Numerical study of boundary conditions for solute transport through a porous medium

A transition region may be defined as a region of rapid change in medium properties about the interface between two porous media or at the interface between a porous medium and a reservoir. Modelling the transition region between different porous media can assist in the selection of the most appropriate boundary conditions for the standard advection–dispersion equation (ADE). An advantage of modelling the transition region is that it removes the need for explicitly defining boundary conditions, though boundary conditions may be recovered as limiting cases. As the width of a transition region is reduced, the solution of the transition region model (TR model) becomes equivalent to the solution of the standard ADE model with correct boundary conditions. In this paper numerical simulations using the TR model are employed to select the most appropriate boundary conditions for the standard ADE under a variety of configurations and conditions. It is shown that at the inlet boundary between a reservoir and porous medium, continuity of solute mass flux should be used as the boundary condition. At the boundary interface between two porous media both continuity of solute concentration and solute mass flux should be used. Finally, in a finite porous medium where the solute is allowed to advect freely from the exit point, both continuity of solute concentration and solute mass flux should be used as the outlet boundary condition. The findings made here are discussed with reference to a detailed review of previous relevant theoretical and experimental observations. Copyright © 2001 John Wiley & Sons, Ltd.     

An interpretation of the semicircular structure of the supernova remnant CTB-109

The supernova exploded at the boundary of a dense molecular cloud in a diffuse gas. The eastern half of the shock wave entered the diffuse gas and is now in the stage of adiabatic expansion, forming the observed semicircular remnant; the western half entered the dense cloud, moved at a much slower speed and is now in the pressure-driven snowplow phase, the radiation it emits is in the uv, and is completely absorbed by the interstellar gas along the line of sight.     

Ultracompact HII regions: the impact of newly formed massive stars on their environment

Summary Ultracompact (UC)HII regions are manifestations of newly formed massive stars that are still embedded in their natal molecular cloud. They are among the brightest and most luminous single objects in the Galaxy at far infrared and radio wavelengths. Recent high spatial resolution studies, particularly at radio wavelengths, have greatly contributed to our understanding of these dynamic objects and the impact they have on their environment. A summary is given of our current understanding of the physical properties, morphologies, dynamics, number and distribution in the Galaxy, and molecular environments of UCHII regions. Recent models of the circumnebular dust imply that the graphite/silicate abundance ratio is about half that of dust in the diffuse interstellar medium. The dust cocoons are large, cool, and optically thick shortward of a few microns. There are apparently between 1700 and 3000 UCHII regions in the Galaxy. This represents 10–20% of the total O star population. There are too many UCHII regions (just counting those studied with the VLA) to be consistent with the short dynamical lifetimes of this very compact stage of evolution. Both the morphologies and the large number can be understood if UC HII regions are bow shocks. Models of stellar wind supported bow shocks are discussed and consequences for the dynamics and morphologies of the ionized and molecular gas are explored.     

Detection of H2 emission towards the wolf-rayet nebula NGC 2359

We report the first detection of molecular hydrogen emission in the vicinity of a Wolf-Rayet star and nebula. The spatial distribution of the excited molecular gas is filamentary and is not correlated with the distribution of the ionised gas as traced by optical emission lines. The typical H₂ surface brightness in the filaments is 5× 10^–5 ergs s^–1 cm^–2 str^–1. We demonstrate that the excitation mechanism can be shocks or fluorescence from the strong ultraviolet flux of the WR star.     

A self-consistent model for the S140/L1204 region

Comparison of submillimetre continuum observations of the L1204/S140 complex with previous high resolution CS, NH₃ and CI observations provides evidence that, for the first time, demonstrates the PDR and outflow are intimately linked. The only scenario that is able to explain all of the available molecular and atomic emission line data and our submillimetre continuum data, is one in which the outflow has expanded towards the edge of the molecular cloud and the edge of the blueshifted outflow lobe is now bounded by the expanding HII region. The NH₃ and continuum emission emanate from the inner edge of the outflow lobe, shielded from the external UV field.