6,873 research outputs found
Instability of Compressible Drops and Jets
We revisit the classic problem of the stability of drops and jets held by
surface tension, while regarding the compressibility of bulk fluids and spatial
dimensions as free parameters. By mode analysis, it is shown that there exists
a critical compressibility above which the drops (and disks) become unstable
for a spherical perturbation. For a given value of compressibility (and those
of the surface tension and density at the equilibrium), this instability
criterion provides a minimal radius below which the drop cannot be a stable
equilibrium. According to the existence of the above unstable mode of drop,
which corresponds to a homogeneous perturbation of cylindrical jet, the
dispersion relation of Rayleigh-Plateau instability for cylinders drastically
changes. In particular, we identify another critical compressibility above
which the homogeneous unstable mode is predominant. The analysis is done for
non-relativistic and relativistic perfect fluids, of which self-gravity is
ignored.Comment: 24 pages, 5 figures, 1 table; v2: typos corrected; v3: final version
to appear in JF
Fault-tolerant and finite-error localization for point emitters within the diffraction limit
We implement an estimator for determining the separation between two
incoherent point sources. This estimator relies on image inversion
interferometry and when used with the appropriate data analytics, it yields an
estimate of the separation with finite-error, even when the sources come
arbitrarily close together. The experimental results show that the technique
has a good tolerance to noise and misalignment, making it an interesting
consideration for high resolution instruments
The 2008 election: A preregistered replication analysis
We present an increasingly stringent set of replications of Ghitza & Gelman
(2013), a multilevel regression and poststratification analysis of polls from
the 2008 U.S. presidential election campaign, focusing on a set of plots
showing the estimated Republican vote share for whites and for all voters, as a
function of income level in each of the states.
We start with a nearly-exact duplication that uses the posted code and
changes only the model-fitting algorithm; we then replicate using
already-analyzed data from 2004; and finally we set up preregistered
replications using two surveys from 2008 that we had not previously looked at.
We have already learned from our preliminary, non-preregistered replication,
which has revealed a potential problem with the published analysis of Ghitza &
Gelman (2013); it appears that our model may not sufficiently account for
nonsampling error, and that some of the patterns presented in that earlier
paper may simply reflect noise.
In addition to the substantive interest in validating earlier findings about
demographics, geography, and voting, the present project serves as a
demonstration of preregistration in a setting where the subject matter is
historical (and thus the replication data exist before the preregistration plan
is written) and where the analysis is exploratory (and thus a replication
cannot be simply deemed successful or unsuccessful based on the statistical
significance of some particular comparison).Comment: This article is a review and preregistration plan. It will be
published, along with a new Section 5 describing the results of the
preregistered analysis, in Statistics and Public Polic
Whispering Gallery States of Antihydrogen
We study theoretically interference of the long-living quasistationary
quantum states of antihydrogen atoms, localized near a concave material
surface. Such states are an antimatter analog of the whispering gallery states
of neutrons and matter atoms, and similar to the whispering gallery modes of
sound and electro-magnetic waves. Quantum states of antihydrogen are formed by
the combined effect of quantum reflection from van der Waals/Casimir-Polder
(vdW/CP) potential of the surface and the centrifugal potential. We point out a
method for precision studies of quantum reflection of antiatoms from vdW/CP
potential; this method uses interference of the whispering gallery states of
antihydrogen.Comment: 13 pages 7 figure
Packing defects and the width of biopolymer bundles
The formation of bundles composed of actin filaments and cross-linking
proteins is an essential process in the maintenance of the cells' cytoskeleton.
