14,452 research outputs found
A comparison of black hole growth in galaxy mergers with Gasoline and Ramses
Supermassive black hole dynamics during galaxy mergers is crucial in
determining the rate of black hole mergers and cosmic black hole growth. As
simulations achieve higher resolution, it becomes important to assess whether
the black hole dynamics is influenced by the treatment of the interstellar
medium in different simulation codes. We here compare simulations of black hole
growth in galaxy mergers with two codes: the Smoothed Particle Hydrodynamics
code Gasoline, and the Adaptive Mesh Refinement code Ramses. We seek to
identify predictions of these models that are robust despite differences in
hydrodynamic methods and implementations of sub-grid physics. We find that the
general behavior is consistent between codes. Black hole accretion is minimal
while the galaxies are well-separated (and even as they "fly-by" within 10 kpc
at first pericenter). At late stages, when the galaxies pass within a few kpc,
tidal torques drive nuclear gas inflow that triggers bursts of black hole
accretion accompanied by star formation. We also note quantitative
discrepancies that are model-dependent: our Ramses simulations show less star
formation and black hole growth, and a smoother gas distribution with larger
clumps and filaments, than our Gasoline simulations. We attribute these
differences primarily to the sub-grid models for black hole fueling and
feedback and gas thermodynamics. The main conclusion is that differences exist
quantitatively between codes, and this should be kept in mind when making
comparisons with observations. However, reassuringly, both codes capture the
same dynamical behaviors in terms of triggering of black hole accretion, star
formation, and black hole dynamics.Comment: 11 pages, 7 figures. Submitted to A&A. Comments welcom
Testing hydrodynamics schemes in galaxy disc simulations
We examine how three fundamentally different numerical hydrodynamics codes follow the evolution of an isothermal galactic disc with an external spiral potential. We compare an adaptive mesh refinement code (RAMSES), a smoothed particle hydrodynamics code (SPHNG), and a volume-discretised meshless code (GIZMO). Using standard refinement criteria, we find that RAMSES produces a disc that is less vertically concentrated and does not reach such high densities as the SPHNG or gizmo runs. The gas surface density in the spiral arms increases at a lower rate for the RAMSES simulations compared to the other codes. There is also a greater degree of substructure in the SPHNG and GIZMOruns and secondary spiral arms are more pronounced. By resolving the Jeans’ length with a greater number of grid cells we achieve more similar results to the Lagrangian codes used in this study. Other alterations to the refinement scheme (adding extra levels of refinement and refining based on local density gradients) are less successful in reducing the disparity between RAMSES and SPHNG/GIZMO. Although more similar, SPHNG displays different density distributions and vertical mass profiles to all modes of gizmo (including the smoothed particle hydrodynamics version). This suggests differences also arise which are not intrinsic to the particular method but rather due to its implementation. The discrepancies between codes (in particular, the densities reached in the spiral arms) could potentially result in differences in the locations and timescales for gravitational collapse, and therefore impact star formation activity in more complex galaxy disc simulations
Comparing Numerical Methods for Isothermal Magnetized Supersonic Turbulence
We employ simulations of supersonic super-Alfvenic turbulence decay as a
benchmark test problem to assess and compare the performance of nine
astrophysical MHD methods actively used to model star formation. The set of
nine codes includes: ENZO, FLASH, KT-MHD, LL-MHD, PLUTO, PPML, RAMSES, STAGGER,
and ZEUS. We present a comprehensive set of statistical measures designed to
quantify the effects of numerical dissipation in these MHD solvers. We compare
power spectra for basic fields to determine the effective spectral bandwidth of
the methods and rank them based on their relative effective Reynolds numbers.
