11,368 research outputs found
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
High accretion rates in magnetised Keplerian discs mediated by a Parker instability driven dynamo
Hydromagnetic stresses in accretion discs have been the subject of intense
theoretical research over the past one and a half decades. Most of the disc
simulations have assumed a small initial magnetic field and studied the
turbulence that arises from the magnetorotational instability. However, gaseous
discs in galactic nuclei and in some binary systems are likely to have
significant initial magnetisation. Motivated by this, we performed ideal
magnetohydrodynamic simulations of strongly magnetised, vertically stratified
discs in a Keplerian potential. Our initial equilibrium configuration, which
has an azimuthal magnetic field in equipartion with thermal pressure, is
unstable to the Parker instability. This leads to the expelling of magnetic
field arcs, anchored in the midplane of the disc, to around five scale heights
from the midplane. Transition to turbulence happens primarily through
magnetorotational instability in the resulting vertical fields, although
magnetorotational shear instability in the unperturbed azimuthal field plays a
significant role as well, especially in the midplane where buoyancy is weak.
High magnetic and hydrodynamical stresses arise, yielding an effective
-value of around 0.1 in our highest resolution run. Azimuthal magnetic
field expelled by magnetic buoyancy from the disc is continuously replenished
by the stretching of a radial field created as gas parcels slide in the linear
gravity field along inclined magnetic field lines. This dynamo process, where
the bending of field lines by the Parker instability leads to re-creation of
the azimuthal field, implies that highly magnetised discs are astrophysically
viable and that they have high accretion rates.Comment: 14 pages, 14 figures, accepted for publication in A&
Computational Contributions to the Automation of Agriculture
The purpose of this paper is to explore ways that computational advancements have enabled the complete automation of agriculture from start to finish. With a major need for agricultural advancements because of food and water shortages, some farmers have begun creating their own solutions to these problems. Primarily explored in this paper, however, are current research topics in the automation of agriculture. Digital agriculture is surveyed, focusing on ways that data collection can be beneficial. Additionally, self-driving technology is explored with emphasis on farming applications. Machine vision technology is also detailed, with specific application to weed management and harvesting of crops. Finally, the effects of automating agriculture are briefly considered, including labor, the environment, and direct effects on farmers
Magnetically Arrested Disk: An Energetically Efficient Accretion Flow
We consider an accretion flow model originally proposed by Bisnovatyi-Kogan &
Ruzmaikin (1974), which has been confirmed in recent 3D MHD simulations. In the
model, the accreting gas drags in a strong poloidal magnetic field to the
center such that the accumulated field disrupts the axisymmetric accretion flow
at a relatively large radius. Inside the disruption radius, the gas accretes as
discrete blobs or streams with a velocity much less than the free-fall
velocity. Almost the entire rest mass energy of the gas is released as heat,
radiation and mechanical/magnetic energy. Even for a non-rotating black hole,
the efficiency of converting mass to energy is of order 50% or higher. The
model is thus a practical analog of an idealized engine proposed by Geroch and
Bekenstein.Comment: 4 pages, 2 figure, new refs added, in print in PAS
Numerical Models of Viscous Accretion Flows Near Black Holes
We report on a numerical study of viscous fluid accretion onto a black hole.
The flow is axisymmetric and uses a pseudo-Newtonian potential to model
relativistic effects near the event horizon. The numerical method is a variant
of the ZEUS code. As a test of our numerical scheme, we are able to reproduce
results from earlier, similar work by Igumenshchev and Abramowicz and Stone et
al. We consider models in which mass is injected onto the grid as well as
models in which an initial equilibrium torus is accreted. In each model we
measure three ``eigenvalues'' of the flow: the accretion rate of mass, angular
momentum, and energy. We find that the eigenvalues are sensitive to r_{in}, the
location of the inner radial boundary. Only when the flow is always supersonic
on the inner boundary are the eigenvalues insensitive to small changes in
r_{in}. We also report on the sensitivity of the results to other numerical
parameters.Comment: 14 pages, 4 figures, 2 tables, to appear in v573 n2 pt1 ApJ July 10,
200
Real Time Wake Computations using Lattice Boltzmann Method on Many Integrated Core Processors
This paper puts forward an efficient Lattice Boltzmann method for use as a wake simulator suitable for
real-time environments. The method is limited to low speed incompressible flow but is very efficient and
can be used to compute flows âon the flyâ. In particular, many-core machines allow for the method to be
used with the need of very expensive parallel clusters. Results are shown here for flows around
cylinders and simple ship shapes
Real Time Wake Computations using Lattice Boltzmann Method on Many Integrated Core Processors
This paper puts forward an efficient Lattice Boltzmann method for use as a wake simulator suitable for
real-time environments. The method is limited to low speed incompressible flow but is very efficient and
can be used to compute flows âon the flyâ. In particular, many-core machines allow for the method to be
used with the need of very expensive parallel clusters. Results are shown here for flows around
cylinders and simple ship shapes
- âŠ