It has also been recreated by in-vitro experiments, where actin networks are
routinely produced to mimic and study the cellular structures. It has long been
observed that these bundles seem to have a well defined width distribution,
which has not been adequately described theoretically. We propose here that
packing defects of the filaments, quenched and random, contribute an effective
repulsion that counters the cross-linking adhesion energy and leads to a well
defined bundle width. This is a two-dimensional strain-field version of the
classic Rayleigh instability of charged droplets
High speed imaging of traveling waves in a granular material during silo discharge
We report experimental observations of sound waves in a granular material
during resonant silo discharge called silo music. The grain motion was tracked
by high speed imaging while the resonance of the silo was detected by
accelerometers and acoustic methods. The grains do not oscillate in phase at
neighboring vertical locations, but information propagates upward in this
system in the form of sound waves. We show that the wave velocity is not
constant throughout the silo, but considerably increases towards the lower end
of the system, suggesting increased pressure in this region, where the flow
changes from cylindrical to converging flow. In the upper part of the silo the
wave velocity matches the sound velocity measured in the same material when
standing (in the absence of flow). Grain oscillations show a stick-slip
character only in the upper part of the silo.Comment: 5 pages, 5 figures, accepted to Phys. Rev.
"Ultimate state" of two-dimensional Rayleigh-Benard convection between free-slip fixed temperature boundaries
Rigorous upper limits on the vertical heat transport in two dimensional
Rayleigh-Benard convection between stress-free isothermal boundaries are
derived from the Boussinesq approximation of the Navier-Stokes equations. The
Nusselt number Nu is bounded in terms of the Rayleigh number Ra according to
uniformly in the Prandtl number Pr. This Nusselt
number scaling challenges some theoretical arguments regarding the asymptotic
high Rayleigh number heat transport by turbulent convection.Comment: 4 page
Asymmetric Wave Propagation in Nonlinear Systems
A mechanism for asymmetric (nonreciprocal) wave transmission is presented. As
a reference system, we consider a layered nonlinear, non mirror-symmetric model
described by the one-dimensional Discrete Nonlinear Schreodinger equation with
spatially varying coefficients embedded in an otherwise linear lattice. We
construct a class of exact extended solutions such that waves with the same
frequency and incident amplitude impinging from left and right directions have
very different transmission coefficients. This effect arises already for the
simplest case of two nonlinear layers and is associated with the shift of
nonlinear resonances. Increasing the number of layers considerably increases
the complexity of the family of solutions. Finally, numerical simulations of
asymmetric wavepacket transmission are presented which beautifully display the
rectifying effect
Coupled Ripplon-Plasmon Modes in a Multielectron Bubble
In multielectron bubbles, the electrons form an effectively two-dimensional
layer at the inner surface of the bubble in helium. The modes of oscillation of
the bubble surface (the ripplons) are influenced by the charge redistribution
of the electrons along the surface. The dispersion relation for these charge
redistribution modes (`longitudinal plasmons') is derived and the coupling of
these modes to the ripplons is analysed. We find that the ripplon-plasmon
coupling in a multielectron bubble differs markedly from that of electrons a
flat helium surface. An equation is presented relating the spherical harmonic
components of the charge redistribution to those of the shape deformation of
the bubble.Comment: 8 pages, 1 figure, E-mail addresses: [email protected],
[email protected], [email protected], [email protected]
Relativistic Models of Galaxies
A special form of the isotropic metric in cylindrical coordinates is used to
construct what may be interpreted as the General Relativistic versions of some
wellknown potential-density pairs used in Newtonian gravity to model
three-dimensional distributions of matter in galaxies. The components of the
energy-momentum tensor are calculated for the first two Miyamoto-Nagai
potentials and a particular potential due to Satoh. The three potentials yield
distributions of matter in which all tensions are pressures and all energy
conditions are satisfied for certain ranges of the free parameters. A few
non-planar geodesic orbits are computed for one of the potentials and compared
with the Newtonian case. Rotation is also incorporated to the models and the
effects of the source rotation on the rotation profile are calculated as first
order corrections by using an approximate form of the Kerr metric in isotropic
coordinates.Comment: 18 pages, 23 eps figures, uses mn2e.cls style file, to be published
in MNRA
- …