4,654 research outputs found
Structures Produced by the Collision of Extragalactic Jets with Dense Clouds
We have investigated how several parameters can affect the results of a
collision between an extragalactic jet and a dense, intergalactic cloud,
through a series of hydrodynamic simulations. Such collisions are often
suggested to explain the distorted structures of some radio jets. However,
theoretical studies of this mechanism are in conflict over whether it can
actually reproduce the observations.
The parameters are the Mach number, and the relative densities of the jet and
the cloud to the ambient medium. Using a simple prescription we have produced
synthetic radio images for comparison with observations. These show that a
variety of structures may be produced from simple jet-cloud collisions. We
illustrate this with a few examples, and examine the details in one case. In
most cases we do not see a clear, sustained deflection. Lighter jets are
completely disrupted. The most powerful jets produce a hotspot at the impact
which outshines any jet emission and erode the cloud too quickly to develop a
deflected arm. It appears that moderate Mach numbers and density contrasts are
needed to produce bends in the radio structure. This explains the apparent
conflict between theoretical studies, as conclusions were based on different
values of these parameters. Shocks are produced in the ambient medium that
might plausibly reproduce the observed alignment of the extended emission line
regions with the radio axis.Comment: 21 pages, 11 figures. Submitted to MNRAS. Also available in html
version at http://www.doc.mmu.ac.uk/STAFF/S.Higgins/jcmnpaper/jc_mn.htm
Design and evaluation of dynamic policy-based flow redirection for multihomed mobile netwotks
This paper presents the design, implementation and evaluation of a solution for dynamic redirection of traffic flows for multihomed mobile networks. The solution was developed for a mobile user that disposes of a Personal Area Network (PAN) with a Personal Mobile Router (PMR), in order to achieve Always Best Connected(ABC) service by distributing flows belonging to different applications among the most appropriate access networks. Designed in a modular way for a NEMO based mobility and multihoming support, the proposed flow redirection solution can be easily coupled with and controlled by dynamic traffic policies that come from advanced network intelligence, according to the currently available network resources and user and application requirements. A prototype implementation was validated and assessed on a testbed as proof-of-concept
Correlations of a bound interface over a random substrate
The correlation function of a one-dimensional interface over a random
substrate, bound to the substrate by a pressure term, is studied by Monte-Carlo
simulation. It is found that the height correlation , averaged
over the substrate disorder, fits a form exp(-(j/b)^c) to a surprising
precision in the full range of j where the correlation is non-negligible. The
exponent c increases from 1.0 to 1.5 when the interface tension is taken larger
and larger.Comment: 7 pages, 5 figure
Fringe spacing and phase of interfering matter waves
We experimentally investigate the outcoupling of atoms from Bose-Einstein
condensates using two radio-frequency (rf) fields in the presence of gravity.
We show that the fringe separation in the resulting interference pattern
derives entirely from the energy difference between the two rf fields and not
the gravitational potential difference. We subsequently demonstrate how the
phase and polarisation of the rf radiation directly control the phase of the
matter wave interference and provide a semi-classical interpretation of the
results.Comment: 4 pages, 3 figure
Multiplexed communication over a high-speed quantum channel
In quantum information systems it is of particular interest to consider the
best way in which to use the non-classical resources consumed by that system.
Quantum communication protocols are integral to quantum information systems and
are amongst the most promising near-term applications of quantum information
science. Here we show that a multiplexed, digital quantum communications system
supported by comb of vacuum squeezing has a greater channel capacity per photon
than a source of broadband squeezing with the same analogue bandwidth. We
report on the time-resolved, simultaneous observation of the first dozen teeth
in a 2.4 GHz comb of vacuum squeezing produced by a sub-threshold OPO, as
required for such a quantum communications channel. We also demonstrate
multiplexed communication on that channel
Direct imaging of a digital-micromirror device for configurable microscopic optical potentials
Programable spatial light modulators (SLMs) have significantly advanced the
configurable optical trapping of particles. Typically, these devices are
utilized in the Fourier plane of an optical system, but direct imaging of an
amplitude pattern can potentially result in increased simplicity and
computational speed. Here we demonstrate high-resolution direct imaging of a
digital micromirror device (DMD) at high numerical apertures (NA), which we
apply to the optical trapping of a Bose-Einstein condensate (BEC). We utilise a
(1200 x 1920) pixel DMD and commercially available 0.45 NA microscope
objectives, finding that atoms confined in a hybrid optical/magnetic or
all-optical potential can be patterned using repulsive blue-detuned (532 nm)
light with 630(10) nm full-width at half-maximum (FWHM) resolution, within 5%
of the diffraction limit. The result is near arbitrary control of the density
the BEC without the need for expensive custom optics. We also introduce the
technique of time-averaged DMD potentials, demonstrating the ability to produce
multiple grayscale levels with minimal heating of the atomic cloud, by
utilising the high switching speed (20 kHz maximum) of the DMD. These
techniques will enable the realization and control of diverse optical
potentials for superfluid dynamics and atomtronics applications with quantum
gases. The performance of this system in a direct imaging configuration has
wider application for optical trapping at non-trivial NAs.Comment: 9 page
A Study of the Role of Organized Structures in Jet Noise Generation
We present analytical, experimental and computational studies aimed at understanding the role of large-scale, organized structures in noise generation from high-speed, compressible jets. Two-point near-field pressure data from experiments are analyzed and used to identify parameters in a wave-packet based model for noise generation from organized, large-scale structures. The statistical spectrum of far field pressure is expressed in terms of two-point space-time correlations of the near-field pressure on a surface enclosing the jet. The surface is assumed to be sufficiently near the turbulent region tobe dominated by non-propagating hydrodynamic disturbances, yet sufficiently far such that linear behavior can be assumed in extending the near-field pressure to the far field. Validity of such assumptions is investigated by interrogating the jet DNS database of Freund [J. Fluid Mech. 438:277-305 2001]. The DNS data analysis is also used to investigate the impact of limited spatial resolution in the experiments. The analytical model is used to study far field noise generation from organized structures. Results show that, for sufficiently short structure lifetime, aft angle far field pressure spectra tend to exhibit frequency scaling with Helmholtz number, rather than Strouhal number
Consistency of Gravity with the Cosmological Observations in Palatini Formalism
In this work we study the dynamics of universe in
modified gravity with Palatini formalism. We use data from recent observations
as Supernova Type Ia (SNIa) Gold sample and Supernova Legacy Survey (SNLS)
data, size of baryonic acoustic peak from Sloan Digital Sky Survey (SDSS), the
position of the acoustic peak from the CMB observations and large scale
structure formation (LSS) from the 2dFGRS survey to put constraint on the
parameters of the model. To check the consistency of this action, we compare
the age of old cosmological objects with the age of universe. In the combined
analysis with the all the observations, we find the parameters of model as
and
.Comment: 12 pages, 7 figure
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