4,177 research outputs found
A Nonparametric Conjugate Prior Distribution for the Maximizing Argument of a Noisy Function
We propose a novel Bayesian approach to solve stochastic optimization
problems that involve finding extrema of noisy, nonlinear functions. Previous
work has focused on representing possible functions explicitly, which leads to
a two-step procedure of first, doing inference over the function space and
second, finding the extrema of these functions. Here we skip the representation
step and directly model the distribution over extrema. To this end, we devise a
non-parametric conjugate prior based on a kernel regressor. The resulting
posterior distribution directly captures the uncertainty over the maximum of
the unknown function. We illustrate the effectiveness of our model by
optimizing a noisy, high-dimensional, non-convex objective function.Comment: 9 pages, 5 figure
The Impact of Dry Midlevel Air on Hurricane Intensity in Idealized Simulations with No Mean Flow
This study examines the potential negative influences of dry midlevel air on the development of tropical cyclones (specifically, its role in enhancing cold downdraft activity and suppressing storm development). The Weather Research and Forecasting model is used to construct two sets of idealized simulations of hurricane development in environments with different configurations of dry air. The first set of simulations begins with dry air located north of the vortex center by distances ranging from 0 to 270 km, whereas the second set of simulations begins with dry air completely surrounding the vortex, but with moist envelopes in the vortex core ranging in size from 0 to 150 km in radius. No impact of the dry air is seen for dry layers located more than 270 km north of the initial vortex center (approximately 3 times the initial radius of maximum wind). When the dry air is initially closer to the vortex center, it suppresses convective development where it entrains into the storm circulation, leading to increasingly asymmetric convection and slower storm development. The presence of dry air throughout the domain, including the vortex center, substantially slows storm development. However, the presence of a moist envelope around the vortex center eliminates the deleterious impact on storm intensity. Instead, storm size is significantly reduced. The simulations suggest that dry air slows intensification only when it is located very close to the vortex core at early times. When it does slow storm development, it does so primarily by inducing outward- moving convective asymmetries that temporarily shift latent heating radially outward away from the high-vorticity inner core
HIIphot: Automated Photometry of HII Regions Applied to M51
We have developed a robust, automated method, hereafter designated HIIphot,
which enables accurate photometric characterization of HII regions while
permitting genuine adaptivity to irregular source morphology. HIIphot utilizes
object-recognition techniques to make a first guess at the shapes of all
sources then allows for departure from such idealized ``seeds'' through an
iterative growing procedure. Photometric corrections for spatially coincident
diffuse emission are derived from a low-order surface fit to the background
after exclusion of all detected sources. We present results for the
well-studied, nearby spiral M51 in which 1229 HII regions are detected above
the 5-sigma level. A simple, weighted power-law fit to the measured H-alpha
luminosity function (HII LF) above log L_H-alpha = 37.6 gives alpha =
-1.75+/-0.06, despite a conspicuous break in the HII LF observed near L_H-alpha
= 10^38.9. Our best- fit slope is marginally steeper than measured by Rand
(1992), perhaps reflecting our increased sensitivity at low luminosities and to
notably diffuse objects. HII regions located in interarm gaps are
preferentially less luminous than counterparts which constitute M51's
grand-design spiral arms and are best fit with a power-law slope of alpha =
-1.96+/-0.15. We assign arm/interarm status for HII regions based upon the
varying surface brightness of diffuse emission as a function of position
throughout the image. Using our measurement of the integrated flux contributed
by resolved HII regions in M51, we estimate the diffuse fraction to be
approximately 0.45 -- in agreement with the determination of Greenawalt et al.
(1998). Automated processing of degraded datasets is undertaken to gauge
systematic effects associated with limiting spatial resolution and sensitivity.Comment: 41 pages, 14 figures, Postscript version with high-resolution figures
at ftp://ftp.aoc.nrao.edu/staff/dthilker/preprint
Development and Persistence of 'Static' or 'Dead' Zones in Flows
Certain ceramic products are formed through extrusion processes, where a slurry is forced through small openings to form such products as filters. At the top is a large tank. The slurry is forced through the tank into an extrusion chamber, and then out through slots to form the finished product.
[The slurry may be thought of as a mixture of clay (or other polymers), water, and other binders. There are many ways to model this mixture, some of which will be discussed in this report. For instance, the slurry can be modeled as a non-Newtonian fluid, a two-phase flow with liquid and solids, or a viscoelastic fluid. One can also model the mixture as an elongated particle suspension in water, where changes in the orientation of the particles could affect the flow.]
After the extrusion process is complete, one finds that ‘dead zones’ of dry paste accumulate in two areas. Most prominently, they occur at the lower corners of the tank. They also occur on the floor of the extrusion chamber near the slots, both near and away from the walls. Since we will consider wall effects in the tank, for the extrusion chamber we consider only flow cells sufficiently far away from the walls. Then we may exploit the periodic nature of the device and consider only a single flow cell.
