1,491 research outputs found
Implicit large eddy simulations of anisotropic weakly compressible turbulence with application to core-collapse supernovae
(Abridged) In the implicit large eddy simulation (ILES) paradigm, the
dissipative nature of high-resolution shock-capturing schemes is exploited to
provide an implicit model of turbulence. Recent 3D simulations suggest that
turbulence might play a crucial role in core-collapse supernova explosions,
however the fidelity with which turbulence is simulated in these studies is
unclear. Especially considering that the accuracy of ILES for the regime of
interest in CCSN, weakly compressible and strongly anisotropic, has not been
systematically assessed before. In this paper we assess the accuracy of ILES
using numerical methods most commonly employed in computational astrophysics by
means of a number of local simulations of driven, weakly compressible,
anisotropic turbulence. We report a detailed analysis of the way in which the
turbulent cascade is influenced by the numerics. Our results suggest that
anisotropy and compressibility in CCSN turbulence have little effect on the
turbulent kinetic energy spectrum and a Kolmogorov scaling is
obtained in the inertial range. We find that, on the one hand, the kinetic
energy dissipation rate at large scales is correctly captured even at
relatively low resolutions, suggesting that very high effective Reynolds number
can be achieved at the largest scales of the simulation. On the other hand, the
dynamics at intermediate scales appears to be completely dominated by the
so-called bottleneck effect, \ie the pile up of kinetic energy close to the
dissipation range due to the partial suppression of the energy cascade by
numerical viscosity. An inertial range is not recovered until the point where
relatively high resolution , which would be difficult to realize in
global simulations, is reached. We discuss the consequences for CCSN
simulations.Comment: 17 pages, 9 figures, matches published versio
Probing Extreme-Density Matter with Gravitational Wave Observations of Binary Neutron Star Merger Remnants
We present a proof-of-concept study, based on numerical-relativity
simulations, of how gravitational waves (GWs) from neutron star merger remnants
can probe the nature of matter at extreme densities. Phase transitions and
extra degrees of freedom can emerge at densities beyond those reached during
the inspiral, and typically result in a softening of the equation of state
(EOS). We show that such physical effects change the qualitative dynamics of
the remnant evolution, but they are not identifiable as a signature in the GW
frequency, with the exception of possible black-hole formation effects. The EOS
softening is, instead, encoded in the GW luminosity and phase and is in
principle detectable up to distances of the order of several Mpcs with advanced
detectors and up to hundreds of Mpcs with third generation detectors. Probing
extreme-density matter will require going beyond the current paradigm and
developing a more holistic strategy for modeling and analyzing postmerger GW
signals.Comment: 5 pages, 3 figures. Matches version accepted on ApJ
Dynamical Mass Ejection from Binary Neutron Star Mergers
We present fully general-relativistic simulations of binary neutron star
mergers with a temperature and composition dependent nuclear equation of state.
We study the dynamical mass ejection from both quasi-circular and
dynamical-capture eccentric mergers. We systematically vary the level of our
treatment of the microphysics to isolate the effects of neutrino cooling and
heating and we compute the nucleosynthetic yields of the ejecta. We find that
eccentric binaries can eject significantly more material than quasi-circular
binaries and generate bright infrared and radio emission. In all our
simulations the outflow is composed of a combination of tidally- and
shock-driven ejecta, mostly distributed over a broad angle from
the orbital plane, and, to a lesser extent, by thermally driven winds at high
latitudes. Ejecta from eccentric mergers are typically more neutron rich than
those of quasi-circular mergers. We find neutrino cooling and heating to
affect, quantitatively and qualitatively, composition, morphology, and total
mass of the outflows. This is also reflected in the infrared and radio
signatures of the binary. The final nucleosynthetic yields of the ejecta are
robust and insensitive to input physics or merger type in the regions of the
second and third r-process peaks. The yields for elements on the first peak
vary between our simulations, but none of our models is able to explain the
Solar abundances of first-peak elements without invoking additional first-peak
contributions from either neutrino and viscously-driven winds operating on
longer timescales after the mergers, or from core-collapse supernovae.Comment: 19 pages, 10 figures. We corrected a problem in the formulation of
the neutrino heating scheme and re-ran all of the affected models. The main
conclusions are unchanged. This version also contains one more figure and a
number of improvements on the tex
Did the ACA's "guaranteed issue" provision cause adverse selection into nongroup insurance? Analysis using a copula-based hurdle model
