3,218 research outputs found
Numerical analysis of a downsized 2-stroke uniflow engine
In order to optimize the 2-stroke uniflow engine performance on vehicle applications, numerical analysis has been introduced, 3D CFD model has been built for the optimization of intake charge organization. The scavenging process was investigated and the intake port design details were improved. Then the output data from 3D CFD calculation were applied to a 1D engine model to process the analysis on engine performance. The boost system optimization of the engine has been carried out also. Furthermore, a vehicle model was also set up to investigate the engine in-vehicle performance
Magnetically Driven Jets in the Kerr Metric
We compute a series of three-dimensional general relativistic
magnetohydrodynamic simulations of accretion flows in the Kerr metric to
investigate the properties of the unbound outflows that result. The overall
strength of these outflows increases sharply with increasing black hole
rotation rate, but a number of generic features are found in all cases. The
mass in the outflow is concentrated in a hollow cone whose opening angle is
largely determined by the effective potential for matter orbiting with angular
momentum comparable to that of the innermost stable circular orbit. The
dominant force accelerating the matter outward comes from the pressure of the
accretion disk's corona. The principal element that shapes the outflow is
therefore the centrifugal barrier preventing accreting matter from coming close
to the rotation axis. Inside the centrifugal barrier, the cone contains very
little matter and is dominated by electromagnetic fields that rotate at a rate
tied closely to the rotation of the black hole. These fields carry an
outward-going Poynting flux whose immediate energy source is the rotating
spacetime of the Kerr black hole. When the spin parameter a/M of the black hole
exceeds ~0.9, the energy carried to infinity by these outflows can be
comparable to the nominal radiative efficiency predicted in the Novikov-Thorne
model. Similarly, the expelled angular momentum can be comparable to that
accreted by the black hole. Both the inner electromagnetic part and the outer
matter part can contribute in significant fashion to the energy and angular
momentum of the outflow.Comment: 43 pages 12 figures To Appear in the Astrophysical Journal replaced
figure 3c with correct imag
The Chromospheric Activity and Ages of M Dwarf Stars in Wide Binary Systems
We investigate the relationship between age and chromospheric activity for
139 M dwarf stars in wide binary systems with white dwarf companions. The age
of each system is determined from the cooling age of its white dwarf component.
The current limit for activity-age relations found for M dwarfs in open
clusters is 4 Gyr. Our unique approach to finding ages for M stars allows for
the exploration of this relationship at ages older than 4 Gyr. The general
trend of stars remaining active for a longer time at later spectral type is
confirmed. However, our larger sample and greater age range reveals additional
complexity in assigning age based on activity alone. We find that M dwarfs in
wide binaries older than 4 Gyr depart from the log-linear relation for clusters
and are found to have activity at magnitudes, colors and masses which are
brighter, bluer and more massive than predicted by the cluster relation. In
addition to our activity-age results, we present the measured radial velocities
and complete space motions for 161 white dwarf stars in wide binaries.Comment: 22 pages including 9 figures and 5 tables. Accepted for publication
in The Astronomical Journa
Numerical evidence for `multi-scalar stars'
We present a class of general relativistic soliton-like solutions composed of
multiple minimally coupled, massive, real scalar fields which interact only
through the gravitational field. We describe a two-parameter family of
solutions we call ``phase-shifted boson stars'' (parameterized by central
density rho_0 and phase delta), which are obtained by solving the ordinary
differential equations associated with boson stars and then altering the phase
between the real and imaginary parts of the field. These solutions are similar
to boson stars as well as the oscillating soliton stars found by Seidel and
Suen [E. Seidel and W.M. Suen, Phys. Rev. Lett. 66, 1659 (1991)]; in
particular, long-time numerical evolutions suggest that phase-shifted boson
stars are stable. Our results indicate that scalar soliton-like solutions are
perhaps more generic than has been previously thought.Comment: Revtex. 4 pages with 4 figures. Submitted to Phys. Rev.
