1,039 research outputs found
Prediction of Nonlinear Propagation of Noise from a Solid Rocket Motor
Nonlinear acoustic wave propagation predictions (Generalized Burgers equation-based) of noise propagation are compared with measurements from a static, horizontally-fired solid rocket motor over a range of 76-1220 m during an 80 s burn time. The modeling suggests the nature of the geometric spreading between 76 and 305 m varies from cylindrical at low-frequencies to spherical at high frequencies. The predicted waveforms and high-frequency spectral slopes associated with significant shock content are in agreement with properties of the measured noise. At 1220 m, the relatively simple nonlinear model again approximates the measured spectrum despite the complexities of the measurement environment and atmospheric propagation
Application of Beamforming Methods to Full-Scale Military Jet Noise
Over the past decade, beamforming in aeroacoustics applications have undergone significant advances. Cross beamforming methods improve upon traditional beamforming in that they relax the assumption of multiple-source incoherence. This paper compares the abilities of three cross beamforming methods to reproduce source and field characteristics for an extended, partially correlated source that mimics supersonic jet noise radiation. Standard cross beamforming and two related methods that involve regularizationâthe hybrid method and improved generalized inverse beamformingâare applied to a numerically generated dataset along a near-field line. Estimated levels and coherence lengths are compared with benchmarks at the source as well as near and far-field locations. All three methods are successful in reproducing the field and source properties in high-amplitude regions. Although regularization generally helps to improve both source and field reconstructions, results are sensitive to regularization parameters, particularly for the generalized inverse method. The successful application of the three methods demonstrate the utility of cross-beamforming in formulating equivalent source models for accurate field prediction of complex sources, including jet noise
X-ray spectral analysis of the jet termination shock in pictor A on subarcsecond scales with Chandra
Hot spots observed at the edges of extended radio lobes in high-power radio galaxies and quasars mark the position of mildly relativistic termination shock, where the jet bulk kinetic energy is converted to the internal energy of the jet particles. These are the only astrophysical systems where mildly relativistic shocks can be directly resolved at various wavelengths of the electromagnetic spectrum. The western hot spot in the radio galaxy Pictor A is an exceptionally good target in this respect, due to the combination of its angular size and high surface brightness. In our previous work, after a careful Chandra image deconvolution, we resolved this hot spot into a disk-like feature perpendicular to the jet axis, and identified it as the front of the jet termination shock. We argued for a synchrotron origin of the observed X-ray photons, which implied electron energies reaching at least 10â100 TeV at the shock front. Here, we present a follow-up on that analysis, proposing, in particular, a novel method for constraining the shape of the X-ray continuum emission with subarcsecond resolution. The method is based on a Chandra hardness map analysis, using separately deconvolved maps in the soft and hard X-ray bands. In this way, we have found there is a systematic, yet statistically significant gradient in the hardness ratio across the shock, such that the implied electron energy index ranges from s 2.2 at the shock front to s > 2.7 in the near downstream. We discuss the implications of the obtained results for a general understanding of particle acceleration at mildly relativistic shocks
Critical Phenomena in Neutron Stars I: Linearly Unstable Nonrotating Models
We consider the evolution in full general relativity of a family of linearly
unstable isolated spherical neutron stars under the effects of very small,
perturbations as induced by the truncation error. Using a simple ideal-fluid
equation of state we find that this system exhibits a type-I critical
behaviour, thus confirming the conclusions reached by Liebling et al. [1] for
rotating magnetized stars. Exploiting the relative simplicity of our system, we
are able carry out a more in-depth study providing solid evidences of the
criticality of this phenomenon and also to give a simple interpretation of the
putative critical solution as a spherical solution with the unstable mode being
the fundamental F-mode. Hence for any choice of the polytropic constant, the
critical solution will distinguish the set of subcritical models migrating to
the stable branch of the models of equilibrium from the set of subcritical
models collapsing to a black hole. Finally, we study how the dynamics changes
when the numerically perturbation is replaced by a finite-size, resolution
independent velocity perturbation and show that in such cases a nearly-critical
solution can be changed into either a sub or supercritical. The work reported
