Or even slopes of snow for tired of cold skiers

For physicists, dunes are not only fantasies for Western hikers enhancers. Or even slopes of snow for tired of cold skiers. What are the pile of sand. Not to conclude not that the subject left marble. On the contrary, some physicists are enamored of the dune. Because far from being trivial, this granular medium puzzles: "we are still not able to predict the angle of a pile of sand", provides Stéphane Douady. Researcher of the Ecole normale supérieure is part of the small community of scientists in the physics of the dunes. It is in their assaults that the waves of sand that haunt the arid deserts seas begin to deliver their mysteries.

On a small scale, the sand is a scientific nightmare because it pastes poorly or not at all the classical theories of mechanics. Its grains are solid, but in large numbers, they defy the ordinary kinematics. Its flows recall liquids but do not comply with the mechanics of the Newtonian fluids using the hydrodynamicians. On a large scale, researchers must take into account the ballet between the dunes and foster winds that result in feedback. Because winds generate and move the dunes but they also affect the flow of air.

Geomorphological modeling and field observations, researchers argue: "We understood the logic of their forms," said Stéphane Douady. For three years, his team has been shedding constraints of observation by the reconstitution of miniature sand dunes in the laboratory. As in a wind tunnel, a hydrodynamic pool simulates the advent of a dune scale 1/1000e.

Physicists were first studied the model of the barchane, a popular wind dune of the physicists for the originality of its form (the Horn of the gourmet gazelle) and mobility. She was born in deserts where reign unidirectional wind and think, for the smaller, 100 metres per year. Shape into growing stems from the movement of the sand, under the action of wind, from the back exposed the dune to the belly. The air stream accumulates snowdrifts of sand on the Ridge, then causing avalanches on the forward slope. The existence of the horns is it a subtle balance: the edge of the dune is more, it moves faster, but is at the same time more erosion of the wind which slows down.

The size of the barchanes varies from ten metres wide for 2 metre up to 100 metres by 10 metres high. It is known that a dune must have a minimum size below which the sand 'flees"by the horns of the dune.

In recent years, researchers have revised their theories that broken not long term behaviour dunes. Their calculations provided the systematic decline of the dunes because of the loss of sand on the sides. In fact, the dune fields cause regular collisions in which the barchanes reload in sand.

At the time, researchers know model the long wandering to fastest dunes. As nomads, they glide tirelessly along the ergs. Some are born to the South of the Morocco and drag their sand on several hundreds of kilometres to Mauritania. This last area is a giant dune cemetery. Satellite photos show that the average of the wind grows the sands of Egypt or Libya to the Southwest. Elsewhere, we have seen, as in the Chile, these barchanes now climb mountains which they then Backstreets flanks.

The researchers identified three other types of dunes. The more massive and more sedentary are dunes of Namibia stars, works from the turning winds. Some have 10,000 years old. There is also the transverse dunes, long ridges pushed by the wind. If the winds change from one season to another, dunes place their Crest longitudinally to the air stream, they change appearance.

Researchers know now reproduce in the laboratory all forms of dunes, by turning their hydrodynamic tunnel currents. They are now interested in more specific phenomena such as the famous song of dunes. In some areas, including the Morocco, the sand sings for its avalanches a form of lament that already one Marco Polo and Guy de Maupassant. The less poetic hikers will speak rather of a grommellement. The melody remains, it is true, rather limited and its frequency reaching sometimes 100 decibels, close to the threshold of pain. It still does not understand why but we know that it must be hot sand and its grain are covered with a film of glaze. Stéphane Douady prepares to exit CD compilation of acoustic events. The sand and its harbingers.