1,789 research outputs found
The evolution of energy in flow driven by rising bubbles
We investigate by direct numerical simulations the flow that rising bubbles
cause in an originally quiescent fluid. We employ the Eulerian-Lagrangian
method with two-way coupling and periodic boundary conditions. In order to be
able to treat up to 288000 bubbles, the following approximations and
simplifications had to be introduced: (i) The bubbles were treated as
point-particles, thus (ii) disregarding the near-field interactions among them,
and (iii) effective force models for the lift and the drag forces were used. In
particular, the lift coefficient was assumed to be 1/2, independent of the
bubble Reynolds number and the local flow field. The results suggest that large
scale motions are generated, owing to an inverse energy cascade from the small
to the large scales. However, as the Taylor-Reynolds number is only in the
range of 1, the corresponding scaling of the energy spectrum with an exponent
of -5/3 cannot develop over a pronounced range. In the long term, the property
of local energy transfer, characteristic of real turbulence, is lost and the
input of energy equals the viscous dissipation at all scales. Due to the lack
of strong vortices the bubbles spread rather uniformly in the flow. The
mechanism for uniform spreading is as follows: Rising bubbles induce a velocity
field behind them that acts on the following bubbles. Owing to the shear, those
bubbles experience a lift force which make them spread to the left or right,
thus preventing the formation of vertical bubble clusters and therefore of
efficient forcing. Indeed, when the lift is artifically put to zero in the
simulations, the flow is forced much more efficiently and a more pronounced
energy accumulates at large scales is achieved.Comment: 9 pages, 7 figure
Effect of the Milky Way on Magellanic Cloud structure
A combination of analytic models and n-body simulations implies that the
structural evolution of the Large Magellanic Cloud (LMC) is dominated by its
dynamical interaction with the Milky Way. Although expected at some level, the
scope of the involvement has significant observational consequences. First, LMC
disk orbits are torqued out of the disk plane, thickening the disk and
populating a spheroid. The torque results from direct forcing by the Milky Way
tide and, indirectly, from the drag between the LMC disk and its halo resulting
from the induced precession of the LMC disk. The latter is a newly reported
mechanism that can affect all satellite interations. However, the overall
torque can not isotropize the stellar orbits and their kinematics remains
disk-like. Such a kinematic signature is observed for nearly all LMC
populations. The extended disk distribution is predicted to increase the
microlensing toward the LMC. Second, the disk's binding energy slowly decreases
during this process, puffing up and priming the outer regions for subsequent
tidal stripping. Because the tidally stripped debris will be spatially
extended, the distribution of stripped stars is much more extended than the HI
Magellanic Stream. This is consistent with upper limits to stellar densities in
the gas stream and suggests a different strategy for detecting the stripped
stars. And, finally, the mass loss over several LMC orbits is predicted by
n-body simulation and the debris extends to tens of kiloparsecs from the tidal
boundary. Although the overall space density of the stripped stars is low,
possible existence of such intervening populations have been recently reported
and may be detectable using 2MASS.Comment: 15 pages, color Postscript figures, uses emulateapj.sty. Also
available from http://www-astro.phast.umass.edu/~weinberg/weinberg-pubs.htm
Production of Milky Way structure by the Magellanic Clouds
Previous attempts at disturbing the galactic disk by the Magellanic Clouds
relied on direct tidal forcing. However, by allowing the halo to actively
respond rather than remain a rigid contributor to the rotation curve, the
Clouds may produce a wake in the halo which then distorts the disk. Recent work
reported here suggests that the Magellanic Clouds use this mechanism to produce
disk distortions sufficient to account for both the radial location, position
angle and sign of the HI warp and observed anomalies in stellar kinematics
towards the galactic anticenter and LSR motion.Comment: 8 pages, uuencoded compressed PostScript, no figures, html version
with figures and mpeg simulations available at
http://www-astro.phast.umass.edu/Preprints/martin/martin1/lmc_online.htm
Studies in the aklu documents of the Middle Babylonian period
The aklu documents are accounts of the commodities like
beer, flour, sheep. But usually we can not find any verb such as
"received" or "gave" in them. Therefore the exact meaning
of the documents remains to be determined. In this study, I made a profile of
the persons appear in the documents according to their seal impressions. By
observing the characteristics of the groups of the seal impressions, we can
find that artisans prepared the commodities and that the members of a few
prominet Akkadian families sealed the documents.
Canon Foundation in Europe (2015)Middle Eastern Studie
Tidal Streams as Probes of the Galactic Potential
We explore the use of tidal streams from Galactic satellites to recover the
potential of the Milky Way. Our study is motivated both by the discovery of the
first lengthy stellar stream in the halo (\cite{it98}) and by the prospect of
measuring proper motions of stars brighter than 20th magnitude in such a stream
with an accuracy of yr, as will be possible with the Space
Interferometry Mission (SIM). We assume that the heliocentric radial velocities
of these stars can be determined from supporting ground-based spectroscopic
surveys, and that the mass and phase-space coordinates of the Galactic
satellite with which they are associated will also be known to SIM accuracy.
Using results from numerical simulations as trial data sets, we find that, if
we assume the correct form for the Galactic potential, we can predict the
distances to the stars as a consequence of the narrow distribution of energy
expected along the streams. We develop an algorithm to evaluate the accuracy of
any adopted potential by requiring that the satellite and stars recombine
within a Galactic lifetime when their current phase-space coordinates are
integrated backwards. When applied to a four-dimensional grid of triaxial
logarithmic potentials, with varying circular velocities, axis ratios and
orientation of the major-axis in the disk plane, the algorithm can recover the
parameters used for the Milky Way in a simulated data set to within a few
percent using only 100 stars in a tidal stream.Comment: Revised version - original algorithm generalised to be applicable to
any potential shape. LaTeX, 12 pages including 3 figures. To be published in
ApJ Letter
Quantal Two-Centre Coulomb Problem treated by means of the Phase-Integral Method I. General Theory
The present paper concerns the derivation of phase-integral quantization
conditions for the two-centre Coulomb problem under the assumption that the two
Coulomb centres are fixed. With this restriction we treat the general
two-centre Coulomb problem according to the phase-integral method, in which one
uses an {\it a priori} unspecified {\it base function}. We consider base
functions containing three unspecified parameters and .
When the absolute value of the magnetic quantum number is not too small, it
is most appropriate to choose . When, on the other hand,
is sufficiently small, it is most appropriate to choose .
Arbitrary-order phase-integral quantization conditions are obtained for these
choices of . The parameters and are determined from the
requirement that the results of the first and the third order of the
phase-integral approximation coincide, which makes the first-order
approximation as good as possible.
In order to make the paper to some extent self-contained, a short review of
the phase-integral method is given in the Appendix.Comment: 23 pages, RevTeX, 4 EPS figures, submitted to J. Math. Phy
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