52,441 research outputs found
Three-dimensional MHD Simulations of Radiatively Inefficient Accretion Flows
We present three-dimensional MHD simulations of rotating radiatively
inefficient accretion flows onto black holes. In the simulations, we
continuously inject magnetized matter into the computational domain near the
outer boundary, and we run the calculations long enough for the resulting
accretion flow to reach a quasi-steady state. We have studied two limiting
cases for the geometry of the injected magnetic field: pure toroidal field and
pure poloidal field. In the case of toroidal field injection, the accreting
matter forms a nearly axisymmetric, geometrically-thick, turbulent accretion
disk. The disk resembles in many respects the convection-dominated accretion
flows found in previous numerical and analytical investigations of viscous
hydrodynamic flows. Models with poloidal field injection evolve through two
distinct phases. In an initial transient phase, the flow forms a relatively
flattened, quasi-Keplerian disk with a hot corona and a bipolar outflow.
However, when the flow later achieves steady state, it changes in character
completely. The magnetized accreting gas becomes two-phase, with most of the
volume being dominated by a strong dipolar magnetic field from which a thermal
low-density wind flows out. Accretion occurs mainly via narrow slowly-rotating
radial streams which `diffuse' through the magnetic field with the help of
magnetic reconnection events.Comment: 35 pages including 3 built-in plots and 14 separate jpg-plots;
version accepted by Ap
Numerical simulations of the magnetorotational instability in protoneutron stars: I. Influence of buoyancy
The magneto-rotational instability (MRI) is considered to be a promising
mechanism to amplify the magnetic field in fast rotating protoneutron stars. In
contrast to accretion disks, radial buoyancy driven by entropy and lepton
fraction gradients is expected to have a dynamical role as important as
rotation and shear. We investigate the poorly known impact of buoyancy on the
non-linear phase of the MRI, by means of three dimensional numerical
simulations of a local model in the equatorial plane of a protoneutron star.
The use of the Boussinesq approximation allows us to utilise a shearing box
model with clean shearing periodic boundary conditions, while taking into
account the buoyancy driven by radial entropy and composition gradients. We
find significantly stronger turbulence and magnetic fields in buoyantly
unstable flows. On the other hand, buoyancy has only a limited impact on the
strength of turbulence and magnetic field amplification for buoyantly stable
flows in the presence of a realistic thermal diffusion. The properties of the
turbulence are, however, significantly affected in the latter case. In
particular, the toroidal components of the magnetic field and of the velocity
become even more dominant with respect to the poloidal ones. Furthermore, we
observed in the regime of stable buoyancy the formation of long lived coherent
structures such as channel flows and zonal flows. Overall, our results support
the ability of the MRI to amplify the magnetic field significantly even in
stably stratified regions of protoneutron stars.Comment: 22 pages, 15 figures, accepted for publication in MNRA
Interstellar MHD Turbulence and Star Formation
This chapter reviews the nature of turbulence in the Galactic interstellar
medium (ISM) and its connections to the star formation (SF) process. The ISM is
turbulent, magnetized, self-gravitating, and is subject to heating and cooling
processes that control its thermodynamic behavior. The turbulence in the warm
and hot ionized components of the ISM appears to be trans- or subsonic, and
thus to behave nearly incompressibly. However, the neutral warm and cold
components are highly compressible, as a consequence of both thermal
instability in the atomic gas and of moderately-to-strongly supersonic motions
in the roughly isothermal cold atomic and molecular components. Within this
context, we discuss: i) the production and statistical distribution of
turbulent density fluctuations in both isothermal and polytropic media; ii) the
nature of the clumps produced by thermal instability, noting that, contrary to
classical ideas, they in general accrete mass from their environment; iii) the
density-magnetic field correlation (or lack thereof) in turbulent density
fluctuations, as a consequence of the superposition of the different wave modes
in the turbulent flow; iv) the evolution of the mass-to-magnetic flux ratio
(MFR) in density fluctuations as they are built up by dynamic compressions; v)
the formation of cold, dense clouds aided by thermal instability; vi) the
expectation that star-forming molecular clouds are likely to be undergoing
global gravitational contraction, rather than being near equilibrium, and vii)
the regulation of the star formation rate (SFR) in such gravitationally
contracting clouds by stellar feedback which, rather than keeping the clouds
from collapsing, evaporates and diperses them while they collapse.Comment: 43 pages. Invited chapter for the book "Magnetic Fields in Diffuse
Media", edited by Elisabete de Gouveia dal Pino and Alex Lazarian. Revised as
per referee's recommendation
Clump morphology and evolution in MHD simulations of molecular cloud formation
Abridged: We study the properties of clumps formed in three-dimensional
weakly magnetized magneto-hydrodynamic simulations of converging flows in the
thermally bistable, warm neutral medium (WNM). We find that: (1) Similarly to
the situation in the classical two-phase medium, cold, dense clumps form
through dynamically-triggered thermal instability in the compressed layer
between the convergent flows, and are often characterised by a sharp density
jump at their boundaries though not always. (2) However, the clumps are bounded
by phase-transition fronts rather than by contact discontinuities, and thus
they grow in size and mass mainly by accretion of WNM material through their
boundaries. (3) The clump boundaries generally consist of thin layers of
thermally unstable gas, but these layers are often widened by the turbulence,
and penetrate deep into the clumps. (4) The clumps are approximately in both
ram and thermal pressure balance with their surroundings, a condition which
causes their internal Mach numbers to be comparable to the bulk Mach number of
the colliding WNM flows. (5) The clumps typically have mean temperatures 20 < T
< 50 K, corresponding to the wide range of densities they contain (20 < n <
5000 pcc) under a nearly-isothermal equation of state. (6) The turbulent ram
pressure fluctuations of the WNM induce density fluctuations that then serve as
seeds for local gravitational collapse within the clumps. (7) The velocity and
magnetic fields tend to be aligned with each other within the clumps, although
both are significantly fluctuating, suggesting that the velocity tends to
stretch and align the magnetic field with it. (8) The typical mean field
strength in the clumps is a few times larger than that in the WNM. (9) The
magnetic field strength has a mean value of B ~ 6 mu G ...Comment: substantially revised version, accepted by MNRAS, 13 pages, 14
figures, high resolution version:
http://www.ita.uni-heidelberg.de/~banerjee/publications/MC_Formation_Paper2.pd
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Lattice Boltzmann simulation of magnetic field effects on nanofluid
This paper was presented at the 4th Micro and Nano Flows Conference (MNF2014), which was held at University College, London, UK. The conference was organised by Brunel University and supported by the Italian Union of Thermofluiddynamics, IPEM, the Process Intensification Network, the Institution of Mechanical Engineers, the Heat Transfer Society, HEXAG - the Heat Exchange Action Group, and the Energy Institute, ASME Press, LCN London Centre for Nanotechnology, UCL University College London, UCL Engineering, the International NanoScience Community, www.nanopaprika.eu.In this paper, the magnetic field effects on natural convection heat transfer in an enclosure filled with nanofluid are numerically investigated by using lattice Boltzmann method. The fluid in the enclosure is a water-based nanofluid containing Al2O3 nanoparticles. A uniform external magnetic field with different angles was applied. A series of simulation cases were carried out for different governing parameters including Hartmann number, Rayleigh number, the nanoparticle volume fractions and magnetic field angles. The results show that the increasing Rayleigh number and nanoparticle volume fraction improve the heat transfer in the enclosure. However, the heat transfer has been suppressed when Hartmann number increases. The results also indicate there are critical values for the Raleigh number and also the magnetic field orientation, at which the impacts of the solid volume fraction and magnetic field effects are the most pronounced
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