348,103 research outputs found
Large eddy simulation of a lifted ethylene flame using a dynamic nonequilibrium model for subfilter scalar variance and dissipation rate
Accurate prediction of nonpremixed turbulent combustion using large eddy simulation(LES) requires detailed modeling of the mixing between fuel and oxidizer at scales finer than the LES filter resolution. In conserved scalar combustion models, the small scale mixing process is quantified by two parameters, the subfilter scalar variance and the subfilter scalar dissipation rate. The most commonly used models for these quantities assume a local equilibrium exists between production and dissipation of variance. Such an assumption has limited validity in realistic, technically relevant flow configurations. However, nonequilibrium models for variance and dissipation rate typically contain a model coefficient whose optimal value is unknown a priori for a given simulation. Furthermore, conventional dynamic procedures are not useful for estimating the value of this coefficient. In this work, an alternative dynamic procedure based on the transport equation for subfilter scalar variance is presented, along with a robust conditional averaging approach for evaluation of themodel coefficient. This dynamic nonequilibrium modeling approach is used for simulation of a turbulent lifted ethylene flame, previously studied using DNS by Yoo et al. (Proc. Comb. Inst., 2011). The predictions of the new model are compared to those of a static nonequilibrium modeling approach using an assumed model coefficient, as well as those of the equilibrium modeling approach. The equilibrium models are found to systematically underpredict both subfilter scalar variance and dissipation rate. Use of the dynamic procedure is shown to increase the accuracy of the nonequilibrium modeling approach. However, numerical errors that arise as a consequence of grid-based implicit filtering appear to degrade the accuracy of all three modeling options. Thus, while these results confirm the usefulness of the new dynamic model, they also show that the quality of subfilter model predictions depends on several factors extrinsic to the formulation of the subfilter model itself
Center to limb observations and modeling of the Ca I 4227 A line
The observed center-to-limb variation (CLV) of the scattering polarization in
different lines of the Second Solar Spectrum can be used to constrain the
height variation of various atmospheric parameters, in particular the magnetic
fields via the Hanle effect. Here we attempt to model non-magnetic CLV
observations of the profiles of the Ca I 4227 A line recorded with the
ZIMPOL-3 at IRSOL. For modeling, we use the polarized radiative transfer with
partial frequency redistribution with a number of realistic 1-D model
atmospheres. We find that all the standard FAL model atmospheres, used by us,
fail to simultaneously fit the observed (, ) at all the limb distances
(). However, an attempt is made to find a single model which can provide a
fit at least to the CLV of the observed instead of a simultaneous fit to
the (, ) at all . To this end we construct a new 1-D model by
combining two of the standard models after modifying their temperature
structures in the appropriate height ranges. This new combined model closely
reproduces the observed at all the , but fails to reproduce the
observed rest intensity at different . Hence we find that no single 1-D
model atmosphere succeeds in providing a good representation of the real Sun.
This failure of 1-D models does not however cause an impediment to the magnetic
field diagnostic potential of the Ca I 4227 A line. To demonstrate this we
deduce the field strength at various positions without invoking the use
of radiative transfer.Comment: 20 pages, 10 figures, Accepted for publication in Ap
More on the determination of the coronal heating function from Yohkoh data
Two recent works have analyzed a solar large and steady coronal loop observed
with Yohkoh/SXT in two filter passbands to infer the distribution of the
heating along it. Priest et al. (2000) modelled the distribution of the
temperature obtained from filter ratio method with an analytical approach, and
concluded that the heating was uniform along the loop. Aschwanden (2001) found
that a uniform heating led to an unreasonably large plasma column depth along
the line of sight, and, using a two component loop model, that a
footpoint-heated model loop (with a minor cool component) yields more
acceptable physical solutions. We revisit the analysis of the same loop system,
considering conventional hydrostatic single loop models with uniformly
distributed heating, and with heating localized at the footpoints and at the
apex, and an unstructured background contribution extrapolated from the region
below the analyzed loop. The flux profiles synthesized from the loop models
have been compared in detail with those observed in both filter passbands with
and without background subtraction; we find that background-subtracted data are
fitted with acceptable statistical significance by a model of relatively hot
loop (~3.7 MK) heated at the apex, with a column depth ~1/10 of the loop
length. In discussing our results, we put warnings on the importance of aspects
of data analysis and modeling, such as considering diffuse background emission
in complex loop regions.Comment: 17 pages, 4 figures, refereed pape
Systematic Study of Accuracy of Wall-Modeled Large Eddy Simulation using Uncertainty Quantification Techniques
The predictive accuracy of wall-modeled large eddy simulation is studied by
systematic simulation campaigns of turbulent channel flow. The effect of wall
model, grid resolution and anisotropy, numerical convective scheme and
subgrid-scale modeling is investigated. All of these factors affect the
resulting accuracy, and their action is to a large extent intertwined. The wall
model is of the wall-stress type, and its sensitivity to location of velocity
sampling, as well as law of the wall's parameters is assessed. For efficient
exploration of the model parameter space (anisotropic grid resolution and wall
model parameter values), generalized polynomial chaos expansions are used to
construct metamodels for the responses which are taken to be measures of the
predictive error in quantities of interest (QoIs). The QoIs include the mean
wall shear stress and profiles of the mean velocity, the turbulent kinetic
energy, and the Reynolds shear stress. DNS data is used as reference. Within
the tested framework, a particular second-order accurate CFD code (OpenFOAM),
the results provide ample support for grid and method parameters
recommendations which are proposed in the present paper, and which provide good
results for the QoIs. Notably, good results are obtained with a grid with
isotropic (cubic) hexahedral cells, with cells per , where
is the channel half-height (or thickness of the turbulent boundary
layer). The importance of providing enough numerical dissipation to obtain
accurate QoIs is demonstrated. The main channel flow case investigated is , but extension to a wide range of -numbers is
considered. Use of other numerical methods and software would likely modify
these recommendations, at least slightly, but the proposed framework is fully
applicable to investigate this as well
Modeling the Halpha line emission around classical T Tauri stars using magnetospheric accretion and disk wind models
Spectral observations of classical T Tauri stars show a wide range of line
profiles, many of which reveal signs of matter inflow and outflow. Halpha is
the most commonly observed line profile due to its intensity, and it is highly
dependent on the characteristics of the surrounding environment of these stars.
