1,951 research outputs found
A lattice mesoscopic model of dynamically heterogeneous fluids
We introduce a mesoscopic three-dimensional Lattice Boltzmann Model which
attempts to mimick the physical features associated with cage effects in
dynamically heterogeneous fluids. To this purpose, we extend the standard
Lattice Boltzmann dynamics with self-consistent constraints based on the
non-local density of the surrounding fluid. The resulting dynamics exhibits
typical features of dynamic heterogeneous fluids, such as non-Gaussian density
distributions and long-time relaxation. Due to its intrinsically parallel
dynamics, and absence of statistical noise, the method is expected to compute
significantly faster than molecular dynamics, Monte Carlo and lattice glass
models.Comment: 4 pages, 3 figures, to appear in Phys. Rev. Let
Notions and subnotions in information structure
Three dimensions can be distinguished in a cross-linguistic account of information structure. First, there is the definition of the focus constituent, the part of the linguistic expression which is subject to some focus meaning. Second and third, there are the focus meanings and the array of structural devices that encode them. In a given language, the expression of focus is facilitated as well as constrained by the grammar within which the focus devices operate. The prevalence of focus ambiguity, the structural inability to make focus distinctions, will thus vary across languages, and within a language, across focus meanings
Luminous radio-quiet sources in the W3(MAIN) cloud core
We have resolved 450 micrometer and 800 micrometer emission from the W3(Main) star forming region into three major peaks, using 8 inch - 14 inch beams with the James Clerk Maxwell Telescope on the summit of Mauna Kea. One of the submillimeter sources is identified with W3 - IRS5, a well-known candidate protostar. However, to our surprise, we find that none of the submillimeter peaks coincides with any of the prominent compact HII regions in the area. We estimate that the three submillimeter sources together contribute 30-50 percent of the total bolometric luminosity of the region, and speculate that the contribution of luminous radio-quiet sources to the total luminosity of HII region/molecular cloud complexes may be larger than is often assumed
Star formation in Perseus
We present a complete survey of current star formation in the Perseus
molecular cloud, made at 850 and 450 micron with SCUBA at the JCMT. Covering 3
deg^2, this submillimetre continuum survey for protostellar activity is second
in size only to that of rho Ophiuchus (Johnstone et al. 2004). Complete above
0.4 msun (5 sigma detection in a 14'' beam), we detect a total of 91 protostars
and prestellar cores. Of these, 80% lie in clusters, representative of star
formation across the Galaxy. Two of the groups of cores are associated with the
young stellar clusters IC348 and NGC1333, and are consistent with a steady or
reduced star formation rate in the last 0.5 Myr, but not an increasing one. In
Perseus, 40--60% of cores are in small clusters (< 50 msun) and isolated
objects, much more than the 10% suggested from infrared studies. Complementing
the dust continuum, we present a C^18O map of the whole cloud at 1' resolution.
The gas and dust show filamentary structure of the dense gas on large and small
scales, with the high column density filaments breaking up into clusters of
cores. The filament mass per unit length is 5--11 msun per 0.1 pc. Given these
filament masses, there is no requirement for substantial large scale flows
along or onto the filaments in order to gather sufficient material for star
formation. We find that the probability of finding a submillimetre core is a
strongly increasing function of column density, as measured by C^18O integrated
intensity, prob(core) proportional to I^3.0. This power law relation holds down
to low column density, suggesting that there is no A_v threshold for star
formation in Perseus, unless all the low-A_v submm cores can be demonstrated to
be older protostars which have begun to lose their natal molecular cloud.Comment: 15 pages, 5 figures, bibtex file scubasf_astroph.bbl, included tex
files SSA-clusters-sorted-tidy.te
Transport Phenomena and Structuring in Shear Flow of Suspensions near Solid Walls
In this paper we apply the lattice-Boltzmann method and an extension to
particle suspensions as introduced by Ladd et al. to study transport phenomena
and structuring effects of particles suspended in a fluid near sheared solid
walls. We find that a particle free region arises near walls, which has a width
depending on the shear rate and the particle concentration. The wall causes the
formation of parallel particle layers at low concentrations, where the number
of particles per layer decreases with increasing distance to the wall.Comment: 14 pages, 14 figure
Fluctuations of elastic interfaces in fluids: Theory and simulation
We study the dynamics of elastic interfaces-membranes-immersed in thermally
excited fluids. The work contains three components: the development of a
numerical method, a purely theoretical approach, and numerical simulation. In
developing a numerical method, we first discuss the dynamical coupling between
the interface and the surrounding fluids. An argument is then presented that
generalizes the single-relaxation time lattice-Boltzmann method for the
simulation of hydrodynamic interfaces to include the elastic properties of the
boundary. The implementation of the new method is outlined and it is tested by
simulating the static behavior of spherical bubbles and the dynamics of bending
waves. By means of the fluctuation-dissipation theorem we recover analytically
the equilibrium frequency power spectrum of thermally fluctuating membranes and
the correlation function of the excitations. Also, the non-equilibrium scaling
properties of the membrane roughening are deduced, leading us to formulate a
scaling law describing the interface growth, W^2(L,T)=L^3 g[t/L^(5/2)], where
W, L and T are the width of the interface, the linear size of the system and
the temperature respectively, and g is a scaling function. Finally, the
phenomenology of thermally fluctuating membranes is simulated and the frequency
power spectrum is recovered, confirming the decay of the correlation function
of the fluctuations. As a further numerical study of fluctuating elastic
interfaces, the non-equilibrium regime is reproduced by initializing the system
as an interface immersed in thermally pre-excited fluids.Comment: 15 pages, 11 figure
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