Prior to the Affordable Care Act (ACA), insurance companies could charge higher premiums, or outright deny coverage, to people with preexisting health problems. But the ACA's “guaranteed issue” provision forbids such price discrimination and denials of coverage. This paper seeks to determine whether, after implementation of the ACA, nongroup private insurance plans have experienced adverse selection. Our empirical approach employs a copula-based hurdle regression model, with dependence modeled as a function of dimensions along which adverse selection might occur. Our main finding is that, after implementation of the ACA, nongroup insurance enrollees with preexisting health problems do not appear to exhibit adverse selection. This finding suggests that the ACA's mandate that everyone acquire coverage might have attracted enough healthy enrollees to offset any adverse selection
Global Trajectory Optimisation : Can We Prune the Solution Space When Considering Deep Space Manoeuvres? [Final Report]
This document contains a report on the work done under the ESA/Ariadna study 06/4101 on the global optimization of space trajectories with multiple gravity assist (GA) and deep space manoeuvres (DSM). The study was performed by a joint team of scientists from the University of Reading and the University of Glasgow
Neutrino-driven Turbulent Convection and Standing Accretion Shock Instability in Three-Dimensional Core-Collapse Supernovae
We conduct a series of numerical experiments into the nature of
three-dimensional (3D) hydrodynamics in the postbounce stalled-shock phase of
core-collapse supernovae using 3D general-relativistic hydrodynamic simulations
of a - progenitor star with a neutrino leakage/heating scheme. We
vary the strength of neutrino heating and find three cases of 3D dynamics: (1)
neutrino-driven convection, (2) initially neutrino-driven convection and
subsequent development of the standing accretion shock instability (SASI), (3)
SASI dominated evolution. This confirms previous 3D results of Hanke et al.
2013, ApJ 770, 66 and Couch & Connor 2014, ApJ 785, 123. We carry out
simulations with resolutions differing by up to a factor of 4 and
demonstrate that low resolution is artificially favorable for explosion in the
3D convection-dominated case, since it decreases the efficiency of energy
transport to small scales. Low resolution results in higher radial convective
fluxes of energy and enthalpy, more fully buoyant mass, and stronger neutrino
heating. In the SASI-dominated case, lower resolution damps SASI oscillations.
In the convection-dominated case, a quasi-stationary angular kinetic energy
spectrum develops in the heating layer. Like other 3D studies, we
find in the "inertial range," while theory and
local simulations argue for . We argue that
current 3D simulations do not resolve the inertial range of turbulence and are
affected by numerical viscosity up to the energy containing scale, creating a
"bottleneck" that prevents an efficient turbulent cascade.Comment: 24 pages, 15 figures. Accepted for publication in The Astrophysical
Journal. Added one figure and made minor modifications to text according to
suggestions from the refere
One-armed spiral instability in neutron star mergers and its detectability in gravitational waves
We study the development and saturation of the one-armed spiral
instability in remnants of binary neutron star mergers by means of
high-resolution long-term numerical relativity simulations. Our results suggest
that this instability is a generic outcome of neutron stars mergers in
astrophysically relevant configurations; including both "stiff" and "soft"
nuclear equations of state. We find that, once seeded at merger, the mode
saturates within \sim 10\ \mathrmms and persists over secular timescales.
Gravitational waves emitted by the instability have a peak frequency
around 1-2\ \mathrmkHz and, if detected, could be used to constrain the
equation of state of neutron stars. We construct hybrid waveforms spanning the
entire Advanced LIGO band by combining our high-resolution numerical data with
state-of-the-art effective-one-body waveforms including tidal effects. We use
the complete hybrid waveforms to study the detectability of the one-armed
spiral instability for both Advanced LIGO and the Einstein Telescope. We
conclude that the one-armed spiral instability is not an efficient
gravitational wave emitter. Its observation by current generation detectors is
unlikely and will require third-generation interferometers
Consequences of asteroid fragmentation during impact hazard mitigation
The consequences of the fragmentation of an Earth-threatening asteroid due to an attempted deflection are examined in this paper. The minimum required energy for a successful impulsive deflection of a threatening object is computed and compared to the energy required to break up a small size asteroid. The results show that the fragmentation of an asteroid that underwent an impulsive deflection, such as a kinetic impact or a nuclear explosion, is a very plausible event.Astatistical model is used to approximate the number and size of the fragments as well as the distribution of velocities at the instant after the deflection attempt takes place. This distribution of velocities is a function of the energy provided by the deflection attempt, whereas the number and size of the asteroidal fragments is a function of the size of the largest fragment. The model also takes into account the gravity forces that could lead to a reaggregation of the asteroid after fragmentation. The probability distribution of the pieces after the deflection is then propagated forward in time until the encounter with Earth. A probability damage factor (i.e., expected damage caused by a given size fragment multiplied by its impact probability) is then computed and analyzed for different plausible scenarios, characterized by different levels of deflection energies and lead times
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