共查询到20条相似文献，搜索用时 31 毫秒
1.
The equation of radiative transfer for an inhomogenous dispersive finite medium subject to general boundary conditions is solved. The Padé approximation technique is used to calculate the angular distribution of radiation. Numerical results for the [0/1] Padé approximant lead to numerical results that compared with the exact results. 相似文献
2.
The particle escape probability from a non-multiplying slab containing an internal source is defined in terms of a functional relation for the scattering function for the diffuse reflection problem. The Padé approximation technique is used to obtain numerical results for particle escape probability for inhomogeneous medium. Numerical results for homogenous and inhomogeneous media are given. 相似文献
3.
Equations for the angular density of radiation, reflected and transmitted intensities associated with radiation scattered by inhomogeneous dispersive media are obtained. The Padé approximant technique is used to calculate these intensities in inhomogeneous and homogeneous media. The results for the [0/1] Padé approximant lead to numerical results that compared with the exact results. 相似文献
4.
Exact relations for radiation heat flux at the boundaries of a slab with diffusely reflecting boundary conditions and internal source are obtained in terms of the reflection and transmission coefficients of a source free slab with isotropic boundary conditions. The integral equation defining the radiation heat flux contains explicitly the internal source. So, the particular solution for radiative transfer equation is not required. Available exact values for albedos give exact values of radiation heat flux. Padé approximant technique is used to obtain numerical values for homogenous media. 相似文献
5.
E. A. Saad S. A. El Wakil M. H. Haggag H. Machali 《Astrophysics and Space Science》1984,100(1-2):53-64
The mean number of particle scattering in a semi-infinite and finite slab for arbitrary energy sources are calculated by Padé approximants. Uniform source distribution is considered for the finite slab. For the semi-infinite medium we calculatethe number of particles due external radiation and at a particular point in the medium. Numerical calculations and comparisons were performed. 相似文献
6.
E. A. Saad S. A. El Wakil H. M. Haggag H. M. Machali 《Astrophysics and Space Science》1986,125(1):1-11
Equations for radiation, scattered anisotropically from dispersive media are obtained for general boundary conditions. The Padé approximant technique is used to calculate the reflection coefficients for these media. Numerical results are given for diffuse reflected boundary condition. 相似文献
7.
The maximum-entropy technique is used to solving three problems in radiative transfer and reactor physics involving spherical geometry. These problems are: i) Luminosity or the total energy emitted by a sphere, ii) Neutron capture probability, iii) The albedo problem. Numerical calculations are done and compared with the exact values as well as with the Padé's approximant results. The comparisons show that the maximum-entropy results are very good and converge to the exact results. 相似文献
8.
S. A. El Wakil H. Machali M. T. Attia E. A. Saad 《Astrophysics and Space Science》1987,137(2):239-250
Radiation transfer problem in the slowing-down region is solved for a spherical geometry. Modelled kernels are used to represent the slowing-down kernel. Calculations are performed for the radiation flux at the boundary of the sphere for two cases: (i) in the presence of internal source, (ii) if the radiation is incident on the sphere. Numerical results are obtained by using the Padé approximant technique. 相似文献
9.
A. R. Degheidy 《Astrophysics and Space Science》1993,202(1):81-87
The maximum entropy method is used to find the energy albedo for half-space, i.e., the classical albedo problem but extended to include energy dependence. The slowing-down kernel is described by elastic scattering and various model representations for dealing with anisotropic scattering. The energy albedo for different values of secondary particles per collisionC is calculated and compared with exact and variational values given by Williams and Padé's approximant given by Machaliet al. (1981). Also the energy albedo for different mass-ratios is found and compared with Williams and Padé's results. 相似文献
10.
