23,806 research outputs found
Nonparametric identification of a class of nonlinear close-coupled dynamic systems
A nonparametric identification technique for the identification of close coupled dynamic systems with arbitrary memoryless nonlinearities is presented. The method utilizes noisy recorded data (acceleration, velocity and displacement) to identify the restoring forces in the system. The masses in the system are assumed to be known (or fairly well estimated from the design drawings). The restoring forces are expanded in a series of orthogonal polnomials and the coefficients of these polynomial expansions are obtained by using least square fit method. A particularly simple and computationally efficient method is proposed for dealing with separable restoring forces. The identified results are found to be relatively insensitive to measurement noise. An analysis of the effects of measurement noise on the quality of the estimates is given. The computations are shown to be relatively quick (when compared say to the Wiener identification method) and the core storage required relatively small, making the method suitable for onboard identification of large space structures
Search for surviving companions in type Ia supernova remnants
The nature of the progenitor systems of type~Ia supernovae is still unclear.
One way to distinguish between the single-degenerate scenario and
double-degenerate scenario for their progenitors is to search for the surviving
companions. Using a technique that couples the results from multi-dimensional
hydrodynamics simulations with calculations of the structure and evolution of
main-sequence- and helium-rich surviving companions, the color and magnitude of
main-sequence- and helium-rich surviving companions are predicted as functions
of time. The surviving companion candidates in Galactic type~Ia supernova
remnants and nearby extragalactic type~Ia supernova remnants are discussed. We
find that the maximum detectable distance of main-sequence surviving companions
(helium-rich surviving companions) is ~Mpc (~Mpc), if the
apparent magnitude limit is 27 in the absence of extinction, suggesting that
the Large and Small Magellanic Clouds and the Andromeda Galaxy are excellent
environments in which to search for surviving companions. However, only five
Ia~SNRs have been searched for surviving companions, showing little support for
the standard channels in the singe-degenerate scenario. To better understand
the progenitors of type Ia supernovae, we encourage the search for surviving
companions in other nearby type Ia supernova remnants.Comment: 25 pages, 5 figures, and 2 tables. Accepted for publication in Ap
Simulations of the symbiotic recurrent nova V407 Cyg. I. Accretion and shock evolutions
The shock interaction and evolution of nova ejecta with a wind from a red
giant star in a symbiotic binary system are investigated via three-dimensional
hydrodynamics simulations. We specifically model the March 2010 outburst of the
symbiotic recurrent nova V407~Cygni from the quiescent phase to its eruption
phase. The circumstellar density enhancement due to wind-white dwarf
interaction is studied in detail. It is found that the density-enhancement
efficiency depends on the ratio of the orbital speed to the red giant wind
speed. Unlike another recurrent nova, RS~Ophiuchi, we do not observe a strong
disk-like density enhancement, but instead observe an aspherical density
distribution with higher density in the equatorial plane than at
the poles. To model the 2010 outburst, we consider several physical parameters,
including the red giant mass loss rate, nova eruption energy, and ejecta mass.
A detailed study of the shock interaction and evolution reveals that the
interaction of shocks with the red giant wind generates strong Rayleigh-Taylor
instabilities. In addition, the presence of the companion and circumstellar
density enhancement greatly alter the shock evolution during the nova phase.
