171 research outputs found
An Implicit Lagrangean Code for Spherically Symmetric General Relativistic Hydrodynamics with an Approximate Riemann Solver
An implicit Lagrangian hydrodynamics code for general relativistic spherical
collapse is presented. This scheme is based on an approximate linearized
Riemann solver (Roe type scheme). This code is aimed especially at the
calculation of the late phase of collapse-driven supernovae and the nascent
neutron star, where there is a remarkable contrast between the dynamical time
scale of the proto-neutron star and the diffusion time scale of neutrinos,
without such severe limitation of the Courant condition at the center of the
neutron star. Several standard test calculations have been done. Two other
adiabatic simulations have also been done in order to test the performance of
the code in the context of the collapse-driven supernovae. It is found that the
time step can be extended far beyond the Courant limitation at the center of
the neutron star. The details of the scheme and the results of these test
calculations are discussed.Comment: AASTeX v4.0, 24 pages, 13 figures on request from
[email protected], submitted to Ap
Equation of State in Numerical Relativistic Hydrodynamics
Relativistic temperature of gas raises the issue of the equation of state
(EoS) in relativistic hydrodynamics. We study the EoS for numerical
relativistic hydrodynamics, and propose a new EoS that is simple and yet
approximates very closely the EoS of the single-component perfect gas in
relativistic regime. We also discuss the calculation of primitive variables
from conservative ones for the EoS's considered in the paper, and present the
eigenstructure of relativistic hydrodynamics for a general EoS, in a way that
they can be used to build numerical codes. Tests with a code based on the Total
Variation Diminishing (TVD) scheme are presented to highlight the differences
induced by different EoS's.Comment: To appear in the ApJS September 2006, v166n1 issue. Pdf with full
resolution figures can be downloaded from
http://canopus.cnu.ac.kr/ryu/ryuetal.pd
Planets opening dust gaps in gas disks
We investigate the interaction of gas and dust in a protoplanetary disk in
the presence of a massive planet using a new two-fluid hydrodynamics code. In
view of future observations of planet-forming disks we focus on the condition
for gap formation in the dust fluid. While only planets more massive than 1
Jupiter mass (MJ) open up a gap in the gas disk, we find that a planet of 0.1
MJ already creates a gap in the dust disk. This makes it easier to find
lower-mass planets orbiting in their protoplanetary disk if there is a
significant population of mm-sized particles.Comment: 5 pages, 3 figures, accepted for publication in A&A Letter
RAM: A Relativistic Adaptive Mesh Refinement Hydrodynamics Code
We have developed a new computer code, RAM, to solve the conservative
equations of special relativistic hydrodynamics (SRHD) using adaptive mesh
refinement (AMR) on parallel computers. We have implemented a
characteristic-wise, finite difference, weighted essentially non-oscillatory
(WENO) scheme using the full characteristic decomposition of the SRHD equations
to achieve fifth-order accuracy in space. For time integration we use the
method of lines with a third-order total variation diminishing (TVD)
Runge-Kutta scheme. We have also implemented fourth and fifth order Runge-Kutta
time integration schemes for comparison. The implementation of AMR and
parallelization is based on the FLASH code. RAM is modular and includes the
capability to easily swap hydrodynamics solvers, reconstruction methods and
physics modules. In addition to WENO we have implemented a finite volume module
with the piecewise parabolic method (PPM) for reconstruction and the modified
Marquina approximate Riemann solver to work with TVD Runge-Kutta time
integration. We examine the difficulty of accurately simulating shear flows in
numerical relativistic hydrodynamics codes. We show that under-resolved
simulations of simple test problems with transverse velocity components produce
incorrect results and demonstrate the ability of RAM to correctly solve these
problems. RAM has been tested in one, two and three dimensions and in
Cartesian, cylindrical and spherical coordinates. We have demonstrated
fifth-order accuracy for WENO in one and two dimensions and performed detailed
comparison with other schemes for which we show significantly lower convergence
rates. Extensive testing is presented demonstrating the ability of RAM to
address challenging open questions in relativistic astrophysics.Comment: ApJS in press, 21 pages including 18 figures (6 color figures
An Efficient Implementation of Flux Formulae in Multidimensional Relativistic Hydrodynamical Codes
We derive and analyze a simplified formulation of the numerical viscosity
terms appearing in the expression of the numerical fluxes associated to several
High-Resolution Shock-Capturing schemes. After some algebraic pre-processing,
we give explicit expressions for the numerical viscosity terms of two of the
most widely used flux formulae, which implementation saves computational time
in multidimensional simulations of relativistic flows. Additionally, such
treatment explicitely cancells and factorizes a number of terms helping to
amortiguate the growing of round-off errors. We have checked the performance of
our formulation running a 3D relativistic hydrodynamical code to solve a
standard test-bed problem and found that the improvement in efficiency is of
high practical interest in numerical simulations of relativistic flows in
Astrophysics.Comment: 10 pages, accepted for publication in Computer Physics Communication
Collimated Jet or Expanding Outflow: Possible Origins of GRBs and X-Ray Flashes
We investigate the dynamics of an injected outflow propagating in a
progenitor in the context of the collapsar model for gamma-ray bursts (GRBs)
through two dimensional axisymmetric relativistic hydrodynamic simulations.
Initially, we locally inject an outflow near the center of a progenitor. We
calculate 25 models, in total, by fixing its total input energy to be 10^{51}
ergs s^{-1} and radius of the injected outflow to be cm while
varying its bulk Lorentz factor, , and its specific
internal energy, . The injected outflow propagates
in the progenitor and drives a large-scale outflow or jet. We find a smooth but
dramatic transition from a collimated jet to an expanding outflow among
calculated models. The maximum Lorentz factor is, on the other hand, sensitive
to both of and ; roughly . Our finding will explain a smooth transition between the
GRBs, X-ray rich GRBs (XRRs) and X-ray Flashes (XRFs) by the same model but
with different values.Comment: Comments 51 pages, 21 figures. accepted for publication in ApJ high
resolution version is available at
http://www.mpa-garching.mpg.de/~mizuta/COLLAPSAR/collapsar.htm
Evolution of clouds in radio galaxy cocoons
This letter presents a numerical study of the evolution of an emission line
cloud of initial density 10 cm, temperature K, and size 200 pc,
being overtaken by a strong shock wave. Whereas previous simple models proposed
that such a cloud would either be completely destroyed, or simply shrink in
size, our results show a different and more complex behaviour: due to rapid
cooling, the cloud breaks up into many small and dense fragments, which can
survive for a long time. We show that such rapid cooling behaviour is in fact
expected for a wide range of cloud and shock properties. This process applies
to the evolution of emission line clouds being overtaken by the cocoon of a
radio jet. The resulting small clouds would be Jeans unstable, and form stars.
Our results thus give theoretical credibility to the process of jet induced
star formation, one of the explanations for the alignment of the optical/UV and
radio axis observed in high redshift radio galaxies.Comment: 4 pages, 2 figures, movies available at
http://www.strw.leidenuniv.nl/TheoryGroup/IG-Cloud.htm
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