Our aim is to analyze how the Halpha line profile is affected by the various
parameters of our model which contains both the magnetospheric and disk wind
contributions to the Halpha flux. We used a dipolar axisymmetric stellar
magnetic field to model the stellar magnetosphere and a modified Blandford &
Payne model was used in our disk wind region. A three-level atom with continuum
was used to calculate the required Hydrogen level populations. We use the
Sobolev approximation and a ray-by-ray method to calculate the integrated line
profile. Through an extensive study of the model parameter space, we have
investigated the contribution of many of the model parameters on the calculated
line profiles. Our results show that the Halpha line is strongly dependent on
the densities and temperatures inside the magnetosphere and the disk wind
region. The bulk of the flux comes, most of the time, from the magnetospheric
component for standard classical T Tauri stars parameters, but the disk wind
contribution becomes more important as the mass accretion rate, the
temperatures and densities inside the disk wind increase. We have also found
that most of the disk wind contribution to the Halpha line is emitted at the
innermost region of the disk wind. Models that take into consideration both
inflow and outflow of matter are a necessity to fully understand and describe
classical T Tauri stars.Comment: 15 pages, 9 figures, accepted for publication in Astronomy &
Astrophysics. Revised version with English correction
Modeling X-ray Emission Line Profiles from Massive Star Winds - A Review
The Chandra and XMM-Newton X-ray telescopes have led to numerous advances in
the study and understanding of astrophysical X-ray sources. Particularly
important has been the much increased spectral resolution of modern X-ray
instrumentation. Wind-broadened emission lines have been spectroscopically
resolved for many massive stars. This contribution reviews approaches to the
modeling of X-ray emission line profile shapes from single stars, including
smooth winds, winds with clumping, optically thin versus thick lines, and the
effect of a radius-dependent photoabsorption coefficient.Comment: to appear in Advances in Space Researc
Deep Near-Infrared Observations of L1014: Revealing the nature of the core and its embedded source
Recently, the Spitzer Space Telescope discovered L1014-IRS, a mid-infrared
source with protostellar colors, toward the heretofore "starless" core L1014.
We present deep near-infrared observations that show a scattered light nebula
extending from L1014-IRS. This nebula resembles those typically associated with
protostars and young stellar objects, tracing envelope cavities presumably
evacuated by an outflow. The northern lobe of the nebula has an opening angle
of ~100 degrees, while the southern lobe is barely detected. Its morphology
suggests that the bipolar cavity and inferred protostellar disk is not inclined
more than 30 degrees from an edge-on orientation. The nebula extends at least
8" from the source at Ks, strongly suggesting that L1014-IRS is embedded within
L1014 at a distance of 200 pc rather than in a more distant cloud associated
with the Perseus arm at 2.6 kpc. In this case, the apparently low luminosity of
L1014-IRS, 0.090 Lsun, is consistent with it having a substellar mass. However,
if L1014-IRS is obscured by a circumstellar disk, its luminosity and inferred
mass may be greater. Using near-infrared colors of background stars, we
investigate characteristics of the L1014 molecular cloud core. We determine a
mass of 3.6 Msun for regions of the core with Av > 2 magnitudes. A comparison
of the radial extinction profile of L1014 with other cores suggests that L1014
may be among the most centrally condensed cores known, perhaps indicative of
the earliest stages of brown dwarf or star formation processes.Comment: Replacement includes revision to mass of core. 22 pages, 6 figures.
Accepted by Ap
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