2,436 research outputs found
On the relation between viscoelastic and magnetohydrodynamic flows and their instabilities
We demonstrate a close analogy between a viscoelastic medium and an
electrically conducting fluid containing a magnetic field. Specifically, the
dynamics of the Oldroyd-B fluid in the limit of large Deborah number
corresponds to that of a magnetohydrodynamic (MHD) fluid in the limit of large
magnetic Reynolds number. As a definite example of this analogy, we compare the
stability properties of differentially rotating viscoelastic and MHD flows. We
show that there is an instability of the Oldroyd-B fluid that is physically
distinct from both the inertial and elastic instabilities described previously
in the literature, but is directly equivalent to the magnetorotational
instability in MHD. It occurs even when the specific angular momentum increases
outwards, provided that the angular velocity decreases outwards; it derives
from the kinetic energy of the shear flow and does not depend on the curvature
of the streamlines. However, we argue that the elastic instability of
viscoelastic Couette flow has no direct equivalent in MHD.Comment: 21 pages, 3 figures, to be published in J. Fluid Mec
A self-sustaining nonlinear dynamo process in Keplerian shear flows
A three-dimensional nonlinear dynamo process is identified in rotating plane
Couette flow in the Keplerian regime. It is analogous to the hydrodynamic
self-sustaining process in non-rotating shear flows and relies on the
magneto-rotational instability of a toroidal magnetic field. Steady nonlinear
solutions are computed numerically for a wide range of magnetic Reynolds
numbers but are restricted to low Reynolds numbers. This process may be
important to explain the sustenance of coherent fields and turbulent motions in
Keplerian accretion disks, where all its basic ingredients are present.Comment: 4 pages, 7 figures, accepted for publication in Physical Review
Letter
Development of a scanning electron mirror microscope
Scanning electron mirrors microscope design and developmen
Viscoresistive MHD Configurations of Plasma in Accretion Disks
We present a discussion of two-dimensional magneto-hydrodynamics (MHD)
configurations, concerning the equilibria of accretion disks of a strongly
magnetized astrophysical object. We set up a viscoresistive scenario which
generalizes previous two-dimensional analyses by reconciling the ideal MHD
coupling of the vertical and the radial equilibria within the disk with the
standard mechanism of the angular momentum transport, relying on dissipative
properties of the plasma configuration. The linear features of the considered
model are analytically developed and the non-linear configuration problem is
addressed, by fixing the entire disk profile at the same order of
approximation. Indeed, the azimuthal and electron force balance equations are
no longer automatically satisfied when poloidal currents and matter fluxes are
included in the problem. These additional components of the equilibrium
configuration induce a different morphology of the magnetic flux surface, with
respect to the ideal and simply rotating disk.Comment: 19 pages, 4 figures. To appear on the Proceedings of the Second
Italian-Pakistani Workshop on Relativistic Astrophysic
A Managerâs Actions? An Exercise for Exploring Sexual Harassment
This exercise is designed to enhance studentsâ understanding of sexual harassment issues in the workplace, as well as to extend these issues to customer interactions in retail settings. Through research and class discussion, participants explore their perceptions and assumptions about a managerâs potentially harassing behaviors. Furthermore, discussions of the managerâs actions provide additional opportunities to explore issues dealing with legal foundations of sexual harassment as well as gender differences in response to harassment. Multiple variations are presented making this scenario/exercise suitable to various courses and all types of participants in order to facilitate a healthy discussion about the issues associated with sexual harassment
Robustly Unstable Eigenmodes of the Magnetoshearing Instability in Accretion Disk
The stability of nonaxisymmetric perturbations in differentially rotating
astrophysical accretion disks is analyzed by fully incorporating the properties
of shear flows. We verify the presence of discrete unstable eigenmodes with
complex and pure imaginary eigenvalues, without any artificial disk edge
boundaries, unlike Ogilvie & Pringle(1996)'s claim. By developing the
mathematical theory of a non-self-adjoint system, we investigate the nonlocal
behavior of eigenmodes in the vicinity of Alfven singularities at
omega_D=omega_A, where omega_D is the Doppler-shifted wave frequency and
