency
(HF) radar measurements in a roughly 60 km 80 km region on the WFS off
Tampa Bay is also used to identify LCSs, which reveal the presence of robust
transient transport barriers. While the HF-radar-derived transport barriers
cannot be unambiguously linked to the boundary of the FZ, this analysis does
demonstrate the feasibility of monitoring transport barriers on the WFS using a
HF-radar-based measurement system. The implications of a persistent cross-shelf
transport barrier on the WFS for the development of harmful algal blooms on the
shoreward side of the barrier are considered.Comment: Submitted to Geophysical Research Letter
Experimental Measurements of Stretching Fields in Fluid Mixing
The mixing of an impurity into a flowing fluid is an important process in many areas of science, including geophysical processes, chemical reactors, and microfluidic devices. In some cases, for example periodic flows, the concepts of nonlinear dynamics provide a deep theoretical basis for understanding mixing. Unfortunately, the building blocks of this theory, i.e. the fixed points and invariant manifolds of the associated Poincaré map, have remained inaccessible to direct experimental study, thus limiting the insight that could be obtained. Using precision measurements of tracer particle trajectories in a two-dimensional fluid flow producing chaotic mixing, we directly measure the time-dependent stretching fields. These quantities, previously available only numerically, attain local maxima along lines coinciding with the stable and unstable manifolds, thus revealing the dynamical structures that control mixing. Contours or level sets of a passive impurity field are found to be aligned parallel to the lines of large stretching at each instant, thus explaining what happens as one stirs milk into coffee. --author-supplied descriptio
Mindfulness- and acceptance-based interventions for psychosis: A Systematic Review and Meta-analysis
Background: Mindfulness- and acceptance-based interventions are increasingly studied as a potential treatment for a variety of mental conditions. Objective: To assess the effects of mindfulness- and acceptance-based interventions on psychotic symptoms and hospitalization in patients with psychosis Methods: MEDLINE/PubMed, Embase, the Cochrane Library, and PsycINFO were screened from inception through April 2015. Randomized controlled trials (RCTs) were analyzed when they assessed psychotic symptoms or hospitalization in patients with psychosis; affect, acceptance, mindfulness, and safety were defined as secondary outcomes. Results: Eight RCTs with a total of 434 patients comparing mindfulness- based (4 RCTs) or acceptancebased interventions (4 RCTs) to treatment as usual or attention control were included. Six RCTs had low risk of bias. Moderate evidence was found for short-term effects on total psychotic symptoms, positive symptoms, hospitalization rates, duration of hospitalization, and mindfulness and for long-term effects on total psychotic symptoms and duration of hospitalization. No evidence was found for effects on negative symptoms, affect, or acceptance. No serious adverse events were reported. Conclusion: Mindfulness- and acceptance- based interventions can be recommended as an additional treatment for patients with psychosis
Craniosacral therapy for chronic pain: a systematic review and meta-analysis of randomized controlled trials.
OBJECTIVES:To systematically assess the evidence of Craniosacral Therapy (CST) for the treatment of chronic pain. METHODS:PubMed, Central, Scopus, PsycInfo and Cinahl were searched up to August 2018. Randomized controlled trials (RCTs) assessing the effects of CST in chronic pain patients were eligible. Standardized mean differences (SMD) and 95% confidence intervals (CI) were calculated for pain intensity and functional disability (primary outcomes) using Hedges' correction for small samples. Secondary outcomes included physical/mental quality of life, global improvement, and safety. Risk of bias was assessed using the Cochrane tool. RESULTS:Ten RCTs of 681 patients with neck and back pain, migraine, headache, fibromyalgia, epicondylitis, and pelvic girdle pain were included. CST showed greater post intervention effects on: pain intensity (SMD = -0.32, 95%CI = [- 0.61,-0.02]) and disability (SMD = -0.58, 95%CI = [- 0.92,-0.24]) compared to treatment as usual; on pain intensity (SMD = -0.63, 95%CI = [- 0.90,-0.37]) and disability (SMD = -0.54, 95%CI = [- 0.81,-0.28]) compared to manual/non-manual sham; and on pain intensity (SMD = -0.53, 95%CI = [- 0.89,-0.16]) and disability (SMD = -0.58, 95%CI = [- 0.95,-0.21]) compared to active manual treatments. At six months, CST showed greater effects on pain intensity (SMD = -0.59, 95%CI = [- 0.99,-0.19]) and disability (SMD = -0.53, 95%CI = [- 0.87,-0.19]) versus sham. Secondary outcomes were all significantly more improved in CST patients than in other groups, except for six-month mental quality of life versus sham. Sensitivity analyses revealed robust effects of CST against most risk of bias domains. Five of the 10 RCTs reported safety data. No serious adverse events occurred. Minor adverse events were equally distributed between the groups. DISCUSSION:In patients with chronic pain, this meta-analysis suggests significant and robust effects of CST on pain and function lasting up to six months. More RCTs strictly following CONSORT are needed to further corroborate the effects and safety of CST on chronic pain. PROTOCOL REGISTRATION AT PROSPERO:CRD42018111975
Friction Drag on a Particle Moving in a Nematic Liquid Crystal
The flow of a liquid crystal around a particle does not only depend on its
shape and the viscosity coefficients but also on the direction of the
molecules. We studied the resulting drag force on a sphere moving in a nematic
liquid crystal (MBBA) in a low Reynold's number approach for a fixed director
field (low Ericksen number regime) using the computational artificial
compressibility method. Taking the necessary disclination loop around the
sphere into account, the value of the drag force anisotropy
(F_\perp/F_\parallel=1.50) for an exactly computed field is in good agreement
with experiments (~1.5) done by conductivity diffusion measurements. We also
present data for weak anchoring of the molecules on the particle surface and of
trial fields, which show to be sufficiently good for most applications.
Furthermore, the behaviour of the friction close to the transition point
nematic isotropic and for a rod-like and a disc-like liquid crystal will be
given.Comment: 23 pages RevTeX, including 3 PS figures, 1 PS table and 1 PS-LaTeX
figure; Accepted for publication in Phys. Rev.
Rain, power laws, and advection
Localized rain events have been found to follow power-law size and duration
distributions over several decades, suggesting parallels between precipitation
and seismic activity [O. Peters et al., PRL 88, 018701 (2002)]. Similar power
laws are generated by treating rain as a passive tracer undergoing advection in
a velocity field generated by a two-dimensional system of point vortices.Comment: 7 pages, 4 figure
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