11,346 research outputs found
Pattern Formation and Dynamics in Rayleigh-B\'{e}nard Convection: Numerical Simulations of Experimentally Realistic Geometries
Rayleigh-B\'{e}nard convection is studied and quantitative comparisons are
made, where possible, between theory and experiment by performing numerical
simulations of the Boussinesq equations for a variety of experimentally
realistic situations. Rectangular and cylindrical geometries of varying aspect
ratios for experimental boundary conditions, including fins and spatial ramps
in plate separation, are examined with particular attention paid to the role of
the mean flow. A small cylindrical convection layer bounded laterally either by
a rigid wall, fin, or a ramp is investigated and our results suggest that the
mean flow plays an important role in the observed wavenumber. Analytical
results are developed quantifying the mean flow sources, generated by amplitude
gradients, and its effect on the pattern wavenumber for a large-aspect-ratio
cylinder with a ramped boundary. Numerical results are found to agree well with
these analytical predictions. We gain further insight into the role of mean
flow in pattern dynamics by employing a novel method of quenching the mean flow
numerically. Simulations of a spiral defect chaos state where the mean flow is
suddenly quenched is found to remove the time dependence, increase the
wavenumber and make the pattern more angular in nature.Comment: 9 pages, 10 figure
Transient spatiotemporal chaos in the complex Ginzburg-Landau equation on long domains
Numerical simulations of the complex Ginzburg-Landau equation in one spatial dimension on periodic domains with sufficiently large spatial period reveal persistent chaotic dynamics in large parts of parameter space that extend into the Benjamin-Feir stable regime. This situation changes when nonperiodic boundary conditions are imposed, and in the Benjamin-Feir stable regime chaos takes the form of a long-lived transient decaying to a spatially uniform oscillatory state. The lifetime of the transient has Poisson statistics and no domain length is found sufficient for persistent chaos
Letter from Geraldine Ferraro to the Red Cross of San Juan de Dios in Costa Rica
Letter from Geraldine Ferraro to the Red Cross of San Juan de Dios in Costa Rica. Letter has handwritten notes.https://ir.lawnet.fordham.edu/vice_presidential_campaign_correspondence_1984_international/1342/thumbnail.jp
Modeling Nonequilibrium Phase Transitions and Critical Behavior in Complex Systems
We comment on some recent, yet unpublished results concerning instabilities
in complex systems and their applications. In particular, we briefly describe
main observations during extensive computer simulations of two lattice
nonequilibrium models. One exhibits robust and efficient processes of pattern
recognition under synaptic coherent activity; the second example exhibits
interesting critical behavior and simulates nucleation and spinodal
decomposition processes in driven fluids.Comment: 6 pages, 4 figure
The Millennium Galaxy Catalogue: the space density and surface brightness distribution(s) of galaxies
We recover the joint and individual space density and surface brightness
distribution(s) of galaxies from the Millennium Galaxy Catalogue. The MGC is a
local survey spanning 30.9 sq deg and probing approximately one--two mag/sq
arcsec deeper than either the Two-Degree Field Galaxy Redshift Survey (2dFGRS)
or the Sloan Digital Sky Survey (SDSS). The MGC contains 10,095 galaxies to
B_mgc < 20 mag with 96 per cent spectroscopic completeness. We implement a
joint luminosity-surface brightness step-wise maximum likelihood method to
recover the bivariate brightness distribution (BBD) inclusive of most selection
effects. Integrating the BBD over surface brightness we recover the following
Schechter function parameters: phi* = (0.0177 +/- 0.0015) h^3 Mpc^{-3}, M_{B}*
- 5 log h = (-19.60 +/- 0.04) mag and alpha =-1.13 +/- 0.02. Compared to the
2dFGRS (Norberg et al 2002) we find a consistent M* value but a slightly
flatter faint-end slope and a higher normalisation, resulting in a final
luminosity density j_{b_J} = (1.99 +/- 0.17) x 10^8 h L_{odot} Mpc^{-3}. The
MGC surface brightness distribution is a well bounded Gaussian at the M* point
with phi* = (3.5 +/- 0.1) x 10^{-2} h^3 Mpc^{-3}, mu^{e*} = (21.90 +/- 0.01)
mag/sq arcsec and sigma_{ln R_e} = 0.35 +/- 0.01. The characteristic surface
brightness for luminous systems is invariant to M_{B} - 5 log h ~ -19 mag
faintwards of which it moves to lower surface brightness. Higher resolution
(FWHM 26 mag/sq arcsec in the B-band)
observations of the local universe are now essential to probe to lower
luminosity and lower surface brightness levels. [abridged]Comment: Accepted for publication in MNRAS, 26 pages with 21 figures (some
degraded). A full pdf version, along with MGC data release, is available from
the MGC website at, http://www.eso.org/~jliske/mg
The WFCAM Multi-wavelength Variable Star Catalog
Stellar variability in the near-infrared (NIR) remains largely unexplored.
