7,519 research outputs found
A Magnetohydrodynamic Nonradiative Accretion Flow in Three Dimensions
We present a global magnetohydrodynamic (MHD) three dimensional simulation of
a nonradiative accretion flow originating in a pressure supported torus. The
evolution is controlled by the magnetorotational instability which produces
turbulence. The flow forms a nearly Keplerian disk. The total pressure scale
height in this disk is comparable to the vertical size of the initial torus.
Gas pressure dominates only near the equator; magnetic pressure is more
important in the surrounding atmosphere. A magnetically dominated bound outflow
is driven from the disk. The accretion rate through the disk exceeds the final
rate into the hole, and a hot torus forms inside 10 r_g. Hot gas, pushed up
against the centrifugal barrier and confined by magnetic pressure, is ejected
in a narrow, unbound, conical outflow. The dynamics are controlled by magnetic
turbulence, not thermal convection, and a hydrodynamic alpha model is
inadequate to describe the flow. The limitations of two dimensional MHD
simulations are also discussed.Comment: 5 pages, 2 figures, submitted to ApJ Letters. For web version and
mpeg animations see http://www.astro.virginia.edu/~jh8h/nraf
The Effect of Resistivity on the Nonlinear Stage of the Magnetorotational Instability in Accretion Disks
We present three-dimensional magnetohydrodynamic simulations of the nonlinear
evolution of the magnetorotational instability (MRI) with a non-zero Ohmic
resistivity. The properties of the saturated state depend on the initial
magnetic field configuration. In simulations with an initial uniform vertical
field, the MRI is able to support angular momentum transport even for large
resistivities through the quasi-periodic generation of axisymmetric radial
channel solutions rather than through the maintenance of anisotropic
turbulence. Simulations with zero net flux show that the angular momentum
transport and the amplitude of magnetic energy after saturation are
significantly reduced by finite resistivity, even at levels where the linear
modes are only slightly affected. This occurs at magnetic Reynolds numbers
expected in low, cool states of dwarf novae, these results suggest that finite
resistivity may account for the low and high angular momentum transport rates
inferred for these systems.Comment: 8 figures, accepted for publication in Ap
Femoral pseudoaneurysms after percutaneous access
The femoral artery has been the primary percutaneous-based arterial access site for coronary artery catheterizations for more than three decades. Noncardiac percutaneous-based procedures have also been performed primarily with femoral access and have increased in number exponentially by vascular specialists in past decades. Groin complications are infrequent in incidence after femoral arterial access for cardiac and peripheral diagnostic and interventional cases, with groin hematomas and pseudoaneurysms being the most common. Until ultrasound-based treatment modalities became the mainstay of treatment, vascular surgeons were the primary specialty managing pseudoaneurysms, but now other specialties also manage these cases. This review outlines the clinical implications and current issues relevant to understanding the ideal treatment strategy for this common complication
Chaos in Turbulence Driven by the Magnetorotational Instability
Chaotic flow is studied in a series of numerical magnetohydrodynamical
simulations that use the shearing box formalism. This mimics important features
of local accretion disk dynamics. The magnetorotational instability gives rise
to flow turbulence, and quantitative chaos parameters, such as the largest
Lyapunov exponent, can be measured. Linear growth rates appear in these
exponents even when the flow is fully turbulent. The extreme sensitivity to
initial conditions associated with chaotic flows has practical implications,
the most important of which is that hundreds of orbital times are needed to
extract a meaningful average for the stress. If the evolution time in a disk is
less than this, the classical formalism will break down.Comment: 6 pages, 8 figures. To be appear in MNRA
Viscous and Resistive Effects on the MRI with a Net Toroidal Field
Resistivity and viscosity have a significant role in establishing the energy
levels in turbulence driven by the magnetorotational instability (MRI) in local
astrophysical disk models. This study uses the Athena code to characterize the
effects of a constant shear viscosity \nu and Ohmic resistivity \eta in
unstratified shearing box simulations with a net toroidal magnetic flux. A
previous study of shearing boxes with zero net magnetic field performed with
the ZEUS code found that turbulence dies out for values of the magnetic Prandtl
number, P_m = \nu/\eta, below P_m \sim 1; for P_m \gtrsim 1, time- and
volume-averaged stress levels increase with P_m. We repeat these experiments
with Athena and obtain consistent results. Next, the influence of viscosity and
resistivity on the toroidal field MRI is investigated both for linear growth
and for fully-developed turbulence. In the linear regime, a sufficiently large
\nu or \eta can prevent MRI growth; P_m itself has little direct influence on
growth from linear perturbations. By applying a range of values for \nu and
\eta to an initial state consisting of fully developed turbulence in the
presence of a background toroidal field, we investigate their effects in the
fully nonlinear system. Here, increased viscosity enhances the turbulence, and
the turbulence decays only if the resistivity is above a critical value;
turbulence can be sustained even when P_m < 1, in contrast to the zero net
field model. While we find preliminary evidence that the stress converges to a
small range of values when \nu and \eta become small enough, the influence of
dissipation terms on MRI-driven turbulence for relatively large \eta and \nu is
significant, independent of field geometry.Comment: Accepted to ApJ; version 2 - minor changes following review; 35 pages
(preprint format), 10 figure
Resistivity-driven State Changes in Vertically Stratified Accretion Disks
We investigate the effect of shear viscosity and Ohmic resistivity on the
magnetorotational instability (MRI) in vertically stratified accretion disks
through a series of local simulations with the Athena code. First, we use a
series of unstratified simulations to calibrate physical dissipation as a
function of resolution and background field strength; the effect of the
magnetic Prandtl number, Pm = viscosity/resistivity, on the turbulence is
captured by ~32 grid zones per disk scale height, H. In agreement with previous
results, our stratified disk calculations are characterized by a subthermal,
predominately toroidal magnetic field that produces MRI-driven turbulence for
|z| < 2 H. Above |z| = 2 H, magnetic pressure dominates and the field is
buoyantly unstable. Large scale radial and toroidal fields are also generated
near the mid-plane and subsequently rise through the disk. The polarity of this
mean field switches on a roughly 10 orbit period in a process that is
well-modeled by an alpha-omega dynamo. Turbulent stress increases with Pm but
with a shallower dependence compared to unstratified simulations. For
sufficiently large resistivity, on the order of cs H/1000, where cs is the
sound speed, MRI turbulence within 2 H of the mid-plane undergoes periods of
resistive decay followed by regrowth. This regrowth is caused by amplification
of toroidal field via the dynamo. This process results in large amplitude
variability in the stress on 10 to 100 orbital timescales, which may have
relevance for partially ionized disks that are observed to have high and low
accretion states.Comment: very minor changes, accepted to Ap
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