14,247 research outputs found

### Sensitivity of lunar particle-detection experiments

The use of the Moon as a detector volume for ultra-high-energy neutrinos and
cosmic rays, by searching for the Askaryan radio pulse produced when they
interact in the lunar regolith, has been attempted by a range of projects over
the past two decades. In this contribution, I discuss some of the technical
considerations relevant to these experiments, and their consequent sensitivity
to ultra-high-energy particles. I also discuss some possible future
experiments, and highlight their potential.Comment: To be published in the Proceedings of the ARENA2016 conference,
Groningen, The Netherland

### Minimal prospects for radio detection of extensive air showers in the atmosphere of Jupiter

One possible approach for detecting ultra-high-energy cosmic rays and
neutrinos is to search for radio emission from extensive air showers created
when they interact in the atmosphere of Jupiter, effectively utilizing Jupiter
as a particle detector. We investigate the potential of this approach. For
searches with current or planned radio telescopes we find that the effective
area for detection of cosmic rays is substantial (~3*10^7 km^2), but the
acceptance angle is so small that the typical geometric aperture (~10^3 km^2
sr) is less than that of existing terrestrial detectors, and cosmic rays also
cannot be detected below an extremely high threshold energy (~10^23 eV). The
geometric aperture for neutrinos is slightly larger, and greater sensitivity
can be achieved with a radio detector on a Jupiter-orbiting satellite, but in
neither case is this sufficient to constitute a practical detection technique.
Exploitation of the large surface area of Jupiter for detecting
ultra-high-energy particles remains a long-term prospect that will require a
different technique, such as orbital fluorescence detection.Comment: 15 pages, 15 figures, 2 tables, accepted for publication in Ap

### Statistical modelling for prediction of axis-switching in rectangular jets

Rectangular nozzles are increasingly used for modern military aircraft propulsion installations, including the roll nozzles on the F-35B vertical/short take-off and landing strike fighter. A peculiar phenomenon known as axis-switching is generally observed in such non-axisymmetric nozzle flows during which the jet spreads faster along the minor axis compared to the major axis. This might affect the under-wing stores and aircraft structure. A computational fluid dynamics study was performed to understand the effects of changing the upstream nozzle geometry on a rectangular free jet. A method is proposed, involving the formulation of an equation based upon a statistical model for a rectangular nozzle with an exit aspect ratio (ARe) of 4; the variables under consideration (for a constant nozzle pressure ratio (NPR)) being inlet aspect ratio (ARi) and length of the contraction section. The jet development was characterised using two parameters: location of the cross-over point (Xc) and the difference in the jet half-velocity widths along the major and minor axes (ÎB30). Based on the observed results, two statistical models were formulated for the prediction of axis-switching; the first model gives the location of the cross-over point, while the second model indicates the occurrence of axis-switching for the given configuration

### Non-equilibrium Phase-Ordering with a Global Conservation Law

In all dimensions, infinite-range Kawasaki spin exchange in a quenched Ising
model leads to an asymptotic length-scale $L \sim (\rho t)^{1/2} \sim t^{1/3}$
at $T=0$ because the kinetic coefficient is renormalized by the broken-bond
density, $\rho \sim L^{-1}$. For $T>0$, activated kinetics recovers the
standard asymptotic growth-law, $L \sim t^{1/2}$. However, at all temperatures,
infinite-range energy-transport is allowed by the spin-exchange dynamics. A
better implementation of global conservation, the microcanonical Creutz
algorithm, is well behaved and exhibits the standard non-conserved growth law,
$L \sim t^{1/2}$, at all temperatures.Comment: 2 pages and 2 figures, uses epsf.st

### Corrections to Scaling in the Phase-Ordering Dynamics of a Vector Order Parameter

Corrections to scaling, associated with deviations of the order parameter
from the scaling morphology in the initial state, are studied for systems with
O(n) symmetry at zero temperature in phase-ordering kinetics. Including
corrections to scaling, the equal-time pair correlation function has the form
C(r,t) = f_0(r/L) + L^{-omega} f_1(r/L) + ..., where L is the coarsening length
scale. The correction-to-scaling exponent, omega, and the correction-to-scaling
function, f_1(x), are calculated for both nonconserved and conserved order
parameter systems using the approximate Gaussian closure theory of Mazenko. In
general, omega is a non-trivial exponent which depends on both the
dimensionality, d, of the system and the number of components, n, of the order
parameter. Corrections to scaling are also calculated for the nonconserved 1-d
XY model, where an exact solution is possible.Comment: REVTeX, 20 pages, 2 figure

### Electrostatic interactions between discrete helices of charge

We analytically examine the pair interaction for parallel, discrete helices
of charge. Symmetry arguments allow for the energy to be decomposed into a sum
of terms, each of which has an intuitive geometric interpretation. Truncated
Fourier expansions for these terms allow for accurate modeling of both the
axial and azimuthal terms in the interaction energy and these expressions are
shown to be insensitive to the form of the interaction. The energy is evaluated
numerically through application of an Ewald-like summation technique for the
particular case of unscreened Coulomb interactions between the charges of the
two helices. The mode structures and electrostatic energies of flexible helices
are also studied. Consequences of the resulting energy expressions are
considered for both F-actin and A-DNA aggregates

### Interface Fluctuations, Burgers Equations, and Coarsening under Shear

We consider the interplay of thermal fluctuations and shear on the surface of
the domains in various systems coarsening under an imposed shear flow. These
include systems with nonconserved and conserved dynamics, and a conserved order
parameter advected by a fluid whose velocity field satisfies the Navier-Stokes
equation. In each case the equation of motion for the interface height reduces
to an anisotropic Burgers equation. The scaling exponents that describe the
growth and coarsening of the interface are calculated exactly in any dimension
in the case of conserved and nonconserved dynamics. For a fluid-advected
conserved order parameter we determine the exponents, but we are unable to
build a consistent perturbative expansion to support their validity.Comment: 10 RevTeX pages, 2 eps figure

### Dynamics and delocalisation transition for an interface driven by a uniform shear flow

We study the effect of a uniform shear flow on an interface separating the
two broken-symmetry ordered phases of a two-dimensional system with
nonconserved scalar order parameter. The interface, initially flat and
perpendicular to the flow, is distorted by the shear flow. We show that there
is a critical shear rate, \gamma_c, proportional to 1/L^2, (where L is the
system width perpendicular to the flow) below which the interface can sustain
the shear. In this regime the countermotion of the interface under its
curvature balances the shear flow, and the stretched interface stabilizes into
a time-independent shape whose form we determine analytically. For \gamma >
\gamma_c, the interface acquires a non-zero velocity, whose profile is shown to
reach a time-independent limit which we determine exactly. The analytical
results are checked by numerical integration of the equations of motion.Comment: 5 page

- âŠ