12,095 research outputs found
Transmission loss predictions for dissipative silencers of arbitrary cross section in the presence of mean flow
A numerical technique is developed for the analysis of dissipative silencers of arbitrary, but axially uniform, cross section. Mean gas flow is included in a central airway which is separated from a bulk reacting porous material by a concentric perforate screen. The analysis begins by employing the finite element method to extract the eigenvalues and associated eigenvectors for a silencer of infinite length. Point collocation is then used to match the expanded acoustic pressure and velocity fields in the silencer chamber to those in the inlet and outlet pipes. Transmission loss predictions are compared with experimental measurements taken for two automotive dissipative silencers with elliptical cross sections. Good agreement between prediction and experiment is observed both without mean flow and for a mean flow Mach number of 0.15. It is demonstrated also that the technique presented offers a considerable reduction in computational expenditure when compared to a three dimensional finite element analysis
Quasi-planar steep water waves
A new description for highly nonlinear potential water waves is suggested,
where weak 3D effects are included as small corrections to exact 2D equations
written in conformal variables. Contrary to the traditional approach, a small
parameter in this theory is not the surface slope, but it is the ratio of a
typical wave length to a large transversal scale along the second horizontal
coordinate. A first-order correction for the Hamiltonian functional is
calculated, and the corresponding equations of motion are derived for steep
water waves over an arbitrary inhomogeneous quasi-1D bottom profile.Comment: revtex4, 4 pages, no figure
Neutron Star Mergers Are the Dominant Source of the r-process in the Early Evolution of Dwarf Galaxies
There are many candidate sites of the r-process: core-collapse supernovae
(including rare magnetorotational core-collapse supernovae), neutron star
mergers, and neutron star/black hole mergers. The chemical enrichment of
galaxies---specifically dwarf galaxies---helps distinguish between these
sources based on the continual build-up of r-process elements. This technique
can distinguish between the r-process candidate sites by the clearest
observational difference---how quickly these events occur after the stars are
created. The existence of several nearby dwarf galaxies allows us to measure
robust chemical abundances for galaxies with different star formation
histories. Dwarf galaxies are especially useful because simple chemical
evolution models can be used to determine the sources of r-process material. We
have measured the r-process element barium with Keck/DEIMOS medium-resolution
spectroscopy. We present the largest sample of barium abundances (almost 250
stars) in dwarf galaxies ever assembled. We measure [Ba/Fe] as a function of
[Fe/H] in this sample and compare with existing [alpha/Fe] measurements. We
have found that a large contribution of barium needs to occur at more delayed
timescales than core-collapse supernovae in order to explain our observed
abundances, namely the significantly more positive trend of the r-process
component of [Ba/Fe] vs. [Fe/H] seen for [Fe/H] <~ -1.6 when compared to the
[Mg/Fe] vs. [Fe/H] trend. We conclude that neutron star mergers are the most
likely source of r-process enrichment in dwarf galaxies at early times.Comment: Accepted to ApJ on 2018 October 2
Bio-linguistic transition and Baldwin effect in an evolutionary naming-game model
We examine an evolutionary naming-game model where communicating agents are
equipped with an evolutionarily selected learning ability. Such a coupling of
biological and linguistic ingredients results in an abrupt transition: upon a
small change of a model control parameter a poorly communicating group of
linguistically unskilled agents transforms into almost perfectly communicating
group with large learning abilities. When learning ability is kept fixed, the
transition appears to be continuous. Genetic imprinting of the learning
abilities proceeds via Baldwin effect: initially unskilled communicating agents
learn a language and that creates a niche in which there is an evolutionary
pressure for the increase of learning ability.Our model suggests that when
linguistic (or cultural) processes became intensive enough, a transition took
place where both linguistic performance and biological endowment of our species
experienced an abrupt change that perhaps triggered the rapid expansion of
human civilization.Comment: 7 pages, minor changes, accepted in Int.J.Mod.Phys.C, proceedings of
Max Born Symp. Wroclaw (Poland), Sept. 2007. Java applet is available at
http://spin.amu.edu.pl/~lipowski/biolin.html or
http://www.amu.edu.pl/~lipowski/biolin.htm
Linking dwarf galaxies to halo building blocks with the most metal-poor star in Sculptor
Current cosmological models indicate that the Milky Way's stellar halo was
assembled from many smaller systems. Based on the apparent absence of the most
metal-poor stars in present-day dwarf galaxies, recent studies claimed that the
true Galactic building blocks must have been vastly different from the
surviving dwarfs. The discovery of an extremely iron-poor star (S1020549) in
the Sculptor dwarf galaxy based on a medium-resolution spectrum cast some doubt
on this conclusion. However, verification of the iron-deficiency and
measurements of additional elements, such as the alpha-element Mg, are
mandatory for demonstrating that the same type of stars produced the metals
found in dwarf galaxies and the Galactic halo. Only then can dwarf galaxy stars
be conclusively linked to early stellar halo assembly. Here we report
high-resolution spectroscopic abundances for 11 elements in S1020549,
confirming the iron abundance of less than 1/4000th that of the Sun, and
showing that the overall abundance pattern mirrors that seen in low-metallicity
halo stars, including the alpha-elements. Such chemical similarity indicates
that the systems destroyed to form the halo billions of years ago were not
fundamentally different from the progenitors of present-day dwarfs, and
suggests that the early chemical enrichment of all galaxies may be nearly
identical.Comment: 16 pages, including 2 figures. Accepted for publication in Nature. It
is embargoed for discussion in the press until formal publication in Natur
Measuring outcomes in complicated intra-abdominal infections
Purpose of review: Complicated intra-abdominal infections (cIAIs) are associated with significant morbidity and mortality. Clinical trials should help guide and improve the management of cIAIs. However, inappropriate selection or measurement of outcomes in cIAIs clinical trials can lead to misleading results on the effectiveness of interventions. This review aims to describe how outcomes are reported in randomized controlled trials evaluating antibiotic treatment for cIAIs and discuss how outcome reporting may be improved.
Recent findings: Commonly used primary outcomes are treatment success or failure, and these outcomes are endorsed by regulatory bodies. However, a consensus objective definition of either is not available and current measures are prone to bias. Variation exists in timing of outcome evaluation and analysis populations, which can lead to further bias. Use of core outcome sets can help standardize outcome reporting.
Summary: Inconsistency in outcome selection and reporting can lead to misleading results and impedes meta-analysis of data. Further progress, engaging clinical trialists, regulatory authorities, clinicians and patients is required to achieve consensus on which outcomes should be reported and how and when to measure them
- …