14,520 research outputs found
Comment on Bramwell et al, "Universal Fluctuations in Correlated Systems"
This is a comment on "Universal Fluctuations in Correlated Systems", by
Bramwell et al, Phys. Rev. Lett., 84, 3744 (2000.Comment: To appear in Phys. Rev. Let
Extreme-value statistics from Lagrangian convex hull analysis for homogeneous turbulent Boussinesq convection and MHD convection
We investigate the utility of the convex hull of many Lagrangian tracers to
analyze transport properties of turbulent flows with different anisotropy. In
direct numerical simulations of statistically homogeneous and stationary
Navier-Stokes turbulence, neutral fluid Boussinesq convection, and MHD
Boussinesq convection a comparison with Lagrangian pair dispersion shows that
convex hull statistics capture the asymptotic dispersive behavior of a large
group of passive tracer particles. Moreover, convex hull analysis provides
additional information on the sub-ensemble of tracers that on average disperse
most efficiently in the form of extreme value statistics and flow anisotropy
via the geometric properties of the convex hulls. We use the convex hull
surface geometry to examine the anisotropy that occurs in turbulent convection.
Applying extreme value theory, we show that the maximal square extensions of
convex hull vertices are well described by a classic extreme value
distribution, the Gumbel distribution. During turbulent convection,
intermittent convective plumes grow and accelerate the dispersion of Lagrangian
tracers. Convex hull analysis yields information that supplements standard
Lagrangian analysis of coherent turbulent structures and their influence on the
global statistics of the flow.Comment: 18 pages, 10 figures, preprin
Sexual and marital trajectories and HIV infection among ever-married women in rural Malawi.
OBJECTIVE: To explore how sexual and marital trajectories are associated with HIV infection among ever-married women in rural Malawi. METHODS: Retrospective survey data and HIV biomarker data for 926 ever-married women interviewed in the Malawi Diffusion and Ideational Change Project were used. The associations between HIV infection and four key life course transitions considered individually (age at sexual debut, premarital sexual activity, entry into marriage and marital disruption by divorce or death) were examined. These transitions were then sequenced to construct trajectories that represent the variety of patterns in the data. The association between different trajectories and HIV prevalence was examined, controlling for potentially confounding factors such as age and region. RESULTS: Although each life course transition taken in isolation may be associated with HIV infection, their combined effect appeared to be conditional on the sequence in which they occurred. Although early sexual debut, not marrying one's first sexual partner and having a disrupted marriage each increased the likelihood of HIV infection, their risk was not additive. Women who both delayed sexual debut and did not marry their first partner are, once married, more likely to experience marital disruption and to be HIV-positive. Women who marry their first partner but who have sex at a young age, however, are also at considerable risk. CONCLUSIONS: These findings identify the potential of a life course perspective for understanding why some women become infected with HIV and others do not, as well as the differentials in HIV prevalence that originate from the sequence of sexual and marital transitions in one's life. The analysis suggests, however, the need for further data collection to permit a better examination of the mechanisms that account for variations in life course trajectories and thus in lifetime probabilities of HIV infection
Searching for Planets in the Hyades. I. The Keck Radial Velocity Survey
We describe a high-precision radial velocity search for jovian-mass
companions to main sequence stars in the Hyades star cluster. The Hyades
provides an extremely well controlled sample of stars of the same age, the same
metallicity, and a common birth and early dynamical environment. This sample
allows us to explore the dependence of the process of planet formation on only
a single independent variable: the stellar mass. In this paper we describe the
survey and summarize results for the first five years.Comment: 8 pages, 3 figures; To appear in the July 2002 issue of The
Astronomical Journa
Maximizing the hyperpolarizability of one-dimensional systems
Previous studies have used numerical methods to optimize the
hyperpolarizability of a one-dimensional quantum system. These studies were
used to suggest properties of one-dimensional organic molecules, such as the
degree of modulation of conjugation, that could potentially be adjusted to
improve the nonlinear-optical response. However, there were no conditions set
on the optimized potential energy function to ensure that the resulting
energies were consistent with what is observed in real molecules. Furthermore,
the system was placed into a one-dimensional box with infinite walls, forcing
the wavefunctions to vanish at the ends of the molecule. In the present work,
the walls are separated by a distance much larger than the molecule's length;
and, the variations of the potential energy function are restricted to levels
that are more typical of a real molecule. In addition to being a more
physically-reasonable model, our present approach better approximates the bound
states and approximates the continuum states - which are usually ignored. We
find that the same universal properties continue to be important for optimizing
the nonlinear-optical response, though the details of the wavefunctions differ
from previous result.Comment: 10 pages, 5 figure
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