We also compare numerical dissipation for solenoidal and dilatational velocity
components to check for possible impacts of the numerics on small-scale density
statistics. Finally, we discuss convergence of various characteristics for the
turbulence decay test and impacts of various components of numerical schemes on
the accuracy of solutions. We show that the best performing codes employ a
consistently high order of accuracy for spatial reconstruction of the evolved
fields, transverse gradient interpolation, conservation law update step, and
Lorentz force computation. The best results are achieved with divergence-free
evolution of the magnetic field using the constrained transport method, and
using little to no explicit artificial viscosity. Codes which fall short in one
or more of these areas are still useful, but they must compensate higher
numerical dissipation with higher numerical resolution. This paper is the
largest, most comprehensive MHD code comparison on an application-like test
problem to date. We hope this work will help developers improve their numerical
algorithms while helping users to make informed choices in picking optimal
applications for their specific astrophysical problems.Comment: 17 pages, 5 color figures, revised version to appear in ApJ, 735,
July 201
Radio emission of SN1993J. The complete picture: II. Simultaneous fit of expansion and radio light curves
We report on a simultaneous modelling of the expansion and radio light curves
of SN1993J. We have developed a simulation code capable of generating synthetic
expansion and radio light curves of supernovae by taking into consideration the
evolution of the expanding shock, magnetic fields, and relativistic electrons,
as well as the finite sensitivity of the interferometric arrays used in the
observations. Our software successfully fits all the available radio data of SN
1993J with an standard emission model for supernovae extended with some
physical considerations, as an evolution in the opacity of the ejecta material,
a radial drop of the magnetic fields inside the radiating region, and a
changing radial density profile of the circumstellar medium beyond day 3100
after explosion.Comment: 12 pages, 12 figures, accepted for publication in A&
Colliding wind binaries and gamma-ray binaries : relativistic version of the RAMSES code
Gamma-ray binaries are colliding wind binaries (CWB) composed of a massive
star a non-accreting pulsar with a highly relativistic wind. Particle
acceleration at the shocks results in emission going from extended radio
emission to the gamma-ray band. The interaction region is expected to show
common features with stellar CWB. Performing numerical simulations with the
hydrodynamical code RAMSES, we focus on their structure and stability and find
that the Kelvin-Helmholtz instability (KHI) can lead to important mixing
between the winds and destroy the large scale spiral structure. To investigate
the impact of the relativistic nature of the pulsar wind, we extend RAMSES to
relativistic hydrodynamics (RHD). Preliminary simulations of the interaction
between a pulsar wind and a stellar wind show important similarities with
stellar colliding winds with small relativistic corrections.Comment: Proceeding of the 5th International Symposium on High-Energy
Gamma-Ray Astronomy (Gamma2012). arXiv admin note: text overlap with
arXiv:1212.404
Production/maintenance cooperative scheduling using multi-agents and fuzzy logic
Within companies, production is directly concerned with the manufacturing schedule, but other services like sales, maintenance, purchasing or workforce management should also have an influence on this schedule. These services often have together a hierarchical relationship, i.e. the leading function (most of the time sales or production) generates constraints defining the framework within which the other functions have to satisfy their own objectives. We show how the multi-agent paradigm, often used in scheduling for its ability to distribute decision-making, can also provide a framework for making several functions cooperate in the schedule performance. Production and maintenance have been chosen as an example: having common resources (the machines), their activities are actually often conflicting. We show how to use a fuzzy logic in order to model the temporal degrees of freedom of the two functions, and show that this approach may allow one to obtain a schedule that provides a better compromise between the satisfaction of the respective objectives of the two functions
A scheme for radiation pressure and photon diffusion with the M1 closure in RAMSES-RT
We describe and test an updated version of radiation-hydrodynamics (RHD) in
the RAMSES code, that includes three new features: i) radiation pressure on
gas, ii) accurate treatment of radiation diffusion in an unresolved optically
thick medium, and iii) relativistic corrections that account for Doppler
effects and work done by the radiation to first order in v/c. We validate the
implementation in a series of tests, which include a morphological assessment
of the M1 closure for the Eddington tensor in an astronomically relevant
setting, dust absorption in a optically semi-thick medium, direct pressure on
gas from ionising radiation, convergence of our radiation diffusion scheme
towards resolved optical depths, correct diffusion of a radiation flash and a
constant luminosity radiation, and finally, an experiment from Davis et al. of
the competition between gravity and radiation pressure in a dusty atmosphere,
and the formation of radiative Rayleigh-Taylor instabilities. With the new
features, RAMSES-RT can be used for state-of-the-art simulations of radiation
feedback from first principles, on galactic and cosmological scales, including
not only direct radiation pressure from ionising photons, but also indirect
pressure via dust from multi-scattered IR photons reprocessed from
higher-energy radiation, both in the optically thin and thick limits.Comment: 25 pages, 13 figures, accepted for publication in MNRAS. Revised to
match published versio
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