The exploitation of public science archives with data-mining methods offers a
perspective for the time-domain exploration of the NIR sky. We perform a
comprehensive search for stellar variability using the optical-NIR multi-band
photometric data in the public Calibration Database of the WFCAM Science
Archive (WSA), with the aim of contributing to the general census of variable
stars, and to extend the current scarce inventory of accurate NIR light curves
for a number of variable star classes. We introduce new variability indices
designed for multi-band data with correlated sampling, and apply them for
pre-selecting variable star candidates, i.e., light curves that are dominated
by correlated variations, from noise-dominated ones. Pre-selection criteria are
established by robust numerical tests for evaluating the response of
variability indices to colored noise characteristic to the data. We find 275
periodic variable stars and an additional 44 objects with suspected variability
with uncertain periods or apparently aperiodic variation. Only 44 of these
objects had been previously known, including 11 RR~Lyrae stars in the outskirts
of the globular cluster M3 (NGC~5272). We provide a preliminary classification
of the new variable stars that have well-measured light curves, but the
variability types of a large number of objects remain ambiguous. We classify
most of the new variables as contact binary stars, but we also find several
pulsating stars, among which 34 are probably new field RR~Lyrae and 3 are
likely Cepheids. We also identify 32 highly reddened variable objects close to
previously known dark nebulae, suggesting that these are embedded young stellar
objects. We publish our results and all light-curve data as the WFCAM Variable
Star Catalog.Comment: 21 pages, 11 figure
The z<=0.1 Surface Brightness Distribution
The surface brightness distribution (SBD) function describes the number
density of galaxies as measured against their central surface brightness.
Because detecting galaxies with low central surface brightnesses is both
time-consuming and complicated, determining the shape of this distribution
function can be difficult. In a recent paper Cross, et al. suggested a
bell-shaped SBD disk-galaxy function which peaks near the canonical Freeman
value of 21.7 and then falls off significantly by 23.5 B mag arcsec-2. This is
in contradiction to previous studies which have typically found flat (slope=0)
SBD functions out to 24 - 25 B mag arcsec^-2 (the survey limits). Here we take
advantage of a recent surface-brightness limited survey by Andreon & Cuillandre
which reaches considerably fainter magnitudes than the Cross, et.al sample (M_B
reaches fainter than -12 for Andreon & Cuillandre while the Cross, et.al sample
is limited to M_B < -16) to re-evaluate both the SBD function as found by their
data and the SBD for a wide variety of galaxy surveys, including the Cross, et
al. data. The result is a SBD function with a flat slope out through the survey
limits of 24.5 B mag arcsec^-2, with high confidence limits.Comment: 5 pages including 5 figures. accepted by A&A
Photoinduced two-proton knockout and ground-state correlations in nuclei
A factorized and analytical form for the A(,pp) and A(e,epp) cross
section is proposed. In the suggested scheme the two-proton knockout cross
sections can be directly analyzed in terms of the ground-state correlation
functions. Central, spin-spin and tensor correlations are considered. In the
longitudinal channel, the (e,epp) cross section is predicted to exhibit a
peculiar sensitivity to ground-state correlation effects.Comment: 11 pages in REVtex with embedded psfigure
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