43,026 research outputs found
On the Anisotropic Nature of MRI-Driven Turbulence in Astrophysical Disks
The magnetorotational instability is thought to play an important role in
enabling accretion in sufficiently ionized astrophysical disks. The rate at
which MRI-driven turbulence transports angular momentum is related to both the
strength of the amplitudes of the fluctuations on various scales and the degree
of anisotropy of the underlying turbulence. This has motivated several studies
of the distribution of turbulent power in spectral space. In this paper, we
investigate the anisotropic nature of MRI-driven turbulence using a
pseudo-spectral code and introduce novel ways to robustly characterize the
underlying turbulence. We show that the general flow properties vary in a
quasi-periodic way on timescales comparable to 10 inverse angular frequencies
motivating the temporal analysis of its anisotropy. We introduce a 3D tensor
invariant analysis to quantify and classify the evolution of the anisotropic
turbulent flow. This analysis shows a continuous high level of anisotropy, with
brief sporadic transitions towards two- and three-component isotropic turbulent
flow. This temporal-dependent anisotropy renders standard shell-average,
especially when used simultaneously with long temporal averages, inadequate for
characterizing MRI-driven turbulence. We propose an alternative way to extract
spectral information from the turbulent magnetized flow, whose anisotropic
character depends strongly on time. This consists of stacking 1D Fourier
spectra along three orthogonal directions that exhibit maximum anisotropy in
Fourier space. The resulting averaged spectra show that the power along each of
the three independent directions differs by several orders of magnitude over
most scales, except the largest ones. Our results suggest that a
first-principles theory to describe fully developed MRI-driven turbulence will
likely have to consider the anisotropic nature of the flow at a fundamental
level.Comment: 13 pages, 13 figures, submitted to Ap
Instabilities of geared couplings: Theory and practice
The use of couplings for high speed turbocompressors or pumps is essential to transmit power from the driver. Typical couplings are either of the lubricated gear or dry diaphragm type design. Gear couplings have been the standard design for many years and recent advances in power and speed requirements have pushed the standard design criteria to the limit. Recent test stand and field data on continuous lube gear type couplings have forced a closer examination of design tolerances and concepts to avoid operational instabilities. Two types of mechanical instabilities are reviewed in this paper: (1) entrapped fluid, and (2) gear mesh instability resulting in spacer throw-out onset. Test stand results of these types of instabilities and other directly related problems are presented together with criteria for proper coupling design to avoid these conditions. An additional test case discussed shows the importance of proper material selection and processing and what can happen to an otherwise good design
Multidimensional simulations of magnetic field amplification and electron acceleration to near-energy equipartition with ions by a mildly relativistic quasi-parallel plasma collision
The energetic electromagnetic eruptions observed during the prompt phase of
gamma-ray bursts are attributed to synchrotron emissions. The internal shocks
moving through the ultrarelativistic jet, which is ejected by an imploding
supermassive star, are the likely source of this radiation. Synchrotron
emissions at the observed strength require the simultaneous presence of
powerful magnetic fields and highly relativistic electrons. We explore with one
and three-dimensional relativistic particle-in-cell simulations the transition
layer of a shock, that evolves out of the collision of two plasma clouds at a
speed 0.9c and in the presence of a quasi-parallel magnetic field. The cloud
densities vary by a factor of 10. The number densities of ions and electrons in
each cloud, which have the mass ratio 250, are equal. The peak Lorentz factor
of the electrons is determined in the 1D simulation, as well as the orientation
and the strength of the magnetic field at the boundary of the two colliding
clouds. The relativistic masses of the electrons and ions close to the shock
transition layer are comparable as in previous work. The 3D simulation shows
rapid and strong plasma filamentation behind the transient precursor. The
magnetic field component orthogonal to the initial field direction is amplified
in both simulations to values that exceed those expected from the shock
compression by over an order of magnitude. The forming shock is
quasi-perpendicular due to this amplification. The simultaneous presence of
highly relativistic electrons and strong magnetic fields will give rise to
significant synchrotron emissions.Comment: 8 pages, 5 figures. This work was presented at 21st International
Conference on Numerical Simulation of Plasmas (ICNSP'09). Accepted for
publication IEEE Trans. on Plasma Scienc
Investigating the Effects of Finite Resolution on Observed Transverse Jet Profiles
Both the emission properties and evolution of Active Galactic Nuclei (AGN)
radio jets are dependent on the magnetic fields that thread them. Faraday
Rotation gradients are a very important way of investigating these magnetic
