Cylindrically symmetric and inhomogeneous cosmological models with viscous fluid,heat flux and electromagnetic field

In this paper cylindrically symmetric, inhomogeneous cosmological models with heat flux, viscous fluid and electromagnetic field are obtained. Various physical and geometrical properties of the models are also discussed.     

Energy albedo for multiregion slabs

An asymptotic solution for the equation of radiative transfer in an inhomogeneous medium was obtained on the basis of the corresponding solutions for homogeneous sub-layers in the slowing down region. Function relations between the reflection and transmission coefficients for the whole slab and those of the sublayers are given. The invariant embedding concepts are used to get the reflection and transmission coefficients for the sub-layers. We assumed different models for the slowing-down kernels. Laplace transform was used to transform the Boltzmann equation to one velocity approximation with re-scaled mean-free path and single-scattering albedo. Numerical results are given for energy albedo as a function of the mass number of the host medium.     

Statistical relationship between sunspots and major flares

In this study, the statistical relationship between sunspots and major flares observed in the descending branch of the 20th and in the ascending branch of the 21st solar cycle is evaluated. It is found that the sunspots which produced these major flares are of the type Dki or Eki with magnetic class D and the largest magnetic field strength between 1600 and 2500 G.     

High resolution spectra of galaxies

We present inversion techniques which aim at recovering the composite nature and the kinematics of a stellar population from its high resolution absorption line spectrum. The originality of the combined inversion is its potential to recover both the stellar content and the kinematics simultaneously. These techniques use new synthetic high resolution spectra produced by PéGASE and minimization algorithms. We apply them to mock data representing the bulge and disk population of the inner region of spiral galaxies. This revised version was published online in August 2006 with corrections to the Cover Date.     

The Near-Nuclear Coma of Comet Halley in March 1986

The cameras carried onboard the flyby missions to comet P/Halleyin 1986 imaged the near nuclear jet activity fromseveral spatial directions. The observed, very structured near nucleardust jets were considered at that timeas the result of dust emission from well localized active surface regions(without supporting 3-D model computations, however).Based on the first, recently developed 3-D gas dynamical model ofP/Halley's activity,we have been shown that jet features can be reproduced assuming ahomogeneous dusty icenucleus surface. The dust in the collisional near nuclear comais concentrated along the gas flow discontinuities resulting from thecomplicated surface orography, creating the visual impression ofdust jets. We present here the results of these calculations forthe near nucleus dust distributions,and we compare them with the direct observations made during thethree Halley flybys (Vega 1, Vega 2, and Giotto).     

Comparative assessment of two distributed watershed models with application to a small watershed

Distributed watershed models are beneficial tools for the assessment of management practices on runoff and water‐induced erosion. This paper evaluates, by application to an experimental watershed, two promising distributed watershed‐scale sediment models in detail: the Kinematic Runoff and Erosion (KINEROS‐2) model and the Gridded Surface Subsurface Hydrologic Analysis (GSSHA) model. The physics behind each model are to some extent similar, though they have different watershed conceptualizations. KINEROS‐2 was calibrated using three rainfall events and validated over four separate rainfall events. Parameters estimated by this calibration process were adapted to GSSHA. With these parameters, GSSHA generated larger and retarded flow hydrographs. A 30% reduction in both plane and channel roughness in GSSHA along with the assumption of Green‐Ampt conductivity K_G‐A = K_s, where K_s is the saturated conductivity, resulted in almost identical hydrographs. Sediment parameters not common in both models were calibrated independently of KINEROS‐2. A comparative discussion of simulation results is presented. Even though GSSHA's flow component slightly overperformed KINEROS‐2, the latter outperformed GSSHA in simulations for sediment transport. In spite of the fact that KINEROS‐2 is not geared toward continuous‐time simulations, simulations performed with both models over a 1 month period generated comparable results. Copyright © 2006 John Wiley & Sons, Ltd.     

Structure of Molecular Clouds

This review, far from exhaustive, focuses on the specific properties of molecular clouds in their least condensed phase. Emphasis is placed on their connection with the diffuse medium, their multi-fractal characteristics and the diversity of their small-scale structure, where cold and dense gas may coexist with dilute and warm gas. Stuctures of essentially pure velocity origin are uncovered and seen as a possible signature of the intermittency of turbulence dissipation.     

The rotation number of bounded orbits in a central field

After reviewing known results on bounded motions in a central field and on the angle between two consecutive apocenters, we investigate the limit of the orbits as the pericenter tends to the origin. A ‘deflection’ of the almost rectilinear orbits is found which depends on the shape of the potential at the origin. We then study the rotation numbers of all, bounded motions in a field homogeneous inr. Finally on a few examples we show how to represent the set of bounded motions in an inhomogeneous field.     

Solute transport in sand and chalk: a probabilistic approach

A probabilistic approach is used to simulate particle tracking for two types of porous medium. The first is sand grains with a single intergranular porosity. Particle tracking is carried out by advection and dispersion. The second is chalk granulates with intergranular and matrix porosities. Sorption can occur with advection and dispersion during particle tracking. Particle tracking is modelled as the sum of elementary steps with independent random variables in the sand medium. An exponential distribution is obtained for each elementary step and shows that the whole process is Markovian. A Gamma distribution or probability density function is then deduced. The relationships between dispersivity and the elementary step are given using the central limit theorem. Particle tracking in the chalky medium is a non‐Markovian process. The probability density function depends on a power of the distance. Experimental simulations by dye tracer tests on a column have been performed for different distances and discharges. The probabilistic approach computations are in good agreement with the experimental data. The probabilistic computation seems an interesting and complementary approach to simulate transfer phenomena in porous media with respect to the traditional numerical methods. Copyright © 2006 John Wiley & Sons, Ltd.