Tides in colliding galaxies

Long tails and streams of stars are the most noticeable upshots of galaxy collisions. Their origin as gravitational, tidal, disturbances has however been recognized only less than fifty years ago and more than ten years after their first observations. This Review describes how the idea of galactic tides emerged, in particular thanks to the advances in numerical simulations, from the first ones that included tens of particles to the most sophisticated ones with tens of millions of them and state-of-the-art hydrodynamical prescriptions. Theoretical aspects pertaining to the formation of tidal tails are then presented. The third part of the review turns to observations and underlines the need for collecting deep multi-wavelength data to tackle the variety of physical processes exhibited by collisional debris. Tidal tails are not just stellar structures, but turn out to contain all the components usually found in galactic disks, in particular atomic / molecular gas and dust. They host star-forming complexes and are able to form star-clusters or even second-generation dwarf galaxies. The final part of the review discusses what tidal tails can tell us (or not) about the structure and content of present-day galaxies, including their dark components, and explains how tidal tails may be used to probe the past evolution of galaxies and their mass assembly history. On-going deep wide-field surveys disclose many new low-surface brightness structures in the nearby Universe, offering great opportunities for attempting galactic archeology with tidal tails.Comment: 46 pages, 13 figures, Review to be published in "Tidal effects in Astronomy and Astrophysics", Lecture Notes in Physics. Comments are most welcom

Influence of variable permeability on combined free and forced convection about inclined surfaces in porous media

The analysis is carried out for mixed convection about inclined surfaces in a saturated porous media incorporating the variation of permeability and thermal conductivity due to packing of particles. Similarity solutions are obtained, for two cases, namely (i) uniform wall temperature, (ii) linear variation of temperature with distance from the leading edge, for both aiding and opposing flows. It is found that the non dimensional parameters Gr/Re2 and Ï 2/Re control the flow. The variation of permeability increases heat transfer rate for all values of Ï 2/Re. Applications to convective flow in a liquid dominated geothermal reservoir are discussed. Criteria are given for flows which are purely forced, purely free and mixed. © 1985

Similarity solutions for buoyancy induced flows in a saturated porous medium adjacent to impermeable horizontal surfaces

In the present paper similarity solutions for the convective flow induced by buoyancy in a saturated porous medium adjacent to horizontal impermeable surfaces are obtained. The analysis incorporates the variation of permeability from the wall and expressions for boundary layer thickness, local and overall surface heat-flux are obtained. Applications of the results to convective flows in a geothermal reservoir are discussed. © 1984 Springer-Verlag