337 research outputs found
Correlation Functions for a Chain of Short Range Oscillators
We consider a system of harmonic oscillators with short range interactions and we study their correlation functions when the initial data is sampled with respect to the Gibbs measure. Such correlation functions display rapid oscillations that travel through the chain. We show that the correlation functions always have two fastest peaks which move in opposite directions and decay at rate t-13 for position and momentum correlations and as t-23 for energy correlations. The shape of these peaks is asymptotically described by the Airy function. Furthermore, the correlation functions have some non generic peaks with lower decay rates. In particular, there are peaks which decay at rate t-14 for position and momentum correlators and with rate t-12 for energy correlators. The shape of these peaks is described by the Pearcey integral. Crucial for our analysis is an appropriate generalisation of spacings, i.e. differences of the positions of neighbouring particles, that are used as spatial variables in the case of nearest neighbour interactions. Using the theory of circulant matrices we are able to introduce a quantity that retains both localisation and analytic viability. This also allows us to define and analyse some additional quantities used for nearest neighbour chains. Finally, we study numerically the evolution of the correlation functions after adding nonlinear perturbations to our model. Within the time range of our numerical simulations the asymptotic description of the linear case seems to persist for small nonlinear perturbations while stronger nonlinearities change shape and decay rates of the peaks significantly
Adiabatic Invariants for the FPUT and Toda Chain in the Thermodynamic Limit
We consider the Fermi\u2013Pasta\u2013Ulam\u2013Tsingou (FPUT) chain composed by N 6b 1 particles and periodic boundary conditions, and endow the phase space with the Gibbs measure at small temperature \u3b2- 1. Given a fixed 1 64 m 6a N, we prove that the first m integrals of motion of the periodic Toda chain are adiabatic invariants of FPUT (namely they are approximately constant along the Hamiltonian flow of the FPUT) for times of order \u3b2, for initial data in a set of large measure. We also prove that special linear combinations of the harmonic energies are adiabatic invariants of the FPUT on the same time scale, whereas they become adiabatic invariants for all times for the Toda dynamics
A review of power electronics equipment for all-electric ship MVDC power systems
Medium Voltage DC (MVDC) distribution Power Systems for all-electric ships (AES) can be regarded as functionally composed of three subsystems, namely the power sources, the load centers and the distribution network. Extensive use of power electronics is required for connecting power sources and load centers to the MVDC bus and for protecting the MVDC power system through properly placed DC circuit breakers. In this paper, an overview is given of the power electronics equipment found in the literature and on the market that could be suitable for use in future AES MVDC power systems. Some industrial experiences regarding DC generator systems, energy storage apparatus and solid-state DC circuit breaker prototypes are reported in the paper as examples of state-of-the-art realizations. Different DC/DC converters, which can be employed as solid-state transformers, are also discussed and a structure obtained by combining them is proposed
Correlation Functions for a Chain of Short Range Oscillators
We consider a system of harmonic oscillators with short range interactions
and we study their correlation functions when the initial data is sampled with
respect to the Gibbs measure. Such correlation functions display rapid
oscillations that travel through the chain. We show that the correlation
functions always have two fastest peaks which move in opposite directions and
decay at rate for position and momentum correlations and as
for energy correlations. The shape of these peaks is
asymptotically described by the Airy function. Furthermore, the correlation
functions have some non generic peaks with lower decay rates. In particular,
there are peaks which decay at rate for position and
momentum correlators and with rate for energy correlators.
The shape of these peaks is described by the Pearcey integral. Crucial for our
analysis is an appropriate generalisation of spacings, i.e. differences of the
positions of neighbouring particles, that are used as spatial variables in the
case of nearest neighbour interactions. Using the theory of circulant matrices
we are able to introduce a quantity that retains both localisation and analytic
viability. This also allows us to define and analyse some additional quantities
used for nearest neighbour chains. Finally, we study numerically the evolution
of the correlation functions after adding nonlinear perturbations to our model.
Within the time range of our numerical simulations the asymptotic description
of the linear case seems to persist for small nonlinear perturbations while
stronger nonlinearities change shape and decay rates of the peaks
significantly.Comment: 25 pages, 6 figure
Equilibrium Spacetime Correlations of the Toda Lattice on the Hydrodynamic Scale
We report on molecular dynamics simulations of spacetime correlations of the
Toda lattice in thermal equilibrium. The correlations of stretch, momentum, and
energy are computed numerically over a wide range of pressure and temperature.
