12,447 research outputs found
On the evolution of non-axisymmetric viscous fibres with surface tension, inertia and gravity
We consider the free boundary problem for the evolution of a nearly straight slender fibre of viscous fluid. The motion is driven by prescribing the velocity of the ends of the fibre, and the free surface evolves under the action of surface tension, inertia and gravity. The three-dimensional Navier-Stokes equations and free-surface boundary conditions are analysed asymptotically, using the fact that the inverse aspect ratio, defined to be the ratio between a typical fibre radius and the initial fibre length, is small. This first part of the paper follows earlier work on the stretching of a slender viscous fibre with negligible surface tension effects. The inclusion of surface tension seriously complicates the problem for the evolution of the shape of the cross-section. We adapt ideas applied previously to two-dimensional Stokes flow to show that the shape of the cross-section can be described by means of a conformal map which depends on time and distance along the fibre axis. We give some examples of suitable relevant maps and present numerical solutions of the resulting equations. We also use analytic methods to examine the coupling between stretching and the evolution of the cross-section shape
Experimental pressure distributions for a family of blunt bodies at Mach numbers from 2.49 to 4.63 and angles of attack from 0 deg to 15 deg
Pressure distributions for blunt body wind tunnel models at supersonic speeds and angles of attack from 0 to 15 degree
On the predictions and limitations of the BeckerDoring model for reaction kinetics in micellar surfactant solutions
We investigate the breakdown of a system of micellar aggregates in a surfactant solution following an order-one dilution. We derive a mathematical model based on the BeckerâDöring system of equations, using realistic expressions for the reaction constants fit to Molecular Dynamics simulations. We exploit the largeness of typical aggregation numbers to derive a continuum model, substituting a large system of ordinary differential equations for a partial differential equation in two independent variables: time and aggregate size. Numerical solutions demonstrate that re-equilibration occurs in two distinct stages over well-separated time-scales, in agreement with experiment and with previous theories. We conclude by exposing a limitation in the BeckerâDöring theory for re-equilibration and discuss potential resolutions
Coldâwater coral assemblages on vertical walls from the Northeast Atlantic
Aim
In this study, we assess patterns of coldâwater coral assemblages observed on deepâsea vertical walls. Similar to their shallowâwater counterparts, vertical and overhanging walls in the deep sea can host highly diverse communities, but because of their geometry, these habitats are generally overlooked and remain poorly known. These vertical habitats are however of particular interest, because they can protect vulnerable coral ecosystems from trawling activities. As such, it is important to understand their ecology and assess their global importance.
Location
Vertical walls on complex geomorphic features, in particular walls of the Rockall Bank Slope Failure Escarpment, Whittard and Explorer Canyons, Northeast Atlantic.
Methods
Video analysis of remotely operated vehicle transects carried out at five sites is used to investigate differences in species composition and diversity across walls and to compare those to nearby coldâwater coral sites on flat terrain. A highâresolution photogrammetric reconstruction is further employed to examine whether wall complexity plays a role in promoting niche differentiation at very fine spatial scales.
Results
The investigated walls showed differences in species assemblage both across walls and in comparison to flat sites, with the fineâscale heterogeneity engendered by walls allowing niche differentiation between closely related taxa.
