715 research outputs found
A study of shock impacts and vibration dose values onboard highspeed marine craft
The shocks and impacts encountered on small high-speed craft exceed the limits set for safe working practice according to current standards. European legislation regarding the exposure to vibration will have far reaching effects on the operators of such craft with respect to the safety of their employees. This paper sets out to highlight the vibration dose values that can be expected during typical transits onboard high-speed craft and attempts to clarify some of the controversy currently surrounding vibration dose measurement in such circumstances. In order to relate vibration dosage to the impacts encountered and to boat motion, an algorithm was developed that identifies the timing and magnitude of impacts
Cosmic String Cusps with Small-Scale Structure: Their Forms and Gravitational Waveforms
We present a method for the introduction of small-scale structure into
strings constructed from products of rotation matrices. We use this method to
illustrate a range of possibilities for the shape of cusps that depends on the
properties of the small-scale structure. We further argue that the presence of
structure at cusps under most circumstances leads to the formation of loops at
the size of the smallest scales. On the other hand we show that the
gravitational waveform of a cusp remains generally unchanged; the primary
effect of small-scale structure is to smooth out the sharp waveform emitted in
the direction of cusp motion.Comment: RevTeX, 8 pages. Replaced with version accepted for publication by
PR
Evolution of cosmic string configurations
We extend and develop our previous work on the evolution of a network of
cosmic strings. The new treatment is based on an analysis of the probability
distribution of the end-to-end distance of a randomly chosen segment of
left-moving string of given length. The description involves three distinct
length scales: , related to the overall string density, , the
persistence length along the string, and , describing the small-scale
structure, which is an important feature of the numerical simulations that have
been done of this problem. An evolution equation is derived describing how the
distribution develops in time due to the combined effects of the universal
expansion, of intercommuting and loop formation, and of gravitational
radiation. With plausible assumptions about the unknown parameters in the
model, we confirm the conclusions of our previous study, that if gravitational
radiation and small-scale structure effects are neglected, the two dominant
length scales both scale in proportion to the horizon size. When the extra
effects are included, we find that while and grow,
initially does not. Eventually, however, it does appear to scale, at a much
lower level, due to the effects of gravitational back-reaction.Comment: 61 pages, requires RevTex v3.0, SUSSEX-TH-93/3-4,
IMPERIAL/TP/92-93/4
Scaling property and peculiar velocity of global monopoles
We investigate the scaling property of global monopoles in the expanding
universe. By directly solving the equations of motion for scalar fields, we
follow the time development of the number density of global monopoles in the
radiation dominated (RD) universe and the matter dominated (MD) universe. It is
confirmed that the global monopole network relaxes into the scaling regime and
the number per hubble volume is a constant irrespective of the cosmic time. The
number density of global monopoles is given by during the RD era and during the MD era. We also examine the peculiar velocity of global
monopoles. For this purpose, we establish a method to measure the peculiar
velocity by use of only the local quantities of the scalar fields. It is found
that during the RD era and during
the MD era. By use of it, a more accurate analytic estimate for the number
density of global monopoles is obtained.Comment: 17 pages, 8 figures, to appear in Phys. Rev.
Cosmic Rays From Cosmic Strings
It has been speculated that cosmic string networks could produce ultra-high
energy cosmic rays as a by-product of their evolution. By making use of recent
work on the evolution of such networks, it will be shown that the flux of
cosmic rays from cosmologically useful, that is GUT scale strings, is too small
to be used as a test for strings with any foreseeable technology.Comment: 11, Imperial/TP/93-94/2
Red imported fire ant impacts on the endangered Florida grasshopper sparrow
Red imported fire ants (Solenopsis invicta) invaded peninsular Florida more
than 30 years ago. Highlands and Polk counties, Florida, were probably first invaded in
the late 1960s. Since then, fire ants have continued both to spread and to increase in
abundance. Experimental evidence has shown that red imported fire ants may have a
detrimental impact on native species, both invertebrate and vertebrate, and that these
impacts may be both direct and indirect. This segment of research was designed to
determine if fire ants have a negative impact on Florida grasshopper sparrow
(Ammodramus savannarumfloridanus) populations at Avon Park Air Force Bombing
Range, Florida. The sampling reported herein was conducted during June and October,
1997, and all analyses are restricted to data collected during those periods. As assessed
