106 research outputs found
A radiation-hydrodynamics scheme valid from the transport to the diffusion limit
We present in this paper the numerical treatment of the coupling between
hydrodynamics and radiative transfer. The fluid is modeled by classical
conservation laws (mass, momentum and energy) and the radiation by the grey
moment system. The scheme introduced is able to compute accurate
numerical solution over a broad class of regimes from the transport to the
diffusive limits. We propose an asymptotic preserving modification of the HLLE
scheme in order to treat correctly the diffusion limit. Several numerical
results are presented, which show that this approach is robust and have the
correct behavior in both the diffusive and free-streaming limits. In the last
numerical example we test this approach on a complex physical case by
considering the collapse of a gas cloud leading to a proto-stellar structure
which, among other features, exhibits very steep opacity gradients.Comment: 29 pages, submitted to Journal of Computational physic
Deterministic Partial Differential Equation Model for Dose Calculation in Electron Radiotherapy
Treatment with high energy ionizing radiation is one of the main methods in
modern cancer therapy that is in clinical use. During the last decades, two
main approaches to dose calculation were used, Monte Carlo simulations and
semi-empirical models based on Fermi-Eyges theory. A third way to dose
calculation has only recently attracted attention in the medical physics
community. This approach is based on the deterministic kinetic equations of
radiative transfer. Starting from these, we derive a macroscopic partial
differential equation model for electron transport in tissue. This model
involves an angular closure in the phase space. It is exact for the
free-streaming and the isotropic regime. We solve it numerically by a newly
developed HLLC scheme based on [BerCharDub], that exactly preserves key
properties of the analytical solution on the discrete level. Several numerical
results for test cases from the medical physics literature are presented.Comment: 20 pages, 7 figure
Improving the analytical assessment of fish stocks by providing parameters of data quality via InterCatch
The last multiannual Community program for the collection, management and use of data in the fisheries sector (Commission Decision 2008/949/EC) stated the provision of precision levels and sampling intensities of the estimates at national level. However, the unequal compliance of this standard has hindered its application in stock assessment and the consequent scientific advice. The cost-benefit analysis of a sampling program, besides addressing logistical and economic constraints, should deepen the potential of the tools currently available. This article proposes to test the calculation, provision and use in stock assessment of extensively collected precision parameters. First, sampling intensities and coefficients of variation of fisheries-dependent parameters are calculated using the COST software, a statistical tool specifically designed to quantify uncertainty in marine sampled data. Secondly, alternative ways are explored to provide precision parameters to the stock assessment coordinators by using InterCatch, the existing ICES web-based system to submit national data and compile international catch matrices. Finally, the incorporation of these precision parameters in the assessment model is tested, through a stock assessed by statistical assessment models (such as SS3) which can account for sampling errors. Thus, it will be possible to quantify how errors in input data propagate through stock assessment models to affect harvest rules, and also to help identify the most cost-effective data collections that adequately support the advisory process
EP-1191: Study and validation of a deterministic model for energetic particles transport. Application in radiotherapy.
Simulations of protostellar collapse using multigroup radiation hydrodynamics. I. The first collapse
Radiative transfer plays a major role in the process of star formation. Many
simulations of gravitational collapse of a cold gas cloud followed by the
formation of a protostellar core use a grey treatment of radiative transfer
coupled to the hydrodynamics. However, dust opacities which dominate extinction
show large variations as a function of frequency. In this paper, we used
frequency-dependent radiative transfer to investigate the influence of the
opacity variations on the properties of Larson's first core. We used a
multigroup M1 moment model in a 1D radiation hydrodynamics code to simulate the
spherically symmetric collapse of a 1 solar mass cloud core. Monochromatic dust
opacities for five different temperature ranges were used to compute Planck and
Rosseland means inside each frequency group. The results are very consistent
with previous studies and only small differences were observed between the grey
and multigroup simulations. For a same central density, the multigroup
simulations tend to produce first cores with a slightly higher radius and
central temperature. We also performed simulations of the collapse of a 10 and
0.1 solar mass cloud, which showed the properties of the first core to be
independent of the initial cloud mass, with again no major differences between
grey and multigroup models. For Larson's first collapse, where temperatures
remain below 2000 K, the vast majority of the radiation energy lies in the IR
regime and the system is optically thick. In this regime, the grey
approximation does a good job reproducing the correct opacities, as long as
there are no large opacity variations on scales much smaller than the width of
the Planck function. The multigroup method is however expected to yield more
important differences in the later stages of the collapse when high energy (UV
and X-ray) radiation is present and matter and radiation are strongly
decoupled.Comment: 9 pages, 5 figures, accepted for publication in A&
Проблеми інноваційного розвитку підприємств вугільної галузі України
Метою статті є визначення проблем інноваційного розвитку вугільної галузі та напрями його забезпечення шляхом упровадження інновацій
Porosity and density of spark-processed silicon
Abstract Spark-processed Si (sp-Si) is a porous solid-state material. Due to the nature of its structure and morphology, the traditional methods for porosity measurements cannot be utilized. Using the measure theory and the expected value theorems of stereology, we have calculated the porosity of sp-Si to be 43%. Stereological analysis was applied to sp-Si specimen, prepared within a fixed set of growth parameters. Over 60 cross-sectional scanning electron micrographs of the specimen were utilized in this work. The sp-Si sample has a characteristic cylindrical symmetry due to the uniform surface resistance of the Si substrate and to the random nature of spark processing. However, sp-Si is not isotropic, uniform and random (IUR), exhibiting radial and axial anisotropy of porosity. To avoid bias in the calculation, we chose random areas of the cross-sectional surface of sp-Si and calculated their porosities. The calculated values entered into a weighted statistical distribution, in which the statistical weights were determined from the symmetry properties of the sample. The statistical approach and the fact that volume is an additive quantity, allowed us to use a two-dimensional population of points in the calculation of the three-dimensional pore volume fraction and to satisfy the requirement for IUR sample. In the course of the present work we examined fourteen sp-Si samples, prepared under different processing conditions. Ten of these samples were studied qualitatively by measuring the area of the pores relative to the total area in a cross-sectional cut of the sample. Four samples were studied quantitatively using the stereological method outlined above and exhibited porosities in the neighborhood of 43%. One of these studies is described in detail in the present paper and provides a consistent value for the porosity of sp-Si materials, processed in air. Small-spot X-ray photoelectron spectroscopy studies of sp-Si were used in the calculation of its density. In the case of inhomogeneous materials, the density is a weighted (with respect to volume) average of the densities of all participating phases. Taking into account the already calculated porosity, we have estimated the density of sp-Si to be 1.36 g/cm 3 . The main contribution to this value comes from amorphous SiO 2 , which occupies most of the volume of sp-Si
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