228 research outputs found
A Continuum Description of Rarefied Gas Dynamics (I)--- Derivation From Kinetic Theory
We describe an asymptotic procedure for deriving continuum equations from the
kinetic theory of a simple gas. As in the works of Hilbert, of Chapman and of
Enskog, we expand in the mean flight time of the constituent particles of the
gas, but we do not adopt the Chapman-Enskog device of simplifying the formulae
at each order by using results from previous orders. In this way, we are able
to derive a new set of fluid dynamical equations from kinetic theory, as we
illustrate here for the relaxation model for monatomic gases. We obtain a
stress tensor that contains a dynamical pressure term (or bulk viscosity) that
is process-dependent and our heat current depends on the gradients of both
temperature and density. On account of these features, the equations apply to a
greater range of Knudsen number (the ratio of mean free path to macroscopic
scale) than do the Navier-Stokes equations, as we see in the accompanying
paper. In the limit of vanishing Knudsen number, our equations reduce to the
usual Navier-Stokes equations with no bulk viscosity.Comment: 16 page
Radiative Transfer in Star Formation: Testing FLD and Hybrid Methods
We perform a comparison between two radiative transfer algorithms commonly
employed in hydrodynamical calculations of star formation: grey flux limited
diffusion and the hybrid scheme, in addition we compare these algorithms to
results from the Monte-Carlo radiative transfer code MOCASSIN. In disc like
density structures the hybrid scheme performs significantly better than the FLD
method in the optically thin regions, with comparable results in optically
thick regions. In the case of a forming high mass star we find the FLD method
significantly underestimates the radiation pressure by a factor of ~100.Comment: 4 Pages; to appear in the proceedings of 'The Labyrinth of Star
Formation', Crete, 18-22 June 201
Vanishing viscosity limits for the degenerate lake equations with Navier boundary conditions
The paper is concerned with the vanishing viscosity limit of the
two-dimensional degenerate viscous lake equations when the Navier slip
conditions are prescribed on the impermeable boundary of a simply connected
bounded regular domain. When the initial vorticity is in the Lebesgue space
with , we show the degenerate viscous lake equations
possess a unique global solution and the solution converges to a corresponding
weak solution of the inviscid lake equations. In the special case when the
vorticity is in , an explicit convergence rate is obtained
Assessing the risk of climate change for buildings: A comparison between multi-year and probabilistic reference year simulations
Copyright © 2011 Elsevier. NOTICE: this is the author’s version of a work that was accepted for publication in Building and Environment . Changes resulting from the publishing process, such as peer review, editing, corrections, structural formatting, and other quality control mechanisms may not be reflected in this document. Changes may have been made to this work since it was submitted for publication. A definitive version was subsequently published in Building and Environment Vol. 46 (2011), DOI: 10.1016/j.buildenv.2010.12.018Given a changing climate, there is a need to provide data for future years so that practicing engineers can investigate the impact of climate change on particular designs and examine any risk the client might be exposed to. In addition, such files are of use to building scientists in developing generic solutions to problems such as elevated internal temperatures and poor thermal comfort. With the release of the UK Climate Projections (UKCP09) [1], and the publication of a methodology for the creation of probabilistic future reference years using the UKCP09 weather generator [2], it is possible to model future building performance. However, the collapse of the distribution of possibilities inherent in the UKCP09 method into a single reference year or a small number of reference years, potentially means the loss of most of the information about the potential range of the response of the building and of the risk occupants might be subject to. In this paper we model for the first time the internal conditions and energy use of a building with all 3000 example years produced by the UKCP09 weather generator in an attempt to study the full range of response and risk. The resultant histograms and cumulative distribution functions are then used to examine whether single reference years can be used to answer questions about response and risk under a changing climate, or whether a more probabilistic approach is unavoidable
Fluctuation-Response Relations for Multi-Time Correlations
We show that time-correlation functions of arbitrary order for any random
variable in a statistical dynamical system can be calculated as higher-order
response functions of the mean history of the variable. The response is to a
``control term'' added as a modification to the master equation for statistical
distributions. The proof of the relations is based upon a variational
characterization of the generating functional of the time-correlations. The
same fluctuation-response relations are preserved within moment-closures for
the statistical dynamical system, when these are constructed via the
variational Rayleigh-Ritz procedure. For the 2-time correlations of the
moment-variables themselves, the fluctuation-response relation is equivalent to
an ``Onsager regression hypothesis'' for the small fluctuations. For
correlations of higher-order, there is a new effect in addition to such linear
propagation of fluctuations present instantaneously: the dynamical generation
of correlations by nonlinear interaction of fluctuations. In general, we
discuss some physical and mathematical aspects of the {\it Ans\"{a}tze}
required for an accurate calculation of the time correlations. We also comment
briefly upon the computational use of these relations, which is well-suited for
automatic differentiation tools. An example will be given of a simple closure
for turbulent energy decay, which illustrates the numerical application of the
relations.Comment: 28 pages, 1 figure, submitted to Phys. Rev.
