11,686 research outputs found
Liquid compressibility effects during the collapse of a single cavitating bubble
The effect of liquid compressibility on the dynamics of a single, spherical cavitating bubble is studied.
While it is known that compressibility damps the amplitude of bubble rebounds, the extent to which
this effect is accurately captured by weakly compressible versions of the Rayleigh–Plesset equation is
unclear. To clarify this issue, partial differential equations governing conservation of mass, momentum,
and energy are numerically solved both inside the bubble and in the surrounding compressible
liquid. Radiated pressure waves originating at the unsteady bubble interface are directly captured.
Results obtained with Rayleigh–Plesset type equations accounting for compressibility effects, proposed
by Keller and Miksis [J. Acoust. Soc. Am. 68, 628–633 (1980)], Gilmore, and Tomita and
Shima [Bull. JSME 20, 1453–1460 (1977)], are compared with those resulting from the full model.
For strong collapses, the solution of the latter reveals that an important part of the energy concentrated
during the collapse is used to generate an outgoing pressure wave. For the examples considered in
this research, peak pressures are larger than those predicted by Rayleigh–Plesset type equations,
whereas the amplitudes of the rebounds are smaller
The Height of a Giraffe
A minor modification of the arguments of Press and Lightman leads to an
estimate of the height of the tallest running, breathing organism on a
habitable planet as the Bohr radius multiplied by the three-tenths power of the
ratio of the electrical to gravitational forces between two protons (rather
than the one-quarter power that Press got for the largest animal that would not
break in falling over, after making an assumption of unreasonable brittleness).
My new estimate gives a height of about 3.6 meters rather than Press's original
estimate of about 2.6 cm. It also implies that the number of atoms in the
tallest runner is very roughly of the order of the nine-tenths power of the
ratio of the electrical to gravitational forces between two protons, which is
about 3 x 10^32.Comment: 12 pages, LaTe
Vortex Fractionalization in a Josephson Ladder
We show numerically that, in a Josephson ladder with periodic boundary
conditions and subject to a suitable transverse magnetic field, a vortex
excitation can spontaneously break up into two or more fractional excitations.
If the ladder has N plaquettes, and N is divisible by an integer q, then in an
applied transverse field of 1/q flux quanta per plaquette the ground state is a
regular pattern of one fluxon every q plaquettes. When one additional fluxon is
added to the ladder, it breaks up into q fractional fluxons, each carrying 1/q
units of vorticity. The fractional fluxons are basically walls between
different domains of the ground state of the underlying 1/q lattice. The
fractional fluxons are all depinned at the same applied current and move as a
unit. For certain applied fields and ladder lengths, we show that there are
isolated fractional fluxons. It is shown that the fractional fluxons would
produce a time-averaged voltage related in a characteristic way to the ac
voltage frequency.Comment: 13 Figures. 10 page
Estimating statistical distributions using an integral identity
We present an identity for an unbiased estimate of a general statistical
distribution. The identity computes the distribution density from dividing a
histogram sum over a local window by a correction factor from a mean-force
integral, and the mean force can be evaluated as a configuration average. We
show that the optimal window size is roughly the inverse of the local
mean-force fluctuation. The new identity offers a more robust and precise
estimate than a previous one by Adib and Jarzynski [J. Chem. Phys. 122, 014114,
(2005)]. It also allows a straightforward generalization to an arbitrary
ensemble and a joint distribution of multiple variables. Particularly we derive
a mean-force enhanced version of the weighted histogram analysis method (WHAM).