fields, and can provide information on the orientation and structure of the
magnetic field in the immediate vicinity of the jet; for example, a toroidal or
helical field component should give rise to a systematic gradient in the
observed Faraday rotation across the jet, as well as characteristic intensity
and polarization profiles. However, real observed radio images have finite
resolution, usually expressed via convolution with a Gaussian beam whose size
corresponds to the central lobe of the point source response function. This
will tend to blur transverse structure in the jet profile, raising the question
of how well resolved a jet must be in the transverse direction in order to
reliably detect transverse structure associated with a helical jet magnetic
field. We present results of simulated intensity, polarization and Faraday
rotation images designed to directly and empirically investigate the effect of
finite resolution on observed transverse jet structures
Galaxies into the Dark Ages
We consider the capabilities of current and future large facilities operating
at 2\,mm to 3\,mm wavelength to detect and image the [CII] 158\,m line
from galaxies into the cosmic "dark ages" ( to 20). The [CII] line
may prove to be a powerful tool in determining spectroscopic redshifts, and
galaxy dynamics, for the first galaxies. We emphasize that the nature, and even
existence, of such extreme redshift galaxies, remains at the frontier of open
questions in galaxy formation. In 40\,hr, ALMA has the sensitivity to detect
the integrated [CII] line emission from a moderate metallicity, active
star-forming galaxy [; star formation rate (SFR) =
5\,\,yr], at at a significance of 6. The
next-generation Very Large Array (ngVLA) will detect the integrated [CII] line
emission from a Milky-Way like star formation rate galaxy (, SFR = 1\,\,yr), at at a significance
of 6. Imaging simulations show that the ngVLA can determine rotation
dynamics for active star-forming galaxies at , if they exist. Based
on our very limited knowledge of the extreme redshift Universe, we calculate
the count rate in blind, volumetric surveys for [CII] emission at
to 20. The detection rates in blind surveys will be slow (of order unity per
40\,hr pointing). However, the observations are well suited to commensal
searches. We compare [CII] with the [OIII] 88m line, and other ancillary
information in high galaxies that would aid these studies.Comment: 11pages, 8 figures, Accepted for the Astrophysical Journa
Particle-in-cell simulation of a mildly relativistic collision of an electron-ion plasma carrying a quasi-parallel magnetic field: Electron acceleration and magnetic field amplification at supernova shocks
Plasma processes close to SNR shocks result in the amplification of magnetic
fields and in the acceleration of electrons, injecting them into the diffusive
acceleration mechanism. The acceleration of electrons and the B field
amplification by the collision of two plasma clouds, each consisting of
electrons and ions, at a speed of 0.5c is investigated. A quasi-parallel
guiding magnetic field, a cloud density ratio of 10 and a plasma temperature of
25 keV are considered. A quasi-planar shock forms at the front of the dense
plasma cloud. It is mediated by a circularly left-hand polarized
electromagnetic wave with an electric field component along the guiding
magnetic field. Its propagation direction is close to that of the guiding field
and orthogonal to the collision boundary. It has a low frequency and a
wavelength that equals several times the ion inertial length, which would be
indicative of a dispersive Alfven wave close to the ion cyclotron resonance
frequency of the left-handed mode (ion whistler), provided that the frequency
is appropriate. However, it moves with the super-alfvenic plasma collision
speed, suggesting that it is an Alfven precursor or a nonlinear MHD wave such
as a Short Large-Amplitude Magnetic Structure (SLAMS). The growth of the
magnetic amplitude of this wave to values well in excess of those of the
quasi-parallel guiding field and of the filamentation modes results in a
quasi-perpendicular shock. We present evidence for the instability of this mode
to a four wave interaction. The waves developing upstream of the dense cloud
give rise to electron acceleration ahead of the collision boundary. Energy
equipartition between the ions and the electrons is established at the shock
and the electrons are accelerated to relativistic speeds.Comment: 16 pages, 18 figures, Accepted for publication by Astron & Astrophy
Human factors analysis of workstation design: Earth Radiation Budget Satellite Mission Operations Room
A human factors analysis addressed three related yet distinct issues within the area of workstation design for the Earth Radiation Budget Satellite (ERBS) mission operation room (MOR). The first issue, physical layout of the MOR, received the most intensive effort. It involved the positioning of clusters of equipment within the physical dimensions of the ERBS MOR. The second issue for analysis was comprised of several environmental concerns, such as lighting, furniture, and heating and ventilation systems. The third issue was component arrangement, involving the physical arrangement of individual components within clusters of consoles, e.g., a communications panel
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