Vortices in Thin, Compressible, Unmagnetized Disks
We consider the formation and evolution of vortices in a hydrodynamic
shearing-sheet model. The evolution is done numerically using a version of the
ZEUS code. Consistent with earlier results, an injected vorticity field evolves
into a set of long-lived vortices, each of which has a radial extent comparable
to the local scale height. But we also find that the resulting velocity field
has a positive shear stress, . This effect appears
only at high resolution. The transport, which decays with time as t^-1/2,
arises primarily because the vortices drive compressive motions. This result
suggests a possible mechanism for angular momentum transport in low-ionization
disks, with two important caveats: a mechanism must be found to inject
vorticity into the disk, and the vortices must not decay rapidly due to
three-dimensional instabilities.Comment: 8 pages, 10 figures (high resolution figures available in ApJ
electronic edition
Let me Google that for you:a time series analysis of seasonality in internet search trends for terms related to foot and ankle pain
BACKGROUND: The analysis of internet search traffic may present the opportunity to gain insights into general trends and patterns in information seeking behaviour related to medical conditions at a population level. For prevalent and widespread problems such as foot and ankle pain, this information has the potential to improve our understanding of seasonality and trends within these conditions and their treatments, and may act as a useful proxy for their true incidence/prevalence characteristics. This study aimed to explore seasonal effects, general trends and relative popularity of internet search terms related to foot and ankle pain over the past decade. METHODS: We used the Google Trends tool to obtain relative search engine traffic for terms relating to foot and ankle pain and common treatments from Google search and affiliated pages for major northern and southern hemisphere English speaking nations. Analysis of overall trends and seasonality including summer/winter differences was carried out on these terms. RESULTS: Searches relating to general foot pain were on average 3.4 times more common than those relating to ankle pain, and twice as common as searches relating to heel pain. Distinct seasonal effects were seen in the northern hemisphere, with large increases in search volumes in the summer months compared to winter for foot (pâ=â0.004, 95Â % CI [22.2â32.1]), ankle (pâ=â0.0078, 95Â % CI [20.9â35.5]), and heel pain (pâ=â0.004, 95Â % CI [29.1â45.6]). These seasonal effects were reflected by data from Australia, with the exception of ankle pain. Annual seasonal effects for treatment options were limited to terms related to foot surgery and ankle orthoses (pâ=â0.031, 95Â % CI [3.5â20.9]; pâ=â0.004, 95Â % CI [7.6â25.2] respectively), again increasing in the summer months. CONCLUSIONS: A number of general trends and annual seasonal effects were found in time series internet search data for terms relating to foot and ankle pain. This data may provide insights into these conditions at population levels. ELECTRONIC SUPPLEMENTARY MATERIAL: The online version of this article (doi:10.1186/s13047-015-0074-9) contains supplementary material, which is available to authorized users
A General Relativistic Magnetohydrodynamics Simulation of Jet Formation
We have performed a fully three-dimensional general relativistic
magnetohydrodynamic (GRMHD) simulation of jet formation from a thin accretion
disk around a Schwarzschild black hole with a free-falling corona. The initial
simulation results show that a bipolar jet (velocity ) is created as
shown by previous two-dimensional axisymmetric simulations with mirror symmetry
at the equator. The 3-D simulation ran over one hundred light-crossing time
units ( where ) which is
considerably longer than the previous simulations. We show that the jet is
initially formed as predicted due in part to magnetic pressure from the
twisting the initially uniform magnetic field and from gas pressure associated
with shock formation in the region around . At later times,
the accretion disk becomes thick and the jet fades resulting in a wind that is
ejected from the surface of the thickened (torus-like) disk. It should be noted
that no streaming matter from a donor is included at the outer boundary in the
simulation (an isolated black hole not binary black hole). The wind flows
outwards with a wider angle than the initial jet. The widening of the jet is
consistent with the outward moving torsional Alfv\'{e}n waves (TAWs). This
evolution of disk-jet coupling suggests that the jet fades with a thickened
accretion disk due to the lack of streaming material from an accompanying star.Comment: 27 pages, 8 figures, revised and accepted to ApJ (figures with better
resolution: http://gammaray.nsstc.nasa.gov/~nishikawa/schb1.pdf
Are the jets accelerated from the disk coronas in some active galactic nuclei?
We use a sample of radio-loud active galactic nuclei (AGNs) with estimated
central black hole masses to explore their jet formation mechanisms. The jet
power of AGNs is estimated from their extended radio luminosity. It is found
that the jets in several AGNs of this sample are too powerful to be extracted
from the standard thin accretion disks or rapidly spinning black holes
surrounded by standard thin disks. If the advection dominated accretion flows
(ADAFs) are present in these AGNs, their bright optical continuum luminosity
cannot be produced by pure-ADAFs due to their low accretion rates and low
radiation efficiency, unless the ADAFs transit to standard thin disks at some
radii . If this is the case, we find that the dimensionless
accretion rates as high as 0.05 and transition from ADAFs to standard thin
disks at rather small radii around 20GM/c^2 are required to explain their
bright optical continuum emission. We propose that the disk-corona structure is
present at least in some AGNs in this sample. The plasmas in the corona are
very hot, and the pressure scale-height of the corona H\sim R. Powerful jets
with Q_jet \sim L_bol (bolometric luminosity) can form by the large-scale
magnetic fields created by dynamo processes in the disk corona of some AGNs.
The maximal jet power extractable from the corona Q_jet^max\le 0.6L_c (L_c is
the corona luminosity) is expected by this jet formation scenario. The
statistic results on the sample of AGNs are consistent with the predictions of
this scenario. Finally, the possibility that the jet is driven from a
super-Keplerian rotating hot layer located between the corona and the cold disk
is discussed. We find that, in principle, this layer can also produce a
powerful jet with Q_jet\sim L_bol.Comment: 9 pages, accepted for publication in Ap
Evidence for Distinct Components of the Galactic Stellar Halo from 838 RR Lyrae Stars Discovered in the LONEOS-I Survey
We present 838 ab-type RR Lyrae stars from the Lowell Observatory Near Earth
Objects Survey Phase I (LONEOS-I). These objects cover 1430 deg^2 and span
distances ranging from 3-30 kpc from the Galactic Center. Object selection is
based on phased, photometric data with 28-50 epochs. We use this large sample
to explore the bulk properties of the stellar halo, including the spatial
distribution. The period-amplitude distribution of this sample shows that the
majority of these RR Lyrae stars resemble Oosterhoff type I, but there is a
significant fraction (26 %) which have longer periods and appear to be
Oosterhoff type II. We find that the radial distributions of these two
populations have significantly different profiles (rho_{OoI} ~ R^(-2.26 +-
0.07) and rho_{OoII} ~ R^(-2.88 +- 0.11). This suggests that the stellar halo
was formed by at least two distinct accretion processes and supports dual-halo
models.Comment: 18 pages, 28 figures, apjemulated, minor corrections and
clarifications. Accepted to ApJ on Jan 21, 200
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