The method can be used to improve distributions computed from molecular
simulations. We illustrate the use in computing a potential energy
distribution, a volume distribution in a constant-pressure ensemble, a radial
distribution function and a joint distribution of amino acid backbone dihedral
angles.Comment: 45 pages, 7 figures, simplified derivation, a more general mean-force
formula, add discussions to the window size, add extensions to WHAM, and 2d
distribution
Evaporation of a Kerr black hole by emission of scalar and higher spin particles
We study the evolution of an evaporating rotating black hole, described by
the Kerr metric, which is emitting either solely massless scalar particles or a
mixture of massless scalar and nonzero spin particles. Allowing the hole to
radiate scalar particles increases the mass loss rate and decreases the angular
momentum loss rate relative to a black hole which is radiating nonzero spin
particles. The presence of scalar radiation can cause the evaporating hole to
asymptotically approach a state which is described by a nonzero value of . This is contrary to the conventional view of black hole
evaporation, wherein all black holes spin down more rapidly than they lose
mass. A hole emitting solely scalar radiation will approach a final asymptotic
state described by . A black hole that is emitting scalar
particles and a canonical set of nonzero spin particles (3 species of
neutrinos, a single photon species, and a single graviton species) will
asymptotically approach a nonzero value of only if there are at least 32
massless scalar fields. We also calculate the lifetime of a primordial black
hole that formed with a value of the rotation parameter , the minimum
initial mass of a primordial black hole that is seen today with a rotation
parameter , and the entropy of a black hole that is emitting scalar or
higher spin particles.Comment: 22 pages, 13 figures, RevTeX format; added clearer descriptions for
variables, added journal referenc
Destruction of Anderson localization by a weak nonlinearity
We study numerically a spreading of an initially localized wave packet in a
one-dimensional discrete nonlinear Schr\"odinger lattice with disorder. We
demonstrate that above a certain critical strength of nonlinearity the Anderson
localization is destroyed and an unlimited subdiffusive spreading of the field
along the lattice occurs. The second moment grows with time , with the exponent being in the range . For small
nonlinearities the distribution remains localized in a way similar to the
linear case.Comment: 4 pages, 5 fig
Effect of the Berendsen thermostat on dynamical properties of water
The effect of the Berendsen thermostat on the dynamical properties of bulk
SPC/E water is tested by generating power spectra associated with fluctuations
in various observables. The Berendsen thermostat is found to be very effective
in preserving temporal correlations in fluctuations of tagged particle
quantities over a very wide range of frequencies. Even correlations in
fluctuations of global properties, such as the total potential energy, are
well-preserved for time periods shorter than the thermostat time constant.
Deviations in dynamical behaviour from the microcanonical limit do not,
however, always decrease smoothly with increasing values of the thermostat time
constant but may be somewhat larger for some intermediate values of ,
specially in the supercooled regime, which are similar to time scales for slow
relaxation processes in bulk water.Comment: 21 pages, 5 figures, To be published in Mol. Phy
The Photometric Period of the Cataclysmic Variable HV Andromedae
We present four nights of time-resolved photometry of the cataclysmic
variable star HV And. Our time series analysis has revealed a prominent period
at 3.368 +/- 0.060 hours, as well as some low frequency power. We interpret
this signal, from saw-tooth waves in the light curve, as evidence of superhumps
in HV And.Comment: 7 pages, 3 figures; accepted for publication in New Astronom
Black Hole-Neutron Star Mergers: Disk Mass Predictions
Determining the final result of black hole-neutron star mergers, and in
particular the amount of matter remaining outside the black hole at late times
and its properties, has been one of the main motivations behind the numerical
simulation of these systems. Black hole-neutron star binaries are amongst the
most likely progenitors of short gamma-ray bursts --- as long as massive
(probably a few percents of a solar mass), hot accretion disks are formed
around the black hole. Whether this actually happens strongly depends on the
physical characteristics of the system, and in particular on the mass ratio,
the spin of the black hole, and the radius of the neutron star. We present here
a simple two-parameter model, fitted to existing numerical results, for the
determination of the mass remaining outside the black hole a few milliseconds
after a black hole-neutron star merger (i.e. the combined mass of the accretion
disk, the tidal tail, and the potential ejecta). This model predicts the
remnant mass within a few percents of the mass of the neutron star, at least
for remnant masses up to 20% of the neutron star mass. Results across the range
of parameters deemed to be the most likely astrophysically are presented here.
We find that, for 10 solar mass black holes, massive disks are only possible
for large neutron stars (R>12km), or quasi-extremal black hole spins (a/M>0.9).
We also use our model to discuss how the equation of state of the neutron star
affects the final remnant, and the strong influence that this can have on the
rate of short gamma-ray bursts produced by black hole-neutron star mergers.Comment: 11 pages, 7 figure
- …