2,310 research outputs found
On the role of numerical diffusivity in MHD simulations of global accretion disc dynamos
Observations, mainly of outbursts in dwarf novae, imply that the anomalous
viscosity in highly ionized accretion discs is magnetic in origin, and requires
that the plasma . Until now most simulations of the magnetic
dynamo in accretion discs have used a local approximation (known as the
shearing box). While these simulations demonstrate the possibility of a
self-sustaining dynamo, the magnetic activity generated in these models
saturates at . This long-standing discrepancy has previously been
attributed to the local approximation itself. There have been recent attempts
at simulating magnetic activity in global accretion discs with parameters
relevant to the dwarf novae. These too find values of . We
speculate that the tension between these models and the observations may be
caused by numerical magnetic diffusivity. As a pedagogical example, we present
exact time-dependent solutions for the evolution of weak magnetic fields in an
incompressible fluid subject to linear shear and magnetic diffusivity. We find
that the maximum factor by which the initial magnetic energy can be increased
depends on the magnetic Reynolds number as . We
estimate that current global numerical simulations of dwarf nova discs have
numerical magnetic Reynolds numbers around 6 orders of magnitude less than the
physical value found in dwarf nova discs of . We suggest that, given the current limitations on computing power,
expecting to be able to compute realistic dynamo action in observable accretion
discs using numerical MHD is, for the time being, a step too far.Comment: 20 pages, 6 figures, accepted for publication in the Journal of
Plasma Physic
Radiation-Driven Warping: The Origin of Warps and Precession in Accretion Disks
A geometrically thin, optically thick, warped accretion disk with a central
source of luminosity is subject to non-axisymmetric forces due to radiation
pressure; the resulting torque acts to modify the warp. In a recent paper,
\cite{pri96} used a local analysis to show that initially planar accretion
disks are unstable to warping driven by radiation torque. Here we extend this
work with a global analysis of the stable and unstable modes. We confirm
Pringle's conclusion that thin centrally-illuminated accretion disks are
generically unstable to warping via this mechanism; we discuss the
time-evolution and likely steady-state of such systems and show specifically
that this mechanism can explain the warping of the disk of water masers in NGC
4258 and the 164-day precession period of the accretion disk in SS 433.
Radiation-driven warping and precession provides a robust mechanism for
producing warped, precessing accretion disks in active galactic nuclei and
X-ray binary systems.Comment: 16 pages, latex, 3 figure
An instability mechanism for particulate pipe flow
We present linear stability analysis for a simple model of particle-laden
pipe flow. The model consists of a continuum approximation for the particles
two-way coupled to the fluid velocity field via Stokes drag (Saffman 1962). We
extend previous analysis in a channel (Klinkenberg et al. 2011) to allow for
the initial distribution of particles to be inhomogeneous and in particular
consider the effect of allowing the particles to be preferentially located
around one radius in accordance with experimental observations. This simple
modification of the problem is enough to alter the stability properties of the
flow, and in particular can lead to a linear instability at experimentally
realistic parameters. The results are compared to the experimental work of
Matas et al. (2004a) and are shown to be consistent with the reported flow
regimes.Comment: 15 pages, 11 figure
Highly-symmetric travelling waves in pipe flow
The recent theoretical discovery of finite-amplitude travelling waves in pipe
flow has re-ignited interest in the transitional phenomena that Osborne
Reynolds studied 125 years ago. Despite all being unstable, these waves are
providing fresh insight into the flow dynamics. Here we describe two new
classes of highly-symmetric travelling waves (possessing rotational,
shift-&-reflect and mirror symmetries) and report a new family of
mirror-symmetric waves which is the first found in pipe flow not to have
shift-&-reflect symmetry. The highly-symmetric waves appear at lower Reynolds
numbers than the originally-discovered non-mirror-symmetric waves found by
Faisst & Eckhardt 2003 and Wedin & Kerswell 2004 and have much higher wall
shear stresses. The first M-class comprises of the various
discrete-rotationally-symmetric analogues of the mirror-symmetric wave found in
Pringle & Kerswell (2007) and have a distinctive double layer structure of fast
and slow streaks across the pipe radius. The second N-class has the more
familiar separation of fast streaks to the exterior and slow streaks to the
interior and looks the precursor to the class of non-mirror-symmetric waves
already known.Comment: 16 pages, 8 figures, for Phil Trans theme issue on pipe flo
Self-Similar Magnetocentrifugal Disk Winds with Cylindrical Asymptotics
We construct a two-parameter family of models for self-collimated, radially
self-similar magnetized outflows from accretion disks. A flow at zero initial
poloidal speed leaves the surface of a rotating disk and is accelerated and
redirected toward the pole by helical magnetic fields threading the disk. At
large distances from the disk, the flow streamlines asymptote to wrap around
the surfaces of nested cylinders. In constrast to previous disk wind modeling,
we have explicitly implemented the cylindrical asymptotic boundary condition to
examine the consequences for flow dynamics. The solutions are characterized by
the logarithmic gradient of the magnetic field strength and the ratios between
the footpoint radius R_0 and asymptotic radius R_1 of streamlines; the Alfven