The ejecta speed after sweeping out most of the circumstellar medium decreases
to km-s, depending on model, which is consistent with the
observed extended redward emission in [N~II] lines in April 2011.Comment: ApJ, In Press. Simulation Animation: https://youtu.be/g5Nu7vDfCO
Monthly mean forecast experiments with the GISS model
The GISS general circulation model was used to compute global monthly mean forecasts for January 1973, 1974, and 1975 from initial conditions on the first day of each month and constant sea surface temperatures. Forecasts were evaluated in terms of global and hemispheric energetics, zonally averaged meridional and vertical profiles, forecast error statistics, and monthly mean synoptic fields. Although it generated a realistic mean meridional structure, the model did not adequately reproduce the observed interannual variations in the large scale monthly mean energetics and zonally averaged circulation. The monthly mean sea level pressure field was not predicted satisfactorily, but annual changes in the Icelandic low were simulated. The impact of temporal sea surface temperature variations on the forecasts was investigated by comparing two parallel forecasts for January 1974, one using climatological ocean temperatures and the other observed daily ocean temperatures. The use of daily updated sea surface temperatures produced no discernible beneficial effect
Modelling the spring ozone maximum and the interhemispheric asymmetry in the remote marine boundary layer 1. Comparison with surface and ozonesonde measurements
Here we report a modelling study of the spring ozone maximum and its
interhemispheric asymmetry in the remote marine boundary layer (MBL). The
modelled results are examined at the surface and on a series of time-height
cross sections at several locations spread over the Atlantic, the Indian, and
the Pacific Oceans. Comparison of model with surface measurements at remote MBL
stations indicate a close agreement. The most striking feature of the
hemispheric spring ozone maximum in the MBL can be most easily identified at
the NH sites of Westman Island, Bermuda, and Mauna Loa, and at the SH site of
Samoa. Modelled ozone vertical distributions in the troposphere are compared
with ozone profiles. For the Atlantic and the Indian sites, the model generally
produces a hemispheric spring ozone maximum close to those of the measurements.
The model also produces a spring ozone maximum in the northeastern and tropical
north Pacific close to those measurements, and at sites in the NH high
latitudes. The good agreement between model and measurements indicate that the
model can reproduce the proposed mechanisms responsible for producing the
spring ozone maximum in these regions of the MBL, lending confidence in the use
of the model to investigate MBL ozone chemistry (see part 2 and part 3). The
spring ozone maximum in the tropical central south Pacific and eastern
equatorial Pacific are less well reproduced by the model, indicating that both
the transport of precursors from biomass burning emissions taking place
in southeastern Asia, Australia, Oceania, southern Africa, and South America
are not well represented in the model in these regions. Overall, the model
produces a better simulation at sites where the stratosphere and biomass
burning emissions are the major contributors.Comment: 24 pages, 8 figure
Shell model description of the 14C dating beta decay with Brown-Rho-scaled NN interactions
We present shell model calculations for the beta-decay of the 14C ground
state to the 14N ground state, treating the states of the A=14 multiplet as two
0p holes in an 16O core. We employ low-momentum nucleon-nucleon (NN)
interactions derived from the realistic Bonn-B potential and find that the
Gamow-Teller matrix element is too large to describe the known lifetime. By
using a modified version of this potential that incorporates the effects of
Brown-Rho scaling medium modifications, we find that the GT matrix element
vanishes for a nuclear density around 85% that of nuclear matter. We find that
the splitting between the (J,T)=(1+,0) and (J,T)=(0+,1) states in 14N is
improved using the medium-modified Bonn-B potential and that the transition
strengths from excited states of 14C to the 14N ground state are compatible
with recent experiments.Comment: 4 pages, 5 figures Updated to include referee comments/suggestion
Estimating the costs and benefits of providing free public transit passes to students in Los Angeles County: lessons learned in applying a health lens to decision-making.
In spite of increased focus by public health to engage and work with non-health sector partners to improve the health of the general as well as special populations, only a paucity of studies have described and disseminated emerging lessons and promising practices that can be used to undertake this work. This article describes the process used to conduct a Health Impact Assessment of a proposal to provide free public transportation passes to students in Los Angeles County. This illustrative case example describes opportunities and challenges encountered in working with an array of cross-sector partners and highlights four important lessons learned: (1) the benefits and challenges associated with broad conceptualization of public issues; (2) the need for more comprehensive, longitudinal data systems and dynamic simulation models to inform decision-making; (3) the importance of having a comprehensive policy assessment strategy that considers health impacts as well as costs and feasibility; and (4) the need for additional efforts to delineate the interconnectivity between health and other agency priorities. As public health advances cross-sector work in the community, further development of these priorities will help advance meaningful collaboration among all partners
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