| Summary: | Underwater avalanches can generate deadly tsunamis and damage oil platforms. But understanding the threat has been difficult because of the complexities of water-grain mixtures. A new two-dimensional model simulates the collapse of a tower of grains immersed in a fluid by computing the forces on each grain from the fluid and from other grains. The results—presented in Physical Review Letters—show that the presence of the fluid has two conflicting effects: it restrains the collapse but then lubricates the grains as they tumble outwards. The model may one day help in predicting risk in underwater environments as well as optimizing wet processing techniques used in the food and steel industries.In 1929, a seafloor avalanche triggered by an earthquake off the coast of Newfoundland swept debris out over 400 miles, snapping 12 transatlantic telegraph cables along its path. Such a long “runout” distance is common in submarine landslides. But the explanation for the long runout is unclear because researchers have focused more on “dry” avalanches and landslides. Wet grains are harder to model, since the fluid can restrain grain motion through cohesion and drag, while also helping the grains slide past each other. Previous studies have often simplified the wet-grain problem by adding a small number of grains (or some granular effects) to a fluid flow, or by adding some fluid effects to a granular flow. However, many grain-fluid mixtures in underwater environments and industrial situations have nearly equal parts solid and liquid. Farhang Radjaï of the University of Montpellier 2 in France and his colleagues have developed a computational model of granular flows in a fluid environment. As in similar work with dry grains, they concentrate on a specific situation in which a tower or column of grains falls under the influence of gravity [1]. The team divided this collapse into discrete time steps at which they calculated the forces on every grain and the overall motion of the fluid. The method builds on earlier simulations of ...
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