The aim of this project is to determine the formation mechanisms of these dead zones, and see how they affect the overall flow
Spitzer 24-micron Time-Series Observations of the Eclipsing M-dwarf Binary GU Bootis
We present a set of {\it Spitzer} 24m MIPS time series observations of
the M-dwarf eclipsing binary star GU Bo\"otis. Our data cover three secondary
eclipses of the system: two consecutive events and an additional eclipse six
weeks later. The study's main purpose is the long wavelength (and thus limb
darkening-independent) characterization of GU Boo's light curve, allowing for
independent verification of the results of previous optical studies. Our
results confirm previously obtained system parameters. We further compare GU
Boo's measured 24m flux density to the value predicted by spectral fitting
and find no evidence for circumstellar dust. In addition to GU Boo, we
characterize (and show examples of) light curves of other objects in the field
of view. Analysis of these light curves serves to characterize the photometric
stability and repeatability of {\it Spitzer's} MIPS 24\micron array over short
(days) and long (weeks) timescales at flux densities between approximately
300--2,000Jy. We find that the light curve root mean square about the
median level falls into the 1--4% range for flux densities higher than 1mJy.
Finally, we comment on the fluctuations of the 24\micron background on short
and long timescales.Comment: ApJ accepted. 10 pages, 12 figure
Quantum steering ellipsoids, extremal physical states and monogamy
A Corrigendum for this article has been published in 2015 New J. Phys. 17 019501Any two-qubit state can be faithfully represented by a steering ellipsoid inside the Bloch sphere, but not every ellipsoid inside the Bloch sphere corresponds to a two-qubit state. We give necessary and sufficient conditions for when the geometric data describe a physical state and investigate maximal volume ellipsoids lying on the physical-unphysical boundary. We derive monogamy relations for steering that are strictly stronger than the Coffman-Kundu- Wootters (CKW) inequality for monogamy of concurrence. The CKW result is thus found to follow from the simple perspective of steering ellipsoid geometry. Remarkably, we can also use steering ellipsoids to derive non-trivial results in classical Euclidean geometry, extending Eulers inequality for the circumradius and inradius of a triangle.The EPSRC and the ARC Centre of Excellence grant no. CE110001027. DJ is funded by the Royal
Society. TR would like to thank the Leverhulme Trust. SJ acknowledges EPSRC grant EP/
K022512/1
A Note on (Meta)stable Brane Configurations in MQCD
We examine the M-theory version of SQCD which is known as MQCD. In the IIA
limit, this theory appears to have a supersymmetry-breaking brane configuration
which corresponds to the meta-stable state of N=1 SU(Nc) SQCD. However, the
behavior at infinity of this non-supersymmetric brane construction differs from
that of the supersymmetric ground state of MQCD. We interpret this to mean that
it is not a meta-stable state in MQCD, but rather a state in another theory.
This provides a concrete example of the fact that, while MQCD accurately
describes the supersymmetric features of SCQD, it fails to reproduce its
non-supersymmetric features (such as meta-stable states) not only
quantitatively but also qualitatively.Comment: 30 pages, 7 figures, harvmac. v2 typo correcte
Tobacco smoke particles and indoor air quality (ToPIQ) - the protocol of a new study
Environmental tobacco smoke (ETS) is a major contributor to indoor air pollution. Since decades it is well documented that ETS can be harmful to human health and causes premature death and disease. In comparison to the huge research on toxicological substances of ETS, less attention was paid on the concentration of indoor ETS-dependent particulate matter (PM). Especially, investigation that focuses on different tobacco products and their concentration of deeply into the airways depositing PM-fractions (PM10, PM2.5 and PM1) must be stated. The tobacco smoke particles and indoor air quality study (ToPIQS) will approach this issue by device supported generation of indoor ETS and simultaneously measurements of PM concentration by laser aerosol spectrometry. Primarily, the ToPIQ study will conduct a field research with focus on PM concentration of different tobacco products and within various microenvironments. It is planned to extend the analysis to basic research on influencing factors of ETS-dependent PM concentration
Phase-field model for grain boundary grooving in multi-component thin films
Polycrystalline thin films can be unstable with respect to island formation
(agglomeration) through grooving where grain boundaries intersect the free
surface and/or thin film-substrate interface. We develop a phase-field model to
study the evolution of the phases, composition, microstructure and morphology
of such thin films. The phase-field model is quite general, describing
compounds and solid solution alloys with sufficient freedom to choose
solubilities, grain boundary and interface energies, and heats of segregation
to all interfaces. We present analytical results which describe the interface
profiles, with and without segregation, and confirm them using numerical
simulations. We demonstrate that the present model accurately reproduces the
theoretical grain boundary groove angles both at and far from equilibrium. As
an example, we apply the phase-field model to the special case of a Ni(Pt)Si
(Ni/Pt silicide) thin film on an initially flat silicon substrate.Comment: 12 pages, 5 figures, submitted to Modelling Simulation Mater. Sci.
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