3,014 research outputs found
Multi-objective engineering shape optimization using differential evolution interfaced to the Nimrod/O tool
This paper presents an enhancement of the Nimrod/O optimization tool by interfacing DEMO, an external multiobjective optimization algorithm. DEMO is a variant of differential evolution – an algorithm that has attained much popularity in the research community, and this work represents the first time that true multiobjective optimizations have been performed with Nimrod/O. A modification to the DEMO code enables multiple objectives to be evaluated concurrently. With Nimrod/O’s support for parallelism, this can reduce the wall-clock time significantly for compute intensive objective function evaluations. We describe the usage and implementation of the interface and present two optimizations. The first is a two objective mathematical function in which the Pareto front is successfully found after only 30 generations. The second test case is the three-objective shape optimization of a rib-reinforced wall bracket using the Finite Element software, Code_Aster. The interfacing of the already successful packages of Nimrod/O and DEMO yields a solution that we believe can benefit a wide community, both industrial and academic
Scheduling aircraft landings - the static case
This is the publisher version of the article, obtained from the link below.In this paper, we consider the problem of scheduling aircraft (plane) landings at an airport. This problem is one of deciding a landing time for each plane such that each plane lands within a predetermined time window and that separation criteria between the landing of a plane and the landing of all successive planes are respected. We present a mixed-integer zero–one formulation of the problem for the single runway case and extend it to the multiple runway case. We strengthen the linear programming relaxations of these formulations by introducing additional constraints. Throughout, we discuss how our formulations can be used to model a number of issues (choice of objective function, precedence restrictions, restricting the number of landings in a given time period, runway workload balancing) commonly encountered in practice. The problem is solved optimally using linear programming-based tree search. We also present an effective heuristic algorithm for the problem. Computational results for both the heuristic and the optimal algorithm are presented for a number of test problems involving up to 50 planes and four runways.J.E.Beasley. would like to acknowledge the financial support of the Commonwealth Scientific and Industrial Research Organization, Australia
Applicability of the Fisher Equation to Bacterial Population Dynamics
The applicability of the Fisher equation, which combines diffusion with
logistic nonlinearity, to population dynamics of bacterial colonies is studied
with the help of explicit analytic solutions for the spatial distribution of a
stationary bacterial population under a static mask. The mask protects the
bacteria from ultraviolet light. The solution, which is in terms of Jacobian
elliptic functions, is used to provide a practical prescription to extract
Fisher equation parameters from observations and to decide on the validity of
the Fisher equation.Comment: 5 pages, 3 figs. include
The Dependence of Galaxy Shape on Luminosity and Surface Brightness Profile
For a sample of 96,951 galaxies from the Sloan Digital Sky Survey Data
Release 3, we study the distribution of apparent axis ratios as a function of
r-band absolute magnitude and surface brightness profile type. We use the
parameter fracDeV to quantify the profile type (fracDeV = 1 for a de
Vaucouleurs profile; fracDeV = 0 for an exponential profile). When the apparent
axis ratio q_{am} is estimated from the moments of the light distribution, the
roundest galaxies are very bright (M_r \sim -23) de Vaucouleurs galaxies and
the flattest are modestly bright (M_r \sim -18) exponential galaxies. When the
apparent axis ratio q_{25} is estimated from the axis ratio of the 25
mag/arcsec^2 isophote, we find that de Vaucouleurs galaxies are flatter than
exponential galaxies of the same absolute magnitude. For a given surface
brightness profile type, very bright galaxies are rounder, on average, than
fainter galaxies. We deconvolve the distributions of apparent axis ratios to
find the distribution of the intrinsic short-to-long axis ratio gamma, assuming
constant triaxiality T. For all profile types and luminosities, the
distribution of apparent axis ratios is inconsistent with a population of
oblate spheroids, but is usually consistent with a population of prolate
spheroids. Bright galaxies with a de Vaucouleurs profile (M_r < -21.84, fracDeV
> 0.9) have a distribution of q_{am} that is consistent with triaxiality in the
range 0.4 < T < 0.8, with mean intrinsic axis ratio 0.66 < gamma < 0.69. The
fainter de Vaucouleurs galaxies are best fit with prolate spheroids (T = 1)
with mean axis ratio gamma = 0.51.Comment: 32 pages, 12 figures, to appear in Ap
Demonstrating Diversity in Star Formation Histories with the CSI Survey
We present coarse but robust star formation histories (SFHs) derived from
spectro-photometric data of the Carnegie-Spitzer-IMACS Survey, for 22,494
galaxies at 0.3<z<0.9 with stellar masses of 10^9 Msun to 10^12 Msun. Our study
moves beyond "average" SFHs and distribution functions of specific star
formation rates (sSFRs) to individually measured SFHs for tens of thousands of
galaxies. By comparing star formation rates (SFRs) with timescales of 10^10,
10^9, and 10^8 years, we find a wide diversity of SFHs: 'old galaxies' that
formed most or all of their stars early; galaxies that formed stars with
declining or constant SFRs over a Hubble time, and genuinely 'young galaxies'
that formed most of their stars since z=1. This sequence is one of decreasing
stellar mass, but, remarkably, each type is found over a mass range of a factor
of 10. Conversely, galaxies at any given mass follow a wide range of SFHs,
leading us to conclude that: (1) halo mass does not uniquely determine SFHs;
(2) there is no 'typical' evolutionary track; and (3) "abundance matching" has
limitations as a tool for inferring physics. Our observations imply that SFHs
are set at an early epoch, and that--for most galaxies--the decline and
cessation of star formation occurs over a Hubble-time, without distinct
"quenching" events. SFH diversity is inconsistent with models where galaxy
mass, at any given epoch, grows simply along relations between SFR and stellar
mass, but is consistent with a 2-parameter lognormal form, lending credence to
this model from a new and independent perspective.Comment: 17 pages, 10 figures; accepted by ApJ; version 2 - no substantive
changes; clarifications and correction
SGAS 143845.1+145407: A Big, Cool Starburst at Redshift 0.816
We present the discovery and a detailed multi-wavelength study of a
strongly-lensed luminous infrared galaxy at z=0.816. Unlike most known lensed
galaxies discovered at optical or near-infrared wavelengths this lensed source
is red, r-Ks = 3.9 [AB], which the data presented here demonstrate is due to
ongoing dusty star formation. The overall lensing magnification (a factor of
17) facilitates observations from the blue optical through to 500micron, fully
capturing both the stellar photospheric emission as well as the re-processed
thermal dust emission. We also present optical and near-IR spectroscopy. These
extensive data show that this lensed galaxy is in many ways typical of
IR-detected sources at z~1, with both a total luminosity and size in accordance
with other (albeit much less detailed) measurements in samples of galaxies
observed in deep fields with the Spitzer telescope. Its far-infrared spectral
energy distribution is well-fit by local templates that are an order of
magnitude less luminous than the lensed galaxy; local templates of comparable
luminosity are too hot to fit. Its size (D~7kpc) is much larger than local
luminous infrared galaxies, but in line with sizes observed for such galaxies
at z~1. The star formation appears uniform across this spatial scale. In this
source, the luminosity of which is typical of sources that dominate the cosmic
infrared background, we find that star formation is spatially extended and well
organised, quite unlike the compact merger-driven starbursts which are typical
for sources of this luminosity at z~0.Comment: 18 pages, 10 figure
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