29,942 research outputs found
A unified framework for Schelling's model of segregation
Schelling's model of segregation is one of the first and most influential
models in the field of social simulation. There are many variations of the
model which have been proposed and simulated over the last forty years, though
the present state of the literature on the subject is somewhat fragmented and
lacking comprehensive analytical treatments. In this article a unified
mathematical framework for Schelling's model and its many variants is
developed. This methodology is useful in two regards: firstly, it provides a
tool with which to understand the differences observed between models;
secondly, phenomena which appear in several model variations may be understood
in more depth through analytic studies of simpler versions.Comment: 21 pages, 3 figure
Multiple Hard Partonic Collisions with Correlations in Proton-Proton Scattering
We propose a simple method for incorporating correlations into the impact
parameter space description of multiple (semi-)hard partonic collisions in high
energy hadron-hadron scattering. The perturbative QCD input is the standard
factorization theorem for inclusive dijet production with a lower cutoff on
transverse momentum. The width of the transverse distribution of hard partons
is fixed by parameterizations of the two-gluon form factor. We then reconstruct
the hard contribution to the total inelastic profile function and obtain
corrections due to correlations to the more commonly used eikonal description.
Estimates of the size of double correlation corrections are based on the rate
of double collisions measured at the Tevatron. We find that, if typical values
for the lower transverse momentum cutoff are used in the calculation of the
inclusive hard dijet cross section, then the correlation corrections are
necessary for maintaining consistency with expectations for the total inelastic
proton-proton cross section at LHC energies.Comment: Typos fixed, Figures 2,9 and 10 added, matches version published in
Phys. Rev.
Observational signatures of convectively driven waves in massive stars
We demonstrate observational evidence for the occurrence of convectively
driven internal gravity waves (IGW) in young massive O-type stars observed with
high-precision CoRoT space photometry. This evidence results from a comparison
between velocity spectra based on 2D hydrodynamical simulations of IGW in a
differentially-rotating massive star and the observed spectra.We also show that
the velocity spectra caused by IGW may lead to detectable line-profile
variability and explain the occurrence of macroturbulence in the observed line
profiles of OB stars. Our findings provide predictions that can readily be
tested by including a sample of bright slowly and rapidly rotating OB-type
stars in the scientific programme of the K2 mission accompanied by
high-precision spectroscopy and their confrontation with multi-dimensional
hydrodynamic simulations of IGW for various masses and ages.Comment: 4 pages, 3 figures, accepted for publication in The Astrophysical
Journal Letter
The Response of Normal Shocks in Diffusers
The frequency response of a normal shock in a diverging channel is calculated for application to problems of
pressure oscillations in ramjet engines. Two limits of a linearized analysis arc discussed: one represents isentropic
flow on both sides of a shock wave; the other may be a crude appr'l'I;imation to the influence of flow separation
induced hy the wave. Numerical results arc given, and the influences of the shock wave on oscillations in the
engine are discus,ed
Modeling pressure oscillations in Ramjets
Pressure oscillations in ramjet engines are approximated as one-dimensional motions and treated within linear acoustics. The exhaust nozzle is represented by the admittance function for a short choked nozzle. New results have been obtained for the quasi-steady response of a
normal shock wave in the diffuser. Acoustic fields in the inlet region and in the combustion chamber are matched to provide an analytical expression of the criterion for linear stability. Combustion processes are accommodated but not treated in detail. As examples, data are discussed
for two liquid-fueled engines, one having axial dump and one having side dumps
Gauge links, TMD-factorization, and TMD-factorization breaking
In this section, we discuss some basic features of transverse momentum
dependent, or unintegrated, parton distribution functions. In particular, when
these correlation functions are combined in a factorization formulae with hard
processes beyond the simplest cases, there are basic problems with universality
and factorization. We discuss some of these problems as well as the
opportunities that they offer.Comment: 9 pages, 10 figures Gluons and the quark sea at high energies:
distributions, polarization, tomograph
On the Tidal Dissipation of Obliquity
We investigate tidal dissipation of obliquity in hot Jupiters. Assuming an
initial random orientation of obliquity and parameters relevant to the observed
population, the obliquity of hot Jupiters does not evolve to purely aligned
systems. In fact, the obliquity evolves to either prograde, retrograde or
90^{o} orbits where the torque due to tidal perturbations vanishes. This
distribution is incompatible with observations which show that hot jupiters
around cool stars are generally aligned. This calls into question the viability
of tidal dissipation as the mechanism for obliquity alignment of hot Jupiters
around cool stars.Comment: 6 pages, 4 figures, accepted at ApJ
Multivariable Repetitive-predictive Controllers using Frequency Decomposition
Repetitive control is a methodology for the tracking of a periodic reference signal. This paper develops a new approach to repetitive control systems design using receding horizon control with frequency decomposition of the reference signal. Moreover, design and implementation issues for this form of repetitive predictive control are investigated from the perspectives of controller complexity and the effects of measurement noise. The analysis is supported by a simulation study on a multi-input multi-output robot arm where the model has been constructed from measured frequency response data, and experimental results from application to an industrial AC motor
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