Our numerical results are compared with the predictions from linearized
generalized hydrodynamics on the Euler scale. The system size is N=3000,4000
and time t =600, at which ballistic scaling is well confirmed. With no
adjustable parameters, the numerically obtained scaling functions agree with
the theory within a precision of less than 3.5%.Comment: 22 pages, 5 figure
FATTY ACIDS AND STEROLS IN SEEDS FROM WILD SPECIES OF BERBERIS IN ARGENTINE PATAGONIA
Abstract Seeds of Berberis heterophylla an
Developing a Mental Health Measurement Strategy to Capture Psychological Problems among Lower Caste Adolescent Girls in Rural, South India.
Adolescent girls vulnerable to early marriage and school dropout in rural India may be at elevated risk of psychosocial problems. However, few screening instruments have been culturally adapted and validated to measure this risk. This paper describes the process by which the Primary Health Questionnaire PHQ-9, a screening instrument for depression, was tested for cultural validity as part of the Samata evaluation - an intervention to support low caste adolescent girls in rural south India to attend and complete secondary school and to delay marriage until adulthood. Three focus groups discussions (FGDs) were held with 20 adolescent girls and six outreach workers of the Samata programme in rural north Karnataka, south India. The FGDs were used to explore local expressions of psychosocial problems and to understand the acceptability and appropriateness of PHQ-9 items. A thematic content analysis was conducted on the transcripts of the FGDs. Descriptions of local expressions of psychosocial problems generally matched the items on the PHQ-9. However, not all representations of psychological symptoms were captured by this tool. Persistent worry, loneliness and isolation, and externalised behaviours were also described by participants as common expressions of psychosocial distress. Based on the limitations of translation methods, local stakeholders must be involved in evaluating the cultural appropriateness of mental health screening tools. The current research demonstrates a strategy by which to assess the cultural validity of Western psychiatric instruments with key stakeholders in low- and middle-income settings
Ages and metallicities of circumnuclear star formation regions from Gemini IFU observations
Aims: We derive the age and metallicity of circumnuclear star formation
regions (CNSFRs) located in the spiral galaxies NGC6951 and NGC1097, and
investigate the cause of the very low equivalent widths of emission lines found
for these regions.
Methods: We used optical two-dimensional spectroscopic data obtained with
Gemini GMOS-IFUs and a grid of photoionization models to derive the the
metallicities and ages of CNSFRs.
Results: We find star formation rates in the range 0.002-0.14 Msun/yr and
oxygen abundance of 12+log(O/H)~8.8 dex, similar to those of most metal-rich
nebulae located in the inner region of galactic disks.
Conclusions: We conclude that the very low emission-line equivalent widths
observed in CNSFRs are caused by the ``contamination'' of the continuum by (1)
contribution of the underlying bulge continuum combined with (2) contribution
from previous episodes of star formation at the CNSFRs.Comment: Accepted for publication in Astronomy and Astrophysic
The NICMOS Snapshot Survey of nearby Galaxies
We present ``snapshot'' observations with the NearInfrared Camera and
MultiObject Spectrometer (NICMOS) on board the Hubble Space Telescope (HST) of
94 nearby galaxies from the Revised Shapley Ames Catalog. Images with 0.2 as
resolution were obtained in two filters, a broad-band continuum filter (F160W,
roughly equivalent to the H-band) and a narrow band filter centered on the
Paschen alpha line (F187N or F190N, depending on the galaxy redshift) with the
51x51 as field of view of the NICMOS camera 3. A first-order continuum
subtraction is performed, and the resulting line maps and integrated Paschen
alpha line fluxes are presented. A statistical analysis indicates that the
average Paschen alpha surface brightness {\bf in the central regions} is
highest in early-type (Sa-Sb) spirals.Comment: Original contained error in flux calibration. Table 1 now has correct
Paschen Alpha fluxes. 14 pages LaTeX with JPEG and PS figures. Also available
at http://icarus.stsci.edu/~boeker/publications.htm
- âŠ