........ published in NEWSLETTER # 67
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........ published in NEWSLETTER # 67
PHYSICS OF DRY GRANULAR MEDIA
By Professors H.J. Herrmann, J.-P. Hovi, and S. Luding, Institute for Computer Applications, University of Stuttgart (Germany)
Granular media are omnipresent in everydays life and play an important role in industry and engineering. Sand, rice, coffee, and pills are examples of the class of `dry', i.e. cohesionless, granular matter - the main subject of the conference proceedings (NATO ASI SERIES E350).
The effects of interstitial fluids, air, cohesive forces or particles in fluids (sedimentation) were only touched on marginally. They appear naturally in the formation of dunes, the hiccup of hour glasses or the moving clogs in pipes.
The NATO ASI summer school in Cargese, France brought together international specialists and students from various disciplines e.g. physics, chemical engineering, and soil mechanics. The aim of the proceedings was to give a didactical introduction to the field and to document the state-of-the-art reasearch.
In the last ten years the physicist's interest in the study of dry granular materials reawakened from a deep hibernation which lasted most of this century. Many new ideas concerning this old subject have emerged from this recent rapid development. The renaissance of the subject has been due to various things. On the one hand new concepts had been developed to describe collected phenomena in disordered media. They include the self-organised criticality introduced by Bak and collaborators in 1987. On the other hand modern computers have enabled the stimulation of many particle systems of sufficient size to be compared to experimental observations. In addition, new algorithms and novel stochastic modelisations have been developed.
Unfortunately, the rapid developments in the last years have often neglected the huge amount of work, published mainly in the engineering literature, that has been done in soil mechanics, chemical engineering and many other applied disciplines. These `classical theories" yield in many cases accurate quantitative predictions and are used in commercial codes in many places. The present school was aimed at bringing together the two efforts. On the one hand we had lectures on classical theories like the non- associate Mohr-Coulomb plasticity used in soil mechanics or the kinetic gas theory developed for rapid granular flow. These lectures gave scientists a solid basis on the existing approaches. On the other hand we had many seminars in which young researchers presented their ideas which were often unconventional and their criticism by the experts in the field gave rise to many discussions.
The school covered roughly all the aspects involved in the statics and dynamics, slow and rapid, of dry granular media. The individual contact between two grains was discussed in detail. Much emphasis was given to the problem of force propagation through a static grain packing, in particular given consideration to the texture. Slow deformations of granular packings were also studied in detailed experiments and simulations, giving information on local rotation and the internal structure of shear bands.
The inelastic nature of collisions is due to dissipation and friction. The first step for many discrete models, or kinetic theories, is thus the two particle interaction. In multi-particle systems, the velocity distributions need corrections to classical fluid (or gas) approaches. Novel modelisations by continuum equations for granular matter flowing down on heap surfaces have given rise to expressions for the shape of the heaps. Vibrated granular media continue to astonish. We discussed in detail convection, segregation, compaction and surface waves that appear in different regimes of amplitude and frequency. Segregation is a central effect in granular materials and was also discussed in shear cells, rotating cylinders or on heaps (stratification). The latest developments of the kinematic gas theory for segregation were presented.
All these subjects are discussed in the various contributions to the book. They convey the scientific content of the NATO-Advanced Study Institute `Physics of Dry Granular Media" in Cargese. What cannot be transmitted through the proceedings are the numerous hands-on experiments, the spontaneous experiments on the beach and the multiple discussions in a relaxed atmosphere. Despite the dense scientific program there was much time for creative brain storming, but also for socialising and enjoying of the beautiful setting of Cargese.
Reference Books: B225, B236, C229, E152, E220, E274, E287, E311, E350