1,373 research outputs found
Dynamical and radiative properties of astrophysical supersonic jets I. Cocoon morphologies
We present the results of a numerical analysis of the propagation and
interaction of a supersonic jet with the external medium. We discuss the motion
of the head of the jet into the ambient in different physical conditions,
carrying out calculations with different Mach numbers and density ratios of the
jet to the exteriors. Performing the calculation in a reference frame in motion
with the jet head, we can follow in detail its long term dynamics. This
numerical scheme allows us also to study the morphology of the cocoon for
different physical parameters. We find that the propagation velocity of the jet
head into the ambient medium strongly influences the morphology of the cocoon,
and this result can be relevant in connection to the origin and structure of
lobes in extragalactic radiosources.Comment: 14 pages, TeX. Accepted for A&
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
Non-linear dynamics of Kelvin-Helmholtz unstable magnetized jets: three-dimensional effects
A numerical study of the Kelvin-Helmholtz instability in compressible
magnetohydrodynamics is presented. The three-dimensional simulations consider
shear flow in a cylindrical jet configuration, embedded in a uniform magnetic
field directed along the jet axis. The growth of linear perturbations at
specified poloidal and axial mode numbers demonstrate intricate non-linear
coupling effects. The physical mechanims leading to induced secondary
Kelvin-Helmholtz instabilities at higher mode numbers are identified. The
initially weak magnetic field becomes locally dominant in the non-linear
dynamics before and during saturation. Thereby, it controls the jet deformation
and eventual breakup. The results are obtained using the Versatile Advection
Code [G. Toth, Astrophys. Lett. Comm. 34, 245 (1996)], a software package
designed to solve general systems of conservation laws. An independent
calculation of the same Kelvin-Helmholtz unstable jet configuration using a
three-dimensional pseudo-spectral code gives important insights into the
coupling and excitation events of the various linear mode numbers.Comment: 10 (+7) pages, 6 figures, accepted for Phys. Plasmas 6, to appear
199
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
A Divergence-Free Upwind Code for Multidimensional Magnetohydrodynamic Flows
A description is given for preserving {\bmsy\nabla}\cdot{\vec B}=0 in a
magnetohydrodynamic (MHD) code that employs the upwind, Total Variation
Diminishing (TVD) scheme and the Strang-type operator splitting for
multi-dimensionality. The method is based on the staggered mesh technique to
constrain the transport of magnetic field: the magnetic field components are
defined at grid interfaces with their advective fluxes on grid edges, while
other quantities are defined at grid centers. The magnetic field at grid
centers for the upwind step is calculated by interpolating the values from grid
interfaces. The advective fluxes on grid edges for the magnetic field evolution
are calculated from the upwind fluxes at grid interfaces. Then, the magnetic
field can be maintained with {\bmsy\nabla}\cdot{\vec B}=0 exactly, if this is
so initially, while the upwind scheme is used for the update of fluid
quantities. The correctness of the code is demonstrated through tests comparing
numerical solutions either with analytic solutions or with numerical solutions
from the code using an explicit divergence-cleaning method. Also the robustness
is shown through tests involving realistic astrophysical problems.Comment: 15 pages of text, 8 figures (in degraded gif format), to appear in
The Astrophysical Journal (Dec. 10, 1998), original quality figures available
via anonymous ftp at ftp://ftp.msi.umn.edu/pub/users/twj/mhddivb5.uu or
ftp://canopus.chungnam.ac.kr/ryu/mhddivb5.u
The Wisconsin Plasma Astrophysics Laboratory
The Wisconsin Plasma Astrophysics Laboratory (WiPAL) is a flexible user
facility designed to study a range of astrophysically relevant plasma processes
as well as novel geometries that mimic astrophysical systems. A multi-cusp
magnetic bucket constructed from strong samarium cobalt permanent magnets now
confines a 10 m, fully ionized, magnetic-field free plasma in a spherical
geometry. Plasma parameters of to eV and
to cm provide an ideal testbed
for a range of astrophysical experiments including self-exciting dynamos,
collisionless magnetic reconnection, jet stability, stellar winds, and more.
This article describes the capabilities of WiPAL along with several
experiments, in both operating and planning stages, that illustrate the range
of possibilities for future users.Comment: 21 pages, 12 figures, 2 table
Dynamics and Structure of Three-Dimensional Trans-Alfvenic Jets. II. The Effect of Density and Winds
Two three-dimensional magnetohydrodynamical simulations of strongly
magnetized conical jets, one with a poloidal and one with a helical magnetic
field, have been performed. In the poloidal simulation a significant sheath
(wind) of magnetized moving material developed and partially stabilized the jet
to helical twisting. The fundamental pinch mode was not similarly affected and
emission knots developed in the poloidal simulation. Thus, astrophysical jets
surrounded by outflowing winds could develop knotty structures along a straight
jet triggered by pinching. Where helical twisting dominated the dynamics,
magnetic field orientation along the line-of-sight could be organized by the
toroidal flow field accompanying helical twisting. On astrophysical jets such
structure could lead to a reversal of the direction of Faraday rotation in
adjacent zones along a jet. Theoretical analysis showed that the different
dynamical behavior of the two simulations could be entirely understood as a
result of dependence on the velocity shear between jet and wind which must
exceed a surface Alfven speed before the jet becomes unstable to helical and
higher order modes of jet distortion.Comment: 25 pages, 15 figures, in press Astrophysical Journal (September
Shocks in relativistic transverse stratified jets, a new paradigm for radio-loud AGN
The transverse stratification of active galactic nuclei (AGN) jets is
suggested by observations and theoretical arguments, as a consequence of
intrinsic properties of the central engine (accretion disc + black hole) and
external medium. On the other hand, the one-component jet approaches are
heavily challenged by the various observed properties of plasmoids in radio
jets (knots), often associated with internal shocks. Given that such a
transverse stratification plays an important role on the jets acceleration,
stability, and interaction with the external medium, it should also induce
internal shocks with various strengths and configurations, able to describe the
observed knots behaviours. By establishing a relation between the transverse
stratification of the jets, the internal shock properties, and the multiple
observed AGN jet morphologies and behaviours, our aim is to provide a
consistent global scheme of the various AGN jet structures. Working on a large
sample of AGN radio jets monitored in very long baseline interferometry (VLBI)
by the MOJAVE collaboration, we determined the consistency of a systematic
association of the multiple knots with successive re-collimation shocks. We
then investigated the re-collimation shock formation and the influence of
different transverse stratified structures by parametrically exploring the two
relativistic outflow components with the specific relativistic hydrodynamic
(SRHD) code AMRVAC. We were able to link the different spectral classes of AGN
with specific stratified jet characteristics, in good accordance with their
VLBI radio properties and their accretion regimes.Comment: 16 pages, 12 figures, accepted for publication in A&
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