Masahiro Nagashima & Naoteru Gouda 《Monthly notices of the Royal Astronomical Society》1998,301(3):849-860
The effects of merging histories of proto-objects on the angular momentum distributions of the present-time dark matter haloes are analysed. An analytical approach to the analysis of the angular momentum distributions assumes that the haloes are initially homogeneous ellipsoids and that the growth of the angular momentum of the haloes halts at their maximum expansion time. However, the maximum expansion time cannot be determined uniquely, because in the hierarchical clustering scenario each progenitor, or subunit, of the halo has its own maximum expansion time. Therefore the merging history of the halo may be important in estimating its angular momentum. Using the merger tree model by Rodrigues &38; Thomas, which takes into account the spatial correlations of the density fluctuations, we have investigated the effects of the merging histories on the angular momentum distributions of dark matter haloes. It was found that the merger effects, that is, the effects of the inhomogeneity of the maximum expansion times of the progenitors which finally merge together into a halo, do not strongly affect the final angular momentum distributions, so that the homogeneous ellipsoid approximation happens to be good for the estimation of the angular momentum distribution of dark matter haloes. This is because the effect of the different directions of the angular momenta of the progenitors cancels out the effect of the inhomogeneity of the maximum expansion times of the progenitors. The contribution of the orbital angular momentum to the total angular momentum when two or more pre-existing haloes merge together was also investigated. It is shown that this contribution is more important than that of the angular momentum of diffuse accreting matter to the total angular momentum, especially when the mergers occur many times. 相似文献
11.
K. Stamnes 《Planetary and Space Science》1980,28(4):427-441
The interaction of a beam of auroral electrons with the atmosphere is described by the linear transport equation, encompassing discrete energy loss, multiple scattering and secondary electrons. A solution to the transport equation provides the electron intensity as a function of altitude, pitch angle (with respect to the geomagnetic field) and energy. A multi-stream (discrete ordinate) approximation to the transport equation is developed. An analytic solution is obtained in this approximation. The computational scheme obtained by combining the present transport code with the energy degradation method of Swartz (1979) conserves energy identically. The theory provides a framework within which angular distributions can be easily calculated and interpreted. Thus, a detailed study of the angular distributions of “non-absorbed” electrons (i.e., electrons that have lost just a small fraction of their incident energy) reveals a systematic variation with incident angle and energy, and with penetration depth. The present approach also gives simple yet accurate solutions in low order multi-stream approximations. The accuracy of the four-stream approximation is generally within a few per cent, whereas two-stream results for backscattered mean intensities and fluxes are accurate to within 10–15%. 相似文献
12.
The problem of wave-propagation in a magnetically structured compressible slab configuration is investigated, allowing for different magnetic field strengths inside and outside the slab and also a general orientation of the field vectors relative to each other and to the propagation vector. Several magnetic field geometries such as equal parallel, and equal orthogonal fields are considered. Properties of body and surface waves both for symmetric and asymmetric modes of perturbation propagating along and normal to the slab field are investigated idealising the slab to be incompressible, or considering the limiting case of wide and slender compressible slab. Numerical results are also obtained for a compressible slab of finite thickness for a specific choice of sound and Alfvén speeds involved. 相似文献
13.
S. S. Ipson V. V. Zharkova S. Zharkov A. K. Benkhalil J. Aboudarham N. Fuller 《Solar physics》2005,228(1-2):399-421
We present an automated technique for comparison of magnetic field inversion-line maps from SOHO/MDI magnetograms with solar
filament data from the Solar Feature Catalogue created as part of the European Grid of Solar Observations project. The Euclidean
distance transform and connected component labelling are used to identify nearest inversion lines to filament skeletons. Several
filament inversion-line characteristics are defined and used to automate the decision whether a particular filament/inversion-line
pair is suitable for quantitative comparison of orientation and separation. The technique is tested on 551 filaments from
14 Hα images at various dates, and the distributions of angles and distances between filament skeletons and line-of-sight
(LOS) magnetic inversion lines are presented for six levels of magnetic field smoothing. The results showed the robustness
of the developed technique which can be applied for a statistical analysis of magnetic field in the vicinity of filaments.