here also lays the basis for the analysis carried in a companion paper, where
the critical behaviour in the the head-on collision of two neutron stars is
instead considered [2].Comment: 15 pages, 9 figure
Investigation of the networking performance of remote real- time computing farms for ATLAS trigger DAQ
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Stability criterion for self-similar solutions with a scalar field and those with a stiff fluid in general relativity
A stability criterion is derived in general relativity for self-similar
solutions with a scalar field and those with a stiff fluid, which is a perfect
fluid with the equation of state . A wide class of self-similar
solutions turn out to be unstable against kink mode perturbation. According to
the criterion, the Evans-Coleman stiff-fluid solution is unstable and cannot be
a critical solution for the spherical collapse of a stiff fluid if we allow
sufficiently small discontinuity in the density gradient field in the initial
data sets. The self-similar scalar-field solution, which was recently found
numerically by Brady {\it et al.} (2002 {\it Class. Quantum. Grav.} {\bf 19}
6359), is also unstable. Both the flat Friedmann universe with a scalar field
and that with a stiff fluid suffer from kink instability at the particle
horizon scale.Comment: 15 pages, accepted for publication in Classical and Quantum Gravity,
typos correcte
3D simulations of Einstein's equations: symmetric hyperbolicity, live gauges and dynamic control of the constraints
We present three-dimensional simulations of Einstein equations implementing a
symmetric hyperbolic system of equations with dynamical lapse. The numerical
implementation makes use of techniques that guarantee linear numerical
stability for the associated initial-boundary value problem. The code is first
tested with a gauge wave solution, where rather larger amplitudes and for
significantly longer times are obtained with respect to other state of the art
implementations. Additionally, by minimizing a suitably defined energy for the
constraints in terms of free constraint-functions in the formulation one can
dynamically single out preferred values of these functions for the problem at
hand. We apply the technique to fully three-dimensional simulations of a
stationary black hole spacetime with excision of the singularity, considerably
extending the lifetime of the simulations.Comment: 21 pages. To appear in PR
Entanglement without nonlocality
We consider the characterization of entanglement from the perspective of a
Heisenberg formalism. We derive an original two-party generalized separability
criteria, and from this describe a novel physical understanding of
entanglement. We find that entanglement may be considered as fundamentally a
local effect, and therefore as a separable computational resource from
nonlocality. We show how entanglement differs from correlation physically, and
explore the implications of this new conception of entanglement for the notion
of classicality. We find that this understanding of entanglement extends
naturally to multipartite cases.Comment: 9 pages. Expanded introduction and sections on physical entanglement
and localit
The Globular Cluster System in the Inner Region of M87
1057 globular cluster candidates have been identified in a WFPC2 image of the
inner region of M87. The Globular Cluster Luminosity Function (GCLF) can be
well fit by a Gaussian profile with a mean value of m_V^0=23.67 +/- 0.07 mag
and sigma=1.39 +/- 0.06 mag (compared to m_V^0=23.74 mag and sigma=1.44 mag
from an earlier study using the same data by Whitmore it et al. 1995). The GCLF
in five radial bins is found to be statistically the same at all points,
showing no clear evidence of dynamical destruction processes based on the
luminosity function (LF), in contradiction to the claim by Gnedin (1997).
Similarly, there is no obvious correlation between the half light radius of the
clusters and the galactocentric distance. The core radius of the globular
cluster density distribution is R_c=56'', considerably larger than the core of
the stellar component (R_c=6.8''). The mean color of the cluster candidates is
V-I=1.09 mag which corresponds to an average metallicity of Fe/H = -0.74 dex.
The color distribution is bimodal everywhere, with a blue peak at V-I=0.95 mag
and a red peak at V-I=1.20 mag. The red population is only 0.1 magnitude bluer
than the underlying galaxy, indicating that these clusters formed late in the
metal enrichment history of the galaxy and were possibly created in a burst of
star/cluster formation 3-6 Gyr after the blue population. We also find that
both the red and the blue cluster distributions have a more elliptical shape
(Hubble type E3.5) than the nearly spherical galaxy. The average half light
radius of the clusters is ~2.5 pc which is comparable to the 3 pc average
effective radius of the Milky Way clusters, though the red candidates are ~20%
smaller than the blue ones.Comment: 40 pages, 17 figures, 4 tables, latex, accepted for publication in
the Ap
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