572 research outputs found
Initial surface deformations during impact on a liquid pool
A tiny air bubble can be entrapped at the bottom of a solid sphere that
impacts onto a liquid pool. The bubble forms due to the deformation of the
liquid surface by a local pressure buildup inside the surrounding gas, as also
observed during the impact of a liquid drop on a solid wall. Here we perform a
perturbation analysis to quantitatively predict the initial deformations of the
free surface of the liquid pool as it is approached by a solid sphere. We study
the natural limits where the gas can be treated as a viscous fluid (Stokes
flow) or as an inviscid fluid (potential flow). For both cases we derive the
spatio-temporal evolution of the pool surface, and recover some of the recently
proposed scaling laws for bubble entrapment. When inserting typical
experimental values for the impact parameters, we find that the bubble volume
is mainly determined by the effect of gas viscosity
Universal mechanism for air entrainment during liquid impact
When a mm-sized liquid drop approaches a deep liquid pool, both the interface
of the drop and the pool deform before the drop touches the pool. The build up
of air pressure prior to coalescence is responsible for this deformation. Due
to this deformation, air can be entrained at the bottom of the drop during the
impact. We quantify the amount of entrained air numerically, using the Boundary
Integral Method (BIM) for potential flow for the drop and the pool, coupled to
viscous lubrication theory for the air film that has to be squeezed out during
impact. We compare our results to various experimental data and find excellent
agreement for the amount of air that is entrapped during impact onto a pool.
Next, the impact of a rigid sphere onto a pool is numerically investigated and
the air that is entrapped in this case also matches with available experimental
data. In both cases of drop and sphere impact onto a pool the numerical air
bubble volume V_b is found to be in agreement with the theoretical scaling
V_b/V_{drop/sphere} ~ St^{-4/3}, where St is the Stokes number. This is the
same scaling that has been found for drop impact onto a solid surface in
previous research. This implies a universal mechanism for air entrainment for
these different impact scenarios, which has been suggested in recent
experimental work, but is now further elucidated with numerical results
The liquid helix
From everyday experience, we all know that a solid edge can deflect a liquid
flowing over it significantly, up to the point where the liquid completely
sticks to the solid. Although important in pouring, printing and extrusion
processes, there is no predictive model of this so-called "teapot effect". By
grazing vertical cylinders with inclined capillary liquid jets, we here use the
teapot effect to attach the jet to the solid and form a new structure: the
liquid helix. Using mass and momentum conservation along the liquid stream, we
first quantitatively predict the shape of the helix and then provide a
parameter-free inertial-capillary adhesion model for the jet deflection and
critical velocity for helix formation.Comment: Accepted in Physical Review Letters, author versio
A search for fast radio burst-like emission from Fermi gamma-ray bursts
We report the results of the rapid follow-up observations of gamma-ray bursts
(GRBs) detected by the Fermi satellite to search for associated fast radio
bursts. The observations were conducted with the Australian Square Kilometre
Array Pathfinder at frequencies from 1.2-1.4 GHz. A set of 20 bursts, of which
four were short GRBs, were followed up with a typical latency of about one
minute, for a duration of up to 11 hours after the burst. The data was searched
using 4096 dispersion measure trials up to a maximum dispersion measure of 3763
pc cm, and for pulse widths over a range of duration from 1.256 to
40.48 ms. No associated pulsed radio emission was observed above for any of the 20 GRBs.Comment: Accepted for publication in Monthly Notices of the Royal Astronomical
Society Main Journa
The Highest Energy Neutrinos
Measurements of the arrival directions of cosmic rays have not revealed their
sources. High energy neutrino telescopes attempt to resolve the problem by
detecting neutrinos whose directions are not scrambled by magnetic fields. The
key issue is whether the neutrino flux produced in cosmic ray accelerators is
detectable. It is believed that the answer is affirmative, both for the
galactic and extragalactic sources, provided the detector has kilometer-scale
dimensions. We revisit the case for kilometer-scale neutrino detectors in a
model-independent way by focussing on the energetics of the sources. The real
breakthrough though has not been on the theory but on the technology front: the
considerable technical hurdles to build such detectors have been overcome.
Where extragalactic cosmic rays are concerned an alternative method to probe
the accelerators consists in studying the arrival directions of neutrinos
produced in interactions with the microwave background near the source, i.e.
within a GZK radius. Their flux is calculable within large ambiguities but, in
any case, low. It is therefore likely that detectors that are larger yet by
several orders of magnitudes are required. These exploit novel techniques, such
as detecting the secondary radiation at radio wavelengths emitted by neutrino
induced showers.Comment: 16 pages, pdflatex, 7 jpg figures, ICRC style files included.
Highlight talk presented at the 30th International Cosmic Ray Conference,
Merida, Mexico, 200
Point-Form Analysis of Elastic Deuteron Form Factors
Point-form relativistic quantum mechanics is applied to elastic
electron-deuteron scattering. The deuteron is modeled using relativistic
interactions that are scattering-equivalent to the nonrelativistic Argonne
and Reid '93 interactions. A point-form spectator approximation (PFSA)
is introduced to define a conserved covariant current in terms of
single-nucleon form factors. The PFSA is shown to provide an accurate
description of data up to momentum transfers of 0.5 , but falls
below the data at higher momentum transfers. Results are sensitive to the
nucleon form factor parameterization chosen, particularly to the neutron
electric form factor.Comment: RevTex, 31 pages, 1 table, 13 figure
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