The method accuracy is limited by the static filament detection which does not distinguish between filaments, fibrils, pre-condensations
and filament barbs and this may increase the asymmetries in magnetic distributions and broadening in angular distributions
that requires the incorporation of a feature tracking technique. 相似文献
14.
Frank C. van den Bosch reas Burkert Rob A. Swaters 《Monthly notices of the Royal Astronomical Society》2001,326(3):1205-1215
We compute the specific angular momentum distributions for a sample of low-mass disc galaxies observed by Swaters. We compare these distributions to those of dark matter haloes obtained by Bullock et al. from high-resolution N -body simulations of structure formation in a ΛCDM universe. We find that although the disc mass fractions are significantly smaller than the universal baryon fraction, the total specific angular momenta of the discs are in good agreement with those of dark matter haloes. This suggests that discs form out of only a small fraction of the available baryons, but yet manage to draw most of the available angular momentum. In addition we find that the angular momentum distributions of discs are clearly distinct from those of the dark matter; discs lack predominantly both low and high specific angular momenta. Understanding these findings in terms of a coherent picture for disc formation is challenging. Cooling, feedback and stripping, which are the main mechanisms to explain the small disc mass fractions found, seem unable to simultaneously explain the angular momentum distributions of the discs. In fact, it seems that the baryons that make up the discs must have been born out of angular momentum distributions that are clearly distinct from those of ΛCDM haloes. However, the dark and baryonic mass components experience the same tidal forces, and it is therefore expected that they should have similar angular momentum distributions. Therefore, understanding the angular momentum content of disc galaxies remains an important challenge for our picture of galaxy formation. 相似文献
15.
Jun-Hwan Choi Yu Lu H. J. Mo Martin D. Weinberg 《Monthly notices of the Royal Astronomical Society》2006,372(4):1869-1874
We consider the sensitivity of the circular-orbit adiabatic contraction approximation to the baryon condensation rate and the orbital structure of dark matter haloes in the Λ cold dark matter (ΛCDM) paradigm. Using one-dimensional hydrodynamic simulations including the dark matter halo mass accretion history and gas cooling, we demonstrate that the adiabatic approximation is approximately valid even though haloes and discs may assemble simultaneously. We further demonstrate the validity of the simple approximation for ΛCDM haloes with isotropic velocity distributions using three-dimensional N -body simulations. This result is easily understood: an isotropic velocity distribution in a cuspy halo requires more circular orbits than radial orbits. Conversely, the approximation is poor in the extreme case of a radial orbit halo. It overestimates the response of a core dark matter halo, where radial orbit fraction is larger. Because no astronomically relevant models are dominated by low angular momentum orbits in the vicinity of the disc and the growth time-scale is never shorter than a dynamical time, we conclude that the adiabatic contraction approximation is useful in modelling the response of dark matter haloes to the growth of a disc. 相似文献
16.
Different methods are proposed and tested for transforming a nonlinear differential system, and more particularly a hamiltonian one, into a map without having to integrate the whole orbit as in the well known Poincaré map technique. We construct piecewise polynomial maps by coarse-graining the phase surface of section into parallelograms using values of the Poincaré maps at the vertices to define a polynomial approximation within each cell. The numerical experiments are in good agreement with the standard map taken as a model problem. The agreement is better when the number of vertices and the order of the polynomial fit increase. The synthetic mapping obtained is not symplectic even if at vertices there is an exact interpolation. We introduce a second new method based on a global fitting . The polynomials are obtained using at once all the vertices and fitting by least square polynomes but in such a way that the symplectic character is not lost. 相似文献
17.