omega_A=k_// v_A is the Alfven frequency. The structure of the spectrum of
discrete eigenmodes is discussed and the magnetic field and wavenumber
dependence of the growth rate are obtained. Exponentially growing modes are
present even in a region where the local dispersion relation theory claims to
have stable eigenvalues. The velocity field created by an eigenmode is
obtained, which explains the anomalous angular momentum transport in the
nonlinear stage of this stability.Comment: 11pages, 11figures, to be published in ApJ. For associated eps files,
see http://dino.ph.utexas.edu/~knoguchi
Magnetohydrodynamic turbulence in warped accretion discs
Warped, precessing accretion discs appear in a range of astrophysical
systems, for instance the X-ray binary Her X-1 and in the active nucleus of
NGC4258. In a warped accretion disc there are horizontal pressure gradients
that drive an epicyclic motion. We have studied the interaction of this
epicyclic motion with the magnetohydrodynamic turbulence in numerical
simulations. We find that the turbulent stress acting on the epicyclic motion
is comparable in size to the stress that drives the accretion, however an
important ingredient in the damping of the epicyclic motion is its parametric
decay into inertial waves.Comment: to appear in the proceedings of the 20th Texas Symposium on
Relativistic Astrophysics, J. C. Wheeler & H. Martel (eds.
The response of a turbulent accretion disc to an imposed epicyclic shearing motion
We excite an epicyclic motion, whose amplitude depends on the vertical
position, , in a simulation of a turbulent accretion disc. An epicyclic
motion of this kind may be caused by a warping of the disc. By studying how the
epicyclic motion decays we can obtain information about the interaction between
the warp and the disc turbulence. A high amplitude epicyclic motion decays
first by exciting inertial waves through a parametric instability, but its
subsequent exponential damping may be reproduced by a turbulent viscosity. We
estimate the effective viscosity parameter, , pertaining to
such a vertical shear. We also gain new information on the properties of the
disc turbulence in general, and measure the usual viscosity parameter,
, pertaining to a horizontal (Keplerian) shear. We find that,
as is often assumed in theoretical studies, is approximately
equal to and both are much less than unity, for the field
strengths achieved in our local box calculations of turbulence. In view of the
smallness () of and we conclude
that for the timescale for diffusion
or damping of a warp is much shorter than the usual viscous timescale. Finally,
we review the astrophysical implications.Comment: 12 pages, 18 figures, MNRAS accepte
Aligning spinning black holes and accretion discs
We consider the alignment torque between a spinning black hole and an
accretion disc whose angular momenta are misaligned. This situation must hold
initially in almost all gas accretion events on to supermassive black holes,
and may occur in binaries where the black hole receives a natal supernova kick.
We show that the torque always acts to align the hole's spin with the total
angular momentum without changing its magnitude. The torque acts dissipatively
on the disc, reducing its angular momentum, and aligning it with the hole if
and only if the angle theta between the angular momenta J_d of the disc and J_h
of the hole satisfies the inequality cos theta > -J_d / 2 J_h. If this
condition fails, which requires both theta > pi/2 and J_d < 2 J_h, the disc
counteraligns.Comment: MNRAS, in pres
Turbulence and angular momentum transport in a global accretion disk simulation
The global development of magnetohydrodynamic turbulence in an accretion disk
is studied within a simplified disk model that omits vertical stratification.
Starting with a weak vertical seed field, a saturated state is obtained after a
few tens of orbits in which the energy in the predominantly toroidal magnetic
field is still subthermal. The efficiency of angular momentum transport,
parameterized by the Shakura-Sunyaev alpha parameter, is of the order of 0.1.
The dominant contribution to alpha comes from magnetic stresses, which are
enhanced by the presence of weak net vertical fields. The power spectra of the
magnetic fields are flat or decline only slowly towards the largest scales
accessible in the calculation, suggesting that the viscosity arising from MHD
turbulence may not be a locally determined quantity. I discuss how these
results compare with observationally inferred values of alpha, and possible
implications for models of jet formation.Comment: ApJ Letters, in press. The paper and additional visualizations are
available at http://www.cita.utoronto.ca/~armitage/global_abs.htm
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