radius must be found as an eigenvalue. Cylindrical solutions require the
magnetic field to drop less steeply than 1/R. We find that the asymptotic
poloidal speed on any streamline is typically just a few tenths of the Kepler
speed at the corresponding disk footpoint. The asymptotic toroidal Alfven speed
is, however, a few times the footpoint Kepler speed. We discuss the
implications of the models for interpretations of observed optical jets and
molecular outflows from young stellar systems. We suggest that the difficulty
of achieving strong collimation in vector velocity simultaneously with a final
speed comparable to the disk rotation rate argues against isolated jets and in
favor of models with broader winds.Comment: 39 pages, Latex (uses AAS Latex macros), 6 eps figures, postscript
preprint with embedded figures available from
http://www.astro.umd.edu/~ostriker/professional/publications.html , to appear
in ApJ 9/1/9
The Stability of Magnetized Rotating Plasmas with Superthermal Fields
During the last decade it has become evident that the magnetorotational
instability is at the heart of the enhanced angular momentum transport in
weakly magnetized accretion disks around neutron stars and black holes. In this
paper, we investigate the local linear stability of differentially rotating,
magnetized flows and the evolution of the magnetorotational instability beyond
the weak-field limit. We show that, when superthermal toroidal fields are
considered, the effects of both compressibility and magnetic tension forces,
which are related to the curvature of toroidal field lines, should be taken
fully into account. We demonstrate that the presence of a strong toroidal
component in the magnetic field plays a non-trivial role. When strong fields
are considered, the strength of the toroidal magnetic field not only modifies
the growth rates of the unstable modes but also determines which modes are
subject to instabilities. We find that, for rotating configurations with
Keplerian laws, the magnetorotational instability is stabilized at low
wavenumbers for toroidal Alfven speeds exceeding the geometric mean of the
sound speed and the rotational speed. We discuss the significance of our
findings for the stability of cold, magnetically dominated, rotating fluids and
argue that, for these systems, the curvature of toroidal field lines cannot be
neglected even when short wavelength perturbations are considered. We also
comment on the implications of our results for the validity of shearing box
simulations in which superthermal toroidal fields are generated.Comment: 24 pages, 12 figures. Accepted for publication in ApJ. Sections 2 and
5 substantially expanded, added Appendix A and 3 figures with respect to
previous version. Animations are available at
http://www.physics.arizona.edu/~mpessah/research
Minimal seeds for shear flow turbulence: using nonlinear transient growth to touch the edge of chaos
We propose a general strategy for determining the minimal finite amplitude
isturbance to trigger transition to turbulence in shear flows. This involves
constructing a variational problem that searches over all disturbances of fixed
initial amplitude, which respect the boundary conditions, incompressibility and
the Navier--Stokes equations, to maximise a chosen functional over an
asymptotically long time period. The functional must be selected such that it
identifies turbulent velocity fields by taking significantly enhanced values
compared to those for laminar fields. We illustrate this approach using the
ratio of the final to initial perturbation kinetic energies (energy growth) as
the functional and the energy norm to measure amplitudes in the context of pipe
flow. Our results indicate that the variational problem yields a smooth
converged solution providing the amplitude is below the threshold amplitude for
transition. This optimal is the nonlinear analogue of the well-studied (linear)
transient growth optimal. At and above this threshold, the optimising search
naturally seeks out disturbances that trigger turbulence by the end of the
period, and convergence is then practically impossible. The first disturbance
found to trigger turbulence as the amplitude is increased identifies the
`minimal seed' for the given geometry and forcing (Reynolds number). We
conjecture that it may be possible to select a functional such that the
converged optimal below threshold smoothly converges to the minimal seed at
threshold. This seems at least approximately true for our choice of energy
growth functional and the pipe flow geometry chosen here.Comment: 27 pages, 19 figures, submitted to JF
A trio of month long flares in the nova-like variable V704 And
We present the discovery of an unusual set of flares in the nova-like
variable V704 And. Using data from AAVSO, ASAS-SN, and ZTF, of the nova-like
variable V704 And, we have discovered a trio of brightening events that
occurred during the high state. These events elevate the optical brightness of
the source from magnitude to magnitude. The events last
for roughly a month, and exhibit the unusual shape of a slow rise and faster
decay. Just after the third event we obtained data from regular monitoring with
Swift, although by this time the flares had ceased and the source returned to
its pre-flare level of activity in the high-state. The Swift observations
confirm that during the high-state the source is detectable in the X-rays, and
provide simultaneous UV and optical fluxes. As the source is already in the
high-state prior to the flares, and thus the disc is expected to already be in
the high-viscosity state, we conclude that the driver of the variations must be
changes in the mass transfer rate from the companion star and we discuss
possible mechanisms for such short-timescale mass transfer variations to occur.Comment: 5 pages + appendix. Accepted for publication in A&A Letter
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