9,431 research outputs found
New numerical solver for flows at various Mach numbers
Many problems in stellar astrophysics feature flows at low Mach numbers.
Conventional compressible hydrodynamics schemes frequently used in the field
have been developed for the transonic regime and exhibit excessive numerical
dissipation for these flows. While schemes were proposed that solve
hydrodynamics strictly in the low Mach regime and thus restrict their
applicability, we aim at developing a scheme that correctly operates in a wide
range of Mach numbers. Based on an analysis of the asymptotic behavior of the
Euler equations in the low Mach limit we propose a novel scheme that is able to
maintain a low Mach number flow setup while retaining all effects of
compressibility. This is achieved by a suitable modification of the well-known
Roe solver. Numerical tests demonstrate the capability of this new scheme to
reproduce slow flow structures even in moderate numerical resolution. Our
scheme provides a promising approach to a consistent multidimensional
hydrodynamical treatment of astrophysical low Mach number problems such as
convection, instabilities, and mixing in stellar evolution.Comment: 16 pages, 8 figures, accepted for publication by A&
X-ray spectroscopy of galaxy clusters: beyond the CIE modeling
X-ray spectra of galaxy clusters are dominated by the thermal emission from
the hot intracluster medium. In some cases, besides the thermal component,
spectral models require additional components associated, e.g., with resonant
scattering and charge exchange. The latter produces mostly underluminous fine
spectral features. Detection of the extra components therefore requires high
spectral resolution. The upcoming X-ray missions will provide such high
resolution, and will allow spectroscopic diagnostics of clusters beyond the
current simple thermal modeling. A representative science case is resonant
scattering, which produces spectral distortions of the emission lines from the
dominant thermal component. Accounting for the resonant scattering is essential
for accurate abundance and gas motion measurements of the ICM. The high
resolution spectroscopy might also reveal/corroborate a number of new spectral
components, including the excitation by non-thermal electrons, the deviation
from ionization equilibrium, and charge exchange from surface of cold gas
clouds in clusters. Apart from detecting new features, future high resolution
spectroscopy will also enable a much better measurement of the thermal
component. Accurate atomic database and appropriate modeling of the thermal
spectrum are therefore needed for interpreting the data.Comment: published in Space Science Review
Stochastic Particle Acceleration in Parallel Relativistic Shocks
We present results of test-particle simulations on both the first- and the
second-order Fermi acceleration for relativistic parallel shock waves. Our
studies suggest that the role of the second-order mechanism in the turbulent
downstream of a relativistic shock may have been underestimated in the past,
and that the stochastic mechanism may have significant effects on the form of
the particle spectra and its time evolution.Comment: Poster at "The XXII Texas Symposium on Relativistic Astrophysics",
Stanford, USA, December 2004, (TSRA04), 6 pages, LaTeX, 5 ps/eps figure
Observability of intermittent radio sources in galaxy groups and clusters
22 pages, 24 figures. This article has been accepted for publication in Monthly Notices of the Royal Astronomical Society © 2018 The Author(s). Published by Oxford University Press on behalf of the Royal Astronomical Society. All rights reserved.We have carried out numerical hydrodynamic simulations of radio jets from active galactic nuclei using the PLUTO simulation code, with the aim of investigating the effect of different environments and intermittency of energy injection on the resulting dynamics and observable properties of the jet-inflated lobes. Initially conical jets are simulated in poor group and cluster environments. We show that the environment into which a radio jet is propagating plays a large role in the resulting morphology, dynamics and observable properties of the radio source. The same jet collimates much later in a poor group compared to a cluster, which leads to pronounced differences in radio morphology. The intermittency of the jet also affects the observable properties of the radio source, and multiple hotspots are present for multiple outburst jets in the cluster environment. We quantify the detectability of active and quiescent phases, and find this to be strongly environment-dependent. We conclude that the dynamics and observational properties of jets depend strongly on the details of energy injection and environment.Peer reviewedFinal Accepted Versio
Computational Eulerian Hydrodynamics and Galilean Invariance
Eulerian hydrodynamical simulations are a powerful and popular tool for
modeling fluids in astrophysical systems. In this work, we critically examine
recent claims that these methods violate Galilean invariance of the Euler
equations. We demonstrate that Eulerian hydrodynamics methods do converge to a
Galilean-invariant solution, provided a well-defined convergent solution
exists. Specifically, we show that numerical diffusion, resulting from
diffusion-like terms in the discretized hydrodynamical equations solved by
Eulerian methods, accounts for the effects previously identified as evidence
for the Galilean non-invariance of these methods. These velocity-dependent
diffusive terms lead to different results for different bulk velocities when
the spatial resolution of the simulation is kept fixed, but their effect
becomes negligible as the resolution of the simulation is increased to obtain a
converged solution. In particular, we find that Kelvin-Helmholtz instabilities
develop properly in realistic Eulerian calculations regardless of the bulk
velocity provided the problem is simulated with sufficient resolution (a factor
of 2-4 increase compared to the case without bulk flows for realistic
velocities). Our results reiterate that high-resolution Eulerian methods can
perform well and obtain a convergent solution, even in the presence of highly
supersonic bulk flows.Comment: Version accepted by MNRAS Oct 2, 2009. Figures degraded. For
high-resolution color figures and movies of the numerical simulations, please
visit
http://www.astro.caltech.edu/~brant/Site/Computational_Eulerian_Hydrodynamics_and_Galilean_Invariance.htm
Cooling Effect of the Richtmyer-Meshkov Instability
We provide numerical evidence that the Richtmyer-Meshkov (RM) instability
contributes to the cooling of a relativistic fluid. Due to the presence of jet
particles traveling throughout the medium, shock waves are generated in the
form of Mach cones. The interaction of multiple shock waves can trigger the RM
instability, and we have found that this process leads to a down-cooling of the
relativistic fluid. To confirm the cooling effect of the instability, shock
tube Richtmyer-Meshkov instability simulations are performed. Additionally, in
order to provide an experimental observable of the RM instability resulting
from the Mach cone interaction, we measure the two particle correlation
function and highlight the effects of the interaction. The simulations have
been performed with an improved version of the relativistic lattice Boltzmann
model, including general equations of state and external forces.Comment: 10 pages, 6 figure
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