I. Stellmacher 《Celestial Mechanics and Dynamical Astronomy》1976,13(2):177-201
Résumé L'auteur étudie le mouvement autour du centre de gravité d'un satellite aimanté dans le champ magnétique terrestre. Le moment magnétique est de l'ordre de 10 Amp m2 et il est dirigé suivant l'axe 0z du satellite portant le plus petit moment d'inertie. La rotation initiale du satellite est lente. L'évolution d'attitude peut être étudiée lorsque l'angle entre le moment cinétiqueL et l'axe 0z reste faible. En utilisant des variables appropriées, on est amené à résoudre un système d'équations différentielles dont la partie principale est un système linéaire à coefficients périodiques.La théorie de Floquet permet de résoudre la partie linéaire; l'application au premier ordre de la méthode de Krilov-Bogolioubov, permet de trouver les fréquences du système non linéaire.
The movement, in the Earth's magnetic field, of a magnetized satellite around its center of gravity is investigated. The magnetic moment is of the order of 10 Amp m2, and its direction is that of the principal axis of smallest inertia 0z of the satellite. The initial spin is low. The movement around the center of mass can be described if the angle between the angular momentL and the axis 0z is small. The use of appropriate variables, leads to a system of differential equations where the main part is a system of linear equations with periodic coefficients.The theory of Floquet enables a solution of the linear system; the application of the first order approximation by Krilov-Bogolioubov allows one to find the frequencies of the nonlinear system.相似文献
18.
Jonas Lundberg 《Solar physics》1994,154(2):215-230
The weakly nonlinear wave propagation of a slow sausage surface wave traveling along a magnetized slab with a thin nonuniform boundary layer is considered. The ideal incompressible MHD equations are used and the nonlinearities are assumed to be due to second harmonic generation. A nonlinear dispersion relation and the related nonlinear Schrödinger equation is derived. The existence of a continuous thin interface leads to sharply peaked field amplitudes due to resonant interaction with local Alfvén waves. It is shown that the nonlinear effects from processes within the thin layer are much more important than those from the main slab. Furthermore, the nonlinear interaction with local Alfvén waves yields a nonlinear damping rate of the wave that is much larger than the linear damping rate when the transition layer is sufficiently thin. 相似文献
19.
Michael Reinhardt 《Astrophysics and Space Science》1971,10(3):363-382
Brown's results (1964, 1968) concerning the distribution of orientation angles of spiral galaxies in different areas of the sky are discussed and a graphical statistical test is applied. The deviations from randomness are found to be significant. It seems difficult to ascribe them to selection effects. It is shown that the observed distributions can be explained, if the angular momenta in greater aggregations of galaxies are distributed at random on congruent precession cones with parallel axes. This hypothesis may apply if the following cosmological conditions hold:
- the matter in the universe was reheated after the recombination at an epoch, when most of the angular momentum was already transferred to the protogalaxies by tidal interaction, and the angular momenta of the protogalaxies in greater aggregations were predominantly parallel at the epoch of reheating;
- the ‘magnetic’ model of the universe is valid and the ‘urfield’ was uniform at least at the epoch of reheating. Under these assumptions, the ‘frozen-in’ magnetic field will give rise to forces, which — apart from slowing down the rotation of the protogalaxies — will cause precession of their angular momenta around the direction of the ‘urfield’.
20.
Wolfgang Glatzel Carsten Obach 《Monthly notices of the Royal Astronomical Society》1999,308(1):147-152
The hydrodynamic interaction of an accretion disc with its central object is reanalysed within the framework of the slim-disc approximation. Arguments are presented against an interpretation of the total angular momentum flux as an eigenvalue of the system. A simple intuitive consideration is provided, which shows that the central object may be in a state of stationary rotation even if the disc imposes the constraint of a finite angular momentum flux into it. It is argued that equilibrium rotation is characterized by vanishing viscous torque rather than by zero total angular momentum flux. As a consequence, the central object can be in a state of stationary rotation below the break-up limit, although its angular momentum increases. Despite accretion, even for positive total angular momentum flux and subcritical rotation, central objects are spun down within a considerable range of their parameters. The results are illustrated by application to FU Orionis systems. 相似文献