9,801 research outputs found
3-D Simulations of Protostellar Jets in Stratified Ambient Media
We present fully three-dimensional hydrodynamical simulations of radiative
cooling jets propagating into stratified isothermal ambient media with
power-law density and pressure distributions. The parameters used are mainly
suitable for protostellar jets but results applicable to extragalactic jets are
also presented. Comparisons are made with previous simulations of jets through
homogeneous media. We find that for radiative cooling jets propagating into
regions where the ambient medium has an increasing density (and pressure)
gradient, the ambient gas tends to compress the cold, low-pressure cocoon of
shocked material that surrounds the beam and destroy the bow shock-like
structure at the head. The compressing medium collimates the jet and promotes
the development of Kelvin-Helmholtz instabilities which cause beam focusing,
wiggling and the formation of internal traveling shocks,
, via pinching along the beam. This remarkably resembles the structure of
some observed systems (e.g. Haro 6-5B northern and HH 24G jets). These effects
are larger for jets with smaller density ratio between jet and environment
(tested for =1, 3, and 10) and larger Mach number
(tested for 12 and 24, where is the jet velocity and the
ambient sound speed). In an ambient medium of decreasing density (and
pressure), the beam is poorly collimated and relaxes, becoming faint. This
could explain ''invisible'' jet sections, like the gap between the parent
source and collimated beam (e.g., in HH30 jet). Although, on average, jets
propagating into an increasing (decreasing) density environment are decelerated
(accelerated) by the increasing (decreasing) ram pressure of the ambient
medium, we find that their propagation velocities have an oscillating pattern.Comment: 33 pp, LaTeX file, 13 figures upon request. To appear in the
Astrophys. J., vol 471, nov. 10t
Effects of Nose Bluntness and Shock-Shock Interactions on Blunt Bodies in Viscous Hypersonic Flows
A numerical study has been conducted to investigate the effects of blunt leading edges on the viscous flow field around a hypersonic vehicle such as the proposed National Aero-Space Plane. Attention is focused on two specific regions of the flow field. Analysis of these flow regions is required to accurately predict the overall flow field as well as to get necessary information on localized zones of high pressure and intense heating.
The forebody is modeled by slender cones and ogives with spherically blunted nose. A combination of Navier-Stokes and parabolized Navier-Stokes equations is used to compute the flow field. The influence of entropy layer thickness on the extent of the leading edge effects is also considered. The extent of downstream effects of leading edge thickness are determined at Mach numbers of 10 and 20 for cone angles of 5°, 10°, and 20°. Three values of nose bluntness are considered with the smallest nose blunting (0.0025m) representing the sharp cone/ogive.
For the flow region around the inlet the forebody shock can interact either with the blunt cowl leading edge shock or with the shock produced by the blunt leading edges of the swept sidewall compression surfaces of the inlet. For the interaction at the cowl leading edge, the forebody shock is assumed planar and the cowl is modeled by a two-dimensional cylindrically blunted wedge of infinite width. Use of the full Navier-Stokes equations is made on the cowl forebody and the thin-layer Navier-Stokes equations are suitably modified for space marching on the cowl afterbody. The results of the study show that the flow around the cowl is significantly altered by the impinging shock. The peak value of pressure is found to be nine times and heating rates eight times the stagnation point value for unimpinged case at Mach 8.03. The peak values were slightly lower for Mach 5.94 calculations. A three-dimensional thin-layer Navier-Stokes code has been used to calculate the flow field. The peak pressure for this case is found to be 2.25 times and the peak heating three times the unimpinged stagnation values. The results of the study are compared with the available experimental and numerical results
Electromagnetic Chirps from Neutron Star-Black Hole Mergers
We calculate the electromagnetic signal of a gamma-ray flare coming from the
surface of a neutron star shortly before merger with a black hole companion.
Using a new version of the Monte Carlo radiation transport code Pandurata that
incorporates dynamic spacetimes, we integrate photon geodesics from the neutron
star surface until they reach a distant observer or are captured by the black
hole. The gamma-ray light curve is modulated by a number of relativistic
effects, including Doppler beaming and gravitational lensing. Because the
photons originate from the inspiraling neutron star, the light curve closely
resembles the corresponding gravitational waveform: a chirp signal
characterized by a steadily increasing frequency and amplitude. We propose to
search for these electromagnetic chirps using matched filtering algorithms
similar to those used in LIGO data analysis.Comment: 13 pages, 5 figures, submitted to Ap
The independent association of overweight and obesity with breathlessness in adults: a cross-sectional, population-based study
Obesity is an independent risk factor for chronic breathlessness and should be assessed in people with this symptom
Magnetic Field Effects on the Head Structure of Protostellar Jets
We present the results of 3-D SPMHD numerical simulations of
supermagnetosonic, overdense, radiatively cooling jets. Two initial magnetic
configurations are considered: (i) a helical and (ii) a longitudinal field. We
find that magnetic fields have important effects on the dynamics and structure
of radiative cooling jets, especially at the head. The presence of a helical
field suppresses the formation of the clumpy structure which is found to
develop at the head of purely hydrodynamical jets. On the other hand, a cooling
jet embedded in a longitudinal magnetic field retains clumpy morphology at its
head. This fragmented structure resembles the knotty pattern commonly observed
in HH objects behind the bow shocks of HH jets. This suggests that a strong
(equipartition) helical magnetic field configuration is ruled out at the jet
head. Therefore, if strong magnetic fields are present, they are probably
predominantly longitudinal in those regions. In both magnetic configurations,
we find that the confining pressure of the cocoon is able to excite
short-wavelength MHD K-H pinch modes that drive low-amplitude internal shocks
along the beam. These shocks are not strong however, and it likely that they
could only play a secondary role in the formation of the bright knots observed
in HH jets.Comment: 14 pages, 2 Gif figures, uses aasms4.sty. Also available on the web
page http://www.iagusp.usp.br/preprints/preprint.html. To appear in The
Astrophysical Journal Letter
On weak convergence of locally periodic functions
We prove a generalization of the fact that periodic functions converge weakly
to the mean value as the oscillation increases. Some convergence questions
connected to locally periodic nonlinear boundary value problems are also
considered.Comment: arxiv version is already officia
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