Critical Protoplanetary Core Masses in Protoplanetary Disks and the Formation of Short-Period Giant Planets
We study a solid protoplanetary core of 1-10 earth masses migrating through a
disk. We suppose the core luminosity is generated as a result of planetesimal
accretion and calculate the structure of the gaseous envelope assuming
equilibrium. This is a good approximation when the core mass is less than the
critical value, M_{crit}, above which rapid gas accretion begins. We model the
structure of the protoplanetary nebula as an accretion disk with constant
\alpha. We present analytic fits for the steady state relation between disk
surface density and mass accretion rate as a function of radius r. We calculate
M_{crit} as a function of r, gas accretion rate through the disk, and
planetesimal accretion rate onto the core \dot{M}. For a fixed \dot{M},
M_{crit} increases inwards, and it decreases with \dot{M}. We find that \dot{M}
onto cores migrating inwards in a time 10^3-10^5 yr at 1 AU is sufficient to
prevent the attainment of M_{crit} during the migration process. Only at small
radii where planetesimals no longer exist can M_{crit} be attained. At small
radii, the runaway gas accretion phase may become longer than the disk lifetime
if the core mass is too small. However, massive cores can be built-up through
the merger of additional incoming cores on a timescale shorter than for in situ
formation. Therefore, feeding zone depletion in the neighborhood of a fixed
orbit may be avoided. Accordingly, we suggest that giant planets may begin to
form early in the life of the protostellar disk at small radii, on a timescale
that may be significantly shorter than for in situ formation. (abridged)Comment: 24 pages (including 9 figures), LaTeX, uses emulateapj.sty, to be
published in ApJ, also available at http://www.ucolick.org/~ct/home.htm
Random Walks on a Fluctuating Lattice: A Renormalization Group Approach Applied in One Dimension
We study the problem of a random walk on a lattice in which bonds connecting
nearest neighbor sites open and close randomly in time, a situation often
encountered in fluctuating media. We present a simple renormalization group
technique to solve for the effective diffusive behavior at long times. For
one-dimensional lattices we obtain better quantitative agreement with
simulation data than earlier effective medium results. Our technique works in
principle in any dimension, although the amount of computation required rises
with dimensionality of the lattice.Comment: PostScript file including 2 figures, total 15 pages, 8 other figures
obtainable by mail from D.L. Stei
Nonequilibrium corrections in the pressure tensor due to an energy flux
The physical interpretation of the nonequilibrium corrections in the pressure
tensor for radiation submitted to an energy flux obtained in some previous
works is revisited. Such pressure tensor is shown to describe a moving
equilibrium system but not a real nonequilibrium situation.Comment: 4 pages, REVTeX, Brief Report to appear in PRE Dec 9
Bispectrum speckle interferometry of the massive protostellar outflow source IRAS 23151+5912
We present bispectrum speckle interferometry of the massive protostellar
object IRAS 23151+5912 in the near-infrared K' band. The reconstructed image
shows the diffuse nebulosity north-east of two point-like sources in
unprecedented detail. The comparison of our near-infrared image with mm
continuum and CO molecular line maps shows that the brighter of the two point
sources lies near the center of the mm peak, indicating that it is a high-mass
protostar. The nebulosity coincides with the blue-shifted molecular outflow
component. The most prominent feature in the nebulosity is a bow-shock-like
arc. We assume that this feature is associated with a precessing jet which has
created an inward-pointed cone in the swept-up material. We present numerical
jet simulations that reproduce this and several other features observed in our
speckle image of the nebulosity. Our data also reveal a linear structure
connecting the central point source to the extended diffuse nebulosity. This
feature may represent the innermost part of a jet that drives the strong
molecular outflow (PA ~80 degr) from IRAS 23151+5912. With the aid of radiative
transfer calculations, we demonstrate that, in general, the observed inner
structures of the circumstellar material surrounding high-mass stars are
strongly influenced by the orientation and symmetry of the bipolar cavity.Comment: accepted by Astronomy & Astrophysics; preprints with high-resolution
images can be obtained from
http://www.mpifr-bonn.mpg.de/staff/tpreibis/iras23151.htm
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