by baits, fire ants were dominant on about half the sites, and absent from others. In
particular, OQ Range sites tended to have fewer fire ants present, while Delta Trail sites
were more heavily infested, although there were exceptions. June fire ant abundance was
strongly negatively correlated with both native ant abundance (r = -0.743, P = 0.006) and
native ant species richness (r = -0.730, P = 0.007). October fire ant abundance was
strongly negatively correlated with both native ant abundance (r = -0.690, P = 0.013) and
native ant species richness (r = -0.736, P = 0.006). The patterns of fire ant abundance as
assessed by pitfalls were very similar to those determined from bait sampling. June fire
ant abundance was negatively correlated (p < 0.05) with native ant richness, but other
June comparisons were not statistically significant (P > 0.05). October fire ant
abundance was negatively correlated (P < 0.05) with native ant richness and abundance,
but other comparisons were not statistically significant (P > 0.05). There was no
significant correlation between overall insect biomass as assessed by light traps and fire
ant abundance as assessed by baits or pitfalls. Total biomass varied considerably among
the two sample periods because of changes in overall insect abundance during different
seasons. There was a negative spatial correspondence between fire ants and native
invertebrates. Over most of the intensive study areas, there was a negative spatial
relationship between fire ants and the abundance of native invertebrates. Over about
50% of the intensive study areas, there was a negative spatial relationship between fire
ants and the abundance of Florida grasshopper sparrows, although the relationship was
not as strong as that between fire ants and native invertebrates. Fire ant and native
invertebrates were negatively correlated at grasshopper sparrow count locations (r =0.347,
P = 0.03). A multiple regression model was fit to the data, using fire ants and
native invertebrates as independent variables, and grasshopper sparrow 100-m population
estimates (n = 39) as the dependent variable. The influence of fire ants on grasshopper
sparrows was negative while the influence of native invertebrates was positive.
However, the overall model, while suggestive, was not significant (r = 0.304, P = 0.17).
Fire ant abundance was a better (negative) predictor of sparrow populations (P = 0.13)
than was invertebrate abundance (P = 0.59). The overall model and influence of fire ants
on sparrow populations was suggestive of a negative influence warranting analyses of
data for 1998 and 1999. (Document has 93 pages
On the size of the smallest scales in cosmic string networks
We present a method for the calculation of the gravitational back reaction
cutoff on the smallest scales of cosmic string networks taking into account
that not all modes on strings interact with all other modes. This results in a
small scale structure cutoff that is sensitive to the initial spectrum of
perturbations present on strings. From a simple model, we compute the cutoffs
in radiation- and matter-dominated universes.Comment: 4 pages, revte
Cosmological Evolution of Global Monopoles
We investigate the cosmological evolution of global monopoles in the
radiation dominated (RD) and matter dominated (MD) universes by numerically
solving field equations of scalar fields. It is shown that the global monopole
network relaxes into the scaling regime, unlike the gauge monopole network. The
number density of global monopoles is given by during the RD era and during the MD
era. Thus, we have confirmed that density fluctuations produced by global
monopoles become scale invariant and are given by during the RD (MD) era, where is the breaking
scale of the symmetry.Comment: 6 pages, 2 figures, to appear in Phys. Rev. D (R
Comment on "Quantum diffusion of 3-He impurities in solid 4- He"
In this comment I show that the experimental data on quantum diffusion of
3-He impurities in solid 4-He can be explained using the adopted quasiparticle
theory. The contention by E.G. Kisvarsanyi and N.S. Sullivan (KS) in Phys.Rev.B
v. 48, 16557 (1993) as well as in their Reply (ibid. v. 55, 3989 (1997)) to the
Grigor'ev's Comment (Phys.Rev. B v. 55, 3987 (1997)) that "Pushkarov's theory
of phonon scattering fails to fit the data by very large factors" is groundless
and may result from their bad arithmetical error. This means that the
phonon-impurity scattering mechanism of diffusion is consistent with experiment
and its neglecting by KS makes their results questionable.Comment: RevTex, 5 pages, no figures, to be published in Phys.Rev.
Lagrangian evolution of global strings
We establish a method to trace the Lagrangian evolution of extended objects
consisting of a multicomponent scalar field in terms of a numerical calculation
of field equations in three dimensional Eulerian meshes. We apply our method to
the cosmological evolution of global strings and evaluate the energy density,
peculiar velocity, Lorentz factor, formation rate of loops, and emission rate
of Nambu-Goldstone (NG) bosons. We confirm the scaling behavior with a number
of long strings per horizon volume smaller than the case of local strings by a
factor of 10. The strategy and the method established here are
applicable to a variety of fields in physics.Comment: 5 pages, 2 figure
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