12,277 research outputs found
Lava Dome Growth and Evolution with an Independently Deformable Talus
Subduction zone volcanism occurs due to partial melting from subducting slabs, which generally results in high-viscosity magma containing a large amount of volatiles. Such volcanic eruptions can form a lava dome, from which collapse events are a common and important part of their evolution. Collapse events can have devastating consequences; in the form of block and ash avalanche deposits, pyroclastic flows, surges and the generation of tsunamis if they enter the sea. In addition to this, once a mass of lava dome has been removed during a collapse event, this results in a drop in pressure in the remaining volatile-rich magma, which may then erupt explosively. The internal structure of a lava dome and the extent of the surrounding carapace/talus is unknown, but likely to be critical for hazard assessment, and is the motivation for this research. Presented is a computational model for the growth and evolution of an endogenous lava dome, including an independently deformable talus, using the Finite Element Method. Dome growth is modelled to occur under two time-scales: continuous dome expansion via the addition of new magma into the molten core interior, and relatively instantaneous talus readjustments due to rockfalls and the disintegration of the solid surface. The continuous deformation of the dome is modelled as a fluid with a yield strength in the talus region. While talus deformation is modelled as a granular material that rests at angles below its angle of repose. Both surfaces, dome and core/talus interface, are displaced using the level-set method. The model is axi-symmetric and assumes that solidification, and therefore talus growth, occurs due to gas exsolution which promotes crystallisation, rather than from surface cooling, appropriate for intermediate composition lava flows. For the purpose of this paper we consider and apply the model to the Soufriere Hills Volcano, Montserrat, but the techniques used are generic, allowing the model to be applied to other dome forming eruptions. The model provides information on the shape of the dome, with the growth and extent of the talus and core found to be predominantly governed by the lava extrusion rate, degree of solidification (i.e. a solidus pressure), the friction angle associated with the talus, and lava dome viscosity
The environment and host haloes of the brightest z~6 Lyman-break galaxies
By studying the large-scale structure of the bright high-redshift Lyman-break
galaxy (LBG) population it is possible to gain an insight into the role of
environment in galaxy formation physics in the early Universe. We measure the
clustering of a sample of bright (-22.7<M_UV<-21.125) LBGs at z~6 and use a
halo occupation distribution (HOD) model to measure their typical halo masses.
We find that the clustering amplitude and corresponding HOD fits suggests that
these sources are highly biased (b~8) objects in the densest regions of the
high-redshift Universe. Coupled with the observed rapid evolution of the number
density of these objects, our results suggest that the shape of high luminosity
end of the luminosity function is related to feedback processes or dust
obscuration in the early Universe - as opposed to a scenario where these
sources are predominantly rare instances of the much more numerous M_UV ~ -19
population of galaxies caught in a particularly vigorous period of star
formation. There is a slight tension between the number densities and
clustering measurements, which we interpret this as a signal that a refinement
of the model halo bias relation at high redshifts or the incorporation of
quasi-linear effects may be needed for future attempts at modelling the
clustering and number counts. Finally, the difference in number density between
the fields (UltraVISTA has a surface density ~1.8 times greater than UDS) is
shown to be consistent with the cosmic variance implied by the clustering
measurements.Comment: 19 pages, 8 figures, accepted MNRAS 23rd March 201
Spray nozzle designs for agricultural aviation applications
Techniques of generating monodisperse sprays and information concerning chemical liquids used in agricultural aviation are surveyed. The periodic dispersion of liquid jet, the spinning disk method, and ultrasonic atomization are the techniques discussed. Conceptually designed spray nozzles for generating monodisperse sprays are assessed. These are based on the classification of the drops using centrifugal force, on using two opposing liquid laden air jets, and on operating a spinning disk at an overloaded flow. Performance requirements for the designs are described and estimates of the operational characteristics are presented
Strong nonlinear optical response of graphene flakes measured by four-wave mixing
We present the first experimental investigation of nonlinear optical
properties of graphene flakes. We find that at near infrared frequencies a
graphene monolayer exhibits a remarkably high third-order optical nonlinearity
which is practically independent of the wavelengths of incident light. The
