4,005 research outputs found
Hierarchical octree and k-d tree grids for 3D radiative transfer simulations
A crucial ingredient for numerically solving the 3D radiative transfer
problem is the choice of the grid that discretizes the transfer medium. Many
modern radiative transfer codes, whether using Monte Carlo or ray tracing
techniques, are equipped with hierarchical octree-based grids to accommodate a
wide dynamic range in densities. We critically investigate two different
aspects of octree grids in the framework of Monte Carlo dust radiative
transfer. Inspired by their common use in computer graphics applications, we
test hierarchical k-d tree grids as an alternative for octree grids. On the
other hand, we investigate which node subdivision-stopping criteria are optimal
for constructing of hierarchical grids. We implemented a k-d tree grid in the
3D radiative transfer code SKIRT and compared it with the previously
implemented octree grid. We also considered three different node
subdivision-stopping criteria (based on mass, optical depth, and density
gradient thresholds). Based on a small suite of test models, we compared the
efficiency and accuracy of the different grids, according to various quality
metrics. For a given set of requirements, the k-d tree grids only require half
the number of cells of the corresponding octree. Moreover, for the same number
of grid cells, the k-d tree is characterized by higher discretization accuracy.
Concerning the subdivision stopping criteria, we find that an optical depth
criterion is not a useful alternative to the more standard mass threshold,
since the resulting grids show a poor accuracy. Both criteria can be combined;
however, in the optimal combination, for which we provide a simple approximate
recipe, this can lead to a 20% reduction in the number of cells needed to reach
a certain grid quality. An additional density gradient threshold criterion can
be added that solves the problem of poorly resolving sharp edges and...
(abridged).Comment: 10 pages, 6 figures. Accepted for publication in A&
AdS/Ricci-flat correspondence
We present a comprehensive analysis of the AdS/Ricci-flat correspondence, a
map between a class of asymptotically locally AdS spacetimes and a class of
Ricci-flat spacetimes. We provide a detailed derivation of the map, discuss a
number of extensions and apply it to a number of important examples, such as
AdS on a torus, AdS black branes and fluids/gravity metrics. In particular, the
correspondence links the hydrodynamic regime of asymptotically flat black
-branes or the Rindler fluid with that of AdS. It implies that this class of
Ricci-flat spacetimes inherits from AdS a generalized conformal symmetry and
has a holographic structure. We initiate the discussion of holography by
analyzing how the map acts on boundary conditions and holographic 2-point
functions.Comment: v3: Minor edits, references added, matches published versio
Legitimating Limburgish: The reproduction of heritage
Limburgish in the Netherlands is another example of a language whose status has risen through protection under European policies of recognition. Formerly viewed as a dialect of Dutch, Diana Camps examines the discourses and practices which legitimate Limburgish as a language in its own right. Beginning with a document analysis of protection policies under the European Charter for Regional and Minority Languages, she notes the role that discourses of heritage play in the legitimation of Limburgish at international and national levels. At the local level of a language classroom, Camps draws on observation data to examine how a teacher of Limburgish legitimates himself and the language through appropriating the discourse of heritage and deploying a discourse of linguistic expertise
Ocean salinity observations with SMOS mission
The purpose of this paper is to present the capabilities of SMOS (Soil Moisture and Ocean Salinity mission) for the global mapping of ocean salinity from space. SMOS has been selected by the European Space Agency as the second Earth Explorer Opportunity with a launch date in June 2005. The sensor embarked on SMOS is MIRAS, a Microwave Imaging Radiometer with Aperture Synthesis. MIRAS works at L-band, in the two-polarisations, and has full polarimetric capability. The measurement of sea surface salinity (SSS) is one of the challenges of SMOS. This paper presents first the scientific requirements for a number of oceanographic applications. The scientific requirements are then translated into instrument accuracy, sensitivity, stability and spatial resolution. Major sources of error in the retrieval of ocean salinity will be addressed through an experimental campaign which is described.Peer ReviewedPostprint (published version
Large and small-scale structures and the dust energy balance problem in spiral galaxies
The interstellar dust content in galaxies can be traced in extinction at
optical wavelengths, or in emission in the far-infrared. Several studies have
found that radiative transfer models that successfully explain the optical
extinction in edge-on spiral galaxies generally underestimate the observed
FIR/submm fluxes by a factor of about three. In order to investigate this
so-called dust energy balance problem, we use two Milky Way-like galaxies
produced by high-resolution hydrodynamical simulations. We create mock optical
edge-on views of these simulated galaxies (using the radiative transfer code
SKIRT), and we then fit the parameters of a basic spiral galaxy model to these
images (using the fitting code FitSKIRT). The basic model includes smooth
axisymmetric distributions along a S\'ersic bulge and exponential disc for the
stars, and a second exponential disc for the dust. We find that the dust mass
recovered by the fitted models is about three times smaller than the known dust
mass of the hydrodynamical input models. This factor is in agreement with
previous energy balance studies of real edge-on spiral galaxies. On the other
hand, fitting the same basic model to less complex input models (e.g. a smooth
exponential disc with a spiral perturbation or with random clumps), does
recover the dust mass of the input model almost perfectly. Thus it seems that
the complex asymmetries and the inhomogeneous structure of real and
hydrodynamically simulated galaxies are a lot more efficient at hiding dust
than the rather contrived geometries in typical quasi-analytical models. This
effect may help explain the discrepancy between the dust emission predicted by
radiative transfer models and the observed emission in energy balance studies
for edge-on spiral galaxies.Comment: 9 pages, 5 figures, accepted for publication in A&
La meteorologÃa en la enseñanza de las ciencias experimentales: una propuesta interdisciplinar e integradora
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