29 research outputs found
Numerical Hydrodynamics of Relativistic Extragalactic Jets
This dissertation describes a multidimensional relativistic hydrodynamic code which solves the special relativistic hydrodynamic equations as a hyperbolic system of conservation laws based on the total variation diminishing (TVD) scheme. Several standard tests and test simulations are presented to demonstrate the accuracy, robustness and flexibility of the code. Using this code we have studied three-dimensional hydrodynamic interactions of relativistic extragalactic jets with two-phase ambient media. The deflection angle of the jet is influenced more by the density contrast of the cloud than by the beam Mach number of the jet, and a relativistic jet with low relativistic beam Mach number can eventually be slightly bent after it crosses the dense cloud. Relativistic jet impacts on dense clouds do not necessarily destroy the clouds completely, and much of the cloud body can survive as a coherent blob due to the combination of the geometric influence of off-axis collisions and the lower rate of cloud fragmentation through the Kelvin-Helmholtz instability for relativistic flows. We find that relativistic jets interacting with clouds can produce synchrotron emission knots similar to structures observed in many VLBI-scale radio sources and the synchrotron emission peaks right before the jet passes through the cloud
A Multidimensional Relativistic Hydrodynamics Code with a General Equation of State
The ideal gas equation of state with a constant adiabatic index, although
commonly used in relativistic hydrodynamics, is a poor approximation for most
relativistic astrophysical flows. Here we propose a new general equation of
state for a multi-component relativistic gas which is consistent with the Synge
equation of state for a relativistic perfect gas and is suitable for numerical
(special) relativistic hydrodynamics. We also present a multidimensional
relativistic hydrodynamics code incorporating the proposed general equation of
state, based on the HLL scheme, which does not make use of a full
characteristic decomposition of the relativistic hydrodynamic equations. The
accuracy and robustness of this code is demonstrated in multidimensional
calculations through several highly relativistic test problems taking into
account nonvanishing tangential velocities. Results from three-dimensional
simulations of relativistic jets show that the morphology and dynamics of the
relativistic jets are significantly influenced by the different equation of
state and by different compositions of relativistic perfect gases. Our new
numerical code, combined with our proposed equation of state is very efficient
and robust, and unlike previous codes, it gives very accurate results for
thermodynamic variables in relativistic astrophysical flows.Comment: 32 pages, 9 figures, accepted by ApJ
From giant gravitons to black holes
We study AdS black holes from a recently suggested giant graviton
expansion formula for the index of maximal super-Yang-Mills theory. We
compute the large entropy at fixed charges and giant graviton numbers
by a saddle point analysis, and further maximize it in . This agrees with
the dual black hole entropy in the small black hole limit. To get black holes
at general sizes, one should note that various giant graviton indices cancel
because gauge theory does not suffer from a Hagedorn-like pathology by an
infinite baryonic tower. With one assumption on the mechanism of this
cancellation, we account for the dual black hole entropy at general sizes. We
interpret our results as analytic continuations of the large free energies
of SCFTs, and based on it compute the entropies of AdS black holes from
M5, M2 giant gravitons.Comment: 27 pages, 4 figure
The shape of non-graviton operators for
The BPS spectrum of AdS/CFT exhibits multi-gravitons at low energies, while
having black hole states at higher energies. This can be studied concretely in
AdS/CFT in terms of classical cohomologies, even in the quantum regimes
at finite . Recently, Chang and Lin found a threshold for non-graviton
states in the maximal super-Yang-Mills theory. We explicitly construct
and present this threshold cohomology.Comment: 8 page
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
Towards quantum black hole microstates
We study the cohomology of local BPS operators in Yang-Mills
theory. The finite cohomologies consist of the graviton part (subject to
the stringy exclusion principle) and the rest which may describe black hole
microstates in quantum AdS/CFT. We construct an infinite tower of non-graviton
cohomologies in the theory and study to what extent they simulate
quantum black holes. We find signals for partial no-hair behaviors by showing
that certain gravitons are forbidden to dress these cohomologies. This is in
qualitative agreement with the perturbative hairs allowed around black holes,
which also leads us to a natural setup to construct hairy BPS black holes. The
cohomologies are simpler to study in the BMN matrix model truncation of the
classical field theory.Comment: 58 page
An Explicit Scheme for Incorporating Ambipolar Diffusion in a Magnetohydrodynamics Code
We describe a method for incorporating ambipolar diffusion in the strong
coupling approximation into a multidimensional magnetohydrodynamics code based
on the total variation diminishing scheme. Contributions from ambipolar
diffusion terms are included by explicit finite difference operators in a fully
unsplit way, maintaining second order accuracy. The divergence-free condition
of magnetic fields is exactly ensured at all times by a flux-interpolated
constrained transport scheme. The super time stepping method is used to
accelerate the timestep in high resolution calculations and/or in strong
ambipolar diffusion. We perform two test problems, the steady-state oblique
C-type shocks and the decay of Alfv\'en waves, confirming the accuracy and
robustness of our numerical approach. Results from the simulations of the
compressible MHD turbulence with ambipolar diffusion show the flexibility of
our method as well as its ability to follow complex MHD flows in the presence
of ambipolar diffusion. These simulations show that the dissipation rate of MHD
turbulence is strongly affected by the strength of ambipolar diffusion.Comment: 25 pages, 5 figures, ApJS accepte
Hydrodynamic Interactions of Relativistic Extragalactic Jets with Dense Clouds
We have studied three-dimensional hydrodynamic interactions of relativistic
extragalactic jets with two-phase ambient media. These jets propagate through a
denser homogeneous gas and then impact clouds with densities 100 to 1000 times
higher than the initial beam density. The deflection angle of the jet is
influenced more by the density contrast of the cloud than by the beam Mach
number of the jet. A relativistic jet with low relativistic beam Mach number
can eventually be slightly bent after it crosses the dense cloud; however, we
have not seen permanently bent structures in the interaction of a high
relativistic beam Mach number jet with a cloud. The relativistic jet impacts on
dense clouds do not necessarily destroy the clouds completely, and much of the
cloud body can survive as a coherent blob. This enhancement of cloud durability
is partly due to the geometric influence of the off-axis collisions we consider
and also arises from the lower rate of cloud fragmentation through the
Kelvin-Helmholtz instability for relativistic jets. To compare our simulations
with observed extragalactic radio jets, we have computed the approximate
surface distributions of synchrotron emission at different viewing angles.
These surface intensity maps show relativistic jets interacting with clouds can
produce synchrotron emission knots similar to structures observed in many
VLBI-scale radio sources. We find that the synchrotron emission increases
steeply at the moment of impact and the emission peaks right before the jet
passes through the cloud.Comment: 32 pages, 7 figures, accepted for publication in Ap