nonlinear optical response can be utilized for imaging purposes, with image
contrasts of graphene which are orders of magnitude higher than those obtained
using linear microscopy.Comment: 4 pages, 5 figure
The bearable lightness of being
How are philosophical questions about what kinds of things there are to be understood and how are they to be answered? This paper defends broadly Fregean answers to these questions. Ontological categories-such as object, property, and relation-are explained in terms of a prior logical categorization of expressions, as singular terms, predicates of varying degree and level, etc. Questions about what kinds of object, property, etc., there are are, on this approach, reduce to questions about truth and logical form: for example, the question whether there are numbers is the question whether there are true atomic statements in which expressions function as singular terms which, if they have reference at all, stand for numbers, and the question whether there are properties of a given type is a question about whether there are meaningful predicates of an appropriate degree and level. This approach is defended against the objection that it must be wrong because makes what there depend on us or our language. Some problems confronting the Fregean approach-including Frege's notorious paradox of the concept horse-are addressed. It is argued that the approach results in a modest and sober deflationary understanding of ontological commitments
Space Representation of Stochastic Processes with Delay
We show that a time series evolving by a non-local update rule with two different delays can be mapped onto a local
process in two dimensions with special time-delayed boundary conditions
provided that and are coprime. For certain stochastic update rules
exhibiting a non-equilibrium phase transition this mapping implies that the
critical behavior does not depend on the short delay . In these cases, the
autocorrelation function of the time series is related to the critical
properties of directed percolation.Comment: 6 pages, 8 figure
Delayed feedback as a means of control of noise-induced motion
Time--delayed feedback is exploited for controlling noise--induced motion in
coherence resonance oscillators. Namely, under the proper choice of time delay,
one can either increase or decrease the regularity of motion. It is shown that
in an excitable system, delayed feedback can stabilize the frequency of
oscillations against variation of noise strength. Also, for fixed noise
intensity, the phenomenon of entrainment of the basic oscillation period by the
delayed feedback occurs. This allows one to steer the timescales of
noise-induced motion by changing the time delay.Comment: 4 pages, 4 figures. In the replacement file Fig. 2 and Fig. 4(b),(d)
were amended. The reason is numerical error found, that affected the
quantitative estimates of correlation time, but did not affect the main
messag
Complete chaotic synchronization in mutually coupled time-delay systems
Complete chaotic synchronization of end lasers has been observed in a line of
mutually coupled, time-delayed system of three lasers, with no direct
communication between the end lasers. The present paper uses ideas from
generalized synchronization to explain the complete synchronization in the
presence of long coupling delays, applied to a model of mutually coupled
semiconductor lasers in a line. These ideas significantly simplify the analysis
by casting the stability in terms of the local dynamics of each laser. The
variational equations near the synchronization manifold are analyzed, and used
to derive the synchronization condition that is a function of the parameters.
The results explain and predict the dependence of synchronization on various
parameters, such as time-delays, strength of coupling and dissipation. The
ideas can be applied to understand complete synchronization in other chaotic
systems with coupling delays and no direct communication between synchronized
sub-systems.Comment: 22 pages, 6 figure
Variable-delay feedback control of unstable steady states in retarded time-delayed systems
We study the stability of unstable steady states in scalar retarded
time-delayed systems subjected to a variable-delay feedback control. The
important aspect of such a control problem is that time-delayed systems are
already infinite-dimensional before the delayed feedback control is turned on.
When the frequency of the modulation is large compared to the system's
dynamics, the analytic approach consists of relating the stability properties
of the resulting variable-delay system with those of an analogous distributed
delay system. Otherwise, the stability domains are obtained by a numerical
integration of the linearized variable-delay system. The analysis shows that
the control domains are significantly larger than those in the usual
time-delayed feedback control, and that the complexity of the domain structure
depends on the form and the frequency of the delay modulation.Comment: 13 pages, 8 figures, RevTeX, accepted for publication in Physical
Review
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