Main Conclusions
Vertical walls represent an important coldâwater coral habitat with differences in species composition across walls within a region, illustrating their role in driving diversity patterns. Based on publicly available bathymetric datasets and a catalogue of broadâscale terrain features, globally over 8,000 features are likely to have vertical walls and coldâwater corals, which highlight the need to consider deepâsea vertical habitats in current conservation efforts
An asymptotic theory for the re-equilibration of a micellar surfactant solution
Micellar surfactant solutions are characterized by a distribution of aggregates comprised predominantly of pre-micellar aggregates (monomers, dimers, trimers, etc.) and a region of proper micelles close to the peak aggregation number, connected by an intermediate region containing a very low concentration of aggregates. Such a distribution gives rise to a distinct two-timescale re-equilibration following a system dilution, known as the 1 and 2 processes, whose dynamics may be described by the BeckerâDöring equations. We use a continuum version of these equations to develop a reduced asymptotic description that elucidates the behavior during each of these processes
X-Shooting EF Eridani: further evidence for a massive white dwarf and a sub-stellar secondary
High resolution spectral observations of the polar EF Eridani obtained in a
low state with X-Shooter revealed narrow emission lines from the irradiated
secondary. The lines were most prominent in the near-IR Ca2-triplet, the more
intensive Halpha line had additional emission likely originating from an
accretion stream. The lines with a radial velocity amplitude, K2' = 385 +- 4
kmps, serve as tracer of the otherwise unseen companion. The mass function
implies a massive white dwarf with Mwd > 0.65 Msun at 3sigma confidence, and a
short distance to the binary, d ~ 111 pc (<145pc at 3sigma confidence). The
spectral energy distribution from the UV to the IR together with the high mass
ratio gives further strong evidence of EF Eri being a post period-minimum
object with M2 < 0.06 MsunComment: Astron, Astrophys., in pres
Laserwire at the Accelerator Test Facility 2 with Sub-Micrometre Resolution
A laserwire transverse electron beam size measurement system has been
developed and operated at the Accelerator Test Facility 2 (ATF2) at KEK.
Special electron beam optics were developed to create an approximately 1 x 100
{\mu}m (vertical x horizontal) electron beam at the laserwire location, which
was profiled using a 150 mJ, 71 ps laser pulse with a wavelength of 532 nm. The
precise characterisation of the laser propagation allows the non-Gaussian
transverse profiles of the electron beam caused by the laser divergence to be
deconvolved. A minimum vertical electron beam size of 1.07 0.06 (stat.)
0.05 (sys.) {\mu}m was measured. A vertically focussing quadrupole just
before the laserwire was varied whilst making laserwire measurements and the
projected vertical emittance was measured to be 82.56 3.04 pm rad.Comment: 17 pages, 26 figures, submitted to Phys. Rev. ST Accel. Beam
The ultra-long GRB 111209A - II. Prompt to afterglow and afterglow properties
The "ultra-long" Gamma Ray Burst GRB 111209A at redshift z=0.677, is so far
the longest GRB ever observed, with rest frame prompt emission duration of ~4
hours. In order to explain the bursts exceptional longevity, a low metallicity
blue supergiant progenitor has been invoked. In this work, we further
investigate this peculiar burst by performing a multi-band temporal and
spectral analysis of both the prompt and the afterglow emission. We use
proprietary and publicly available data from Swift, Konus Wind, XMM-Newton,
TAROT as well as from other ground based optical and radio telescopes. We find
some peculiar properties that are possibly connected to the exceptional nature
of this burst, namely: i) an unprecedented large optical delay of 410+/-50 s is
measured between the peak epochs of a marked flare observed also in gamma-rays
after about 2 ks from the first Swift/BAT trigger; ii) if the optical and
X-ray/gamma-ray photons during the prompt emission share a common origin, as
suggested by their similar temporal behavior, a certain amount of dust in the
circumburst environment should be introduced, with rest frame visual dust
extinction of AV=0.3-1.5 mag; iii) at the end of the X-ray "steep decay phase"
and before the start of the X-ray afterglow, we detect the presence of a hard
spectral extra power law component never revealed so far. On the contrary, the
optical afterglow since the end of the prompt emission shows more common
properties, with a flux power law decay with index alpha=1.6+/-0.1 and a late
re-brightening feature at 1.1 day. We discuss our findings in the context of
several possible interpretations given so far to the complex multi-band GRB
phenomenology. We also attempt to exploit our results to further constrain the
progenitor nature properties of this exceptionally long GRB, suggesting a
binary channel formation for the proposed blue supergiant progenitor.Comment: ApJ accepted. Revised version with substantial adjustments, the main
results remain unchange
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