2,429 research outputs found
Regions of star formation: chemical issues. Discussion session
This is an electronic version of the lecture presented at the Astronomical Society of the Pacific Conference held in Granada, Spain, 24-28 September, 2002
Smilansky's model of irreversible quantum graphs, II: the point spectrum
In the model suggested by Smilansky one studies an operator describing the
interaction between a quantum graph and a system of K one-dimensional
oscillators attached at different points of the graph. This paper is a
continuation of our investigation of the case K>1. For the sake of simplicity
we consider K=2, but our argument applies to the general situation. In this
second paper we apply the variational approach to the study of the point
spectrum.Comment: 18 page
The effects of turbulence intensity on the downstream performance of horizontal axis tidal stream turbines
This study focuses on a comparison of model results from a blade element momen-
tum computational fluid dynamics (BEM-CFD) steady state RANS model, and a BEMT model that accounts
for the wake generated from upstream turbines using analytic expressions. Rotor forces are
calculated using 3D hydrofoil profile data. Both techniques are validated against ex- isting
experimental data, and then used to assess the power extraction of downstream turbines. Turbulent
inflow conditions of 3% and 7% are applied, and the results of power extraction as- sessed.
Particular attention is paid to the velocity field, with respect to the downstream wake, to assess
how the turbulence characteristics effect the recovery rate and downstream power
potential. Both models highlight the different recovery rates of the two turbulent conditions
Gemini Spectroscopic Survey of Young Star Clusters in Merging/Interacting Galaxies. II. NGC 3256 Clusters
We present Gemini optical spectroscopy of 23 young star clusters in NGC3256.
We find that the cluster ages range are from few Myr to ~150 Myr. All these
clusters are relatively massive (2--40)x 10^{5} \msun$ and appear to be of
roughly 1.5 \zo metallicity. The majority of the clusters in our sample follow
the same rotation curve as the gas and hence were presumably formed in the
molecular-gas disk. However, a western subsample of five clusters has
velocities that deviate significantly from the gas rotation curve. These
clusters may either belong to the second spiral galaxy of the merger or may
have formed in tidal-tail gas falling back into the system. We discuss our
observations in light of other known cluster populations in merging galaxies,
and suggest that NGC 3256 is similar to Arp 220, and hence may become an
Ultra-luminous Infrared Galaxy as the merger progresses and the star-formation
rate increases.
Some of the clusters which appeared as isolated in our ground-based images
are clearly resolved into multiple sub-components in the HST-ACS images. The
same effect has been observed in the Antennae galaxies, showing that clusters
are often not formed in isolation, but instead tend to form in larger groups or
cluster complexes.Comment: 20 pages, 10 figures, 3 tables; Accepted Ap
A comparison of numerical modelling techniques for tidal stream turbine analysis
To fully understand the performance of tidal stream turbines for the creation of
ocean ren wable energy, a range of computational models is required. We review and
compare results from several models at different length scales. Models under review include blade
element momentum theory (BEMT), blade element actuation disk RANS-CFD, blade- resolved RANS-CFD and
coastal models based on the shallow water equations. Three sets of
experimental results are used for model validation
Spatial Analyses of Oral Polio Vaccine Transmission in an Community Vaccinated With Inactivated Polio Vaccine.
Background: Understanding the spatial dynamics of oral polio vaccine (OPV) transmission will improve resource targeting. Mexico provides a natural laboratory, as it uses inactivated polio vaccine routinely as well as OPV bi-annually. Methods: Using geospatial maps, we measured the distance and density of OPV vaccinees' shedding in the areas nearest to unvaccinated households in 3 Mexican villages. Comparison of transmission to unvaccinated households utilized a mixed effects logistic regression with random effects for household and time, adjusted for age, gender, area, and running water. Results: The median distance from an unvaccinated household to its nearest OPV-shedding household was 85 meters (interquartile range, 46-145) and the median number of vaccinees shedding OPV within 200 m was 3 (2-6). Transmission to unvaccinated households occurred by day 1. There was no association (odds ratio [OR] 1.04; 95% credible interval [CrI] 0.92-1.16) between the distance from OPV shedding and the odds of transmission. The number of OPV vaccinees shedding within 200 m came close to a significant association with unvaccinated transmission (OR 0.93; CrI 0.84-1.01), but this was not the case for households 100 or 500 m apart. Results were consistent across the 3 villages. Conclusions: Geospatial analysis did not predict community transmission from vaccinated to unvaccinated households, because OPV use resulted in rapid, low transmission levels. This finding supports the global cessation of OPV
Spherical coupled-cluster theory for open-shell nuclei
A microscopic description of nuclei is important to understand the nuclear
shell-model from fundamental principles. This is difficult to achieve for more
than the lightest nuclei without an effective approximation scheme. The purpose
of this paper is to define and evaluate an approximation scheme that can be
used to study nuclei that are described as two particles attached to a closed
(sub-)shell nucleus. The equation-of-motion coupled-cluster formalism has been
used to obtain ground and excited state energies. This method is based on the
diagonalization of a non-Hermitian matrix obtained from a similarity
transformation of the many-body nuclear Hamiltonian. A chiral interaction at
the next-to-next-to-next-to leading order using a cutoff at 500 MeV was used.
The ground state energies of Li and He were in good agreement with
a no-core shell-model calculation using the same interaction. Several excited
states were also produced with overall good agreement. Only the
excited state in Li showed a sizable deviation. The ground state energies
of O, F and Ne were converged, but underbound
compared to experiment. Moreover, the calculated spectra were converged and
comparable to both experiment and shell-model studies in this region. Some
excited states in O were high or missing in the spectrum. It was also
shown that the wave function for both ground and excited states separates into
an intrinsic part and a Gaussian for the center-of-mass coordinate. Spurious
center-of-mass excitations are clearly identified.Comment: 23 pages, 17 figures. Accepted for publication in PR
Traditional turbulence methods and novel visualisation techniques for coastal flow model in order to deploy tidal stream turbines
Characteristics of flow in the coastal regions are strongly influenced by the
topography of the seabed and understanding of these features is necessary before installation of
tidal stream turbines (TST). In this paper, the bathymetry of a potential TST deployment site is
surveyed using an echosounder and the resulting data is used in the development of the geometric
model. The steady state k-É› and transient Large Eddy Simulation (LES) turbulence methods are
employed.
The stream surface visualisation method employed has important inherent characteristics that can
enhance the visual perception of complex flow structures [1]. In this method lighting and shading
reinforce the perception of shape and depth, images or textures can be mapped to the surface
primitives providing additional visual information, colour and transparency can be used to convey
additional data attributes.
The results of all cases are compared with the flow data transect gathered by
Acoustic Doppler Current Profiler (ADCP). It has been understood that the k-É› method can predict
the flow pattern with relatively good accuracy near the main features of the domain and the LES
model has the ability to simulate some important flow patterns because of the bathymetry
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