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 regularization—the hybrid method and improved generalized inverse beamforming—are 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 10–100 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 $\leq$ 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

Researching Environments for Early Learning (REEL) Study

No description supplie

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 $P=\rho$. 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